# Rationalizing the Superior Catalytic Efficiency of Nickel Nitride vs Nickel Sulfide for Alkaline Hydrogen Evolution Reaction from Bubble Dynamics Study and Density Functional Theory (DFT) Calculations

Chinkit Tyagi, Pooja Basera, Corinne Lagrost, Valérie Bouquet, Franck Tessier, Marie-Caroline Jullien, Bruno Fabre

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Chinkit Tyagi, Pooja Basera, Corinne Lagrost, Valérie Bouquet, Franck Tessier, et al. Rationalizing the Superior Catalytic Efficiency of Nickel Nitride vs Nickel Sulfide for Alkaline Hydrogen Evolution Reaction from Bubble Dynamics Study and Density Functional Theory (DFT) Calculations. ACS Catalysis, 2025, 15, pp.3823-3835. 10.1021/acscatal.4c05777. hal-04975408

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Rationalizing the Superior Catalytic Efficiency of Nickel Nitride vs. Nickel Sulfide for
Alkaline Hydrogen Evolution Reaction From Bubble Dynamics Study and Density
Functional Theory (DFT) Calculations

Chinkit Tyagi,¹ Pooja Basera²,³, Corinne Lagrost,¹,⁴ Valérie Bouquet,¹ Franck Tessier,¹ Marie-Caroline Jullien,⁵ and Bruno Fabre¹,*

¹ CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR6226, Univ Rennes, Rennes
F-35000, France

² Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

³ SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory,
Menlo Park, California 94025, United States

⁴ ScanMAT – UAR 2025, F-35000 Rennes, France

⁵ CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, Univ Rennes, Rennes F-35000,
France.

ABSTRACT. Due to their high electronic conductivity, high catalytic activity and superior chemical stability, nickel nitrides and sulfides have been demonstrated to be cost-effective and robust electrocatalysts for achieving HER under alkaline conditions. Herein, we report on a simple and optimized approach to directly grow single-phase nickel nitride (Ni₃N) and nickel sulfide (Ni₃S₂) on a flat nickel substrate. Different preparation conditions are tested in order to
achieve materials exhibiting the best electrocatalytic efficiency for HER. The optimized Ni₃N and Ni₃S₂ on nickel are obtained at 700°C for 30 min under ammonia gas flow and at 350°C for 1 h in 10 % H₂S/H₂ gas mixture, respectively. Ni₃N operates HER in 1M KOH more efficiently than Ni₃S₂, as supported by overpotential values of 0.189, 0.291 and 0.342 V measured at 10, 100 and 200 mA cm⁻², respectively, which are lower than those measured for Ni₃S₂, i.e. 0.204, 0.351 and 0.417 V. Remarkably, it is worth noticing that its HER activity competes with that of Pt for current densities higher than 200 mA cm⁻². Its superior catalytic activity is corroborated by additional cyclic voltammetry (Tafel slopes) and electrochemical impedance spectroscopy measurements. Moreover, Ni₃N and Ni₃S₂ are found to be stable over 45 h electrolysis at 10 mA cm⁻² with a potential change of only 24 and 31 mV, respectively. To gain further understanding on the electrocatalytic HER activities of both materials, Density Functional Theory (DFT) calculations and *in situ* bubble dynamics study are performed. Owing to the more hydrophilic character of Ni₃N, smaller H₂ bubbles form and detach more rapidly from the surface which leads to a fast renewal of the active surface for HER. In contrast, for Ni₃S₂, both the bubble size and the retention time increase, leading to the adverse blockage of the active sites and requiring higher overpotential for HER. This observation perfectly aligns with DFT calculations, which show that H₂O adsorption is predominantly favored on the Ni₃N surface. Our work highlights that the use of a planar electrocatalyst support instead of foam-type porous one is essential to evaluate the alone contribution of the electrocatalyst on the electrogenerated gas bubble dynamics and consequently the impact on the catalytic performance.

KEYWORDS. Water Splitting, Hydrogen Evolution Reaction, Electrocatalysis, Bubble Dynamics, Nickel Nitrides, Nickel Sulfides
# 1. Introduction.

The production of dihydrogen ($H_2$) by electrolytic water splitting constitutes a carbon-free, clean and sustainable alternative to traditional steam reforming based on fossil fuels which is emitting greenhouse gases ($CO_2$ and CO) and is highly energy intensive.¹,²,³ In that context, alkaline water electrolysis has aroused a tremendous interest because it allows the use of cost-effective and Earth-abundant transition metals (e.g. Ni, Mo, Fe and Co) as efficient electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).⁴,⁵,⁶ In particular, transition metal chalcogenides, phosphides, borides, nitrides, oxides and carbides have been intensively studied as electrocatalysts for these two half-reactions of interest, essentially owing to their high electronic conductivity, excellent catalytic activity and superior chemical stability.⁷,⁸,⁹,¹⁰,¹¹ Among these materials, nickel nitrides and sulfides have been demonstrated to be catalytically efficient for HER and other energy conversion reactions.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷ They can be prepared from the direct nitridation or sulfidation of nickel under $NH_3$ or $H_2S-H_2$ mixtures using various conditions (time, temperature, and gas mixture composition).¹⁸,¹⁹,²⁰ These electrocatalysts can be also produced in the form of nanostructures (nanosheets, nanorods and nanowires) using various methods, including hydrothermal, electrodeposition, chemical vapor deposition, and microwave-assisted methods.²¹,²²,²³,²⁴,²⁵,²⁶,²⁷,²⁸,²⁹ Such a catalyst nanostructuring was found to be an effective strategy to enhance the electrocatalytic performance due to the high surface area and the facilitated release of the electrogenerated gas bubbles.³⁰ For the latter, it is worth recalling that gas bubbles evolving at the electrode surface may result in undesired blockage of active sites and negatively impact the performance of the electrocatalytic process.³¹ This is a critical issue for industrial electrolyzers as bubble films formed at large scale (several hundred cm²) electrodes can be more
than 1 cm thick, leading to high Ohmic resistances and mass transfer barriers of the electrode
reactions. As a result, a strong enhancement in the electrical power consumption of the
electrolyzer may occur.³² Such detrimental effects can be minimized by acting on the bubble
dynamics, i.e. by facilitating and controlling the formation of small H₂ bubbles and shortening
the retention time at the electrode surface before their release.³³,³⁴ Besides the morphology of the
electrocatalysts, their surface-wetting properties can also greatly influence the detachment of the
gas bubbles and consequently the overall performance of the water splitting.³⁵,³⁶,³⁷,³⁸,³⁹

