Unveiling the cooperative roles of pyrrolic-N and carboxyl groups in biomass-derived hierarchical porous carbon nanosheets for high energy-power Zn-ion hybrid supercapacitors

The low energy density of Zn-ion hybrid supercapacitors (ZHSCs) remains to be one of the key challenges against its commercialization due to the lack of cathode materials with high capacity, particularly at high current density. Herein, biomass-derived hierarchical porous carbon nanosheets functionalized with oxygen groups and nitrogen dopants used as the cathode for ZHSCs are synthesized via a consecutive two-step pyrolysis-activation strategy, which manifest a high capacity of 220.1 mAh g−1 at 0.2 A g−1, and maintain 118.2 mAh g−1 when the current density increases 100 times to 20 A g−1. Benefitting from the promising performance of the as-obtained cathode materials, the ZHSC finally demonstrates an ultra-high energy density of 181.6 Wh kg−1 at the power density of 165.0 W kg−1, which surpasses the vast majority of state-of-the-art reported ZHSCs. Accordingly, the experimental results and theoretical calculations reveal that the hierarchically porous ultrathin nanosheet architecture with macro-, meso- and micropores is favorable for boosting the physical adsorption of Zn2+, while the introduction of surface pyrrolic-N and carboxyl (–COOH) groups synergistically boosts the chemical adsorption of Zn2+. The present work provides insights into the synergistic effect of nitrogen doping and oxygen functional groups modification hierarchical porous carbon for Zn-ion storage.