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D1 = 6.68 106k2 D2 = 1.08 105k2 (3) kinetic control region. The semicircle is extended by the moving charge-complexes close to the Helmholtz plane and is represented by an interfacial contact capacitance (Cc) and charge transfer (4) resistance (Rct). In the quiet low frequency range, the charge- where kstands for the slope of the line which the variables are the current density (ip) and the square root of scan rate (n1/2). Consequently, the slope k is fitted and obtained from Fig. 5(i), and the diffusion coefficient D1 and D2 of the positive electrolyte at different temperatures are obtained and listed in Table 1. It is observed that, no matter what the oxidation process or the reduction reaction, the diffusion coefficient D increases with the temperature increasing. Besides, the diffusion coefficient D of the oxidation reaction is bigger than that of the reduction reaction for the positive electrolyte. In other words, a higher temperature can elevate the diffusivity of the vanadium ions and intensify the processes of mass transfer and charge transfer, and finally improve the activity of the positive redox reaction. The effect of temperature on electrochemical behaviors of positive electrolyte is further investigated through EIS. Fig. 6(a) shows the Nyquist spectrum of 20C at different polarization potentials, and all spectra contain a semi-circle in the high frequency region and a straight line in the low frequency. For the polarization potential can bring in influence on the test [49], a potential near the start potential of the oxidation reaction in the CV curve (see Fig. 5) is chosen to take the following temperature influence test. Usually, a Nyquist spectrum can be explained in the following way. The electrical double layer in the pores of the electrode is composed of the charge-complexes diffusing towards the Helmholtz plane inside the pores. The first cross point on the real axis of the plot in the high-frequency region is represented as the bulk solution resistance of the electrolyte, electrode resistance and the contact resistance (Rb). It is followed by a semicircle in the range of high frequency to medium frequencies, a region where only small number of charge-complexes can get over the activation energy to shift with the alternating potential, which is called the complexes have enough time to get over the activation energy and reach the region near the Helmholtz plane. These charge- complexes finally occupy all surfaces inside the micro-pores of the electrodes and consist of the electrical double layer capacitance (Cdl). The diffusion control regime is directed by the low frequency range which is contributed by Cdl. All the circuit elements related to the Nyquist plot can be arranged to form an equivalent circuit as shown in the inset of Fig. 6(c). It is used to fit the Nyquist spectra measured for the electrolyte at different temperatures. In the equivalent circuit where the interfacial contact capacitance Cc and the electrical double layer Cdl are replaced by a constant phase element (CPE) in order to compensate for the depressed semicircle in the kinetic control and fully fit the line in the diffusion control [50]. The diffusion resistance that be interpreted using the Warburg diffusion element (W) in the diffusion control regime, is a main element to influence the performance of the circuit. The Nyquist plots of positive electrolyte at different temper- atures are shown in Fig. 6(b). The plot indicates that the redox reaction of V(IV)/V(V) couple is simultaneously controlled by the charge transfer process at high frequency and the diffusion process at low frequency. In other words, the reaction is a mixture of kinetic and diffusion-controlled process. It's obviously seen in Fig. 6(c) that, Rb and Rct clearly decrease with the increase of temperature which corresponds well with the former test results. On the basis of the data which was fitted by ZsimpWin software with the equivalent circuit, the temperature gives slight influence on the W, for the electrolyte is aqueous phase, and the diffusion in solution can satisfy the reaction need. However, the change of the Rct indicates that the temperature could evidently affect the charge transfer process, which may be caused by the loss of activity of the electrode. The temperature dependence of the charge transfer resistance Rct can be better understood in the form of the activation S. Xiao et al. / Electrochimica Acta 187 (2016) 525–534 531 Fig. 6. (a) EIS results of V(IV)/V(V) redox couple record under different polarization potentials at 20 C; (b) EIS results of V(IV)/V(V) redox couple record at different temperatures under 0.800 V; (c) Values of Rb and Rct at different temperatures; (d) An Arrhenius plot of the temperature-dependence of Rct.PDF Image | Broad temperature adaptability vanadium redox flow battery
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