Alexandr OSHCHEPKOV

Étude des réactions d'électrodes de l'hydrogène sur des électrocatalyseurs de Ni en milieu alcalin

Résumé La thèse présentée traite principalement de l'influence de la composition et de l'état de surface d'électrodes à base de Ni sur la cinétique et le mécanisme des réactions d'oxydation/dégagement de l'hydrogène (HOR/HER) en milieu alcalin. En combinant les résultats de mesures électrochimiques avec une modélisation microcinétique, il a pu être montré que l'activité spécifique du Ni pour l'HOR/HER augmente jusqu'à 10 fois en présence à la fois d'oxydes de Ni et de Ni métallique à la surface de l'électrode. En outre, l'influence de l'addition d'un second métal aux électrocatalyseurs à base Ni sur leurs activités pour l'HOR/HER a été étudiée dans le cas des systèmes NiMo/C et NiCu/C. Dans les deux cas, une augmentation de l'activité spécifique a été observée par rapport à l'échantillon Ni/C de référence et a été attribuée à une diminution de l'énergie d'adsorption de l'hydrogène adsorbé sur Ni, espèce intermédiaire de l'HOR/HER. DE MOTS-CLÉS : Nickel, Électrocatalyseurs, Réaction d'oxydation de l'hydrogène, Réaction de d'évolution de l'hydrogène, Milieu alcalin, Modèle cinétique, Pile à combustible

Résumé en anglais

The present thesis is mainly focused on the influence of the surface state of Ni electrodes on the kinetics and the mechanism of the hydrogen oxidation/evolution reactions (HOR/HER) in alkaline medium. By combining the results of electrochemical measurements with microkinetic modeling, it was shown that specific activity of Ni in the HOR/HER increases up to 10 times if along with metallic Ni, Ni oxide species are present on the electrode surface. In addition, the effect of the addition of a second metal to Ni electrocatalysts on their activity in the HOR/HER was investigated for NiMo/C and NiCu/C systems. In both cases an enhancement of specific activity was observed in comparison with the reference Ni/C sample, which was assigned to a decrease of the adsorption energy of the hydrogen intermediate on Ni participating in the HOR/HER. KEYWORDS : Nickel, Electrocatalysts, Hydrogen oxidation reaction, Hydrogen evolution reaction, Alkaline media, Kinetic modeling, Fuel cell

Acknowledgement

Taking the opportunity, I would like to thanks all the people and organizations without whom this PhD work would not be possible. A great thanks to European Doctoral College and, in particularly, the educational director of PDI program Christopher Mueller for having given me the opportunity to be a part of this community. Many thanks also to Virginie Herbasch, Joëlle Hubé, Christine Fromholtz, Laurence Barondeau and all the members of PDI program and StrasAIR for making the PhD life more interesting and filling it with lots of remarkable events.

Last but certainly not least, I would like to thanks my parents, my brother and especially my darling wife who always believe in me and support me allowing to stay motivated for hard working. Thanks also to my friends for their patience and warm welcoming every times I came back to Russia.

General Introduction

Increasing energy demand and diminishing of fossil fuel resources have largely stimulated development of renewable and non-traditional energy sources during the past decades. Hydrogen is perceived as one of the most promising energy carriers, because of its high (per unit mass) energy density (ca. 3 times higher compared to gasoline). By use of fuel cell (FC) technology, the chemical energy stored in H2 can be electrochemically converted to electric energy with high efficiency and zero emissions (the only product of hydrogen oxidation being water). It is essential that hydrogen can be produced from renewable energy sources such as solar or wind (through the water splitting process). Besides, conversion of surplus electricity in the form of chemical energy of a fuel (e.g. hydrogen) allows to mitigate the intermittent nature of renewable power sources. Among hydrogen-fed FCs, the technology of the proton exchange membrane fuel cell (PEMFC) has had the most progress until now.

Significant decrease of the total content of platinum-group metals (PGMs) in the PEMFC stacks has been achieved during the past decades. However despite of this, the necessity to use the PGMs still remains one of the reasons for the high price of PEMFC, which is hindering their wide commercialization. One of the promising approaches to reduce the cost of FCs, intensively developed during the past decade, is based on the transition from acidic to alkaline media. The latter is achieved by the replacement of proton exchange membrane by anion exchange membrane and the development of alkaline membrane fuel cell (AMFC). The main advantage of the FC with alkaline electrolyte is the possibility to use non-noble metal (thus widely available and cheaper) catalysts, which would be unstable in acidic media.

Among non-noble metals, Ni is perceived as one of the most promising electrocatalysts for the anodes of AMFCs. It has low price, shows good corrosion stability in alkaline environments and possesses fair activity in the hydrogen evolution reaction (HER, i.e. the reverse reaction of the hydrogen oxidation (HOR), occurring at the anode of a FC). Due to utilization of Ni at the cathodes of alkaline water electrolyzers, the HER on Ni has been widely investigated. However the values of electrocatalytic activity of Ni reported in the literature may differ by nearly two orders of magnitude. There are two likely reasons for this inconsistency. First, in many studies, the activity is normalized to the geometric surface area of the electrode, while its real surface area could be significantly different from the geometric one. Second, the kinetics of the HER might depend on the methodologies used for preconditioning of the Ni electrode surface prior to measurements, which differ in these studies.

Meanwhile, Ni is prone to the formation of hydrides upon cathodic polarization and surface oxide species after anodic polarization and/or contact with the oxygen from air. The influence of Ni hydrides, formed at the cathode during water electrolysis, has been investigated in detail. On the contrary, even if some authors have pointed to a noticeable influence of surface oxidation on the activity of Ni electrodes in the HOR/HER, there have been no systematic studies of this effect yet.

Activity of Ni in the HOR/HER, being high compared to other non-noble metals, is, however, significantly lower with respect to the activity of Pt. It is known from the literature, that addition of a second metal (for example, Co, Zn, Fe, Cu, Mo, Cr, etc.) results in a noticeable increase of the activity of Ni electrocatalysts in the HER.

Recently several works devoted to the study of bi-and polymetallic catalysts in the HOR have also appeared. However the values of electrocatalytic activity reported by different authors are often contradictory (which could either be assigned to the above mentioned difficulties in the determination of the real surface area of the electrode or obtaining a reproducible surface state). Besides, the reasons for the observed activity increase are still not clearly understood and remain the subject of discussions.

In summary, we note that the literature data concerning electrocatalysis of the Chapter 4 is focused on the synthesis of nanostructured Ni electrocatalysts immobilized on either a flat surface of glassy carbon or a porous carbon support with the aim to get high metal loading at relatively small particle sizes. An influence of the surface state on the activity of these electrodes in the HOR/HER is discussed as well.

Chapter 5 is dedicated to the study of carbon supported bimetallic NiCu/C and NiMo/C catalysts and understanding how their composition influences the activity of Ni in the HOR/HER.

Finally, the General conclusions and the Outlook of the work are presented.

At the end of the thesis, supplementary information can be found in the Appendix section.

Chapter 1: Literature Review

Hydrogen electrode reactions

Hydrogen electrode reactions, which include the processes of cathodic evolution and anodic oxidation of hydrogen, are among the best-understood electrocatalytic processes up to now [1,2]. Relative simplicity of their mechanism has allowed to use the HOR/HER as model reactions in electrochemistry. Already in 1905 Tafel established the famous phenomenological equation relating the currents to the electrode potential [3]. Later in 1930, Volmer and Erdey-Gruz for the first time pointed out the retarded electrochemical step of the discharge of hydrogen ions during the HER [4,5], while Kobosev and Nekrasova demonstrated the influence of the adsorption energy of Had atoms on the kinetics of this process [6]. It was the study of the HER, which inspired Kobosev and Monblanova to invent the term "electrocatalysis" for the description of the catalytic effect of the electrode material on the kinetics of electrode reactions. Whereas development of polarography by Heyrovsky [7][8][9] as well as development of the thermodynamic theory of surface processes across the metal/solution interface and theory of the double layer structure by Frumkin [10][11][12] provided great impact to further progress in electrochemical kinetics.

Kinetics of the HOR/HER

The overall reactions of anodic oxidation (forward direction) and cathodic evolution (backward direction) of hydrogen in acidic or alkaline media can be expressed as follows:

H2 ! 2H + + 2e - (in acidic media) (eq. 1.1) H2 + 2OH -! 2H2O + 2e - (in alkaline media) (eq. 1.

2)

Notwithstanding their simplicity, compared to the outer-sphere electrode reactions the HOR/HER are complicated by the adsorption with the formation of adsorbed Had atoms as surface intermediate. The HOR/HER usually proceed according to either Heyrovsky-Volmer or Tafel-Volmer mechanism. The corresponding reaction steps can be written in acidic media as H2 ! 2Had (Tafel step) (eq. 1.3) H2 ! Had + H + + e - (Heyrovsky step) (eq. 1.4) Had ! H + + e - (Volmer step) (eq. 1.5) or in alkaline media as H2 ! 2Had (Tafel step) (eq. 1.6) H2 + OH -! Had + H2O + e -(Heyrovsky step) (eq. 1.7)

Had + OH -! H2O + e - (Volmer step) (eq. 1.8) In contrast to catalysis, in electrocatalysis the reaction rate depends on the electrode potential. For simplicity, let us first discuss the influence of the potential on the rate of a one-electron outer-sphere electrode reaction, which is non-complicated by adsorption:

R ! O + e - (eq. 1.9)

Here both oxidized (O) and reduced (R) species are present in the solution. The reaction rate can be written as follows [2]: " = # $ %& ' ( ) * $ +% ' , ) (eq. 1.10) where $ %& and $ +% are heterogeneous rate constants (in cm s -1 ) of the anodic and cathodic reactions, respectively; CO and CR are the concentrations (in mol cm -3 ) of the reacting species close to the metal/solution interface (superscript 's').

The observed overall currents (in amperes (A)) can be then written as -= -& * -+ = "./ = $ %& ./' ( ) * $ +% ./' , ) (eq. 1.11) where F is Faraday constant (in C mol -1 ), A is the electrode surface area (in cm 2 ).

According to the theory of activated complex the rate constant k depends exponentially on the standard free energy of activation (∆G # ) as $ = $0exp#1 234 5 67 8 (eq. 1.12) In electrochemical reactions, ∆G # for the cathodic (39 + 5 ) and anodic (39 & 5 ) reactions depends on the electrode potential E as follows [2]:

39 & 5 = 39 & 5:; * <> * ?@.<A * A ;B @ (eq. 1.13a)

39 + 5 = 39 + 5:; C ?.<A * A ;B @ (eq. 1.13b) where A ;B is the formal electrode potential and α is the charge-transfer coefficient.

By inserting eq. 1.12 and 1.13 into eq. 1.11 and introducing the standard rate constant k 0 , which corresponds to the case of equal bulk concentrations of oxidized and reduced species (' , D = ' ( D ), one would obtain the following equation for the overall current density: ) .$ ; exp 1

E = F G = ' ( ) .$ ; exp 1 <H2I@JKL2L MN O 67 8 * ' ,
2IJKL2L MN O 67 8 (eq. 1.14) At equilibrium (i = 0), the partial anodic and cathodic current densities have the same absolute value, which is denoted as the exchange current density (i0). Taking into account that surface and bulk concentrations under equilibrium conditions are also equal (' P = ' Q ), i0 can be written as:

E ; = .$ ; ' R Q <H2I@ ' 6 Q I
(eq. 1.15) Finally, by assuming rapid mass transport (i.e. when the reaction is controlled only by kinetics of the process) at equal bulk concentrations of oxidized and reduced species and introducing the overpotential (η = E -Eeq, where Eeq is the electrode potential at equilibrium conditions) instead of E, we obtain the well-known Butler-Volmer equation [2]:

E = E ; Sexp 1 <H2I@JT UV 8 * exp 1 2WJT UV 8X (eq. 1.
16)

The eq. 1.16 can be simplified at "low" (Y Z These equations yield a straight line, if we plot them in semi-logarithmic scale, also known as Tafel plots [3]:

η = a + b•ln|i| (eq. 1.19)
where a and b are called Tafel constants. Evaluation of the slope b allows to estimate α, while extrapolation of the linear segment of ln|i|/η curve up to the equilibrium potential (η = 0) yields i0. The Tafel plots are very useful tool for discussing kinetics of the electrode processes.

It should be noted that in contrast with outer-sphere electrode reactions the description of the kinetics of electrocatalytic processes is complicated by the formation of adsorbed intermediates. In particular, the surface coverages by these species, which generally also depend on the electrode potential, should be included in the Butler-Volmer equation (see Section 2.4). The latter complicates interpretation of the Tafel slopes in such electrocatalytic reactions as the HER (at negative η) or the HOR (at positive η).

Depending on the rate determining step, theory predicts at 25 °C a different value for the Tafel slope in the potential region of the HER [START_REF] Conway | Electrochemical reaction orders: Applications to the hydrogen-and oxygen-evolution reactions // Electrochim[END_REF][START_REF] Trasatti | Electrocatalysis of Hydrogen Evolution: Progress in Cathode Activation[END_REF], i.e. 120 mV for the limiting Volmer reaction, 30 mVfor the Tafel reaction and 40 mVfor the Heyrovsky reaction. However, for the majority of materials the Tafel slope of 120 mV is usually observed at practical current densities, due to either a slow discharge of hydrogen ions, or a slow electrochemical desorption of Had atoms from the electrode surface [START_REF] Conway | Electrochemical reaction orders: Applications to the hydrogen-and oxygen-evolution reactions // Electrochim[END_REF][START_REF] Trasatti | Electrocatalysis of Hydrogen Evolution: Progress in Cathode Activation[END_REF]. A low Tafel slope of 30 mV is mostly observed for expensive precious metals and only in acidic media, when the Volmer reaction is extremely rapid (being one of the most rapid elementary reactions). In particular, on Pt electrodes at pH close to 0 its rate is calculated to be ca. 0.42 A cm -2 [START_REF] Notoya | Determination of the rate of the discharge step of hydrogen ion on a hydrogen-platinum electrode in aqueous solutions by the galvanostatic transient method[END_REF]. The situation when various steps proceed with comparable rates has also been suggested [START_REF] Saraby-Reintjes | The hydrogen evolution reaction under mixed kinetic control[END_REF][START_REF] Divisek | Determination of the kinetics of hydrogen evolution by analysis of the potential current and potential coverage curves[END_REF]. In this case analysis of the Tafel slope becomes questionable. Besides, activated adsorption following Temkin's isotherm (instead of Langmuir isotherm usually used for the description of adsorption) has been postulated in order to explain the Tafel slope of ca.

60 mV observed in some studies [START_REF] Conway | Electrochemical reaction orders: Applications to the hydrogen-and oxygen-evolution reactions // Electrochim[END_REF].

Activities of different electrodes in the HOR/HER can be compared by estimation of corresponding exchange current densities, i0, at equal operating reactive intermediates neither too weakly, nor too strongly [START_REF] Sabatier | Hydrogénations et déshydrogénations par catalyse // Berichte der[END_REF][START_REF] Boreskov | Heterogeneous catalysis[END_REF]. As can be seen from the Fig. 1.1 the highest HER activity is observed for PGMs (Pt, Pd, Ir, Rh) with

ΔadGH close to zero, being in agreement with the prediction of Parson [START_REF] Parsons | The rate of electrolytic hydrogen evolution and the heat of adsorption of hydrogen // Trans[END_REF]. Some ,c) [START_REF] Trasatti | Work function, electronegativity, and electrochemical behaviour of metals III. Electrolytic hydrogen evolution in acid solutions[END_REF][START_REF] Nørskov | Trends in the Exchange Current for Hydrogen Evolution[END_REF][START_REF] Sheng | Correlating the hydrogen evolution reaction activity in alkaline electrolytes with the hydrogen binding energy on monometallic surfaces[END_REF]. Despite the observed correlations, the volcano-plots of the HOR/HER have been subject of justified criticism due to neglecting possible difference in the mechanisms of the hydrogen oxidation and evolution reactions on the same catalyst as well as a possible existence of several types of hydrogen adsorbates with different adsorption energy [START_REF] Santos | Hydrogen electrocatalysis on single crystals and on nanostructured electrodes[END_REF][START_REF] Petrii | Electrocatalytic activity prediction for hydrogen electrode reaction: intuition[END_REF]. Besides, the presence of oxide species on the surface of non-noble metals further complicates the utilization of volcano-plots, being one of the likely reasons of existing inconsistencies in the values of M-Had binding energy and exchange currents in the HOR/HER [START_REF] Quaino | Volcano plots in hydrogen electrocatalysis -uses and abuses[END_REF]. Notwithstanding some limitations, there have been a number of successful attempts to use volcano-plots for creating bimetallic systems with enhanced electrocatalytic activity in the HOR/HER by combination of metals located on the opposite branches of the curve [START_REF] Greeley | Computational high-throughput screening of electrocatalytic materials for hydrogen evolution[END_REF][START_REF] Sheng | Non-precious metal electrocatalysts with high activity for hydrogen oxidation reaction in alkaline electrolytes[END_REF]. Along with the use of adsorption energy ΔadGH, some other descriptors, such as electron work functions, melting points, etc. have also been proposed to correlate with the activity of catalysts in the HOR/HER [START_REF] Trasatti | Electrocatalysis of Hydrogen Evolution: Progress in Cathode Activation[END_REF][START_REF] Petrii | Electrocatalytic activity prediction for hydrogen electrode reaction: intuition[END_REF][START_REF] Jakšić | Advances in electrocatalysis for hydrogen evolution in the light of the Brewer-Engel valence-bond theory[END_REF][START_REF] Leonard | Pattern Recognition Correlating Materials Properties of the Elements to Their Kinetics for the Hydrogen Evolution Reaction[END_REF][START_REF] Kita | Electrocatalysis by d-and sp-metals[END_REF] Fig. 1.1 Exchange current density of the HER (in acidic (a, b) and alkaline (c) medium) plotted as a function of M-Had binding energy (a, given from the Ref. [START_REF] Trasatti | Work function, electronegativity, and electrochemical behaviour of metals III. Electrolytic hydrogen evolution in acid solutions[END_REF]) or free energy of adsorption of Had atoms on the catalyst (b and c, given from the Refs. [START_REF] Nørskov | Trends in the Exchange Current for Hydrogen Evolution[END_REF][START_REF] Sheng | Correlating the hydrogen evolution reaction activity in alkaline electrolytes with the hydrogen binding energy on monometallic surfaces[END_REF]).

Comparison of the HOR/HER in acidic and alkaline medium

As it has been already pointed out, the catalytic activity of Pt in the HOR/HER is extremely high in acidic media [START_REF] Sheng | Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes[END_REF][START_REF] Durst | New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism[END_REF][START_REF] Marković | Temperature-Dependent Hydrogen Electrochemistry on Platinum Low-Index Single-Crystal Surfaces in Acid Solutions[END_REF] and even little amount of the catalyst is enough for the reactions to occur. However, the HOR/HER activity of PGMs decreases by almost two orders of magnitude in alkaline media [START_REF] Durst | New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism[END_REF], being in the range of 0.55 -0.69 mA cm -2 for polycrystalline Pt and Pt/C catalysts in diluted alkaline electrolytes [START_REF] Sheng | Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes[END_REF][START_REF] Rheinlander | Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure[END_REF]. The fact that the activity of Pt, Pd and Ir changes by a similar factor prompted Durst et al. [START_REF] Durst | New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism[END_REF] to propose that adsorption energy of Had is the only factor which affects the HOR/HER activity on various catalysts and determines the differences in the activity values depending on the pH [START_REF] Durst | New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism[END_REF]. This assumption is in agreement with observations of Zheng et al., who reported close to linear dependence between the values of M-Had binding energy (estimated from the potential of Hupd a desorption peak in cyclic voltammograms (CV)) and pH [START_REF] Zheng | Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy[END_REF].

Notably, similar slopes of the dependence were obtained for supported Pt/C, Pd/C, Ir/C and Rh/C catalysts. Moreover, log(i0) was found to decrease linearly with increase of pH and M-Had binding energy [START_REF] Zheng | Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy[END_REF]. Sheng at al. [START_REF] Sheng | Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy[END_REF] reached the same conclusion while studying polycrystalline Pt disk electrode at various pH and assuming that the position of the Hupd desorption peaks is determined by the binding energy of Pt-Had. However, Koper et al. have recently shown that the potential of these peaks also depends on the adsorption potential of oxygenated species on the surface of Pt [START_REF] Van Der Niet | Water dissociation on well-defined platinum surfaces: The electrochemical perspective[END_REF].

Markovic et al. have proposed that along with the adsorption energy of Had

atoms two other descriptors, namely the nature of the proton source (H3O + or H2O) and the presence of surface spectators are required to understand the role of pH in the HOR/HER [START_REF] Strmcnik | Design principles for hydrogen evolution reaction catalyst materials[END_REF]. The authors suggested a hypothesis according to which in alkaline media the HOR/HER require two types of sites on the electrode surface for the adsorption of Had as well as OHad (formed by the reaction OH -↔ OHad + e -) species, while in acidic media Had is the only important intermediate [START_REF] Strmcnik | Design principles for hydrogen evolution reaction catalyst materials[END_REF][START_REF] Strmcnik | Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption[END_REF]. Following the a Hupd corresponds to the hydrogen adsorption/desorption in the potential region above an equilibrium potential of reversible hydrogen electrode hypothesis, higher (compared to Au (111) and Pt (111)) activity of Ir (111) in the HER in alkaline medium was assigned by Markovic et al. to larger strength of OHad adsorption on the surface of Ir [START_REF] Strmcnik | Design principles for hydrogen evolution reaction catalyst materials[END_REF][START_REF] Strmcnik | Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption[END_REF]. However, this conclusion was based on the analysis of the very first anodic scan of corresponding CVs, while comparison of the steady-state curves, performed by Durst et al. [33], indicates that the HOR/HER activity decreases in the order of Pt/C > Ir/C > Pd/C. Bi-functional mechanism was also suggested by Markovic et al. to explain a promotion effect of Ru on the activity of Pt, according to which the adsorption of OHad and Had takes place on Ru and Pt atoms respectively [START_REF] Strmcnik | Design principles for hydrogen evolution reaction catalyst materials[END_REF]. However, the enhancement of the activity in PtRu systems has been later shown to be mostly due to the weakening of Pt-Had bond in the presence of Ru [START_REF] Wang | Pt-Ru catalyzed hydrogen oxidation in alkaline media: oxophilic effect or electronic effect?[END_REF]. In addition, the adsorption of OHad was found to occur on PGMs at lower potentials when the pH is increased [START_REF] Zheng | Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy[END_REF][START_REF] Rizo | Towards the understanding of the interfacial pH scale at Pt(111) electrodes // Electrochim[END_REF][START_REF] Martínez-Hincapié | Exploring the interfacial neutral pH region of Pt(111) electrodes[END_REF]. It means that if the OHad would be the intermediate of the HOR in alkaline media, then the rate of this reaction should increase at higher pH, while in fact an opposite tendency is observed [START_REF] Zheng | Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy[END_REF].

Besides the above mentioned reaction between adsorbed Had and OHad, Markovic et al. also proposed that presence of low-coordinated metal sites or transition metal hydroxides on the surface of noble metals should promote the Volmer step of water dissociation, which is often assumed to be the rate-determining step of the HOR [START_REF] Strmcnik | Design principles for hydrogen evolution reaction catalyst materials[END_REF][START_REF] Strmcnik | Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption[END_REF][START_REF] Subbaraman | Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts[END_REF][START_REF] Subbaraman | Enhancing hydrogen evolution activity in water splitting by tailoring Li + -Ni(OH)₂-Pt interfaces[END_REF][START_REF] Zeng | Stabilization of ultrathin (hydroxy)oxide films on transition metal substrates for electrochemical energy conversion[END_REF]. Another explanation for the promoting effect of Ni(OH)2 on the HOR/HER activity of Pt has been recently suggested by Koper et al., according to which the presence of Ni(OH)2 on the Pt surface lowers the energetic barrier for the reorganization of water molecules in the double layer, thus facilitating transfer of OH - [START_REF] Ledezma-Yanez | Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes[END_REF]. The authors proposed that in acidic media, in the case of transfer of H + protons, the energetic barrier is insignificant, but increases in alkaline solution, what might be the reason for a decrease of the HOR/HER activity with an increase of pH [START_REF] Ledezma-Yanez | Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes[END_REF].

According to Wang et al., the addition of a more oxophilic metal (such as Ru or Ni) to Pt could also shift the onset potential of OHad adsorption on the Pt surface to more positive values, thus enhancing the overall HOR/HER activity [START_REF] Wang | Pt-Ru catalyzed hydrogen oxidation in alkaline media: oxophilic effect or electronic effect?[END_REF].

On the basis of the above analysis, we conclude that activity of PGMs in the HOR/HER at various pH is mainly determined by the value of M-Had binding energy, while the presence of surface spectators (for example, transition metal hydroxides) might change the strength of the Had adsorption as well as affect the adsorption of water molecules, thus influencing the rate of the Volmer step.

Electrochemical properties of Ni-based electrocatalysts

Non-noble metals have been used for many years as cathode catalyst of the HER in alkaline water electrolysis. Iron utilized at the beginning was later replaced by steel, while recently the interest has been shifted towards application of Ni-based catalysts.

The latter is mainly caused by higher corrosion stability of Ni in alkaline solutions compared to steel, even if its activity is slightly lower [START_REF] Pletcher | Prospects for alkaline zero gap water electrolysers for hydrogen production[END_REF].

Electrochemistry of metallic Ni electrodes has been studied for almost 100 years, starting, likely, from the works of Emelianova [START_REF] Emelianova | Researches with the dropping mercury cathode: Part VII. Nickel and Cobalt // Recl[END_REF], Frumkin et al. [50,[START_REF] Frumkin | Hydrogen overvoltage // Discuss[END_REF], Legran and Levina [START_REF] Legran | Hydrogen overvoltage on Ni in acid solutions[END_REF], , Breiter et al. [START_REF] Weininger | Effect of Crystal Structure on the Anodic Oxidation of Nickel[END_REF][START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF], Bockris and Potter [63] as well as many others. The most significant results, obtained in alkaline media, have been recently summarized by Hall et al. [64,[START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF] and Jerkiewicz et al. [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 2: Formation of β-Ni(OH)2 and NiO in Relation to the Polarization Potential[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF], who paid special attention to the discussion of various processes, which take place on the surface of Ni electrodes depending on the applied potential. However some questions, for example, an influence of the degree of surface oxidation were barely discussed. In addition, there are still several controversial issues that require more detailed study.

Electrochemical behavior of Ni in alkaline medium

Typical cyclic voltammograms obtained on a freshly polished polycrystalline Ni disk in an alkaline electrolyte are presented in the Fig. 1.2 [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF]. In order to facilitate the discussion about characteristic features of the CV of the Ni electrode, we marked out two potential regions, namely the "low-potential region" at E ≤ 0.5 V b and the "high-potential region" at 0.5 < E < 1.6 V.

b If not otherwise stated, the potential is referred to the reversible hydrogen electrode (RHE). [-0.15 -0.50 V], while two other CVs (first scan -blue curve and tenth scan -black curve) cover the potential range of [-0.15 -1.55 V]. Given from the Ref. [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF].

In the low-potential region anodic and cathodic peaks are generally assigned with the formation and reduction of surface α-Ni(OH)2, respectively [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Grden | A quartz crystal microbalance study on a metallic nickel electrode[END_REF][START_REF] Melendres | On the composition of the passive film on nickel: a surface-enhanced Raman spectroelectrochemical study[END_REF][START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF].

Anodic dissolution and cathodic redeposition of Ni, predicted on the basis of the Pourbaix diagram [START_REF] Beverskog | Revised Pourbaix diagrams for iron at 25-300??[END_REF] and observed in acidic media, were shown (based on electrochemical quartz crystal microbalance measurements and experiments with rotating Ni disk electrode) to be insignificant in alkaline electrolytes [START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF][START_REF] Grdeń | EQCM studies on oxidation of metallic nickel electrode in basic solutions[END_REF]. Several models have been suggested for the mechanism of low-potential Ni oxidation, among which the nucleation and growth model and the place exchange model have the best agreement with experimental observations [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF]. The latter model was proposed by Conway et al. [74] and later improved by Jerkiewicz et al. [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF] by adding the influence of applied potential on the structure and physical properties of the surface hydroxides. According to that model, the formation of nickel hydroxide proceeds through the following mechanism: [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF]), (b) 1.0 M NaOH (after subtraction of the currents corresponding to the HER, given from the Ref. [START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF]) and (c) 0.5 M NaOH electrolyte (given from the Ref [START_REF] Conway | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF]).

Ni + OH -→ Ni-OHad + e -(fast) Ni-OHad → (OH-Ni)quasi-3D lattice (rate determining step) (OH-Ni)quasi-3D lattice + OH -→ Ni(OH)2,ad + e -(fast)
Existence of intermediate Ni-OHad species was proposed on the basis of CVs obtained at various sweep rates (v), when two cathodic peaks were observed at high v (Fig. 1.3a). However, two cathodic peaks (even at relatively low v) were also noticed by D'Alkaine and Santanna [START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF], who assigned the one at more negative potentials to the reduction of an oxide film formed on the electrode surface during its preparation to the experiment (Fig. 1.3b). On the contrary, only one cathodic peak was observed by Conway et al. [76] independent of the sweep rate (Fig. 1.3c), which makes the question about the nature of the second cathodic peak disputable. It should be noted that the presence of highly mobile adsorbed OHad species, characterized by low binding energy with the Ni surface, was detected by in situ scanning tunneling microscopy at potentials around 0.1 V, i.e. before the formation of α-Ni(OH)2 [START_REF] Seyeux | In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)[END_REF].