Herein, we propose a simple and original approach to directly grow HER electrocatalysts,
namely single-phase nickel nitride (Ni₃N) and nickel sulfide (Ni₃S₂), on a flat nickel substrate.
These two catalysts were selected because (i) they exhibited different hydrophilic/aerophobic
properties, and (ii) they showed the highest activities for alkaline HER among the other Ni
nitride (Ni₃N > Ni₄N)⁴⁰,⁴¹,⁴²,⁴³,⁴⁴ and sulfide (Ni₃S₂ > NiS₂ > NiS)⁴⁵,⁴⁶,⁴⁷ phases. Moreover, a flat
Ni substrate was preferred to porous Ni one (e.g. Ni foam) in order to eliminate the effect of the
porosity of the substrate on the dynamics of electrogenerated H₂ bubbles⁴⁸ for investigating only
the intrinsic effects of the prepared catalyst. After optimization of the preparation conditions of
these materials, the best performing electrodes were further characterized using multiple surface
characterization techniques to get more insights on their morphological, topographical, and
wetting properties. Finally, the in-situ bubble formation was monitored by videography to
understand the effects of surface hydrophilicity or aerophobicity on the electrochemical HER
activity of the catalytic electrodes in 1 M KOH solution. Complementary Density Functional
Theory (DFT) calculations provided essential information on the thermodynamics of alkaline
HER at both materials.
## 2. Materials and Methods

**2.1. Chemicals.** Nickel foil (NF) with a thickness of 0.5 mm, 20% platinum (Pt) on graphitized carbon, poly(vinylidene fluoride) (PVDF) binder, and *N*-methyl-2-pyrrolidone (NMP) were purchased from Sigma-Aldrich. VULCAN carbon black and KOH were bought from Cabot and Thermo Fisher Scientific, USA, respectively. All chemicals were used as received, and ultra-pure water (Veolia Water-STI) with a resistivity of 18.2 MΩ cm was used to prepare the aqueous solutions.

**2.2. Direct growth of Ni₃N (Ni₃N@NF) and Ni₃S₂ (Ni₃S₂@NF) on NF.** Before the thermal treatment, the NF piece (1 x 1.5 cm²) was successively cleaned under sonication for 10 min in acetone, absolute ethanol, and ultrapure water to remove impurities and was dried using an argon flow gun. Cleaned NF was thermally treated at various temperatures and times in a tubular furnace with a heating rate of ~10 °C/min under ammonia (NH₃, ≥ 99.96 %) and H₂S/H₂ (10%/90 %) gas flow to generate Ni₃N (Ni₃N@NF) and Ni₃S₂ (Ni₃S₂@NF), respectively, on NF. The Ni₃N@NF electrodes were obtained at 700 °C for 30 min, 1 h or 2 h under ammonia gas flow. To prepare Ni₃S₂@NF, cleaned NF was heated at different temperatures (300, 350, 400, 450, and 500 °C) for 30 min under H₂S/H₂ gas mixture to grow the nickel sulfide phase on NF. Ni₃S₂@NF electrodes were further optimized for the sulfidation time (from 30 min to 4 h) at an optimized temperature of 350 °C.

**2.3. Instrumentation.** X-ray diffraction (XRD) patterns of synthesized materials on NF were obtained using Bruker D8 Advance diffractometer (Cu Kα1 radiation, λ = 1.5406 Å) with a LynxEye fast detector.

JSM-7100F JEOL Scanning electron microscope (SEM) equipped with an X-Max 50 mm² EDS detector (Oxford Instruments) was used for images and elemental mapping.
The profilometry measurements were conducted at three different areas (500 µm x 500 µm) for each electrode using a Bruker Dektak X profilometer having a Stylus B-type tip with a curvature radius of 12.5 µm, and average roughness was calculated after tilt angle correction.

An automated Krüss Easy Drop goniometer was used for contact angle measurements using a 1M KOH solution. Contact angles were determined using the Tangent 2 fitting model with 2.0 µL droplets of 1M KOH at three different spots for each electrode.

XPS data have been collected by a NEXSA G2 (ThermoFischer Scientific) spectrometer using the Al Kα X-ray source working at 1486.6 eV with a spot size of 200 µm². Survey spectra (0-1000 eV) were acquired with an analyzer pass energy of 200 eV (1 eV/step); high-resolution spectra used pass energy of 50 eV (0.1 eV/step). Binding energies were referenced to C 1s peak at 284.8 eV (adventitious carbon). The core level spectra were peak-fitted using the CasaXPS Software, Ltd. Version 2.3.25PR1.0. U2 Tougaard or Shirley background subtraction was used for the spectral analysis. The peak areas were normalized by the manufacturer-supplied sensitivity factor (S<sub>C1s</sub> = 1, S<sub>O1s</sub> = 2.88, S<sub>Ni2p</sub> = 20.76, S<sub>N1s</sub> = 1.68 and S<sub>S2p</sub> = 1.88). For each sample, XPS survey spectra were recorded at three different locations, and we did not observe any difference.

