Abstract: Supercapacitors (SCs) play a crucial role in flexible electronics, necessitating innovative approaches to enhance surface faradaic reactions and minimize faradaic diffusion while using aqueous electrolytes. Thus, the urea treatment of cobalt oxide (CoO_x)-decorated carbon nanofibers (CNFs) is proposed in this study to decrease the contribution of faradaic diffusion-limited current. Flexible CoO_x/CNF electrodes were obtained by annealing ZIF-67-grafted polyacrylonitrile (PAN) fibers via a wet chemical method. The urea treatment of CoO_x/CNFs increased the content of sp²-hybridized carbon and pyridinic nitrogen, as confirmed by X-ray photoelectron spectroscopy, effectively enhancing conductivity and pseudocapacitive charge storage capability via nitrogen doping. Notably, urea-treated CoO_x/CNF electrode samples exhibited a capacitance of 750 mF cm^-2 at a scan rate of 10 mV s^-1, while retaining more than 81 % capacitance at a higher scan rate of 100 mV s^-1. The cyclic voltammetry curves during variable bending angle testing (0°, 45°, and 90°) exhibited negligible changes, indicating the excellent flexibility of the SCs. The CoO_x/CNFs and urea-treated CoO_x/CNFs exhibited 80 % and 91 % capacitance retentions, respectively, after 10,000 galvanostatic charge and discharge cycles. Furthermore, the attained energy densities of 76 and 61 µWh cm^-2 at the respective power densities of 2 and 20 mW cm^-2 indicated the excellent electrochemical performance of the optimal urea-treated CoO_x/CNF electrode.

Key words: Electrospinning, ZIF-67, Carbon nanofiber, Freestanding electrode, Flexible supercapacitor, Urea