Electrocatalytic Hydrogen Evolution by Nickel, Cobalt, and Molybdenum Phosphide-Based Materials

Efficient electrocatalysts made from earth-abundant materials are crucial for industrial scale hydrogen generation from electrocatalytic water splitting. At present, developing a highly efficient hydrogen evolution reaction (HER) electrocatalyst from cheap raw materials is a challenging task. In this work, it is found that, nickel phosphide-based hybrid nanosheet arrays electrocatalyst synthesized by single-step thermal phosphorization of commercial nickel foam exhibits very stable and fast hydrogen evolution kinetics. The as-prepared Ni/Ni5P4/NiP2 nanosheet arrays electrocatalyst requires overpotentials of 61 and 121 mV to achieve current densities of 10 and 100 mA cm-2, respectively, in 0.5 M H2SO4. The impressive HER activity of the Ni/Ni5P4/NiP2 catalyst is attributed to its largely increased active sites in nanosheet morphology and 3D porous feature of the electrode support. In addition, cobalt phosphide (CoP) decoration on nickel phosphide (Ni5P4) nanosheet arrays results into a unique hierarchical CoP/Ni5P4 microsheet arrays catalyst, which shows a promising HER activity in wide range of pH. It requires overpotentials of merely 33 and 85 mV to achieve current densities of 10 and 100 mA cm-2, respectively, along with impressive operational durability at current density as high as 1 A cm-2 in 0.5 M H2SO4. The hierarchical CoP/Ni5P4 electrocatalyst is very active in alkaline electrolyte as well, achieving current densities of 10 and 100 mA cm-2 at overpotentials of 71 and 140 mV, respectively, in 1 M KOH. The excellent HER activity of the hierarchical CoP/Ni5P4 catalyst is mainly due to its extremely rough surface and very good electrical contact between the catalyst and electrode support. Moreover, ternary Ni2(1-x)Mo2xP nanowire arrays catalysts grown on nickel foam also show impressive HER activity at higher current densities for alkaline HER catalysis. The Ni2(1-x)Mo2xP catalyst requires overpotentials of 72, 240, and 294 mV at current densities of 10, 500, and 1,000 mA cm-2, respectively, together with outstanding operational durability in 1 M KOH. The density functional theory (DFT) calculations reveal that partial Mo substitution of Ni in Ni2P, that gives rise to Ni2(1-x)Mo2xP, results into optimal free energy of hydrogen adsorption on the Ni2(1-x)Mo2xP catalyst surface, thereby enhancing its HER catalytic activities.