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) TBA(3)[PW11O39(SiC6H5)(2)O] (TBA(3)W(11)SiPh) 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 TBA(3)W(11)SiPh vs <1 mmol dm(-3) for TBA(3)[PW12O40]). Electrochemical analysis of TBA(3)W(11)SiPh reveals four one-electron, quasi-reversible, redox couples between -0.60 and -2.50 V vs Ag+vertical bar Ag, prompting us to explore its application as a dual-function charge carrier in symmetric RFBs. The stability of TBA(3)W(11)SiPh 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.