The electrochemical measurements were performed at room temperature in a three-electrode configuration on a BioLogic SP-200 single-channel electrochemical workstation equipped with an AC frequency response analyzer (FRA). An alkaline solution (1.0 M KOH, pH = 14 ± 0.1) was used as the electrolytic solution. Bare or modified NF was masked from one side with Teflon tape and used as the working electrode (1 cm² geometric surface area). A graphite rod and Hg/HgO (filled with 1.0 M KOH) were used as the counter and reference electrodes, respectively. A 20 nm-thick Pt film-coated NF electrode was used for comparison. It was
obtained by magnetron sputtering of a Pt target on NF. All HER polarization curves were recorded by linear sweep voltammetry (LSV) at 5 mV s⁻¹ with 95% iR compensation using the automated iR-correction function of the potentiostat. The current densities (j) were normalized by the geometric surface area (1 cm²) of NF electrodes, and overpotential vs. log (current density) (η vs. log j) linear curves were plotted to determine the corresponding Tafel slopes. All potentials measured vs. Hg/HgO were converted versus the reversible hydrogen electrode (RHE) according to the equation given below:

$$E_{\text{RHE}} = E_{\text{Hg/HgO}} + 0.059 \text{ pH} + E_{\text{Hg/HgO}}^{0} \text{ with } E_{\text{Hg/HgO}}^{0} = +0.102 \text{ V} \quad (1)$$

where E<sub>Hg/HgO</sub> is applied potential versus the used reference electrode.

Electrochemical impedance spectroscopy (EIS) measurements were recorded at an applied overpotential of 0.2 V over a frequency range from 100 kHz to 100 mHz (50 frequencies, logarithmic distribution) at 10 mV amplitude. The double-layer capacitance (C<sub>dl</sub>) was calculated using cyclic voltammograms (CVs) recorded in the non-Faradaic region at various scan rates (20 to 100 mV s⁻¹). C<sub>dl</sub> (in mF cm⁻²) corresponds to the slope value of the Δj/2 vs. v plots where Δj is the difference between the anodic and cathodic current densities and v is the potential scan rate. Electrochemically active surface area (ECSA) was calculated from C<sub>dl</sub> and the specific capacitance (C<sub>s</sub>) of a flat metallic Ni surface that was assumed to be 20 µF cm⁻² (ECSA = C<sub>dl</sub>/C<sub>s</sub>).<sup>49</sup> The long-term stability tests of the prepared electrodes were examined using chronopotentiometry at an applied current density of -10 mA cm⁻².

Prior to electrochemical measurements, Ni<sub>3</sub>S<sub>2</sub> was activated by successively cycling the Ni<sub>3</sub>S<sub>2</sub>@NF electrode at 20 mV/s between 0.1 and -0.57 V vs RHE in 1 M KOH solution (50
scans). As previously reported, such a procedure led to an increase in the surface area of the electrode and in the number of exposed active sites.⁵⁰,⁵¹,⁵²

For studying bubble dynamics, H₂ bubbles were produced in a three-electrode cell system with vertical configuration under the same conditions (1.0 M KOH, pH = 14 ± 0.1) at the current density of -5 and -10 mA cm⁻² without any convection. The bubble dynamics was monitored using a Fastcam UX50 (Photron) through a stereomicroscope (Leica MZ FLIII) placed aside of the cell. The bubbles were mapped manually using ImageJ freeware, and the average bubble density and size distribution were estimated for all the electrodes during HER.

**2.4. Computational details.** DFT calculations were performed as implemented in the Vienna Ab Initio Simulation package. Projector-augmented wave (PAW) pseudopotentials were employed to describe the ion-electron interactions.⁵³ The Perdew-Burke-Ernzerhof (PBE) exchange-correlational functional was used for optimizing the atomic position of the structures.⁵⁴,⁵⁵ The DFT optimized bulk structures of Ni, Ni₃N and Ni₃S₂ were used to construct the initial slabs, with a 2 x 2 in-plane supercell employed for slab construction. During the relaxation of the surface layers, only atomic positions were allowed to relax. A five-layer thick slab was started, with the top two layers relaxed while the internal layers were kept frozen to mimic the bulk. The surfaces were optimized using a 4 × 4 × 1 Monkhorst-Pack grid,⁵⁶ and a kinetic energy cutoff was set to 500 eV. Structural relaxation was performed until the forces were smaller than 0.02 eV/Å. For single-point energy calculations, the PBE functional was used with the energy convergence tolerance set to 0.01 meV. The Gibbs free energies (ΔG) of each step along the reaction pathway at T = 298 K were estimated using the equation (2):

$$[\Delta G = \Delta E - T\Delta S + \Delta EZPE] \qquad (2)$$
where ΔE, ΔS, and ΔEZPE are the changes in enthalpy (obtained from DFT total energy), entropy, and zero-point energy, respectively. Zero-point energies for the adsorbed and the free molecules were computed using DFT calculations, while the entropic contributions for free molecules were obtained from the thermodynamic tables.⁵⁷ For the adsorption of water molecules on surfaces, charge transfer was computed by examining electronic accumulation or depletion upon molecular adsorption by comparing the charge densities in the combined system and individual components. This analysis yielded the expression for charge transfer between the slab and the adsorbate (or vice versa), represented by the equation (3):

$$ \Delta\rho = \rho_{\text{slab + adsorb}} - (\rho_{\text{slab}} + \rho_{\text{adsorb}}) \quad (3) $$

where ρslab + adsorb, ρslab, and ρadsorb denote the total electronic charge density of the entire system containing the Ni, Ni₃N and Ni₃S₂ slab and the adsorbate, the electronic charge density of the slab, and the electronic charge density of the adsorbate (H₂O), respectively.

### 3. Results and Discussion.

3.1. Preparation/Optimization and Characterization of Ni₃N@NF and Ni₃S₂@NF. Different temperatures and durations for nitridation or sulfidation of NF have been tested and their effects on HER electrocatalytic properties of the resulting materials have been examined. The Ni₃N phase was directly grown on NF (Ni₃N@NF) from the thermal treatment of cleaned NF at 700°C for 30 min, 1 h or 2 h under ammonia (≥ 99.96%) gas flow. For the preparation of Ni₃S₂, cleaned NF was heated at different temperatures (300, 350, 400, 450, and 500°C) for 30 min under H₂S/H₂ (10 %/90 %) gas mixture to grow the nickel sulfide phase on NF. To further optimize the
$Ni_3S_2$ composition, the effect of the sulfidation time (from 30 min to 4 h) at an optimized temperature of 350°C was also examined.

