Huali Wu

Amrita Singh-Morgan

Kun Qi

Zhiyuan Zeng

Victor Mougel
email: mougel@inorg.chem.ethz.ch

Damien Voiry
email: damien.voiry@umontpellier.fr

Electrocatalyst Microenvironment Engineering for Enhanced Product Selectivity in Carbon Dioxide and Nitrogen Reduction Reactions

Keywords: Carbon Dioxide, Nitrogen, Electrochemical Reduction, Microenvironment, Selectivity, Electrocatalyst, Electrolyte, Three-phase Interface

Carbon and nitrogen fixation strategies are regarded as alternative routes to produce valuable chemicals used as energy carriers and fertilizers that are traditionally obtained from unsustainable and energy-intensive coal gasification (CO and CH4) Fischer-Tropsch (C2H4) and Haber-Bosch (NH3) processes. Recently, the electrocatalytic CO2 reduction reaction (CO2RR) and N2 reduction reaction (NRR) have received tremendous attention with the merits of being both efficient strategies to store renewable electricity while providing alternative preparation routes to fossil-fuel driven reactions. To date, the development of the CO2RR and NRR processes is primarily hindered by the competitive hydrogen evolution reaction (HER), however the corresponding strategies for inhibiting this undesired side reaction are still quite limited. Considering such complex reactions involving three gas-liquidsolid phases and successive proton-coupled electron transfers, it appears meaningful to review the current strategies for improving product selectivity in light of their respective reaction mechanisms, kinetics, and thermodynamics. By examining the developments and understanding in catalyst design, electrolyte engineering and three phase interface modulation, we discuss three key strategies in improving product selectivity for the CO2RR and NRR: i) targeting molecularly defined active sites, ii) increasing the local reactant concentration at the active sites, and iii) stabilizing and confining product intermediates.

Introduction

Many of today's environmental, economic, and societal issues are related to the transformation of two inert gases, N2 and CO2.The transformation of N2 via the Haber-Bosch process accounts for ~2% of the world energy consumption, providing nitrogen fertilizers required to sustain the current global food production. Meanwhile, the amount of CO2 released into the atmosphere from the combustion of fossil fuels has reached unprecedented levels, further accelerating climate change. [START_REF] Li | Exclusive Ni-N4 sites realize near-unity CO selectivity for electrochemical CO2 reduction[END_REF][START_REF] Battin | The boundless carbon cycle[END_REF][START_REF] Wan | Heterogeneous electrocatalysts design for nitrogen reduction reaction under ambient conditions[END_REF][START_REF] Foster | Catalysts for nitrogen reduction to ammonia[END_REF][START_REF] Zhang | Global nitrogen cycle: critical enzymes, organisms, and processes for nitrogen budgets and dynamics[END_REF] Both CO2 and nitrogen undergo complex environmental cycles (Figure 1a-b), increasing the challenges associated with their capture and conversion. Implementing CO2 and N2 sustainable cycles and minimizing their environmental impact is critical, as recently highlighted in the latest IPCC report or in Europe in the EU green deal and Fit for 55 package. [START_REF]IPCC Climate Change 2022: Impacts, Adaptation and Vulnerability[END_REF][7][START_REF]Council of the European Union Fit[END_REF] The electrochemical conversion of CO2 and N2 into value-added products or net zero commodities such as materials, renewable fuels and energy vectors, appears as an appealing solution in this context, as it can utilize sustainable sources of electricity powered by solar, wind, wave, and hydro energy to promote reactions currently carried out using fossil fuels. This approach would provide a carbonneutral route to C-and N-containing products whilst enabling the efficient storage of intermittent renewable sources of electricity as chemical bonds, largely overperforming battery storage energy efficiency. [START_REF] Tackett | Opportunities and challenges in utilizing metal-modified transition metal carbides as low-cost electrocatalysts[END_REF] The main hurdle to developing energy efficient processes for converting nitrogen to ammonia and carbon dioxide to energy dense products such as hydrocarbons is selectivity. The chemical inertness of these reactants disadvantage their transformation compared to more kinetically facile reactions such as the hydrogen evolution reaction (HER). Furthermore, selectivity is one of the most challenging aspects to address when developing electrocatalysts to mediate the CO2 reduction reaction (CO2RR) and the nitrogen reduction reaction (NRR). In both cases multiple reaction products are typically observed, resulting from the reduction of CO2 and N2 themselves as well as from the proton sources used to mediate these reduction reactions, which involve successive coupled electron-proton transfers.

In this regard, the electrocatalyst microenvironment plays a vital role and can be engineered to improve selectivity through three key strategies: i) targeting a narrow distribution of molecularly defined active sites, ii) increasing the reactant/proton ratio at the three-phase interface where the reaction takes place to lower the undesired formation of H2 and iii), the stabilisation and confinement of reaction intermediates in the electrode vicinity to favour the formation of multi-electron reduction products.

While there exists an extensive amount of literature in both the CO2RR and NRR fields, including several recent reviews of specific subtopics, [START_REF] Ren | Strategies to suppress hydrogen evolution for highly selective electrocatalytic nitrogen reduction: challenges and perspectives[END_REF][START_REF] Deng | Interfacial Electrolyte Effects on Electrocatalytic CO2 Reduction[END_REF][START_REF] Ozden | Carbon-efficient carbon dioxide electrolysers[END_REF][START_REF] Wen | Engineering Electrochemical Surface for Efficient Carbon Dioxide Upgrade[END_REF] we target in this review to illustrate through a handful of selected examples the key strategies for increasing selectivity towards value-added products.

After a brief explanation of the kinetic and thermodynamic origins of multiple product generation in the CO2RR and NRR, we discuss the key factors in catalyst design in steering product selectivity, namely nano-structuring, surface functionalisation, control of crystal size and facets, and single site engineering. We then explore the impact of the electrolyte on activity at the electrode surface, including aspects such as pH, the alkali metal cation, and the use of novel electrolytes. The final section focuses on the implementation and optimisation of triple-phase interfaces to improve local reactant concentration and mass transport. We conclude with our perspectives on this rapidly growing topic and where we envisage future challenges and opportunities to lie.

It is important to note that the NRR field has been strongly affected by a series of false positives, and a standardized set of experiments have been outlined to identify, quantify and eliminate experimental artefacts. [START_REF] Andersen | A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements[END_REF] Ammonia contamination may arise from sources such as the air, chemicals and the experimental set-up, which is particularly significant when the quantity of ammonia produced in NRR is very low. Additionally, labile nitrogen-containing compounds such as nitrates, nitrites, nitrogen oxides and amines are often present in the N2 gas stream, the air, and the catalyst itself. To reliably attribute ammonia production to the NRR, quantitative isotope measurements with [START_REF] Deng | Electrocatalytic nitrogen reduction at low temperature[END_REF] N2 gas and the removal of impurities from the gas stream are imperative. To preserve a fair comparison of performance between catalytic materials, in this review we present only examples that follow the guidelines provided in the above reference 14, unless clearly stated otherwise. Cycle of biologically driven N-transformations that occur in natural and human-influenced terrestrial and marine environments.

Reproduced from Chem. Rev. 2020, 120 (12), 5308-5351. Copyright 2020 American Chemical Society.

Mechanistic and thermodynamic origin of multiple product generation in CO2RR and NRR

Both the CO2RR and NRR to value-added products involve multiple successive proton-coupled electron transfers (Table 1), which represent a significant kinetic challenge to overcome to achieve high selectivity, particularly compared to the more kinetically facile two-electron hydrogen generation reaction. [START_REF] Deng | Electrocatalytic nitrogen reduction at low temperature[END_REF][START_REF] Chen | Water-dispersible CsPbBr3 perovskite nanocrystals with ultra-stability and its application in electrochemical CO2 reduction[END_REF][START_REF] Qi | Enhancing the CO2-to-CO conversion from 2D silver nanoprisms via superstructure assembly[END_REF] This kinetic challenge is further complexified by the low availability of the reactants, as both CO2 (~33 mM at PCO2=1atm) and N2 (~0.7 mM at PN2=1 atm) have typically poor solubility in water. [START_REF] Weiss | In The solubility of nitrogen, oxygen and argon in water and seawater[END_REF] In the context of the CO2RR to multi-carbon products, the low solubility of the primary reaction products such as CO also decreases the overall catalyst selectivity for multi-carbon products which result from the subsequent reduction of these primary products.

