Alexandr G Oshchepkov

Guillaume Braesch

Antoine Bonnefont

Elena R Savinova

Marian Chatenet

Antoine Bonnefont #

Recent advances in the understanding of Ni-based catalysts for the oxidation of hydrogen-containing fuels in alkaline media

Keywords: Nickel, Hydrogen oxidation reaction (HOR), Borohydride Oxidation Reaction (BOR), Ammonia Borane Oxidation Reaction (ABOR), Hydrazine hydrate oxidation reaction

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Recent advances in the understanding of Ni-based catalysts for the oxidation of hydrogen-containing fuels in alkaline media

Introduction

Recent years have witnessed tremendous increase in the interest towards electrocatalysis in alkaline medium. This is largely due to the success in the development of more potent anion exchange membranes, but also to the desire to replace scarce and expensive noble metals by more abundant platinum group metal (PGM)-free materials in fuel and electrolysis cells. [START_REF] Holdcroft | Sterically-Encumbered Ionenes as Hydroxide Ion-Conducting Polymer Membranes[END_REF][START_REF] Ramaswamy | Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer[END_REF][START_REF] Campos-Roldán | The Hydrogen Oxidation Reaction in Alkaline Medium: An Overview[END_REF][START_REF] Hu | Recent Progress Made in the Mechanism Comprehension and Design of Electrocatalysts for Alkaline Water Splitting[END_REF] Ni is a ubiquitous element widely applied in both heterogeneous catalysis and electrocatalysis due to its low cost, high abundance and high corrosion stability in alkaline media. [START_REF] Vesborg | Addressing the Terawatt Challenge: Scalability in the Supply of Chemical Elements for Renewable Energy[END_REF] In heterogeneous catalysis Ni is mostly used for hydrogenation and reforming of hydrocarbons, largely owing to its ability to dissociatively adsorb (similar to Pd and Pt) hydrogen providing M-Had active species, while being a much cheaper alternative. [START_REF] De | Ni-Based Bimetallic Heterogeneous Catalysts for Energy and Environmental Applications[END_REF] Ni is indispensable in energy storage/conversion systems, such as batteries and electrochemical capacitors, [START_REF] Ouyang | Progress of Hydrogen Storage Alloys for Ni-MH Rechargeable Power Batteries in Electric Vehicles : A Review[END_REF][START_REF] Salunkhe | Large-Scale Synthesis of Coaxial Carbon Nanotube/Ni(OH)2 Composites for Asymmetric Supercapacitor Application[END_REF][START_REF] Zhang | High-Performance Aqueous Rechargeable Li-Ni Battery Based on Ni(OH)2/NiOOH Redox Couple with High Voltage[END_REF] Li-ion batteries, [START_REF] Li | High-Voltage Positive Electrode Materials for Lithium-Ion Batteries[END_REF][START_REF] Yuan | Mixed Transition-Metal Oxides: Design, Synthesis, and Energy-Related Applications[END_REF] solar cells [START_REF] You | Improved Air Stability of Perovskite Solar Cells via Solution-Processed Metal Oxide Transport Layers[END_REF] where it is used in the form of (oxy)hydroxides, simple and complex oxides. In electrocatalysis, Ni-based materials are used for catalyzing oxygen [START_REF] Kim | Prospect and Reality of Ni-Rich Cathode for Commercialization[END_REF][START_REF] Roger | Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting[END_REF][START_REF] Mccrory | Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction[END_REF][START_REF] Moureaux | Timely-Activated 316L Stainless Steel: A Low Cost, Durable and Active Electrode for Oxygen Evolution Reaction in Concentrated Alkaline Environments[END_REF] and hydrogen evolution [START_REF] Wang | Precious Metal-Free Approach to Hydrogen Electrocatalysis for Energy Conversion: From Mechanism Understanding to Catalyst Design[END_REF][START_REF] Gong | A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction[END_REF][START_REF] Zeng | Recent Advances in Heterogeneous Electrocatalysts for the Hydrogen Evolution Reaction[END_REF] reactions. Ni is also widely encountered as a current collector/substrate for catalyst' deposition (see e.g. Refs. [START_REF] Miliauskas | Investigation of Borohydride 55 Oxidation at Ultrafine Gold Layer Supported on Nickel[END_REF][START_REF] Cao | A Direct NaBH4-H2O2 Fuel Cell Using Ni Foam Supported Au Nanoparticles as Electrodes[END_REF][START_REF] Celik | Investigation of Ni Foam Effect for Direct Borohydride Fuel Cell[END_REF] ). This review considers electrocatalytic properties of Ni as a promising anode material for fuel cells fed either with hydrogen or complex fuels (borohydride, boranes and hydrazine) in alkaline medium. We start by discussing electrochemical properties of Ni in alkaline medium. Under ambient conditions, Ni is prone to oxidation resulting in the formation of ca. 1-2 nm thick surface oxide layer composed of NiO and Ni(OH)2. [START_REF] Medway | In Situ Studies of the Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Lambers | Room-Temerature Oxidation of Ni(110) at Low and Atmospheric Oxygen Pressures[END_REF][START_REF] Holloway | Chemisorption and Oxide Formation on Metals: Oxygen-Nickel Reaction[END_REF] The presence of oxides on the Ni surface can greatly affect its catalytic properties, especially in low-temperature electrocatalysis, where, contrary to high temperature heterogeneous catalysis, their reduction may not be facile. Section 2.1. is devoted to the electrochemical oxidation of Ni in different potential intervals as well as hydrogen adsorption/absorption. Section 2.2. discusses the determination of the surface area of Ni electrocatalysts. This issue is of paramount importance for heterogeneous catalysis and electrocatalysis, since it forms the basis for benchmarking Ni-based electrocatalysts. Section 3 of the review summarizes recent advances in the understanding of electrocatalysis of the hydrogen oxidation reaction (HOR) on Ni, with a specific emphasis on how it is affected by the surface state of Ni. It considers the development of Ni and Ni-based electrocatalysts for the HOR and strategies used to prepare highly active materials for the anodes of alkaline fuel cells (AFC) and anion exchange membrane fuel cells (AEMFCs). Meanwhile, the hydrogen evolution reaction (HER) on Ni-based electrodes, which has been widely discussed and reviewed in the past [START_REF] Wang | Precious Metal-Free Approach to Hydrogen Electrocatalysis for Energy Conversion: From Mechanism Understanding to Catalyst Design[END_REF][START_REF] Gong | A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction[END_REF][START_REF] Lasia | Mechanism and Kinetics of the Hydrogen Evolution Reaction[END_REF][START_REF] Wang | Non-Noble Metal-Based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications[END_REF][START_REF] Zhu | Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles[END_REF] , is only briefly discussed, primarily in the potential region in the vicinity of the equilibrium potential of the hydrogen electrode, when it enters in competition with the oxidation of complex fuels. Section 4 is devoted to the oxidation of complex fuels, such as borohydride, boranes and hydrazine, which can be used in direct liquid fuel cells (DLFC). Finally, the stability issues of Ni-based electrocatalysts are briefly presented in Section 5.

Electrochemical properties of Ni electrocatalysts

Electrochemical behavior of Ni in alkaline media

Electrochemical properties of Ni electrodes have been widely studied since the beginning of the XX th century, starting from the works of Emelianova, [START_REF] Emelianova | Researches with the Dropping Mercury Cathode: Part VII. Nickel and Cobalt[END_REF] Frumkin et al., [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF][START_REF] Frumkin | Hydrogen Overvoltage[END_REF] Conway et al., [START_REF] Conway | The Electrochemical Behavior of the Nickel -Nickel Oxide Electrode: Part I. Kinetics of Self-Discharge[END_REF][START_REF] Bourgault | The Electrochemical Behavior of the Nickel Oxide Electrode: Part II. Quasi-Equilibrium Behavior[END_REF][START_REF] Conway | Electrochemistry of the Nickel Oxide Electrode: Part III. Anodic Polarization and Self-Discharge Behavior[END_REF][START_REF] Conway | Electrochemistry of the Nickel Oxide Electrode: Part IV. Electrochemical Kinetic Studies of Reversible Potentials as a Function of Degree of Oxidation[END_REF][START_REF] Conway | Electrochemistry of the Nickel Oxide Electrode[END_REF][START_REF] Conway | Electrochemistry of the Nickel-Oxide Electrode-V. Self-Passivation Effects in Oxygen-Evolution Kinetics[END_REF][START_REF] Sattar | Electrochemistry of the Nickel-Oxide Electrode-VI. Surface Oxidation of Nickel Anodes in Alkaline Solution[END_REF][START_REF] Conway | Electrochemistry of the Nickel-Oxide Electrode-VII. Potentiostatic Step Method for Study of Adsorbed Intermediates[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 [START_REF] Bockris | The Mechanism of Hydrogen Evolution at Nickel Cathodes in Aqueous Solutions[END_REF] as well as some others. The behavior of Ni in alkaline media has been recently summarized by the groups of MacDougall [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] and Jerkiewicz, [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature[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, Polarization Time, and Temperature[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[END_REF] who discussed in detail various potential-dependent processes taking place on the surface of Ni electrodes. In addition, Oshchepkov et al. have recently pointed out to the influence of the surface state of Ni electrodes on their HOR/HER activity in the potential interval -0.2 < E < 0.5 V vs RHE. [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][START_REF] Salmazo | On the Feasibility of Bifunctional Hydrogen Oxidation on Ni and NiCu Surfaces[END_REF][START_REF] Kuznetsov | Influence of the NaOH Concentration on the Hydrogen Electrode Reaction Kinetics of Ni and NiCu Electrodes[END_REF] Below the most relevant information regarding electrochemistry of Ni electrodes in alkaline media is briefly summarized.

In order to facilitate the discussion about characteristic features of cyclic voltammograms (CVs) of a Ni electrode (Figure 1a) as well as its electrocatalytic properties in a number of oxidation reactions, it is convenient to mark out two potential intervals, depending on the reversibility of the Ni oxidation. The latter include the "low-potential region" at -0.2 < E ≤ 0.5 V vs RHE, where the surface of a Ni electrode can be reversibly oxidized and reduced, and the "high-potential region" at 0.5 < E < 1.6 V vs RHE, in which the formation of irreversible, passivating, Ni oxides occurs, the degree of irreversibility depending on the anodic potential limit. The high-potential region can be additionally split into two sub-regions, namely 0.5 < E < 1.2 V vs RHE and 1.2 < E < 1.6 V vs RHE depending on the degree of Ni oxidation, as discussed further in the text. Below E = 0 V vs RHE and above ca. E = 1.5 V vs RHE the evolution of hydrogen and oxygen take place, respectively. In what follows, the processes taking place on Ni electrodes in the lowand the high-potential regions will be briefly discussed, mainly focusing on the former one, due to controversies existing in the literature. -1 and T = 25°C. The first CV in panel a (dash-dotted blue curve) covers the potential range from E = -0.2 to 0.5 V vs RHE, while two other CVs (first scan -solid black curve and second scandashed red curve) cover the potential range from E = -0.3 to 1.6 V vs RHE. The figure is inspired by some previous works 47,53-56 (b) CVs obtained for metallic and partially oxidized polycrystalline Ni electrodes in N2-saturated 0.1 mol L -1 NaOH at a sweep rate v = 20 mV s -1 and T = 25°C. Adapted from Ref. [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF] with permission from Springer.

