Alexandr G Oshchepkov

Guillaume Braesch

Salem Ould-Amara

Gholamreza Rostamikia

Gaël Maranzana

Antoine Bonnefont

Vasiliki Papaefthimiou

Michael J Janik

Marian Chatenet

Elena R Savinova

Antoine Bonnefont #

Nickel Metal Nanoparticles as Anode Electrocatalysts for Highly Efficient Direct Borohydride Fuel Cells

Keywords: Borohydride Oxidation Reaction (BOR), Direct Borohydride Fuel Cell (DBFC), Nickel, Platinum, Palladium, Density Functional Theory

published or not. The documents may come

INTRODUCTION

Sodium borohydride (NaBH4) is a promising carbon-free energy carrier, owing to its high energy density, non-toxicity, facile storage and transportation (in liquid or solid form), and wide availability 1 . BH4 -can be directly used as a fuel in direct borohydride fuel cells (DBFCs), by feeding base-stabilized NaBH4 solution to the anode [START_REF] Wang | Efficient PH-Gradient-Enabled Microscale Bipolar Interfaces in Direct Borohydride Fuel Cells[END_REF] , where the borohydride oxidation reaction (BOR) occurs:

BH 4 -+ 8 OH -→ BO 2 -+ 6 H 2 O + 8 e - E 0 = -0.41 V vs RHE (1)
Alternatively, NaBH4 can be used as a source of hydrogen, which can be released upon reaction with water (Equations 1, 2) and further used in a hydrogen-fed fuel cell, e.g. a proton exchange membrane FC (PEMFC).

BH 4 -+ H 2 O → BH 3 OH + H 2 (2)
BH 3 OH + H 2 O → BO 2 -+ 3 H 2 (3)
Compared to state-of-the-art PEMFCs, DBFCs exhibit high theoretical open-circuit voltage (1.64 V for a DBFC vs. 1.23 V for a PEMFC), and are compatible with non-noble metals, while platinum-group metal (PGM) electrocatalysts are typically required in PEMFCs. This makes DBFCs a competitive alternative to PEMFCs for mobile and portable applications, where high energy-density is targeted.

However, state-of-the-art DBFCs are currently using PGM electrocatalysts, at least at one of their electrodes [START_REF] Dekel | Review of Cell Performance in Anion Exchange Membrane Fuel Cells[END_REF][START_REF] Olu | Evaluation of Anode (Electro)Catalytic Materials for the Direct Borohydride Fuel Cell: Methods and Benchmarks[END_REF] , and cannot deliver an open-circuit voltage exceeding that of PEMFCs, due to the competing faradaic hydrogen evolution reaction (HER, Equation 4), which occurs at low anode potentials, and to the non-faradaic heterogeneous hydrolysis of BH4 -(Equations 2, 3). These processes result in hydrogen escape [START_REF] Olu | Evaluation of Anode (Electro)Catalytic Materials for the Direct Borohydride Fuel Cell: Methods and Benchmarks[END_REF][START_REF] Olu | Influence of the Concentration of Borohydride towards Hydrogen Production and Escape for Borohydride Oxidation Reaction on Pt and Au Electrodes -Experimental and Modelling Insights[END_REF][START_REF] Braesch | Borohydride Oxidation Reaction Mechanisms and Poisoning Effects on Au , Pt and Pd Bulk Electrodes : From Model ( Low ) to Direct Borohydride Fuel Cell Operating ( High ) Concentrations[END_REF] and a decrease of the faradaic efficiency of the fuel conversion.

2 H 2 O + 2 e -→ 2 OH -+ H 2 E 0 = 0 V vs RHE (4) The competition between BOR, HER, and borohydride hydrolysis depends on: (i) the composition of the electrocatalyst and (ii) the anode potential. High anode overpotentials favor BOR kinetics (unless the electrocatalyst is poisoned by reaction intermediates or passivated by oxides [START_REF] Olu | Influence of the Concentration of Borohydride towards Hydrogen Production and Escape for Borohydride Oxidation Reaction on Pt and Au Electrodes -Experimental and Modelling Insights[END_REF][START_REF] Braesch | Borohydride Oxidation Reaction Mechanisms and Poisoning Effects on Au , Pt and Pd Bulk Electrodes : From Model ( Low ) to Direct Borohydride Fuel Cell Operating ( High ) Concentrations[END_REF][START_REF] Finkelstein | Self-Poisoning during BH4 -Oxidation at Pt and Au, and in Situ Poison Removal Procedures for BH4 -Fuel Cells[END_REF] ) but are of little practical interest. The strength of BH4 -adsorption and thus the electrocatalytic activity depend on the nature of the catalytic material. Noble metals like Pt, Pd, Ir, etc. [START_REF] Rostamikia | Direct Borohydride Oxidation: Mechanism Determination and Design of Alloy Catalysts Guided by Density Functional Theory[END_REF] , strongly adsorb BH4 -, resulting in its dissociation and formation of BHx,ad and (4-x) Had.

However, high electrocatalytic activity of PGMs in the HER as well as intensive hydrolysis of borohydride on these materials result in BOR overpotentials above 0.4 V, a severe drawback for the practical energetic efficiency of a DBFC. Molecular adsorption of BH4,ad with much less favorable adsorption energy was proposed for Au, Ag and Cu [START_REF] Rostamikia | Direct Borohydride Oxidation: Mechanism Determination and Design of Alloy Catalysts Guided by Density Functional Theory[END_REF][START_REF] Escaño | Borohydride Electro-Oxidation on Metal Electrodes: Structure, Composition and Solvent Effects from DFT[END_REF] . These metals, being in theory more selective for the direct BOR, with a very low rate of borohydride hydrolysis [START_REF] Escaño | Electrocatalysis of Borohydride Oxidation: A Review of Density Functional Theory Approach Combined with Experimental Validation[END_REF][START_REF] Merino-Jiménez | Developments in Direct Borohydride Fuel Cells and Remaining Challenges[END_REF] , however, show poor activity towards the BOR, owing to their inability to activate B-H bond breaking.

Nickel is a popular material for a variety of electrochemical applications in alkaline media, owing to its low cost, high availability and high corrosion stability: Ni is used in various energy storage/conversion systems, e.g. 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] Cao | Synthesis of Hierarchical Porous NiO Nanotube Arrays for 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][START_REF] Yuan | Mixed Transition-Metal Oxides: Design, Synthesis, and Energy-Related Applications Changzhou[END_REF][START_REF] Li | High-Voltage Positive Electrode Materials for Lithium-Ion Batteries[END_REF] , as well as to electrocatalyze oxygen evolution/reduction reactions [START_REF] Roger | Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting[END_REF][START_REF] Song | Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis[END_REF][START_REF] Zhao | Tuning Electronic Push/Pull of Ni-Based Hydroxides to Enhance Hydrogen and Oxygen Evolution Reactions for Water Splitting[END_REF][START_REF] Schalenbach | An Alkaline Water Electrolyzer with Nickel Electrodes Enables Efficient High Current Density Operation[END_REF][START_REF] Diaz-Morales | Guidelines for the Rational Design of Ni-Based Double Hydroxide Electrocatalysts for the Oxygen Evolution Reaction[END_REF][START_REF] Mccrory | Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction[END_REF] and hydrogen evolution/oxidation reactions (HER/HOR) [START_REF] Gong | A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction[END_REF][START_REF] Oshchepkov | Nanostructured Nickel Nanoparticles Supported on Vulcan Carbon as a Highly Active Catalyst for the Hydrogen Oxidation Reaction in Alkaline Media[END_REF][START_REF] Wang | Dominating Role of Ni 0 on the Interface of Ni/NiO for Enhanced Hydrogen Evolution Reaction[END_REF][START_REF] Danilovic | Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts[END_REF] . However, the literature remains very uncertain regarding the activity of Ni towards the BOR: some papers point to low activity [START_REF] Ma | Facile Fabrication of Gold Coated Nickel Nanoarrays and Its Excellent Catalytic Performance towards Sodium Borohydride Electro-Oxidation[END_REF][START_REF] Li | A New Application of Nickel-Boron Amorphous Alloy Nanoparticles: Anode-Catalyzed Direct Borohydride Fuel Cell[END_REF][START_REF] Zhang | Three-Dimensional Functionalized Graphene Networks Modified Ni Foam Based Gold Electrode for Sodium Borohydride Electrooxidation[END_REF] , whereas others suggest the BOR to proceed already at relatively low overpotentials [START_REF] Dong | Electrooxidation Mechanisms and Discharge Characteristics of Borohydride on Different Catalytic Metal Surfaces[END_REF][START_REF] Geng | Ni-Pt/C as Anode Electrocatalyst for a Direct Borohydride Fuel Cell[END_REF][START_REF] Zhang | Nickel Particles Supported on Multi-Walled Carbon Nanotubes Modified Sponge for Sodium Borohydride Electrooxidation[END_REF][START_REF] Santos | Nickel 29 and Nickel-Cerium Alloy Anodes for Direct Borohydride Fuel Cells[END_REF] . As of today, no electrocatalyst that enables both high BOR rate, high faradaic efficiency and low BOR onset potential has been isolated with certitude.

