Electrochemical activation induced the transition of NiCoOx nanoneedles for boosting supercapacitance

Electrochemical activation has been adopted for optimizing the microstructure and composition of electrode materials. However, the phase transition process of bimetallic oxides and related activation mechanisms have not yet known. Herein, NiCoOx nanoneedles are depositing on nickel foam (NF) by hydrothermal and annealing treatment, and the relationship between the phase transition of NiCoOx, activation condition and electrochemical performance is revealed in KOH electrolyte. The phase transition process is divided into three steps. Initial activation stage is the granulation process of NiCoOx nanoneedles to form crystalline NiCo2O4 nanoparticles and few Ni(OH)2 nanosheets. The second activation stage involves the growth and reassembly of NiCo-hydroxides to generate optimized NiCo2O4/NiCoOOH heterostructure, with NiCo2O4 nanoparticles dispersed on NiCoOOH nanosheets. The optimized product delivers the highest capacitance of 2857.4 F g−1 at 1 A g−1, about three times the initial value of NiCoOx (969.4 F g−1), with a superior rate capability. The last activation process involves the fusion of NiCoOOH nanosheets, and the reduced surface area causes the performance attenuation of activation product. When acted as a positive electrode in an asymmetric supercapacitor, the NF/NiCoOx exhibits an obvious activation phenomenon and a superior electrochemical performance, including a high discharge capacitance, cycling performance and energy density.