α-Ni(OH)2 formation

It is generally accepted that the main process occurring in the low-potential region is the reversible oxidation of Ni with the formation of surface α-Ni(OH)2 during the anodic sweep, followed by its reduction in the cathodic sweep. [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, Polarization Time, and Temperature[END_REF][START_REF] Grdeń | A Quartz Crystal Microbalance Study on a Metallic Nickel Electrode[END_REF][START_REF] Conway | Realization of Monolayer Levels of Surface Oxidation of Nickel by Anodization at Low Temperatures[END_REF][START_REF] Melendres | On the Composition of the Passive Film on Nickel: A Surface-Enhanced Raman Spectroelectrochemical Study[END_REF] The common use of the term 'α-Ni(OH)2' (suggesting formation of a 3D structure with H2O molecules incorporated between Ni(OH)2 layers [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF] ) is explained by the fact that it likely grows on the Ni surface through formation of 3D islands of α-Ni(OH)2. This has been evidenced by 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] 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 M NaOH(Aq)[END_REF] as well as by the analysis of CVs at low (< 263 K) temperatures. [START_REF] Conway | Realization of Monolayer Levels of Surface Oxidation of Nickel by Anodization at Low Temperatures[END_REF] According to the place-exchange model proposed by Conway et al. [START_REF] Conway | A Surface-Electrochemical Basis for the Direct Logarithmic Growth Law for Initial Stages of Extension of Anodic Oxide Films Formed at Noble Metals[END_REF] and later adapted by Alsabet 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, Polarization Time, and Temperature[END_REF] The existence of intermediate Ni-OHad species was suggested on the basis of electrochemical (mainly cyclic voltammetry) [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] D'alkaine | The Passivating Films on Nickel in Alkaline Solutions I. General Aspects of the Ni (II) Region[END_REF] and in situ scanning tunneling microscopy [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 M NaOH(Aq)[END_REF] measurements.

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] Beden | A Voltammetric Study of the Formation on Hydroxides and Oxyhydroxides on Nickel Single Crystal Electrodes in Contact with an 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 M NaOH(Aq)[END_REF][START_REF] Keddam | Transpassive Dissolution of Ni in Acidic Sulfate Media: A Kinetic Model[END_REF][START_REF] Zoltowski | The Capacity of Monocrystalline Nickel Electrode in Potassium Hydroxide Solution at Low Hydrogen Overpotentials[END_REF][START_REF] Floner | Electrocatalytic Oxidation of Hydrogen on Polycrystal and Single-Crystal Nickel Electrodes[END_REF][START_REF] Esau | Controlled-Atmosphere Flame Fusion Growth of Nickel Poly-Oriented Spherical Single Crystals-Unraveling Decades of Impossibility[END_REF] . While early studies [START_REF] Weininger | Effect of Crystal Structure on the Anodic Oxidation of Nickel[END_REF][START_REF] Beden | A Voltammetric Study of the Formation on Hydroxides and Oxyhydroxides on Nickel Single Crystal Electrodes in Contact with an Alkaline Solution[END_REF][START_REF] Floner | Electrocatalytic Oxidation of Hydrogen on Polycrystal and Single-Crystal Nickel Electrodes[END_REF] did not find noticeable differences in the α-Ni(OH)2 formation for low-index Ni planes ( 111), ( 110), (100), recent work of Esau et al. clearly demonstrates the differences in the CV profile depending on the crystallographic orientation of the Ni surface. [START_REF] Esau | Controlled-Atmosphere Flame Fusion Growth of Nickel Poly-Oriented Spherical Single Crystals-Unraveling Decades of Impossibility[END_REF] In particular, the anodic peak potential of α-Ni(OH)2 formation shifts positive following the order (100) < ( 110) < ( 111), and an additional (pre-)peak at lower potential values shows up in the case of Ni (100) and Ni (110) planes (with a less intense pre-peak in the latter case). This observation agrees with DFT calculations, [START_REF] Mohsenzadeh | DFT Study of the Water Gas Shift Reaction on Ni(111), Ni(100) and Ni(110)[END_REF][START_REF] Kresse | First-Principles Study of the Adsorption of Atomic H on Ni (111), (100) and (110)[END_REF] which point to structure-sensitive adsorption of OH and other species on the Ni surface (vide infra). Meanwhile, the potential of the cathodic peak is very similar for the three Ni single crystal planes.

In addition to α-Ni(OH)2 discussed above, some works mention possible formation of NiO species sandwiched between the metal and the outer layer of α-Ni(OH)2. [START_REF] Medway | In Situ Studies of the Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Visscher | Anodic Oxide Films of Nickel in Alkaline Electrolyte[END_REF][START_REF] Grdeń | EQCM Studies on Oxidation of Metallic Nickel Electrode in Basic Solutions[END_REF] The amount of NiO formed during potential cycling in the low-potential region can be considered insignificant, as evidenced by in situ

Raman [START_REF] Melendres | On the Composition of the Passive Film on Nickel: A Surface-Enhanced Raman Spectroelectrochemical Study[END_REF] and IR spectroscopy [START_REF] Beden | The Anodic Layer on Nickel in Alkaline Solution: An Investigation Using in Situ IR Spectroscopy[END_REF] as well as ex situ 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[END_REF] Note however, that NiO might be also formed on the surface of Ni electrode upon contact with oxygen species, followed by partial transformation of this NiO layer into α-Ni(OH)2 after either contact with humid air or immersion in an aqueous alkaline electrolyte, thus resulting in a three-layer structure Ni | NiO | α-Ni(OH)2. [START_REF] Medway | In Situ Studies of the Oxidation of Nickel Electrodes in Alkaline Solution[END_REF] Finally, it is worth mentioning that, contrary to acidic medium, anodic dissolution and cathodic redeposition of Ni, which might be expected from Pourbaix diagram, [START_REF] Beverskog | Revised Pourbaix Diagrams for Nickel at 25-300[END_REF] have not been observed in alkaline electrolytes in the low-potential interval. [START_REF] Conway | 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] It is remarkable that for a Ni electrode, whose surface is partially covered by stable Ni oxides (e.g. NiO or -Ni(OH)2), the latter formed either through oxidation of Ni in air (i.e. chemical oxidation) or via excursions to high anodic potentials (i.e. electrochemical oxidation), the shape of CV changes substantially, an additional anodic peak showing up for 0.05 < E < 0.15 V vs RHE (Figures 1a and1b).

Such changes are reproducible and have been reported in several publications. [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Floner | Electrocatalytic Oxidation of Hydrogen on Polycrystal and Single-Crystal Nickel Electrodes[END_REF][START_REF] Visscher | Anodic Oxide Films of Nickel in Alkaline Electrolyte[END_REF][START_REF] Hu | Effects of the Nickel Oxide on the Hydrogen Evolution and Para-Nitroaniline Reduction at Ni-Deposited Graphite Electrodes in NaOH[END_REF][START_REF] Pshenichnikov | Electrocatalytic Properties of Nickel and Nickel-Based Alloys[END_REF][START_REF] Machado | The Hydrogen Evolution Reaction on Nickel Surfaces Stabilized by H-Absorption[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[END_REF][START_REF] Visscher | Absorption of Hydrogen in Reduced Nickel Oxide[END_REF][START_REF] Shumilova | Oxygen Ionization on Nickel in Alkaline Solutions[END_REF] The peak potential and the corresponding current density significantly depend on the surface state of the electrode, as well as on the cathodic potential limit of the CV. [START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][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[END_REF] The origin of that second anodic peak is still debated, some authors attributing it to the formation of Ni-OHad,, [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Floner | Electrocatalytic Oxidation of Hydrogen on Polycrystal and Single-Crystal Nickel Electrodes[END_REF] the other -to the oxidation of adsorbed/absorbed hydrogen, [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Hu | Effects of the Nickel Oxide on the Hydrogen Evolution and Para-Nitroaniline Reduction at Ni-Deposited Graphite Electrodes in NaOH[END_REF][START_REF] Pshenichnikov | Electrocatalytic Properties of Nickel and Nickel-Based Alloys[END_REF][START_REF] Karimi Shervedani | Evaluation of the Surface Roughness of Microporous Ni-Zn-P Electrodes by in Situ Methods[END_REF] as discussed below.

Hydrogen adsorption

Hydrogen adsorption is of paramount importance for the HOR/HER on metal electrodes (vide infra), and thus has widely been discussed in the literature (see Refs. [START_REF] Jerkiewicz | Hydrogen Sorption at/in Electrodes[END_REF][START_REF] Markovic | Surface Science Studies of Model Fuel Cell Electrocatalysts[END_REF][START_REF] Quaino | Volcano Plots in Hydrogen Electrocatalysis -Uses and Abuses[END_REF] and references therein). On Pt and other PGM electrodes, there has been convincing evidence for the existence of two types of phenomena, usually addressed as "under-potential" deposition of hydrogen (Hupd) at potentials above E = 0 V vs RHE and "over-potential" deposition of hydrogen (Hopd) at potentials below E = 0 V vs RHE. [START_REF] Jerkiewicz | Hydrogen Sorption at/in Electrodes[END_REF][START_REF] Markovic | Surface Science Studies of Model Fuel Cell Electrocatalysts[END_REF] While the presence of Hopd on Ni is commonly accepted, there are significant controversies in the literature regarding the existence of Hupd on Ni electrodes. Some researchers insist that, above E = 0 V vs RHE, a Ni electrode is covered by adsorbed oxygenated species (OHad / OH -ad / H2Oad) which prevent adsorption of Had. [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature[END_REF][START_REF] Grdeń | EQCM Studies on Oxidation of Metallic Nickel Electrode in Basic Solutions[END_REF][START_REF] Jerkiewicz | Hydrogen Sorption at/in Electrodes[END_REF][START_REF] Choquette | Investigation of Hydrogen Evolution on Raney-Nickel Composite-Coated Electrodes[END_REF][START_REF] Kreysa | Kinetic and Thermodynamic Analysis of Hydrogen Evolution at Nickel Electrodes[END_REF] Other publications present evidence for the existence of Hupd on the surface of Ni electrodes (even if it is still being debated). More specifically, Hu and Wen [START_REF] Hu | Effects of the Nickel Oxide on the Hydrogen Evolution and Para-Nitroaniline Reduction at Ni-Deposited Graphite Electrodes in NaOH[END_REF] This is in agreement with the adsorption isotherms calculated by Pshenichnikov for the hydrogen and oxygen adsorption from the gas phase. [START_REF] Pshenichnikov | Electrocatalytic Properties of Nickel and Nickel-Based Alloys[END_REF] Finally, some of us [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][START_REF] Salmazo | On the Feasibility of Bifunctional Hydrogen Oxidation on Ni and NiCu Surfaces[END_REF] studied the HOR on Ni by combining electrochemical measurements with microkinetic modeling, and concluded that assuming the existence of Hupd on Ni allows the most consistent explanation of a large amount of experimental data for the HOR on

Ni electrodes subjected to various surface treatments (reductive vs oxidative), as discussed in more detail in section 3 of this review.

Along with experimental works, there has been significant number of computational studies devoted to the adsorption of H, OH, O and other species on Ni. The binding energy values computed using density functional theory (DFT) and reported by different research groups, vary depending on the density functional used for the calculations. However, some general trends can be identified. Firstly, it is widely accepted that oxygen atoms (Oads) adsorb much stronger that the hydrogen atoms. [START_REF] Mohsenzadeh | DFT Study of the Water Gas Shift Reaction on Ni(111), Ni(100) and Ni(110)[END_REF][START_REF] Juarez | The Initial Stage of OH Adsorption on Ni(111)[END_REF][START_REF] Bai | Atomic and Molecular Adsorption on Ni(111)[END_REF] Ni(111) only slightly decreases from 0 to 1 monolayer coverage. [START_REF] Greeley | A First-Principles Study of Surface and Subsurface H on and in Ni(111): Diffusional Properties and Coverage-Dependent Behavior[END_REF] Meanwhile, Juares et al. [START_REF] Juarez | The Initial Stage of OH Adsorption on Ni(111)[END_REF] found that because of their high negative charge, OHad on Ni strongly repel each other, leading to a significant decrease of the adsorption strength with the coverage.