The X-ray diffraction (XRD) patterns and the respective HER activity of thermally treated NF at 700°C for 30 min, 1 h, and 2 h under $NH_3$ are shown in Figures S1. All three diffraction patterns exhibited characteristic peaks indexed to hexagonal nickel nitride ($Ni_3N$) phase (JCPDS card no. 01-089-5144 or 00-010-0280) and cubic nickel (Ni) (JCPDS card no. 00-004-0850) coming from NF substrate. It was found that the nitridation duration had only a limited impact on the HER activity of the resulting materials.

The diffraction patterns and HER activity of NF thermally treated at 300, 350, 400, 450, and 500°C for 30 min in a 10 % $H_2S/H_2$ atmosphere are shown in Figure S2. At the temperature of 350°C, the XRD pattern confirmed the growth of the $Ni_3S_2$ phase as the main phase (JCPDS card no. 00-044-1418 or 00-030-0863). Furthermore, the material prepared at 350°C and 400°C for 30 min showed the highest HER activity among all the sulfidation temperatures (Figure S2b). Additionally, the XRD patterns and HER activity of materials prepared at an optimized temperature of 350°C using different sulfidation times are shown in Figure S3. NF treated at 350°C for 1 h showed characteristic peaks attributed to the desired $Ni_3S_2$ phase (JCPDS card no. 00-044-1418) along with a further improvement of HER activity. To sum up, best HER performing $Ni_3N$ and $Ni_3S_2$ on NF were obtained at 700°C for 30 min under ammonia gas flow and at 350°C for 1 h in 10 % $H_2S/H_2$ gas mixture, respectively. Their corresponding XRD patterns along with that of bare NF (BNF) are shown in Figure 1. These $Ni_3N@NF$ and $Ni_3S_2@NF$ electrodes were selected for further characterizations and their alkaline HER activity was examined in more details (vide supra).
<figure><img src="image_2.png" /><figcaption>**Figure 1.** XRD diffraction patterns of bare nickel foil (BNF) and NF thermally treated at 700°C for 30 min under $NH_3$ ($Ni_3N@NF$), and at 350°C for 1 h under $H_2S$ ($Ni_3S_2@NF$).</figcaption></figure>

$Ni_3N@NF$ and $Ni_3S_2@NF$ materials were further characterized by scanning electron microscopy (SEM), profilometry, and contact angle measurements to get more insights into the morphological, topographical, and wetting properties, respectively. As shown in Figures 2 and S4, the SEM images of $Ni_3N@NF$ and $Ni_3S_2@NF$ showed a rough and porous structure uniformly covering Ni. Consistent with that, the average root-mean-square roughness measured by profilometry was 176 ± 6, 732 ± 19, and 902 ± 55 nm for BNF, $Ni_3N@NF$, and $Ni_3S_2@NF$, respectively. Globally, the morphological characteristics of both materials were relatively
comparable. Additionally, water contact angles measured with 1 M KOH showed that the hydrophilic character of the materials increased in the following order: BNF (97 ± 5°) < Ni₃S₂@NF (70 ± 2°) < Ni₃N@NF (40 ± 1°) (Figures 2 and S5). Furthermore, the elemental mapping and the energy dispersive X-ray spectroscopy (EDS) analysis of these electrodes evidenced the homogeneous distribution of the expected elements, namely, Ni, N, and S (Figures S6 and S7) with a very small quantity of detected oxygen.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** SEM images, contact angle, and profilometry measurements of (a) bare nickel foil (BNF), (b) Ni₃N@NF and (c) Ni₃S₂@NF.</figcaption></figure>
In order to complete the XRD analyses, X-ray photoelectron spectroscopy (XPS) was also employed to probe in more details the chemical composition at the outermost surface of the materials (5-10 nm depth). It is worth noting that XPS measurements also investigate amorphous parts of the materials in contrast to XRD. Comparison of the survey spectrum of BNF with those of optimized Ni₃N@NF and Ni₃S₂@NF revealed as expected the presence of new photoelectrons peaks for the elemental species N or S, respectively (Figure S8). Core level spectra in the Ni 2p, N 1s and S 2p regions were subsequently recorded for Ni₃N@NF and Ni₃S₂@NF to probe the chemical states of the corresponding species. The Ni 2p spectra displayed the 2p doublet structure with an energy separation of 17.5 eV (Figure 3). Peak-fitting of the Ni 2p₃/₂ signals for the three samples led to two main lines and a minor contribution with their corresponding satellite peaks (dashed and dotted lines in Fig. 3) that could be attributed to Ni-Ni bonding, Ni(OH)₂ and NiO species, respectively.²²,⁵⁸,⁵⁹,⁶⁰,⁶¹,⁶² The presence of NiO and Ni(OH)₂ species also supported by the peak-fitting of the O 1s photoelectron signal (Figure S9) indicates that the materials' surface was partially oxidized. The Ni₃N@NF and Ni₃S₂@NF materials presented nearly the same spectra than that of metal BNF. However, the peak due to Ni-Ni bond (in BNF) was shifted to higher binding energy values by 0.5 eV for Ni₃N@NF and Ni₃S₂@NF due to the formation of Ni-N and Ni-S bonds (Table S1).⁶²,⁶³ For Ni₃N@NF, the nitridation of the nickel materials was further assessed by the decomposition of the signal of the N 1s photoelectron peak (Figure 3c). The major component at 397.7 eV is in agreement with Ni-N bonding.⁵⁹,⁶⁴ Signals due to oxidized nitrogen species were also observed as minor contributions. Such observations were consistent with either the partial oxidation of the surface or the presence of residual species due to the incomplete transformation of ammonia. Regarding Ni₃S₂@NF, the S 2p spectrum brought a very interesting piece of information. The decomposition of the signal was quite
complex with several pairs of doublets due to the spin orbit splitting at the 2p level with a peak-to-peak separation of 1.2 eV between both S 2p<sub>3/2</sub> and S 2p<sub>1/2</sub> components. As expected, all these doublets exhibit intensity ratio of 2:1. On the higher binding energy side, S 2p<sub>3/2</sub> peaks at 168.4 and 166.3 eV were related to sulfur oxidized species that contributed in a minor portion. Elemental sulfur, probably due to the incomplete transformation of H<sub>2</sub>S, could also be detected at 164 eV at low content. A main doublet was observed at 161.2 and 162.3 eV. A second doublet at 162.5 and 163.7 eV also contributed to the signal but in lesser extent. This latter doublet could be assigned to Ni<sub>3</sub>S<sub>2</sub> while the signal at 161.2 eV corresponded to NiS if one refers to XPS data reported for millerite (NiS) and heazlewoodite (Ni<sub>3</sub>S<sub>2</sub>) compounds.<sup>65,66</sup> This observation indicates that the outermost surface of optimized Ni<sub>3</sub>S<sub>2</sub>@NF mainly consisted of NiS. NiS could be the initial product during the sulfidation reaction and could transfer to the nickel surface in a non-stoichiometric Ni<sub>3</sub>S<sub>2</sub> zone as an amorphous phase being not detected by XRD. Alternatively, the NiS amorphous phase could be produced upon exposure to ambient air.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Peak-fitted high-resolution core-level spectra of Ni $2p_{3/2}$ region (left panel) for BNF (a), Ni$_3$N@NF (b) and Ni$_3$S$_2$@NF (d), of N 1s and S 2p regions (right panel) for Ni$_3$N@NF (c) and Ni$_3$S$_2$@NF (e), respectively. Experimental data are the black solid lines, fittings are represented by colored solid (main peaks) and dashed/dotted (satellites) lines, respectively. The resulting fitting envelopes are the dark yellow dashed lines. For Ni 2p, the gray component at 858.4 ± 0.4 eV corresponds to interband losses.</figcaption></figure>
### 3.2. Electrocatalytic Activity for Alkaline HER.
The electrocatalytic HER activity of the optimized Ni₃N@NF and Ni₃S₂@NF was evaluated in an alkaline aqueous solution (1.0 M KOH, pH 14) along with the reference BNF electrode. As shown in Figure 4a, Ni₃N@NF exhibited the highest HER activity, as supported by the lowest onset potential of -0.075 V vs RHE (set arbitrarily at 1 mA cm⁻²) and the lowest iR-corrected overpotential values of 0.189 (η₁₀), 0.291 (η₁₀₀) and 0.342 V (η₂₀₀) measured at 10, 100 and 200 mA cm⁻², respectively (Table 1). These values are 125, 65, and 24 mV higher than those determined for benchmark 20 nm-thick Pt film-coated NF (20nm Pt@NF). Remarkably, it is worth noticing that its HER activity competes with that of Pt for current densities higher than 200 mA cm⁻². In comparison, Ni₃S₂@NF yielded overpotentials η₁₀ = 0.204 V, η₁₀₀ = 0.351 V and η₂₀₀ = 0.417 V which were expectedly lower than those measured for BNF, i.e. 0.292, 0.422 and 0.503 V, respectively (Table 1).

