Aqueous All-Polyoxometalate Redox-Flow-Batteries

Abstract

Lithium-Ion Batteries (LIBs) are widely discussed and investigated as versatile energy storage systems. However, LIBs may pose safety risks, such as flammability of the electrodes and the electrolytes, which is a problem that may be amplified when transitioning from small-scale to medium- or large-scale energy storage. Yet, large-scale low-cost energy storage is crucial for the transition from a fossil fuel-based energy economy to renewable energy sources and in order to utilise the already existing energy sources like wind and solar power more efficiently.A promising technology for this task are Redox-Flow-Batteries (RFBs). The RFB is the only type of battery where power output and energy capacity can be scaled independently, allowing it to be specifically tailored to a variety of applications. However, the most mature RFB so far, the All-Vanadium RFB, suffers from low energy density and low power density.In order to overcome these challenges while maintaining the advantages of an aqueous RFB, like non-flammability and minimal self-discharge, we are investigating new redox chemistries. As redox systems, polyoxometalates (POMs) and specifically [SiW12O40]4- and [PV14O42]9- as nano-sized electron shuttles were investigated.1 These POMs exhibit fast redox kinetics (electron transfer constant k 0 ≈ 10-2 cm s-1 for [SiW12O40]4-) which together with their high solubility in water and multiple redox-centres per molecule provides the potential for for high power densities and high energy densities. The POMs that were used also exhibit high electrochemical and chemical stability, thus providing long cycle lifetimes. Other POMs were investigated as well.The system was scaled up from a lab-sized cell of 25 cm2 membrane area to a cell of 1400 cm2 in order to assess the implications on efficiency and operational parameters.2 The cell was operated for 1400 cycles over a time of nearly three months, providing some very promising results; the Coulombic efficiency was nearly 100% with the energy efficiency dropping only from 86.1% to 85.1% during the whole period, indicating a highly stable system. The observed capacity loss of 0.011% per cycle could be attributed to ambient air leakage leading to oxidation of SiW12. Post-cycling analysis showed no sign of degradation of the electrolytes. Acknowledgement We acknowledge funding and the fruitful cooperation with SIEMENS AG. Also, funding from NECEM, the North East Centre for Energy Materials (EP/R021503/10) is thankfully acknowledged.