Mixed-Metal, Structural, and Substitution Effects of Polyoxometalates on Electrochemical Behavior in a Redox Flow Battery

A pair of redox flow batteries containing polyoxometalates was tested as part of an ongoing program in stationary energy storage. The iron-containing dimer, (SiFe3W9(OH)(3)O-34)(2)(OF)(3)(11-), cycled between (SiFe3W9(OH)(3)O-34)(2)(OF)(3)(11-)/(SiFe3W9(OH)(3)O-34)(2)(OH)(3)(14-)and (SiFe3W9(OH)(3)O-34)(2)(OH)(3)(17-)/(SiFe3W9(OH)(3)O-34)(2)(OH)(3)(14-) for the positive and negative electrode, respectively. This compound demonstrated a coulombic efficiency of 83% after 20 cycles with an electrochemical yield (measured discharge capacity as a percentage of theoretical capacity) of 55%. Cyclic voltammetry on the Lindqvist ion, cis-V2W4O194-, showed quasi-reversible vanadium electrochemistry, but tungsten reduction was mostly irreversible. In a flow cell configuration, cis-V(2)W(4)O(19)(4-)had a coulombic efficiency of 45% (for a two-electron process) and an electrochemical yield of 16% after 20 cycles. The poor performance of cis-V(2)W(4)O(19)(4-)was attributed primarily to its higher charge density. Collectively, the results showed that both polyoxometalate size and charge density are both important parameters to consider in battery material performance. Published by Elsevier Ltd.