Electrochemically induced surface reconstruction of Ni-Co oxide nanosheet arrays for hybrid supercapacitors

Transition metal oxides (TMOs) are promising materials for supercapacitors (SCs) because of their high theoretical capacity. However, their finite active sites and poor electrical conductivity lead to reluctant electrochemical performance. Herein, we report a facile electrochemical activation (ECA) method to boost the electrochemical activity of Ni-Co oxide nanosheet arrays (NiCoO NSA) for SCs. Specifically, honeycomb-like NiCoO NSA that was made through a solvothermal method followed by air annealing was activated by simply exerting certain cyclic voltammetry scans (the activated sample is named ac-NiCoO NSA). We have found this treatment results in dramatic surface structure change, forming numerous sub-nanostructures (nanoparticles and nano-leaves) on the NiCoO nanosheets. Rich antisite defects and oxygen vacancies in the NiCoO spinel phase were also created by the ECA treatment. Consequently, the ac-NiCoO NSA delivered a maximum capacity of 206.5 mAh g^-1 (0.5 A g^-1), which is about three times of the NiCoO NSA without treatment. A hybrid SC based on the ac-NiCoO NSA demonstrated excellent energy storage capacity (power density of 17.3 kW kg^-1 and energy density of 45.4 Wh kg^-1) and outstanding cyclability (>20,000 cycles, 77.4% retention rate). Our method provides a simple strategy for fabricating high performance TMOs for electrical energy storage devices like SCs.