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Investigations on the self-discharge process in vanadium flow battery
Journal of Power Sources 294 (2015) 562e568
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Investigations on the self-discharge process in vanadium flow battery Jiawei Sun a, Dingqin Shi a, Hexiang Zhong a, Xianfeng Li a, b, *, Huamin Zhang a, b, ** a b
Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, PR China
h i g h l i g h t s 3þ The diffusion coefficients of vanadium ions were in order of V2þ > VO2þ > VOþ 2 >V . The self-discharge behavior of VFB was investigated in detail for the first time. Nafion 115 was selected to investigate the self-discharge behavior. The experiment was done under argon to eliminate the influence of oxygen. The mechanism was clarified during self-discharge process.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 2 June 2015 Accepted 19 June 2015 Available online xxx
The self-discharge process of vanadium flow battery (VFB) assembled with Nafion 115 is investigated in very detail for the first time. The self-discharge phenomenon of VFB is closely related to the diffusion 3þ coefficients of the vanadium ions, which are found to be in the order of V2þ > VO2þ > VOþ 2 > V . Five regions on the change of open circuit voltage (OCV) are clearly found during the self-discharge process. The regions include three platforms and two obvious decreasing regions. VOþ 2 disappears in the second region, while the V2þ disappears in the fourth one. In the first three regions, the self-discharge reactions at the positive and negative side are different, owing to the crossover of vanadium ions. In the last two regions, the changes of vanadium ions are derived from the diffusion of V3þ and VO2þ at positive and negative electrolyte. The self-discharge process at different flow rates or different state of charge (SOC) is also investigated, indicating that the self-discharge time shortens with increasing of flow rate between 40 and 80 mL/min or decreasing of the initial SOC. This paper will provide very valuable information for the relaxation or elimination of self-discharge phenomenon of VFB, which is one of the most troublesome issues in VFB application. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vanadium flow battery Diffusion coefficients Self-discharge Chemistry reactions
1. Introduction Renewable energy sources like solar power and wind power attracted more and more attention due to the critical issues of energy shortage and air pollution [1e3]. However, the renewable energy sources are instable and discontinuous, they need to be combined with a large-scale energy storage device to realize their smooth output and to improve their stability [1,4,5]. A vanadium
* Corresponding author. Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China. ** Corresponding author. Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2015.06.123 0378-7753/© 2015 Elsevier B.V. All rights reserved.
flow battery, which was invented by Skyllas-Kazacos and coworkers in 1980s [6,7], is one of the most suitable candidates for the large-scale energy storage, owing to its attractive features like high energy efficiency, long cycle life, high safety and environmentally friendly, etc [8e11]. So far, many VFB demonstrations have been carried out in different fields, which proved the promising features of VFB. However, there are still some critical issues need to be clarified. Among all the issues, the self-discharge phenomenon of the VFB stacks is one of the most critical problems. Especially for the backup power, the self-discharge will seriously affect the backup time and final battery performance. However, the real mechanism of selfdischarge in VFB was not very clear yet, only part of research was focused on the self-discharge process. It is well known that the different diffusion vanadium ions from
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one half-cell to the other will induce self-discharge reactions, and further lead to an imbalance between the SOC of the two half-cell electrolytes and a subsequent drop in capacity [12]. You et al., established a simple mathematical model to predict the selfdischarge process in a kilowatt-class vanadium redox flow battery stack [13]. Even though, very seldom research was carried out to systemically investigate the detailed mechanism of selfdischarge in a VFB, which is very important for a VFB system. In addition, an overwhelming majority of the current researches were accomplished under the atmosphere or with the protection of inert gas like nitrogen or argon [14]. In this case the V2þ ions can be easily oxidized even at very low concentration of oxygen gas [15], which makes the results more confusing. In this article, the mechanism self-discharge will be investigated in detail under the oxygen-free condition. In this paper, Nafion 115 is selected as a membrane for the selfdischarge experiment under VFB operating condition. The effect of flow rate or initial SOC on self-discharge process is investigated as well. Besides, the permeability of different vanadium ions across a Nafion 115 was detected using a diffusion cell. To eliminate the influence of oxygen, all the experiments were carried out in a glove box with 99.999% argon. 2. Experiments 2.1. The preparation of the vanadium electrolyte and membrane treatment
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membrane between two carbon felt electrodes, clamped by two graphite polar plates. The effective area of electrode was 48 cm2. Nafion 115 was selected as ion exchange membrane, carbon felts were used as electrodes. The original volume of the electrolytes at each side was 100 mL. And the electrolyte was reserved in graduated cylinder. The state of VFB was kept at open circuit and the electrolytes were cycling pumped into the cell by peristaltic pump (BT 100M, Baoding Chuangrui Precision Pump Co. Ltd) during selfdischarge process. The flow rate of electrolyte was controlled in the range between 40 and 80 mL/min. The self-discharge performances of the VFB were conducted by using a chargeedischarge controller (Model BT 2000, Arbin Instruments Corp., USA). The open circuit voltage of VFB was recorded by the chargeedischarge controller till the voltage was lower than 0.08 V. The volume change of positive and negative electrolyte during discharge process was recorded at a regular time interval, meanwhile, a 0.5 mL sample was collected from the positive and negative electrolyte at a regular time interval, respectively. The concentration of vanadium ions with different valences was measured via auto potentiometric titrator (905 titrando, Metrohm, Switzerland). The sample was titrated by using standard potassium hypermanganate solution as titrant and mixed acid (sulphuric acid and phosphoric acid) as medium. The titration finished when the potential saltation appeared. And the vanadium concentration was calculated by the volume of standard potassium hypermanganate solution corresponding to the potential saltation. 3. Results and discussion
The vanadium electrolyte was prepared by dissolving VOSO4$4H2O (Haizhongtian Fine Chemical Factory, Shenyang) in 3 M sulfuric acid (Kemiou Chemical Reagent Co. Ltd, Tianjin) to form a 1.7 M solution. The V (Ⅱ), V (Ⅲ)and V (Ⅴ) electrolytes was prepared from V (Ⅳ) electrolyte via an electrolytic cell. V (Ⅳ) electrolytes with volume ratio of 2/1 were added into positive and negative side respectively. Then the cell was charged to theoretical capacity to prepare 1.7 M V (Ⅴ) and 1.7 M V (Ⅱ) respectively, the current density is 80 mA cm2 and then gradually decrease to 10 mA cm2. While V (Ⅲ) was prepared by oxidation of V (Ⅱ). 75% SOC of electrolyte was prepared by charging the above cell to 75% of theoretical capacity. In addition, the Nafion 115 membranes (Du Pont Company, CEM) were pretreated before the experiments. The membranes were first boiled in a 3% H2O2/H2O solution of hydrogen peroxide, followed by rinsing in boiling distilled water. Then the membranes were boiled in a 1 mol L1 sulfuric acid solution and finally washed with boiling distilled water. The time of each treatment was 1 h [16].
