Functionalized Graphene-Lignosulfonate Hydrogel Cathode for Zn-Ion Hybrid Capacitors

Cathode materials, being pivotal to the performance of Zn-ion hybrid capacitors (ZIHCs), have garnered significant attention and research. However, the simple physical combination of components often causes phase interfaces, thereby impeding the optimization of electrode material performance. Herein, a functionalized graphene-lignosulfonate hydrogel (FRGO-Lig) is developed by a simple hydrothermal process using graphene oxide (GO) and biomass lignosulfonate (Lig) as raw materials with p-phenylenediamine (PPD) as the functionalizing modifier. The high electroactivity of Lig and PPD components can provide high capacitance contribution. In addition, the amine groups of PPD can simultaneously react with the epoxy groups of GO to functionalize graphene and the phenolic units of Lig to connect graphene and Lig by the formed –NH– covalent bonds during the hydrothermal process. This is beneficial for eliminating the phase interfaces to further improve charge transfer efficiency and electrochemical performance. When used as a supercapacitor electrode, the specific capacitance of FRGO-Lig can reach 425.3 F g–1 at a high current density of 5 A g–1 and still maintain 297 F g–1 at 50 A g–1, demonstrating a high-rate performance of 69.8%. Notably, the assembled ZIHCs using FRGO-Lig as the cathode achieved a high specific capacitance of 342.7 F g–1 at 1 A g–1 and an excellent energy density of 129.6 Wh kg–1 at a power density of 825 W kg–1, showcasing favorable practical application potential in advanced energy storage devices.