Mixed-metal, structural, and substitution effects of polyoxometalates on electrochemical behavior in a redox flow battery
Electrochimica Acta
Pratt, Harry P.; Pratt, William R.; Fang, Xikui; Hudak, Nicholas H.; Anderson, Travis M.
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)3O34) 2(OH)311-, cycled between (SiFe 3W9(OH)3O34)2(OH) 311-/(SiFe3W9(OH)3O 34)2(OH)314-and (SiFe 3W9(OH)3O34)2(OH) 317-/(SiFe3W9(OH)3O 34)2(OH)314- 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-V2W4O 194-, showed quasi-reversible vanadium electrochemistry, but tungsten reduction was mostly irreversible. In a flow cell configuration, cis-V2W4O194-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-V2W 4O194-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.