3.1. The permeability of different vanadium ions across Nafion 115 The vanadium ions could transfer across Nafion 115 from enrichment side to deficiency side under the drive force of concentration. The diffusion coefficient of vanadium ion across the membrane abides by the Fick law:
VB
dCB ðtÞ P ¼ A ðCA ðtÞ CB ðtÞÞ dt L
(1)
where VB is the volume of electrolyte at deficiency side (L); CA is the concentration of vanadium ions at enrichment side (molL1); CB is the concentration of vanadium ions at deficiency side (molL1); A is the effective area of the membrane (m2); P is the diffusion coefficients of vanadium ions (m2s1); L is the thickness of the membrane (m); t is the test time (s). CB is vanadium ion
2.2. The permeability of different vanadium ions across Nafion 115 The permeability of vanadium ions was detected by using a diffusion cell, as described earlier [17]. The diffusion cell was separated by a membrane. The left cell was filled with 1.7 M vanadium ions with different valences in 3 M H2SO4 solution, while the right one was filled with the mixture of MgSO4 and H2SO4 in order to equalize the ionic strengths and to minimize the osmotic pressure effect before the test of vanadium permeability. Both sides were vigorously stirred by magnetic stirrers to avoid concentration polarization. Samples from the right cell were collected at regular time interval. The concentration of vanadium ion was detected by using UVeVis spectrometer. 2.3. The self-discharge experiment of Nafion 115 A single VFB was employed to investigate the self-discharge process. A single VFB cell was assembled by sandwiching a
Fig. 1. The concentration of vanadium ions with different valence at different time at the deficiency side.
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concentration at deficiency side as function of time. Fig. 1 shows the concentration of vanadium ion with different valences at different time at the deficiency side. The concentration of vanadium ion at the deficiency side is much lower than that at the enrichment side, therefore, the change of vanadium ions concentration at the enrichment side is negligible, i.e. ðCA ðtÞ CB ðtÞÞzCA ð0Þ, Eq. (1) can be changed to:
CB ¼ A
CA ð0ÞP t VB L
(2)
The CB is liner with t, the slope k ¼ A CAVð0ÞP . The concentration of BL vanadium ions is in line with the permeation time from Fig. 1, and the k can be figured out by method of least squares, so the P can be expressed as:
P¼
kVB L ACA ð0Þ
(3)
Consequently, the diffusion coefficient P can be calculated by Eq. (3) and the results are listed in Table 1. It can be seen that the diffusion coefficient of V2þ across Nafion 115 is the highest among all the vanadium ions. The diffusion coefficient of vanadium ions is 3þ in the order of V2þ > VO2þ > VOþ 2 >V .
Fig. 2. The curve of voltage versus time during the process of self-discharge when the initial SOC of electrolyte is 100%.
3.2. The self-discharge process of VFB assembled with Nafion 115 3.2.1. The self-discharge process of VFB at an initial SOC of 100% when the flow rate is 60 mL/min As described above, the vanadium ions with different valence state can transfer through the cation exchange membrane in a VFB. However, the non-adjacent vanadium ions will react immediately once they meet in the same container, and this will lead to the selfdischarge phenomenon under the VFB medium. Since the diffusion rate of vanadium ions with different valence states across Nafion 115 is different, the side reactions are also different during selfdischarge process. The related experiments were done to investigate how the self-discharge process occurred in the VFB. The self-discharge experiment was accomplished when the initial SOC of electrolyte is 100% (1.7 M VOþ 2 in positive electrolyte and 1.7 M V2þ in negative electrolyte) with the flow rate of 60 mL/ min , and the curve of OCV versus time is shown in Fig. 2. The original voltage is 1.71 V in the preliminary stage of self-discharge process and the voltage decreases gradually. In the whole curve, there are five regions of the OCV value, which contain three platforms regions (1.71 Ve1.15 V, 0.90 Ve0.50 V and 0.25 Ve0 V) and two dramatically decreasing regions (1.15 Ve0.90 V and 0.50 Ve0.25 V). To clarify all the regions mentioned above, a sample was collected from the positive and negative electrolytes at a regular time interval, respectively, to further analyze the change of vanadium ions during self-discharge (Figs. 3 and 4). From Fig. 3, only 2þ VOþ in the negative electrolyte 2 in the positive electrolyte and V exist at the initial state since the initial SOC of the electrolytes is 100%. At the beginning of self-discharge (region 1), the amount of 2þ VOþ appears in the positive electrolyte, while, 2 decreases and VO the amount of V2þ decreases and V3þ appears in the negative electrolyte. The decreasing rate of VOþ 2 at the positive side is much Table 1 Diffusion coefficients of vanadium ions with different valence. Vanadium ions 4
Slope k/10 Diffusion coefficient P/107 cm2 min1
V (Ⅱ)
V (Ⅲ)
V (Ⅳ)
V (Ⅴ)
5.589 6.290
0.987 1.111
2.276 2.561
1.292 1.454
Fig. 3. The change of vanadium ions in positive (a) and negative (b) half-cell during the process of self-discharge.
faster than the rate of V2þ at the negative side, and the increasing rate of VO2þ at the positive side is much faster than the rate of V3þ at the negative side. At 65.6 h (region 2), VOþ 2 is exhausted and only VO2þ can be detected at the positive side, while, V2þ and V3þ coexist at the negative side. After 65.6 h (region 3), the amount of VO2þ decreases and V3þ appears at the positive side, while, the amount of V2þ decreases and V3þ increases continuously at the negative side. At 99.3 h (region 4), V2þ is exhausted and only V3þ can be detected at the negative side, while, V3þ and VO2þ co-exist at
J. Sun et al. / Journal of Power Sources 294 (2015) 562e568
2þ Positive : V3þ þ VOþ 2 /2VO
(8)
Negative : VO2þ þ V2þ þ 2Hþ /2V3þ þ H2 O
(9)
3þ
Fig. 4. The change of total vanadium ions in positive and negative half-cell during the process of self-discharge.