2𝐻 + + 2𝑒 -→ 𝐻 2 (𝐸 0 = 0 𝑉 𝑣𝑠. 𝑆𝐻𝐸) (1)
𝑁 2 + 8𝐻 + + 6𝑒 -→ 2𝑁𝐻 4 + (𝐸 0 = 0.274 𝑉 𝑣𝑠. 𝑆𝐻𝐸) (2)
𝐶𝑂 2 + 2𝐻 + + 2𝑒 -→ 𝐶𝑂 + 𝐻 2 𝑂 (𝐸 0 = -0.106 𝑉 𝑣𝑠. 𝑆𝐻𝐸) (3)
𝐶𝑂 2 + 2𝐻 + + 2𝑒 -→ 𝐻𝐶𝑂𝑂𝐻 (𝐸 0 = -0.250 𝑉 𝑣𝑠. 𝑆𝐻𝐸) (4)
2𝐶𝑂 2 + 12𝐻 + + 12𝑒 -→ 𝐶 2 𝐻 4 + 4𝐻 2 𝑂 (𝐸 0 = 0.064 𝑉 𝑣𝑠. 𝑆𝐻𝐸) (5)
2𝐶𝑂 2 + 12𝐻 + + 12𝑒 -→ 𝐶 2 𝐻 5 𝑂𝐻 + 3𝐻 2 𝑂 (𝐸 0 = 0.084 𝑉 𝑣𝑠. 𝑆𝐻𝐸) (6)
In addition, a thermodynamic challenge is associated with the CO2RR, since proton reduction (HER) is more thermodynamically favourable than the reduction of CO2 to most products (Figure 2a and Equations 3-6 ). [START_REF] Schneider | Thermodynamics and kinetics of CO2, CO, and H + binding to the metal centre of CO2 reduction catalysts[END_REF][START_REF] Qiao | A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels[END_REF][START_REF] Wakerley | Bio-inspired hydrophobicity promotes CO2 reduction on a Cu surface[END_REF] Although less critical in the case of NRR, the standard electrochemical potential for the proton reduction reaction is yet close to that of the nitrogen reduction reaction (NRR) at 0.057 V vs. SHE (Equation 2). [START_REF] Guo | Electrochemical nitrogen fixation and utilization: theories, advanced catalyst materials and system design[END_REF] The intrinsic stronger binding of H atoms over N2 on most metal surfaces, highlighted in Figure 2b, further illustrates the challenge to increase NRR selectivity vs. HER. [START_REF] Skulason | A theoretical evaluation of possible transition metal electro-catalysts for N2 reduction[END_REF] This illustrates the three main challenges (thermodynamic, kinetic, or related to the mass-transport of the reactants and primary reaction products) that must be overcome to reach high selectivity in the CO2RR and NRR. We will review in the next sections the three main axes currently explored toward that goal, focusing on catalyst design, electrolyte engineering and three-phase interface modulation.

Increasing selectivity via catalyst design

Catalyst nano-structuring for improved mass transport

Advancements in nanotechnology and characterisation techniques have enabled a plethora of morphologies to be explored to improve catalytic activity and product selectivity. Porous materials have attracted particular attention due to their effect on the local chemical environment, including local pH and the mass transport of the reactant and intermediates. [START_REF] Pan | Designing CO2 reduction electrode materials by morphology and interface engineering[END_REF][START_REF] Wagner | Towards molecular understanding of local chemical environment effects in electro-and photocatalytic CO2 reduction[END_REF] The ability to increase the number of effective active sites, both by maximising surface area and facilitating the accessibility of such sites, makes porosity useful and interesting across a broad range of fields. [START_REF] Li | Recent advances in the nanoengineering of electrocatalysts for CO2 reduction[END_REF] Such effects are especially crucial when considering the poor solubility of CO2 and N2 in aqueous electrolytes, which cause mass transport limitations and barriers to high activity and selectivity.

Hierarchical porous networks are found commonly in biological organisms as a strategy to mitigate mass transport limitations in the utilisation of nutrients. [START_REF] Trogadas | Nature-inspired electrocatalysts and devices for energy conversion[END_REF] The three-dimensional networks were replicated in early work by Huan et al. who used gold nanodendrites for electrochemical sensing. [START_REF] Huan | A three-dimensional gold nanodendrite network porous structure and its application for an electrochemical sensing[END_REF] Their application in catalysis has recently appeared as an efficient strategy to increase current densities and catalyst selectivity in small molecule electroreduction and oxidation.

The dynamic hydrogen bubble templating (DHBT) method has been the most prominent technique to create such hierarchical porosity, which was recently comprehensively reviewed by the Bhargava group [START_REF] Plowman | Building with bubbles: the formation of high surface area honeycomb-like films via hydrogen bubble templated electrodeposition[END_REF] and specifically for CO2RR materials by the Broekmann group. [START_REF] Vesztergom | Hydrogen Bubble Templated Metal Foams as Efficient Catalysts of CO2 Electroreduction[END_REF] The process involves the electrodeposition of a metal from aqueous solutions of the respective cations, while co-generated hydrogen bubbles act as a dynamic template to create a metal foam. As the bubbles nucleate, grow and detach, a hierarchical pore structure forms with layers of pores of increasing diameter (Figure 3a), including micropores in the submicron range and macropores 10-100 μm. [START_REF] Plowman | Building with bubbles: the formation of high surface area honeycomb-like films via hydrogen bubble templated electrodeposition[END_REF] The DHBT technique is relatively simple, requiring aqueous solutions and no need for organic or inorganic templates (as in traditional metal foam synthesis), [START_REF] Du | Engineering Self-Supported Noble Metal Foams Toward Electrocatalysis and Beyond[END_REF] high temperatures, high pressures or uncommon equipment.

Nonetheless, additives such as citrate are common to influence crystal growth. [START_REF] Dutta | Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam Catalysts[END_REF][START_REF] Wang | Electrodeposited porous Pb electrode with improved electrocatalytic performance for the electroreduction of CO2 to formic acid[END_REF][START_REF] Qin | Effect of the surface roughness of copper substrate on three-dimensional tin electrode for electrochemical reduction of CO2 into HCOOH[END_REF] Bi-and multimetallic catalysts are also possible by co-electrodeposition, galvanic replacement, stepwise electrodeposition or spontaneous decoration. [START_REF] Plowman | Building with bubbles: the formation of high surface area honeycomb-like films via hydrogen bubble templated electrodeposition[END_REF] For example, many studies for CO2RR have coupled copper with one other metal such as Ag, Sn, In or Zn. [START_REF] Dutta | Activation of bimetallic AgCu foam electrocatalysts for ethanol formation from CO2 by selective Cu oxidation/reduction[END_REF][START_REF] Lee | Nanostructured Ag/In/Cu foam catalyst for electrochemical reduction of CO2 to CO[END_REF][START_REF] Zeng | Advanced Cu-Sn foam for selectively converting CO2 to CO in aqueous solution[END_REF][START_REF] Rahaman | Suppression of the Hydrogen Evolution Reaction Is the Key: Selective Electrosynthesis of Formate from CO2 over Porous In55Cu45 Catalysts[END_REF][START_REF] Lamaison | High-Current-Density CO2-to-CO Electroreduction on Ag-Alloyed Zn Dendrites at Elevated Pressure[END_REF] By fine-tuning parameters such as proton source and concentration, applied overpotential or current density, substrate material, and the metal source and concentration, the nanostructure can be carefully controlled and optimised. Broekmann and co-workers produced a dendritic Cu-based DHBT foam, and demonstrated a strong dependence of the C2-product selectivity on the surface pore size diameter, with the optimal size being between 50 and 100 μm. [START_REF] Dutta | Morphology Matters: Tuning the Product Distribution of CO2 Electroreduction on Oxide-Derived Cu Foam Catalysts[END_REF] They identified the temporal trapping of gaseous intermediates inside these pores as the key to product selectivity. Intermediates such as CO and C2H4, which would otherwise be released into the bulk electrolyte, were entrapped in the pores of the foam catalyst, causing them to further react to form C2H6 (Figure 3b). At -0.8 V vs RHE they achieved a 55 % Faradaic efficiency for C2 -products.

Such dendritic structures with large surface areas are common in this synthesis due to the deposition taking place at high current densities and therefore in the diffusion limited regime. Copper and oxide derived copper dendrites have had particular interest due to their apparent selectivity for multicarbon products. [START_REF] Shin | Copper Foam Structures with Highly Porous Nanostructured Walls[END_REF][START_REF] Zhang | Effect of Bubble Behavior on the Morphology of Foamed Porous Copper Prepared via Electrodeposition[END_REF][START_REF] Rashid | Dendritic copper microstructured electrodeposits for efficient and selective electrochemical reduction of carbon dioxide into C1 and C2 hydrocarbons[END_REF][START_REF] Malik | Effect of catalyst layer on electrochemical reduction of carbon dioxide using different morphologies of copper[END_REF] Huan et al. produced a dendritic CuO material from DHBT that could be used both as a CO2R and OER catalyst. [START_REF] Huan | A Dendritic Nanostructured Copper Oxide Electrocatalyst for the Oxygen Evolution Reaction[END_REF][START_REF] Huan | Low-cost high-efficiency system for solar-driven conversion of CO2 to hydrocarbons[END_REF] It consisted of a triple layer structure with a metallic Cu core covered by layers of Cu2O and CuO (Figure 3c). In electrocatalytic conditions, the CuO material is reduced to metallic Cu, generating nano-Kirkendall voids within the dendrite structures. Such voids, that appear at the copper/copper oxides interface upon reduction, are termed nano-Kirkendall voids as they appear as a consequence of the very different diffusivities of Cu and O atoms. The overall external shape of the material is maintained upon reduction but cavities are generated under its external layer due to the lower density of Cu with respect to the original copper oxides. [START_REF] Fan | Formation of Nanotubes and Hollow Nanoparticles Based on Kirkendall and Diffusion Processes: A Review[END_REF] These gas-accessible voids were proposed to enhance the confinement of secondary CO2RR products, such as CO, resulting in FEC2+ over 50 %. By using the catalyst in a continuous flow electrolyzer, they were able to reach a stable current of 25 mA/cm 2 with 2.95 V, equating to 21 % energy efficiency for hydrocarbon production. By coupling the cell to a photovoltaic cell, they achieved a 2.3 % solar-to-hydrocarbon efficiency.

DHBT foams for single-carbon products such as CO and formate have also been reported. A silverfoam with needle-shaped features in the mesopores was produced by using a citrate additive to control growth on the nanometer scale. [START_REF] Dutta | Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam Catalysts[END_REF] Between -0.3 to -1.2 V vs RHE 90% Faradaic efficiency for CO was observed, however at higher over-potentials they produced C2-products, with 51% CH4 at -1.5V

(Figure 3d). This unusual activity for Ag was attributed to the catalyst morphology and nanostructure increasing *CO surface concentration and residence time. Recent work by Mayer and co-workers exemplifies the advantages of the simplicity of the DHBT method. In a one-step synthesis they used waste industrial Cu-Sn bronze as a material precursor to deposit a mesoporous Cu10Sn foam. [START_REF] Stojkovikj | Electrocatalyst Derived from Waste Cu-Sn Bronze for CO2 Conversion into CO[END_REF] They achieved over 85 % Faradaic efficiency for CO at -0.8 V vs RHE, over double that of the plain Cu-Sn bronze, with partial current densities three times higher. Other morphology-based strategies have been utilised to modulate mass transport in CO2 reduction, including the application of nanostructures such as nano-wires, sheets, needles, cones or tubes.

