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In situ X-Ray Absorption Near Edge Spectroscopy (XANES) and Extended X-Ray Absorption Fine Structure (EXAFS) techniques are applied to a metal center ionic liquid undergoing oxidation and reduction in a three electrode spectroscopic cell. Determination of the extent of reduction under negative bias on the working electrode and the extent of oxidation are determined after pulse voltammetry to quiescence. While the ionic liquid undergoes full oxidation, it undergoes only partial reduction, likely due to transport issues on the timescale of the experiment. Nearest neighbor Fe-O distances in the fully oxidized state match well to expected values for similarly coordinated solids, but reduction does not result in an extension of the Fe-O bond length, as would be expected from comparisons to the solid phase. Instead, little change in bond length is observed. We suggest that this may be due to a more complex interaction between the monodentate ligands of the metal center anion and the surrounding charge cloud, rather than straightforward electrostatics between the metal center and the nearest neighbor grouping. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]