(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 (XMmO40n-) in acetonitrile were examined. The 1A and 2A groups of counter-cation exchanged POMs – Li3, Na3, Mg1.5 and Ca1.5 salts – were studied. Heteroatom-exchanged POMs including XMo12O403-/4- and XW12O403-/4- forms (Li3PMo12O40, Li3AsPMo12O40, Li4SiMo12O40, Li3PW12O40, Li4SiW12O40.) were also tested. Lastly, two series of framework-substituted POMs, Li4PMo11VO40, Li5PMo10V2O40, and Li6PMo9V3O40 and Li3PMo12O40, 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.