As a result, under certain conditions, adsorption of hydrogen may become more favorable compared to that of OHad species. Note also that, according to the DFT calculations, [START_REF] Dong | First-Principles Determination of Active Sites of Ni Metal-Based Electrocatalysts for Hydrogen Evolution Reaction[END_REF][START_REF] Liu | First-Principles Study of Oxygen Coverage Effect on Hydrogen Oxidation on Ni(1 1 1) Surface[END_REF] the state of the Ni surface (namely the presence of adsorbed oxygen in the vicinity of Ni 0 sites) largely affects the adsorption energy of the Had. This conclusion is also supported by microkinetic modeling [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF] of the HOR on Ni electrodes as discussed in the following sections.

In the end, if one acknowledges the existence of Hupd on Ni in the potential interval from E = 0 to 0.2 V vs RHE, the anodic CV peak at ca. E = 0.3 V vs RHE in Figure 1a should not be interpreted as α-Ni(OH)2 formation on free Ni surface sites but rather as Hupd replacement by OHad followed by the conversion of the latter into a surface α-Ni(OH)2 layer (Eqs. 2.2 and 2.3). In this context, it is instructive to recall hydrogen UPD on Pt, which until recently had been considered as a well-understood phenomenon.

Meanwhile, new evidence provided by electrochemical experiments, DFT calculations and surface science studies (see Janik et al. [START_REF] Janik | On the Presence of Surface Bound Hydroxyl Species on Polycrystalline Pt Electrodes in the "Hydrogen Potential Region" (0-0.4 V-RHE)[END_REF] and references therein) confirms that certain peaks observed in the potential interval of hydrogen UPD in fact do not correspond to Hupd, but rather to the replacement of adsorbed H atoms by OH on low-co-ordinated Pt sites.

Hydrogen absorption

There are numerous evidence of hydrogen absorption in the Ni lattice, which occurs at potentials significantly below E = 0 V vs RHE, especially upon prolonged water electrolysis conditions. 43,69,75,77,78,94- 96 Since hydrogen diffusion in the Ni lattice under ambient conditions is slow, in a transient (e.g. fast CV)

experiment the release of these hydrogen species from the Ni lattice may be detected as an anodic current at potentials above E = 0 V vs RHE, [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF][START_REF] Soares | Hydride Effect on the Kinetics of the Hydrogen Evolution Reaction on Nickel Cathodes in Alkaline Media[END_REF][START_REF] Cheng | Electrochemical Mechanics of Metal Thin Films: Charge-Induced Reversible Surface Stress for Actuation[END_REF] that is in the potential interval of α-Ni(OH)2 formation, further complicating interpretation of the CVs. Such a release of absorbed hydrogen at positive potentials has been evidenced by electrochemical quartz crystal microbalance measurements [START_REF] Grdeń | EQCM Studies on Oxidation of Metallic Nickel Electrode in Basic Solutions[END_REF] and in situ measurement of surface stress of a thin Ni film. [START_REF] Cheng | Electrochemical Mechanics of Metal Thin Films: Charge-Induced Reversible Surface Stress for Actuation[END_REF] It has also been noticed that oxidation of the Ni surface facilitates hydrogen absorption and related formation of Ni hydrides [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Floner | Electrocatalytic Oxidation of Hydrogen on Polycrystal and Single-Crystal Nickel Electrodes[END_REF][START_REF] Machado | The Hydrogen Evolution Reaction on Nickel Surfaces Stabilized by H-Absorption[END_REF][START_REF] Visscher | The Anodic Oxidation of Nickel in Alkaline Solution[END_REF][START_REF] Visscher | Absorption of Hydrogen in Reduced Nickel Oxide[END_REF] (in essence Ni-oxides are more disordered and thus less dense than Ni-metal, and therefore facilitate hydrogen insertion). However, in most cases, the contribution of absorbed hydrogen to the anodic currents at potential above E = 0 V vs RHE is negligible, especially when the cathodic limit of CVs lies above E = -0.2 V vs RHE.

Hydrogen absorption in Ni lattice has also been studied in surface science and heterogeneous catalysis.

For example, surface science studies of Comsa et al. provided a convincing evidence for the absorption and bulk diffusion of hydrogen (deuterium) into the Ni lattice. [START_REF] Comsa | The Angular Dependence of Flux, Mean Energy and Speed Ratio for D2 Molecules Desorbing from a Ni(111) Surface[END_REF] More recently, Ceyer's group performed high precision surface science experiments in order to imbed hydrogen atoms in the Ni lattice, detect their formation, and study their reactivity. [START_REF] Haug | Distinctive Reactivities of Surface-Bound H and Bulk H for the Catalytic Hydrogenation of Acetylene[END_REF] An excellent overview of the literature, published before 2001 and related to the hydrogen absorption in metals was performed by Ceyer. 100 In addition, Greeley and

Mavrikakis performed first-principles study and calculated the phase diagram of the hydrogen/Ni system, which suggest the formation of one monolayer of subsurface hydrogen at high H2 gas pressure. [START_REF] Greeley | A First-Principles Study of Surface and Subsurface H on and in Ni(111): Diffusional Properties and Coverage-Dependent Behavior[END_REF] Considering this phase diagram and by using the Nernst equation, it is possible to estimate an equilibrium potential for the conversion of hydrogen from the adsorbed into a sub-surface state. At 300 K this potential lies around E = -0.15 V vs RHE. This estimation is in agreement with the electrochemical studies evidencing hydrogen absorption in the Ni lattice at high cathodic polarization.

α-/β-Ni(OH)2 and Ni(OH)2/NiOOH transition

In the high-potential region, increase of the potential from E = 0.5 to 1.2 V vs RHE leads to dehydration of α-Ni(OH)2 and its irreversible transformation into β-Ni(OH)2, along with the formation and/or thickening of the NiO. [START_REF] Medway | In Situ Studies of the Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][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, Polarization Time, and Temperature[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[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 M NaOH(Aq)[END_REF]101 Formation of a stable β-Ni(OH)2 layer leads to an attenuation of the cathodic peak in the low-potential region (Figure 1a), firstly followed by its shift towards 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, Polarization Time, and Temperature[END_REF][START_REF] D'alkaine | The Passivating Films on Nickel in Alkaline Solutions I. General Aspects of the Ni (II) Region[END_REF]102 and finally, once β-Ni(OH)2 fully covers the electrode surface (red curve on Figure 1a), complete disappearance of the peak in the low-potential region. [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[END_REF][START_REF] Shumilova | Oxygen Ionization on Nickel in Alkaline Solutions[END_REF] Above E = 1.2 V vs RHE, α/β-Ni(OH)2 undergoes further transformation into γ/β-NiOOH, resulting in an emergence of the anodic peak around E = 1.4 V vs RHE (Figure 1a). The corresponding cathodic peak appears around E = 1.3 V vs RHE and at low sweep rates is often split into two components supposedly due to the presence of two distinct (β and γ) phases of the surface NiOOH oxyhydroxide [103][104][105] . At potentials above E = 1.5 V vs RHE, the surface of the Ni electrode is covered by NiOOH and NiO as evidenced by in situ Raman spectroscopy 101,[106][107][108][109] and XPS measurements (both ex situ and in situ) [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[END_REF]110,111 . The crystal plane-orientation dependence of Ni oxidation in the high-potential region has not been observed, likely because of the fast structure disordering at high potentials. 65

Determination of the active surface area of Ni

Correct determination of the electrochemical surface area (ECSA) of an electrode is of paramount importance for evaluation of its intrinsic electrocatalytic activity and comparison between various materials. However, facile oxidation of Ni and possible presence of various oxides (reversible or irreversible, vide supra) on its surface complicate its ECSA determination. Thus, there is still no generally accepted way to determine the ECSA, the latter depending on several factors discussed below. For electrocatalytic reactions occurring in the low-potential region (for example, the HOR), the adsorption of reagents typically takes place at metallic Ni sites, whose number is thus essential for determination of the specific electrocatalytic activity. As a consequence, the surface fraction covered by stable oxide species as well as the double layer capacitance between E = 0.5 and 1.2 V vs RHE significantly decrease, while the charge under the peaks corresponding to the Ni(OH)2/NiOOH transition changes only slightly, due to a small modification of the Ni surface roughness caused by cycling in a wide potential range [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF]112 .

At present, one of the most suitable approaches developed to estimate the ECSA of metallic Ni is based on the use of the full anodic charge under the peak of α-Ni(OH)2 formation [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature[END_REF][START_REF] Machado | The Hydrogen Evolution Reaction on Nickel Surfaces Stabilized by H-Absorption[END_REF] and the corresponding specific charge of 514 µC cm -2 experimentally determined with the aid of low index single crystal Ni(hkl) electrodes [START_REF] Beden | A Voltammetric Study of the Formation on Hydroxides and Oxyhydroxides on Nickel Single Crystal Electrodes in Contact with an Alkaline Solution[END_REF] . This value assumes transfer of 2 e -per Ni surface site and hence corresponds to the formation of a monolayer of α-Ni(OH)2. The cathodic charge of α-Ni(OH)2 reduction may also be considered to determine the ECSA [113][114][115] , but this requires correct subtraction of currents corresponding to the HER (which is not an easy task). In order to minimize the influence of side processes on the estimation of ECSA, it is recommended to record CVs with a cathodic potential limit between E = -0.06 and -0.12 V vs RHE (to minimize the formation of Ni hydrides) and anodic potential limit between E = 0.40 and 0.50 V vs RHE (to avoid formation of irreducible β-Ni(OH)2), at sweep rates between v = 10 and 50 mV s -1 . Note however that the described procedure has some inherent uncertainties. Firstly, the α-Ni(OH)2 layer may grow beyond a monolayer thickness, and its growth depends on the experimental conditions (sweep rate, upper potential limit, temperature, etc.), which affects the accuracy of the surface area determination [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature[END_REF] .

Furthermore, assuming that formation of surface α-Ni(OH)2 occurs via replacement of Hupd (see Section 2.1), the charge under the anodic peak in the low-potential region must also involve hydrogen desorption, further questioning the accuracy of the described procedure of the Ni surface area determination.

Another way to estimate the ECSA of metallic Ni is based on the measurement of the electrochemical double layer capacitance (CDL), either by electrochemical impedance spectroscopy (EIS) or by CV in a narrow potential interval (< 100 mV) slightly positive of E = 0 V vs RHE. Specific capacitance values of metallic Ni electrodes range from 20 to 25 μF cm -2 . [START_REF] Weininger | Hydrogen Evolution and Surface Oxidation of Nickel Electrodes in Alkaline Solution[END_REF][START_REF] Karimi Shervedani | Evaluation of the Surface Roughness of Microporous Ni-Zn-P Electrodes by in Situ Methods[END_REF]114,[116][117][118] Once the surface of a Ni electrode is (partially) oxidized, the CDL value changes. Thus, Grdeń et al. estimated the CDL value of a fully-oxidized Ni surface to be around 8 µF cm -2 114 in the potential interval between E = -0.2 and -0.25 V vs RHE.

