Regulating metal active sites of atomically-thin nickel-doped spinel cobalt oxide toward enhanced oxygen electrocatalysis

Two-dimensional (2D) transition metal oxides are promising bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, atomically-thin nickel-doped spinel cobalt oxide (NCO) was prepared as OER/ORR catalyst through a simple hydrothermal method. Electrochemical test results show that the NCO nanosheets exhibit bifunctional catalytic performances with the overpotential of 275 mV for OER and the onset-potential of 0.92 V for ORR. Moreover, the assembled Zn−air battery based on the material exhibit excellent performance among transition metal oxides based oxygen electrocatalysts. Characterization results demonstrate that the Ni dopants contribute to creating the oxygen vacancies and promoting the electron transfer from nickel to cobalt. More importantly, the ratio of Co2+ and Ni3+ in the octahedral oxygen gap are significantly increased, which serve as the active sites for ORR and OER, respectively. In addition, the 2D nanostructure and Ni dopants help to facilitate the charge transfer during the reaction and the real active species of OER process is more biased towards Ni ions which were proven by in-situ Raman. This work offers a strategy for preparing highly active and durable spinel transition metal oxides materials for bifunctional oxygen catalysts.