Influence of Cr3+ concentration on the electrochemical behavior of the anolyte for vanadium redox flow batteries

Science Bulletin, Nov 2012

The composition of electrolyte affects to a great extent the electrochemical performance of vanadium redox flow batteries (VRB). The effects of Cr3+ concentration in the anolyte on the electrode process of V(V)/V(IV) couple have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that Cr3+ causes no side reactions, but affects the electrochemical performance of V(V)/V(IV) redox reaction, including the reaction activity, the reversibility of electrode reaction, the diffusivity of vanadium ions, the interface film impedance, and the electrode reaction impedance. The experimental results show that Cr3+ within a certain concentration range can improve the reversibility of electrode reaction and the diffusion of vanadium ions. With the Cr3+ concentration increasing from 0 to 0.30 g L−1, the reversibility of V(V)/V(IV) reaction increases, while the diffusion resistance decreases. Correspondingly, the diffusion coefficient of vanadium ions increases from (5.48–6.77) × 10−7 to (6.82–8.44) × 10−7 cm2 s−1, an increase of ∼24%. However, the diffusion resistance increases and the diffusion coefficient decreases when Cr3+ concentration is over 0.30 g L−1, while the impedances of the interface, the film as well as the charge transfer increase continuously. As a result, Cr3+ with a certain concentration improves the diffusion and mass transfer process, but the resistances of the film, the interface, and the charge transfer rise. Furthermore, Cr3+ concentration of no more than 0.10 g L−1 has few effect on the electrode reaction process, and that of no more than 0.30 g L−1 is favorable to the diffusion of vanadium ions.

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Influence of Cr3+ concentration on the electrochemical behavior of the anolyte for vanadium redox flow batteries

HUANG Fei 1 ZHAO Qiang 1 LUO ChunHui 1 WANG GuiXin ) 1 YAN KangPing 1 LUO DongMei 0 0 PanGang Group Panzhihua Iron & Steel Research Institute , Panzhihua 617000, China 1 College of Chemical Engineering, Sichuan University , Chengdu 610065, China The composition of electrolyte affects to a great extent the electrochemical performance of vanadium redox flow batteries (VRB). The effects of Cr3+ concentration in the anolyte on the electrode process of V(V)/V(IV) couple have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that Cr3+ causes no side reactions, but affects the electrochemical performance of V(V)/V(IV) redox reaction, including the reaction activity, the reversibility of electrode reaction, the diffusivity of vanadium ions, the interface film impedance, and the electrode reaction impedance. The experimental results show that Cr3+ within a certain concentration range can improve the reversibility of electrode reaction and the diffusion of vanadium ions. With the Cr3+ concentration increasing from 0 to 0.30 g L1, the reversibility of V(V)/V(IV) reaction increases, while the diffusion resistance decreases. Correspondingly, the diffusion coefficient of vanadium ions increases from (5.48-6.77) 107 to (6.82-8.44) 107 cm2 s1, an increase of ~24%. However, the diffusion resistance increases and the diffusion coefficient decreases when Cr3+ concentration is over 0.30 g L1, while the impedances of the interface, the film as well as the charge transfer increase continuously. As a result, Cr3+ with a certain concentration improves the diffusion and mass transfer process, but the resistances of the film, the interface, and the charge transfer rise. Furthermore, Cr3+ concentration of no more than 0.10 g L1 has few effect on the electrode reaction process, and that of no more than 0.30 g L1 is favorable to the diffusion of vanadium ions. - Vanadium redox flow batteries (VRB), a high efficient and clean rechargeable device for energy conversion and storage, have promising prospects in the fields of generating electricity using renewable power sources, peak regulation of electric networks, emergency power, remote supply and electric vehicles. They have unique advantages such as high power density, excellent cycling stability, high reliability, simple operation and convenient maintenance [1]. The process of energy storage and release of VRB is generally achieved through an interconversion of soluble redox couples like V(V)/V(IV) and V(III)/V(II) in a electrolyte. Thus, the electrolyte is both the electroactive species and the reservoirs for energy storage, and has a determining influence on the electrochemical performance of VRB. The anolyte can be prepared directly by