However much higher values of 40 to 60 µF cm -2 have been reported at positive potentials (0.8 < E < 1.2

V vs RHE). [START_REF] Mccrory | Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction[END_REF][119][120][121] Such significant uncertainties in the specific capacitance values raise a question regarding the applicability of the estimated CDL values to measure the ECSA. [START_REF] Mccrory | Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction[END_REF]122 In order to determine the total surface area of a Ni electrode (i.e. including contribution both from beyond a monolayer, its thickness depending on the growth conditions. [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[END_REF] To overcome this complication, Hall et al.,123 suggested to use oxalate salt (acting as a stabilizing agent due to the formation of an oxalate ad-layer) and sufficiently high sweep rate v ≥ 150 mV s -1 to limit the thickness of surface Ni(OH)2 and

NiOOH to a single layer. This approach was later adopted to measure the ECSA of a Ni foam. 112 In this case sharp and narrow peaks for the Ni(OH)2/NiOOH oxidation-reduction processes were observed in the CVs, resulting in higher precision of the determined ECSA value of Ni assuming a specific charge of 195 µC cm -2 . However, the latter method can only be applied either after measuring the catalytic activity of an electrode, or in a separate experiment, since the presence of the oxalate salt in the electrolyte will most likely affect the electrocatalytic properties of Ni. Alternatively, Ho and Piron [START_REF] Soares | Hydride Effect on the Kinetics of the Hydrogen Evolution Reaction on Nickel Cathodes in Alkaline Media[END_REF] and later Lyons and

Brandon [START_REF] Szklarska-Smialowska | Electrochemical Study of the Nickel-Hydrogen System[END_REF] suggested to galvanostatically charge a Ni electrode in the potential interval of the oxygen evolution reaction (occurring on the NiOOH-covered surface) and then calculate charge passing during a cathodic current-time transient. This method assumes a specific charge of 420 µC cm -2 , although, as mentioned by the authors, the exact charge passing during the desorption of a full monolayer of the oxygen evolution reaction intermediates is uncertain, and may vary, in particular, depending on the type of the NiOOH surface phase (β or γ) at stake, which present somewhat different unit cell parameters and symmetries [START_REF] Hall | Nickel Hydroxides and Related Materials: A Review of Their Structures, Synthesis and Properties[END_REF]124 (not to mention that the nature of active oxygen evolution intermediates is still debated).

Recently, Watzele et al. 125 proposed a yet another strategy to determine the ECSA of metal oxide catalysts (including Ni) by measuring the so-called adsorption pseudocapacitance at the onset of the oxygen evolution reaction.

Finally, chemical methods to measure the surface area, while not giving the exact information about the ECSA, are still useful as they allow to calculate the full surface area of Ni. Among various techniques described by Trasatti and Petrii,126 CO and H2 chemisorption measurements for metallic Ni, or N2 physisorption measurements for Ni oxides can be mentioned. 122,127-129

Electrocatalytic properties of Ni-based electrocatalysts in the HOR in alkaline media

Introduction: electrocatalysis of hydrogen electrode reactions on Ni

The HOR/HER are among the most widely studied electrode processes. The mechanism of the hydrogen electrode reactions on metal electrodes ("M" in the Eqs. below stands for a metal surface site)

includes Volmer, Heyrovsky and Tafel steps, which in alkaline media can be written as follows [START_REF] Lasia | Mechanism and Kinetics of the Hydrogen Evolution Reaction[END_REF]130 : Since atomically adsorbed hydrogen (Had) acts as a reaction intermediate, the electrocatalytic activity in hydrogen electrode reactions is closely related to its strength of adsorption. [131][132][133][134] There has been convincing evidence that on Pt and other PGM electrodes, Hupd, which is adsorbed in multi-fold hollow sites, cannot act as the HOR/HER intermediate due to its strong binding to the surface, and that it is rather weakly adsorbed hydrogen (Hopd), likely adsorbed on-top, which participates in hydrogen electrocatalysis. [START_REF] Jerkiewicz | Hydrogen Sorption at/in Electrodes[END_REF][START_REF] Markovic | Surface Science Studies of Model Fuel Cell Electrocatalysts[END_REF][START_REF] Quaino | Volcano Plots in Hydrogen Electrocatalysis -Uses and Abuses[END_REF] For the most catalytically-active metals like Pt and Pd, the Gibbs energy of adsorption of this weakly-adsorbed hydrogen is close to zero. 131,135,136 On Ni, hydrogen in multi-fold hollow sites is adsorbed stronger than either on Pt or Pd. 131,135,136 Santos et al., using DFT, compared strongly-and weakly-adsorbed hydrogen on Ni, Pt and some other surfaces. 133 According to these calculations, and in agreement with other studies, strongly-adsorbed hydrogen binds to Ni(111) stronger than to Pt(111). In the meantime, for weakly-adsorbed hydrogen on Ni(111), adsorbed on top of a monolayer of the strongly-adsorbed hydrogen, GH  0.9 eV, which makes its participation in the HOR rather unlikely. In what follows, when discussing the HOR on Ni, it will be assumed that it is stronglyadsorbed hydrogen, which acts as a reaction intermediate on Ni, and it will simply be denoted as Had.

H2 + 2 M ⇄ 2 M-Had ( 3
While this conjecture requires additional confirmation, it agrees with an increase of the HOR on Ni-based electrodes with the decrease of the strength of hydrogen adsorption, the fact widely acknowledged in the literature. [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][137][138][139][140][141][142] Besides, experimental data (collected for PGM-based electrodes) evidence much slower hydrogen electrode kinetics in alkaline media than in acid. [143][144][145] To explain this experimental observation, the so called 'bifunctional' mechanism has been proposed, [START_REF] Gong | A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction[END_REF]146 which postulates that in order to be active in the HOR/HER in alkaline media, an electrode must provide two types of sites: one for Had adsorption and another one for water dissociative adsorption to form Had and OHad. We will come back to the feasibility of this 'bifunctional' mechanism on Ni.

Owing to the use of alkaline water electrolysis at the industrial level, studies of Ni-based electrocatalysts have been primarily carried out in the potential interval of the HER, while, until recently, the interest in the HOR on Ni has been rather limited. HER starts with the Volmer step, which provides hydroxyl ion and an adsorbed Had. Then, hydrogen molecule is produced via either recombination of two Had atoms (Tafel step) or interaction of Had and a water molecule (Heyrovsky step). The contribution of individual steps (3.1 -3.3) to the overall reaction mechanism depends on many factors such as the Ni surface state, the presence of a second element (metal or nonmetal) as well as the applied overpotential.

In particular, the HER on polycrystalline Ni follows Volmer-Heyrovsky mechanism if the electrode surface is dominated by metallic Ni sites [START_REF] Lukovtsev | Hydrogen Overpotential on Nickel[END_REF][START_REF] Diard | Etude de l'activation Du Degagement d'hydrogene Sur Electrode d'oxyde de Nickel Par Spectroscopie d'impedance[END_REF][START_REF] Machado | The Hydrogen Evolution Reaction on Nickel Surfaces Stabilized by H-Absorption[END_REF]118 , while the Volmer-Tafel mechanism was proposed in some publications where Ni electrodes were used likely in a partially-oxidized state of the surface. [START_REF] Kreysa | Kinetic and Thermodynamic Analysis of Hydrogen Evolution at Nickel Electrodes[END_REF]147,148 The latest progress in the field of the HER on Ni and Ni-based electrocatalysts, including the reaction mechanism, is summarized in several reviews [START_REF] Wang | Precious Metal-Free Approach to Hydrogen Electrocatalysis for Energy Conversion: From Mechanism Understanding to Catalyst Design[END_REF][START_REF] Gong | A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction[END_REF][START_REF] Lasia | Mechanism and Kinetics of the Hydrogen Evolution Reaction[END_REF][START_REF] Wang | Non-Noble Metal-Based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications[END_REF] and is beyond the scope of this paper. In this review we are primarily interested in the HOR and will mention the HER only when relevant (e.g. when it enters in competition with reactions of interest for this work). The HOR comprises the same elementary steps as the HER but in the reverse direction, the H2 molecule first dissociating with the formation of surface Had (through the Heyrovsky and/or Tafel step), the latter undergoing electrochemical oxidation with the formation of H2O (Volmer step). It should be noted however that the nature of the rate-determining step depends on the electrode potential, thus resulting in differences in the detailed reaction mechanisms for the HOR and the HER if considered at high anodic or cathodic potentials, correspondingly.

HOR on monometallic Ni electrodes

Literature data regarding the kinetics of the HOR on Ni (and HER at low cathodic overpotentials) are very contradictory, [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][START_REF] Floner | Electrocatalytic Oxidation of Hydrogen on Polycrystal and Single-Crystal Nickel Electrodes[END_REF][START_REF] Kreysa | Kinetic and Thermodynamic Analysis of Hydrogen Evolution at Nickel Electrodes[END_REF]118,147,[149][150][151] which may be attributed to the strong influence of the electrode pre-treatment and thus the state of the surface on the reaction kinetics. [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF]146,152 In the vicinity of the equilibrium potential, a purposely-reduced metallic Ni electrode (see black solid trace in Figure 2b) shows a very small HOR current, which then increases peaking at ca. E = 0.25 V vs RHE (i.e. close to the anodic peak position observed under N2 atmosphere), and then decays again. Oshchepkov et al. attributed this low activity of metallic Ni to the slow kinetics of the Volmer step likely due to the strong adsorption of Had species already mentioned above. [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF]131,132 Indeed, according to the results of kinetic modeling, surface coverage of metallic Ni with Had (H) is high under N2 atmosphere, and further increases in the presence of H2, adsorbed hydrogen atoms dominating on the surface until ca. E = 0.2 V vs RHE (Figures 2a and2b, for details a reader is referred to Ref. [START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF] ). Note that the obtained data are in a good agreement with hydrogen adsorption isotherm (from the gas phase) reported by Pshenichnikov. [START_REF] Pshenichnikov | Electrocatalytic Properties of Nickel and Nickel-Based Alloys[END_REF] Besides, according to Taylor et al. at pH = 13 the equilibrium potential for Had/H + is similar to 0.2 V, which is in agreement with the data of Figure 2a. [START_REF] Taylor | Theoretical Analysis of the Nature of Hydrogen at the Electrochemical Interface Between Water and a Ni(111) Single-Crystal Electrode[END_REF] At higher potential values, formation of -Ni(OH)2, which proceeds through the OH adsorption (see OH and (OH)2 traces in Figures 2), results in blocking of the Ni active sites. [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature[END_REF][START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Mohsenzadeh | DFT Study of the Water Gas Shift Reaction on Ni(111), Ni(100) and Ni(110)[END_REF][START_REF] Liu | First-Principles Study of Oxygen Coverage Effect on Hydrogen Oxidation on Ni(1 1 1) Surface[END_REF] Thus, considering the kinetics of the HOR on Ni electrocatalysts also requires accounting for the processes adsorption at the surface, both leading to an enhancement of the kinetics of the Volmer step (Figure 2). [START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][START_REF] Dong | First-Principles Determination of Active Sites of Ni Metal-Based Electrocatalysts for Hydrogen Evolution Reaction[END_REF] As a result, the HOR starts at lower overpotential, as evidenced by an emergence of the second anodic peak (labeled as a2 in Figure 3a) in CVs in H2 atmosphere. This peak shifts towards E = 0 V vs RHE with the NiOx coverage and an ensuing decrease of Ni-Had binding energy (Figure 3). It is instructive to compare H for a metallic and partially oxidized Ni surface (Figure 2). Indeed, H significantly decreases in the latter case (NiOx surface coverage is estimated to be ca. 14 % based on the data presented in Ref. [START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF] ), which goes along with the increase of the HOR current. While further (preferably in situ spectroscopic)

studies are required to unveil the surface state of Ni electrodes during the HOR and thus unambiguously confirm its influence on the reaction kinetics, it should be emphasized that the model proposed by

Oshchepkov et al. currently presents the most consistent explanation of the available literature data. [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Effect of Temperature and Surface Oxidation on the Kinetics of Hydrogen Electrode Reactions on Nickel in Alkaline Media[END_REF][START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][START_REF] Salmazo | On the Feasibility of Bifunctional Hydrogen Oxidation on Ni and NiCu Surfaces[END_REF][START_REF] Kuznetsov | Influence of the NaOH Concentration on the Hydrogen Electrode Reaction Kinetics of Ni and NiCu Electrodes[END_REF] Armed with this knowledge, 1). 137 It has been demonstrated that the existence of surface NiOx species (their estimated coverage varies between 48 and 58%) enhances the surface-and mass-normalized HOR exchange current densities from 6.2 to 56 μA cmNi -2 and from 5. Ni3N phase provides significant enhancement of the activity in the HOR with respect to a pure Ni electrocatalyst (Table 1). 139,140,142 DFT calculations suggest that in the presence of an interstitial N dopant, the absolute value of GH decreases, hence approaching zero (i.e. an optimal value for the HOR). 139,142 In addition, it was proposed that the activation energy for both water formation and water dissociation decrease at the interface between Ni3N and Ni follows, we will only discuss NiM systems with a non-precious second component (Table 1).

