Elucidating Catalytic Sites Governing the Performance toward the Hydrogen Evolution Reaction in Ternary Nitride Electrocatalysts

Proton exchange membrane electrolyzers are considered the most advanced devices for producing green hydrogen by water electrolysis. Their development requires catalytic materials that are stable under acidic conditions and drive the hydrogen evolution reaction (HER) forward efficiently which makes research into the identification of the catalytic sites important. We report that free-standing Co2Mo3N and Ni2Mo3N achieve overpotentials of 149 +/- 8 and 158 +/- 10 mV (in 0.5 M H2SO4) at a benchmark current density of 10 mA cm-2. Both nitrides remained stable and consistently deliver current densities >500 mA cm-2 at a potential as low as 308 +/- 22 mV when they were immobilized on nickel foam. Replacing Ni for Fe in Ni2Mo3N leads to FexNi2-xMo3N (0.5 <= x <= 1.25) that show a decrease in catalytic activity as the value of x increases which confirms that Ni (rather than Mo and N) sites are catalytically active. The X-ray photoelectron spectroscopy data additionally suggests that preserving the low oxidation states of transition metals in the nitrides is important for achieving good catalytic performance toward the HER in acidic electrolytes.