Meanwhile the place exchange model is not able to explain some observed phenomena. In particular, in spite of widely accepted in literature belief that in the low-potential region the surface of Ni is covered by a monolayer of surface Ni(OH)2 (see, for example, [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]), experiments carried out with the use of in situ scanning tunneling microscopy [START_REF] Nakamura | Surface structures at the initial stages in passive film formation on Ni(111) electrodes in acidic electrolytes[END_REF][START_REF] Seghiouer | Electrochemical oxidation of nickel in alkaline solutions: a voltammetric study and modelling[END_REF] as well as analysis of the CVs obtained at low temperatures (< 263 K) [START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF] predict a formation of 3D islands of Ni(OH)2 before full coverage of the electrode surface (Fig. 1.4). The latter likely explains the common use of the term 'α-Ni(OH)2' for the oxide species formed on Ni electrode surface in the low-potential region. [START_REF] Nakamura | Surface structures at the initial stages in passive film formation on Ni(111) electrodes in acidic electrolytes[END_REF]).

In some works the formation of NiO was proposed to occur after oxidation of Ni at Е < 0.5 V instead of α-Ni(OH)2 [START_REF] Grdeń | EQCM studies on oxidation of metallic nickel electrode in basic solutions[END_REF][START_REF] Visscher | Anodic oxide films of nickel in alkaline electrolyte[END_REF]. However, in situ experiments with the use of Raman [START_REF] Melendres | On the composition of the passive film on nickel: a surface-enhanced Raman spectroelectrochemical study[END_REF] or IR spectroscopy [START_REF] Beden | The anodic layer on nickel in alkaline solution: an investigation using in situ IR spectroscopy // Electrochim[END_REF] as well as X-ray photoelectron spectroscopy (XPS) [START_REF] Hoppe | XPS and UPS examinations of the formation of passive layers on Ni in 1 M sodium hydroxide and 0.5 M sulphuric acid // Surf[END_REF] have shown that the amount of NiO on the surface of Ni in the lowpotential region is insignificant (or NiO is completely absent). Besides, several authors stated that along with the oxidation of Ni surface the oxidation of adsorbed on the surface and/or absorbed in the Ni metal hydrogen contributes to the anodic peak at Е < 0.5 V [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Grdeń | EQCM studies on oxidation of metallic nickel electrode in basic solutions[END_REF][START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF][START_REF] Burke | Voltammetric behaviour of nickel in base with particular reference to thick oxide growth[END_REF][START_REF] Macdougall | Anodic Oxidation of Nickel in Neutral Sulfate Solution[END_REF]. Ni hydrides are known to be produced at high cathodic overpotentials (see below). It was shown that contribution of the oxidation of absorbed hydrogen and/or Ni hydrides to the anodic peak currents increases in the case of long cathodic polarization or cycling up to high negative potentials [START_REF] Burke | Voltammetric behaviour of nickel in base with particular reference to thick oxide growth[END_REF]. At the same time recording of CVs in a narrow potential range results in insignificant contribution of these processes [START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF][START_REF] Grdeń | EQCM studies on oxidation of metallic nickel electrode in basic solutions[END_REF][START_REF] Schrebler Guzmán | The kinetics and mechanism of the nickel electrode-III. The potentiodynamic response of nickel electrodes in alkaline solutions in the potential region of Ni(OH)2 formation[END_REF]. Note however that the contribution of the oxidation of adsorbed hydrogen to the anodic peak currents have to be considered.

Based on electrochemical quartz crystal microbalance measurements Grden et al. concluded, that oxidation of adsorbed hydrogen is occurring just before the lowpotential Ni oxidation [START_REF] Grdeń | EQCM studies on oxidation of metallic nickel electrode in basic solutions[END_REF]. More detailed discussion about the influence of cathodic polarization on the CV shape of Ni electrode can be found in the works [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF] and references therein.

In the high-potential region increase of the applied potential leads to transformation of α-Ni(OH)2 into β-Ni(OH)2 with decrease of the degree of hydration as schematically shown in Fig. 1.5a, along with the thickening of NiO layer [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 2: Formation of β-Ni(OH)2 and NiO in Relation to the Polarization Potential[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF][START_REF] Melendres | On the composition of the passive film on nickel: a surface-enhanced Raman spectroelectrochemical study[END_REF][START_REF] Seyeux | In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)[END_REF][START_REF] Desilvestro | Characterization of Redox States of Nickel Hydroxide Film Electrodes by In Situ Surface Raman Spectroscopy[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]. Oxidation of Ni electrode at more positive potentials than 0.5 V results in a shift of the cathodic peak in the low-potential region towards negative values, suggesting formation of more stable Ni oxide species on the electrode surface [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF][START_REF] Samoilov | The study of oxygen adsorption on Ni by electrochemical method // Kinetika i Kataliz[END_REF]. Moreover, cycling up to E > ~0.9 V (at v = 100 mV•s -1 ) leads to disappearance of the cathodic peak due to irreversible character of β-Ni(OH)2 formation (unlike α-Ni(OH) 2), what can be clearly seen in Fig. 1.2 [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Visscher | The anodic oxidation of nickel in alkaline solution // Electrochim[END_REF][START_REF] Shumilova | Oxygen ionization on nickel in alkaline solutions // Electrochim[END_REF]. Note that partial reduction of β-Ni(OH)2 is possible upon application of high cathodic overpotentials. The first identification of two pseudopolymorphs of Ni(OH)2 was done by Bode et al. [93], who also suggested a scheme, describing the processes of Ni oxidation in the high-potential region (Fig. 1.5b). According to that scheme increase of the applied potential leads to further oxidation of both α-and β-Ni(OH)2 with the formation of Ni oxyhydroxide [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF][START_REF] Bode | Zur kenntnis der nickelhydroxidelektrode-I.Über das nickel (II)-hydroxidhydrat // Electrochim[END_REF][START_REF] Oliva | Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides[END_REF][START_REF] De Souza | Spectroscopic ellipsometry study of nickel oxidation in alkaline solution // Electrochim[END_REF][START_REF] Guzmán | Non-equilibrium effects in the nickel hydroxide electrode[END_REF]. The anodic peak corresponding to this process is located at potentials around 1.4 V (Fig. 1.2), while reduction of Ni oxyhydroxide (NiOOH → Ni(OH)2) occurs in the potential interval of 1.2 -1.4 V with appearance of, usually, two cathodic peaks, whose exact position depends on the involved phases of NiOOH (β or γ) as well as Ni(OH)2 (α or β) [START_REF] Guzmán | Non-equilibrium effects in the nickel hydroxide electrode[END_REF][START_REF] Kim | A Study on the Phase Transformation of Electrochemically Precipitated Nickel Hydroxides Using an Electrochemical Quartz Crystal Microbalance[END_REF]. Besides, two anodic peaks can also be visible at low sweep rates (v ≤ 10 mV•s -1 ), while temperature and oxide film thickness affect the position of both anodic and cathodic peaks in the high-potential region [START_REF] Burke | Voltammetric behaviour of nickel in base with particular reference to thick oxide growth[END_REF][START_REF] Guzmán | Non-equilibrium effects in the nickel hydroxide electrode[END_REF][START_REF] Meier | The influence of temperature on the current peak multiplicity related to the nickel hydroxide electrode[END_REF]. Above ca. 1.5 V the oxygen evolution reaction begins, where, according to in situ Raman experiments [START_REF] Desilvestro | Characterization of Redox States of Nickel Hydroxide Film Electrodes by In Situ Surface Raman Spectroscopy[END_REF][START_REF] Trzesniewski | In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity[END_REF][100][101][102] and XPS measurements [START_REF] Hoppe | XPS and UPS examinations of the formation of passive layers on Ni in 1 M sodium hydroxide and 0.5 M sulphuric acid // Surf[END_REF]103], the surface of Ni electrode is covered by NiOOH and NiO. At negative applied potentials the HER and hydrogen absorption into Ni metal lattice (with further formation of Ni hydrides) proceed along with the reduction of Ni surface oxide species. At present, two bulk phases, namely α and β, are known for Ni hydrides, similarly to these for Pd hydrides [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF]105]. The lattice parameters of α-NiH and β-NiH are ca.!1 % and ca.!6 %, respectively, greater compared to Ni metal [106][107][108]. According to Ref. [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF], the equilibrium potentials for the formation of α-NiH and β-NiH may be estimated as -0.075 --0.095 V and -0.12 --0.13 V, respectively.

As shown by Hall et al. [64] and Soares et al. [109], undisputed existence of bulk α-or β-NiH is observed (by XRD) only for the electrodes treated galvanostatically at currents above I < -0.3 A cm -2 . Meanwhile one cannot exclude the presence of small amounts of absorbed hydrogen or α-NiH even at potentials close to 0 V vs RHE.

Influence of the oxidation of the Ni electrode surface on its electrochemical behavior

Oxidation of Ni is well known to occur rapidly after its exposure to oxygencontaining atmosphere even at room temperature. According to the ''island-growth'' model proposed by Holloway and Hudson, this process proceeds in three stages: (i) oxygen chemisorption; (ii) nucleation and lateral growth of nickel oxide islands; (iii) slow thickening of the oxide layer up to ca. 2 -4 atomic layers of NiO [110]. If the oxidation takes place in the presence of water (under humid air or oxygen atmosphere), then three-layered structure consisting of Ni(OH)2 upper layer and NiO layer between the metal substrate and Ni(OH)2 is formed [START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF][111][112][113]. After immersion of the air-formed oxide into an alkaline solution, Ni hydroxide layer becomes thicker, while overall three-layered Ni/NiO/Ni(OH)2 structure generally remains the same [START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]112].

It should be noted that unlike chemical oxidation in air, which at room temperature is stopped at ca. 1-2 nm thickness of the oxide layer, electrochemical oxidation by anodic polarization results in much thicker oxide with larger crystalline domains [START_REF] Burke | Voltammetric behaviour of nickel in base with particular reference to thick oxide growth[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]111,112,114].

It has been observed that in CVs, obtained after either chemical (under contact with air or concentrated acid solutions) or electrochemical (for example, after cycling up to the potentials above 0.8 V) oxidation, a second anodic peak appears at around E ≈ 0.10 V (Fig. 1.6) [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF][START_REF] Visscher | Anodic oxide films of nickel in alkaline electrolyte[END_REF][START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF][START_REF] Burke | Voltammetric behaviour of nickel in base with particular reference to thick oxide growth[END_REF][START_REF] Visscher | The anodic oxidation of nickel in alkaline solution // Electrochim[END_REF][START_REF] Shumilova | Oxygen ionization on nickel in alkaline solutions // Electrochim[END_REF][115][116][117]. The amplitude and the position of that peak significantly depend on the values of anodic and cathodic potential limits of the CV [START_REF] Burke | Voltammetric behaviour of nickel in base with particular reference to thick oxide growth[END_REF][START_REF] Shumilova | Oxygen ionization on nickel in alkaline solutions // Electrochim[END_REF]. There is no common view concerning the origin of the peaks observed in CVs of oxidized Ni electrodes in the potential interval of [0 -0.15 V].

Several processes have been suggested in different works, such as (i) adsorption of hydroxide-ions with the formation of , (ii) oxygen adsorption [START_REF] Shumilova | Oxygen ionization on nickel in alkaline solutions // Electrochim[END_REF], (iii) dissolution of Ni with the formation of HNiO2 - [START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF], (iv) oxidation of adsorbed hydrogen [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][116][117][118], (v) oxidation of absorbed hydrogen and/or Ni hydrides [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF][START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF][START_REF] Visscher | The anodic oxidation of nickel in alkaline solution // Electrochim[END_REF]115]. The impact of each of these processes, likely, depends on the methodology used for preconditioning of the electrode surface as well as the measurement protocol. In particular, utilization of a highly passivated electrode subjected to prolonged cathodic polarization at high overpotentials results in significant contribution of the process (v) [START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF][START_REF] Visscher | The anodic oxidation of nickel in alkaline solution // Electrochim[END_REF]. On the contrary, oxygen adsorption (ii) is unlikely if under experiments are performed in an inert atmosphere. The same conclusion is valid for the dissolution and further redeposition of Ni in alkaline media (iii) especially if the concentration of hydroxide-ions is higher than 0.1 M, as it was shown in Refs. [START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF][START_REF] Grdeń | EQCM studies on oxidation of metallic nickel electrode in basic solutions[END_REF]. The influence of crystallographic orientation of the Ni surface on its electrochemical behavior has been studied in several works both in acid and in alkaline media [START_REF] Weininger | Effect of Crystal Structure on the Anodic Oxidation of Nickel[END_REF][START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Seyeux | In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)[END_REF]115,[119][120][121]. The shape of the CVs (at v ≥ 50 mV•s -1 ) was found to be similar for low-index Ni planes (111), ( 110), (100), but the potential of the anodic peak corresponding to α-Ni(OH)2 formation shifts following the order (110) < (100) < (111) [START_REF] Weininger | Effect of Crystal Structure on the Anodic Oxidation of Nickel[END_REF]115,120]. This observation confirms structure-sensitive adsorption of hydroxide ions on Ni surface. Meanwhile the potential of the cathodic peak is similar for three Ni single crystals.

An additional anodic peak in the potential interval of [0.1 -0.2 V] was observed in CVs, obtained at low sweep rates (v ≤ 5 mV•s -1 ) (Fig. 1.7a) [115]. The position of that peak depends on the crystallographic orientation of Ni surfaces. In particular, the peak maximum is located around 0.07 V on Ni (110) plane, being as high as the main peak of α-Ni(OH)2 formation. The same peak on Ni (100) plane is observed at more positive potentials (around 0.12 V), while on Ni (111) plane its contribution is less visible (Fig. 1.7a). The dependence of the processes of Ni oxidation in the highpotential region on the crystallographic orientation of its surface was noticed only for first three scans of CVs (Fig. 1.7b). Meanwhile continuous cycling leads to the structure disordering and, as a result, similar shape of CVs for three types of Ni planes (Fig. 1.7b) [START_REF] Weininger | Effect of Crystal Structure on the Anodic Oxidation of Nickel[END_REF]120].

Determination of the electrochemical surface area of Ni

Estimation of the electrochemical surface area (SEC) of the electrode is one of the key stages for determination of its specific electrocatalytic activity and comparison of several materials between each other. At present, the most widely used approach for the determination of SEC is described in the work of Machado and Avaca [78], who suggested to use the charge under the peak of α-Ni(OH)2 formation and the corresponding conversion coefficient of 514 µC cm -2 . This is an average value determined from the measurements performed using low-index Ni single crystals [120]. Note that there is no strong theoretical background of this value unlike the values used in the case of noble metals [122]. Transfer of 2 electrons is usually assumed in the calculations [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]. However, since one cannot exclude a possible impact of the electrochemical hydrogen desorption in the measured anodic charge, then it is better to consider a number of involved electrons between 2 and 3. In some works the charge of the cathodic peak of α-Ni(OH)2 reduction (after subtraction of the currents corresponding to the HER) is used instead of the charge under the anodic peak [123,124]. Despite its wide use, the SEC values obtained by this method should be considered as approximate due to the used assumptions. First, as discussed above, along with the Ni/α-Ni(OH)2 redox transition other processes may occur in the lowpotential region, giving an indefinite contribution to the charge obtained by integration. Therefore, in order to minimize an impact of 'side' processes the potential limits should be chosen in such a way that, on the one hand, provide complete reduction of α-Ni(OH)2 formed during the anodic scan, while avoiding formation of irreducible β-Ni(OH)2 and, on the other hand, minimize the formation of absorbed hydrogen and Ni hydrides, whose oxidation contributes to the anodic charge. Second, although a monolayer coverage of Ni surface by α-Ni(OH)2 during its oxidation is suggested, in fact the thickness of the surface oxide layer is unknown. Moreover its thickness may vary depending on the structure and the composition of the electrode surface. Finally, as it has been shown above, the way of pretreatment of the surface of the Ni electrode may significantly influence its voltammetric behavior in the lowpotential region. Therefore it is not clear which charge should be used for the estimation of SEC.

In addition to the method, suggested by Machado and Avaca, in some works, mostly related to the study of Ni-based alloys, the value of SEC was recalculated from the value of the electrochemical double layer capacitance (Cdl), obtained using electrochemical impedance spectroscopy [124][125][126][127][128]. However, analysis of the literature points out that the value of Cdl depends on the surface state of Ni electrode leading to the variation of specific capacitance from 20 to 60 µF•cm -2 [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][124][125][126]129,130]. Therefore, in the majority of works authors preferred to record impedance spectra in the HER region, where contribution of the oxide phases is believed to be insignificant. However an increased probability of the formation of Ni hydrides in this case can also affect the measured values.

The value of SEC can also be determined in the high-potential region. In this case, the value of Cdl is usually estimated either by electrochemical impedance spectroscopy under the assumption that the surface of Ni electrode is covered by Ni(II)/Ni(III) oxide species or on the basis of CVs recorded at various sweep rates in the double layer potential region (with typically 0.1 V potential window) [131,132]. In the latter case plotting of the current values measured at a certain potential as a function of sweep rate gives a straight line with the slope equal to Cdl. Along with the use of the Cdl value, the charge of monolayer formation of NiOOH was suggested to be used for an estimation of SEC as well. Corresponding measurements should be performed in the following way. First, anodic currents are applied in order to form NiOOH. Second, after interruption of the charging currents cathodic current transients are recorded [130,133]. Then, if one would plot the estimated total transient charge (Qcath) against the overpotential of the anodic charging step (ηan), at sufficiently high values of ηan, Qcath will reach a plateau, which corresponds to a monolayer coverage by NiOOH. Theoretically derived conversion coefficient of 420 µC cm -2 is proposed by authors as an "universal accepted standard" for approximate estimation of SEC.

However, a possible formation of β-and γ-NiOOH, whose unit cell parameters and symmetries are somewhat different, may result in slightly different values of SEC [START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF][START_REF] Oliva | Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides[END_REF].

A yet another method has been proposed by Hall et al. [134], who suggested to use an oxalate salt to limit the thickness of surface Ni(OH)2 by a single layer. Sharp and narrow peaks for the Ni(II)/Ni(III) oxidation-reduction processes were observed in CVs in the presence of oxalate, giving constant cathodic charge at v ≥ 150 mV•s -1 (analysis of the anodic charge is complicated by the involvement of the oxygen evolution reaction). The authors then assumed a Coulombic charge of 195 µC cm -2 estimated from the lattice parameters of α-Ni(OH)2 (for β-Ni(OH)2 the difference is only around 3%) and obtained the roughness factor for mechanically polished Ni electrode equal to 1.78.

It should be noted that all the methods for SEC determination, which require cycling of Ni electrode up to high positive potentials (> 0.5 V), lead to the formation of irreducible oxide species on the surface and in some cases may even change the surface roughness [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 2: Formation of β-Ni(OH)2 and NiO in Relation to the Polarization Potential[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF][START_REF] Melendres | On the composition of the passive film on nickel: a surface-enhanced Raman spectroelectrochemical study[END_REF][START_REF] Seyeux | In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)[END_REF][START_REF] Desilvestro | Characterization of Redox States of Nickel Hydroxide Film Electrodes by In Situ Surface Raman Spectroscopy[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]. Therefore, it is reasonable to determine the value of SEC from the high-potential region only when studying processes, taking place at high anodic potentials, for example, anodic oxygen evolution/reduction reactions.

Electrocatalytic properties of Ni-based electrocatalysts in the HOR/HER 1.3.1. Analysis of adsorption energies of the HOR/HER intermediates on Ni

According to Sabatier principle, one of the main parameters, which determines the activity of electrocatalysts, is the adsorption energy of reactive compounds (educts, intermediates and products) on the electrode surface, which depends on its nature [2,[START_REF] Sabatier | Hydrogénations et déshydrogénations par catalyse // Berichte der[END_REF][START_REF] Boreskov | Heterogeneous catalysis[END_REF]. It is generally assumed (and experimentally observed in many cases), that the strength of adsorption decreases going from more open planes to the closepacked ones, which for face-centered cubic lattice of Ni would mean the order of Ni (110) > Ni (100) > Ni (111) (corresponding coordination numbers of surface atoms are 7, 8 and 9, respectively). In this section we analyze adsorption energies, which were obtained using DFT calculations for the main species involved in the HOR/HER in base, namely: molecular and atomic hydrogen (H2, Had), water molecules (H2Oad), hydroxo-species (OHad) and, given the tendency of nickel towards oxidation, atomic oxygen (Оad). These compounds can adsorb on metal surface in various positions, such as on top, bridge, and hollow, being coordinated by one, two, and three surface atoms, respectively. Hydrogen molecules were found to adsorb on the surface of low-index Ni planes in the on top position [135][136][137][138] with their further dissociation. The lowest energy barrier for dissociation was obtained for Ni (110) [139][140][141][142]. Adsorbed H2Oad molecules occupy the same position, binding with the surface through the oxygen atom, while adsorbed Had, OHad and Oad species occupy hollow sites [135,136,143,144]. DFT calculations predict that adsorption energies for H2Oad and OHad on different Ni planes decrease following the order Ni (110) > Ni (100) > Ni (111) [135,144-147], while for the adsorption energies of Had and Oad atoms the observed order is Ni (100) > Ni (111) > Ni (110) [135,141,144,[146][147][148]. Note, however, that the difference between the adsorption energies of Had atoms on Ni (111) and Ni (100) can be considered insignificant and in some studies a bit larger values were obtained for Ni (111) compared to Ni(100) [141,149]. It was also found that adsorption of Had on stepped surfaces ( 210) and ( 531) is characterized by just a little higher energy compared to the adsorption on low-index Ni planes [141]. Besides, it was concluded that surface diffusion of Had atoms between different surface sites is facile, while their diffusion into subsurface sites (necessary for the formation of Ni hydrides) is expected only after occupation of all available surface sites [136].

Adsorption of water molecules H2Oad, which along with Had atoms participate in the HOR/HER through the Volmer and Heyrovsky steps (eq. 1.7, 1.8), can also significantly affect the kinetics of the HOR/HER. According to the DFT calculations, adsorption energy of H2Oad molecules is significantly lower compared to the other intermediates of the processes [135,143,144,150]. In particular, eV for the adsorption of Oad, OHad, Had and H2Oad on Ni (110) plane, respectively [144]. Besides, an energetic barrier for water splitting was found to be significant in the case of Ni (111) surface [151,152], but is decreasing for more open Ni (100) and Ni (110) planes as well as stepped or other low-coordinated surfaces, due to an increase of the adsorption energy of H2Oad molecules [144,150,153]. The latter may stimulate the Heyrovsky step and, as a result, lead to an increase of the activity in the HER. It is worth to note, that pre-adsorption of strongly bonded species on Ni surface significantly affects subsequent adsorption of water molecules. It was found that existence of Had atoms on Ni surface leads to the formation of multilayer islands of water, surrounded by chains of adsorbed Had atoms already at small surface coverage [151,154]. On the contrary, pre-covering of Ni surface with Oad atoms increases the binding energy of Ni-OH2 due to the formation of hydrogen bonds between adsorbed water molecule and nearby Oad atom [135,150,155]. The latter also facilitates water splitting with the formation of two OHad molecules, which was observed on Ni (111) surface in the combined XPS-TPD (TPDtemperature programmed desorption) experiments [156].

DFT calculations also showed that the binding energies of both Ni-Had and Ni-OHad decrease at the vicinity of pre-adsorbed Oad atoms [142,155,[157][158][159]. It was found that the adsorption of Had atoms occurs only at a considerable distance from the adsorbed Oad atoms [157]. A repulsive interaction, although to a much lesser extent, was also observed for the adsorption of OHad nearby Oad atoms [159].

Activity of monometallic Ni electrocatalysts in the HOR/HER

Owing to the use of alkaline water electrolysis at the industrial level, studies of Ni-based electrocatalysts in the HOR/HER have been primarily carried out in the potential region of the HER, while the interest to the HOR has been quite small until recently. Early studies were largely dedicated to Raney Ni electrodes, while during the past decades the interest has shifted to bimetallic Ni-based catalysts [START_REF] Sheng | Non-precious metal electrocatalysts with high activity for hydrogen oxidation reaction in alkaline electrolytes[END_REF][160][161][162][163][164][165].

However, even without influence of a second metal the exchange current densities of Ni electrodes reported in the literature may differ by nearly two orders of magnitude [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Bockris | The Mechanism of Hydrogen Evolution at Nickel Cathodes in Aqueous Solutions[END_REF]126,[166][167][168]. As it was mentioned in the Introduction, this diversity can be caused by the use of various methods for pretreatment of Ni electrodes before carrying out electrocatalytic measurements. In this section we present a short analysis of published works, which allows one to observe the influence of the structure and the composition of the surface of Ni electrodes on their activity in the HOR/HER.

Influence of Ni structure on its activity in the HOR/HER

Very few works were devoted to the study of the HOR/HER on Ni depending on the crystallographic orientation of its surface [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF]115,121]. Weininger et al. and Zoltowski et al. [62,121] observed very small or even no effect of the crystallographic orientation of the surface, because of, likely, poor quality of used single crystals or chosen experimental procedure, during which the electrodes lost their initial surface structure after oxidation in alkaline electrolyte (see Section 1.2.1,Fig. 1.7). On the contrary, Floner et al. observed significant difference in the shape of CVs, obtained for low-index Ni planes as well as polycrystalline Ni disk electrode in the lowpotential region [115]. It was found, that electrocatalytic activity of Ni (in terms of exchange current densities) in the HOR decreases following the order Ni (110) > Ni (100) > Ni (111) >> Nipoly (Fig. 1.8). To explain the observed differences, the authors proposed a hypothesis that the onset potential of hydrogen oxidation on Ni is determined by the strength of the adsorption of water molecules, which hinder the adsorption of Had atoms on a particular surface. However, according to the analysis of theoretical studies (see Section 1.3.1), adsorption energy of Had and OHad species is much higher compared to the one for H2Oad molecules [135,143,144,150]. Therefore blockage of the active sites on the Ni surface by H2Oad molecules seems to be unlikely. At the same time, analysis of the DFT calculations allows to suggest that the activity of Ni single crystals in the HOR/HER is mainly determined by the strength of Ni-Had bond, which is decreased in the order Ni (111) > Ni (100) > Ni (110) [115,135,141,144,149]. The latter correlates with the observed by Floner et al. dependence of Ni activity on the crystallographic orientation of its surface. The presence of adsorbed hydrogen atoms at E < ~0.15 V is in agreement with the observation of p-nitroaniline hydrogenation currents in this potential interval [116].

From the control experiments on Pt the authors showed that this process is occurring in the region of underpotential deposition of hydrogen [116]. Besides, Hu and Wen showed that full oxidation of Ni electrode surface at Т = 400 °C leads to complete suppression of p-nitroaniline hydrogenation, due to the absence of adsorbed hydrogen on the surface [116]. Comparison of Figs. 1.7 and 1.8 clearly shows that Ni oxidation overlaps with the HOR in a wide potential range [115,169]. This results in a rapid decrease of the HOR currents and complete suppression of HOR at E > ~0.3 V due to the blockage of the electrode surface by Ni hydroxide (Fig. 1.8). On the reverse sweep (cathodic direction), the HOR begins with the onset of Ni(OH)2 reduction [115]. The influence of the Ni structure on its activity in the HOR/HER can be further considered on the basis of works devoted to the study of such materials as porous Ni films [170][171][172], nanowires [173,174], dendrites [168,175], nanosheets [176] and nanospikes [177]. It was found that normalizing HER currents to the geometric surface area results in much higher activity for these materials with respect to a polycrystalline Ni electrode. However, the enhanced apparent catalytic activity is mainly due to an increase of the number of active sites on the electrode surface (i. e. an increase of the roughness factor), while the specific activity attributed to the SEC was found to be almost the same, for example, for a Ni nanospike array,

Influence of the oxidation of Ni surface on its activity in the HOR/HER

Despite the fact that systematic study of the influence of Ni surface oxidation on its activity in the HOR/HER has not been performed yet, in some works the authors observed a decrease of the Tafel slope and the HER overpotential after electrochemical oxidation (by cycling up to E > 1.0 V) of the surface of Ni disk [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF]126,169,170,178,179]. In particular, Lasia et al. reported

Activity of bimetallic Ni-based electrocatalysts in the HOR/HER

To date several reviews have been published summarizing the studies of bimetallic NiM systems, as well as compounds of Ni with various non-metals (selenides, phosphides, carbides, nitrides, sulfides) in the HER [START_REF] Trasatti | Electrocatalysis of Hydrogen Evolution: Progress in Cathode Activation[END_REF][START_REF] Pletcher | Prospects for alkaline zero gap water electrolysers for hydrogen production[END_REF]131,160,185,[188][189][190][191]. Considering the fact that the description of all the systems studied is too large for the current review, in this section we mainly focus on the evaluation of the activity enhancement factor after an addition of a second metal to

Ni catalyst as well as the possible causes of the observed changes. Most of the works reviewed in this section are related to the study of the HER on Ni-based electrocatalysts, while some recent works devoted to the HOR are also considered.