dissolving VOSO4 in a dilute sulfuric acid solution [2]. However, it is usually prepared by dissolving V2O5 in a dilute sulfuric acid with a reduction treatment because of the high cost of VOSO4 [3]. V2O5 is an intermediate product in the smelting process for producing steel in Pangang Group, but it usually contains small amounts of impurities, such as Cr3+, Mn2+ ions, which exist also in the VOSO4 prepared by using such V2O5. We have investigated the influence of Mn2+ concentration on the electrochemical behavior of VOSO4 anolyte for vanadium redox flow batteries in detail [4], however, to our knowledge, no research about the electrochemical behavior of the electrolyte with different Cr3+ concentrations has been reported. The Author(s) 2012. This article is published with open access at Springerlink.com Herein, VOSO4 anolyte was prepared by electrolyzing V2O5 in a sulphuric acid solution. The electrochemical behavior of the anolytes with different Cr3+ concentrations was investigated by CV and EIS. The effects of Cr3+ concentration on the electrochemical characteristics and the mechanism of the redox reaction were discussed. Experimental 1.1 Preparation of VOSO4 anolytes with different Cr3+ concentrations The reference anolyte (1.45 mol L1 VOSO4 + 3.0 mol L1 H2SO4) was prepared by electrolyzing the sulfuric acid solution containing V2O5 (analytical reagent, AR) at a constant voltage. The ion content in the reference anolyte was determined by ICP-AES (IRIS Intrepid II XSP, Thermo Electron Corporation, USA) and the potential titration. The total content of vanadium in the reference anolyte was determined as 74.14 g L1 [4]. Adulterant solution consisting of 1 mol L1 Cr3+ and 3 mol L1 H2SO4 was prepared by the Cr2(SO4)3 (AR) solution of 50 wt.% and the concentrated sulphuric acid of 98 wt.%. The VOSO4 anolytes with different Cr3+ concentrations were obtained by adding different amounts of adulterant solution prepared above to the reference anolyte. The determined composition is presented in Table 1. As the graphite electrode was dissolved into the acid solution in the process of electrolysis, many carbon particles come into existence in the electrolyte. To eliminate the effect of carbon, the as-obtained electrolyte was filtered after being rested for about 7 d, which is different from our previous work [4]. Electrochemical performance measurement To avoid the graphite electrode degrading the measurements, the platinum foils of ~0.14 cm2, ~15 cm2, and the saturated calomel electrode (SCE) were applied as the working electrode, auxiliary electrode, and reference electrode, respectively. The platinum electrodes were treated with the chromium acid lotion and the dilute hydrochloric acid before each measurement [4]. The electrochemical behavior was investigated at room temperature by using the threeelectrode system on an electrochemical station consisting of PAR273A and 5210 lock-in-amplifier. The potential range of the CV curves was 0.401.15 V. The scan rates were 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV s1, sequently. In Table 1 Concentration of the metal ions in the testing anolytes (g L1) the frequency range from 100 kHz to 10 mHz, EIS measurements were performed with a sinusoidal excitation voltage of 10 mV. Results and discussion The cyclic voltammograms of the VOSO4 anolytes with different Cr3+ concentrations at a scan rate of 50 mV s1 are shown in Figure 1. The curves are similar to each other in shape, and exhibit evidently only one couple of redox peaks. However, the potential difference between the oxidation peak and the reduction peak as well as the peak current intensity changes greatly as Cr3+ concentration increases. The results show that the main redox reactions in all anolytes exist as V(V)/V(IV) couple, and addition of Cr3+ has a great effect on the electrode reaction process. The oxidization peak at ~0.95 V is related to the oxidation of VO2+ to VO2+, VO2++H2O VO2++2H e whereas the reduction peak at ~0.76 V is associated with the reduction of VO2+ to VO2+ [59], VO2++2H++e VO2++H2O In order to investigate the influence of Cr3+ concentration on the electrode polarization and reaction reversibility, the separation of peak potentials, Vp, and the ratio of peak current intensities of the oxidization peak (IpO) to the reduction peak (IpR), IpO/IpR, obtained from the CV measurements at different scan rates are summarized in Figure 2. With Cr3+ concentration increasing from 0 to 0.35 g L1, Vp shows a quasi decrease tendency. It indicates that Cr3+ with a certain concentration reduces the electrode polarization, and improves the reaction activity [1012]. On the other hand, the increase of Cr3+ concentration results in the ratio of IpO/IpR near to 1. It suggests that Cr3+ can improve the reversibility of the V(V)/V(IV) redox reaction and the efficiency of VRB. To study the influence of Cr3+ concentration on the reaction activity of electrodes, the relationship between Cr3+ concentration and the current intensities of the redox peaks is summarized in Figure 3. The absolute values of the peak current intensities obtained at different scan rates exhibit in whole an increase tendency with the increase of Cr3+ concentration. The current intensity increases by ~3% with Cr3+ Figure 2 Influence of Cr3+ concentration on the voltage difference (a) and the current intensity ratio (b) of the oxidization peak and the reduction peak. concentration rising from 0 to 0.10 g L1, whereas a fast growth of ~9% from 0.15 to 0.40 g L1. It suggests that Cr3+ with high concentration increases the reaction capability of V(V)/V(IV) couple. Correspondently, the electrode activity is improved [10,11]. The CV curves of the anolyte with 0.05 g L1 Cr3+ obtained at different scan rates are shown in Figure 4. With the scan rates increasing, the peak values of the current intensity are quite different, but the redox curves are comparatively symmetric in shape. The potential difference between the oxidation peak and the reduction peak is about 140 mV, in the range of 59200 mV, indicating the quasi-reversibility of the V(V)/V(IV) reaction on the Pt electrode [1214]. Theoretically, the value of the diffusion coefficient for a quasi-reversible reaction (D) is between that for a reversible (D1) one and an irreversible (D2) one. According to the experimental results of the CV curves obtained at different scan rates, D1 and D2 were determined by applying eqs. (25C, irreversible reaction), where Ip, n, A, D, , Co and v refer to the current density of the reduction peak (A), the number of transferred electrons, the surface area of the working electrode (cm2), the diffusion coefficient (cm2 s1), the transfer coefficient, the electrolyte concentration (mol mL1), and the scan rate (V s1), respectively. Based on the above experimental results, eqs. (3) and (4) are transformed as follows: Figure 3 Influence of Cr3+ concentration on the current intensities of the oxidation peaks and the reduction peaks at different scan rates. In point of the linear function, k refers to the slope of the line, where the variables are the current density Ip/A and the square root of scan rate v1/2, and can be determined by the linear regression. D1 and D2 calculated by using the following eqs. (7) and (8) are listed in Table 2, According to eqs. (7) and (8), with the slope k increasing, the diffusion coefficient D increases, which refers to the enhanced diffusivity of vanadium ions. Figure 5 indicates the relationship of k to Cr3+ concentration. With the increase of Cr3+ concentration, k increases firstly, and then decreases. The diffusion coefficient, D, rises to the maximum, (6.82 8.44) 107 cm2 s1, which is about 24% higher than that of the reference anolyte, when Cr3+ concentration is 0.3 g L1. Apparently, Cr3+ with a certain concentration improves the diffusivity of the vanadium ions and intensifies the processes of mass transfer and charge transfer. As a result, the reaction activity is improved. However, the diffusion process is blocked due to the increase of the polarization degree when Cr3+ concentration is over 0.30 g L1, which agrees well with the increase of the potential difference at the corresponding concentration shown in Figure 2. (3) and (4) [5,8,10,1318]. Ip 2.69 105 An3 2Co D11 2v1 2 (25C, reversible reaction), Ip 2.99 105 An3 2Co 1 2 D21 2v1 2 Table 2 Influence of Cr3+ concentration on the diffusion coefficient D1 and D2 of vanadium ions The influence of Cr3+ concentration on the electrode reaction process of the anolyte was further investigated by EIS. The Nyquist plots are shown in Figure 6. In the range of 0 Figure 6 Influence of Cr3+ concentration on the EIS of the anolyte. 