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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, H 3 PMo 12 O 40 .nH 2 O/Nafion 117/H x RuO 2 .nH 2 O 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.