Burdyny et al. explored the effect of nanomorphology of a silver catalyst on gas-evolution and subsequently bubble-induced mass transport. [START_REF] Burdyny | Nanomorphology-Enhanced Gas-Evolution Intensifies CO2 Reduction Electrochemistry[END_REF] By combing mathematical modelling and experimental observations using a dark field microscope, they compared bubble formation on nanoparticles, nanorods and nanoneedles, and found a mean bubble diameter of 97, 31 and 23 µm respectively. They illustrated that the generation of smaller bubbles improved long-range mass transport of CO2, resulting in a small diffusion thickness and a 4-fold increase in limiting current density of CO production (Figure 4a). Surendranath and co-workers synthesised gold inverse opal thin films and found that changing the mesostructure by increasing porous film thickness could diminish HER 10-fold whilst maintaining activity for CO2 to CO, enhancing the faradaic efficiency for CO from less than 5 % to over 80 %. [START_REF] Yoon | Tuning of Silver Catalyst Mesostructure Promotes Selective Carbon Dioxide Conversion into Fuels[END_REF] They attributed this to the formation of diffusional gradients. Studies into nanocavities and their performance and mechanism of action have emerged in recent years. Yang et al. utilised finite-element method simulations and experimental measurements on a multihollow cuprous oxide catalyst. [START_REF] Yang | Protecting Copper Oxidation State via Intermediate Confinement for Selective CO2 Electroreduction to C2+ Fuels[END_REF] Analysis from X-ray absorption studies and operando Raman spectra indicated that the pore cavities confined *CO intermediates, which bound to Cu + sites and locally protected them against reduction during CO2RR (Figure 4b), as well as promoted C-C coupling. They achieved a C2+ product Faradaic efficiency of 75 % and partial current density of 267 mA cm -2 .

As N2 electroreduction is a comparatively less mature field with its own unique challenges, studies into morphological effects on catalytic activity and selectivity are less extensive. Although a range of nanostructures exist amongst the literature, [START_REF] Yang | Recent progress in electrochemical synthesis of ammonia from nitrogen: strategies to improve the catalytic activity and selectivity[END_REF] specific insight into the role morphology plays in catalysis is limited. Wei et al. loaded ruthenium nanoparticles onto carbon nanotubes, which were also applied as the gas diffusion electrode. [START_REF] Wei | Microtubular Gas Diffusion Electrode Based on Ruthenium-Carbon Nanotubes for Ambient Electrochemical Nitrogen Reduction to Ammonia[END_REF] Despite using a typical H-cell set-up, the GDE structure allowed N2 gas to be flowed through the GDE and porous catalyst, instead of being solely solubilised in the electrolyte as illustrated in Figure 4c. They achieved a NH3 yield rate of 2.1 nmol/cm 2 s and Faradaic efficiency of 13.5 %.

A great range of nanostructures have been applied to the CO2RR and NRR to regulate mass transport, and although strong correlations between structure and performance have been made, their mechanisms of action are often highly complex and difficult to define. Most theories focus on the mass transport of reactants and intermediates either through improved diffusion and convection or through their physical confinement in the catalyst pores. Considerable progress has been made by combining

Surface functionalization

Functionalization of the electrode or catalyst surface with organic or inorganic ligands has been explored as a strategy to tune the interaction between adsorbed intermediates and catalysts, inhibiting HER and improving product selectivity. In addition to the decoration of the surface of a catalytic material with surface-bound ligands, the covalent grafting of molecular co-catalysts onto the surface of a catalytic material has also been explored as a strategy to further tune the catalyst selectivity. [START_REF] Nam | Molecular enhancement of heterogeneous CO2 reduction[END_REF] In this section, we will outline some key examples in the diverse field of catalyst surface functionalization, which has been comprehensively reviewed for the CO2RR by Reisner and coworkers. [START_REF] Wagner | Towards molecular understanding of local chemical environment effects in electro-and photocatalytic CO2 reduction[END_REF] To date, many organic additives such as amino acids, [START_REF] Xie | Amino acid modified copper electrodes for the enhanced selective electroreduction of carbon dioxide towards hydrocarbons[END_REF] amines, [START_REF] Kim | Insight into electrochemical CO2 reduction on surface-molecule-mediated Ag nanoparticles[END_REF][START_REF] Zhao | Engineering surface amine modifiers of ultrasmall gold nanoparticles supported on reduced graphene oxide for improved electrochemical CO2 reduction[END_REF] aminothiols, [START_REF] Kim | Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles[END_REF] pyridiniums, [START_REF] Han | CO2 Reduction Selective for C≥2 Products on Polycrystalline Copper with N-Substituted Pyridinium Additives[END_REF][START_REF] Li | Molecular tuning of CO2-to-ethylene conversion[END_REF] N-heterocyclic carbenes (NHCs), [START_REF] Cao | Chelating N-Heterocyclic Carbene Ligands Enable Tuning of Electrocatalytic CO2 Reduction to Formate and Carbon Monoxide: Surface Organometallic Chemistry[END_REF][START_REF] Cao | A Molecular Surface Functionalization Approach to Tuning Nanoparticle Electrocatalysts for Carbon Dioxide Reduction[END_REF] imidazolium ligands, [START_REF] Pankhurst | Molecular tunability of surfacefunctionalized metal nanocrystals for selective electrochemical CO2 reduction[END_REF] porphyrin-based metallic complexes, [START_REF] Gong | Supramolecular Porphyrin Cages Assembled at Molecular-Materials Interfaces for Electrocatalytic CO Reduction[END_REF][START_REF] Li | Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule-metal catalyst interfaces[END_REF] polymers, [START_REF] Wang | Selective CO2 electrochemical reduction enabled by a tricomponent copolymer modifier on a copper surface[END_REF][START_REF] Xia | Surface-functionalized palladium catalysts for electrochemical CO2 reduction[END_REF] and inorganic additives, [START_REF] Parada | CO2 Electroreduction on Silver Foams Modified by Ionic Liquids with Different Cation Side Chain Length[END_REF][START_REF] Varela | Tuning the catalytic activity and selectivity of Cu for CO2 electroreduction in the presence of halides[END_REF] have been proposed to control the binding energy of CO2RR reaction intermediates (Figure 5a). For instance, Kim et al. demonstrated a 94.2% FE for the production of CO from amine-capped Ag supported on carbon, thanks to the effective suppression of the HER and the intrinsic high selectivity towards the CO2RR from Ag (Figure 5b). [START_REF] Kim | Insight into electrochemical CO2 reduction on surface-molecule-mediated Ag nanoparticles[END_REF] DFT calculations suggested that the amine-capped Ag nanoparticles stabilize the *COOH intermediate depress the hydrogen evolution reaction on ultrasmall Au NPs and enhance CO2-to-CO conversion. [START_REF] Zhao | Engineering surface amine modifiers of ultrasmall gold nanoparticles supported on reduced graphene oxide for improved electrochemical CO2 reduction[END_REF] The amine groups, as well as the molecular configuration, were found to play important roles in tuning the electrocatalytic activity of low-coordinated sites of the nanoparticles. The authors claimed that strong interactions between the Au surface and the amine ligands combined with the peculiar configuration were responsible for the improved CO2RR performance. Remarkably, linear amines promoted the formation of CO, an effect, which was enhanced by increasing the length of the alkyl chain, whereas the branched polyamine greatly depressed it. Wang et al. demonstrated a 55% and 77% selectivity for ethylene and C2+ products, respectively, using a tricomponent copolymer to modify the surface of Cu electrodes (Figure 5d). [START_REF] Wang | Selective CO2 electrochemical reduction enabled by a tricomponent copolymer modifier on a copper surface[END_REF] Systematic studies indicated that the three components of the copolymer control electrostatic interactions, gas diffusion, hydrophilicity, which were found to be necessary to improve selectivity. The copolymer was obtained by ring-opening metathesis polymerization, thereby offering a new degree of freedom for tuning the selectivity.

Applying a molecular design approach to tune heterogeneous catalysts has also proved effective in the functionalization of palladium foil with chelating N-heterocyclic carbene (NHC) ligands, demonstrating a 32-fold increase in activity for CO2 to C1 products. [START_REF] Cao | Chelating N-Heterocyclic Carbene Ligands Enable Tuning of Electrocatalytic CO2 Reduction to Formate and Carbon Monoxide: Surface Organometallic Chemistry[END_REF] N-aryl-pyridinium salts have also proved effective in tuning electronic properties to stabilize intermediates for CO2RR to ethylene. [START_REF] Li | Molecular tuning of CO2-to-ethylene conversion[END_REF] Porphyrin-based metallic complexes have been used to functionalize copper surfaces to increase the concentration of CO intermediates and promote C-C coupling; a Faradaic efficiency of 41 % for ethanol was achieved at 124 mA/cm 2 at -0.82 V vs RHE. [START_REF] Li | Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule-metal catalyst interfaces[END_REF] Modifying the catalyst surface indirectly has also been implemented by Varela et al. through the addition of halides to the electrolyte. [START_REF] Varela | Tuning the catalytic activity and selectivity of Cu for CO2 electroreduction in the presence of halides[END_REF] They hypothesized that the adsorption of halides onto copper increased the negative charge of the catalyst surface, altering selectivity. In the case of iodide, the induced negative charge favored the protonation of CO, enhancing CH4 production.

