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[SiV(IV)3W(VI)9O40]10- / [SiV(IV)3W(V)3W(VI)6O40]13-) were investigated for application as electrolyte in aqueous or non-aqueous media for RFBs. The CE in aqueous solution was greater than 95% with low capacity loss observed during more than 100 charge-discharge cycles and no decomposition of the molecule was reported [120]. The POM also dissolved in a non-aqueous electrolyte (0.5 M TBAOTf in propylene carbonate) and the non-aqueous system had a higher operating voltage (1.1 V, 0.3 V higher than the aqueous system) but a drop of CE (initially 87%, after 10 cycles dropped by half) occurred. With a concentration of 20 mM POM and 0.5 M H2SO4, the observed current densities were one order of magnitude lower than in conventional RFBs. A reasonable approach to increase the current would be to enhance the POM concentration. Stability and costs are not reported. In the paper the dimerization and eventual deposition of POMs containing W-ions at negative potentials was not discussed [120]. Keita and Nadjo reported that at negative potentials (approx. 1 V vs. SHE) in acidic solutions POMs will modify the electrode surface [124]. The deposited material is usually a catalyst for the HER, thereby reducing the stability window of the electrolyte [125]. For [SiW12O40]4-, the precursor for the redox active material used in [120], electrode coverages of 33 to 120 monolayers are reported [124]. We have reported a potential candidate for a POM catholyte: [MnII3SiW9O34]7- [122,126]. While this molecule was able to transfer six electrons at high potentials, by oxidising three Mn(II) to Mn(IV), the POM adsorbed on the electrode and was too difficult to synthesise to make upscaling reasonable. Another symmetric POM RFB was presented by Liu et al. [127]. They employed H6[CoW12O40] in both anolyte and catholyte of an aqueous battery. The reactions for the anolyte were two two-electron waves at -0.04 V vs. SHE and -0.16 V vs. SHE (in 1 M H2SO4, recalculated form the SCE) [128]. As catholyte the single one-electron redox reaction of the Co(II)-heteroatom at 1.1 V vs. SHE was used. The POM was exceptionally soluble, up to 0.8 M, and 14 Ah L-1 were reached as capacity. However, for that result four times more catholyte volume than anolyte volumes was used (and above volumetric capacity is given for the anolyte) which is necessitated by the imbalance in charge transfer reactions at low and high potential. 6.7 Metal-free organic-inorganic aqueous RFBs based on anthraquinones Page 35 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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