Oshchepkov et al. have studied NiCu/XC-72 electrocatalysts with a Cu fraction from 0.05 to 0.50 prepared by the incipient wetness impregnation. In this series, samples containing 5 wt.% of Cu showed the highest specific and mass activities in the HOR, which was at least 3 times higher compared to a pure

Ni/C electrocatalyst prepared by the same method. 127 By using freeze-drying, the same group prepared more homogeneous alloyed NiCu/XC-72 electrocatalysts with relatively narrow particle size distribution, in which the highest activity was again observed for the Ni0.95Cu0.05/XC-72 sample. 158 The specific activity of Ni0.95Cu0.05/XC-72 was found to be the same in both series (ca. 14 μA cmNi -2 ), while the mass activity of the sample prepared using freeze-drying increased by a factor of 1.5 (up to 2.5 A gM -1 ) due to a smaller average particle size. Similar electrocatalyst composition was also explored by Roy et al. 159 , who reported two times higher exchange current density (25 μA cmNi -2 ) for their Ni95Cu5/Ketjenblack electrocatalyst at however 2.5 times lower mass-normalized activity (Table 1). The latter is likely arising from the low ECSA of NiCu nanoparticles in Ref. 159 The authors also evaluated the performance of an AEMFC consisting of a Ni95Cu5/Ketjenblack anode and a Pd/XC-72R cathode separated by a Tokuyama A201 membrane, possessing the power density of ca. 160 and 350 mW cm -2 at 70 and 80°C, respectively, and 100 % relative humidity. 159 In addition, Wang et al. have studied Ni-Cu alloy films with different composition prepared using a combinatorial magnetron co-sputtering method. 160 The authors have found that the highest (4 times higher compared to a pure Ni sample) exchange current density in the HOR (34 μA cmNi -2 ) is reached for NiCu alloys with ca. 40 % Cu content. 160 The large difference in the optimum composition of NiCu nanoparticles and NiCu alloy films towards the HOR may be related to the particlesize dependent segregation of Cu atoms to the surface. 127,161 Indeed, Monte Carlo simulations revealed massive segregation of Cu to the surface of NiCu nanoparticles. [START_REF] Salmazo | On the Feasibility of Bifunctional Hydrogen Oxidation on Ni and NiCu Surfaces[END_REF] For example, for ca. 18 nm diameter NiCu nanoparticles with the 0.04 Cu fraction in the bulk, Cu surface fraction was as high as 0.5. ) being at least 2 times higher than the ones obtained for the reference Ni/C sample (Table 1). On the basis of DFT calculations the authors attributed the enhanced HOR performance of the CeO2(r)-Ni/XC-72 electrocatalyst to a bifunctional mechanism at the interface between Ni and CeO2 (Figure 5). It

Another additive used to tailor

3.4.Perspectives

Data presented in section 3.3 revealed significant progress in the development of PGM-free Ni-based electrocatalysts for the HOR in alkaline media within the last 5 years. The activities of the best materials approach those reported for much more expensive Pd/C electrocatalysts, while being only 10 times lower compared to those of Pt/C electrocatalysts. The most popular strategies used to enhance the performance of Ni in the HOR are based on its alloying with a second metal or incorporation of heteroatoms in either the Ni structure or in the carbon support. In both strategies the electronic effect of the additives on Ni is proposed as the key factor, which leads to the enhanced HOR activity due to weakening of the Ni-Had bond. At the same time, the values of the surface-and especially mass-normalized HOR activities of these alloyed and/or heteroatom-doped materials are inferior to those of the Ni-NiOx/XC-72 electrocatalyst with an optimized surface state of the Ni electrode. 137 This suggests that addition of a second metal and/or dopant, along with a possible electronic effect, might indirectly affect the energy of hydrogen and oxygen bonding to the surface through stabilization of a particular Ni-NiOx surface state, which possesses high activity in the HOR. In order to better distinguish between these two possibilities, in situ studies for monitoring the Ni surface state along with the measurement of the HOR activity are highly desirable.

Oxidation of complex fuels on Ni-based electrocatalysts

Section 3 of this review made it clear that Ni (and its alloys/composites) is a very popular electrocatalyst in hydrogen oxidation and evolution reactions, and that its activity greatly depends on the state of the Ni surface. Considering this, one may expect similar behavior of Ni in other electrocatalytic reactions occurring on the surface of metallic Ni sites and/or involving adsorbed Had species in the reaction mechanism. In the current section we discuss more "exotic" oxidation reactions in basic electrolytes, such as the oxidation of complex fuels: borohydride, boranes and hydrazine. All these molecules contain hydrogen in their structure and have been studied as promising fuels for DLFCs.

Borohydride

The borohydride (BH4 -) oxidation reaction (BOR, Equation 4.1) is thermodynamically possible 0.41

V negative of the equilibrium potential of the hydrogen electrode (Equation 4.2). This conveys large theoretical energy density and specific energy to direct borohydride fuel cells (DBFC), but requires complete valorization of the BH4 -fuel (recovering 8 electrons per BH4 -species) at potentials close to its reversible potential. In practice, this requires electrocatalysts which are very active for the BOR (Equation 4.1), but at the same time are poorly active for the heterogeneous hydrolysis of BH4 -(Equation 4.3 and 4.4) and for the HER (Equation 4.2).

BH4 -+ 8 OH -⇄ BO2 -+ 6 H2O + 8 e - E 0 = -0.41 V vs RHE (4.1)

H2 + 2 OH -⇄ 2 H2O + 2 e - E 0 = 0 V vs RHE (4.2) BH4 -+ H2O ⇄ BH3OH -+ H2 (4.3) BH3OH -+ H2O ⇄ BO2 -+ 3 H2 (4.4)
The first study dealing with the borohydride oxidation reaction at Ni electrodes was published by Elder in 1962. 168 As for the two other complex fuels mentioned above (hydrazine and boranes), the practical interest of such studies was linked to electroplating reactions. 169 Two decades later, the group of Suda published the first papers dealing with the BOR at Ni electrodes (based on commercial µm-size Ni powders mixed with a PTFE binder pasted on a Ni foam) in the context of the DBFC development. 170,171 They immediately spotted the main advantages and drawbacks of the BOR at Ni: "The open-circuit potential was found to be about 0.15-0.2 V more negative than the hydrogen potential, depending on the concentration of borohydride. The results of polarization measurements indicated that a high power density can be achieved for the borohydride/Ni system. However, the coulombic efficiency was found to be 50% or less due to hydrogen evolution (…) hydrogen gas was not only generated from the hydrolysis reaction, but also from the electrochemical reaction. The actual anodic reaction of borohydride on the Ni electrode was proved to be a four-electron process rather than an eight-electron one." 170 Wang et al.

later on confirmed these findings 172 : Ni fails to completely valorize the BH4 -fuel and leads to nonnegligible hydrogen escape 173 in the course of the BOR.

Based on this statement, there have been multiple studies exploring Ni-based (sometimes nanostructured) catalysts for the BOR. Many focused on alloying Ni with other metals, like lanthanum in hydrogen storage alloys, 174,175 boron, 176 cobalt, 177,178 coper, 179 cerium 180 or other rare Earths, 181 ruthenium, 182,183 palladium, either in composites 184,185 or alloys, 186 gold, either in composites, [START_REF] Cao | A Direct NaBH4-H2O2 Fuel Cell Using Ni Foam Supported Au Nanoparticles as Electrodes[END_REF]187 coreshell 188 or alloyed 189 nanoparticles, and platinum in composite 185,190 or alloyed [191][192][193][194][195][196] nanoparticles.

Most of these strategies have been reviewed in Refs. 197,198 , but none of them enabled to reach significant BOR currents at potentials below that of the reversible hydrogen electrode, and in many cases (except for NiCo alloys), the co-metal was governing the overall catalyst's behavior. Besides, the issue of Ni passivation upon incursion to positive (E > 0.4 V vs RHE) potentials or contact with oxygen (either during the electrode storage/preparation or while in an electrolyte) was often ignored, even for pure Ni catalysts,199 which renders comparison between different materials inconclusive. In essence, the existing literature prior to 2019 did not really enable to state whether nanostructured Ni could be a viable catalyst for DBFC anode.

The picture recently changed, when Oshchepkov et al. demonstrated that the state of surface of electrodeposited nanostructured Ni electrodes (NiED/C) had a tremendous influence on their BOR activity. 200 The authors took benefit of the slow HER at a reduced Ni surface (see Section 3), to exploit electrocatalytic activity of carbon-supported electrodeposited NiED/C towards the BOR: in other words, the net BOR oxidation current at potentials below E = 0 V vs RHE is larger when the electrode is less HER active (Figure 6). In result, significant BOR currents are monitored below E = -0.2 V vs RHE on a reduced metallic NiED/C (curve 3 in Figure 6b and 6a), the limiting current being already reached at E = 0 V vs RHE (a potential where most of the catalysts listed above as well as their PGM-based counterparts barely initiate the BOR). On the contrary, a partially-oxidized NiED/C demonstrates significantly lower BOR activity, which goes along with its higher HER activity (curve 2 in Figure 6b support. The latter was further modified by electrodeposition of Ni nanoparticles providing high active surface area and ensuring its metallic state (Figure 8). The maximum power density of the DBFC achieved using NiED/eNFT (Ni nanoparticles electrodeposited on an etched NFT) anode was as high as 470 mW cm -2 at ca. 0.58 A cm -2 , while the current density at the cell voltage of 1.0 V was more than two times higher compared to NiED/GDE anode (23 mA cm -2 vs 11 mA cm -2 after 12 hours, respectively).

These results clearly demonstrate that Ni-based catalysts, provided that they are duly-prepared and employed, can outperform PGM-based ones for the oxidation of complex fuels in alkaline conditions, which is promising for the development of PGM-free DLFCs.

Hydrazine

The use of Ni-based electrocatalysts (namely Ni salts and Ni boride "impregnated on carbon") for hydrazine oxidation dates back to 1970. 207 Note that as the oxidation of hydrazine for fuel cell applications is studied in aqueous electrolytes, this reactant is likely oxidized as hydrazine hydrate (hence the notation hydrazine hydrate oxidation reaction, HHOR, eq. 4.6).

