Benefits of Organo‐Aqueous Binary Solvents for Redox Supercapacitors Based on Polyoxometalates

A novel redox electrolyte is proposed based on organo‐aqueous solvent and a polyoxometalate (POM) redox moiety. The presence of dimethyl sulfoxide (DMSO) plays multiple roles in this system. Firstly, it enhances the cathodic electrochemical stability window by shifting the H(2) evolution to lower potentials with respect to pure aqueous systems; secondly, it improves the reversibility of the redox reaction of the PW(12)O(40) (3−) anion at low potentials. The presence of DMSO suppresses the Al corrosion, thus enabling the use of this metal as the current collector. An activated carbon‐based supercapacitor is investigated in 1 M LiNO(3)/10 mM H(3)PW(12)O(40) in a mixed DMSO/H(2)O solvent and compared with a POM‐free electrolyte. In the presence of POMs, the device achieves better stability under floating conditions at 1.8 V. At 1 kW kg(−1), it delivers a specific energy of 8 Wh kg(−1) vs. 4.5 Wh kg(−1) delivered from the POM‐free device. The H(2) evolution is further shifted by the POMs adsorbed on the activated carbon, which is one reason for the improved stability. The POM‐containing cell demonstrates a mitigated self‐discharge, owing to strong POMs adsorption into the carbon pores.