The superior activity of Ni₃N@NF for HER was also confirmed by its Tafel slope of 92 mV dec⁻¹ which was smaller than those of Ni₃S₂@NF (98 mV dec⁻¹) and BNF (124 mV dec⁻¹) (Fig. 4b). Such Tafel slopes are significantly higher compared to the value obtained for the Pt@NF electrode (59 mV dec⁻¹). These findings suggest that the HER on Ni-based materials is controlled by the Volmer reaction step, which involves the electrochemical adsorption of hydrogen atoms onto the catalyst's active sites. In contrast, for Pt, the Tafel reaction, which is the combination of two adsorbed hydrogen atoms to form molecular hydrogen (H₂), appears to be the rate-determining step.⁶⁷

To gain further insights on the electron-transfer kinetics, EIS measurements were performed for all electrodes at an applied overpotential of 0.2 V and in the frequency range 10⁵ to 10⁻¹ Hz.
The data are presented in the form of Nyquist diagrams, i.e. by plotting the imaginary part (Z")
vs the real part (Z') of the complex impedance (Z = Z' + iZ") as a function of frequency f. All
Nyquist plots are characterized by a semicircle and can be adequately fitted by assuming the
simplified Randles equivalent electrical circuit comprising a resistance (the cell resistance R_s) in
series with the charge-transfer resistance (R ct) and a constant phase element (CPE) in parallel
(Figure 4c). For this circuit model, the semicircle's diameter on the Z' axis corresponds to R ct.
Consistent with previous electrochemical data, the R ct of Ni_3N@NF (7.8 Ω) was found to be
smaller than that calculated for Ni_3S_2@NF (9.7 Ω) and BNF (49.3 Ω), confirming faster charge-
transfer kinetics for HER.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** a) *iR*-corrected HER polarization curves in 1 M KOH solution at 5 mV/s of BNF, Ni₃N@NF and Ni₃S₂@NF compared with benchmark Pt@NF, b) corresponding Tafel slopes, c) EIS Nyquist plots and fitted curves using the simplified Randles circuit (inset) at an applied overpotential of 0.2 V, d) Δj/2 vs *v* plots where Δj is the difference between the anodic and cathodic charging current densities (extracted from cyclic voltammograms shown in Figure S10)</figcaption></figure>
used for determining C<sub>dl</sub> for the different electrodes and e) chronopotentiometric curves at an applied current density of -10 mA cm<sup>-2</sup>.

<table><caption>**Table 1.** HER Electrocatalytic Parameters for Different Modified NF Cathodes in 1 M KOH.</caption><thead><tr><th>Electrode</th><th>Onset potential / V vs RHE <sup>a</sup></th><th>η<sub>10</sub> / V <sup>b</sup></th><th>η<sub>100</sub>, η<sub>200</sub> / V <sup>b</sup></th><th>Tafel slope / mV dec<sup>-1</sup></th><th>R<sub>ct</sub> / Ω <sup>c</sup></th><th>C<sub>dl</sub> / mF cm<sup>-2</sup></th><th>ECSA / cm<sup>2</sup></th></tr></thead><tbody><tr><td>BNF</td><td>-0.212</td><td>0.292</td><td>0.422, 0.503</td><td>124</td><td>49.3</td><td>0.03</td><td>1.5</td></tr><tr><td>Ni<sub>3</sub>N@NF</td><td>-0.075</td><td>0.189</td><td>0.291, 0.342</td><td>92</td><td>7.8</td><td>0.75</td><td>37.5</td></tr><tr><td>Ni<sub>3</sub>S<sub>2</sub>@NF</td><td>-0.109</td><td>0.204</td><td>0.351, 0.417</td><td>98</td><td>9.7</td><td>0.53</td><td>26.5</td></tr><tr><td>Pt@NF</td><td>-0.005</td><td>0.064</td><td>0.226, 0.318</td><td>59</td><td>1.9</td><td>n.a.</td><td>n.a.</td></tr></tbody></table>

<sup>a</sup> Set arbitrarily at 1 mA cm<sup>-2</sup>. <sup>b</sup> iR-corrected overpotential at a current density of 10, 100 and 200 mA cm<sup>-2</sup>. <sup>c</sup> Determined from Nyquist plots at 200 mV overpotential.

