Porous and graphitic carbon nanosheets with controllable structure for zinc-ion hybrid capacitor

The imbalances of storage capacity and reaction kinetics between carbonaceous cathodes and zinc (Zn) anodes restrict the widespread application of Zn-ion hybrid capacitor (ZIHC). Structure optimization is a promising strategy for carbon materials to achieve sufficient Zn²⁺ storage sites and satisfied ion-electron kinetics. Herein, porous graphitic carbon nanosheets (PGCN) were simply synthesized using a K₃[Fe(C₂O₄)₃]- and urea-assisted foaming strategy with polyvinylpyrrolidone as carbon precursor, followed by activation and graphitization. Sufficient pores with well-matched pore sizes (0.80-1.94 nm) distributed across the carbon nanosheets can effectively shorten mass-transfer distance, promoting accessibility to active sites. A partially graphitic carbon structure with high graphitization degree can accelerate electron transfer. Furthermore, high nitrogen doping (7.2 at.%) provides additional Zn²⁺ storage sites to increase storage capacity. Consequently, a PGCN-based ZIHC has an exceptional specific capacity of 181 mAh g^-1 at 0.5 A g^-1, superb energy density of 145 Wh kg^-1, and excellent cycling ability without capacity decay over 10,000 cycles. In addition, the flexible solid-state device assembled with PGCN exhibits excellent electrochemical performances even when bent at various angles. This study proposes a straightforward and economical strategy to construct porous graphitic carbon nanosheets with enhanced storage capacity and fast reaction kinetics for the high performance of ZIHC.