M Vincent

Keywords: Urea electro-oxidation, thermodynamic standard potential, NiO/NiOOH anode, anode deactivation, surface stability, Electrochemical Impedance Spectroscopy, spectroelectrochemistry

The presence of urea in wastewater effluents presents a significant environmental challenge, requiring effective treatment to ensure safe water discharge into the environment. This challenge offers an opportunity for the circular economy as urea electrolysis can be employed for green hydrogen production. The electrochemical oxidation of urea (UOR) on Ni-based catalysts, has shown promising results in alkaline solution. In particular, NiO nanoparticles (NPs) exhibit high stability in strong alkaline solution, but very few examples of their application into UOR has been reported so far.

Moreover, the thermodynamic standard potential (E 0 ) is one of the primary criteria to evaluate the performance of any electrocatalytic reaction and different values are reported in the literature for UOR. Thus, we have included here our own calculated E 0 values, considering the production of highly oxidized N-species as well as N2 gas.

This thesis focuses on studying UOR on Ni-based catalysts in strong alkaline media (1 M or 5 M NaOH), and primarily on NiO NPs after electrochemical activation, where a layer of NiOOH is formed. Several parameters such as urea and electrolyte concentration, as well as mass transport during UOR were evaluated in detail by the rotating disk electrode (RDE). Various electrochemical techniques such as cyclic voltammetry (CV), chronoamperometry (CA), and coupled spectroelectrochemistry (SEC) are employed to gain valuable insights into the electro-oxidation kinetics of urea on NiO/NiOOH NPs. Our findings indicate that UOR is primarily limited by kinetics, while the formation of cyanate/carbonate ( -OCN / CO3 2-) and nitrite (NO2 -) have been detected and monitored during UOR by SEC. Additionally, previous studies have highlighted the significant and progressive deactivation of the NiO/NiOOH anode during urea electro-oxidation, which is attributed to the adsorption of reaction intermediates or by-products. Thus, this thesis is focused on identifying the source of anode deactivation, since this will play a crucial role in designing more efficient electrocatalysts for urea electrolysis in the future. Recent reports have shown significant NH3 production and proposed an alternative reaction pathway involving -OCN and NO2 -species for UOR. Thus, NH3, -OCN, NO2 -, as well as CO3 2- are studied here as by-products potentially responsible of the deactivation process.

Consequently, the impact of these by-products has been investigated for the first time, by evaluating the surface stability of NiO/NiOOH NPs using Electrochemical Impedance vi Spectroscopy (EIS). The obtained results have ruled out the responsibility of any of those urea electro-oxidation by-products and pointed out the slow kinetics in the regeneration of the active NiOOH film as limiting factor provoking catalyst deactivation.

The urea degradation into NH3 and the formation of some unwanted by-products (mainly NO2 -) observed during UOR have highlighted certain obstacles for H2 production from urea electrolysis. Thus, we include as perspective in this work another approach for wastewater treatment involving the electro-oxidation of ammonia on NiO/NiOOH. Consequently, initial investigations have been carried out on the electro-oxidation of ammonia as an alternative approach to produce H2 and convert it into N2 and H2O.

vii Résumé La présence d'urée dans les effluents d'eaux usées représente un défi environnemental important, nécessitant un traitement efficace pour garantir un rejet sûr de l'eau dans l'environnement. Ce défi offre une opportunité pour l'économie circulaire car l'électrolyse de l'urée peut être utilisée pour la production d'hydrogène vert. L'oxydation électrochimique de l'urée (UOR) sur des catalyseurs à base de Ni a donné des résultats prometteurs en solution alcaline. En particulier, les nanoparticules de NiO présentent une grande stabilité dans les solutions alcalines fortes, mais très peu d'exemples de leur application à l'UOR ont été rapportés jusqu'à présent. En outre, le potentiel standard thermodynamique (E 0 ) est l'un des principaux critères d'évaluation de la performance de toute réaction électrocatalytique et certaines valeurs controversées ont été rapportées dans la littérature pour l'UOR. Nous avons donc inclus ici nos propres valeurs E 0 calculées, en tenant compte de la production d'espèces N hautement oxydées ainsi que de gaz N2.

Cette thèse se concentre sur l'étude de l'UOR sur des catalyseurs à base de Ni dans des milieux fortement alcalins (NaOH 1 M ou 5 M), et principalement sur des NPs de NiO après activation électrochimique, où une couche de NiOOH est formée. Plusieurs paramètres tels que la concentration d'urée et d'électrolyte, ainsi que le transport de masse pendant l'UOR ont été évalués en détail par l'électrode à disque tournant (RDE). Diverses techniques électrochimiques telles que la voltammétrie cyclique (CV), la chronoampérométrie (CA) et la spectroélectrochimie couplée (SEC) sont utilisées pour obtenir des informations précieuses sur la cinétique de l'électro-oxydation de l'urée sur les NP de NiO/NiOOH. Nos résultats indiquent que l'UOR est principalement limitée par la cinétique, tandis que la formation de cyanate/carbonate ( -OCN / CO3 2-) et de nitrite (NO2 -) a été détectée et suivie pendant l'UOR par la SEC. En outre, des études antérieures ont mis en évidence la désactivation significative et progressive de l'anode NiO/NiOOH pendant l'électro-oxydation de l'urée, qui est attribuée à l'adsorption d'intermédiaires ou de sous-produits de la réaction. Cette thèse se concentre donc sur l'identification de la source de désactivation de l'anode, car cela jouera un rôle important dans l'élaboration d'électrocatalyseurs plus efficaces pour l'électrolyse de l'urée à l'avenir. Des rapports récents ont montré une production significative de NH3 et ont proposé une voie de réaction alternative impliquant les espèces -OCN et NO2 -pour l'UOR. Ainsi, NH3, -OCN, NO2 - , ainsi que CO3 2-sont étudiés ici en tant que sous-produits potentiellement responsables viii du processus de désactivation. Par conséquent, l'impact de ces sous-produits a été étudié pour la première fois, en évaluant la stabilité de la surface des NPs NiO/NiOOH à l'aide de la spectroscopie d'impédance électrochimique (EIS). Les résultats obtenus ont exclu la responsabilité de l'un de ces sous-produits de l'électro-oxydation de l'urée et ont mis en évidence la lente cinétique de régénération du film actif de NiOOH comme facteur limitant provoquant la désactivation du catalyseur.

La dégradation de l'urée en NH3 et la formation de certains sous-produits indésirables (principalement NO2 -) observées pendant l'UOR ont mis en évidence certains obstacles à la production de H2. Ainsi, nous incluons comme perspective dans ce travail une autre approche pour le traitement des eaux usées impliquant l'électro-oxydation de l'ammoniac sur NiO/NiOOH. Par conséquent, des recherches initiales ont été menées sur l'électro oxydation de l'ammoniac en tant qu'approche alternative pour produire du H2 et le convertir en N2 et H2O.

Mots-clés : Électro-oxydation de l'urée, potentiel standard thermodynamique, anode NiO/NiOOH, désactivation de l'anode, stabilité de la surface, Spectroscopie d'impédance électrochimique, Spectroélectrochimie. by Photoassisted Urea Oxidation, aims to develop a Photoelectrochemical (PEC) reactor. The primary objective is to apply the PEC reactor for the urea electro-oxidation (UOR) in order to convert a contaminated effluent, containing mainly urea CO(NH2)2 from separated urine at the source, into clean water by simultaneously producing useful H2 gas. The main focus of the project was on the anodic reaction, which involves the photoassisted UOR. The anode consists of nanostructured semi-conducting α-Fe2O3 layers deposited onto a transparent substrate of fluorine doped tin oxide (FTO), decorated with Ni-based catalysts. The use of photoanodes will decrease the UOR overpotential. Figure 1.1 shows the main reactions involved to achieve the goals established with the PEC reactor. The project is a collaborative effort involving four different laboratories (LISE UMR 8235-Paris, LRS UMR 7197-Paris, LGC UMR 5503-Toulouse, and ICMPE UMR 7182-Thiais) and the enterprise SIAAP (Syndicat Interdépartemental pour l'Assainissement de l'Agglomération Parisienne). In particular, this thesis is devoted to study the mechanism of urea electrooxidation on Ni-based catalysts by our team (LISE/LRS) and exchanging information with the ICMPE-Thiais team to improve catalyst performance and LGC-Toulouse to improve the reactor performance. The ICMPE-Thiais team main task is synthesizing different catalyst materials to enhance the performance and selectivity in UOR. Meanwhile, the LGC-Toulouse group is in charge of optimizing the pre-pilot reactor setup and implementing all the elements for testing the UOR in long-term electrolysis. To ensure effective communication and progress during the project, a meeting involving all partners takes place every six months, allowing the teams to share the new findings and draw conclusions.

Initially, we reviewed the literature devoted to the UOR and we noticed that there were several controversial issues regarding the reaction mechanism. For instance, Ni-based catalysts face deactivation during UOR, but the reasons for that behavior are not clear at present. Most of the previous studies have only considered CO2 or CO3 2-as potential byproducts of UOR formed on NiOOH anodes, which could compete with urea for active sites and, therefore, be responsible for the observed deactivation process. In addition, the intermediate by-product during UOR, CO was considered also in some studies as responsible for the catalysts deactivation, as mentioned later in section 1.2.11. Another example, mainly electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements have been conducted in the literature to study the effect of CO2 adsorption on Ni-based catalysts; however, those results have been misinterpreted in some cases (presented in section 1.2.11), mainly because the change in the solution pH provoked by CO2 bubbling in an alkaline solution was not properly considered. In addition to this, contradictory values of the thermodynamic standard potential for UOR were reported in the literature.

Different in situ/operando characterization techniques, such as Infrared (IR), Raman and, UV-visible spectroscopies, as well as microscopies such as transmission electron microscopy (TEM) and scanning electrochemical microscopy (SECM) have recently gained prominence for studying reaction mechanisms in electrochemistry, since they allow tracking formation Chapter 1 -Introduction Figure 1.2. Status in 2015 of the control variables for seven of the planetary boundaries [3].

The majority of nitrogen (N) found in urine wastewater is attributed to urea (H2NCONH2) [4], a nitrogen-rich compound and the largest constituent in urine present at high concentrations (240-470 mM urea) [5,6]. While the remaining nitrogen content is derived from other nitrogenous compounds present in urine such as creatinine (C4H7N3O), uric acid (C5H4N4O3) and ammonium chloride (NH4Cl) [7]. During storage, urine undergoes a natural degradation process that originates from urea hydrolysis, which leads to the conversion of at least 90% of nitrogen into ammonia (NH3) [4,[8][9][10][11][12]. This chemical decomposition reaction is catalyzed by the enzyme urease, which is widely present in nature and produced by microorganisms, plants, fungi and algae [8]. As shown in Figure 1.3, the half-time period of urea significantly drops from 40 years to few milliseconds when urease is present, due to the fast biological decomposition of urea into ammonia and carbamate (H2NCOOH), while the resulting carbamate spontaneously breaks down into carbonic acid (H2CO3) and another molecule of ammonia. Besides catalyzed urea decomposition reaction by urease, it can also undergo an uncatalyzed elimination reaction [8,9,13] , that gives cyanate ( -OCN) and ammonia (NH3), but this reaction is known to be 10 10 slower than the catalyzed urea hydrolysis [13]. Under physiological pH, the carbonic acid formed from catalyzed and uncatalyzed chemical decomposition reaction of urea, dissociates to release protons (equation 1.1), and the ammonia molecules become protonated after equilibrating with water, and form ammonium (NH4 + ). This ultimately results in an overall increase in pH from pH 6 to pH 9 [8,10], which triggers the precipitation of calcium (Ca) and magnesium (Mg) in the form of carbonates and phosphates, causing pipeline clogging in the selective urine collection systems. Urine: from a problematic pollutant to a valuable liquid gold

The conventional treatment technologies of urine wastewater such as adsorption, chemical, biological, and physico-chemical methods are still widely used due to their simplicity and ease of control [11,15]. However, these conventional methods are known to have certain drawbacks such as low efficiencies and limited capacities [16]. In recent decades, the Chapter 1 -Introduction electrochemical methods have received increased attention as promising alternatives for urine wastewater treatment. Figure 1.4 shows some of the most relevant electrochemical approaches.

The technique involves the oxidation of urea at the anode into environmentally benign products N2 and CO2. Meanwhile, water is reduced at the cathode to generate hydrogen (H2).

This method of electrolysis, which uses an eco-friendly approach to produce hydrogen, is rapidly gaining attraction as a replacement for carbon-based techniques. As a result, the increasing attention towards urea electrochemical oxidation is due to its potential for simultaneously treating wastewater and generating hydrogen.

The electrochemical oxidation of urea has been investigated in the presence of chlorides at neutral pH [17][18][19][20] using Pt, and in the presence of sodium or potassium hydroxides (NaOH, KOH) at alkaline pH using Ni-based catalysts. Chloride containing solutions in the form of "active chlorine" (hypochlorite ions), have been used to boost the electrochemical oxidation of urea [17][18][19][20] at neutral pH due to their ability to react with urea to form N2 and CO2, through Reaction 1.3: NH2-CO-NH2 + 3 ClO - N2 + CO2 + 3 Cl -+ 2H2O (1.3) However, no quantitative measurements were performed to provide evidence for the formation of N2. Li et al. [21] compared the performance of boron doped diamond (BDD) and IrO2 anodes for urine electrochemical oxidation. Generally, the generated hydroxyl radicals are weakly physisorbed onto BDD, while they are chemisorbed onto IrO2, leading to the availability of active hydroxyl radicals in the case of BDD, but not for IrO2. In the case of BDD, the electrolysis analysis showed that active chlorine remained stable during the urea oxidation, indicating that active chlorines were not responsible for the process, and the majority of organic compounds were oxidized through the hydroxyl radicals generated by BDD. Hypochlorite ions (ClO -) were responsible for the indirect oxidation of ammonia that was produced by urea hydrolysis, resulting in the formation of NO3 -. In the case of IrO2, only hypochlorite ion (ClO -) is present for the oxidation process. Consequently, an incomplete oxidation occurs due to the low oxidation activity of hypochlorite ions, and N2H4 and NO2 -are formed. In conclusion, these results show that the active hydroxyl radical is the main Chapter 1 -Introduction contributor for direct urine electro-oxidation, while hypochlorite ions are responsible for the indirect oxidation of NH3.

The use of alkaline solutions for urea electrolysis is much more prevalent than that of neutral solutions. This is because alkaline solutions offer many advantages, such as inhibition of urease activity that leads to ammonia production [12,22] and high ionic conductivity. In general, -1.23 V is required for hydrogen production via water electrolysis (Eqs.1.4 to 1.6).

In contrast, the thermodynamic potential for hydrogen production via UOR, which requires only 0.02 V (Eqs. 1.7,1.5 and 1.8), makes this approach increasingly appealing due to its costeffectiveness for hydrogen production. Electrocatalysts for urea electro-oxidation Urea electro-oxidation is a sluggish anodic reaction because it requires the transfer of 6 electrons (Eq.1.7). Thus, finding an effective catalyst is a key practical requirement for UOR in order to promote the oxidation process. In the past few years, a significant amount of scientific investigations reporting tests with different materials (Pt, Pt-Ir, Ti, Rh, Cu, Co, Ni…) in alkaline solution have been published [5,[24][25][26][27][28]. Sodium or potassium hydroxides (NaOH/KOH) have been employed to assess their influence on the efficiency of UOR. The first attempt to break down urea from urine involved noble metals, such as platinum [29], which studied the possibility of developing artificial kidneys for portable dialysis devices, but the high cost associated with such a noble metal prompted researchers to seek out a more affordable alternative.

Active Ni-species for urea electro-oxidation

To further confirm that NiOOH is the active form of nickel for the electro-oxidation of urea, in situ surface-enhanced Raman spectroscopy (SERS) was used [35] to observe the catalyst surface in real time and identify the composition of the active phases while applying a Chapter 1 -Introduction potential. Figure 1.7 illustrates potential-dependent SERS spectra of Ni(OH)2 catalyst in 5 M KOH in the absence (a) and presence (b) of 1 M urea. In Figure 1.7a, the potential-dependent SERS spectra obtained in 5 M KOH show a pair of broad, overlapping bands between 200 and 600 cm -1 below 0.34 V vs. Hg/HgO, which intensify and transform into two distinct peaks at 479 and 559 cm -1 above 0.36 V vs. Hg/HgO. These are characteristic of NiOOH [36,37] suggesting its formation from Ni(OH)2 oxidation at anodic potentials. When 1 M urea is added to the 5 M KOH solution (Figure 1.7b), the original NiOOH doublet peaks shifted to 476 and 558 cm -1 , and new peaks corresponding to the symmetric C-N stretch of urea appear at 1003 cm -1 [38]. The intensity of the symmetric C-N stretch remained stable in presence of Ni(OH)2, while it decreased with increasing the applied potential, indicating that the NiOOH catalyzes the electrochemical oxidation of urea. Several techniques have been employed to investigate the mechanism in detail, including SERS [35] and X-ray diffraction (XRD) [39], allowing to observe the catalyst surface changes in real time, thus enabling a better understanding of the reaction mechanism pathways and facilitating a more conclusive analysis. The combination of XRD and electrochemistry provides insights into the structural modifications of the nickel catalyst during the electrochemical urea reaction [39]. In conclusion, the study suggests that the electro-oxidation of urea on Ni(OH)2 catalyst in alkaline medium follows the electrochemical (EC) mechanism, which is mediated through the NiOOH/Ni(OH)2 redox pair. ), there is a linear relationship between the limiting current and the square root of the rotation rate up to 500 rpm, suggesting that the urea oxidation reaction is primarily controlled by mass transport under those conditions. In contrast, beyond 500 rpm, the reaction is rotation rate independent, indicating that it is mostly governed by kinetics.

However, there is not a clear current "plateau" that is typical in voltammetry with RDE, suggesting that the surface blocking of the NiOOH catalyst during the urea oxidation reaction may occur. Nevertheless, it is crucial to highlight that in order to build a Levich plot, the current density should be selected at the point where the electrochemical system reaches a limiting current, characterized by a plateau on the CVs. However, in Figure 1.10 of the present study, such a current plateau was not observed in the obtained CVs. As a result, the Levich plot presented in this study needs further validation. Observing the current literature, only a limited number of studies have been conducted utilizing a RDE. Consequently, additional research in this area is necessary. In this thesis, we aim to contribute to this field of study by performing a deeper investigation of urea electrooxidation with a rotating disk electrode.

Effect of urea concentrations on urea electro-oxidation

The first studies performed by Botte et al. [34] showed the main focus on observing the variation of the cathodic peak associated with the reduction of Ni(OH)2 to NiOOH in CV at different concentrations of urea.

