utilization CE Counter electrode CoPc Cobalt pthalocyanine CO2RR Carbon dioxide reduction reaction CNT Carbon nanotube Cp* Pentamethylcyclopentadienyl CPE Constant potential electrolysis CV Cyclic voltammetry

CCE

Constant current electrolysis CCS Carbon capture and storage CCU Carbon capture and

SUMMARY

Carbon dioxide (CO2) electroreduction by molecular catalysts constitutes a promising method to generate high-value added products such as CO and HCOOH. Ionic Liquids (ILs) as promoters for CO2 electroreduction has only been extensively addressed in heterogeneous catalysis and in contrast, little study has been devoted to the elucidation of their interaction with molecular electrocatalysts. This thesis studies the application of electrolyte engineering to CO2 conversion catalyzed by two model molecular catalysts [Re(bpy)(CO)3Cl] and [Rh(bpy)(Cp*)Cl]Cl. Electrolyte engineering strategy allows to modify the electrostatic field present in the double layer at the electrode/solution interface by adding different ionic liquids in solution, which strongly impacts CO2RR activity and selectivity.

Complex [Re(bpy)(CO)3Cl] as a model catalyst for CO2 conversion to CO was studied in acetonitrile.

The presence of imidazolium-based ILs in solution under CO2 catalytic conditions causes a decrease in the CO2 reduction overpotential of about 330 mV, proving the existence of a co-catalytic role of the IL, though this effect was less pronounced when an external proton source was added. The energy efficiency for CO2 conversion to CO reached by this catalyst in the presence of ILs was lower than the one reached using the benchmark electrolyte. Complex [Rh(bpy)(Cp*)Cl]Cl as a model catalyst for CO2 conversion to HCOOH was studied in the presence of ILs in both acetonitrile and acidic aqueous solutions, displaying in both cases a significant decrease in overpotential and tuning of its selectivity towards formate production. Thus, a very selective CO2 to formate conversion (FEHCOO-> 90 %) with a maximal energy efficiency of 66 % is reached in acetonitrile solution. CO2 conversion exhibits less selectivity for formate production in acidic aqueous solution than in acetonitrile, but it allows suppressing the competitive hydrogen evolution reaction (HER) and CO2 conversion at a very low overpotential (0.28 V). For both catalysts mechanistic explanations of the observed interaction with ILs are proposed and in the case of [Rh(bpy)(Cp*)Cl]Cl they also include density functional theory (DFT) calculations. An original protocol for imidazolium-based ILs immobilization at the electrode surface is presented and the enhanced catalytic effect produced during CO2 electroreduction is evaluated with the aid of model catalyst [Rh(bpy)(Cp*)Cl]Cl in both acetonitrile and aqueous conditions. This immobilization of ILs at the electrode surface exhibits a superior selectivity towards formate than previously stuided ILs in solution. Further characterization by spectroscopic and electrochemical methods strongly suggests that the nature of this immobilized layer of IL is cationic. Finally, the feasibility of the aforementioned CO2 catalytic system in larger scale is successfully proven by experiments in a flow electrolyzer of 10 cm 2 of electrode area using a gas diffusion electrode as a cathode.

In conclusion, this thesis provides an original approach to increase activity and selectivity in CO2RR based on electrolyte engineering instead of the conventional catalyst engineering approach, which could be easily extrapolated to other molecular electrocatalytic systems.

Keywords: electrochemical CO2 reduction reaction; molecular catalysts; imidazolium and pyrrolidinium based ionic liquids; electrolyte engineering; electrodeposited ionic liquid layer; flow electrolyzer.

RÉSUMÉ

L'électroréduction du dioxyde de carbone (CO2) par des catalyseurs moléculaires constitue une voie prometteuse pour générer des produits à haute valeur ajoutée tels que le CO et le HCOOH. Les liquides ioniques (ILs) en tant que promoteurs de l'électroréduction du CO2 n'ont été largement abordés que dans la catalyse hétérogène et, en revanche, peu d'études ont été consacrées à l'élucidation de leur interaction avec les électrocatalyseurs moléculaires. Cette thèse étudie l'application de l'ingénierie électrolytique à la conversion du CO2 catalysée par deux catalyseurs moléculaires modèles : [Re(bpy)(CO)3Cl] et [Rh(bpy)(Cp*)Cl]Cl. La stratégie de l'ingénierie électrolytique permet de modifier le champ électrostatique présent dans la double couche à l'interface électrode/solution en ajoutant différents liquides ioniques en solution, ce qui impacte fortement l'activité et la sélectivité du CO2RR.

Le complexe [Re(bpy)(CO)3Cl] comme catalyseur modèle pour la conversion du CO2 en CO a été étudié dans l'acétonitrile. La présence d'ILs à base d'imidazolium en solution dans des conditions catalytiques de réduction du CO2 provoque une diminution du surpotentiel de réduction de CO2 d'environ 330 mV, prouvant l'existence d'un rôle co-catalytique de l'IL, bien que cet effet soit moins prononcé lorsqu'une source externe de protons était ajoutée. Le rendement énergétique de conversion du CO2 en CO atteint par ce catalyseur en présence d'ILs était inférieur à celui atteint en utilisant l'électrolyte de référence. Le complexe [Rh(bpy)(Cp*)Cl]Cl en tant que catalyseur modèle pour la conversion du CO2 en HCOOH a été étudié en présence d'ILs dans l'acétonitrile et dans des solutions aqueuses acides, montrant dans les deux cas une diminution significative du surpotentiel et une orientation de la sélectivité vers la production de formiate. Ainsi, une conversion très sélective du CO2 en formate (FEHCOO -> 90 %) avec une efficacité énergétique maximale de 66 % est atteinte dans une solution électrolytique d'acétonitrile.

La conversion du CO2 présente moins de sélectivité pour la production de formiate en solution aqueuse acide qu'en acétonitrile, mais elle permet de supprimer la réaction compétitive d'évolution d'hydrogène (HER) et la conversion de CO2 à un très faible surpotentiel (0,28 V). Pour les deux catalyseurs, des explications mécanistiques de l'interaction observée avec les IL sont proposées et dans le cas de [Rh(bpy)(Cp*)Cl]Cl, elles incluent également des calculs de la théorie de la fonctionnelle de la densité (DFT). Un protocole original pour l'immobilisation des IL à base d'imidazolium à la surface de l'électrode de travail est présenté et l'effet catalytique accru produit pendant l'électroréduction du CO2 est évalué à l'aide du catalyseur modèle [Rh(bpy)(Cp*)Cl]Cl dans les deux conditions d'acétonitrile et d'eau .

Cette immobilisation des IL à la surface de l'électrode présente une sélectivité supérieure envers le formiate que les IL précédemment étudiées en solution. Une caractérisation plus poussée par des méthodes spectroscopiques et électrochimiques suggère fortement que la nature de cette couche immobilisée d'IL est cationique. Enfin, l'applicabilité de ce système catalytique à plus grande échelle est prouvée avec succès par des expériences dans un électrolyseur à flux de 10 cm 2 de surface d'électrode en utilisant une électrode à diffusion de gaz comme cathode.

En conclusion, cette thèse propose une approche originale pour augmenter l'activité et la sélectivité en CO2RR basée sur l'ingénierie électrolytique au lieu de l'approche conventionnelle de l'ingénierie des catalyseurs, qui pourrait être facilement extrapolée à d'autres systèmes électrocatalytiques moléculaires.

Mots-clés : électroréduction de CO2, catalyseurs moléculaires, liquides ioniques à base d'imidazolium et pyrrolidinium, ingénierie électrolytique, couche de liquide ionique électrodéposée, électrolyseur à flux.

LIST OF ABBREVIATIONS AND CHEMICAL FORMULAS

Acronyms Meaning

Carbon Dioxide and the greenhouse effect

Energy demand has been rising over the past decades and meeting this demand has led to a drastic increase in fossil fuel consumption. As a consequence, the produced quantities of global mean monthly carbon dioxide have also been on an upwards trajectory, reaching an average value of 417 ppm in July of 2021 1 . CO2 is a well-known greenhouse gas for its capacity to firstly absorb and subsequently re-emit thermal radiation. Average increasing concentrations of CO2 constitute a trend that surpasses seasonality and have been correlated with changes on the planet's climate and especially the temperature on the Earth's surface and in the oceans 2 . In Figure 1.1 (a) we see the deviation of average temperatures of the past 140 years from the average temperature of the 20 th century. Blue bars represent years where the average temperature was lower than the overall average of the examined 140-year time period, while on the opposite, red bars represent higher average temperature values. Plotted alongside them, as a grey line, is the average carbon dioxide concentration measured in ppm 3 . While both temperature and CO2 concentration increase are moderate over the first 7 decades shown, from the 1950s forwards, however, the rate of CO2 increase is five times higher and it is reflected in the average + 0.14 °C per decade compared to the previously measured + 0.04 °C value. The result of such increase is not only witnessed by an increase in average temperature on Earth, a phenomenon also referred to as climate change, but is also felt particularly in marine ecosystems. In this case, increasing CO2 causes an acidification of oceanic pH, disrupting the quantities of dissolved calcium carbonate, an essential element for organisms that utilize it to construct their skeletons 4 . Furthermore, some studies suggest that CO2 uptake by terrestrial and marine ecosystems is affected by their climate and existing atmospheric CO2 concentration, which means that in the present conditions of positive carbon-feedback, the effects could be accelerated 5 .

Given the detrimental effects of overproduction of CO2, several strategies to contain and reduce carbon dioxide have been in development over the last few years, known as carbon capture and storage (CCS) and carbon capture and utilization (CCU), respectively. CCS is principally exploring ways of injection and sequestration of CO2 and some industrial plants are already in existence 6 . CCU has the added advantage over CCS of transforming a waste material into useful feedstock chemicals and can be used for storing energy from intermittent renewable energy sources in the form of chemical bonds.

Meanwhile, CCS also has the risk of inadvertent CO2 leaking in the atmosphere from long-term storage.

Considering the aforementioned points, CCU appears to be the preferable option 5,7,8 . Currently, CO2 has some industrial uses, including the food and beverage industries, where it is mainly used as a carbonating agent, as well as a preservative or packaging gas. It also has uses within the pharmaceutical industry, either during the synthetic pathway of drugs, or as a respiratory stimulant. Nevertheless, the aim of these applications is not the decrease of atmospheric CO2 levels and as a consequence, CO2 is still liberated at the end of these processes 9 .

Carbon Dioxide Conversion

In order for the excess of CO2 to be used and the carbon cycle (shown here in Figure 1.2) to be closed, we need to convert CO2 to other chemicals in a manner that constitutes carbon recycling. More particularly, in order to promote a carbon-neutral outcome, CO2 that is captured at industrial sources undergoes reductive conversion powered by renewable energy sources to result in added-value products that can be themselves used to satiate the energy demand, as shown in the scheme of Figure 1.2 8,9 .

Figure 1.2 A model of a carbon-neutral cycle

, where CO2 is electrochemically transformed to fuels and other chemicals utilizing energy from renewable sources 8 .

CHAPTER 1: INTRODUCTION

The principal ways to approach CO2 conversion are biochemically, thermochemically, photochemically, and electrochemically. Thermochemical CO2 conversion has long been used for methanol production and during biomass gasification and pyrolysis, where it has a beneficial role in increasing methanol content in the resulting syngas ratio. However, these processes are limited by their intrinsic conditions, such as endothermic nature of the reactions and the necessary high temperature and pressure, which render them overall expensive [10][11][12] . Meanwhile, biochemical CO2 conversion by plants, algae and cyanobacteria already occurs naturally as photosynthesis, but by applying genetic engineering we can vastly expand the range of produced molecules to include bio-plastics and bio-fuels 13 . Another approach is taking inspiration from natural systems' capacity of using solar energy to drive CO2 conversion and applying this concept to molecular systems, known as artificial photosynthesis. The basic principle in photochemical CO2 conversion, involves light irradiation being absorbed by molecules (photosensitizers) who transfer the absorbed energy to photocatalysts that results in the reduction of tethered CO2 molecules. While it is a very promising strategy that utilizes a renewable energy source, it still faces challenges in in terms of producing inexpensive and catalytically robust photocatalysts [14][15][16][17] .

Electrochemical CO2 Reduction Reaction (CO2RR)

Other than the aforementioned conversion approaches, CO2 can also be electrochemically converted to several different chemical compounds. This method, on which this thesis will focus on, is reliant on an electrical energy input to drive the CO2 reduction 8,[18][19][20][21][22][23][24][25] . However, the CO2 molecule is intrinsically thermodynamically stable and as a consequence has a significant energetical barrier associated with its initial activation. Furthermore, in order to produce valuable reduction products, it is necessary to go through Proton Coupled Electron Transfer (PCET) reactions, that as their name suggests necessitate the presence of a certain number of protons and electrons. More specifically, the higher the necessary number of protons and electrons are, is also reflected on the kinetic barriers of the reaction. Another challenge related to CO2 electroreduction is the presence of competing Hydrogen Evolution Reaction (HER), a side-reaction that especially in aqueous media occurs at similar potentials with CO2RR and at appreciable catalytic rates. The thermodynamic potentials corresponding to CO2RR to various products in aqueous and in some instances organic solutions are shown in Table 1.1.

CHAPTER 1: INTRODUCTION Table 1.1 Thermodynamic CO2 reduction potentials to various products and HER standard potential in aqueous conditions and in a dry acetonitrile solution, adapted from the literature 26 . a Potential value reported in reference 27 . b Potential value reported in reference 28 .

Those values are indicative of the minimum potential necessary for each reaction to take place. However, in reality due to kinetic parameters, such as overpotentials, much more negative potentials are required in order to form the reduction intermediates with multiple PCET that lead to these reduction products.

In order to overcome these barriers, catalysts are employed in order to change the mechanistic pathway by lowering the activation energy to produce the necessary CO2 reduction intermediates. A schematic representation of CO2 electrocatalytic reduction with and without a catalyst is shown in Figure 1

Important Parameters in CO2 Electroreduction

The catalytic performance of any system is subject to various factors that govern the observed activity of CO2RR, as well as its selectivity towards added value products. They include first and foremost the choice of catalyst, but equally the electrolyte composition. Other catalysis defining parameters are the electrode shape and configuration within the cell, the presence of co-factors, the CO2 solubility, the potentials, the electrolyte and local pH, the temperature and pressure under which the electrochemical tests are conducted among others. All the aforementioned parameters have been duly investigated, sometimes even in combination to one another, but there is still room for improvement in our understanding of their effect on CO2 electroreduction mechanism 18,29,30 . Here below special mention will be made to the factors that were studied during the thesis, as well as the literature necessary to put these studies into context.

Catalytic materials and their catalytic criteria

Electrocatalysts for CO2RR are the primary defining characteristic of any catalytic system. Their vast majority can be sorted in three categories, either metallic, non-metallic or molecular catalysts 29 . In the first two cases, the electrode itself constitutes the catalytic material and as a consequence the CO2RR is performed heterogeneously. Many metals have been explored for CO2RR, often with appreciable catalytic performances and many have been sorted into categories based on the CO2 reduction product they produce. For example, Ag, Au and Zn are known for CO2 to CO conversion [31][32][33] , while Sn, Pb, Bi and In selectively result in formate evolution [34][35][36][37][38] . Copper is a unique case, as it is the only element that can produce multi-carbon products, such as hydrocarbons and oxygenates 24,38 . Non-metallic materials have also possessed interesting features for CO2RR. Under this umbrella fall many porous carbon materials, which through variable porosity and high surface area often exhibit admirable conductivity, good stability all while remaining highly competitive from an economic perspective 39,40 . The final main category of electrocatalysts are molecular catalysts that by either being added in the electrolyte solution or immobilized on the electrode surface can respectively reduce CO2 homogeneously or heterogeneously 41,42 .

Electrolyte composition, CO2 solubility and pH effect

The catalytic environment is also a crucial parameter when optimizing CO2RR. On the one hand water is often the solvent of choice because it is the most environmentally friendly and most inexpensive option, both key attributes for industrial applications. It is also in itself a proton source that helps the PCET necessary for decreasing CO2 reduction barriers to take place. Appropriate conductivity and pH control can be assured by using buffer solutions as supporting electrolytes. Many of the CO2RR CHAPTER 1: INTRODUCTION heterogeneous catalytic systems in aqueous conditions operate at neutral or slightly basic pH values. 29,43 The pH value of an aqueous solution is crucial as it can affect CO2RR in different ways. Firstly, it regulates CO2 solubility by determining the carbonic species present in solution according to equation 1, with increasing pH values shifting the balance firstly towards bicarbonate and then carbonate 43 . This is particularly problematic, since crossover of these species to the anode compartment during electrolysis will only regenerate CO2 44 . CO2(aq) + H2O ↔ H2CO3 ↔ H + + HCO3 -2 ↔ 2H + + CO3 -2 eq 1

Secondly, pH affects the thermodynamics of CO2RR, as demonstrated by its Pourbaix diagram 45 , with lower pH values reducing the necessary applied potential for CO2 reduction. However, more acidic conditions equally favor the competing HER. In general, pH values are critical for the product selectivity of CO2RR. For instance, in the case of copper it has been shown that highly basic conditions aid the C-C coupling of adsorbed *CO species on the electrodes, therefore leading in higher yields of C2 products, such as ethylene and ethanol. 24,38,46,47 Moreover, considering the overall cell potential values resulting from CO2RR in the cathode coupled with oxygen evolution reaction (OER) in the anode, it was recently shown that a pH gradient between the two was beneficial in minimizing cell overpotential 48 . More specifically, since the most efficient OER catalytic materials are only stable in very alkaline conditions, it would be optimal in terms of energy efficiency to perform CO2RR in neutral and even acidic conditions 48 . As a consequence, performing CO2RR under acidic conditions would not only be of particular interest in terms of minimizing carbon loss as carbonate and bicarbonate, but could also help decrease cell potential values, if only under those conditions HER could also be suppressed. Recently, a couple of studies have demonstrated efficient and very selective CO2RR in very acidic media (pH<1.5), using potassium cations (K + ) from the supporting electrolyte to suppress HER. 44,49 One focuses on copper in 1 M H3PO4 and 3 M KCl aqueous solution for multicarbon reduction products 44 , while the second studies three carbon deposited catalytic materials SnO2, Au and Cu in 0.1 M H2SO4 + 0.4 M K2SO4 aqueous solution resulting in mostly formate, CO and mixed carbon products respectively 49 .

Nevertheless, use of water as an electrolyte comes with some drawbacks as well. First amongst them is the limited value of CO2 solubility, only 33 mM at 25 °C50 . Another limitation of aqueous conditions is that it often promotes HER at the expense of CO2RR due to the large accessibility of protons. Finally, when working with molecular electrocatalysts, water as solvent is often not a viable option, because only few of them are soluble in aqueous solutions 42,51 . The aforementioned reasons lead to choosing nonaqueous electrolyte systems, such as various organic solvents (acetonitrile, dimethyl formamide or dimethyl sulfoxide etc.) and Ionic Liquids (ILs). Although organic solvents are inherently toxic and flammable, they also have significantly larger CO2 solubility values than water, for example in acetonitrile that was used as solvent in this study CO2 solubility is 280 mM at 25°C 52 . Other advantages CHAPTER 1: INTRODUCTION of non-aqueous solvents besides their ability to dissolve most molecular catalysts is that they often increase CO2 reduction product selectivity by suppressing HER 53 .

Other parameters (Cell design, temperature, pressure)

The efficiency of CO2RR is subject to many more factors, one of which is the nature of the electrodes in terms of geometry, morphology and configuration within the cell where the electrocatalytic measurements take place. The reason for their importance is that they govern the current density, the product selectivity, as well as physical and electrochemical characteristics, such as the mass transfer regime of the system and the system's resistance while current is applied, also known as ohmic drop.

The cell configuration most often used for bulk electrochemical CO2RR as is the case during electrolysis experiments, is an H-type cell 54 . In this configuration there are two distinct compartments, one where reduction reaction takes place and another where species are oxidized, named cathode and anode, respectively. The two compartments have a physical separator between them, which in use with organic solvents is usually a frit and in aqueous conditions a membrane, that also adds resistance to the system.

For this reason, the smallest possible distance between the working and counter electrodes, leads to minimized resistance (ohmic drop). The catholyte and anolyte compositions can be the same or not. In our case the same solvent and supporting electrolyte was used, but the molecular catalyst was only added in the cathodic compartment. In some cases, it is also possible to correct up to 85 % of the intrinsic IR drop by using the appropriate module of the potensiostat, making the actual applied potential as close in value to what we define for the electrochemical test 54,55 . Also, stirring both compartments of the H-type cell help minimize mass transport limitations. In the aforementioned conditions we are performing liquid-phase CO2RR, where the CO2 gas is either bubbled in the catholyte before electrolysis (static conditions), or the cell is not closed and CO2 is continuously bubbled. An alternative to addressing the mass transport limitations and ohmic drop is to opt for gas-phase CO2RR through the use of Gas Diffusion Electrodes (GDEs). In this case CO2 does not need to be bubbled into the catholyte, but is fed instead through the electrode, ensuring that the reactant (CO2), catalyst and electrolyte meet only on the electrode interface 56 . Finally, temperature and pressure factor in CO2RR as well. According to Henry's law, CO2 solubility increases along with increasing pressure, leading to improved CO2 electrocatalysis 57 .

In a CO2 to CO reduction study on a silver electrode it was shown that increasing CO2 pressure also led to higher current density values and favored CO evolution over HER 58 . Meanwhile, in the case of formate evolution from high-pressure CO2 on tin, lead and indium electrodes, temperature was shown to affect selectivity, though the optimal temperature value that produced maximum formate faradaic efficiency (FE) was not the same for all electrodes tested 59 .

CHAPTER 1: INTRODUCTION

Homogeneous molecular catalysts for CO2 reduction

Molecular catalysts for CO2 reduction have been developed over the past decades and are still evolving today thanks to their often-admirable activities, extraordinarily high selectivities towards CO2RR and to the fact that they provide invaluable mechanistic insight into CO2RR pathways.

In this thesis, two model molecular catalysts were used, one that selectively reduces CO2 to CO and another that results in a mixture of formate and H2. In Figure 1.4 we see some characteristic examples of molecular catalysts containing different metals and ligands that are all highly selective for CO2 to CO conversion 41,42,[START_REF] Beley | Electrocatalytic Reduction of Carbon Dioxide by Nickel Cyclam2+ in Water: Study of the Factors Affecting the Efficiency and the Selectivity of the Process[END_REF][START_REF] Costentin | Efficient and Selective Molecular Catalyst for the CO2-to-CO Electrochemical Conversion in Water[END_REF][START_REF] Hawecker | Electrocatalytic Reduction of Carbon Dioxide Mediated by Re(Bipy)(CO)3Cl (Bipy = 2,2′-Bipyridine)[END_REF] . They all share the similarity that their ligands are bound through the N atom, in most cases the ligands themselves are aromatic in nature and they exhibit their catalytic activity in either organic and/or aqueous electrolyte, based on their respective solubilities. Meanwhile, other studies target electrocatalysts with non-noble metal centers like the Ni cyclam [START_REF] Beley | Electrocatalytic Reduction of Carbon Dioxide by Nickel Cyclam2+ in Water: Study of the Factors Affecting the Efficiency and the Selectivity of the Process[END_REF] and the substituted iron tetraphenylporphyrin [START_REF] Costentin | Efficient and Selective Molecular Catalyst for the CO2-to-CO Electrochemical Conversion in Water[END_REF] shown in Figure 1.4, we chose to work with the well-established model electrocatalyst [Re(bpy)(CO) 3 Cl], known also as Lehn's catalyst [START_REF] Hawecker | Electrocatalytic Reduction of Carbon Dioxide Mediated by Re(Bipy)(CO)3Cl (Bipy = 2,2′-Bipyridine)[END_REF][START_REF] Hawecker | Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′-Bipyridine)Tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts[END_REF] . Not only has its mechanism for CO2RR been fully elaborated [START_REF] Hawecker | Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′-Bipyridine)Tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts[END_REF][START_REF] Clark | Kinetic and Mechanistic Effects of Bipyridine (Bpy) Substituent, Labile Ligand, and Brønsted Acid on Electrocatalytic CO2 Reduction by Re(Bpy) Complexes[END_REF] , but it has also been studied in different electrolytes, with several proton sources and various electron-withdrawing or electron-donating substitutions on its bipyridine ligand [START_REF] Clark | Kinetic and Mechanistic Effects of Bipyridine (Bpy) Substituent, Labile Ligand, and Brønsted Acid on Electrocatalytic CO2 Reduction by Re(Bpy) Complexes[END_REF][START_REF] Smieja | Re(Bipy-TBu)(CO)3Cl-improved Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical and Mechanistic Studies[END_REF] .

Finally and more importantly, the aim of this thesis being mainly to decipher the interactions that occur between molecular catalysts and ILs in order to optimize a CO2RR catalytic system, this catalyst was particularly interesting because it had already been tested for CO2RR in a pure IL [START_REF] Grills | Electrocatalytic CO2 Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential[END_REF] .

Figure 1.4 Structures of molecular catalysts for CO2 to CO conversion alongside their electrolyte

conditions and faradaic efficiency value for CO. The second model molecular catalyst we have selected reduces CO2 to formate and it is complex [Rh(bpy)(Cp*)Cl]Cl, represented in Figure 1.5 (b). In Figure 1.5 we see some characteristic examples of molecular catalysts whose principal reduction product is also formate 51,[START_REF] Ishida | Selective Formation of HCOOin the Electrochemical CO2 Reduction Catalysed by [Ru(Bpy)2(CO)2] 2+ (Bpy = 2,2′-Bipyridine)[END_REF][START_REF] Bolinger | Electrocatalytic Reduction of CO2 Based on Polypyridyl Complexes of Rhodium and Ruthenium[END_REF][START_REF] Caix | Electrocatalytic Reduction of CO2 into Formate with [(η 5 -Me5C5)M(L)Cl] + Complexes (L = 2,2′-Bipyridine Ligands; M = Rh(III) and Ir(III))[END_REF][START_REF] Collin | Electrocatalytic Properties of (Tetraazacyclotetradecane)Nickel(2+) and Ni2(Biscyclam)4+ with Respect to Carbon Dioxide and Water Reduction[END_REF][START_REF] Kang | Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by a Water-Soluble Iridium Pincer Catalyst[END_REF][START_REF] Kang | Rapid Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by an Iridium Pincer Catalyst Immobilized on Carbon Nanotube Electrodes[END_REF][START_REF] Taheri | An Iron Electrocatalyst for Selective Reduction of CO2 to Formate in Water: Including Thermochemical Insights[END_REF] . The characteristic of these electrocatalysts is their ability to form hydride intermediates that result in C-H bonds, rather than HER which results from either recombination of two hydrides or protonation of one hydride. In the firstgeneration of electrocatalysts we find noble-metal complexes with bipyridine ligands [START_REF] Ishida | Selective Formation of HCOOin the Electrochemical CO2 Reduction Catalysed by [Ru(Bpy)2(CO)2] 2+ (Bpy = 2,2′-Bipyridine)[END_REF][START_REF] Bolinger | Electrocatalytic Reduction of CO2 Based on Polypyridyl Complexes of Rhodium and Ruthenium[END_REF][START_REF] Caix | Electrocatalytic Reduction of CO2 into Formate with [(η 5 -Me5C5)M(L)Cl] + Complexes (L = 2,2′-Bipyridine Ligands; M = Rh(III) and Ir(III))[END_REF] that can produce formate in a non-selective way. A newer generation of catalysts have since emerged that use second or third row metal centers. These metal centers are less nucleophilic, which results in suppressing HER. They also suppress C-O bond cleavage that results in CO production, therefore these complexes favor formate evolution. They include Iridium pincer complexes [START_REF] Kang | Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by a Water-Soluble Iridium Pincer Catalyst[END_REF][START_REF] Kang | Rapid Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by an Iridium Pincer Catalyst Immobilized on Carbon Nanotube Electrodes[END_REF] and iron carbonyl clusters [START_REF] Taheri | An Iron Electrocatalyst for Selective Reduction of CO2 to Formate in Water: Including Thermochemical Insights[END_REF] that selectively produce formate in some cases even in aqueous media at appreciable rates. was for a long time neglected until its use in CO2 photocatalysis [START_REF] Todorova | Controlling Hydrogen Evolution during Photoreduction of CO2 to Formic Acid Using [Rh(R-Bpy)(Cp*)Cl] + Catalysts: A Structure-Activity Study[END_REF] and incorporation in higher-order systems [START_REF] Benseghir | Co-Immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction[END_REF] brought it again to the forefront. While other examples of electrocatalysts that yield formate more selectively exist [START_REF] Kang | Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by a Water-Soluble Iridium Pincer Catalyst[END_REF][START_REF] Taheri | An Iron Electrocatalyst for Selective Reduction of CO2 to Formate in Water: Including Thermochemical Insights[END_REF] , it still constitutes an interesting model catalyst in terms of IL interaction, especially because it has a well-known catalytic mechanism and its product selectivity is heavily influenced by the reaction environment.

Ionic Liquids: From alternative solvents to catalysts for CO2 reduction

Ionic Liquids (ILs) are salts in liquid state below 100 °C entirely comprised of organic cations and organic or inorganic anions. A subcategory of ILs are Room Temperature Ionic Liquids (RTILs), which are in liquid form below 25 °C. In contrast, normal salts that also consist entirely of anions and cations also possess high melting points due to their strong ionic bond. Therefore, liquidifying them by heating, for example reaching 801 °C for NaCl, renders them molten salts [START_REF] Alvarez-Guerra | Ionic Liquids in the Electrochemical Valorisation of CO2[END_REF][START_REF] Sanchez-Sanchez | Electrocatalytic Reduction of CO2 in Imidazolium-Based Ionic Liquids[END_REF] . On the contrary, in ILs the cation and anion are typically asymmetric and lead to sterical hinderance that weakens their bonding and as a result lowers their melting point. Although the first IL, [CH3CH2NH3][NO3], was synthesized in the early 20 th century [START_REF] Walden | Molecular Weights and Electrical Conductivity of Several Fused Salts[END_REF] , their involvment in many industrial applications and especially the field of electrochemistry was after the dicovery of the air and water stable RTIL 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM] [BF4], in the early nineties by Wilke's group [START_REF] Plechkova | Applications of Ionic Liquids in the Chemical Industry[END_REF]80 . Their huge number estimated to exceed 10 6 , [START_REF] Plechkova | Applications of Ionic Liquids in the Chemical Industry[END_REF] necessitates their classification. This can be based on their ability to be water-miscible or not, or their ability to act as a proton source by possessing an available proton in their cation, in which case they are characterized as protic, while if they do not, they are considered aprotic [START_REF] Hapiot | Electrochemical Reactivity in Room-Temperature Ionic Liquids[END_REF][START_REF] Greaves | Protic Ionic Liquids: Properties and Applications[END_REF] . Another way to sort them into categories is based on the family of the cation, which can be base on an aromatic ring like imidazolium, pyrrolinium, pyridinium etc., a non-aromatic ring such as pyrrolidinium, piperidinium etc. or a heteroatom-cetrered polyaliphatic compound, as is the case with ammonium, sulfonium or phosphonium to name a few. Equally, we can categorize ILs based on their anions, whether they are inorganic, like the following examples: PF6 -, BF4 -, BCN4 -, Br -, Cl -, etc. or organic, such as CH3COO -, N(SO2CF3)2 -, amongst others. In this thesis, we will concentrate our study on imidazolium and pyrrolidinium based ILs seen here in Figure 1.6, which is the reason why we should also define their nomeclature. Especially imidazolium-based ILs have several possible abbreviations, that have been regrouped in three main systems [START_REF] Sanchez-Sanchez | Electrocatalytic Reduction of CO2 in Imidazolium-Based Ionic Liquids[END_REF] . For example 1-ethyl-3-methylimidazolium can be abbreviated as

[C2mim] + , [C2C1Im] + or as it will be abbreviated in this thesis, [EMIM] + . In such examples "E", "B" and "M" stand for "ethyl" and "butyl" and "methyl" alkyl chain groups respectively, the first letter representing the R1 and the second the R2 group on the imidazolium ring. Based on this abbreviation CHAPTER 1: INTRODUCTION system, a pyrrolidinium based IL with R1 = "butyl" and R2 = "methyl" groups would be abbreviated

[ΒΜPyrr] + .

Ionic Liquids as solvents

ILs have been considered as alternative non-aqueous solvents compared to organic solvents that though valuable, also come with a host of inconveniences such as toxicity and flammability. ILs as solvents on the other hand, possess many interesting properties. Especially considering the immense number of possible cation and anion combinations, it is also possible to tune their physicochemical properties to the needs of many electrochemical systems [START_REF] Hapiot | Electrochemical Reactivity in Room-Temperature Ionic Liquids[END_REF][START_REF] Farrán | Green Solvents in Carbohydrate Chemistry: From Raw Materials to Fine Chemicals[END_REF] . ILs are generally stable over a large thermal window ranging from approximatively -40 °C up to 300 °C84 and an electrochemical window wide enough to perform many electrochemical processes, such as CO2RR. Their electrochemical window is confined by the reduction of some IL cations and likewise the oxidation some anions and any present impurities, such as water, can further diminish the IL's range of electrochemical stability [START_REF] Hapiot | Electrochemical Reactivity in Room-Temperature Ionic Liquids[END_REF][START_REF] Schröder | Water-Induced Accelerated Ion Diffusion: Voltammetric Studies in 1-Methyl-3-[2,6-(S)-Dimethylocten-2-Yl]Imidazolium Tetrafluoroborate, 1-Butyl-3-Methylimidazolium Tetrafluoroborate and Hexafluorophosphate Ionic Liquids[END_REF][START_REF] Michez | Electroreduction of 1-Butyl-3-Methylimidazolium Bis(Trifluoromethanesulfonyl)Imide Ionic Liquid: Oriented Product Selectivity through the Electrode Material[END_REF] . However, their inherent ionic nature renders them highly conductive, therefore they could act as supporting electrolyte as well as solvent [START_REF] Galiński | Ionic Liquids as Electrolytes[END_REF] . This fact is furthermore linked to their high dissolving ability, that means ILs have the capacity to dissolve a variety of organic and inorganic compounds including gases like CO2. More specifically, the CO2-solubilities of some conventional ILs have been determined and as an example the commonly used imidazolium-based [BMIM] [BF4] can have up to 111 mM of CO2 dissolved [START_REF] Kelley | Understanding Carbon Dioxide Solubility in Ionic Liquids by Exploring the Link with Liquid Clathrate Formation[END_REF] , which would place it rather in the middle between the poor CO2 solubility of water (33 mM) [START_REF] Sanchez-Sanchez | Electrocatalytic Reduction of CO2 in Imidazolium-Based Ionic Liquids[END_REF] and the superior one of organic solvents, such as acetonitrile (280 mM) 52 . Moreover, ILs by definition have low melting points and are non-volatile compounds, which is useful in terms of preventing loss of solvent and for applications that require high vacuum conditions [START_REF] Rogers | Ionic Liquids-Solvents of the Future?[END_REF] . However, despite all the aforementioned advantageous features, ILs suffer from high viscosity. High viscosity is problematic in terms of electrochemistry because viscosity is inversely correlated to the conductivity and diffusion of soluble species in the solvent and as a result leads to significant decrease in the maximum current density possible [START_REF] Sanchez-Sanchez | Electrocatalytic Reduction of CO2 in Imidazolium-Based Ionic Liquids[END_REF] . As a consequence, use of ILs as supporting electrolytes in organic or aqueous conditions is a way to circumvent their high viscosity while maintaining to some degree some of their interesting CHAPTER 1: INTRODUCTION properties and it is the strategy chosen in this thesis. Another inconvenience sometimes encountered when using ILs is their hydricity. Water uptake may vary based on the structure of their cations and anions of ILs, but it has been associated with altering their redox behavior [START_REF] Montiel | Electrocatalytic Studies on Imidazolium Based Ionic Liquids: Defining Experimental Conditions[END_REF] .