The first example of Ni-based catalysts is Raney Ni, which can be produced by leaching of Al (or Zn) from Ni-Al(Zn) precursor alloys in concentrated alkaline solution at elevated temperatures, thus providing high porosity and large surface area of Ni [129,[192][193][194]. However, the second metal may partly remain in the material after leaching and, therefore, influence the catalyst properties. This can be a likely reason for an enhanced activity in the HER usually observed for Raney Ni with respect to pure Ni [START_REF] Pletcher | Prospects for alkaline zero gap water electrolysers for hydrogen production[END_REF]129,192,194,195]. Later, especially during the past 20 years, many other bi-and polymetallic Ni-based systems have been studied, among which NiAg [196], NiBi [197], NiCo [128,[198][199][200], NiCu [201][202][203], NiFe [127,204,205],

NiMo [205][206][207][208][209][210][211][212][213][214][215][216][217][218][219][220], NiMn [132,221,222], NiRu [223], NiSn [224], NiTi [225][226][227], NiV [228,229], NiCr [164,184,230] and NiW [127,212,220,228,231,232]. In addition, modification of Raney Ni by an addition of other transition metals has been also investigated [161,162,192,195,[233][234][235]. Absolute majority of these samples were prepared in the form of thick deposits by electrodeposition techniques. Although such methods as dip pyrolysis [206], high energy alloying [208], melting [210,211], wet impregnation [164] and some other were also employed for the catalyst preparation.

It should be noticed that direct comparison of the HER activity of various NiM electrocatalysts is almost impossible due to a lack of information about the real Ni surface area SEC, which is necessary for estimation of specific catalytic activity.

Comparison of the activity values, normalized by geometric surface area, for porous samples, is incorrect since an enhancement of an apparent activity can be due to an increase of the real metal surface area [START_REF] Trasatti | Electrocatalysis of Hydrogen Evolution: Progress in Cathode Activation[END_REF]129,198,205,219,224,225] [211,212,214,216,217,220], NiMn [132,221,222],

NiW [220,231] and NiCr [164,184]. Significant decrease of the HER overpotential was also observed after simple addition of Mo ions (MoO4 2-) to the alkaline electrolyte and was attributed to the adsorption of Mo on the electrode surface with the formation of Ni-Co-Mo [236] or 238]. Ternary alloys on the basis of NiMo, namely NiMoCo [START_REF] Sheng | Non-precious metal electrocatalysts with high activity for hydrogen oxidation reaction in alkaline electrolytes[END_REF] and NiMoZn [239], have also shown high activity in the HOR/HER.

In order to explain synergistic effect of an increase of the HOR/HER activity observed for bimetallic NiM electrocatalysts, the well-known Sabatier principle and based on it volcano dependence of the HOR/HER exchange current density on the hydrogen adsorption energy ΔadGH are usually used (see Section 1.1.2). Despite some limitations, it allows one to predict an increase of the activity, in particular, for the alloys of Ni, strongly adsorbing hydrogen, with metals, which adsorb hydrogen weakly (for example, Ag or Cu). In the case of transition metals such as Mn, Mo, Ti, Cr, W, utilization of volcano-plots is, however, complicated by the tendency of these metals to form oxides under operating conditions (which exclude the possibility to use

ΔadGH calculated for pure metals). To explain an enhancement of the specific activity observed for these systems, some authors suggested to use the Brewer-Engel valence bond theory [START_REF] Jakšić | Advances in electrocatalysis for hydrogen evolution in the light of the Brewer-Engel valence-bond theory[END_REF]240]. According to this theory the electrons of the metal with more filled d-bands are shared with the metal with less filled or empty d-orbitals, while the highest activity is observed for systems with nearly filled d-orbital [START_REF] Kita | Electrocatalysis by d-and sp-metals[END_REF]212]. In Modification of the electronic structure of Ni by the second metal presumably leads to a decrease of ΔadGH value on Ni and, as a result, to an increase of its activity in the HOR/HER [START_REF] Sheng | Non-precious metal electrocatalysts with high activity for hydrogen oxidation reaction in alkaline electrolytes[END_REF]213]. Besides, the presence of transition metal oxides (Mn, Cr, Mo, Ti)

on the electrode surface is believed to facilitate the coordination of water molecules and, as a result, accelerate the Volmer step, which in some cases is the ratedetermining step for the HOR/HER [132,211,221,227].

Stability of Ni-based electrocatalysts in the HOR/HER

Due to the fact that Ni-based catalysts have been primarily studied in the region of the cathodic hydrogen evolution, their stability can be discussed only with regard to this process. It was found, that during prolonged use of Ni catalysts in alkaline water electrolysis, the value of the cathodic overpotential, which must be applied to the electrode to achieve a given current density, increases [175,241]. Taking into consideration the works, where an increase of the HER activity of Ni was observed in the presence of a small amount of Ni oxide, one can assume that the first overpotential decay is caused by the reduction of these surface oxide species [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF]170,179,185]. In addition, some phase separation between NiO and Ni may occur, likely, due to high resistance in a thick oxide layer [185], which leads to a decrease of the active surface area. Further decrease in the HER activity is believed to be due to the formation of Ni hydrides [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]109,[242][243][244][245], the presence of which after prolonged cathodic polarization was confirmed by XRD [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF]109]. It is worth to note, that Conway et al. The first significant studies of the HOR/HER kinetics on the surface of Ni electrode were, likely, carried out by Tafel [3], Frumkin et al. [50,[START_REF] Frumkin | Hydrogen overvoltage // Discuss[END_REF] and Bockris and

Potter [START_REF] Bockris | The Mechanism of Hydrogen Evolution at Nickel Cathodes in Aqueous Solutions[END_REF]. It is worth to note, that even at that time it was assumed that the presence of surface oxides may significantly affect the kinetics of the HOR/HER. Therefore experiments were performed in such a way as to minimize their formation. In particular, Frumkin et al. annealed Ni electrode at 400 -420 °C in H2 atmosphere inside of specially constructed electrochemical cell just before filling it with alkaline electrolyte [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF]. Bockris et al. used Ni electrodes, which were preliminarily annealed in H2 atmosphere and sealed in fragile glass bulbs in the absence of oxygen [START_REF] Bockris | The Mechanism of Hydrogen Evolution at Nickel Cathodes in Aqueous Solutions[END_REF]250].

Later the kinetics of the HER on Ni has been widely investigated by many others [START_REF] Divisek | Determination of the kinetics of hydrogen evolution by analysis of the potential current and potential coverage curves[END_REF][START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]126,166,241,244,[251][252][253][254][255][256][257], while the kinetics of the HOR on Ni was considered in only few cases [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF]115,258].

Based on the methods used for preconditioning of Ni electrode surface prior to the investigation of the HOR/HER kinetics, the published works can be divided into two groups. The first group contains studies in which the surface of Ni electrodes was subjected to cathodic polarization with the purpose to remove possible traces of Ni oxide species. As was shown by Conway et al. [246,256,259] as well as Machado and

Avaca [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF] such treatment may result in the formation of Ni hydrides, therefore the catalyst composition can be labeled as Ni/NiHx (Table 1.1). Note also, that the formation of Ni hydrides under application of cathodic overpotentials is facilitated if

Ni electrode was first subjected to anodic polarization for a short time [START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF][START_REF] Visscher | The anodic oxidation of nickel in alkaline solution // Electrochim[END_REF]. The second group contains studies in which the surface of Ni electrodes was either subjected to continuous cycling between high anodic and high cathodic potential limits or cleaned by its contact with acid solutions [166,[251][252][253]. Such preconditioning, likely, results in the formation of surface oxide species NiOx, which may coexist with metallic Ni sites even at negative overpotentials (at least for a short period of time), while the corresponding catalyst composition can be labeled as

Ni/NiOx (Table 1.1).

A careful analysis of the published works in accordance with the current level of understanding of the electrochemical behavior of Ni in alkaline medium (see Section 1.2) allows one to suggest that the HER usually follows the Volmer-Heyrovsky mechanism on metallic Ni electrodes even in the presence of Ni hydrides (Ni/NiHx, Table 1.1) [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF][START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]126,[255][256][257]. On the contrary, for systems where existence of NiOx species on the surface of Ni electrodes is expected from either the pretreatment procedure employed or utilization of high anodic potentials (E > 0.9 V) during the measurements, the reported mechanism of the HER was usually attributed to the Vomer-Tafel mechanism (Ni/NiOx, Table 1 Ni rod was heated at 400-420 °C in H2 atmosphere and then cooled down inside the cell before immersing in the electrolyte. Then it was cathodically polarized until stabilization of the potential.

Ni metal with possible traces of NiHx In addition to the experimental study we also performed kinetic modeling which allows us to further advance in understanding the effects of the state of Ni electrode surface as well as the presence of a second component on it on the kinetics and mechanism of the HOR/HER.

Preparation of electrocatalysts and electrodes

Polycrystalline Ni rod and foil

To inspect an influence of Ni surface state on its electrochemical and electrocatalytic (in the HOR/HER) properties in 0. or porous Vulcan XC-72 carbon (Cabot; BET surface area 254 m 2 g -1 , total pore volume 0.43 cm 3 g -1 ) as a catalyst support. In the latter case, a thin layer of carbon (ca.

100 µg cm -2 geom) was first deposited from its suspension with isopropanol : water (3:2, vol.) mixture on a clean and dry surface of GC and then dried under N2 flow for at least 60 min. Prior to the use, the top surface of GC cylinders was thoroughly polished with Al2O3 slurries (1.0 and 0.3 μm), after that they were thoroughly rinsed by water and consecutively sonicated in acetone, ethanol and water (10 min in each solvent).

Finally, the GC cylinders were dried in the oven at Т ≈ 80 °C for ca. 1 h. and then cooled dawn at room temperature. The electrodeposition procedure will be discussed in detail in Chapter 4. h. After reduction, the reactor was cooled down in H2 atmosphere, which was further replaced by helium. Before taking the reduced catalysts out, their surfaces were passivated by introducing short air pulses to the helium flow.

Synthesis of bimetallic NiCu/C electrocatalysts

In addition, monometallic Ni/C and bimetallic NiCu/C catalysts with Ni:Cu molar ratio from 0.90:0.10 to 0.98:0.02 were prepared by the same method, but using freeze-drying (lyophilization) instead of drying in vacuum with the objective to obtain more homogeneous catalysts (it will be discussed in detail in Chapter 5).

Synthesis of bimetallic

Scanning Electron Microscopy (SEM)

The In the spectra deconvolution procedure, the same peak parameters for Ni 0 , NiO and Ni(OH)2 components were employed for all processed samples after Shirley background subtraction. The charge effect arising in the process of photoemission of electrons was taken into account by correcting the spectra with respect to C1s of the carbon line (284.8 eV). Quantitative calculations were performed using normalized Ni2p, O1s, C1s peak areas, taking into account the atomic sensitivity factors of each element [262]. Spectra analysis and their deconvolution were performed using CasaXPS software.

Taking into account the overlapping of the Ni 0 , NiO and Ni(OH)2 peaks in a narrow interval of binding energies, a self-consistent fitting of Ni2p, C1s and O1s XP spectra was undertaken by assuming, according to the literature [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]110,263] At the first stage of deconvolution the contribution of NiO was determined.

Lattice oxygen peak of O1s spectra has the most noticeable and characteristic peak at low value of binding energy (529.6 eV), which is not overlapping with the other components and corresponds to NiO phase. On the basis of the intensity of this peak and the atomic ratio for nickel oxide (Ni:O=1:1), the intensity of the NiO component was adjusted and then followed by fitting of the other components (Ni 0 , Ni(OH)2) for Ni2p spectra. The inverse procedure was applied for fitting the hydroxyl contribution to the O1s spectra, recalculated from the previously fitted Ni(OH)2 intensity and the atomic ratio Ni:O=1:2. It should be mentioned that the intensity of the Ni2p peak was calculated from the Ni2p3/2 value multiplied by 1.5. The O1s deconvolution correction was finalized by comparing the atomic ratios of C:O for alcohol group.

Nanostructured NiED/XC-72 electrocatalysts

Surface state of NiED/XC-72 electrocatalysts (deposited on GC cylinder) was also analyzed using XPS in an ultra-high vacuum chamber (residual gas pressure

Inductively Coupled Plasma -Atomic Emission Spectrometry (ICP-AES)

Determination of the mass of Ni in the NiED/XC-72 samples and estimation of the Faradaic efficiency of the electrodeposition were performed by ICP-AES technique for several samples using a Varian 720 spectrometer. The sample probe was obtained by dissolving the catalyst layer from the top surface of GC cylinder in 30% HNO3 solution. For that the GC cylinder was immersed in glass jars containing 2 ml of 30% HNO3 solution for 60 min at T ≈ 50 °C. The mass of the jar was measured before and after this procedure to avoid mistakes caused by evaporation of a part of the solution. The instrument was calibrated using standards (1000 mg•l -1 , CPI).

Electrochemical techniques 2.3.1. Materials

For electrochemical experiments, all solutions were prepared using ultrapure water produced by either Milli-Q system or ELGA Ultra Analytic apparatus. 0.

Setup for electrochemical measurements

Electrochemical experiments were carried out using either a Biologic SP-300 potentiostat or an Autolab PGSTAT 30 potentiostat equipped with an analog Scangen module to apply a linear potential ramp, and a rotating disk electrode Autolab Rotator.

The temperature was controlled using laboratory thermostat/cryostat Julabo F25 (Julabo Labortechnik GMBH, Germany) within ± 0.1 °C accuracy.

A = A d * -d `ij (eq. 2.1.)
where Em and Im correspond to the obtained experimental values of the current and the potential, while Ru is the resistance of the electrolyte between the working and the reference electrodes.

Electrode preparation

To perform electrochemical measurements either a Ni rod or a GC cylinder were fixed in a holder and their lateral surfaces were sealed with the Teflon tape so that only the flat top surface of the electrodes was accessible to the electrolyte. Ni foil, utilized for XPS measurements, was fully immersed in the electrolyte using thin Au wire as a current collector.

To study electrochemical properties of carbon supported catalysts (Ni/C, NiCu/C and NiMo/C) a thin film approach was utilized. First, the catalyst powder was ground with a pestle in a mortar to break down agglomerates and then a desired amount of the powder was mixed with isopropanol to get the suspension with the concentration of 1.0 mg ml -1 . Second, the suspension was sonicated in ultrasonic bath (Branson 1510 and Bandelin) for 30-60 min and just after that several aliquots (typically, 5 ´ 4 μl) of the suspension were pipetted on a clean surface of GC cylinder (the cleaning procedure is described in Section 2.1.2). The obtained on the GC surface catalyst layer was dried under gentle Ar or N2 flow for 5-10 min between consecutive aliquots and for at least 60 min after final deposition.

Methodology of electrochemical measurements

Electrochemical pretreatment of the Ni electrodes Polycrystalline Ni rod

To inspect an influence of the oxidation of Ni electrode surface on its electrochemical and electrocatalytic properties, the polycrystalline Ni rod was tested in 0.10 M NaOH electrolyte either immediately after polishing of its top surface (hereinafter, "non-activated Ni rod") or after a preliminary oxidation of it in air for ca.

Determination of electrochemical surface area of Ni

As discussed in the literature review (see Section 1. Based on the value of SEC, the roughness factor of the surface of Ni electrodes could be determined as:

i k = l mn l oqrs (eq. 2.2)

Determination of the electrocatalytic activity of Ni in the HOR/HER Determination of the exchange current density

To compare the electrocatalytic activity of Ni electrodes in the HOR/HER, the values of the exchange currents were determined by analyzing the micropolarization region (|η| ≤ 10 mV) of CVs (2 nd scan) recorded in H2-saturated electrolyte at a low sweep rate (5 or 2 mV•s -1 ), according to the eq. 1.17 (see Section 1.1.1). The use of potentiodynamic regime instead of potentiostatic measurements was motivated by the fact that it is better adapted for separation of the contributions of various surface processes, such as oxidation/reduction of hydrogen, formation of surface NiOx, and formation of NiHx. Typical potential interval of [-0.06 -0.40 V] was used in most studies, except for the study of Ni/C and NiCu/C catalysts when the cathodic potential limit was extended up to -0.12 V. In the study of polycrystalline Ni rod at various temperatures, the cathodic potential limit was placed at -0.20 V in order to be able to analyze both the micropolarization region and the region of "high overpotentials". As a result, the exchange currents, the Tafel slopes and the apparent charge transfer coefficient α were determined by linearization of polarization curves plotted in Tafel coordinates (eq. 1.19, see Section 1.1.1). The analysis was performed in the overpotential regions of -0.20 < η < -0.07 V and 0.06 < η < 0.12 V for the HER and the HOR, respectively. From Fig. 2.3 one can see, that exchange current densities determined either using the Tafel equation or from the analysis of the micropolarization region are consistent with each other.

Determination of the HOR kinetic current density -rotating disk electrode study

For a number of systems, where it was possible, the kinetics of the HOR was also investigated using the method of rotating disk electrode, the schematic cross view of which is shown in Fig. 2.4a [273,274]. Stirring of the electrolyte by rotation of the RDE accelerates the mass transport of reagents and products of a reaction, due to a decrease of the thickness of the diffusion layer (δ), defined as:

t = >fu> `vHwy " Hw_ z 2Hw{ # (eq. 2.3)
where D is a diffusion coefficient, v -kinematic viscosity of the electrolyte, ωrotation rate. With an increase of overpotential and, as a consequence, acceleration of the charge transfer process, the reaction rate begins to be limited by the diffusion of reagents to the electrode surface. As a result, on CVs one can see stationary currents (Fig. 2.4b), which, according to Levich equation [273,274], can be written as follows:

-a = }fu~`•v € • /." 2 ' ƒ ' " € z ' € = … `z' € (eq. 2.4)
where In the potential region, where both mass transport and charge transfer contribute to the rate of the reaction, the kinetic currents (Ik) can be calculated according to the Koutecky-Levich equation:

H F = H F † C H F ‡ = H F † C H ˆ`‰ MfŠ (eq. 2.5)
There are two main approaches to extract Ik. The first one is based on direct application of the eq. 2.5 at a given rotation rate provided that the value of ID is known. The second method consists in plotting I -1 at selected potentials as a function of ω -1/2 . According to the eq. 2.5, the obtained curve should be linear with a slope (B -1 ) directly linked to the number of involved electrons, while the intercept gives the value of Ik -1 . The second approach was preferred in this work due to the fact that experimental values of the limiting diffusion currents may differ slightly from the theoretical one.

Kinetic modeling

Mean The kinetics of the HOR/HER in an alkaline medium is described on the basis of the Tafel (eq. 2.6), the Heyrovsky (eq. 2.7) and the Volmer (eq. 2.8) steps incorporated into the mathematical model:

1) H2 + 2Ni ! 2 Ni-Had (eq. 2.6)

2) H2 + OH -+ Ni ! Ni-Had + H2O + e - (eq. 2.7)

3) Ni-Had + OH -! H2O + e -+ Ni (eq. 2.8)

Here 'Ni' denotes a free nickel surface site.

In addition, in order to take into account the effect of nickel oxidation in the lowpotential region, a two-step formation of surface Ni-(OH)2,ad was considered in the model (based on the Refs. [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF]115]):

4) OH -+Ni ! Ni-OHad + e - (eq. 2.9)

5) Ni-OHad + OH -! Ni-(OH)2,ad + e - (eq. 2.10)
The total Faradaic current density is due to the Heyrovsky and Volmer reactions and to the Ni hydroxide formation/reduction reactions:

oe J = •• ž <‹ { C ‹ y C ‹ " C ‹ -@ (eq. 2.19)
Here St is the mole number of active Ni sites per electrode geometric surface area (in mol cm -2 ).

As it has been discussed in the literature review, the Had adsorption free energy ∆adG 0 H is one of the key factors affecting the activity of the catalyst in the HOR/HER.

As discussed by Bonnefont et al. [276], the rate constants of the Tafel, Heyrovsky and

Volmer steps exponentially depend on the ∆adG 0 H, if there is a linear relation between the value of ∆adG 0 H and the activation energies of the corresponding steps. An energy diagram for the non-activated Ni electrode is schematically shown in Fig. 2.5.

The value of ∆adG 0 H is directly related to the equilibrium potential of the Volmer step, and then to the ratio between the forward and backward rate constants of the Volmer step:

3 Ÿ-9 " ; = i ¡¢ £ • M ^¤¥ ' £ '• M (eq. 2.20)
Furthermore, since the thermodynamic equilibrium potential of the H2O/H2 redox couple has to be kept at 0V vs. SHE, ∆adG 0 H is also related to the ratios between the forward and the backward rate constant of the Heyrovsky and Tafel steps by the expressions: In this chapter we have attempted to understand how the presence of stable Ni oxide species on the electrode surface affects the kinetics of the HOR/HER in alkaline media. For this, we used polycrystalline Ni rod, subjected to various surface treatments (see Section 2.3.4.1) in order to obtain two types of the surface: (i) preferentially metallic (hereinafter, "non-activated"), and (ii) partially oxidized (hereinafter, "Ch activated" or "EC activated" depending on the way of surface oxidation). As will be shown below, this terminology originates from the fact of significant enhancement of the activity of "Ch activated" and "EC activated" Ni samples in the HOR/HER compared to "non-activated" samples. Further increase of the anodic potential limit leads to the appearance of an anodic (a1) and cathodic (c1) peaks, which, according to the literature [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF], correspond to electrochemical formation and reduction of surface α-Ni(OH)2, respectively. Note that setting the anodic potential limit close to the maximum of the peak a1 leads to a splitting of cathodic peak c1 into two components, which merge into one during potential cycling to E ≈ 0.4 V (Fig. 3.1b).

3 Ÿ-9 " ; = i ¡¢ £ '€ M £ € M ^¤¥ '^¥ € = H { i ¡¢ £ '' M £ ' M ^¥€ (eq. 2.21)
Since no cathodic peaks were detected in the CV registered in the potential interval of [-0.2 -0.2 V], it is unlikely, that one of the two components of the peak c1 originates from the reduction of Ni oxide species formed upon exposure of the electrode to air, as it was proposed by D'Alkaine and Santanna [START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF]. More plausible hypothesis was proposed by Jerkiewicz et al. [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF], according to which two cathodic peaks are due to an existence of intermediate NiOHad species before the formation of Ni(OH)2. This hypothesis will be used later in the development of the kinetic model. Nevertheless, it should be noted that in situ spectroscopic measurements are required for unambiguous assignment of the observed peaks. As noted in the literature review (see Section 1.2.1), an increase of the anodic potential limit to E > ~0.5 V leads to the formation of an electrochemically irreducible β-Ni(OH)2 [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF]. To avoid uncontrolled a to β-Ni(OH)2 conversion and its influence on electrochemical properties of Ni, the anodic potential limit was set at 0.40 (at v ≤ 20 mV•s -1 ) or 0.50 V (at v > 20 mV•s -1 ). To choose the cathodic potential limit (EC), which is necessary for reducing the oxides formed at positive potentials, a series of CVs with varied cathodic limit (from -0.20 to -0.02 V) was obtained (Fig. 3.2a).

For convenience, Fig. 3.2b shows the comparison of the absolute values of anodic (QA) and cathodic (QC) charges, determined at different cathodic limits of CV. The value of QC was calculated after subtraction of the anodic sweep at negative potentials, in order to minimize the contribution of the HER currents. One may notice that shifting the cathodic potential limit from -0.02 V to -0.20 V results in an increase of the absolute value of both the anodic and the cathodic charge, the latter growing to higher extent especially at EC < -0.12 V. The difference between QA and QC observed in this case is likely caused by an increasing contribution of Ni hydrides, the formation of which is expected at E < -0.07 V [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF], to the value of QC. At the same time, increase of anodic currents at -0.20 < EC < -0.12 V is believed to be due to the oxidation of hydrogen incorporated in the Ni metal [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF]. Analysis of the obtained data suggests that setting the cathodic limit to -0.06 V is sufficient for the reduction of the Ni electrode surface after recording of an anode scan of the CV. Besides, this allows one to minimize possible contribution of Ni hydrides and, as a consequence, their effect on electrochemical and electrocatalytic properties of Ni electrodes.

On the basis of the above analysis, CVs were usually registered in the potential range of [-0.06 -0.40 V] for the estimation of electrochemically active surface area of Ni electrodes as well as investigation of the kinetics of the hydrogen electrode reactions (mainly the HOR). For the study of the HOR/HER kinetics at elevated temperatures, this potential interval was extended up to [-0.2 -0.4 V] in order to be able to analyze both the HER and the HOR, which was required for kinetic modeling.

Influence of surface Ni oxide species on the electrochemical behavior of Ni electrodes in alkaline media

As it has been discussed in the literature review (see Section 1. In order to be sure that the change of CV shape is caused by the partial oxidation of the Ni electrode surface and exclude a possible influence of some impurities, which might adsorb on the electrode surface during its storage under air, we then inspected the effect of electrochemical oxidation of the Ni rod. For that, freshly polished Ni rod (after removal of air formed Ni oxide species, as described in Section 3.2.1) was immersed in N2-purged 0.1 M NaOH and then cycled three times in the potential region of [-0.30 -1.0 V] at a sweep rate of 50 mV•s -1 (Fig. 3.3c). The final scan was stopped at 0 V during the anodic sweep of the CV. The positive potential limit for these CVs was chosen in such a way as to ensure oxidation of a part of the Ni surface with likely formation of NiO and β-Ni(OH)2 (based on the literature data [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Trzesniewski | In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity[END_REF]), but avoiding full coverage of the electrode surface by electrochemically irreversible β- Finally, it was interesting to compare the values of SEC of Ni calculated for the three types of electrodes (the method of determination is described in Section 2.3.4.2). The latter was found to decrease by a factor of ca. 1.2 and 1.5 for EC activated and Ch activated Ni rod, respectively, compared to non-activated Ni rod (Table 3.1, see Section 3.3.1). This confirms the assumption of blocking a part of the surface (~20 and ~33%, respectively) of the activated electrodes by stable (under experimental conditions) Ni oxide species.

Reversibility of Ni oxidation in the low-potential region

Influence of the sweep rate

As has been shown in the literature review (see Section 1.2.1), there is still no consensus regarding the nature of the processes, which occur in the potential region of the peaks a1 and c1. However, the main contribution in inert atmosphere is believed to be related to Ni/α-Ni(OH)2 redox transition [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]. To inspect the reversibility of processes, contributing to peaks a1 and c1, a series of CVs was recorded for nonactivated and Ch activated Ni rod at different potential sweep rates (Fig. 3.4). For clarity the curves in Fig. 3.4 are normalized by the sweep rate. One can see, that the maxima of the peaks a1 and c1, present in all CVs, shift toward more positive or more negative potentials, respectively, with an increase of the sweep rate. This behavior can be related to the slow (compared to the sweep rate) charge transfer [273,274]. The latter suggests that α-Ni(OH)2 formation, even if chemically reversible, is an electrochemically irreversible surface process for both non-activated and activated surface state of Ni electrode. At the same time, the nature of the processes contributing to the peak a2 (observed for activated electrodes) is more complicated and will be analyzed later after studying the behavior of the Ni electrodes in the presence of H2 (see Section 3.3.1).

Investigation of the Ni electrode surface by physicochemical techniques X-ray photoelectron spectroscopy (XPS)

In order to unveil the differences in the surface state of Ni samples, nonactivated and Ch activated Ni foil (pre-treatment procedure is clarified in the Section 2.3.4.1) were characterized by XPS. Owing to the fact that we could only make ex situ analysis, the presence of surface oxides was expected, even if special care (protection of the electrode surface by a drop of water) was taken to minimize the contact of the electrode with air during its rapid (< 5 min) transferring to the XPS entry chamber.

Therefore the purpose of the measurements was to check whether the oxide coverage and the type of detected oxide species change after subjecting the electrode to different pretreatment protocols. as well as the literature data [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Melendres | On the composition of the passive film on nickel: a surface-enhanced Raman spectroelectrochemical study[END_REF][START_REF] Seyeux | In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)[END_REF], we tentatively attribute Ni hydroxide, observed on the Ni2p XP spectra, to the chemically reversible a-Ni(OH)2.

Considering the fact that a-Ni(OH)2 is electrochemically reduced close to the equilibrium potential of the hydrogen electrode (Fig. 3.1), it is reasonable to assume that in situ the surface of non-activated electrode predominantly consists of metallic Ni 0 . Fig. 3.7 shows that the contributions of individual components to the XP spectra depend on the take-off angle (two take-off angles were utilized: 0 (sub-surface sensitive angle) and 50 degrees (surface sensitive angle)) in agreement with the Ni oxidation model described in the literature [START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]110]. According to this model, a three- It should be stressed that since the XPS analysis was performed ex situ, it allows only some tentative conclusions regarding the state of the Ni surface in electrolyte under the potential control.

X-ray photoelectron spectroscopy (in situ)

In order to reach unambiguous conclusions regarding the state of the Ni surface in electrolyte under the potential control, we have also tried to perform in operando measurements by the method of near ambient pressure XPS. For that a special facility mounted on SOLEIL (France) and BESSY II (Germany) synchrotron work stations was used. The measurements were carried out at pressures from 3 to 6 mbar.

Membrane electrode assemblies were fabricated using alkaline polymer membrane Tokuyama A201 by the procedures described, for example, in [277,278]. Several types of Ni electrodes were tested. These include Ni film (obtained by vacuum deposition on the membrane), NiCu/C, NiMo/C and NiED/C nanoparticles immobilized on the surface of the membrane in a mixture with anion-exchange ionomer in the form of a thin layer. Unfortunately, Ni oxide species, which are present on the surface of Ni electrodes taken from air, were found to be very stable preventing us from reduction of the electrode surface by applying any electrochemical treatment in the pressure interval of 3 -6 mbar, even when the measurements were performed in gas/liquid cell (the cell device is described in [278]). Therefore the performed experiments did not allow us to obtain any reliable data regarding the surface state of the Ni electrode depending on the applied potential.

