Studying polyoxometalates for application in redox flow batteries

Abstract

Energy storage is important to an energy sector in which renewable energy sources become more prevalent and the use of energy from fossil fuels is minimised. As energy from renewable sources is produced intermittently (e.g. from wind and solar photovoltaic systems), energy storage can be used to stabilise these energy sources by storing excess energy produced during times of low demand and releasing it later. Redox Flow Batteries (RFBs), a type of secondary battery, are an energy storage technology that is of interest to both large-scale energy storage and smaller off-grid energy storage systems. RFBs have differences in operation compared to other secondary batteries. For instance, all of the active material of an RFB is dissolved in the electrolytes and no plating or intercalation reactions occur at the electrodes during charge and discharge. Further, the electrolytes are stored in external tanks and transported to the cells of an RFB by using pumps. This decouples the power and energy content of an RFB; power and energy can be adjusted independently by varying the size of the cell stack or the volume of electrolyte. In RFB research, a common endeavour is the investigation of active materials that could target some of the limitations of the most mature RFB, the all-vanadium RFB, which uses vanadium ions as the active species. Polyoxometalates (POMs) are one type of compound that have been studied as RFB active materials. POMs are anionic metal-oxygen clusters containing early transition metals, termed the addenda atoms of the POM, which are often tungsten, molybdenum and vanadium. These elements can exhibit reversible redox reactions when incorporated into a POM. An additional element, such as silicon or phosphorus, is sometimes present in the core of a POM and is termed a heteroatom. In this work, aspects of POMs for application in symmetric and asymmetric RFBs are investigated. Symmetric RFBs use the same active species in the positive and negative halfcell whereas asymmetric RFBs use a different species in each half-cell. [SiV3W9O40] 7- , a mixed-addenda POM containing tungsten and vanadium, is studied as an active species for a symmetric RFB. Selected electrochemical properties of two POMs containing one type of addenda atom but a different heteroatom, [SiW12O40] 4- and [CoW12O40] 6- , are also investigated in relation to their application as the active material in the negative electrolyte of an asymmetric RFB. In symmetric flow cell testing of [SiV3W9O40] 7- , coulombic efficiencies were >98% with 65% of the theoretical capacity of the battery accessed and a capacity fade of 2.7% over 50 cycles observed. Analysis of cycled [SiV3W9O40] 7- electrolytes did not indicate decomposition had occurred, indicating stability of [SiV3W9O40] 7- to the charge/discharge testing conditions in this study. In a study of the kinetics of the electron transfer of