In situ two-step electrochemical preparation of fluoride-free nickel-based compound film on nickel plate for supercapacitors

A nickel-based compound layer was prepared on a nickel plate by anodization in a 75 wt% H₃PO₄ solution containing NH₄F. This layer was then treated by galvanostatic charge/discharge (GCD) until a black outer layer was detached, leaving behind a film on the nickel plate as a binder-free electrode material for supercapacitors. The microstructural characterization shows that the film consists of Ni(OH)₂ and NiO, and no fluoride is found in the as-obtained film. Electrochemical tests demonstrate that this fluoride-free film electrode exhibits a high capacitance of 954 F·g⁻¹ at 7.5 A·g⁻¹, excellent rate capability (a 19.5 % capacitance reduction with the current density increasing to 120 A·g⁻¹) and cycling stability. Within 3500 cycles, the specific capacitance does not decrease, but rather increases from 840 F·g⁻¹ to approximately 1092 F·g⁻¹ in the first 100 cycles at 60 A·g⁻¹, and remains stable until the aforementioned layer is detached.