ARTICLE pubs.acs.org/EF
Study of the Ce3þ/Ce4þ Redox Couple in Mixed-Acid Media (CH3SO3H and H2SO4) for Redox Flow Battery Application Zhipeng Xie,†,‡ Fengjiao Xiong,† and Debi Zhou*,† † ‡
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, People’s Republic of China College of Chemistry and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, People’s Republic of China ABSTRACT: The present paper first reports a kind of supporting electrolyte, mixed-acid media (CH3SO3H and H2SO4), used in redox flow battery (RFB) technology. Experimental work is performed with the aim of evaluating the Ce3þ/Ce4þ redox couple in mixed-acid electrolyte for use in RFB technology. The mixed-acid media have their special advantages: (i) the Ce3þ/Ce4þ electrode reaction appears to be more reversible in mixed-acid solution; the peak splittings in mixed acid are significantly less than in sulfuric acid; the diffusion coeffcient in mixed acid is larger than in single acid (CH3SO3H or H2SO4); the exchange current density and standard rate constant of the Ce3þ/Ce4þ redox reaction in mixed-acid media on graphite electrode are 8.05 103 A cm2 and 4.17 104 cm s1; (ii) the solubility of cerium salt in mixed acid is larger than in sulfuric acid; a solution of 1 mol dm3 cerium containing 2 mol dm3 MSA and 0.5 mol dm3 H2SO4 is sufficiently stable for more than 1 month at temperatures up to 313 K; (iii) Coulombic and energy efficiencies of the test cell using mixed-acid electrolyte are both higher than that using single CH3SO3H electrolyte; the average Coulombic and energy efficiencies of the cell using mixed acid are 87.1 and 73.5%, respectively, which are comparable to that of the all-vanadium RFB.
’ INTRODUCTION Extensive fossil fuel consumption in our activities led to environmental problems. We should replace fossil fuel usage as much as possible with renewable energy sources to prevent the situation from deteriorating even further. Redox flow battery (RFB) technology1,2 has received wide attention in the application for renewable energy storage systems. Much research has been focused on all-vanadium RFB35 for its excellent performance, where there is no decrease in capacity caused by the cross-mixing of the positive electrolyte and negative electrolyte. However, the standard electrode potential of the V4þ/V5þ redox couple is relatively low (about 1.0 V versus NHE). The Ce3þ/Ce4þ redox couple has a high positive potential (about 1.7 V versus NHE), which helps result in a higher cell voltage and a greater energy storage capacity. Therefore, it is attractive for use in RFB technology. A Ce/Zn system with a cell using organic acid as positive electrolyte media was patented by Clarke et al.68 The electrochemical behaviors of Ce3þ/Ce4þ in sulfuric acid,911 methanesulfuric acid (MSA),12 and nitric acid13 and its application14 were reported by different researchers. Hydrochloric acid15 and nitric acid16 are not suitable as the supporting electrolytes for use in a cerium RFB because of the oxidation of Cl and the reduction of NO3. The relatively low solubility of cerium salts in sulfuric acid9,13 makes it difficult to prepare Ce3þ solution with a concentration higher than 0.1 mol dm3 in the sulfuric acid media with a concentration higher than 3 mol dm3. On the other hand, working with a lower acid concentration brings the problem of Ce4þ hydrolysis9 to solid HCe(OH)3(SO4)3. Therefore, sulfuric acid may also be unsuitable as the supporting electrolyte for application in cerium RFB technology. The above-mentioned investigations are all limited to singleacid media. To this day, there is no literature describing mixed-acid r 2011 American Chemical Society
media (CH3SO3H and H2SO4) used in RFB technology. The mixed-acid electrolyte has many advantages. The Ce3þ/Ce4þ electrode reaction exhibits better reversibility in mixed-acid solution. The diffusion coeffcient, exchange current density, and standard rate constant of the Ce3þ/Ce4þ redox reaction in mixed-acid media are all larger than in MSA. The solubility of cerium salt in mixed acid is significantly larger than in sulfuric acid. Coulombic and energy efficiencies of the test cell using mixed acid are all higher than using single MSA, and they are comparable to that of all-vanadium RFB. The present research consists of the following parts: measurement of the kinetic parameter, the stability of cerium mixed-acid electrolyte, and chargedischarge experiment.
