Hybrid energy storage: high voltage aqueous supercapacitors based on activated carbon–phosphotungstate hybrid materials

Abstract

The use of hybrid electrodes prepared from phosphotungstic acid (H₃PW₁₂O₄₀, PW₁₂) and activated carbon (AC) allows the fabrication of highly durable aqueous supercapacitors operating at a superior voltage (1.6 V) thanks to the high overpotential of PW₁₂ towards H₂ generation. The combination of the double-layer capacitance plus the redox activity of the polyoxometalate clusters leads to an intrinsic increase in specific capacitance (from 185 F g⁻¹ for AC to 254 F g⁻¹ for AC–PW₁₂) and a 60% increase in the operating voltage (compared to conventional AC supercapacitors with aqueous electrolytes, e.g. 1 M sulphuric acid). The hybrid energy storage mechanism and the increased operating voltage converge to yield improved specific energy and power. Moreover, the hybrid AC–PW₁₂ electrode material showed an outstanding stability even after 30 000 cycles (0 to 1.6 V) with 98% retention of the initial capacitance, much superior to the stability of the parent supercapacitor based on plain AC under the same conditions.