N2H4 + 4 OH -⇄ N2 + 4 H2O + 4 e - E 0 = -0.33 V vs RHE (4.6)

In his seminal paper, Wiesener noted that the HHOR performance surpassed that of PGM catalysts, provided the Ni surface was "appropriately" activated (treatment with gaseous NH3 was the best solution according to the author). Many studies followed this initial report, aiming at optimizing the HHOR This selected literature review demonstrates that there is some merit to combine Ni with other elements, notably to improve its capability to remain in "reduced" state of surface. The strategy was further followed by preparing binary or ternary Ni-based catalysts in a combinatorial chemistry approach, 219 or oxide-based 220,221 catalysts, but the latter materials showed more positive reaction onset potentials and did therefore not enable to surpass catalysts where Ni was in its reduced state.

Then, many other materials were developed for the HHOR, including strategies to enhance the bubble release (N2 is the final product of the HHOR), via shape optimization (super-hydrophobicity was targeted) of the electrode material, when maintained in prolonged hydrogen evolution regime 245,246 ; the leaching of their residual impurities (e.g. Al and Zn) when the electrode is maintained at open-circuit (i.e. in discontinuous HER operation) or positive (e.g. HOR) polarization also accounts for their gradual loss of activity over time. 247 To counter these adverse effects, several doped Raney Ni alloys 243,244,248,249 and composites between Raney Ni and metal hydride alloys 250,251 have been tested, as well as addition of metal ions/complexes in the electrolyte (e.g. iron cations from the electrolyte deposit at the Ni surface and inhibit Ni hydrides formation, 246,252 while vanadium salts in the electrolyte enhance Ni hydride decomposition, 253 a positive effect in both cases). To date, these strategies were only partly successful, and there remains room for improving the durability and long-term activity of Ni-based electrodes in operation.

Figure 1 .

 1 Figure 1. (a) CVs of a polycrystalline Ni electrode in N2-saturated 0.1 mol L -1 NaOH at a sweep rate v

  the formation of -Ni(OH)2 proceeds through three consecutive steps: firstly, the discharge of OH -anions at the Ni surface and formation of adsorbed OH species (Ni-OHad), secondly, the incorporation of OH into subsurface between the first and the second layer of Ni atoms ((OH-Ni)quasi-3D lattice) and thirdly, chemisorption of the second monolayer of OH groups on the (OH-Ni)quasi-3D lattice: Ni + OH -⇄ Ni-OHad + e - (2.1, fast) Ni-OHad ⇄ (OH-Ni)quasi-3D lattice (2.2, rate determining step) (OH-Ni)quasi-3D lattice + OH -⇄ Ni(OH)2,ad + e - (2.3, fast)

  investigated electrocatalytic hydrogenation of p-nitroaniline in alkaline medium, and found out that the reaction occurs on Ni and on Pt electrodes roughly in the same potential interval, that is below ca. E = 0.2 V vs RHE. Considering formation of Hupd on Pt in this potential interval, they inferred the existence of adsorbed hydrogen on the surface of Ni under these conditions as well. Note that the authors of Ref.[START_REF] Hu | Effects of the Nickel Oxide on the Hydrogen Evolution and Para-Nitroaniline Reduction at Ni-Deposited Graphite Electrodes in NaOH[END_REF] demonstrated that prior oxidation of the Ni electrode surface at Т = 400°C leads to complete suppression of p-nitroaniline hydrogenation, and attributed this to the blocking of hydrogen adsorption by the formed passive layer of NiO, which cannot be electrochemically reduced due to likely insufficient electronic conductivity. In addition, several authors documented intersection (named "isopotential") points in the forward and backward scans of CVs recorded until different positive potential limits [for example, Hu and Wen,[START_REF] Hu | Effects of the Nickel Oxide on the Hydrogen Evolution and Para-Nitroaniline Reduction at Ni-Deposited Graphite Electrodes in NaOH[END_REF] Katic et al.[START_REF] Katić | The Electronic Structure of the α-Ni(OH)2 Films: Influence on the Production of the High-Performance Ni-Catalyst Surface[END_REF] ], which they attributed to α-Ni(OH)2 formation and hydrogen adsorption/desorption occurring in the same potential interval (above E = 0 V vs RHE) and competing for the Ni surface sites.

(

  such as β-Ni(OH)2 or NiO) may be discarded for the determination of the ECSA value. In contrast, for reactions occurring in the high-potential region (for example oxidation of various organic molecules as well as the oxygen evolution reaction), the active sites generally comprise oxidized Ni. In this case, utilization of the Ni/α-Ni(OH)2 potential region for the ECSA determination (as it is frequently done in the literature) may result in its significant underestimation, if the electrode surface has been (at least partially) covered by stable Ni oxides. This is evident from the comparison of CVs of a Ni disk (Figure.

  1a): after partial oxidation of the Ni electrode, the charge under the anodic peak of α-Ni(OH)2 formation

  metallic and from oxidized Ni sites),Grdeń et al. have suggested to complement CV (or CV plus EIS) measurements with XPS experiments.114 In their approach the XPS data are used to determine the ratio between the surface areas of oxidized and metallic Ni (r = Aox/Am), which is then applied to calculate the entire surface area of Ni. Note however that ex situ XPS data might not necessarily reflect the surface state of a Ni electrode established in situ after conditioning in a specific potential region. Besides, to the best of the authors' knowledge, XPS does not allow to differentiate between α-Ni(OH)2 and β-Ni(OH)2, the former species being reducible in the low-potential region, while the latternot.When Ni electrocatalysts are used for reactions occurring in the high-potential region (organics oxidation or oxygen evolution reaction), the active sites consist of Ni(II)/Ni(III) species. In this case, the most straightforward way to estimate the ECSA of a Ni electrode could be the determination of the charge associated with the Ni(OH)2/NiOOH transition. The difficulty, however, lies in the NiOOH layer growth

. 1 ,

 1 Tafel step) H2 + OH -+ M ⇄ M-Had + H2O + e - (3.2, Heyrovsky step) M-Had + OH -⇄ H2O + e -+ M (3.3, Volmer step)

  of the Ni surface hydroxide formation and reduction, since they occur in the same potential range as the HOR. Besides, the coexistence of both Ni-Had and Ni-OHad species on the electrode surface might result in their recombination with the formation of water through the following reaction ('bifunctional' of the latter step to the mechanism of the HOR has been shown to be unlikely on a pure metallic Ni surface because of the very strong adsorption of Had on Ni and thus the high activation barrier for the recombination reaction.[START_REF] Salmazo | On the Feasibility of Bifunctional Hydrogen Oxidation on Ni and NiCu Surfaces[END_REF] Nevertheless, the recombination step might contribute to the HOR on bimetallic NiM electrocatalysts and/or partially oxidized Ni electrodes, when the adsorption energies of Had and OHad are weakened.[START_REF] Oshchepkov | On the Influence of the Extent of Oxidation on the Kinetics of the Hydrogen Electrode Reactions on Polycrystalline Nickel[END_REF][START_REF] Salmazo | On the Feasibility of Bifunctional Hydrogen Oxidation on Ni and NiCu Surfaces[END_REF]

Figure 2 . 3 . 3 .

 233 Figure 2. Experimental anodic scans of CVs (black solid curves) for (a, b) metallic and (c, d) partially

7 to 22 . 4 A

 224 gNi-1 , respectively, compared to a reduced Ni/C electrocatalyst. The HOR kinetic current density of Ni-NiOx/XC-72 normalized by mass of Ni was estimated as 32.1 A gNi -1 at E = 50 mV vs RHE, which makes it the current state-of-the-art in the field of PGM-free electrocatalysts for the HOR. An improved HOR specific activity was also reported for Ni/NiO/C composites derived from the Ni-based metalorganic frameworks and ascribed by the authors to the optimal binding energies of both Had and OHad at the Ni/NiO interface as well as to the high level of carbon graphitization, the latter resulting in high conductivity.141 It has also been demonstrated, that the Ni/NiO/C composite possesses much better CO tolerance than a 20 wt.% Pt/C catalyst, the HOR activity of the latter in the presence of CO decaying below that of Ni/NiO/C within a couple of hours. Since most of hydrogen is currently produced through natural gas reforming, the presence of CO in the fuel stream cannot be fully avoided, not to mention that the carbon support itself can lead to CO-like species adsorption at the metal nanoparticles (see Section 5), hence to their "poisoning" at low potentials. Therefore, synthesis of CO-tolerant electrocatalysts is an important task in the development of AEMFCs.

Figure 3 .

 3 Figure 3. Comparison of the anodic scans of (a) experimental and (b) simulated CVs for Ni/NiOx

Figure 4 .

 4 Figure 4. Electrocatalytic performance of Ni3N/Ni/NF, Ni/NF, and Pt/NF (NF is a Ni foam) catalysts

  Ni activity in the HOR is molybdenum. Roy et al. reported a 50% Ni9Mo1/Ketjenblack electrocatalyst, which demonstrated high activity in the HOR in terms of both surface-(27 μA cmNi-2 ) and mass-normalized (4.5 A gM -1 ) exchange current densities.162 The authors assigned the high activity to the decrease of the binding energy of the hydrogen intermediate to the NiMo surface as compared to that of metallic Ni. In addition, they reported power density of 120 mW cm -2 for an AEMFC with the Ni9Mo1/Ketjenblack anode and 30 wt.% Pd/XC-72R cathode separated by Tokuyama A201 membrane at 70 °C and 70 % relative humidity. Note however that post-mortem XPS analysis revealed almost total disappearance of Mo (initially dominating the electrocatalyst surface) from the surface of a membrane electrode assembly after the AEMFC test. The fact that Mo leaching did not result in a noticeable decrease in the fuel cell performance raises doubts regarding the role of Mo in the electrocatalyst and the actual nature of active sites. Leaching Mo from NiMo samples has been also observed bySchalenbach et al. 163 , who studied NiMo alloys with different composition prepared by arc melting (more emphasis on the durability of Ni-based catalysts will be given in Section 5).Sheng et al. reported a ternary Co0.17Ni4.49Mo1 alloy prepared by electrodeposition with the HOR exchange current density of 15 μA cmNi -2 .164 The much higher activity of the latter compared to pure Ni electrocatalyst has been attributed to a favorable Ni-Had binding energy, while the effect of OHad species on the HOR was considered as insignificant. It has also been proposed that MoO2 species (if surviving at the surface of a Ni/MoO2 electrocatalyst) should accelerate the Volmer step of the HOR by promoting the OHad formation, while the nearby Ni sites serve for the hydrogen adsorption. 165 Tang et al. reported Ni-Ag alloys with various compositions prepared by electron-beam vapor codeposition. 166 DFT calculations predict a decrease of the Had binding energy on Ni3Ag hollow sites as compared to pure Ni, which should result in an enhanced kinetics of the hydrogen electrode reactions. The latter was indeed observed by the authors for the Ni0.75Ag0.25 sample in the HER, while the HOR current densities of NiAg alloys were found to be either similar (for Ni-rich samples) or inferior (for Agrich samples) to the pure Ni sample. Finally, by using CeO2(r)-Ni/XC-72 electrocatalyst prepared through decoration of Ni particles with partially reduced CeO2 during hydrothermal synthesis, Yang et al. investigated the influence of the oxygen vacancies in CeO2 on the kinetics of the HOR on Ni. 115 The CeO2(r)-Ni/XC-72 electrocatalyst demonstrated an improved activity in the HOR with the values of exchange current density (38 μA cmNi -2 ) and mass-normalized kinetic HOR current at E = 50 mV vs RHE (12.8 A gNi -1

