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

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 degrees C for 30 min under ammonia gas flow and at 350 degrees C for 1 h in 10% H2S/H2 gas mixture, respectively. Ni3N operate HER in 1 M 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 of 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 an 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 a 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.