the positive side. Afterward (region 5), the amount of V3þ decreases and VO2þ appears at the negative side, the amount of VO2þ decreases and V3þ increases continuously at the positive side. In region 5, V3þ and VO2þ ions co-exist at both positive and negative electrolyte, respectively, where they permeate each other until the concentration balance of both sides is reached. In addition, from Fig. 4, the total vanadium amount at the positive side first increases then decreases and reaches the maximum in region 2, while, the total vanadium amount at the negative side first decreases then increases and reaches the minimum in region 2 during the whole self-discharge process. As shown in Figs. 2 and 3, the OCV value decreases gradually and the component of the electrolyte changes at both positive and negative sides due to the crossover of vanadium ions. This is because that the electrolyte continuously flows through the surfaces of Nafion 115 membrane, V2þ and VOþ 2 permeate across the membrane each other, and react with the other ion immediately. In region 1, the correlative reactions at the positive and negative sides are described as: 2þ þ Positive : V2þ þ 2VOþ þ H2 O 2 þ 2H /2VO
(4)
2þ Negative : VOþ þ 4Hþ /3V3þ þ 2H2 O 2 þ 2V 2þ
(5) 3þ
So VO appears in the positive electrolyte and V appears in the negative electrolyte according to Eqs. (4) and (5), meanwhile, the amount of VO2þ and V3þ increases in this region. Moreover, the generated VO2þ (Eq. (4)) at the positive side may react with V2þ, which transfer from negative side, to form V3þ. And the generated V3þ (Eq. (5)) at the negative side may react with VOþ 2 , which transfer from positive side, to VO2þ. The reactions are described as:
Positive : V2þ þ VO2þ þ 2Hþ /2V3þ þ H2 O
(6)
2þ 3þ Negative : VOþ 2 þ V /2VO
(7)
However, the generated V3þ (Eq. (6)) will react with VOþ 2 to form VO2þ immediately due to the excess VOþ 2 at the positive side, similarly, the generated VO2þ (Eq. (7)) will react with V2þ to form V3þ immediately due to the excess V2þ at the negative side. At the same time, the generated V3þ (Eq. (5)) and VO2þ (Eq. (4)) will transfer across Nafion 115 membrane and take part in the reactions 3þ like V2þ and VOþ and VO2þ are 2 ions do. The reactions about V described as:
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Notably, V at the positive side (Eq. (8)) can be divided into two parts: the one generated from Eq. (6) and the one transferred from negative side, which generated from Eq. (5). Similarly, VO2þ at the negative side (Eq. (9)) can be divided into two parts as well: the one generated from Eq. (7) and the one transferred from positive side, which generated from Eq. (4). In summary, VO2þ and VOþ 2 co-exist in the positive electrolyte; V2þ and V3þ co-exist in the negative electrolyte in region 1. Therefore, the VO2þ amount in the positive electrolyte and the V3þ amount in the negative electrolyte increase, the VOþ 2 amount in the positive electrolyte and the V2þ amount in the negative electrolyte decrease in region 1 during as described above. In addition, as shown in Fig. 4, the total amount of vanadium ions increases at the positive side and decreases at the negative side in region 1, due to the different diffusion rates of vanadium ions (Table 1). The permeation rate of vanadium ions with lower valence is faster than that of vanadium ions with higher valance, so the vanadium ions transferred from negative to positive side is more than that from positive to negative side. Consequently, the total amount of vanadium ions increases at the positive side and decreases at the negative side in region 1. Besides, the diffusion rate of 2þ V2þ is much faster than that of VOþ transfers 2 , so the amount of V from negative electrolyte is much more than that of VOþ 2 transfers from positive electrolyte, which results in more V2þ taking part in þ Eq. (4) than VOþ 2 taking part in Eq. (5). Meanwhile, more VO2 will be reduced by V2þ at the positive side during self-discharge process. For example, if 1 mol V2þ ions diffuse from negative to positive 2þ side, 2 mol VOþ according to Eq. (4). 2 ions will be reduced by V þ Then the amount of VO2 at the positive side is less than the amount of V2þ at the negative side [18]. At the same time, the amount of VO2þ at the positive side is more than the amount of V3þ at the negative side in region 1. This regular phenomenon remains unchanged till a certain amount of vanadium ions disappear in the system. During the self-discharge, the amount of vanadium ions with different valances changes slowly in the positive and negative electrolyte and the OCV drops slowly as well. When the OCV gradually reduce to 1.15 V, the region 1 finished and the selfdischarge process transferred to region 2. In this region, the OCV drops quickly from 1.15 V to 0.90 V in 20 min (Fig. 2), V2þ and V3þ still co-exist in the negative electrolyte, while VOþ 2 in the positive electrolyte finally exhausted at 65.6 h (Fig. 3). The interrelated selfdischarge reactions, like Eqs. (4), (5), (7) and (8), finished in region 2, however, the reactions of Eqs. (6) and (9) will continue as a result of the existence of V2þ. And the amount of V3þ increases all the time in the negative electrolyte in this region. Besides, due to the fact 2þ that VOþ to form VO2þ at the positive side, the 2 could react with V 2þ amount of VO increases all the time and reaches its maximum in the positive electrolyte till VOþ 2 is exhausted completely. When VOþ 2 in the positive electrolyte is exhausted, the selfdischarge switches to region 3. In this region, the OCV decreases gradually again, the component of the electrolyte changes and V3þ appears in the positive electrolyte. In this region, only the reaction Eq. (6) exists in the positive electrolyte and Eq. (9) exists in the negative electrolyte as a result of the disappearance of VOþ 2 . According to Eq. (6), VO2þ in the positive electrolyte is consumed by V2þ, which transferred from the negative side and V3þ appears in the positive electrolyte. So the amount of VO2þ decreases in the positive electrolyte in region 3. According to Eq. (9), V2þ in the negative electrolyte is consumed by VO2þ, which transferred from
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the positive side, so the amount of V3þ increases and V2þ decreases continually in negative electrolyte. In addition, the majority vanadium ions is VO2þ in the positive electrolyte and V3þ in the negative electrolyte in this region, so the total vanadium amount decreases at the positive side and increases at the negative side according to the different permeation rate of vanadium ions in Table 1. The self-discharge continually goes on, the OCV continuously decreases. When the OCV gradually reduce to 0.50 V, the region 3 finished and the self-discharge process transferred to region 4. In this region, the OCV drops quickly to 0.25 V from 0.50 V in 10 min (Fig. 2), V2þ in the negative electrolyte is finally exhausted at 99.3 h (Fig. 3). The interrelated self-discharge reactions, like Eqs. (6) and (9), finished accordingly by disappearance of the V2þ in the negative electrolyte. All the reactions during self-discharge process have finished so far. Besides, due to the fact that V2þ could react with VO2þ to form V3þ at the negative side according to Eq. (9), the