Ionic Liquids as CO2RR catalysts and co-catalysts

Besides any possible benefits of using ILs as solvent or electrolyte, in the context of CO2RR one of their main functions is to enhance the catalytic effect. The initial study on the topic was conducted by Rosen et. al. [START_REF] Rosen | Ionic Liquid-Mediated Selective Conversion of CO2 to CO at Low Overpotentials[END_REF] group demonstrating the beneficial role of using an IL on CO2RR overpotential diminution. More specifically, they reported that the presence of 18 mol % of [EMIM] [BF4] in water significantly reduced the overpotential of a silver electrode for CO2RR, as represented in Figure 1.7 (a). They proposed that the decrease in the energy of anion radical intermediate CO2 [START_REF] Sun | Switching the Reaction Course of Electrochemical CO2 Reduction with Ionic Liquids[END_REF][START_REF] Zhu | Efficient Reduction of CO2 into Formic Acid on a Lead or Tin Electrode Using an Ionic Liquid Catholyte Mixture[END_REF][START_REF] Wang | Effect of Alkyl Chain Length of Imidazolium Cation on the Electroreduction of CO2 to CO on Ag Electrode in Acetonitrile[END_REF][START_REF] Lau | New Insights Into the Role of Imidazolium-Based Promoters for the Electroreduction of CO2 on a Silver Electrode[END_REF][START_REF] Zhao | Is the Imidazolium Cation a Unique Promoter for Electrocatalytic Reduction of Carbon Dioxide?[END_REF] .

However, other types of ILs, such as guanidinium ILs have also been proven to act as co-catalysts in acetonitrile solutions and having a CO2RR promoting effect [START_REF] Vasilyev | The Role of Organic Promoters in the Electroreduction of Carbon Dioxide[END_REF] . Furthermore, low concentrations of ILs, as much as 20 mM 96 or 2 mM 97 have been demonstrated to produce a co-catalytic effect on silver electrodes. In those studies, it is also shown that the IL catalytic effect varies based on the nature of the imidazolium cation, but the anion is apparently not significant in tuning their CO2RR-promoting properties [START_REF] Lau | New Insights Into the Role of Imidazolium-Based Promoters for the Electroreduction of CO2 on a Silver Electrode[END_REF][START_REF] Zhao | Is the Imidazolium Cation a Unique Promoter for Electrocatalytic Reduction of Carbon Dioxide?[END_REF] .

Concerning the mechanism by which ILs promote CO2RR many studies have been conducted. In the case of imidazolium-based ILs, many endorse Rosen's theory of an {EMIM-CO2} complex, but different mechanisms have been proposed based on experimental and calculated intermediates and products 99,100 .

Important positions of the imidazolium ring are the C2 position, which is the more electron depleted atom and as a consequence considered the reduction site 101 and the C4, C5 atoms that can anchor adsorbed CO2 by hydrogen bonding [START_REF] Lau | New Insights Into the Role of Imidazolium-Based Promoters for the Electroreduction of CO2 on a Silver Electrode[END_REF] . An important distinction has however been made between imidazolium and other families of ILs in a study where 20 different ILs were tested for promoting the CO2RR catalytic effect on a silver electrode. It concluded that only imidazolium-based ILs are in fact acting as co-catalysts, while other IL cations promote CO2RR through modification of the double layer (Figure 1.7 b) [START_REF] Zhao | Is the Imidazolium Cation a Unique Promoter for Electrocatalytic Reduction of Carbon Dioxide?[END_REF] . An important note should also be made in regards to ILs HER suppression effect. It has been demonstrated on various metallic electrodes, that the addition of 0.1 M [EMIM]Cl in acidic aqueous conditions (pH=1) can lead to significant HER inhibition 102 . On the contrary, under basic conditions (pH=13) this effect is not observed. This effect under acidic conditions was rationalized by the ability of imidazolium cations to interact with the electrodes themselves and therefore displacing hydronium

Ionic Liquid reduction and immobilization

Another interesting feature of imidazolium-based ILs is their ability to be reduced and result in different catalytic species. This reduction feature is dependent on the electrode material, as we can see in Figure 1.8 a and can occurs at potentials as low as -2.5 V vs. Fc + /Fc in some cases. It is then followed by a oxidation wave around -0.65 V, which has been attributed to the reoxidation of these reduced species 103 .

Understanding the nature of these new species derived from the reduced and reoxidized imidazolium can be based on NMR studies of the liquid and gas phase before and after running short electrolysis on Cu, Pt, Au and Hg under an inert atmosphere [START_REF] Michez | Electroreduction of 1-Butyl-3-Methylimidazolium Bis(Trifluoromethanesulfonyl)Imide Ionic Liquid: Oriented Product Selectivity through the Electrode Material[END_REF]103 . Possible forms of the produced molecules are shown in Figure 1.8 b 86 , though their distribution varies based on the electrode material. This has been correlated

to each electrode's ability to adsorb hydrogen, as well as the availability of protons in the solution. In the theoretical scheme presented in Figure 1.8 b, an initial one-electron reduction of an imidazolium leads to an imidazol-2-yl radical (2), which can either decompose towards a carbene (3) and hydrogen, recombine with another imidazol-2-yl radical to result in a neutral dimer (4) or disproportionate that leads to the neutral hydrogenated monomer (5). From all these compounds the intermediate and product analysis led to the conclusion that the most present species are the two diastereomers of the neutral dimer (4), with a smaller contribution of the hydrogenated monomer (5) and the carbene (3) [START_REF] Michez | Electroreduction of 1-Butyl-3-Methylimidazolium Bis(Trifluoromethanesulfonyl)Imide Ionic Liquid: Oriented Product Selectivity through the Electrode Material[END_REF]103 . This study also mentions that no carbenes were detected, not any of the species named 6-9 [START_REF] Michez | Electroreduction of 1-Butyl-3-Methylimidazolium Bis(Trifluoromethanesulfonyl)Imide Ionic Liquid: Oriented Product Selectivity through the Electrode Material[END_REF] . Moreover, [EMIM] [BF4] has been reported to form a electrochemical layer on the Cu surface that can suppress HER in favor of CO2RR 104 . In this study, the HER suppression effect was not solely observed on an initial CV, that is in accordance with [EMIM] + adsorption on the electrode surface 102 , but persisted and even strengthened in consecutive scans, which the authors attribute to a reaction of electrochemical nature between the electrode surface and the IL 104 .

Co-catalytic effect of molecular catalysts and Ionic Liquids

While both molecular catalysts and ILs have been extensively explored independently in catalytic CO2RR, much less effort has been devoted to studying their combined effect. This is especially evident in homogeneous CO2RR, where both molecular catalyst and IL coexist in the electrolyte solution. To the best of our knowledge, so far only two types of complexes have been used thus in conjuncture with ILs, Lehn's catalyst ([Re(bpy)(CO)3Cl]) and Iron tetraphenylporphyrin (FeTPP) [START_REF] Grills | Electrocatalytic CO2 Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential[END_REF]105,106 . In the case of the former, it was initially demonstrated that when used in pure IL electrolyte, more particularly in

[ΕΜΙΜ][TCB],
where TCB stands for tetracyanoborate, the onset potential for CO2RR was shifted anodically by approximatively 450 mV in comparison to when acetonitrile is the solvent and [TBA][PF6] the supporting electrolyte. Furthermore, while the viscosity of the IL did impair the catalytic current density, the rate constant for CO2RR was still estimated to have increased by as much as forty times.

Both the diminution in overpotential and the increase in the rate constant were attributed to the presence of the IL and the synergistic effect it exerted with Lehn's catalyst [START_REF] Grills | Electrocatalytic CO2 Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential[END_REF] . Further study was then conducted In it we can see that the addition of the IL in conjuncture with a proton source leads to an overpotential diminution of approximatively 200 mV and a quadrupling of the observed catalytic current density. Furthermore, the fact that the mere addition of the IL without a proton source present or vice versa does not lead to a significant increase in catalytic current suggests that the IL does not act as a proton source, nor is its beneficial effect due to direct interaction between the imidazolium and CO2.

Instead, it is shown that this IL shifts the potentials of FeTPP's reduction waves less negatively and especially the third electron reduction from Fe I to Fe 0 that is associated with CO2 reduction. This cocatalytic effect is then attributed to electrostatic stabilization of the negatively-charged reduced catalyst forms [Fe I TPP] -and [Fe 0 TPP] 2-by the imidazolium cation. Finally, another interesting note is the effect of this imidazolium IL on the molecular catalyst selectivity by bulk electrolysis experiments. This demonstrates that when electrolysis was run in presence of the IL, not only could the applied potential be decreased by 190 mV, but it also resulted to more CO produced, the average FE with IL being 93 % versus 89 % when there was no addition of IL 106 . Therefore, as we can also see in Figure 1.6, the presence of an imidazolium-based IL led to positive shifts in overpotential in both molecular catalysts and it was attributed to favorable interactions of the imidazolium cation with the catalyst itself. However, the nature of the IL used and its role on the synergistic effect with molecular catalysts for CO2RR is still to be fully elucidated and will be the subject of study in Chapter 3.

However, other strategies to achieve the combined effect of molecular catalysts and ILs outside having both those components in solution also exist. Though these approaches were not explored in the present thesis, it would be remiss to not mention them. One of them consists in incorporating an IL moiety and more specifically an imidazolium ring, into the structure of the molecular catalyst itself leading to the synthesis of a novel catalyst. Interestingly and also understandably, the two examples of this technique use as scaffold the model catalysts already described with an IL in solution, namely Lehn's catalyst and FeTPP 107,108 . Both structures are represented in Figure 1.7. In the case of the Rhenium catalyst, the functionalization with the imidazolium ring results in anodic shift in the catalysts reduction potentials of approximatively 170 mV compared to the Lehn's catalyst in a CO2-saturated acetonitrile solution.

Moreover, when electrolysis was run for both imidazolium-functionalized and catalyst depicted in Figure 1.10 a at the same potential (-1.77 V vs. Fc + /Fc) for the span of one hour in presence of 9.4 M of H2O as proton source, the former consumed more charge and resulted in higher CO selectivity, namely 73 % against only 54 % from the catalyst 107 .

CHAPTER 1: INTRODUCTION

The other example of imidazolium-functionalized molecular catalyst is a FeTPP containing four methylimidazolium units appended on the porphyrin macrocycle that result in four positively charged 'arms', as depicted in Figure 1.7 b 108 . The presence of these imidazolium units produces significant positive shifts in the catalyst's potential under inert and CO2 atmospheres in a 9/1 DMF/H2O with 0.1 M

[TBA][PF6] electrolyte mixture. More specifically, under CO2 the decrease in overpotential can reach up to 375 mV compared to the non-functionalized FeTPP under the same catalytic conditions. More interesting still, is the fact that the functionalized FeTPP is not only rendered water-soluble by the imidazolium units, but it also displayed higher values of catalytic current density in water than in the DMF/H2O conditions. This effect, though counter intuitive considering the diminished CO2 solubility in H2O compared to DMF, the authors suggest it is explained through the relative permittivity values of both solvents. Since the beneficial effect of imidazolium units has been assigned to electrostatic stabilization of the negatively charged [FeTPPCO2] 2-adduct by the imidazolium cations, their degree of availability is key. If the solvent permittivity is higher, as is the case in water, the dissociation of the IL units comprised of imidazolium cations and chloride anions in solution is higher and therefore the imidazolium groups are more available to participate in Coulombic stabilizing interactions with the catalyst's negative intermediates 108 . Most examples of molecular catalysts in flow electrolyzers concern heterogenized catalysts [109][110][111][112][113][115][116][117] .

The process of immobilizing molecular catalysts on solid supports, which are then used in flow electrolyzers. This strategy is advantageous, as it results in higher catalytic efficiency for these molecular systems, that combine elements of molecular and heterogeneous catalysis. Nevertheless, using heterogenized molecular catalysts in flow electrolyzers also presents a disadvantage when considered at an industrial scale. That is the result of increased cost and energy expenditure from having to regularly turn the electrolyzer off-line for catalyst replacement. On the contrary, by using a homogeneous catalyst, the reaction can take place continuously, which is not only economically preferable, but also integral to green chemistry 118 .

Purpose and objectives of the thesis

Performing efficient CO2RR under environmentally-friendly conditions is a challenge that must be addressed. When approaching this question through electrochemistry, both the choice of catalyst and reaction environment arise as important issues, that should be discussed in relation to one another in order to provide information necessary to develop optimized catalytic systems. The well-defined mechanistic pathways of some molecular catalysts make them ideal models to study this interaction.

Meanwhile, the use of ILs as potential alternative electrolytes and CO2RR co-catalysts is also promising.

Nevertheless, rationalization of the interactions of ILs with molecular catalysts was yet to be performed at the beginning of this thesis. The few studies present [START_REF] Grills | Electrocatalytic CO2 Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential[END_REF]105,106 demonstrated beneficial catalytic effects from combining molecular catalysts and ILs, but without providing all necessary background on how the nature of the catalyst and the IL impact their interaction.

The objectives of this thesis are thus:

1. To investigate the nature of interaction between ILs and model molecular catalysts. 3. To provide proof of scaling-up the most interesting catalytic system within a flow electrolyzer.

The outline of the thesis is as following:

In Chapter 2, all the reagents and materials involved, as well as instruments used throughout this thesis will be introduced. Besides, electrochemical processes and general operational procedures will be described. Furthermore, the characterization methods and techniques used will be introduced, providing information on the instrument type and configuration. Finally, some inorganic complex synthesis protocols will be included.

In • Gold Electrode (AuE), diameter = 3 mm, purchased from BioLogic. Used as WE in CV experiments.

• Gold Electrode plate, 0.5 mm thick, 99.95 % purity, purchased from BioLogic. Used for XPS analysis.

• Platinum wire electrode, 0.5 mm diameter, 99.9 % purity, purchased from Alfa Aesar. Used as CE for CV experiments.

• Platinum mesh electrode geometric area = 2-4 cm 2 , 0.

Electrolyzer components:

Flow Electrolyzer parts, all purchased from ElectroCell:

• Cathodes: Graphite electrode, Sigrafine® R8710, geometric area of 10 cm 2 , purchased by The Carbon Company or Gas Diffusion Electrode (GDE), Sigracet 28 BC, geometric area of 10 cm 2 , purchased from Fuel Cell Store. The GDE must undergo an activation protocol described here in section 2.3.4.

• Anodes: Dimensionally Stable Anode (DSA) electrode, made from a IrO2-Ta2O5 mixed oxide, which also has a 10 cm 2 electrode geometric area or the aforementioned graphite electrode.

• Polytetrafluoroethylene (PTFE) End Frames, each containing 4 inlet/outlet openings of 0.32 cm and another 6 peripheral holes to accommodate the bolts that keep the cell together.

• Stainless Steel Endplates, that provide additional support for the cell.

• Hardware for Cell Assembly, such as nuts, bolts and washers all made from stainless steel and guide pins made from PTFE.

• Flow frames of 2 mm thickness made from PTFE.

• Turbulence Mesh, typically made from PVDF (Polyvinylidene difluoride).

• Gaskets, to separate all cell compartments with 0-4 holes for inlet/outlet based on the position used and/or a center cut-out corresponding to the 10 cm 2 electrode area.

• Nafion 117 membrane of 10 cm 2 geometric area and 183 μm thickness was purchased from

ElectroCell. This Nafion perfluorosulfonic (PFSA) acid membrane has improved stability due to lower fluoride ion release and is compatible with water electrolyzers. It was already pre-treated and soaked, so its activation protocol within the electrolyzer involved only an hour-long run with water at 22.1 mL/min prior to use with the electrolyte.

• Tubing used to link the flow electrolyzer with the solution recipients is C-Flex Ultra 14 (Internal Diameter (ID) = 1.6 mm), purchased from Cole-Parmer. It has physical properties similar to silicone with chemical compatibility of Tygon® and suitable for pumping from -40 to 100 °C.

• Catholyte/anolyte recipients are glass bottles of 100 mL, purchased from Dewar.

Instruments

I.

A VSP 300 and a SP 300 Potensiostat were used for all cyclic voltammetry and bulk electrolysis experiments and were purchased from BioLogic Instruments.

II.

The Gas Chromatography instrument used to detect and quantify all gaseous products during bulk electrolysis was purchased from SRI Instruments, Model 8610 C. H2 gas was quantified using a thermal conductivity detector (TCD) separated from other gases by a HaySepD precolumn attached to a 3 mm molecular sieve column. Carbon-based products, like CO, are detected by a flame ionization detector (FID) equipped with a methanizer and are separated using a 3 mm molecular sieve column. Calibration of the GC was performed from pure CO and H2 gases or from a custom standard gas mixture.

III

Catalytic parameters for the molecular catalysts

In order to compare catalytic performance of various catalysts we need a set of criteria. Here below are mentioned some catalytic parameters that have been used throughout the thesis:

[1] Faradaic Efficiency (FE): In order to evaluate product selectivity, we use this term to represent the percentage of each reduction product by a set amount of charge passed through the system.

technique and can provide valuable information not solely on reduction and oxidation reactions of molecular species, but equally on transient surface dependent reactions that involve electron transfers. It consists in scanning the potential from an initial value E1 to a final value E2 and then a return sweep back to the initial potential using a triangular potential waveform. All while the potential is swept, the potensiostat registers the current obtained from all electrochemical processes taking place at the electrode interface due to the potential application. The resulting cyclic voltammogram provides on the y axis the response, which is the current recoded, as a function of the parameter imposed, that being the potential represented on the x axis 3 .

The cyclic voltammetry (CV) experiments were carried out in a classical three-electrodes electrochemical cell sealed with a Teflon cap (BioLogic), with a 3 mm diameter GCE/AuE as a working electrode, which was polished on a polishing cloth on a 1 μm diamond suspension (Struers), sonicated for 10 seconds, thoroughly rinsed with ethanol and dried under a stream of pressurized air prior to experiments. A platinum wire was used as a counter electrode and was previously flame annealed. The reference electrode was either a silver chloride coated silver wire in saturated KCl solution reference electrode or simply a silver chloride coated silver wire, in which case potentials were calibrated using the Ferrocenium/Ferrocene (Fc + /Fc) redox couple as internal standard, which was added in solution at the end of each measurement. Only the last cycle of all CVs is shown unless explicitly mentioned otherwise, although in most cases no difference in consecutive scans has been observed.

Constant Potential Electrolysis (CPE) and Constant Current Electrolysis (CCE)

In Constant Potential Electrolysis (CPE), a constant potential is applied for a defined time frame and measure the current response produced on the electrochemical system. The obtained response of currenttime association is characteristic of the concentration of electroactive species at the electrode interface.

CPE can be used for determining the diffusion coefficient of electroactive species that are adsorbed on the working electrode's surface , as well as a way to investigate the mechanism of different electrode processes. Here this method is used as electrolysis permitting to define the catalytic reduction products within the systems studied, since products accumulate within the electrolysis cell throughout the duration of CPE and therefore reach detectable range concentrations. An alternative technique used for electrolysis is constant current electrolysis (CCE), which consists of keeping a stable current value applied between the working and counter electrodes and registering the potential of the working electrode as a function of time. It is similar in principle to CPE, with the exception that current steps are replacing potential steps, however, it can also provide information concerning the kinetics of the CHAPTER 2: MATERIALS AND METHODS electrode. Nevertheless, it is often more widely used in industrial settings for CO2RR catalysis and it was a better way to compare different catalytic conditions when the maximum catalytic potential values were dissimilar.

CPE and CCE experiments were conducted in a gas-tight two-compartment electrochemical cell with a glass frit (porosity level 4) and a PTFE needle valve separating the anodic and cathodic compartments in liquid phase and gas phase, respectively. The volume of anolyte and catholyte were approximatively 5 and 10 mL respectively. The working electrode was either a GCE or RVC plate, the counter electrode was a platinum mesh or GCE rod and the reference electrode was either Ag/AgCl/KClsat reference or a silver chloride coated silver wire, which was calibrated with Fc + /Fc as an internal redox reference.

Surface Coverage of Chemically Modified Electrodes

The CV measurements used to extrapolate the surface coverage of the working electrodes with a electrodeposited IL layer were measured in an acetonitrile solution containing 0.5 M [EMIM][PF 6 ] under an inert atmosphere. The electrodeposited IL layer was formed under the same conditions and comparative tests were carried out on both GCE and AuE of the same area. By measuring the reoxidation wave peak area that is linked to the IL quantity reduced, the surface coverage of electrodeposited layer on the electrodes was calculated based on the folowing formula:

Γ = Q (C) n F (C/mol) A (cm 2 )
where Q is the charge (measured in Coulomb), n is the number of electrons involved in the process (according to the literature 4 n=1), F is the Faraday constant (96500 C/mol) and A is the working electrode geometric area (measured in cm 2 ) 5,6 .

GDE activation protocol

In the flow electrolyzer setup, the GDE cathode had to first undergo an activation protocol in order to produce optimal faradaic efficiency for formate. This consisted of initially running water through it for at least an hour and subsequently running experiments amassing a total charge of approximatively 3000 C. This value was experimentally determined as subsequent experiments before the 3000 C mark led to progressively increasing formate FE values, which stabilized past that point. According to the literature on the topic 7 , it is considered that a thin layer of electrolyte forms on top of the GDE as the electrode is progressively wetted. This is likely caused by the creation of oxygen groups on the electrode surface rendering it more hydrophilic and as a consequence more active towards the electrolyte solution. This may also in some cases result in promoting HER, but it was not observed experimentally in this case, since formate efficiency increased instead. It is acknowledged that the nature of the electrolyte as well as the applied potential or current in the case of electro-wetting, to which our system belongs, affect the eventual catalytic activity and selectivity of the electrode 7 .

Physical characterization: X-ray photoelectron spectroscopy on gold plates

X-ray photoelectron spectroscopy (XPS) is a very widely used technique in material science to characterize chemical bonding on solid surfaces by identifying the chemical state of different atoms based on binding energy (BE) values they exhibit. Its principle relies on irradiating a sample kept under ultra-high vacuum with soft X-rays that produce ionizations of electrons from the analyzed atoms or molecules. The binding energy values are obtained from the difference between the energy of the X-ray irradiation and the kinetic energy of excited core level photoelectrons 8,9 .XPS spectra in this study were collected on gold plates containing electrodeposited and covalent IL layers using a Omicron Argus Xray photoelectron spectrometer, equipped with a monochromated Al Kα radiation source (hν = 1486.6

eV) and a 280 W electron beam power. The diameter of analyzed area was 1 mm. The emission of photoelectrons from the sample was analyzed at a takeoff angle of 45° under ultra-high vacuum conditions (≤10-9 mBar). Spectra were carried out with a 100 eV pass energy for the survey scan and Miche (Laboratoire de Réactivité de Surface -UMR 7197).

Analytical Methods for product quantification

In order to quantify the amount of CO2 reduction products are being produced during electrolysis we have recourse to analytical techniques, that provide us the values necessary to calculate each product's faradaic efficiency.

Gaseous product detection

Gas products were quantified by gas chromatography from 50 µL aliquots of the headspace of both compartments. Hydrogen (H2) and carbon monoxide (CO) were detected by a thermal conductivity detector (TCD) and a flame ionization detector (FID) coupled to a methanizer, respectively. The column in both cases was a 3 mm molecular sieve column and in the case of H2 the sample also passed through a HaySepD precolumn.

Liquid product detection

Liquid products were evaluated using an ionic exchange chromatograph equipped with a Metrosep A Supp 5 column and a conductivity detector. A typical measurement requires the sampling of 50 μL of solution, followed by a 200-400 times dilution based on the observed formate concentration in ultrapure water and injection of 20 μL into the IC chromatograph. For better accuracy it is suggested that recipients for the dilute samples be made from plastic and discarded after one use, that dilution in MiliQ water be done by weighing instead of volumetric measurement and that the sample is filtered or centrifuged prior to analysis.

ILs drying process and water quantification

Before use in solution with molecular catalyst [Re(bpy)(CO) 3 Cl], all ILs were vacuum-dried overnight under stirring using a vacuum pump and then stored in a glove box under argon. The drying procedure followed was reported by Montiel et al. 10

Density Functional Theory (DFT) calculations

DFT calculations were carried out at the ωB97X-D level 11 using the Gaussian 16 (rev. C.01) quantum chemistry software 12 . The LANL2DZ 13 basis set and associated pseudopotentials were used to describe Rh ions, which were supplemented by one shell of f-type polarization functions 14 . Remaining atoms were treated with the Pople-type 6-31G(d,p) basis set [15][16][17] . Solvent effects of acetonitrile were included in the geometry optimizations and energy calculations by means of the IEF-PCM implicit solvent model 18 , as implemented in Gaussian 16. The nature of the stationary points on the potential energy surface was confirmed via normal-mode analysis calculations. The standard-state correction to switch from the reference state of 1 atm used in the Gaussian code to 1.0 M in solution at 25 °C (+1.89 kcal/mol) was applied to the free energy of all the species. A dataset collection of the computational results is available in the ioChem-BD repository 19 and can be accessed via: DOI: 10.19061/iochem-bd-6-109.

Synthesis of complexes [Rh(bpy-X2)(Cp*)Cl] and [Re(bpy-X2) (CO)3Cl], where X=H, OCH3, COOCH3

Syntheses of complexes [Re(bpy-X2)(CO)3Cl] were performed according to previously described protocols [20][21][22] and syntheses of complexes [Rh(bpy-X2)(Cp*)Cl] were adapted from existing protocols in the literature [23][24][25] .

Synthesis of [Rh(2,2'-bipyridine)(Cp*)Cl]Cl :

Protocol adapted from the literature [23][24][25]

Synthesis of [Rh (2,2'-bipyridine-carboxylic acid methyl ester)(Cp*)Cl] :

Protocol adapted from the literature 24 . A methanol solution ( 10

Synthesis of fac-[Re(4,4'-dimethoxy-2,2'-bipyridine)(CO)3Cl]:

Synthesized according to protocols described in the literature 22 . A toluene solution (25 mL) containing [Re(CO)5Cl] (100 mg, 0.27 mmol) and 4,4'-dimethoxy-2,2'-bipyridine (60 mg, 0.27 mmol) was refluxed under an Ar atmosphere for 3 h in dim light. After the solution cooled to room temperature, the resulting yellow precipitates were filtered and washed with toluene, hot hexane, and then n-pentane. Yield = 95

%. The purity of the final precipitate was verified by 1 H NMR spectroscopy.

Synthesis of [Re(2,2'-bipyridine-carboxylic acid methyl ester)(CO)3Cl]:

Synthesized according to protocols described in the literature 22 . [Re(CO)5Cl] (100 mg, 0.27 mmol) was dissolved in 25 mL of methanol under a nitrogen (inert) atmosphere. 2,2'-bipyridine-carboxylic acid methyl ester (76 mg, 0.28 mmol) was added as a solid, and the obtained colorless solution was heated for 2 h at reflux temperature under inert atmosphere. During this time, the solution became deep yellow.

The volume was reduced at 20 mL by rotary evaporation and left to cool at rt, where upon the product precipitated as a pure, yellow solid. Yield = 93 %. The purity of the final precipitate was verified by 1 H NMR spectroscopy.

1 H NMR (300 MHz, CD3CN): δ/ppm, 4.02 (s, 6H), 8.01 (dd, J = 7.5 Hz, 2H), 8.95 (d, J = 0.9 Hz, 2H), 9.18 (t, J = 6.0 Hz, 2H).

CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

Introduction

Carbon dioxide electroreduction reaction (CO2RR) is considered an environmentally friendly approach that not only allows exploiting a waste material to produce valuable fuels or other chemicals, but also constitutes a promising solution for storing the intermittent energy originating from renewable sources.

A first step in that direction would be the electroreduction of CO2 to CO, which could be used as feedstock in Fischer-Tropsch technologies that result in hydrocarbon products. In addition to this, CO2 conversion to formate also represents an interesting alternative, since formate is industrially used and considered as a commodity chemical 1 . However, the intrinsic inertness of the CO2 molecule is an obstacle to its direct reduction, which necessitates the use of a catalyst. Many metal complexes have been identified as efficient homogeneous catalysts for CO2 reduction, mainly contributing to ameliorating the activity, selectivity and efficiency of this reduction [2][3][4][5] . Molecular electrocatalysts have been extensively studied since they offer a high degree of tunability through: i) the metal center, traditionally composed of a noble-metal (Re, Rh, Ru, Pd,…) [6][7][8][9][10] , although more recently substituted by earth-abundant transition metals (Ni, Co, Mn, Fe,…) 8,9,[11][12][13] and ii) the choice of ligand, which tunes the electronic (thus the redox potential) and the geometric features 14 . An example of a model molecular catalyst to selectively convert CO2 to CO is the rhenium (I) complex [Re(bpy)(CO)3Cl], (where bpy is 2,2′-bipyridine), also known as Lehn's catalyst and here seen in Figure 3.1, hitherto referred to as complex [1]. A detailed mechanism for complex [1] has already been well-established 6,7,15 . Furthermore, studies have been carried out incorporating electron donating or electron withdrawing groups on the bpy ligand [16][17][18] , as well as studies where this complex was attached to higher-order systems or immobilized on surfaces 19 . Many of these studies aimed at increasing of efficiency of this catalyst in terms of energy, that is to say maintaining high Faradaic efficiencies for CO2 to CO conversion at the lowest possible overpotential, while not compromising on current density. Another example of a model molecular catalyst, this time for formate production, is [Rh(bpy)(Cp*)Cl]Cl (where Cp*= pentamethylcyclopentadienyl), also shown in Figure 3.1 and referred to as complex [2]. This catalyst presents three main features of interest in comparison with other molecular catalysts; i) it is soluble in aqueous solution, ii) it can be used as both CO2RR and hydrogen evolution reaction (HER) electrocatalyst 10,20 ; and iii) its reaction mechanism is well-stablished in the literature 10,20 . Moreover, it has been further explored in CO2 photoreduction 21 and within higherorder systems, such as metal-organic frameworks (MOFs) with appreciable catalytic performance 22 .

CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

.1 Structures of the [Re(bpy)(CO)3Cl

] complex [1] and [Rh(bpy)(Cp*)Cl]Cl complex [2],

where bpy = 2,2'-bipyridine and Cp* = pentamethylcyclopentadienyl, and the supporting electrolytes:

(

a) 1-Butyl-3-methylimidazolium hexafluorophosphate ([ΒΜΙΜ][PF6]), (b) 1-Ethyl-3methylimidazolium hexafluorophosphate ([ΕΜΙΜ][PF6]), (c) 1-Ethyl-3-methylimidazolium tetrafluoroborate ([ΕΜΙΜ][BF4]), (d) 1-Butyl-1-methylpyrrolidinium hexafluorophosphate ([ΒΜΡyrr][PF6]), (e) 1,2-Dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([DiΜΙΜ][N(SO2CF3)2]) and (f) Tetrabutyl ammonium hexafluorophosphate ([TBA][PF6]).

Both complex [1] and [2] are model CO2RR catalysts, whose shared similarities include having a pelectron rich bipyridine ligand and sharing HER as the competitive process. Nevertheless, they present different mechanisms for CO2RR, which are described in scheme 3.1. Complex [1] assumed its active form after having undergone an initial two electron reduction with simultaneous chlorine ligand dissociation and was then capable to coordinate a CO2 molecule to form a monoanionic intermediate (species B in schema 3.1a) 16 . The CO2RR mechanism followed beyond that point was however entirely depended on the availability of protons in the reaction medium. In the absence of a specific proton source, an electron-first mechanistic pathway was favored (species C in scheme 3.1 (a)) that led to the formation of dianionic intermediate. On the other hand, in presence of a proton source in solution the mechanism proceeded through a proton-first pathway that led to a neutral catalyst intermediate (species D in scheme 3.1 (a)) 16 . HER from complex [1] is also possible and even thermodynamically favored under certain conditions 23 , but due to kinetic considerations the formation of species B was favored over the Rehydride, therefore conferring to complex [1] its high selectivity towards CO production 23,24 . In contrast, complex [2] undergoes an initial two-electron reduction at the Rh center and chloride ligand dissociation followed by the formation of a metal-hydride intermediate and a subsequent ligand-centered reduction step for generating the active form of the catalyst 20 . Then, two competitive reactions could take place, either CO2 reduction to formate by its insertion in the Rh-H bond (green pathway in Scheme 3.1 (b)) or formation of H2 by protonation of the hydride (red pathway in Scheme 3.1 (b)) 10,20 . Generally, molecular catalysts are seldom soluble in aqueous solution and for this reason most studies were reported in organic . This was the case of complex [1], but complex [2] is a water-soluble catalyst. Altogether, their common points and divergencies make both complex [1] and [2] suitable models for understanding basic aspects of molecular CO2 electroreduction catalysis. [1] for either CO2 reduction in the absence and presence of an external proton source 16 or HER 23,24 and (b) Electrocatalytic cycle of complex [2] for CO2 to formate conversion (green pathway) or HER (red pathway) 10,20 .

