Crystalline Multi-Metal Nanosheets Array with Enriched Oxygen Vacancies as Efficient and Stable Bifunctional Electrocatalysts for Water Splitting

The electrolysis of water on a large scale for hydrogen production needs low cost, efficient, and stable electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report the synthesis of a crystalline multimetal (Co, Ni, Fe) nanosheets array (NSA) enriched with oxygen vacancies by a simple electrosynthesis method, which can serve as efficient and stable electrocatalysts for both OER and HER. The oxygen vacancies are generated by electro-oxidation or electro-reduction of CoFeNiSe precursor via the replacement of selenium by oxygen. For HER, as synthesized CoFeNi-NSA achieved a 400 mA cm –2 current density at a low overpotential of −211 mV. For OER, CoFeNi-NSA was in situ oxidized to form CoFeNiO-NSA, which achieved a 400 mA cm –2 current density at a low overpotential of 270 mV. The activities of CoFeNi-NSA and CoFeNiO-NSA remained stable after 150 h HER or OER test at high current densities, showing higher activity and stability than noble-metal electrocatalysts Pt/C and IrO 2, respectively. The electrolytic cell of CoFeNiO-NSA∥CoFeNi-NSA can achieve a high current density of 400 mA cm –2 at a moderate voltage of 1.98 V. During a long-term water splitting test, the voltage of the electrolytic cell remained stable at 1.76 V for 150 h at a current density of 120 mA cm –2, indicating that these catalysts have both high activity and stability for OER and HER reactions.