Bifunctional surface engineering of reduced graphene oxides to overcome electrode mismatch in sodium-ion hybrid capacitors

Sodium-ion hybrid capacitors (SICs) exhibit promise for energy storage owing to their integration of high power and energy densities with an exceptional cycle life into a single device. However, their practical application is hindered by their inadequate energy density stemming from the inherent disparities in energy storage mechanisms between anodes and cathodes. Herein, a bifunctional surface-engineering strategy is demonstrated to develop both cathode and anode materials using graphene oxide (GO). The defect-rich reduced graphene oxide (DRGO) anode is characterized by numerous defects on its layer surface, which offers shortcuts for rapid Na-ion insertion and desertion. The partially reduced graphene oxide (PRGO) cathode is equipped with precisely regulated oxygen-containing functional groups (OFGs), which function as active sites for PF₆⁻ absorption and subsequent storage via surface redox reactions, thereby markedly contributing to the pseudocapacitance. The enhanced charge diffusion kinetics of the DRGO anode and improved capacity of the PRGO cathode notably mitigate the mismatch between the electrodes in SICs. Consequently, this work, for the first time, realizes an all-graphene SIC with remarkable energy density of 187 Wh kg⁻¹ at 49 W kg⁻¹ and power density of 9232 W kg⁻¹at 117 Wh kg⁻¹, along with an ultralong lifespan of 12000 cycles.