’ EXPERIMENTAL SECTION A graphite sheet (0.20 cm2), a platinum sheet (0.15 cm2), and a rotating platinum disk electrode (0.12 cm2) were used as working electrodes. A titanium sheet (5 cm2) was used as a counter electrode. A saturated calomel electrode (SCE) was used as a reference electrode. Prior to test, the working electrodes were pretreated as follows: after grinding with emery paper (1000 grade), the graphite electrode was washed by ultrasonic cleaning in doubly distilled water for 10 min. The Pt electrodes was cycled in 2 mol dm3 H2SO4 solution between 0.6 and 2 V versus SCE for 25 min at a scan rate of 50 mV s1. Reagents of analytical reagent (AR) grade and doubly distilled water were used throughout. The Ce3þ solutions in mixed acid were prepared by neutralizing Ce2(CO3)3 (Alfa Corporation, Tianjin, China) in water by adding concentrated MSA (Alfa Corporation, Tianjin, China) and Received: November 24, 2010 Revised: April 17, 2011 Published: April 19, 2011 2399
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Figure 2. Effect of the scan rate on the cyclic voltammogram of the Ce3þ/Ce4þ redox couple on the platinum electrode in mixed-acid media. Scan rates (mV s1): (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50. Concentration: [Ce3þ] = 0.02 mol dm3 and [MSA] = [H2SO4] = 1 mol dm3. Temperature = 293 K. width and 1 mm in thickness) negative electrode was polished and washed with doubly distilled water before experiments. The performance of the test cell was evaluated at a constant current (300 mA) with 2.8 V as the upper limit of charging and 0.5 V as the bottom limit of discharging. An eight-channel BTS-5 V3A (Neware Ltd., China) was employed for the chargedischarge experiments. The positive electrolyte was 18 mL 0.3 mol dm3 Ce3þ solution. The negative electrolyte was 18 mL 0.3 mol dm3 ZnSO4 solution.
’ RESULTS AND DISCUSSION
Figure 1. (A) Front view of the unit Zn/Ce redox flow cell. (B) Vertical view of the unit Zn/Ce redox flow cell. (a) Zinc negative electrode. (b) Carbon felt positive electrode. (c) Nafion 115 ion-exchange membrane. Volume of each tank, 25 mL; flow rate, 11.5 cm min1. sulfuric acid. Desired concentrations of mixed acid were obtained by adding the required amount of MSA and H2SO4 to this solution. The Ce4þ solution was obtained by the electro-oxidation of Ce3þ at carbon felt at a constant current. The concentration of Ce4þ in the electrolyte was determined by redox potential titration using Fe2þ in 1 mol dm3 HNO3 as the reductant. The Fe2þ solution concentration was determined using potassium permanganate standard solution. The total concentration of cerium in the electrolyte was determined by inductively coupled plasma (ICP) spectroscopy.10 The cerium solutions of different states of charge were placed in hermetic glass jars in water baths set at 293, 313, and 333 K for 1 month. Every solution was inspected visually every day, and the time taken for a slight precipitate to emerge was recorded. At the end of the 1 month test period, the cerium concentration was mensurated. A test cell (made of polycarbonate) is shown in Figure 1. The total volume of the cell is 3 5 3 cm3, which is divided into two equal parts of 3 5 1.5 cm3 by a Nafion 115 membrane (DuPont, Wilmington, DE). The electrolytes were stored in two external reservoirs (each 25 mL). The electrolyte was pumped (flow rate of 11.5 cm min1) through the electrode where the electrochemical reactions occurred. The electrodes were immersed in solution approximately 2 cm. Porous carbon felt (5 cm in width and 3 mm in thickness, LZC Works, China) thermally treated at 723 K for 16 h was used as a positive electrode. A titanium sheet was used as a current collector. A zinc sheet (5 cm in