Here we propose design principles for highly-active BOR electrocatalysts enabling the reaction to proceed at relatively low overpotentials and outperforming all currently-known materials. We capitalize on the strong dependence of the HER electrocatalysis on the extent of the oxidation of the Ni surface [START_REF] Oshchepkov | Nanostructured Nickel Nanoparticles Supported on Vulcan Carbon as a Highly Active Catalyst for the Hydrogen Oxidation Reaction in Alkaline Media[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] and demonstrate that by controlling the surface state of Ni, it is possible to suppress the competing HER and enable fast BOR at potentials below 0 V vs RHE. NiED/C electrocatalysts are prepared by electrodeposition and compared with commercially-available Pt/C, Pd/C, Au/C electrocatalysts (the most studied BOR materials to date) [START_REF] Wang | Efficient PH-Gradient-Enabled Microscale Bipolar Interfaces in Direct Borohydride Fuel Cells[END_REF][START_REF] Braesch | Borohydride Oxidation Reaction Mechanisms and Poisoning Effects on Au , Pt and Pd Bulk Electrodes : From Model ( Low ) to Direct Borohydride Fuel Cell Operating ( High ) Concentrations[END_REF][START_REF] Olu | Investigation of Platinum and Palladium as Potential Anodic Catalysts for Direct Borohydride and Ammonia Borane Fuel Cells[END_REF] . The results of this comparison show outstanding performances of NiED/C both in half-cell and in unit fuel cell configurations, in terms of activity and stability of the BOR current at low overpotentials. The experimental data are rationalized by using density functional theory (DFT) calculations and comparing adsorption energies of reactive species at various metal surfaces. [START_REF] Oshchepkov | Nanostructured Nickel Nanoparticles Supported on Vulcan Carbon as a Highly Active Catalyst for the Hydrogen Oxidation Reaction in Alkaline Media[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] . To perform the XPS measurements, the GC cylinders with NiED/C electrocatalysts were pulled out from the electrolyte (either 1 M NaOH or 1 M NaOH + 50 mM NaBH4) at 0 V vs RHE (the final scan was stopped when going from negative to positive values), rinsed with water, fixed on the sample holder and protected with a drop of purified water (aiming to minimize the contact with the ambient atmosphere). After that the holder was quickly placed into the entry chamber of the XP spectrometer, degassed down to P ≈ 10 -5 Pa and only then the sample was transferred to the main chamber of the XP spectrometer. Typically, the duration of the transfer procedure did not exceed 5 min.

MATERIALS AND METHODS

Synthesis of NiED

Electrochemical

Fuel cell study.

A unit cell with the active electrode surface area of 8 cm 2 developed at LEMTA was used for the DBFC tests. Anodic and cathodic flow fields were single serpentine channels (1 mm width, 0.7 mm deep) machined in a brass block and coated with gold [START_REF] Mainka | On the Estimation of High Frequency Parameters of Proton Exchange Membrane Fuel Cells via Electrochemical Impedance Spectroscopy[END_REF] . The membrane electrode assembly was constituted of: (i) an anodic active layer made of either a commercial Ptblack/GDE with a loading of 2 mgPt cm -2 (Fuel Cells Etc) or a NiED/GDE with a loading of 0.60 ± 0.05 or 3.6 ± 0.1 mgNi cm -2 for NiED/GDE-L and NiED/GDE-H, respectively (according to ICP-AES), (ii) a commercial Nafion ® NRE-212 membrane exchanged for the Na + form, (iii) a commercial air cathode Pt-black/GDE (2 mgPt cm -2 ). PTFE seals were used to adjust the thickness of the electrodes to get 20% of compression of the initial thickness. The anodic fuel was composed of 0.5 M NaBH4 in 4.0 M NaOH (VMR Analar ® Normapur ® ). Pure oxygen (Messer, 99.99%) was used at the cathode. No sensitivity to oxygen flow rate was noticed. Temperature regulation was provided thanks to a circulating bath driven by a Pt (100) probe inserted in the anodic plate. Before entering the cell, the cooling water passed through a heat exchanger, the function of which was to preheat the borohydride solution to the desired temperature.

Computational methods. All calculations were performed using the ab initio total energy and molecular-dynamics Vienna ab initio simulation program (VASP) developed at the Institute for Material Physics at the University of Vienna [START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | Ab Initio Molecular Dynamics for Liquid Metals[END_REF] . The projector augmented wave method, Perdew-Wang [START_REF] Perdew | Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy[END_REF] form of the generalized gradient approximation (GGA) and a cut-off energy of 450 eV were used to represent the electronic structure. A 5×5×1 Monkhorst-Pack k-point grid [START_REF] Monkhorst | Special Points for Brillonin-Zone Integrations[END_REF] 1d,e). Note however that due to ex situ character of the XPS analysis one cannot exclude that a part of Ni 2+ species was formed during the sample transfer to the XPS chamber. The extraordinary activity of the metallic NiED/C can be attributed to the sluggish kinetics of the competing HER, which is evidenced by small cathodic currents in Figure 2a. In agreement with previous findings [START_REF] Gong | A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction[END_REF][START_REF] Danilovic | Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts[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] , the HER current increases several times on the partially-oxidized NiED/C electrode (Figure 2a), due to a decrease of the hydrogen binding-energy on the Ni surface in the presence of closely-located Ni oxide species [START_REF] Wang | Dominating Role of Ni 0 on the Interface of Ni/NiO for Enhanced Hydrogen Evolution Reaction[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] . This HER increase translates in a significant decrease of the BOR currents and a positive shift of the OCP (open-circuit potential). Interestingly, the BOR and the HER currents are not in direct anti-correlation (cf. Figure 2a,b) suggesting a complex interplay between the adsorption energies, the Ni site availability and the electrochemical kinetics of BOR/HER. The onset-potential for the oxidation of 5 mM NaBH4 is -0.052 V vs RHE for strongly-oxidized NiED/C, while it shifts favorably to -0.152 V vs RHE and -0.235 V vs RHE in the case of partially-oxidized and metallic NiED/C electrodes, respectively (Table 1). To sum up, by suppressing the rate of electrochemical HER and increasing the metallic character of the Ni surface, we significantly improve the performance of Ni electrocatalyst in the BOR (Figure 2b).

In particular, metallic NiED/C demonstrates the highest BOR current densities measured at 0.1 V vs RHE, being ca. 1.5 and ca. 15 times larger compared to partially-and strongly-oxidized NiED/C, respectively (Figure 2c). The current densities of metallic NiED/C sample measured below 0.3 V vs RHE, in both potentiostatic and potentiodynamic modes, were much larger compared to any Nibased electrocatalysts (including those consisting of noble metals) published in the literature (Table S2). Thus, the surface state of Ni is of paramount importance for its BOR activity, which accounts for the contradictory conclusions in the literature regarding the BOR at Ni, since the oxidation state of Ni surfaces has not been controlled in previously published measurements 28- 34,44 . In this respect, it is extremely important to avoid applying anodic potentials higher than 0.3

V vs RHE, during electrokinetic studies of the BOR on Ni-based materials. Applying higher potentials leads to continuous passivation of the Ni electrode surface, resulting in rapid decrease of the BOR activity of metallic NiED/C (Figure S6). in agreement with Oshchepkov et al. [START_REF] Oshchepkov | Nanostructured Nickel Nanoparticles Supported on Vulcan Carbon as a Highly Active Catalyst for the Hydrogen Oxidation Reaction in Alkaline Media[END_REF] . Finally, the activity of Pd/C and Au/C is much smaller than for NiED/C and Pt/C in the studied potential range, especially at 50 mM NaBH4.

While potentiodynamic measurements show higher BOR currents on Pt compared to Ni above E = 0.2 V vs RHE, potentiostatic measurements (Figure 3c) reveal that the BOR activity of Pt is unstable and promptly decays, the loss exceeding 90 % after only 1 hour. A similar decrease, although to lesser extent, was observed for Pd/C, with ca. 80 % current decay after 1 hour, while Au/C is almost inactive at potentials below 0.3 V vs RHE (Figure S9). Contrary to the Pt/C and Pd/C electrocatalysts, the activity of NiED/C is relatively stable with only ca. 12 and 18 % current decay after 1 hour at 5 and 50 mM NaBH4, respectively. Besides, the BOR currents on NiED/C are almost proportional to the concentration of NaBH4, whereas the BOR performances of the Pt/C and Pd/C electrocatalysts at higher NaBH4 concentrations are worse than expected (Figures 3c).

To some extent, the current decay on NiED/C is related to the poisoning of Ni active sites by the formed Ni-B and B 3+ species as evidenced from the XPS analysis (Figure S10, Table S1).

However, no modification in the size/shape of the Ni nanoparticles after the treatment in borohydride electrolyte was found compared to the Ni nanoparticles obtained right after their synthesis by electrodeposition (TEM images on Figure S11), which demonstrates to some extent the robustness of these materials.

The effect of poisoning becomes more important below E = 0 V vs RHE, with an ability of Ni to regenerate active surface sites already at 0.1 V vs RHE (Figure S7), while noble metals require application of much larger potentials. Poisoning and likely mass-transport limitations become more important at high concentrations of NaBH4, resulting in a decrease of the concentrationnormalized current densities of NiED/C (Figure S12), as was previously shown for Pt, Pd and Au massive electrodes [START_REF] Braesch | Borohydride Oxidation Reaction Mechanisms and Poisoning Effects on Au , Pt and Pd Bulk Electrodes : From Model ( Low ) to Direct Borohydride Fuel Cell Operating ( High ) Concentrations[END_REF] . Meanwhile, higher stability of the BOR currents on NiED/C over Pt/C and Pd/C electrocatalysts is related to a weaker adsorption of poisoning boron species on the surface of Ni as discussed below (cf Computational modeling). V, respectively (Table 1), thus reflecting the difference observed in RDE studies for Pt/C and NiED/C samples. More importantly, the cell with NiED/GDE-L anode significantly outperforms the Elrespectively, Figure 4g). Remarkably, relatively high and stable current densities were obtained for the first time at cell voltages as high as 1.0 V using NiED/GDE anodes (Figure 4h), i.e. the voltage at which noble metals are not able to oxidize BH4 -. Again, we emphasize that the reduced state of the Ni surface is the key factor for the high performance of a DBFC. For this reason, the cell voltage should not be lowered below U = 0.5 V during the test, to avoid irreversible Ni oxidation and ensure repeatable results. Deliberate partial oxidation of NiED/GDE either electrochemically or under prolonged contact with air results in ca. 12% decrease of the performance, respectively (Figure S14). Operation of DBFCs with Ni anodes at high voltages would also be better in avoiding secondary H2 evolution, for which the rate increases with a decrease of cell voltage (Figure S15). It is well known that Had is strongly adsorbed on metallic Ni which results in "low" HER (both from water and BH4 -) at high voltages. However when anodic potential increases (cell voltage decreases) the strength of Ni-Had bond becomes lower, thus stimulating the HER, since formation of adsorbed Had (at these potentials) mainly proceeds through the dissociation of BH4 -molecule and not from H2O. When Pt is used at the anode of a DBFC, the production of hydrogen instead decreases at lower cell voltages (Figure S15), due to high activity of Pt in the HOR. Surprisingly, both NiED/GDE-L and NiED/GDE-H show similar performance (Figure 4g), despite a 6-fold difference in the Ni-loading. This suggests that the high-performing

Ni-based DBFCs may be limited by either (i) the cathode used, (ii) partial operation of the anode or (iii) the construction of the cell; this effect was not observed for (lower-performances) Pt-based DBFCs.

