Publications

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Nonaqueous redox flow batteries (RFB) leverage nonaqueous solvents to enable higher operating voltages compared to their aqueous counterparts. Most commercial components for flow batteries, however, are designed for aqueous use. One critical component, the ion-selective membrane, provides ionic conductance between electrodes while preventing crossover of electroactive species. Here we evaluate the area-specific conductances and through-plane conductivities of commercially available microporous separators (Celgard 2400, 2500) and anion exchange membranes (Neosepta AFX, Neosepta AHA, Fumasep FAP-450, Fumasep FAP-PK) soaked in acetonitrile, propylene carbonate, or two imidazolium-based ionic liquids. Fumasep membranes combined with acetonitrile-based electrolyte solutions provided the highest conductance values and conductivities by far. When tested in ionic liquids, all anion exchange membranes displayed conductivities greater than those of the Celgard microporous separators, though the separators decreased thickness-enabled conductances on par with the most conductive anion exchange membranes. Ionic conductivity is not the only consideration when choosing an anion exchange membrane; testing of FAP-450 and FAP-PK membranes in a nonaqueous RFB demonstrated that the increased mechanical stability of PEEK-supported FAP-PK minimized swelling, in turn decreasing solvent mediated crossover and enabling greater electrochemical yields (40% vs. 4%) and Coulombic efficiencies (94% vs. 90%) compared to the unsupported, higher conductance FAP-450.

SNL has developed a series of ionic-liquid electrolytes with accompanying non- Aqueous compatible membranes and flow cell designs for improved energy density redox flow batteries targeted to support increasing demands for stationary energy storage. The new electrolytes yield a higher energy density by chemically incorporating an electro- Active transition metal element into the solvent's molecular formula. Although ionic liquids have higher viscosities than conventional non- Aqueous electrolytes, they are promising for higher energy densities due to higher metal concentrations and wider voltage windows. We have addressed high viscosity by developing new materials through careful ligand and anion selection. We have also developed tunable membranes for non- Aqueous compatibility and rapid laboratory-scale prototyping to quickly screen materials and cell designs. We are projecting a four-fold improvement in energy density over the next two years.

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.

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A redox flow battery utilizing two, three-electron polyoxometalate redox couples (SiV^V₃W^VI₉O₄₀^7–/SiV^IV₃W^VI₉O₄₀^10- and SiV^IV₃W^VI₉O₄₀^10-/SiV^IV₃W^V₃W^VI₆O₄₀^13-) was investigated for use in stationary storage in either aqueous or non-aqueous conditions. The aqueous battery had coulombic efficiencies greater than 95% with relatively low capacity fading over 100 cycles. Infrared studies showed there was no decomposition of the compound under these conditions. The non-aqueous analog had a higher operating voltage but at the expense of coulombic efficiency. The spontaneous formation of these clusters by self-assembly facilitates recovery of the battery after being subjected to reversed polarity. Polyoxometalates offer a new approach to stationary storage materials because they are capable of undergoing multi-electron reactions and are stable over a wide range of pH values and temperatures.

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