Seamless All‐Solid‐State Supercapacitor Fabricated Using a Proton‐Conducting Methanesulfonic Acid‐Intercalated Graphene Oxide Film as an Electrolyte

Abstract

An all‐solid‐state supercapacitor with no boundary between the electrode/electrolyte interface is prepared using methanesulfonic acid (MSA)‐intercalated graphene oxide (GO) membranes as a proton‐conducting electrolyte. The electrodes (reduced GO) are formed within the surface of the solid GO electrolyte by a combination of self‐reduction of the GO under UV‐light illumination and electrochemical reduction. In this process, the surface of the GO film is converted to an electrode material with mixed electron/proton conduction, which results in the formation of a seamless capacitor structure. The resultant capacitor shows a large capacitance of 33.8 mF cm−2, 11.9 F g–1 (g: total weight of device including electrodes, electrolyte, separator and current collector), which is 15 times higher than the capacitance retention of an all‐solid‐state supercapacitor fabricated using proton‐conducting GO film. The seamless structures for the electrode/electrolyte interface suppress the decomposition of the GO electrolyte by the local concentration of applied voltage, resulting in improved cycle stability. The very large capacitance is likely derived not only from the seamless structure but also from the high proton conductivity of the MSA‐intercalated GO electrolyte (4.2 × 10−3 S cm−1).