Although additional studies are necessary to optimize the preparation parameters of the membrane electrode assembly with NiED electrocatalysts, DBFCs with NiED/GDE anodes already show great promise and outperform DBFCs with Pt anodes in terms of maximum power density normalized by the mass of the active anode component (Table S2). Besides, they show better performance compared to hydrogen-fed alkaline membrane FC (AMFC) with state-of-the-art PGM-free anode electrocatalyst NiMo/KB 45 . The latter system shows power density around P = 120 mW cm -2 at j = 0.22 A cm -2 , while it is as high as P = 155 mW cm -2 at the same current density for a DBFC with NiED/GDE-L anode, although lower metal loading and operating temperature were used in the latter case (0.6 vs 2 mg cm -2 and 60 vs 70ºC for DBFC with NiED/GDE and AMFC with NiMo/KB, respectively). This leads to fast poisoning of Pt active sites with BH* species (likely responsible for the current decay for Pt/C in Figure 3c) and formation of H* that contributes to the non-selective H2 evolution.

Oxidation of the BH* species resulting from BH4 -dissociation is slow on Pt at potentials below 0

V vs RHE, resulting in slow BOR kinetics [START_REF] Rostamikia | Direct Borohydride Oxidation: Mechanism Determination and Design of Alloy Catalysts Guided by Density Functional Theory[END_REF][START_REF] Rostamikia | First Principles Mechanistic Study of Borohydride Oxidation over the Pt(111) Surface[END_REF]

  measurements. Electrochemical measurements were carried out in a threeelectrode Teflon cell thermostated at T = 25°C with a Pt wire as a counter-electrode and Hg/HgO as a reference electrode (EHg/HgO = 0.93 V vs RHE). Although the use of Pt as a counter electrode may cause contamination of the working electrode, for example, when studying hydrogen evolution or oxygen reduction reactions, this should not be an issue when testing BOR electrocatalysts. Indeed, the Pt counter electrode will experience reduction conditions (the HER) and therefore negligibly corrode. Besides, characterization of the NiED/C and NiED/GDE electrodes either after the synthesis or RDE measurements, confirmed an absence of any sign of Pt contamination (FiguresS1-S3). The measurements were performed in 1.0 M NaOH solution (prepared from 50% NaOH solution in water, Premium quality, Aldrich) using either Gamry REF 600 or Autolab PGSTAT302N potentiostats.After immersion in the electrochemical cell and before the measurement of their electrocatalytic activities, NiED/C electrodes were conditioned in N2-saturated 1 M NaOH by cycling between -0.20 and 0.40 V vs RHE at a sweep rate (v) of 20 mV s -1 . The cyclic voltammetry (CV) curves acquired afterwards in the potential interval from -0.06 to 0.40 V vs RHE were employed to estimate the electrochemical surface area (ECSA) of Ni considering the entire value of the anodic charge and the 0.514 mC cm -2 conversion coefficient. The catalytic performance in the BOR was derived from the RDE voltammetric data obtained in N2-saturated 5 (50) mM NaBH4 + 1.0 M NaOH electrolyte at ω = 1600 rpm. Positive-feedback IR compensation (80 %) was used during the measurements with typical Ohmic resistance of the electrolyte measured to be between 1 and 5 Ohm.

  and spin polarized calculations were used during structural optimizations. Structural optimization minimized the forces on all atoms to less than 0.02 eV Å -1 . Harmonic vibrational modes were calculated to determine the zero-point vibrational energy (ZPVE) corrections to the total energy of adsorbed species and isolated molecules. The Ni(111) surface was modeled using a 3×3, four layer slab with the bottom two layers constrained to the Ni face centered cubic (fcc) lattice positions. A vacuum region of 15 Å was inserted between the slabs.RESULTS AND DISCUSSIONBOR activity of NiED/C vs. its surface state. NiED/C electrocatalysts synthesized by electrodeposition of Ni on Vulcan XC-72R were characterized by scanning transmission electron microscopy (STEM); STEM revealed formation of metal nanoparticles with a mean size of ca. 10 nm, homogeneously distributed over the support surface (Figures1a-c). The estimated interplanar spacing of 0.205 nm corresponds to the (111) plane of Ni (0.2034 nm, calculated for a theoretical lattice constant a = 0.35238 nm for crystalline Ni [ICDD PDF-2#00-004-0850]). Ni content was estimated as ca. 40 wt.% by inductively-coupled plasma atomic emission spectroscopy (ICP-AES).

Figure 1 .

 1 Figure 1. (a) Low and (b) high-resolution STEM images of the as-prepared NiED/C electrocatalyst,

Figure 2 and

 2 Figure 2 and S5 indicate that, indeed, the extent of the surface oxidation has a major

Figure 2 .

 2 Figure 2. (a, b) CV and (c) chronoamperometry curves obtained for various NiED/C rotating disc

3 V

 3 NiED/C is compared with the commercially-available 40 wt. % Pt/C, Pd/C and Au/C electrocatalysts -the benchmarks in the field. Two concentrations of NaBH4 solution, 5 and 50 mM, in 1.0 M NaOH, were chosen to compare the electrocatalysts, to evaluate the influence of electrode surface-poisoning by boron-containing species. The measurements were performed under potentiodynamic and potentiostatic conditions in the potential range between -0.2 < E < 0.vs RHE, as larger potentials are not attractive for a practical application (they would correspond to a low DBFC voltage). Potentiodynamic measurements (Figure3a,b) evidence unmatched activity for NiED/C at potentials below 0 V vs RHE, both in 5 and 50 mM NaBH4 solution.

Figure 3 .

 3 Figure 3. CV curves obtained for metallic NiED/C (solid curve), Pt/C (dashed curve), Pd/C (dash-

Fuel

  cell performance. Electrodeposited Ni was further evaluated in a unit DBFC. For this, Ni particles were deposited directly on a carbon cloth at low (NiED/GDE-L, 0.60 ± 0.05 mgNi cm -2 ) or high (NiED/GDE-H, 3.6 ± 0.1 mgNi cm -2 ) metal loading and compared with commercial Ptblack/GDE anode. Separated metal particles consisting of smaller crystallites were mainly found in NiED/GDE-L (Figures4c, e, S13), while in the NiED/GDE-H electrode, Ni particles fully cover the external surface of the support (Figures4d, f). The size of Ni crystallites was estimated from XRD patterns as 13.4 and 16.3 nm for NiED/GDE-L and NiED/GDE-H, respectively. Unit DBFCs using Pt-black/GDE and NiED/GDE anodes displayed open-circuit voltages of Uj=0 = 1.0 and 1.2

Figure 4 .

 4 Figure 4. Characterization of NiED/GDE samples. SEM images of pure SGL carbon (a) and after

Table 1 .

 1 Comparison of kinetic parameters for various carbon-supported metal electrocatalysts in

	the BOR.
		Half-cell open-circuit		Unit-cell open-circuit voltage /
	Electrocatalyst	potential / RDE ( V vs RHE )	Electrocatalyst	DBFC ( V )
		5 mM BH4 -	50 mM BH4 -		50 mM BH4 -500 mM BH4 -
	Strongly-oxidized NiED/C	-0.052		NiED/GDE-L	1.10	1.18
	Partially-oxidized NiED/C	-0.152		NiED/GDE-H		1.21
	Metallic NiED/C	-0.235	-0.258	Pt-black/GDE		1.03
	Pt/C	-0.041	-0.051
	Pd/C	-0.140	-0.189
	Au/C	-0.121	-0.015

Comparison of NiED/C with noble metal electrocatalysts. The

BOR activity of metallic

Table 2

 2

shows, the binding of borohydride varies from strong on Pt to weak on Au surfaces, with Pd and Ni metals having intermediate values. The inability of Au to break B-H bonds at low BOR overpotentials

[START_REF] Olu | Influence of the Concentration of Borohydride towards Hydrogen Production and Escape for Borohydride Oxidation Reaction on Pt and Au Electrodes -Experimental and Modelling Insights[END_REF][START_REF] Finkelstein | Self-Poisoning during BH4 -Oxidation at Pt and Au, and in Situ Poison Removal Procedures for BH4 -Fuel Cells[END_REF][START_REF] Rostamikia | Borohydride Oxidation over Au(111): A First-Principles Mechanistic Study Relevant to Direct Borohydride Fuel Cells[END_REF]

makes this metal inappropriate for practical applications. On the Pt (111) surface, the adsorption of BH4 - species is detrimentally-strong, adsorbing dissociatively, with no barrier for B-H bond breaking

[START_REF] Rostamikia | First Principles Mechanistic Study of Borohydride Oxidation over the Pt(111) Surface[END_REF]

.