  was suggested that CeO2 moieties on the surface serve as active sites for OHad adsorption (due to the presence of oxygen vacancies), while the nearby Ni sites are characterized by a weaker adsorption of Had as compared to pure Ni. Both of these factors should facilitate the HOR kinetics. 115 affects d-orbitals of Ni leading to weakening of the Ni-Had binding energy. Similar enhancement of the HOR activity of Ni through metal-support interaction was observed by Yang et al., who systematically evaluated performance of Ni nanoparticles supported on N-, B-and S-doped XC-72 carbon. 167 The activity trend was found to follow the sequence of Ni/SC > Ni/NC > Ni/BC > Ni/C (

  and 6a). Finally, a strongly-oxidized NiED/C is neither BOR nor HER active. This observation essentially explains the very contradictory results reported for Ni-based electrodes in the literature: in essence, most of these catalysts had an uncontrolled (likely strongly oxidized) state of the surface and could not lead to consequent BOR currents below E = 0 V vs RHE.To better understand the origin of high BOR activity of metallic Ni, DFT calculations were used to determine binding of key BOR intermediates onNi(111) and compare them with those of Pt(111), Au(111) and Pd(111).200 It was found that the binding of adsorbed BH4 with the metallic Ni surface is intermediate to that of Pt and Au. It is strong enough to allow for the B-H bond breaking at low overpotential but not too strong to result in surface poisoning by BHad species as on the Pt surface.173 These promises of reduced NiED/C catalysts were then confirmed in a DBFC configuration (Figure7); Ni nanoparticles could be electrodeposited directly on a carbon-based commercial gas diffusion electrode (GDE), mounted as the anode in a membrane electrode assembly with a Nafion cation-exchange membrane and a commercial Pt-based cathode, and tested in a DBFC unit cell.200 In these tests, the NiED/GDE outperformed a Ptblack anode in identical operating conditions: the Ni-based DBFC presented a higher open circuit voltage (OCV = 1.2 V against 1.0 V for the Pt-based DBFC), hereby confirming the possibility to catalyze BOR at potential below E = 0 V vs RHE, but also led to larger current densities at a given cell voltage. Overall, the power density of the Ni-based DBFC was ca. 2.5 times superior to that of the Pt-based DBFC, but larger hydrogen escape was monitored, owing to the inability of Ni to completely valorize produced H2. More recently Braesch et al. have further improved the DBFC performance by preparing Ni-based anodes with an open structure to facilitate mass-transfer of either borohydride or evolved hydrogen. 201 For this purpose, the authors used electrochemically-assisted etching in order to depassivate the surface of an initially-oxidized (because of storage under air) Ni felt (NFT)

Figure 6 .

 6 Figure 6. (a, b) CVs and (c) chronoamperommetry curves obtained for various NiED/C catalysts

Figure 7 .

 7 Figure 7. Characterization of NiED/GDE samples. SEM images of pure carbon cloth (SGL carbon,

Figure 9 .

 9 Figure 9. Comparison of the ABOR activity measured in 5 mmol L -1 AB + 0.1 mol L -1 NaOH at E =

  performance.Meibuhr et al. evaluated NiB/Ni electrodes for the HHOR208 and found that surfaces reduced for 1 hour in a mixture of 6% H2 in Ar at mild temperature (93°C) were the most durable in operation, which was explained by their optimal retention of boron in such a reduced NiB layer.209 Considering the above-cited study of the BOR (see section 4.1.), strong dependence of the HHOR activity on the extent of oxidation of the Ni surface might represent an alternative explanation. Ni (studied as a mechanically-polished Ni rod) is itself active for the HHOR, and shows two domains of activity: the first at low potentials around the Ni/α-Ni(OH)2 transition (0 < E < 0.4 V vs RHE), and the second one at higher potentials (above E = 1 V vs RHE) 203 . This suggests that reversible redox transitions of Ni/Ni(II) and Ni(II)/Ni(III) are involved in the HHOR at low and high potentials, respectively (the second domain being useless for electricity generation in a direct hydrazine fuel cell). Because practical fuel cell electrodes are necessarily porous, it soon became obvious that nanostructured Ni-based electrodes were mandatory. So, several studies evaluated carbon-supported Nibased electrodes for the HHOR in either neutral 210 or alkaline 211 conditions, and it became clear that the irreversible oxidation of Ni was a severe impediment for nanostructured materials. As such, many strategies have been employed to make Ni-based HHOR catalysts more robust. Amongst these strategies, forming composites (e.g. carbon-supported Ni + Pd 212 ) or alloys (e.g. unsupported NiZn 213 , NiCo 214 , NiLa 215 and NiMo 216 alloys) could be promising solutions. Their success was explained by a retardation of the Ni passivation 212 ; for example, in the case of the NiZn alloy, Ni alloying with Zn creates a Lewisbase catalyst in which Ni acts as an electron-pair donor inducing the adsorption and partial dehydrogenation of hydrazine, which was proposed to prevent (or at least retard) its passivation 213 (it could also be suggested that Zn provides anodic protection of Ni). For the NiCo alloy, the authors noted that Ni was more reduced at a given potential than in a monometallic Ni electrode, here again pointing to the importance of the state of the Ni surface for electrocatalysis of oxidation reactions. The importance of initially metallic Ni for high-activity HHOR was confirmed by Jeon et al.217 and Finkelstein et al.218 . In the case of NiLa "alloys", a positive influence of some La(OH)3 phase (the material is therefore a composite, at least in part) enhances the reaction kinetics by providing OH-species at lower potential values. Finally, for the NiMo catalysts, the non-negligible (< 15 at.%) molybdenum content was proposed "to stabilize the hydrazine N-N bond, preventing the chemical decomposition of hydrazine into ammonia, and improving the catalyst selectivity towards the complete (and desired) hydrazine oxidation into N2 gas".216

5 .

 5 Durability of Ni-based catalysts in alkaline environmentsStudies dealing with the degradation of Ni-based catalysts in alkaline environments are scarce. In the 1980s,Ross et al. performed systematic studies of corrosion of carbon black substrates to be used in oxygen evolution anodes in concentrated potassium hydroxide at a moderate temperature (T = 50°C).They noticed that while the presence of Co oxides (which were used as oxygen evolution catalysts) accelerated irreversible oxidation of an acetylene black support into CO and/or soluble organic compounds, 232 the presence of Ni oxide nanoparticles led to little or no effect on the corrosion rate of a graphitized carbon black.233 More recently, Davydova et al. investigated chemical and electrochemical stability of tailored Vulcan (VXCMAX22) carbon-supported monometallicNi and bimetallic Ni3M (M = Co, Fe, Cu, Mo) nanoparticles in supporting alkaline electrolytes, using a combination of in situ on-line ICP-MS (inductively coupled plasma mass spectrometry) with a flow cell and ex situ TEM (transmission electron microscopy), XPS (X-ray photoelectron spectroscopy) and XRD (X-ray diffraction).234 The authors noticed different behavior of the studied materials (Figure10a). On the one hand, two catalysts have shown non-negligible instability in potentiodynamic conditions: Mo intensely dissolves from Ni3Mo/C, while Cu in Ni3Cu/C is stable below E = 0.4 V vs RHE but undergoes noticeable electrochemical transient dissolution above this potential. On the other hand, pure Ni/C, Ni3Co/C and Ni3Fe/C showed negligible dissolution up to E = 0.7 V vs RHE. This absence of dissolution does however not grant stability of their HOR performance, since all catalysts lose their HOR activity upon incursions to the potential interval 0.05 mol L -1 KOH, T = 25 °C, v = 1 mV s -1 , ω = 1600 rpm. Reproduced from 234 with permission from the American Chemical Society.These results confirmed those of Zadick et al., obtained on a similar class of materials (Ni3M/ Ketjenblack 600J nanoparticles, with M = Pd, Co and Ag).205 The Ni-based nanoparticles showed neither sign of consequent metal dissolution nor of severe modification of their shape or of detachment from the carbon support, as revealed by identical-location transmission electron microscopy after potentiostatic (at E =100 mV vs RHE in a 5 mmol L -1 AB +0.1 mol L -1 NaOH electrolyte) or potentiodynamic (between 0.1 < E < 1.23 V vs RHE in 0.1 mol L -1 NaOH) polarization at T = 25°C (Figure11), for Ni3Co/ Ketjenblack 600J nanoparticles). While the borane oxidation activity did decrease after incursion of the electrodes to high potential values, it could be restored provided a proper re-activation procedure (incursion to low potential values in presence of the strong borane reducer) was adopted. This shows that combining a suitable reducing-potential and the presence of strong reducer in the electrolyte can depassivate Ni3Co nanoparticles (and even more easily noble-based Ni3Ag and Ni3Pd catalysts); the comparison with the results ofDavydova et al. 234 , shows that molecular hydrogen is not a sufficiently strong reducer to enable depassivation of this class of non-noble carbon-supported Ni-based catalysts (nanostructured Ni3M, M = Co, Fe, Cu, etc.).In any case, carbon-supported Ni-based nanoparticles do not seem to suffer consequent nanoparticles detachment from their carbon support, owing to the rupture of their binding to the carbon substrate by nanoparticles-assisted local corrosion of the carbon into carbonate species. While this mechanism of degradation seriously hampers the long-term stability of PGM-based nanoparticles in alkaline electrolytes,[235][236][237][238] it seems rather less operant in interface with an anion-exchange polymer electrolyte, 239 but no such data exists for Ni-based catalysts, at least to the authors' knowledge.

Figure 11 :

 11 Figure 11: Representative ILTEM micrographs of a Ni3Co/Ketjenblack 600J catalyst obtained before

Finally

  , although Ni electrodes show some material's stability in operation in alkaline environments, their progressive deactivation (e.g. by surface oxide or hydride formation) may pose a problem. Future studies should therefore aim at stabilizing such Ni surfaces for real applications, i.e. in conditions where the materials could be subjected to oxygen contact, alternation between low and high potential values, open-circuit holds, etc.6. ConclusionsIn this paper, the electrochemical properties of Ni electrocatalysts have been reviewed. Since Ni and its (oxy)(hydr)oxides are stable at high pH (while being unstable at low pH), the electrochemical behavior of Ni in alkaline media was particularly emphasized. As one moves from low to high potentials, the Ni surface is (i) covered by adsorbed hydrogen (Had) in the hydrogen evolution and likely also in the hydrogen oxidation potential region, (ii) covered by surface α-Ni(OH)2 and then (iii) β-Ni (OH)2, the latter being (iv) further oxidized into γ/β-NiOOH in the oxygen evolution region. The occurrence of Ni/α-Ni(OH)2 and α/β-Ni(OH)2/γ/β-NiOOH transitions in the "low" and "high" potential regions provide practical tools to determine the active surface area of Ni. Besides, these different potential-dependent states of the surface largely determine the activity of Ni electrodes towards fuel oxidation reactions. This is the case for the HOR in alkaline media, which is strongly accelerated on partially-oxidized Ni surfaces (mixed Ni/NiOx surface). Alloying/combining Ni with other elements or supporting Ni on well-chosen substrates, can also lead to a better HOR performance, and both strategies have been practically demonstrated in AEMFCs.Oxidation of complex fuels (borohydride, boranes, hydrazine) is also possible on Ni-based electrocatalysts, paving the way to PGMs-free direct liquid alkaline fuel cells. In these cases, Ni is interesting because it is possible to tune its surface state so that it catalyzes the desired reaction (e.g. the borohydride oxidation reaction) without promoting possible competing reactions (e.g. the hydrogen evolution reaction or the hydrolysis of the fuel species). It should be noted however that much work still needs to be done by applying in situ spectroscopic tools (e.g. FTIR), ab initio calculations, and kinetic modeling in order to better understand the nature of adsorbed species on Ni surface depending on the applied potential and mechanisms of oxidation of these fuel molecules on Ni electrodes, and propose viable solutions for the development of more active catalysts.Lastly, this selected review also addresses the durability of Ni-based catalysts in alkaline environments.It is notably shown that, unlike carbon-supported PGM-based electrocatalysts, Ni/C and NiM/C materials are rather robust against the detachment of the metal nanoparticles from the carbon support, and, depending on their composition, can be resistant to metal dissolution. Stability of Ni-based electrodes is however strongly dependent on the explored potential domain, and more work is necessary to clarify whether they are sufficiently stable under the AEMFCs and DLFCs operation conditions. In addition, it is clear that materials stability of Ni-based catalysts must not be confused with the performance stability of the corresponding electrodes. Indeed, passivation by Ni oxides or hydride formation is possible if the Ni-based electrode is brought to "inadequate" potential intervals during the operation. This means that,

  Oshchepkov et al. have recently reported a partially oxidized Ni-NiOx/XC-72 electrocatalyst prepared by electrodeposition technique, which possesses HOR specific activity at the level of a commercial Pd/C electrocatalyst (Table

  139,142 . As a result, the reported carbon-supported Ni3N electrocatalysts provide very high mass-normalized HOR kinetic current densities at E = 50 mV vs RHE , respectively from the Refs. 140 and 139 ), thus being close to the state-of-the-art Ni-NiOx/XC-72 electrocatalyst (32.1 A gNi -1 ).137 Notably, the Ni3N nanoparticles supported on either carbon or on Ni foam demonstrated very high resistance towards CO poisoning (Figure4)139,142 .