Effect of the [electrolyte]/[urea] ratio

As previously stated, the concentration of urea in urine is approximately 0.33 M, which is why this concentration is commonly used in urea electrolysis studies. However, varying the concentration of urea can provide additional insights into its mechanism, namely whether it is limited by kinetics or diffusion. Additionally, studying the effect of the Celectrolyte/Curea ratio may provide information about the role of the OH -ions present in the electrolyte and the effect of the solution viscosity on the electrocatalytic activity of urea oxidation [34,[40][41][42].

The electrolytes used in alkaline solutions were mostly sodium or potassium hydroxides (NaOH/KOH). Figure 1.13a and b illustrate the influence of urea concentration on the electrocatalytic activity of the Ni2P@Ni foam catalyst for the UOR [41]. The KOH concentration was held constant at 5 M in that case, while the urea concentration ranged from 0.02 M to 0.10 M (Figure 1.13a) and from 0.2 M to 1 M (Figure 1.13b). In Figure 1.13a, the addition of urea significantly enhanced the anodic current density, and this increase continued until the urea concentration reached 0.60 M (Figure 1.13b), and started to decrease beyond 0.8 M urea [41].

Chapter 1 -Introduction

This can be explained by the fact that, significant surface coverage of urea molecules on nickel, results in a decrease in the oxidation rate of urea, as necessary OH -species become locally depleted [34].

To achieve the optimal hydroxide OH -and CO(NH2)2 ratio, (Eq. 1. In conclusion, the presence of higher concentrations of hydroxide ions seems to promote more rapid regeneration of Ni(III), resulting in increased catalytic activities and a shift in the onset potential. If the ratio [electrolyte]/[urea] is below 8, the adsorbed OH -ion coverage on the catalyst surface decreases due to the increased surface coverage of urea and its intermediates, leading to a decay in urea electro-oxidation rate [34,41,42].

Effect of by-products from urea electro-oxidation

Several studies have reported that the rate of urea electrolysis on nickel-based electrodes decreases over time. Figure 1.14a shows the chronoamperometry curve of UOR on LaNiO3, highlighting a decreasing current over time. Effect of CO2/CO3 2- CO2 has been already considered among the species that are potentially responsible for catalyst deactivation. Forslund et al. [40] reported that in a strong alkaline solution, CO2 can react with Ni-based perovskite electrodes, leading to the formation of a new stable surface phase such nickel carbonate NiCO3, which was confirmed through Raman spectroscopy. In addition, Tewari et al. [43] suggested that the precipitation of K2CO3 salt blocks the pores of the gas diffusion electrode (GDE), thus it can be considered as responsible for blocking the nickel catalyst surface. Furthermore, Guo et al. [44] conducted a study on urea electro-oxidation using EIS as shown in Figure 1.16. According to those authors, this is evidenced from the EIS diagrams, where a negative polarization resistance value is observed, resulting in a reverse loop at potentials relevant to urea oxidation (between 0.55 V and 0.59 V vs. Ag/AgCl), suggesting catalyst deactivation. And, at potentials higher than 0.59 V vs. Ag/AgCl, the impedance resistance increases again, forming a positive loop, likely due to the concurrent oxygen evolution Chapter 1 -Introduction reaction (OER), which releases gas bubbles and clears the electrode. However, as noted in literature [START_REF] Macdonald | Review of mechanistic analysis by electrochemical impedance spectroscopy[END_REF][START_REF] Keddam | Whitney Award Lecture: Application of Advanced Electrochemical Techniques and Concepts to Corrosion Phenomena[END_REF] and as we discuss in detail in Chapter 4, a negative value of polarization resistance (Rp) occurs when the applied potential is located in the negative slope region of the CV, indicating a decrease in current with increasing potential. This negative value results in a reverse loop, irrespective of the presence or absence of any adsorption. Thus, the impact of CO2 in the urea electro-oxidation deactivation needs again to be reviewed.

Effect of CO

A similar analysis was conducted for studying CO effect by Guo et al. [START_REF]Preparation of nickel nanowire arrays electrode for urea electro-oxidation in alkaline medium[END_REF] as well. Those authors suggested that the reverse loop displayed by EIS in the Nyquist plot is caused by the blockage of CO on the nickel nanowires surface (Figure 1.17). However, as previously mentioned, the presence of the reverse loop is not necessarily linked to an adsorption process. Furthermore, Forslund et al [40] demonstrated that the current density associated with UOR decreased by the same percentage with and without CO in solution, indicating that catalyst deactivation/poisoning is not solely caused by CO. Wang et al. [4] performed in situ FTIR analysis and found no CO bands on the electrode surface during urea electro-oxidation, which weakens the evidence of the role of CO in catalyst poisoning/deactivation. Deep studies for understanding the formation of products/by-products during urea electro-oxidation During urea electrolysis under neutral and acidic pH conditions, researchers have observed the generation of various by-products including CO2, N2, H2, -OCN, NO3 -, NO2 -, [N2O2] -, N2O, and NO2 [15]. In contrast, in alkaline pH, N2 has been considered the primary product of UOR.

However, the evidence for N2 formation has been limited, with only a few qualitative analyses conducted in the past decades, while most studies have focused on finding catalysts to reduce overpotential and enhance urea oxidation rates, rather than placing significant importance on selectivity towards N2 production. Only recently, in 2021, a study [START_REF] Li | Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis[END_REF] mentioned for the first time the formation of -OCN, NO2 -, NO3 -and CO3 2-as by-products during urea electrolysis in alkaline medium. Subsequently, in 2022 [START_REF] Tatarchuk | Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution[END_REF] another research article also confirmed the presence of these previously mentioned by-products. In 2023, our collaborator from LGC-Toulouse [START_REF] Hopsort | Progress toward a better understanding of the urea oxidation by electromediation of Ni(III)/Ni(II) system in alkaline media[END_REF] reported further findings and identified a new by-product, which is ammonia (NH3). Figure 1.18 shows the results obtained by LGC-Toulouse from long-term urea electrolysis on nickel electrode in 0.33 M urea and 1.5 M KOH solution [START_REF] Hopsort | Progress toward a better understanding of the urea oxidation by electromediation of Ni(III)/Ni(II) system in alkaline media[END_REF]. Ion chromatography (IC) analysis of the electrolyte composition showed the presence of different C-and N-species, including cyanate ( -OCN), nitrite (NO2 -), carbonate (CO3 2-), and ammonium (NH4 + ) in the form of ammonia (NH3) since the pKa of NH4 + /NH3 = 9.25. The Chapter 1 -Introduction nitrogen mass balance presented in Figure 1.18b shows that the primary nitrogen byproducts from urea oxidation in an alkaline environment are NH4 + , NO2 -, and -OCN. These byproducts accumulate over time, and ammonium is the main species at more than 44 mol% at the end of 45h of electrolysis. On the other hand, the carbon mass balance presented in Figure 1.18c indicates that all the consumed urea is converted into either CO3 2-or -OCN. Chapter 1 -Introduction So far, two research studies [START_REF] Li | Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis[END_REF][START_REF] Tatarchuk | Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution[END_REF] have suggested new insights into mechanism pathways involved in urea electro-oxidation. These studies shed light on the formation of the aforementioned by-products namely, cyanate ( -OCN), nitrite (NO2 -), nitrate (NO3 -), and carbonate (CO3 2-). The proposed pathways in both studies are quite similar and lead to the formation of nitrite (NO2 -), nitrate (NO3 -), cyanate( -OCN), carbonate (CO3 2-) and nitrogen (N2).

Finally, it has been also confirmed by IC analysis [START_REF] Tatarchuk | Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution[END_REF] that regardless of the potential applied during urea electrolysis, the concentration of -OCN produced was almost the same as the concentration of NOx products all along urea electrolysis. This almost one-to-one ratio implies that the formation of -OCN is closely linked to the mechanism of NOx -production, whereas the N2 pathway does not contribute to the generation of -OCN [START_REF] Tatarchuk | Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution[END_REF]. Based on those results, two mechanism pathways have been proposed in the literature. A clear illustration of both mechanism pathways for UOR are presented in Figure 1. 19, where either intramolecular N-N coupling leading to N2 and carbonate (CO3 2-) products Figure 1.19b or C-N cleavage leading to nitrogenous oxides (N2O, NO2 -and NO3 -) and cyanate ( -OCN) Figure 1.19a are described. Figure 1.19a shows the C-N cleavage pathway towards NOx -and -OCN formation. As a result, when cyanate ( -OCN) and nitric oxide (NOx -) are formed, only one nitrogen atom is oxidized by the electrocatalyst surface to produce NOx -, since the cyanate and urea exhibit the same oxidation state. Thus, the bond between the second nitrogen atom and a carbon atom in urea undergoes C-N cleavage to form cyanate, which is then released into the solution. Meanwhile, the remaining *NH species on the surface undergo further oxidation steps to form nitrogenous oxides NOX -. Figure 1.19b shows the pathway towards N2 and CO3 2-The formation of N2 occurs due to the oxidation of both N centers in urea. This process needs to bind both N centers in urea to the catalytic active sites, which allows the electron transfer from both N centers to the anode.

It should be noted that the over-oxidation pathway to form NOx -is more energetically favorable than the N2 pathway, as has been already demonstrated by DFT calculations [START_REF] Li | Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis[END_REF].

Consequently, during UOR on Ni-based catalysts, the NOx -product is highly favored. However, it is noteworthy that there is currently no explanation provided regarding the pathway

The thesis Summary

Nowadays, the treatment of wastewater has become a critical environmental concern due to the harmful impact of pollutants on the environment. One pollutant that poses a significant threat is NH3, which is known to contribute to the harmful phenomenon of eutrophication [1]. Notably, urine is a significant contributor of 80% of nitrogen present in wastewater [4].

More specifically, the main nitrogen compound in urine is urea CO(NH2)2, but its biological degradation by urease enzyme leads to its transformation into NH3. Consequently, urea electro-oxidation to produce innocuous N2 represents a sustainable technology for achieving nitrogen removal from polluted water. UOR is typically conducted in an alkaline medium; however, its sluggish kinetics needs the use of a suitable electrocatalyst as anode. Extensive research has identified Ni-based catalysts as highly effective for UOR, but suffers deactivation. Thus, studying the UOR mechanism and particularly the reaction pathway and the individual role of each by-product in the catalyst deactivation is of major interest. In addition to this, production of green H2 at the cathode from electrochemical processes represents a very hot topic nowadays. Urea electro-oxidation has already been proposed as an alternative to water splitting for hydrogen production, as the electrolysis of urea requires Chapter 1 -Introduction a lower cell potential (0.02 V) in comparison with water electrolysis (-1.23 V). However, contradictory values of the thermodynamic standard potential for UOR have been reported in the literature and need to be verified.

Motivation

Numerous research studies on the electro-oxidation of urea have been conducted using Nibased catalysts. However, to the best of our knowledge, research on the use of NiO nanoparticles (NPs) for electro-oxidation of urea is very limited, despite their high stability in alkaline solution and their strong activity for UOR. Furthermore, few mechanistic studies have been conducted using the rotating disk electrode (RDE) combined with operando EIS and operando UV-vis SEC. Those characterization techniques can provide valuable insights about the reaction mechanism. Thus, the performance of commercially available NiO NPs for the electro-oxidation of urea was investigated in this thesis by operando EIS and UV-vis SEC using a RDE.

Recent studies have indicated the formation of various products during the electro-oxidation of urea, extending beyond the traditionally studied N2 and CO3 2-. These studies have clarified the pathways leading to the formation of cyanate ( -OCN), carbonate (CO3 2-), nitrite (NO2 -), nitrate (NO3 -), and nitrogen (N2) as by-products of UOR. However, the mechanistic pathway for the formation of ammonia (NH3) during urea electrolysis [START_REF] Hopsort | Progress toward a better understanding of the urea oxidation by electromediation of Ni(III)/Ni(II) system in alkaline media[END_REF] remains unclear. In this thesis, we will investigate the origin of NH3 formation. Furthermore, according to the literature, the NiOOH anode exhibits significant and progressive catalytic deactivation during the electro-oxidation of urea [22,23]. Previous studies have only considered CO2 or CO3 2-and CO as potential-responsible for the NiOOH deactivation, but leading to inconclusive results. Therefore, to clarify this matter, this thesis examines the impact of CO3 2-as well as all other detected by-products (NH3, -OCN, NO2 -) using operando EIS.

The thesis Objectives

1) The primary objective of this study is to gain a comprehensive understanding of the mechanism underlying UOR on Ni-based catalysts, primarily focusing on the utilization of NiO NPs. These NPs are known for their exceptional stability and catalytic activity, making them suitable candidates for potential usage in the final reactor setup. To achieve this, a combination of techniques including cyclic voltammetry, RDE, EIS and spectroelectrochemistry are employed to analyze the electrochemical behavior and performance of the NiO NPs.

2) To investigate the performance and stability of the NiO NPs in alkaline medium by chronoamperometry (CA) and EIS.

3) The by-products formation from UOR on NiO NPs are monitored by UV-vis SEC. 4) To examine the impact of the products and by-products generated during UOR on the stability and performance of NiO NPs by using operando EIS. Additionally, to investigate the electrode aging by EIS to determine any potential degradation or deactivation mechanism that may hinder its performance over time.

5) Due to the discrepancy in the literature regarding the proposed values for the standard thermodynamic potential of urea (E 0 ), we conducted our own thermodynamic calculation to determine the more accurate value. As E 0 value is very relevant and plays a crucial role in evaluating the catalyst performance, it provides valuable insights into the energy required for simultaneous wastewater treatment and hydrogen production.

Chapter 2 Materials and Experimental methods

This chapter discusses the reagents utilized to prepare the catalyst ink, electrolyte solution, the by-products of urea electro-oxidation reaction and the artificial urine matrix evaluated for directing future research on this topic. It also covers the various electrochemical techniques employed to evaluate the performance of Ni-based catalysts for urea electrooxidation, and to identify the by-products or products generated during the reaction. RDE is a commonly used as convective electrode system in an electrochemical analysis, and primarily utilized to investigate electrocatalytic reactions [1][2][3][4][5]. It consists of a disk-shaped working electrode that rotates at a controlled speed by a motor. Figure 2.1a shows a welldefined liquid flow pattern in a laminar regime induced by the electrode rotation, that consistently transports material from the bulk solution to the electrode surface, resulting in a controlled diffusion layer (δ). RDE is usually coupled with other electrochemical methods, such as cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS), since it offers a well-defined hydrodynamic behavior of the electrochemical system under study.

Reagents and Materials

The RDE contains a conductive planar disk embedded (WE) into an inert insulating material (PTFE). The working disk electrode can be made from a range of conductive materials, including glassy carbon (GC), graphite carbon, nickel (Ni), platinum (Pt), or gold (Au), among others, depending on the specific needs and requirements of the application.

Levich approach (diffusion control)

An electrochemical reaction studied by RDE under conditions where the mass transport is limited by diffusion. Then, it is possible to generate the Levich plot by plotting the limiting current (𝐼 D ) of an electrochemical reaction against the square root of the rotation speed (𝜔 1/2 ). Levich plot provides valuable information about the reaction kinetics by calculating the diffusion coefficient of the electroactive species, the thickness of the diffusion layer and the number of electrons involved in the electron transfer reaction. Levich was the first to solve the equations for the rotating disk electrode, and the limiting current is given by the Levich Equation 2.1:

𝐼 𝐷 = (0.62 𝑛𝐹𝐴𝐷 0 2/3 𝐶𝑣 -1/6 )𝜔 1/2 (2.1)
where 'ID' is the diffusion-controlled limiting current (A), 'n' is the number of electrons transferred in the reaction, 'F ' is Faraday constant (96485 C mol -1 ), 'A ' is the electrode area (cm 2 ); 'D0' is the diffusion coefficient (m 2 s -1 ); 'υ' is the kinematic viscosity (m 2 s -1 ); 'C ' is the reactant concentration in bulk solution (mol m -3 ); 'ω' is the angular rotation rate of the RDE (rad s -1 ).

And the diffusion layer thickness (𝛿) is given by Equation 2.2:

𝛿 = (1.61 𝐷 0 1/3 𝐶𝑣 1/6 )𝜔 -1/2 (2.2)

Koutecký-Levich approach (diffusion-kinetics control)

Levich behavior applies when the current in an electrochemical reaction is only limited by diffusion. However, when the current is controlled by both diffusion and electrode kinetics, Koutecký-Levich approach can be applied. It involves plotting the reciprocal limiting current

(1/IL) against the reciprocal square root of the angular rotation rate (1/ω 1/2 ). This method can reveal whether the reaction is limited by sluggish kinetics or mass transport. In particular, if the line of the plot doesn't intersect the vertical axis at zero, it indicates that the reaction is limited by sluggish kinetics, rather than mass transport. Thus the current IL (A) is

given by the Koutecký-Levich Equations 2.3 and 2.4:

1 𝐼 𝐿 = 1 𝐼 𝑘 + 1 𝐼 𝐷 (2.3)
where 'IL' is the limiting current (total current) and Ik is the kinetic controlled current (A).

1 𝐼 𝐿 = 1 𝐼 𝑘 + 1 (0.62 𝑛𝐹𝐴𝐷 0 2/3 𝐶𝑣 -1/6 )𝜔 1/2 (2.4)

RDE setup used in this thesis

In this thesis, Ni-based catalysts for urea electro-oxidation were evaluated using the rotating disk electrode. As shown in Various in situ/operando characterization techniques have become increasingly prominent for investigating reaction mechanisms in electrochemistry, since their ability in monitoring the real-time formation of intermediates/products and/or the tracking of the electrocatalysts surface modifications under realistic reaction conditions. In our mechanistic study of UOR, we primarily used two of those in situ/operando techniques i ) operando EIS that will be explained in detail in the following section and ii ) operando UV-vis spectroelectrochemistry that will be explained in detail in section 2.4.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical reactions at the electrode-electrolyte interfaces involve different processes that occur on different time scales. These processes include mass transport of Chapter 2 -Materials and experimental methods reactants/products to/from the electrode surface, charge transfer between the electrode and the electrolyte, and adsorption of species onto the electrode surface. However, DC (direct current) electrochemistry poses a difficulty in identifying individual processes due to their simultaneous occurrence. But, as the individual steps are time-dependent and occur at different time scales, EIS allows time-dependent mechanisms to be analyzed based on the current or potential response of the electrochemical system at selected frequencies [6].

Moreover, EIS involves the application of two different sinusoidal modes, namely potentiostatic and galvanostatic mode. In potentiostatic mode, a periodic potential perturbation E(t)=E0 sin(ωt) is applied, whereas in galvanostatic mode, a periodic current perturbation i(t)=i0 sin(ωt) is applied. Where ω is the angular frequency, related to the frequency 'f ' (Hz) by ω = 2πf. The current or potential response of the system to this perturbation is measured by the magnitude of impedance Z (Ω), which is the ratio of the frequency-dependent potential over frequency-dependent current (Equation 2.5):

𝑍 = 𝐸 (𝜔) 𝑖(𝜔) (2.