amount of V3þ increases all the time and reaches its maximum in the negative electrolyte till V2þ is exhausted completely. When V2þ in the negative electrolyte is exhausted, the selfdischarge switches to region 5. In this region, the OCV drops continually and is close to 0 V, the component of the electrolyte changes and VO2þ appears in the negative electrolyte, In the initial state of this region, V3þ and VO2þ co-exist in the positive electrolyte, only V3þ exists in the negative electrolyte. And the amount of V3þ in the negative electrolyte is more than that in the positive electrolyte from Fig. 3. Therefore, the net transfer direction of V3þ is from negative to positive side, and the net transfer direction of VO2þ is from positive to negative side. In this region, the OCV will drop continually to 0 V until the concentration of V3þ and VO2þ is equal at the positive and negative electrolyte respectively. Consequently, only the permeation of vanadium ions, but not selfdischarge reactions, exists at the positive and negative sides in the region 5. According to the above discussion, the state of vanadium ions and interrelated self-discharge reactions at the positive and negative sides are summarized in Table 2 during self-discharge process. To note that, this is only an overall result, in the reality, the crossover of vanadium ions with different valances together with the self-discharge process are far more complicated. 3.2.2. The self-discharge process of VFB at different flow rate when the initial SOC is 100% To investigate the effect of the flow rate on the crossover of ions, the self-discharge process was accomplished at different flow rates (40, 80 mL/min ). The OCV curves versus time are shown in Fig. 5. It can be seen that the self-discharge time shortens with increasing of flow rate. The second region appears at 82.3 h, 65.6 h, 62.5 h and the fourth one appears at 104 h, 99.3 h, 87 h when the flow rate is 40 mL/min , 60 mL/min , 80 mL/min respectively. The phenomenon indicates that the crossover of vanadium ions intensifies with increasing flow rate between 40 and 80 mL/min , and the selfdischarge reactions were accelerated due to the increase of vanadium crossover. The changes of vanadium ions during these
Table 2 The state of vanadium ions and interrelated self-discharge reactions at the positive and negative sides during the whole self-discharge process. Region
Region Region Region Region Region
Positive
1 2 3 4 5
Negative
State
Reaction
State
Reaction
V V V V V
4, 6, 8 6 6
V V V V V
5, 7, 9 9 9
(Ⅳ), V (Ⅴ) (Ⅳ) (Ⅲ), V (Ⅳ) (Ⅲ), V (Ⅳ) (Ⅲ), V (Ⅳ)
(Ⅱ), V (Ⅲ) (Ⅱ), V (Ⅲ) (Ⅱ), V (Ⅲ) (Ⅲ) (Ⅲ), V (Ⅳ)
Fig. 5. The curve of voltage versus time during the self-discharge process at different flow rate when the initial SOC is 100%.
processes are shown in Fig. 6. The changing tendency of vanadium ions, which the flow rate is 40 or 80 mL/min , is the same as that the flow rate at 60 mL/min (Fig. 3). VOþ 2 in the positive electrolyte is exhausted in the second region and V2þ in the negative electrolyte is exhausted in the fourth one as well. The only difference is the crossover of ions intensifies with increasing flow rate, which corresponding with the OCV. The self-discharge reactions in each region are the same as that the flow rate at 60 mL/min, due to the same changing tendency of vanadium ions. 3.2.3. The self-discharge process of VFB at an initial SOC of 75% when the flow rate is 60 mL/min To further investigate the effect of SOC on self-discharge process, self-discharge experiment was carried out when the initial SOC is 75% (0.425 M VO2þ/1.275 M VOþ 2 in positive electrolyte and 1.275 M V2þ/0.425 M V3þ in negative electrolyte) with the flow rate of 60 mL/min. The curve of OCV versus time is shown in Fig. 7. The original voltage is at 1.49 V in the preliminary stage of selfdischarge process and the voltage decreases gradually during the process. The second region appears at 61.5 h and the fourth one appears at 77.6 h. The change of vanadium ions during the selfdischarge process is shown in Fig. 8. It indicates that VOþ 2 in the positive electrolyte is exhausted in the second region and V2þ in the negative electrolyte is exhausted in the fourth one as well, which shows the same tendency with the initial SOC of 100%. The selfdischarge reactions in each region are also the same with the initial SOC of 100% due to the same changing tendency of vanadium ions. 4. Conclusions The self-discharge process of a single VFB assembled with Nafion 115 was studied in detail. The self-discharge phenomenon of VFB is closely related to the diffusion coefficient of the vanadium ions, which is detected and found to be in the order of 3þ V2þ > VO2þ > VOþ 2 > V . The results of self-discharge experiment indicate that, the OCV value always decreases once the electrolytes flow continually through the two surfaces of membrane due to the crossover of vanadium ions, and there are five regions in the whole process of self-discharge, which contain three platforms regions and two dramatically decreasing regions. VOþ 2 disappeared in the second region and V2þ disappeared in the fourth one. In the first three regions, the self-discharge reactions at the positive and negative side are different owing to the crossover of vanadium ions. In the last two regions, only the transfer of vanadium ions, but not
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Fig. 6. The change of vanadium ions during the process of self-discharge (a: 40 mL/min and positive, b: 40 mL/min and negative, c: 80 mL/min and positive, d: 80 mL/min and negative).
Fig. 7. The curve of voltage versus time during the self-discharge process at an initial SOC of 75% when the flow rate is 60 mL/min.
self-discharge reactions, exists at the positive and negative sides in the system because V3þ and VO2þ ions co-exist in the positive and negative electrolyte. The composition of electrolyte at positive and negative side is also different in these five regions. The selfdischarge process at different flow rates or different SOC is also investigated, indicating that the self-discharge time shortens with the increasing of flow rate between 40 and 80 mL/min or with decreasing of the initial SOC. It means that the crossover of vanadium ions intensifies with increasing flow rate, and the selfdischarge reactions were accelerated due to the increasing of vanadium crossover amount.
Fig. 8. The change of vanadium ions in positive (a) and negative (b) half-cell during the self-discharge process at an initial SOC of 75%.
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Acknowledgments The authors acknowledge the financial support from China Natural Science Foundation (No. 21206158, 51361135701 and 21476224), the Outstanding Young Scientist Foundation, CAS and Dalian Municipal Outstanding Young Talent Foundation (2014J11JH131), Key Research Program of the Chinese Academy of Sciences (KG2D-EW-602-2). References [1] B. Dunn, H. Kamath, J.-M. Tarascon, Science 334 (2011) 928e935. € rissen, C. Kolbeck, F. Philippi, G. Tomazic, [2] C. Fabjan, J. Garche, B. Harrer, L. Jo F. Wagner, Electrochim. Acta 47 (2001) 825e831. [3] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Chem. Rev. 111 (2011) 3577e3613. [4] X. Ma, H. Zhang, C. Sun, Y. Zou, T. Zhang, J. Power Sources 203 (2012) 153e158.
