Factors affecting performance of electrochemical capacitors operating in Keggin-type silicotungstic acid electrolyte

The key factors influencing the application of Keggin-type silicotungstic acid electrolyte, H4SiW12O40 (SiW12), in electrochemical capacitors are considered here by means of electrochemical, N2 adsorption, and Differential Scanning Calorimetry analysis. A series of activated carbons has been selected in order to match the porous structure of the electrode allowing to accommodate ca. 1.1 nm in diameter SiW12O404− anions. It has been shown that for carbons with adequately wide pore size distribution, the electrical parameters of capacitor can be as high as for the standard acidic H2SO4 electrolyte but the system is characterized by better discharge dynamics and, consequently, by higher specific power. The observation can be attributed to high population and mobility of protons in the Keggin structure and to a great ionic (molar) conductivity of SiW12. It has been also shown that a water/methanol mixture can be used to permit application of the SiW12 electrolyte at low temperatures with a performance exceeding the H2SO4 counterpart. Additionally, the electrochemical behaviour of SiW12 has been compared to the characteristics of phosphate-coordinated analogue, H3PW12O40, namely by exploring the influence of the POMs redox processes on the systems’ charge storage properties and self-discharge rates.