Properties of supporting electrolytes and solvents were examined for use with vanadium acetylacetonate – a member of the class of metal(β-diketonate) active species – in non-aqueous redox flow batteries. Twenty supporting-electrolyte/solvent combinations were screened for ionic conductivity and supporting-electrolyte solubility. Hexane, tetrahydrofuran, and dimethylcarbonate solvents did not meet minimal conductivity and solubility criteria for any of the electrolytes used, which included tetraethylammonium tetrafluoroborate, tetrabutylammonium tetrafluoroborate, tetrabutylammonium hexafluorophosphate, and (1-butyl, 3-methyl)imidazolium bis(trifluoromethanesulfonyl)imide. Ionic conductivities and solubilities for solutions of these electrolytes passed screening criteria in acetonitrile and dimethylformamide solvents, in which maximum supporting-electrolyte and active-species solubilities were determined. Active-species electrochemistry was found to be reversible in several solvent/support systems; for some systems the voltammetric signatures of unwanted side reactions were suppressed. Correlations between supporting-solution properties and performance metrics suggest that an optimal solvent for a vanadium acetylacetonate RFB should have a low solvent molar volume for active-species solubility, and a high Hansen polarity for conductivity.

A single-metal redox flow battery employing manganese(III) acetylacetonate in tetraethylammonium tetrafluoroborate and acetonitrile has been investigated. Cyclic voltammetry was used to evaluate electrode kinetics and reaction thermodynamics. The Mn(II)/Mn(III) and Mn(III)/Mn(IV) redox couples appeared to be quasi-reversible. A cell potential of 1.1 V was measured for the one-electron disproportionation of the neutral Mn(III) complex. The diffusion coefficient for manganese acetylacetonate in the supporting electrolyte solution was estimated to be in the range of 3-5 x 10(-6) cm(2) s(-1) at room temperature. The charge-discharge characteristics of this system were evaluated in an H-type glass cell. Coulombic efficiencies increased with cycling suggesting an irreversible side reaction. Energy efficiencies for this unoptimized system were similar to 21%, likely due to the high cell-component overpotentials. (C) 2011 Elsevier B.V. All rights reserved.

Three electrode materials (glassy carbon, gold, and platinum) were investigated for application in a non-aqueous single-metal redox flow battery based on vanadium (III) acetylacetonate, supported by tetraethylammonium tetrafluoroborate in acetonitrile. Redox couples associated with the one-electron disproportionation of V(acac)3 were observed in voltammetry for each metal tested. An elementary kinetic model was created and used to determine rates for oxidation or reduction of the vanadium complex. The oxidation rates for V(acac)3 were mass-transfer limited on all electrode materials, suggesting reversible kinetics. For the V(acac)3 reduction reaction, exchange-current densities of 1.3, 3.8, and 8.4 A m−2 were observed on glassy carbon, platinum, and gold electrodes, respectively.

Electrochemical and physical measurements elucidate several thermodynamic properties and chemical factors that affect the performance of a non-aqueous all-vanadium flow battery. An H-type test cell was constructed that demonstrates stable coulombic efficiencies of 70% without flow after several weeks of slow cycling, with a steady plateau voltage near 1.7V during most of the discharge step. Environmental oxygen and water are associated with side reactions that affect long-term charge/discharge response of the battery. Oxygen passivates the electrode and may react with the solvent or supporting electrolyte, while water can cause the formation of oxovanadium complexes. Reversible cycling of the vanadyl acetylacetonate complex appears possible.

A single-metal redox flow battery employing chromium(III) acetylacetonate in tetraethylammonium tetrafluoroborate and acetonitrile has been investigated using electrochemical techniques. Cyclic voltammetry was used to evaluate electrode kinetics. Four redox couples were observed in the stable potential window. The CrII/CrIII, CrI/CrII, CrIII/CrIV and CrIV/CrV redox couples all appeared to be quasi-reversible, with the CrIII/CrIV couple exhibiting comparatively slow kinetics. A cell potential of 3.4V was measured for the one-electron disproportionation of the neutral CrIII complex. The diffusion coefficient for chromium acetylacetonate in the supporting electrolyte solution was estimated to be in the range of 5.0–6.2×10−7cm2s−1 at room temperature. The charge–discharge characteristics of this system were evaluated in an H-type glass cell, and coulombic and energy efficiencies of approximately 55% and 20%, respectively, were obtained.

The electrochemistry of a single-component redox flow battery employing vanadium(III) acetylacetonate in acetonitrile and tetraethylammonium tetrafluoroborate has been investigated. The electrode kinetics of the anodic and cathodic reactions were studied using cyclic voltammetry. The V(II)/V(III) and V(III)/V(IV) couples were quasi-reversible and together yielded a cell potential of 2.2V. The diffusion coefficient for vanadium acetylacetonate was estimated to be in the range of 1.8–2.9×10−6cm2s−1 at room temperature. The charge–discharge characteristics of this system were evaluated in an H-type glass cell, and coulombic efficiencies near 50% were achieved.

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