Linearity and Causality verification

Choosing the appropriate amplitude for EIS is crucial. The amplitude should be high enough to impede the system, but not so high that the system deviates from linearity during the measurement. Kramers-Kronig relationships [6] can be used to test whether the impedance response of the system is consistent with causality and linearity, which are essential requirements for accurate interpretation of impedance data and modeling of electrochemical Chapter 2 -Materials and experimental methods processes. Moreover, RDE can be coupled with EIS to ensure a defined transport of reactants and products towards the electrode surface, thereby regulating the diffusion layer.

EIS setup used in this thesis

In this thesis, the surface of the working electrode was assessed using EIS to determine the impact of urea, by-products and products generated during UOR on the working electrode [8]. The by-products detected included cyanate ( -OCN), carbonate (CO3 2-), ammonia (NH3)

and nitrite (NO3 -).

In an alkaline medium, ammonium (NH4 + ) predominantly exists in the form of NH3 (pKa = 9.2) thus ammonium sulfate is used as a source for ammonia, in order to avoid direct usage of ammonia due to its chemical risk. As previously mentioned (section 2.2.3) the solution is previously saturated with Argon. In this case, argon gas used for purging the solution was passed through a bubbler containing the same concentration of ammonia and electrolyte in order to prevent the evaporation of ammonia in the studied solution. This approach ensured that the argon gas remained saturated with ammonia, effectively preventing any loss of ammonia through evaporation into the surrounding atmosphere.

The setup employed is shown in Figure 2.2, which is identical to the RDE setup, except for the addition of a shunt that is connected in parallel to the RE. The reason for incorporating this shunt will be explained in detail later on the next section (2.3.3). All EIS measurements were performed using RDE for controlling the diffusion layer. EIS was used in potentiostatic mode, within a frequency range of 100 kHz -10 mHz, using a sinewave perturbation of 10 mVrms, and 12 points per decade.

Chapter 2 -Materials and experimental methods

Reference electrode contribution at high frequency

One of the challenges in EIS is the effect of the reference electrode, which can cause inaccurate impedance results, especially at high frequencies. At high frequencies the reference electrode impedance may decrease, leading to increased current flow through it and resulting artefacts in impedance measurements. To address this issue, a shunt (small capacitor that is in series with a platinum wire) can be placed in parallel with the reference electrode, as shown in Figure 2.3b and is described in literature [6]. The choice of capacitor value is influenced by the resistivity of the electrolyte. As demonstrated in Figure 2.3a, the Nyquist plot of NiO NPs in 5 M NaOH at high frequency reveals a small semi-circle at high frequency (black plot), which is attributed to the RE. Initially, a capacitor of 0.68 µF (red plot) was utilized, but the results indicated that the RE contribution remained. Subsequently, a 1.6 µF capacitor (green plot) was tested, and it led to the complete correction of the RE contribution.

Two operation modes of EIS

The appropriate mode of EIS to be used depends on the magnitude of the impedance (Z) being measured. If the impedance Z is large (in the kΩ range), then a potentiostatic mode with an amplitude of 5 to 15 mV can be used. In this mode, the resulting current response can be calculated using the formula i=E/Z, and it will be in an appropriate range (mA). However, if the impedance Z is small (in the mΩ range), the resulting current response will be very high (A) for a potentiostat, and it may not be possible to use a potentiostatic mode. In this case, galvanostatic mode can be utilized, and by the formula E=Z×i we can say that the potential response will be in an appropriate range (mV). Overall, the choice of the operation mode depends on the impedance range of the system being tested. Most systems tend to have high impedance, which means that the potentiostatic mode is typically used in EIS measurements.

UV-vis spectroelectrochemistry

Spectroelectrochemistry is an effective technique for investigating reaction mechanisms and detecting reaction products in real time and operando, making it a valuable tool in electroanalysis [9][10][11]. The coupling of electrochemistry with the UV-vis absorption spectroscopy is particularly successful among SEC techniques; it shows changes in the absorption spectrum during electrochemical oxidation or reduction, as a result of products/by-products generation and the consumption of the reactant molecule. UV-Vis spectroscopy implies high-energy electromagnetic radiation, which is divided into two wavelength ranges: ultraviolet (200-400 nm) and visible (400-800 nm). Many compounds have absorption features in the UV-visible light range, which accounts for the broad applicability of this technique with and without electrochemistry.

SEC setup used in this thesis

The SEC technique was conducted using Metrohm (SPELEC) [9,10] equipment, which includes a bi-potentiostat connected to a spectrometer that is connected to a light source.

Disk electrode cleaning

Glassy carbon disk electrode was mainly used in this thesis, and a few experiments were done on Ni disk electrode (PINE, 5 mm OD). Both electrodes were previously polished before each experiment to ensure a clean and smooth surface, free from any contaminants or rough spots that may have accumulated from previous experiments. The polishing involves gently abrading the electrode surface with a small amount of 0.05 µm alumina powder on a polishing cloth for 3 minutes. Afterwards, the electrode is sonicated for 5 minutes with ultrapure water (18.2 MΩ cm) to remove any remaining alumina particles.

To ensure the removal of any oxide layer that may have formed from previous experiments, the GC electrode is immersed in 0.5 M sulfuric acid (H2SO4) solution for 5 minutes. Chapter 2 -Materials and experimental methods Hg/HgO, respectively, which may be attributed to some NiO remaining from previous experiments. This is difficult to remove through polishing alone. However, CV (blue line)

shows that, after immersing the electrode in diluted sulfuric acid solution, the oxidation and reduction peaks disappear. and after (blue) diluted sulfuric acid solution treatment.

Catalyst ink preparation

After polishing the GC disk electrode, a catalytic ink was prepared to be dropcasted on the surface of the electrode. The NiO catalytic ink consisted of 9 mg of NiO NPs that were dispersed in 1 mL of isopropanol through 20 minutes of sonication. To ensure that the NPs remained attached to the GC electrode during the measurements, 7 µL of a 5 wt. % Nafion™ perfluorinated resin solution (5 wt. %) was added as a binder [12,13]. Then, the mixture was re-sonicated for an additional 15 minutes and 4 µL of the resulting mixture was drop-casted onto the polished GC disk electrode, which was dried in an open atmosphere. The catalyst loading was 0.225 mg cm 2 .

Chapter 3 Urea electro-oxidation on Ni-based anodes 3.1. Introduction

In the face of energy shortage and the pressing need to combat environmental pollution, urea-rich wastewater or urine (240-470 mM urea) [1] electro-oxidation in alkaline medium emerges as a promising solution. It addresses two critical challenges simultaneously:

effectively treating wastewater to produce clean water at anode by urea electrolysis (Eq.3.3 in Table 3.1) and generating hydrogen as a renewable energy source at the cathode by water reduction (Eq. 3.9 in Table 3.1). This approach offers a sustainable alternative to alleviate energy shortage while reducing the harmful impact of wastewater discharge, such as eutrophication caused by the release of excessive ammonia (NH3).

It is important to note that the literature reported different values for the urea electrooxidation standard potential (E 0 ), prompting the need for clear calculations to determine the appropriate value. One aspect of this chapter will focus on addressing this matter by presenting a clear calculation of the standard potential for urea electro-oxidation. This parameter holds significant importance as it serves as a crucial factor in evaluating the catalyst performance and provides valuable insights into the energy required for wastewater treatment and hydrogen production. Furthermore, recent studies have indicated the formation of various products during urea electro-oxidation that go beyond the traditionally studied N2 and CO2 [2][3][4][5]. Therefore, these calculations will take into account a new range of products that have been recently detected during urea electro-oxidation. The additional products to be considered include nitrite (NO2 -), nitrate (NO3 -), cyanate ( -OCN) and nitrous oxide (N2O). Figure 3.1 summarizes the different detected by-products during UOR. As generally known, the water electrolysis for hydrogen production by Eqs. 3.8 and 3.9

requires (E 0 cell = -1.23 V), while urea electrolysis by Eqs. 3.3 and 3.9 requires (E 0 cell = E 0 cathode -E 0 anode = + 0.02 V) for hydrogen production. The positive value of the standard cell potential indicates that the reaction should be spontaneous from a thermodynamic point of view. However, as stated in Chapter 1, urea electro-oxidation suffers from a sluggish anodic reaction, primarily attributed to the 6-electrons transfer process (through reaction Eq. 3.3).

Botte et al. and subsequent researchers [6][7][8][9][10][11], have found that nickel-based catalysts in alkaline media exhibit a superior performance in the electro-oxidation of urea when compared to other noble and non-noble catalysts. Thus, numerous investigations on urea electro-oxidation have been conducted using nickel-based catalysts. However, to the best of our knowledge, there is a shortage or very limited research on the use of NiO NPs for urea electro-oxidation, despite the high stability of NiO in strong alkaline medium compared to Ni 0 . Thus, this chapter focuses on examining the performance and stability of commercially available NiO NPs for urea electro-oxidation.

A glassy carbon (GC) rotating disk electrode (RDE) was used as an inert substrate to deposit and study the performance of NiO NPs during UOR. This technique provides a well-defined hydrodynamic behavior of the electrochemical system, since the rotation rate controls the thickness of the diffusion layer. In addition, by employing analytical approaches such as Levich or Koutecký-Levich analysis [12][13][14][15], it becomes possible to gain insights into the reaction kinetics. Additionally, the impact of electrolyte concentration and the urea-toelectrolyte ratio were examined in this study in order to provide additional insights into the mechanism, and help to determine the optimal ratio of electrolyte to urea that maximizes the reaction rate of urea while minimizing the amount of electrolyte required. The current density was calculated by considering the geometric area of the RDE (0.196 cm 2 ).

Furthermore, recent publications [2,3] have provided new data to clarify the pathways leading to the formation of cyanate ( -OCN), carbonate (CO3 2-), nitrite (NO2 -), nitrate (NO3 -), nitrous oxide (N2O) and nitrogen (N2) as by-products from UOR. However, a reaction pathway for the formation of ammonia (NH3) during urea electrolysis [5] remains unclear.

Thus, in this chapter we present new operando UV-vis spectroelectrochemistry (SEC) data, which have been collected thanks to the collaboration with Dr. J.V. Perales-Rondón and Pr. A Colina at Burgos University in Spain. This research team is well recognized for their experience in spectroelectrochemical studies [16][17][18][19], but they never studied before the UOR. The operando SEC technique offers an enhanced understanding of the formation of byproducts during an electrochemical reaction, either adsorbed on the surface of the electrode or within the diffusion layer (the setup is described in section 2.4.1). Operando SEC enables the observation of changes in the absorption spectra when the polarization potential of the electrode reaches the point of product formation or reactant consumption. Consequently, operando SEC was applied here for the first time to study UOR on NiO NPs, providing valuable insights into the dynamics of the electrochemical reaction and shedding light on the intermediates and pathways involved in this process. In addition to this, the chemical stability of CO(NH2)2 and -OCN in aqueous solution was investigated by ion chromatography (IC) analysis and have been conducted within the framework of the ANR HYUREA project and particularly, in collaboration with the LGC team -Toulouse, France. The thermodynamic standard potential (E 0 ) is one of the primary criteria to evaluate the performance of an electrocatalytic reaction, since it indicates the minimum energy (as applied potential) required to make an electrochemical reaction take place. The overpotential (η) of an electrochemical reaction such as the urea oxidation reaction corresponds to the difference between the applied potential (Eapp) necessary for reaching a given current and the thermodynamic standard potential E 0 . Thus, the overpotential represents a key parameter for assessing electrocatalyst activity based on reaction efficiency [13]. The presence of an electrocatalyst is crucial in most electrochemical reactions for diminishing the reaction overpotential and promoting the reaction efficiency. Reporting accurate E 0 values for the electro-oxidation of urea is therefore very important.

Results and Discussion

Unfortunately, different E 0 values have been reported in the literature in the case of urea electro-oxidation. Moreover, the conversion of both nitrogen atoms within the urea molecule Remarkably, E 0 value for urea electro-oxidation to N2 in alkaline medium have been differently reported by Boggs et al. [6] -0.46 V and by Yang et al. [20] -0.746 V, which means almost 300 mV of difference. In addition to this, Guo et al. [21] also reported -0.746 V, despite these authors have considered the generation of CO3 2-in alkaline medium as a by-product rather than CO2. However, the same E 0 value is reported in both articles despite the Gibb's free energy for CO3 2-is different from the one for CO2 (aqueous or gas). In our opinion, it is evident the need of some additional work in this area, since as mentioned before, the E 0 value is an essential parameter to evaluate the electrochemical reactions efficiency, and literature (including some review articles [22][23][24][25][26][27]) shows dispersion of E 0 values. In addition to this, the production of highly oxidized N-species different from N2 has not been considered in detail so far. For all those reasons, we have included our own calculated E 0 values for urea electro-oxidations in Table 3.1. All Gibb's free energy values necessary to perform these calculations applying Eqs. 3.13 and 3.14 were obtained from the literature [28]. In which:

ΔG 0 is the Gibbs free energy (J mol -1 ) ν is the stoichiometric coefficient

E 0 cell is the standard cell potential (V)
n is the number of electrons transferred F is the Faraday constant (96485 C mol -1 )

Chapter 3 -Urea electro-oxidation on Ni-based anodes Initially, Table 3.1. presents the calculated E 0 values for urea electro-oxidation to N2 involving 6 electrons by considering different possible forms of CO2 in alkaline media (aqueous (Eq.

3.1), gas (Eq. 3.2), and CO3 2-(Eq. 3.3)). This fact provokes up to 110 mV of dispersion in E 0 values, being E 0 = -0.85 V vs. SHE the most likely one, because it considers the realistic approach of forming CO3 2-in alkaline solution (pH > 13.5

). An alternative reaction pathway for urea electro-oxidation [2], which considers the formation of two different N-species also involving 6 electrons is reported in equation 3. 4. In particular, one of the N atoms contained in urea remains without any change in oxidation state ( -OCN) and the other N atoms gets oxidized (NO2 -), being E 0 = -0.43 V vs. SHE in this case. In addition to this, the production of highly oxidized N-species (N2O, NO2 -and NO3 -) from urea electro-oxidation is considered as well. Thus, formation of N2O involving 8 electrons (Eq. 3.5), two NO2 -by 12 electrons (Eq.

3.6) or two NO3 -by 16 electrons (Eq. 3.7), exhibit E 0 values -0.40, -0.22 and -0.16 V vs. SHE, respectively, which are between 450 and 690 mV less negative than the E 0 value corresponding to the production of N2 (Eq. 3.3). Some discrepancies appear again when comparing with the E 0 values for Eqs. 3.6 and 3.7 in the literature, which were reported -0.166 and -0.126 V vs. SHE [3]. Urea electro-oxidation reactions display, independently of the N-product formed, a significantly more negative E 0 value than H2O electro-oxidation to produce oxygen gas (O2) (Eq.3.8). This fact should allow producing green H2 in alkaline solution (Eq. 3.9) by urea electro-oxidation at an overall cell potential (Ecell) between +0.02 and -0.67 V (depending on the oxidation pathway). Those thermodynamic Ecell values are significantly lower than green H2 production from H2O electrolysis (-1.23 V). However, the sluggish kinetics of urea oxidation reactions cause the appearance of high overpotentials.

NiOOH is one of the few electrocatalytic materials able to efficiently oxidize urea. For this reason, the E 0 value for the formation of NiOOH is also included in Eqs. 3.10 and 3.11 of Table 3.1 (0.49 V vs. SHE). Then, production of green H2 on NiOOH from urea electro-oxidations requires an Ecell of -1.32 V, a bit larger than the one required for H2O electrolysis. Thus, finding new highly active catalysts for urea electro-oxidations is crucial to minimize the overpotential and consequently the cost of the urea effluent treatment, as well as H2

production. In most cases NiO/NiOOH anode is used with NaOH as the electrolyte for UOR due to its cost effectiveness compared to KOH. However, in certain cases, such as in the section 3.2.4 of this thesis, KOH was used instead of NaOH because NaOH was not available. To explore the effect of the different cations Na + and K + , a comparative study was done.

t =0 h t =1 h t =2 h t =3 h t =4 h

Urea electro-oxidation on NiO/NiOOH anode

The investigations shown in this chapter were mainly performed by cyclic voltammetry and chronoamperometry with and without electrode rotation and either coupled or not to ultraviolet-visible spectroscopy (operando SEC). However, the results studying UOR by the electrochemical impedance spectroscopy (EIS) are reported in Chapter 4. In particular, we primarily focused on two urea concentrations: 0.01 M and 0.33 M, as well as two different To determine the optimal electrolyte concentration and its impact on the UOR mechanism, the effect of electrolyte (NaOH) concentration was investigated. In literature, few studies have been conducted on rotating disk electrode [36] for urea electro-oxidation, justifying further investigation in this work. In the present study, we explored urea electro-oxidation at different rotation rates (ω). As shown in CVs of Figure 3.11a, the current density of 0.33 M urea in 1 M NaOH at ω =0 (no rotation) and ω =800 rpm exhibited a similar range, indicating that the electro-oxidation process is independent of the rotation rate. In addition, Figure 3.11b presents the CA curve, which shows the current density evolution with time on NiO/NiOOH NPs in a solution containing 1 M NaOH and 0.33 M urea (R = 3) at 0.53 V vs. Hg/HgO, along with the influence of varying electrode rotation rate during CA. At this potential (0.53 V vs. Hg/HgO) the urea electro-oxidation reaction is under mass-transfer control conditions. Thus, it is typically expected that higher rotation rates lead to an increase in the limiting current density from urea electro-oxidation associated with a decrease in the thickness of the diffusion layer. However, the observed i-t curve in Figure 3.11b shows that the current density only increases when the rotation rate of the electrode increases from 0 to 400 rpm, but then gradually decreases over time regardless of the increase in the electrode rotation rate. This behavior showing mass-transfer control of urea electro-oxidation only at low rotation rates is in agreement with the literature [36] and

shows that the UOR is mainly limited by kinetics. Additionally, the current decrease observed in Figure 3.11 will be explained in detail in chapter 4. Figure 3.12. illustrates the influence of CO(NH2)2 concentration and the ratio (R) on the evolution of current density during urea electro-oxidation in CA. It is noteworthy that the current density achieved with a 0.33 M urea solution (R = 3) after 1600 s of electrolysis at constant potential (green plot in Figure 3.12) is equivalent to the current density obtained from 0.02 M urea solution with R = 50 (orange plot in Figure 3.12). Moreover, increasing the urea concentration from 0.01 to 0.1 M leads to a modest increase in current density of approx.