Scheme 3.1 (a) Electrocatalytic cycle of complex

An alternative strategy to improve catalytic performances, other than the modification of the molecular catalyst itself, would be the modulation of the local electric field in the double layer by electrolyte engineering. The latter corresponds to the control of the ions present at the electrode-electrolyte interface when the electrical double layer is built up. This has been known to impact on the selectivity and the energy efficiency of CO2RR by stabilizing reaction intermediates and/or blocking competitive mechanistic pathways [26][27][28] .

Pure ionic liquids (ILs) as a solvent or a supporting electrolyte for CO2RR have attracted a lot of attention in the field of catalysis due to the wide electrochemical potential window provided by ILs [29][30][31][32] . However, the high viscosity of most ILs limits their applications as solvents in electrochemistry, since it is inversely proportional to the conductivity of the solvent, it limits diffusion of soluble species and can lead to significant limitations in terms of maximum current density achieved 33 . However, this issue is not as prevalent when they are used as supporting electrolytes mixed with molecular (organic or aqueous)

solvents. The positive effect of ILs at the electrode/solution interface either by stabilizing reaction demonstrated on heterogeneous catalysts [34][35][36][37] . In contrast, very few studies using ILs in solution have been devoted to molecular catalytic systems and all those studies were conducted in organic solvents 34,38,39 . Notably for the present study, Matsubara et al. 38 combined linear sweep voltammetry (LSV) and density functional theory (DFT) calculations to study the role of a unique imidazolium-based IL [EMIM] + in acetonitrile for CO2 electroreduction catalyzed by complex [1]. They reported a significant positive shift of the catalytic onset potential in the presence of ILs 38 . To the best of our knowledge, complex [2] has not yet been used alongside ILs besides the present study.

Taking all this into consideration, we aimed at further studying the effects of different types of ILs in promoting CO2 electroreduction, however not as solvents but as supporting electrolytes, using the two model molecular catalysts presented. We particularly wanted to address the role of the structure of the IL used and the type of interaction between IL and catalyst, that were at the root of either enhanced catalysis or altered product selectivity. For that purpose, we chose two different molecular catalysts with well-established catalytic mechanisms, complexes [1] and [2], as models and five different ILs (protic and aprotic) as shown in

Results

[Re(bpy)(CO)3Cl] with ILs as electrolyte in acetonitrile under inert conditions

In ). A constant electrolyte concentration of 0.5 M was used for studying the electrochemical behavior of complex [1] and to rule out any effect from increasing ionic strength. Moreover, this concentration proved a good compromise by providing a relevant overpotential diminution and enough conductivity when ILs were CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS in solution, as can be seen in Figure 3.3. Furthermore, all ILs were dried before use as explained in Chapter 2.6, since H2O content in ILs has been demonstrated to play a relevant role 40,41 . According to previous reports 6,16,42 , the first reduction wave shown in Figure 3.2 was attributed to a one-electron reversible reduction centered on the bpy ligand (bpy to bpy -) and the second more cathodic wave to a one-electron irreversible or quasi-reversible metal centered reduction (Re I to Re 0 ).

(a) [TBA][PF6] (black plot), [BMPyrr][PF6] (green plot) and [BMIM][PF6] (blue plot) and (b) [EMIM][BF4] (red plot), [EMIM][PF6] (grey plot) and [BMIM][PF6] (blue plot)

. Scan rate 0.1 V/s. The reason why we had dried the ILs before use resided in the observation that the H2O content in the IL/CH3CN solutions was significant and had a relevant impact on the potentials displayed by the two reduction waves of complex [1]. Table 3.1 shows the H2O concentration in the IL/CH3CN solutions quantified by Karl Fischer titration using dried and not dried ILs. In some cases, H2O concentration greatly decreased after drying, even by an order of magnitude. In Figure 3.5, CVs of complex [1] in CH3CN solutions using dried ILs and not dried ILs are compared and in all 4 cases the reduction waves of complex [1] were cathodically shifted and the current densities were larger when not dried ILs were used as supporting electrolyte, proving that the drying process of ILs proposed here, in coordination with adopting specific filters that blocked humidity in the gas inlet were important in order to reach reliable results. Furthermore, it indicated that not only the interactions between the complex and the IL that

IL present: (a) [EMIM][BF4], (b) [EMIM][PF6], (c) [BMPyrr][PF6] and (d) [BMIM][PF6]

. Scan rate 0.1 V/s.

[Re(bpy)(CO)3Cl] with ILs as electrolyte under CO2 reduction catalytic conditions

We subsequently studied the catalytic properties of complex [1] for CO2 electroreduction in the presence of all different ILs investigated. Thus, Figure 3.6 shows CVs of complex [1] under catalytic CO2 conditions without an added proton source. A catalytic wave for CO2 electroreduction, as expected, was observed at the second complex reduction process, where the catalytically active 2-electron reduced species was generated. Thus, a significant increase in current density (jcat) was observed at the second potential peak in all CVs of Figure 3.6 when CO2 was present in solution. 16 . These values showed that catalysis occurred at more positive potentials in the presence of ILs, with positive shifts ranging from 200 to 330 mV depending on the IL used. Thus, Figure 3.9 shows CVs of complex [1] under catalytic CO2 conditions in the presence of 1.5 M TFE as a proton source and 0.5 M of all different supporting electrolytes studied here. Table 3.3 reports an estimation of catalytic current densities obtained in each case by comparison versus inert conditions (jcat/jp) and the characteristic potentials for the catalytic reduction peaks shown in Figure 3.9. As expected from the literature 16 giving the highest value, in the vicinity of 10 mA/cm 2 . However, [BMPyrr][PF6] exhibited the highest catalytic current ratio due to its lower current under inert conditions.

(a) [TBA][PF6] (black plot), [BMPyrr][PF6] (green plot) and [BMIM][PF6] (blue plot) and (b) [EMIM][PF6] (grey plot), [EMIM][BF4] (red plot) and [BMIM][PF6] (blue plot

Figure 3.9 Cyclic Voltammograms of 1 mM complex [1] and 0.5 M of different supporting electrolytes in acetonitrile solution under Ar (dashed plots) and CO2 (solid plots) colored as follows: [TBA][PF6] (black), [BMPyrr][PF6] (green), [BMIM][PF6] (blue), [EMIM][PF6] (grey), and [EMIM][BF4] (red), and CO2 with 1.5 M TFE (solid purple plots in all cases

). Scan rate 0.1 V/s.

Electrolysis for CO2 reduction with ILs as electrolyte catalyzed by [Re(bpy)(CO)3Cl]. Impact on reaction selectivity

The catalytic performance of complex [1] with respect to electrochemical CO2 reduction in the presence of TFE as a proton source and different ILs or conventional benchmark [TBA][PF6] electrolyte, was also studied by constant potential electrolysis (CPE). Some of the results can be seen in Figure 3.10 and a global overview in Table 3.4. The applied potential was -2.05 V vs. Fc + /Fc for all CPE experiments reported, except for an additional CPE carried out in the absence of TFE at 300 mV more positive potential (-1.75 V vs. Fc + /Fc). The reduction of imidazolium cations on the GC surface under those conditions was completely excluded, since more negative potentials were required for this reduction process (approximatively -2.5 V vs. Fc + /Fc under Ar atmosphere) and they were never reached during experiments with complex [1] 44 . Carbon monoxide (CO) and hydrogen (H2) were the only reduction products detected, with no evidence of any additional product in solution. In all cases, CO remained the major reaction product and the overall faradaic efficiency was nearly 90 % (Table 3

[Rh(bpy)(Cp*)Cl]Cl with ILs as electrolyte in acetonitrile under inert conditions

Figure 3.11 shows the electrochemical response of complex [2] under inert conditions, a similar study to the one initially represented in Figure 3.2 for the case of complex [1]. The molecular catalyst, which in this case corresponds to 1 mM of complex [2], was tested by cyclic voltammetry in an acetonitrile solution under an argon atmosphere in presence of a series of different supporting electrolytes. These all supporting electrolytes was chosen in order to ensure appropriate ionic strength. In the case of complex [2], drying the ILs was abandoned as a practice, since water was eventually used as the optimal proton source reported in the catalytic CO2 reduction tests 10 . It should also be stressed that, contrarily to the CVs shown previously for complex [1], in the case of complex [2] in order to obtain a better definition of all redox features in the voltammograms a slower scan rate was used, more particularly one at 0.01 V/ s.

According to the literature 10,20,21 , the first reduction wave observed in all plots represented in Figure 3.11 and centered at -1.21 V vs. Fc + /Fc has been attributed to a quasi-reversible peak of the metal center reduction from Rh III into Rh I . This wave was characterized by a peak-to-peak separation (ΔEp) of 150 mV. This step was also coupled with the dissociation of the labile chloride ligand, as was shown in Scheme 3.1 (b). A second reduction wave at more negative potentials, that was centered around -2.7 V vs. Fc + /Fc in the absence of IL in solution, corresponded to a one-electron reversible wave located on the bipyridine ligand 10,20,21 . On the one hand, ). In all three cases, the metal-centered reduction peak remained unchanged very close to -1.21 V vs. Fc + /Fc, but the second reduction wave of complex [2] could not be observed because imidazolium cations got reduced at about -2.5 V vs. Fc + /Fc, hindering any process occurring at more negative potentials 30,45,46 , as is shown in Figure 3.12. As a consequence, it was not possible to evaluate the impact of imidazolium-based cations in solution on the bipyridine ligand reduction under those inert conditions. The cyclic voltammograms that contained 0.5 M of ILs were also shown to differ from the one with the conventional electrolyte benchmark, since they presented small reduction waves in the interim between the Rh-centered reduction wave and the more cathodic bpy-centered third electron reduction. Those features were only clearly visible at low scan rate, which is the primary reason this scan rate was selected. Waves in this zone of the voltammogram had already been reported in the initial studies of this catalyst for H 2 evolution and CO2RR in presence of added acid in solution and they could be attributed to the formation of the hydrido complex and/or the poorly reversible one-electron reduction of said hydrido complex (peaks B and C in Scheme 3.1 (b)) 10,20 . Finally, one IL tested with complex [1] was hereby not included and it was the

protic IL [DiΜΙΜ][N(SO 2 CF 3 ) 2 ]
. This IL was abandoned after the poor catalytic results displayed with complex [1]. In Table 3.5 all potentials represented in Figure 3.11 are listed.

(a) [TBA][PF6] (black plot) and [BMPyrr][PF6] (green plot) and (b) [TBA][PF6] (black plot), [EMIM][PF6] (grey plot), [BMIM][PF6] (blue plot) and [EMIM][BF4] (red plot)

. Scan rate 0.01 V/s. (red plot). Scan rate 0.01 V/s.

[Rh(bpy)(Cp*)Cl]Cl with ILs as electrolyte in acetonitrile under CO2 catalytic conditions

Under CO2 in the presence of H2O (5 % vol.) as a proton source, an evident catalytic wave was observed in all cases, which was visible within the electrochemical window where the ILs are stable, thus reflecting a large anodic shift of the bpy-centered redox event, as seen in Figure 3.13. In particular, the catalytic wave was shifted from -2.56 V to -2.

(a) [TBA][PF6] (black plot) and [BMPyrr][PF6] (green plot) and (b) [TBA][PF6] (black plot), [EMIM][PF6] (grey plot), [BMIM][PF6] (blue plot) [EMIM][BF4] (red plot)

. Scan rate 0.01 V/s. Next, taking advantage of complex [2]'s solubility in both organic and aqueous conditions, we studied the CO2RR in acetonitrile/aqueous solvent mixtures, as well as different aqueous solutions. Firstly, we tested the catalytic effect of adding an IL in solution in an acetonitrile/H2O 50/50 (vol.) solvent mixture, as seen in Figure 3.14. However, the supporting electrolytes previously used in the acetonitrile/H2O 95/5 (vol.) solvent with complex [2] were not adapted to those conditions, as they contained the hydrophobic to acetonitrile/H2O 50/50 (vol.) solution was also followed by a decrease in CO2 solubility from 240 mM to approximatively 116 mM 48 . However, the values of maximum current density were not significantly decreased, as both the benchmark and the IL as electrolytes produce maximum current density values that exceeded 3 mA/cm 2 . 3.8) also presented an advantage over basic aqueous conditions. That consisted of avoiding CO2 loss in the form of bicarbonate or carbonate 49 , which could be transformed back into CO2 by the anode 50 .

However, the unbuffered aqueous solutions used might not maintain the pH stable during long experiments. This led us to search for a buffer solution that could be stable in this specific acidic region and for that reason an acetate buffer was selected. Also, acetic acid had also previously been reported to have beneficial effect in increasing the current density in catalytic studies with complex [1] 16 . In the red dashed plots of Figure 3.15, we therefore see the resulting CVs of complex [2]

Electrolysis for CO2 reduction with ILs as electrolyte catalyzed by [Rh(bpy)(Cp*)Cl]Cl. Impact in reaction selectivity

The effect on products selectivity and energy efficiency of the CO2RR due to the presence of ILs in the electrical double layer was studied from both potentiostatic and galvanostatic electrolysis, namely constant potential (CPE) and constant current (CCE) electrolysis, respectively. In all experiments, only formate was detected as product in the liquid phase and only H2 was observed in the gas phase. Table 3.7 shows the overpotential, faradic efficiencies obtained for both products and energy efficiency for CO2 conversion to formate as a function of the electrolyte, at different values of applied potential during electrolysis (CPE) in acetonitrile/H2O 95/5 (vol.). As formate can partly migrate from the catholyte to the anolyte 1 , a systematic analysis of both catholyte and anolyte solutions was performed in all electrolysis reported here proving that between 15 % and 20 % of the total formate generated during the electrolysis was detected within the anolyte solution. Thus, analyzing the presence of reaction products in both compartments allowed closing efficiently the mass balance of the electrolysis reaching in most cases a total FE (FEHCOO-+ FEH2) between 87 % and 100 %. 3.8. In Figure 3.17 are presented the plots of cathode potential vs. electrolysis time (corresponding to Table 3.8, entries 1 and 2), as well as the thermodynamic potential for CO2 reduction to formate, which for those conditions had been calculated at -1.32 V vs. Fc + /Fc 47 . The presence of the [EMIM] + cation led to a significant reduction in the observed overpotential of at least 800 mV. In addition to this, the products ratio (FEHCOO-/FEH2) doubled from (1.5/1) in presence [TBA] + (Table 3.8, entry 1) to (3/1) when

[EMIM] + is present at the electrode/solution interface (Table 3.8, entry 2). This increase in product selectivity for formate in the presence of the IL cation was also reflected on the energy efficiency values, which pass from 25 to 47 % for entries 1 and 2, respectively. Finally, by increasing the % of H2O present in solution from 5 % (entry 1 of Table 3.8) to 50 % (entry 3 of Table 3.8), the products ratio (FEHCOO-/FEH2) remained almost identical (1.5/1). We also performed CCE (applied current density -3.33 mA/cm 2 ) with complex [2] in different aqueous CO2-saturated solutions. These results are depicted in Figure 3.17 and Table 3. of a molecular complexes performing CO2 conversion to formate in both organic and aqueous solutions [51][52][53] . The blue and red plots of Figure 3.17, which corresponded to entries 1 and 2 of Table 3.9

respectively, showed that a relevant decrease in overpotential (600 mV) together with a slight increase in the selectivity towards formate was achieved when [EMIM] [BF4] was present in the solution instead of [TBA] [BF4]. However, in those unbuffered conditions the initial acidic pH was not stable throughout the electrolysis and as can be seen in Table 3.9, the final pH of entry 1 had increased from 3.8 to 5.5. As a consequence, we then opted for an acetate buffered solution, which successfully maintained pH stable at a value of 3.8. Using the buffered solution in combination with either [TBA] + or [EMIM] + led to even lower overpotential values at a constant current density of -3.33 mA/cm 2 , as demonstrated in the corresponding green and purple plots of Figure 3.17, as well as entries 3 and 4 of Table 3.9, respectively.

Notably, the electrolyte composed by [EMIM] [BF4] and acetate buffer (Table 3.9, entry 4) reached one of the lowest overpotential values reported so far for a homogenous molecular catalyst producing formate in aqueous solution (0.28 V) resulting in a relevant energy efficiency (32 %). In particular, the combination of [EMIM] + at the electrical double layer together with the acetate buffer solution provoked a significant HER suppression (entry 4 of Table 3.9). Finally, it is also demonstrated that both complex [2] and CO2 are necessary for CO2 conversion to formate (entries 5 and 6 of Table 3.9). kcal mol -1 in the absence of IL. This generated a formate ion in addition to the Rh II species B, which tended to undergo disproportionation to generate a Rh I and a Rh III species 54,55 .This, together with the protonation of the formate to give formic acid, provided the thermodynamic driving force to the reaction and made it irreversible. On the other hand, the HER in the absence of IL involved the initial formation of a slightly stabilizing adduct followed by the H-H coupling process, which occurred through TS2 (represented in Figure 3.19 (c)) and involved a very smooth barrier of 0.5 kcal mol -1 . The formation of the H2 product releasing a water dimer and species B is highly exergonic (by > 40 kcal mol -1 ). The first feature that may draw one's attention is that the standard-state free-energy barrier for the HER was significantly lower than that associated to the hydrogenation of CO2, while experimental results using benchmark electrolyte pointed towards formate as a main product (entries 1 and 3 in Table 3.7).

However, it is important to note that those experiments were carried out in a CO2-saturated solution of acetonitrile containing only 5 % (vol.) of H2O. As such, the concentration of protons is expected to be several orders of magnitude lower than that of CO2, balancing the preference of the system to proceed through either of these two pathways and explaining the experimentally observed product distribution.

Importantly, as shown in red lines in Figure 3 With the gained knowledge about the catalyst-IL interactions and the parameters influencing the selectivity of the reaction, we sought to rationalize the experimental outcomes in water solvent. First, the shape of the free-energy profile in water was found not to differ significantly from that calculated in acetonitrile, as shown in Table 3.10. Thus, the decrease of the FEHCOO-when increasing the water content should be ascribed to concentration effects, as the concentration of protons was directly proportional to that of water, whereas that of CO2 dropped as the polarity of the solvent increased.

Discussion

The role of ILs in CO2 electroreduction has been largely investigated in previous studies with heterogeneous catalysts where ILs have demonstrated to promote CO2 electroreduction by means of inhibiting hydrogen evolution reaction and increasing the concentration of dissolved CO2 in aqueous based solutions. Overall, by diminishing the energetic barrier of the reaction by formation of adducts 33,56,57 with CO2 ILs lead to CO2 activation. Studies with homogeneous catalysts for CO2 electroreduction in the presence of ILs are much fewer 32,38,39 and the role of the structure of different ILs has never been addressed before in CO2 conversion by molecular electrocatalysts. Here, we evaluated for the first time the effect of different ILs used as supporting electrolytes in binary mixtures with acetonitrile or totally aqueous solutions on the electroreduction of CO2 catalyzed by two model molecular complexes, namely complex [1] and complex [2], respectively. In particular, within this structure-activity relationship study, we varied the nature of the cation, anion and cation alkyl chain by a choice of 5 different ILs and in the case of complex [2] varied the solvent/electrolyte system as well.

Firstly, for complex [1] we demonstrated that the presence of IL cations in solution under inert conditions (argon) greatly favored its reduction, as shown by the large impact on its redox potentials, with positive shifts of both one-electron redox events: up to 210 mV for the first reversible reduction wave corresponding to bipyridine reduction and up to 370 mV for the second one, which corresponded to the reduction of Re I to Re 0 . Furthermore, when CO2 was present without an added proton source in solution (Figure 3.6), almost the same potential shifts were reported for both reduction waves together with a catalytic wave developed at the second one-electron process (Table 3. Particularly, the aforementioned potential shifts, more relevant on the second electron transfer, could stem from the electrostatic stabilization of the negatively-charged reduced form of complex [1] (equation 1) by the IL cations present in solution (equation 2). This was in agreement with the electrostatic stabilization previously suggested by Choi et al. 39 for the case of FeTPP and Matsubara et al. 38 for the interaction between [EMIM] + and the negatively charged reduced form of complex [1]. In addition to this, Matsubara et al. 38 proposed several theoretical structures for the interaction between the CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS imidazolium cation and the reduced form of complex [1] based on DFT calculations. One of those calculated structures suggested that the imidazolium ring lies above the bipyridine ligand due to π+-π interactions. It must be noted that the first one-electron reduction of complex [1] was centered on the bpy ligand, which is an aromatic system containing delocalized  electrons in their  orbitals. Thus, our results showing potential diminution under both Ar and CO2 in the presence of ILs, which greatly depends on the nature of the IL cation (Figure 3 Finally, electrochemical CO2 reduction in the presence of TFE as a proton source and ILs was studied by CV (Figure 3.9 and Table 3.3) and CPE experiments (Table 3 Comparing the co-catalyst effect of all ILs studied here under catalytic CO2 reduction conditions, in the presence and the absence of TFE as a proton source in solution, we demonstrated that the presence of TFE seemed to weaken, at least partially, the electrostatic interaction between the reduced form of complex [1] (equation 1) and IL cations present in solution as described by equation 2, which was responsible for the positive potential shift observed for both electron transfers in Figures 3.2 and 3.6.

The overpotential was between 330 and 200 mV lower than in the case of benchmark electrolyte in the absence of TFE (Table 3.2), but only between 70 and 30 mV lower in the presence of TFE (Table 3.

3).

As reported by Kubiak's group 16 , CO2 reduction catalytic cycle for complex [1] follows two different pathways whether or not a proton source is present in solution, seen in scheme 3.1 (a). Thus, once the reduced form of the catalyst is generated by 2 electron transfers and the CO2 molecule had been incorporated in complex [1] (equation 1), two possible pathways were described. On the one hand, in the absence of a proton source, a new electron transfer took place forming a di-anion catalytic species:

[Re(bpy)(CO) 3 CO2] 2-(equation 3), which could be greatly stabilized thanks to the IL cations (equation 4). On the other hand, when an explicit proton donor is added, such as TFE, a "proton first" step controlled the reaction initially forming a non-charged intermediate (equation 5), which then became a mono-anionic catalytic species [Re(bpy)(CO) 3 CO2H] -after the following electron transfer (equation 6).

As a result, these two intermediates on the "proton first" pathway were much less stabilized by the presence of IL cations (equation 7) and for this reason the ILs exhibited a much weaker co-catalytic effect when a proton donor is present in solution.

[Re(bpy)(CO

) 3 CO2] -+ e -⇔ [Re(bpy)(CO) 3 CO2] 2- eq. 3 [Re(bpy)(CO) 3 CO2] 2-+ 2IL + ⇔ [Re(bpy)(CO) 3 CO2] 2-[IL] + [IL] + eq. 4 [Re(bpy)(CO) 3 CO2] -+ H + ⇔ [Re(bpy)(CO) 3 CO2H] eq. 5 [Re(bpy)(CO) 3 CO2H] + e -⇔ [Re(bpy)(CO) 3 CO2H] - eq. 6 [Re(bpy)(CO) 3 CO2H] -+ IL + ⇔ [Re(bpy)(CO) 3 CO2H] -[IL] + eq. 7
When comparing the effect of the presence of ILs and especially imidazolium-based ones on molecular catalysts in acetonitrile a clear distinction between complexes [1] and [2] was observed. By comparing Figures 3.2 and 3.11, we saw that the differing effect of ILs produced on each catalyst under inert conditions. As mentioned before, in the case of complex [1], the potential for both reduction peaks were shifted less negatively, while for complex [2] the Rh-centered peak remained unchanged and the more CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS negative bpy-centered peak was not even visible with the IL, because it exceeded the point at which imidazolium itself is reduced (see Figure 3.12). Nevertheless, in a CO2-saturated atmosphere in the presence of a proton source, the catalytic wave of complex [2] was visible in all cases, as it was shifted less negatively than the imidazolium IL reduction. Figure 3.13 demonstrates that all ILs tested here favor CO2 electroreduction by decreasing the Ecat/2 between 80 and 30 mV (Table 3.6) as compared to the benchmark electrolyte. This overpotential shift was in the same magnitude as the one observed with catalyst [1] in the presence of TFE, even though these catalysts do not share the same mechanistic pathway for CO2 reduction. The rationale behind the importance of the cation or anion of an imidazolium IL though was consistent in both cases, displaying that the anion is relatively insignificant in determining catalytic parameters besides hydrophilicity, while that of the cation is comparatively more important, with the [EMIM] + cation displaying the best catalytic results in both cases.

Complex [2] also presented the opportunity to work in aqueous solution, which had not been possible

with complex [1], which was insoluble in purely aqueous unbuffered and subsequently buffered solutions. We were therefore able to explore for the first time the effect of an aqueous soluble IL

([EMIM][BF4]
) as supporting electrolyte in CO2RR catalyzed by complex [2] in unbuffered aqueous solution. However, a relevant drop in CO2 solubility was associated with the solvent switch from acetonitrile/H2O 95/5 (vol.) to aqueous solution, which strongly impacted the catalytic current density achieved (solid red plots in Figure 3.15). Interestingly, we demonstrated the effect of imidazolium-based ILs promoting CO2 conversion to formate in unbuffered aqueous solution, since a gain of 80 mV in overpotential was achieved in the presence of [EMIM][BF4] (solid red plot in Figure 3.15 (b)). This tied nicely with the overpotential gain already observed under catalytic conditions with complex [2] and

[EMIM] + based ILs in acetonitrile (Table 3.6). Nevertheless, no proton source optimization was previously performed for catalyzing CO2RR by complex [2] in aqueous solution, since the catalytic CO2RR behavior of complex [2] in purely aqueous solution remained unexplored thus far. An additional electrolyte effect in CO2RR activity was reached by modifying the proton donor source in solution. Thus, acetic acid from an acetate buffer solution (pH = 3.8) acting as a more efficient proton donor source than Another point where both complexes' interaction with ILs diverged was in product selectivity. Whereas, we saw that the presence of the IL had only a minor and slightly detrimental role on CO2 to CO conversion on complex [1], it drastically impacted complex [2] reaching as high as FEHCOO-≥ 90 % (entries 2 and 3, Table 3.7), taking place at lower overpotentials and improving the energy efficiency of CO2 conversion between +15 % and +22 % as compared to

[TBA][PF6]. This selective conversion of CO2 to formate places complex [2] among the top selective molecular catalysts reported in the literature, which exhibited FEHCOO-= (80 -97 %) 25 . Actually, previously reported results using complex [2] and the benchmark electrolyte ([TBA][PF6]) in electrocatalysis 10 or photocatalysis 58 , never reached FEHCOO-≥ 50 %. CCE results in unbuffered aqueous solution reported in Table 3.9 showed a weaker impact enhancing FEHCOO-in the presence of 3.9). On the one hand, this buffered solution system reached a significant current density (-3.33 mA/cm 2 ) and exhibited one of the lowest overpotentials (0.28 V, entry 4 of Table 3.9) reported so far for a homogenous molecular catalyst producing formate in aqueous solution. On the other hand, this buffered solution also inhibited HER compared to [TBA] [BF4] electrolyte in the same acetate buffered solution (entries 3 and 4 of Table 3.9). For the sake of comparison, we have taken the CPE data reported in the literature 53,59 to calculate the overpotential for two of the scarce examples of a molecular catalyst for formate production in neutral aqueous solution. Specifically, these were an Ir pincer complex 59 and an iron carbonyl cluster [Fe4N(CO)12] -53 , which displayed overpotentials of 0.8 V at -0.60 mA/cm 2 and 0.35

V at -4 mA/cm 2 , respectively. In addition to this, Ir pincer complex required small amounts (ca. 1 %) of acetonitrile in solution, which was not the case for complex [2], working at much higher current density (-3.33 mA/cm 2 ) in purely acidic aqueous solution. As far as we are aware, not a single example of electrocatalytic formate production using a molecular catalyst in that acidic pH has been reported in the literature. This fact is very relevant in order to avoid significant CO2 losses due to carbonate and bicarbonate generation in alkaline and neutral aqueous solutions.

CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

In terms of theoretically interpreting the molecular catalyst-IL interaction by DFT, we only had as initial reference the Matsubara et. al. 38 study done on complex [1]. Thus, DFT calculations herein provide consistent theoretical support to understand the impact in formate production catalyzed by complex [2] by adding [EMIM] + in solution. They illustrate the formation of a π cation•••π adduct between the reduced form of the catalyst and the imidazolium-based cation, which exhibited an overall decrease of the calculated reduction potential matching the experimental results obtained by cyclic voltammetry (Figure 3.13). Moreover, the incorporation of [EMIM] + at the electrode interface was found to increase the free-energy barriers for both CO2RR and HER pathways, this effect being much more pronounced in the HER pathway, which proved HER inhibition by the presence of [EMIM] + at the interface in line with the experimental results in electrolysis (Tables 3.7, 3.8 and 3.9). The stronger impact into the HER pathway demonstrated by DFT calculations can be ascribed to the cationic nature of the [Rh III (bpy •- )(Cp*)H]•••[EMIM] + complex that might have prevented to some extent the approach of other positively charged species such as free protons (H3O + ), decreasing the local concentration of protons around the catalyst and thus disfavoring the HER process via electrostatic repulsion. Thus, based on these calculations and the aforementioned DFT study on complex [1] 38 , we observed that the beneficial effect on the catalytic performance for CO2RR displayed by complexes [1] and [2] in presence of imidazolium based ILs was due in both cases to electrostatic interactions at the electrical double layer between the bipyridine ligand within the molecular catalyst and the imidazolium cation.

Conclusions

In conclusion, for the first time an electrolyte engineering strategy based on comparing the role as cocatalyst of different ILs, including pyrrolidinium and imidazolium based ILs, for CO2 electroreduction with two model molecular catalysts is presented here. For the case of CO2 conversion to CO catalyzed by complex [1], we firstly proved by CV and electrolysis that the use of ILs as a supporting electrolyte in acetonitrile did not only ensure appropriate conductivity, but also displayed co-catalyst features. More particularly, we have demonstrated that under both inert and CO2 atmosphere, the negatively-charged reduced forms of complex [1] (mono-and di-anion intermediates) were electrostatically stabilized by the IL cations. This stabilization gives as a result an important IL cation-dependent overpotential diminution when compared to a conventional benchmark supporting electrolyte. However, this co-catalyst effect of indicated an additional effect of ILs improving the catalytic reaction rate. In the case of complex [1] overall, [BMPyrr][PF6] and [BMIM][PF6] exhibited the lowest overpotential and the highest CO selectivity among the ILs studied in the presence of TFE as a proton source. However, the energy efficiency reached by the conventional benchmark electrolyte (61%, Table 3.4) was never overcome by catalytic performance of complex [1] in presence of ILs.

For the case of CO2 conversion to formate catalyzed by complex [2], ILs were found to interact in a different manner than they previously had with complex [1]. However, studying complex [2] interacting with ILs, we demonstrated a significant impact on the CO2RR performance and particularly its effect on overpotential, current density and products selectivity. The effect of ILs was not as evident under inert atmosphere as the only observable reduction peak of the catalyst remained at the same potential. Even under CO2 reduction conditions, the gain in terms of overpotential and current density was modest. The great difference was actually observed by CCE in terms of product selectivity. We proved that aromatic imidazolium-based IL cations, and particularly [EMIM] + , but not pyrrolidinium-based ILs, played a significant role in suppressing HER, enhancing catalytic current (jcat) and decreasing overpotential (η)

for the CO2RR to formate production catalyzed by complex [2] in both acetonitrile and aqueous solutions. To sum up, complex [2] exhibited FEHCOO-≥ 90 % at low overpotentials (0.5 -0.6 V) and a maximal energy efficiency for CO2 conversion to formate of 66 % in acetonitrile solution in the presence

of [EMIM][BF4] or [EMIM]

[PF6] (Table 3.7). Remarkably, this was the first time that FEHCOO-≥ 50 % is reported for CO2RR catalyzed by complex [2] in acetonitrile solution. Furthermore, complex [2] represents one of the rare molecular catalysts soluble in aqueous solution. However, its catalytic behavior for CO2RR in purely aqueous solution was never reported before. Comparing CO2RR results in acetonitrile and aqueous solutions (Tables 3.8 and 3.9, respectively) revealed that complex [2] became less selective FEHCOO-≤ 49 % and (FEHCOO-/FEH2) decreased to (1/1) in unbuffered aqueous solution.

The use as electrolyte of [EMIM] [BF4] in an acetate buffer solution exhibited the lowest overpotential value (0.28 V, entry 4 of Table 3.9) reported so far for a homogenous molecular catalyst producing formate in aqueous solution. Moreover, this represented the first example of electrocatalytic formate production using a molecular catalyst in acidic aqueous solution (pH = 3.8). The significant role of acetic acid at the electrode/solution interface enhancing catalytic activity for CO2RR might result from its lower pKa and higher concentration than water protons and/or its neutral character, which avoids electrostatic repulsions at the electrode/solution interface.

Thus, the combination of experimental results and DFT calculations provided here allowed us to reach a deeper mechanistic understanding of the promoter role of [EMIM] + at the electrode/solution interface enhancing either selective CO2 conversion to CO or formate and suppressing HER catalyzed by complexes [1] and [2], respectively. This mainly involved the reduction of the potential required to of free protons at the electrode/solution interface, despite both complexes following a different reaction mechanism for CO2RR. In conclusion, the impact of the imidazolium-based IL cation was linked to the interaction between the imidazolium cation and the reduced bipyridine ligand, which was present within the active form of both complexes. Finally, we suggest that other active molecular catalysts for CO2RR containing bipyridine ligands will probably get a significant promoting effect by incorporating imidazolium-based ILs at the electrode/solution interface.

CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

Introduction

The electrocatalytic carbon dioxide reduction reaction (CO2RR) on metal-based heterogeneous catalysts is currently exhibiting high activities at low overpotentials, although key parameters such as product selectivity, stability and low current density still need to improve. Nevertheless, there is an increasing interest in merging homogeneous molecular catalysts to tune heterogenous catalysts to address structurefunction relationship 1 . Various molecular additives have been explored, immobilized on different electrode materials and enhanced catalytic responses have been registered, even though more structureactivity studies are often necessary to be able to control such effects. These molecular additives are usually small organic molecules, including but not restricted to: amines 2,3 , thiols [2][3][4][5] , amino acids 6 , polypyrrole 7 , N-heterocyclic carbenes 8 , N-substituted bipyridines [9][10][11][12] . The effect of the resulting coated electrodes is either an enhancement of CO2RR by stabilization of reduction intermediates that leads to lowered overpotentials and better selectivities 1 , or a suppression of the main parasitic reaction, HER.

