Electrochemical activation of NiCo-hydroxysulfides in high concentration KOH for constructing ultra-high capacitance electrodes

Cyclic voltammetry (CV) activation strategy can be used to enhance the charge storage capability of electrode materials. To explore the activation mechanism of transition metal hydroxysulfides, herein, NiCo-hydroxysulfide (NC-SO) is synthesized on Ni foam by hydrothermal reaction followed by partially sulfurization, and the CV activation mechanism is revealed in different KOH electrolytes. With an increase of scanning cycle, the specific capacitance gradually enhances. When activated for 300 cycles in 6M KOH, ultra-high capacitance of 5660 F g -1 or 2830 C g - 1 is obtained at 1 A g - 1, much higher than that activated in 1M KOH (2540 F g - 1), due to the transition of active substance and the positive contribution of high concentration electrolyte. The activation mechanism of NC-SO is actually the dissolution of S2- firstly, followed by the generation, reassembly and optimization of hydroxide nanosheets. Besides the transition from hydroxysulfides to hydroxide nanosheets, sufficient OH- ions in 6M KOH reduce the internal resistance of electrode, accelerate the ion diffusion and enhance the redox-pseudocapacitive contribution. When assembled in an asymmetric supercapacitor, the device delivers a high capacitance of 343.8 F g -1 at 1 A g -1, and the energy density reaches 121.9W h kg-1 at 839.4W kg- 1.