  . The interaction of BH4* with the Ni surface is intermediate to that of Au and Pt. The binding is strong-enough to allow for subsequent oxidation, but not too strong to lead to surface poisoning. DFT calculations predict molecular (nondissociative) adsorption of BH4 -on the Ni (111) surface, although the stretching of the B-H bond suggests facile dissociation. B-H bond dissociation is further promoted by strong adsorption of formed H* on the surface of pure Ni. Although the detailed data regarding adsorption of various BHxOy species are not included for the Pd surface, the electrochemical results point to a similar behavior than for Pt surface. Subsequent reaction steps also suggest the complete borohydride oxidation process can occur at low overpotentials on Ni (111), and via an alternative mechanism than found on Pt (111). Binding of the BH* species is weaker on the Ni (111) surface compared to the Pt (111) surface. Alternatively, BH2OH* binds stronger to the Ni (111) surface compared to the Pt (111) surface, causing B-OH bond formation to precede complete B-H dissociation, as BH2OH* is the significantly-preferred 4-electron intermediate, compared to BH*, on Ni (111), in agreement with the experimental data. The reaction energy for BH4* conversion to BH* and BH2OH* is calculated on the Ni (111) surface at 0 V vs RHE: Therefore, unlike the Pt (111) surface, the Ni (111) surface will not be poisoned by BH* species at low potentials, suggesting that Ni (111) can outperform Pt (111) below E = 0 V vs RHE. The expected higher coverage of OH* on Ni (111) at BOR-relevant potentials (see stronger OH* binding in Table2) will also help to promote B-OH bond formation.The HER competes with the BOR for E < 0 V vs RHE. The weaker binding of H* on Pt (relative to Ni) results in intensive H2 escape, while H* adsorption is very strong on metallic Ni[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 explains its low activity in the HER. Based on DFT results, the BOR on Ni (111) is expected to be faster than on Pt (111) at low potentials, though one cannot differentiate whether this is the leading cause of the observed higher oxidation current on Ni at low potentials, rather than the lower competing reduction current for the HER on Ni.

	BH 4 * + 3OH -→ BH 2 OH * + 2H 2 O + 3e -	ΔG = -0.47 eV	(5)
	BH 4 * + 3OH -→ BH * + 3H 2 O + 3e -	ΔG = +0.17 eV	(6)

Table 2

 2 Binding energies of species bound to the M (111) surface at optimal binding sites.

	Species	Ni (111)	Binding energy (eV) Pt (111) Au (111)	Pd (111)
	BH4*	-3.68; -3.50 9	-	-1.73 46 ; -1.96 9	-
	BH+3H*	-	-4.73 47 ; -4.56 9	-	-3.63 9
	BH3*	-2.19	-1.94 47	-0.55 46
	BH2*	-4.10	-3.40 47	-2.64 46
	BH*	-5.39	-6.14 47	-4.40 46
	B*	-5.75	-6.50 47	-4.43 46
	BH3OH*	-2.77		-1.08 46
	BH2OH*	-1.27	-0.06 47	-0.06 46
	BHOH*	-3.46	-3.31 47	-2.34 46
	BOH*	-3.74	-4.41 47	-2.76 46
	BH(OH)2*	-0.49	-0.17 47	-0.05 46
	B(OH)2*	-2.36	-3.09 47	-2.20 46
	B(OH)3*	-0.22	-0.16 47
	H*	-2.86; -2.94 48	-2.72 47 ; -2.72 48	-2.12 46 ; -2.18 48	-2.88 48
	OH*	-3.37;

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. The authors express their gratitude to Dr. C. Ulhaq-Bouillet, Dr. T. Dintzer and Dr. A. Boss for their assistance with (S)TEM, SEM and ICP-AES analysis, and to Prof. U. Demirci for fruitful discussions.

outperform all the noble-metal electrocatalysts tested as benchmarks under the same conditions.

The experimental data reveal that controlling the surface oxidation state rgreatly impacts the activity of Ni in the BOR; the highest activity is measured for reduced metallic Ni, and substantially drops after partial oxidation of the electrode surface. Experimental data reinforced by DFT calculations suggest that a good electrocatalyst enabling the BOR at low overpotentials should allow: (i) dissociative but not too strong BH4 -adsorption, (ii) relatively strong adsorption of Had (to decrease the HER) and (iii) formation of OHad species at low potentials (to enable the oxidation of poisoning B-containing species).

AUTHOR INFORMATION

Corresponding Author

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

Author Contributions

AO, GB and SOA contributed to the experimental work, data analysis and writing of the manuscript.

GR and MJJ performed the DFT calculations and contributed to the writing of the manuscript.

VP performed XPS analysis and contributed to the writing of the manuscript. GM, AB, ERS and MC contributed to the project planning, experimental work, data analysis, writing and review of the manuscript.

Notes

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


D Lallias

E Harté

M Marchand

A Chaulot-Talmon

A Frambourg

L Jouneau

L Goardon

L Labbé

T Terrenne

H Jammes

M Houssier

L Marandel

H Kiefer

M Dupont-Nivet

IMPACT OF EARLY COLD TEMPERATURE ON GENOMEWIDE DNA METHYLATION PATTERNS IN JUVENILE RAINBOW TROUT DIVERGENT LINES FOR MUSCLE FAT CONTENT

Introduction

Recent studies have shown that early exposure to environmental stimuli (such as hypoxia or temperature) could impact fish physiology, growth, metabolism and nutrition at midor long term. There are several possible mechanisms underlying this programming phenomenon, among which epigenetic regulations such as DNA methylation [START_REF] Best | Epigenetics in teleost fish: From molecular mechanisms to physiological phenotypes[END_REF]. The objective of this study was to understand by which molecular mechanisms early cold exposure (at eyed-eggs stage) can impact fish physiology later in life (at juvenile stage), by analysing DNA methylation patterns in rainbow trout. This study was based on 2 experimental divergent lines for muscle lipid content [START_REF] Quillet | Two-way selection for muscle lipid content in pan-size rainbow trout (Oncorhynchus mykiss)[END_REF]) that have been shown to utilise differently feed and to possess a well differentiated intermediary and energetic metabolism. Our hypothesis is that they will react differently to the cold exposure during incubation.

Materials and methods

At 17 days post fertilization (dpf), eyed-eggs from two experimental lines selected for high or low muscle lipid content, fat line (FL) and lean line (LL), were either incubated at normal temperature (11°C) or incubated at 3°C for 15 days in 12 tanks (2 lines x 2 incubation temperatures x 3 tanks). Hatching and the rest of the rearing was performed at standard temperature (11°C). Eyed eggs were collected at a similar stage of development (220 degree days; 19 and 31 dpf for the control and cold conditions respectively) and analysis of gene expression was performed by qPCR on 5 eggs per line, per incubation temperature and per tank, for 12 genes: hsp47 (Heat Shock Protein 47), two cirbp (coldinducible RNA-binding protein), ucp2 (Uncoupling protein 2) and eight dnmt3 (DNA methyltransferase 3). Genome-wide patterns of DNA methylation was assessed by RRBS (Reduced Representation Bisulfite Sequencing) on liver samples of 48 juvenile trout (2 lines x 2 incubation temperatures x 3 tanks x 4 fish per tank) collected at 189 dpf, snap frozen in liquid nitrogen and kept at -80°C until DNA extraction. Liver was chosen as it is the central organ for intermediary metabolism. RRBS libraries were prepared using MspI and size selection of 40-290 bp fragments and then sequenced on an Illumina NovaSeq6000 sequencer to produce 100 bp paired-end reads (Integragen SA, France). Trimmed reads were aligned to the current Arlee rainbow trout reference genome with the bisulfite mapping tool Bismark. Differential methylation analyses were performed using methylKit (qvalue<0.01, minimal methylation differences between temperature groups>20%). Identified DMCs (Differentially Methylated Cytosines) and DMRs (Differentially Methylated Regions) were finally annotated relative to gene features.

Results

Globally, there were significant but low differences in gene expression for 8 of the 12 targeted genes, with a minor impact of incubation temperature or line at eyed-stage. For the RRBS sequencing data, an average of 47 million paired-end reads were obtained per individual (lowest: 28 million; highest: 74 million). Bisulfite conversion rates were very high (>99.7%). Sequences that aligned to unique positions of the genome represented 50% of all reads on average and were used for subsequent analysis. Numbers of CpGs tested by methylKit (i.e. CpGs with a minimal number of 9 samples in each group satisfying the coverage range of 10-500 reads per CpG) were 1,273,685 for FL line and 1,240,712 for LL line. Differential methylation analyses between control and cold exposure groups revealed 3187 DMCs and 89 DMRs for the FL line; 3442 DMCs and 79 DMRs for the LL line. Annotation of DMCs and DMRs revealed that different genes were involved in the two genetic lines, with a certain degree of overlap (Figure 1).

Discussion

Using two divergent lines for muscle fat content allowed testing the impact of the genetic background on the establishment of DNA methylation patterns in response to early cold exposure. Preliminary results suggest that the two lines responded differently, although some genes with DNA methylation varying according to cold exposure were also found in common. Ongoing interpretation of the biological pathways involved will lead to a finer understanding of underlying mechanisms. This study will contribute to use early programming as a lever to improve long term performances of animals.

Figure 1 .

 1 Figure 1. Venn diagrams of geneIDs found in DMCs and DMRs in the two experimental lines, FL (fat line) and LL (lean line).

Acknowledgements

This study was supported by the European Maritime and Fisheries Fund and FranceAgrimer (EpiCOOL project, n° PFEA470018FA1000015). RRBS data analysis used a pipeline previously described by Perrier et al. 2018. We are grateful to the Genotoul bioinformatics platform Toulouse Occitanie (Bioinfo Genotoul, doi: 10.15454/1.5572369328961167E12) for providing computing and storage resources.