Notable examples of the former are the electron donating effect of carbenes on a Pd electrode that tunes selectivity towards formate evolution by stabilizing *HCOO intermediates 8 and the stabilization of atopbound *CO reduction on a copper electrode by arylpyridiniums, that favors further reduction to C2 products and more specifically leads to 72 % ethylene yield at appreciable current densities 11 . On the other hand, molecular additives have been found to dramatically alter selectivity by suppressing HER.

A notable case is that of planar silver electrodes, normally very active towards HER, after coated by pyridinium-based molecules exhibit a CO2-to-CO conversion FE that is over 99 % 12 .

Ionic Liquids (ILs) themselves have often been employed in solution in order to promote CO2RR and tune the reaction selectivity more towards desirable products by the intrinsic ability of their cations, usually belonging to the imidazolium family, to stabilize CO2 reduction intermediates [13][14][15][16] . More recently, a study has also demonstrated their potential in suppressing HER in transition metal electrodes, the chief side-reaction occurring during CO2RR, in very acidic pH by using 0.1 M of [ΕΜΙΜ]Cl in aqueous solutions. 17 More specifically, it was shown that they have this effect only under acidic conditions, during which the imidazolium cations repel the protons approaching the electrode, while, on the contrary, under alkaline conditions the hydrogen source is water that cannot be electrostatically repelled. 17 As far as the ILs immobilization on electrodes is concerned, there have been examples using either diazonium chemistry 18 , thiol groups within self-assembled monolayers (SAM) 19 or covalent grafting. 20 All these methods produce covalently bound IL moieties on the electrode, but to the best of our knowledge they have not been explored in CO2 electroreduction.

Meanwhile, fundamental studies, where electrochemical IL reduction was studied on different electrode materials and the formed products were analyzed by NMR (liquid phase) or GC (gas phase), resulted in many interesting observations 21,22 . The first information was that IL reduction on different materials CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS occurs at different potentials and though the primary imidazolium reduction products are the same, their distribution may vary based on the material's ability to absorb hydrogen. In general, the main imidazolium reduction species are a neutral dimer resulting from one-electron transfer between two already reduced imidazol-2-yl radicals and the disproportionation product of the aforementioned radicals to form a neutral monomer. Possible pathways that result in charged monomers and dimers are proposed, but those species were not observed 21,22 . Moreover, some additional studies have demonstrated the electrochemical reduction of [EMIM] [BF4] on a copper electrode to form an electrochemical layer on the Cu surface that can suppress HER in favor of CO2RR 23 . In particular, the HER suppression effect was not solely observed on an initial CV, that is in accordance with [EMIM] + adsorption on the electrode surface 17 , but persisted and even strengthened in consecutive scans, which the authors attribute to a reaction of electrochemical nature between the electrode surface and the IL 23 .

Having observed the ways in which surface engineering may alter the catalytic performance of heterogeneous catalysts, in this chapter we attempted to apply this principle to ILs. In the previous chapter, we reported the reduction of imidazolium-based cations beyond -2.5 V vs. Fc + /Fc in acetonitrile solution and thus here we attempted to electrochemically modify the working electrode with an imidazolium layer. This layer was then assessed using the molecular catalyst [Rh(bpy)(Cp*)Cl]Cl (referred as complex [2]) as a catalytic model under different CO2 catalytic conditions as they were defined in the previous chapter and characterized by electrochemistry and different surface analysis techniques.

Results

IL Layer formation and optimization by testing on [Rh(bpy)(Cp*)Cl]Cl model catalyst

The reduction of imidazolium-based cations in acetonitrile solution has been reported in literature 21,22 as well as witnessed in the previous chapter. As can be seen in Figure 4.1, we observed a reduction wave when cycling either a GCE or a AuE in 0.5 M [EMIM][PF6] solution in acetonitrile under an inert atmosphere beyond their reduction limits (-2.50 V vs. Fc + /Fc in GCE) or (-2.10 V vs. Fc + /Fc in AuE), respectively. A reoxidation peak is observed when the potential reaches -0.7 V, but is only observed as long as the imidazolium is first reduced. ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS Subsequently, the modified GCE was removed from the solution, rinsed by acetonitrile and placed in a solution containing the model molecular catalyst [2] in acetonitrile solution containing 0.5 M

[TBA][PF6] as supporting electrolyte under catalytic conditions: saturation of CO2 and 5 % (vol.) H2O.

The aim was to compare the CO2 catalytic response of complex [2] by this IL-modified electrode towards a bare electrode with 0.5 M of the same IL in solution. Furthermore, a CV of the bare electrode in standard benchmark conditions, i.e. 0.5 M [TBA][PF6] was also shown. As seen in Figure 4.2, we immediately remarked that the CVs of the IL-modified electrode and the bare electrode containing IL in the solution had identical shapes with the same onset potentials, which were more anodic than those of the bare electrode in benchmark conditions. Moreover, the presence of the IL provoked an enhancement of almost 1.5 times larger current, independently if the IL is in solution or immobilised on the electrode. This sort of electrode modification by IL reduction will be henceforth referred to as an electrodeposited IL layer.

Cyclic voltammograms under catalytic conditions of 1 mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on: a bare GCE (black plot) and an IL-modified CGE by electrodeposited layer (CPE of 10 s at -3.95 V in 0.5 M of [EMIM][PF6] in acetonitrile) (red plot). Also, CV of bare GCE in a solution containing 0.5 M [EMIM][PF6] in acetonitrile as supporting electrolyte (green plot). Catalytic conditions: acetonitrile solution under COwith 5 % H2O (vol.).

Scan rate 0.01 V/s. WE=GCE.

A deeper optimization study of the IL-immobilization was carried out. Different parameters were tested and assessed by the best catalytic response of complex [2] towards CO2 in terms of simultaneously lowering the overpotential and increasing the current density. First of all, we studied the IL reduction potential window, as presented in as supporting electrolyte in argon saturated acetonitrile solution, rinsing the electrode and then study of the modified electrode in a new solution of complex [2] under CO2 catalytic conditions. Comparing the catalytic behavior displayed by complex [2] in Figure 4.4, Ecat did not vary significantly whereas, on the contrary, the maximal current density appeared to have an inverted relation with the time of CPE; producing the higher current density value the shorter electrolysis. Thus, the selected conditions for the rest of the optimization experiments were 10 s CPE at -3.95 V.

M [EMIM][PF6] acetonitrile solution (red plot) and in 0.5 M of [EMIM][BF4] acetonitrile solution (blue plot) or (b) 0.5 M [EMIM][PF6] acetonitrile solution (red plot) and in 0.5 M of [BMIM][PF6] acetonitrile solution (blue plot). Catalytic conditions: acetonitrile solution under CO2

with 5 % H2O (vol.). Scan rate 0.01 V/s. WE=GCE.

M [EMIM][PF6] acetonitrile solution) : Under Argon (red plot) and under CO2 (blue plot). CV of bare GCE (black plot). Catalytic conditions: acetonitrile solution under CO2 with 5 % H2O (vol.). Scan rate 0.01 V/s. WE=GCE.

The reduction and electrodeposition of imidazolium-based ILs is also possible in aqueous solutions, as can be seen in Figure 4.8. In this case, a GCE was modified by CV until -2.5 V vs. Ag/AgCl/KClsat in an aqueous solution containing 0.5 M [EMIM] [BF4] as supporting electrolyte under an inert atmosphere, since salts containing [PF6] -anion are not soluble in water. The CV limit chosen was less cathodic in these conditions, since beyond that point H2 bubbles were evident on the electrode surface. Subsequently, this GCE was removed from that solution, rinsed and its catalytic activity for CO2RR was evaluated in a new acetonitrile solution of complex [2] (black plot in Figure 4.9). In addition to this, we also explored the catalytic effects on CO2RR of immobilizing ILs following a different synthetic route already described in the literature 20 , which gives as a result the formation of a covalently grafted IL layer on the electrode. Basically, this synthetic route follows two steps: firstly, the electrochemical grafting of 2bromoethylamine to the electrode surface by oxidation, which leads to the attachment of a layer bearing a Br end group and an amine grafted to the electrode. Thus, a second step based on the chemical reaction between the Br terminated layer and the 1-ethylimidazole in acetonitrile solution. The result is a layer bearing imidazolium-based IL moiety attached on the electrode surface. The catalytic effect for CO2RR of this covalent layer was never explored before with molecular catalysts. Therefore, in Figure 4.9, the effect of both electrodeposited [EMIM] + layer in aqueous solution and covalent [EMIM] + layer was evaluated under CO2 catalytic conditions of complex [2]. We can observe that the onset catalytic ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS potential is unaltered regardless of the solvent in which the electrodeposition took place, as well as being nearly identical to the one belonging to the covalent IL layer. However, in the case of the covalent layer, the maximum current density observed is larger than the electrodeposited layer.

M [EMIM][PF6] acetonitrile solution (red plot), an IL-modified GCE (electrodeposited IL layer grown by CPE of 10 s at -2.50 V vs. Ag/AgCl/KClsat in 0.5 M [EMIM][BF4] aqueous solution) (black plot) and an IL-modified GCE by a covalent grafting of IL layer as described in the literature 20 (blue plot). Also, CV of bare GCE in a solution containing 0.5 M [EMIM][PF6] in acetonitrile as supporting electrolyte (green plot). Catalytic conditions: under CO2 with 5 % H2O

(vol.). Scan rate 0.01 V/s. WE=GCE.

CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

Electrodeposited IL Layer testing in different solvents

We subsequently explored the effects of the aforementioned optimized electrodeposited IL layer in the CO2RR catalytic behavior of complex [2] using different solvents, always exploiting complex [2]'s solubility in both organic and aqueous media. In Figure 4.10 we explore a 50/50 % (vol.) acetonitrile/water mixture. We compared the catalytic response on a bare GCE without any IL in solution (black plot), a bare electrode in solution containing IL as supporting electrolyte (green plot), the electrodeposited IL layer (red plot) and the covalent IL layer (blue plot) under the same catalytic conditions. We first remarked that in terms of onset potential both IL layers closely resemble each other, as well as the IL in solution, being all three less cathodic than the control conditions (bare WE without IL). In terms of the current density observed however, both IL-modified electrodes produced a larger catalytic density, which was similar than in acetonitrile/water 95/5 % (vol.), than the bare GCE whether or not IL is present in solution. complex [2] on either a bare GCE (black plot) or an IL-modified GCE by an electrodeposited layer (red plot). Firstly, we observe that the passage from an acetonitrile/H2O 95/5 % (vol.) to an aqueous solution entails a significant decrease in current density. This is consistent with the decrease in CO2 solubility from 240 mM to 33 mM 13,15,24 . Aside from this effect however, the addition of an electrodeposited IL layer on the GCE surface leads to both a decrease in the catalytic onset potential and a modest increase in terms of current density in regards to the bare electrode. In

Electrodeposited IL layer impact on CO2RR catalyzed by [Re(CO)3(bpy)Cl] in acetonitrile

Even though the main work on layer optimization was done using complex [2] as shown above, complex [1] was also tested with the electrodeposited layer to demonstrate whether such a surface modification strategy could produce an effect with other molecular catalysts. For this reason, in Figure 4.12 we compare the effect of an IL electrodeposited layer on the CO2 catalytic activity of 1 mM complex [1] in acetonitrile solution, with 0.5 M of the same IL in solution and the control conditions (bare GCE, no IL either in solution nor immobilized). This figure shows the CVs under argon (Figure 4.12 (a)), under CO2 (Figure 4.12 (b)) and finally upon explicit addition of 1.5 M TFE as a proton source (Figure 4.12 (c)).

CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

We can observe that the electrodeposited IL layer produced an enhanced catalytic effect by decreasing the overpotential compared to control conditions with this molecular catalyst as well. Under inert conditions the reduction waves of complex [1] shift to less cathodic potentials when an electrodeposited IL layer is present, however the effect is not as prominent as with the same IL in solution on a bare electrode. Under CO2 the bpy-centered initial wave is superposable for both ILs, in solution and electrodeposited layer, even if the second Re-centered reduction wave is significantly lower in terms of current density for the electrodeposited IL layer than for the IL in solution. Finally, in the presence of the external proton source the onset potential for immobilized IL and IL in solution is essentially identical. The physicochemical characterization of the IL layer was done by X-ray photoelectron spectroscopy (XPS). In order to obtain clear spectral signature analysis of the IL layer, gold was selected as a more proper substrate electrode material than GCE. Given the similarities in CV redox features (Figure 4.1) and IL surface coverage, we considered gold to be a well-adapted model to perform XPS analysis on.

For this purpose, three gold plates were prepared and sent for analysis, two with electrodeposited IL layers grown in an acetonitrile or aqueous environment and one with a covalent grafted IL layer. The survey XPS spectra shown in

M of [EMIM][BF4] in water) and (c) covalent grafted IL layer. Bond attributions colored as follows: C-C, C-H (red plot), C-OH, C-O-C, C-N (yellow plot), C=N (blue plot), C=O (green plot) and O-C=O (pink plot). Also XPS high-resolution spectra for N (1s) on (d) ILmodified AuE (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M of [EMIM][PF6] in acetonitrile), (e) ILmodified AuE (CPE of 10 s at -2.5 V vs. Ag/AgCl/KClsat in 0.5 M of [EMIM][BF4] in water) and (f) covalent IL layer. Bond attributions colored as follows: pyridinic N (red plot), pyrrolic N (blue plot) and quaternary N (green plot).

Two redox probes were tested for reaching a deeper characterization of the electrodeposited IL layer. In particular, dopamine and ferrocene, an inner-and outer-sphere redox probe, respectively. Figure 4.17 (a)

shows the electrochemical behaviour of dopamine studied by CV in acidic aqueous solution. Dopamine presents a reversible redox signal and follows an inner-sphere mechanism. As a consequence the presence of any immobilized species on the electrode surface provokes an increase in the redox peak-topeak separation. This is indeed what we observed when comparing the bare GCE ( peak-to-peak separation value of 0.08 V) and the electrodeposited IL-modified GCE (0.2 V). In addition to this, the covalent IL layer (blue plot) trumps the electrodeposited one (red plot) in terms of peak-to-peak separation by exhibiting a value of 0.32 V, that may be indicative of a more efficient immobilization.

Electrodeposited IL layer impact on HER catalyzed by [Rh(bpy)(Cp*)Cl]Cl

Complex [2] exhibits the same catalytic features by CV in the presence of the electrodeposited imidazolium layer at the electrode surface than upon the presence of imidazolium-based ILs in solution, as has been shown in Figure 4.2. Thus, according to the experimental data and DFT calculations already presented in Chapter 3, we must not exclude that the electrodeposited IL layer remains positively charged, since it mimics the impact of imidazolium cations in the electrical double layer. Thus, our next goal was to understand the nature of the electrodeposited IL layer whether it was positively charged or resulted in a neutral molecular layer. To investigate that we used a positively charged inner-sphere redox probe whose electrochemical response would be able to distinguish between the presence (repulsion electrostatic effects) and absence (no repulsion electrostatic effects) of a positively charged layer on the electrode surface. For this purpose, HER was selected, since we could perform it with different added proton sources (trifluoroethanol (TFE), acetic acid (AcOH)) and it is also a reaction that complex [2] successfully catalyzes.

CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

The next step was to test the electrodeposited IL layer alongside complex [2] for HER. For this purpose, 0.1 M of acetic acid was selected as the proton source. The catalytic behavior of complex [2] for HER was evaluated with and without the electrodeposited IL layer in

Electrolysis for CO2 reduction with immobilized ILs and catalyzed by [Rh(bpy)(Cp*)Cl]Cl.

Impact in reaction selectivity

The effect of the electrodeposited IL layer on product distribution and energy efficiency for CO2RR was studied by CCE electrolysis in an H-type cell at -3.33 mA/cm 2 of applied current density firstly in acetonitrile/H2O 95/5 % (vol.) solutions. The results of these electrolysis were compared to those already reported in Chapter 3 on bare electrodes with and without ILs in solution. In all experiments, the only liquid product detected was formate and only H2 was detected in the gas phase. Table 4.1 shows the CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS average cathode potential during electrolysis, overpotential, faradic efficiencies for both products and energy efficiency for CO2 conversion to formate as a function of the presence or absence of different IL layers. In Figure 4.20 are represented the plots of electrolysis time vs. cathode potential (corresponding to Table 4.1, entries 1-4), as well as the thermodynamic potential for CO2 reduction to formate, which for those conditions has been calculated at -1.32 V vs. Fc + /Fc 30 . We observe that the presence of either an electrodeposited or a covalent grafted IL layer led to a reduction in overpotential very similar to the one displayed by a bare electrode with imidazolium-based IL in solution already reported in Chapter 3 (Table 3.8). This overpotential diminution is in the range of 800 mV. In addition to this, the formate production, already enhanced from 53 % (entry 1), to 69 % (entry 2) by the presence of [EMIM] + instead of [TBA] + in solution, is further enhanced by an additional (12-16) % in the presence of either electrodeposited (entry 3) or covalent (entry 4) IL layer and [TBA] + in solution. This increase in product selectivity for formate in the presence of the IL layers is also reflected on the energy efficiency values, which reached 59 % as a maximum value reported in this thesis in acetonitrile solution at -3.33 mA/cm 2 obtained either by electrolyte engineering or electrode surface engineering strategies. Finally, by increasing the % of H2O present in solution to 50 % (entries 5 and 6 of Table 4.1), the beneficial effect of the electrodeposited IL layer was still evidenced and resulted in a product ratio (FEHCOO-/FEH2) enhancement of more than double from 1.5/1 on the bare electrode (entry 5) to 3.75/1 on the IL-modified GCE (entry 6). a Determined based on Ecat/2 using E 0 CO2/HCOOH (CH3CN, H2O) = -1.32 V vs. Fc + /Fc. b Faradaic efficiency for each product (FEHCOOH and FEH2). c Energy Efficiency = ET/E ⋅ FE, where ET = E o CO2/HCOOH (CH3CN, H2O) = -1.32 V vs. Fc + /Fc.

We also performed CCE (applied current density -3.33 mA/cm 2 ) with complex [2] and either the electrodeposited or covalent IL layer on the electrode in purely aqueous CO2-saturated solution. These electrolysis results are shown in Table 4.2 and were compared to some of those already reported in Chapter 3 (Table 3.9) on bare electrodes with and without ILs in solution. We tested three different the change in product selectivity was drastic. The products ratio (FEHCOO-/FEH2) did pass from 0.6/1 on the bare electrode (entry 1) to almost double than that at 1.2/1 (entry 2) and 1/1 (entry 3) on the modified RVC electrodes with electrodeposited and covalent IL layers, respectively. As a consequence, the formate EE increased up to 25 % with an electrodeposited layer (entry 2), 1.5 times higher than the EE on the bare electrode.

At this point, we decided to combine both strategies developed in this thesis for enhancing CO2RR. On the one hand, electrolyte engineering by selecting 0.5 M [EMIM] [BF4] as unique electrolyte in solution (entry 4) and on the other hand, electrode surface engineering strategy by testing the different IL layers immobilized on the cathode (entries 5 and 6). A decrease of 100 and 200 mV in overpotential compared to the bare RVC cathode was observed for the electrodeposited and covalent layers, respectively (entries 5, 6). However, their overall overpotential was 400-500 mV higher than the one containing 0.1 M AcOH at the same pH value. In addition to this, a significant change in product selectivity was observed. The products ratio (FEHCOO-/FEH2) did pass from 1.1/1 on the bare electrode (entry 4) to a maximum of 1.7/1 on the electrodeposited IL layer modified electrode (entry 5). Nevertheless, even though the combination of ILs in solution and immobilized at the electrode surface favored formate production over HER in acidic aqueous solution, the resulting EE values were generally lower than those obtained in the presence of a proton source, such as AcOH (entries 2-3). For this reason, we also tested the addition of AcOH in solution by using an acetate buffer solution at pH=3.8 (entries 7 and 8), which was already successfully tested in Chapter 3 upon evaluation of the electrolyte engineering strategy. Thus, entry 8 in Table 4

Electrolysis for CO2 reduction with immobilized ILs and catalyzed by [Rh(bpy)(Cp*)Cl]Cl.

Performance stability evaluation.

The results displayed in Table 4.2 have demonstrated that electrodeposited IL layer on the electrode surface impacted complex [2]'s performance for CO2RR and in particular, the product selectivity by enhancing formate production. However, all electrolysis described in Table 4.2 lasted for 10 C (~ 17 min). For this reason, some additional longer electrolysis in H-type cell were performed to evaluate the performance stability of the IL-modified cathode before scaling up the process in a flow electrolyzer, which will be described in the next chapter of this thesis (Chapter 5). To assess that, we performed three (either a bare RVC foam or an IL-modified RVC foam) and using a fresh solution before the first and the third electrolysis (0.1 M [TBA][BF 4 ] and 0.1 M acetate buffer (pH=3.8)). Moreover, to overcome the limited solubility of CO2 in aqueous solution we decided to use an open configuration of the H-type cell for electrolysis, meaning that CO2 was continuously bubbled through the solution during the experiment, even if it rendered H2 measurements impossible. Figure 4.22 shows that the formate FE values were fairly repeatable when comparing the first and the third electrolysis in both electrodes, but decreased significantly during the second electrolysis. That decrease in performance was attributed to the accumulation of formate in solution after 50 C and for this reason, the solution was substituted by a fresh one before starting the third electrolysis. Then, the IL modified-cathode exhibited FEHCOO-of 52 % and 53 % in the first (10 C) and the third (10 C) electrolysis, respectively, while the bare electrode produced 41 % and 39 % of formate. Therefore, we consider that a single electrodeposited layer could withstand at least for three consecutive electrolysis and accumulate 60 C without losing any of the benefits it provides in terms of selectivity towards formate production.

Discussion

Other methods of electrode surface engineering have been previously reported in heterogeneous electrocatalysis using a range of usually small organic molecules with the goal of tuning product selectivity and decreasing energetic barriers for CO2RR [1][2][3][4][5][6][7][8][9][10][11][12]31 . ILs themselves have in some cases exhibited the ability to decrease overpotentials and to drive certain systems towards CO2RR [13][14][15] by suppressing HER 17 , which makes them interesting candidates for surface modification. In particular, the work from T. Agapie group in Caltech (USA) [10][11][12] has demonstrated the relevant impact in products selectivity achieved by electrodeposition of an IL layer on copper and silver electrodes for CO2RR.

Actually, those authors use a strategy based on electro-dimerization of IL cations, which is very similar to the one presented in this thesis, but using arylpyridinium instead of imidazolium IL cations. However, the role of electrodeposited IL layers has never been addressed before in CO2 conversion by molecular electrocatalysts. Thus, this fact highlights the originality of the work presented in this thesis.

In this chapter, we present a rapid, in-situ and efficient electrochemical deposition method of imidazolium-based IL cations. This is based on a protocol that relies in imidazolium-based ILs's intrinsic reduction and subsequent reoxidation features and can be performed in both organic and aqueous solutions. This electrodeposited IL layer was tested for CO2 electroreduction in different electrolytes alongside model molecular catalyst [Rh(bpy)(Cp*)Cl]Cl (complex [2]) and managed to increase catalytic activities. We also demonstrated that this surface modification strategy could produce a positive effect on CO2RR catalysis with other molecular catalysts such as complex [1]. More pertinent though was its effect in driving complex [2]'s selectivity towards formate versus H2 evolution. Actually, the electrodeposited IL-modified electrode exhibited a superior electrochemical performance catalyzing CO2RR by complex [2] than the bare electrode upon presence of the same IL in solution, which seems to indicate that the arrangement of the IL cations layer at the electrode interface is a little bit more efficient than the free cations arrangement at the electrical double layer. Thus, throughout this chapter we have compared the catalytic performance for CO2RR of the imidazolium based electrodeposited layer with the effect played by the imidazolium-based ILs in solution reported in Chapter 3 and an additional covalently grafted IL layer synthesized from previous reports in the literature. The nature of the electrodeposited IL layer was evaluated by surface coverage evaluation, XPS analysis and CVs in the presence of 3 different redox probes (dopamine, ferrocene and protons).

Firstly, an extensive optimization study of the synthetic parameters to obtain the electrodeposited IL layer was carried out using complex [2] catalytic performance for CO2RR in acetonitrile solution as evaluation tool. In particular, the production of formate was used as key parameter to select the optimal CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS synthetic conditions. Two synthetic protocols were established either based on scanning the applied potential by CV or applying a constant potential by CPE. Studies of the cathodic potential limit for the IL layer deposition by CV (Figure 4.3) show that the electrodeposited layer is more efficient the further we scan cathodically and as a consequence the more efficiently imidazolium was reduced. On the contrary, in terms of CPE length, longer times of applied constant potential led to electrodeposited layers with lower catalytic performance (Figure 4.4), pointing towards a certain optimal thickness to promote CO2RR catalytic activity. The nature of the IL used for synthesizing the IL layer ( that the role of the IL anion is minor on the catalytic performance of the IL layer, but that the [EMIM] + cation produces superior catalytic performance than the [BMIM] + cation, probably due to a more closely packed layer achieved using less sterically impeded [EMIM] + cations. Finally, the electrodeposition protocol necessitates an inert atmosphere, as it is not efficient under a CO2 atmosphere (see Figure 4.7). This is probably due to the interaction of reduced imidazolium species with CO2 molecules to form adducts that are liberated towards the solution and as a consequence cannot be immobilized on the electrode surface. Subsequently, we studied the optimized electrodeposited IL layer in different electrolyte compositions alongside the covalent IL layer and a bare electrode with IL in its solution. The blatant similarity in terms of catalytic activity of the electrodeposited IL layer with a bare electrode when IL is in solution (see Figure 4.2) and with the covalent IL layer (see Figures 4.9 and 4.10) led us to consider that they may also have some structural similarities. It should be mentioned that in the case of the covalent grafted IL layer we know from the literature data that the attached molecule at the solution side is an imidazolium ring with a positive charge (quaternary N). XPS results further corroborated the hypothesis that both the electrodeposited and the covalent layers are similar in nature, because in both layers the same band attributions were present (Figure 4.16). This led us to infer that some sort of imidazole moiety was bound on the electrode surface. Moreover, the integration of the oxidation wave shown in the reverse CV scan after electrodeposition allowed us to evaluate the surface coverage. In particular, the IL surface coverage determined for the flat GCE case was 2.3 10 -7 mol/cm 2 . A further confirmation of an electrode surface modification came from the use of the inner-sphere redox probe dopamine, whose peak-to-peak separation was visibly larger for the electrodes with IL layers compared to the bare electrode. This effect was comparatively larger for the covalent grafted IL layer, possibly due to a higher surface coverage due to a higher packing of the less hindered amine grafting to the electrode. Unfortunately, the surface coverage evaluation done for the electrodeposited IL layer on both GCE and AuE (Figure 4.15), was not applicable to the electrodes with a covalent IL layer, since there was no reoxidation peak to measure in that case. The other interesting point witnessed, was the electrodeposited layer's effect on HER catalyzed by complex [2]. Our working hypothesis in that study was that if the electrodeposited IL layer was positively charged, the approach of protons in close proximity to the cathode would be hampered due to electrical repulsion between two positive charges and this could be represented by either HER onset being shifted to more cathodic potentials, overall current drop or both. On the other hand, considering the electrodeposited IL layer as a neutral molecular layer, no particular difference would be visible in the CVs of HER catalyzed by complex [2] with and without an electrodeposited IL layer. Therefore, the observed HER suppression effect (see Figures 4.18 and 4.19) could be indicative of positive charges present on the coated electrodes' surface that repel protons from the cathode surface. However, this difference observed in aqueous acidic conditions cannot be witnessed in neutral or alkaline media, as the proton source will no longer be protons (H3O + ), but instead water molecules or hydroxyl anions.

This HER suppression effect demonstrated by the electrodeposited IL layer translates also in terms of product selectivity, increasing formate production compared to control conditions (bare electrode) by +32 %, +28 % and +13 % in 95/5 % (vol.) acetonitrile/water, 50/50 % (vol.) acetonitrile/water and 100 % 0.1 M AcOH aqueous solutions, respectively. Theoretical studies described in section 3.2.8 of Chapter 3 of this thesis also corroborated the suggestion that enhanced selectivity for formate is due to decreased concentration of protons in the electrode's vicinity induced by repulsion from the positively charged layer. This also leads to the highest EE values for complex [2] at -3.33 mA/cm 2 (59 %) with comparison to the previous chapter as well (Chapter 3), demonstration that immobilizing an IL at the electrode surface enhances +12 % in EE for formate production in comparison with adding the same IL in solution (entries 2 and 3 in Table 4.1). The relevant increase in FEHCOO-between a bare electrode and one with an electrodeposited IL layer in acetonitrile/water 50/50 % (vol.) solution containing [TBA][PF6] together with a significant decrease in overpotential of approximatively 500 mV, led to an increase in EE of +23 % (entries 5 and 6 of Table 4.1). Finally, in aqueous solution the presence of acetic acid in solution was shown to be vital in order to observe the beneficial effect of the IL layer in terms of formate selectivity, as well as overall overpotential minimization. As a consequence, using both buffered acetic conditions and an electrodeposited IL layer (entry 8 in Table 4.2) we managed to get the best yet EE of complex [2] in acidic aqueous solution, at 33 %. Furthermore, the catalytic performance of this electrodeposited layer was proven to be stable, as it could maintain its FE for formate and even increase its EE over consecutive CCE.

So far, we have not enough evidences to propose an unambiguous structure and composition for the chemical species synthesized by imidazolium-based IL electrodeposition. Nevertheless, supported by previous studies reported in the literature using arylpyridinium [10][11][12] and imidazolium IL cations 21,22 , electrodimerized species are the most likely species formed during the IL cation electroreduction

Conclusion

In conclusion, in the present chapter we have presented a method to synthesize an IL layer based on electrodeposition, applicable to at least two different electrode material surfaces (GCE and AuE). It was also shown that this in-situ, simple and electrochemically induced layer deposition protocol can be performed in either acetonitrile or aqueous solutions. A schematic representation of this electrodeposited layer and its catalytic role are shown in Scheme 4.1. The formation of a layer at the electrode surface containing immobilized imidazolium moieties was confirmed and characterized by cyclic voltammetry and XPS analysis. It was furthermore compared and contrasted with a known covalent grafted IL layer to draw information of the layer's nature based on their similarities and differences, the latter proving minimal. The effect of this electrodeposited IL layer on CO2RR was studied and optimized when in use alongside molecular catalyst [Rh(bpy)(Cp*)Cl]Cl and it produced a catalytic effect similar to the one of imidazolium ILs in solution. However, this electrodeposited layer was also tested with another molecular catalyst, [Re(CO)3bpyCl], to prove that its interaction with different catalytic systems was not excluded.

It was also demonstrated that this layer effectively suppressed HER in different electrolyte conditions based on its presumed cationic nature. Finally, through electrolysis we were able to assess its beneficial effect in driving product distribution towards formate rather than H2 to varying extends in different electrolytes, exhibiting a more marked effect in acetonitrile than in aqueous solutions. Finally, a singular electrodeposited IL layer was proved to result in the same product selectivity for several consecutive electrolysis, leading to the conclusion that its catalytic performance was stable.

CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION

Introduction

In this chapter, we test the feasibility of using the homogeneous molecular catalyst [Rh(bpy)(Cp*)Cl]Cl (referred to as complex [2]), which has been already studied in Chapters 3 and 4 as a catalytic model for CO2 conversion to formate in an H-type cell, but scaling the electrodes size up to 10 cm 2 in a flow electrolyzer. This approach combines the high selectivity of a molecular catalyst with the scalability of a flow electrolyzer, which should allow a significant increase in the catalytic current density achieved.

The development of efficient, selective and stable catalysts for CO2 electroreduction has long been a subject of study 1,2 . For these catalysts to be able to be considered economically viable for CO2 electroreduction at an industrial scale they need to simultaneously meet a few criteria. Those are exhibiting high activity, as illustrated by partial current density values exceeding 150 mA/cm 2 , for CO2RR products like CO, while maintaining overall cell overpotential (|ηcell|) below 1 V and overall good catalyst stability 3 . However challenging it may be fitting into these criteria 4 , testing electrocatalysts in a flow electrolyzer, remains the way of assessing a catalyst's potential application at larger scale. Flow electrolyzers are also often called flow-cell or small-scale reactor. This is the case herein, because screening tests performed in H-type cells in the previous chapters, though very useful in pinpointing the optimal catalytic conditions, cannot substitute actual experiments at much higher partial current densities achieved within a flow electrolyzer 5 . In particular, the electrolyte engineering strategy developed within Initially, the benchmark conditions without the presence of ILs were explored in different flow electrolyzer configurations to reach optimal CO2RR catalytic performance and demonstrate the system's stability and product efficiency at charge values up to hundred-fold higher than previously tested in Htype cell. Finally, the effect of an electrodeposited IL layer on the cathode within the optimized flow electrolyzer configuration was investigated and compared to the results obtained in benchmark conditions. However, in order to better organize this study and facilitate comparison of catalytic performance with other flow electrolyzers with molecular catalysts we needed a set of criteria. Here below are mentioned some parameters that are relevant to the flow electrolyzer tests presented throughout this chapter:

CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION [1] Galvanostatic versus potensiostatic tests. Throughout this chapter we have opted for galvanostatic control by using an external power source connected between cathode and anode that maintains the cathodic current value stable for a set time period, while simultaneously tracking the cell potential with the aid of a voltmeter. The alternative would have been to apply a controlled cathode potential and follow the evolution of the observed current, which requires an additional reference electrode and a potentiostat.

Thus, galvanostatic approach was considered closer to industrial scale.

[2] One compartment versus two compartments electrolyzer configuration. Either CO2RR is performed in a one compartment cell sharing the same solution for both cathode and anode or a separator is used, such as an ion selective membrane or a polymeric porous separator, that creates two separate compartments for the cathode and anode respectively. In this latter case CO2RR is performed in a two compartment configuration and the catholyte and anolyte compositions can be kept the same of be different.

[3] Recirculating flow versus single-pass flow electrolyzer configuration. During recirculating flow mode, the electrolyte solution is in a constant feedback loop through the electrolyzer for the duration of the experiment, which modifies the solution composition as a function of electrolysis time. On the contrary, in single-pass configuration, there are separate input and output electrolyte recipients ensuring all electrolyte passes through the electrolyzer only once and only initial solution composition is in contact with the electrodes.

