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532 S. Xiao et al. / Electrochimica Acta 187 (2016) 525–534 energy E following an Arrhenius-type equation [50]: between oxidation and reduction peak potential (DV) is increasing with the decreasing of temperatures. Thirdly, Fig. 7(h) shows the value of –Ipc/Ipa with various scan rates at different temperatures, from which we could see the value are closer to 1 at high temperatures, while farther away to 1 at low temperatures. These results also suggest that the activity and reversibility of the negative electrolyte is greatly affected by temperature, and a higher temperature can facilitate the redox reactions of V(II)/V(III) couple. Fig. 7(i) illustrates the relationship between the redox peak current density and the square root of the scan rate, for which a linear relationship could be seen from the two parameters, and the slope of the line is decreasing with the reducing of temperature. The diffusion coefficient D could be calculated from Eqs. (3) and (4) which is listed in Table 1. Apparently, the diffusion coefficient Dis going up with the increasing of the temperature, and the D of reduction reaction is bigger than that of the oxidation reaction which is opposite to that of the positive electrolyte. Therefore a higher temperature can elevate the diffusivity of the vanadium ions and intensify the processes of mass transfer and charge transfer, and as well finally improve the activity of the negative reaction. In addition, the Nyquist plot of the negative electrolyte is shown in Fig. 8. The plot indicates that the redox reaction of V(II)/V(III) couple is also a mixture of kinetic and diffusion- controlled process. It's clearly in Fig. 8(c) that, the Rb and Rct are decreasing with the temperature, the Rct of the negative electrolyte is larger than that of the positive electrolyte. The activation energy could also been calculated by Eq. (5) as shown in 1 R 1⁄4 Aexp DE RT ð5Þ 0 where A is a constant,DE is the activation energy, R is the gas constant, and T is the absolute temperature. Replacing R0 by the value of Rct and plotting lnRct versus 1/T, we can get the activation energy Ea from the slope of the line shown in Fig. 6(d), which is to overcome the charge transfer resistance Rct in the kinetic control regime. As illustrated in Fig. 6(d), an activation energy of 28.5 kJ mol1 can be received, which explains the potential energy difference resulted from the temperature change. 3.4.2. Negative electrolyte The negative electrolyte is composed of 1.5 M V(III) + 3.25 M H+ + 3.875 M SO42, which is equal to 1.5 M V(III) + 1.625 M H2SO4. In order to study the electrochemical influence of temperature to the negative electrolyte, CV and EIS test are also conducted to see the change with temperature. Fig. 7(a)-(g) expresses the CV curves of the negative electrolyte at various temperatures with different scan rates. As a matter of fact, the variation tendency is the same as the positive electrolyte (see Fig. 5). Firstly, the oxidation and reduction peak current density are decreasing with the tempera- ture going down, e.g. the ipa decreases from 94.5mA cm2 to 37.8 mA cm2 when the temperature decreases from 50 C to -10 C at the scan rate of 200mV s1, but the oxidation peak current density is smaller than the reduction peak current density which is opposite to that of the positive electrolyte. Secondly, the separation Fig. 7. (a)-(g) CV curves of V(II)/V(III) redox couple record at 50 C–-10 C with different scan rates; (h) Values of –Ipc/Ipa with different scan rates at different temperatures; (i) Plots of peak current density versus the square root of scan rates at different temperatures.PDF Image | Broad temperature adaptability vanadium redox flow battery
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