Polyoxometalates for Non-Aqueous Redox Flow Battery Applications

Scalable energy storage technologies are needed to enable integration of high levels of integration of intermittent energy sources such as wind and solar into the grid. Redox Flow Batteries (RFBs) designed to store energy in chemical form in the electrolyte can help to address the energy storage problem. However, most traditional RFBs are aqueous systems for which the cell voltage is limited by water splitting. Non-aqueous RFBs could permit higher cell voltages by removing this limitation. Polyoxometalates (POMs), with versatile, tunable properties, undergo multi-electron redox reactions which may meet the requirements of high performance Redox Flow Batteries.

Research conducted to date has focused on the electrochemical characteristics of different combinations of Keggin-type polyoxometalates (XM_mO₄₀^n-). Metals substituted into the Keggin framework play a key role in altering the redox properties of polyoxometalates. Based on the result, the effects of 1A or 2A counter  cations on cell potentials were fairly minor. And Keggin ions containing different central heteroatoms (phosphorous, arsenic and silicon) were compared. While the charge on the Keggin ion does affect the telative positions of redox couples in the CV spectrum, the number of redox couples is not affected.  Last mixed addenda POMs were also compared. The mechanism of addenda element effects on POM redox properties is still unclear, but the mixed addenda elements play an important role in determining the redox properties. Investigation of the mix addenda POMs is the focus of current efforts.

This research aims to select suitable polyoxometalate species that can be used for non-aqueous Redox Flow Batteries. Preliminary results have shown high coulombic efficiency (~90%), reflecting the reversibility of polyoxometalate redox reactions.