Three-Dimensional Mesoporous Phosphide–Spinel Oxide Heterojunctions with Dual Function as Catalysts for Overall Water Splitting

Highly efficient electrocatalysts for oxygen evolution (OER) and hydrogen evolution reactions (HER) are critical in the development of efficient and sustainable alternative energy. Toward this goal, we report on the nanocasting synthesis of a three-dimensionally (3D) mesoporous CoP/CoCr2O4 heterojunction as efficient bifunctional electrodes for use in overall water splitting in alkaline media. The phosphorization of mesoporous Co2CrO4 nanocast from a KIT-6 hard silica template leads to phase transformation and separation, spontaneously forming a heterojunction between CoP and spinel CoCr2O4 with attractive nanostructured properties (large surface area of 83 m2/g with 4.8 nm diameter accessible mesopores). This phosphide–spinel oxide heterojunction catalyst yields excellent catalytic activity for both OER and HER with overpotentials of only 290 and 212 mV, respectively, to achieve the benchmark current density of 10 mA cm–2 in a 1.0 M KOH electrolyte. Moreover, CoP/CoCr2O4 shows small Tafel slopes with outstanding durability and long-life stability in alkaline media. Finally, the bifunctionality of CoP/CoCr2O4 for both OER and HER was shown by producing symmetric electrodes in an alkaline water electrolyzer, resulting in a low cell voltage of 1.68 V with 24 h durability, making it among the best Co-based electrocatalysts for overall water splitting. Such bifunctionality suggests a cost-effective heterojunction-based system for water electrolysis using inexpensive earth-abundant metals. The simple route to achieving an effective bifunctional material, which can be applied to various transition-metal-based homo- or heterojunctions, offers a paradigm for templated bifunctional electrode materials.