Dual-salt synthesis of oxygen-rich lignin-based porous carbon nanosheets with large-sized micropores for fast zinc-ion storage

Porous carbon represents a highly promising cathode material for zinc-ion hybrid capacitors (ZIHCs). Nevertheless, conventional activation methods typically produce narrow micropores that are mismatched with the dimensions of hydrated Zn 2+ ions, limiting the electrochemical performance of ZIHCs. Herein, a dual-salt activation strategy employing potassium citrate and potassium oxalate was proposed to synthesize lignin-derived oxygen-rich porous carbon nanosheets (OPC-Kc-Ko) featuring large-sized micropores exceeding 0.86 nm. The optimized OPC-Kc-Ko-700 exhibited a high specific surface area of 1762 m 2 /g, a significant proportion of large micropores (38 %), and abundant oxygen functionalities (12.24 at.%). The gas exfoliation of potassium oxalate and the self-templating effect of potassium citrate facilitated the formation of nanosheet morphology. Meanwhile, the persistent etching by potassium carbonate contributed to the high surface area and the presence of large-sized micropores. These architectural characteristics endowed OPC-Kc-Ko-700 with dense electroactive sites and rapid ion-sieving nanochannels tailored to hydrated Zn 2+ dimensions (≈0.86 nm), enhancing energy storage and charge-transfer kinetics in ZIHCs. When utilized as a cathode in ZIHCs, OPC-Kc-Ko-700 delivered a remarkable specific capacitance of 324 F/g at 0.1 A/g and an energy density of 115.52 Wh/kg. The material exhibited outstanding cycling stability, preserving 93.5 % capacitance retention after undergoing 12,000 charge–discharge cycles under a high current density of 5 A/g. This superior Zn 2+ storage capability can be attributed to the electric double-layer capacitance enabled by fast ion adsorption/desorption within the large-sized micropores and pseudocapacitance arising from reversible oxygen-mediated redox reactions. This study elucidates innovative strategies for tailoring pore architectures in carbon-based cathode materials for high-performance ZIHCs.