Renewable energy generation around the world continues to see dramatic growth. According to a recent U.S. Energy Information Administration (EIA) report, 13% of US energy demand in 2013 was met by renewables and the amount is still increasing. Large scale energy storage systems promise to increase the effectiveness of renewable energy sources. Redox Flow Batteries (RFB) represent a highly scalable approach. Traditional RFBs are mostly aqueous which limits their operating voltage range due to water splitting. Non-aqueous systems offer a larger voltage window. Polyoxometalates (POMs), which exhibit versatile redox properties and multiple electron transfers were used as the active species in non-aqueous RFBs. POMs generally contain group 5 or group 6 transition metals linked by oxygen atoms and centered around a heteratom. Common POMs are of the Keggin type ([XM12O40]q, where X = P, Si, Ge, etc., and M = Mo, W, V, etc. ) or Wells-Dawson type ([X2M18O62]q-). The redox properties of POMs can be altered by exchanging the framework metal atoms or the central heteroatom very easily.
Li3PMo12O40, Li4PMo11VO40 and Li6P2W18O62 were tested in acetonitrile and the lithium was chosen as the counter-cation in order to avoid H2 generation. Owing to limitations on the transport rate of Li+ through conventional proton-exchange membranes such as Nafion, an aramid nano-fiber (ANF) membrane was utilized for charge-discharge experiments. The low permeability of ANF prevents the POM crossover, with less than 1% crossover observed after one week. Greater than 85% coulombic efficiency was observed for symmetric cells containing the POMs above, with good reversibility and stability. Both symmetric and asymmetric cells (different POMs utilized for the anolyte and catholyte) were tested. Asymmetric POM-based RFBs hold promise for increasing the voltage window and overall energy density.