Surface-enhanced Raman spectroscopy (in situ)

Another technique for in operando study of the surface state is based on the use of surface-enhanced Raman spectroscopy (SERS) in liquid electrolyte simultaneously with electrochemical measurements in such a way as described in detail, for example, in the Refs. [START_REF] Trzesniewski | In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity[END_REF]279]. These experiments were done during one-week internship at the University of Leiden (The Netherland) under the guidance of Dr. Oscar Diaz-Morales and Prof. Marc T. M. Koper. Ni nanoparticles electrodeposited on Au substrate (NiED/Au, the preparation procedure is similar to that described in Section 2.1.2 for NiED/GC) were used as the working electrode. Comparison of the SER spectra of the NiED/Au electrode, primarily held in air for 2 hours (i. e. chemically activated) and then immersed in the alkaline electrolyte at an open-circuit potential (~0.7 V vs RHE), confirms the formation of surface NiO in air (Fig. 3.9a) in agreement with the literature data [START_REF] Trzesniewski | In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity[END_REF]. At higher potentials, the formation of β-Ni(OH)2 (an appearance of the oscillations at a frequency of 450 cm -1 at E = 0.9 and 1.1 V) and NiOOH (an appearance of the oscillations at a frequency of 560 cm -1 at E = 1.55 V) can be observed as well. However, these characteristic oscillations disappear, when the potential is shifted to the region of [-0.06 -0.40 V] (Figs. 3.9b, c). Similar obstacle also appeared when the electrode was studied immediately after immersing in the electrolyte, avoiding application of high anodic potentials. This observation is probably due to the fact that in the low-potential region the oxide film becomes very thin and, likely, disordered, which lead to a decrease in the intensity of the oscillations in the wavelength region from 400 to 700 cm -1 to the background level. Despite the fact that essential differences in the SER spectra of non-activated and Ch activated NiED/Au electrodes were observed in the wavelength region of 1300 -1700 cm -1 (Figs. and cathodic (c1) peaks observed under N2 atmosphere may be also noticed in H2 atmosphere (although the positions of these peaks are not exactly the same as under N2). An increase of anodic currents in H2 compared to N2 atmosphere the occurrence of the HOR. This difference in currents is larger in the case of Ch activated electrode.

Besides, it becomes more pronounced at lower sweep rates (comparison of Figs. 3.4 and 3.11). Moreover, the relative contribution of the a2 peak was found to increase with a decrease of the sweep rate (Fig. 3.11b). Experiments performed at different rotation rates in the presence or absence of H2 (Fig. 3.12) suggest that the HOR on polycrystalline Ni rod is kinetically rather than mass-transport (diffusion of H2 to the electrode surface) controlled. It should be noticed, however, that increase of a number of active sites on the electrode surface (see Chapter 4, Section 4.3.4) allowed us to observe the rotation rate dependence for NiED/C samples. As noted in Section 1.2.1, various explanations have been proposed in the literature for the appearance of the peak a2 in CVs in an inert atmosphere. Comparison of the experimental and literature data indicates that anodic currents under the peak a2 observed in this work are likely due to the formation of Ni-OHad and/or oxidation of adsorbed hydrogen [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][115][116][117][118]. Additional support for this will be presented later in the discussion of the results of kinetic modeling (see Section 3.3.3). Meanwhile, rotation rate independence of the currents in CVs obtained in an inert atmosphere (Fig. 3.12a) allows us to conclude that the peak a2 cannot be associated with the oxidation of molecular hydrogen evolved during the electrode cycling at negative potentials. Besides, oxidation of absorbed hydrogen and/or Ni hydrides, which could be formed at potentials below ca. -0.07 V [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF][START_REF] Visscher | Absorption of hydrogen in reduced nickel oxide[END_REF], also seems to be an unlikely reason for the appearance of the peak a2 in CVs obtained in the potential interval of [-0.06 -0.40 V] at relatively fast sweep rates v (20 mV•s -1 ) (Figs. 3.3 and 3.10). For the comparison of electrocatalytic activities of Ni electrodes in the HOR/HER the exchange currents were normalized to either electrochemical (μA•cm - 2 Ni) or geometric (μA•cm -2 geom) surface area of polycrystalline Ni rod, as described in Section 2.3.4.3 (Table 3.1). As it has been noted in the experimental part, every single CV curve for nonactivated Ni rod presented in this chapter was obtained in a separate experiment just after polishing of the electrode surface (see Section 2.3.4.1). This was found to be an

Influence of the temperature on the kinetics of the HOR/HER

For a more detailed study of the kinetics of the HOR/HER, including the effect of the surface state of Ni electrode, a series of experiments was performed at different temperatures. Comparison of CVs obtained in H2 and N2 atmosphere (Figs. 3.5 and 3.15) shows that with increase of temperature the HOR currents increase to a greater extent than the currents related to the oxidation of Ni electrode. Similarly, the HER currents also increase with temperature, confirming the results of earlier studies [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]241,244,254]. It is worth to note, that an abrupt increase in the currents (at E < 0.15 V) at the cathodic sweep of the CVs for non-activated Ni rod is also observed at elevated temperatures. Besides, the difference between the anodic and cathodic currents increases with an increase of the temperature (Fig. 3.16a). On the contrary, almost the same slope was observed for anodic and cathodic sweeps of CVs obtained for EC activated Ni rod (Fig. 3.16b), the latter suggesting similar state of the electrode surface in both cases. [START_REF] Bockris | The Mechanism of Hydrogen Evolution at Nickel Cathodes in Aqueous Solutions[END_REF]), Kibria et al. (35 and 27 kJ mol -1 , at low and high cathodic overpotentials, respectively [254]) and Huot (21 kJ mol -1 [244]), while in some other studies much higher values of Ea were obtained (58 kJ mol -1 [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF], 60 kJ mol -1 [241], 60 and 73 kJ mol -1 [261]). The values of Ea determined for polycrystalline Ni rod are also close to the activation energies reported for Pt/C, Ir/C and Rh/C in alkaline media (29.6 ± 0.4, 32.8 ± 0.4, 26.6 ± 0.7 kJ mol -1 , respectively [START_REF] Zheng | Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy[END_REF]), while being a bit smaller compared to the values reported for Pd/C (38.9 ± 3.0 [36]). 18 with temperature and agrees with the literature data [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF]126,166,244,256,257] (Table 3.3). The apparent charge transfer coefficient α is almost constant and equal to 0.52 ± 0.01. In addition, the exchange current densities calculated by the use of eq. 1.18 are in line with the values calculated from the analysis of micropolarization region of CVs. On the contrary, analysis of the potential region of anodic hydrogen oxidation (0.06 < η < 0.12 V) clearly shows strong variation of the Tafel slope with temperature. This is likely due to significant impact of the currents related to the oxidation of Ni in a given potential region, especially at low temperatures. The latter follows from the comparison of CVs obtained in N2 and H2 atmosphere (Fig. 3.16a). At higher temperatures the HOR starts at more negative potentials and an impact of this process to the overall currents increases. As a result, the apparent Tafel slope of the HOR decreases with the temperature, tending to the values obtained in the interval of the HER (Table 3.3).

Mean-field kinetic modeling of the HOR/HER

In the final part of this Chapter we present the analysis of the data of kinetic modeling performed with the aim to unveil the mechanism of the HOR/HER on metallic and partially oxidized Ni electrodes as well as understand the origin of the HOR/HER kinetics enhancement in the presence of surface Ni oxides. The kinetic model was developed using the mean-field approximation, which a priori imposes certain limitations on the discussion of the obtained results. While the surface of the non-activated Ni rod can be considered relatively uniform, in the case of activated electrodes, partial reduction of the electrode surface may result in the formation of Ni oxide islands [START_REF] Nakamura | Surface structures at the initial stages in passive film formation on Ni(111) electrodes in acidic electrolytes[END_REF]280,281]. This can lead to the appearance of surface reaction centers with different properties and, as a consequence, different kinetics of the HOR/HER on them. Meanwhile, within the frame of the developed model, the surface was considered homogeneous even in the case of activated electrode. This approximation is dictated by the lack of in situ information about the state of the surface and the distribution of various centers on it, as well as the need to simplify the model.

Given the complexity of the electrochemical processes at Ni electrodes, the purpose of the model is not to accurately determine the rate constants of the reaction steps, but rather to propose a self-consistent interpretation of the experimental trends, which does not contradict the literature data regarding the HER kinetics [126,241,253,257].

Based on the results of XPS measurements we assumed that it is the presence of NiO (together with metallic Ni 0 ) on the surface of an activated Ni electrode, which is responsible for the changes of the CV in the supporting electrolyte, and an increase of the activity of Ni in the HOR/HER. The suggested hypothesis can be further supported by the analysis of computational studies devoted to the adsorption of various species (Had, OHad, Oad and H2Oad) on the Ni surface [135,[142][143][144]150,155,[157][158][159],

presented in Section 1. [135,150,155]. Besides, hydrogen is also strongly adsorbed on Ni, which may be considered as one of the likely reasons for its lower (compared to Pt) HOR/HER activity [START_REF] Greeley | Computational high-throughput screening of electrocatalytic materials for hydrogen evolution[END_REF]135,136]. However, the binding energies of Ni-Had and Ni-OHad were shown by DFT calculations to decrease in the presence of adsorbed oxygen atoms [150,155]. In accordance with Sabatier principle, this effect should increase the activity of Ni towards the HOR/HER. In addition, DFT calculations showed that the To explain the experimentally observed changes in the shape of CV and the HOR/HER activity enhancement upon oxidation of the Ni electrode, we assumed weakening of the Ni-Had bond strength in the presence of surface NiO, and the ensuing increase of ∆adG 0 H by ca. 0.08 eV. This allowed us to reproduce the emergence of the peak a2 (cf Figs. 3.17b and d) which is now attributed to the Had oxidation and Ni-OHad formation in agreement with the assumptions in some earlier studies [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF]115]. At the same time, oxidation of Ni(OH)ad with the formation of Ni(OH)2,ad still occurs at the potentials of the peak a1. Note however that to explain the strong HOR/HER activity enhancement for the Ch activated Ni the ∆adG 0 H increase alone was not sufficient (see Fig. 19), but also required an increase of the Volmer rate constant k3 0 . The latter implies that the presence of NiO on the surface results in changes in the double layer structure on the metal/electrolyte interface [START_REF] Ledezma-Yanez | Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes[END_REF]. This is in agreement with the DFT calculations, which suggest that pre-adsorbed Oad not only influences the binding energies of Ni-Had and Ni-OHad, but also the adsorption of water molecules on the Ni surface [150,155]. Taking into account this as well as stronger shift of the potential of the peak a1 with temperature compared to that of the peak c1 (Fig. 3.5), it is reasonable to assume that the position of the peak a1 is mainly determined by the kinetics of electrochemical desorption of Had. Therefore, the rate constants for Ni oxidation and reduction were roughly adjusted from the position of the cathodic peak c1 on the CVs obtained in inert atmosphere, allowing us at the same time to reproduce the abrupt decrease of the HOR current observed during the anodic sweep of CVs in H2 atmosphere. As before, the same set of rate constants k4 0 , k-4 0 , k5 0 , k-5 0 has been used for non-activated and EC activated Ni electrodes, with the aim to minimize the number of adjustable parameters (Appendix 3). [START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]253,257]. It should be noted that present work for the first time shows that the Heyrovsky-Volmer mechanism is valid not only for the HER but also for the HOR on metallic Ni. Besides, on the bases of data listed in Table 3.4 we conclude that the rate of the HER is determined by the backward Heyrovsky step (k-2 0 ), while the rate of the HOR is controlled by the forward Volmer step (k3 0 ). Remarkably, the calculated activation energy for the k-2 0 is fairly close to the experimentally obtained value of Ea (ca. 32 and 30 kJ mol -1 , respectively, Tables 3.2 and3 .4). In contrast, the calculated activation energy for the k3 0 , according to the modeling, was found to be much higher (ca. 42 kJ mol -1 , Table 3.4). Table 3. 4 The activation energies and the pre-exponential factors, determined for the rate constants of the Tafel (k±1), Heyrovsky (k±2) and Volmer (k±3) steps by the analysis of Fig. 3.22 The results of this study, among other things, allow one to reconcile the inconsistencies in the data reported in the literature regarding the electrocatalytic activity of Ni in the HOR/HER and the mechanism of this process (Heyrovsky-Volmer vs. Tafel-Volmer). The observed contradictions are attributed to differences in the surface state of Ni electrodes, which originate from the differences in the methodology used for the pretreatment of the surface of Ni electrode.

k1 0 , cm 3 s -1 mol -1 k-1 0 , s -1 k2 0 , cm 6 s -1 mol -2 k-2 0 , s -1 k3 0 , cm 3 s -1 mol -1 k-3 0 , s -1 Non- activated Ni Ea, kJ mol -1 - - 17±1 32±1 42±1 27±1 Ai 0 - - 3.

Introduction

For practical applications (e.g. at the anodes of AMFCs), carbon (or other conductive material) supported catalysts with high dispersion and loading of an active component are preferable. Such materials provide high specific surface area of the active component, which is necessary for preparation of compact and highly efficient FC systems. However, incorporation of Ni metal nanoparticles on carbon surface by traditional methods, such as, for example, wet impregnation or adsorption, usually results in formation of large agglomerates and, as a consequence, low Ni dispersion.

An alternative method for preparation of supported catalysts is based on the method of electrodeposition, the main advantage of which is the possibility to tune the structure of the deposits and the particle size via modification of the deposition parameters, such as applied potential and the composition of the deposition bath. In addition, this method allows one to prepare nanostructured materials with high density of intergrain boundaries, which have proven to enhance the activity of metals in a number of catalytic [282,283] and electrocatalytic [284][285][286][287][288][289] processes.

Therefore the main tasks in the current chapter were formulated as follows:

(i) To develop an electrodeposition procedure allowing one to obtain Ni nanoparticles of relatively small size with a high density of them on the surface of carbon support;

(ii) To compare the electrochemical behavior of the obtained samples, as well as their electrocatalytic activity in the HOR/HER with the results of the study of polycrystalline Ni electrode;

(iii) To find out how the oxidative surface pre-treatment of the obtained electrodes affects their activity in the HOR/HER.

The synthesis of Ni particles was first performed using flat GC support, with the aim to determine the parameters of electrodeposition. After developing the procedure of electrodeposition and revealing the main characteristics of the obtained samples, a series of samples with a commercial carbon support Vulcan XC-72 was also obtained.

This allowed us for the first time to prepare a supported catalyst with a high content (ca. 40 wt.%) of small Ni nanoparticles (ca. 10 nm). In order to obtain a narrower size distribution of Ni particles, the time of the first electrodeposition step in the synthesis of model NiED/GC samples in what follows was set before reaching tmax (t1 = 8 s). This allows one to reduce the contribution of the secondary nucleation and the growth of Ni particles when the potential E1 is applied.

In addition, with the aim to increase the density of sites accessible for the primary nucleation, the surface roughness factor of the GC support was increased. The latter was achieved by excluding the final polishing of the surface of GC support with 0.05 µm Al2O3 slurry (i. e. the GC rods were polished only with 1.0 and 0.3 µm Al2O3 slurries). Almost seven times increase of the number of particles per unit surface area was observed, if comparing the samples NiED/GC-1 (ca. 7.7•10 9 particles/cm 2 geom) and NiED/GC-4 (ca. 5.5•10 10 particles/cm 2 geom). In the latter case, with the aim to minimize agglomeration of the particles, the time for the third electrodeposition step was decreased from 300 to 120 s, which resulted in a decrease of the surface mean diameter of Ni particles (Table 4.1,Fig. 4.3a).

It is well known, that electrodeposition often results in formation of nanostructured deposits due to the existence of secondary nucleation processes [284,[287][288][289]291,292]. Analysis of the NiED/GC-4 particles by TEM (after transferring the deposits from the GC surface to the copper mesh) evidences their nanostructured nature. The defect structure and the presence of intergrain boundaries can be observed in the HR-TEM image, shown in Fig. 4.3b. However particle mobility under the electron beam did not allow us to obtain the images at higher resolution, thus preventing us from their detailed analysis.

Fig. 4.3 SEM and HR-TEM images of NiED/GC-4 sample (the deposition parameters are listed in Table 4.1).

Subsequently, for the electrochemical and electrocatalytic measurements, a series of NiED/GC samples was obtained using the parameters listed in Table 4.1 for the NiED/GC-4 sample.

Electrochemical properties of NiED/GC electrocatalysts

Fig. 4.4a shows the CVs registered just after the immersion of freshly prepared NiED/GC electrode in N2-purged 0.10 M NaOH. As it has been noticed for nonactivated Ni rod (see Section 3.2.1), the cathodic peak attributed to the reduction of Ni oxide species, which are formed on the electrode surface during its short exposure to the ambient, appears also on the first scan of the CV obtained for non-activated NiED/GC electrode. However, this peak is broader compared to that observed for Ni rod and located at more negative potentials (ca. -0.08 and ca. 0.02 V for the nonactivated NiED/GC and Ni rod, respectively) (Figs. 4.4a and 3.1a). The latter is likely related to a stronger oxidation of the surface of NiED/GC particles during the electrode transfer to the electrochemical cell with 0.1 М NaOH through air, even if this procedure was performed fast (less than 15 s). This assumption is in agreement with the fact that the value of the open-circuit potential measured for NiED/GC just after its immersion of in the electrolyte was ca. 0.2 V more positive than that for the nonactivated Ni rod (0.4 -0.5 and 0.2 -0.3 V, respectively). Notwithstanding this difference, already after the first scan of the CV, registered in the potential range of [-0.20 -0.40 V], the air-formed Ni oxide species are believed to be reduced. As a result, the shape of the subsequent CVs becomes similar to that observed for a nonactivated Ni rod (Fig. 4.4a) and discussed in Chapter 3. In order to inspect whether the electrochemical and electrocatalytic properties of NiED/GC nanoparticles change after their partial oxidation, as described in the previous chapter (see Chapter 3), the freshly prepared NiED/GC electrode was dried under a N2 flow and subjected to oxidation in air for quite a long time (typically 24 h.) ("Ch activated", see Section 2.3.4.1). The open-circuit potential, measured after the immersion of Ch activated NiED/GC sample in 0.1 M NaOH, was found to be in the range of 0.55 -0.70 V, which is ca. 0.15 V higher than the value measured for a Ch activated Ni rod (see Section 3.

Kinetics of the HOR/HER on NiED/GC electrocatalysts

Electrocatalytic activities of the NiED/GC catalysts in the HOR/HER were estimated from the analysis of CVs registered in H2-purged 0.10 M NaOH at v = 5 mV•s -1 (Fig. 4.5). Analysis of the data listed in the Table 4.2 shows that the activity of non-activated NiED/GC is almost 3 times higher than the value measured for non-activated Ni rod (Table 3.1). However the observed difference is most likely due to the longer contact of the NiED/GC sample with alkaline media during the measurements. Indeed, the CVs of non-activated Ni rod in H2 atmosphere were obtained just after polishing of the electrode surface. On the contrary, before measuring the activity of non-activated NiED/GC, CVs in N2-saturated 0.1 M NaOH were registered (with the aim to determine the value of SEC). To further confirm this assumption, the same experiments, namely consecutive measurements in, first, N2 atmosphere and then in H2 atmosphere, were carried out using freshly polished Ni rod (hereinafter, 'polished Ni rod'). It was found that the specific activities of nonactivated NiED/GC and polished Ni rod in the HOR/HER are essentially similar (Table 4.2), even if the shape of the corresponding CVs is not exactly the same (Fig. 4.5b). The study of Ch activated NiED/GC electrode in H2 atmosphere showed that its specific activity in the HOR/HER is almost 4 times as high as the activity of nonactivated NiED/GC, while the value of SEC decreases by less than 20% (Table 4.2) after partial oxidation of the surface of NiED/GC particles. However, same as for the EC activated Ni rod (see Section 3.3.1), the activity of the Ch activated NiED/GC sample decreases during prolonged cycling of the electrode in the potential region of [-0.06 -0.4 V] (by about 30% for 10 scans). This is likely due to continuous reduction of the surface Ni oxide species (Fig. 4.5a).

Table 4.2 The values of the exchange current densities in the HOR/HER obtained for NiED/GC and polycrystalline Ni rod after various pre-treatment of their surface

Catalyst SEC / cm 2 Ni j0 / µA cm -2 Ni
Non-activated NiED/GC 0.20 ± 0.09 6.4 ± 0.6

Ch activated

NiED/GC 0.17 ± 0.04 25 ± 3

Polished Ni rod 0.52 ± 0.03 6.0 ± 0.8

Ni electrodeposited on porous XC-72 carbon support 4.3.1. Choice of electrodeposition parameters and investigation of the synthesized samples by physicochemical techniques

To prepare electrocatalysts, which could be potentially applied at the anode of an AMFC, electrodeposition of Ni was also undertaken with the use of the high surface area Vulcan XC-72 carbon as a support. The latter was immobilized on the GC surface in the form of a thin layer (ca. 100 µg cm -2 geom). The electrodeposition parameters were slightly modified in comparison with those described above. First, with the aim to obtain high metal loading on the porous carbon support, the time for the third deposition step (t3) was increased from 120 to 210 s. Second, the rotation of the electrode at a speed of 400 rpm was found to increase the currents of the deposition transients. The latter is probably due to the removal of hydrogen bubbles formed during electrodeposition from the electrode surface, as well as a decrease in the thickness of the diffusion layer. An increase in the amount of Ni electrodeposited on a porous carbon support (compared to NiED/GC) allowed us to measure the mass of the deposits for several NiED/XC-72 samples using the ISP-AES technique (see Section 2.2.6). According to these measurements, faradaic efficiency (under the given conditions) of Ni electrodeposition was estimated to be ca. 35% (confirming the high contribution of the HER currents), while the typical metal loading was calculated to be 42 ± 3 wt.% (with respect to the total mass of the NiED/XC-72 catalyst).

Investigation of the microstructure of NiED/XC-72 samples

The microstructure of the metal particles in NiED/XC-72 samples was investigated by TEM and STEM techniques. TEM survey images (Fig. 4.7a) revealed a fairly homogeneous distribution of Ni nanoparticles over the surface of XC-72 carbon support, while their surface mean diameter was estimated to be 10 ± 2 nm (under the assumption of the spherical particle shape). Moreover, the high resolution STEM images obtained in bright and dark fields (Figs. 4.7b,c)

Investigation of the surface state of NiED/XC-72 samples

As will be shown later, prolonged cycling of the NiED/XC-72 electrode in 0.10 M NaOH in the potential range of [-0.06 -0.40 V] results in significant increase of its activity in the HOR/HER (see Section 4.3.4). To explain the observed changes, the surface state of the NiED/XC-72 electrocatalyst was investigated using XPS before and after such treatment. As it has been noticed in Section 3.2.5, due to ex situ character of the measurements, the presence of surface Ni oxides formed during the transfer of the electrodes to the XPS chamber through the air was expected. Because of this, the obtained XP spectra can be used only for comparative analysis of activated and nonactivated samples.

The Ni 2p spectra (Fig. 4.8) obtained for NiED/XC-72 samples were deconvoluted into three components, namely metallic Ni 0 , NiO and Ni(OH)2, using the procedure described for the experiments with the Ni foil (see Section 2.2.3). Analysis of the obtained XP spectra allows one to make several conclusions. First, we found that the surface of non-activated NiED/XC-72 sample is oxidized to a higher extent compared to that of non-activated Ni foil. This is likely related to smaller size of Ni particles in NiED/XC-72 sample and their defective structure, both known to facilitate adsorption of oxygen containing species [293]. Second, continuous cycling of the NiED/XC-72 electrode in 0.10 M NaOH leads to a further decrease in the contribution of metallic Ni 0 component to the XP spectra (Fig. 4.8b). As for the Ni foil, the surface of the freshly prepared NiED/XC-72 particles is predominantly covered with Ni(OH)2, which can be presumably attributed to the α-form, given the possibility of its reduction under the conditions of electrochemical measurements (see Section 4.3.2). At the same time, an increase in the amount of Ni(OH)2 on the electrode surface after prolonged cycling in the potential range of [-0.06 -0.40 V] appears to be likely due to the formation of irreversible β-Ni(OH)2. This is in agreement with a decrease of SEC for the NiED/XC-72 electrocatalyst (Table 4.3). The presence of NiO in the Ni2p spectra is likely associated with its formation during the transfer of the samples to the XPS entry chamber through the air. The available data are insufficient to unambiguously explain the greater contribution of the oxide to the XP spectrum of the non-activated sample. Finally, it is interesting to compare electrocatalytic activities of NiED/XC-72 samples in the HOR with the values reported for PGM-free catalysts as well as finely dispersed Pd/C and Pt/C catalysts (Appendix 1). It was found that the specific surfaceweighted activity of the activated NiED/XC-72 nanoparticles is either comparable (for the sample obtained after cycling) or exceeds (for the Ch activated sample) the values reported for the best PGM-free catalysts of this process. Besides, the mass-weighted activity of non-activated NiED/XC-72 electrocatalyst is more than 1.5 times higher compared to the values reported for state-of-the-art Ni/N-CNT (3.5 A gNi -1 , Appendix 1), which can be attributed to the small size of Ni nanoparticles (10 ± 2 nm) and, likely, their special nanostructured nature. Moreover, activation of the NiED/XC-72 electrocatalyst by partial oxidation of its surface allows to further improve its specific mass-weighted activity up to ca. 14 and ca. 22 A gNi -1 for the sample obtained after cycling and Ch activated sample, respectively. The mass-weighted activity of the Ch activated NiED/XC-72 is just 1.7 and 16 times lower compared to commercial 20%

Electrochemical properties of NiED/ XC-72 electrocatalysts

Pd/C (Premetek Co) and 46% Pt/C (TKK) catalysts, respectively. Finally, kinetic current densities of the HOR were calculated for the activated NiED/XC-72 samples at an overpotential of 50 mV in accordance with the eq. 2.5 (see Section 2.3.4.3). The obtained values were found to be about 3.5 times higher than the values reported for the best PGM-free Ni/N-CNT catalyst described in the literature (Appendix 1). Carbon supported 40% NiED/XC-72 electrocatalyst prepared by electrodeposition demonstrates mass-weighted activity in the HOR, which is 6 times as high as the activity of the best PGM-free Ni/N-CNT catalyst reported up to now. This high activity might be presumably attributed to the low size of Ni particles, their defective nature as well as the synergistic effect between metal Ni sites and nearby "irreversible" oxide species present on the surface of the NiED/XC-72 electrocatalyst.

Taking into account an insignificant difference between the mass-weighted activity of the activated NiED/XC-72 catalyst and the commercial Pd/C catalyst, the sample obtained in this work can be considered as a promising alternative to noble metals at the anode of AMFC.

Introduction

Analysis of the literature presented in Section 1.3.3 shows that many bi-and polymetallic Ni-based catalysts have been investigated in the HER, demonstrating an increase of the catalytic activity and, importantly, stability in comparison with monometallic Ni electrocatalysts. However only few materials have been studied in the potential interval of the HOR in alkaline media [START_REF] Sheng | Non-precious metal electrocatalysts with high activity for hydrogen oxidation reaction in alkaline electrolytes[END_REF]192,195,294] However the carbon supported metal particles have not been studied in the HOR/HER yet. Besides, the optimum Ni:Cu ratio for the anodic hydrogen oxidation has not been known. According to the literature, addition of Mo allows one to significantly increase the activity of Ni in the HER [211,212,214,216,217,220]. However the origin of this catalytic enhancement is still not fully understood. Besides, there is no information concerning the activity of carbon supported NiMo/C electrocatalysts in the HOR.

Bimetallic NiCu/C electrocatalysts

Monometallic Ni/C and bimetallic Ni1-xCux/C catalysts with 0.05 ≤ x ≤ 0.50 prepared by traditional wetness impregnation technique (as described in Section 2. 1.3) were studied with the aim to inspect the effect of Cu on the activity of Ni in the HOR/HER and define the optimum Ni:Cu ratio. However, as will be shown below, the resulting samples had a high degree of heterogeneity, which impede the estimation of the optimum Ni:Cu ratio. To obtain more homogeneous samples the synthetic procedure was modified by replacing the step of vacuum drying with freeze-drying (lyophilization). For the second series of catalysts (for convenience, denoted as Ni1-xCux/C (lyo)), the Cu content was varied in a narrower range (0.02 ≤ x ≤ 0.10) near the maximum activity found for Ni1-xCux/C catalysts. As will be shown in this section, the main conclusions are valid for both series of samples. Elemental mapping using energy dispersive X-ray spectroscopy revealed that the composition of bimetallic particles is also inhomogeneous and depends on the particle size (Table 5.1). The highest Cu content was detected in small particles with dTEM < 5 nm, while large particles (dTEM > 20 nm) mostly consist of Ni. This observation allows one to suppose that the presence of metal particles with dTEM < 10 nm in bimetallic Ni1-xCux/C samples is probably due to significant content of Cu in them.

Investigation of the

Modification of the synthetic procedure by the use of freeze-drying instead of traditional drying in vacuum allows one to obtain samples with a narrower particle size distribution (Fig. 5.1b) and, in addition, more homogeneous composition (Table 5.1).

Electrochemical properties of NiCu/C electrocatalysts

Despite the fact that utilization of freeze-drying allows one to prepare more homogeneous catalysts, the electrochemical behavior of the Ni1-xCux/C and Ni1-xCux/C (lyo) samples was found to be essentially similar. Therefore in this section we preferentially discuss the first series of samples, in which the Cu content varied over a wider range.