(10-15) % (orange plot in Figure 3.12). The optimal conditions of NaOH/CO(NH2)2 concentrations ratio for UOR have been previously investigated in the literature [37] and indicate that an R value greater than 8 provides the most favorable NaOH/CO(NH2)2 concentrations ratio. Otherwise, OH -in solution required as reactant for urea electrooxidation (Eqs. 3.3 and 3.4), but also as supporting electrolyte, will not be enough to avoid the migration of the electroactive species and provide fast regeneration of the consumed OH - species in the diffusion layer at the anode during the urea oxidation reaction (Eqs. However, at all other wavelengths within the studied range, the absorbance is independent of the applied potential. This behavior is also observed in the presence of urea is Figure Lastly, within the orange circle region (at 1.3 V vs. RHE in the reverse scan), a significant decrease in absorbance is observed starting from that point. This substantial reduction in absorbance aligns with a sharp and abrupt decrease in the derivative, highlighting a pronounced alteration in the system's behavior.

It is evident from Figure 3.14b that the absorbance spectra closely resemble the CV profile, this means that the peaks observed on the absorbance spectrum correspond to specific Chapter 3 -Urea electro-oxidation on Ni-based anodes electrochemical processes occurring on the electrode surface. Thus, in our case, it is the oxidation and reduction of Ni(OH)2 and NiOOH on the forward and backward scan, respectively. These processes give rise to peaks and inflection points in the absorbance, leading to similarities between the derivative of absorbance and the CV. Figure 3.15a shows that CO3 2-mainly absorbs at 217 nm, CO(NH2)2 at 220 nm and -OCN at 220 nm and 280 nm. Moreover, Figure 3.15b shows the spectra of both NO2 -and NO3 -, which display a common absorbance peak centered at 240 nm. However, additional distinct peaks at different wavelengths help to distinguish between them. Nitrate (NO3 -) displays an extra peak at 295 nm while nitrite (NO2 -) displays it at 350 nm. Thus, the absorbance peak Chapter 3 -Urea electro-oxidation on Ni-based anodes observed at 220 nm in Figure 3.16a, which rises up with applied potential could correspond to CO3 2-, -OCN and/or CO(NH2)2. However, the fact that these compounds exhibit absorbance at such a close proximity of wavelengths (220 nm) poses a challenge in distinguishing between them. This observation shows some limitations of using UV-vis SEC. In addition to this, Figure 3.16b shows an additional absorbance peak at a wavelength of 350 nm, which also rises up with the applied potential that could correspond to NO2 -formation during UOR. Thus, for studying UOR by UV-vis SEC, the primary emphasis was placed on the parallel configuration due to the absence of any significant changes in absorbance signals in the normal configuration, except the described previously at wavelength of 400 nm, which corresponds to the redox couple Ni(OH)2/NiOOH. The data presented in Figure 3.17 includes current-potential and absorbance-potential plots at a single wavelength 220 nm, which could refer to CO(NH2)2, CO3 2-and/or -OCN. The results indicate that the absorbance at 220 nm increases when the potential reaches 1.4 V vs. RHE , and reaches a maximum at 1.36 V vs. RHE, during the backward scan, as shown in the derivative of the absorbance spectrum (dA/dt, Figure 3.17b). This is probably due to some small temporal delay between the electrochemical product generation and the spectroscopic detection. This increase of absorbance during UOR corresponds to the formation of new compounds during the electrochemical reaction, and specifically CO3 2-and/or -OCN. In contrast, this potential-dependent increase in absorbance at 220 nm cannot be attributed to CO(NH2)2, because the absorbance spectra of CO(NH2)2 would decrease due to its consumption as reactant during the electrochemical reaction. This fact will provide a negative peak in absorbance, which is not observed in Figure 3.16a. At lower potentials in the backward scan (E < 1.25 V vs. RHE), the absorbance continuously decreases due to the diffusion of CO3 2-/ -OCN from the diffusion layer towards the bulk solution. Likewise, the data presented in Figure 3.18 includes current-potential (i-E ) and absorbancepotential (A-E ) plots at 350 nm. These results indicate that the absorbance associated to NO2 - increases starting from 1.4 V vs. RHE, where the UOR occurs, which suggests its formation as a product from UOR. A maximum in absorbance is observed at 1.35 V vs. RHE in the backward scan (dA/dt, Figure 3.18) and a continuous decrease in absorbance due to the diffusion of NO2 -from the diffusion layer towards the bulk solution is also observed at lower potentials in the backward scan (E < 1.25 V vs. RHE). The observed behavior is very similar to the one displayed in Figure 3.17 by CO3 2-and/or -OCN, but the spectroscopic signal associated to the production of NO2 -is much weaker (2 orders of magnitude) than the one for CO3 2-and/or -OCN. This could indicate a higher production rate of CO3 2-and/or -OCN than NO2 -during UOR. However, we do not report quantitative data from UV-vis SEC in this thesis. Moreover, the magnitude of the spectroscopic signal recorded depends on the concentration, but also on the molar absorption coefficient (α) of each compound. In conclusion, this study by UVvis SEC confirms that NO2 -is one of the by-products formed during urea electro-oxidation together with the formation of -OCN and/or CO3 2-, which are successfully monitored for the first time by operando UV-vis SEC. Recent publications [2,3] have clarified the pathways leading to the formation of cyanate ( - OCN), carbonate (CO3 2-), nitrite (NO2 -), nitrate (NO3 -), and nitrogen (N2) as by-products from urea electro-oxidation. However, an explanation for the formation of ammonia (NH3) remains unclear. The work done by our collaborators at LGC-Toulouse [5] showed that a notable amount of NH3 is accumulated all along the electrolysis of 0.33 M urea solution in 1.5 M KOH using a Ni electrode at 550 mV vs. Hg/HgO, being NH3 concentration 0.18 M after 18 h and 0.28 M after 45 hours of urea electrolysis. Furthermore, the total concentration of -OCN (0.14 M) formed during the course of 45 hours of urea electrolysis was nearly equivalent to the total concentration of CO3 2-(0.13 M) quantified. This nearly equivalent stoichiometric ratio (1:1) led us to speculate that the formation of -OCN may be mechanistically linked to the production of CO3 2 . However, according to literature [2,3,38,39], producing both -OCN and CO3 2-simultaneously is not described yet, because each one occurs at different pathways.

The generation of -OCN occurs during the incomplete UOR (pathway 3, Eq. 3.4 in Table 3.1), while the generation of CO3 2-only occurs during the complete UOR to N2, N2O, NO2 -or NO3 - (pathway 1, Eqs. 3.3, 3.5, 3.6 and 3.7 in Table 3.1). Moreover, the fact that the oxidation state of nitrogen in both NH3 and CO(NH2)2 is the same (-3) (see Figure 3.2) shows that the reaction producing NH3 is more likely chemical rather than purely electrochemical.

Consequently, there is still a lack of understanding regarding the mechanism of urea, and the objective of this chapter is to elucidate the pathway responsible for the simultaneous formation of NH3, -OCN and CO3 2-.

We propose a new UOR pathway, which includes production of NH3 potentially arising from the chemical decomposition of urea into -OCN and NH4 + through Eq. 3.15 [1,[40][41][42][43].

Moreover, under acidic conditions, the -OCN can undergo further hydrolysis by Eq. 3.16 [44] to produce CO2 and another ion NH4 + , which in alkaline solution reacts with OH -and forms CO3 2-(Eq. 3.17), as well as the NH4 + is in the form of NH3 (pKa (NH4 + /NH3) = 9.25). It is wellknown that electro-oxidation reactions provoke the generation of a local acidic pH at the electrode surface due to the consumption of OH -. Thus, we propose for the new UOR pathway the development of an acidic interfacial pH at the electrode surface under anodic polarization due to the consumption of OH -during UOR and/or OER. This electrochemically induced decrease in the interfacial pH catalyzes the chemical reaction of -OCN hydrolysis (Eq. 3.16).

Thus, we suggest in this thesis two plausible new UOR pathways for the simultaneous formation of NH3, -OCN and CO3 2-: i ) We start from the published incomplete UOR mechanism [2,3] (pathway 3, Eq. 3.4 in Table 3.1), which produces NO2 -and -OCN. Then, we incorporate the electrochemically induced decrease in the interfacial pH during urea electrolysis, which catalyzes -OCN hydrolysis (Eq. Thus, based on results shown in Figure 3.19, we can conclude that chemical decomposition of CO(NH2)2 into -OCN and NH3 (Eq. 3.15) is not relevant under either alkaline or acid pH conditions. Then, the production of -OCN during UOR comes from the incomplete electrooxidation of CO(NH2)2 as described by pathway 3 in Figure 3.2 [2,3] (Eq. 3.4 in Table 3.1). In contrast, the chemical decomposition of -OCN producing NH3 and CO3 2-(Eq. 3.16) is significant in alkaline solution, but it is accelerated at pH 6. Then, this chemical reaction is electrochemically assisted by the local acidic pH generated at the electrode surface during urea electrolysis, which supports our new UOR pathway (i ), where NO2 -, -OCN, NH3 and CO3 2- are, for the first time, successfully put together as by-products in one single reaction pathway. This is illustrated in a new reaction scheme presented in Figure 3.20, which completes the initial understanding presented in Figure 3.2. Additionaly, these findings are able to prove that the detection of CO3 2-as a by-product in urea electrolysis does note necessarily indicate a complete UOR, as was previously suggested in the literature [2,3,38,39]. We demonstrated that CO3 2-can also result from the chemical degradation of -OCN (Eq.3.16). Consequently, the acidic interfacial pH at the electrode surface reached during UOR plays a crucial role in products selectivity, since it impacts the quantity of -OCN and NH3 generated as products. As

Conclusions

In this chapter, a clear calculation of the standard potential (E 0 ) for UOR was presented to address the different values reported in the literature. Our calculations took into account a range of products beyond the traditionally studied N2 and CO2, including nitrite (NO2 -), nitrate (NO3 -), cyanate ( -OCN), and nitrous oxide (N2O). These calculated values of the Chapter 3 -Urea electro-oxidation on Ni-based anodes thermodynamic potentials for UOR allow an accurate assessment of the energetic cost of the H2 production from urea electrolysis.

Nickel-based catalysts, particularly NiO NPs were investigated for their performance and stability in urea electro-oxidation. It was found that NiO NPs exhibited excellent stability in alkaline medium compared to metallic Ni NPs. However, prior electrochemical activation of NiO NPs was required to induce the formation of an external porous hydroxide layer of Ni(OH)2/NiOOH on the surface. This activation process improved the stability of NiO NPs and enabled efficient urea electro-oxidation.

The electro-oxidation of urea on NiO/NiOOH NPs was analyzed using a rotating disk electrode and various analytical approaches. The results suggested an indirect pathways EC' mechanism, where urea oxidation occurs once the polarization potential reaches the point of oxidizing the inactive Ni(OH)2 species into the active form NiOOH. The chemical reaction between NiOOH and urea then occurs, followed by reduction back to Ni(OH)2. The presence of urea in solution affected the cathodic peak current associated with the reduction of NiOOH, leading to a decrease in the amount of NiOOH available for reduction. The impact of electrolyte concentration and urea-to-electrolyte ratio on urea electrolysis efficiency was investigated, aiming to determine the optimal conditions that maximize urea reaction rate while minimizing the amount of electrolyte required.

To gain a comprehensive understanding of the by-products formed during urea electrooxidation, operando UV-vis SEC was applied. This technique offers valuable insights into the formation of reaction by-products, enabling the observation of changes in absorption spectra at the point of product formation or reactant consumption. This technique shed light on the intermediates and pathways involved in the electrochemical reaction, providing a deeper understanding of the dynamics of the process. UV-vis SEC showed the production of -OCN and/or CO3 2-, as well as NO2 -during electro-oxidation of urea. Operando UV-vis SEC, although a powerful technique, has certain limitations when it comes to distinguishing between chemical species displaying a similar absorption spectrum. In our case, both CO3 2-and -OCN possess a similar absorption band centered at 220 nm, which makes it challenging to differentiate them solely based on spectral changes. To overcome this limitation and obtain more conclusive results, additional analytical techniques are employed. Thus, the chemical stability of two individual 0.33 M CO(NH2)2 and 0.33 M -OCN solutions under different pH conditions were evaluated by ion chromatography. Based on those data we conclude that the chemical decomposition of CO(NH2)2 into -OCN and NH3 (Eq. 3.15) is not relevant under either alkaline or acid pH conditions. But the chemical decomposition of -OCN producing NH3 and CO3 2-(Eq.3.16) is significant in alkaline solution and is accelerated at pH 6. The combination of the identified species by UV-vis SEC and the chemical stability of urea and cyanate in solution obtained by ion chromatography allow us to propose a new UOR pathway (i) and update the mechanistic understanding of UOR (Figure 3.20). The UOR pathway (i) starts from the incomplete UOR mechanism (Eq. 3.4 in Table 3.1), which produces NO2 -and -OCN and is followed by an electrochemically induced step of acidification, which catalyzes - OCN hydrolysis (Eq. 3.16) and produces NH3 and CO3 2-. Thus, NO2 -, -OCN, NH3 and CO3 2-are, for the first time, successfully put together as by-products from UOR in one single reaction pathway.

Finally, the performance of bulk Ni electrode was investigated for comparing it with the performance of NiO/NiOOH NPs. Initially, the stability of the bulk Ni electrode in an alkaline medium was examined, revealing satisfactory stability despite a lower oxidation current density peak compared to NiO/NiOOH NPs, due to its smaller active surface area. The electrooxidation of urea on the bulk Ni electrode was observed once the polarization potential reached the point of NiOOH formation, similar to the behavior observed on NiO/NiOOH NPs.

However, an interesting difference was observed in the reduction peak corresponding to the conversion of NiOOH into Ni(OH)2, which only remained identical in the presence and absence of urea on bulk Ni electrode. This fact suggests a different electrochemical regeneration reaction rate of NiOOH on the bulk Ni electrode surface than on NiO NPs surface.

Chapter 4 Effect of by-products during urea electro-oxidation 4.1. Introduction

According to the literature, NiOOH anode shows a significant and gradual decrease in catalytic activity during UOR [1,2]. This decrease was primarily attributed to the adsorption of reaction intermediates or by-products generated during urea electrooxidation. So far, most of the mechanistic studies have primarily focused on considering N2, CO2 / CO3 2-as potential UOR by-products formed on NiOOH anodes. And suggesting that CO2 / CO3 2-by-products could compete with urea for active sites leading to the observed deactivation process [1,3].

Recently, a significant amount of NH3 generated as a by-product from urea electrolysis has been reported [4]. This finding challenges the previous understanding that CO2 or CO3 2-were the primary by-products. Moreover, an alternative reaction pathway has been recently proposed in the literature [5], suggesting the formation of two distinct N-species, namely cyanate ( -OCN) and nitrite (NO2 -) during urea electro-oxidation (Eq. 3.4 Table 3.1). This highlights the need to evaluate the mechanistic impact of these newly identified nitrogen compounds, as they differ from the commonly studied N2 by-products. In this chapter we investigate the potential contributions of CO3 2-, NH3, -OCN and NO2 -that have been reported as by-products [4][5][6][7][8]. Consequently, understanding the source of anode deactivation is crucial for designing more efficient electrocatalysts for urea electrolysis in the future.

The performance and stability of the nanostructured NiO/NiOOH anode for UOR has been evaluated by EIS, CV and CA. Through cyclic voltammetry, we examined the performance of NiO/NiOOH both before and after conducting urea electro-oxidation, by evaluating the oxidation and reduction current densities of Ni/NiOOH. This allowed us to determine if there was a decrease in the catalytic activity of Ni/NiOOH due to catalyst degradation.

While EIS, renowned for its sensitivity to surface modification [9], it was used to investigate the impact of the products/by-products on the electrode surface, and determine whether any surface modifications occurred before and after exposure to UOR.

When a product/by-product is adsorbed on the electrode surface, a distinct time constant (τ) emerges at low frequencies that corresponds to the relaxation process of the adsorbed species and can be observed on Nyquist and Bode plots. In addition to that, the thickness Chapter 4 -Effect of by-products during urea electro-oxidation of the thin hydroxide layer formed during the electrochemical activation process on NiO NPs (Figure 3.7) was calculated by EIS at high frequencies.

Results and Discussion

EIS of glassy carbon and NiO NPs

EIS results are presented in Nyquist and Bode plot representations. The Nyquist plot is widely used due its simplicity and ability to provide a clear visualization of the data. It displays the real part of impedance (Zr) on the x-axis and the negative imaginary part of impedance (Zi) on the y-axis. Zr and Zi are normalized by multiplying them by the geometric surface area (0.196 cm 2 ). On the other hand, the Bode plot is used for its sensitivity to changes at high frequencies, and easily identifies the time constants. It consists of two logarithmic plots: magnitude (Z ) vs. frequency (f ) and phase (ϕ) vs. frequency (f ). While the Nyquist plot will always be included, the Bode plot will be selectively presented to enhance visualization when needed. It is confirmed that the time constant at the characteristic frequency of 0.05 Hz on the GC electrode dropcasted with NiO NPs is a faradaic capacitance and is not originating from the contribution of the oxide layer formed on the surface of the NP. This confirmation is based on the calculated capacitance (C ), which can provide information about the thickness of a layer. By using Eq. 4.1 (with R = 1000 Ω cm 2 and f=5x10 -2 Hz) the capacitance value is C= 4 mF. Additionally, if it is the case of an oxide layer, the thickness of the oxide layer (d) formed at the electrode surface can be easily calculated from Eq. 4.2 [14]. Assuming a permittivity (𝜀) of 12 for the NiO/NiOOH layer, where 𝜀 0 corresponds to the vacuum permittivity. Thus, this capacitance value corresponds to a thickness of approximately 0.2 nm, which does not align with the characteristics of an oxide layer film. line expected for blocking electrode behavior [10], which implies that there is no Faraday process occurring at the electrolyte/electrode interface. Since it represents the interfacial capacitance, which includes the electric double layer and the contribution of the oxide film formed on the surface of the NP, the various slopes 45° and 22.5° observed in HF must be explained. Furthermore, generally with a planar electrode, the high frequency part of impedance on the Nyquist plot exhibits an angle of 90° with respect to the real axis.

