Keggin‐Type Heteropolyacids as Electrode Materials for Electrochemical Supercapacitors

An inexpensive electrochemical capacitor system with an asymmetric configuration is demonstrated. The system includes heteropolyacids and a proton‐exchange polymer membrane. Heteropolyacid electrodes show facile redox processes and host large amounts of easily accessible mobile protons required for charge balance. The proton‐conducting polymer electrolyte provides chemical stability of the heteropolyacids which would otherwise dissolve into the aqueous liquid electrolyte. In this context, /Nafion 117/ is a model system. A very reversible specific capacitance of 112 F/g and an energy density of 36 J/g, based on the active materials only, were measured for the optimized cold‐pressed single cell with an operating voltage of 0.8 V. This approach can be extended to other transition‐metal oxides chemically unstable in aqueous solution that have mixed‐valence redox energies between the highest occupied and lowest unoccupied molecular orbital of the electrolyte. An electronic energy diagram for advanced design considerations is presented.