•Molybdenum polyoxometalates show promise for non-aqueous RFBs.•Aramid nanofiber membranes provide good separation and conductivity.•High coulombic efficiencies were observed.The Keggin-type polyoxometalate Li3PMo12O40 was evaluated for energy storage applications in non-aqueous media. In acetonitrile, using a static H-cell, high coulombic efficiency (∼90%) and stable (>500 h) charge and discharge results were obtained. Different separators taking advantage of charge and size-based differentiation were compared for this system. Celgard®2325, Nafion®117 and aramid nanofiber-based membranes (ANF) were examined. Both Li-exchanged Nafion®117 and ANF membranes enabled stable charge/discharge cycles with high coulombic efficiency for this non-aqueous polyoxometalate system. The ANF membrane, which separates based on size, permitted operation at higher current densities than the Nafion®117 ion-exchange membrane.Download high-res image (103KB)Download full-size image

The electrochemical characteristics of several Keggin-type polyxometalates (POMs) in acetonitrile were investigated by cyclic voltammetry. The effects of three types of substitutions (countercation, heteroatom, and framework metal element) were investigated. The spectra for group IA or IIA POM salts showed three redox couples, with only slightly different peak positions (±0.1 V). Lithium salts were selected for exploration of other POM compositions in order to prevent H2 evolution in electrochemical experiments. CV spectra of heteroatom-exchanged POMs showed that the overall charge on the POM affects reduction potentials. Substitution of the framework metal atoms (Mo, W, and V) had the greatest effect on reduction potentials. The results of this study demonstrate that reduction potentials in water may provide a useful guide to relative reduction potentials for POMs in nonaqueous media.