Implications of Polyoxometalate Properties on the Performance Characteristics of Redox Flow Battery

Abstract

Implications of rate coefficients, concentration ratio, and electron-transfer number of the redox species present in the anolyte and catholyte on the performance characteristics of a redox flow battery (RFB) were investigated. Toward this, a polyoxometalate (POM)-V4+/V5+ RFB (with order 104 of magnitude difference in their rate coefficients) was assembled by replacing the anolyte (V2+/V3+) of a well-established vanadium RFB (VRFB); the redox potential of POM is comparable to that of V2+/V3+. The performance of the POM-V4+/V5+ RFB improves by ~100 mV at reasonable operating current densities (~ 200 mA cm-2) as compared to that of a VRFB, in a 5 cm2 cell, even though the solubility of tungstosilicic acid (TSA) is 300 mM as compared to 1.8 M VOSO4. Only four electrons can be reversibly extracted from the POM, although extraction of 14 electrons (theoretical) is possible from TSA, limiting the charging voltage to 1.4 V. Overcharging leads to capacity loss and concentration ratio (Catholyte: Anolyte) impacts the overall performance of RFB. Significant vanadium crossover loss is also observed at the anode side.