Cyclic Voltammograms. A study of the Ce3þ/Ce4þ redox couple at carbon glassy electrode using cyclic voltammetry by Paulenova et al.9 confirmed the slow kinetics of the Ce3þ/Ce4þ redox couple in sulfuric acid media. Quite large peak splittings (400 mV in 4 mol dm3 H2SO4 to almost 1200 mV in 1 mol dm3 H2SO4 at 293 K) were observed by them. They suggested that the especially slow kinetics of the Ce3þ/Ce4þ redox reaction was the result of an especially large structural change associated with a conversion between Ce3þ and Ce4þ in sulfuric acid media. A similar study by Liu et al.16 also confirmed the slow kinetics of the Ce3þ/Ce4þ redox reaction in sulfuric acid media. They observed large peak splittings, i.e., more than 462 mV for 0.5 2 mol dm3 H2SO4 solutions containing 0.3 mol dm3 Ce4þ on platinum and glassy carbon electrodes at a scan rate of 50 mV s1. The cyclic voltammograms for a mixed-acid solution containing 1 mol dm3 each of MSA and H2SO4 as well as 0.02 mol dm3 Ce(CH3SO3)3 on the platinum electrode at various scan rates at 293 K are shown in Figure 2. The cathodic peak corresponds to the reduction of Ce4þ to Ce3þ, and the anodic peak corresponds to the oxidation of Ce3þ to Ce4þ. As seen in Figure 2, the anodic and cathodic peak potentials change slightly with the scan rates. The peak potential difference is 103 mV at a scan rate of 50 mV s1. The peak splittings in Figure 2 are significantly less than those depicted in the literature.9,16 The ratio of the cathodic peak current to the anodic peak current (ipc/ipa) is found to be about 0.80 at 293 K. Fang et al.11 reported that the ratio of the cathodic peak current to the anodic peak current is about 0.45 in sulfuric acid at a glassy carbon electrode at 298 K for the Ce3þ/Ce4þ couple. It indicates that the Ce3þ/Ce4þ electrode reaction appears to be more reversible in mixed-acid media. Current-Overpotential Curve. To determine the kinetic parameters of the Ce3þ/Ce4þ redox reaction, linear sweep voltammetry (LSV) was employed to obtain the current-overpotential 2400
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Table 1. Kinetic Parameters for Different Supporting Electrolytes
3þ
Figure 3. Current-overpotential curve for the Ce /Ce redox couple in mixed-acid media on the graphite electrode at 293 K. Scan rate = 1 mV s1. Concentration: [Ce3þ] = [Ce4þ] = 0.2 mol dm3 and [MSA] = [H2SO4] = 1 mol dm3. Open-circuit potential = 1.162 V versus SCE.
standard rate constant, k0
(103 A cm2)
(104 cm s1)
electrolyte
this
literature
(mol dm3)
work
values
2 Ma 2 M þ 0.25 S
1.53b 2.28b
0.79b 1.18b
2 M þ 0.50 S
2.76b
1.43b
2 M þ 0.75 S
b
1.48b
c
4.17c
1Mþ1S
4þ
exchange current density, j0
literature this work
2.86 8.05
a
values
1.6d 19
1S
4.2 S
11
e4
0.05f 4
4.2 S
0.01f 4
a
curve. At very small overpotential η, the current-overpotential relation17 is virtually linear (see Figure 3). It can be expressed as i ¼ i0 Fη=RT
ð1Þ
Rct ¼ η=i
ð2Þ
j0 ¼ i0 =A
ð3Þ
j0 ¼ RT=ARct F
ð4Þ
Because
M, MSA; S, H2SO4. The number before M and S is the value of the concentration; for example, 2 M = 2 mol dm3 MSA and 1 S = 1 mol dm3 H2SO4. b The values are obtained using a platinum working electrode for a solution containing 0.2 mol dm3 each of Ce3þ and Ce4þ. c The values are obtained using a graphite working electrode for a solution containing 0.2 mol dm3 each of Ce3þ and Ce4þ. d The value is obtained using a platinum working electrode for a solution containing 0.1 mol dm3 Ce4þ. e The value is obtained using a glassy carbon as the working electrode for a solution containing 0.013 mol dm3 V4þ and 0.017 mol dm3 V5þ. f The values are obtained using a glassy carbon as the working electrode for a solution containing 0.017 mol dm3 V2þ and 0.016 mol dm3 V3þ.
