(405a) Polyoxometalate Application in Redox Flow Battery

The impact of renewable energy has increased over the past decade. Based on a recent U.S. Energy Information Administration (EIA) report, renewable energy sources supplied 13% of the energy consumed in the US. The need for improved methods of renewable energy storage has also increased. Redox flow batteries (RFBs), having both high energy capacity and a flexible layout, are a promising option to help solve the storage problem. Polyoxometalates (POMs) are attractive active species for use in RFBs, owing to their versatile, tunable properties and multielectron redox reactions.

            The object of this research is to select POMs which can be used in non-aqueous RFBs in a systematic way and to demonstrate their performance for energy storage. Electrochemical characteristics of different keggin POMs (XM_mO₄₀^n-) in acetonitrile were examined. The 1A and 2A groups of counter-cation exchanged POMs – Li₃, Na₃, Mg_1.5 and Ca_1.5 salts – were studied. Heteroatom-exchanged POMs including XMo₁₂O₄₀^3-/4- and XW₁₂O₄₀^3-/4- forms (Li₃PMo₁₂O₄₀, Li₃AsPMo₁₂O₄₀, Li₄SiMo₁₂O₄₀, Li₃PW₁₂O₄₀, Li₄SiW₁₂O₄₀.) were also tested. Lastly, two series of framework-substituted POMs, Li₄PMo₁₁VO₄₀, Li₅PMo₁₀V₂O₄₀, and Li₆PMo₉V₃O₄₀ and Li₃PMo₁₂O_40, Li₃PW₁₂O₄₀, Li₃PMo₁₁WO₄₀ and Li₃Mo₆W₆O₄₀, were examined.

            The 1A and 2A groups of counter-cation exchanged POMs generally showed 3 redox couples with slightly different peaks and peak positions. The effect of 1A or 2A cation exchange resulted in minor differences in observed POM redox potentials. The effect of heteroatom exchange on the POM redox potentials was observed to vary with the negative charge density of the P-, As-, and Si-substituted Keggin ions, however the number of accessible redox couples was not affected. In contrast, framework metal atom variation

affected the redox potentials and the numbers of redox couples. However, the mechanism of how these metal substitutions into the Keggin structure alters the POM electrochemical properties is still unknown.

            The most promising POM materials of those studied were used in charge/discharge tests in acetonitrile, with Li₃PMo₁₂ O₄₀ chosen as the model.  90% coulombic efficiency of the Li₃PMo₁₂O₄₀ charge/discharge testwas found, suggesting both high stability and reversibility of the POMs under these conditions. Our results suggest that POMs in non-aqueous media warrant further study as active species for the development of RFBs with high energy density and high stability.