Sulfur-Doped Dicobalt Phosphide Outperforming Precious Metals as a Bifunctional Electrocatalyst for Alkaline Water Electrolysis

Metallic dicobalt phosphide (Co2P) is doped with electronegative sulfur (S:Co2P) by using an economical and eco-friendly thiourea-phosphate-assisted strategy. Density functional theory calculation in conjunction with X-ray photoelectron spectroscopy reveals that S-doping decreases the electron density near the Fermi level to reduce the metallic nature of Co2P. Thus, a more positive charge is induced onto Co to balance between hydride (Coδ+ Hδ−) and proton (S/Pδ− Hδ+) acceptors. As a result, it increases the number of active Co2+ sites as well as the turnover frequency of a single site. The hybrid electrodes obtained by loading S:Co2P nanoparticles on N-doped carbon cloth or nickel foam (NF) exhibit an outstanding activity and stability of hydrogen and oxygen evolution reactions in alkaline electrolytes outperforming conventional, precious metal-based Pt/C and IrO2 catalysts, and most of the other state-of-the-art nonprecious metal electrocatalysts reported so far. An alkaline electrolyzer with S:Co2P@NF as both cathode and anode produces a stable current density of 100 mA/cm2 at 1.782 V, which is superior to the IrO2–Pt/C electrolyzer (1.823 V).