we have Table 2. Effect of the MSA Concentration on the Saturation Ion Product of Cerous Sulphate at 298 K
where R is the universal gas constant, T is the temperature (Kelvin), A is the area of the electrode, F is the Faraday constant, i0 is the exchange current, j0 is the exchange current density, and Rct is the charge-transfer resistance, which is the negative reciprocal slope of the current versus overpotential curve (η/i). The value of Rct can be obtained using the CHI electrochemical analyzer software. When the bulk concentration (C*) of oxidized species is equal to that of reduced species, the standard rate constant, k0, is given by k0 ¼ j0 =FC 3þ
quasi-reversible 15 g Λ g 102ð1 þ RÞ ; 0:3υ1=2 g k0 g 2 105 υ1=2 cm=s Λ e 102ð1 þ RÞ ;
k0 e 2 105 υ1=2 cm=s
Ce3þ
MSA 3
4þ
Λ g 15; k0 g 0:3υ1=2 cm=s
totally irreversible
(mol dm3)a
ð5Þ
Some kinetic parameters of the Ce /Ce redox reaction calculated using expressions 4 and 5 are shown in Table 1. As seen in Table 1, the exchange current and standard rate constant in mixed-acid media are significantly larger than that in MSA media but very close to that in sulfuric acid.19 In addition, at the graphite electrode, the kinetic parameters are calculated to be 8.05 103 A cm2 and 4.17 104 cm s1, which are obviously larger than those at the platinum electrode. Matsuda and Ayabe17 suggested the following zone boundaries for LSV: reversible
total SO42/HSO4
(mol dm )
(mol dm3)
no ppt
slight ppt
Ksipb
0 1
0.5 0.5
1.51c 2.02c
1.54d 2.05d
0.86 2.06
2
0.5
1.98c
2.02d
1.94
3
0.5
c
2.23
2.27d
2.77
4
0.5
2.88c
2.92d
5.97
5
0.5
2.52c
2.55d
4.00
The total SO42 and HSO4 concentration was calculated according to the amount of sulfuric acid consumed. b The value of the saturation ion product (Ksip) was calculated according to the “no ppt” values; therefore, it also had a negative deviation from the true value. c The values were calculated according to the amount of sulfuric acid consumed before a precipitation emerged (but very close to the saturation state), which had a negative deviation from the saturation value. d The values were calculated according to the amount of sulfuric acid consumed until a slight precipitation emerged, which had a positive deviation from the saturation value. a
where Λ is the equivalent conductivity, R is the transfer coefficient, υ is the scan rate, and k0 is the rate constant. Therefore, it can be concluded that the Ce3þ/Ce4þ electrode reaction in mixed-acid media is quasi-reversible for its k0 value, satisfying the quasi-reversible condition. Stability of the Electrolyte. As mentioned in the Introduction, the solubility of cerium sulfate is low in terms of use in RFB 2401
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Table 3. Stability of 1 mol dm3 Cerium in a Mixed-Acid Solution Containing 2 mol dm3 MSA and 0.5 mol dm3 H2SO4 at Different Temperatures and States of Chargea precipitation time (days) electrolyte 100% Ce3þ
state of charge (%)
293 K
313 K
333 K
0
no ppt after 30 days
no ppt after 30 days
no ppt after 30 days
30% Ce3þ þ 70% Ce4þ
70
no ppt after 30 days
no ppt after 30 days
no ppt after 30 days
20% Ce3þ þ 80% Ce4þ 10% Ce3þ þ 90% Ce4þ
80 90
no ppt after 30 days no ppt after 30 days
no ppt after 30 days no ppt after 30 days
18 6
100
no ppt after 30 days
no ppt after 30 days
3
100% Ce4þ
Stability refers to time taken (in days) for a slight precipitate to appear in solution. The mixed acid initially involved 2 mol dm3 MSA and 0.5 mol dm3 H2SO4 and was not analyzed after charging. a
technology. For a saturation Ce2(SO4)3 solution, the following dissolving equilibrium can occur: Ce2 ðSO4 Þ3 ðsÞ h 2Ce3þ þ 3SO4 2
ð6Þ
To calculate the solubility product of Ce2(SO4)3, it is necessary to determine the equilibrium concentrations of Ce3þ and SO42. However, it is very difficult to distinguish between SO42 and HSO4 in the solution. Therefore, in the following discussion, the solubility product (Ksp) is replaced by the “saturation ionic product (Ksip)” defined as Ksip ¼ ½Ce3þ 2 ð½SO4 2 þ ½HSO4 Þ3
ð7Þ