Applying well-defined molecular approaches to heterogeneous systems can give important insights into catalytic mechanisms and help to fine-tune active sites and product selectivity. Some functionalization strategies operate through molecular coordination and can therefore be carefully controlled by altering functional and side groups so that specific CO2RR intermediates can be stabilized. Other strategies, such as the addition of halides or ionic liquids, affect the charge on the catalyst surface, increasing COads coverage for example. [START_REF] Varela | Tuning the catalytic activity and selectivity of Cu for CO2 electroreduction in the presence of halides[END_REF][START_REF] Wang | Advanced electrocatalysts with single-metal-atom active sites[END_REF] Both have proven effective in improving product selectivity in the CO2RR, and similar approaches could be applied to the NRR to help overcome the dominance of HER (Figure 5e). The exact surface binding motifs of ligands and the mechanism for altered selectivity are still unclear. Understanding the precise nature of the interface remains a key challenge for attaining the desired catalytic properties.

Crystal size and facet control

Tremendous advances have recently been made to engineer catalysts in order to lower the HER during the CO2RR and NRR processes. [START_REF] Wang | Advanced electrocatalysts with single-metal-atom active sites[END_REF] Compared with their bulk counterparts, nanostructured catalysts show original and often enhanced activity owing to their unique surface electronic and chemical properties. These properties can be finely adjusted to tune the activity and selectivity of electrocatalytic reactions. The surface of a nanomaterial catalyst typically consists of planar areas with single-crystalline orientations separated by steps and kink sites with lower coordination numbers.

Complex atomic structures are therefore present at the interface between different grains in polycrystalline and/or nanostructured surfaces. Buonsanti et al. investigated the catalytic properties of exposed facets of Cu nanocatalysts at commercially relevant current densities (Figure 6a). [START_REF] De Gregorio | Facet-dependent selectivity of Cu catalysts in electrochemical CO2 reduction at commercially viable current densities[END_REF] The study revealed that facet-dependent selectivity is retained in a gas-fed flow cell, showing greater HER suppression than in a conventional H-cell. The (100) facets of Cu nanocubes have been identified to be selective for the evolution of C2H4, whereas the (111) facets of Cu octahedra are selective towards CH4. Conversely, Cu spheres do not exhibit any specific product selectivity, suggesting that randomly mixed facets cannot depress the HER during the CO2RR. Chorkendorff et al. systematically investigated the structure-selectivity relationship of Au single crystals for electrocatalytic CO2 reduction (Figure 6b). [START_REF] Mezzavilla | Structure sensitivity in the electrocatalytic reduction of CO2 with Gold catalysts[END_REF] Remarkably, they found that the kinetics for the formation of CO strongly depend on the surface structure. Under-coordinated sites, for instance, on the surface of Au(110) or at the step edges of Au(211), show at least 20fold higher activity than more coordinated configurations -such as Au(100). By selectively poisoning under-coordinated sites with Pb, they identified the selectivity of these active sites towards the reduction of CO2, effectively suppressing the HER.

Roldan Cuenya, Strasser and coworkers investigated the role of particle size in CO2 electroreduction using size-controlled Cu nanoparticles (NPs). [START_REF] Reske | Particle size effects in the catalytic electroreduction of CO2 on Cu nanoparticles[END_REF] A dramatic increase in the catalytic activity and selectivity of CO against H2 was observed once the particle size was decreased, particularly for NPs smaller than 5 nm, as shown in Figure 6c. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to H2 and CO selectivity. As shown in the inset of Figure 6c, a drastic increase in undercoordinated atoms is observed below a particle size of 2 nm with a coordination number lower than 8. These peculiar sites accelerate both hydrogen evolution and CO2 reduction to CO via an increase in binding energy. However, the undercoordinated sites are unfavourable for the subsequent hydrogenation of CO, which lowers the hydrocarbon selectivity of the NPs. A plausible explanation for the observed trend is the reduced mobility of intermediate reaction species (CO and H) on the small NPs due to stronger bonding, which decreases the possibility of further recombination to form hydrocarbons. At intermediate particle sizes, the spherical particle model predicts low and constant populations of (100) and (111) facets, which is consistent with the reduced yet constant hydrocarbon selectivities observed for Cu NPs between 5 and 15 nm compared to Cu bulk surfaces. For these larger NPs, weaker binding of CO and H is expected, favouring hydrocarbon formation.

Another critical parameter for suppressing the HER with metal NP catalysts is the interparticle spacing. Mesoscale phenomena, such as interparticle reactant diffusion and readsorption of intermediates, can play an important role in the product selectivity for multistep reactions. [START_REF] Seidel | Mesoscopic mass transport effects in electrocatalytic processes[END_REF][START_REF] Ono | Effect of interparticle interaction on the low temperature oxidation of CO over size-selected Au nanocatalysts supported on ultrathin TiC films[END_REF] In this context, Mistry et al. showed that for CO2 electroreduction, decreasing the interparticle spacing for a constant nanoparticle size can suppress the HER, which further increases the selectivity for CH4 and C2H4 owing to the increased possibility of the *CO intermediate re-adsorbing on a neighbouring particle and being further reduced (Figure 6d and6e). [START_REF] Mistry | Tuning Catalytic Selectivity at the Mesoscale via Interparticle Interactions[END_REF] More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering the size and distance. These principles guide the rational design of mesoscopic catalyst architectures to enhance the production of the desired reaction products. 78

Single site engineering

One of the main hurdles to the rational improvement of selectivity using metallic or metaloxide/sulfide catalysts is the large distribution of accessible sites that may result in different favoured reaction products and decreased selectivities. Single atom catalysts (SACs) hence represent an attractive strategy to increase selectivity via a narrower distribution of active sites and an improved control of the first coordination sphere of the active site, bridging the gap between well-defined molecular catalysts and complex heterogeneous materials. The catalytic properties of SACs hence result from the combination between the molecular tuning of the coordination environment of the active sites and its interaction with the support. [START_REF] Qi | Single atom is not alone: Metal-support interactions in singleatom catalysis[END_REF] Different types of supports for SACs have been explored to date and include metals, carbon-based materials, and metal (hydr)oxides, nitrides, and carbides. Metal-supported SACs, also called single-atom alloys (SAAs) have also been explored. They generally yield thermodynamically more stable interactions than other atom-support interactions due to strong metal-metal interactions. [START_REF] Ji | Chemical Synthesis of Single Atomic Site Catalysts[END_REF] Advantageously SAAs can offer different active sites on the host metal (i.e. the support) and the individual atoms, providing further opportunities to modulate reaction pathways. [START_REF] Darby | Elucidating the Stability and Reactivity of Surface Intermediates on Single-Atom Alloy Catalysts[END_REF][START_REF] Lucci | Selective hydrogenation of 1,3-butadiene on platinum-copper alloys at the single-atom limit[END_REF][START_REF] Kyriakou | Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations[END_REF] Zhang et al. demonstrated the control of the CO2RR products between formate and CO by varying the Cu/Sn composition. [START_REF] Zhang | Tunable Selectivity for Electrochemical CO2 Reduction by Bimetallic Cu-Sn Catalysts: Elucidating the Roles of Cu and Sn[END_REF] They reported that the use of Cu1Sn1 comprising a core-shell structure doped with a small amount of Cu using CuSn and SnO alloy as core and shell, respectively, leads to the preferential formation of formate with an FE greater than 95% at -1.2 V. In contrast, single atoms of Sn supported on Cu: Cu20Sn1 show a high selectivity for CO with a maximum FECO of 95.3% at -1.0 V.

Carbon substrates have been widely explored in the form of graphite, graphdiyne as well as graphene and its derivatives including heteroatom (N, O, S and P)-doped sp 2 carbon materials. Carbon supports indeed offer several advantages such as high surface area, high electronic conductivity and strong thermal stability, while they possess numerous coordination environments to stabilize the single atom sites. [START_REF] Xiong | Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation[END_REF][START_REF] Zheng | Large-Scale and Highly Selective CO2 Electrocatalytic Reduction on Nickel Single-Atom Catalyst[END_REF] The different behaviours of transition metals in the form of nanoclusters or metal-nitrogendoped carbon catalysts (MNCs) were examined by the Chan and Strasser groups. [START_REF] Vijay | Unified mechanistic understanding of CO2 reduction to CO on transition metal and single atom catalysts[END_REF] The results of their calculations revealed that *CO2 adsorption is the limiting step on metals, whereas for nitrogencoordinated SACs, the reaction can be limited either by *CO2 adsorption or by the formation of *COOH via a proton-electron transfer (Figure 7a). Pan and co-authors reported the design of MNC SACs with atomically dispersed Co sites anchored on polymer-derived hollow N-doped porous carbon spheres. [START_REF] Pan | Design of single-atom Co-N5 catalytic site: a robust electrocatalyst for CO2 reduction with nearly 100% CO selectivity and remarkable stability[END_REF] The single-atom Co-N5 sites were identified as the main active centers for CO2 activation, and the rapid formation of *COOH as a critical reaction intermediate followed by a rapid desorption of CO. A similar behaviour has also been reported on carbon nanosheet-supported Ni-N4 sites, which resulted in near-utility selectivity for CO and a single-pass conversion of 2.6% cm -2 when implemented in a flow cell. [START_REF] Zhang | 2.6% cm-2 Single-Pass CO2-to-CO Conversion Using Ni Single Atoms Supported on Ultra-Thin Carbon Nanosheets in a Flow Electrolyzer[END_REF] Huan et al. investigated a series of iron-based catalysts synthesized by pyrolysis of Fe-, N-, and C-containing precursors for the electroreduction of CO2 to CO in aqueous medium and demonstrated that the selectivity of these materials for CO2 reduction is governed by the proportion of isolated FeN4 sites compared to Fe-based nanoparticles. [START_REF] Huan | Electrochemical Reduction of CO2 Catalyzed by Fe-N-C Materials: A Structure-Selectivity Study[END_REF] They demonstrated that the nature of the metal species modulates the selectivity of the reaction pathways and suggested that FeN4 sites are responsible for CO2RR whereas the Fe cluster are responsible for HER. In a following work, they demonstrated the strong influence of the electrode support on the catalyst selectivity, highlighting the importance of reducing mass transport limitation to promote a higher selectivity towards CO2 reduction. [START_REF] Karapinar | FeNC catalysts for CO2 electroreduction to CO: effect of nanostructured carbon supports[END_REF] In recent years, metal (hydr)oxides, nitrides, carbides and sulfides have become very popular supports of SACs thanks to their high specific surface areas, abundant vacancies, and surface functional groups. [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF] Thanks to their strong corrosion resistance, metal nitrides/carbides with metal centres exposed on their surface are good supports to stabilize isolated metal atoms via strong metalsupport interactions. In this context, electrically conducting MXenes such as Mo2C have been explored

