Activation of carbon fiber for enhancing electrochemical performance

Abstract

Carbon fiber (CF) sequentially undergoes thermal activation (TCF), electrochemical oxidation activation (E_OTCF), electrochemical reduction activation (E_ROTCF) and a secondary thermal activation process to form a highly activated CF (TE_ROTCF) electrode material. E_OTCF undergoes a thermal activation process to form TE_OTCF. E_ROTCF, TE_OTCF and TE_ROTCF show specific capacitance values of 1455, 1438 and 1426 mF cm⁻² at 1 mA cm⁻² and capacitance retention values of 0.97%, 35.62%, and 59.61% when the current density increases from 1 to 20 mA cm⁻². E_ROTCF, TE_OTCF and TE_ROTCF show similar capacitance values, but much superior to CF (709 mF cm⁻²). TE_ROTCF shows much higher rate capability than E_ROTCF and TE_OTCF. Carbonyl, epoxy and hydroxyl groups are introduced on the TCF by electrochemical oxidation activation. Carbonyl groups existing at the edge of the basal plane contribute towards improving the specific capacitance of CF. Hydroxyl and epoxy groups on the basal plane of the carbon skeleton lead to the destruction of the conjugated system. The electrochemical reduction and secondary thermal activation cause the removal of the non-electroactive epoxy and hydroxyl groups for recovering the sp² structure, accordingly enhancing the rate capability of CF. All-solid-state flexible supercapacitors using two TE_ROTCF electrodes achieve 26.0 μW h cm⁻² at 125.0 μW cm⁻² and good cycling performance. So, CF with thermal and electrochemical activation treatment shows highly enhanced capacitance for high-performance supercapacitor application.