The parameter Ksip was first used to explain the solubility of VOSO4 in sulfuric acid by Rahman and Skyllas-Kazacos.18 The saturation ion product involves the following assumptions: (a) the first dissociation constant of H2SO4 is infinite, which means that there are no neutral H2SO4 species, and (b) the Ce3þ and SO42/HSO4 ion complexes are unstable and result in unassociated Ce3þ ions. To evaluate the effect of the MSA concentration on the solubility of Ce2(SO4)3 in mixed-acid solution, experiments were carried out as follows. First, Ce2(SO4)3 solution was prepared by neutralizing Ce2(CO3)3 in water by adding sulfuric acid. Second, a certain amount of MSA was added to Ce2(SO4)3 solution. Third, concentrated sulfuric acid was added to the solution obtained at the second step until a slight precipitate was observed. In the calculation of Ksip(Ce2(SO4)3), the total SO42/HSO4 concentration was approximate and not analyzed. The results show that MSA can improve the solubility of Ce2(SO4)3 (see Table 2). As shown in Table 2, the total SO42 and HSO4 concentration can only reach 1.51 mol dm3 before a slight precipitation appears in a 0.5 mol dm3 Ce3þ solution in the absence of MSA at 298 K. That is to say, the approximate saturation ion product is 0.86 evaluated from expression 7. However, a mixed-acid solution involving 4 mol dm3 MSA, 2.88 mol dm3 SO42/HSO4, and 0.5 mol dm3 Ce3þ shows no signs of precipitation. In other words, the total SO42 and HSO4 concentration can reach 2.88 mol dm3 for a 0.5 mol dm3 Ce3þ mixed-acid solution in the presence of 4 mol dm3 MSA. The value of the saturation ion product (approximately 5.97) is markedly larger than 0.86. MSA can improve the solubility of Ce2(SO4)3 mainly because of the increase of the hydrogen ion concentration. Hydrogen ion can promote the transformation of SO42 into HSO4, leading to the decrease of SO42 ions and, thus, allowing the solubility of Ce2(SO4)3 to increase. On the other hand, the saturation ion
Figure 4. (A) Current versus time curves and (B) i versus t1/2 plots. Concentration: [Ce3þ] = [Ce4þ] = 0.2 mol dm3 and (a) 2 mol dm3 MSA, (b) 2 mol dm3 MSA þ 0.75 mol dm3 H2SO4, (c) 2 mol dm3 MSA þ 0.5 mol dm3 H2SO4, and (d) 2 mol dm3 MSA þ 0.25 mol dm3 H2SO4. Temperature = 293 K. Working electrode = platinum sheet.
product of Ce2(SO4)3 decreased with the further increase of the MSA concentration from 4 to 5 mol dm3, suggesting intricate interplay among Ce3þ, SO42, and CH3SO3. Cerium mixed-acid solutions (25 mL) of different states of charge were allowed to stand for 30 days at 293, 313, and 333 K. The test solution involved 1 mol dm3 total cerium, 2 mol dm3 MSA, and 0.5 mol dm3 H2SO4. The time taken for a slight precipitate to emerge was recorded (see Table 3). At a lower Ce4þ concentration, a longer time is required before a slight precipitate appears in the solution. Therefore, the precipitation from the charged electrolyte could be a problem in certain applications where the system is likely to keep fully charged for extended periods and high temperatures are experienced. A preliminary test shows that a 1 mol dm3 Ce mixed-acid solution containing 2 mol dm3 MSA and 0.5 mol dm3 H2SO4 at any 2402
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Table 4. Ce4þ Diffusion Coefficients and the Ce4þ/Ce3þ Reaction Charge-Transfer Resistances at 298 Ka D (cm2 s1) [Ce4þ] = [Ce3þ] (mol dm3)
MSA (mol dm3)
H2SO4 (mol dm3)
RDE
Rct (Ω)
CA 6
6
0.20
2.00
0
2.68 10
2.56 10
0.20
2.00
0.25
8.35 106
8.27 106
75
0.20
2.00
0.50
6.98 106
6.92 106
62
0.20
2.00
0.75
5.93 106
5.87 106
60
112
RDE, rotating platinum disc electrode (0.12 cm2); CA, chronoamperometry. Values of CA and Rct are obtained using the platinum sheet working electrode (0.15 cm2). a
state of charge is a perfectly stable system at temperatures up to 313 K. Chronoamperometry. At a planar electrode, the diffusionlimited current following the application of a potential step follows the familiar Cottrell equation.17 iðtÞ ¼
nFAD1=2 C π1=2 t 1=2
ð8Þ
For electrodes with pure linear diffusion, the diffusion coefficient can be calculated from a plot of i versus t1/2. The slope of the best-fit line is then nFAD1/2C*/π1/2, and D can be found from D¼
ðslopeÞ2 π ðnFACÞ2
ð9Þ