as supports for the CO2RR. [START_REF] Zhao | Active facet regulation of highly aligned molybdenum carbide porous octahedrons via crystal engineering for hydrogen evolution reaction[END_REF] The pH value of the electrolyte greatly influences the equilibrium potential of the CO2RR and NRR, as highlighted in the partial Pourbaix diagrams for the CO2RR and NRR provided in Figure 8a and Figure 8b. [START_REF] Wang | Electrocatalysis for CO2 conversion: from fundamentals to value-added products[END_REF][START_REF] Chen | Beyond fossil fueldriven nitrogen transformations[END_REF][START_REF] Hori | Electrochemical CO2 reduction on metal electrodes[END_REF][START_REF] Varela | pH effects on the selectivity of the electrocatalytic CO2 reduction on graphene-embedded Fe-N-C motifs: Bridging concepts between molecular homogeneous and solidstate heterogeneous catalysis[END_REF] A high local pH typically disfavor HER, thus enabling higher faradaic efficiencies for multicarbon products in the context of CO2RR and for ammonia in the context of NRR. [START_REF] Billy | Experimental parameters influencing hydrocarbon selectivity during the electrochemical conversion of CO2[END_REF]100 The groups of Sinton and Sargent have achieved remarkable results for the CO2RR in highly alkaline media; using 7 M KOH they achieved a 1.3 A cm -2 partial current density for ethylene in a flow cell. 101 Engineering of the triple-phase interface was key to these results and will be discussed further in Section 5. Unfortunately for CO2 electrolysis, the use of alkaline electrolyte is complicated by the fatal exergonic formation of carbonate (𝐶𝑂 2 + 2𝑂𝐻 -→ 𝐶𝑂 3 2-+ 𝐻 2 𝑂/ 𝐶𝑂 2 + 𝑂𝐻 -→ 𝐻𝐶𝑂 3 -) which is detrimental to both energy and carbon efficiency. 102 Neutral bicarbonate electrolytes have been applied

to reduce electrolyte consumption and to buffer the local pH, although at high currents CO3 2-is still formed from CO2 and electrogenerated OH -. Several studies have explored the dependence of product distribution on local pH at the electrode/electrolyte interface, as well as the concentration and buffer capability of the electrolyte. In that line, a fine tuning of the product selectivity for CO2RR on Cu electrodes was achieved via the modulation of local pH upon variation of the electrolyte buffer capacity, CO2 pressure, and current density. 103 Varela et al. proposed that electrolytes with a high buffer capacity could facilitate the transfer of coupled electrons/protons, thus being beneficial for the evolution of hydrogen. 104 By comparison, they found that electrolytes with a low buffer capacity could suppress the formation of H2 owing to the low concentration of protons near the electrode surface, favouring selectivity towards the formation of C2H4 (Figure 8c). Conversely, applying a higher current density can also lead to a higher local pH. This is due to a high consumption rate of local protons compared to the rate of mass transport of protons from the bulk electrolyte. Huang et al. modelled an electrode surface and found that even in highly acidic electrolytes (pH=1), local neutrality and alkalinity could be created above 200 mA/cm 2 . 105 They required at least 400 mA/cm 2 to produce multicarbon products.

This improved carbon efficiency considerably, although energy efficiency remains problematic. While

Optimizing the components of the electrolyte: alkali metal cation effects

Bicarbonate or carbonate are the most investigated electrolyte salts employed for the CO2RR as they provide a near-neutral pH but most importantly allow to maintain a stable and high dissolved CO2 concentration upon operation. 111,112 Hence, while the nature of the anions are rarely explored in electrochemical studies, a wide range of studies has investigated the variation of the alkali cations. In CO2RR, while the influence of alkali cations on product selectivity and catalyst efficiency are commonly accepted 72, 113 114 the origin of this effect is still largely debated in the literature. The influence of the used alkali metal cations on the CO2RR activity and selectivity is generally attributed to the relatively high concentration of alkali cations in the outer Helmholtz plane (OHP). Early work from Monteiro et al. proposed that large cations are specifically adsorbed more easily on the catalyst surface because of the fewer coordinated water molecules. 115 Adsorbed cations can also elevate the potential at the OHP and decrease the local proton concentration, suppressing HER. 116 Alternatively, it was suggested that the cation size can significantly affect the rate of water hydrolysis by tuning the hydration energy. 117 For instance, the pKa value of Li + was calculated to be three times higher than that of Cs + . The hydrated Cs + acts as a buffer, maintaining a locally low pH near the electrode and increasing the local CO2 concentration compared to Li + by 28 times (Figure 9a). To gain more insight into the role of cations in electrocatalysis, Ringe et al. developed a combined ab initio/continuum model of cation and electric double layer field effects based on a continuum modified Poisson-Boltzmann approach (Figure 9b). 118 By applying a single set of cation sizes derived from experimental data, the model showed quantitative agreement with the experiments for the catalyst system on both Ag and Cu. Their theoretical model and experimental results indicate that the repulsive interactions derived from the hydrated cations in the Helmholtz layer should be responsible for the change of surface charge and their electric field. The use of high-valent cations with a small hydration radius also increases the potential of zero charges or capacitance, which maximizes the surface charge density and the corresponding interfacial electric fields. 119 Bell's group provided insights regarding the beneficial effect of cations, particularly at relatively low overpotentials, for which the reaction rate does not perturb the local pH. 120,121 Notably, the hydrogen and CH4 partial currents remained steady, while formate, C2H4, and C2H5OH formation rates increased when using large alkali cations. The cation sizeindependent production of H2 and CH4 was attributed to the zero dipole moment of * H and * CHO, which are the corresponding reaction intermediates of the reactions (Figure 9c).

Alkali metal cations have also been used to promote the CO2RR in strongly acidic medium. A key advantage to operating at a low pH is the improved carbon utilisation efficiency, which is limited in neutral and alkaline media due to the formation of carbonate. Sargent and co-workers utilised a cationaugmenting layer to sustain a high K + concentration at the copper catalyst surface. 105

The search for novel electrolytes: ionic liquids and non-aqueous electrolytes

Ionic liquids (ILs), which are defined as salts that remain liquid below 100 °C, have been proven to be a promising new class of environmentally benign solvents. 123 By tuning the molecular structure and polarity of the IL, the CO2 and N2 absorption capacity and the ability to stabilize charged CO2 and N2 species can be tuned and optimized. ILs also possess several advantages, such as a wide electrochemical windows, thermal and chemical stability, negligible volatility and electron transfer mediation for redox catalysis, which make them an interesting alternative to promote the CO2RR and NRR. 124 As they are nonaqueous by nature, ILs allow control of the aqueous content to an optimum level to provide protons for hydrocarbon formation while suppressing the HER. [125][126][127][128][129] ILs have been extensively investigated for the CO2RR because the cations of ILs can form a complex with CO2 and further activates it. Rosen et al. reported the use of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) as an IL electrolyte for the electrochemical conversion of CO2 to CO on silver (Figure 10a). 130 The IL system lowers the energy of the * CO2 intermediate via the formation of a complex intermediate, which lowers the energy associated with the initial step of the reduction reaction. 131 The formation of CO occurred at a very low onset overpotential, and the IL system demonstrated sustained production of CO for 7 hours with a FECO of more than 96%. ILs have also been applied with transition metal dichalcogenides, which are known to be more prone to promote the HER over other reduction reactions. Remarkably, Asadi et al. exfoliated WSe2 nanoflakes to perform the electroreduction of CO2 to CO using a 50 vol.% [Emim]BF4/H2O solution. 132 The current density, FE, and TOF in producing CO were all superior at lower overpotentials, suggesting a high selectivity for the CO2RR (Figure 10b). Copper selenide nanocatalysts have been identified to convert CO2 to CH3OH at low overpotentials in a [Bmim]PF6/acetonitrile-H2O mixed electrolyte. 133 In addition, in a

[Bmim]BF4-H2O electrolyte, MoTe2 could also be used as a catalyst for CO2 reduction to CH4 with a high FE of 83% at a relatively low overpotential. 134 Atifi et al. demonstrated that protic ionic liquids (PILs) derived from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) effectively promote the electrochemical reduction of CO2 to formate (HCOO -) with high selectivity (Figure 10c). 135 The use of PILs composed of the conjugate acid of DBU, [DBU-H] + , efficiently catalysed the reduction of CO2 to HCOO -(FEHCOOH ≈ 80%) with significant suppression of CO and H2 production (FECO + FEH2 ≈ 20%) in either acetonitrile or an acetonitrile/H2O mixed electrolyte.

