Optimizing oxygen substituents of a carbon cathode for improved capacitive behavior in ethanol-based zinc-ion capacitors

Zinc ion capacitors (ZICs) have been widely studied in recent years due to their high energy density, excellent rate capability, long cycling life and low cost. The incorporation of oxygen functional groups (OFGs) on the surface of the carbon-based cathodes is an effective strategy for improving the capacitive performance of aqueous ZICs. However, whether their presence helps improve the capacitance of ethanol (EtOH)-based ZICs has not been investigated. In this work, a combination of nitric acid oxidation and thermal treatment was used to regulate the OFGs on the activated surface of the carbon cathode. The optimized sample had a high specific capacitance of 195 F g⁻¹ at 1 A g⁻¹ using ZnCl₂/EtOH as the electrolyte, i.e., a 56% increase compared to an unmodified cathode (125 F g⁻¹). ZICs also shown excellent stability for more than 16 000 cycles at 3 A g⁻¹, while maintaining 100% coulombic efficiency. This significantly improved performance is attributed to the presence of OFGs, especially carboxyl and ester groups, which provide abundant electrochemical active sites for redox reaction with the zinc ions. This study reports a significant improvement in the specific capacitance of carbon cathodes for commercial EtOH-based ZIC systems.