[5] C. Jizhong, X. Ziqiang, L. Bei, J. Power Sources 241 (2013) 396e399. [6] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A. Fane, M. Green, J. Electrochem. Soc. (United States) 133 (1986) 1057e1058. [7] M. Skyllas-Kazacos, M. Rychick, R. Robins, in, Google Patents, 1988. [8] C. Ding, H. Zhang, X. Li, H. Zhang, C. Yao, D. Shi, Chem. Sus. Chem. 6 (2013) 1262e1269. [9] A. Parasuraman, T.M. Lim, C. Menictas, M. Skyllas-Kazacos, Electrochim. Acta 101 (2013) 27e40. [10] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, J. Electrochem. Soc. 158 (2011) R55eR79. [11] A. Tang, J. Bao, M. Skyllas-Kazacos, J. Power Sources 248 (2014) 154e162. [12] A. Tang, J. Bao, M. Skyllas-Kazacos, J. Power Sources 196 (2011) 10737e10747. [13] D. You, H. Zhang, C. Sun, X. Ma, J. Power Sources 196 (2011) 1578e1585. [14] Q. Luo, L. Li, W. Wang, Z. Nie, X. Wei, B. Li, B. Chen, Z. Yang, V. Sprenkle, Chem. Sus. Chem. 6 (2013) 268e274. [15] N.H. Choi, S.k. Kwon, H. Kim, J. Electrochem. Soc. 160 (2013) A973eA979. [16] N. Yoshida, T. Ishisaki, A. Watakabe, M. Yoshitake, Electrochim. Acta 43 (1998) 3749e3754. [17] H. Zhang, H. Zhang, X. Li, Z. Mai, J. Zhang, Energy Environ. Sci. 4 (2011) 1676e1679. [18] J. Sun, X. Li, X. Xi, Q. Lai, T. Liu, H. Zhang, J. Power Sources 271 (2014) 1e7.
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Investigations on the self-discharge process in vanadium flow battery Jiawei Sun a, Dingqin Shi a, Hexiang Zhong a, Xianfeng Li a, b, *, Huamin Zhang a, b, ** a b
Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, PR China
h i g h l i g h t s 3þ The diffusion coefficients of vanadium ions were in order of V2þ > VO2þ > VOþ 2 >V . The self-discharge behavior of VFB was investigated in detail for the first time. Nafion 115 was selected to investigate the self-discharge behavior. The experiment was done under argon to eliminate the influence of oxygen. The mechanism was clarified during self-discharge process.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 2 June 2015 Accepted 19 June 2015 Available online xxx
The self-discharge process of vanadium flow battery (VFB) assembled with Nafion 115 is investigated in very detail for the first time. The self-discharge phenomenon of VFB is closely related to the diffusion 3þ coefficients of the vanadium ions, which are found to be in the order of V2þ > VO2þ > VOþ 2 > V . Five regions on the change of open circuit voltage (OCV) are clearly found during the self-discharge process. The regions include three platforms and two obvious decreasing regions. VOþ 2 disappears in the second region, while the V2þ disappears in the fourth one. In the first three regions, the self-discharge reactions at the positive and negative side are different, owing to the crossover of vanadium ions. In the last two regions, the changes of vanadium ions are derived from the diffusion of V3þ and VO2þ at positive and negative electrolyte. The self-discharge process at different flow rates or different state of charge (SOC) is also investigated, indicating that the self-discharge time shortens with increasing of flow rate between 40 and 80 mL/min or decreasing of the initial SOC. This paper will provide very valuable information for the relaxation or elimination of self-discharge phenomenon of VFB, which is one of the most troublesome issues in VFB application. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vanadium flow battery Diffusion coefficients Self-discharge Chemistry reactions
1. Introduction Renewable energy sources like solar power and wind power attracted more and more attention due to the critical issues of energy shortage and air pollution [1e3]. However, the renewable energy sources are instable and discontinuous, they need to be combined with a large-scale energy storage device to realize their smooth output and to improve their stability [1,4,5]. A vanadium
* Corresponding author. Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China. ** Corresponding author. Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2015.06.123 0378-7753/© 2015 Elsevier B.V. All rights reserved.
flow battery, which was invented by Skyllas-Kazacos and coworkers in 1980s [6,7], is one of the most suitable candidates for the large-scale energy storage, owing to its attractive features like high energy efficiency, long cycle life, high safety and environmentally friendly, etc [8e11]. So far, many VFB demonstrations have been carried out in different fields, which proved the promising features of VFB. However, there are still some critical issues need to be clarified. Among all the issues, the self-discharge phenomenon of the VFB stacks is one of the most critical problems. Especially for the backup power, the self-discharge will seriously affect the backup time and final battery performance. However, the real mechanism of selfdischarge in VFB was not very clear yet, only part of research was focused on the self-discharge process. It is well known that the different diffusion vanadium ions from
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one half-cell to the other will induce self-discharge reactions, and further lead to an imbalance between the SOC of the two half-cell electrolytes and a subsequent drop in capacity [12]. You et al., established a simple mathematical model to predict the selfdischarge process in a kilowatt-class vanadium redox flow battery stack [13]. Even though, very seldom research was carried out to systemically investigate the detailed mechanism of selfdischarge in a VFB, which is very important for a VFB system. In addition, an overwhelming majority of the current researches were accomplished under the atmosphere or with the protection of inert gas like nitrogen or argon [14]. In this case the V2þ ions can be easily oxidized even at very low concentration of oxygen gas [15], which makes the results more confusing. In this article, the mechanism self-discharge will be investigated in detail under the oxygen-free condition. In this paper, Nafion 115 is selected as a membrane for the selfdischarge experiment under VFB operating condition. The effect of flow rate or initial SOC on self-discharge process is investigated as well. Besides, the permeability of different vanadium ions across a Nafion 115 was detected using a diffusion cell. To eliminate the influence of oxygen, all the experiments were carried out in a glove box with 99.999% argon. 2. Experiments 2.1. The preparation of the vanadium electrolyte and membrane treatment
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membrane between two carbon felt electrodes, clamped by two graphite polar plates. The effective area of electrode was 48 cm2. Nafion 115 was selected as ion exchange membrane, carbon felts were used as electrodes. The original volume of the electrolytes at each side was 100 mL. And the electrolyte was reserved in graduated cylinder. The state of VFB was kept at open circuit and the electrolytes were cycling pumped into the cell by peristaltic pump (BT 100M, Baoding Chuangrui Precision Pump Co. Ltd) during selfdischarge process. The flow rate of electrolyte was controlled in the range between 40 and 80 mL/min. The self-discharge performances of the VFB were conducted by using a chargeedischarge controller (Model BT 2000, Arbin Instruments Corp., USA). The open circuit voltage of VFB was recorded by the chargeedischarge controller till the voltage was lower than 0.08 V. The volume change of positive and negative electrolyte during discharge process was recorded at a regular time interval, meanwhile, a 0.5 mL sample was collected from the positive and negative electrolyte at a regular time interval, respectively. The concentration of vanadium ions with different valences was measured via auto potentiometric titrator (905 titrando, Metrohm, Switzerland). The sample was titrated by using standard potassium hypermanganate solution as titrant and mixed acid (sulphuric acid and phosphoric acid) as medium. The titration finished when the potential saltation appeared. And the vanadium concentration was calculated by the volume of standard potassium hypermanganate solution corresponding to the potential saltation. 3. Results and discussion
The vanadium electrolyte was prepared by dissolving VOSO4$4H2O (Haizhongtian Fine Chemical Factory, Shenyang) in 3 M sulfuric acid (Kemiou Chemical Reagent Co. Ltd, Tianjin) to form a 1.7 M solution. The V (Ⅱ), V (Ⅲ)and V (Ⅴ) electrolytes was prepared from V (Ⅳ) electrolyte via an electrolytic cell. V (Ⅳ) electrolytes with volume ratio of 2/1 were added into positive and negative side respectively. Then the cell was charged to theoretical capacity to prepare 1.7 M V (Ⅴ) and 1.7 M V (Ⅱ) respectively, the current density is 80 mA cm2 and then gradually decrease to 10 mA cm2. While V (Ⅲ) was prepared by oxidation of V (Ⅱ). 75% SOC of electrolyte was prepared by charging the above cell to 75% of theoretical capacity. In addition, the Nafion 115 membranes (Du Pont Company, CEM) were pretreated before the experiments. The membranes were first boiled in a 3% H2O2/H2O solution of hydrogen peroxide, followed by rinsing in boiling distilled water. Then the membranes were boiled in a 1 mol L1 sulfuric acid solution and finally washed with boiling distilled water. The time of each treatment was 1 h [16].