Ionic liquids and nonaqueous electrolytes with high N2 solubility under ambient conditions can also increase the local concentration of N2 near the catalyst surface by as much as 20 times compared to water on a volumetric basis. 136 MacFarlane and co-workers reported the use of ionic liquids with high N2 solubility for the electroreduction of N2 to ammonia at room temperature and atmospheric pressure. 137 As presented in

Solid-state electrolyte designs

Conventional liquid electrolytes used in the CO2RR and NRR, such as KHCO3, Na2SO4, or KOH, mainly have three purposes: i) to transport ions between the cathode and anode for efficient current flow, ii) to provide protons for successive PCET and iii) to solvate liquid products. The mixture of liquid products and ion impurities requires energy-and cost-intensive downstream separation steps to obtain pure products, which complicates the infrastructure for delocalized production. 141 To tackle this problem, the concept of solid-state electrolytes was proposed, inspired by progress in solid-state electrolytes for batteries. 142 A solid-state electrolyte is typically placed between ion-exchange membranes with close contact to efficiently transport the generated ions and minimize the ohmic loss of the device. 143 Remarkably, solid-state electrolytes were found to be very effective in suppressing the HER by limiting the flow of protons to the catalyst active sites during the electrochemical CO2RR. 144 The Wang group have reported the continuous electrocatalyic conversion of CO2 to pure liquid fuels using two electrode systems with solid electrolytes. 145,146 They applied a porous solid electrolyte (PSE) layer composed of styrenedivinylbenzene copolymer microspheres with sulfonic acid functional groups for proton conduction. Using a formic-acid-selective bismuth catalyst (FEHCOOH ~97%), the electrochemically generated protons and formate anions could combine at the PSL to produce formic acid (Figure 11a). By directly flowing a carrier gas instead of deionized water through the PSL, they were able to collect product vapours that could be condensed to form the pure product (almost 100 wt.% formic acid), alongside impressive current density and stability (Figure 11b).

Three-phase interface engineering

The abundance of protons near the catalyst active sites constitutes a significant challenge for the catalyst selectivity vs. the competing HER in aqueous electrolytes, resulting in low selectivity and activity of the CO2RR and NRR. A mitigation strategy resides in facilitating the accessibility of the catalyst to high concentrations of CO2 or N2 molecules. While protons (H + ) are readily available in aqueous solutions via water ionization, the supply of CO2 and N2 molecules to the catalyst surface is limited by their low concentration and slow diffusibility. In saturated aqueous electrolytes, the solubility of CO2 in H2O is 33 mmol L -1 at 298 K and 1 atm pressure, whereas the value for N2 in H2O remains as low as 0.7 mmol L -1 . [START_REF] Weiss | In The solubility of nitrogen, oxygen and argon in water and seawater[END_REF] By comparison, the concentration of protons in a neutral aqueous electrolyte is typically 2.7-fold and 132-fold higher than the concentrations of CO2 and N2, respectively.

In coupling. This study led to the identification of the role of hydrophobicity and the formation of gaseous voids as effective levers to orient the reaction pathway towards the formation of multicarbon products.

Khan and coworkers explored the idea of gas-trapping further by using a gasphilic silicon substrate in proximity to the catalyst layer. Creating a CO2 plastron adjacent to the catalyst improved mass transfer, enriching and maintaining the local CO2 concentration. Using a smooth copper catalyst they recorded improved activity and a decrease in FEH2 (13 % compared to 29 % with bulk CO2 bubbling). These trends were replicable using nanostructured copper, demonstrating the transferability of such an approach to different catalysts. 152 Moreover, Xing et al. showed that a hydrophobic microenvironment can significantly enhance CO2 electrolysis by facilitating reactant diffusion (Figure 12b). 153 Using commercial copper nanoparticles dispersed with hydrophobic polytetrafluoroethylene (PTFE) nanoparticles, they reported improved activity and Faradaic efficiency for CO2 reduction with a partial current density >250 mA cm -2 and a single-pass conversion of 14% at moderate potentials. Importantly, this performance was approximately twice as large as that of regular electrodes without added PTFE.

Similar findings were also observed from a Bi-based catalyst modified with PTFE nanoparticles in the catalyst layer to demonstrate a partial current density of 677 mA cm -2 for formate and 35% single-pass CO2 conversion at -0.7 V vs. RHE (Figure 12c). 154 Pham et al. compared various ionomeric binders on a Cu catalyst, and achieved a 77 % Faradaic efficiency and 600 mA cm -2 partial current density for C2+ products at -0.76 V vs RHE using a fluorinated ethylene propylene (FEP) binder. 155 They attributed these results to the hydrophobic properties of FEP. The Sinton and Sargent groups have also done notable work on modulating the three-phase interface in continuous flow and membrane electrode assembly (MEA) electrolyzers, enabling high current densities (e.g. > 1 A cm -2 ) to be achieved. 101,144 For example, they presented a catalyst:ionomer bulk heterojunction (CIBH) architecture, which had both hydrophilic and hydrophobic functionalities. By having different domains that favoured gas and ion transport routes, they were able to decouple gas, ion and electron transport, extending the reaction interface from the submicrometer to the several micrometer range. 101 These examples illustrate that moderate hydrophobicity of the catalyst layer can establish a microenvironment with a balance between gaseous CO2 and liquid electrolytes inside the catalyst layer. Such microenvironmentsequivalent to microreactors -reduce the thickness of the diffusion layer, accelerate CO2 mass transport and link highly active reaction zones at the interfaces between the three phases involved in the reaction. 156 The triple-phase interface can also be further tuned by applying ionomers to control pH and CO2/H2O concentrations. Bell and co-workers postulated that anion-exchange ionomers (e.g. 12d). 159 The hydrophobic carbon fibres facilitated the formation of three-phase contact points (TPCPs) for N2, the liquid electrolyte and the Au NPs. Xiao et al. successfully modified the d-band structure of a self-supporting nanoporous Mo4P3 catalyst by capping with a fluorosilane hydrophobic layer. 160 This approach aims at weakening the ability of the material surface to adsorb protons while simultaneously preventing lowering the amount of water available at the active sites, thus further lowering competitive HER. This hydrophobic Mo4P3 material exhibits decent NRR performances, with a FE of 10.1% and an NH3 yield rate of 17.3 μg h -1 cm -2 . According to Wang and co-workers, excessive suppression of the HER is not, however, beneficial to NRR activity, although it can lead to higher Faradic efficiency (Figure 12e). A sharp decrease in the local concentration of protons does not benefit the NRR process, as protons are necessary for the successive PCET steps associated with the formation of ammonia. These investigations point out that although the release of hydrogen is a competitive reaction, protons are paradoxically essential to increase the ammonia yield rates. 161

Conclusions and Perspectives

The industrial development of the CO2RR and NRR is currently plagued by low Faradaic and energy efficiencies. The successive PCET steps associated with the corresponding reaction intermediates increase the complexity and complicate the search for an ideal catalyst. In contrast, the simplicity of the HER mechanism and the abundant presence of protons in traditional electrolytes make the production of hydrogen a competitive and parasitic reaction that consumes a significant amount of electrons to the detriment of the fixation of CO2 and N2. Additionally, the selectivity towards a single product, particularly important in the context of CO2RR is a central point to be considered. Multiple strategies have shown promise but still require the elaboration of a robust and rational framework; they have demonstrated that optimal activity and selectivity can be obtained upon modulating thermodynamics and kinetics of the reaction. By engineering the catalyst, electrolyte and reaction interface, three main strategies have been applied towards that goal (Table 2): i) targeting a narrow distribution of molecularly defined active sites, ii) increasing the reactant/proton ratio at the threephase interface where the reaction takes place to lower the undesired formation of H2 and iii), the stabilisation and confinement of reaction intermediates in the electrode vicinity to favour the formation of multi-electron reduction products.

The complexity of the parameters involved to address these challenges simultaneously further highlights the interest in combining experimental and theoretical approaches to guide the design of both catalysts and electrolyzers for the CO2RR and NRR. From this perspective, machine learning will help rapid screening of catalysts with high selectivity based on massive data in the silico database by focusing on near-optimal bond energy with adsorbates, such as *CO and *N2H. In addition, enabling a better understanding and control of the PCET steps, notably via the elaboration of a robust framework to link the relative contribution of charge transfer and protonation steps on overpotential and on the distribution of surface species will be key to further rationally improve electrocatalysts.

This review illustrated several examples displaying industry-relevant performances in terms of selectivity and current densities, highlighting the potential of electrochemical approaches for the preparation of carbon and nitrogen containing molecules. However, for the CO2RR, many studies have been performed in alkaline or neutral media, resulting in carbonate formation in the electrolyte. This is detrimental to carbon utilization and energy efficiency, especially considering the energy that would be required to regenerate spent electrolyte. This problem has been considerably underestimated and overlooked for some time, however, an increasing number of studies over recent years have attempted to tackle this issue. Using acidic electrolyte prevents carbonate crossover to the anode and regenerates CO2 close to the cathode surface, improving carbon utilization efficiency. Naturally this media poses challenges regarding hydrogen evolution, and the application of strategies covered in this review will be pivotal in overcoming this.

