Organic–Inorganic Hybrid Polyoxotungstates As Configurable Charge Carriers for High Energy Redox Flow Batteries

We describe the synthesis and electrochemical analysis of the phenyl siloxane-hybridized phosphotungstate Keggin-type polyoxometalate (POM) TBA3[PW11O39(SiC6H5)2O] (TBA3W11SiPh) and its performance as the charge carrier in nonaqueous redox flow batteries (RFBs). The hybridized POM is synthesized by modification of the parent POM [PW12O40]3–, increasing its saturation concentration in acetonitrile by 2 orders of magnitude over that of the parent compound (600 mmol dm–3 for TBA3W11SiPh vs <1 mmol dm–3 for TBA3[PW12O40]). Electrochemical analysis of TBA3W11SiPh reveals four one-electron, quasi-reversible, redox couples between −0.60 and −2.50 V vs Ag+|Ag, prompting us to explore its application as a dual-function charge carrier in symmetric RFBs. The stability of TBA3W11SiPh is investigated in several symmetric RFBs, and the system demonstrates high coulombic efficiency (98%), voltage efficiency (89%), and energy efficiency (87%) during redox cycling. We show that capacity fade due to oxidation-state imbalance can be counteracted by rereduction of electrolytes. These results demonstrate that organic–inorganic hybridization of POMs offer opportunities for the development of highly soluble multielectron redox electrolytes that operate across a wide range of potentials, expanding the available range of charge carriers for high-energy RFBs.