Moreover, because the considered electrodes showed a certain roughness, the estimation of the electrochemically active surface area (ECSA) for each electrode is strongly recommended to account for differences in the density of electrocatalytically active sites. ECSA can be commonly determined using ECSA = C<sub>dl</sub>/C<sub>s</sub> where C<sub>dl</sub> is the double-layer capacitance extracted from cyclic voltammograms recorded in a non-Faradaic potential window at different scan rates (Figure S10) and C<sub>s</sub> is the specific capacitance of flat Ni (see the Materials and Methods section for details).<sup>49</sup> Consistent with aforementioned electrochemical data, Ni<sub>3</sub>N@NF exhibited C<sub>dl</sub> and ECSA values slightly larger than those determined for Ni<sub>3</sub>S<sub>2</sub>@NF, namely 0.75 mF cm<sup>-2</sup> and 37.5 cm<sup>2</sup> against 0.53 mF cm<sup>-2</sup> and 26.5 cm<sup>2</sup> (Fig. 4d), highlighting a higher density of active sites. This demonstrates that the superior density of active sites in Ni<sub>3</sub>N@NF contributes to the
enhanced HER activity. Furthermore, we have checked that the ECSA value calculated for BNF
(1.5 cm²) was close to 1 cm², as expected for a perfectly flat surface.

In addition to the electrocatalytic activity, the long-term stability of BNF, Ni₃N@NF, and Ni₃S₂@NF electrodes was also examined in 1 M KOH solution at an applied current density of -10 mA cm⁻². The corresponding chronopotentiometric curves revealed that Ni₃N@NF and Ni₃S₂@NF were stable over 45 h electrolysis with a potential change of only 24 and 31 mV, respectively (Fig. 4e). In comparison, a much higher potential shift of 153 mV was measured for BNF electrode.

After the HER stability test, the high-resolution core level spectra for the Ni 2p, N 1s and S 2p regions (Figure S13), as well as the EDS-estimated atomic ratios of the characteristic elements (Figures S11 and S12), were consistent with a decrease in the amounts of N and S for both electrodes concomitantly with an increase in the content of O. Such a result can be ascribed to the surface hydroxylation phenomenon during the stability test which was probably caused by chemical interactions of the electrocatalytic surface with hydroxide ions (OH⁻) present in the strongly alkaline electrolyte.

3.3. *DFT Calculations.* The electrocatalytic HER activities were better understood by performing DFT calculations to explore the thermodynamics of HER in an alkaline environment (Figure 5a). In contrast to the acidic HER mechanism, the rate-determining step in alkaline HER is likely the barrier related to water dissociation steps (Volmer).⁶⁸ The Ni(111), Ni₃N(111), and Ni₃S₂(110) facets were modeled based on the XRD results (Fig. 1). The Gibbs free energy change toward HER for these surfaces was then calculated using these models. For Ni(111), a high energy barrier of 1.01 eV for the water dissociation step was observed, mainly attributed to the weak
adsorption of OH* on Ni sites. Similarly, the water dissociation energy for Ni₃S₂ was found to be as high as 0.70 eV due to the weak bonding strength between Ni and adsorbed OH (OH*). The adsorption energy of H (H*) on the S site at Ni₃S₂ was calculated to be -0.19 eV. On the other hand, for Ni₃N, the stronger adsorption of OH* at Ni sites and H* at N sites significantly reduces the energy barrier for water dissociation to 0.49 eV. These findings align with You *et al.*, where nitrogen acts as a promoter on an N-doped Ni(111) surface, facilitating hydrogen extraction from adsorbed water.⁶⁹ Herein, the DFT calculations show that the Ni₃N (111) surface exhibits the lowest water activation energy and near-zero energy for hydrogen evolution, making it highly favorable for alkaline HER, consistent with the experimental results. Next, the optimized surfaces with H₂O adsorbed on Ni sites are presented in Figure 5b. It was found that H₂O preferentially adsorbs to the Ni top site on all surfaces via the O atom. The adsorption energy, distance between H₂O and the surface metal atom (Ni-O), O-H bond length, bond angles of water, and Bader charges of the Ni active site before and after H₂O adsorption are listed in Table 2. The Ni-O bond lengths between the O atom and the surface metal atom Ni are 2.18 Å, 2.06 Å, and 2.15 Å for the Ni, Ni₃N, and Ni₃S₂ surfaces, respectively. The shorter Ni-O bond length observed for the Ni₃N surface results in a more negative adsorption energy compared to the Ni₃S₂ and Ni surfaces (Fig. 5c). This reduction in bond length, along with O-H distances measured at 0.98 Å and a bond angle of 106°, indicates a weakening of O-H bonds, which contributes to an enhanced HER. The charge density difference analysis, also shown in the insets of Figure 5c, reveals charge depletion (cyan color) at the Ni surface and charge accumulation (yellow) at the O atom, which is more prominent for the Ni₃N surface. The Bader charge analysis supports these findings, showing an enhancement of charge at the Ni active site after water adsorption (Table 2). Figure 5d presents the density of states (DOS) for the Ni, Ni₃N, and Ni₃S₂
surfaces. The projected DOS of Ni₃N and Ni₃S₂ reveals that the energy bands surrounding the Fermi level are composed of Ni 3d, N 2p, and S 2p orbitals. While Ni has low DOS at the Fermi level, Ni₃N and Ni₃S₂ exhibit a metallic state with a high density of occupied electrons around the Fermi level. These occupied electrons are transferred to H₂O upon adsorption on the surface, leading to charge depletion at the interface, and enhanced adsorption energy.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** a) Gibbs free energy diagrams for alkaline HER pathway on Ni(111), Ni₃N(111), and Ni₃S₂(110) surfaces. The right panel shows the modeled transition state (OH* + H*) and the H* adsorbed state. b) Optimized side and top views of Ni(111), Ni₃N(111), and Ni₃S₂(110) with H₂O. c) H₂O adsorption energy and charge density differences (cyan: depletion, yellow: accumulation) with an iso-surface value of 0.05 e/Å³. d) Total and partial density of states for Ni(111), Ni₃N(111), and Ni₃S₂(110) surfaces.</figcaption></figure>
<table><caption>**Table 2.** Calculated adsorption energies (*E*<sub>ads</sub>), Ni-O distances, O-H bond lengths, and bond angles in water. Bader charges of the Ni active site [*q*(*Ni*)] are computed for both the pristine and H<sub>2</sub>O-adsorbed surface.</caption><thead><tr><th rowspan="2">Surface</th><th rowspan="2">E<sub>ads</sub> (eV)</th><th rowspan="2">d(Ni-O) (Å)</th><th rowspan="2">d(O-H) (Å)</th><th rowspan="2">H-O-H (°)</th><th colspan="2">q(Ni) (e<sup>-</sup>)</th></tr><tr><th>Pristine</th><th>H<sub>2</sub>O<sub>ads</sub></th></tr></thead><tbody><tr><td>Ni(111)</td><td>-0.26</td><td>2.18</td><td>0.983</td><td>104.9</td><td>-0.03</td><td>0.12</td></tr><tr><td>Ni<sub>3</sub>N(111)</td><td>-0.54</td><td>2.06</td><td>0.981, 0.980</td><td>106.1</td><td>0.19</td><td>0.34</td></tr><tr><td>Ni<sub>3</sub>S<sub>2</sub>(110)</td><td>-0.38</td><td>2.15</td><td>0.982, 0.977</td><td>104.9</td><td>0.40</td><td>0.48</td></tr></tbody></table>