[4] CO2 solubilized in liquid-phase versus direct gas-phase feeding. In the first case the cathode is typically a planar electrode, CO2 reaches it after first being solubilized in the electrolyte solution and the reduction reaction takes place on the cathode surface. This process is illustrated in Figure 5.1 (a) and it is limited by the CO2 solubility in each particular solvent (33 mM at 25 ºC in aqueous solution). On the other hand, direct gas-phase feeding allows CO2 to be fed directly through the electrode without necessarily being bubbled in the electrolyte solution. The cathode used in this case is referred to as Gas Diffusion Electrode (GDE) 6 , it results in minimizing CO2 mass transport limitations within the electrolyzer. Therefore, the cathode acts as a three-phase interface where the electrode itself encounters on each side the electrolyte and the CO2 gas, the catalytic reaction taking place only within its confines, as is illustrated in Figure 5.1 (b). The CO2RR products are then released according to their nature, gaseous products towards the gas phase and liquid products accumulating in the electrolyte 6 . Many electrolyzers that contain GDEs with heterogeneous precious metal catalysts are approaching industrially interesting catalytic activity 7 . Moreover, there are an increasing number of examples in recent years where heterogenized molecular catalysts are also supported on GDEs [8][9][10][11] . However, according to the available literature at this time, there are no examples of molecular catalysts in solution tested with ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION GDEs, nor any examples of IL electrodeposition on GDEs. Therefore, the present study aimed to explore these topics for the first time. The vast majority of molecular CO2 electrocatalysts used in flow electrolyzers described in the literature are in fact heterogenized molecular catalysts 7,[9][10][11][13][14][15] . Heterogenization of molecular catalysts especially on carbon supports and subsequent use of them in flow electrolyzers presents advantages. These include enhancing their catalytic efficiency in some cases 7,[9][10][11][13][14][15] and permitting their use in aqueous solution instead of less green organic solvents. Both these considerations are often pertinent in an industrial setting. However, using grafted molecular catalysts in flow electrolyzers also has one inherent limitation when reviewed from an industrial point of view. That consists of the necessity to force a flow electrolyzer to regularly go off-line whenever the heterogenized catalyst system needs replacement due to degradation issues. This process results in increased cost, resources and energy expenditure that counterbalances the advantages of continuous reactions, which are not only economically preferable, but also integral to green chemistry 16 .

To the best of our knowledge, the only not grafted molecular catalyst for CO2RR that has been tested in a flow electrolyzer to date is [Ni(cyclam)] 2+ in organic solvents (ACN and N, N-DMF) containing 0.5

M of [ΝΗ4]

[PF6] as both supporting electrolyte and sacrificial proton donor in the catholyte. Graphite felt electrodes were used as both cathode and anode. Meanwhile, in the anodic compartment the reaction taking place was the oxidation of 0.1 M ferrocene (Fc), which acted as a sacrificial electron donor. This system produced CO and their best FE for CO observed was 83 % at an |Ecell| value of 1.7 V, but only at the moderate current density value of 12.7 mA/cm 2 . The maximum current density of 18.9 mA/cm 2 reported in this study was achieved at an |Ecell| of 1.8 V and resulted in 80 % of CO, with H2 being the CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION only other detected product. It should also be mentioned that these results were observed in a twocompartment recirculating flow electrolyzer configuration using 10 mM of [Ni(cyclam)] 2+ in solution 17 .

In order to assess the scalability of the catalytic conditions already reported using a H-type cell in Chapters 3 and 4 , those conditions were transposed in a commercially available flow electrolyzer, seen here assembled and disassembled in Figure 5.2. This flow electrolyzer that can accommodate electrodes of 10 cm 2 geometric area or others of similar size have been used in several previous studies 9,17,18 to illustrate the applicability of such systems in even bigger scale, that may be of industrial interest. In this chapter, we explored different electrolyzer configurations using the [Rh(bpy)(Cp*)Cl]Cl catalyst in acidic aqueous catholyte. This model catalyst was selected since it provided two main advantages.

Namely, it was soluble in purely aqueous solutions allowing us to scale-up the aqueous conditions already detailed in the previous chapters and its selectivity towards formate and H2 was highly tunable by its environment, which made it a model system with which to assess the impact of the IL electrodeposited layer at larger scale. The reaction conditions were initially optimized on bare carbonbased electrodes (graphite plate or GDE) and eventually, a cathode with an electrodeposited IL layer was also tested. ). Overall, this configuration for all conditions tested led to formate faradaic efficiency (FE) values that ranged from 1.7 to 6.5 %. It was probably due to either the oxidation of electrogenerated formate or the rapid oxidation of the active form of complex [2] before performing CO2RR, in both cases at the anode. Another note should be made on the |Ecell| values, which ranged from 2.2 to 3.5 V for all tests. Based on the assumption that only oxygen evolution reaction (OER) from water oxidation takes place in the anode (2H2O(l) ↔ 4H + +O2(g) +4e -E°=1.01 V) 20 and CO2RR to formate in the cathode (CO2(g) + 2H + + 2e -↔ HCOOH(l) E°= -0.42 V vs. NHE) 1 both at pH=3.8, the calculated absolute thermodynamic cell potential would be 1.43 V. Finally, the final value of the solution pH was not significantly modified during the electrolysis by comparing the average 3.6 versus the initial 3.8 value. It should also be stressed that no other CO2 reduction products were detected in the liquid phase, while the experimental setup did not allow characterization of the gas phase. The assumption made in this case was that the only other undetected product is hydrogen, since the same setup had been tested in an H-type cell configuration and the only products were formate and hydrogen.

Two-compartments recirculating flow configuration for gas-phase CO2RR

Having adopted the two-compartment recirculating flow configuration, we subsequently turned our attention to performing gas phase CO2RR, in order to be able to optimize the performance of the system and especially increase the applied current density. The catholyte and anolyte compositions remained 0.1 M acetate buffer at pH=3.8 saturated with [TBA][BF4] containing 1 mM [Rh(bpy)(Cp*)Cl]Cl and 0.5 M KOH respectively. The anode remained a 10 cm 2 DSA, while as cathode a commercially available carbon-based GDE (Sigracet 28BC, built by a hydrophobized gas diffusion layer (5 wt. % PTFE) and a microporous carbon layer) of the same geometrical area was employed. In this configuration, CO2 flow was directly pumped in gas phase from the back side of the GDE through a different entry than the catholyte in order to ensure their combined presence only on the electrode interface. However, prior to starting a test, CO2 was also directly bubbled in the catholyte for 5 minutes to remove oxygen. The direct CO2 flow rate through the electrolyzer had not undergone any "wetting" process and was manually controlled to correspond to 190 mL/min. The entire configuration is depicted in Figure 5.5 (a).

Using this flow electrolyzer configuration, we were able to apply current densities of 10 and 20 mA/cm 2 . Furthermore, in Figure 5. 5 % and 6 % of FEHCOO-, respectively. Finally, the increase of the catalyst concentration to 5 mM in test 10 did however not produce any significant effect on formate production. The |Ecell| values decrease along the electrolysis and an average value is reported in Figure 5.5 while the final catholyte pH measured was approximatively at a value of 4.5.

Discussion

Testing a catalytic system in a flow electrolyzer configuration is a laboratory available strategy to investigate the system's efficiency and productivity at larger scale than an H-type cell setup and provides invaluable information about the feasibility of further scaling up at industrial level. In recent years, an increasing number of CO2RR electrolyzers with heterogeneous metallic catalysts, usually Ag and Cu, report CO2 reduction product generation at commercially relevant values of current density 6 .

Heterogenized molecular catalysts grafted on carbon support materials and also often incorporated in GDEs have shown catalytic performances that rival those of noble-metal nanocatalysts 9 . However, testing the effects of a GDE with an electrodeposited IL layer on a molecular catalyst with tunable selectivity towards formate in a flow electrolyzer in acidic aqueous conditions remains, to the best of our knowledge, a completely novel approach. Furthermore, the only other example in literature where a molecular catalyst is used homogeneously presents many differences that complicate direct comparisons 17 .

In this chapter we have elucidated how several parameters pertaining to the reactor configuration impact the system, which was monitored in terms of formate faradaic efficiency, as well as |Ecell|. Firstly, we saw that in a single-compartment recirculating flow configuration on a graphite cathode the formate production was far inferior than the expected values based on the previous studies of this system in an H-type cell with porous carbon cathode, even though tuning of several parameters was tested. More particularly, a CCE for 10 C using the benchmark electrolyte had previously been tested in a singlecompartment of the H-type cell, that led to a FEHCOO-of 39 % and FEH2 of 40 %. Nevertheless, the greater proximity of cathode and anode in the flow electrolyzer resulted in very low FEHCOO-(< 7 %) and led to the adoption of a two-compartment configuration. As expected, this passage from a single to a twocompartment configuration led to a fourfold increase in formate production (< 30 %). This observation allowed us to infer that the main limitation of single-compartment reactor setup derived from having the electrolyte containing the catalyst and the accumulated product in contact with the anode, which could reoxidize both formate and the active form of the catalyst.

The two-compartment configuration also provided the opportunity to employ two different solution compositions as catholyte and anolyte, thus allowing to maintain the weakly acidic conditions in the cathodic compartment that lead to better performance of the electrodeposited IL layer, while using a KOH solution as anolyte resulting in lower |Ecell| values. Though acidic, neutral and basic conditions have all been used for OER, more basic pH values have been shown to promote OER at lower potentials because O2 is produced from OH -oxidation that in basic conditions occurs at 0.404 V vs. RHE instead of water oxidation which is energetically more costly (1.23 V vs. RHE) 20,23 . The other important point CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION in having a basic anolyte pH is that the presence of a pH gradient between a neutral cathode and basic anode compartment was proven beneficial in terms of energy efficiency in a flow electrolyzer for CO2RR and water splitting 23 . Therefore ideally, we want to perform CO2RR under acidic conditions in order to minimize the cell overpotential, which is also one of the reasons that led us to adopt the pH=4 buffered conditions. Another benefit of acidic catholyte was avoiding the inevitable loss of CO2 as bicarbonate and carbonate when working in very basic conditions 24 , which would have only regenerated CO2 when they crossed over to the anode 25 . However, to date only few examples of efficient systems for CO2RR have been reported in acidic media 25,26 .

Subsequently, we compared the same two-compartment recirculation setup for gas-phase CO2RR using a bare GDE instead of the liquid-phase CO2RR performed with the 2D graphite plate as cathode. As expected, formate production remained stable, but at higher current density values, due to the fact that in GDEs we were not limited by the moderate CO2 solubility in aqueous conditions, as we had been when using the graphite plate cathode.

However, gas-phase CO2RR was limited by the recirculation solution, since beyond 100 C the system's product selectivity drastically deteriorated. A possible explanation for this deleterious effect is that when formate accumulates at high concentrations, [Rh(bpy)(Cp*)Cl]Cl can catalyze the conversion of HCOOH to CO2 and H2 27 . However, we were able to circumvent it by utilizing a single-pass flow configuration, that by definition ensures the catholyte containing the catalyst only goes through the reactor once. With this configuration we observed stable FEHCOO-over a span of 1000 C for a partial current density value of 20 mA/cm 2 . Another advantage of employing a single-pass flow configuration was the extinction of all formate crossover from catholyte to anolyte. Despite knowing that theoretically formate should not be able to traverse a cation exchange Nafion membrane, this crossover was apparent in all recirculation configurations tested and necessitated analysis of both catholyte and anolyte in order to obtain complete product quantification. In contrast, with the in-flow configuration formate quantification was simplified to only testing the catholyte in the outlet reservoir.

Another point to be stressed is that this catalytic system was not inherently limited to only 20 mA/cm 2 because of the molecular catalyst, but merely because of technical limitations in the setup. More particularly, with the employed low solution flow rate of 1.1 mL/min, higher current densities like 30 mA/cm 2 resulted in an overproduction of O2 bubbles on the anode surface that could not be removed fast enough and led to power shortcuts. We are confident this is truly due to the anode limitation, since increasing the flow rate from 1.1 to 4.4 mL/min led to a return to usual |Ecell| values. A promising perspective in order to test higher current densities would therefore be to separate the anolyte and CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION catholyte flow by using separate peristatic pumps, allowing for a slow and so economically sustainable catholyte flow rate and simultaneously a fast anolyte rate to remove O2 from the anode surface.

Finally, when the flow electrolyzer using GDE was proven to be stable and reproducible, we prepared an electrodeposited IL layer on a GDE and compared its activity with the optimized bare GDE conditions in the two-compartment single pass flow configuration for gaseous CO2RR. What emerged was that the selectivity tuning of the [Rh(bpy)(Cp*)Cl]Cl towards formate in presence of the electrodeposited IL layer previously witnessed in an H-type cell configuration, was also maintained in the single-pass electrolyzer configuration used. More outstanding still was the fact that though FEHCOO-in the reactor was at an average 7-8 % inferior to that in the H-type cell for both the bare electrode and the one with the electrodeposited layer, the increment between the electrodeposited layer-containing electrode and the bare one was almost identical in both H-type cell and flow electrolyzer (13 %). This effect is shown in Table 5.1 and renders credibility to the hypothesis that the electrodeposited layer had the same effect on enhancing formate production selectivity regardless of the scale of application. The majority of systems containing molecular catalysts described in literature have heterogenized catalysts and are therefore too dissimilar to be directly compared with our system

Conclusions

In conclusion, for the first time, a study of an IL electrodeposited layer in molecular electrocatalysis of CO2 was scaled-up from an H-type cell into a flow electrolyzer in aqueous acidic catholyte solution .

The optimal catalytic conditions with a homogeneous molecular catalyst with modulable selectivity towards formate were obtained when the layer was grown on a GDE. Moreover, the most efficient setup for the flow electrolyzer itself was a two-compartment single-pass flow configuration. We have demonstrated the necessity of having a membrane separating the two compartments in order to achieve appreciable FEHCOO-and the importance of using highly basic anolyte to ensure the |Ecell| values remain at minimal values. Furthermore, we have provided evidence that a single-pass flow configuration is preferable when the [Rh(bpy)(Cp*)Cl]Cl used is in solution, since employing a recirculating flow configuration led to rapid formate production decrease over time, probably due to reoxidation of produced formate and/or the reduced active form of the catalyst. Besides, by implementing a bare GDE for gas-phase CO2RR we were able to reach current densities up to 6 times higher than those in H-type cells, as well as maintain a stable FEHCOO-(32%) over the span of 1000 C, only 8 % inferior than the same value in an H-type cell with a porous carbon electrode at only 10 C. Finally, an electrodeposited IL layer was formed in aqueous solution on a GDE, as was evident by the system's increased selectivity towards formate that reached an average of 45% and was stable for 3 independent electrolysis of 1000 C, thus accumulation the same electrode a total of 3000 C without exhibiting any evident degradation in its performance. To sum up, the increment in FEHCOO-observed between the GDE with electrodeposited layer and the one without, which exhibits the same value as the one remarked in the H-type cell tests, is accompanied by an evident improvement in full-cell energy efficiency and significant decrease in energy consumption, proving that the effect of the electrodeposited IL layer is applicable in larger scale systems in weakly acidic aqueous solution.

CHAPTER 6: GENERAL CONCLUSIONS AND OUTLOOK Overall, in order to optimize a catalytic system, considerations about activity, selectivity and stability must be taken into account. Because of the recently reported cocatalytic activity between molecular catalysts and ionic liquids for CO2RR, their combined study is considered appealing in this context.

However, no in-depth study of the structure-activity relationship between different molecular catalysts and a variety of ILs had been reported at the beginning of this thesis, which is the reason we investigated them in depth experimentally, as well as with some theoretical calculations. We explored in this work the further improvement of catalytic performance by combining molecular catalysts with ILs in solution or immobilized on surfaces to favor CO2RR activity and selectivity and scaled-up the most pertinent catalytic system by testing it within a flow electrolyzer.

General Conclusions

The role of ionic liquids (ILs) as promoters for CO2 electroreduction (CO2RR) has been studied in this thesis following two different strategies:

1) Electrolyte engineering strategy:

• The interaction of a series of ILs used as supporting electrolytes with two model molecular catalysts with distinct CO2RR mechanisms is shown and explained by experimental, as well as theoretical data.

• Selective CO2-to-CO reduction catalyst [Re(bpy)(CO) 3 Cl] displayed a cocatalytic interaction with both imidazolium-based and pyrrolidinium-based ILs in acetonitrile. This was demonstrated by significant decrease in its characteristic reduction potentials by as much as 370 mV under inert and CO2 atmospheres. Imidazolium-based ILs produced a more prominent effect than pyrrolidinium-based ILs. The only protic IL tested did not interact with the catalyst and instead produced H2 directly on the electrode surface.

• Under CO2 and in the absence of an explicit proton source, ILs reduce the overpotential of [Re(bpy)(CO) 3 Cl] by 200 -330 mV compared to the benchmark supporting electrolyte. This is possibly due to an "electron-first" reported mechanistic pathway 1 that forms mono-anionic and di-anionic catalyst intermediates that can be successfully stabilized electrostatically by IL cations present in the electrical double layer. The superior effect produced by imidazolium over pyrrolidinium ILs can be attributed to their aromatic nature and they have been reported 2 to interact with the bipyridine ligand of catalyst [Re(bpy)(CO) 3 Cl] through π + -π interaction.

• However, in the presence of an explicit proton source (1.5 M TFE) the decrease in overpotential for CO2RR by catalyst [Re(bpy)(CO) 3 Cl] is down to 30 -70 mV. In those conditions, a "protonfirst" mechanistic pathway is favored 1 , that involves formation of a neutral and a mono-anionic catalyst intermediate, which are less efficiently involved in electrostatic stabilization by IL cations in the electrical double layer.

• Overall, the beneficial effect of ILs on [Re(bpy)(CO) 3 Cl] catalyst by overpotential diminution was offset by slight loss of selectivity for CO over H2. This led to high energy efficiency values (47 % -53 %) for the entire series of ILs in presence of an external proton source, but they were lower than the value obtained using the benchmark electrolyte (61 %).

• Catalyst [Rh(bpy)(Cp*)Cl]Cl, that produces a mixture of formate and H2 under reductive conditions, was explored with the series of ILs and tested in both acetonitrile and aqueous conditions.

• Electrolysis experiments performed with catalyst [Rh(bpy)(Cp*)Cl]Cl and ILs in acetonitrile solution displayed an almost 800 mV decrease in overpotential compared to the benchmark electrolyte for the appreciable current density value of 3.33 mA/cm 2 . Meanwhile, the selectivity was tuned in favor of formate evolution, resulting in faradaic efficiency values equal to or exceeding 90 %, leading to a maximum EE value of 66 %.

• [Rh(bpy)(Cp*)Cl]Cl was also tested alongside imidazolium IL [EMIM] [BF4] in acidic aqueous solution. The decrease of current density due to decreased CO2 solubility (33 mM in 25 °C) was compensated by the addition of acetic acid as an additional proton source. This allowed not only to restore high current densities, but also to obtain a CO2RR product distribution of 1/1 for FEHCOO-/FEH2. Overall, this catalytic system operated at the exceptionally low overpotential of 280 mV and reached a maximum EE value of 32 %.

• DFT calculations corroborated the decrease in overpotential seen in experimental results, as they proved the formation of an adduct between the active form of catalyst [Rh(bpy)(Cp*)Cl]Cl and the [EMIM] + . They also explain the HER suppression and CO2RR promoting effect of ILs, because in the presence of ILs the free-energy of the HER pathway is disproportionately increased in comparison to the free-energy for formate production.

2) Electrode surface engineering strategy:

• A simple and convenient method for imidazolium-based IL electrodeposition on different electrode materials (GCE, AuE) is presented and its resulting catalytic effect for CO2RR is assessed on the model catalyst [Rh(bpy)(Cp*)Cl]Cl system in both acetonitrile and aqueous conditions. Optimization studies of this electrodeposited IL layer are presented on this catalytic system, as well as catalytic comparisons with a IL layer covalently attached to the electrode described in the literature 3 , but never applied before for a catalytic purpose.

• Elucidation of the nature of this electrodeposited IL layer is achieved through XPS and electrochemical characterization. More particularly, the presence of N atoms bound on the electrode surface and N from an imidazole ring in XPS spectra, proved the presence of this IL layer on the electrode surface.

• The ability of the electrodeposited IL layer to suppress HER is also demonstrated in various electrolyte compositions with and without catalyst [Rh(bpy)(Cp*)Cl]Cl. This could be attributed to electrostatic repulsion between the protons and the apparently cationic nature of this IL layer.

• In electrolysis experiments with [Rh(bpy)(Cp*)Cl]Cl catalyst, the electrodeposited IL layer led to even higher formate production over HER than using the same IL in solution. In acetonitrile conditions with the electrodeposited IL layer, we obtained a maximum FEHCOO-of 85 % at a current density value of 3.33 mA/cm 2 and EE of 59 % in comparison with FEHCOO-of 69 % and • In order to broaden the scope of IL interactions with molecular catalysts would also be of interest to target other CO2RR products different than CO and formate, such as methane 6 .

• Another strategy that would permit studying IL interactions with molecular catalysts consists of incorporating an IL moiety and more specifically an imidazolium ring, into the structure of the molecular catalyst itself leading to the synthesis of a novel catalyst. A couple of examples in the literature have already demonstrated that such inclusions led to enhanced catalytic performance 7,8 .

• Considering the outstanding catalytic performance of the presented electrodeposited IL layer with molecular catalysts, studies could also be envisioned about its potential cocatalytic effect with heterogeneous catalysts. Some studies have already shown the possibility of growing an ILbased layer on Cu electrodes for enhanced CO2RR 9 . Building on our own results, alongside other studies performed on layers from pyridinium cations 10 , it could open many original possibilities in the field of heterogeneous surface modification.

• Meanwhile, the effect of this electrodeposited IL layer could also be tested in other electrochemical reduction reactions besides CO2RR, such as N2O reduction 11 .

• Finally, the selected catalytic system already tested in laboratory scale within a flow electrolyzer could be further optimized and scaled up to industrial scale. This would lead to additional tests from an engineering perspective, such as more extensive design of the flow electrolyzer.

divers solvants et électrolytes. Les mécanismes catalytiques de la réduction de CO2 de ces deux catalyseurs moléculaires sont montrés sur la Figure 1. la solubilité de CO2 par rapport à l'eau [16][17][18] . Leur désavantage principal est leur grande viscosité, inversement proportionnelle à la conductivité du solvant. La faible conductivité se traduit en une limitation de transfert de masse et, par conséquent, mène à la baisse la densité maximale du courant observé 6 . Une possibilité alternative sera alors d'utiliser les ILs en tant que le sel de fond dans des mélanges binaires avec d'autres solvants.

Chapitre 1

Dans le chapitre 1, l'état de l'art servant la base théorique de cette thèse est présenté en détail.

Il comprend une brève description des enjeux autour de la réduction catalytique du CO2, ainsi que des exemples de catalyseurs moléculaires. Ensuite, les caractéristiques des liquides ioniques, des solvants alternatifs et cocatalyseurs de CO2RR, sont détaillées. Finalement, la possibilité de la combinaison des catalyseurs moléculaires et des liquides ioniques en solution est discutée.

Chapitre 2

Le chapitre 2, nommé « matériaux et méthodes » comprend la liste des réactifs chimiques utilisés pendant la thèse, des matériaux des électrodes, des composants au sein de l'électrolyseur en flux et des

Chapitre 3

Le chapitre 3 est consacré à l'étude de l'activité catalytique des catalyseurs moléculaires en présence d'une série de liquides ioniques utilisés en tant que sels de fond afin de rationaliser leur rôle cocatalytique. La La Figure 3a montre les voltammogrammes cycliques sous atmosphère inerte des solutions d'acétonitrile contenant du catalyseur [1] à 1 mM en présence de 0,5 M de trois sels de fond différents, ; (ii) le déplacement de la deuxième réduction est bien plus marqué par rapport à celui de la première ;

(iii) ces déplacements sont dépendants de la nature du liquide ionique utilisé, avec l'imidazolium produisant un effet supérieur par rapport au pyrrolidinium. La même tendance se dessine quand la comparaison est faite sous une atmosphère de CO2 (Fig. 3b). On constate que le deuxième pic de la réduction est plus haut en termes de densité de courant. Cela reflète la présence d'une vague catalytique , ce qui nous a conduit à ajouter un tampon acétate à pH=4, capable de maintenir le pH du catholyte stable après les électrolyses à 10 C de charge (expériences 5 et 6.) L'addition de l'acide acétique favorise l'électrolyse avec une surtension très faible (0,28 V) et conduit à une valeur de EE de 32 %.

Tableau 1. Electrolyses à courant constant de 1 mM catalyseur [2] Ces résultats expérimentaux sont complétés par des calculs théoriques sur le mécanisme d'action du catalyseur [2] pour la CO2RR, ainsi que l'évolution de H2. La Figure 6 montre que la forme active du catalyseur [2], ici nommé 1, peut soit réduire le CO2 à travers l'état de transition (TS) TS1 en surpassant la barrière énergétique de 14,3 kcal/mol, soit produire du H2 à travers l'intermédiaire TS2 avec une barrière de 0,5 kcal/mol. Pourtant, dans l'acétonitrile le produit principal de réduction est l'acide formique, même si la barrière énergétique pour la réduction de CO2 est significativement plus grande que celle de l'évolution de H2. Ceci est raisonnable vu que dans la solution 95/5 (vol.) acétonitrile/H2O, la concentration des protons est inférieure à celle de CO2 de plusieurs ordres de grandeur, favorisant la voie du CO2RR. Ensuite, l'incorporation du cation [EMIM] + sur l'interface de l'électrode conduit, selon les calculs théoriques, à l'augmentation de l'énergie libre pour les deux voies de réduction. Ceci est le résultat de la réduction du caractère nucléophile de l'hydrure de Rh par la polarisation du catalyseur vers le cation organique. Cet effet affecte pourtant beaucoup plus l'évolution de H2. Par conséquent, la différence énergétique entre TS1 et TS2 diminue de 13,8 à 10,3 kcal/mol dans l'acétonitrile et 9,8 RÉSUMÉ EN FRANÇAIS kcal/mol dans l'eau, ce qui est en accord avec l'augmentation de sélectivité vers l'acide formique expérimentalement constatée en présence de [EMIM] + en solution. Tableau 2. Electrolyses à courant constant avec du catalyseur [2]

Chapitre 5

Dans ce chapitre, on présente l'exploitation des travaux déjà décrits dans les chapitres 3 et 4 sur une échelle plus grande, en utilisant un électrolyseur en flux. Cet électrolyseur commercial permet l'installation d'électrodes à diffusion de gaz (GDE) de 10 cm 2 et permet donc de compenser la limitation de densité de courant dans l'eau due à la basse solubilité de CO2. L'illustration schématique de cet électrolyseur est présentée sur la Figure 8a. En utilisant une GDE, le CO2 n'est en contact avec la solution qu'au sein de la triple interface gaz-électrode-électrolyte. Cet électrolyseur est utilisé pour tester la réduction de CO2 par le catalyseur [2] dans un milieux aqueux acide avec 0,1 M de tampon acétate à

2. 5 . 2 1 [

 521 Liquid product detection 2.6 ILs drying process and water quantification 2.7 Density Functional Theory (DFT) calculations 2.8 Synthesis of complexes [Rh(bpy-X 2 )(Cp*)Cl] and [Re(bpy-X 2 )(CO) 3 Cl], where X=H, Re(bpy)(CO) 3 Cl] with ILs as electrolyte in acetonitrile under inert conditions 3.2.2 [Re(bpy)(CO) 3 Cl] with ILs as electrolyte under CO2 reduction catalytic conditions 3.2.3 Electrolysis for CO2 reduction with ILs as electrolyte catalyzed by [Re(bpy)(CO) 3 Cl]. Impact on reaction selectivity 3.2.4 [Rh(bpy)(Cp*)Cl]Cl with ILs as electrolyte in acetonitrile under inert conditions 3.2.5 [Rh(bpy)(Cp*)Cl]Cl with ILs as electrolyte in acetonitrile under CO2 catalytic conditions 3.2.6 [Rh(bpy)(Cp*)Cl]Cl with ILs as electrolyte in organic-aqueous mixture and aqueous solution under CO2 catalytic conditions 3.2.7 Electrolysis for CO2 reduction with ILs as electrolyte catalyzed by [Rh(bpy)(Cp*)Cl]Cl. Impact in reaction selectivity 3.2.8 Density Functional Theory (DFT) calculations on the interaction between [Rh(bpy)(Cp*)Cl]Cl and [EMIM] + in CO2RR and HER (Collaborators' Layer formation and optimization by testing on [Rh(bpy)(Cp*)Cl]Cl model catalyst 4.2.2 Electrodeposited IL Layer testing in different solvents 4.2.3 Electrodeposited IL layer impact on CO2RR catalyzed by [Re(bpy)(CO) 3 Cl] in acetonitrile 4.2.4 IL layer characterization. Surface coverage evaluation, XPS analysis and CVs with redox probes 4.2.5 Electrodeposited IL layer impact on HER catalyzed by [Rh(bpy)(Cp*)Cl]Cl 4.2.6 Electrolysis for CO2 reduction with immobilized ILs and catalyzed by [Rh(bpy)(Cp*)Cl]Cl. Impact on reaction selectivity 4.2.7 Electrolysis for CO2 reduction with immobilized ILs and catalyzed by [Rh(bpy)(Cp*)Cl]Cl. Performance stability evaluation 4-compartment recirculating flow configuration for liquid-phase CO2RR 5.2.2 Two-compartments recirculating flow electrolyzer configuration for liquidphase CO2RR 5.2.3 Two-compartments recirculating flow configuration for gas-phase CO2RR 5.2.4 Two-compartments single pass flow configuration for gas-phase CO2RR 5.2.5 Two-compartments single pass flow configuration for gas-phase CO2RR on a GDE with an electrodeposited IL layer 5Résumé de la Thèse en Français LIST OF PUBLICATIONS AND CONFERENCES ACKNOWLEDGEMENTS

CHAPTER 1 :

 1 Figure 1.1 (a) The variation of average global temperature throughout the last 140 years as a correlation of CO2 concentration 3 and (b) the monthly average CO2 values of recent years as registered by the Mauna Loa Observatory in Hawaii as averages at the middle of each month (red lines) and after correction according to the seasonal cycle (black lines) 1 .

  .

  3.

Figure 1 . 3

 13 Figure 1.3 Schematic representation of a catalyst's effect on activation energy (Ea) for an endothermic reaction.

Figure 1 . 5

 15 Figure 1.5 Structures of molecular catalysts for CO2 to CO conversion alongside their electrolyte conditions and faradaic efficiency value.

Figure 1 . 6

 16 Figure 1.6 Structure of imidazolium and pyrrolidinium IL cations.

CHAPTER 1 :

 1 INTRODUCTIONcations, which leads to fewer active sites for HER. Meanwhile, under basic conditions the positively charged imidazolium cations cannot displace the diffusion of neutral water molecules 102 .

Figure 1 . 7

 17 Figure 1.7 Schematic effect of ILs in CO2 electroreduction. (a) Free energy profile on a silver electrode in either water or acetonitrile (solid plot) or in a water/IL ([EMIM][BF4]) mixture (dashed plot) 91 and (b) Diagram of the effect of 2 mM of different ILs as CO2 reduction promoters on a silver electrode in an acetonitrile solution containing 0.1M [TBA][PF6] 97 .

CHAPTER 1 :

 1 Figure 1.8 (a) Cyclic voltammograms of pure IL [BMIm][NTf2] on Hg, Cu, Au and Pt electrodes under a nitrogen atmosphere. Scan rate = 0.05 V/s, 5 th scan shown 86 and (b) Schematic representation of all suggested cathodic pathways of converting dialkylimidazolium ILs 103 .

CHAPTER 1 :

 1 INTRODUCTIONwith the same catalyst and varying concentrations of the same IL, but this time in acetonitrile already containing the conventional supporting electrolyte [TBA][PF6], as well as a benzoic acid (BzOH) used as a proton source. These conditions were investigated by linear sweep voltammetry (LSV) and the observations were that though the gain in overpotential was still apparent as in the purely IL reaction environment, there was no particular effect of the IL on the catalytic current density105 . Moreover, this study also explored the effect of another IL, [EMIM][PF6], in order to determine whether the IL anion also affected catalytic performance. This comparison of Lehn's catalyst in acetonitrile with either [EMIM][PF6] or [EMIM][TCB] versus [TBA][PF6] is shown in the adapted Figure 1.6 (a) and clearlyshows that the co-catalytic effect with this molecular catalyst is due to the imidazolium cation and not on the anion present. Further theoretical calculations were conducted to determine the form of the CO2RR intermediates in presence of IL and benzoic acid and concluded that most likely the imidazolium firstly stabilizes the 2-electron reduced form of the catalyst. This is achieved by the positive charge delocalized on the imidazolium cycle that aids the formation of metallocarboxylate (Re-CO2) by diminishing the energy of its π orbitals. More particularly, the lowest energy configuration of the catalyst has the imidazolium stacked on top of it, with the imidazolium interacting by π-π stacking with the bipyridine ligand of the molecular complex 105 .

Figure 1 . 9

 19 Figure 1.9 Voltammograms of molecular catalysts in organic solvents containing ILs: (a) LSVs (scan rate = 0.35 V/s) of 0.5 mM [Re(bpy)(CO) 3 Cl] in a CO2-saturated acetonitrile solution containing 1.08M [TBA][PF6], 0.02 M of both BzOH and [TBA][BzO] and 0.1 M of the following supporting electrolytes: [EMIM][TCB] (solid red plot), [EMIM][PF6] (dashed red plot), [TBA][PF6] (black plot) and [EMIM][TCB] without the molecular catalyst in solution (dashed grey plot).Adapted from Matsubara et. al. 105 and (b) CVs (scan rate = 0.05 V/s) of 0.5 mM FeTPP in a CO2-saturated DMF solution containing: 0.1 M [TBA][PF6] (black plot), 0.1 M [TBA][PF6] and 1 M TFE (red plot), 0.1 M [TBA][PF6] and 0.3 M [BMIM][BF4] (green plot) and 0.1 M [TBA][PF6], 0.3 M [BMIM][BF4], 1 M TFE (blue plot). Adapted from Choi et. al. 106

Figure 1 .