Jieun Yang

Abdul Rahman Mohmad

Yan Wang

Raymond Fullon

Xiuju Song

Fang Zhao

Ibrahim Bozkurt

Mathias Augustin

Elton J G Santos
email: e.santos@qub.ac.uk

Hyeon Suk Shin

Damien Voiry

Young Hu

Jeong

Manish Chhowalla

Ultrahigh current density niobium disulfide catalysts for hydrogen evolution

The Nb 1+x S 2 could therefore be a viable non-precious metal catalyst for practical electrolyzers used to generate hydrogen.

Ultra-thin layers of TMD catalysts exhibit improved electrocatalytic performance due to efficient charge injection and transfer to active sites [START_REF] Merki | Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts[END_REF][START_REF] Benck | Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials[END_REF] . Increasing the conductivity via engineering of metallic phases [START_REF] Voiry | Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction[END_REF][START_REF] Voiry | The role of electronic coupling between substrate and 2D MoS 2 nanosheets in electrocatalytic production of hydrogen[END_REF][START_REF] Yin | Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets[END_REF] and edges [START_REF] Benck | Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials[END_REF][START_REF] Kibsgaard | Engineering the surface structure of MoS 2 to preferentially expose active edge sites for electrocatalysis[END_REF][START_REF] Kong | Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers[END_REF][START_REF] Tsai | Tuning the MoS2 Edge-Site Activity for Hydrogen Evolution via Support Interactions[END_REF] also leads to substantial improvement in catalytic properties. To this end, metallic 1T phase of semiconducting MoS 2 induced by lithium chemistry [START_REF] Lukowski | Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS 2 Nanosheets[END_REF][START_REF] Voiry | Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction[END_REF][START_REF] Eda | Photoluminescence from Chemically Exfoliated MoS2[END_REF] has been widely studied for HER catalysis. Recently, multi-layers of thin metallic TMDs such as NbS 2 [START_REF] Liu | Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution[END_REF] , TaS 2 [START_REF] Shi | Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst[END_REF] , and VS 2 [START_REF] Yuan | Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction[END_REF] have also been studied for HER. The key advantage of metallic TMDs according to theory is that the entire basal plane is catalytically active for the HER [START_REF] Liu | Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution[END_REF][START_REF] Pan | Metal Dichalcogenides Monolayers: Novel Catalysts for Electrochemical Hydrogen Production[END_REF][START_REF] Tsai | Theoretical insights into the hydrogen evolution activity of layered transition metal dichalcogenides[END_REF] , unlike in semiconducting MoS 2 where only the metallic edges are active [START_REF] Jaramillo | Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts[END_REF] . However, in contrast with noble metal catalysts such as Pt nanoparticles, the electronic structure of atomically thin materials is strongly influenced by local electrochemical reactions. For example, in the case of metallic 1T phase MoS 2 , the adsorption of protons on the surface -the first step in the HER -can dramatically reduce the conductivity of the 2D nanosheets [START_REF] Pan | Metal Dichalcogenides Monolayers: Novel Catalysts for Electrochemical Hydrogen Production[END_REF] , which can slow down the reaction kinetics and reduce the overall current density. Thus, a fine balance must be achieved between reducing the thickness of catalysts and maintaining metallic nature of 2D catalysts are multilayered and therefore electrons injected from the glassy carbon support must overcome the van der Waals gaps between the layers to reach the active sites on the surface. This increases the charge transfer resistance and thus the catalytic activity has been demonstrated to be inversely related to the number of layers in TMDs [START_REF] Yu | Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution[END_REF] . In the case of single layers, while the charge transfer kinetics are improved, the current carrying capability is lower and therefore the overall current density that can be achieved is also lower. Furthermore, the recombination of protons with free electrons on the surface of the single layer catalysts (the first step of the reaction) decreases the concentration of free carriers and therefore the conductivity [START_REF] Pan | Metal Dichalcogenides Monolayers: Novel Catalysts for Electrochemical Hydrogen Production[END_REF] , reducing the kinetics of the HER and lowering the current density.

We have tested the catalytic performance of metallic 2H Nb 2a.

It can be clearly seen from the polarization curves that the current density for the 2H Nb 1.35 S 2 is exceptionally large, reaching 1000 mA-cm -2 at ~ 370 mV and 5,000 mA-cm -2 at ~ 420 mV. This current density is obtained from careful analyses of the catalytic active area. We have examined of numerous scanning electron microscope images to determine the areal coverage of catalyst material on the glassy carbon surface. We found that 20% ± 5% of the electrode surface is covered by the catalyst particles, see Extended Data Fig. 4. In addition to the areal coverage, we also carefully measured the electrochemical surface area (see Extended Data Fig. 5) to find the roughness factor (RF = 11 ± 3). Using this value, we were able to accurately determine the active area of the catalyst and use it to calculate the current density. To complement the current density measurements, we measured the actual amount of evolved hydrogen using a gas chromatograph (GC, Agilent 7890 B) equipped with molecular sieve 5 Å capillary column and a thermal conductivity detector (TCD). The measurements were performed in a gas-tight 3-electrode cell.

Ar gas was purged in the cell to remove air. The TCD cell was calibrated using known hydrogen amounts (5% and 10 % H 2 balance Ar certified standard gas). The volume of hydrogen evolved by the 2H-Nb 1.35 S 2 electrodes was measured at two overpotentials. We found that 2H-Nb 1.35 S 2 can evolve H 2 up to 150 µmol-h -1 and 300 µmol-h -1 at 300mV and 400 mV, respectively. When normalizing the performance to the surface area, the rate of H 2 evolution reaches ~ 30 L-cm -2 -h -1 equivalent to 6 mmol-h -1 -cm -2 at 400 mV. the Faradaic efficiency for the HER is typically 100% and therefore the current density is a direct indication of the amount of hydrogen evolved. Thus, careful measurement of the electrochemical surface area to obtain current density is a good indicator of catalyst performance. To confirm this, we measured the Faradaic efficiency using,

FE =

where n is the moles of hydrogen measured from GC, F is the Faraday constant, Q is the charge obtained from measurements in Figure 2a of the MS ( = × , where I is the current and t is the time). The obtained efficiencies are 96% and 100% at 300 mV and 400 mV, respectively. Thus, the GC measurements confirm the electrochemical measurements and provide additional evidence for the high performance of 2H-Nb 1.35 S 2 .

It can be seen from Figure 2b, which provides an expanded view of the low overpotential region in Figure 2a, that the cathodic potential at which the HER starts is less than 100 mV (overpotential of < 150 mV) for the 2H Nb suggests that the rate limiting reaction is the desorption of hydrogens. At higher potentials (120 -250 mV), the Tafel slope increases to ~ 70 mV-dec -1 (or 65 mV-dec -1 after iR correction), suggesting that hydrogen adsorption or diffusion of protons to active sites limits the reaction. To differentiate between these two mechanisms, we made HER measurements under agitation to increase diffusion of protons and found that the results are largely unchanged. These observations are consistent with theoretical results that show that, unlike Pt, metallic TMD catalysts such as NbS 2 possess dilute adsorption of hydrogens on the surface at low potentials but the high activity per site is retained at higher potentials where coverage of adsorbed hydrogens increases [START_REF] Liu | Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution[END_REF] . In addition to the overpotential and Tafel slope, we have also measured the number of hydrogen molecules evolved per second (the turnover frequency, TOF) for the catalysts. We have found that the TOF for the 2H phase Nb 1.35 S 2 catalysts to be in excess of 100 s -1 at 280 mV.

The TOF from this work compares favorably with other reports, as summarized in Figure 2d.

The exchange current density at 0 V for the Nb 1.35 S 2 phase catalysts was found to be ~ 0.8 mAcm -2 which corresponds to a TOF of 0.17 s -1 . For comparison, the exchange current density for Pt (111) is 0.5 -1 mA-cm -2 corresponding to TOF of ~ 1 s -1 (Ref [START_REF] Jaramillo | Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts[END_REF] ) and for metallic 2D TMDs the values are 0.02 mA-cm -2 and 0.043 s -1 (Ref To elucidate the fundamental mechanisms responsible for the catalytic performance of 2H [START_REF] Kibsgaard | Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate [Mo 3 S 13 ] 2clusters[END_REF][START_REF] Xie | Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution[END_REF] . MoS 2 basal planes with sulfur vacancies (blue) [START_REF] Li | Activating and optimizing MoS 2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies[END_REF] , strained MoS 2 basal planes with sulfur vacancies (green) [START_REF] Li | Activating and optimizing MoS 2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies[END_REF] and MoS 2 with low charge transfer resistance, R c (magenta) [START_REF] Voiry | The role of electronic coupling between substrate and 2D MoS 2 nanosheets in electrocatalytic production of hydrogen[END_REF] . Sigma-Aldrich) mixed with alkali halide (potassium iodine, KI) and sulfur powder (99.5%, Alfa Aesar). Since niobium oxide has a high melting point, alkali halide is used to create a new eutectic point for the reaction to produce more volatile oxides. [START_REF] Li | Halide-assisted atmospheric pressure growth of large WSe2 and WS2 monolayer crystals[END_REF] The mixture of 150 mg of Synthesis of 3R-NbS 2 : 3R-NbS 2 crystals were grown by a chemical vapor transport (CVT) method which has been employed to prepare other layered compounds. [START_REF] Suh | Doping against the Native Propensity of MoS2: Degenerate Hole Doping by Cation Substitution[END_REF] Prior to crystal growth, a quartz tube containing high purity source (Nb 99.99% and S 99.99% with a molar ratio of Nb:S is 1:2) was evacuated at 10 -6 Torr and sealed. The sealed quartz tube was then inserted into the tube furnace. The furnace was heated up to 900 °C with a ramp rate of 3°C/min. The reaction time was 18 h at 900 °C and the furnace was cooled down naturally. The CVT grown 3R crystals and the corresponding Raman spectrum are shown in Extended Data Fig. 8c,d.

