Benefits of Organo‐Aqueous binary solvents for redox supercapacitors based on polyoxometalates

Abstract

Present and correct: The presence of dimethyl sulfoxide (DMSO) in water can drastically improve the properties of an electrochemical system: the hydrogen evolution shifts to lower potential due to the strong interaction of DMSO with water. DMSO suppresses the corrosion of aluminum and stabilizes the reduced form of polyoxometalates (POMs). Organo-aqueous electrolytes based on DMSO, H2O and POMs are thus applied in a supercapacitor device. 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 PW12O403− 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.