EIS response in a Nyquist plot representation shown in

However, in our case in Figure 4.3, the slope of the high frequency line against the real axis is approx. 45°. This value may be linked to a porous electrode in which the NiO electrode undergoes a residual reaction with diffusion (slope 22.5°). These values suggest that the electrode behaves as porous, in agreement with the theory of de Levie [11,12]. As far as we know, the thickness of the NiOOH layer generated through electrochemical activation (shown in Figure 3.7b) has not been estimated before in the literature. Thus, assuming an average radius of the NiO NPs of 20 nm, the NiO density of 6.7 g cm -3 and taking into account the catalytic ink properties, it is estimated that about 1.6 x 10 11 particles of NiO are deposited on the glassy carbon electrode, thus allowing to roughly estimate the active surface area. At high frequencies, the current passes through the capacitor as the resistor offers greater resistance. Thus, the representation of complex capacitances known as (Cole-Cole plot) has emerged as a useful tool for determining specific interface properties, particularly in the high-frequency domain, such as the layer thickness. Thus, the charge-transfer process associated with the NiO/Ni(OH)2 oxidation Chapter 4 -Effect of by-products during urea electro-oxidation into NiOOH at the electrode surface (characteristic frequency of 146 Hz) observed at 0.53 V vs. Hg/HgO (Figure 4.3) allows estimating the thickness of the formed NiOOH film from the EIS data. The complex capacitance is obtained using the relationship in Eq. 4.3 [13,14] allowing to generate the Cole-Cole plot presented in Figure 4 where 𝐶(𝜔) is the complex capacitance, 𝜔 is the angular frequency, 𝑍(𝜔) is the impedance magnitude Ω cm 2 , 𝑗 is the imaginary number and 𝑅𝑒 is the electrolyte resistivity. adsorption. Instead, it is only associated to the current decrease with increasing potential [19,20]. However, urea adsorption cannot be ruled out as a cause of electrode deactivation. Since EIS is renowned for its sensitivity to surface modification, it is applied in the upcoming section to determine whether any surface modifications occurred before and after exposure to urea electro-oxidation reaction. However, it is demonstrated that urea competes with OH -for the active sites at the electrode surface for its electro-oxidation and the catalyst regeneration NiOOH might be the limiting step which is responsible for the current decrease [21,22]. Operando EIS measurements in the presence of urea, demonstrated the presence of two time constants in the low-frequency domain, which are attributed to the relaxation of the adsorbed OH -and CO(NH2)2. In addition, it showed the absence of adsorption of UOR byproducts (CO3 2-, -OCN, NH3 and NO2 -). Thus they do not affect the stability of NiO/NiOOH anode. Furthermore, the EIS data obtained at a potential of 0.2 V vs. Hg/HgO, in which the Chapter 4 -Effect of by-products during urea electro-oxidation process is purely capacitive, have demonstrated that the deactivation NiO/NiOOH anode is not influenced by urea adsorption, and there are no significant aging effects during the initial hour of urea electro-oxidation at 0.53 V vs. Hg/HgO, which was consistent with CV measurements. The combination of CV and EIS measurements allow us to propose that the decrease in current density may be attributed to the inefficient catalyst regeneration process rather than the adsorption of the by-products generated during UOR on the electrode surface. The decrease in the cathodic peak of NiOOH reduction and the anodic peak of urea oxidation during the initial hour of UOR at 0.53 V vs. Hg/HgO, followed by an increase of NiOOH reduction and UOR rate after "catalyst reactivation", suggests that the slow regeneration of NiOOH might be responsible to the UOR rate decrease. Overall, these findings contribute to a better understanding of the behavior of NiO/NiOOH catalysts in urea electro-oxidation and highlight the importance of efficient catalyst regeneration for improved performance.

Chapter 5 -Conclusions and Perspectives UOR. This chapter provides an overview of the initial mechanistic understanding of UOR on Ni-based catalysts by including the pathways for by-products generation and the catalyst stability. Thus, it sets the foundation for the following chapters, which will focus into the experimental methods, results, and discussion.

In Chapter 2, a theoretical and experimental overview of various characterization techniques and methods used for the evaluation of NiO nanoparticles (NPs) as catalyst for UOR is provided. It introduces the different reagents used in preparing the catalyst ink and electrolyte solutions. This chapter also covers the description of the electrochemical techniques and experiments employed, such as RDE, CV, CA, EIS, and UV-vis SEC. In particular, the main principles and equations related to RDE, including Levich and Koutecký-Levich approaches are described in detail. The setup configurations for RDE, EIS, and UV-vis SEC experiments are described, highlighting their key components and parameters.

Chapter 3, presents a thorough investigation of UOR mechanism mainly on NiO/NiOOH anode. Firstly, a comprehensive calculation of the standard potential (E 0 ) for UOR is presented, resolving discrepancies previously found in the literature and considering all possible products formed during UOR. This allows a more accurate assessment of the energetic cost of H2 production from urea electrolysis. Secondly, the performance of NiO/NiOOH NPs and bulk Ni electrodes in UOR are investigated using RDE at various rotation rates and at different ratios (R) of Celectrolyte/Curea, which demonstrates that the limiting current for UOR is independent of the rotation scan (mass transport) beyond 400 rpm and points out a different electrochemical regeneration reaction rate of NiOOH on bulk Ni electrode surface than on NiO NPs surface. Thirdly, the formation of NO2 -, -OCN and/or CO3 2- as product from UOR is successfully monitored for the first time by Operando UV-vis SEC.

Fourthly, the initial mechanistic understanding of UOR existing in the literature lacks a mechanistic pathway that could explain the formation of NH3 and simultaneouslly CO3 2-, -OCN and NO2 -from urea electrolysis, despite their confirmed detection during urea electrolysis.

We present thanks to the combination of ion chromatography (IC) and UV-vis SEC results, a new UOR pathway (i), which incorporates an additional electrochemically assisted chemical step to the initial pathway 3 (Figure 5.1). This step involves the electrochemically induced conversion of -OCN into NH3 and CO3 2-. As a result, this new proposed UOR pathway allows, Chapter 5 -Conclusions and Perspectives

Perspectives

Up to this point in this thesis, the primary approach for treating urban wastewater and generating energy simultaneously is through the UOR. However, there are challenges related to the enzymatic decomposition of urea into ammonia and carbonic acid during storage and transport [5], which can significantly affect the nitrogen content reaching the sewage treatment plant in the form of urea. As a result, it becomes necessary to investigate both UOR and ammonia electro-oxidation (AOR) as concurrent reactions and conduct a comparative study. In this thesis, the focus was solely on studying the electro-oxidation of urea, which was carried out in a highly alkaline environment to inhibit enzymatic activity and exclusively target urea for treatment. However, research conducted by LGC-Toulouse revealed that urea electro-oxidation produces high concentrations of NH3, as well as NO2 -and -OCN [6].

Consequently, we started considering the possibility of electro-oxidizing NH3 from the beginning, rather than blocking the enzymatic reaction that converts urea into ammonia, allowing the treatment of nitrogen in the form of both ammonia and urea. Furthermore, this study avoids the need for high alkalinization of the solution since the pH of urine, due to enzymatic activity, is naturally adjusted between 9 and 10. This natural pH adjustment Chapter 5 -Conclusions and Perspectives eliminates the requirement for costly alkalinization treatment. Moreover, Nickel-based catalysts have demonstrated high effectiveness for the electro-oxidation of ammonia [7,8].

Hence, initial investigations have been conducted on NiO/NiOOH catalysts to study ammonia electro-oxidation.

The standard potential of ammonia electro-oxidation under alkaline conditions (pH = 14) was calculated to be E 0 = -0.77 V vs SHE, as indicated in Equation 5.1. Thus, combined with the hydrogen evolution reaction (HER) in alkaline medium at the cathode E 0 = -0.83 V vs SHE (Equation 5.2), it results in an overall cell potential of E 0 cell = 0.06 V, as stated in Equation 5.3. This value is considerably lower than the potential required for water electrolysis (E 0 cell = -1.23 V) and slightly higher than UOR (E 0 cell = 0.02). However, the slow kinetics of the AOR lead to a significant overpotential. Consequently, a Nickel-based catalyst was considered as one of the potential electrocatalysts for ammonia electro-oxidation. A few studies on AOR were carried out following similar experimental conditions to the one already described in the thesis for UOR. The setup involved utilizing a rotating disk electrode with a NiO/NiOOH anode. Initially, AOR was studied using NaOH in solution at two distinct pHs (14 and 13). that the electro-oxidation reaction of urea is primarily controlled by kinetics. It is noteworthy that, according to literature, the electro-oxidation of ammonia follows a direct pathway mechanism referred to as E mechanism [8], while urea follows an indirect mechanism pathway referred to as the EC mechanism [10,11]. Finally, initial studies have been done by the Master's 2 student to perform electrochemical oxidation of artificial urine on NiO/NiOOH NPs. The artificial urine contains a large variety of compounds besides urea and ammonia [12]. Thus, the main interest of this approach is to identify potential interfering compounds. Results presented in Figure 5.7 show that the CV of artificial urine electro-oxidation is nearly similar to the one from UOR (Figure 3.9). The limiting current is independent of the rotation rate suggesting a kinetics control process in both cases. In conclusion, more work is necessary to reach a complete understanding of the urine electrolysis. (2 2 2) of bulk NiO, respectively. This correspondence is in agreement with existing literature [1]. While, the blue spectrum represents an XRD pattern after the electrochemical activation, which remained unchanged before and after the electrochemical activation. This process involves the formation of a hydroxide layer, and according to the literature [2] the NiO structure in alkaline medium consists of a crystalline NiO core surrounded by an amorphous hydroxide layer which explains why the XRD pattern remained unchanged. La majorité de l'azote (N) présent dans les eaux usées urinaires est attribuée à l'urée (H2NCONH2) [3], un composé riche en azote et le plus grand constituant de l'urine présent à des concentrations élevées (240-470 mM d'urée) [4,5]. Alors que le reste de la teneur en azote provient d'autres composés azotés présents dans l'urine tels que la créatinine (C4H7N3O), l'acide urique (C5H4N4O3) et le chlorure d'ammonium (NH4Cl) [6]. Pendant le stockage, l'urine subit un processus de dégradation naturelle, qui provient de l'hydrolyse de l'urée, conduisant à la conversion de 90% de l'azote en ammoniac (NH3) [3,[7][8][9][10][11]. Cette décomposition chimique est catalysée par l'enzyme uréase, largement présente dans la nature et produite par des micro-organismes, des plantes, des champignons et des algues [7]. Néanmoins, l'UOR souffre d'une réaction anodique lente due au processus de transfert d'électrons (Eq.1). Ainsi, trouver un catalyseur efficace est une exigence pratique clé pour l'oxydation de l'urée afin de promouvoir le processus d'oxydation. Au cours des dernières années, de nombreuses investigations scientifiques ont été menées pour tester différents matériaux de composition (Pt, Pt-Ir, Ti, Rh, Cu, Co, Ni..) en solutions alcalines [4,[12][13][14][15][16],

principalement des hydroxydes de sodium ou de potassium (NaOH/KOH), pour évaluer leur influence sur l'efficacité de l'UOR. La première tentative de l'oxydation de l'urée à partir de l'urine impliquait l'utilisation de métaux nobles, tels que le platine [17], mais les coûts élevés associés à ces métaux ont incité les chercheurs à rechercher une alternative plus abordable.

Finalement, Botte et al [4] ont découvert que les catalyseurs à base de Ni, qui sont plus économiques, offrent une activité exceptionnelle pour l'électro-oxydation de l'urée en milieu alcalin [4]. L'inspiration initiale pour l'utilisation de catalyseurs à base de Ni semble avoir été tirée de la décomposition biocatalytique de l'urée qui se produit dans les sites actifs des enzymes uréases, qui contiennent des centres Ni(II) pontés par des hydroxydes.

Les motivations chromatographie ionique nous permet de proposer un nouveau mécanisme de l'UOR (i) et de mettre à jour la compréhension mécanistique de l'UOR (Figure 2). Le mécanisme de l'UOR de l'urée augmente à nouveau, passant de 7,7 mA cm -2 à 9,7 mA cm -2 à 0,6 V vs. Hg/HgO. La densité de courant atteint presque le même niveau que celle obtenue avec une électrode neuve (10,2 mA cm -2 à 0,6 V vs. Hg/HgO, ligne noire).

Figure 1 . 1 .

 11 Figure 1.1. Scheme of the main reactions involved to achieve the goals established with the PEC reactor in HYUREA project.

Chapter 1 -Figure 1 . 3 .

 113 Figure 1.3. Scheme showing chemical (un-catalyzed elimination reaction) and enzymatic decomposition pathways of urea at physiological pH.

Figure 1 . 4 . 6 )

 146 Figure 1.4. Schematic illustration of the main sources of urea as well as the different electrochemical technologies for urea conversion including photoelectrochemical urea splitting, urea fuel cells and urea electrolysis [23].

Chapter 1 -

 1 IntroductionNi-based catalysts for urea electro-oxidation Botte et al.[5] discovered that Ni-based catalysts, which are cost-effective, offer exceptional activity for UOR in an alkaline medium. The initial inspiration for using Ni-based catalysts seems to have been drawn from the biocatalytic decomposition of urea that occurs in the active sites of urease enzyme, which contains Ni 2+ centers that are bridged by hydroxides[30,31]. Their study compared the cyclic voltammetry of various anodes (Pt, PtIr, Ni and Rh) with (#1 in Figure1.5) and without (#2Figure 1.5) the presence of 0.33 M urea in 5 M NaOH, using Ti-foil supported electrodes. As shown in Figure1.5, CVs of Ni-based catalysts generated the highest current density for UOR. Figure1.5b presents CVs in the presence and in the absence of urea on a Ni electrode. It shows that the anodic peak (identified as a1) in 5 M KOH solution corresponds to the oxidation of nickel (additional information will be discussed in the following section), while the onset potential of CV (#1) for urea oxidation coincides with that of nickel oxidation. This implies that the oxidation of Ni must occur prior to urea oxidation.

Figure 1 . 5 .

 15 Figure 1.5. Anodic catalyst analysis at 25 °C: (a) CVs obtained in 5 M KOH with (#1) and without (#2) the presence of 0.33 M urea on different metals supported on a Ti-foil electrode at 10 mV s -1 ; (b) CVs of Ni supported on Ti-foil electrode in the absence (dashed grey) and presence (black) of 0.33 M urea in 5 M KOH solution [5].

Figure 1 . 6 .

 16 Figure 1.6. CVs obtained in 5 M KOH with and without the presence of 0.33 M urea on Nibased anodes at a 10 mV s -1 [34].

Figure 1 . 7 .

 17 Figure 1.7. Potential-dependent in situ SERS of Ni(OH)2 in 5 M KOH in absence (a) and presence (b) of 1 M urea [35].

Figure 1 .

 1 8 displays the variation of XRD reflections of Ni(OH)2 at various potentials, in the absence (a-b) and presence (c-d) of 1 M urea in 5 M KOH. In Figure 1.8a, the intensity of Ni(OH)2 diffraction peaks decrease with increasing potential, while a new peak at around 12.7° emerges (Figure 1.8b), characteristic to NiOOH reflection. These findings suggest that Ni(OH)2 undergoes electrochemical oxidation to NiOOH in an alkaline solution through Equation 1.13. However, in the presence of urea, all Ni(OH)2 diffraction peaks remain stable (Figure 1.8c), and no new phase (NiOOH) is observed (Figure 1.8d). This indicates that both the electrochemical oxidation of Ni(OH)2 and reduction of NiOOH by urea occur throughout the process. In conclusion, the absence of NiOOH reflection on XRD is due to its instant chemical reduction to Ni(OH)2 by urea, which explains the stability of Ni(OH)2 diffraction. So, XRD was able to prove for the first time, that the mechanism of UOR on Ni(OH)2 follows the indirect pathway (catalyst regeneration Chapter 1 -Introduction mechanism), and that the chemical reaction between NiOOH and urea into Ni(OH)2 and products (Eq. 1.14) is faster than the transition of Ni(OH)2 to NiOOH (Eq. 1.13).

Figure 1 . 8 .

 18 Figure 1.8. Evolution of XRD diffractograms of Ni(OH)2 electrode under different applied potentials from 1.2 to 1.6 V vs. Pt cathode in the absence (a-b) and presence (c-d) of 1 M urea in 5 M KOH [39].

Figure 1 . 9 .

 19 Figure 1.9. Time-resolved SERS spectra at Ni(OH)2 catalyst that was previously electrochemically oxidized to NiOOH for 60s. (a) in 5 M KOH and (b) in 1 M urea and 5 M KOH [35].

Chapter 1 -

 1 Figure 1.10. (a) CVs of nickel (Ni) electrode in 5 M KOH and 0.33 M urea at 10 mV s -1 and various rotation rates: 100, 300, 500, 1000, and 1500 rpm. (b) Levich plot obtained from CVs in (a) [34].

Figure 1 .

 1 11a shows CVs of Ni in 5 M KOH solution for different concentrations of urea from 0 to 0.5 M. The peak associated with the oxidation of urea showed an increase in current upon the addition of urea, which is attributed to the increased availability of urea molecules in solution for oxidation. Additionally, the anodic peak current density exhibits a linear increase up to 0.2 M urea, indicating a diffusioncontrolled process at low urea concentrations. However, beyond 0.2 M urea, the increase in anodic current density becomes independent of the urea concentration, suggesting a kinetics controlled process. Figure1.11b. shows that the corresponding cathodic charge exhibited a decrease with increasing urea concentration. This can be due to the fact that urea molecules Chapter 1 -Introduction are more abundant in the solution at high concentrations, leading to a higher reduction of NiOOH to Ni(OH)2 according to (Eq. 1.14) during the forward scans. As a result, the available amount of NiOOH for electrochemical reduction in the cathodic region decreases, causing a decrease in cathodic charge in the CV. This phenomenon is attributed to the EC mechanism proposed by Botte[5,30,35].

Figure 1 .

 1 Figure 1.11.(a) CVs of Ni electrode in 1 M KOH solution at 10 mVs -1 at various concentrations of urea such (1) 0 M, (2) 0.05 M, (3) 0.1 M, (4) 0.2 M, (5) 0.3 M, (6) 0.4 M, and (7) 0.5 M (b) enlarged view of figure (a) at the cathodic peak [34].

Figure 1 . 12 .

 112 Figure 1.12. CVs on LaNiO3 at 10 mV s -1 in a 0.33 M urea solution containing different concentrations of KOH [40].

[Figure 1 . 13 .

 113 Figure 1.13. CVs of the Ni2P@Ni foam electrode at low concentrations (a) and high concentrations (b) of urea in 5 M KOH solution. (c) Different concentrations of KOH with [urea] constant at 0.60 M; (d) the plot of onset oxidation potential vs. KOH concentration at a scan rate of 15 mV s -1 [41].

Figure 1 .