Device fabrication of 2H-NbS 2 :

To demonstrate the HER performance of 2H-NbS 2 , we carried out HER test using an electrochemical microcell. [START_REF] Voiry | The role of electronic coupling between substrate and 2D MoS 2 nanosheets in electrocatalytic production of hydrogen[END_REF] We performed e-beam lithography to put gold contacts on as-grown 2H-NbS 2 on SiO 2 (300 nm) substrate. The gold electrodes were deposited via e-beam evaporation under high vacuum conditions (10 -7 Torr). After this process, we performed another e-beam lithography step to open a window on 2H-NbS 2 and cover the gold electrodes to avoid contact with electrolyte. The device is shown in Extended Data Fig. 10.

Electrochemical measurements were performed in 0.5 M H 2 SO 4 and glassy carbon and Ag/AgCl electrodes were used as counter and reference electrodes, respectively.

Electrochemical surface area measurements:

The estimation of electrochemically active surface area was conducted by measurement of the double-layer capacitance in a potential region with no faradaic response. Cyclic voltammetric (CV) measurements were performed between 285 mV and 315 mV vs. RHE at various scan rates from 5 mVs -1 to 100 mVs -1 in order to estimate the double layer capacitance (C dl ), see Extended Data Fig. 5. Roughness factor (RF) was estimated from the ratio of the measured double layer capacitance with respect to the specific capacitance of glassy carbon electrode (0.87 mF/cm 2 ). Our image analysis of the electrode surface (Extended Data Fig. 4) shows that only ~ 20% of the glassy carbon electrode is covered by the catalyst. Therefore, to calculate the RF, we take the areal capacitance of the glassy carbon after catalyst deposition to be (0.87 mF-cm -2 )(0.8) = 0.7 mF-cm -2 . Thus, the RF is given by:

RF = 9.6 -2 0.7 -2 = 13.7
Turnover frequency calculation: According to previous literature, turnover frequency can be obtained following equation.

TOF ( ) = ( ) × × × (1.602 × 10 )

To determine the density of active sites (N), we assume the lattice constant of 2H-Nb 1.35 S 2 is 3.31 Å (Figure 1c, STEM image). n is the number of electrons involved in the reaction. The surface area of unit cell can be 9.4 x 10 -16 cm 2 . We assumed that entire basal plane can be catalytically active. Therefore, the density of active is estimated to be about 1.06 x 10 15 cm -2 . The density of the surface active sites of 2H Nb 1.35 S 2 on geometric area : 1.06 x 10 15 sites/cm 2 x 13.7 = 1.4 x 10 [START_REF] Yin | Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets[END_REF] sites/cm [START_REF] Voiry | Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction[END_REF] To get TOF at exchange current density, the exchange current density was extrapolated linearly from the Tafel slope. The exchange current density of 2H-Nb 1.35 S 2 is 800 µA/cm 2 which is remarkable value among the reported TMD based catalysts as listed in Extended Data Table 1.

Electrical conductivity measurements:

The conductivity of the intercalated phases was measured and by evaporating gold contacts and is shown in Extended Data Fig. 11. We find the 2H Nb 1.35 S 2 phase to be highly conducting compared to other reports in the literature. [START_REF] Huang | Transport properties in semiconducting NbS2 nanoflakes[END_REF][START_REF] Molenda | Electrical and electrochemical properties of niobium disulphide[END_REF][START_REF] Niazi | Low-temperature resistance minimum in non-superconducting 3R-Nb 1+ x S 2 and 3R-Ga x NbS 2[END_REF][START_REF] Zhao | Two-dimensional metallic NbS 2 : growth, optical identification and transport properties[END_REF] Extended Data Fig. 1 7.

Extended Data

  abundant electrocatalysts for the hydrogen revolution reaction (HER). The performance of HER catalysts is typically evaluated in terms of overpotential at which the reaction starts and the Tafel slope -a measure of the over-potential required to increase a reaction by a factor of 10. The overpotential and Tafel slope values of metallic phases and edges 9 of 2D TMDs approach those of Pt nanoparticles -the best HER catalyst. However, despite substantial progress the overall current density of 2D TMD catalysts remains orders of magnitude lower (~ 10 -100 mA-cm -2 ) than industrial Pt and Ir electrolyzers (> 1,000 mA-cm -2 ) 10,11 . Here, we report the synthesis of three dimensional (3D) niobium disulfide (Nb 1+x S 2 where x is ~ 0.35) 12 as a HER catalyst that is capable of evolving hydrogen at current densities of > 5,000 mA-cm -2 at an overpotential of ~420 mV versus reversible hydrogen electrode (RHE). We find the exchange current density at 0 V for 2H phase Nb 1.35 S 2 catalysts to be ~ 0.8 mA-cm -2 (comparable to that of noble metals), corresponding to a turnover frequency of ~ 0.2 s -1 . We demonstrate a proof of concept electrolyzer based on 2H Nb 1.35 S 2 cathode that is capable of generating current densities of 1000 mA-cm -2 . Our theoretical results reveal that the Nb 1.35 S 2 with Nb terminated surface has free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER.

  S 2 crystals synthesized by chemical vapor deposition is shown in Figure1a. Films of varying thicknesses ranging from 2 nm to 50 nm with lateral dimensions of ~ 0.5 -1 μm were realized by CVD on substrates such as SiO 2 and glassy carbon. We find that the phase of NbS 2 is dependent on the thickness of the material (Extended Data Fig.2a-c). The electron diffraction patterns corresponding to the phases as a function of thickness are provided in Extended Data Fig.2d,e. In particular, 2H -Nb 1+x S 2 is predominantly observed in crystals with thickness of less than ~ 20 nm while the 3R phase is primarily stable for thicker samples. High-resolution annular dark field (ADF) cross-sectional STEM images of Nb 1+x S 2 clearly reveal Nb in both 2H phase (Figure1b) and the 3R phase (Figure1c). The d (0002) spacing for the 2H excess Nb phase (6.35 Å, c = 12.60 Å) was found to be close to the theoretical value of the Nb 1.35 S 2 composition. The lower contrast in ADF image of the intercalated Nb planes is attributed to partial occupation of these sites (occupancy of 0.35 in excess layers versus 1 in the NbS 2 2H and 3R layers). Additional evidence for intercalated Nb atoms in the form of Z contrast scans of atoms and chemical analyses are provided in Extended Data Fig.3a,b. X-ray diffraction (XRD) spectrum shown in Extended Data Fig.3cprovides additional evidence for the intercalated hexagonal phase of NbS 2 . Direct synthesis of intercalated layered compounds with CVD is highly beneficial for preserving the high quality of the material.

  1.35 S 2 catalysts. The reaction for metallic 2H Nb 1.35 S 2 phase starts immediately upon application of the potential and proceeds slowly initially but as the potential is increased, it proceeds rapidly with very high current densities. The Tafel slopes shown in Figure 2c provide additional information about the rate limiting step in the catalytic performance. Tafel slope values can vary from 30 mV-dec -1 for Pt -indicating that HER is limited by the recombination of adsorbed hydrogens -to > 120 mV-dec -1 where adsorption of protons limits the catalytic activity. It can be seen that the Tafel slope for 2H phase Nb 1.35 S 2 catalysts is 43 mV-dec -1 (and ~ 38 mV-dec -1 after iR correction) at low overpotentials (100 -120 mV), which is comparable to what has been measured for metallic 1T phase MoS 2 and NbS 2 and

Nb 1 .

 1 35 S 2 , we calculated the thermodynamics of the limiting reaction -the adsorption of hydrogen on the catalyst surface using density functional theory (DFT) including van der Waals interactions. It is well known that the best HER catalysts possess Δ ~ 0 (that is, thermoneutral condition) so that there is a driving force for hydrogen adsorption on the active site but the binding energy is low to allow efficient hydrogen evolution. The results of the calculations are shown in Figure4. Our theoretical results suggest that both the Nb and S terminations are stable for the Nb 1.35 S 2 phases. We have observed intercalated Nb terminated surface by STEM imaging (see Extended Data Fig.3d). It can be seen in Figure4athat the free energy for hydrogen adsorption for the 2H Nb 1.35 S 2 is close to being thermoneutral (~ 0.11 eV) when the surface is terminated by Nb under 0.25 monolayer coverage (Figure4b). In contrast, the free energy of adsorbed hydrogen is ~ 1eV when the surface is terminated by sulfur in 2H Nb 1.35 S 2 . The calculations also reveal that the reaction is localized to the surface of the catalysts, as indicated by the charge density diagrams in Figure4c. The better HER properties of the 2H Nb 1.35 S 2 relative to the 3R Nb 1.35 S 2 is consistent with the higher density of states at the Fermi level (Figure4e,f), which translates into higher measured conductivity and better charge transfer kinetics. Thus, our work provides new insight into materials design for achieving high current densities with non-precious metal catalysts.

Figure captions Figure 1 |

 captions1 Figure captionsFigure 1 | Images of Nb 1+x S 2 crystals and atomic structure. a, Atomic force microscopy image of a grown Nb 1+x S 2 (x ~ 0.35) crystals on high quality glassy carbon (scale bar = 1 μm). b, Cross-sectional ADF STEM image of 2H phase Nb 1.35 S 2 and 3R phase Nb 1+x S 2 , c. The dashed white rectangle represents the unit cell of the structure and the corresponding ball and stick model of the unit cell is shown on the right. The red dashed rectangles in the STEM image and the schematic indicate the positions of the intercalated Nb. Since these sites are partially occupied, the contrast of these planes is lower than of the fully occupied Nb sites. The arrows correlate the locations of the atoms in the STEM image with the atoms in the unit cell. The dark green arrows and spheres in the schematic indicate fully occupied Nb sites and light green arrows and spheres in unit cell indicate partially occupied Nb atoms. Z. A. refers to zone axis of the crystal structure. Scale bar = 5 Å.