3.1. The permeability of different vanadium ions across Nafion 115 The vanadium ions could transfer across Nafion 115 from enrichment side to deficiency side under the drive force of concentration. The diffusion coefficient of vanadium ion across the membrane abides by the Fick law:
VB
dCB ðtÞ P ¼ A ðCA ðtÞ CB ðtÞÞ dt L
(1)
where VB is the volume of electrolyte at deficiency side (L); CA is the concentration of vanadium ions at enrichment side (molL1); CB is the concentration of vanadium ions at deficiency side (molL1); A is the effective area of the membrane (m2); P is the diffusion coefficients of vanadium ions (m2s1); L is the thickness of the membrane (m); t is the test time (s). CB is vanadium ion
2.2. The permeability of different vanadium ions across Nafion 115 The permeability of vanadium ions was detected by using a diffusion cell, as described earlier [17]. The diffusion cell was separated by a membrane. The left cell was filled with 1.7 M vanadium ions with different valences in 3 M H2SO4 solution, while the right one was filled with the mixture of MgSO4 and H2SO4 in order to equalize the ionic strengths and to minimize the osmotic pressure effect before the test of vanadium permeability. Both sides were vigorously stirred by magnetic stirrers to avoid concentration polarization. Samples from the right cell were collected at regular time interval. The concentration of vanadium ion was detected by using UVeVis spectrometer. 2.3. The self-discharge experiment of Nafion 115 A single VFB was employed to investigate the self-discharge process. A single VFB cell was assembled by sandwiching a
Fig. 1. The concentration of vanadium ions with different valence at different time at the deficiency side.
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concentration at deficiency side as function of time. Fig. 1 shows the concentration of vanadium ion with different valences at different time at the deficiency side. The concentration of vanadium ion at the deficiency side is much lower than that at the enrichment side, therefore, the change of vanadium ions concentration at the enrichment side is negligible, i.e. ðCA ðtÞ CB ðtÞÞzCA ð0Þ, Eq. (1) can be changed to:
CB ¼ A
CA ð0ÞP t VB L
(2)
The CB is liner with t, the slope k ¼ A CAVð0ÞP . The concentration of BL vanadium ions is in line with the permeation time from Fig. 1, and the k can be figured out by method of least squares, so the P can be expressed as:
P¼
kVB L ACA ð0Þ
(3)
Consequently, the diffusion coefficient P can be calculated by Eq. (3) and the results are listed in Table 1. It can be seen that the diffusion coefficient of V2þ across Nafion 115 is the highest among all the vanadium ions. The diffusion coefficient of vanadium ions is 3þ in the order of V2þ > VO2þ > VOþ 2 >V .
Fig. 2. The curve of voltage versus time during the process of self-discharge when the initial SOC of electrolyte is 100%.
3.2. The self-discharge process of VFB assembled with Nafion 115 3.2.1. The self-discharge process of VFB at an initial SOC of 100% when the flow rate is 60 mL/min As described above, the vanadium ions with different valence state can transfer through the cation exchange membrane in a VFB. However, the non-adjacent vanadium ions will react immediately once they meet in the same container, and this will lead to the selfdischarge phenomenon under the VFB medium. Since the diffusion rate of vanadium ions with different valence states across Nafion 115 is different, the side reactions are also different during selfdischarge process. The related experiments were done to investigate how the self-discharge process occurred in the VFB. The self-discharge experiment was accomplished when the initial SOC of electrolyte is 100% (1.7 M VOþ 2 in positive electrolyte and 1.7 M V2þ in negative electrolyte) with the flow rate of 60 mL/ min , and the curve of OCV versus time is shown in Fig. 2. The original voltage is 1.71 V in the preliminary stage of self-discharge process and the voltage decreases gradually. In the whole curve, there are five regions of the OCV value, which contain three platforms regions (1.71 Ve1.15 V, 0.90 Ve0.50 V and 0.25 Ve0 V) and two dramatically decreasing regions (1.15 Ve0.90 V and 0.50 Ve0.25 V). To clarify all the regions mentioned above, a sample was collected from the positive and negative electrolytes at a regular time interval, respectively, to further analyze the change of vanadium ions during self-discharge (Figs. 3 and 4). From Fig. 3, only 2þ VOþ in the negative electrolyte 2 in the positive electrolyte and V exist at the initial state since the initial SOC of the electrolytes is 100%. At the beginning of self-discharge (region 1), the amount of 2þ VOþ appears in the positive electrolyte, while, 2 decreases and VO the amount of V2þ decreases and V3þ appears in the negative electrolyte. The decreasing rate of VOþ 2 at the positive side is much Table 1 Diffusion coefficients of vanadium ions with different valence. Vanadium ions 4
Slope k/10 Diffusion coefficient P/107 cm2 min1
V (Ⅱ)
V (Ⅲ)
V (Ⅳ)
V (Ⅴ)
5.589 6.290
0.987 1.111
2.276 2.561
1.292 1.454
Fig. 3. The change of vanadium ions in positive (a) and negative (b) half-cell during the process of self-discharge.