0.30 g L1 of Cr3+ concentration, the plots consist of a semicircle part in the high frequency region and a linear part in the low frequency region. When the concentration of Cr3+ increases to 0.35 and 0.40 g L1, the curves change to a uncompleted semicircle part in the high frequency region, Table 3 Influence of Cr3+ concentration on the parameters of EIS Cr3+ concentration (g L1) Rs ( cm2) Ri ( cm2) while the linear part in the low frequency region disappears. The semicircle part in the high frequency region corresponds to the impedances of the solution, the interface, the electrode reaction and the electrolyte film altogether, while the linear part in the low frequency region corresponds to the Warburg impedance resulted from vanadium ions diffusion [18]. The equivalent circuit is illustrated in Figure 7, where the circuit model is described as Rs(C1(Ri(Q1(Rf(C2 (RctW)))))), where Rs, Ri, Rf, Rct, and W refer to the solution resistance, the interface resistance, the electrolyte film resistance on electrode surface, the charge transfer resistance, and the Warburg diffusion impedance, respectively. Because of the nonhomogeneity of the electrode surface, the interface capacitance C is substituted by the constant phase element (CPE). The parameters of the equivalent circuit are listed in Table 3, which were determined by simulating the EIS plots with ZSimpWin and Zview. With Cr3+ concentration increasing, Rs is nearly constant, while Ri, Rf, and Rct, as a whole, increase. The increase degrees of Ri, Rf, and Rct are small below 0.10 g L1 Cr3+ concentration, and become notably large when the concentration is higher. The Warburg impedance W shows a minimum value at 0.30 g L1 Cr3+ concentration. When the Cr3+ concentration is less than 0.30 g L1, the Warburg impedance W decreases with Cr3+ concentration increasing, which is consistent with the CV results discussed above, which indicates Cr3+ enhances the diffusivity of vanadium ions. However, The Warburg impedance W increases when the Cr3+ concentration reaches 0.35 g L1. The semicircle part in the high frequency region becomes uncompleted when the Cr3+ concentration is 0.40 g L1, where the diffusion part disappears, as shown in Figure 6, indicating the charge transfer impedance is extremely high. Thus, the electrochemical reaction and the vanadium ion diffusion become difficult due to the increased electrolyte Figure 7 Equivalent circuits of the EIS fitting. Rf ( cm2) W ( cm2) film resistance and interface resistance resulted from high Cr3+ concentration. As a result, increasing Cr3+ concentration to a great extent (especially over 0.10 g L1) has negative effects on the charge transfer reaction, the characteristics of the interface and the electrolyte film, and the vanadium ions diffusivity, which interfere with the electrode reaction and the power characteristics of VRB. Mechanism analysis Based on the results discussed above, it is suggested that Cr3+ affects the electrode process in respect of the charge transfer reaction, the electrode interface behavior, the surface electrolyte film impedance, and the vanadium ion diffusion. The electrode process of V(V)/V(IV) redox reaction is commonly controlled by the ion diffusion and the charge transfer reaction, where the influence factors of the electrochemical behavior are complicated, including the reactant concentration difference between the electrode surface and the bulk solution, the electric double-layer capacitance of the two-phase interface, the charge transfer and the electrolyte film on the electrode surface [1924]. Consequently, the possible reasons for the influence of Cr3+ ions on the electrode process of V(V)/V(IV) redox reaction are as follows: (1) The valence electron configurations of V and Cr are 3d34s2 and 3d54s1, respectively. Vanadium ion is thermodynamically active due to the existence of the empty d orbitals, which is easy to associate with each other in the anolyte [2527]. However, Cr is comparably stable in view of thermodynamics because all d orbitals are occupied by electrons. On the other hand, addition of Cr3+ changes the original ionic circumstance, and leads to the change of the electric field interaction of vanadium ions. Therefore, the addition of Cr3+ in anolyte can hinder to some extent the association of vanadium ions, which results in the intensified diffusion process and the decreased concentration polarization. Because the number of the active vanadium ions for the electrode reaction increases, the electrode reaction activity is improved correspondingly. (2) Cr3+ makes the vanadium ions easy to diffuse by limiting the ion association. However, the collision probability of VO2+ with Cr3+ increases due to the larger radius of Cr3+ (0.63 ) [27] compared to that of V(IV) (0.46 ) [28]. Thus, the diffusion resistance of vanadium ions increases when large amount of Cr3+ ions exist in anolyte. (3) The valence of Cr3+ is lower than that of vanadium ions, and Cr3+ is competitive with vanadium ions for the adsorption on the electrode surface, leading to the composition of the electrolyte film on the electrode surface changes [29]. Simultaneously, the substitution of Cr3+ with low valence for vanadium ions with high valence