**3.4. Bubble Dynamics at the Electrocatalytic Cathodes.** The visualization of H<sub>2</sub> bubbles electrogenerated at the catalytic electrodes was monitored by videography using a high-speed camera (Figure S14). Therefore, photographs have been captured from videos recorded during the electrolysis of cathodes in 1 M KOH in order to determine the average bubble density and size distribution. A current density of -10 mA cm<sup>-2</sup> was selected to monitor the formation of H<sub>2</sub> bubbles at the electrode surface. Figure 6 displays snapshots of the H<sub>2</sub> bubbles produced on the different substrates (additional photographs and movie are shown in Figures S15-S17 and Movie S1). Table 3 summarizes the physico-chemical properties and the quantitative data extracted from these snapshots, i.e. bubble diameter and bubble density. At first glance, the bubble diameter decreased upon decreasing contact angle and Ni<sub>3</sub>N@NF thus produced the smallest bubbles. This can be well understood if we consider that the more hydrophilic the substrate, the
more the liquid tends to spread and consequently the less bubbles stay attached. A deeper study
on the bubble statistics is given on Figure 7 which displays the bubble diameter histograms for
the three substrates. It is clear that the bubble diameter distribution is well centered around a
mean value for Ni₃N@NF, i.e. the bubble radius is well controlled, while the distributions are
almost equi-distributed for the two other electrodes. The properties of the interface, hydrophilic
vs. hydrophobic, have here a direct impact on the control of the size of the bubbles produced, but
also on their residence time on the electrodes. In fact, the distribution centred around a mean
value of 50-60 µm for Ni₃N@NF indicates a much shorter residence time than for the other two
electrodes for which the distributions are spread out, indicating an uncontrolled residence time
on the electrodes.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Photographs of H₂ bubbles (highlighted by yellow circles) evolved at BNF (a, d), Ni₃N@NF (b, e) and Ni₃S₂@NF (c, f) electrodes at -10 mA cm⁻² in 1M KOH. Numbers in the white boxes (d-f) correspond to the average size of the bubbles.</figcaption></figure>
<table><caption>**Table 3.** Evolved H₂ Bubble Size and Density for Different Modified NF Cathodes in 1 M KOH.</caption><thead><tr><th>Electrode</th><th>Contact angle /°</th><th>Roughness /nm</th><th>Bubble diameter /μm</th><th>Bubble density / per mm²</th></tr></thead><tbody><tr><td>BNF</td><td>97 ± 5</td><td>176 ± 6</td><td>98 ± 35</td><td>25 ± 3</td></tr><tr><td>Ni₃N@NF</td><td>40 ± 1</td><td>732 ± 19</td><td>53 ± 15</td><td>99 ± 9</td></tr><tr><td>Ni₃S₂@NF</td><td>70 ± 2</td><td>902 ± 55</td><td>77 ± 34</td><td>24 ± 4</td></tr></tbody></table>

More surprising was the bubble density for the three substrates. Despite the Ni₃N@NF was more hydrophilic, it showed the highest bubble density with a factor of 4 compared to the two other substrates. Actually, if we measure the contact area of the bubble, i.e. the wetting area A<sub>w</sub> for a single bubble, it is given by:

$$A_w = \pi R_w^2 = \pi [R \sin(\theta)]^2 \quad (4)$$

with R<sub>w</sub> the wetting radius (Figure S18). This successively gives in our range of parameters for the mean radii: A<sub>W,Ni₃N@NF</sub> = 3650 µm², A<sub>W,Ni₃S₂@NF</sub> = 16500 µm² and A<sub>W,BNF</sub> = 29700 µm². A single H₂ bubble on Ni₃N@NF occupied 4 to 5 times less area of the electrode than on Ni₃S₂@NF, and 8 times less area than on the BNF substrate. This leads to the following total wetted areas: A<sub>Ni₃N@NF</sub> = 0.36 mm², A<sub>Ni₃S₂@NF</sub> = 0.39 mm² and A<sub>BNF</sub> = 0.74 mm². The more hydrophilic substrate is the less covered by H₂ bubbles and thus screens less the electrodes.
Finally, a higher hydrophilicity leads to smaller bubbles, larger bubble densities and less H₂
wetted areas.

