Controllable in situ synthesis of Ni(OH)2 and NiO films on nickel foam as additive-free electrodes for electrochemical capacitors

Nickel hydroxide nanosheet films are prepared via a hydrothermal treatment of nickel foam at a low temperature using Fe(NO3)3 as an oxidant without any templates and nickel salts. The as-formed Ni(OH)2 nanosheets are vertically self-grown on the Ni foam substrate. They are inter-connected and free-standing to form a homogeneously porous coating on the substrate with a thickness about 1 μm. The mass loading of Ni(OH)2 film on nickel foam can be precisely determined by thermogravimetric analysis on base of phase transformation from nickel hydroxide to oxide. The mass loading of Ni(OH)2 is strongly related with the concentration of Fe(NO3)3. The Ni(OH)2 film grown on nickel foam can directly serve as additive-free electrodes and display a much high specific capacitance, 1100 F g−1 at 0.5 A g−1, excellent rate-capability performance owing to fast electron and ion transport. The Ni(OH)2 film can transform into NiO by annealing under a moderate temperature. However, the NiO film shows a low specific capacitance of 79 F g−1 at 0.5 A g−1. The preparation method concerns limited chemicals and produces little residual, implying a green chemical process to fabricate Ni(OH)2 and NiO films on Ni substrate.