Early Transition-Metal-Based Binary Oxide/Nitride for Efficient Electrocatalytic Hydrogen Evolution from Saline Water in Different pH Environments

Using abundant seawater can reduce reliance on freshwater resources for hydrogen production from electrocatalytic water splitting. However, seawater has detrimental effects on the stability and activity of the hydrogen evolution reaction (HER) electrocatalysts under different pH conditions. In this work, we report the synthesis of binary metallic core-sheath nitride@oxynitride electrocatalysts [Ni-(ETM)](delta+) -[O-N](delta-), where ETM is an early transition metal V or Cr. Using NiVN on a nickel foam (NF) substrate, we demonstrate an HER overpotential as low as 32 mV at -10 mA cm(-2) in saline water (0.6 M NaCl). The results represent an advancement in saline water HER performance of earth-abundant electrocatalysts, especially under near-neutral pH range (i.e., pH 6-8). Doping ETMs in nickel oxynitrides accelerates the typically rate-determining H2O dissociation step for HER and suppresses chloride deactivation of the catalyst in neutral-pH saline water. Heterointerface synergism occurs through H2O adsorption and dissociation at interfacial oxide character, while adsorbed H* proceeds via Heyrovsky or Tafel step on the nitride character. This electrocatalyst showed stable performance under a constant current density of -50 mA cm(-2) for 50 h followed by additional 50 h at -100 mA cm(-2) in a neutral saline electrolyte (1 M PB + 0.6 M NaCl). Contrarily, under the same conditions, Pt/C@NF exhibited significantly low performance after a mere 4 h at -50 mA cm(-2). The low Tafel slope of 25 mV dec(-1) indicated that the reaction is Tafel limited, unlike commercial Pt/C, which is Heyrovsky limited. We close by discussing general principles concerning surface charge delocalization for the design of HER electrocatalysts in pH saline environments.

Automated summary

The study focuses on using abundant seawater for hydrogen production through electrocatalytic water splitting, but highlights challenges related to seawater's effects on stability and activity of hydrogen evolution reaction (HER) electrocatalysts. The synthesized binary metallic core-sheath nitride@oxynitride electrocatalysts show promising advancements in HER performance in saline water, especially under near-neutral pH conditions. Doping early transition metals in nickel oxynitrides aids in accelerating the rate-determining H2O dissociation step and suppressing chloride deactivation, leading to stable performance even at high current densities in a neutral saline electrolyte.