Furthermore, both the H₂ bubble size and retention time increased upon increasing the hydrophobic character of the catalytic material, which led to the blockage of active sites and increased the dead surface area. This resulted in low HER activity and current fluctuations as observed on the LSV curves of Ni₃S₂@NF and BNF at current densities higher than 100 mA cm⁻² (Figure 4a). Note that this retention time can be reinforced by triple line pinning that depends on the roughness of the substrate, leading to coupled effects that are out of the scope of the present paper. Due to the more hydrophilic nature of Ni₃N@NF, smaller H₂ bubbles formed and detached more rapidly from the surface which provided large surface area for ongoing hydrogen production. This observation perfectly aligns with DFT calculations, which show that H₂O adsorption is predominantly favored on the Ni₃N surface (Fig. 5c). As a result, this electrode achieved HER with lower overpotential and superior catalytic efficiency.

In a nutshell, based on our observations, it can be concluded that the release of H₂ bubbles is more efficient for the most hydrophilic substrate, Ni₃N@NF, than for the other electrodes (Figures S15-S17 and Table 3).
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Size distribution of H₂ bubbles electrogenerated on BNF (a), Ni₃N@NF (b) and Ni₃S₂@NF (c) electrodes.</figcaption></figure>

**4. Conclusion**

To sum up, nickel nitride (Ni₃N) and nickel sulfide (Ni₃S₂) were conveniently grown in one step on nickel foil (NF) through the thermal treatment of cleaned NF under NH₃ and 10% H₂S/H₂, respectively. The hydrophilic character of these materials in 1 M KOH solution was found to
increase in the following order: NF < Ni₃S₂@NF < Ni₃N@NF, which is corroborated by DFT
results. The most hydrophilic electrode Ni₃N@NF showed the best HER activity, in terms of
onset potential and overpotential at 10, 100 and 200 mA cm⁻². Complementary *in situ* bubble
dynamics study has enabled us to understand the impact of surface hydrophilicity on the
electrochemical HER activity in 1 M KOH solution. Owing to its hydrophilic and probably
aerophobic character, Ni₃N@NF produced a high density of about 50 µm-diameter H₂ bubbles
which were rapidly released from the electrode surface, leading to a fast renewal of the active
surface for HER. Such an efficient release of H₂ bubbles from Ni₃N@NF can explain why its
HER activity competes with that of Pt for current densities higher than 200 mA cm⁻². In contrast,
for BNF and rougher Ni₃S₂@NF, both the bubble size and the retention time increased, leading
to the adverse blockage of the active sites and requiring higher overpotential for HER. Our
findings highlight that the impact of material roughness on both the bubble size and density is
therefore somewhat weak.

This work constitutes a first step toward advanced and high-performance catalytic electrodes.
In that context, combining the electrocatalyst nanostructuring with a porous or textured electrode
substrate represents an effective strategy to significantly enhance the HER performance of our
systems thanks to a further enhanced release of H₂ bubbles. Finally, it would be also valuable to
investigate the effects of both the electrolyte countercation and concentration on the *in-situ*
bubble dynamics since it has been demonstrated by Koper and co-workers that such
experimental factors may significantly affect bubble coalescence and detachment due to the
solutal Marangoni effect.⁷⁰,⁷¹
## ASSOCIATED CONTENT

**Supporting Information.** Additional XRD patterns, SEM images, EDS and XPS spectra, electrochemical data, angle contact measurements, photographs of the evolved $H_2$ bubbles and movie on the $H_2$ bubble release at -5 and -10 mA cm$^{-2}$. The Supporting Information is available free of charge on the ACS Publications website.

## AUTHOR INFORMATION

### Corresponding Author

* Bruno Fabre — CNRS, ISCR (Institut des Sciences Chimiques de Rennes)-UMR6226, Univ Rennes, Rennes F-35000, France. E-mail: bruno.fabre@univ-rennes1.fr

### Author Contributions

B. F. and C. T. conceived the project, designed the experiments, analyzed the data, and wrote the manuscript. V. B. and F. T. contributed in the synthesis of $Ni_3S_2@NF$ and $Ni_3N@NF$, respectively. F. T. performed and analyzed the XRD data. C. L. performed and analyzed the XPS data. M.-C. J. provided the experimental environment for the bubbles dynamics and contributed to the data analysis. P. B. carried out the DFT theoretical calculations and analyzed the data. All authors contributed to the discussion of the results. All authors participated in editing the paper and have given approval to the final version of the manuscript.

### Funding Sources

This project has received financial support from the CNRS through the 80|Prime program.
ACKNOWLEDGMENT

Bruno Fabre, Marie-Caroline Jullien and Chinkit Tyagi acknowledge the Centre National de la
Recherche Scientifique (CNRS) for the research funding and Ph.D. research grant (80|Prime
program). The XRD and XPS measurements have been performed on OSIRIX, THEMIS, and
ASPHERYX platforms, respectively (ScanMAT, UAR 2025 University of Rennes-CNRS;
CPER-FEDER 2015-2020). The authors are grateful to Stéphanie Lamy and Sophie Ollivier for
their assistance in the cutting of NF and preparation of Ni₃S₂@NF, respectively. Loic Joanny and
Francis Gouttefangeas are fully acknowledged for SEM images and EDS analyses performed at
CMEBA (ScanMAT, University of Rennes). Laurent Courbin, Thomas Mabit and Florian
Scholkopf (IPR, UMR 6251, Univ Rennes, France) are thanked for their help in bubble dynamics
study. Pooja Basera thanks Dr. Viswanath Pasumarthi from Purdue University for helpful
discussions.
Table of Contents (TOC)

<figure><img src="image_9.png" /></figure>
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