Figure 2 |

 2 Figure 2 | HER catalytic activities of different TMDs. a, Polarization curves for 2H-MoS 2 , 1T-MoS 2 and WS 2 , 2H-Nb 1.35 S 2 , 3R-Nb 1.35 S 2 , 2H-NbS 2 and 3R NbS 2 and Pt measured in 0.5M H 2 SO 4 with a scan rate of 5 mVs -1 . b, Expanded view of the low overpotential region in a. The reaction starts immediately upon application of voltage, indicating that the onset potential of 2H-Nb 1.35 S 2 is < 100 mV and overpotential is 150 mV. c, Tafel slopes of different catalysts obtained from polarization curves in a. 2H-Nb 1.35 S 2 shows slope of 43 mV-dec -1 (38 mV-dec -1 after iR correction) at low overpotentials (100-130 mV, green) and 70 mV-dec -1 (65 mV-dec -1 after iR correction) at high overpotentials (130-200 mV, red). d, TOF values of the 2H-Nb 1.35 S 2 (black line) and other MoS 2 -based catalysts reported in the literature[START_REF] Kibsgaard | Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate [Mo 3 S 13 ] 2clusters[END_REF][START_REF] Xie | Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution[END_REF] . MoS 2 basal planes with sulfur

Figure 3 |

 3 Figure 3 | Electrochemical impedance spectroscopy (EIS), electrochemical stability of Nb 1.35 S 2 and proof of concept electrolyzer demonstration. a, Nyquist plots of 2H phase (red curve) and 3R phase Nb 1.35 S 2 . Magnification of 2H-Nb 1.35 S 2 plot is shown in the inset. The plots (dash line) were fitted using an equivalent circuit to extract the series and charge transfer resistances. b, Polarization curves of 2H-Nb 1.35 S 2 before (black curve) and after (red curve) 10,000 cycles. The insets show the percentage of change in overpotential. The error bars are

Figure 4 |

 4 Figure 4 | Thermodynamic stability and free-energy calculations for hydrogen evolution for 2H-Nb 1.35 S 2 and 3R-Nb 1.35 S 2 phases. a, The free-energy diagram for hydrogen evolution at standard conditions (1 bar of H 2 and pH=0 at 300 K). The energies of the intermediate states are calculated using BEEF-vdW functional as described in the Methods. A coverage of 0.25 ML was used for all calculations. b, Optimized geometries for S-and Nb-terminated surfaces of 2H-Nb 1.35 S 2 and 3R-Nb 1.35 S 2 phases. c, Charge density difference for H adsorbed on top of Nbterminated 2H-and 3R-Nb 1.35 S 2 phases. Most of the charge is localized on the surface, with clear differences between 2H-and 3R-Nb 1.35 S 2 phases with the former having larger magnitudes. e-f, Density of states (DOS) per atom for Nb-terminated 2H-and 3R-Nb 1.35 S 2 phases projected on different Nb sites. Surface Nb, inner Nb and deep bulk Nb atoms are shown in red, blue and orange, respectively. For a better contrast between the different Nb's atoms, the DOS of surface Nb atoms is highlighted with a filled faint red. Fermi level is set to 0 eV.

Figure 1 2 :Fig. 7 .

 127 Figure 1:

Nb 2 O

 2 5 and 100 mg of KI powders in alumina boat was loaded at the center of the tube furnace and SiO 2 substrates were placed above the powder with its polished side facing down. 100 mg of S powder was loaded in the upstream region of the tube. Then, 80 sccm carrier gas (10 % of H 2 in Ar) was introduced for 20 min to remove oxygen in the furnace. The furnace was heated with a ramp rate of 33 °C/min to the growth temperatures (1000 °C) and held at 1000 °C for 30 min before cooling down. The upstream region reached 200 °C during the growth. The thin 2H NbS 2 crystals and their corresponding Raman spectrum are shown in Extended Data Fig.8a,b.

Electrochemical measurements:

  Electrochemical measurements were performed in a threeelectrode cell using a Versa Stat 3 potentiostat from Princeton Applied research. All measurements were made in 0.5 M H 2 SO 4 electrolyte purged with Ar gas. A saturated calomel electrode (Pine Instrument) and glassy carbon were used as the reference electrode and counter electrode, respectively. Extended Data Fig. 9 shows the polarization curves obtained from 2H Nb 1.35 S 2 using platinum and carbon counter electrode. No noticeable difference was observed, and this excludes any Pt contamination during the measurements. The glassy carbon plate (Ted pella) loaded with 3R-and 2H-Nb 1.35 S 2 was used as a working electrode. 3R-NbS 2 was dispersed into toluene and sonicated for 1h. The solution was deposited onto glassy carbon electrode (0.1 mg/cm 2 ) and dried. 1T-MoS 2 was prepared by the exfoliation of 2H-MoS 2 by n-BuLi following literature. 20 1T-MoS 2 solution was loaded onto glassy carbon electrode (4 µg/cm 2 ) and 2H-MoS 2 powder is dispersed into the mixed solution with DI water/IPA (the volume ratio 4:1) and sonicated for 1h to drop onto the glassy carbon electrode. All polarization curves were measured at the scan rate of 5 mVs -1 . All potentials are referenced to RHE. In 0.5 M H 2 SO 4 , E(RHE)=E(SCE) + 0.254 V. Impedance measurements were performed at -0.22 V versus RHE from 1MHz to 0.1 Hz with a 5 mV a.c. amplitude.

. 2 |

 2 | Phases of NbS 2 . (a, b) Side view and (d, e) top view of 2H hexagonal (P63/mmc symmetry) and 3R rhombohedral (R3m symmetry) phases of NbS 2 . (c, f) Side and top views of 2H phase Nb 1.35 S 2 phase. The excess of Nb (labelled as Nb II in the c) occupies octahedral configuration while the Nb I are in trigonal prismatic sites. The Nb 1.35 S 2 phase can also occur in the 3R phase. TEM of Nb 1.35 S 2 and corresponding diffraction images. a, Low magnification BFTEM image of cross-sectional FIB sample (thickness = 40 nm) prepared from CVD grown samples on SiO 2 . b, c HRTEM images taken from top of the sample and bottom of the sample as indicated by square dotted regions on the left image. d, e Fast Fourier transform (FFT) pattern indicating that the top (thicker) region is rhombohedral while the bottom (thin) region is in hexagonal configuration based on their very good agreement with simulated patterns shown in f (simulated ED patterns for intercalated 3R phase) and g (simulated ED for intercalated 2H phase). h, Low magnification of image of the FIB cross section and the corresponding Energy Dispersive Spectroscopy map showing Nb (i) and S (j) in the sample.

. 3 |. 4 |

 34 Line scan and chemical analyses in STEM. a, Annular dark field image (left) and the corresponding (right) intensity line scan of 2H Nb 1.35 S 2 phase showing that Nb atoms are present between the layers. Scale bar = 5Å. b, Black and white unfiltered STEM image and corresponding elemental maps of Nb, S, and both Nb and S obtained using energy dispersive x-ray spectroscopy. The mapping and the line scan below the Nb map confirm that the intercalated atoms are Nb. c, X-ray diffraction of Nb 1.35 S 2. The spectrum matches JCPDS#97-004-3699 for hexagonal phase Nb 1.35 S 2 , indicated by the triangles. In particular, the presence of additional diffraction peak at ~14.3 o indicates excess Nb . The structure is a combination of layered NbS2 and intercalated 2H Nb 1.35 S 2 -completely consistent with our STEM results. d, STEM image of 2H phase Nb 1.35 S 2 with an enlargement of the surface region shown on the right. Faint images of Nb atoms terminating the surface can be observed as indicated by the dotted red circle. Image analyses of areal coverage of catalysts on electrode surface. a, SEM image of 2H-Nb 1.35 S 2 . b, Image analyses was used to highlight (in green) the Nb 1.35 S 2 flakes on the glassy carbon electrode (black background). Then the percent of area covered by the flakes was calculated. A total of 15 images were taken and the average area covered by the catalyst Nb 1.35 S 2 flakes was approximately 20 % ± 3% of the electrode surface.

. 5 |. 9 |

 59 Electrochemical surface area measurements. a, CV curves of Nb 1.35 S 2 electrodes measured between 285 and 315 mV vs. RHE. b, Current density at 300 mV plotted against cyclic voltammetry scan rate. Pt vs glassy carbon as counter electrode. Polarization curves for glassy carbon plate and 2H Nb 1.35 S 2 using Pt or carbon as counter electrode. Extended data Fig. 10 | Electrochemical microcell devices. Optical image of an electrochemical microcell with 2H-NbS 2 as the catalyst. The blue region is the NbS 2 flake and rectangular gold contacts are patterned onto it. A window is opened lithographically so that catalysis measurements can be made. The entire area is covered by PMMA except the window so the electrolyte can only interact with the catalyst. Extended data Fig. 11 | Electrical properties of different NbS 2 . a, I-V curve measured for the 2H Nb 1.35 S 2 nanoflakes. The inset shows a SEM image of the device. An average value of 1800 S/cm was obtained with the highest value being > 2400 S/cm. b, Comparison of conductivities for NbS 2 . NbS 2 is known as a metallic TMD material. However, the conductivity values in literature are scattered. 30-33 Extended data Fig. 12 | Definition of geometrical parameters used at Extended Data Table 3 at regular 2H-NbS 2 and 3R-NbS 2 phases as well as in the intercalated 2H-Nb 1.35 S 2 , 3R-Nb 1.35 S 2 phases. monolayer NbS 2 with an additional Nb layer into the system. This would correspond to the smallest "intercalated phase" before it established in a 2H or 3R phases. The energetics for these systems are shown in Extended Data Table

  That is, intercalation involving wet chemistry can lead to undesirable local reactions and deterioration of the material properties.The catalytic properties of NbS 2 and other metallic TMDs such as TaS 2 and VS 2 have been measured in several studies[START_REF] Liu | Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution[END_REF][START_REF] Shi | Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst[END_REF][START_REF] Yuan | Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction[END_REF] . The results generally reveal Tafel slopes of ~ 40 mV-dec -1 and low overpotential values of tens of millivolts vs RHE. For comparison, the overpotential at which HER proceeds for Pt nanoparticles is 0 V vs RHE with a corresponding Tafel slope of ~ 30 mVdec -1 . Despite the promising overpotentials and Tafel slopes achieved with metallic TMD catalysts, the maximum reported current density value has remained around ~ 100 mA-cm -2 . The current density is limited by the multiple layers of metallic TMDs. That is, in contrast to phase transformed 1T metallic MoS 2 nanosheets that are single layered, it is generally challenging to synthesize or exfoliate metal TMDs down to the monolayer. Thus, most metallic NbS 2 and TaS 2

Table 1 .