 1 14b compares CVs at 1 st , 2 nd and 50 th cycle, and it shows that the current density decreases from 25 mA cm -2 to 13 mA cm -2 when Chapter 1 -Introduction going from 2 nd to 50 th cycle. This decline is likely attributed to catalyst instability or poisoning. This might involve the adsorption of reaction intermediates and products on the catalyst surface, which subsequently block the active sites of the catalyst. In particular, extensive research has been conducted on the effect of carbon dioxide (CO2) and carbon monoxide (CO) on the deactivation of nickel-based catalysts. CO2 desorption from the catalyst surface (Ni) was identified as the rate-limiting step during urea electro-oxidation reaction according to DFT calculations[30].

Figure 1 . 14 .

 114 Figure 1.14. Plot of normalized decreasing peak current densities for successive cyclic voltammograms in 5 M KOH and 0.33 M urea solution, with and without vulcan carbon (VC) support for LaNiO3. (b) CVs of LaNiO3 in 5 M KOH and 0.33 M urea at a scan rate of 100 mV s -1 [40].

Figure 1 .Figure 1 . 15 .

 1115 Figure 1.15 demonstrates that the peaks associated with the oxidation and reduction of Ni species vanish after purging the solution with CO2, which suggest the presence of a blockage

Figure 1 .

 1 Figure 1.16. (a) Cyclic voltammograms on Ni catalyst in 1 M KOH in the absence (dashed curve) and presence (solid curve) of 0.1 M urea; at scan rate of 10 mV s -1 . (b) Nyquist plots of Ni catalyst in 1 M KOH and 0.1 M urea at various polarization potentials with an amplitude of 5 mV, and frequency range from 10 5 to 10 -2 Hz [44].

Chapter 1 -Figure 1 . 17 .

 1117 Figure 1.17. Nyquist plots corresponding to EIS spectra of Ni nanowires at 0.37 V from 100 kHz to 10 mHz with 5 mV potential amplitude, in 5 M KOH solution in the presence and the absence of 0.33 M urea in solution, RE: saturated Ag/AgCl [47].

Figure 1 . 18 .

 118 Figure 1.18. Results of a urea electrolysis on nickel electrode. S = 4 cm 2 in 50 mL of 1.5 M KOH and 0.33 M (CO(NH2)2)t=0 at 0.55 V vs. Hg/HgO. (a) Urea molar concentration evolution, as well as the obtained by-products evolution all along the electrolysis. (b) Nitrogen mass balance and (c) carbon mass balance. Labels: ①-▢ CO(NH2)2, ②-△ NH4 + , ③-◯ OCN -, ④-✩ NO2 -and ⑤-⬠ CO3 2-[50].

Chapter 1 -

 1 Introduction responsible for the formation of NH3, despite Figure 1.18 showing significant quantities of NH3 being generated during urea electrolysis.

Figure 1 . 19 .

 119 Figure 1.19. Schematic illustration of two proposed urea electro-oxidation pathways on NiOOH surface to produce either NOx -and NCO -(a) or N2 and CO3 2-(b) [49].

2 -

 2 Materials and experimental methods

Figure 2 .

 2 1b, the experimental setup involved a cell filled with 100 mL of either 1 M or 5 M NaOH solution that was previously saturated with argon for 20 minutes. The working electrode (AFE5T050GC, Pine research) consisted of a 5 mm OD glassy carbon disk electrode embedded into a 15 mm OD Teflon. The counter electrode is a graphite rod (PINE research, AFCTR3B, diameter = 0.64 cm), the reference electrode Hg/HgO directly Chapter 2 -Materials and experimental methods immersed in solution, since it is the most stable reference electrode in strong alkaline media. Cyclic voltammetry, linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy experiments were conducted to study the performance of Ni-based catalysts. The potential (E ) is applied between the working and reference electrode, while the current response is measured between the working and counter electrode. The current density was calculated by considering the geometric area of the RDE (0.196 cm 2 ).

Figure 2 . 1 .

 21 Figure 2.1. (a) Disk electrode under rotation. (b) Full setup of the electrochemical cell used for analysis with RDE.

5 )

 5 This is analogous to Ohm's law in a DC circuit system where the resistance 𝑅 = 𝐸 𝑖 , however for an alternating current/potential the R is replaced by Z as the generic term for the impedance. As shown in Figure2.2, in a potentiostatic mode (E(t) = E0 sin(ωt)), the input sinusoidal potential E(t) has three main characteristic. (1) time dependency (t), meaning that the potential fluctuates as a function of time, (2) amplitude E0, (3) and angular frequency (ω). The corresponding output current has these same attributes, but with an added phase shift or phase angle (ϕ), resulting in i(t)=io sin (ωt -ϕ) due to the offset from the input and output signals. EIS is graphically represented by Bode and Nyquist plots for a quick visualization[6,7].

Figure 2 . 2 .

 22 Figure 2.2. Representation of the EIS applied on the RDE setup. Phase angle (ϕ) and amplitude (E0).

Figure 2

 2 Figure 2.3. (a) Nyquist plots corresponding to EIS spectra of NiO NPs at 800 rpm in 5 M NaOH at 0.53 V vs. Hg/HgO, without capacitor (black), with capacitor of 0.68 µF (red) and 1.6 µF (green). (b) Schematic representation showing the capacitor in series with Pt wire, and in parallel with the RE.

Figure 2 .

 2 Figure 2.4 represents the experimental setup featuring the two different configurations: parallel and normal. The normal configuration or the reflection mode, is represented by the laser and reveals information on processes taking place on the electrode surface of a GC disk electrode (diameter = 3 mm) dropcasted with NiO NPs. For the parallel configuration, represented by optical fibers (orange wires in Figure 2.4) incorporated adjacent to the surface of a GC disk electrode (diameter = 3 mm) dropcasted with NiO NPs. This configuration allows the detection of products present only in solution, but in close proximity to the electrode surface, during the electrochemical measurements. The fibers are located at approximately 200 µm from the solution interface. In order to perform the two different modes at the same time, two potentiostats and two spectrometers are implanted in the experimental setup, since each mode requires a potentiostat, light source and a spectrometer.However, only one potentiostat applied a potential to the WE. Cyclic voltammetry was performed during the UV-vis measurements, using a reference electrode of reversible hydrogen electrode (RHE), platinum as counter electrode (diameter = 3 mm), and NiO NPs drop-casted on the GC disk electrode as working electrode. The reference spectrum of the sample is obtained prior to launching any experiment, and is used as the baseline for

Figure 2 . 4 .

 24 Figure 2.4. Scheme of the spectroelectrochemistry setup.

  Figure 2.5 shows a comparison between the CV results of the GC electrode (polished) in 5 M NaOH before (black line) and after cleaning with 0.5 M H2SO4 (blue line). Prior to acid cleaning, CV plotted in black line shows a small oxidation and reduction peak at 0.5 V and 0.35 V vs.

Figure 2 . 5 .

 25 Figure 2.5. Cyclic voltamogramms of NiO NPs by RDE at 800 rpm and 10 mV s -1 , before (black)

Figure 3 . 1 .

 31 Figure 3.1. Schematic illustration showing the list of by-products detected during UOR [2,3,5].

Chapter 3 -

 3 Urea electro-oxidation on Ni-based anodes into nitrogen gas (N2), reactions (3.1 to 3.3), was considered almost exclusively as the reaction pathway in the literature.

Figure 3 . 2 .

 32 provides an overview of the 3 different reaction pathways considered at present: Pathway (1) based on successive electrochemical oxidation reactions, which is only able to justify N2, N2O, NO2 -and NO3 -production, pathway (2) based on chemical decomposition of urea, which does not involve any electron transfer and justify the detection of OCN -and NH3 and pathway (3) where only one of the two nitrogen atoms present in urea gets oxidized and the other one remains at the same oxidation state, which justify the detection of NO2 -, NO3 -and -OCN. Thus, it is probably the combination of those 3 pathways simultaneously responsible for detecting that large variety of products during urea electrooxidation.

Figure 3 . 2 .Figure 3 . 3 .

 3233 Figure 3.2. Reaction pathways during urea electro-oxidation showing all possible N-based products and their oxidation state in each case.

Figure 3 . 4 .

 34 Figure 3.4. CVs obtained on Ni NPs and with an RDE in 1 M NaOH at 800 rpm and 10 mV s -1 in the presence of Nafion at progressive immersion times.

Figure 3 . 5 .Figure 3 . 6 .Figure 3 .Figure 3 . 7 .

 3536337 Figure 3.5. Ni Pourbaix diagram [29].Initially, there were doubts about whether the low stability of the catalyst was caused by the small amount of Nafion added as a binder in the catalytic ink. Thus CVs were performed in the absence and presence of Nafion in the catalytic ink. Figure3.6 shows the CV of the Ni NPs in a 1 M NaOH solution at two different immersion times, t = 0 (continuous line) and t = 3 hours (dash line), both in the absence (orange line) and presence of Nafion (black line). The results indicate that the anodic peaks shift in the same manner regardless of the presence of Nafion, suggesting that the instability of the catalyst is not caused by Nafion. The presence of an unstable surface represents an important limitation in our studies, especially with EIS, as nickel must have sufficient stability, in order to ensure minimal changes in the electrode properties during a single measurement of the impedance spectrum.

Figure 3 .

 3 8 shows CV of NiO/NiOOH anode in the two different electrolytes 5 M NaOH and 5 M KOH. Results show that regardless of the electrolyte used, NiO/NiOOH exhibit the same anodic and cathodic current densities. Additionally, both electrolytes show the same anodic (0.46 V vs. Hg/HgO) and cathodic (0.34 V vs. Hg/HgO) peak potentials of NiO/NiOOH oxidation and reduction,respectively. Thus, we can conclude that the use of either KOH or NaOH does not have any significant effect on the obtained results.

Figure 3 . 8 .

 38 Figure 3.8. CV obtained on NiO/NiOOH NPs with a RDE at 800 rpm and 10 mV s -1 in 5 M KOH (red line) and 5 M NaOH (blue line).

Chapter 3 -Figure 3 . 9 .

 339 Figure 3.9. CVs of NiO/NiOOH NPs in 5 M NaOH at 800 rpm and 10 mV s -1 in the absence (blue line) and presence of 0.01 M urea (green line).

Figure 3 .Figure 3 . 10 .

 3310 Figure 3.10. CVs of NiO/NiOOH NPs at 800 rpm and 10 mV s -1 , in 1 M NaOH without (black line) and with (orange line) 0.01 M urea. In 5 M NaOH with (green line) and without (dark blue line) 0.01 M urea (a) at potentials E vs. Hg/HgO and (b) E vs. RHE.

Figure 3 .

 3 Figure 3.11. (a) CVs of NiO/NiOOH NPs at 800 rpm and 10 mV s -1 , in 1 M NaOH without (black line) and with 0.33 M urea at 0 (green line) and 800 rpm (red line). (b) Chronoamperometry curve of NiO/NiOOH NPs in 5 M NaOH + 0.33 M urea at 0.53 V vs. Hg/HgO and different rotation rates (from 0 to 2000 rpm).

3 .3 and Chapter 3 -Figure 3 . 12 . 3 . 2 . 4 .

 33312324 Figure 3.12. Chronoamperometry curve of NiO/NiOOH NPs deposited on RDE in 1 M NaOH at 0.45 V vs. Hg/HgO, different ratios [NaOH]/[CO(NH2)2] and at a rotation rate of 800 rpm. Each vertical arrow in the black plot indicates the addition of more urea in the solution.

Figure 3 .

 3 Figure 3.13 presents an example of the obtained absorbance spectra from 200 to 800 nm under normal configuration in 1 M KOH in the absence (Figure 3.13a) and presence of 0.33 M urea (Figure 3.13b), while varying the applied potential during the forward scan of a cyclic

3 .

 3 13b, implying that within the normal configuration, only the interconversion of Ni(OH)2 and NiOOH redox couple can be observed, and no other compounds are identified to be adsorbed on the electrode surface during UOR. The correlation between the absorbance and the applied potential is more clearly shown in Figure 3.14.

Figure 3 . 13 .

 313 Figure 3.13. Normal configuration UV-Vis SEC of NiO/NiOOH NPs deposited onto a GC electrode at different applied potentials between 0.9 and 1.5 V vs. RHE, at 10 mV s -1 in 1 M KOH solution (a) in the absence and (b) presence of 0.33 M urea.

Figure 3 .

 3 Figure 3.14a presents SEC results under normal configuration, which includes currentpotential and absorbance-potential plots at a single wavelength of 400 nm in 1 M KOH solution without urea. The blue line represents a typical Ni(OH)2/NiOOH CV, illustrating the current -potential plot ranging from 0.9 to 1.5 V vs. RHE, where the typical peaks associated to the formation of NiOOH and Ni(OH)2 are observed. The orange line represents the absorbance spectrum collected at 400 nm as a function of the same applied potential. The absorbance varies with the applied potential, which is associated with the oxidation and reduction of Ni(OH)2 and NiOOH, respectively. It can be observed that the absorbance increases in the forward scan until it reaches a maximum current value at 1.5 V vs. RHE (formation of NiOOH), after which presents a plateau between 1.5 V and 1.25 V vs. RHE

Figure 3 .

 3 Figure 3.14. (a) Normal configuration UV-Vis SEC of NiO/NiOOH NPs deposited onto a GC electrode in 1 M KOH solution and absorbance collected at 400 nm, i-E plot (blue line) at scan rate 10 mV s -1 and A-E plot (orange line). (b) i-E plot (blue line) at scan rate 10 mV s -1 and derivative A-E plot (orange line).

Figure 3 . 15 .

 315 Figure 3.15. Absorbance spectrum measured on UV-vis of individual solutions containing (a) Carbonate (black spectrum), Cyanate (blue spectrum) or Urea (red spectrum), (b) Nitrate (grey spectrum) and Nitrite (green spectrum).

Figure 3 .Figure 3 .

 33 Figure 3.16 show that a significant change of absorbance at wavelengths of 220 nm (Figure 3.16a) and 350 nm (Figure 3.16b) is observed when different potentials are applied. The correlation between the absorbance and the applied potential is more clearly shown in Figure 3.17 and Figure 3.18.

Figure 3 .

 3 Figure 3.16. (a) Parallel configuration UV-Vis SEC of NiO/NiOOH NPs deposited onto a GC electrode at different applied potentials between 1.02 and 1.40 V vs. RHE in forward scan and 1.40 and 0.94 V vs. RHE in backward scan at 10 mV s -1 in 1 M KOH + 0.33 M urea solution. (b) zoom of Figure 3.16a centered at wavelength 350 nm.

Figure 3 .

 3 Figure 3.17. (a) Parallel Configuration UV-Vis SEC of NiO/NiOOH NPs deposited onto a GC electrode in 1 M KOH + 0.33 M urea solution and absorbance collected at 220 nm, i-E plot (blue line) at scan rate 10 mV s -1 and A-E plot (orange line), (b) i-E plot (blue line) at scan rate 10 mV s -1 and derivative A-E plot (orange line).

Figure 3 .

 3 Figure 3.18. (a) Parallel Configuration UV-Vis SEC of NiO/NiOOH NPs deposited onto a GC electrode in 1 M KOH + 0.33 M urea solution and absorbance collected at 350 nm, i-E plot (blue line) and A-E plot (orange line), (b) i-E plot (blue line) and derivative A-E plot (orange line).

3. 16 )Figure 3 . 19 .

 16319 Figure 3.19. Left y-axis: Remaining reactant percentage after 16 h of two solutions containing: 0.33 M CO(NH2)2 (dashed orange bars) or 0.33 M -OCN (blue bars) at pH=14 and pH=6. Secondary Y-axis, molar concentration of [NH4 + ] detected by ion chromatography.

Chapter 3 -

 3 Urea electro-oxidation on Ni-based anodes a result, different ratios of -OCN to NH3 can be expected depending on the diffusion layer thickness and hydrodynamic conditions during UOR.

Figure 3 . 20 . 5 MFigure 3 . 21 .Figure 3 . 22 .

 3205321322 Figure 3.20. Updated mechanistic understanding of UOR by adding our new UOR pathway (i). Reaction pathways during urea electro-oxidation showing all possible N-based products and their oxidation state in each case.

Figure 4 . 1 ,

 41 presents the behavior of an inert GC electrode (represented by the black plot) and a GC electrode dropcasted with NiO NPs (represented by the orange plot) in a 5 M NaOH solution at 0.33 V vs. Hg/HgO, where only capacitive processes were involved. The results indicate a significant reduction in the magnitude of the impedance (Zr), decreasing from 1.5x10 4 to 5x10 2 , upon the presence of NiO NPs on the GC electrode. Such a behavior is expected since the electrode material now shows different properties.

Figure 4 .

 4 Figure 4.1. (a) Nyquist plots corresponding to EIS response obtained in 5 M NaOH and 0.33 V vs. Hg/HgO on GC and GC modified with NiO NPs at 800 rpm. (b) Zoom of Figure 4.1a at HF domain.

Figure 4 . 2 .

 42 Figure 4.2. shows Nyquist and Bode plots representations corresponding to an electrochemical response at 0.53 V vs. Hg/HgO of an inert GC electrode and GC electrode modified on NiO NPs. At this potential, a faradaic process is taking place in the case of NiO/NiOOH (blue line of CV, Figure 3.9), while only capacitive behavior occurs on the GC electrode (blue line CV, Figure 2.5) Similarly to the previous results, both Nyquist and Bode plots clearly show a decrease in impedance magnitude from 10 6 to 10 3 when NiO is present. Moreover, a new time constant is observed at high frequencies on Bode plot in Figure 4.2b for GC electrode modified with NiO NPs (orange plot), which corresponds to a characteristic frequency of 146 Hz, indicating a charge-transfer process associated with the oxidation of NiO/Ni(OH)2 into NiOOH at the electrode surface. Furthermore, at lower frequencies, time constants with characteristic frequencies in the order of 10 -2 Hz, for both the inert GC electrode (0.03 Hz, black plot in Figure 4.2a) and the GC electrode modified with NiO/NiOOH (0.05 Hz, orange plot in Figure 4.2a) are observed. These time constants can be attributed to the relaxation of adsorbed OH -ions on the GC (black plot) and the NiO NPs (orange plot).

Chapter 4 -

 4 Effect of by-products during urea electro-oxidation

Figure 4 . 2 .Figure 4 . 3 .

 4243 Figure 4.2. (a) Nyquist plot and (b) Bode plot corresponding to EIS response obtained at 800 rpm in 5 M NaOH solution at 0.53 V vs. Hg/HgO on GC electrode (black plot) and on GC electrode modified with NiO/NiOOH NPs (orange plot).

Figure 4 . 3 .

 43 Figure 4.3. Nyquist plot corresponding to EIS spectra at the HF domain of NiO NPs in 1 M NaOH at 0.35 V vs. Hg/HgO.

Figure 4 . 4 .

