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3H2O at elevated temperatures. The solubility and stability of V3+ and VO2+ is also thought to be increased by the lower sulphate concentration of the mixed electrolyte relative to standard sulphate concentrations for VRFBs. When a 2.5 M vanadium solution in the mixed sulphate-chloride electrolyte was tested in a RFB cell, a stable performance with 87% energy efficiency over 20 days was achieved. Energy densities of > 36 Wh L-1 were demonstrated for 2.5 M vanadium solutions in mixed electrolyte compared to around 22 Wh L-1 for 1.6 M vanadium solutions in 4.5 M sulphate electrolyte [96,97]. The formation of Cl2 gas during cycling is a concern, but it was reported that no significant gas evolution was observed. Our group was able to show that k0 of the VO2+/VO2+ reaction in 1 M H3PO4 is up to 67 higher than in 1 M H2SO4, which was attributed to a different chemical coordination in the electrolytes [98]. Electrolyte additives are also a possible measure that can be implemented to increase vanadium concentration and achieve higher energy densities for the VRFB. For instance, Roe et al. recently investigated various stabilising additives for the purpose of preventing the thermal precipitation of VO2+ species [97,99]. Several inorganic additives were studied as it was noted that finding effective organic additives that have long-term stability is a challenge due to the high oxidising power of VO2+. Sodium pentapolyphosphate, K3PO4, H3PO4 and (NH4)2SO4 were thus screened as stabilisers for 3 M supersaturated VO2+ solutions. H3PO4 (1 wt%) was found to be most effective at maintaining VO2+ concentration at 30°C while a 1 wt% H3PO4 + 2 wt% ammonium sulphate formulation performed best at 50°C and was investigated further in cell cycling tests. Such stabilising behaviour could be attributed to the formation of V(V)-phosphate complexes and the increase in H+ concentration due to phosphoric acid addition causing thermal precipitation of V(V) as V2O5 to become disfavoured. A 3 M VO2+ solution with 5 M total sulphate concentration and containing 1 wt% H3PO4 + 2 wt% ammonium sulphate demonstrated stable efficiencies over 90 charge/discharge cycles with a slight decrease in cell capacity observed [99]. No precipitation was evident during the experiments. A viable 3 M vanadium electrolyte could allow a 60-90% increase in energy density relative to the practical concentrations of 1.6 M 1.8 M that are currently used in VRFBs. A number of organic additives were proposed [100] and researchers found that amino acids [101], coulter dispersant [102], polyacrylic acid and its mixture with CH3SO3H [103] as well Page 23 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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