 1 Figure 1.10 Structures of molecular catalysts functionalized with IL moieties: (a) {Re[bpyMe(ImMe)](CO) 3 Cl}[PF6] catalyst, where bpyMe(ImMe) is an ortho methyl-bipyridine bearing a methylimidazolium arm on one side 107 and (b) Iron tetraphenyl porphyrin with four methylimidazolium units attached on the periphery of the porphyrin ligand 108 .

CHAPTER 1: INTRODUCTION 2 .

 2 To explore electrolyte engineering and IL immobilization to improve CO2RR with molecular catalysts.

  Anolyte and catholyte contained acetonitrile and 0.5 M of supporting electrolyte (either individual ILs or [TBA][PF 6 ]), but only in the catholyte 1 mM of complex [Re(bpy)(CO) 3 Cl] or complex [Rh(bpy)(Cp*)Cl]Cl was added. Ohmic losses in the cell were minimized by placing the electrodes as close as possible to each other and stirring both solution compartments during the experiment. Moreover, 85 % of the electrolyte resistance was compensated by the ohmic drop compensation module of the potentiostat. Both solution compartments were saturated with CO 2 during at least 20 minutes before starting the electrolysis, but no more gas was bubbled during the electrolysis. In both cases, FE is corrected to account for the initial 2 or 3 electron reduction of molecular catalyst [Re(bpy)(CO) 3 Cl] or [Rh(bpy)(Cp*)Cl]Cl, respectively (1 mM in solution) necessary to generate its active form. Catalyst activation charge = [number of electrons ⋅ Faraday constant ⋅ mol of catalyst] = [n ⋅ 96485 ⋅ 6.06 10 -6 ].

  20 eV pass energy for the C 1s and N 1s regions. Binding energies were calibrated against the C 1s (C-C) binding energy at 284.8 eV and element peak intensities were corrected by Scofield factors. The peak areas were determined after subtraction of a linear background. The spectra were fitted using Casa XPS v.2.3.15 software (Casa Software Ltd, U.K.) and applying a GAUSSIAN/LORENTZIAN RATIO G/L equal to 70/30. XPS analysis was carried out in the available platform of Sorbonne Université by A.

57 CHAPTER 2 :

 572 All solutions of ILs in acetonitrile (ILs/CH 3 CN) were freshly prepared in the glove box before each experiment. Water content in all the resulting acetonitrile solutions was quantified by Karl Fischer titration, which consists of an electrochemical coulometric method that MATERIALS AND METHODS consumes water in the oxidation reaction and allows quantifying even traces of humidity in samples. The titration samples mass was of (0.3 -0.5) g and the measurement range was of 20 -3000 ppm (detection limit for H 2 O quantification 2 ppm). The final concentration of water present in each IL/CH 3 CN solution was: 88 ppm for 0.5 M [ΒΜΙΜ][PF 6 ], 250 ppm for 0.5 M [ΕΜΙΜ][PF 6 ], 151 ppm for 0.5 M [ΕΜΙΜ][BF 4 ], 158 ppm for 0.5 M [ΒΜΡyrr][PF 6 ]. No water quantification was performed for [DiΜΙΜ][N(SO 2 CF 3 ) 2 ]. Pure acetonitrile water content was 77 ppm.

  mL) of 1 equivalent [Rh(Cp*)Cl]2 (100 mg, 0.16 mmol) and 2 equivalents 2,2'-bipyridine-carboxylic acid methyl ester (102.55 mg, 0.38 mmol) was stirred at RT for 12 h. The solvent was removed in vacuo and the residue was dissolved in a minimum amount of acetonitrile. Addition of ethyl acetate gave a pale-yellow solid which was collected on a Buchner funnel, washed with ether, and dried in vacuo. An analytical sample was obtained by recrystallization from CH3CN/AcOEt (yield = 91 %) and characterized by 1 H NMR spectroscopy. 1 H NMR (300 MHz, DMSO-d6): δ/ppm, 1.01 (s, 15H), 4.03 (s, 6H), 8.26 (m, 2H), 9.21 (t, J = 10.5 Hz ,4H).

CHAPTER 3 :

 3 IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS solvents 2,25

CHAPTER 3 :

 3 IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSISintermediates and binding the CO2 molecule or suppressing competitive HER has been already

Figure 3 . 1 ,

 31 allowing to compare four different cations and three different anions, as supporting electrolytes in acetonitrile and one IL in aqueous solution.Figure 3.1 presents three ILs from the imidazolium cation family, namely 1-Ethyl-3-methylimidazolium with either hexafluorophosphate [PF 6 ] -([ΕΜΙΜ][PF 6 ]) or tetrafluoroborate [BF 4 ] -([ΕΜΙΜ][BF 4 ]) as the counter-anion, 1-Butyl-)imide ([DiΜΙΜ][N(SO 2 CF 3 ) 2 ]), a protic IL containing an acidic proton, and one IL from the pyrrolidinium cation family, namely 1-Butyl-1-methylpyrrolidinium hexafluorophosphate ([ΒΜΡyrr][PF 6 ]).

Figure 3 .

 3 2 are shown the cyclic voltammograms (CVs) of an acetonitrile solution containing 1 mM complex [1] under inert conditions (Argon saturated) in the presence of different supporting electrolytes including conventional benchmark [TBA][PF 6 ], one pyrrolidinium based IL [BMPyrr][PF 6 ] and 3 different imidazolium based ILs ([BMIM][PF 6 ], [EMIM][PF 6 ], and [EMIM][BF 4 ]

  Figure 3.2 (a) presents the behavior of complex [1] in the presence of [TBA][PF 6 ] and 2 different IL cations (pyrrolidinium and imidazolium), which kept constant the alkyl chain length in the cation. The anion [PF6] -was also kept unchanged in all 3 cases to evaluate only the role of the cation. We observed that: (i) both redox processes were significantly shifted anodically in the presence of both ILs as compared to [TBA][PF6]; (ii) the potential shift of the second reduction wave was much larger than that of the first reduction wave; (iii) these shifts greatly depended on the nature of the IL cation, being much more relevant when the imidazolium cation was present in solution, with the trend in potentials being [IM] + > [Pyrr] + > [TBA] + .

Figure 3 .

 3 Figure 3.2 (b) shows the complex[1] performance in the presence of 3 different imidazolium based ILs by studying the effect of the alkyl chain length in the cation and the nature of the anion. It shows that those two parameters had only a minor effect on the CVs of complex[1]. Table3.1 reports the characteristic potentials for all reduction peaks shown in Figure3.2. Thus, the first reduction potential was shifted positively between 60 and 210 mV by replacing [TBA][PF6] with an IL. Meanwhile the second reduction peak was also shifted positively between 190 and 370 mV depending on the IL used, with [EMIM][BF4] being the one that displayed the most important potential shift in Figure 3.2 (b). We equally observed that the current density displayed in Figure 3.2 remained very similar in the presence and absence of ILs and did not vary according to the nature of the IL.

Figure 3 . 2

 32 Figure 3.2 Cyclic Voltammograms of 1 mM complex [1] and 0.5 M of different supporting electrolytes in acetonitrile solution under Ar: (a) [TBA][PF6] (black plot), [BMPyrr][PF6] (green

CHAPTER 3 :Figure 3 . 3

 333 Figure 3.3 CVs of 1 mM complex [1] at increasing concentrations of [EMIM][PF6] in acetonitrile: 0.1 M (black), 0.3 M (red), 0.5 M (blue), 0.7 M (purple) and 1 M (green): (a) under Ar and (c) under a CO2 atmosphere in the presence of 1.5 M TFE. Scan rate 0.1 V/s. Also, representation of the variation of the second reduction peak potential of complex [1] and its current density vs. the concentration of [EMIM][PF6]: (b) under Ar and (d) under a CO2 atmosphere in the presence of 1.5 M TFE.

Figure 3 .

 3 Figure 3.4 shows that complex [1] in the presence of [DiΜΙΜ][N(SO2CF3)2], the only protic IL tested in this work, whose structure is also shown in Figure 3.1 (e), did not show its two characteristic reduction waves in the mixture IL/ CH3CN.Figure 3.4 seemed to indicate the direct proton reduction present in [DiΜΙΜ] + at the GCE surface.

CHAPTER 3 :

 3 IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSISfacilitated the reduction were partly suppressed in the presence of H2O, but also under these conditions some proton reduction catalysis may occur. As a consequence, all experiments with complex[1],including those reported in Figure3.2, had thus been carried out with dried ILs exclusively.

Figure 3 .

 3 Figure 3.4 CV of 1 mM complex [1] in 0.5 M [DiΜΙΜ][N(SO2CF3)2] acetonitrile solution under Ar (black solid line). Blank CV of 0.5 M [DiΜΙΜ][N(SO2CF3)2] in acetonitrile under Ar (black dashed line). Scan rate 0.1 V/s.

Figure 3 . 5

 35 Figure 3.5 CVs of 1 mM complex [1] and 0.5 M of different ILs in acetonitrile solution under Ar after following the drying treatment of ILs (colored plots) or without drying (black plots). The drying protocol is described in Chapter 2. IL present: (a) [EMIM][BF4], (b) [EMIM][PF6], (c)

  Figure 3.6 (a) illustrates the catalyst behavior in the presence of conventional benchmark [TBA][PF6] electrolyte and 2 different IL cations (pyrrolidinium and imidazolium), which kept constant the alkyl chain length in the cation. The anion [PF6] -was kept unchanged in all 3 cases. A relevant positive shift due to the presence of ILs in solution was displayed for both reduction waves following the same trend already observed in Figure 3.2 (a) under inert conditions ([IM] + > [Pyrr] + > [TBA] + ).

  Figure 3.6 (b) shows the catalyst behavior in the presence of 3 different imidazolium based ILs by studying the effect of the alkyl chain length in the CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS cation and the nature of the anion. Both ILs containing [EMIM] + cation showed the largest jcat value.

[

  EMIM][BF4] was the IL that displayed the most important potential shift and therefore, the lowest overpotential (1.19 V). Nevertheless, all three imidazolium-based ILs exhibited very close overpotential values. Finally, Figure 3.7 shows that in the case of using [DiΜΙΜ][N(SO2CF3)2] in solution, the same reduction current was observed when the CV was recorded in the absence or in the presence of CO2, reflecting that reduction of protons provided by the IL was the predominant reaction.

Figure 3 . 6

 36 Figure 3.6 Cyclic Voltammograms of 1 mM complex [1] and 0.5 M of different supporting electrolytes in CO2-saturated acetonitrile solution. (a) [TBA][PF6] (black plot), [BMPyrr][PF6] (green plot) and

Figure 3 . 7

 37 Figure 3.7 CVs of 1 mM complex [1] in 0.5 M [DiΜΙΜ][N(SO2CF3)2] in acetonitrile under Ar (black solid line) and CO2 (red solid line). Scan rate 0.1 V/s.

CHAPTER 3 :

 3 IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSISIt has been previously demonstrated in the literature that CO2 electroreduction catalyzed by molecular complexes required the presence of a proton source in solution, in order to facilitate the proton-coupled electron transfers (PCET) that are associated with CO2 reduction. Following previous studies,16 the weak Brønsted acid 2,2',2''-trifluoroethanol (TFE) was identified as an appropriate proton source and 1.5 M an optimal TFE concentration to give an enhanced activity for CO2 electroreduction, which was also the case when ILs were in solution, as can be witnessed in Figure3.8.

Figure 3 . 8

 38 Figure 3.8 CVs of 1 mM complex [1] in 0.5 M [EMIM][PF6] and different concentrations of TFE in CO2 saturated acetonitrile solution with no added TFE (black dotted plot), 0.015 M TFE (red dotted plot), 0.15M TFE (blue dotted plot) and 1.5 M TFE (black solid plot). Scan rate 0.1 V/s.

  , when using [TBA][PF6] as supporting electrolyte, a significant shift (130 mV) towards more positive potentials was observed in the catalytic CO2 reduction wave due to the addition of a weak acid, such as TFE. In contrast, in the case of pyrrolidinium based IL, the Ecat/2 in CO2-saturated solution with and without TFE were about the same (see Ecat/2, Tables3.2 and 3.3) and in the case of all 3 imidazolium-based ILs, we observed a significant shift (~ 150 mV) towards more negative potentials when TFE was present in solution. Nevertheless, catalysis still occurred at potentials slightly more positive in the presence of ILs as compared to benchmark supporting electrolyte [TBA][PF6] by 30-70 mV, [BMIM][PF6] and [BMPyrr][PF6] being the ones displaying the most positive Ecat/2 and the lowest overpotential (0.50 V and 0.52 V, respectively). Another effect of the ILs in solution appeared to be a CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS moderate enhancement of maximum current density as compared to [TBA][PF6], with [EMIM][BF4]

Finally, CPE

  Figure 3.10 Constant potential electrolysis (CPE) at -2.05 V vs. Fc + /Fc of 1 mM complex [1] in acetonitrile under CO2 in presence of 1.5 M TFE: in 0.5 M [TBA][PF6] (black plot), 0.5 M [EMIM][BF4] (red plot) and [BMPyrr][PF6] (green plot). Room temperature: 20 ± 2 °C, stirring rate: 400 rpm.

  were the benchmark electrolyte [TBA][PF 6 ], the pyrrolidinium-based IL [BMPyrr][PF 6 ] and the three imidazolium-based ILs, [EMIM][PF 6 ], [EMIM][BF 4 ] and [BMIM][PF 6 ]. The concentration of 0.5 M for

Figure 3 .

 3 11 (a) compares the benchmark electrolyte cation [TBA] + and the non-reducible pyrrolidinium cation [BMPyrr] + with [PF6] -as the counter anion in both cases. The metal-centered reduction peak remained unchanged at -1.21 V vs. Fc + /Fc, but the bipyridine ligand-centered peak was shifted anodically by 80 mV. On the other hand, Figure 3.11 (b) compares the benchmark electrolyte cation [TBA] + and 3 different imidazolium-based ILs, comprising 2 different cations ([EMIM] + and [BMIM] + ) and two different counter anions ([PF6] -and [BF4] -

CHAPTER 3 :

 3 IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

Figure 3 .

 3 Figure 3.11 Cyclic voltammograms on GCE of 1 mM complex [2] and 0.5 M of different supporting electrolytes in acetonitrile solution under Ar. (a) [TBA][PF6] (black plot) and [BMPyrr][PF6] (green

Figure 3 .

 3 Figure 3.12 Cyclic voltammograms on GCE electrode of 1 mM complex [2] and 0.5 M of different supporting electrolytes in acetonitrile solution under Ar: [TBA][PF6] (black plot) and [EMIM][PF6]

  Figure 3.13 Cyclic voltammograms on GCE of 1 mM complex [2] and 0.5 M of different supporting electrolytes in acetonitrile solution containing 5 % (vol.) H2O under CO2. (a) [TBA][PF6] (black plot)

[

  PF6] -anion. For this reason, we chose to add instead [EMIM][BF4] and [TBA][BF4] as electrolytes, since the [BF4] -anion is more hydrophilic and as a consequence exhibits higher solubility in aqueous solution. In Figure3.14, the presence of 0.5 M of the IL led to a modest 40 mV decrease in catalytic half current (Ecat/2) compared to the benchmark electrolyte. The passage from a acetonitrile/H2O 95/5 (vol.)

Figure 3 .

 3 Figure 3.14 Cyclic voltammogram on GCE of 1 mM complex [2] in an acetonitrile/H2O 50/50 (vol.) solution containing 0.1 M of [TBA][BF4] (black plot) or 0.5 M [EMIM][BF4] (red plot) under CO2. Scan rate 0.01 V/s.

  Figure 3.15 Cyclic voltammograms on GCE of 1 mM complex [2] in: (a) 0.1 M [TBA][BF4] as electrolyte or (b) 0.5 M [EMIM][BF4] as electrolyte in unbuffered aqueous solution under Ar (black plot), unbuffered aqueous solution under CO2 (red plot) and 0.1 M acetate buffer solution (pH = 3.8) under CO2 (red dashed plot). Scan rate = 0.01 V/s.

CHAPTER 3 :

 3 Figure 3.16 Constant current electrolysis (CCE) at -3.33 mA/cm 2 of 1 mM complex [2] in acetonitrile solution under CO2 in presence of 5 % (vol.) H2O and: 0.5 M [TBA][PF6] (black plot) and 0.5 M [EMIM][PF6] (red plot). Stirring rate: 400 rpm.

9 .

 9 The electrolyte system used was either an unbuffered aqueous solution containing 0.1 M of [TBA][BF4] (entry 1) or 0.5 M of [EMIM][BF4] (entry 2) as supporting electrolyte or a 0.1 M acetate buffer solution at pH = 3.8 with an additional 0.1 M [TBA][BF4] (entry 3) or 0.1 M [EMIM][BF4] (entry 4). Interestingly, this catalyst CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS exhibited significant FEHCOO-even in those acidic aqueous solutions, representing one of few examples

Figure 3 .

 3 Figure 3.17 Constant current electrolysis (CCE) at -3.33 mA/cm 2 of 1 mM complex [2] under CO2 with: 0.1 M [TBA][BF4] as electrolyte in unbuffered aqueous solution (blue plot) or 0.1 M acetate buffer solution (green plot) and 0.5 M [EMIM][BF4] as electrolyte in unbuffered aqueous solution (red plot) or 0.1 M acetic acetate buffer solution (purple plot). Stirring rate: 400 rpm.

3. 2 . 8

 28 Density Functional Theory (DFT) calculations on the interaction between [Rh(bpy)(Cp*)Cl]Cl and [EMIM] + in CO2RR and HER (Collaborators' contribution)Finally, we collaborated with Dr. Albert Solé-Daura and Dr. Caroline Mellot-Draznieks who performed DFT calculations to investigate the role of ILs at the electrode/solution interface in both acetonitrile and aqueous solution taking the representative case of the [EMIM] + cation. Previous computational studies on the reduction of CO2 catalyzed by complex[2] 21 had already shown that the Rh I species generated after the two-electron reduction of the initial Rh III in complex[2] could reduce a proton to form a Rh IIIhydride, which tended to evolve towards the more stable Rh I species bearing a protonated Cp* ligand21 .Accordingly, our DFT calculations predicted that the [Rh I (bpy)(HCp*)] + complex was 4.6 kcal/mol more stable than the [Rh III (bpy)(Cp*)H] + one in terms of Gibbs free energy. Then, the third reduction of the catalyst that generated its active form is known to be bipyridine-centered. For this reason, we analyzed how the interactions between the [Rh I (bpy)(HCp*)] + species and [EMIM] + in solution could be related with the experimentally observed shift in the reduction potential that led to the catalyst's active form. Notably, our calculations revealed that the formation of the π cation•••π adduct between the catalyst and [EMIM] + represented in Figure 3.18 (a) lowers the LUMO energy of the [Rh I (bpy)(HCp*)] + complex by CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS 0.22 eV (Figure 3.18 (b)). Thus, despite that the formation of this π cation•••π adduct was slightly endergonic (by 2.5 kcal/mol), it facilitated the entrance of an extra electron, resulting in an overall decrease of the calculated reduction potential of 170 mV (Figure 3.18 (b)). This was in fair agreement with the experimental shift (110 mV) observed in the catalytic peak potential (Ecat) when comparing [TBA][PF6] and [EMIM][PF6] as supporting electrolytes in solution (Table 3.4). After the reduction of the bipyridine ligand, the dissociation of the [Rh I (bpy •-)(HCp*)]•••[EMIM]+ adduct became thermodynamically unfavorable (by 3.8 kcal/mol) due to the increased charge density supported by the bipyridine ligand, which strengthened its interaction with the electron-deficient [EMIM] + cation.Likewise, the less stable although catalytically active [Rh III (bpy •-)(Cp*)H] species interacted with[EMIM] + with a binding free energy of -3.5 kcal/mol and therefore, we carried out additional DFT calculations to evaluate whether the favorable formation of such adducts could be responsible for the selectivity variations observed experimentally upon the addition of IL.

Figure 3 .Figure 3 .

 33 Figure 3.18 (a) Optimized geometry for the non-bonding adduct between the [Rh I (bpy)(HCp*)] + catalyst and [EMIM] + . Key distances are shown in Å and carbon atoms of [EMIM] + are colored in green for clarity. (b) Schematic frontier MO diagram showing the impact of the interaction with [EMIM] + on the energy levels and reduction potential of the [Rh I (bpy)(HCp*)] + species. Orbital energies computed at the ωB97X-D level are given in eV and reduction potentials are referred to the Fc + /Fc reference electrode.

  Figure 3.19 (a) Calculated Gibbs free energy profile (kcal•mol -1 ) for the CO2RR and the HER (solid and dashed lines, respectively) promoted by the active form of complex [2] (A) in the absence (black lines) or presence (red lines) of an explicit [EMIM] + cation. (b) and (c) DFT-optimized geometries for the transition states TS1 and TS2 in the presence of [EMIM] + , respectively. Main distances are shown in Å and relative free energies are given in parentheses in kcalmol -1 .

2 CHAPTER 3 :

 23 EMIM] + → 1•••[EMIM] + -3.5 -3.0 1•••[EMIM] + + CO2 → TS1•••[EMIM] + 15.6 15.0 1•••[EMIM] + + H5O2 + →TS2•••[EMIM] + 5.3 5.IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

  2, under CO2 catalytic conditions). This clearly demonstrated that ILs favor CO2 electroreduction as compared to [TBA][PF 6 ]. An in-detail view of all positive shifts on the CO2 reduction potential led to the following conclusions: (i) those shifts were very sensitive to the nature of the IL cation and all 3 imidazolium-based ILs exhibited a much larger positive potential shift than pyrrolidinium based IL and benchmark electrolyte, following the trend [IM] + > [Pyrr] + > [TBA] + ; (ii) there was almost no effect on the CO2 reduction potential with respect to the size of the cation alkyl chain and the nature of the associated anion (Figure 3.6). These observations strongly indicated that the IL cation took part in a direct interaction with the molecular catalyst.

  water has been proved in the present work to enhance up to 5 times the catalytic current density in the presence of either [TBA][BF4] or [EMIM][BF4] (dashed red plot in Figure3.15 (b)). This might result from the acetic acid pKa, concentration and/or its neutral character in comparison with water at pH = 3.8, where it is actually positively charged (H3O + ), which allows acetic acid to avoid repulsion electrostatic effects from the cations present at the cathodically polarized electrical double layer.

and 3 .

 3 18 respectively. Especially in acetonitrile and to a lesser degree in water complex[2] became more selective in formate production in the presence of [EMIM] + in comparison with the benchmark electrolyte. In addition to this, the combination of a acetate buffer and [EMIM][BF4] as electrolyte played a dual role on increasing CO2 conversion activity and selectivity towards formate production (entry 4 of Table

  ILs became less relevant in the presence of a proton source in solution due to the formation of a noncharged intermediate as a result of protonation (equation 5). CPE experiments in the presence of TFE demonstrated a slight decrease of selectivity for CO production when ILs were in solution. Nevertheless, higher current densities during CPEs were reached in the presence of ILs as supporting electrolyte, which CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

CHAPTER 3 :

 3 IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS generate the active form of the catalyst via π + -π interactions with [EMIM] + and the electrostatic repulsion

Figure 4 . 1

 41 Figure 4.1 Cyclic voltammograms of the electroreduction of 0.5 M [EMIM][PF6] solution in acetonitrile on a 3 mm GCE (black plot) and AuE (red plot) under Ar. Scan rate of 0.1 V/s.

CHAPTER 4 :

 4 IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

Figure 4 .

 4 Figure 4.2 Cyclic voltammograms under catalytic conditions of 1 mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M

Figure 4 .

 4 3 (a). The electrode was placed in an acetonitrile solution containing 0.5 M [EMIM][PF6] as supporting electrolyte saturated in argon, and cycled towards subsequently more cathodic potentials up to -3.95 V vs. Fc + /Fc. After each CV for IL layer electrodeposition, the electrode was removed, rinsed with acetonitrile and placed in a solution containing the model molecular catalyst[2] under catalytic CO2RR conditions (Figure4.3 (b)). The outcome suggested that the more cathodically we scanned (corresponds to CV in red in Figure4.3 (a)), which coincided with more imidazolium reduction, the larger the catalytic effect we obtained. Thus, the potential limit to immobilize the IL by CV was selected to be -3.95 V vs. Fc + /Fc. Next, we studied the IL immobilization by constant potential electrolysis (CPE) varying the duration (ranging from 10 s CPE to 15 min CPE) at this selected potential to obtain an optimal electrodeposited IL layer. Based on the CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS catalytic onset potentials and current density values shown in Figure 4.4, the optimal layer from a catalytic point of view was obtained by 10 sec CPE (red plot in Figure 4.4) and it was almost identical to the effect produced by cycling only once by CV up to -3.95 V (black plot in Figure 4.4). The procedure applied was the same as previously described: first immobilization of the IL using 0.5 M [EMIM][PF6]

Figure 4 .

 4 Figure 4.3 (a) Cyclic voltammograms at a scan rate of 0.1 V/s of electrodeposited layer formation on GCE in 0.5 M [EMIM][PF6] acetonitrile solution with reductive potential limits until: -3.45 V (black plot), -3.75 V (blue plot), -3.95 V (red plot) and (b) the corresponding catalytic condition CVs of 1 mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on: IL-modified GCE by 1 CV until -3.45 V electrodeposited layer (black plot), ILmodified CGE by 1 CV until -3.75 V electrodeposited layer (blue plot) and IL-modified GCE by 1 CV until -3.95 V electrodeposited layer (red plot). Catalytic conditions: acetonitrile solution under CO2 with 5 % H2O (vol.). Scan rate 0.01 V/s. WE=GCE.

Figure 4 . 4

 44 Figure 4.4 Cyclic voltammograms under catalytic conditions of 1 mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on an IL-modified GCE by electrodeposited layer formed by CPE at -3.95 V in 0.5 M [EMIM][PF6] acetonitrile solution by: 10 s (red plot), 30 s (blue plot), 60 s (purple plot), 15 min (green plot) and by 1 CV until -3.95 V at a scan rate of 0.1 V/s (black plot). Catalytic conditions: acetonitrile solution under CO2 with 5 % H2O (vol.). Scan rate 0.01 V/s. WE=GCE.

Figure 4 . 5 ,

 45 shows the CVs on complex[2] under CO2 catalytic conditions with electrodes where different electrodeposited IL layers have been formed using different concentrations of [EMIM][PF6], from 0.01 M to 1 M by applying a CPE of 10 s at -3.95 V. These results showed that 0.5 M [EMIM][PF6] was the concentration that resulted in the highest catalytic current density. In the same line, we also decided to test if there was any effect on the anion used to electrodeposit the IL layer (CPE of 10 s at -3.95 V).

Figure 4 .

 4 6 (a) shows that there is no effect on the anion used to generate the layer, showing that both [EMIM][PF6] and [EMIM][BF4] gave similar results in terms of electrodeposition of an IL layer. Then, we decided to study the effect of a more bulky imidazolium cation and we tested the electroreduction of [BMIM][PF6] as shown in 4.6 (b), which exhibited the same decrease in overpotential displayed by [EMIM][PF6], but was less catalytically efficient. Finally, in order to evaluate the effect of CO2 during the IL layer formation, we performed the electroreduction of [EMIM][PF6] in non inert conditions.

Figure 4 .

 4 7 shows the CVs of complex[2] under CO2 catalytic conditions when comparing the electrodeposition of the [EMIM][PF6] layer in argon and CO2 atmospheres in GCE, respectively. It is evident that the electrodeposited layer CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS grown under CO2 does not produce any clear impact on the catalytic behaviour of complex [2], since it closely resembles the response of the bare electrode also shown in Figure 4.7 for the shake of comparison.

Figure 4 . 5

 45 Figure 4.5 Cyclic voltammograms under catalytic conditions of 1mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on an IL-modified GCE (electrodeposited IL layer by CPE for 10 s at -3.95 V) using different concentrations of [EMIM][PF6] in acetonitrile: 0.01 M (black plot), 0.1 M (blue plot), 0.5 M (red plot) and 1 M (green plot) and CV on a bare GCE (purple plot). Catalytic conditions: acetonitrile solution under CO2 with 5 % H2O (vol.). Scan rate 0.01 V/s. WE=GCE.

Figure 4 . 6

 46 Figure 4.6 Cyclic voltammograms under catalytic conditions of 1mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on an IL-modified GCE (electrodeposited IL layer by CPE of 10 s at -3.95 V) in: (a) 0.5 M [EMIM][PF6] acetonitrile solution (red plot) and in 0.5 M of [EMIM][BF4]

CHAPTER 4 :

 4 IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

Figure 4 . 7

 47 Figure 4.7 Cyclic voltammograms under catalytic conditions of 1mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on an IL-modified GCE (electrodeposited IL layer by CPE of 10 s at -3.95 V in 0.5 M [EMIM][PF6] acetonitrile solution) : Under Argon (red plot) and under CO2 (blue

Figure 4 . 8

 48 Figure 4.8 Cyclic voltammogram of 0.5 M [EMIM][BF4] aqueous solution under argon. Scan rate 0.1 V/s. WE= GCE.

Figure 4 . 9

 49 Figure 4.9 Cyclic voltammograms under catalytic conditions of 1 mM [Rh(bpy)(Cp*)Cl]Cl in 0.5 M [TBA][PF6] acetonitrile solution on: an IL-modified CGE (electrodeposited IL layer by CPE of 10 s at -3.95 V in 0.5 M [EMIM][PF6] acetonitrile solution (red plot), an IL-modified GCE (electrodeposited

Figure 4 .

 4 Figure 4.10 Cyclic voltammograms under catalytic conditions of 1mM [Rh(bpy)(Cp*)Cl]Cl in acetonitrile/water mixture solution on: a bare GCE in 0.5 M [TBA][BF4] solution (black plot), a bare GCE in 0.5 M [EMIM][BF4] solution (green plot), an IL-modified GCE with an electrodeposited layer (CPE of 10 s at -3.95 V in 0.5 M [EMIM][PF6] in acetonitrile solution) in 0.5 M [TBA][BF4] solution (red plot) and an IL-modified GCE with a covalent [EMIM] + layer in 0.5 M [TBA][BF4] solution (blue plot). Catalytic conditions: acetonitrile/water 50/50 % (vol.) solution under CO2. Scan rate 0.01 V/s. WE=GCE.

Figure 4 .

 4 Figure 4.11 (a) are presented the catalytic CVs in a 0.1 M [TBA][BF4] aqueous solution of 1 mM

Figure 4 .

 4 11 (b) though, where 0.5 M [EMIM][BF4] is present as the supporting electrolyte, the effect of an additional electrodeposited IL layer on the catalytic activity seems to be insignificant from a CV point of view.

Figure 4 .

 4 Figure 4.11 Cyclic voltammograms under catalytic conditions of 1mM [Rh(bpy)(Cp*)Cl]Cl in unbuffered aqueous solution under CO2 atmosphere on a bare GCE (black plot) or IL-modified GCE by an electrodeposited IL layer (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution) (red plot) in (a) 0.1 M [TBA][BF4] and (b) 0.5 M [EMIM][BF4]. Scan rate 0.01 V/s. WE=GCE.

Figure 4 .

 4 Figure 4.12 Cyclic voltammograms of 1mM [Re(CO)3bpyCl] in a 0.5 M [TBA][PF6] acetonitrile solution on a bare GCE (black plot), in a 0.5 M [EMIM][PF6] acetonitrile solution on a bare GCE (blue plot) and in a 0.5 M [TBA][PF6] acetonitrile solution on an IL-modified GCE with an electrodeposited layer (CPE of 10 s at -3.95 V in 0.5 M [EMIM][PF6] in acetonitrile solution) (red plot): (a) under Argon, (b) under CO2 and (c) under CO2 with 1.5 M TFE. . Scan rate 0.1 V/s.

Figure 4 .

 4 Figure 4.13 Cyclic voltammograms of an IL-modified GCE by an electrodeposited layer (CPE of 10 s at -3.95 V in 0.5 M of [EMIM][PF6] in acetonitrile under argon) tested in a 0.5 M [TBA][PF6] acetonitrile solution under Ar. First cycle (black plot) and second cycle (blue plot). Scan rate 0.01 V/s. WE= GCE.

Figure 4 .

 4 Figure 4.15 (a) Cyclic voltammograms of the electroreduction of 0.5 M [EMIM][PF6] solution in acetonitrile on a 3 mm AuE in different potential windows under Ar. Inlet zoom on the resulting reoxidation peaks with their corresponding surface coverage values and (b) Cyclic voltammogram up to -3.95 V vs. Fc + /Fc of the electroreduction of 0.5 M [EMIM][PF6] solution in acetonitrile on a 3 mm GCE under Ar. Inlet zoom on the resulting reoxidation peak and its surface coverage value. Scan rate of 0.1 V/s.

Figure 4 .

 4 16 confirm the coexistence of C, O and N elements in all the samples. The XPS spectra of C(1s) (Figure 4.16 (a), (b) and (c)) present five main contributions at binding energies of 284.7, 285.6, 286.6, 287.4 and 288.5 eV. The second (285.6 eV) and third (286.6 eV) of these contributions correspond to amide and aromatic C-N bands, respectively, thus corroborating the presence of imidazolium moieties on the electrode surface. Moreover, the same bands are present for all three electrodes, though the intensities of the bands may vary. As far as the attribution of the XPS spectra of N(1s) (Figure 4.16 (d), (e) and (f)) is concerned, we observed three bands at 398.7, 400.1 and 401.7 eV. The first (398.7 eV), second (400.1 eV) and third (401.7 eV) are attributed to a pyridinic N, a CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS pyrrolic N and a quaternary N, respectively 26-28 . The electrodeposited IL layer grown in aqueous solution (Figures 4.17

  (b) and (e)) exhibits a poorer signal to noise ratio than the one grown in acetonitrile (Figures 4.17

  (a) and (d)). Overall, the XPS spectra corroborates the presence of a layer containing imidazole moieties immobilized on the electrode surface in all cases.

Figure 4 .

 4 Figure 4.16 XPS high-resolution spectra for C (1s) on (a) IL-modified AuE (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M of [EMIM][PF6] in acetonitrile), (b) IL-modified AuE (CPE of 10 s at -2.5 V vs. Ag/AgCl/KClsat in 0.5 M of [EMIM][BF4] in water) and (c) covalent grafted IL layer. Bond

Figure 4 .

 4 Figure 4.17 (b) compares the electrochemical behaviour of ferrocene on bare GCE and on the

Figure 4 .

