Regulating surface oxygen species on activated carbon via plasma treatment to boost zinc-ion adsorption for zinc-ion hybrid capacitors

Activated carbon (AC) is an ideal cathode for zinc-ion hybrid capacitors (ZIHCs) due to its high specific surface area, rational pore structures, good electrical conductivity, and chemical stability. However, the specific capacity of AC remains far below theoretical limits, primarily due to its poor wettability, which leads to inadequate penetration into deep micropores/mesopores, leaving active sites inaccessible and hindering ion diffusion and adsorption, thus degrading performance. Herein, we report a room-temperature air plasma strategy for precisely tuning oxygen functional groups on AC cathodes while keeping their bulk structure intact. By controlling the plasma treatment time, the surface content of the groups is precisely modulated. These oxygen functional groups significantly enhance the electrolyte wettability of the AC electrode, increasing the effective interface area accessible to electrolyte ions. This promotes efficient ion adsorption and diffusion, leading to markedly improved specific capacitance, rate capability, and cycling stability. Moreover, the assembled large-area pouch cells demonstrate excellent stability and practical applicability. This work highlights the importance of carbon electrode surface properties in enhancing the electrochemical performance and will inspire further surface modification strategies for advanced energy storage devices.