Ni-Doped Cobalt Phosphite, Co₁₁(HPO₃)₈(OH)₆, with Different Morphologies Grown on Ni Foam Hydro(solvo)thermally for High-Performance Supercapacitor

Ni-doped Co11(HPO3)8(OH)6 with different morphologies was directly grown on Ni foam hydro(solvo)thermally under different synthetic conditions. The optimum condition is solvothermal reaction for 6 h in an ethanol/water (EW) mixed solution, the molar ratio of NaH2PO2/Co(NO3)2 being 0.5:0.1, and the obtained S0.5-6 h-EW shows three-dimensional (3D) porous nanowire bundles. Whereas in the water-only solution, microrods are obtained, suggesting that the nanowires in bundles are aggregated together via the lateral (400) direction. Long reaction time and low molar ratio of reactants are all beneficial for the lateral growth of the nanowires, and the possible formation mechanism is proposed. All the obtained Ni-doped Co11(HPO3)8(OH)6/Ni foam samples are directly used as supercapacitor electrodes, and S0.5–6 h-EW shows the best electrochemical performance with a specific capacity of 159 mAh g–1 at 0.5 A g–1, which is close to the theoretical value of 212 mAh g–1 for Co11(HPO3)8(OH)6, and it is the largest reported value so far. The excellent capacitive behavior of S0.5–6 h-EW is ascribed to the 3D porous nanowire bundles directly grown on a Ni foam collector without an additive and a binder, as well as to the doping of Ni into the cobalt phosphite. The S0.5–6 h-EW//activated carbon asymmetrical supercapacitor shows a maximum energy density of 58.7 Wh kg–1 at a power density of 532 W kg–1 and good cycling stability with the capacity retention of 90.5% after 10 000 charging–discharging cycles at 5.5 A g–1.