Activating and stabilizing hydrogen evolution on trace-iridium-incorporated nickel hydroxide for energy-efficient hydrogen production

Dissociation of water molecules determines the efficiency of water reduction to hydrogen molecules in alkaline media. Nickel hydroxide (Ni(OH)₂) shows strong adsorption to the hydroxyl group, which helps to dissociate H₂O to provide protons for the subsequent hydrogen evolution reaction (HER). However, Ni(OH)₂ is not an efficient HER catalyst due to its intrinsically unfavorable adsorption of hydrogen. Herein we report a method to introduce trace iridium atoms as the catalytic site into the Ni(OH)₂, which activates its catalytically inert basal planes for HER in alkaline electrolytes. Though the incorporated Ir tends to dissolve during the HER, an electrochemical oxidation well stabilizes the incorporation of Ir into the Ni(OH)₂ matrix, and the resulting NiIr(OH)₂-EO exhibits high activity and robust stability for HER. The NiIr(OH)₂-EO affords HER current density of 10 and 100 mA cm⁻² at overpotential of 26 and 69 mV, and shows excellent Ir-mass-specific activity and durability, which are among the best of electrocatalysts reported. Theoretical calculations ascribe the high activity to that Ni(OH)₂ facilitates the dissociation of water molecules while the Ir atoms serve to optimize the energetics of hydrogen adsorption. Experimental measurement of the electrochemical activation energy implies that the Heyrovsky step, which determines the rate of HER, is greatly facilitated. The NiIr(OH)₂-EO also exhibits excellent performance in overall water splitting and urea-assisted water electrolysis, which energy-efficiently produce hydrogen at low voltage powered by photovoltaics. This work sheds light on exploiting hydroxides as efficient electrocatalysts for alkaline water splitting and provide insights into the mechanism of HER happening on hydroxides from thermodynamic and kinetic aspects.