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528 S. Xiao et al. / Electrochimica Acta 187 (2016) 525–534 concentration in five types of vanadium electrolytes should be between the two cases (the first dissociation and fully dissociation) while maintain the same trend. To simplify the process, we assumed that the sulfuric acid was fully dissociated and did not consider other ionic effect. The detailed reactions and the transfer of electron and proton during charge and discharge processes in the positive and negative half-cells of VRFB are illustrated in Fig. 1(b). As observed, in the process of precharge, the V3+ is oxidized into VO2+ and there have protons and electrons been produced at the positive half-cell, an inverse reaction happened simultaneously at the negative half-cell. If careful analysis of this process, 0.75 M electrons transfer from the positive to the negative half-cell through external circuit; and meanwhile, 0.75 M protons transfer from the positive to the negative half-cell through the membrane. The final consequence of this step is equivalent to that 0.75M protons transfer from negative electrolyte to positive electrolyte, which leads to that the concentration of proton in positive electrolyte is higher than that in negative electrolyte. The similar situation occurs during the charge process, but in reverse during discharge. Fig. 1(c) illustrates the ionic compositions of the five types of vanadium electrolytes. These five vanadium electro- lytes are corresponding to different state of charge of the cell, including different proton concentration. The following test and analysis are performed to study the performance of these five vanadium electrolytes at various temperatures. 3.2. Broad temperature stability analysis In previous research [38–40], it had been confirmed that high acid concentration was good to the stability of V(V) ion at high temperature, but bad to the stability of V(II), V(III), and V(IV) ions at low temperature. This will limit the operating temperature range in 10-40 C for VRFB. In order to extend the operating temperature range, especially under extreme temperatures (-35 C - 50 C), 1.5 M vanadium concentration was chosen to test the stability of the electrolyte and the operating performance of VRFB in this series of work. The five types of vanadium electrolyte are kept in the thermostat to test the low temperature stability in various temperatures. Fig. 2(a) shows the photographs of the electrolytes, which are taken at 0 h, 5 h, 10 h, 24 h and 48 h at temperatures from -20 C to -35 C, respectively. It is observed that all the electrolytes are stable at -20 C and -25 C through 48 hours, with no precipitation or freezing. The V(III) solution precipitates within less than 24 hours at -30 C, while the other electrolytes are stable for more than 48 hours. When the electrolytes are kept at -35 C, the V(II) and V(III) solutions precipitate or freeze within less than one hour, and the V3.5+ mixed solution precipitates in less than 10hours. In the purpose of testing the reversibility of the precipitation, the solutions are subsequently warmed to 25 C. After 30minutes, the precipitation dissolves completely and recovers to the initial state, as is shown in Fig. 2(b). Accordingly, the precipitation of the V(II), V(III) and V3.5+ solution at low temperatures could redissolve when the temperature goes up, which can be considered as a reversible process. Moreover, the stability of the electrolytes at high temperatures from 30C to 50C is shown in Fig. 3. Obviously, the V(V) precipitates out at 35 C, 40 C, 45 C and 50 C in 80 hours, 60 hours, 10 hours and 7 hours respectively, and the precipitations can't redissolve at low temperatures. According to previous report [7], the precipitation is V2O5 formed via a deprotonation reaction as following: [VO2(H2O)3]+ ! H3VO4 + H3O+ 2 H3VO4 ! V2O5 + 3H2O 3.3. Conductivity and viscosity analysis Ionic conductivity of the vanadium electrolyte will have great effect on the performance of VRFB, such as the electrode kinetics and the resistance of the battery. S. Corcuera et al. had studied the conductivity of 1.6 M V in 4.2 M sulphate electrolyte corresponding to different SOC of the VRFB positive half-cell at 10 C, 22 C and 43 C and found that the conductivity increases with temperature [41]. M. Skyllas-Kazacos et al. had studied the effect of composition Fig. 3. Photographs of five types of electrolytes at high temperature.PDF Image | Broad temperature adaptability vanadium redox flow battery
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