faster than the rate of V2þ at the negative side, and the increasing rate of VO2þ at the positive side is much faster than the rate of V3þ at the negative side. At 65.6 h (region 2), VOþ 2 is exhausted and only VO2þ can be detected at the positive side, while, V2þ and V3þ coexist at the negative side. After 65.6 h (region 3), the amount of VO2þ decreases and V3þ appears at the positive side, while, the amount of V2þ decreases and V3þ increases continuously at the negative side. At 99.3 h (region 4), V2þ is exhausted and only V3þ can be detected at the negative side, while, V3þ and VO2þ co-exist at
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2þ Positive : V3þ þ VOþ 2 /2VO
(8)
Negative : VO2þ þ V2þ þ 2Hþ /2V3þ þ H2 O
(9)
3þ
Fig. 4. The change of total vanadium ions in positive and negative half-cell during the process of self-discharge.
the positive side. Afterward (region 5), the amount of V3þ decreases and VO2þ appears at the negative side, the amount of VO2þ decreases and V3þ increases continuously at the positive side. In region 5, V3þ and VO2þ ions co-exist at both positive and negative electrolyte, respectively, where they permeate each other until the concentration balance of both sides is reached. In addition, from Fig. 4, the total vanadium amount at the positive side first increases then decreases and reaches the maximum in region 2, while, the total vanadium amount at the negative side first decreases then increases and reaches the minimum in region 2 during the whole self-discharge process. As shown in Figs. 2 and 3, the OCV value decreases gradually and the component of the electrolyte changes at both positive and negative sides due to the crossover of vanadium ions. This is because that the electrolyte continuously flows through the surfaces of Nafion 115 membrane, V2þ and VOþ 2 permeate across the membrane each other, and react with the other ion immediately. In region 1, the correlative reactions at the positive and negative sides are described as: 2þ þ Positive : V2þ þ 2VOþ þ H2 O 2 þ 2H /2VO
(4)
2þ Negative : VOþ þ 4Hþ /3V3þ þ 2H2 O 2 þ 2V 2þ
(5) 3þ
So VO appears in the positive electrolyte and V appears in the negative electrolyte according to Eqs. (4) and (5), meanwhile, the amount of VO2þ and V3þ increases in this region. Moreover, the generated VO2þ (Eq. (4)) at the positive side may react with V2þ, which transfer from negative side, to form V3þ. And the generated V3þ (Eq. (5)) at the negative side may react with VOþ 2 , which transfer from positive side, to VO2þ. The reactions are described as:
Positive : V2þ þ VO2þ þ 2Hþ /2V3þ þ H2 O
(6)
2þ 3þ Negative : VOþ 2 þ V /2VO
(7)
However, the generated V3þ (Eq. (6)) will react with VOþ 2 to form VO2þ immediately due to the excess VOþ 2 at the positive side, similarly, the generated VO2þ (Eq. (7)) will react with V2þ to form V3þ immediately due to the excess V2þ at the negative side. At the same time, the generated V3þ (Eq. (5)) and VO2þ (Eq. (4)) will transfer across Nafion 115 membrane and take part in the reactions 3þ like V2þ and VOþ and VO2þ are 2 ions do. The reactions about V described as:
565
Notably, V at the positive side (Eq. (8)) can be divided into two parts: the one generated from Eq. (6) and the one transferred from negative side, which generated from Eq. (5). Similarly, VO2þ at the negative side (Eq. (9)) can be divided into two parts as well: the one generated from Eq. (7) and the one transferred from positive side, which generated from Eq. (4). In summary, VO2þ and VOþ 2 co-exist in the positive electrolyte; V2þ and V3þ co-exist in the negative electrolyte in region 1. Therefore, the VO2þ amount in the positive electrolyte and the V3þ amount in the negative electrolyte increase, the VOþ 2 amount in the positive electrolyte and the V2þ amount in the negative electrolyte decrease in region 1 during as described above. In addition, as shown in Fig. 4, the total amount of vanadium ions increases at the positive side and decreases at the negative side in region 1, due to the different diffusion rates of vanadium ions (Table 1). The permeation rate of vanadium ions with lower valence is faster than that of vanadium ions with higher valance, so the vanadium ions transferred from negative to positive side is more than that from positive to negative side. Consequently, the total amount of vanadium ions increases at the positive side and decreases at the negative side in region 1. Besides, the diffusion rate of 2þ V2þ is much faster than that of VOþ transfers 2 , so the amount of V from negative electrolyte is much more than that of VOþ 2 transfers from positive electrolyte, which results in more V2þ taking part in þ Eq. (4) than VOþ 2 taking part in Eq. (5). Meanwhile, more VO2 will be reduced by V2þ at the positive side during self-discharge process. For example, if 1 mol V2þ ions diffuse from negative to positive 2þ side, 2 mol VOþ according to Eq. (4). 2 ions will be reduced by V þ Then the amount of VO2 at the positive side is less than the amount of V2þ at the negative side [18]. At the same time, the amount of VO2þ at the positive side is more than the amount of V3þ at the negative side in region 1. This regular phenomenon remains unchanged till a certain amount of vanadium ions disappear in the system. During the self-discharge, the amount of vanadium ions with different valances changes slowly in the positive and negative electrolyte and the OCV drops slowly as well. When the OCV gradually reduce to 1.15 V, the region 1 finished and the selfdischarge process transferred to region 2. In this region, the OCV drops quickly from 1.15 V to 0.90 V in 20 min (Fig. 2), V2þ and V3þ still co-exist in the negative electrolyte, while VOþ 2 in the positive electrolyte finally exhausted at 65.6 h (Fig. 3). The interrelated selfdischarge reactions, like Eqs. (4), (5), (7) and (8), finished in region 2, however, the reactions of Eqs. (6) and (9) will continue as a result of the existence of V2þ. And the amount of V3þ increases all the time in the negative electrolyte in this region. Besides, due to the fact 2þ that VOþ to form VO2þ at the positive side, the 2 could react with V 2þ amount of VO increases all the time and reaches its maximum in the positive electrolyte till VOþ 2 is exhausted completely. When VOþ 2 in the positive electrolyte is exhausted, the selfdischarge switches to region 3. In this region, the OCV decreases gradually again, the component of the electrolyte changes and V3þ appears in the positive electrolyte. In this region, only the reaction Eq. (6) exists in the positive electrolyte and Eq. (9) exists in the negative electrolyte as a result of the disappearance of VOþ 2 . According to Eq. (6), VO2þ in the positive electrolyte is consumed by V2þ, which transferred from the negative side and V3þ appears in the positive electrolyte. So the amount of VO2þ decreases in the positive electrolyte in region 3. According to Eq. (9), V2þ in the negative electrolyte is consumed by VO2þ, which transferred from
