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on conductivity at 22 C and found that the conductivity changes with the concentration of vanadium ion and total sulfate [42]. Therefore, the impact of temperature on the conductivity of vanadium electrolyte was also studied in this work. The ionic conductivity (s) of five types of vanadium electrolytes as a function of temperature are shown in Fig. 4(a). Obviously, the s of all electrolytes is increasing with temperature. The conductivity of V(V) electrolyte ranks highest, followed by the V(II) electrolyte and V(IV) electrolyte, the V3.5+ electrolyte and V(III) electrolyte in namely array. It's worth noting that the V(II) electrolyte and V(IV) electrolyte almost have the same conductivity at all tested temperatures. This tendency is in accordance with the proton concentration of the electrolyte as shown in the inset of Fig. 4(a) (also see Fig. 1(c)). Recently, S. Suarez had found out that the conductivity of the electrolyte is related to the vanadium and proton concentration [43]. However, the vanadium concentrations of these five electro- lytes are the same (1.5 M) and so the s may be decided mainly by the proton concentration. As a significant feature of VRFB, the electrolytes are cyclically pumped through the battery (stack) when it is working. Therefore, the viscosity of the electrolyte is an important parameter which affects the uniform distribution of electrolyte in the battery and the energy consumption of the pump. F. Rahman and M. Skyllas- Kazacos had taken a specific research on the viscosity of the positive electrolyte and found that the viscosity increases with the Fig. 4. (a) Impact of temperature on the conductivity of five types of electrolytes. Inset shows the concentration of proton in different vanadium electrolyte; (b) Impact of temperature on the viscosity of five types of electrolytes. concentration of vanadium (V) and sulphate [25]. A. Mousa and M. Skyllas-Kazacos had also investigated the possible factors like the sulphate concentration and temperature (15 C - 40 C), which will affect the properties of negative electrolyte [44]. Both temperature and concentration will affect the viscosity of vanadium electrolyte. Variable temperature solution viscosity (h) data for different types of electrolyte are shown in Fig. 4(b). The viscosity of pure water is shown as benchmark. For each electrolyte, the viscosity is observed to decrease with temperature increasing, and apparently, the viscosity in low temperature is much larger than that in high where kB and T are the Boltzmann constant and temperature of the solution, respectively. For a established temperature, hwill increase with the decrease of the diffusion coefficient. According to previous report [45,46], the V(III) electrolyte was supposed to have the biggest viscosity for its lowest diffusion coefficient, which is in accordance with the experiment result here. The differences in s and h for the electrolyte may be due to a number of factors. The conductivity of electrolyte is mainly decided by the quantity and electromigration velocity of ions. On one hand, the velocity of the ions increases with the temperature. On the other hand, these five electrolytes are different in aspects of the proton concentration and the vanadium valence. The existence forms of the vanadium species in acidic conditions are usually regarded as VO2+ (V(V)), VO2+ (V(IV)), V(H2O)63+ (V(III)) and V(H2O)62+ (V(II)) [45]. The difference in existence form leads to the different diffusion coefficient and Stokes radius of the vanadium species. These elements give combined influence to the s and h. Anyhow, temperature has great influence on the conductivity and viscosity of the electrolyte, and then temperature could strongly affects the VRFB cell performance. 3.4. Electrochemical performance 3.4.1. Positive electrolyte As discussed above, the positive electrolyte consists of 1.5 M V(IV) + 4.75M H+ + 3.875 SO42, which equals to 1.5M V(IV) + 2.375 M H2SO4. The CV results of the positive electrolyte in various temperatures with different scan rates are shown in Fig. 5. It reveals that, for a given scan rate, firstly, both the anodic peak (ipa) and the cathodic peak (ipc) current densities of the V(IV)/V(V) couple decreases apparently with the decrease of temperature. For example, the ipa decreases from 199.6 mA cm2 to 32.3 mA cm2 when the temperature decreases from 50C to -10C at the scan rate of 200 mV s1. Secondly, the separation between oxidation and reduction peak potential (DV) increases with the temperature drops. For instance, the DVincreases from 0.18 V to more than 0.9 V when temperature decreases from 50 C to -10 C at the scan rate of 20mV s1, and apparently, the redox peak at -10C is not clearly perhaps mainly for the reason that electrode's activity was poor. The value of –Ipc/Ipa with various scan rates in different temperature is shown in Fig. 5(h). It can be seen that the value of –Ipc/Ipa increases with temperature from -10 C to 30 C and then maintain relatively stable from 30C to 50C, suggests a more complex reaction mechanism that requires further investigation. These results indicate that the activity and reversibility of the electrolyte is greatly affected by temperature, and a higher S. Xiao et al. / Electrochimica Acta 187 (2016) 525–534 529 more, it increases suddenly below 0 C, and temperature. What's the value at -20 C is almost 10 times larger than that at 50 C. However, the viscosity of V(II), V(IV) and V(V) electrolytes are very close to each other at a definite temperature, except that of the V(III) electrolyte. According to the Stokes-Einstein equation as shown below [43]: D1⁄4kBT hPDF Image | Broad temperature adaptability vanadium redox flow battery
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