Controllable synthesis of NixCo3−xO4-rGO with enhanced oxygen reduction/evolution activity

The composition of nickel cobalt oxide has a great influence on its electrochemical performance. A series of three-dimensional porous honeycomb-like nickel cobalt oxides with different compositions were fabricated by varying the molar ratio of Ni to Co and tested by various characterizations. The electrochemical activity was evaluated by oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrochemical measurements showed that the nickel–cobalt oxide with a molar ratio of Ni/Co to be 1:2 demonstrates the superior electrochemical performance with lower onset potential, smaller Tafel slope, higher electron transfer number, longer cycling stability for ORR, lower overpotential for OER, and better overall bifunctional activity. The enhanced performance could be attributed to the synergistic effect of the porous structure, rich defect oxygen, and a proper molar ratio of Ni2+/Ni3+ and Co2+/Co3+.