 44 Figure 4.4. Capacitance graph (Cole-Cole) of the impedance calculated for the oxide film of NiO NPs in 1 M NaOH at 0.53 V vs. Hg/HgO.

4. 2 . 3 .Figure 4 . 5 and

 2345 Figure 4.5 and Figure 4.6 display the Nyquist plots corresponding to EIS spectra obtained with the NiO/NiOOH NPs electrode in both absence (Figure 4.5) and presence (Figure 4.6)of urea in solution at different polarization: 0.33 V, 0.43 V and 0.53 V vs. Hg/HgO. At 0.33 V vs. Hg/HgO, in the absence of urea(Figure4.5a and Figure4.5b green plots), a capacitive behavior is observed in the high frequency domain, which corresponds to the interfacial capacitance (electric double layer and the contribution of the oxide film formed at the NP surface), which is in agreement with the CV results shown in Figure3.9 (blue line), where non-Faradaic processes are taking place. This behavior is typically observed at low frequencies and is represented by a vertical line on the Nyquist plot.In contrast, at 0.43 V and 0.53 V vs. Hg/HgO (grey and orange plots, respectively in Figure 4.5a and Figure 4.5b), a semicircle loop with a characteristic frequency of 146 Hz is clearly observed in the Nyquist plot zoomed at HF domain (Figure 4.5b). This is attributed to the charge-transfer process associated with the NiO/Ni(OH)2 oxidation into NiOOH at the electrode surface.In addition to this, at 0.53 V vs. Hg/HgO (orange plots in Figure4.5a and Figure4.5b), an additional time-constant is observed with a characteristic frequency of 0.04 Hz, which may be attributed to the relaxation of the adsorbed OH -.

Figure 4 . 5 .

 45 Figure 4.5. (a) Nyquist plots corresponding to EIS spectra obtained on NiO/NiOOH NPs at 800 rpm in 5 M NaOH solution at different polarization potentials: 0.33 V (green), 0.43 V (grey), and 0.53 V (orange). (b) Zoom of Figure 4.5a on the HF domain.

  The Nyquist plots in Figure4.6a and Figure4.6b show the impact of different polarization potentials applied in the presence of urea in solution.

Figure 4 .

 4 6b, which zooms the HF domain of Figure4.6a, shows no difference compared to Figure4.5b . This demonstrates that the capacitive behavior observed at 0.33 V vs. Hg/HgO (green plot) and the chargetransfer process (characteristic frequency of 146 Hz) associated with the NiO/Ni(OH)2 oxidation into NiOOH at the electrode surface, are not affected by the presence of urea in solution. In contrast, at 0.43 V vs. Hg/HgO, an additional time-constant appeared with a characteristic frequency of 0.46 Hz (grey plot in Figure4.6a), suggesting urea electrooxidation which is in agreement with CV results reported in Figure3.9 of Chapter 3 (green line). At 0.53 V vs. Hg/HgO, during UOR as well but under mass-transfer control, a reverse semicircle loop is observed in the Nyquist plot (blue plot in Figure4.6a), indicating a negative polarization resistance (Rp < 0). This negative Rp value results in a reverse capacitive loop, regardless of either the presence or absence of any adsorption process, which sometimes has been confusedly attributed to adsorption on the electrode surface in previous EIS studies related to UOR[15,16].

Figure 4 . 6 .

 46 Figure 4.6. (a) Nyquist plots corresponding to EIS spectra obtained on NiO/NiOOH NPs at 800 rpm in 5 M NaOH solution in the presence of 0.01 M urea and at different polarization potentials: 0.33 V (green), 0.43 V (grey) and 0.53 V (blue). (b) Zoom of Figure 4.6a on the HF domain.

Chapter 4 -Figure 4 . 7 .

 447 Figure 4.7. Theoretical Lissajous plot of a linear EIS response, and experimental CV of NiO/NiOOH anode in 5 M NaOH and 0.01 M urea at 800 rpm and 10 mV s -1 . Green arrowsshow the direction of the potential scan, while the red and blue arrows represent the applied potentials during EIS at 0.43 V and 0.53 V vs. Hg/HgO, respectively.

Figure 4 .

 4 Figure 4.8a. As previously mentioned, the process at 146 Hz shown in Figure 4.8b remains constant, as well as that at 0.04 Hz (Figure 4.8a), which is associated to the relaxation of the adsorbed OH -. In contrast, when urea is present in solution (blue plot in Figure 4.8a) an additional time constant appears at 0.15 Hz, which is associated to the relaxation of an adsorbed intermediates from either the products/by-products generated during the electro-oxidation of urea or by the adsorption of urea itself on the surface of the electrode.

Figure 4 . 8 . 4 . 2 . 4 .

 48424 Figure 4.8. (a) Nyquist plots corresponding to EIS spectra obtained on NiO/NiOOH NPs at 800 rpm in 5 M NaOH solution at 0.53 V vs. Hg/HgO, in the absence (orange plot) and the presence (blue plot) of 0.01 M urea in solution and (b) Zoom of Figure 4.8a on the HF domain.

Figure 4 .9 and Figure 4 . 2 Z

 442 10 show the corresponding EIS diagrams, presented on a Nyquist plot and Bode plot. Nyquist plots in Figure 4.9a and Figure 4.10a are normalized by the low frequency value r / W.cm 2 146Hz of the real part in order to provide the easiest comparison and to avoid any effect of the active surface area. Additionally, the Bode plot is included to offer a clearer observation of the results (Figure 4.9b and Figure 4.10b).Those EIS measurements were performed at 0.53 V vs. Hg/HgO to specifically enhance the deactivation process of the catalyst that was previously observed during UOR. The results presented in Figure 4.9a and b (in the absence of urea), in both the Nyquist and Bode plots respectively, demonstrate that the electrochemical response of NiO/NiOOH NPs in 5 M NaOH (black plot) is not affected by the presence of CO3 2-(red plot), NO2 -(blue plot), -OCN (green plot) or NH3 (orange plot).

Figure 4 . 9 .

 49 Figure 4.9. (a) Normalized Nyquist plots and (b) Bode plots corresponding to EIS spectra obtained on NiO/NiOOH NPs at 800 rpm and 0.53 V vs. Hg/HgO. Solution composition: 5 M NaOH. In the presence of 0.01 M CO3 2-(red plot), 0.01 M NO2 -(blue plot), 0.01 M -OCN (green plot) or 0.01 M NH3 (orange plot) in solution.

Figure 4 . 5 M

 45 Figure 4.10. (a) Normalized Nyquist plots and (b) Bode plots corresponding to EIS spectra obtained on NiO/NiOOH NPs at 800 rpm and 0.53 V vs. Hg/HgO. Solution composition:5 M NaOH and 0.01 M urea (black plot). In the presence of 0.01 M CO3 2-(red plot), 0.01 M NO2 -(blue plot), 0.01 M -OCN (green plot) or 0.01 M NH3 (orange plot) in solution.

Chapter 4 -

 4 Effect of by-products during urea electro-oxidation vs.Hg/HgO before 1h CA of 0.33 M urea after 1h CA of 0.33 M urea

Figure 4 . 11 .

 411 Figure 4.11.Three consecutive CVs of NiO/NiOOH NPs deposited on a RDE at rotation speed 800 rpm and scan rate 10 mV s -1 in 5 M NaOH before (black line) and after (orange line) 1h of CA at 0.53 V vs. Hg/HgO in 5 M NaOH and 0.33 M urea solution.

Figure 4 .

 4 Figure 4.12 shows the Nyquist plots corresponding to EIS spectra obtained on the NiO/NiOOH NPs electrode at 0.2 V vs. Hg/HgO, a potential region where only purely capacitive processes are expected. Those EIS data allow evaluating the electrode aging and any surface modification resulting from being exposed to urea. To investigate this, the electrode is examined by EIS before and after undergoing immersion in urea solution and urea electro-oxidation either by CV or electrolysis.Figure 4.12a and b compare the

Figure 4 .

 4 12a and b compare the Nyquist and Bode plots of the electrode immersed in a urea-free solution (black plot), immersed 1 h in a 0.33 M urea solution (green plot) and after performing 1 CV cycle up to 0.6 V vs. Hg/HgO in a 0.33 M urea solution. All these EIS results are almost identical.Additionally, Figure4.12c and d show that the electrode surface is not modified after 1 hour of electrolysis at 0.53 V vs. Hg/HgO in a 0.33 M urea solution (orange plot). This suggests that none of the by-products generated during urea electro-oxidation remains adsorbed on the electrode surface, which is consistent with the observations from the CV analysis in Figure4.11. Consequently, Figure4.12 also suggests that NiO/NiOOH NPs deactivation presented in Chapter 3 (Figures 3.9 and 3.11) by a significant decrease in current density at 0.53 V vs. Hg/HgO is most likely not due to the adsorption of urea.

Figure 4 . 12 . 5 M 2 (Figure 4 .Figure 4 . 14 .

 412524414 Figure 4.12. Nyquist plots corresponding to EIS spectra obtained on NiO/NiOOH NPs at 800 rpm and 0.2 V vs. Hg/HgO in 5 M NaOH solution (black curves). (a and b) After doing a cyclic voltammogram up to 0.6 V vs. Hg/HgO (scan rate 10 mV s -1 ) in 5 M NaOH and 0.33 M urea solution (red curve), and after electrode immersion for 1 h in 5 M NaOH and 0.33 M urea solution (green plot). (c and d) After 1 h of electrolysis at 0.53 V vs. Hg/HgO in 5 M NaOH and 0.33 M urea solution (orange plot).

4. 3 .

 3 ConclusionsIn conclusion, the presented experimental results have evaluated the stability and performance of a nanostructured NiO/NiOOH anode in the electro-oxidation of urea. The data obtained from cyclic voltammetry and chronoamperometry in chapter 3 have indicated that the catalyst undergoes a deactivation process during the electrochemical reaction of urea. EIS measurements have been applied in chapter 4 to investigate the catalyst surface. The Nyquist plots obtained from EIS measurements in 5 M NaOH showed a significant reduction in the impedance magnitude upon modifying the GC electrode with the catalytic ink of NiO NPs. At high frequencies, EIS measurements demonstrated the porosity of the NiO/NiOOH anode, additionally the complex capacitance plot allowed the estimation of the thickness of the NiOOH film formed (4 nm) at the electrode surface resulted from the electrochemical activation.

Figure 5 . 3 .

 53 Figure 5.3. Graphical representation showing the natural chemical decomposition of urea into ammonia with the release of ammonia into the atmosphere represented by the red arrows.

Figure 5 . 4 displays

 54 CVs of the NiO/NiOOH anode under two different ammonia concentrations (0.01 M and 0.02 M) in 1 M NaOH, corresponding to pH 14 (Figure 5.4a),and in 0.1 M NaOH, corresponding to pH 13 (Figure 5.4b). These results show the absence of any anodic or cathodic peak associated with an ammonia redox process, which Chapter 5 -Conclusions and Perspectives limiting current associated with UOR remained unaffected by the rotation rate, indicating

Figure 5 . 6 .Chapter 5 -

 565 Figure 5.6. (a) AOR on NiO/NiOOH NPs deposited on a RDE studied by LSV at a scan rate 10 mV s -1 in 1 M NaClO4 and 0.01 M ammonia solution at pH = 10 and at different rotation rates. (b) AOR and UOR on NiO/NiOOH NPs deposited on a RDE studied by CA at different rotation rates in 1 M NaClO4 and 0.01 M ammonia solution and 1 M NaOH and 0.33 M urea solution at pH = 14, respectively.

Figure 5 . 7 .Annex 1 .

 571 Figure 5.7. CVs of artificial urine in 5 M NaOH at 10 mV s -1 and different rotation rates.

Figure 1 .

 1 Figure 1. XRD pattern of the catalyst ink of NiO NPs.

Face à la pénurie

  d'énergie et à la nécessité pressante de lutter contre la pollution environnementale, l'électro-oxydation des eaux usées riches en urée ou de l'urine en milieu alcalin apparaît comme une solution prometteuse. Ce processus innovant Ce processus innovant répond simultanément à deux défis critiques : traiter efficacement les eaux usées pour produire de l'eau propre à l'anode par électrolyse de l'urée (Équation 1) et générer de l'hydrogène comme source d'énergie renouvelable à la cathode par réduction de l'eau (Équation 2). Cette approche offre une alternative durable pour pallier la pénurie d'énergie tout en réduisant l'impact néfaste du rejet des eaux usées, tel que l'eutrophisation causée par le relâchement excessif d'ammoniac NH3.Électrolyse de l'urée pour la production d'hydrogène en milieu alcalin : A l'anode: CO(NH2)2(aq) + 8 OH - N2 + 6 H2O(l) + CO3 2-(aq)+ 6 e - E 0 = -0,85 V vs. SHE (1) A la cathode: 6 H2O + 6 e - 3 H2 + 6 OH - E 0 = -0,83 V vs. SHE(2)

( 3

 3 Toutes les limitations mentionnées ci-dessous seront traitées au Chapitre 3).De nombreuses investigations sur l'UOR ont été menées en utilisant des catalyseurs à base de nickel. Cependant, à notre connaissance, il y a une pénurie ou une recherche très limitée sur l'utilisation des NPs de NiO pour l'UOR, malgré la grande stabilité du NiO dans un milieu fortement alcalin par rapport au Ni 0 . De plus, dans la littérature, peu d'études ont été menées sur l'UOR à l'aide de l'électrode à disque tournant (RDE)[18], justifiant ainsi une investigation plus approfondie dans ce travail. Cependant la performance des NPs de NiO pour l'UOR a été examinée dans cette thèse en utilisant la RDE. Il est important de noter que la littérature rapporte différentes valeurs pour le potentiel standard (E 0 ) de l'UOR, ce qui soulève la nécessité de calculs clairs pour déterminer la valeur appropriée. Un aspect de ce chapitre se concentrera sur la résolution de cette question en présentant un calcul précis du E 0 pour l'UOR. Ce paramètre revêt une importance significative car il joue un rôle crucial dans l'évaluation des performances du catalyseur et fournit des informations précieuses sur l'énergie nécessaire au traitement des eaux usées et à la production d'hydrogène. De plus, expérimentaux pour les expériences RDE, EIS et UV-vis SEC sont décrites, en mettant en évidence leurs composants clés et leurs paramètres. Chapitre Ce chapitre présente un calcul complet du potentiel standard E 0 pour l'électro-oxydation de l'urée, qui résout les divergences constatées dans la littérature. Les NPs de NiO ont montré une excellente stabilité en comparaison avec les Ni NPS dans les milieux alcalins. Cependant, les NPs de NiO nécessitent une activation électrochimique préalable, impliquant plusieurs cycles courant-potentiel atteignant des potentiels élevés pour induire la réaction d'évolution de l'oxygène (OER), comme rapporté ailleurs [23-26]. Ce processus d'activation enrichit la surface initiale de NiO avec une coquille externe de Ni(OH)2/NiOOH, créant ainsi une structure coeur-coquille de NiO et de Ni(OH)2/NiOOH [24-26]. Les investigations présentées dans ce chapitre ont été principalement réalisées par voltammétrie cyclique et chronoampérométrie, avec ou sans rotation de l'électrode, et couplées ou non à la spectroscopie UV-vis (opérando SEC). En particulier, nous nous sommes principalement concentrés sur deux concentrations d'urée : 0,01 M et 0,33 M, ainsi que sur deux solutions alcalines différentes : une solution de NaOH de 1 M ou 5 M, préalablement saturées de gaz argon. Le choix de 0,33 M d'urée a été fait pour se rapprocher étroitement de la concentration maximale d'urée dans l'urine [4,10]. En revanche, l'urée à 0,01 M a été principalement utilisée pour atteindre un excès important d'OH -dans la solution et maintenir un rapport élevé (R) de concentrations de NaOH et CO(NH2)2 (R = [NaOH] / [CO(NH2)2]), ce qui évite tout effet potentiel de migration pendant l'UOR. La performance des NPs de NiO/NiOOH et de l'électrode vrac de Ni dans l'UOR est étudiée en utilisant la RDE à différentes vitesses de rotation et à différents ratios (R) de Célectrolyte/Curée. Les résultats démontrent que le courant limite pour la réaction d'oxydation de l'urée est indépendant de la vitesse de rotation (transport de masse) au-delà de 400 rpm, et ils mettent en évidence une différence dans le taux de régénération électrochimique du NiOOH entre la surface de l'électrode de Ni et la surface des NPs de NiO.Des expériences d'UV-vis SEC en mode opératoire ont été menées en collaboration avec l'Université de Burgos (Espagne) afin d'étudier le mécanisme de réaction d'UOR et de suivre en temps réel la formation des produits de réaction. Les deux configurations possibles de l'UV-vis SEC en mode opératoire ont été étudiées à l'aide de l'équipement de spectroélectrochimie couplée SPELEC de Metrohm. La configuration parallèle de la SEC permet la détection des produits présents à proximité immédiate de la surface de l'électrode et en solution, tandis que la configuration normale révèle des informations sur les produits attachés et les processus se déroulant à la surface de l'électrode (NPs de NiO/NiOOH).Les données présentées dans la Figure1acomprennent des courbes courant -potentiel (i-E ) et dérivée d'absorbance-potentiel (dA/dE ) à 220 nm, pourraient se référer au CO3 2-et/ou - OCN sur la base des mesures à blanc. De même, les données présentées dans la Figure1bcomprennent des graphiques courant-potentiel (i-E ) et la dérivée d'absorbance-potentiel (dA/dE ) à 350 nm, qui correspond à la longueur d'onde associée au nitrite NO2 -, sur la base des mesures à blanc. Ces résultats indiquent que les absorbances associées au CO3 2-et/ou - OCN (Figure1a) et NO 2-( Figure1b) augmentent à partir de 1,4 V vs. RHE, où se produit l'UOR, ce qui suggère leur formation en tant que produits de l'UOR. Un maximum d'absorbance est observé à 1,35 V vs. RHE dans le balayage inverse et une diminution continue de l'absorbance due à la diffusion de ces produits (CO3 2-et/ou -OCN et NO 2-) depuis la couche de diffusion vers la solution en vrac est également observée à des potentiels inférieurs dans le balayage inverse (E < 1,25 V vs. RHE). Cette étude par UV-vis SEC confirme que NO2 -, -OCN et/ou CO3 2- sont l'un des sous-produits formés lors de l'électro-oxydation de l'urée, et ils sont surveillés avec succès pour la première fois par opérando UV-vis SEC.

Figure 1 .

 1 Figure 1. Configuration parallèle UV-Vis SEC de NiO/NiOOH NPs déposés sur une électrode GC dans 1 M KOH + 0.33 M urée, i-E (ligne bleue) et la dérivée A-E (ligne orange) (a) 220 nm, (b) 350 nm.

Figure 3 .