Moreover, most of the presented strategies introduced in the present review enable improving catalyst selectivity for a relatively short period of time, but have not been investigated over industrially relevant time scales. Maintaining high selectivity for the CO2RR and over long operation time remains the largest challenge to date, as rapid loss in activity and selectivity is observed for most of the systems reported. This notably results from the fact that in operation undesirable intermediates or poisonous byproducts preferably deposit on the catalyst surface and affect the catalysis process. This phenomenon may decrease the effective area of the electrocatalyst, accelerate cathodic degradation and increase selectivity towards competitive HER. The demonstration of catalysts with ultralong stability of > 5000 hours constitutes in our view the last milestone to be reached in order to validate the industrial potential of the CO2RR and NRR.

Figure 1 .

 1 Figure 1. (a) Scheme of the carbon cycle. Reproduced from Nat. Geosci. 2009, 2 (9), 598-600. Copyright 2009 Springer Nature. (b)

Figure 2 .

 2 Figure 2. (a) Kinetic versus thermodynamic requirements of various CO2 reduction reactions. The plotted values are based on the reaction equation given above the graph, made stoichiometric according to the product composition. Reproduced from Nat. Mater. 2019, 18 (11), 1222-1227. Copyright 2019 Springer Nature. (b) Combined volcano diagrams (lines) for the flat (black) and stepped (red) transition metal surfaces for the reduction of nitrogen with a Heyrovsky-type reaction, without (solid lines) and with (dotted lines) Hbond effects. Reproduced from Phys. Chem. Chem. Phys. 2012, 14 (3), 1235-1245. Copyright 2012 Royal Society of Chemistry.

Figure 3 .

 3 Figure 3. (a) Schematic illustration and SEM image of a copper DHBT foam, demonstrating the hierarchical pore structure. Reproduced from Chemical Communications 2015, 51 (21), 4331-4346, copyright 2015 Royal Society of Chemistry, and Journal of The Electrochemical Society 2013, 160 (10), D441-D445, copyright 2013 IOP Publishing. (b) Schematic illustration of gaseous CO2R intermediates (CO and C2H4) and by-products (H2) trapped within the porous Cu foam catalyst. Reproduced from ACS Catalysis 2016, 6 (6), 3804-3814. Copyright 2016 American Chemical Society. (c) Schematic illustration of a dendritic CuO DHBT-foam before (top) and after (bottom) CO2 electroreduction in 0.1 M CsHCO3, showing the material reduction to metallic Cu and the formation of nano-Kirkendall voids. Reproduced from Proc Natl Acad Sci U S A 2019, 116 (20), 9735-9740. Copyright 2019 Proceedings of the National Academy of Sciences. (d) Potential dependent product distribution of the CO2RR using a Ag-DHBT-foam catalyst by faradaic efficiency, showing the formation of hydrocarbons at potentials more negative than -1.2 V vs RHE. Reproduced from ACS Catalysis 2018, 8 (9), 8357-8368. Copyright 2018 American Chemical Society.

  computational and experimental research to define and improve catalyst nanomorphology, especially in the CO2RR field, however their application to new materials and fields such as NRR is still an open area of research.

Figure 4 .

 4 Figure 4. (a) Schematic showing the effect of catalyst nanostructure on bubble departure diameter and its impact on the diffusion boundary layer thickness and CO2 mass transport. Reproduced from ACS Sustainable Chemistry & Engineering 2017, 5 (5), 4031-4040. Copyright 2017 American Chemical Society. (b) Schematic of a cuprous oxide catalyst with nanocavities that confine carbon intermediates such as CO and C2H4. White: hydrogen; grey: carbon; red: oxygen; pink: copper. Reproduced from Journal of the American Chemical Society 2020, 142 (13), 6400-6408. Copyright 2020 American Chemical Society. (c) Schematic illustration (left to right) and picture (middle-top) of the NRR in an H-cell with a microtubular Ru-CNT (carbon nanotube) gas diffusion electrode. Reproduced from ChemElectroChem 2020, 7 (22), 4679-4684. Copyright 2020 European Chemical Societies Publishing.

  while destabilizing *H. Conversely, thiol-capped Ag nanoparticles exhibited superior reaction rates towards both the HER and CO2 reduction by indiscriminately increasing ∆G*H and ∆G*COOH. As presented in Figure 5c, Zhao et al. developed a simple modification strategy using amines to

  [START_REF] Nam | Molecular enhancement of heterogeneous CO2 reduction[END_REF]

Figure 5 .

 5 Figure 5. (a) Surface modifiers grouped into different classes used to modulate the local chemical environment around the catalytic site. (amino acids, amines, N-heterocyclic carbenes, thiols, imidazolium, three-dimensional cavities, N-arylpyridinium salts and derivatives). Reproduced from Nature Catalysis 2020, 3 (10), 775-786. Copyright 2020 Springer Nature. (b) Schematic of the product selectivity, depending on the Ag NPs immobilized with an amine (or thiol)-containing anchoring agent. Reproduced from ACS Catal. 2017, 7 (1), 779-785. Copyright 2017 American Chemical Society. (c) FECO (column) and jCO (circle) of gold catalysts with different surface amine modifications in CO2-saturated 0.1 M KHCO3 at -0.7 V vs. RHE. Reproduced from Adv. Energy Mater. 2018, 8 (25), 1801400. Copyright

Figure 6 . 1 V

 61 Figure 6. (a) Relation between the Faradaic efficiencies and potentials for different Cu morphologies (sphere, cube and octahedra). Reproduced from ACS Catal. 2020, 10 (9), 4854-4862. Copyright 2020 American Chemical Society. (b) Relation between the Faradaic efficiencies and potentials with the exposure of different Au facets. Reproduced from Angew. Chem. Int. Ed. 2019, 58 (12), 3774-3778. Copyright 2019 Wiley. (c) Particle size effect during catalytic CO2 electroreduction. The Faradaic current densities at -1.1 and -1.0 V vs. RHE are plotted against the size of the Cu NP catalysts, and the inset shows the population (relative ratio) of surface atoms with a specific coordination number (CN) as a function of particle diameter. Reproduced from J. Am. Chem. Soc. 2014, 136 (19), 6978-6986. Copyright 2014 American Chemical Society. (d) Simulation results of the CO2 concentration distribution based on diffusion equations. The red arrows show the reactant flux towards the NPs. The colour scale shows the concentration of CO2 at a given distance from the NPs as a percentage of its value in the bulk of the electrolyte. A diffusion layer thickness of 100 nm was assumed. Reproduced from ACS Catalysis 2016, 6 (2), 1075-1080. Copyright 2016 American Chemical Society. (e) Faradaic selectivity during the electroreduction of CO2 at -1.1 V vs. RHE with a Cu interparticle distance of 4.7 nm. Reproduced from ACS Catalysis 2016, 6 (2), 1075-1080. Copyright 2016 American Chemical Society.

  Zhang et al. demonstrated an efficient approach to produce single atom copper immobilized on MXene for the electrosynthesis of methanol from CO2. The SACs were obtained via selective etching of hybrid A layers (Al and Cu) in quaternary MAX phases (Ti3(Al1-xCux)C2).[START_REF] Zhao | Selective Etching Quaternary MAX Phase toward Single Atom Copper Immobilized MXene (Ti3C2Clx) for Efficient CO2 Electroreduction to Methanol[END_REF] Combining X-ray absorption spectroscopy analysis and density functional theory calculations, they proposed that the Cu single atoms in the form of Cu δ+ with 0 < δ < 2 have a low energy barrier for the rate-determining step corresponding to the conversion of HCOOH* to CHO*, a key reaction intermediate for the reduction of CO2 to CH3OH (Figure7b).

Figure 7 . 4 . 4 . 1

 7441 Figure 7. (a) Rate map for CO2R to CO at -0.8 VSHE and pH=2 obtained from the (211) TM scaling line. The annotated points show MNC SACs either at single or double vacancies. Reproduced from Nature Catalysis 2021, 4 (12), 1024-1031. Copyright 2021 Springer Nature. (b) DFT calculations proposed a reaction pathway for the functionalization of CO2 to methanol on isolated Cu of SA-Cu-MXene. Reproduced from ACS Nano 2021, 15 (3), 4927-4936. Copyright 2021 American Chemical Society.

aFigure 8 .

 8 Figure 8. (a) Partial Pourbaix diagram for CO2 reduction in aqueous solutions that describes the relationship between the equilibrium potential of the associated reaction and pH, which is plotted based on thermodynamic data. Reproduced from Chem. Soc. Rev. 2021, 50, 4993-5061. Copyright 2021 Royal Society of Chemistry. (b) Partial Pourbaix diagram for the N2-H2O system. Solid lines correspond to N2 reduction to NH4 + or NH3 (red) and N2 oxidation to NO3 -(blue). Dotted lines a and b straddle the region of water reduction to H2 and oxidation to O2, respectively. Reproduced from Science 2018, 360 (6391), eaar6611. Copyright 2018 AAAS. (c) Formation rates of gas products as a function of applied electrode potentials in CO2 saturated electrolytes with different buffer capacities. Reproduced from Catal. Today 2016, 260, 8-13. Copyright 2016 Elsevier. (d) NH3 yield rate and Faradaic efficiency of Pd/C processed in N2-saturated electrolytes with different pH values. Reproduced from Nat. Commun. 2018, 9 (1), 1-7. Copyright 2018 AAAS.

They achieved 61 %

 61 Faradaic efficiency for CO2RR products and 40 % for C2+ products at 1.2 A/cm 2 , and by lowering the CO2 flow they reached a single pass conversion efficiency of 77%. Gu et al. explored the effect of alkali cations on the CO2RR in acid with tin oxide, gold and copper catalysts, achieving 90% faradaic efficiencies for formic acid and CO.122 Using a simulation based on the Poisson-Nernst-Planck (PNP) model, they predicted that the origin of such striking effects was the modulation of electric fields, which inhibited the migration of hydrononium ions.

Figure 9 .

