Surveying of electrocatalytic performance on water splitting to metal stoichiometry in iron-cobalt and nickel ternary transition metal phosphides

Hydrogen production by water electrolysis is efficient and has high throughput. But the scarcity of current noble metal catalysts limits its scalability. In contrast, transition metals are rich in reserves. Replacing noble metal catalysts with them would significantly cut the cost of water electrolysis for hydrogen production. However, a single transition metal's catalytic performance is far from that of noble metals. Studies show multi-transition metallic elements often have better catalytic properties due to coupling effects, like those of iron, cobalt, nickel enhancing catalyst activity and selectivity. Also, iron, cobalt, and nickel phosphides easily get partially oxidized in air, forming an oxide layer with thickness order FeP > CoP > NiP. Adjusting the proportion of these elements can control the oxide film thickness, and a proper one can improve catalyst activity. This article explores the mechanism and coupling effect of iron, cobalt, and nickel phosphides as overall water splitting catalysts. The chosen Fe 0.2 Co 0.6 Ni 0.2 P with an optimal oxide film thickness shows good electrochemical performance for oxygen evolution reaction (280 mV in 1.0 M KOH at 100 mA cm⁻ 2 ), good hydrogen evolution reaction performance in both acidic and alkaline environments, and has the lowest overall hydrolytic potential in alkaline electrolyte with excellent 300-h stability.