 1 Impedance measurements on 2H and 3R Nb 1.35 S 2 are shown in Figure3a. It can be seen that the series resistance for the 2H phase Nb 1.35 S 2 obtained at 1MHz where the phase angle is close to zero is 3.5 Ω due to its high electrical conductivity and indicating good contact with glassy carbon electrode. The charge transfer resistance is 7.4 Ω, facilitating charge injection and transport to active sites. In contrast, the 3R Nb 1.35 S 2 shows higher resistances and diffusion limited activity. This is in agreement with our observations that the 3R phase is less conducting and is stable at higher film thicknesses. Finally, we have measured the electrochemical stability of the electrodes by carrying out over 10000 cycles. The high current density shows remarkable stability with negligible difference in polarization curves and overpotential values after 10,000 cycles (Figure3b). The above experimental results suggest that intercalation of Nb between 2H phase NbS 2 layers to realize Nb 1.35 S 2 allow realization of practical current densities of > 5,000 mA-cm -2 -making them potentially useful alternatives to Pt and Ir. To translate the high current densities of 2H phase Nb 1.35 S 2 catalysts into practical device, we fabricated a proof of concept two electrode electrolyzer for water splitting. The device shown in Figure3cconsists of 2H Nb 1.35 S 2 on glassy carbon cathode and a commercial Pt anode. The measurements were carried out at room temperature in 0.5 M H 2 SO 4 . The device characteristics are plotted in Figure 3d, which show that the reaction for the 2H Nb 1.35 S 2 starts around 1.3 V and the current densities reach 1000 mA-cm -2 at 2.0 V. Proof of concept electrolyzers of this type typically report current densities of 10 -100 mA-cm -2 at similar potential values 24 .

[START_REF] Voiry | Enhanced catalytic activity in strained chemically exfoliated WS 2 nanosheets for hydrogen evolution[END_REF]

), suggesting that the turnover for hydrogen evolution is high for 2H phase Nb 1.35 S 2 catalysts. Summary of the obtained values is provided in Extended Data

Table 1 |

 1 Comparison of TOF and exchange current density of TMDs catalysts

	2H-Nb 1.35 S 2 2H-MoS 2 /Au 1T-WS 2 2H-TaS 2 Pt	J 0 (µA/cm 2 ) 800 ± 100 7.9 20 100-179.47 450	TOF at J 0 0.17s -1 ± 0.03 -0.043 -0.9	ref Our work 9 3 5 9
	Extended Data

Table 2 |

 2 Parameters used for synthesis of Nb 1+x S 2

	3R-Nb 1+x S 2 2H-Nb 1+x S 2	NbCl 5 35 mg 20 mg	S 200 mg 100 mg	Temperature of NbCl 5 300 °C 260 °C

Table 4 | Thermodynamic properties of S terminated surface using opt-B88 functional.

 4 Energetics of the intercalated phase 2H-Nb 1.35 S 2 with adsorption of H at S atoms at different coverages using opt-B88 functional. The large values of ΔG H (eV) at different coverages rule out any HER using S-terminate surfaces. 2H-Nb 1.35 S 2 (H-S adsorption)

	Coverage	0.11 ML	0.25 ML	1 ML
	ΔE (eV)	1.011	0.804	1.459
	ZPE (eV)	0.203	0.203	0.162
	ΔG H (eV)	1.245	1.077	1.694

Extended Data Table 5 | Bulk lattice parameters using BEEF-vdW functional. Simulations of

  the geometrical parameters using BEEF-vdW functional including vdW interactions. Lattice parameter, a(Å), interlayer distance via Nb-Nb distance, d(Å), and supercell length, c(Å) are defined as in Extended Data Figure12. Sound comparison with experimental data at different

	Extended
	phases is obtained.
		2H-NbS 2	3R-NbS 2	2H-Nb 1.35 S 2	3R-Nb 1.35 S 2
		Theory	Exp.	Theory	Exp.	Theory	Exp	Theory
	a( )	3.35	3.31	3.35	3.33	3.30	3.30	3.22
	d( )	6.34	5.91	6.36	5.98	6.65	6.35	6.65
	c( )	12.67	11.89	19.10	17.81	13.30	12.6	19.94

Data Table 6 | Thermodynamic properties of S terminated surface using BEEF- vdW functional.

  Energetics of the intercalated phases 2H-Nb 1.35 S 2 and 3R-Nb 1.35 S 2 with adsorption of H at S atoms at different coverages using BEEF-vdW functional. Similarly as in the opt-B88 functional, the S-terminated surface seems inert for HER as long as H atoms bind on

	S.
			2H-Nb 1.35 S 2 (H-S adsorption)		3R-Nb 1.35 S 2 (H-S adsorption)
	Coverage	0.11 ML	0.25 ML	1 ML	0.25 ML
	ΔE (eV)	1.005	0.766	1.41	0.619
	ZPE (eV)	0.226	0.226	0.16	0.226
	ΔG H (eV)	1.302	0.989	1.64	0.915
	Extended Data

Table 7 | Thermodynamic properties of a monolayer NbS 2 with additional Nb atoms in the structure.

 7 Energetics of the monolayer NbS 2 with additional Nb atoms incorporated into the structure (see Extended Data Fig.13). This system corresponds to the smallest situation where the effect of additional Nb atoms could be appreciated on the free energies and HER of monolayer NbS 2 . The adsorption of H is simulated either at S-terminated the surface or at Nb-terminated. The large variations in energy between both configurations indicate the H adatoms would prefer binding at the Nb sites rather than on S. This suggests that additional Nb atoms have a strong effect on the chemical kinetics of the reaction changing the preferable site of H adsorption for efficient HER. Similar results as those in Ref. 22 are compared for the monolayer NbS 2 without additional Nb atoms. Calculations at the level of BEEF-vdW functional.

		Monolayer NbS 2	Monolayer NbS 2 Nb terminated (H-S adsorption)	Monolayer NbS 2 Nb terminated (H-Nb adsorption)
	Coverage	0.0625 ML	0.25 ML	0.25 ML
	ΔE (eV)	-0.185	1.039	0.072
	ZPE (eV)	0.226	0.215	0.098
	ΔG H (eV)	0.11	1.324	0.24

Acknowledgments: MC, JY acknowledge financial support from AFOSR FA9550-16-1-0289. MC, YW acknowledge support from NSF ECCS-1608389. MC and XS acknowledge support from Shenzhen Peacock Plan (Grant No. KQTD2016053112042971). MC and ARM acknowledge financial support from the Ministry of Higher Education Malaysia. HYJ acknowledges the support from Creative Materials Discovery Program through the National Research Foundation of Korea (NRF-2016M3D1A1900035). E.J.G.S. acknowledges the use of computational resources from the UK national high-performance computing service (ARCHER) for which access was obtained via the UKCP consortium (EPSRC grant ref EP/K013564/1); the UK Materials and Molecular Modelling Hub for access to THOMAS supercluster, which is partially funded by EPSRC (EP/P020194/1). The Queen's Fellow Award through the grant number M8407MPH, the Enabling Fund (A5047TSL), and the Department for the Economy (USI 097) are also acknowledged.

samples and characterized them with the help of FZ. HYJ prepared the FIB samples, performed the STEM analyses on the samples. MA and EJGS provided theoretical insight for the experimental results. MC, HYJ, JY, DV, RF and HS analyzed the data. MC wrote the paper with JY and all of the authors edited the manuscript before submission.

Computational Methods: The calculations were done using the plane wave density functional theory (DFT) Vienna ab-initio simulation package (VASP) [START_REF] Kresse | Ab initio molecular dynamics for liquid metals[END_REF][START_REF] Kresse | Ab initio molecular-dynamics simulation of the liquid-metal\char21{}amorphous-semiconductor transition in germanium[END_REF][START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF][START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF] The van der Waals interactions were taken into account at the level of opt-B88 functional [START_REF] Klimeš | Chemical accuracy for the van der Waals density functional[END_REF] and the Bayesian error estimation exchange-correlation functional with long-range interactions [START_REF] Wellendorff | Density functionals for surface science: Exchange-correlation model development with Bayesian error estimation[END_REF] (BEEF-vdW). A 500 eV planewave cutoff was used, and the Brillouin zone was sampled with a Monkhorst-Pack 40 sampling of 3x3x1 k-points grid for the relaxation and a 15x15x1 k-points for the energy calculations. The ∆G H were calculated using the formula:

Where E(surface + H) is the total energy of the system with the H-adatom bound at the basal plane, E(surface) is the total energy of the pristine system, E(H 2 ) is the total energy of the H 2 molecule. ∆(ZPE) is the difference between the zero-point energy (ZPE) of the H-adatom and the zero-point energy of the H 2 molecule, calculated using the normal mode analysis. We will use the entropy of the molecular hydrogen in the gas phase at standard conditions (1 bar of H 2 , pH=0 and T=300 K).

The bulk lattice parameters calculated using ab initio opt B88-vdW and BEEF-vdW functionals are shown in Extended Data Tables 3 and5