Elucidating the charge storage mechanism of high-performance vertical graphene cathodes for zinc-ion hybrid supercapacitors

Aqueous zinc-ion hybrid supercapacitors (ZHSs) are gaining enormous attention due to intrinsic safety, low cost and potential for acquiring both high energy density and high power density, but their electrochemical properties are realistically restricted by improper physicochemical characteristics and ambiguous charge storage mechanism of carbon cathodes. Herein, we report an advanced carbon cathode of activated vertical graphene (A-VGN) for high-performance ZHSs. For the A-VGN cathode, vertical graphene array morphology and hierarchically porous structure effectively shorten ion diffusion distance, and meanwhile, large specific surface and oxygen doping offer abundant active sites for ion adsorption. Consequently, the A-VGN cathode-based ZHS presents exceptional electrochemical performance such as a high capacity of 246 mAh/g, fast charge/discharge capability, superior cycling stability with 97.4% capacity retention over 10,000 charge/discharge cycles, as well as preponderant energy density among various carbon cathode-based ZHSs. Furthermore, in-operando Raman spectroscopy and in-operando pH monitoring techniques, being supplemented by ex-situ methods, are applied to investigate the charge storage mechanism of the A-VGN cathode. Dynamic adsorption/desorption behaviors of different ions on the carbon cathode during charge/discharge processes are elucidated. This work is expected to facilitate the development of ZHSs by providing new insights into the design and charge storage mechanism of carbon cathodes.