Boosting effects of hydroxyl groups on porous carbon for improved aqueous zinc-ion capacitors

•Oxygen functional groups can largely enhance the performance of zinc-ion hybrid capacitors (ZIC) and the corresponding energy storage mechanism is proposed.•The reaction mechanism between Zn2+ and the hydroxyl groups on active carbon is proposed.•The assembled solid-state ZIC is flexible enough to withstand various deformations. Aqueous zinc-ion capacitors (ZIC) are promising energy storage devices due to their high specific capacitance, wide voltage window, and good safety. It is well known that the oxygen functional groups on carbon surface have a significant role for the capacitance performance in ZIC, however, as for the various oxygen functional groups, it is not clear which functional group plays an important role and what is the energy storage mechanism for ZIC. Herein, activated carbon (AC) was treated by a gradient acid to induce surface oxygen functional groups, mainly hydroxyl groups, as cathode materials in ZIC, which lead to capacitance increase by 33.3% from 120 to 160 F•g−1. Further experimental results and DFT calculation demonstrated that the secondary alcohols hydroxyl groups play a very important role in storage Zn2+, showing better electrochemical reversibility than tertiary alcohols. The modified AC cathodes also work well at different bending angles, manifesting huge potential for use in flexible and smart wearable portable electronic products. Overall, this improvement strategy for the capacity of AC cathode is of great significance and guidance value for the large-scale development of ZIC. [Display omitted]