Advances in vanadium and polyoxometalate redox flow batteries

Abstract

Electrochemical energy storage is one of the few options to store the energy from intermittent renewable energy sources like wind and solar. Redox flow batteries are such an energy storage system, which has favourable features over other battery technologies, for example, solid state batteries, due to their inherent safety and the independent scaling of energy and power content. However, because of their low energy density, low power density, and the cost of components such as redox species and membranes, commercialised RFB systems like the all-vanadium chemistry cannot make full use of the inherent advantages over other systems. This thesis shows a comparison of promising cell chemistries with the aim to elucidate which redox system is most favourable in terms of energy density, power density and capital cost. Additionally, the choice of solvent and the selection of inorganic or organic redox couples with the entailing consequences are discussed. The sluggish redox kinetics of the VO2+/VO2 + couple limit the power density of the VRFB, which increases the footprint of the power converters and increases capital costs. During recent years, much research has been carried out in the field of heterogeneous catalysis, but this is now approaching its limits. In this work, homogeneous catalysis was conducted to improve the system. The kinetics of the VO2+/VO2 + redox reaction have been investigated in 1M sulphuric and 1 M phosphoric acid by cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy and flow battery tests. It was found that in 1 M phosphoric acid the electron transfer constant k0 is up to 67 times higher than in 1 M sulphuric acid and an over-potential dependent difference in electron transfer constant was observed and explained. This study shows that the redox kinetics of the VO2+/VO2 + can be considerably accelerated by altering the chemical environment of the vanadium ions, and that this effect can also be transferred into a flow battery. However, the prevailing technology, the all-vanadium system, comprises low energy and low power densities, therefore, the chemistry of polyoxometalates, [SiW12O40] 4- and [PV14O42] 9- , as nano-sized electron shuttles was investigated. It is shown that these POMs exhibit fast redox kinetics (electron transfer constant k0 ≈ 10-2 cm s-1 for [SiW12O40] 4- ), thereby enabling high power densities; in addition, they feature multi-electron transfer, realizing a high capacity per molecule; they do not cross cation exchange membranes, thus eliminating self-discharge through the separator; and they are chemically and electrochemically stable as shown by in- iv situ nuclear magnetic resonance spectroscopy. In flow battery studies the theoretical capacity (10.7 Ah L-1 ) could be achieved under operating conditions with Coulombic efficiency of 94%. Very small losses occurred due to residual oxygen in the system. Options to improve the energy density of the system are discussed.