In accordance with the results of TEM and XPS studies, electrochemical measurements also showed that the surface of Ni/C and Ni1-xCux/C catalysts is largely oxidized. The latter is evidenced in particular by the value of the open-circuit potential (0.75 -0.85 V), measured just after the immersion of the electrode in the electrolyte.

That is why, prior to the main measurements, the electrode surface was subjected to electrochemical reduction as described in Section 2.3.4.1. However, the presence of some amount of unreduced β-Ni(OH)2 on the surface of Ni/C and Ni1-xCux/C catalysts after such treatment cannot be excluded.

Kinetics of the HOR/HER on NiCu/C electrocatalysts

Electrocatalytic activity in the HOR/HER as a function of the catalyst composition

To measure the catalytic activity of the Ni/C and Ni1-xCux/C catalysts in the HOR/HER, CVs were registered in H2-purged 0.10 M NaOH at v = 2 mV•s -1 (Fig.

5.8a

). The catalytic activities were compared in terms of exchange currents, which, considering low activity of Cu in the HOR/HER [START_REF] Sheng | Correlating the hydrogen evolution reaction activity in alkaline electrolytes with the hydrogen binding energy on monometallic surfaces[END_REF]203], were normalized either to the mass (i0) or to the surface (j0) of Ni (Figs. 5. 8b andc).

Inspection of the Fig. 5.8 allows us to conclude that an addition of small amount of Cu (0.05 ≤ x ≤ 0.30) leads to a notable increase of the mass-weighted activity of Ni in the HOR/HER. There are two factors leading to the enhancement, namely (i) an increase of the specific surface area of Ni (Figs. 5.7), and (ii) an increase of the specific (surface-weighted) electrocatalytic activity (Fig. 5.8b). Besides, we note that preparation of more homogeneous Ni1-xCux/C (lyo) catalysts allowed us, first, to confirm that the maximum activity in the HOR/HER is achieved at 5 at% of Cu fraction in the catalyst and, second, enhance the mass-weighted activity due to an increase of the dispersion of the active component.

Bimetallic NiMo/C electrocatalyst

Analysis of the literature showed (see Section 1.2.2.3) that among studied NiMo catalysts the highest activity in the HER was achieved at about 10 at.% of Mo fraction [212]. In order to answer the question about the prospects of using NiMo electrocatalysts in AMFCs, we investigated Ni0.9Mo0.1/KB sample with a total metal content of 50 wt.% (the synthesis is described in Section 2.1.4).

Investigation of the NiMo/C electrocatalyst by physicochemical techniques

Morphology and dispersion of NiMo/C particles TEM analysis of synthesized Ni0.90Mo0.10/C electrocatalyst revealed that notwithstanding the high (50 wt.%) loading of the active component, the metal particles are quite uniformly distributed on the surface of the support with no sign of particle agglomeration (Fig. 5.9). This can be attributed to the high specific surface area of the KB support utilized for the NiMo catalyst preparation. The surface mean diameter of the NiMo particles was estimated to be 22 ± 6 nm (under the assumption of the spherical particle shape). 3.4.1. Fig. 5.12 shows that this procedure results in considerable increase of measured currents and an emergence of typical a1 and c1 peaks as well as a shoulder in the potential interval of the peak a2. It was also found that, despite the high (50 wt.%) metal content on the KB support, the contribution of the currents arising from the carbon support is significant, as evidenced by the comparison of the CVs presented in Fig. 5.12b. This is consistent with the high specific surface area of the used support. Therefore, the estimation of the SEC was done after subtraction of the charge corresponding to carbon (Table 5.3). A mean diameter of the Ni particles estimated from the SEC value (under the assumption that the metal particles have spherical shape) was found to be 36 ± 2 nm. The obtained value is reasonably close to the one calculated from TEM (22 ± 6 nm), which suggests that under the applied electrochemical conditions Ni surface is essentially reduced.

Kinetics of the HOR/HER on NiMo/C electrocatalyst

Electrocatalytic activity as a function of the catalyst loading

To measure the catalytic activity of Ni0.90Mo0.10/C in the HOR/HER, CVs were registered in H2-purged 0.10 M NaOH in the potential region of [-0.06 -0.40 V] at v = 5 mV•s -1 and rotating rate of 1600 rpm (Fig. 5.13a). The values of the exchange currents referred to SEC (j0) or to the mass of metals (Ni+Mo, i0) are listed in the Table 5.3. It was found that both specific and mass-weighted activities of the Ni0.90Mo0.10/C catalyst exceed the values obtained for the Ni0.95Cu0.05/C catalyst, as well as those reported in the literature for Ni/N-CNT (Appendix 1). However, at a small (100 µg cm -2 ) catalyst loading on the surface of the GC electrode, the measured currents were found to be essentially independent on the rotating rate. Meanwhile, increase of the catalyst loading from 100 to 500 µg cm -2 allowed us to significantly increase the measured currents (Fig. 5.13a) and observe their rotating rate dependence (Fig. 5.13b). However, as can be seen from the Fig. 5.13a and Table 5.3, currents do not increase proportionally to the catalyst loading.

The latter could be likely attributed to the inaccessibility of a part of the catalyst surface for the electrolyte at increasing thickness of the catalyst layer. This assumption is supported by the lack of proportionality between the catalyst loading and the electrochemical surface area of Ni (see Table 5.3). Considering this fact, determination of the kinetic current density from the rotation rate dependence obtained for the electrode with high catalyst loading is incorrect and therefore was not performed.

Conclusions of Chapter 5

In this chapter we studied carbon supported monometallic Ni/С and bimetallic The present work has clarified a number of important questions regarding the electrochemical properties of Ni electrodes and their electrocatalytic activity in the HOR/HER. However several aspects could be further improved.

General conclusions and Outlook

First of all, in situ studies of Ni electrodes by physicochemical methods are highly desirable and could allow one to better understand the surface state of Ni depending on the applied potential and unambiguously answer the question about its influence on the mechanism and the kinetics of the HOR/HER on Ni. These might be performed using in situ X-ray absorption spectroscopy and X-ray photoelectron spectroscopy if one would be able to deposit Ni particles directly on the polymer membrane, assemble the electrochemical cell and conduct an experiment avoiding influence of oxygen-containing atmosphere at each of the preparation steps. This would allow one to overcome the difficulties encountered in the current study, when the surface of the Ni electrodes prepared for NAP-XPS measurements was oxidized in air, its reduction under applied potential being prevented by the low water pressure in the measurement chamber.

It would be also interesting to prepare carbon supported bimetallic Ni-based electrocatalysts using the method of electrodeposition. This will require development of the synthetic protocol and adjusting a number of parameters, among which are the composition of the deposition bath, the value of the applied potential and time, etc.

These materials are expected to have a number of advantages, such as high metal loading, high dispersion and the possibility to avoid a passivation step (necessary for chemical methods of preparation), which could make them promising candidates for the application in practical AMFCs. Another interesting task in this instance is to design a cell allowing one to scale up the synthesis of electrocatalysts by the method of electrodeposition.

In order to validate the application of Ni-based systems as AMFC anode materials, fuel cell tests are required. Besides, the stability tests are also of great importance. For these, optimization of several parameters such as the thickness of the catalytic layer, the type of the polymer membrane, the quantity of an ionomer and the water management have to be optimized. Furthermore, it would be necessary to optimize the procedure of the preparation of the membrane electrode assembly.

Finally, the test protocol for the catalytic activity and stability measurements in alkaline environment should be developed. i These values of k3 0 and k-3 0 correspond to ∆adG 0 Н=-RT ln(10000*k-3 0 /k3 0 )=-0.1 eV. j These values of k3 0 and k-3 0 correspond to ∆adG 0 Н =-RT ln(10000*k-3 0 /k3 0 )=-0.02 eV. k These values of k3 0 and k-3 0 correspond to ∆adG 0 Н =-RT ln(10000*k-3 0 /k3 0 )=0.04 eV. électrochimiques se produisant sur des sites de Ni libres à proximité de sites de Ni oxydé (Fig. 3).

Afin d'étudier le mécanisme du HOR / HER sur des électrodes de Ni métalliques et partiellement oxydées et pour comprendre les raisons de l'augmentation de la cinétique de l'HOR/HER en présence d'oxydes de Ni de surface, les courbes expérimentales de voltampérométrie cyclique ont été analysées à l'aide d'un modèle cinétique champ moyen. Le modèle est basé sur le mécanisme Tafel-Heyrovsky-Volmer pour l'HOR/HER en milieu alcalin (eq. 1 -3, respectivement):

• { # C #~¦ §# $ H ! $ 2H
#~¦ § * • ¨© (eq. 1)

• { C Ž• 2 C ¦ §# $ { ! $ 2{ #¦ § * • ¨© C • { Ž C e 2
(eq. 2)

¦ § * • ¨© C Ž• 2 # $ y ! $ 2y #¦ § C • { Ž C e 2
(eq. 3)

Étant donné que l'HOR se produit dans la même gamme de potentiel que l'oxydation de la surface du Ni, la formation en deux étapes de Ni(OH)2 superficiel a également été considérée dans le modèle (selon les Réfs. [2,5,6]):

¦ § C Ž• 2 # $ " ! $ 2

II.2. Électrocatalyseurs de Ni nanostructurés

First

  and foremost, I wish to express my deepest gratitude to my advisors: Professor Elena Savinova, for her passion, patience, motivation and great help in the understanding where to go; Dr. Antoine Bonnefont for his enthusiasm, encouragement and for teaching me in the kinetic modeling; Professor Valentin Parmon for his guidance, which helped me to choose the right direction in the scientific life and for supporting all of my initiatives. I am proud to work with you and had the opportunity to learn many things from you during the PhD study. Thank you very much! I am very grateful to the University of Strasbourg (Doctoral School "Sciences Chimiques"), Institut de Chimie de Strasbourg (IC), Institut de Chimie des Procédés pour l'Energie, l'Environnement et la Santé (ICPEES) in Cronenbourg and Boreskov Institute of Catalysis in Novosibirsk (RU) for providing the possibility to perform all the required experiments. I also would like to thanks the French Ministry of Foreign Affairs and International Development for attributing me the Eiffel and Vernadski PhD fellowships as well as CNRS and RFBR organizations for the financial support of the research projects. I wish to express my warmest gratitude to my dissertation committee members, Professor Galina Tsirlina, Professor Alexander Maslii and Dr. Frederic Maillard for their time, insightful comments and questions on the PhD study. It is a great honor for me, that you accepted to be a part of the jury of this thesis. Many thanks to all the people who helped me with materials preparation and characterization: Dr. Olga Cherstiouk, Dr. Sergey Pronkin and Dr. Alexandr Simonov for the help with performance of some electrocatalytic measurements; Dr. Pavel Simonov for the synthesis of Ni/C and NiCu/C electrocatalysts; Dr. Alexey Serov and Dr. Katerina Artyushkova for the preparation of NiMo/C electrocatalyst and its characterization by TEM, XRD and XPS; Dr. Corinne Ulhaq-Bouillet and Dr. Vladimir Zaikovskii for TEM, HRTEM and STEM analysis; Dr. Vasiliki Papaefthimiou, Dr. Spiros Zafeiratos, Dr. Ren Kvon and Viktoriia Saveleva for XPS analysis; Dr. Thierry Dintzer, Dr. Nina Rudina for SEM analysis; Dr. Tatyana Kardash and Dr. Alexander Shmakov for XRD analysis; Dr. Anne Boss for ICP-AES analysis. Thanks also to Veronique Verkruysse and Francine Jacky for the vast administrative staff they realized. I would like to thank Sebastien, Gwénaëlle, Anya, Pierre-Alex, Wen, Thomas, Ivan, Denis and Pablo with whom I shared most free time of laboratory life in France and all the nice people I met during my PhD. Thank you for creating a warm and kindly atmosphere in the lab.

1 . 2 .

 12 HER on Ni are often contradictory, thus complicating comparison of the results obtained in different laboratories. Moreover, there are only a few studies devoted to the investigation of the HOR on Ni and Ni-based electrodes, or the influence of their composition and surface state on the activity. Meanwhile, understanding of these phenomena is essential for disclosing the origins of the catalytic enhancement in bimetallic Ni-based systems as well as the development of more efficient AMFC anodes. The objectives of the present work were to unveil the key factors, which determine electrocatalytic activity of Ni in the HOR, and to develop efficient monoand bimetallic Ni-based electrocatalysts for this process.These objectives were achieved by solving the following problems: Determination of specific electrocatalytic activity of polycrystalline Ni in the HOR/HER in alkaline medium, and understanding its dependence on the temperature; Uncovering the dependence of specific electrocatalytic activity of Ni in the HOR/HER on the extend of its surface oxidation; 3. Developing a mathematical microkinetic model allowing to describe in a semi-quantitative way the influence of the main parameters affecting the kinetics of the HOR/HER on mono-and bimetallic Ni-based electrocatalysts, including the influence of surface oxide species; 4. Synthesis and physicochemical characterization of nanostructured Ni electrocatalysts supported on porous carbon and determination of their specific activity in the HOR/HER in dependence of the surface state; 5. Determination of specific electrocatalytic activity of NiMo/C and NiCu/С electrocatalysts in the HOR/HER, defining an optimal composition of the NiCu/С electrocatalysts, and unveiling the influence of the second metal on physicochemical properties of Ni. The structure of the thesis The thesis comprises 205 pages and contains 5 chapters, 61 figures, 11 tables, 3 appendixes and 299 references. Chapter 1 is the literature review. It provides an overview of the HOR/HER kinetics and mechanism, including the dependence of the activity on the type of metal as well as the electrolyte pH. It also summarizes the data on the electrochemical properties of Ni electrodes in alkaline media and their electrocatalytic activity in the HOR/HER. Chapter 2 describes the materials and methods used in this work to study the Ni-based electrocatalysts. Chapter 3 is devoted to the study of model polycrystalline Ni electrodes. By combining electrochemical measurements with microkinetic modeling, the influence of the surface state of Ni electrodes on the kinetics of the HOR/HER is discussed.

  the first case, in the so-called micropolarization region (|η| ≤ 10 mV at T = 298 K) the exponents in the eq. 1.16 can be expended in a Taylor series, leading to a linear relation between current density and overpotential: values of η (|η| ≥ 50 mV at T = 298 K), one of the exponents in the eq. 1.16 becomes much smaller compared to the other and can be neglected, resulting in the equations 1.18a and b for large negative and large positive η,

1 . 1 . 2 .

 112 conditions and normalizing them by either surface area or mass of the active component. However, one should also consider the value of the Tafel slope. If it significantly differs for different electrodes, then an inverse order of their activities might be obtained depending on the desired range of η [2]. Electrocatalysts for the HOR/HER Studying different metal electrodes in the HOR/HER has shown that the values of electrocatalytic activities might differ up to six orders of magnitude depending on the electrode composition. Many researchers have tried to make a link between physicochemical properties of the electrodes and their activities in the HOR/HER [18]. Among possible correlations the dependence of the HOR/HER exchange current density on the value of Gibbs energy (∆adGH) of adsorption of the hydrogen intermediate (Had) on the catalyst surface (or on the value of M-Had binding energy) is the most commonly used now. This is mainly due to the fact that adsorbed Had atoms are the key intermediate of the HOR/HER, according to the equations 1.3 -1.8. The observed dependence shows volcano-type behavior (Fig. 1.1) in accordance with the Sabatier principle proposed in the early 1900s, and well-known in heterogeneous catalysis. The Sabatier principle states that the most active catalyst should adsorb

  differences between the dependences plotted by different authors mainly arose from the utilization of different approaches for the determination of ΔadGH value. They are based on measuring the M-Had binding energy in corresponding metal hydrides (in early works, Fig. 1.1a) or DFT calculations (in modern works, Figs. 1.1b

Fig. 1 . 2

 12 Fig. 1.2 CVs obtained for a polycrystalline Ni electrode in 0.5 M KOH at a sweep rate v = 100 mV•s -1 and T = 293 K. The first CV (red curve) covers the potential range of [-0.15 -0.50 V], while two other CVs (first scan -blue curve and tenth scan -black curve) cover the potential range of[-0.15 -1.55 V]. Given from the Ref.[START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature // Electrocatalysis[END_REF].

Fig. 1 . 3

 13 Fig. 1.3 CVs obtained for polycrystalline Ni electrode at various sweep rates in (a) 0.5 M KOH (given from the Ref.[START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF]), (b) 1.0 M NaOH (after subtraction of the currents corresponding to the HER, given from the Ref.[START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF]) and (c) 0.5 M NaOH

Fig. 1 . 4

 14 Fig. 1.4 Sequence of STM images for the formation of Ni(OH)2 film (shown by arrows) on Ni (111) at 150 mV vs SHE in 0.05 M Na2SO4: (a) immediately after stepping the potential from -300 to 150 mV vs SHE; (b) 3 min after (a) (given from the Ref.[START_REF] Nakamura | Surface structures at the initial stages in passive film formation on Ni(111) electrodes in acidic electrolytes[END_REF]).

Fig. 1 .

 1 Fig. 1.5 (a) Schematic representation of the α/β-phase transition. Blue circles represent water molecules located between the NiOH layers in α-Ni(OH)2. Given from the Ref. [104]; (b) A general scheme of the chemical and electrochemical processes that occur on the surface of Ni electrode in the high-potential region. Given from the Ref. [65].

Fig. 1 .

 1 Fig. 1.6 (a) CVs, obtained for polycrystalline Ni in 0.1 M KOH after registration of 11 potential sweeps from -0.8 to 1.2 V at v = 2 mV•s -1 . Given from the Ref. [84]; (b) CVs, obtained for Ni/C electrode in 0.2 M NaOH at v = 20 mV•s -1 and various anodic limits. Given from the Ref. [116] Influence of the structure of Ni electrode surface on its electrochemical behavior

Fig. 1 . 7

 17 Fig. 1.7 CVs, obtained for Ni (100), Ni (110), Ni (111) single crystals in deaerated 0.1 M NaOH at T = 25 °C in (a) the low-potential region at v = 5 mV•s -1 (given from the Ref. [115]) and (b) the high-potential region at v = 50 mV•s -1 (given from the Ref. [120]).

Fig. 1 . 8

 18 Fig. 1.8 CVs, obtained for (a) Ni single crystals (Ni (110) -solid line, Ni (100)dashed line, Ni (111) -dotted line) and (b) polycrystalline Ni in H2-saturated 0.1 M NaOH at v = 1 mV•s -1 and T = 25 °C. Given from the Ref. [115].

electrodeposited

  Ni and Ni plate [177] or Ni particles, electrodeposited Ni film and commercial Ni foil [168]. Meanwhile a bit higher specific activity, normalized to the SEC, in the HER was observed for Ni dendrites compared to Ni particles [168]. Recently, Zhuang et al. have shown that the HOR activity of Ni can be significantly enhanced after doping of the carbon nanotube (CNT) support with nitrogen (from 9.2 to 28 µA cm -2 for Ni/CNT and Ni/N-CNT, respectively) [123]. On the basis of DFT calculations, the enhanced activity of Ni was assigned by the authors to a decrease of Ni-Нad bond strength in the vicinity of nitrogen atoms.

  about an increase of the exchange current density of Ni in the HER from 1.4 to 29 µA cm -2 geom after cycling of the electrode in the potential region of [-0.35 -1.6 V] at v = 50 mV•s -1 [126]. Noticeably, the roughness factor of the electrode is believed to remain unaffected by such treatment [62]. On the contrary, prolonged anodic polarization leads to a decrease of the HER activity of Ni, likely, due to the formation of irreversible β-Ni(OH)2 which strongly passivates the surface of the electrode, decreasing its fractional coverage by hydrogen [166]. It is known, that bulk Ni oxide and hydroxide are inactive in the HER due to the low adsorption energies of hydrogen atoms on their surfaces [180]. Therefore, an enhanced activity compared to that of the reduced electrode surface can be observed only if Ni oxide species coexist on the electrode surface with metallic Ni sites (necessary for hydrogen adsorption) [181]. Danilovic et al. have shown, that the activity of Ni in the HER increases by a factor of four after modification of its surface by deposited Ni(OH)2 [182]. The authors proposed that Ni(OH)2 serves to enhance the water dissociation step by weakening the H-OH bond of the adsorbed water, while Had adsorption takes place on nearby metallic Ni sites [182]. The same approach but with the use of NiO instead of Ni(OH)2 allowed Gong et al. to obtain NiO/Ni-CNT sample, which outperforms two reference Ni/CNT and, especially, NiO/CNT samples [183-185]. The synergistic effect was also proposed to explain the high activity of Ni/NiO/CoSe2 hybrid [186], Ni/NiO nanosheets [187] and single-crystalline Ni nanosheet arrays subjected to partial oxidation [176]. Besides, DFT calculations performed by Kuang et al. revealed that desorption of H2 possesses a lower energy barrier on the Ni/NiO (111) surface than on pure Ni (111) surface (3.228 and 3.763 eV, respectively) [176].

  . Besides determination of the SEC of Ni by different methods further complicates the comparison of results obtained in different laboratories. Therefore, comparison of several NiM electrocatalysts studied in the same work (thus, using the same methods) seems to be more reliable. In particular, Raj et al. [165] reported that the HER activity decreases in the row Ni-Mo > Ni-Zn > Ni-Co > Ni-W > Ni-Fe > Ni-Cr > Ni-plated steel, which agrees with the observation of Navarro-Flores et al. [127], who compared NiMo, NiW, NiFe and pure Ni. However Bates et al. have recently reported that activity of NiCr/C exceeds the values measured for NiMo/C and NiFe/C [164]. This discrepancy, likely, originates from an ignorance of possible difference in the values of SEC of Ni, while comparing different systems. Meanwhile, careful analysis of the works in which bimetallic NiM catalysts were studied taking into account an estimation of the real surface area, allowed us to determine the systems for which an increase of the specific electrocatalytic activity of Ni in the HER is the most reliable. These systems can be divided into two groups depending on the enhancement effect caused be second metal: (i) the activity increases less than 3 times for NiCo [128], NiCu [201-203], NiFe [205], NiTi [227], NiAg [196]; (ii) the activity increases up to two orders of magnitude for NiMo

  particular, addition of Mo or W was shown to decrease the d-band vacancy of Ni, which becomes nearly zero at about 11 and 8 at.% content of Mo and W, respectively, being in good agreement with the observed optimum surface composition of NiMo and NiW catalysts, possessing the highest activity in the HER [210,212,228].

1 . 3 . 5 .

 135 suggested a promoting role of Ni hydrides in the HER instead, which may serve as protectors of Ni-based cathodes from oxidation[246]. Meanwhile, this conclusion is, likely, valid only for metastable α-NiHx, while the formation of β-phase has negative effect[243,247].Soares et al. proposed that formation of Ni hydrides decreases the HER activity due to the modification of the density of states at the Fermi level [109]. Along with an enhancement of the activity of Ni in the HER, addition of a second metal was found to increase the stability of the catalyst during long-term operation of the electrolyzer (Cu-[248], Mn3O4-[132], Mo-[206,228], doped Raney Ni -[233,234]) as well as shutdown cycles [228,249]. It is believed that addition of a second metal prevents formation of β-NiHx [204] as well as deep oxidation of Ni [230,249]. Recently, after discovering of the positive synergetic effect in the Ni-NiO system on the HER activity and considering instability of Ni oxide species during water electrolysis, it was proposed that the oxide of the second metal may act as a stabilizing agent for the Ni-NiO system [164,184,185]. In particular, Gong et al. reported about an increase of the stability of Ni/NiO composites in the HER after an addition of Cr2O3 [184,185]. Mechanism of the HOR/HER on the surface of Ni electrocatalysts The study of the mechanism of the HOR/HER is an important stage in the study of the kinetics of these processes on various electrocatalysts. Along with the fundamental significance, unveiling of the reaction mechanism and discovering the rate-determining step allow one to propose possible ways for the enhancement of the catalytic activity. The mechanism of the HER on Ni electrodes has been studied since the beginning of the XX th century, but there is still no consensus concerning either the contribution of individual steps of the process (Tafel-Volmer vs Heyrovsky-Volmer mechanism) or the rate-determining step in the literature. These discrepancies are largely due to differences in the experimental protocols and the methodologies used for preconditioning of the Ni electrode surface prior to the experiment, i.e. the same factors which affect the activity of Ni in the HOR/HER.

  annealed in H2 atmosphere and sealed in fragile glass bulbs. The latter were broken inside the electrolyte just before measurements. scraped with grease-free blade to give a bright surface beforeTafel [251] Ni rod was cleaned in a mixture of conc. chromic and sulfuric acid, washed in boiling water and polarized cathodically for 5 min at j = 2 mA cm -2 (a); the electrode was additionally oxidized by cycling up to E > 1.0 V (b).Ni/NiOx 303 0.1 M NaOH 125 (a); 90 (b) 25 (a); 10 (b) -Volmer* -Tafel [166]Ni wires were annealed in H2 atmosphere and sealed in fragile glass bulbs. The latter were broken inside the electrolyte just before measurements. was pulse polarized (5 cycles) by application of an anodic pulse to reach E = 1.6 V after 3 s and then the current direction was changed until the electrode reached E = -0Cathodic polarization of polished Ni disk at j = 1 mA cm -2 for 30 min (b) cycling 10 times in the potential interval of[-0.35 -1.6 V] at v = 50 mV•s -1 disk electrode was introduced in the electrolyte at E ≈ 0.03 V and then either stabilized at E ≈ -0.87 V (a) or at E ≈ -0.37 V after first anodic polarization at E ≈ 1.33 V for 10 min (bfirst oxidized at E = 1.4 V for 5s and then cathodically polarized for 1h at E = -0obtained by galvanostatic deposition, then washed by water, dried in air flow and immersed in the electrolyte. Analysis of the studies devoted to the investigation of Ni-based catalysts in the HOR/HER evidences considerable interest of the scientific community to this field during the past years. Due to an industrial use of Ni-based catalysts in alkaline water electrolysis, the researchers have been largely focused on the study of the HER and development of more active Ni-based catalysts for this process. However growing interest in AMFC has led to an increasing number of publications devoted to the study of the HOR on Ni-based catalysts in recent years.It was found that electrochemical properties of Ni in alkaline electrolytes significantly depend on the methodology used for the pretreatment of the electrode surface, which can be seen, in particular, by the change of the CV shape. In addition, there is a serious discrepancy in the literature concerning the values of specific electrocatalytic activity of Ni in the HER, which originates from, first, utilization of the geometric surface area for normalization of the activity and, second, dependence of the activity on the state of Ni surface (for example, the degree of its oxidation). It should be noted that there is also no consensus in the literature concerning the mechanism of the HOR/HER on Ni electrodes (Heyrovsky-Volmer or Tafel-Volmer).Therefore, the first problems, which were solved within the frames of this work, were to obtain reproducible data of specific (normalized to the real surface area) electrocatalytic activity of Ni in the HOR/HER and to determine how and why the oxidation of the surface of Ni electrode affects its activity in the HOR/HER. The analysis of the works published in the literature also does not allow one to make an unambiguous conclusion about the influence of the microstructure of the nickel catalyst on its activity in the HOR/HER. To better understand this issue, in the current work we prepared nickel nanoparticles of various sizes and determined their specific (real surface-weighted) activities in the HOR/HER.Finally, the analysis of the works devoted to the study of Ni-based bimetallic catalysts allows one to conclude that (i) these materials were investigated primarily in the potential range of the HER, while their activity in the HOR has been studied insufficiently; (ii) normalization of the activity on the geometric surface area does not allow one to make unambiguous conclusions about the effect of a second metal on the specific electrocatalytic activity of Ni in the HOR/HER; (iii) most of the studied NiM electrocatalysts were in the form of either thick deposits or polycrystalline alloys, whereas for application in AMFC it is necessary to prepare nanoparticles of metals supported on high surface area (e.g. carbon) supports. Aiming to better understand the influence of a second metal on electrochemical and electrocatalytic properties of Ni in the HOR/HER in the current work we studied carbon supported NiCu/C and NiMo/C catalysts.

10 M 2 . 1 . 2 . 72 electrocatalysts

 1021272 NaOH electrolyte, polycrystalline Ni rod (Ø5 mm, Mateck, 99.99 %) and Ni foil (10 x 10 mm, Mateck, 99.99 %) were employed. Prior to use the top flat surface of the Ni rod and both sides of the Ni foil were thoroughly polished with Al2O3 slurries (Alfa Aesar) from 1.0 down to either 0.3 or 0.05 μm, after that they were rinsed by ultrapure water produced by either Milli-Q system (Milli Pore, USA, 18.2 MΩ•cm, TOC < 5 ppb) or ELGA Ultra Analytic apparatus (18.2 MΩ•cm, TOC < 3 ppb). Ni foil was additionally sonicated in a jar of water for 2 min to remove the traces of Al2O3. Synthesis of nanostructured NiED/GC and NiED/XC-Nanostructured nickel electrodes were prepared by potentiostatic electrodeposition of Ni from the bath containing 0.01 M NiSO4 + 0.10 M (NH4)2SO4 solution (performed from NiSO4•6H2O (99.99%, Aldrich) and (NH4)2SO4 (99.0%, Aldrich) granules) at Т = 298 K using either nonporous glassy carbon (GC) cylinder

  NiMo/C electrocatalysts Bimetallic NiMo/C catalyst with 50 wt.% total metal loading and Ni:Mo atomic ratio close to 0.9:0.1 was synthesized by A. Serov (Center for Micro-Engineered Materials, University of New Mexico, Albuquerque, USA) by the incipient wetness impregnation of the support with aqueous solutions of Ni(NO3)2 and (NH4)2MoO4.KetjenBlack-600 carbon (KB, AkzoNobel, BET surface area ~ 1200 m 2 g -1 ) with larger compared to XC-72 BET surface area was used as a support to increase total metal loading. After reaching the incipient wetness point the mixture was dried in oven at T = 85 °C and then dry composite was ground with mortar and pestle. The fine powder was reduced in a hydrogen atmosphere (flow rate of 100 ccm) at T = 550 °C for 1 h. Before exposing the catalyst to the ambient it was passivated in a He flow containing 2% of O2 at room temperature for 8 h.

