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 H₂ evolution to lower potentials with respect to pure aqueous systems; secondly, it improves the reversibility of the redox reaction of the PW₁₂O₄₀ ^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 LiNO₃/10 mM H₃PW₁₂O₄₀ in a mixed DMSO/H₂O 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 H₂ 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.