 9 Figure 9. (a) Effect of cation hydrolysis on the electrochemical reduction of CO2 over Ag. Distribution of pH and CO2 concentration in the boundary layer. Hydrated Cs + buffers the cathode to maintain the pH close to 7 and to increase the CO2 concentration. Reproduced from J. Am. Chem. Soc. 2016, 138 (39), 13006-13012. Copyright 2016 American Chemical Society. (b) Illustration of the origin of cation effects in field-driven electrocatalysis. Repulsive interactions between hydrated cations at the outer Helmholtz plane reduce the local concentration of cations, the surface charge density (depicted by the red-coloured region) and the electric double layer field. The diffuse layer that is explicitly modelled by the size-modified Poisson-Boltzmann (MPB) model is depicted, as well as the Helmholtz gap capacitance region and the interfacial ion diameter. Reproduced from Energy Environ. Sci. 2019, 12 (10), 3001-3014. Copyright 2019 Royal Society of Chemistry. (c) Average current densities obtained during bulk electrolysis as a function of metal cations at different potentials. Reproduced from J. Am. Chem. Soc. 2017, 139 (32), 11277-11287. Copyright 2017 American Chemical Society.

Figure 10d ,Figure 10 .

 10d10 Figure 10. (a) Schematic of how the free energy of the system changes during the CO2 + 2H + + 2e -⇌ CO + H2O reaction in water, acetonitrile (solid line) or EMIM-BF4 (dashed line). Reproduced from Science 2011, 334 (6056), 643-644. Copyright 2011 AAAS. (b)

Sheets

  et al. proposed a novel polymer gel approach to convert N2 to NH3 at mild temperatures (30-60 °C) and pressures (20 psig).147 As illustrated in Figure11c, the polymer gel electrolyte helped to control the rate of the HER by limiting water transport and boosting N2 transport, thus improving the selectivity towards the NRR.

Figure 11 .

 11 Figure 11. (a) Schematic illustration of the CO2 reduction cell with a solid electrolyte. Reproduced from Nat. Energy 2019, 4 (9), 776-785. Copyright 2019 Nature Publishing Group. (b) Electrochemical performance of a all-solid-state CO2RR reactor compared with previous literature. Reproduced from Nat. Commun. 2020, 11 (1), 1-9. Copyright 2020 Springer Nature. (c) Cathode species transport diagram illustrating the advantage of the polymer gel electrolyte to limit water transport. Reproduced from Chem. Comm. 2018, 54 (34), 4250-4253. Copyright 2018 Royal Society of Chemistry.

  the context of CO2RR, Raciti et al. demonstrated that the local concentration of CO2 at the catalyst surface can reach zero under strong reaction driving force conditions, hence lowering selectivity by limiting the supply of the reactant.148 Significant advances to minimize this reactant supply issue at the electrode have been made thanks to the implementation of efficient three-phase interfaces between gaseous CO2, the liquid electrolyte and the solid catalyst. The most typical realisation of such a threephase interface involve porous gas diffusion layer (GDL) electrodes, which allow the delivery of gasphase CO2 directly to the catalyst active sites. Such a strategy, resulting in higher CO2 and lower H + surface concentrations, has the potential to improve CO2RR performances while significantly lowering competitive HER. The properties of the GDL can affect CO2 and water transport heavily, and main advances in this field have been recently reviewed. 149, 150 and will not be extensively reviewed here in the context of CO2RR. Thinner GDL/catalyst layers shorten the CO2 diffusion distance, raising the relative CO2 concentration; however, excessively high concentrations can decrease multicarbon product formation by competing with intermediates such as CO for binding sites. Tan et al. found that by adjusting the catalyst layer structure and the CO2 feed concentration and flow rate, they could establish a moderate local CO2 concentration that was optimal for multi-carbon product selectivity.151 Alternatively to requiring GDL-based electrodes, the catalyst support itself can be modulated to modulate the three-phase interface, via a fine-tuning of the local microenvironment near the catalyst surface through nanostructuring and surface functionalisation. Inspired by biological strategies to entrap a gas layer at the surface of a solid, and in particular by plastron effect enabling the diving bell spider to breathe underwater, Wakerley et al. functionalized porous dendritic Cu electrodes generated via the DHBT strategy mentioned above in section 3 with long-chain alkanethiols. The resulting superhydrophobic Cu electrodes demonstrated a 6 fold decrease of HER upon treatment with the alkanethiol and a subsequent drastic increase in CO2 reduction selectivity.[START_REF] Wakerley | Bio-inspired hydrophobicity promotes CO2 reduction on a Cu surface[END_REF] They proposed that the hydrophobicity establishes triple phase interfaces at the electrode where CO2 mass transport is omnidirectional and H + mass transport is unilateral (Figure12a). This increases the local CO2 concentration and thereby the surface concentration of Cu-COOH* and Cu-CO*, enhancing C-C

Figure 12 .

 12 Figure 12. (a) Operation of the hydrophobic dendrite, illustrating the enhanced CO2 mass transport from the triple-phase boundary between the electrolyte, the electrode and gaseous CO2 and the resultant formation of key products on the surface. Reproduced from Nat. Mater. 2019, 18 (11), 1222-1227. Copyright 2019 Springer Nature. (b) Faradaic efficiencies for the CO2RR on the two electrodes (dash: AvCarb MGL370 + Cu/C; solid: AvCarb GDS2230 + Cu/C) at -1.0 V vs. RHE with various CO2 flow rates. Reproduced from Nat. Commun. 2021, 12 (1), 1-11. Copyright 2021 Springer Nature. (c) Schematic illustration of CO2 mass transport inside the catalyst layer with added PTFE, including gas-phase diffusion (solid red arrows) and aqueous-phase diffusion (dashed blue arrows). The dashed rectangles indicate catalyst areas that are only exposed to the electrolyte, exposed to both electrolyte and gaseous CO2, and only exposed to gaseous CO2. Reproduced from ACS Energy Lett. 2021, 6, 1694-1702. Copyright 2021 American Chemical Society. (d) Schematic illumination of three-phase contact for N2 (gas), the electrolyte (liquid), and the catalyst (solid) at the hydrophobic interface. Reproduced from Adv. Sci. 2020, 7 (22), 2002630. Copyright 2020 Wiley. (e) NRR catalytic mechanism of Mo2C/C under proton-suppressed and proton-enriched conditions. Reproduced from Adv. Mater. 2018, 30 (46), 1803694. Copyright 2018 Wiley.

Table 1 .

 1 Selected standard potentials of CO2 and N2 in aqueous solutions (V vs. SHE) at 1.0 atm and 25 °C, calculated according to the standard Gibbs energies of the reactants in reactions. Reproduced from Chem. Soc. Rev. 2014, 43 (2), 631-675, copyright 2014 Royal Society of Chemistry, and Chem. Soc. Rev. 2019, 48 (24), 5658-5716, copyright 2019 Royal Society of Chemistry.

  Du et al. prepared a nanoporous tin DHBT foam on a tin substrate and achieved a Faradaic efficiency for formate of 90 % with current densities of 23 mA/cm 2 .[START_REF] Du | Achieving Both High Selectivity and Current Density for CO2 Reduction to Formate on Nanoporous Tin Foam Electrocatalysts[END_REF]

  By optimising a bilayer ionomer coating and coupling to pulsed electrolysis, they achieved 90 % Faradaic efficiency for C2+ products and just 4 % for H2.In the case of NRR, when applying large potentials at the electrodes, the kinetically facile HER becomes preferable to the reduction of N2 due to the relatively low energy barrier associated with the reaction. It was suggested that the HER should always dominate at normal proton concentrations near the metal electrode surface. However, when few protons or electrons are provided, the NRR may preferentially occur, as recently observed experimentally. Designing a triple-phase interface for NRR can increase the local N2 concentration and improve *N2 adsorption, whilst limiting the availability of protons by reducing contact with the electrolyte.158 Using this strategy, Zhang et al. realized triplephase electrolysis via in situ fabrication of Au nanoparticles located on hydrophobic carbon fibre paper

sustainion) increase CO2 solubility, cation-exchange ionomers (e.g. nafion) increase local pH by trapping OH -ions, and both types increase water concentration.

157

(Au/CFP) (Figure

Table 2 .

 2 Reference numbers of key examples of the three strategies for enhanced product selectivity in carbon dioxide and nitrogen reduction reactions and how they are implemented. References related to CO2RR are in normal text while references related to NRR are underlined.

			Strategy	i) Molecularly	ii) High	iii) Stabilizing
				Defined	Local Reactant	and Confining
				Active Sites	Concentration	Intermediates
			Porous Networks		30	32, 40, 45
	Catalyst Design		Nanostructures (e.g. wires, films, needles etc.) Surface functionalization Control of crystal size, facet & spacing	61, 62, 66 72-74, 77	50, 51, 54	52 57, 58, 61, 66, 70
			Single-site engineering	81, 86-91		81
	Electrolyte Engineering		Adjusting local pH Alkali metal cation effects Ionic liquids Solid-state electrolyte		101, 103-105, 107 105, 122 137, 139 143-145, 147	106, 108 120 130, 131, 138
	Three-Phase Interface	Engineering	Gas diffusion layer electrodes Gas trapping Utilizing ionomers		150-152 21, 152-154, 159, 160 101, 155, 157

Acknowledgment: D. V., H. W., K. Q. and V. M. acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreements No 804320 (D. V., H. W., K. Q.) and No 853064 (V. M.)). V.M. and A. S. M. acknowledge funding from the Swiss National Science Foundation (SNSF) project funding (grant No 200021_197153/1). K. Q. acknowledge financial support from the China Postdoctoral Science Foundation (2018M633127) and Natural Science Foundation of Guangdong Province (2018A030310602).