  Nanostructured NiED/GC and NiED/XC-72 electrocatalysts TEM (including high resolution (HR-TEM)) and STEM were used to characterize, respectively, the microstructure of Ni nanoparticles in NiED/XC-72 catalyst and their distribution over the carbon support. To do this the catalyst layer was transferred from the GC surface to a copper grid. Analogously, Ni nanoparticles directly deposited on GC support (NiED/GC) were carefully scrapped from its surface and their microstructure was analyzed using HR-TEM. Microscopic measurements were performed by C. Ulhaq-Bouillet in Institut de Physique et Chimie des Materiaux de Strasbourg (IPCMS, Strasbourg, France). STEM was performed with a LaB6-JEOL 2100 microscope operating at 200 kV with a point to point resolution of 0.1 nm. TEM and HR-TEM were carried out with JEOL 2100F instrument operating at 200kV with a 0.2 nm resolution. Bimetallic NiCu/C and monometallic Ni/C electrocatalysts High Angle Annular Dark Field STEM (HAADF-STEM) analysis, EDX spectroscopy, elemental mapping and conventional HR-TEM measurements of Ni/C and Ni1-xCux/C catalysts were undertaken using a JEM-2200FS (JEOL) microscope. The measurements were performed by V. I. Zaikovskii in the BIC (Novosibirsk, Russia). The elemental maps were obtained using Ni K (E = 7.471 keV) and Cu K (E = 8.040 keV) lines. Lattice spacing was calculated by Fast Fourier Transform (FFT) using DigMicrograph (GATAN) soft. Bimetallic NiMo/C electrocatalyst The structure of Ni0.9Mo0.1/C catalyst and the metal particles distribution over carbon support surface were studied by TEM using a JEOL 2010 microscope with an accelerating voltage of 200 keV. The measurements were performed by A. Serov (UNM CMEM, Albuquerque, USA).

  morphology of Ni nanoparticles in NiED/GC samples was studied by SEM using microscopes of either a JEOL JSM-6700F at accelerating voltage of 6 kV or a JEOL JSM-6490LV. The measurements were performed by T. Dintzer in Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé (ICPEES, Strasbourg, France) or by N. A. Rudina in the BIC (Novosibirsk, Russia), respectively.2.2.3. X-Ray Photoelectron Spectroscopy (XPS)Polycrystalline Ni foil Surface state of the Ni foil (subjected to various pretreatments) was analyzed using XPS in an ultra-high vacuum chamber (residual gas pressure <5•10 -6 Pa) equipped with a Clam2 (VG Microtech) electron analyzer and a dual anode (Al and Mg Kα) X-ray source. The measurements and XP spectra characterization were performed by V. Papaefthimiou and V. A. Saveleva in ICPEES (Strasbourg, France).After electrochemical treatment (see Section 2.3.4) the Ni foil was taken out from the electrolyte (0.10 M NaOH) and immediately fixed on the XPS sample holder with a drop of water completely covering the surface of Ni foil in order to minimize its contact with the ambient atmosphere. Then the holder was quickly (less than 5 min) placed into the entry chamber of the XP spectrometer, after that gazes were evacuated up to P ≈ 10 -5 Pa (during this step a drop of water evaporates from the surface of the Ni foil) and only then the sample was transferred to the main chamber of the XP spectrometer. Survey and narrow scan Ni2p, O1s, C1s spectra were recorded using the excitation photon energy of 1486.6 eV. Characterization of the Ni foil was carried out by means of normal (0 degrees) and grazing (50 degrees) take-off angle measurements. The temperature was maintained constant throughout the experiment at 30°C. NiO produced by thermal oxidation of Ni foil in air at Т = 500 ºC for 1 h., and pure metallic Ni foil (cleaned by several Ar + sputtering and annealing cycles) were used as reference samples.

  Electrochemical measurements were performed in a standard three-electrode cell made either from Teflon or Pyrex glass (schematically shown on Fig. 2.1). The compartments of the reference and counter electrodes were connected with the working electrode chamber via a Luggin capillary and a glass (or Teflon) frit, respectively. Depending on the type of experiments, polycrystalline Ni rod, polycrystalline Ni foil or GC cylinder with catalyst thin layer on its top surface were employed as the working electrode. The counter electrode was a high surface area platinum. The type of the reference electrodeto which the applied potential was referredwas dependent on the type of experiments. In alkaline electrolyte either a Mercury-Mercury oxide electrode (Hg|HgO|0.10 M NaOH, IJ Cambria Scientific, E = 0.928 V vs RHE at 25 °C) or Silver-Silver chloride electrode (Ag|AgCl|KCl(sat.), E = 0.954 V vs RHE at 25 °C) were utilized as the reference electrode, while electrodeposition of Ni from the solution of NiSO4 was carried out using Mercury-Mercury sulfate (Hg|Hg2SO4|K2SO4(sat.), Schott, E = 0.960 V vs RHE at 25 °C) reference electrode. A salt bridge was introduced between the compartments of the working and the reference electrodes, while using Ag|AgCl|KCl(sat.) RE, to prevent eventual Cl -contamination of the main compartment. If not otherwise stated, all electrode potentials in this work are referred to the RHE. Prior to electrochemical measurements all Teflon-and glassware were filled by a solution containing Н2О2(30 %) + H2SO4 (conc.) (1:1, vol.) for at least 12 h. and then multiply washed by ultrapure (Milli-Q or Elga) water.

Fig. 2 . 1

 21 Fig. 2.1 Schematic representation of three-electrode cell utilized in the work for performing of electrochemical measurements. The electrolyte resistance between the working and the reference electrode was determined by electrochemical impedance spectroscopy at constant potential around 0 V vs RHE in the frequency range from 100 kHz to 1 Hz using high-frequency component. The obtained values varied in the range between 10 and 35 Ω depending on the configuration of the cell and utilized sample holder. Considering the value of measured currents during electrochemical experiments, the Ohmic drop in most cases

  48 h. (hereinafter, "Ch activated Ni rod"). In some cases, the surface of Ni rod was oxidized electrochemically (hereinafter, "EC activated Ni rod") by applying three potential cycles in N2-saturated 0.10 M NaOH between -0.30 and 1.0 V at a sweep rate of 50 mV•s -1 .Before performing the main measurements, non-activated Ni rod was cycled in the potential region of [-0.20 -0.30 V] at v = 20 mV•s -1 in order to remove oxide species formed on its surface during the transfer to the cell when the Ni rod was shortly exposed to the ambient. It should be noted that due to the tendency of Ni towards oxidation, each CV presented in Chapter 3 for the non-activated Ni rod (in both N2 and H2 atmosphere) was obtained in a separate experiment immediately after polishing of the electrode surface. Prior to the main electrochemical measurements, the surface of activated Ni rod was partially reduced by recording CVs in the potential range of [-0.06 -0.40 V] at v = 20 mV•s -1 in order to obtain an active surface containing both metallic and an oxide sites (see Chapter 3).Polycrystalline Ni foil for XPS measurementsNi foil pretreated in two different ways was used to study the surface state of Ni by XPS. In the first case, the surface of a freshly polished Ni foil (see Section 2.1.1) was reduced in 0.10 M NaOH by applying cathodic polarization (E = -0.20 V) for 5 min, and then cycled once between -0.20 and 0.40 V and twice between -0.60 and 0.40 V at v = 20 mV•s -1 (hereinafter, "non-activated Ni foil"). In the second case, the surface of Ni foil was oxidized in air for ~48 h (producing a passive oxide film of ca.2 nm thickness[110]) and then partially reduced by cycling several times between -0.06 and 0.40 V at v = 20 mV•s -1 (hereinafter, "Ch activated Ni foil"). The final scan in both cases was stopped at 0 V on the anodic (i.e. going from negative to positive values) sweep of the CV. Immediately after that the electrodes were removed from the solution at open-circuit potential, rinsed with water and rapidly transferred to the XPS entry chamber as described in Section 2.2.3.Nanostructured NiED/GC and NiED/XC-72 electrocatalystsTo study the influence of surface oxide species on electrochemical and electrocatalytic properties of nanostructured NiED/GC and NiED/XC-72, two states of their surfaces were considered. In the first case the experiments were performed immediately after the preparation of the sample and quick rinsing of the electrode by water (to remove the Ni salt left on the surface). Such electrodes in what follows are named "non-activated NiED/GC and NiED/XC-72". In the second case before conducting the measurements NiED/GC and NiED/XC-72 electrodes were rinsed by water, dried under N2 flow and oxidized under the contact with air for ~24 h (this time was enough to reach similar to polycrystalline Ni degree of oxidation). These electrodes in what fallows are named "Ch activated NiED/GC and NiED/XC-72". After immersion of the electrodes in 0.10 M NaOH electrolyte they were conditioned by applying the same procedure as for non-activated and Ch activated Ni rod. Supported Ni/C, NiCu/C and NiMo/C electrocatalysts Prior to study of electrochemical and electrocatalytic properties of carbon supported Ni/C, NiCu/C and NiMo/C electrocatalysts in 0.10 M NaOH, their surface was reduced by cycling in inert atmosphere in the potential interval of [-0.30 -0.40 V] for Ni/C and NiCu/C or [-0.20 -0.40 V] for NiMo/C and at v = 20 mV•s -1 until a stable voltammetric response was attained.

  2.2), several methods have been proposed in the literature for determination of the electrochemically active surface area of Ni electrodes (SEC). These include (i) analysis of the anodic and cathodic peaks in the low-potential region, which correspond to the formation and/or reduction of α-Ni(OH)2; (ii) utilization of Cdl values, estimated either by using electrochemical impedance spectroscopy or from CVs obtained at various sweep rates in the double layer potential interval; (iii) analysis of the anodic and cathodic peaks in the high-potential region, which correspond to the Ni(II)/Ni(III) oxidation-reduction processes.We have chosen the first method, proposed in the work of Machado and Avaca[START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF], since it allows one, on the one hand, to avoid irreversible oxidation of the surface of Ni electrodes (which occurs in the case of the electrode cycling in the highpotential region) and, on the other hand, to use the same conversion coefficient (0.514 mC cm -2 ) independently of the surface state of the electrodes (the value of Cdl can change after oxidation, as shown in Section 1.2.2). However this approach also has some limitations. In particular, a possible contribution of the charge related to the oxidation of adsorbed/absorbed hydrogen in the measured anodic currents should be considered. Besides, the specific surface area of Ni could be underestimated due to an incomplete reduction of α-Ni(OH)2 formed on the surface. These will be discussed in more detail in the Chapter 3. The charge required to form a surface α-Ni(OH)2 was estimated by integrating CVs acquired in the potential interval of [-0.06 -0.40 V] ([-0.02 -0.40 V] for Ni/C and NiCu/C) in 0.10 M NaOH solution, saturated by N2 or Ar, at v = 20 mV•s -1 as shown in Fig. 2.2. The potential limits were selected on the basis of the analysis presented in the Section 3.2.1.

Fig. 2 . 2

 22 Fig. 2.2 CVs obtained for non-activated Ni rod in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 and T = 298 K. Patterned area was used for estimation of the surface area of the electrode.

Fig. 2 . 3

 23 Fig. 2.3 An example of analysis of (a) micropolarization region and (b) overpotential region of -0.20 < η < -0.07 V (where the Tafel equation is applicable) of CV obtained for non-activated Ni rod in H2-saturated 0.10 M NaOH at v = 5 mV•s -1 at T = 298 K.

Fig. 2 . 4

 24 Fig. 2.4 (a) Schematic cross view of a rotating disk electrode: 1 -working electrode; 2 -insulator; 3 -electrolyte; 4-current collector. Given from the Ref. [274]; (b) CVs obtained in H2-saturated 0.10 M NaOH for Pt/C electrocatalyst at various rotation rates and v = 10 mV•s -1 .

  n is a number of involved electrons, A -geometric surface area of the electrode, F -Faraday constant, CH2 -the concentration of H2 in saturated electrolyte and other parameters are the same as above. The corresponding parameters determined in 0.1 M NaOH at T = 298°K [275]: DH2 = 5.11•10 -5 cm 2 s -1 , v = 0.010 cm 2 s -1 , CH2 = 7.8•10 -7 mol cm -3 . Therefore, for Ni and GC rods used in the work (A = 0.196 cm 2 , n = 2, F = 96485 C mol -1 ) theoretical value of the curve slope B -1 equals to 11.16 mA -1 cm 2 disk rpm. 0.5 .

  -field microkinetic modeling of the experimental current-potential curves obtained for several Ni electrodes (non-activated and activated Ni rod as well as NiMo/C electrocatalyst) was performed with the purpose to better understand the mechanism of the HOR/HER on the Ni surface as well as possible causes of the change of Ni activity in the HOR/HER. The modeling was carried out with a great help of Dr. A. Bonnefont from Institut de Chimie de Strasbourg (IC, Strasbourg, France).

Fig. 2 . 5

 25 Fig. 2.5 Schematic energy diagram for the HOR/HER on non-activated Ni electrode following the Heyrovsky-Volmer mechanism. Ea,i correspond to the activation energies of the Heyrovsky and Volmer steps listed in the Table3.4 (seeSection 3.3.3).

  Analysis of the literature presented in Section 1.3 shows that Ni has been mainly studied in the potential region of the HER, whereas the interest to the HOR has appeared during the past decade, stimulated by the development of AMFCs. However, despite a considerable amount of works carried out since the beginning of the 20 th century, some inconsistencies concerning the mechanism of the HOR/HER as well as the activity of Ni in this process still exist in the literature. This discrepancy is likely caused by the use of various procedures for pretreatment of the Ni electrode surface prior to the catalytic measurements, which results in different states of the electrode surface during investigation of the HOR/HER kinetics. It is known that the formation of Ni hydrides leads to a decrease of the activity of Ni in the HER. On the other hand, the effect of Ni surface oxidation on its electrocatalytic activity has not been studied in detail yet. However, the analysis of the literature suggests that significant discrepancy regarding the specific electrocatalytic activity of Ni in the HOR/HER is caused by uncontrolled formation of surface oxides and their effect on the kinetics of the HOR/HER.

  Electrochemical and electrocatalytic measurements were carried out in the temperature range from 298 to 338 K on both types of Ni electrodes. In addition, the experimental study was complemented with kinetic modeling with the aim to better understand the kinetics of the HOR/HER depending on the surface state of Ni electrode. This allowed us to propose the mechanism of the HOR/HER, evaluate the contribution of individual reaction steps (Heyrovsky, Volmer, Tafel) and put forward a hypothesis concerning the origin of the change of the activity of Ni in the HOR/HER in the presence of stable Ni oxide species on the surface of the electrode.

Fig. 3 .

 3 Fig.3.1a shows the first CVs obtained for freshly polished Ni rod in N2-purged 0.10 M NaOH just after immersion of the electrode in the electrolyte. As one may see, the cathodic peak (c1) usually attributed to the reduction of α-Ni(OH)2[START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF] appears already on the first scan. This fact suggests formation of some amount of surface Ni oxide species during the transfer of the electrode to electrochemical cell, when it is shortly exposed to air, after polishing. Considering the value of an opencircuit potential (0.2 -0.3 V), one may assume that formed surface oxides mostly consist of α-Ni(OH)2. Meanwhile, cycling up to E = -0.2 V is sufficient for (almost) complete reduction of the electrode surface, as evidenced by the absence of any peaks on subsequently registered CV in the potential interval of [-0.2 -0.2 V], i.e. when the anodic potential limit is placed before electrochemical oxidation of the surface of the Ni electrode(Fig. 3.1b). Further increase of the anodic potential limit leads to the

Fig. 3 . 1

 31 Fig. 3.1 CVs obtained for non-activated Ni rod in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 (a) just after the polishing and (b) after reduction of air formed surface Ni oxide species. Inset on the panel (b) zoom in the potential interval of [-0.1 -0.15 V].

Fig. 3 .

 3 Fig. 3.2 (a) CVs for non-activated Ni rod registered with various cathodic potential limits (EC = -0.20, -0.16, -0.12, -0.06, -0.02 V) in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 and (b) corresponding to these curves absolute values of anodic (QA) and cathodic (QC) charges.

  2.1), oxidation of Ni electrode surface results in changing the shape of CV and might be the reason for change of the HOR/HER activity as well as the mechanism of the reactions (see Sections 1.3.2). In order to further progress in understanding of this phenomenon, a part of experiments was carried out using a partially oxidized Ni electrode. Note that the presence of metallic Ni sites on the electrode surface is an essential condition for the adsorption of Had atoms and the occurrence of the HOR/HER [181], while complete oxidation of the surface leads to deactivation of the Ni electrode, as will be shown later (see Section 3.3.1). Oxidation of the surface of Ni rod was performed in two ways: (i) under contact with oxygen from air for ~48 h. (Ch activated electrode), or (ii) by cycling to the high-potential region (EC activated electrode). After that, the electrode surface was partially reduced by conditioning in the potential interval of [-0.06 -0.40 V]. Based on the literature data, such pretreatment is expected to result in the formation of three-layered structure Ni/NiO/Ni(OH)2 on the surface of the electrode, with α-Ni(OH)2 being on the top [89,110-113]. It was found that prolonged oxidation of Ni leads to, first, higher values of the open-circuit potential after immersion of the electrode in the electrolyte (between 0.4 and 0.55 V, i.e. ~0.2 V higher than in the case of freshly polished Ni rod) and, second, a change in the shape of CV, registered in the potential range of [-0.06 -0.40 V] (Figs. 3.3a, b). Besides, the absence of a cathodic peak on the first scan of the CV indicates a difference in the nature of Ni oxide species initially present on the surface of non-activated and Ch activated Ni rod. Further cycling, however, leads to at least partial reduction of the surface of Ch activated electrode, which is accompanied by the growth of anodic and cathodic currents, appearance of the characteristic a1 and c1 peaks as well as an additional anodic peak a2 in the potential range of [0 -0.15 V] (Fig. 3.3b).

  Ni(OH)2. On the CVs subsequently obtained in the typical potential interval [-0.06 -0.40 V] (Figs. 3.3a, d) one can see two anodic (a1, a2) and one cathodic (c1) peaks similarly to that observed for Ch activated Ni rod.

Fig. 3 .

 3 Fig. 3.3 (a) Comparison of CVs obtained for non-activated (curve 1, black), EC activated (curve 2, red) and Ch activated (curve 3, blue) Ni rod in N2-saturated 0.10 М NaOH for at v = 20 mV•s -1 ; (c) CVs of Ni rod, obtained in the potential interval of [-0.30 -1.00 V] at v = 50 mV•s -1 ; Panels (b) and (d) shows evolution of the CVs, obtained in the potential interval of [-0.06 -0.40 V] at v = 20 mV•s-1 for Ch activated and EC activated Ni rod, respectively.

Fig. 3 . 4

 34 Fig. 3.4 CVs obtained for non-activated (a) and Ch activated (b) Ni rod in N2saturated 0.10 М NaOH at various sweep rates v = 100 (curve 1, magenta), 50 (curve 2, green), 20 (curve 3, blue), 10 (curve 4, red), and 5 (curve 5, black) mV•s -1 . For convenience the currents were normalized by the sweep rates. Insets show the shift of peaks potential relative to the logarithm of the sweep rate.

Fig. 3 . 5

 35 Fig. 3.5 CVs for (a) non-activated and (b) EC activated Ni rod obtained in N2saturated 0.10 М NaOH at v = 20 mV•s -1 and various temperatures. Insets represent the shift of the potential of maximum of anodic (a1 -black squares, a2 -blue triangles) and cathodic (c1 -red circles or blue stars (before and after correction, respectively, see the text for details)) peaks with temperature.

Fig. 3 . 6

 36 Fig. 3.6 CVs for non-activated Ni rod obtained in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 and various positive potential limits (EA). Inset shows the shift of the potential of cathodic peak c1 with increase of the EA.

Fig. 3 .

 3 Fig. 3.7 shows Ni2p XP spectra obtained for two types of Ni foil at different take-off angles. The deconvolution of the spectra into individual components (Ni 0 ,NiO and Ni(OH)2) was done under the assumption that (according to the literature data[START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Medway | In situ studies of the oxidation of nickel electrodes in alkaline solution[END_REF]110,263]) oxidation of Ni in contact with oxygen and/or alkaline electrolyte mainly results in the formation of NiO and Ni(OH)2. The curve-fitting procedure is described in Section 2.2.3.

Fig. 3 .

 3 Fig. 3.7 X-Ray photoelectron Ni 2p3/2 spectra for non-activated (a, b) and Ch activated (c, d) Ni foil at two take-off angles: (a, c) 0 and (b, d) 50 degrees. Spectra were curve-fitted with Ni 0 (solid line, green), NiO (dash-dotted line, red) and Ni(OH)2 (dashed line, blue) components, their resulting atomic concentrations are indicated in the panels. According to the ex situ XPS characterization, surfaces of both samples contain Ni oxide and hydroxide species. While for the non-activated sample the surface is dominated by Ni(OH)2, for the Ch activated one NiO prevails (see Figs. 3.7a, c). Therefore we assume that activation of the Ni surface under its contact with air results in the formation of surface NiO. Conditioning of the activated electrode in the potential interval of [-0.06 -0.40 V] leads to only a partial reduction of the NiO layer. The existence of some amount of NiO on non-activated Ni foil is believed to be due to its incomplete removal during sample polishing and subsequent electrochemical treatment of the electrode (Fig. 3.8). Besides, one cannot exclude a possible formation of a small amount of NiO during the sample transfer to the XPS entry chamber through air. Meanwhile, the presence of Ni(OH)2 on the electrode surfaces can be explained by its formation during the emersion of the electrode from 0.10 M NaOH at open-circuit potential (0.1 -0.25 V) and its further transferring through air with a drop

  layer structure Ni/NiO/Ni(OH)2 is formed with metallic Ni 0 located in the bulk of the sample, Ni(OH)2 -in the upper layer and NiObetween them.

Fig. 3 . 8

 38 Fig. 3.8 Comparison of CVs obtained for non-activated Ni rod (curve 1, black), non-activated Ni foil (curve 2, blue) and Ch activated Ni foil (curve 3, red) in N2saturated 0.10 М NaOH at v = 20 mV•s -1 and Т = 298 К. The last two electrodes were utilized for the XPS measurements.

  3.9b, c), the absence of any peaks in the interval between 400 and 600 cm-1 precludes us from unambiguous conclusions about the surface state of activated and non-activated NiED/Au electrodes in the potential region of [-0.06 -0.40 V].

Fig. 3 . 9

 39 Fig. 3.9 SER spectra recorded for Ch activated (a, c) and non-activated (b) NiED/Au particles in (a) N2-or (b, c) H2-saturated 0.10 М NaOH. The electrochemical measurements were performed potentiostatically.

Fig. 3 .

 3 Fig. 3.10 CVs for non-activated (curve 1, black) and Ch activated (curve 2, red) Ni rod obtained in (a) N2-and (b) H2-saturated 0.10 М NaOH at v = 20 mV•s -1 .

Fig. 3 .

 3 Fig. 3.11 CVs for (a) non-activated and (b) Ch activated Ni rod obtained in H2saturated 0.10 М NaOH at various sweep rates v = 100 (curve 1, magenta), 50 (curve 2, green), 20 (curve 3, blue), 10 (curve 4, red), and 5 (curve 5, black) mV•s -1 . Insets shows the shift of the potential of the maximum of peaks a1 and c1 relative to the logarithm of the sweep rate.

Fig. 3 .

 3 Fig. 3.12 CVs for Ch activated Ni rod obtained in (a) N2-and (b) H2-saturated 0.10 М NaOH at the electrode rotation rate of 0 (curve 1, black) and 1600 (curve 2, red) rpm and v = 20 mV•s -1 .Electrocatalytic activity of Ni electrodes in the HOR/HER

Fig. 3 .

 3 Fig. 3.13 CVs for (a) non-activated and (b) Ch activated Ni rod obtained in H2saturated 0.10 М NaOH at v = 2 (curve 1, black) or 5 (curve 2, red) mV•s -1 .

Fig. 3 .

 3 Fig. 3.14 shows typical CVs obtained in H2 atmosphere for the Ni rod, whose surface was subjected to different pre-treatments. One can see that the HOR on nonactivated Ni starts at relatively high anodic overpotentials (E > 0.15 V), which is due to the low activity of metallic Ni in the HOR/HER (2.3 ± 0.1 μA cm -2 Ni). At the same time, on the activated Ni the HOR begins already at the thermodynamic equilibrium potential of the hydrogen electrode, reaching maximum current densities at 0.10 < E < 0.15 V. Analysis of the data in the Table 3.1 shows that the specific activity of Ch activated and EC activated Ni electrode is almost 9 and 7 times larger, respectively, than for non-activated Ni electrode. It should be stressed that the observed enhancement is not an artifact arising from the normalization to a smaller surface. Indeed, normalization of the activity to the geometric surface area of the Ni rod also shows more than 5 times enhancement for partially oxidized Ni electrode. Some differences in the activity values for Ch and EC activated Ni electrode are likely caused by the differences in their surface state.

Fig. 3 .

 3 Fig. 3.14 CVs for non-activated (curve 1, black), Ch activated (curve 2, red) and EC activated (curve 3, blue) Ni rod obtained in H2-saturated 0.10 М NaOH at v = 5 mV•s -1 . CV for fully oxidized Ni rod (curve 4, dark yellow) is presented on the panel (b) for comparison. The currents are normalized by either (a) SEC or (b) Sgeom.

  essential condition to obtain a clean reproducible surface with minimum contribution of stable Ni surface oxides. It is thus interesting to see an evolution of the CV profile for non-activated and activated surface during recording of several subsequent scans at v = 5 mV•s -1 under H2 atmosphere. Fig.3.15a shows that in the case of nonactivated Ni rod the HOR and the HER currents increase from one scan to the next, in particular, in the potential interval of the peak a2. This increase in currents can be presumably attributed to the formation of stable surface oxides, which are not reduced in the used potential region. Finally, a steady-state is established after ca. 10 scans, while the HOR/HER activity increases up to 1.5 times compared to the initial values for non-activated Ni rod. On the contrary, prolonged cycling of EC activated Ni rod leads to a decrease of its specific activity in the HOR/HER (Fig.3.15b). The latter is accompanied by a decrease of the currents at potentials of the peak a2 and is likely related to continuous reduction of surface Ni oxide species formed during the activation of the electrode. After 10 scans specific activity of EC activated Ni rod in the HOR/HER decreases by a factor of 1.5 in comparison with the initial value, while further drop of activity proceeds with a much lower rate.

Fig. 3 .

 3 Fig. 3.15 CVs obtained by recording of 10 consecutive scans for (a) nonactivated and (b) EC activated Ni rod in H2-saturated 0.10 М NaOH at v = 5 mV•s -1 .

Fig. 3 .

 3 Fig. 3.16 CVs for (a) non-activated and (b) EC activated Ni rod obtained in H2saturated 0.10 М NaOH at v = 5 mV•s -1 and 298 ≤ T ≤ 338 К; CVs obtained in N2 atmosphere at v = 5 mV•s -1 and T = 298 K are presented for comparison by dashed curves. Insets represent Arrhenius plots of exchange current densities versus reciprocal temperature. Specific catalytic activities of non-activated and EC activated Ni rod in the HOR/HER normalized to the geometric surface area of the Ni rod are presented in Table3.2. By plotting the activity values calculated from the anodic sweep of the CVs in Arrhenius coordinates (insets in Fig.3.16), the apparent activation energy (Ea) was estimated to be 30 ± 1 and 26 ± 1 kJ mol -1 for non-activated and EC activated Ni rod, respectively. Deviation of the activity value j0 for EC activated Ni rod at Т = 338 K from the Arrhenius dependence correlates with the modification of the shape of the CVs presented in Fig.3.16b and is likely caused by two factors: (i) stronger (probably excessive) oxidation of the electrode surface during its cycling up to E = 1.0 V and (ii) a possible limitation of the rate of the HOR/HER by slow H2 mass-transport, rather than kinetics of the processes as in the case of lower values of j0. The obtained values of Ea are comparable with those reported for the HER by Bockris and Potter (24 kJ mol -1[START_REF] Bockris | The Mechanism of Hydrogen Evolution at Nickel Cathodes in Aqueous Solutions[END_REF]),Kibria et al. (35 and 27 kJ mol -1 , at low and high cathodic overpotentials,

Fig. 3 .

 3 Fig. 3.17 shows anodic sweeps of CVs plotted in Tafel coordinates and further used for the analysis of both anodic and cathodic potential intervals in accordance with the Tafel equation (eq. 1.18). It was found that the apparent Tafel slope calculated for non-activated Ni rod at low cathodic overpotentials (-0.20 < η < -0.07 V) changes in the range of 115 -130 mV/decade, which essentially corresponds to a change in the RT/F term in the eq. 1.18 with temperature and agrees with the literature

Fig. 3 .

