Physisorbed Hydroquinone on Activated Charcoal as a Supercapacitor: An Application of Proton-Coupled Electron Transfer

High surface area carbon materials have high double layer capacitances because of their enhanced internal surface area and hence are attractive materials for supercapacitor applications. In this work, we demonstrate that utilizing a simple shaking experiment, hydroquinone can be physisorbed inside the pores of activated charcoal, and the material can be used as a supercapacitor having a highest specific capacitance of ∼200 F/g in 1 M H₂SO₄. Nearly 40% of the specific capacitances were pseudocapacitance in nature because of the observed reversible redox chemistry of hydroquinone/benzoquinone couples, where hydroquinone underwent proton-coupled electron transfer (2H⁺/2e^–) to form benzoquinone. The redox chemistry of hydroquinone/quinone is chemically irreversible, but the same chemistry has been found to be chemically reversible, and fast electron transfer kinetics at the electrode surface was observed in this study, presumably because of proton-coupled electron transfers that were catalyzed by oxide sites present on the activated charcoal. Due to the observed reversible electrochemistry, the material also showed excellent capacitance retention in the long term cyclic tests. The approach presented in this study conceptually brings a new dimension to improve the chemistry of energy storage systems by simultaneous introduction of physisorption and proton-coupled electron transfer.