Unconventional Nickel Nitride Enriched with Nitrogen Vacancies as a High‐Efficiency Electrocatalyst for Hydrogen Evolution

Abstract Development of high‐performance and cost‐effective non‐noble metal electrocatalysts is pivotal for the eco‐friendly production of hydrogen through electrolysis and hydrogen energy applications. Herein, the synthesis of an unconventional nickel nitride nanostructure enriched with nitrogen vacancies (Ni3N1− x) through plasma‐enhanced nitridation of commercial Ni foam (NF) is reported. The self‐supported Ni3N1− x/NF electrode can deliver a hydrogen evolution reaction (HER) activity competitive to commercial Pt/C catalyst in alkaline condition (i.e., an overpotential of 55 mV at 10 mA cm−2 and a Tafel slope of 54 mV dec−1), which is much superior to the stoichiometric Ni3N, and is the best among all nitride‐based HER electrocatalysts in alkaline media reported thus far. Based on theoretical calculations, it is further verified that the presence of nitrogen vacancies effectively enhances the adsorption of water molecules and ameliorates the adsorption–desorption behavior of intermediately adsorbed hydrogen, which leads to an advanced HER activity of Ni3N1− x/NF.


EXPERIMENTAL SECTION
Material synthesis. The synthesis of Ni3N1-x/NF self-supported electrocatalysts was carried out in a 1.5 kW ASTeX MPCVD system. Before the synthesis, Ni foams (Changsha Lyrun New Material Co., Ltd, China) with an area of 1.5 cm × 3.0 cm were first washed in acetone, alcohol, and deionized (DI) water.
After dried with nitrogen gas, the Ni foams were placed into the MPCVD system and subjected to the nitrogen plasma for the in-situ growth of nickel nitride. Nitrogen plasma was produced using a microwave power of 450 W. During plasma nitridation, the pressure was maintained at 14 Torr with a nitrogen flow rate of 30 sccm, the substrate temperature was 300 o C, and the duration of treatment was 90 s. After the plasma was switched off, the samples were cooled down to room temperature naturally. For the synthesis of the control Ni3N/NF sample, the NF was placed in a tubular furnace and heated at 450 º C for 1 h under ammonia flow.
Material characterization. X-ray diffraction (XRD; Philips X' Pert, Cu-Kα radiation) was utilized to characterize the crystal structure of the as-grown samples. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using an ESCALAB 220i-XL spectrophotometer with Al-Kα radiation. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) mapping pictures of the samples were collected on a Philips XL30 FEG with the accelerating voltage of 20 kV. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were acquired on JEM-2100F operated at 200 kV. Contact angle measurements were conducted with a DKSH system. Electrochemical measurements. All the electrochemical tests were carried out in a typical three-electrode system at an electrochemical station (Germany, Zahner). Linear sweep voltammetry (LSV) with a scan rate of 5 mV s -1 was conducted in 1 M KOH using a Ag/AgCl electrode (3 M KCl) as the reference electrode, a graphite rod plate as the counter electrode, and the sample electrocatalysts as the working electrode.
Before measurements, high-purity N2 gas was used to purge the system for at least 30 min, so as to ensure the saturation of N2 in the electrolyte solution; and during the tests, the system was continuously purged with N2. All the potentials in this work were calibrated to a reversible hydrogen electrode (RHE) according to the formula E(RHE) = E(Ag/AgCl) + 0.194 + 0.05916 × pH. The electrochemical stability of the catalyst was evaluated by chronoamperometry test under a constant overpotential of 100 mV. Electrochemical impedance spectroscopy was performed when the working electrode was biased at a constant overpotential of 120 mV and the frequency was swept from 100 kHZ to 10 mHZ with a 10 mV AC dither. The impedance data were fit to a simplified circuit to extract the series and charge transfer resistances.
DFT calculations. Plane-wave density functional theory (DFT) calculations of the electronic properties of the electrocatalysts were performed using the DMOL module in Material Studio. GGA with PBE functional was used for the DFT exchange correlation energy, and 300 eV of kinetic energy cutoff was assigned to the planewave basis set. The self-consistent field (SCF) tolerance was 2×10 −6 eV. The Brillouin zone was sampled by 2×2×1 k-points. The core electrons were replaced with ultrasoft pseudo-potentials. For calculations of adsorption energy, the (0001) facet was modeled with vacuum widths of 15 Å. We adopted slabs with 6 layers Ni3N layers consisting of 48 atoms (Ni36N12). The periodically repeated slabs were separated from their neighboring images by a 15-Å-wide vacuum in the direction perpendicular to the surface.
The Gibbs free-energy (ΔGH*) is expressed as: [1] ΔGH* = ∆EH* + ∆EZPE -T∆S where ∆EH* is the adsorption energy of atomic hydrogen on the given unit cell, ∆EZPE is the difference corresponding to the zero point energy between the adsorbed hydrogen and hydrogen in the gas phase, and ∆S is entropy change of H * adsorption. As the entropy of hydrogen in absorbed state is negligible, ∆S can be calculated as -1/2 S0 (S0 is the entropy of H2 in the gas phase at standard conditions, 1 bar of H2 and pH = 0 at 300 K). Therefore the free energy of the adsorbed state can be taken as: ∆GH* = ∆EH* + 0.24 eV HER active sites for electrocatalysts were performed by adding a hydrogen atom at a distance of 1.5 Å on the surface of electrocatalysts. Pt was selected as standard electrode for free energy calculation. Figure S1. SEM image of a pristine Ni foam. Figure S2. Photos of the NF before and after nitrogen plasma treatment.