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6. Review of battery chemistries The development of novel chemistries for RFBs is a research area that attracts a tremendous amount of attention at the moment. Therefore, it is impossible to present a holistic compendium of all the investigated (half-cell) chemistries. The present article should be seen complimentary to recently published reviews on the topic of chemistries for RFBs [40 46]. In our selection, we focus on chemistries that target the capital costs of RFBs which is currently the main inhibitor for their wide-spread application. Avenues to achieve these cost-savings are high capacity chemistries that allow for a reduction in footprint of the battery, high power chemistries that allow for a reduction in size of the cells and therefore reduce the footprint and costly materials such as membranes and low-cost molecules. 6.1 High energy density all-vanadium RFBs 6.1.1 VRFBs with increased concentration As described, for VRFBs, V2+ and V3+ species in the negative half-cell precipitate below 10°C while VO2+ precipitates as V2O5 above 40°C in the positive half-cell which allows a maximum vanadium concentration of 2 M in the sulphuric acid electrolyte and limits the energy density to 25 Wh kg-1 [96]. Reformulation of the electrolyte is a possible way to access higher vanadium concentrations and increase energy density. Indeed, Li et al. demonstrated use of a mixed sulphate and chloride electrolyte that allowed a 2.5 M concentration of vanadium to be achieved [97]. This represented about a 70% increase in energy density relative to current sulphate-only systems. It was found that vanadium in all four oxidation states was stable in a solution of 2.5 M SO42- and 6 M Cl- from -5 to 40°C, and a subsequent study demonstrated the stability of VO2+ in solution at 50°C. The mixed electrolyte could therefore provide an extended operational temperature range of -5 to 50°C, compared to the 10 to 40°C temperature window that is allowable for VRFBs that use a supporting electrolyte of sulphuric acid only. The enhanced stability of V(V) is attributed to the formation of soluble, neutral vanadium-containing complexes of formula VO2Cl(H2O)2 as the temperature approaches 20°C [97]. This was evidenced by use of the Amsterdam Density Functional program and by 51V and 35Cl NMR analysis. Quantum calculations also indicated that in sulphate solution, V(V) exists as [VO2(H2O)3]+ which is converted to insoluble V2O5- Page 22 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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