 3 Figure 3. CVs des NiO/NiOOH NPs deposés sur une RDE avec une vitesse de rotation 800 rpm une vitesse de balayage 10 mV s -1 dans NaOH 5 M + urée 0.33 M, avant (ligne noire) et après (ligne orange) 1 h de CA à 0.53 V vs. Hg/HgO dans NaOH 5 M et urea 0.33 M solution, accompagné par une illustration schématique de la surface du catalyseur.

Table 1 .

 1 Chemical reagents used in experiments.

	2.1.1. Chemical Reagents
	Reagent Name	Reagent formula	Reagent Grade	Brand
	Nickel Oxide nanoparticles	NiO NPs (<50 nm)		Sigma-Aldrich
	Ni nanoparticles	Ni NPs (<100 nm)		Sigma Aldrich
	Isopropanol anhydrous	C3H8O	99.5%	VWR Prolabo
	Nafion™ perfluorinated resin		5 wt. % in lower	Sigma-Aldrich
	solution		aliphatic alcohols
			and water,
			contains 15-20%
			water
	Urea	CO(NH2)2	99.5 %	Sigma-Aldrich
	Sodium hydroxide	NaOH (pellets)	>98%	VWR Prolabo
	Sodium carbonate	Na2CO3	99.5%	Sigma Aldrich
	Sodium cyanate	NaOCN	96%	Sigma Aldrich
	Ammonium sulfate	(NH4)2SO4	99.5%	VWR Prolabo
	Sodium nitrite	NaNO2	>97%	Sigma Aldrich
	Creatinine	C4H7N3O	>98%	Merck
	Uric acid	C5H4N4O3	>99%	Sigma Aldrich
	Potassium chloride	KCl	100%	VWR Prolabo
	Calcium chloride	CaCl2	>93%	Sigma Aldrich
	Sodium bicarbonate	NaHCO3	>99.7%	Sigma Aldrich

All solutions were prepared by using ultrapure water with resistivity >18.2 MΩ cm at 25 °C (Elga Purelab Flex, Veolia Water Technologies).

2.1.2. Experimental Materials and Instruments

 Rotating disk electrode (WaveVortex 10) was purchased from PINE research.  Bipotentiostat Workstation is 760E from CH Instruments connected to RDE. Cyclic voltammetry experiments have been conducted, starting from the OCP and scanning towards an anodic polarization.  pH meter (Five Easy) from Mettler Toledo.  SPELEC UV-vis (200-900 nm) from Metrohm.  SPELEC RAMAN from Metrohm.  Ultrasonic cleaners from Thermo-Fisher. 2.1.3. Data acquisition and analysis software  CHI software was used for performing cyclic voltammetry, chronoamperometry, linear sweep voltammetry, and electrochemical impedance spectroscopy. Chapter 2 -Materials and experimental methods  DropView SPELEC software (Metrohm-DropSens) was used for performing operando, time-resolved SEC experiments.  Data were treated by OriginPro 2018 2.2. Rotating disk electrode (RDE)

Table 3 .

 3

1. Thermodynamic standard potentials calculated in alkaline media (pH=14).

nm

Chapter 3 -Urea electro-oxidation on Ni-based anodes 3.4. References et donc être responsables du processus de désactivation observé. Des mesures de spectroscopie d'impédance électrochimique (EIS) ont été réalisées dans la littérature pour étudier l'effet de l'adsorption du CO2 sur les catalyseurs à base de Ni, cependant les résultats ont été mal interprétés. De plus, étant donné que d'autres sous-produits ont été détectés ( - OCN, NO2 -, NH3) nous étudierons l'effet de tous ces sous-produits en plus du CO3 2-afin de confirmer l'absence ou la présence d'une adsorption. Par conséquent, l'identification de la source de désactivation de l'anode aidera à concevoir la prochaine génération d'électrocatalyseurs plus efficaces pour l'électrolyse de l'urée.

Résumé des chapitres :

Chapitre 1 Le contexte de recherche sur la réaction d'électro-oxydation de l'urée est présenté en détail.

Le chapitre commence par souligner l'importance du traitement des eaux usées en tant que préoccupation environnementale pressante, en mettant l'accent sur l'élimination des polluants tels que l'ammoniac qui contribuent à l'eutrophisation. L'électro-oxydation de l'urée représente une approche prometteuse pour le traitement simultané des eaux usées, à l'anode, et la production d'hydrogène à la cathode. Différents catalyseurs à base de nickel ont été identifiés comme des catalyseurs très efficaces pour l'UOR. Ce chapitre donne un aperçu de la compréhension mécanistique initiale de UOR sur des catalyseurs à base de nickel en incluant les voies de génération de sous-produits et la stabilité du catalyseur. Ainsi, il pose les bases pour les chapitres suivants, qui se concentreront sur les méthodes expérimentales, les résultats et les discussions. Chapitre 2 Une vue d'ensemble théorique et expérimentale de différentes techniques de caractérisation et méthodes utilisées pour l'évaluation des NPs de NiO en tant que catalyseur pour l'UOR est fournie. Il présente les différents réactifs utilisés pour la préparation de l'encre catalytique et des solutions d'électrolyte. Ce chapitre couvre également la description des techniques électrochimiques et des expériences utilisées, telles que RDE, CV, CA, EIS et UV-vis SEC. En particulier, les principes et équations principaux liés à RDE, y compris les approches de Levich et Koutecký-Levich, sont décrits en détail. Les configurations des dispositifs Acknowledgements I am immensely grateful to have had the opportunity to pursue a PhD in chemistry, a path that unfolded during my undergraduate studies and became an unforgettable journey.

This remarkable undertaking, though at times challenging, has been a profoundly enlightening experience, and I owe its success to the invaluable help and support of numerous individuals, whom I would like to express my heartfelt gratitude.

First, my deepest gratitude goes to my supervisor, Carlos Sánchez-Sánchez, whose constant support and motivation have been the driving force behind my four-year-long endeavor, from the beginning of my M2 project to my internship and culminating in this thesis. I am truly grateful for the invaluable guidance and lessons he provided in the field of electrochemistry, his expertise has been instrumental in deepening my understanding.

Beyond his role as a supervisor, I cherish the moments we spent with his family, where meaningful discussion about life were shared. Your support and belief in me have made all the difference, and I will carry the knowledge and lessons I've gained from you throughout the rest of my career. Your mentorship has had a profound impact on both my academic and personal growth, and for that, I am truly grateful. I would like to extend my sincere gratitude to my co-supervisor, Vincent Vivier, for his invaluable expertise in chemistry, particularly in Electrochemical Impedance Spectroscopy. Knowing that I have such a resourceful co-supervisor provided me with a sense of security and confidence in my work. I am truly fortunate to have had him as a cosupervisor, always available to answer my questions and offer valuable guidance. I also want to thank the jury committee members: Prof. Dodzi Zigah, Prof. Ignacio Sirés Dr. Conchi Ania and Dr. Encarnación Torralba-Peñalver for their efforts and time devoted to read and revise my PhD thesis. I would like to express my heartfelt gratitude to Dr. Juan Victor Perales, Prof. Alvaro Colina and M. Luis Ramey at Burgos Universidad in Spain, for their invaluable guidance in teaching me SEC experiments, which was really nutritive for my thesis. I extend my sincere gratitude to the collaborators of the ANR-HYUREA project, especially Dr. Stéphane Bastide, Prof. Christine Cachet-Vivier, Prof. Théodore Tzedakis, Prof. Karine Groenen Serrano and Dr. Karine Loubière, for their valuable scientific discussions and

This thesis work is in the framework of a collaborative project financed by the French National Research Agency (ANR). The HYUREA project, also known as Hydrogen Production Chapter 1 -

Chapter 3 Urea electro-oxidation on Nibased anodes

Chapter 3 -Urea electro-oxidation on Ni-based anodes alkaline solutions: either 1 M or 5 M NaOH solution, previously saturated with Argon gas. The choice of 0.33 M urea was made to closely resemble the maximal concentration of urea in urine [6,34]. Conversely, 0.01 M urea was predominantly utilized to reach large excess of OH - in solution, and keep a high ratio (R) of NaOH and CO(NH2)2 concentrations (R =

[NaOH]/[CO(NH2)2]), which avoids any potential migration effect during UOR. Figure 3.9 shows the electro-oxidation of 0.01 M urea in a 5 M NaOH solution on NiO NPs (green line). The CV in the presence of urea shows that UOR occurs once the polarization potential of the electrode reaches the point (E > 0.4 V vs. Hg/HgO) for oxidizing the inactive Ni(OH)2 species into the active Ni form (NiOOH). Then, the NiOOH species facilitates the chemical reaction with urea and subsequently reduces back into Ni(OH)2, which is regenerated under polarization potential (E > 0.4 V vs. Hg/HgO). The described behavior suggests an indirect pathways EC (electrochemical-chemical reaction) mechanism.

Additionally, it is noteworthy that the cathodic peak centered at 0.35 V vs. Hg/HgO in Figure 3.9, which is attributed to the electrochemical reduction of NiOOH into Ni(OH)2 decreases in charge in the presence of urea. This observation seems to indicate that the chemical oxidation of urea, which reduces NiOOH to Ni(OH)2 through reaction 3.11 in the forward scan and the early phases of the reverse scan, is faster than the electron transfer required to regenerate Ni(OH)2 into NiOOH. Consequently, this leads to a decrease in the amount of NiOOH available for reduction in the cathodic region (E < 0.4 V vs. Hg/HgO).

Chapter 3 -Urea electro-oxidation on Ni-based anodes and solution pH with time. In particular, the concentration of CO(NH2)2, -OCN and NH3 (analyzed in ion chromatography as NH4 + ) were quantified after 16 hours. The main purpose of this study was trying to prove whether the two new proposed UOR pathways based on combining chemical and electrochemical steps are possible or not. Figure 3.19 compares the theoretical remaining percentage of each reactant intially present in solution (CO(NH2)2 and -OCN) assuming no chemical decomposition reaction (100%) and after 16 hours in solution at both pH 14 and 6. The concentration of CO(NH2)2 (orange bars) remained stable (0.33 M) in both acidic and alkaline media, indicating no significant chemical decomposition of urea, and thus ruling out the participation of this reaction (Eq. 3.15) in the production of -OCN and NH3, which discard our new UOR pathway (ii ). However, in the case of -OCN solution, significant amounts of NH3 were detected in both media (pH = 14 and pH = 6), but at different ratios. Quantification of CO3 2-was only possible at pH 14, but the concentration was too high and not reliable due to the important carbonatation of the solution after 16 h. Therefore, the decomposition reaction of -OCN seems to happen with NH4 + present at a ratio of ([ -OCN]:[NH4 + ]) (2:1) and (1.4:1) at pH= 14 and pH=6, respectively. In addition to this, it is worth noting that during the solution preparation of 0.33 M -OCN and 0.5 M Na2SO4, the pH increased from pH = 6 to pH= 9, and it was re-adjusted to pH =6, by adding H2SO4. This increase in solution pH during the -OCN solution preparation may be attributed to the formation of NH4 + from the chemical hydrolysis of -OCN (Eq. 3.16). In Chapter 3, Figure 3.9 shows 3 consecutive potential cycles on a NiO/NiOOH electrode in the absence of urea in solution (Figure 3.9, blue line) and in the presence of 0.01 M (R=500) urea (Figure 3.9, green line). NiO/NiOOH NPs in a 5 M NaOH solution exhibit good stability and the characteristic oxidation-reduction current peaks for Ni(OH)2/NiOOH remained unchanged along the potential cycles. In contrast, in the presence of 0.01 M urea, the oxidation current is amplified due to the UOR reaction, but it slightly decreases after each new cycle (green line, Figure 3.9). This deactivation behavior of the catalyst is more evident at higher concentration of urea (0.33 M, R = 15). For instance, Figure 3.11a presents 6 consecutive potential cycles on NiO/NiOOH NPs in the presence of 0.33 M urea (blue and red plots), and shows that the current density associated with UOR decreases after each new cycle, which has been previously attributed to the catalyst deactivation in the literature [2,3,7,[15][16][17]. Moreover, Figure 3.11b presents a CA curve at 0.53 V vs. Hg/HgO of 5 M NaOH + 0.33 M urea, results indicate a continuous decrease in current density from 2.5 mA cm -2 at 200 s to 1.6 mA cm -2 at 1200 s. This significant decrease clearly demonstrates a fast catalyst deactivation at 0.53 V vs. Hg/HgO. Furthermore, it highlights that UOR exhibits higher activity at this applied potential compared to 0.45 V vs. Hg/HgO, as shown in Figure 3.12 (blue plot) where a current density of 0.96 mA cm -2 is observed at 1200 s.

In the following section, EIS measurements were performed in two different potential regions to investigate specific phenomena taking place on the electrode surface. The first set of measurements, performed EIS under operando conditions within a potential range of 0.33 V vs. Hg/HgO, where capacitive processes were involved, as well as at 0.43 V and 0.53 V vs. Hg/HgO, where the faradaic UOR reaction takes place. This allows for the analysis of the reaction mechanism and the study of by-product adsorption on the catalyst surface using EIS. In the presence of adsorption, the Nyquist plot exhibited an inductive loop at low frequencies, indicating the relaxation process of the adsorbed species [18].

The second set of EIS measurements focused on a potential where only purely capacitive processes are involved (0.2 V vs. Hg/HgO), indicating a blocking electrode behavior [10]. This range of potential was chosen to characterize and investigate the aging of the electrode. Initially, AOR on NiO/NiOOH anode was studied in 1 M NaClO4 at pH = 12 in the absence and presence of 0.01 M ammonia (Figure 5.5a). Similar to the results obtained in Figure 5.4 at pH 13 and pH 14, no oxidation peak associated with AOR was observed in the presence of 0.01 M ammonia at pH=12 (orange line). Figure 5.5b presents AOR studied by CV at pH 10, in the absence (blue line) and the presence of 0.01 M ammonia (orange line). These results show that an oxidation current rises (E > 0.9 V vs. Hg/HgO, orange line) when 0.01 M ammonia is present in solution, indicating its oxidation. The absence of an oxidation current for ammonia at higher pH values can be attributed to the fact that the oxygen evolution reaction (OER) becomes more favorable at higher pH. Thus, the OER takes place at less positive potentials, approximately 0.7 V vs. Hg/HgO, which coincides with the onset potential of ammonia oxidation. Consequently, the oxidation peaks related to ammonia are not observable at higher pH levels. On the other hand, at lower pH values such as 10, the OER occurs at a later potential, around 1.27 V vs. Hg/HgO, allowing for the observation of AOR at 0.98 V vs. Hg/HgO. Furthermore, as stated at the beginning of the thesis, the solution of urine may contain a Chapter 5 -Conclusions and Perspectives combination of ammonia and urea, depending on the urea stockage conditions. Therefore, we conducted a few experiments to investigate whether urea can undergo oxidation at pH 10. Additional research has been conducted to explore and compare the electro-oxidation of ammonia and urea by varying the rotation rate (ω). Figure 5.6 displays LSV of the NiO/NiOOH anode in a solution containing 1 M NaClO4 and 0.01 M ammonia at different rotation rates.

The results indicate that the current associated with AOR increases with increasing the rotation rate. In addition, CA of AOR was performed at 1.15 V vs. Hg/HgO, a potential where the ammonia electro-oxidation reaction takes place under mass transfer control according to the CV in Figure 5.5b. The results are shown in Figure 5.6b and compared with the UOR data presented within a small box (reported in Chapter 3 Figure 3.11b). For the UOR experiments, the applied potential was also set where UOR is under mass transfer control according to the corresponding CV. This comparison in Figure 5.6b shows a contrasting behavior between AOR and UOR. The limiting current associated with AOR increases by increasing the rotation rate suggesting that the reaction is controlled by mass transfer. On the other hand, the des études récentes ont indiqué la formation de divers produits lors de l'électro-oxydation de l'urée qui vont au-delà du N2 et du CO2 traditionnellement étudiés [19][20][21][22]. Par conséquent, ces calculs prendront en compte une nouvelle gamme de produits récemment détectés lors de l'électro-oxydation de l'urée. Les produits supplémentaires à prendre en compte incluent le cyanate ( -OCN), le carbonate (CO3 2-), le nitrite (NO2 -), le nitrate (NO3 -) et l'oxyde nitreux (N2O).

De plus, des publications récentes [19,20] ont fourni de nouvelles données pour clarifier les voies de formation du -OCN, CO3 2-, NO2 -, NO3 -, N2O et N2 en tant que sous-produits de UOR.

Cependant, la voie de réaction pour la formation d'ammoniac (NH3) pendant l'électrolyse de l'urée [22] reste incertaine. Ainsi, dans ce chapitre, nous présentons de nouvelles données de UV-vis opérando SEC, qui ont été collectées grâce à la collaboration avec le Dr J.V. Perales-Rondón et le Pr. A Colina de l'Université de Burgos en Espagne. Cette équipe de recherche est reconnue pour son expérience dans les études spectroélectrochimiques [16][17][18][19], mais elle n'avait jamais étudié auparavant l'UOR. La technique opérando SEC offre une compréhension approfondie de la formation des sous-produits pendant une réaction électrochimique, soit adsorbés à la surface de l'électrode, soit dans la couche de diffusion (la configuration est décrite dans la section 2.4.1). Par conséquent, l'opérando SEC a été appliquée ici pour la première fois pour étudier l'UOR sur des NPs de NiO, fournissant des informations précieuses sur la dynamique de la réaction électrochimique et éclairant les intermédiaires et les voies impliquées dans ce processus. De plus, la stabilité chimique du CO(NH2)2 et du -OCN en solution aqueuse a été étudiée par analyse en chromatographie ionique (IC) et a été réalisée dans le cadre du projet ANR HYUREA et en collaboration avec l'équipe LGC -Toulouse, France.

(Toutes les limitations mentionnées ci-dessous seront traitées au Chapitre 4).

Selon la littérature, l'anode NiOOH présente une désactivation catalytique significative et progressive lors de l'électro-oxydation de l'urée [18,23]. Notamment, la désactivation a été attribuée à l'adsorption d'intermédiaires de réaction ou de sous-produits formés pendant l'UOR. La plupart des études mécanistiques rapportées jusqu'à présent n'ont considéré que le CO2 ou le CO3 2-comme des sous-produits potentiels de l'électro-oxydation de l'urée formés sur les anodes de NiOOH, qui pourraient entrer en compétition avec l'urée pour les sites actifs (i) commence par le mécanisme incomplet de l'oxydation de l'urée (voie 3, Figure 2

List of Publications, trainings and conferences

Publications [1] My heartfelt thanks to the countless individuals who have contributed to this thesis in various ways. Your involvement and encouragement have been invaluable, and I am truly grateful for your support.