Hierarchically crystalline copper borate nanosheets as a freestanding electrode for a hybrid supercapacitor

In this study, we have suggested a straightforward approach to fabricating a binder-free electrode of hierarchically crystalline copper borate (Cu-B_i) nanosheets grown on nickel foam using the Successive Ionic Layer Adsorption Reaction (SILAR) method. The best-performing electrode shows a remarkable specific capacitance (Cs) of 2002 Fg^-1 at 1 Ag^-1 and Cs retention of 85 % for 10,000 GCD cycles. The assembled CB/NF-2//AC device exhibits high cycling stability and achieves a high energy of 52.2 Whkg^-1 at a power density of 2622.4 Wkg^-1. Remarkably, it reached 152.7 Fg^-1 at 2 Ag^-1, and the device still has a high capacitance retention of 85 % for 10,000 cycles. This remarkable electrochemical activity could be attributed to: (i) the inherent defects of the prepared electrode via the existence of borates, providing straightforward interaction between the electrolyte and the active species; (ii) designing the hierarchical architecture that could provide a porous nanosheet structure, which generates accessible active sites for quick ion transport, boosting the Cs; (iii) the formation of crystalline Cu-B_i nanosheets, resulting in high stability and superior electrochemical performance compared to the previous amorphous borides; and (iv) the hybridization between the B 2p state and the multiple d-orbitals of transition metals, increasing the electron flow between the atoms. The current work suggests that the growth of hierarchically crystalline Cu-B_i on Ni foam using a low-cost and simple procedure could provide a practical approach to designing hybrid supercapacitors with outstanding electrochemical performance.