Polyoxometalate Related Redox Flow Batteries

Abstract

Scalable energy storage technologies are needed to integrate high levels of intermittent energy sources, such as wind and solar, into the grid. Redox Flow Batteries (RFBs), designed to store energy in chemical form known as the electrolyte, can help address the energy storage problem. However, most traditional RFBs are aqueous systems in which the cell voltage is limited by undesired water splitting. Non-aqueous RFBs could permit higher cell voltages by removing this limitation. One proposed non-aqueous RFB system is based on polyoxometalates (POMs). POMs have versatile, tunable properties and can undergo multi-electron redox reactions which may meet the requirements of high performance Redox Flow Batteries. This dissertation presents work investigating the electrochemical characteristics of the different combinations of Keggin-type polyoxometalates (XMmO40n-). Effects of the counter cations, central heteroatoms, and framework metal atoms on the electrochemical characteristics were compared. The basic characteristics of POMs including FTIR, XRD, TGA and bulk electrolysis were examined and compared in this research in order to have a better understanding of Keggin-type polyoxometalates. With understanding of the influences of different Keggin-type polyoxometalates on the electrochemical characteristics, suitable lithium polyoxometalate salts were selected that can be used for non-aqueous Redox Flow Batteries in a static H cell. Conditions of POMs charge/discharge including membrane selection were determined there. The aramid nanofiber (ANF) membrane showed a high separation ability and low resistance, and was therefore selected as the test membrane for battery charge/discharge experiments. Additionally, the POMs were applied to different types of redox flow batteries with some of the results yielding high coulombic efficiency (~90%), reflecting the reversibility of polyoxometalate redox reactions. Different kinds of RFBs were compared in this research and the performance of a RFB with a high concentration (0.1 M Li3PMo12O40) was also demonstrated to be successful, surpassing the concentration range of previous POM RFB studies in the literature. Last, an asymmetric POM RFB charge/discharge was demonstrated for the purpose of achieving a higher voltage window. Overall, these studies provide proof of concept for polyoxometalate applications in non-aqueous redox flow batteries.