 4 Figure 4.17 Cyclic voltammograms of : (a) 1 mM dopamine hydrochloride in a 0.1 M H2SO4 aqueous solution under Ar on a bare GCE (black plot), on an IL-modified GCE by an electrodeposited layer (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution) (red plot) and on an IL-modified GCE by a covalent IL layer formed as previously described in the literatire 20 (blue plot) and (b) 5 mM ferrocene in a 0.1 M [TBA][PF6] acetonitrile solution under Ar on a bare GCE (black plot) and on an IL-modified GCE by an electrodeposited layer (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution) (red plot). Scan rate 0.1 V/s.

Figure 4 .

 4 19 (a) in 100 % acetonitrile solution and 50/50 % (vol.) acetonitrile/water solution, Figure4.19 (b). In Figure4.19 (a) we can see a clear suppression on HER activity by comparing the catalytic response of a bare GCE (black plot) and the one produced by a GCE with an electrodeposited IL layer (red plot). Furthermore, we can still observe the characteristic reduction waves associated with the formation of the active form of complex[2]. In contrast in Figure4.19 (b) in the acetonitrile/water 50/50 % (vol.) mixture, we only observe a welldefined reduction wave for the first Rh-centered reduction process. Nevertheless, the catalytic HER response is visibly hindered in the presence of the electrodeposited IL layer (red plot).

Figure 4 .

 4 Figure 4.19 Cyclic voltammograms of a bare GCE (black plot) and an IL-modified GCE by an electrodeposited layer (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution) (red plot) under an argon atmosphere in an electrolyte system containing 1 mM complex [2] and 0.1 M AcOH that is: (a) acetonitrile solution with 0.1 M of [TBA][PF6] or (b) acetonitrile/water 50/50 % (vol.) solution with 0.1 M of [TBA][BF4]. Scan rate 0.1 V/s.

Figure 4 .

 4 Figure 4.20 Constant current electrolysis (CCE) at -3.33 mA/cm 2 of 1 mM complex [2] in acetonitrile solution under CO2 in presence of 5 % (vol.) H2O on: a bare GCE in 0.5 M [TBA][PF6] (black plot), a bare GCE in 0.5 M [EMIM][PF6] (red plot), an IL-modified GCE by an electrodeposited layer (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution) in 0.5 M [TBA][PF6] (blue plot) and on an IL-modified GCE by a covalent grafted IL layer formed as previously described in the literature 20 in 0.5 M [TBA][PF6] (green plot). Stirring rate: 400 rpm.

CHAPTER 4 :

 4 IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

  electrolytes: i) 0.1 M [TBA][BF4] together with 0.1 M AcOH as a proton source (pH = 2.5) (entries 1-3), ii) 0.5 M [EMIM][BF4] without any additional proton source (entries 4-6) (pH = 3.1) and iii) 0.1 M [TBA][BF4] together with 0.1 M acetate buffer solution (pH = 3.8) (entries 7-8). All those electrolysis were performed using 3D RVC foam electrodes in order to reach the desired current density of 3.33 mA/cm 2 . In Figure 4.21 are depicted the plots of electrolysis time vs. cathode potential (corresponding to Table4.2, entries 1-3 and 8), as well as the thermodynamic potential for CO2 reduction to formate, which has been calculated at -0.55 V vs. Ag/AgCl/KClsat for pH=2.5, -0.58 V vs. Ag/AgCl/KClsat for pH=3.1 and -0.62 V vs. Ag/AgCl/KClsat for pH=3.8. The cathode potential evolution during electrolysis ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS displayed in Figure4.21 was not very different in all 4 cases, which pointed out to that the cathode potential was not limited by the presence or absence of an IL layer on the electrode surface. In contrast,

  consecutive CCE electrolysis of different duration (10 C, 40 C and 10 C) keeping the same cathode CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

Figure 4 .

 4 Figure 4.22 Faradaic efficiency for formate production results of three consecutive CCE (for 10, 40 and 10 C respectively) at -3.33 mA/cm 2 of 1 mM complex [2] in aqueous solution in presence of 0.1 M [TBA][BF4] and 0.1 M acetate buffer solution (pH=3.8). CCE performed in an open H-type cell under constant CO2 bubbling on an IL-modified RVC electrode by an electrodeposited layer (CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution) and on a bare RVC electrode.

CHAPTER 4 :

 4 IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

Figure 4 .

 4 6) is confirming, what was previously reported in Chapter 3 concerning imidazolium-based ILs in solution;

CHAPTER 4 :

 4 IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS

CHAPTER 4 :

 4 IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSISprocess. However, we think it is not just a molecular layer, we think those dimeric species undergo spontaneous oxidation at open circuit potential an become positively charged, which is supported by the HER suppressing observed on the IL-modified electrodes only in acidic solution and the almost identical catalytic behavior for CO2RR catalyzed by complex[2] displayed by imidazolium-ILs in solution and immobilized on the electrode surface.

.

Chapter 3

 3 and based on comparing the role as co-catalyst of different ILs in solution was not selected for scale up tests. In contrast, the electrode surface engineering strategy developed within Chapter 4 and based on the immobilization by electrodeposition of imidazolium-based ILs at the electrode surface was scaled up in acidic aqueous solution due to its potential industrial interest.

Figure 5 .

 5 Figure 5.1 (a) Schematic illustration of (a) liquid-phase CO2RR, where CO2 reaches the planar electrode surface through the electrolyte and (b) gas-phase CO2RR, where CO2 is vapor-fed through a GDE 12 .

Figure 5 . 2 9 CHAPTER 5 :

 5295 Figure 5.2 Illustration of the CO2 flow electrolyzer. (a) Simplified cross-sectional view graphic scheme featuring the two-compartment configuration with a GDE cathode * , (b)Frontal view of the reactor with illustrated all visible elements and (c) Side view of the reactor with illustrated the electrodes and the electrolyte inlets and outlets. * Adapted from Wang et. al.9

Figure 5 .

 5 Figure 5.3 (a) Picture of the single-compartment recirculating flow reactor configuration and (b) Results of formate Faradaic efficiency on a series of tests performed in a one-compartment recirculating flow reactor configuration on a bare 10 cm 2 graphite cathode at varying current density, catholyte volume, solution flow rate and anode material with the catholyte under continuous stream of "wet" CO2.

Figure 5 .

 5 Figure 5.5 (a) Picture of the two-compartment in-flow reactor configuration and (b) Representation of formate Faradaic efficiency for 50 C (black bar), 100 C (red bar) and in one case 367 C (grey bar) on a series of tests performed in a two-compartment recirculating flow reactor configuration on a bare 10 cm 2 GDE cathode at varying current density and catalyst concentration. The solution flow rate was kept constant at 22.1 mL/min and the CO2 flow rate of 190 mL/min.

Figure 5 . 8 Formate

 58 Figure 5.8 Formate Faradaic efficiency over 1000 C charge passed for a bare GDE cathode (black curve) and an IL-modified by electrodeposited layer GCE cathode (red curve) in a two-compartment single pass flow reactor configuration for a current density of 20 mA/cm 2 , at a solution flow rate of 1.1 mL/min and CO2 flow rate of 190 mL/min. Also mentioned are the average values of cell potential, as well as initial and final pH value of the catholyte, while anolyte was stable at pH=13 throughout. Energy Efficiency (EE) = (ETcell/Ecell ) • FEHCOO-, where ΕTcell is the theoretical cell potential for CO2RR/OER in those conditions (ΕTcell = 0.82 V), Ecell is the average cell potential and FEHCOO-is the faradaic efficiency for formate production. Energy Consumption (EC): EC = (Q • |Ecell| / N) • 2.78 10 - 4 (kWh/kmol), where Q represents the total charge circulated measured in Coulombs, |Ecell| represents the absolute cell potential in volts and N is the number of moles of each product. 21 a Averages of three independent experiments. b Average of values taken during each experiment.

  by electrodeposited layer cathode in an H-type cell (cathode electrode is RVC) for a charge of 10 C at a current density of 3.4 mA/cm 2 and in a single pass flow electrolyzer in two-compartment in-flow configuration on a GDE cathode for a charge of 1000 C at a current density of 20 mA/cm 2 and a solution flow rate of 1.1 mL/min. Catholyte composition in both cases was 1 mM [Rh(bpy)(Cp*)Cl]Cl in a 0.1 M acetate buffer solution at pH=3.8 saturated with [TBA][BF4] and CO2.

Figure 6 . 1

 61 Figure 6.1 Outlook of this thesis

CHAPTER 6 :Figure 6 . 2

 662 Figure 6.2 Structures of molecular catalysts with one or multiple bpy ligands: (a) Manganese monomer and dimer complexes with bpy and carbonyl ligands 4 and (b) Polyriridyl Rhodium or Iridium complexes 5 .

Figure 1 .

 1 Figure 1. Mécanisme catalytique de la réduction de CO2 par : (a) le catalyseur de Lehn [Re(bpy)(CO)3Cl] en présence et en absence d'une source des protons externe 13 et (b) le catalyseur [Rh(bpy)(Cp*)Cl]Cl. Le chemin (b) est adapté de Caix et. al. 15 et des études théoriques présentées dans la thèse.

  équipements utilisés. Le chapitre inclut également les brèves descriptions des techniques électrochimiques utilisées pendant ces travaux, ainsi que des techniques physicochimiques de caractérisation de surface, tels que la spectrométrie de photoélectrons induits par rayons X. En outre, la description des techniques d'analyses des produits de réduction de CO2 après des expériences d'électrolyse est présentée. Ce chapitre comprend également les détails des calculs théoriques autour de l'interaction entre le catalyseur [Rh(bpy)(Cp*)Cl]Cl et les cations imidazolium, qui ont été réalisés par nos collaborateurs.

Figure 2

 2 montre les structures des deux catalyseurs [Re(bpy)(CO)3Cl] [1] et [Rh(bpy)(Cp*)Cl]Cl[2], où bpy est 2,2'-bipyridine et Cp* est 1,2,3,4,5-pentacyclopentadienyl, ainsi que tous les liquides ioniques utilisés pendant cette étude. Les mécanismes catalytiques de deux catalyseurs modèles choisis sont bien compris[12][13][14][15] . Parmi cette série des liquides ioniques on a testé quatre cations et trois anions distincts. La plupart des cations appartiennent à la famille des imidazolium (Fig.2a, b, c, e), l'un appartient à la famille des pyrrolidinium (Fig.2d). La Figure2fmontre la structure du sel de fond conventionnel utilisé.

Figure 2 .

 2 Figure 2. Structures des complexes [Re(bpy)(CO)3Cl] [1] et [Rh(bpy)(Cp*)Cl]Cl [2], dont bpy = 2,2'bipyridine et Cp* = 1,2,3,4,5-pentacyclopentadienyl, et la série des sels de fond: a) 1-Butyle-3methylimidazoliumhexafluorophosphate ([ΒΜΙΜ][PF6]), b) 1-Ethyle-3-methylimidazolium hexafluorophosphate ([ΕΜΙΜ][PF6]), c) 1-Ethyle-3-methylimidazolium tetrafluoroborate ([ΕΜΙΜ][BF4]), d) 1-Butyle-1-methylpyrrolidinium hexafluorophosphate ([ΒΜΡyrr][PF6]), e) 1,2-Dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([DiΜΙΜ][N(SO2CF3)2]) et f) Tetrabutyle ammonium hexafluorophosphate ([TBA][PF6]).

  le conventionnel [TBA][PF6] et les deux liquides ioniques, l'imidazolium ([ΒΜΙΜ][PF6]) et le pyrrolidinium ([ΒΜΡyrr][PF6]). L'anion est le même dans ces trois cas, ce qui permet d'évaluer exclusivement l'importance du cation. Le premier pic de la réduction quasi-réversible est attribué à la réduction à un électron situé sur le ligand bipyridine, tandis que le deuxième pic est attribué à la réduction du centre métallique de Re I à Re 0 . Notons que : (i) les deux processus redox sont déplacés vers des valeurs de potentiel plus anodiques quand des liquides ioniques sont présents par rapport au [TBA][PF6]

  de réduction de CO2 quand le catalyseur est réduit à deux électrons. En fonction du liquide ionique utilisé la réduction de surpotentiel de CO2RR est plus ou moins importante, ses valeurs allant de 200 mV à 330 mV. En revanche, une fois qu'une source des protons, 1,5 M de trifluoroéthanol, est ajoutée au système, la diminution du surpotentiel est moins marquée, comme le montre la Figure 3c. Néanmoins, la vague catalytique est plus intense dans les trois cas, parce que l'abondance des protons accessibles facilite les réactions de transfert d'électrons qui abaissent les barrières énergétiques pour la CO2RR. Le fait que les liquides ioniques diminuent plus drastiquement le surpotentiel en absence de source des protons peut RÉSUMÉ EN FRANÇAIS être expliqué par le mécanisme d'action du catalyseur. Selon le mécanisme représenté sur la Figure 1a, en absence de source des protons l'ajout d'un électron précède celui d'un proton et, par conséquent, les intermédiaires du catalyseur sont chargés négativement à un ou deux électrons. Ces formes peuvent être électrostatiquement stabilisées par les cations des liquides ioniques et si ceux-là sont aromatiques, tels que les imidazolium, ils peuvent avoir des interactions de type π-π stacking. Par contre, si une source de protons est présente dans le milieu catalytique, l'ajout d'un proton est prioritaire. Cela produit un intermédiaire neutre et un autre chargé -1, qui ne peut pas être si efficacement stabilisé par les cations imidazolium. Sur la Figure 3d l'efficacité faradique en CO et H2 d'une série des électrolyses à potentiel -2,05 V vs. Fc + /Fc est montrée. Dans toutes les conditions le produit majoritaire est le CO et le seul autre produit minoritaire détecté est du H2 26 .

Figure 3 .

 3 Figure 3. Voltammogrammes cycliques de 1 mM catalyseur [1] et 0,5 M de sels de fond différents dans l'acetonitrile, [TBA][PF6] est en noir, [BMPyrr][PF6] est en vert et [BMIM][PF6] est en bleue : (a) sous atmosphère inerte, (b) sous CO2 et (c) sous CO2 avec 1,5 M de trifluoroéthanol ajouté. Vitesse de balayage est 0,1 V/s. (d) Efficacité faradique des électrolyses dans l'acetonitrile avec 0,5 M de chaque sel de fond et 1,5 M de trifluoroéthanol sous CO2 à potentiel appliqué de -2,05 V vs. Fc + /Fc pour une charge passée de 15 C.

Figure 4 .

 4 Figure 4. (a) et (b) Voltammogrammes cycliques de 1 mM catalyseur [2] et 0,5 M de sels de fond différents dans la solution d'acétonitrile qui contient 5 % (vol.) de l'eau sous atmosphère de CO2 : [TBA][PF6] (en noir), [EMIM][PF6] (en rouge), [EMIM][BF4] (en vert) et [BMIM][PF6] (en bleu).Potentiels rapportés versus le standard interne Fc + /Fc (V).

Figure 5 .

 5 Figure 5. (a) et (b) Voltammogrammes cycliques de 1 mM catalyseur [2] dans une solution aqueuse contenant : (a) 0,1M de [TBA][BF4] sous Ar (en noir), 0,1 M de [TBA][BF4] sous CO2 (en bleu solide) et 0,1 M de tampon acétate à pH=4 saturé en [TBA][BF4] sous CO2 (en bleu pointillé) et (b) 0,5 M de [EMIM][BF4] sous Ar (en noir), 0,5 M de [EMIM][BF4] (en bleu solide) et 0,1 M de tampon acétate à pH=4 avec 0,1 M de [EMIM][BF4] (en bleu pointillé). Potentiels rapportés versusl'électrode de référence Ag/AgCl/KClsaturé. Vitesse de balayage 0,01 V/s.

Figure 6 . 4

 64 Figure 6. (a) Energies de Gibbs libres (kcal/mol) calculées pour la réduction de CO2 et l'évolution de H2 (lignes solides et pointillées respectivement) par l'espèce 1 en absence (lignes noires) et en présence (lignes rouges) d'un cation [EMIM] + . (b) et (c) Géométries optimisées selon la théorie de la fonctionnelle de la densité (DFT) pour les états de transition TS1 et TS2 en présence de [EMIM] + .

Figure 7 .

 7 Figure 7. (a) Voltammogrammes cycliques de 0,5 M [EMIM][PF6] dans l'acétonitrile sous atmosphère inerte sur électrode de carbon vitreux (en noir) et sur électrode d'or (en rouge) de 3 mm diamètre et (b) Voltammogrammes cycliques de catalyseur [2] à 1 mM dans une solution d'acétonitrile sous CO2 contenant : 0,5 M de [TBA][PF6] (en noir), 0,5 M de [EMIM][PF6] (en vert) et une couche électrodéposée de [EMIM][PF6] testée dans du 0,5 M [TBA][PF6]. Couche électrodéposée par 10 sec de chronoampérométrie à -3,95 V vs. Fc + /Fc dans une solution d'acétonitrile contenant 0,5 M de [EMIM][PF6]. Vitesse de balayage 0,01 V/s.

Figure 8 .

 8 Figure 8. (a) Représentation schématique de l'électrolyseur en flux en configuration de deux compartiments et (b) Efficacité faradique en formiate produite par le catalyseur [2] sur un GDE sans couche (en noir) et avec couche électrodéposée (en rouge).

1.8 Flow electrolyzers: State of the art and incorporation of molecular catalysts

	CHAPTER 1: INTRODUCTION
	into the structure-activity relationship at play that is crucial for optimizing CO2RR 109 . In contrast,
	molecular catalysts are by nature well-defined, often extremely selective towards one CO2RR product,
	easily tunable through simple synthetic pathways. We can distinguish two different approaches: i)
	molecular catalysts in solution, in which the CO2 is bubbled in solution only, and ii) molecular catalysts
	heterogenized supported on the electrode, in which the CO2 is bubbled in solution or GDE. So far only
	a few other examples of molecular CO2 electrocatalysts in flow electrolyzers exist and most of them
	involve heterogenized catalysts 109-113 . To the best of our knowledge, the only homogeneous catalyst used
	in a flow electrolyzer to date for CO2RR was [Ni(cyclam)] 2+ . This catalyst was used in two organic
	solutions (ACN and N, N-DMF) in a flow electrolyzer with recirculating flow of the catholyte
	solution 114 .
	Flow electrolyzers are commonly used to prove the scalability of electrochemical systems from the
	laboratory environment to the industrial scale. Most studies in flow electrolyzers for CO2RR use
	heterogeneous catalysts but the use of those materials leads to questions about the exact nature and
	density of the active sites that result in CO2RR products and cannot provide detailed mechanistic insight

.1 Reagents, Materials and Instruments 2.1.1 Chemical Reagents Chemical

  reagents used in the following chapters are of analytical grade and are listed in Table2.1.

	CHAPTER 2: MATERIALS AND METHODS
	In the present chapter are detailed all reagents and other materials used throughout this thesis. Moreover,
	presented here are all experimental procedures, characterization techniques, syntheses and other methods
	used.
	Chapter 3, two model molecular catalysts are used for homogeneous CO2RR in electrolyte solutions containing 0.5 M of different ILs. Initially, [Re(bpy)(CO) 3 Cl] is chosen as a model for its high selectivity 2Water used in all experiments is MiliQ grade with a resistivity value of 18.2 MΩ cm.
	for CO evolution and its well-established reaction mechanism for CO2RR. It is subsequently tested in
	acetonitrile solutions containing a series of ILs in order to determine what features of ILs govern their
	interaction with the catalyst. Product selectivity is also assessed by electrolysis to determine whether
	there is an IL effect. Secondly, another model catalyst is chosen, [Rh(bpy)(Cp*)Cl]Cl, this time because
	its product selectivity is very sensitive to changes in the reaction environment and is active towards both
	HER and CO2RR. Cyclic voltammetry studies with ILs in solution are followed by selectivity studies to
	decipher how ILs impact the activity of this catalyst. The solubility of this catalyst in water, also allows
	to explore this effect in different electrolyte compositions. Finally, theoretical calculations are provided
	to corroborate mechanistic explanations of how ILs impact both those catalysts in different ways.
	In Chapter 4 an immobilization method for ILs is presented and its CO2RR effect is assessed with the
	help of model catalyst [Rh(bpy)(Cp*)Cl]Cl. Optimization studies of this electrodeposited IL layer are
	presented, while many parameters are investigated. The catalytic effect of said layer on
	[Rh(bpy)(Cp*)Cl]Cl catalyst is also compared to the effect produced by IL in solution and a covalently
	attached IL layer. This test is shown in different electrolyte compositions and the comparison is used to
	draw conclusions on the nature of the electrodeposited IL layer. Its effect on HER is equally investigated,
	alongside characterization by XPS and CV. Finally, its effect on the product selectivity of the molecular
	catalyst is assessed by electrolysis.
	In Chapter 5 a flow electrolyzer setup is used to scale-up the previously described system of molecular
	catalyst [Rh(bpy)(Cp*)Cl]Cl in acidic aqueous conditions and in the presence of the electrodeposited IL
	layer. Through investigation of different electrolyzer configurations, a final setup is presented that allows
	testing the electrodeposited layer on GDE compared to a bare GDE with the molecular catalyst in
	solution for charge passed up to 1000 C.
	Finally, in Chapter 6 are presented the overall conclusions drawn from the ensemble of the thesis, as
	well as future directions for this research.

Table 2 .

 2 1 Chemical reagents used throughout the PhD thesis.

	Reagent name	Reagent formula	Reagent Grade	Brand
	Anhydrous acetonitrile	CH 3 CN	99.99 %	Sigma-Aldrich
	Tetrabutyl ammonium hexafluorophosphate	[TBA][PF 6 ]	> 99 %	Sigma-Aldrich
	Tetrabutyl ammonium tetrafluoroborate	[TBA][BF 4 ]	> 99 %	Sigma-Aldrich
	2,2,2-trifluoroethanol (TFE)	CF3CH2OH	> 99 %	Sigma-Aldrich
	Acetic acid (AcOH)	CH3CO2H	> 99.5 %	TCI chemicals
	Sodium acetate	CH3CO2Na	> 99 %	Sigma-Aldrich
	Sodium formate	HCOONa	> 99 %	Sigma-Aldrich
	Ferrocene (Fc)	(C5H5)2Fe	98 %	Merck
	1-Butyl-3-methylimidazolium hexafluorophosphate	[ΒΜΙΜ][PF 6 ]	99 %	Io-li-tec
	1-Ethyl-3-methylimidazolium hexafluorophosphate	[ΕΜΙΜ][PF 6 ]	99 %	Io-li-tec
	1-Ethyl-3-methylimidazolium tetrafluoroborate	[ΕΜΙΜ][BF 4 ]	> 98 %	Io-li-tec
	1-Butyl-1-methylpyrrolidinium hexafluorophosphate	[ΒΜΡyrr][PF 6 ]	99 %	Io-li-tec
	1,2-Dimethylimidazolium bis(trifluoromethylsulfonyl)imide	[DiΜΙΜ][N(SO 2 CF 3 ) 2 ]	98 %	Io-li-tec
	1-Ethylimidazole	C5H8N2	> 98%	Io-li-tec
	Chlorotricarbonyl(2,2'-bipyridine)rhenium(I)	[Re(bpy)(CO) 3 Cl]	99 %	STREM chemicals
	Dichloro(pentamethylcyclopentadienyl)rhodium(III) dimer	[Rh(Cp*)Cl]2	99 %	STREM chemicals

Figure 2.1 Chemical structure, name and the corresponding abbreviation of the ILs used in this work.

2.1.2 Experimental Materials Electrodes:

	• Glassy Carbon Electrode disk (GCE), diameter = 3 mm, purchased from BioLogic. Used as
	working electrode (WE) for Cyclic voltammetry (CV) experiments.
	• GCE plate, geometric area = 2-3 cm 2 , 1 mm thick, purchased from Alfa Aesar. Used as WE for
	Constant Potential Electrolysis (CPE) and Constant Current Electrolysis (CCE) experiments in
	acetonitrile solutions.
	• GCE rod, diameter = 1.2 cm 2 , length = 8 cm, purchased from Alfa Aesar. Used as counter
	electrode (CE) for CPE and CCE experiments with catalyst [Rh(bpy)(Cp*)Cl]Cl.
	• Reticulated vitreous carbon (RVC) foam electrode, Duocel® RVC foam, density 45 PPI,
	geometric area = 3-6 cm 2 , purchased from ERG Materials and Aerospace Co. Used as WE for
	CCE experiments in aqueous solutions with catalyst [Rh(bpy)(Cp*)Cl]Cl.

  Protocol adapted from the literature24 . A methanol solution (10 mL) of 1 equivalent [Rh(Cp*)Cl]2 (100 mg, 0.16 mmol) and 2 equivalents 4,4'-dimethoxy-2,2'-bipyridine (82.17 mg, 0.38 mmol) was stirred at RT for 12 h. The solvent was removed in vacuo and the residue was dissolved in a minimum amount of acetonitrile. Addition of ethyl acetate gave a pale-yellow solid which was collected on a Buchner

. A methanol solution (30 mL) of 1 equivalent [Rh(Cp*)Cl]2 (200 mg, 0.32 mmol) and 2 equivalents 2 2'-bipyridine (100 mg, 0.64 mmol) was stirred at RT for 2h in the dark. The resulting clear orange-yellow solution was evaporated until dry. The yellow solid was purified by recrystallization. It was dissolved in a minimal quantity of acetonitrile (CH3CN) and precipitated upon the addition of ethyl acetate (AcOEt), then collected on a Buchner funnel and dried under vacuo. Yield = 88 %. The purity of the final precipitate was verified by 1 H NMR spectroscopy. 1 H NMR (300 MHz, DMSO-d6): δ/ppm, 1.01 (s, 15H), 7.89 (t, J = 6.2 Hz, 2H), 8.32-8.34 (m, 2H), 8.72 (d, J = 7.8 Hz, 2H), 8.98 (d, J = 5.4 Hz, 2H). Synthesis of [Rh(4,4'-dimethoxy-2,2'-bipyridine)(Cp*)Cl] : funnel, washed with ether, and dried under vacuo. An analytical sample was obtained by recrystallization from CH3CN/AcOEt (yield = 90 %) and characterized by 1 H NMR spectroscopy. 1 H NMR (300 MHz, DMSO-d6): δ/ppm, 1.01 (s, 15H), 4.07 (s, 6H), 7.42 (dd, J = 9.1 Hz, 2H), 8.31 (d, J = 2.4 Hz ,2H), 8.73 (d, J = 6.3 Hz, 2H).

Table 3 .

 3

1 H2O content quantification in 0.5 M IL CH3CN solutions using Karl-Fischer titration before and after ILs drying treatment. 0.

5 M IL in CH3CN solution [H2O] (ppm) before drying ILs [H2O] (ppm) after drying ILs [BMIM][PF6]

		652	88
	[BMPyrr][PF6]	2557	158
	[EMIM][PF6]	435	250
	[EMIM][BF4]	1821	151

CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

Table 3 .

 3 2 reports an estimation of catalytic current densities obtained in each case by comparison with the corresponding current densities obtained under inert conditions (using the ratio jcat/jp) and the characteristic potentials for the catalytic peaks shown in Figure3.6. Thus, the largest jcat/jp ratios were

	obtained in 0.5 M [EMIM][PF6] and 0.5 M [EMIM][BF4] in CH3CN. Furthermore, Table 3.2 reports Ecat/2
	values, potentials corresponding to half of the catalytic peak current, as proposed by Kubiak's group when
	using complex [1]

Table 3 .

 3

	CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS
	COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR
		ELECTROCATALYSIS
	2 Descriptors for 1 mM complex [1] under inert conditions (Ar) and CO2 catalytic conditions
	in 0.5 M of each supporting electrolyte in CH3CN. Potential of the Rh-centered reduction peak (Ep1/2),
	potential of the bpy-centered reduction peak (Ep), catalytic reduction potential (Ecat/2), overpotential
	(η), and ratio of the catalytic peak current density and the peak current density under inert conditions
			(jcat/jp).
		Under inert conditions	Under catalytic CO2 conditions
	Supporting Electrolyte	Ep1/2 (V) a Ep (V) a Ecat/2 (V) a	η (V) b	jcat/jp
	[TBA][PF6]	-1.70	-2.07	-2.06	1.52	3.2
	[BMIM][PF6]	-1.52	-1.76	-1.75	1.21	4.0
	[BMPyrr][PF6]	-1.64	-1.88	-1.86	1.32	4.6
	[EMIM][PF6]	-1.57	-1.75	-1.74	1.20	9.9
	[EMIM][BF4]	-1.49	-1.70	-1.73	1.19	5.3

). Scan rate 0.1 V/s. a Potentials are referred vs. Fc + /Fc (V), b Determined based on Ecat/2 using E 0 CO2/CO (CH3CN) = -0.541 V vs. Fc + /Fc.

16,43

Table 3 .

 3

	Supporting Electrolyte	Ecat/2 (V) a	η (V) b	jcat/jp
	[TBA][PF6]	-1.93	0.57	16.0
	[BMIM][PF6]	-1.86	0.50	20.2
	[BMPyrr][PF6]	-1.88	0.52	27.0
	[EMIM][PF6]	-1.90	0.54	19.5
	[EMIM][BF4]	-1.90	0.54	20.9
	a Potentials are referred vs. Fc + /Fc (V), b Determined based on Ecat/2 using E 0	CO2/CO (CH3CN, TFE) = -
		1.36 V vs. Fc + /Fc. 16,43

3

Comparison of the catalytic parameters of 1 mM complex

[1]

under a CO2 atmosphere in the presence of 1.5 M of TFE in 0.5 M of each supporting electrolyte in CH3CN: catalytic potential (Ecat/2), overpotential (η), and ratio of the catalytic peak current density and the peak current density under inert conditions (jcat/jp).

Table 3 .

 3

	Entry Solvent	Supporting Electrolyte	Electrolysis applied potential (V) vs. Fc + /Fc	FECO (%) a FEH2 (%) a	Energy Efficiency (%) c
	1	CH3CN 0.5 M [TBA][PF6]	-2.05	92	2	61
	2	CH3CN 0.5 M [BMIM][PF6]	-2.05	77	8	51
	3	CH3CN 0.5 M [BMPyrr][PF6]	-2.05	80	10	53
	4	CH3CN 0.5 M [EMIM][BF4]	-2.05	71	18	47
	5	CH3CN 0.5 M [EMIM][PF6]	-2.05	77	12	51
	6	CH3CN 0.5 M [EMIM][PF6] b	-1.75	96	1	30 d
	a Faradaic efficiency for each product (FECO and FEH2).

4

Electrolysis of 1 mM of complex

[1]

in 0.5 M supporting electrolyte in CH3CN in the presence of 1.5 M of TFE under a CO2 saturated atmosphere. Total electrolysis duration to circulate 14 C. b Electrolysis of 1 mM of complex

[1]

in 0.5 M [EMIM][PF6] in CH3CN under a CO2 saturated atmosphere to circulate 10 C. c Energy Efficiency = ET/E ⋅ FE, where ET = E°CO2/CO (CH3CN, TFE) = -1.36 V vs. Fc + /Fc. d ET = E°CO2/CO (CH3CN) = -0.541 V vs. Fc + /Fc.

Table 3 .

 3

	Under inert conditions

5 Descriptors for 1 mM complex

[2]

under inert conditions (Ar) in 0.5 M of each supporting

electrolyte in CH3CN. Potential of the Rh-centered reduction peak (Ep A), potential of the intermediate peaks (Ep B) and (Ep C) and potential of the bpy-centered reduction peak (Ep D).

CHAPTER 3: IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS

Table 3 .

 3

	Supporting Electrolyte	Ecat (V) a	Ecat/2 (V) a	jcat	jcat/jp	η (V) b
	[TBA][PF6]	-2.14	-2.00	-3.51	17.5	0.68
	[BMPyrr][PF6]	-2.04	-1.95	-1.64	3.1	0.63
	[BMIM][PF6]	-2.09	-1.97	-3.70	-	0.65
	[EMIM][PF6]	-2.03	-1.94	-4.81	-	0.62
	[EMIM][BF4]	-2.04	-1.93	-4.15	-	0.61
			Determined based on Ecat/2 using E 0	CO2/HCOOH (CH3CN, H2O)
			= -1.32 V vs. Fc + /Fc 47 .

6

Comparison of the catalytic parameters of 1 mM complex

[2]

and 0.5 M of different supporting electrolytes in acetonitrile solution containing 5 % (vol.) H2O under CO2. Catalytic potential (Ecat), the half wave catalytic potential (Ecat/2), peak current density exhibited under catalytic conditions (jcat), peak current density exhibited under inert conditions (jp) and the overpotential (η).

a Potentials are referred vs. Fc + /Fc (V).

b

Rh(bpy)(Cp*)Cl]Cl with ILs as electrolyte in organic-aqueous mixture and aqueous solution under CO2 catalytic conditions

Table 3

 3

	.7 shows that H2 formation was

very selective CO2 to formate conversion (FEHCOO-> 90 % and a maximal energy efficiency of 66 %), taking place at much more positive potentials than with [TBA][PF6] (Table

3

.7, entries 1-3). Thus, it was evidenced from the results reported in Table

3

.7, the significant role of [EMIM] + cation at the electrode/solution interface enhancing formate production and suppressing HER.

Table 3 .

 3 7 Constant potential electrolysis in acetonitrile using 5 % of H2O (vol.) as proton source in an electrochemical two compartments H-type cell. Total electrolysis duration to circulate 15 C.

	Entry Solvent	Electrolyte		%H 2 O (vol.)	Electrolysis applied potential (V) vs. Fc + /Fc	η (V)	a	FE HCOO -(%) b	FE H2 b (%)	Energy efficiency (%) c
	1	CH 3 CN	[TBA][PF 6 ]		5		-2.10	0.78		77	18	48
	2	CH 3 CN [EMIM][PF 6 ]	d	5		-1.89	0.57		90	9	63
	3	CH 3 CN [EMIM][BF 4 ]	5		-1.83	0.51		91	9	66
	a Determined based on Ecat/2 using E 0	CO2/HCOOH (CH3CN, H2O) = -1.32 V vs. Fc + /Fc. b Faradaic
	efficiency for each product (FEHCOOH and FEH2). c Energy Efficiency = ET/E ⋅ FE, where ET =
		E o CO2/HCOOH (CH3CN, H2O) = -1.32 V vs. Fc

+ /Fc. d Total charge transferred 10 C. We subsequently performed CCE experiments in the same H-type cell at -3.33 mA/cm 2 of applied current density in acetonitrile/H2O 95/5 (vol.) and acetonitrile/H2O 50/50 (vol.) solutions. The results of these CCEs in terms of overpotential, FE for formate and H2 and energy efficiency values are shown in Table

Table 3 .