Moreover, the Ni3N/C shows a limiting HOR current densities up to E = 0.26 V vs RHE, which is ca. 0.1 V higher the values typically observed with Ni-based electrocatalysts.

[137][138][139][140][141][142]154

On the basis of ultraviolet photoemission spectroscopic measurements, this was attributed to a weaker oxygen adsorption on Ni3N as compared to pure Ni.

140

A decrease of the propensity of Ni towards oxidation was also reported by Gao et al. for a Ni core-hexagonal boron nitride (h-BN) shell (Ni/h-BN/KB electrocatalyst).

129

This was achieved via formation of a few layers of a defective hexagonal BN shell, which protected the Ni core from an excessive oxidation in air, yet allowing H, OH, and H2O to interact with the Ni surface. In addition, it was suggested that the h-BN shell weakened Had adsorption on the Ni surface, resulting in a six times higher HOR exchange current density measured for Ni/h-BN/KB as compared to pure Ni/KB sample (Table

1

).

  Ni nanoparticles is based on alloying them with other metals. In order to take full advantage of the AEMFC concept, noble metal additives to Ni electrocatalysts should be avoided. Besides, alloying Ni with metals like Pt, Pd and Ir does not allow to decipher the influence of the noble metal (being more active than Ni) on its activity in the HOR. Instead, Ni serves as an additive helping to further improve the activity of noble metals in the HOR, presumably by providing surface sites for adsorbing hydroxyl species[155][156][157] or by tuning the hydrogen binding energy.134 Considering the above-mentioned reasons, in what

	Bimetallic NiM electrocatalysts
	Another (and the most popular) approach to modify the catalytic properties of materials and, in
	particular,
	1 mol L -1 KOH saturated with H2 or a mixture of H2 and 2% CO (v/v). (d) Chronoamperometry
	curves of Ni3N/Ni/NF and Pt/NF in 0.1 mol L -1 KOH saturated with a mixture of H2 and 2% CO (v/v)
	measured at E = 0.09 V vs. RHE. All results were collected without iR compensation. The current densities
	were calculated on the basis of the geometric electrode areas. Reproduced from Ref. 142 with permission
	from Springer Nature

Table 1

 1

	) and was

Table 1 .

 1 Comparison of the HOR activity of PGM-free Ni-based electrocatalysts in

alkaline media reported between 2014 and 2019. Notations: dNiM (TEM) is the mean size of Ni(M) nanoparticles determined by TEM, T is the temperature (RT means room temperature), ECSA is the electrochemically active surface area normalized by the mass of Ni, j0 and i0 are the surface-and massnormalized exchange currents, jk 50mV and ik 50mV are the surface-and mass-normalized HOR kinetic currents at E = 50 mV vs RHE

Electro- catalyst Loading, mgNiM cmgeom -2 dNiM (TEM), nm Electrolyte T, °C ECSA, m 2 gNi -1 j0, µA cmNi -2 i0, A gM -1 ik 50mV , A•gNi -1 jk 50mV , µA•cmNi -2 Ref. Influence of the surface state and doping by heteroatoms

			27% Ni/KB	-	10-15	0.1 mol L -1 NaOH	25	15.34 (H2 chemisorp tion)	4	0.61	-	-	129
			32.5% Ni/h-BN/KB	0.25	10-15	0.1 mol L -1 NaOH	25	15.20 (H2 tion) chemisorp	23	3.5	-	-
			45% Ni/C	0.17	33.1	0.1 mol L -1 KOH	30	7.1±0.8	13.8 ± 5.0 0.97 ± 0.25	-	-
			54% Ni/BC	0.17	17.7	0.1 mol L -1 KOH	30	15.4±0.5 13.0 ± 4.7 1.99 ± 0.73	-	-	167
			55% Ni/NC	0.17	16.5	0.1 mol L -1 KOH	30	16.1±0.3 29.9 ± 5.1 4.82 ± 0.83	-	-
			55% Ni/SC	0.14	14.2	0.1 mol L -1 KOH	30	16.9±0.9 40.2 ± 2.8 7.44 ± 0.88	11	-
			60% Ni/KB	0.06	12	0.1 mol L -1 NaOH	25	24.5 ± 0.6 28.6 ± 0.3 7.02 ± 0.1			128
			52.3% Ni/XC-72	0.16	15.1	0.1 mol L -1 KOH	30	-	16 ± 2	-	5.89±0.71	-	115
			46.1% CeO2(r)-Ni/XC-72	0.14	11.9	0.1 mol L -1 KOH	30	-	38 ± 2.2	-	12.8±0.27	-
			50% Ni9Mo1/KB	0.26	5-50	0.1 mol L -1 NaOH	25	16.9 ± 1.1	27 ± 2	4.5 ± 0.2	-	-	162
			Co0.17Ni4.49Mo1	-	-	0.1 mol L -1 KOH	20	-	15	-	-	44 ± 5	164
	Influence of a second metal	25% Ni/XC-72 25% Ni0.95Cu0.05/XC-72 50% Ni0.95Cu0.05/KB Ni Ni6Cu4	0.025 0.025 0.51 --	36.5 29.8 22±9 --	0.1 mol L -1 NaOH 0.1 mol L -1 NaOH 0.1 mol L -1 KOH 0.1 mol L -1 KOH 0.1 mol L -1 KOH	25 25 RT 25 25	5.1 ± 0.6 10.8 ± 0.1 0.55 ± 0.06 20.6 14 ± 1 2.5 ± 0.2 3.7 ± 0.2 25 ± 1 0.9 ± 0.1 1.81 cm 2 16 -3.3 cm 2 34 -	-----	-----	158 159 160
			43% Ni/VXCMAX2 2	0.09	-	1 mol L -1 KOH	25	1.3±0.2	39	0.83	-	-
			37% Ni3Fe1/ VXCMAX22	0.07	ca. 10	1 mol L -1 KOH	25	1.2±0.6	60	0.96	-	-	154
			45% Ni3Cu1/ VXCMAX22	0.09	-	1 mol L -1 KOH	25	1.2±0.2	51	0.98	-	-
			47% Ni3Co1/ VXCMAX22	0.09	-	1 mol L -1 KOH	25	1.5±0.3	20	0.66	-	-
			42% Ni/XC-72	0.07 ± 0.01	10	0.1 mol L -1 NaOH	25	92.4 ± 6.3 6.2 ± 0.6	5.7 ± 0.2	-	-	137
			42% Ni-NiOx/XC-72	0.07 ± 0.01	10	0.1 mol L -1 NaOH	25	46.7 ± 8.6	56 ± 10	22.4 ± 4.3 32.1 ± 4.8	83 ± 12
			63.2% Ni/NiO/C	0.32	0-60	0.1 mol L -1 KOH	-	-	26	-	5	-	141
			Ni3N	0.28	-	0.1 mol L -1 KOH	-	-	1.73	-	-	-	140
			51.4% Ni3N/XC-72R	0.16	4.6 ± 1	0.1 mol L -1 KOH	-	-	14	12	24.38	-
			71.7% np-Ni3N/C	0.16	5-20	0.1 mol L -1 KOH	RT	-	-	10.3	29.75	-	139
	Influence of a	support	Ni NPs 70% Ni/CNT 70% Ni/N-CNT	0.25 0.25 0.25	--20	0.1 mol L -1 KOH 0.1 mol L -1 KOH 0.1 mol L -1 KOH	RT RT RT	-10.4 12.4	1.3 9 28	0.15 1 3.5	0.28 1.9 9.3	2.4 18 75	113

  222,223 or combination of Ni with Se,224,225 noble metal,226,227 phosphorus, 228,229 or sulfur.224,230 As a matter of fact, none of these strategies resulted in catalysts which really outperform those prepared by the more conventional routes presented above. As such, real direct hydrazine fuel cell tests were performed in automotive conditions with the unsupported Ni-Zn catalysts, moreover, a demonstration vehicle was launched with this catalyst 231 , clearly highlighting that Ni-based catalysts are practical and good candidates to replace PGM ones for automotive applications.

ACKNOWLEDGMENTS

This work has been performed in the frame of the MobiDiC project, funded by the French National Research Agency (ANR, grant No. ANR-16-CE05-0009-01) and RSF project No. 18-73-00143, funded by the Russian Science Foundation. Some of the work presented herein has been performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" No. ANR-10-LABX-44-01.

although Ni catalysts do really present strong assets for use in fuel oxidation reactions, their practical deployment in long-lasting real devices still requires research and development.

Corresponding Authors

* E-mail: Oshchepkov@catalysis.ru * E-mail: Marian.Chatenet@grenoble-inp.fr

Extended and unsupported Ni surfaces, also show some degree of instability upon electrochemical operation in alkaline electrolytes. As detailed in Section 2, Ni surfaces are subjected to complementary adverse effects, which can affect their electrocatalytic properties [START_REF] Hall | The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution[END_REF] : below E = 0 V vs RHE, they form Ni hydrides (α-NiHx and β-NiHx depending on the applied conditions) [START_REF] Bernardini | Formation of Nickel Hydrides by Hydrogen Evolution in Alkaline Media: Effect of Temperature[END_REF]240 ; above E = 0 V vs RHE (and in particular at open-circuit conditions), Ni (hydr)oxides are formed (e.g. reversible α-Ni(OH)2, which mostly reduces back to Ni on subsequent cathodic polarization, but also β-Ni(OH)2, which cannot be electrochemically reduced back into metallic Ni, and at higher potentials undergoes reversible oxidation into β/-NiOOH [START_REF] Alsabet | Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature[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, Polarization Time, and Temperature[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[END_REF]114 ), which may deactivate the electrode if uncontrolled. Sections 3 and 4 presented the means to optimize the electrode activity for the HOR [START_REF] Oshchepkov | Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media[END_REF]137 oxide formation led to their progressive deactivation, overall gradually depreciating the HOR performance in operation. Raney Ni is particularly used for the HER in alkaline environments. 243,244 This catalyst, obtained from dealloying NiAl or NiZn alloys, is known to deactivate owing to the formation of hydrides

Author Contributions

All authors contributed to the analysis of the referenced papers, writing and reviewing of the manuscript.

Notes

The authors declare no financial and non-financial competing interests.