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

Graphical abstract: - Highlights: • Testing of a flow battery with polyoxometalates. • Coulombic efficiency of 83% for an iron-based compound. • Both size and charge density influence battery performance. - Abstract: 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 (SiFe₃W₉(OH)₃O₃₄)₂(OH)₃¹¹⁻/(SiFe₃W₉(OH)₃O₃₄)₂(OH)₃¹⁴⁻and (SiFe₃W₉(OH)₃O₃₄)₂(OH)₃¹⁷⁻/(SiFe₃W₉(OH)₃O₃₄)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-V₂W₄O₁₉⁴⁻, 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-V₂W₄O₁₉⁴⁻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