 3

	Entry Solvent	Electrolyte	%H 2 O

8 Constant current electrolysis in acetonitrile using different % of H2O as proton source in an electrochemical two compartments H-type cell. Total electrolysis duration to circulate 15 C.

(vol.) Electrolysis applied current density (mA/cm 2 ) Electrolysis E cat average (V) vs.

						Fc	+ /Fc	η (V)	a	FE HCOO -(%) b	FE H2 b (%)	Energy efficiency (%) c
	1	CH 3 CN	[TBA][PF 6 ]	5	-3.33	-2.81	1.49	53	34	25
	2	CH 3 CN [EMIM][PF 6 ]	5	-3.33	-1.95	0.63	69	22	47
	3	CH 3 CN	[TBA][BF 4 ]	50	-3.33	-2.60	1.28	47	31	24
		a Determined based on Ecat/2 using E 0	CO2/HCOOH (CH3CN, H2O) = -1.32 V vs. Fc + /Fc. b Faradaic
		efficiency for each product (FEHCOOH and FEH2). c Energy Efficiency = ET/E ⋅ FE, where ET =
			E o CO2/HCOOH (CH3CN, H2O) = -1.32 V vs. Fc + /Fc.

Table 3 .

 3 9 Constant current electrolysis at -3.33 mA/cm 2 in aqueous solution in an electrochemical two compartments H-type cell. Total electrolysis duration to circulate 10 C.

	Entry	Electrolyte		pH 0	pH f	Electrolysis E cat average (V) vs. Ag/AgCl/KClsat	η (V)	FE HCOO -(%) a	FE H2 a (%)	Energy efficiency (%)
	1	0.1 M [TBA][BF 4 ]	b	3.8	5.5	-2.2	1.58	c	41	42	12
	2	0.5 M [EMIM][BF 4 ]	3.1	3.4	-1.6	1.02	d	49	46	18
	3	0.1 M [TBA][BF 4 ] CH 3 COONa/CH 3 COOH b + 0.1 M	3.8	3.8	-1.0	0.38	c	40	55	25
	4	0.1 M [EMIM][BF 4 ] + 0.1 M CH 3 COONa/CH 3 COOH	3.8	3.9	-0.9	0.28	c	46	40	32
	5	e	0.1 M [TBA][BF 4 ] CH 3 COONa/CH 3 COOH b + 0.1 M	3.8	3.8	-	-			2	84
	6	f	0.1 M [TBA][BF 4 ] CH 3 COONa/CH 3 COOH b + 0.1 M	3.8	3.7	-	-			2	87

a Faradaic efficiency for each product

(FEHCOOH and FEH2

). b 0.1 M is the maximum solubility of [TBA][BF4] in aqueous solution. c E o CO2/HCOOH = -0.62 V vs. Ag/AgCl (pH=3.8). d E o CO2/HCOOH = -0.58 V vs. Ag/AgCl (pH=3.1). e No complex [2] in solution. f No CO2 in solution (Ar bubbling). pH0 and pHf represent initial and final electrolysis solution pH, respectively. Energy Efficiency = ET/E ⋅ FEHCOO-, where ET = E o CO2/HCOOH (H2O).

Table 3 .

 3 10 Key thermodynamic and kinetic parameters of the reaction computed in acetonitrile and water solvent.

  IMIDAZOLIUM-AND PYRROLIDINIUM-BASED IONIC LIQUIDS AS COCATALYSTS FOR CO2 ELECTROREDUCTION IN MODEL MOLECULAR ELECTROCATALYSIS ILs studied (80/20). Finally, with [DiΜΙΜ][N(SO 2 CF 3 ) 2 ], the only protic IL tested in this work, reduction of the protons provided by the IL resulted in the production of H 2 as the only product.

	catalytic current ratio displayed by imidazolium and pyrrolidinium IL solutions were in all cases at least
	25 % higher than [TBA][PF 6 ] containing solutions, [BMPyrr][PF 6 ] being the one exhibiting the highest
	catalytic current ratio. Furthermore, CPE experiments showed that in most cases shorter electrolysis were
	needed to transfer 14 C when ILs were present in solution, since higher current densities were reached
	(Figure 3.10) in agreement with previous CVs (Figure 3.9). However, the catalyst was slightly less
	selective in CO production in the presence of ILs in comparison with benchmark [TBA][PF6] electrolyte.

.4 and Figure 3.10) leading to the following observations from CVs: (i) as expected, addition of TFE resulted in all cases in larger catalytic current densities for CO2 reduction; (ii) a small but significant and stimulating effect of ILs in decreasing overpotential was observed, reflecting the activating role of ILs also in the presence of TFE; (iii) the This was also reflected on the energy efficiency values, which being proportional to the observed FE for CO, decreased in the presence of ILs. [BMIM][PF 6 ] and [BMPyrr][PF 6 ] kept a very high FECO/FEH 2 ratio, 91/9 and 89/11 respectively, meanwhile [EMIM][BF 4 ] led to the lowest selectivity ratio among the CHAPTER 3:

Table 4 .

 4 1 Constant current electrolysis for CO2RR at an applied current density of 3.33 mA/cm 2 in acetonitrile using different % of H2O as proton source in an electrochemical two compartments H-type cell. Total electrolysis duration to circulate 15 C. Electrodeposited layer on GCE electrode by CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution and covalent grafted IL layer on GCE according to the protocol as previously described in the literature 20 .

						Electrolysis	Electrolysis
	Entry	IL layer	Solvent	Electrolyte	%H 2 O (vol.)	applied current density (mA/cm 2 )	E cat average (V) vs. Fc + /Fc	η (V) a	FE HCOO -(%) b	FE H2 b (%)	Energy efficiency (%) c
	1	None	CH 3 CN	[TBA][PF 6 ]	5	-3.33	-2.81	1.49	53	34	25
	2	None	CH 3 CN [EMIM][PF 6 ]	5	-3.33	-1.95	0.63	69	22	47
	3	Electrodeposited	CH 3 CN	[TBA][PF 6 ]	5	-3.33	-1.90	0.58	85	15	59
	4	Covalent	CH 3 CN	[TBA][PF 6 ]	5	-3.33	-1.85	0.53	81	12	58
	5	None	CH 3 CN	[TBA][PF 6 ]	50	-3.33	-2.60	1.28	47	31	24
	6	Electrodeposited	CH 3 CN	[TBA][PF 6 ]	50	-3.33	-2.10	0.78	75	20	47

Table 4 .

 4 .2 shows the combination of the electrodeposited IL layer with the acetate buffer solution, which reached the lowest cathode potential displayed in Figure 4.21 (red plot) and the highest EE value out of the entire 2 Constant current electrolysis for CO2RR at -3.33 mA/cm 2 in aqueous solution in an electrochemical two compartments H-type cell. Total electrolysis duration to circulate 10 C. Electrodeposited layer on RVC foam electrode by CPE of 10 s at -3.95 V vs. Fc + /Fc in 0.5 M [EMIM][PF6] acetonitrile solution and Covalent grafted IL layer on RVC foam electrode according to the protocol as previously described in the literature 20 .

	series, at 33 %.

a Faradaic efficiency for each product (FEHCOOH and FEH2). b 0.1 M is the maximum solubility of [TBA]

[BF4]

in aqueous solution.

c

  are shown the results of a series of gas-phase CO2RR tests performed in this CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION electrolyzer configuration in order to determine relevant catalytic parameters. In test 8, a current density value of 3.4 mA/cm 2 using a GDE as cathode was tested and resulted in FEHCOO-of 23 % and 16 % at 50 C and 100 C, respectively. Meanwhile, increasing the applied current density to 10 mA/cm 2 in test 9 has a slight beneficial effect for the FEHCOO-at both analyzed points, while further increase at 20 mA/cm 2 in test 11 resulted in similar FE values of 28 % at 50 C and 24 % for 100 C. The evolution of FEHCOO-for test 9 is also presented in Figure 5.6, where 10 mA/cm 2 was applied to the system. It resulted in 30 % FEHCOO-at 50 C of charge passed. However, we clearly observed a decrease in formate production over electrolysis time, where further analyzed points that corresponded to 100 C and 367 C afforded only 19

Table 5 .

 5

1 Comparison of formate Faradaic efficiency on a non-modified cathode and on a IL-modified

Table 5 .

 5 Fc → 2 Fc + +2e -. As a consequence, the |Ecell| reported are not easily comparable with our conditions of CO2RR to formate coupled with OER. Though the aforementioned sacrificial donors do not themselves produce other CO2RR products and some of their decomposition products can be isolated and recycled (it is not the case of Fe3O4 formed from oxidized ferrocene reacting with O2), this process nevertheless is restrictive to the system's stability since they occasionally need replacement, especially in the recirculating flow configuration that they are implementing. In contrast, our system works in acidic aqueous conditions which are more environmentally-friendly, overcomes the CO2 solubility limitation (0.033 M) by implementing gas-phase CO2RR and does not need any further addition of sacrificial electron donor in the anolyte. As far as their best catalytic conditions are concerned, it is difficult to contrast, since their CO2RR product is CO and they have selected a catalyst selective towards it, whereas we have deliberately opted for a catalyst that gives a mixture of products because our primary aim was always to study its selectivity in relation to the electrodeposited IL layer. Nevertheless, their highest CO selectivity of 83 % is observed at a moderate current density value of 12.7 mA/cm 2 , whereas our highest formate production (average of 45 % with the electrodeposited layer) was obtained at 20 mA/cm 2 . SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION grafted CO2RR catalyst in a flow electrolyzer setup. Most importantly, despite our catalyst's inherent low selectivity for formate, it is still highly promising that almost all of its catalytic features were clearly ameliorated through the implementation in the flow electrolyzer using an acidic aqueous catholyte composition.

	CHAPTER 5: SCALE-UP TEST OF A HOMOGENEOUS CATALYST IN A FLOW
	ELECTROLYZER FOR ELECTROCATALYTIC CO2 REDUCTION
	reaction: 2

7,[9][10][11][12][13]15

. The only other case where a molecular catalyst reduces CO2 homogeneously is the case of [Ni(cyclam) 2+ ] in a nonaqueous flow electrolyzer

17

, which despite our dissimilarities constitutes the most pertinent comparison for our conditions. In this study, [Ni(cyclam) 2+ ] is used to catalyze CO2 reduction to CO in organic solvents (ACN or N, N'-DMF), using the supporting electrolyte [NH4][PF6] simultaneously as sacrificial proton donor. Their overall cathodic reaction is therefore: CO2 + 2 [NH4] + +2e -→ CO + NH3 + H2O.

Meanwhile, in the anode they add ferrocene (Fc) as a sacrificial electron donor according to the half-Another significant difference is the catalyst concentration used. Where our test at 5 mM of [Rh(bpy)(Cp*)Cl]Cl does not produce any more formate than the 1 mM originally used since the H-type cell tests presented in previous chapters, they report that increasing from 1 to 10 mM lead the optimal observed CO FE. Finally, contrary to our promising stability results over a span of 1000 C, they have observed poor stability of their catalyst in an environment that contains high amounts of CO. More particularly, [Ni(cyclam)] 2+ has been proven spectroscopically to decompose during the experiments and this effect is exacerbated by high overpotential values. This comparison is summarized in Table

5

.2. 2 Comparison of catalytic parameters for CO2RR by homogeneous molecular catalysts in flow electrolyzers, according to cathode material, solvent, anodic half-reaction, configuration of flow electrolyzer, applied or measured current density, overall cell potential, CO2 reduction product and FE for aforementioned product.

Though our system would not fare well in comparison to some heterogenized molecular catalysts in terms of activity and energy efficiency, it is in many ways superior to the other existing example of not CHAPTER 5:

  dans l'acétonitrile ou dans l'eau réalisés dans une cellule électrochimique de type H sous atmosphère CO2. Charge totale circulée : 10-15 C.

	Expérience	Solvant		Electrolyte	% H2O (vol.)	Densité de courante appliquée (mA/cm2)		FY HCOO -(%)	FY H2 (%)	Efficacité Energétique (%)
	1	CH3CN		0.5 M [TBA][PF6]	5		3.3		53	34	25 a
	2	CH3CN		0.5 M [EMIM][PF6]	5		3.3		69	22	47 a
	3	H2O		0.1M [TBA][BF4]	100		3.3		41	42	12 b
	4	H2O		0.5M [EMIM][BF4]	100		3.3		49	46	18 c
	5	H2O	0.1M CH3COO-/CH3COOH + 0.1M [TBA][BF4]	100		3.3		40	55	25 b
	6	H2O	0.1M CH3COO-/CH3COOH + 0.1M [EMIM][BF4]	100		3.3		46	40	32 b
	Efficacité Energétique (EE) % = ET/E ⋅ FEHCOO-, où ET = E o	CO2/HCOOH.	a E o CO2/HCOOH (CH3CN, H2O) = -
	1.32 V vs. Fc + /Fc 28 . b E o	CO2/HCOOH = -0.62 V vs. Ag/AgCl (pH=3.8). c E o	CO2/HCOOH = -0.58 V vs. Ag/AgCl
					(pH=3.1).

  à 1 mM dans l'acétonitrile ou dans l'eau réalisées dans une cellule électrochimique de type H sous atmosphère CO2. Durée totale d'électrolyse correspond à la circulation de 10 C de charge. Couches électrodéposées par 10 sec de chronoampérométrie à -3,95 V vs. Fc + /Fc dans une solution d'acétonitrile contenant 0,5 M de [EMIM][PF6]. Couches covalentes formées selon les protocoles décrits par Bouden et. al.33

	Expérience	Solvant		Electrolyte	Couche IL	%H2O (vol.)	Densité de courant (mA/cm 2 )	FE HCOO -(%)	FE H2 (%)	Efficacité Energétique (%)
	1	CH3CN		0.5 M [TBA][PF6]	Aucune		5		3.33	53	34	25	a
	2	CH3CN	0.5 M [EMIM][PF6]	Aucune		5		3.33	69	22	47	a
	3	CH3CN		0.5 M [TBA][PF6]	Electrodéposée		5		3.33	85	15	59	a
	4	CH3CN		0.5 M [TBA][PF6]	Covalente		5		3.33	81	12	58	a
	5	H2O	0.1 M [TBA][BF4] + 0.1 M CH3COOH	Aucune	100		3.33	38	61	18 b
	6	H2O	0.1 M [TBA][BF4] + 0.1 M CH3COOH	Electrodéposée	100		3.33	53	44	27 b
	7	H2O	0.1 M [TBA][BF4] + 0.1 M CH3COOH	Covalente	100		3.33	49	46	26	c
			0.1 M [TBA][BF4] +
	8	H2O	0.1 M tampon pH=3,8	Electrodéposée	100		3.33	53	42	33	c
			CH3COOH/CH3COONa
	Efficacité Energétique (EE) % = ET/E ⋅ FEHCOO-, où ET = E o	CO2/HCOOH.	a E o CO2/HCOOH (CH3CN, H2O) = -
	1.32 V vs. Fc + /Fc 28 . b E o	CO2/HCOOH = -0.58 V vs. Ag/AgCl (pH=3.1). c E o	CO2/HCOOH = -0.62 V vs. Ag/AgCl
					(pH=3.8).

ACKNOWLEDGEMENTS

Undertaking a PhD in chemistry had been one of my goals ever since my undergraduate studies. While it was at times challenging, it was an incredibly edifying learning experience, that was made possible with the help and support of several people, to whom I owe my sincere gratitude.

Firstly, I would like to thank my PhD directors, Dr. Carlos Sanchez-Sanchez and Prof. Marc Fontecave for selecting and entrusting me with a project in a domain I previously had little experience. Thanks to their invaluable guidance and constant support throughout my thesis, I was able to gain useful insight into not only my topic of research, but also into different scientific mentalities. Their patient feedback and constructive criticism coupled with encouragement helped me and kept me motivated in my work.

My collaboration with two such highly regarded scientists has been a determining experience in my life that I will look back on for the rest of my career. I feel very fortunate to have been able to learn by their example about life and science. I am equally grateful to my co-supervisor Dr. Maria Gomez-Mingot, who shared with me not only an office, but also her knowledge and expertise. During these three years she was always by my side with suggestions, advice and her generous nature, making every day in the lab better. I would also like to express my gratitude to the rest of the members of my thesis committee: Dr. Sylvie Chardon-Noblat, Dr. Sophie Griveau, Prof. Claude Jolivalt and Prof. Thomas Doneux for kindly accepting to devote time and effort into revising my thesis. This work would not have been possible without the financial support from the doctoral program "Génie des Procédés" and the contribution of the Onassis Foundation.

My heartfelt thanks to Dr. Yun Li, for her advice on molecular catalysis and organic chemistry throughout my thesis. Special thanks also to our collaborators Dr. Albert Solé-Daura and Dr. Caroline Mellot-Draznieks for performing the theoretical calculations and all the edifying conversations that helped me better comprehend our catalytic system. I am also grateful to Dr. Huan Tran, for the help in the laboratory and in discussions, I will always consider him a valuable friend. I would also like to thank Dr. Charles Creissen for our recent collaboration and for always being very helpful in the lab. I would equally like to thank Dr. Antoine Miche for performing the XPS characterization tests.

A big thanks goes to all members of LCPB and LISE who made my time there very enjoyable. I would particularly like to mention Phong, Adèle, Youven, Alessandro, Sophia, Gabriel, Alice and to my former colleagues Sophie, Lin, Mathis, Alex, Dilan, Cécilia, Dave, Dona, Tanya, Cameron, Subal and Hemlata.

CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS Encyclopedia of Interfacial Chemistry : Surface Science and Electrochemistry; Wandelt, K., Ed.; Elsevier, 2018; pp 539-551. (14) Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I. Ionic Liquid-Mediated Selective Conversion of CO₂ to CO at Low Overpotentials. Science 2011, 334 (6056), 643-644. (15) Hanc-Scherer, F. A.; Montiel, M. A.; Montiel, V.; Herrero, E.; Sánchez-Sánchez, C. M. Surface Structured Platinum Electrodes for the Electrochemical Reduction of Carbon Dioxide in Imidazolium Based Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17 (37), 23909-23916. https://doi.org/10.1039/C5CP02361K. (16) Zhao, S.-F.; Horne, M.; Bond, A. M.; Zhang, J. Is the Imidazolium Cation a Unique Promoter for Electrocatalytic Reduction of Carbon Dioxide? J. Phys. Chem. C 2016, 120 (42), 23989-24001. https://doi.org/10.1021/acs.jpcc.6b08182. (17) Feaster, J. T.; Jongerius, A. L.; Liu, X.; Urushihara, M.; Nitopi, S. A.; Hahn, C.; Chan, K.; Nørskov, J. K.; Jaramillo, T. F. Understanding the Influence of [EMIM]Cl on the Suppression of the Hydrogen Evolution Reaction on Transition Metal Electrodes. Langmuir 2017, 33 (37), 9464-9471. https://doi.org/10.1021/acs.langmuir.7b01170. (18) Fontaine, O.; Ghilane, J.; Martin, P.; Lacroix, J.-C.; Randriamahazaka, H. Ionic Liquid Viscosity Effects on the Functionalization of Electrode Material through the Electroreduction of Diazonium. Langmuir 2010, 26 (23), 18542-18549. https://doi.org/10.1021/la103000u. (19) Chi, Y. S.; Hwang, S.; Lee, B. S.; Kwak, J.; Choi, I. S.; Lee, S. Anion Exchange-Promoted Ru3+/2+ Redox Switch in Self-Assembled Monolayers of Imidazolium Ions on a Gold Electrode. Langmuir 2005, 21 (10), 4268-4271. https://doi.org/10.1021/la046806k. (20) Bouden, S.; Gómez-Mingot, M.; Randriamahazaka, H.; Ghilane, J. Surface Initiated Immobilization of Molecules Contained in an Ionic Liquid Framework. Anal. Chem. 2016, 88 (1), 1017-1021. https://doi.org/10.1021/acs.analchem.5b03922. (21) Michez, R.; Doneux, T.; Buess-Herman, C.; Luhmer, M. NMR Study of the Reductive Decomposition of [BMIm][NTf2] at Gold Electrodes and Indirect Electrochemical Conversion of CO2. ChemPhysChem 2017, 18 (16), 2208-2216. https://doi.org/10.1002/cphc.201700421. (22) Michez, R.; Vander Steen, J.; Doneux, T.; Luhmer, M.; Buess-Herman, C. Electroreduction of 1-Butyl-3-Methylimidazolium Bis(Trifluoromethanesulfonyl)Imide Ionic Liquid: Oriented Product Selectivity through the Electrode Material. Electrochimica Acta 2018, 270, 434-439. (23) Wang, Y.; Hayashi, T.; He, D.; Li, Y.; Jin, F.; Nakamura, R. A Reduced Imidazolium Cation Layer Serves as the Active Site for Electrochemical Carbon Dioxide Reduction. Appl. Catal. B Environ. 2020, 264, 118495. https://doi.org/10.1016/j.apcatb.2019.118495. (24) Tomita, Y.; Teruya, S.; Koga, O.; Hori, Y. Electrochemical Reduction of Carbon Dioxide at a Platinum Electrode in Acetonitrile-Water Mixtures. J. Electrochem. Soc. 2000, 147 (11), 4164. (25) Pugliese, S.; Huan, N. T.; Forte, J.; Grammatico, D.; Zanna, S.; Su, B.-L.; Li, Y.; Fontecave, M. Functionalization of Carbon Nanotubes with Nickel Cyclam for the Electrochemical Reduction of CO2. ChemSusChem 2020, 13 (23), 6449-6456. https://doi.org/10.1002/cssc.202002092. (26) Bermúdez, M.-D.; Jiménez, A.-E.; Sanes, J.; Carrión, F.-J. Ionic Liquids as Advanced Lubricant Fluids. Molecules 2009, 14 (8), 2888-2908. https://doi.org/10.3390/molecules14082888. (27) Efthimiadis, J.; Neil, W. C.; Bunter, A.; Howlett, P. C.; Hinton, B. R. W.; MacFarlane, D. R.; Forsyth, M. Potentiostatic Control of Ionic Liquid Surface Film Formation on ZE41 Magnesium Alloy. ACS Appl. Mater. Interfaces 2010, 2 (5), 1317-1323. https://doi.org/10.1021/am900889n. (28) Wu, J.-B.; Lin, Y.-F.; Wang, J.; Chang, P.-J.; Tasi, C.-P.; Lu, C.-C.; Chiu, H.-T.; Yang, Y.-W. Correlation between N 1s XPS Binding Energy and Bond Distance in Metal Amido, Imido, and Nitrido Complexes. Inorg. Chem. 2003, 42 (15), 4516-4518. https://doi.org/10.1021/ic034261w. (29) Clark, M. L.; Cheung, P. L.; Lessio, M.; Carter, E. A.; Kubiak, C. P. Kinetic and Mechanistic Effects of Bipyridine (Bpy) Substituent, Labile Ligand, and Brønsted Acid on Electrocatalytic CO 2 Reduction by Re(Bpy) Complexes. ACS Catal. 2018, 8 (3), 2021-2029. https://doi.org/10.1021/acscatal.7b03971.

, where n is the number of electrons involved in the process, F is the Faraday constant (F = 96485 C/mol), N is the number of moles of each product and Q is the amount of charge measured in Coulombs (C).

[2] Overpotential = (applied potential in order for CO2RR to take place) -(thermodynamic potential of the same reaction).

[3] Catalytic Potential, referred as E cat max or E cat/2 is the potential that corresponds to the value of maximum or half of the catalytic current respectively. E cat max and E cat/2 are both used within the thesis.

[4] Current Density (j) is the current divided by the electrode surface and is measured in mA/cm 2 .

[5] Catalytic response (j cat /j p ) is the ratio of the current density value under catalytic conditions (for example under CO2 with/out the presence of an added proton source) divided by the current density value for the same system under inert conditions.

[6] Energy Efficiency (EE): EE = ΕT/E ⋅ FE (%), where ΕT is the theoretical cathodic potential to reduce CO2 to the product of interest in Volts, E represents the actual cathodic potential applied in Volts and FE is the Faradaic Efficiency for said product, calculated according to equation [1] 1,2 .

[7] Energy Consumption (EC): EC = (Q ⋅ |Ecell|/ N) ⋅ 2.78⋅10 -4 (kWh /kmol), where Q represents the total charge circulated measured in Coulombs, |Ecell| represents the absolute cell potential in volts and N is the number of moles of each product 1 .

Electrochemical characterization

All electrochemical experiments were performed on a VSP-300 or SP-300 potensiostat (BioLogic Science Instruments SAS) and were conducted at room temperature (20 ± 2 °C) in CH3CN, CH3CN/H2O mixtures or aqueous solutions as specified. Either benchmark electrolytes or one of the ILs studied was used as supporting electrolyte in solution. Ar and CO2 gases used to saturate solutions were purchased from Air Liquide.

Cyclic Voltammetry (CV)

Cyclic voltammetry (CV) experiments were used throughout this thesis in order to obtain quantitative data concerning the electrochemical reactions studied. CV is the most widely used electrochemical We have already demonstrated in Figure 4.1 the formation by CV of an IL electrodeposited layer on different electrode materials, in particular GCE and AuE. The imidazolium reduction, as well as the reoxidation wave potentials were shifted to different values based on the electrode material used, as has already been reported in the literature 21,22 . In order to evaluate the IL surface coverage (Γ) achieved on the electrode surface by IL layer electrodeposition, we used the integration of the oxidation wave shown in the reverse CV scan after electrodeposition. This oxidation wave is centered at -0.7 V on GCE and -0.4 V vs. Fc + /Fc on AuE. As we can observe in Figure 4.15 (a), applying more cathodic potential limits during IL electrodeposition provoked an increase in the maximum current displayed by the oxidation wave in the reverse scan and thus, higher IL surface coverage was achieved. However, by keeping the maximum current density of imidazolium reduction constant at almost 50 mA/cm 2 , we established a proper comparison between those two electrode materials, which reached very similar surface coverage values for both electrodes, namely 1.3 10 -7 mol/cm 2 and 1.4 10 -7 mol/cm 2 for the AuE and GCE electrodes, respectively. This demonstrated that the electrodeposited layer was independent of the chemical nature of the electrode surface where it was deposited and did not follow an epitaxial growth.

The electrodeposited IL layer used in most of the catalytic studies reported in this chapter was achieved by CV up to -3.95 V vs. Fc + /Fc on either flat GCE or RVC foam electrodes. In particular, the IL surface CHAPTER 4: IMMOBILIZATION OF IONIC LIQUIDS FOR TUNING CO2 ELECTROREDUCTION SELECTIVITY IN MODEL MOLECULAR ELECTROCATALYSIS Figure 4.18 shows the electrodeposited IL layer, the covalent grafted IL layer and a control bare GCE tested under HER conditions in the absence of complex [2], more particularly in an acetonitrile solution under inert atmosphere containing a lower (0.01 M) and a higher (0.1 -0.5 M) quantity of two different proton sources, TFE or AcOH. These two proton sources vary in acidity, TFE (pKa = 35.4) and AcOH (pKa = 23.5) in acetonitrile 29 . In Figure 4.18 (a) and (b) we see the effect of both proton sources. Even at the lowest concentration it is evident that the presence of an IL layer on the electrode surface suppresses HER activity compared to the control bare electrode by causing a more cathodic onset potential and a smaller maximum current density. However, this effect is much more marked at 0.

Results

Single-compartment recirculating flow configuration for liquid-phase CO2RR

The first setup of the flow electrolyzer tested was also the simplest, the single-compartment configuration. Under those conditions both the cathode and the anode are situated in a unique compartment without any membrane or other physical barrier between them and their distance was approximatively 4 mm. The cathode and anode used were a 2D graphite plate and a Dimensionally Stable Anode (DSA) electrode of 10 cm 2 active geometric area, respectively. The DSA consists of a IrO2-Ta2O5 mixed oxide and is known to promote Oxygen Evolution Reaction (OER) 19 Finally, an external power source with the capacity of applying up to 1 Ampere (over the span of 150 Volts) was used, though for this study we applied between 1 and 3.4 mA/cm 2 . The gaseous products were not collected and only liquid phase products were quantified. The entire single compartment configuration is shown in Figure 5.3 (a).

In Figure 5.3 (b) we observe the results of tests using the single-compartment flow electrolyzer configuration, where at any time only one parameter was altered in order to assess its impact. At first (test 1), a current density of 3.4 mA/cm 2 was applied, as a 10 mL electrolyte solution was pumped at

Two-compartments recirculating flow electrolyzer configuration for liquid-phase CO2RR

This flow electrolyzer configuration is similar to the one discussed in 5.2.1, with the exception that the addition of a cation selective Nafion membrane created two distinct compartments within the flow electrolyzer, the cathodic and the anodic compartment. As a consequence, there was a possibility to have different catholyte and anolyte solutions. We took the experimental conditions already studied for singlecompartment configuration as starting point and we modified parameters one by one. Thus, the flow rate it must be noted that using the two-compartment configuration (test 6) resulted in an increased cell potential value of 7.3 V. In order to mitigate this effect, we switched the anolyte composition to a 0.5 M solution of KOH. In this case (test 7) the reaction taking place in the anode was OH -oxidation, which is favored in basic conditions and occurs at 0.404 V vs. RHE. Therefore, the calculated absolute thermodynamic cell potential would be smaller (0.82 V), taking into consideration the CO2RR to formic acid as cathodic reaction. The FEHCOO-observed in that case was 30 % and 23 % for 50 C and 100 C, respectively. These results were almost identical to those obtained using the previous anolyte composition, while the |Ecell| significantly decreased from test 6 to test 7, from 7.3 V to a value of 2.8

V, thus demonstrating that a pH gradient between the anolyte and catholyte was highly beneficial in terms of decreasing |Ecell|. For that reason, 0.5 M KOH aqueous solution as anolyte was used for all subsequent studies. It should also be mentioned that the change of anolyte incurred a slight increase in the final values of the catholyte pH, which after each experiment reached an average pH value of 4.5

versus the initial 3.8.

Two-compartments single pass flow configuration for gas-phase CO2RR

The recirculating flow conditions used up to this point provided a formate efficiency that decayed over time and were inconclusive as to the system's stability towards degradation over time. We needed to overcome the obstacles linked with recycling the solution during the electrolysis, which could distort the effect due to the use of a GDE in a flow electrolyzer. For this reason, we opted for a single-pass flow configuration with separate inlet and outlet reservoirs. Then, the catalyst would only be passing through the electrolyzer once. In order to avoid needless catalyst waste and to increase the catalyst residence time in contact with the cathode, the solution flow rate was reduced to 1.1 mL/min , which would allow reaching 1000 C without surpassing 100 mL of catholyte solution. The anode and cathode electrodes, as well as both catholyte and anolyte compositions, remained unaltered from the previous configuration.

The current density applied for this test was 20 mA/cm 2 and sampling for formate quantification was performed every 250 C for a total charge value of 1000 C.

In Figure 5.7, we observe the formate faradaic efficiency over time in a single pass flow configuration.

The apparent stability of FEHCOO-at approximatively 32 % throughout the test allowed us to test for a greater charge value than in any of the previous configuration, reaching 1000 C. Attempts to run at higher current densities were also undergone, more precisely at 30 mA/cm 2 . However, after an initial smooth run of a few minutes the |Ecell| values skyrocketed, indicating a power short cut due to gas accumulating at the anode surface. The cause was identified to be the small solution flow rate also used in the anolyte flow (1.1 mL/min), which was not able to remove fast enough the O2 bubbles generated at the anode

Two-compartments single pass flow configuration for gas-phase CO2RR on a GDE with an electrodeposited IL layer

Since adopting the single pass flow configuration allowed for an average formate production of 32 % for the whole span of points analyzed at a current density of 20 mA/cm 2 , we proceeded to create an electrodeposited layer on another piece of the same GDE again measuring 10 cm 2 . The electrodeposition was performed in a 0. points the GDE with electrodeposited layer produced higher amounts of formate than the bare GDE, which resulted in an average FE of formate of 45 % in the presence of the electrodeposited layer.

Energy Efficiency (EE) and Energy Consumption (EC) were also calculated for these two conditions. The simultaneous increase in FEHCOO-alongside the small decrease in cell potential in the presence of the electrodeposited layer led to a higher EE value than the bare electrode. The same • Overall, this IL layer proved to be stable and obtain appreciable formate yields in acidic aqueous conditions, that are crucial in avoiding CO2 loss as bicarbonate or carbonate, as is the case at alkaline pH values.

In addition to the fundamental knowledge accumulated, a significant effort to scale up the CO2 conversion to formate process have been also carried out in this thesis. Then, from 10 C electrolysis in an H-type cell at 3.33 mA/cm 2 , we have been able to reach 1000 C electrolysis by scaling the electrodes size up to 10 cm 2 in a flow electrolyzer working at 20 mA/cm 2 . This approach combining the high selectivity of a molecular catalyst with the scalability of a flow electrolyzer reaches a significant catalytic current density and keeps the product selectivity control.

In conclusion, extensive study and rationalization of the interactions between molecular catalysts and ILs were shown in this thesis. We have used different model catalysts, varied the nature and components of the ILs used, as well as immobilizing them and tested them in different electrolyte conditions. Those studies permitted us to determine the factors governing the catalytic activity, illustrated by increasing values of current density in aqueous solution, and the product selectivity at very low overpotential values by simultaneous HER suppression and enhanced efficiency for CO2RR products. Finally, the stability of the catalytic system containing an electrodeposited IL layer was demonstrated over long-term electrolysis (over 3000 C) performed within the flow electrolyzer.

Outlook

From the aforementioned results, many new and exciting research pathways shine strongly and will be worthy to explore them. Listed here are a few outlooks related to the results mentioned above:

• Since the bipyridine ligand of both of the catalysts used in the thesis was reported to present π +π interaction with imidazolium-based ILs, the cocatalytic effect of ILs could be explored with other molecular catalysts containing one or more bpy ligands, such as the ones shown in Figure 6.2.

RÉSUMÉ EN FRANÇAIS

Résumé de la Thèse en Français

LIST OF PUBLICATIONS AND CONFERENCES

I would also like to extend my thanks to the administrative personnel of both laboratories, especially Jessica Duvoisin, Johnny Tondeleir, Isabelle Lefebvre and Martine Chaduc for their help. Finally, thanks to Iulia and Renaud, two amazing interns I had the pleasure to work with.