 3 Fig. 3.17 CVs for (a) non-activated and (b) EC activated Ni rod obtained in H2saturated 0.10 М NaOH at v = 5 mV•s -1 and various temperatures and plotted in Arrhenius coordinates. The behavior of the EC activated electrode (Fig. 3.17b) differs from that described above. The value of the apparent Tafel slope varies with the temperature in accordance with the change in the RT/F component for both the HER and the HOR, being in the range of 125 -130 mV/decade and 210 -220 mV/decade, respectively. The high values obtained at anodic overpotentials are likely related to the effect of the oxidation of Ni surface on the magnitude of the HOR currents, even at relatively low potentials [0.05 -0.15 V].

3 . 1 .

 31 Confirming great tendency of Ni towards oxidation, DFT calculations have shown strong adsorption of Oad and OHad on metallic Ni

Fig. 3 .

 3 Fig. 3.18 Comparison of the experimental (a, b) and simulated (c, d) CVs obtained for non-activated (a, c) and Ch activated (b, d) Ni rod in N2-(curve 1, black) and H2-(curve 2, red) saturated 0.10 М NaOH at v = 20 mV•s -1 and T = 298 K.

Figs. 3 .

 3 Figs. 3.18a and c show the comparison of the simulated and experimental current-potential curves in N2-and H2-purged electrolytes for non-activated Ni. The essential experimental features, namely the presence of one anodic (a1) and one cathodic (c1) peaks in the CVs, under N2 atmosphere, and the low HOR/HER activity could be qualitatively reproduced by assuming ∆adG 0 = -0.10 eV (other parameters could be found in the Appendix 2). The low HOR/HER activity of non-activated Ni in the potential interval of [0 -0.15 V] results from, on the one hand, the surface blocking by strongly adsorbed hydrogen and, on the other hand, the relatively slow Volmer step. The rate of electrocatalytic hydrogen oxidation increases with the potential and begins to appreciably exceed background currents at E > ~0.15 V, while at potentials E > ~0.25 V there is a drop in the HOR currents, which is caused by the formation of a monolayer of Ni hydroxide at the electrode surface. According to the developed model, in inert atmosphere the peak a1 results from the contribution of both the electrochemical desorption of Had and the Ni(OH)2,ad formation (Fig. 3.18c), being in agreement with the conclusions of Hall et al. [64].

Fig. 3 .

 3 Fig. 3.19 Simulated CVs obtained after either changing of ∆adG 0 H from -0.10 to -0.02 eV (a) or 10 times increase of k3 0 (b) regarding the parameters determined for non-activated Ni in the absence (curve 1, black) or presence (curve 2, red) of H2 at v = 20 mV•s -1 and Т = 298 K. Influence of the temperature on the kinetics of the HOR/HER

Fig. 3 .

 3 Fig. 3.20 Comparison of the experimental (solid lines) and simulated (dashed lines) CVs for (a) non-activated and (b) EC activated Ni rod in H2-saturated 0.10 М NaOH at v = 5 mV•s -1 and 298 ≤ Т ≤ 338 К. Insets represent the coverage of Ni surface by adsorbed hydrogen depending on the applied potential.

Fig. 3 .

 3 Fig. 3.20a compares simulated and experimental current-potential curves obtained in H2-saturated 0.10 M NaOH for non-activated Ni rod. The developed model allows to reproduce well the experimental curves in all potential interval studied by assuming ∆adG 0 Н = -0.15 eV c and an increase of the rate constants for the

Fig. 3 .

 3 Fig. 3.21 Simulated CVs for non-activated (a) and EC activated (b) Ni rod obtained in the presence of H2 at v = 5 mV•s -1 and T = 298 K. Dashed curves represent the coverage of the electrode surface by reactive intermediates. Careful analysis of calculated rate constants (Appendix 3, Fig. 3.22) allows to conclude that on the surface of non-activated Ni electrode the HOR/HER proceed according to the Heyrovsky-Volmer mechanism in agreement with several other studies, where authors assumed (almost) full coverage of Ni surface by adsorbed Had in the potential interval of the HER[START_REF] Machado | The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption // Electrochim[END_REF]253,257]. It should be noted that present work

Fig. 3 .

 3 Fig. 3.22 Arrhenius plots of the calculated rate constants for the Volmer (k±3 0 ), Heyrovsky (k±2 0 ) and Tafel (k±1 0 ) elementary steps versus reciprocal temperature for (a) non-activated and (b) EC activated Ni rod. In accordance with the analysis presented above, the HOR/HER activity enhancement on partially oxidized Ni electrode can be described by assuming an increase of the ∆adG 0 H in the presence of oxide species on the surface of the Ni electrode. Based on the analysis of CVs obtained at various temperatures, the change of ∆adG 0 H was found to be ca. 0.10 eV (from -0.15 to -0.05 eV). A reasonable fit of the experimental current-potential curves in the potential region of [-0.2 -~0.2 V] was obtained for the activated Ni electrode (Fig. 3.20b). At the same time, deviations of the simulated curves from the experimental ones observed at Е > 0.2 V are likely related to the approximations used in the development of the kinetic model. In reality the oxidation of Ni surface might be more complicated than that assumed in the model due to possible formation of multilayered islands of Ni hydroxide before full coverage of the Ni surface by Ni(OH)2 is established[START_REF] Nakamura | Surface structures at the initial stages in passive film formation on Ni(111) electrodes in acidic electrolytes[END_REF][START_REF] Seghiouer | Electrochemical oxidation of nickel in alkaline solutions: a voltammetric study and modelling[END_REF]. Besides, the surface of partially oxidized Ni electrode likely contains several types of reactive Ni sites with different activity in the HOR, while in the mean-field approximation, it is considered to be homogeneous.

3. 4 .

 4 Conclusions of Chapter 3Significant variations in the electrochemical and electrocatalytic properties of Ni in the hydrogen oxidation and hydrogen evolution reactions were observed depending on the pretreatment of the electrode surface. If electrochemical tests are performed shortly after polishing of the surface of Ni rod and care is taken to minimize its contact with air, then electrocatalytic activity of Ni in the HOR/HER is low and amounts to ca. 2 μA cm -2 Ni or ca. 6 μA cm -2 geom for the normalization by electrochemically active or geometric surface area, respectively. If, however, Ni electrode is "activated" by either its exposure to an ambient atmosphere or under controlled electrochemical conditions, then its activity in the HOR/HER increases up to ca. 20 μA cm -2 Ni (32 μA cm -2 geom) or ca. 15 μA cm -2 Ni (29 μA cm -2 geom), respectively. On the basis of ex situ angle-resolved XPS measurements this difference is tentatively explained by the presence of a significant amount of NiO on the surface of partially oxidized Ni.By combining the experimental observations with microkinetic modeling we conclude that the enhancement of the electrocatalytic activity of Ni in the presence of surface Ni oxide species is due to a decrease of the adsorption energy of the hydrogen intermediate participating in the HOR/HER, along with an increase of the rate constant of the Volmer step. This conclusion agrees with the published results of DFT calculations suggesting decrease of the Ni-Had binding energy and an increase of the Ni-H2Oad binding energy in the presence of oxygen adsorbed on Ni surface. We infer that not only the rate of the HOR/HER, but also its mechanism is affected by the surface state of Ni electrode. On the surface of metallic Ni, the HOR/HER follow the Heyrovsky-Volmer mechanism with the forward Volmer step being the rate determining one for the HOR, while the rate of the HER is controlled by the backward Heyrovsky steps. On the surface of "activated" Ni electrode, where Ni metal sites coexist with the "irreversible" surface oxide species, the contribution of the Tafel step in the mechanism of the reactions has to be taken into account. In this case the rates of the backward Volmer and Heyrovsky steps become comparable in the potential interval of the HER, while the HOR is still limited by the slow Volmer step.

4. 2 . 4 . 2 . 1 .

 2421 Ni electrodeposited on flat GC support Choice of electrodeposition parameters and investigation of the synthesized samples by physicochemical techniques Synthesis of nanostructured catalysts was done by using the method of potentiostatic deposition from the electrolytic bath, containing 0.01 M NiSO4 and 0.10 M (NH4)2SO4. The range of Ni deposition potentials was determined on the basis of CVs obtained in an inert atmosphere at a sweep rate of 20 mV•s -1 , and using GC rod as a working electrode (Fig. 4.1a). One can see that on the first scan (cathodic sweep) Ni deposition begins at potentials E < -1.2 V vs MSE e , while at the anodic sweep of the CV a so-called "nucleation loop" is observed, which is typical for the processes of nucleation and growth. At the second scan, the deposition starts at more positive potentials, which is associated with the growth of previously formed Ni particles. It should be also noted that the deposition of Ni proceeds simultaneously with the HER. Electrodeposition of Ni was performed in three steps using a pulse potentiostatic mode. The corresponding pulse potentials were chosen in such a way that nucleation of Ni nanoparticles occurs during the first applied potential (E1 = -1.50 V vs. MSE), while their growth takes place mainly at the potential E3 = -1.25 V vs. MSE. The second step (E2 = -0.93 vs. MSE) was added with the aim to replenish the electrolyte layer in the vicinity of the electrode surface with Ni 2+ ions. e MSE = Mercury-Mercury Sulphate Electrode, EMSE = 0.96 V vs RHE at T = 298 K, pH (0.01 M NiSO4 + 0.10 M (NH4)2SO4) ≈ 5.4

Fig. 4 .

 4 Fig. 4.1 (a) CVs for GC electrode in N2-saturated 0.01 M NiSO4 + 0.10 M (NH4)2SO4 solution at v = 20 mV•s -1 and T = 298 K; (b) Ni deposition transients obtained for NiED/GC-1 (curve 1, black) and NiED/GC-2 (curve 2, red) (the parameters of deposition are listed in Table 4.1). Inset on the panel (b) shows reduced transients for NiED/GC-2 along with model transients for instantaneous (dash dot line) and progressive (dashed line) nucleation. To inspect the influence of the electrodeposition parameters on the morphology of obtained Ni nanoparticles, a series of NiED/GC samples with varied time of the nucleation step or concentration of Ni 2+ ions in the electrolyte (Table 4.1) was obtained. Experimental transients for Ni deposition on GC support are shown in Fig. 4.1b. A well-defined current maximum (Imax achieved at tmax) was observed when the first deposition potential was applied for t > ~20 s, allowing us to plot the I vs t dependence in dimensionless (i. e. normalized on Imax and tmax) coordinates. However the analysis of obtained transients and their comparison with model transients for instantaneous and progressive nucleation [290,291] is complicated by the concomitant hydrogen evolution. The low amount of Ni electrodeposited on GC support did not allow us to precisely determine its mass, thus preventing us from the estimation of the Faradaic efficiency of the electrodeposition. Meanwhile, as will be shown later for the electrodeposition of Ni on the Vulcan carbon support (see Section 4.3.1), the contribution of the HER currents is significant and cannot be ignored. This might explain significant deviation of the experimental transients from the model ones at t < tmax (see the inset to Fig. 4.1b). Indeed, high contribution of the HER currents results in overestimation of the Imax value and thus underestimation of the value of I/Imax. Similarly, despite the fairly good agreement between experimental and model

Fig. 4 . 2

 42 Fig. 4.2 SEM images of Ni particles electrodeposited on GC support: NiED/GC-1 (a), NiED/GC-2 (b), NiED/GC-3 (c), and corresponding size distribution of Ni globules.

Fig. 4 . 4

 44 Fig. 4.4 CVs obtained in N2-saturated 0.10 М NaOH and v = 20 mV•s -1 for (a) non-activated and (b) Ch activated NiED/GC just after an immersion of the electrodes in the electrolyte; (c) comparison of conditioned CVs obtained for non-activated (curve 1, black) and Ch activated (curve 2, red) NiED/GC electrodes.

  2.2). The latter likely indicates a deeper oxidation of Ni nanoparticles compared to a polycrystalline Ni rod. Similarly to Ch activated Ni rod, no cathodic peak was observed on the first scan of the CV recorded in the potential interval of [-0.06 -0.40 V] (Figs. 4.4b and 3.3b), while partial reduction of the electrode surface resulted in an appearance of characteristic anodic (a1) and cathodic (c1) peaks (Fig. 4.4b). The peak a2 was less defined compared to the one observed for the Ch activated Ni rod, although its contribution to the anodic currents is visible if CVs registered for non-activated and Ch activated NiED/GC are compared (Fig. 4.4c).

Fig. 4 .

 4 Fig. 4.5 (a) CVs obtained in H2-saturated 0.10 М NaOH at v = 5 mV•s -1 for nonactivated (curve 1, black) and Ch activated (1 st scan -curve 2, red and 10 th scan -

Fig. 4 .

 4 6 compares the deposition transients registered for Ni electrodeposition on GC and on XC-72/GC surface. As one can see, a much larger amount of Ni is deposited in the latter case, most likely due to an increase in the surface area of the support and, as a consequence, an increase in the number of nucleation sites. The corresponding values of the total deposition charge (including the one related to the HER, which accompanies Ni deposition) were calculated to be 3.6 ± 1.8 and 140 ± 20 mC for GC and XC-72/GC supports, respectively.

Fig. 4 . 6

 46 Fig. 4.6 Nickel deposition transients obtained for NiED/XC-72 (curve 1, black) and NiED/GC (curve 2, red) electrodes in N2-saturated 0.01 M NiSO4 + 0.10 M (NH4)2SO4 solution at Т = 298 K.

  clearly show the nanostructured nature of Ni nanoparticles, which consist of crystallites of smaller size connected through the intergrain boundaries. The value of the interplanar spacing was calculated to be ca. 0.21 nm, which is consistent with the Ni (111) interplanar distances.

Fig. 4 . 7

 47 Fig. 4.7 High resolution TEM (a) and STEM (bright (b) and dark (c) field) images of the NiED/XC-72 electrocatalyst.

Fig. 4 .

 4 Fig. 4.8 X-Ray photoelectron Ni 2p3/2 spectra obtained for non-activated NiED/XC-72 (a) before and (b) after cycling of the electrode in the potential interval of [-0.06 -0.40 V] at 0 take-off angle. Spectra were curve-fitted with Ni 0 (gray), NiO (red) and Ni(OH)2 (green) components, their resulting atomic concentrations are indicated in corresponding panels. The curve-fitting procedure is described in Section 2.2.3.

Fig. 4 .

 4 Fig. 4.9a shows CVs registered for freshly prepared non-activated NiED/XC-72 sample just after its immersion in N2-saturated 0.10 M NaOH. Unlike the previously studied samples, only a small cathodic peak was observed in the first scans of the CVs. Besides, much smaller value of the open-circuit potential was obtained for nonactivated NiED/XC-72 (0.1 -0.15 V) in comparison with either non-activated NiED/GC particles or non-activated polycrystalline Ni rod (0.4 -0.5 and 0.2 -0.3 V, respectively). The observed lower degree of oxidation of the surface of NiED/XC-72 during the electrode transfer to the electrochemical cell with 0.1 M NaOH (the procedure usually took less than 15 s) might be explained by porous structure of the sample. As a result, only Ni particles located on the outer surface of the carbon support are accessible to the oxygen from air. The apparent contradiction with the results of XPS measurements then may be reconciled by taking into account the small depth of the XPS analysis (< 10 nm), according to which oxidized Ni particles located at the external surface of carbon support mostly contribute to the XP spectra. At the same time, Ni particles within the pores of the XC-72 carbon are also available for electrochemical measurements. On the second scan of the CVs the typical anodic (a1)

Fig. 4 .

 4 Fig. 4.9b shows the comparison of CVs obtained for the non-activated NiED/XC-72 and XC-72/GC (electrode before the electrodeposition of Ni), after normalization of the currents to the geometric surface area of the GC electrode. Due to high metal loading on the surface of XC-72 support (42 ± 3 wt.%), the currents originating from the carbon support were found to be insignificant compared to those corresponding to the electrochemical processes on the surface of Ni nanoparticles. Therefore the carbon contribution was not taken into account in further discussions. In addition, comparison of CVs registered for non-activated NiED/GC and NiED/XC-72 (Fig. 4.9b) confirms deposition of significantly larger amount of Ni on the surface of porous XC-72 support, than on the bare GC surface.

Fig. 4 . 9

 49 Fig. 4.9 CVs for non-activated NiED/XC-72 (curve 1, black), NiED/XC-72 (curve 2, red) and XC-72/GC (curve 3, blue) obtained in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 . The currents are normalized to (a) SEC or (b) Sgeom of GC electrode. In order to inspect the influence of the partial oxidation of the surface of NiED/XC-72 nanoparticles on their electrochemical and electrocatalytic properties, a Ch activated NiED/XC-72 samples (i. e. measured after chemical oxidation of freshly prepared sample under air for typically ca. 24 h. (cf. Section 2.3.4.1)) were studied.

Fig. 4 .

 4 Fig. 4.10a shows CVs obtained for the NiED/XC-72 sample taken from air. The open-

Fig. 4 . 4 . 3 . 4 .

 4434 Fig. 4.10 (a) CVs obtained for Ch activated NiED/XC-72 just after an immersion of the electrode in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 ; (b) Comparison of the CVs registered for non-activated NiED/XC-72 electrode before (curve 1, black) and after (curve 2, red) cycling in the potential interval of [-0.06 -0.40 V] and Ch activated (curve 3, blue) NiED/XC-72 electrode. 4.3.4. Kinetics of the HOR/HER on NiED/XC-72 electrocatalysts

Fig. 4 .

 4 Fig. 4.11a shows CVs obtained for NiED/XC-72 electrocatalysts in H2 atmosphere at v = 5 mV•s -1 . Unlike either polycrystalline Ni rod or NiED/GC, the currents registered for NiED/XC-72 samples were found to depend on the rotation rate of the electrode (Fig. 4.12). The latter indicates the influence of mass transfer of hydrogen to the

Fig. 4 .

 4 Fig. 4.11 (a) Cyclic voltammograms for non-activated (curve 1, black), EC activated (curve 2, red) and Ch activated (curve 3, blue) NiED/XC-72 registered in H2saturated 0.1 M NaOH at v = 5 mV/s and ω = 1600 rpm; (b) evaluation of cyclic voltammograms obtained for non-activated NiED/XC-72 during continuous cycling in the potential interval of [-0.06 -0.40 V] at v = 5 mV s -1 and ω = 1600 rpm. It was found that specific surface-weighted activity of freshly prepared NiED/XC-72 is in line with the values obtained for freshly prepared NiED/GC and polished Ni rod (Appendix 1), despite the difference in the size of Ni particles deposited on the surface of GC and XC-72/GC support (25 ± 5 nm and 10 ± 2, respectively). This observation confirms the earlier assumption that the specific activity of Ni catalysts in the HOR/HER does not depend on their morphology in the case of (essentially) reduced surface, at least at a particle size of more than 10 nm. At the same time the size of Ni particles and/or their nanostructure affect the process of oxidation of electrodeposited samples and, probably, stability of formed Ni oxides against reduction. As a result, specific surface-weighted activity of Ch activated NiED/XC-72 is 2 times as high as the activity of Ch activated NiED/GC (Appendix 1). Furthermore, the tendency of Ni nanoparticles towards oxidation resulted in

Fig. 4 .

 4 Fig. 4.12 Polarization curves of the HOR obtained using rotating disk electrode in H2-saturated 0.10 M NaOH at v = 5 mV•s -1 and various rotation speeds for (a) Ch activated NiED/XC-72 electrode and (b) NiED/XC-72 electrode after prolonged cycling. Insets show the Koutecky-Levich plot at overpotential of 50 mV.

4. 4 .

 4 Conclusions of Chapter 4This work shows that using electrodeposition technique it is possible to synthesize nanostructured Ni particles of small size deposited on the surface of either smooth (glassy carbon) or porous (Vulcan XC-72) support. The main findings, namely the change of the shape of cyclic voltammograms and an increase of the specific activity of Ni in the HOR/HER observed for massive polycrystalline Ni after partial oxidation of its surface, were also detected in the case of electrodeposited Ni particles of different diameters. Hence we conclude that the observed changes are universal and do not depend on the morphology of the Ni catalyst.

  . The rapid development of AMFCs and the need to replace Pt catalysts with inexpensive PGMfree analogues stimulate the interest in the investigation of bimetallic Ni-based catalysts for the anodes of AMFC. Besides, despite a significant progress during the past decades, the question about the optimal composition in bimetallic NiM system still remains open even for the cathodic hydrogen evolution process. In this chapter we analyze the carbon supported NiCu/C and NiMo/C electrocatalysts in alkaline media with the aim to determine the effect of Cu and Mo on the electrochemical properties of Ni, as well as its activity in the HOR. The positive effect of Cu on the activity of Ni in the HER has been already shown in the literature with the use of electrodeposited films of various compositions [201-203].

  NiCu/C electrocatalysts by physicochemical techniques Morphology and dispersion of Ni/C and NiCu/C particles TEM analysis of the Ni/C and Ni1-xCux/C electrocatalysts synthesized using wetness impregnation technique in conventional manner revealed high heterogeneity of the metal particles with the size ranging from 1 to 100 nm in size (Fig. 5.1a). Nevertheless, comparison of the TEM data for monometallic Ni/C and bimetallic Ni1-xCux/C samples allowed us to draw several important conclusions. It was found that the surface of "large" particles (dTEM > ca. 20 nm) is covered by NiO layer independent of the catalyst composition. However the thickness of the oxide shell decreases in the presence of Cu (Fig. 5.2). Moreover, while "small' particles (dTEM < 10 nm) in the monometallic Ni/C sample are fully oxidized, in Ni1-xCux/C catalysts they remain in the metallic state (Fig. 5.2b).

Fig. 5 . 1

 51 Fig. 5.1 TEM images of the Ni0.95Cu0.05/C catalysts prepared by wetness impregnation technique in either conventional manner (a) or with the use of lyophilization (b). Insets show corresponding particles size distribution.

Fig. 5 . 2

 52 Fig. 5.2 HRTEM images of the (a) Ni/C and (b) Ni0.95Cu0.05/C catalysts; (c) HAADF-STEM image of Ni0.95Cu0.05/C catalyst and (d) corresponding to it elemental map (Ni -green and Cu -red).

  Crystalline structure of Ni/C and NiCu/C particles XRD patterns obtained for selected Ni1-xCux/C catalysts and for the monometallic Ni/C reference are displayed in Fig. 5.3. It was found that despite oxidation of the samples under ambient conditions (as it is evidenced by TEM and XPS measurements), no diffraction peaks of nickel oxide/hydroxide phases were detected. This observation suggests that Ni (hydr)oxide phase is amorphous or consists of either very small or highly distorted crystallites. Analysis of the XRD patterns also revealed some differences in the structure of bimetallic Ni1-xCux/C and Ni1-xCux/C (lyo) catalysts. While the cell parameter of metallic Ni phase in the first series of samples (Ni/C and Ni1-xCux/C) was a = 3.524 Å, which agrees well with the tabulated data [ICDD PDF-2 #00-004-0850], in the second series of the samples (Ni1g Ni fraction strongly increases with the particle size in the interval from 5 to 20 nm h Ni fraction below the detection limit xCux/C (lyo)) it slightly exceeded the tabulated values (a = 3.527(2) Å). This increase of the lattice parameter of Ni is manifested in a small but measurable shift of the 220 reflection (see inset in Fig. 5.3b). Consequently, an enlarged cell parameter suggests formation of Ni-Cu alloy particles and according to the Vegard's law corresponds to ca. 5 at.% Cu content for Ni0.95Cu0.05/C (lyo) and Ni0.90Cu0.10/C (lyo) samples. The lower Cu content than the nominal composition of the Ni0.90Cu0.10/C (lyo) could be tentatively attributed to the Cu segregation on the surface of the particles due to its smaller surface energy, as it was found in a number of experimental and computational studies [295-297]. Based on the XRD analysis we conclude that drying of the catalyst precursors in vacuum favors Ni and Cu phase separation (Fig. 5.3a), while the use of freeze-drying results in formation of more homogeneous NiCu alloy particles. The average size of Ni crystallites estimated from the (111) peak broadening using the Scherrer equation varies between 11 and 17 nm depending on the catalyst composition.

Fig. 5 . 3

 53 Fig. 5.3 XRD patterns for (a) Ni/C (1), Ni0.95Cu0.05/C (2), Ni0.80Cu0.20/C (3), Ni0.50Cu0.50/C (4) and (b) Ni/C (lyo) (1), Ni0.95Cu0.05/C (lyo) (2), Ni0.90Cu0.10/C (lyo) (3). Insets on the panels zoom into selected areas corresponding to the Cu (220) and Ni (220) diffraction peaks. Surface state of Ni/C and NiCu/C particles

Fig. 5 .

 5 Fig. 5.4 X-Ray photoelectron Cu 2p spectra for the Ni/C (1, black), Ni0.95Cu0.05/C (2, red), Ni0.90Cu0.10/C (3, blue) and Ni0.80Cu0.20/C (4, dark yellow) catalysts at 0 take-off angle.

Fig. 5 .

 5 Fig. 5.5 X-Ray photoelectron Ni 2p3/2 spectra for the Ni/C, Ni0.95Cu0.05/C, Ni0.90Cu0.10/C and Ni0.80Cu0.20/C catalysts at 0 take-off angle. Spectra were curve-fitted with Ni 0 (solid line, blue), NiO (dash-dotted line, green) and Ni(OH)2 (dashed line, red) components, their resulting atomic concentrations are indicated in the panels. The curve-fitting procedure is described in Section 2.2.3.

Fig. 5 . 7

 57 Fig. 5.7 Comparison of specific surface areas of Ni in Ni1-xCux/C samples, derived from the CO-chemisorption measurements, SCO (black squares) and from the electrochemical measurements, SEC, based on the CVs registered in the potential interval of [-0.02 -0.40 V] (down triangles, red) and [-0.12 -0.40 V] (up triangles, blue) at v = 20 mV•s -1 . Dark yellow circles refer to Ni1-xCux/C (lyo) as measured using CO-chemisorption.

Fig. 5 .

 5 Fig. 5.8 (a) Anodic sweep of CVs for Ni/C and Ni1-xCux/C samples obtained in H2-saturated 0.10 М NaOH at v = 2 mV•s -1 . Specific (b) mass-and (c) surfaceweighted activity as a function of the Cu fraction in the catalyst.

Fig. 5 . 9

 59 Fig. 5.9 TEM images of the Ni0.90Mo0.10/C electrocatalyst at different magnifications. Crystalline structure of NiMo/C particles

Fig. 5 .

 5 Fig. 5.11 X-Ray photoelectron Ni2p3/2 (a) and Mo3d (b) spectra for the Ni0.90Mo0.10/C electrocatalyst at 0 take-off angle. Spectra were curve-fitted with the components listed in the graphs, according to the procedure described in Section 2.2.3.

Fig. 5 .

 5 Fig.5.12 shows CVs of the Ni0.90Mo0.10/KB electrocatalyst in N2-saturated 0.10 M NaOH. In agreement with the XPS data, electrochemical measurements showed that the surface of the electrode taken from air is covered by metal oxide species, as it is evidenced by the value of the open-circuit potential (0.50 -0.60 V) measured after immersion of the electrode in the electrolyte. Besides, no peaks were observed in CVs registered in the potential interval of [-0.06 -0.40 V] (Fig.5.12a). That is why the Ni0.90Mo0.10/C electrocatalyst was subjected to electrochemical reduction as described in Section 2.3.4.1. Fig. 5.12 shows that this procedure results in considerable increase

Fig. 5 .

 5 Fig. 5.12 (a) CVs obtained in N2-saturated 0.10 М NaOH at v = 20 mV•s -1 for Ni0.90Mo0.10/C sample after its immersion in the electrolyte (black curve) and after electrochemical reduction of its surface (red curve); (b) Comparison of CVs obtained for Ni0.9Mo0.1/C (100 µg cm -2 geom, curve 1, black) and KB carbon support (50 µg cm - 2 geom, curve 3, blue); in addition, the CV obtained after subtraction of the currents corresponding to the KB support (curve 2, red) is presented.

Fig. 5 .

 5 Fig. 5.13 (a) CVs for Ni0.90Mo0.10/C obtained at various loadings of the catalyst on the surface of GC electrode in H2-saturated 0.10 M NaOH at v = 5 mV•s -1 and ω = 1600 rpm; (b) Polarization curves of the HOR registered for Ni0.90Mo0.10/C (with a loading of 500 µg cm -2 ) in H2-saturated 0.10 M NaOH at v = 5 mV•s -1 and various rotating rates of the electrode.

Fig. 5 .

 5 Fig. 5.14 compares experimental and simulated CVs obtained in both N2 and H2 atmosphere. Good agreement was obtained in the potential interval of [-0.06 -0.15 V], while at higher potentials simulated currents are much inferior of the experimental ones. This difference is likely related to a possible diversity of the Ni active sites on the catalyst surface, while in the mean-field approximation the surface is considered to be homogeneous. Despite some discrepancies with the experimental data, kinetic modeling predicts a significant increase of the Gibbs energy of adsorption of the hydrogen intermediate (∆adG 0 Н) from -0.15 eV for non-activated polycrystalline Ni to 0.04 eV for the Ni0.90Mo0.10/C catalyst (Appendixes 2 and 3). Therefore the high activity of the Ni0.90Mo0.10/C catalyst in the HOR/HER may be attributed to a decrease of the Ni-Had binding energy. Modification of the hydrogen adsorption energy is in agreement with the work of Paloukis et al. [213], who concluded that the shift of the metal Ni 0 peak on Ni 2p3/2 XP spectrum from 852.4 eV to 852.6 eV is caused by an influence of Mo on the electronic properties of Ni. In addition, the presence of Mo oxides on the surface of Ni0.90Mo0.10/C catalyst might also contribute to the enhanced HOR activity by facilitating the Volmer step[START_REF] Subbaraman | Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts[END_REF]132].

Fig. 5 .

