Ternary Mo₂NiB₂ as a Superior Bifunctional Electrocatalyst for Overall Water Splitting

Transition metal borides are considered as promising electrocatalysts for water splitting due to their metallic conductivity and good durability. However, the currently reported monometallic and noncrystalline multimetallic borides only show generic and monofunctional catalytic activity. In this work, the authors design and successfully synthesize highly crystalline ternary borides, Mo₂NiB₂, via a facile solid-state reaction from pure elemental powders. The as-synthesized Mo₂NiB₂ exhibits very low overpotentials for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), that is, 280 and 160 mV to reach a current density of 10 mA cm⁻², in alkaline media. These values are much lower from the ones observed over monometallic borides, that is, Ni₂B and MoB, and the lowest among all nonprecious metal borides. By loading Mo₂NiB₂ onto Ni foams as both cathode and anode electrode for overall water splitting applications, a low cell voltage of 1.57 V is required to achieve a current density of 10 mA cm⁻², comparable with the value required from the Pt/C||IrO₂/C couple (1.56 V). The proposed synthesis strategy can be used for the preparation of cost-effective, multi-metallic crystalline borides, as multifunctional electrocatalysts.