will bring on the charge imbalance on the electrode surface, which alters the electric double-layer at the interface. Therefore, the electrolyte film impedance and the interface impedance increase with the increase of Cr3+ concentration. (4) The increased film impedance and interface impedance will hinder the charge transfer and the diffusivity of vanadium ions. So, the high film impedance polarization and the electrochemical polarization will affect the diffusion impedance and the electrochemical reaction impedance. (5) Before the electrochemical reaction, the step of diffusion is the determined factor for the electrode process. Cr3+ intensifies the diffusion process and decreases the concentration polarization, resulting in the increase of the reaction current intensities, so the reaction activity is enhanced. However, the step of electrochemical reaction becomes the determined factor for the electrode process when the vanadium ions reach the Pt electrode. Cr3+ ions change the film composition and the interface status, inducing the increase of film impedance polarization and the electrochemical polarization. Consequently, the electrochemical reaction changes difficult. According to the above possible reasons, it is not difficult to understand the influence of Cr3+ on the electrochemical behavior of the anolyte for VRB in a certain concentration. When Cr3+ concentration is as low as 0.10 g L1, the ionic atmosphere changes little, and the competitive adsorption between Cr3+ and vanadium ions is not severe. Therefore, the electrode surface status changes slightly and the increase of impedances of electrolyte film, interface and electrochemical reaction is not obvious. However, more Cr3+ ions will enhance the competitive adsorption and change the electric field. The large change of interface status will affect the processes of mass transfer and charge transfer, which hinders the electrochemical reaction. When Cr3+ concentration is no more than 0.30 g L1, the limited function for the association of vanadium ions is dominant. The decreased concentration polarization makes the diffusion coefficient of vanadium ions increase. However, when Cr3+ concentration is higher than 0.30 g L1, the changes of the collision probability, the ionic atmosphere and the competitive adsorption during the diffusion process become dominant. The increased diffusion resistance makes the diffusion coefficient of vanadium ions decline. The effects of Cr3+ concentration on the electrode process of V(V)/V(IV) redox reaction were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that Cr3+ affects to a great extent the electrochemical performance, including the electrode reaction activity and reversibility, the charge transfer process, the interface behaviour, the electrolyte film on the electrode surface and the ions diffusion. CV results show that Cr3+ with a certain concentration improves the diffusivity of vanadium ions, and lowers the electrode concentration polarization degree. Consequently, the reaction activity and reversibility are improved. Compared to the reference anolyte, ~24% decrease of the potential difference of redox peaks, ~24% increase of vanadium ions diffusion coefficient, and ~10% increase of the current peaks are obtained when Cr3+ concentration is 0.30 g L1. EIS results suggest that the impedances of the electrolyte film, the charge transfer and the interface, increase with Cr3+ concentration increasing, but the increase extent is small when Cr3+ concentration is no more than 0.10 g L1. On the other hand, the diffusion impedance declines with the increase of Cr3+ concentration when the concentration is less than 0.30 g L1. According to the results of CV and EIS obtained in this work, it was concluded that Cr3+ with a certain concentration can enhance the processes of the diffusion and the mass transfer, but excessive amount of Cr3+ will increase the impedances of electrolyte film, charge transfer, ions diffusion and interface. Consequently, Cr3+ concentration of no more than 0.10 g L1 has few effect on the electrode reaction process, while Cr3+ concentration of no more than 0.30 g L1 facilitates the diffusion of vanadium ions. This work was supported by Youth Foundation of Sichuan University (07046) and PanGang Group Panzhihua Iron & Steel Research Institute (11H0762).


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Fei Huang, Qiang Zhao, ChunHui Luo, GuiXin Wang, KangPing Yan, DongMei Luo. Influence of Cr3+ concentration on the electrochemical behavior of the anolyte for vanadium redox flow batteries, Science Bulletin, 2012, 4237-4243, DOI: 10.1007/s11434-012-5302-0