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the positive side, so the amount of V3þ increases and V2þ decreases continually in negative electrolyte. In addition, the majority vanadium ions is VO2þ in the positive electrolyte and V3þ in the negative electrolyte in this region, so the total vanadium amount decreases at the positive side and increases at the negative side according to the different permeation rate of vanadium ions in Table 1. The self-discharge continually goes on, the OCV continuously decreases. When the OCV gradually reduce to 0.50 V, the region 3 finished and the self-discharge process transferred to region 4. In this region, the OCV drops quickly to 0.25 V from 0.50 V in 10 min (Fig. 2), V2þ in the negative electrolyte is finally exhausted at 99.3 h (Fig. 3). The interrelated self-discharge reactions, like Eqs. (6) and (9), finished accordingly by disappearance of the V2þ in the negative electrolyte. All the reactions during self-discharge process have finished so far. Besides, due to the fact that V2þ could react with VO2þ to form V3þ at the negative side according to Eq. (9), the amount of V3þ increases all the time and reaches its maximum in the negative electrolyte till V2þ is exhausted completely. When V2þ in the negative electrolyte is exhausted, the selfdischarge switches to region 5. In this region, the OCV drops continually and is close to 0 V, the component of the electrolyte changes and VO2þ appears in the negative electrolyte, In the initial state of this region, V3þ and VO2þ co-exist in the positive electrolyte, only V3þ exists in the negative electrolyte. And the amount of V3þ in the negative electrolyte is more than that in the positive electrolyte from Fig. 3. Therefore, the net transfer direction of V3þ is from negative to positive side, and the net transfer direction of VO2þ is from positive to negative side. In this region, the OCV will drop continually to 0 V until the concentration of V3þ and VO2þ is equal at the positive and negative electrolyte respectively. Consequently, only the permeation of vanadium ions, but not selfdischarge reactions, exists at the positive and negative sides in the region 5. According to the above discussion, the state of vanadium ions and interrelated self-discharge reactions at the positive and negative sides are summarized in Table 2 during self-discharge process. To note that, this is only an overall result, in the reality, the crossover of vanadium ions with different valances together with the self-discharge process are far more complicated. 3.2.2. The self-discharge process of VFB at different flow rate when the initial SOC is 100% To investigate the effect of the flow rate on the crossover of ions, the self-discharge process was accomplished at different flow rates (40, 80 mL/min ). The OCV curves versus time are shown in Fig. 5. It can be seen that the self-discharge time shortens with increasing of flow rate. The second region appears at 82.3 h, 65.6 h, 62.5 h and the fourth one appears at 104 h, 99.3 h, 87 h when the flow rate is 40 mL/min , 60 mL/min , 80 mL/min respectively. The phenomenon indicates that the crossover of vanadium ions intensifies with increasing flow rate between 40 and 80 mL/min , and the selfdischarge reactions were accelerated due to the increase of vanadium crossover. The changes of vanadium ions during these
Table 2 The state of vanadium ions and interrelated self-discharge reactions at the positive and negative sides during the whole self-discharge process. Region
Region Region Region Region Region
Positive
1 2 3 4 5
Negative
State
Reaction
State
Reaction
V V V V V
4, 6, 8 6 6
V V V V V
5, 7, 9 9 9
(Ⅳ), V (Ⅴ) (Ⅳ) (Ⅲ), V (Ⅳ) (Ⅲ), V (Ⅳ) (Ⅲ), V (Ⅳ)
(Ⅱ), V (Ⅲ) (Ⅱ), V (Ⅲ) (Ⅱ), V (Ⅲ) (Ⅲ) (Ⅲ), V (Ⅳ)
Fig. 5. The curve of voltage versus time during the self-discharge process at different flow rate when the initial SOC is 100%.
processes are shown in Fig. 6. The changing tendency of vanadium ions, which the flow rate is 40 or 80 mL/min , is the same as that the flow rate at 60 mL/min (Fig. 3). VOþ 2 in the positive electrolyte is exhausted in the second region and V2þ in the negative electrolyte is exhausted in the fourth one as well. The only difference is the crossover of ions intensifies with increasing flow rate, which corresponding with the OCV. The self-discharge reactions in each region are the same as that the flow rate at 60 mL/min, due to the same changing tendency of vanadium ions. 3.2.3. The self-discharge process of VFB at an initial SOC of 75% when the flow rate is 60 mL/min To further investigate the effect of SOC on self-discharge process, self-discharge experiment was carried out when the initial SOC is 75% (0.425 M VO2þ/1.275 M VOþ 2 in positive electrolyte and 1.275 M V2þ/0.425 M V3þ in negative electrolyte) with the flow rate of 60 mL/min. The curve of OCV versus time is shown in Fig. 7. The original voltage is at 1.49 V in the preliminary stage of selfdischarge process and the voltage decreases gradually during the process. The second region appears at 61.5 h and the fourth one appears at 77.6 h. The change of vanadium ions during the selfdischarge process is shown in Fig. 8. It indicates that VOþ 2 in the positive electrolyte is exhausted in the second region and V2þ in the negative electrolyte is exhausted in the fourth one as well, which shows the same tendency with the initial SOC of 100%. The selfdischarge reactions in each region are also the same with the initial SOC of 100% due to the same changing tendency of vanadium ions. 4. Conclusions The self-discharge process of a single VFB assembled with Nafion 115 was studied in detail. The self-discharge phenomenon of VFB is closely related to the diffusion coefficient of the vanadium ions, which is detected and found to be in the order of 3þ V2þ > VO2þ > VOþ 2 > V . The results of self-discharge experiment indicate that, the OCV value always decreases once the electrolytes flow continually through the two surfaces of membrane due to the crossover of vanadium ions, and there are five regions in the whole process of self-discharge, which contain three platforms regions and two dramatically decreasing regions. VOþ 2 disappeared in the second region and V2þ disappeared in the fourth one. In the first three regions, the self-discharge reactions at the positive and negative side are different owing to the crossover of vanadium ions. In the last two regions, only the transfer of vanadium ions, but not
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Fig. 6. The change of vanadium ions during the process of self-discharge (a: 40 mL/min and positive, b: 40 mL/min and negative, c: 80 mL/min and positive, d: 80 mL/min and negative).
Fig. 7. The curve of voltage versus time during the self-discharge process at an initial SOC of 75% when the flow rate is 60 mL/min.
self-discharge reactions, exists at the positive and negative sides in the system because V3þ and VO2þ ions co-exist in the positive and negative electrolyte. The composition of electrolyte at positive and negative side is also different in these five regions. The selfdischarge process at different flow rates or different SOC is also investigated, indicating that the self-discharge time shortens with the increasing of flow rate between 40 and 80 mL/min or with decreasing of the initial SOC. It means that the crossover of vanadium ions intensifies with increasing flow rate, and the selfdischarge reactions were accelerated due to the increasing of vanadium crossover amount.
Fig. 8. The change of vanadium ions in positive (a) and negative (b) half-cell during the self-discharge process at an initial SOC of 75%.
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