 5 Fig. 5.14 Comparison of the experimental (solid line) and simulated (dashed line) CVs for Ni0.90Mo0.10/C catalyst (with a loading of 100 µg cm -2 ) in N2-(black curve) or H2-saturated (red curve) 0.10 M NaOH at v = 5 mV•s -1 and T = 298 K. All the curves are presented after the subtraction of the KB carbon contribution

Fig. 5 .

 5 Fig. 5.15 The change of the specific mass-weighted activity referred to the maximum obtained for Ni0.9Mo0.1/C (black squares), EC activated polycrystalline Ni rod (red circles) and Ch activated NiED/GC particles (blue triangles) during cycling of the electrodes in H2-saturated 0.10 М NaOH in the potential interval of [-0.06 -0.4 V] at v = 5 mV•s -1 .

  NiCu/С (with varied amounts of Cu) as well as NiMo/С electrocatalysts. It was found that adding of a small Cu fraction to Ni has a number of consequences, namely (i) increase of the tolerance of the Ni component towards oxidation under ambient conditions; (ii) increase of the specific surface area of Ni particles during the synthesis; and (iii) increase of the mass-weighted catalytic activity of Ni in the HOR/HER. This catalytic activity enhancement was linked to two factors: first, increase of the specific surface area of Ni and second, growth of the specific electrocatalytic activity attributed to an electronic and/or ensemble effect of Cu, which are the most pronounced in the case of Ni0.95Cu0.05/C catalyst. Investigation of the Ni0.90Mo0.10/C catalyst showed that, similarly to Cu, adding Mo facilitates an electrochemical reduction of Ni oxide species present on the surface of the electrode taken from air. Besides, the specific activity of Ni0.90Mo0.10/C electrocatalyst was found to be ca. 2 times as high as the activity of Ni0.95Cu0.05/C (the best among studied NiCu/C samples). By combining the experimental observations with microkinetic modeling the high activity of Ni0.90Mo0.10/C catalyst in the HOR/HER was attributed to a decrease of the adsorption energy of the hydrogen atoms, being intermediate of the HOR/HER, in comparison with non-activated polycrystalline Ni. Furthermore, it was observed that addition of Mo to Ni catalyst allows one to reduce the HOR/HER activity decay during potential cycling.

1 . 4 .

 14 It was found that partial oxidation of the surface of Ni electrocatalysts significantly affects their electrochemical properties in an alkaline electrolyte. Specific activity of metallic Ni in the HOR/HER does not depend on the morphology of the catalyst, but increases up to 10 times after partial oxidation of its surface. On the contrary, full oxidation of the electrode surface results in suppression of the activity of Ni in the HOR/HER. 2. The developed kinetic model allows one to describe the kinetics of both the hydrogen evolution and the hydrogen oxidation reactions taking into account the process of Ni surface oxidation with the formation of Ni hydroxide with the same set of parameters. Based on the data of mathematical modeling we conclude that: • the enhancement of the electrocatalytic activity of Ni in the presence of surface Ni oxides is related to a decrease of the adsorption energy of the hydrogen atoms on Ni, along with an increase of the rate constant of the Volmer step; • the mechanism of the HOR/HER on Ni is mainly described by the Heyrovsky and the Volmer steps, while the contribution of the Tafel step becomes noticeable only after partial oxidation of the electrode surface. The rate of the HOR is controlled by the Volmer step independently on the extent of oxidation of the surface of a Ni electrode. 3. For the first time a highly dispersed 40 wt.% NiED/XC-72 electrocatalyst was prepared using electrodeposition technique and Vulcan XC-72 carbon as a support. Its mass-weighted electrocatalytic activity in the HOR significantly exceeds the values reported in the literature for non-noble metal catalysts and approaches the activity of commercial 20% Pd/C electrocatalyst. Investigation of the series of carbon supported NiCu/C electrocatalysts showed that addition of Cu to the active component leads to: • an increase of specific activity of Ni in the HOR/HER compared to monometallic Ni/C sample, which was prepared by the same method. The highest activity was observed at ca. 5 at.% fraction of Cu in the sample; • an enhancement of the tolerance of Ni towards oxidation. 5. Investigation of carbon supported Ni0.9Mo0.1/C electrocatalyst showed that adding Mo to the active component allows one to increase the specific activity of the catalyst in the HOR/HER. With the help of kinetic modeling this enhancement was attributed to a decrease of the adsorption energy of the hydrogen atoms on Ni as well as an increase of the rate constant of the Volmer step.

LFig. 1 Tableau 1 .

 11 Fig. 1 Les symboles sur le graphique correspondent aux valeurs des activités de l'HER rapportées pour le Ni polycristallin dans la littérature. La zone hachurée correspond aux données obtenues dans la thèse et montre la dispersion des valeurs des activités spécifiques du Ni pour l'HOR/HER en fonction de l'état de surface de l'électrode de Ni.

  ' désigne des sites métallique à la surface de l'électrode. La dépendance en potentiel des constantes de vitesse des étapes impliquant un transfert d/désorption à la surface sont modélisées par des isothermes d'adsorption de Langmuir.

Fig. 2

 2 Fig. 2 Comparaison des CVs expérimentaux (courbes solides) et simulés (courbes pointillées) obtenues pour un disque de Ni non actif (a) et EC activé (b) dans 0.1 M NaOH saturé en N2 (courbes noires) ou H2 (courbes rouges) à une vitesse de balayage de 5 mV s -1 et T = 298 K. Les courbes en pointillé sur les panneaux (c) et (d) représentent les taux de recouvrement simulés pour la surface du Ni par Нad (θН), OHad (θОН) et (OH)2,ad (θ(OH)2) adsorbées pour les électrodes de Ni non activées et EC activées, respectivement. Les courbes solides correspondent aux balayages de potentiel anodiques des CV simulés. L'électrode d'hydrogène réversible (RHE) est utilise comme référence.

Fig. 3

 3 Fig. 3 Illustration schématique de la structure de l'électrode Ni activée in situ et un mécanisme d'activation HOR provisoire. Codes de couleurs: gris -Ni 0 , rouge -NiOx.

  La deuxième partie de la thèse a été consacrée à la synthèse et à l'étude des nanoparticules de Ni supportées sur carbone vitreux (GC) ou carbone poreux Vulcan XC-72. Ces catalyseurs ont été préparés par électrodéposition (ED) à partir de l'électrolyte contenant 0.01 M NiSO4 et 0.10 M (NH4)2SO4. Le support GC a été utilisé pour le développement de la procédure d'électrodéposition et la synthèse des échantillons modèles NiED/GC. Ensuite, une série de catalyseurs NiED/XC-72 qui peuvent être potentiellement utilisés aux anodes de l'AMFC a été obtenue. La caractérisation des échantillons électrodéposés par microscopie électronique (SEM, TEM, STEM) a montré que les particules de Ni sont réparties de manière homogène sur la surface du support, tandis que leur diamètre moyen en surface (sous l'hypothèse que la forme des particules est sphérique) est estimé à 25 ± 5 et 10 ± 2 nm pour NiED/GC et NiED/XC-72 respectivement. En outre, les images STEM haute résolution (figures 4b, c) montrent clairement la nature nanostructurée des nanoparticules de Ni : les particules sont constituées de cristallites de la taille d'environ. 3 nm, connectées par des joints de grain. De plus, par l'utilisation de la technique ICP-AES (spectrométrie d'émission atomique à plasma à couplage inductif), la charge typique de métal dans le catalyseur NiED/XC-72 a été estimée à 42 ± 3 % en poids.

Fig. 4 II. 3 .

 43 Fig. 4 Images haute résolution de TEM (a) et STEM (champ clair (b) et sombre (c)) de l'électrocatalyseur NiED/XC-72

Fig. 5 III. Conclusions générales 1 . 2 .

 512 Fig. 5 Activités pondérées en masse (a) et en surface (b) obtenues pour les catalyseurs Ni/C et Ni1-xCux/C en fonction de la fraction de Cu. Les échantillons ont été préparés par séchage conventionnel (carrés noirs) soit par lyophilisation (triangles rouges) pendant la synthèse. De plus, les valeurs d'activité obtenues pour le catalyseur Ni0.9Mo0.1/C sont indiquées par des cercles bleus.

Table 1 . 1

 11 .1)[166,[251][252][253]260]. Besides,Krstajic et al. shown that the mechanism of the HER can switch from the Volmer-Tafel at -0.2 < Kinetic parameters and the mechanism of the HER (* denotes the rate-determining step), depending on the pretreatment of the surface of nickel electrodes

	Methodology of Ni electrode pretreatment	Possible surface state	T, K	Electrolyte	Tafel slope, mV	j0, 2 geom µA cm -	j0, 2 Ni µA cm -	Proposed the HER mechanism of	Ref.

η < 0 V to the Volmer-Heyrovsky at η < -0.2 V

[257,261]

. Changing of the Tafel slope at different overpotentials was also reported by

Kibria et al. [254]

and

Conway et al. [246,256]

and most likely reflects the changes in the surface composition of the Ni electrode at different hydrogen overpotentials.

  Bimetallic nickel-copper catalysts with 25 wt.% total metal loading and Ni:Cu molar ratio from 0.50:0.50 to 0.95:0.05 were prepared by P. A. Simonov in the

	Boreskov Institute of Catalysis (BIC, Novosibirsk, Russia) by the incipient wetness
	impregnation of the support with aqueous solutions of Ni(OAc)2 and Cu(OAc)2 taken
	in a desired proportion. Commercial furnace black Vulcan XC-72 (Cabot; BET
	surface area 210 m 2 g -1 , total pore volume 0.36 cm 3 g -1 ) was used as a catalyst support.
	Monometallic Ni/C was also synthesized for the comparison. After reaching the
	incipient wetness point the mixture was dried in vacuum (Т =100 °C, Р ≈ 2.7•10 3 Pa),
	then calcined in a He flow by increasing the temperature from 50 to 250 °C in a
	stepwise manner (50° steps, 15 min each) and further reduced with H2 at 250 ºC for 1

  The measurement was performed by A. Serov in the Center for Micro-Engineered Materials (CMEM UNM, Albuquerque, USA). The data were collected in the 2θ range between 3 and 60° with a 0.02° step size and 15 s per step acquisition time. Data were analyzed using MDI JADE software and the ICSD database. Average

	Bimetallic NiMo/C electrocatalyst
	Surface state of NiMo/C electrocatalyst was analyzed by XPS using a Kratos
	Axis Ultra DLD XP spectrometer equipped with a monochromatic Al Kα (150 W)
	source with no charge compensation. The measurements and XP spectra
	characterization were performed by K. Artyushkova in the Center for Micro-crystallite sizes were estimated from the coherent scattering region using Scherrer
	<5•10 -6 Pa) equipped with a VSW Class WA hemispherical electron analyzer and a Engineered Materials (CMEM UNM, Albuquerque, USA). Survey and Ni2p, Mo3d, equation.
	dual anode (Al and Mg Kα) X-ray source. The measurements and XP spectra C1s and O1s high-resolution spectra were acquired at pass energies of 80 eV and 20 2.2.5. CO chemisorption measurements
	characterization were performed by V. Papaefthimiou and V. A. Saveleva in the eV, respectively. A linear background subtraction was used for quantification of C1s CO chemisorption measurements were employed to estimate the dispersion of Ni
	ICPEES (Strasbourg, France). and O1s spectra, while a Shirley background was applied to Ni2p and Mo3d spectra. particles in the Ni/C and NiCu/C electrocatalysts. The measurements were performed
	collected in the 2θ range between 15 and 100° with a 0.05° step size and 2 s per step
	spectrometer was calibrated against Au4f7/2 (84.00 eV) and Cu2p3/2 (932.67 eV) acquisition time. The phase composition was determined using Powder Diffraction
	peaks. Catalyst powders (without additional grinding) were pressed tightly onto a File (PDF-2) ICDD and Inorganic Crystal Structure Database as well as Topas v. 4.3
	double sided adhesive conducting tape (3MTM). Residual gas pressure during the (Bruker AXS, Karlsruhe, Germany) software. Average crystallite sizes were estimated
	spectra acquisition was lower than 5•10 -6 Pa. A self-consistent spectra deconvolution from the coherent scattering region using Scherrer equation.
	employing the same peak parameters for Ni 0 , NiO and Ni(OH)2 components was

Survey and high resolution Ni2p, O1s, C1s spectra were recorded in constant pass energy mode

(44 and 22 eV, respectively)

. The charge effect arising in the process of photoemission of electrons was taken into account by correcting the spectra with respect to C1s of the carbon line (284.8 eV). XP spectra were curve fitted after Shirley background subtraction, following the procedure described above for polycrystalline Ni foil. Spectra analysis and their deconvolution were performed using CasaXPS software.

Bimetallic

NiCu/C and monometallic Ni/C electrocatalysts Surface state of Ni/C and NiCu/C electrocatalysts was analyzed by XPS using a SPECS electron spectrometer equipped with a PHOIBOS-150 MCD-9 hemispherical energy analyzer and monochromatic Al Kα source (200 W) with no charge compensation. The measurements and XP spectra characterization were performed by R. I. Kvon in the BIC (Novosibirsk, Russia). Prior to the measurements, the achieved for all processed samples. The values of binding energies and the areas of the peaks were determined after Shirley background subtraction. Spectral analysis and data processing were performed with XPS Peak 4.1 software package. Sensitivity factors provided by the manufacturer were utilized. Ni2p spectrum was fitted using constrained line shapes for Ni metal, NiO and Ni(OH)2 developed by Biesinger et al. [264] and peaks for NiMoO4 reported in the literature [268,269].

Mo3d spectrum was fitted using reference positions for metallic Mo, various Mo oxides and NiMoO4 reported in the literature

[270,271]

. Data analysis and quantification were performed using CasaXPS software.

2.2.4. X-Ray Diffraction (XRD)

Bimetallic NiCu/C and monometallic Ni/C electrocatalysts XRD data for Ni/C and NiCu/C catalysts were recorded on a Bruker D8 New (Germany) diffractometer equipped with a multistrip LynxEye detector using Cu Kα radiation (λ = 0.15418 nm). The measurements were performed by T. Yu. Kardash in the Boreskov Institute of Catalysis (BIC, Novosibirsk, Russia). The data were Bimetallic NiMo/C electrocatalyst XRD pattern for NiMo/C catalyst was obtained using a Scintag PAD V diffractometer in Bragg-Brentano geometry with a Mo X-ray source (λ Kα1 = 0.7093 nm). by P. A. Simonov in the Boreskov Institute of Catalysis (BIC, Novosibirsk, Russia).

In order to ensure complete reduction of the metal surface the samples were calcined in H2 flow at 250 °C immediately before the measurements. After that, the temperature was lowered to 20 °C and the sample was saturated with chemisorbed CO by introducing of 10% (vol.) CO + H2 mixture in the gas flow in a short pulse. Dispersion of Ni (DNi), defining as a ratio of surface Ni atoms capable to chemisorb CO to the total number of metal atoms in the sample, was calculated from the amount of chemisorbed CO, assuming that one surface Ni atom binds one CO molecule [272]. The dispersion can be then recalculated to the surface area of Ni (SCO, cm 2 ) following the equation: ] ^R = # _`a bc d bc H;fg`h bc . Dedicated experiments using pure Cu sample confirmed that Cu does not adsorb CO under the applied experimental conditions.

Table 3 . 1

 31 The values of the exchange current densities in the HOR/HER obtained for polycrystalline Ni rod after various pre-treatments of its surface

	Catalyst	SEC,

cm 2 Exchange current density j0, μA•cm -2 Ni j0, μA•cm -2 geom

	Non-activated Ni rod	0.52 ± 0.03	2.3 ± 0.1	6.0 ± 0.6
	Ch activated Ni rod	0.35 ± 0.03	20 ± 2	33 ± 5
	EC activated Ni rod	0.42 ± 0.04	14 ± 2	29 ± 3

Table 3 . 2

 32 The values of the exchange current densities in the HOR/HER obtained for non-activated and EC activated Ni rod at various temperatures

	Temperature, K		j0, µA cm -2 geom		Ea, kJ mol -1
		298	308	318	328	338
	Non-activated Ni	6.8	9.3	13.4	18.2	26.8	30 ± 1
	EC-activated Ni	25.3	37.5	52.1	66.4	76.7	26 ± 1

Table 3 . 2

 32 The values of the apparent Tafel slope, determined (with an accuracy of ± 1 mV)

at various temperatures Potential interval Tafel slope as a function of temperature, mV/decade 298 К 308 К 318 К 328 К 338 К

	Non-activated Ni	-0.20 < η < -0.07 V 0.06 < η < 0.12 V	115 246	116 214	116 203	123 180	129 172
	EC activated Ni	-0.20 < η < -0.07 V 0.06 < η < 0.12 V	125 211	128 214	130 212	131 219	130 218

Table 4 . 3

 43 The values of the specific electrocatalytic activities in the HOR/HER obtained for NiED/XC-72 particles after various pre-treatments of the electrode surface

	Catalysts	SEC /

cm 2 Exchange current density Kinetic current densities j0 / μA•cm -2 Ni io / A gNi -1 ik 50mV / A gNi -1 jk 50mV / µA cmNi -2

	Non-activated NiED/XC-72	13.0 ± 1.1	6.2 ± 0.6	5.7 ± 0.2	-	-
	After cycling NiED/XC-72	6.1 ± 1.4	29 ± 7	14.2 ± 3.1	27.8 ± 1.4	50 ± 4
	Ch activated NiED/XC-72	4.9 ± 0.9	56 ± 10	22.4 ± 4.3	32.1 ± 4.8	83 ± 12

Table 5 . 1

 51 Approximate fraction of Ni in the particles of different size determined for Ni1-xCux/C on the basis of the analysis of STEM images in dark field

		Ni fraction (at. %) in particles
	Catalyst	depending on their diameter dTEM (nm)
		dTEM < 5 5 ≤ dTEM ≤ 20	dTEM > 20
	Ni0.95Cu0.05	14-18	62-99 g	98-99
	Ni0.90Cu0.10	-h	10-83g	71-89
	Ni0.70Cu0.30	-h	2-6	83-89
	Ni0.50Cu0.50	-h	2-14	75-83
	Ni0.95Cu0.05 (lyo)	92	93	94

Table 5 . 2

 52 The concentrations (in at.%) of different types of Ni and Mo species present on the surface of Ni0.90Mo0.10/C as revealed by XPS

	Ni2p3/2	Ni 0 19.0	NiO 30.2	Ni(OH)2 39.4	NiMoO4 11.4
	Mo3d	Mo 0	MoO2	MoO2 / Mo(OH)2	Mo (V)	NiMoO4 / MoO3
		2.7	13.6	18.1	7.2	58.4
	5.3

.2. Electrochemical properties of NiMo/C electrocatalyst

Table 5 . 3

 53 The values of the exchange current densities in the HOR/HER obtained for Ni0.9Mo0.1/C electrocatalyst at various loadings on the surface of GC electrode

mMe+C / µг cm -2 geom SEC / cm 2 Ni Exchange current density j0 / µA cm -2 Ni io

				/ A gMe -1
	100	1.6 ± 0.2	27 ± 2	4.5 ± 0.2
	200	2.4	26	3.1
	400	4.7	25	2.9
	500	4.8	27	2.6
	Influence of Mo on the activity of Ni in the HOR/HER
	In order to better understand the origin of high HOR/HER activity of
	Ni0.90Mo0.10/C electrocatalyst, experimental CVs were analyzed with the help of
	microkinetic modeling (see Section 2.4). The employed parameters are shown in
	Appendix 2.

  It will allow one to compare different electrode materials providing guidelines for the development of systems with activities comparable to or even exceeding those of Pt-based electrocatalysts. Parameters used for the kinetic modeling at Т = 298 К

	Appendix 2. Parameter	Meaning	Dimension	Non-activated	Value Ch activated	Bimetallic
				Ni rod	Ni rod	Ni0.90Mo0.10/C
	CH2	H2 concentration	M	7 10 -4	7 10 -4	7 10 -4
	COH-	OH -concentration	M	0.1	0.1	0.1
	St	Density of Ni surface active sites	mol•cm -2	2.2 10 -9	2.2 10 -9	2.0 10 -8
	α2	Charge transfer coefficient of the Heyrovsky step	-	0.4	0.4	0.4
	α3	Charge transfer coefficient of the Volmer step	-	0.5	0.5	0.5
	k1 0	H2 adsorption rate constant by the Tafel step	cm 3 •s -1 •mol -1	3.94 10 7	1.53 10 6	6.3 10 4
	k-1 0	H2 formation rate constant by the Tafel step	s -1	0.010	0.220	1
	k2 0	H2 adsorption rate constant by the Heyrovsky step	cm 6 •s -1 •mol -2	8.7 10 9	2.5 10 9	1.1 10 9
	k-2 0	H2 desorption rate constant by the Heyrovsky step	s -1	0.016	0.1	0.3
	k-3 0	Had formation rate constant by the Volmer step	s -1	0.21 i	0.44 j	0.11 k
	k3 0	Had oxidation rate constant by the Volmer step	cm 3 •s -1 •mol -1	4.3 10 1 i	2.0 10 3 j	5.4 10 3 k
	k4 0	Ni-OHad formation rate constant	cm 3 •s -1 •mol -1	1.9 10 4	1.9 10 4	10 5
	k-4 0	Ni-OHad reduction rate constant	s -1	19.3	19.3	10
	E4 0	Ni-OHad formation potential	V	0.06	0.06	0.0
	k5 0	Ni-(OH)2,ad formation rate constant	cm 3 •s -1 •mol -1	30	30	11
	k-5 0	Ni-(OH)2,ad reduction rate constant	s -1	3.3	3.3	0.82
	E5 0	Ni-(OH)2,ad formation potential	V	0.18	0.18	0.17
	T	Temperature	K	298	298	298
	γ	Frumkin parameter		0	0	4

Appendix 3 .

 3 Parameters used for the kinetic modeling of CVs obtained for polycrystalline Ni rod at various temperatures

		Non-activated Ni rod			EC activated Ni rod
	Parameter
	298 К	308 К	318 К	328 К	338 К	298 К	308 К	318 К	328 К	338 К
	k1 0 ,

cm 3 s -1 mol -1

  4.74•10 8 3.24•10 8 2.27•10 8 1.63•10 8 1.19•10 8 4.42•10 6 4.95•10 6 5.16•10 6 5.11•10 6 5.31•10 6 •10 -3 6.3•10 -2 8.0•10 -2 9.4•10 -2 1.0•10 -1 1.2•10 -1 k2 0 , cm 6 s -1 mol -2 3.33•10 10 4.38•10 10 5.37•10 10 6.37•10 10 7.72•10 10 3.51•10 9 5.15•10 9 7.29•10 9 9.90•10 9 1.17•10 10 k-2 0 , s -1 6.76•10 -3 1.08•10 -2 1.58•10 -2 2.21•10 -2 3.13•10 -2 3.51•10 -2 5.48•10 -2 8.23•10 -2 1.18•10 -1 1.47•10 -1 k3 0 , cm 3 s -1 mol -1 1.71•10 1 2.98•10 1 5.18•10 1 7.74•10 1 1.26•10 2 9.44•10 1 1.60•10 2 2.49•10 2 3.34•10 2 3.95•10 2 •10 -2 1.05•10 -1 1.54•10 -1 1.96•10 -1 2.20•10 -1

	k-1 0 , s -1 2.8k-3 0 , s -1 2.8•10 -3 2.8•10 -3 2.8•10 -3 2.8•10 -3 0.56 0.85 1.24 1.56 2.17	6.62

k4 0 , cm 3 s -1 mol -1

  6.22•10 4 6.46•10 4 6.69•10 4 6.92•10 4 7.14•10 4 6.22•10 4 6.46•10 4 6.69•10 4 6.92•10 4 7.14•10 4 k-4 0 , s -1 6.43•10 1 6.19•10 1 5.98•10 1 5.78•10 1 5.60•10 1 6.43•10 1 6.19•10 1 5.98•10 1 5.78•10 1 5.60•10 1

	E4 0 , V	0.06	0.06	0.06	0.06	0.06	0.06	0.06	0.06	0.06	0.06

k5 0 , cm 3 s -1 mol -1 5

  .39•10 1 5.93•10 1 6.48•10 1 7.04•10 1 7.62•10 1 5.39•10 1 5.93•10 1 6.48•10 1 7.04•10 1 7.62•10 1

	Résumé de la thèse
	I. Introduction
	k-5 0 , s -1	1.86	1.69	1.54	1.42	1.31	1.86	1.69	1.54	1.42	1.31
	E5 0 , V	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15

List of abbreviations

List of abbreviations

Chapter 2: Materials and Methods

A Butler-Volmer dependence (eq. 1.16, see Section 1.1.1) on the electrode potential is assumed for the rate constants of charge transfer steps (eq. 2. 7 -2.10).

Langmuir (the Frumkin interaction parameter γ = 0) or Frumkin (γ = 4) adsorption isotherms were used for the surface coverages of Had, OHad and OH2,ad on Ni or NiMo, respectively. This allowed us to obtain good agreement between the experimental and the simulated curves.

The rates of the reactions ( 1) - (5) in alkaline electrolyte can be expressed as: ‹ " = $ " ; exp 1

(eq. 2.13) When the potential E is scanned at a sweep rate v, the simulated current-potential curves can be obtained by solving the following set of differential equations:

Influence of the temperature

In order to further advance in understanding of the processes occurring on the surface of a Ni electrode in the low-potential region, we inspected an influence of the temperature on the CV profile of both non-activated and EC activated Ni rod. If the anodic a1 and the cathodic c1 peaks correspond solely to the Ni/α-Ni(OH)2 redox transition (as suggested in a number of publications [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] Grden | A quartz crystal microbalance study on a metallic nickel electrode[END_REF][START_REF] Melendres | On the composition of the passive film on nickel: a surface-enhanced Raman spectroelectrochemical study[END_REF][START_REF] Simpraga | Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures[END_REF]), then the peak positions are expected to shift toward more negative or more positive potentials, respectively, with an increase of the temperature (due to acceleration of the charge transfer reaction [273]). As can be seen from the Fig. 3.5, shifting of the maximum of the peak a1 agrees with the expectation. At the same time, the maximum of the peak c1 in the case of non-activated Ni rod remains almost unaffected by the temperature.

Note that an increase of the temperature may lead to a deeper oxidation of the surface of the Ni rod during cycling. The latter may result in a shift of the cathodic peak c1

toward more negative values [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential[END_REF][START_REF] D'alkaine | Passivating films on nickel in alkaline solutions I. General aspects of the Ni (II) region[END_REF][START_REF] Samoilov | The study of oxygen adsorption on Ni by electrochemical method // Kinetika i Kataliz[END_REF] (i.e., in the opposite direction relative to the one expected with the temperature). To analyze a possible effect of this phenomenon a series of CVs was obtained at Т = 298 К in such a way as to reproduce the difference between the anodic potential limit and the maximum of the peak a1 observed in CVs obtained at various temperatures (Fig. 3.6). Inset in Fig. 3.5a shows that accounting of this factor allows one to observe a slight shift of the peak c1 in the expected direction with the temperature. However, the magnitude of the shift is still significantly different from that observed for the peak a1. Besides, it is interesting to note that in the case of EC activated Ni rod (see Fig. 3.5b), the peak a1 shifts up with temperature to a larger extent compared to non-activated electrode.

Considering the above analysis and the value of the standard potential for the

), it is reasonable to assume that there could be some other processes except of Ni/α-Ni(OH)2

redox transition, which influence the position of the anodic peak a1. More detailed analysis of this question will be presented in Section 3.3.3 with the help of kinetic modeling.

binding energy of H2Oad, which is weakly adsorbed on metallic Ni, is increased in the vicinity of adsorbed oxygen atoms [135,150,155]. This can affect the kinetics of the Volmer step, thus changing the activity of Ni in the HOR/HER.

Comparison of metallic and partially oxidized Ni electrodes

To

Estimation of the Ni surface area

The surface area of Ni in monometallic Ni/C and bimetallic Ni1-xCux/C catalysts was determined using two approaches: (i) by gas-phase CO chemisorption performed just after (before their passivation) the preparation of the samples (SCO, the procedure is described in Section 2.2.5) and (ii) by measuring of the anodic charge of the CVs (SEC) obtained in the potential range of [-0.02 -0.40 V] or [-0.12 -0.40 V] at a sweep rate of 20 mV•s -1 after subtraction of the charge related to the XC-72 carbon support.

Comparison of the obtained values is shown on the Fig. 5.7. First, we note that all methods essentially provide similar values for the specific surface area of Ni.

However, depending on the negative potential limit, the values of SEC are either slightly inferior (for EC = -0.02 V) or slightly superior (for EC = -0.12 V) to those of SCO. We believe that this discrepancy is caused by an underestimation of the SEC values in the first case (due to an incomplete reduction of formed Ni (hydr)oxide) or their overestimation in the second case (due to a contribution of currents corresponding to the oxidation of formed Ni hydrides), as was shown in Section 3.2.1.

Nevertheless, regardless the method of the determination, the comparison of the data in Fig. 5.7 allows to make several conclusions. First, addition of Cu in a small fraction allows one to increase specific surface area of Ni, while it decreases at high Cu content. Second, higher values of SCO were obtained for the samples prepared using

Appendices

Appendix 1 Summarized data regarding the electrocatalytic activity of various catalysts used in the thesis, as well as from the literature (values obtained at pH = 13)

Catalyst

SM / cm 2 j0 / µA cm -2 M io / A gM -1 ik 50mV / A gNi -1 jk 50mV / µA cmNi