where n is the number of electrons transferred in the reaction and C* is the bulk concentration of the diffusing species. The other symbols have their usual meaning. Chronoamperometric curves and i versus t 1/2 plots for mixed-acid solutions containing 2 mol dm3 MSA and sulfuric acid with different concentrations as well as 0.2 mol dm3 each of Ce3þ and Ce4þ are demonstrated in Figure 4. Diffusion coefficients of Ce4þ ion in mixed-acid solutions calculated according to expression 9 are shown in Table 4. From Table 4, it can be seen that all of the values of the Ce4þ diffusion coefficient in mixedacid media are larger than that in 2 mol dm3 MSA media, indicating that mixed-acid media are more suitable for application in Ce/Zn RFB than single-acid media. The Ce4þ diffusion coefficient underwent a maximum in 2 mol dm3 MSA þ 0.25 mol dm3 H2SO4 solution and then decreased with the increase of the H2SO4 concentration. The result is in agreement with that obtained from the rotating disk electrode (shown in Table 4). Modiba and Crouch19 reported a value of 2.4 106 cm2 s1 for Ce4þ in H2SO4 solution. On the other hand, the charge-transfer resistance diminished with the increase of the H2SO4 concentration. The values of charge-transfer resistance shown in Table 4 were obtained using the method depicted in the Current-Overpotential Curve section. Therefore, a mixed-acid solution containing 2 mol dm3 MSA and 0.5 mol dm3 H2SO4 was used in the chargedischarge test as a compromise of the diffusion coeffecient, charge-transfer resistance, and solubility of cerium salts. ChargeDischarge Curves. A Ce/Zn system with a cell using MSA as positive electrolyte media was patented20 in 2004. As mentioned previously, sulfuric acid is not well-suitable as the supporting electrolyte for use in cerium RFB because of the low solubility of cerium sulfate. Therefore, a comparison was made only between MSA and mixed acid in the following charge discharge experiments. The composition of the positive half-cell electrolyte was 0.3 mol dm3 Ce(CH3SO3)3 in (a) 2.5 mol dm3
Figure 5. Typical chargedischarge curves of the Zn/Ce cell using different supporting electrolytes at 293 K. Positive electrolyte (18 mL) = 0.3 mol dm3 Ce3þ in the following acid solutions: (a) 2.5 mol dm3 MSA, (b) 3 mol dm3 MSA, and (c) 2 mol dm3 MSA þ 0.5 mol dm3 H2SO4. Negative electrolyte (18 mL) = 0.3 mol dm3 ZnSO4 solution.
MSA, (b) 3 mol dm3 MSA, and (c) 2 mol dm3 MSA þ 0.5 mol dm3 H2SO4. The negative half-cell electrolyte was 0.3 mol dm3 ZnSO4 aqueous solution. The typical plot of cell voltage versus time for the first chargedischarge cycle is given in Figure 5. The upper limit of charging is 2.8 V, and the bottom limit of discharging is 0.5 V. As seen in Figure 5, the time taken for charging the cell using 2.5 mol dm3 MSA as positive supporting electrolyte media is 19 min, corresponding to 65.6% extent of charge. It is 21 min for 3 mol dm3 MSA, corresponding to 72.5% extent of charge. For mixed acid containing 2 mol dm3 MSA and 0.5 mol dm3 H2SO4, it is 22.5 min, reaching a 77.7% extent of charge. The high extent of charge indicates a high use rate of the electrolyte. It helps to improve the practical capacity of the cell. In addition, from this plot, the Coulombic and voltage efficiency values were calculated as (a) 85.1 and 80.3% for 2.5 mol dm3 MSA, (b) 85.7 and 84.1% for 3 mol dm3 MSA, and (c) 86.7 and 84.5% for 2 mol dm3 MSA þ 0.5 mol dm3 H2SO4. The Coulombic and energy efficiencies of the Ce/Zn cell using mixed acid as the supporting electrolyte are higher than those using single MSA. The change in Coulombic and energy efficiencies of the Ce/Zn cell using mixed acid as the positive supporting electrolyte in the first 10 cycles is given in Figure 6. As shown in Figure 6, the average Coulombic and energy efficiencies of this cell are 87.1 and 73.5%, respectively, which are comparable to those (about 80 and 62.6%) for the all-vanadium RFB.21 This indicates that the self-discharge because of diffusion of the species of cerium through the membrane is small. Clarke et al.22 reported that such ceriumzinc RFB may be operated without a separator or with a separator that allows for at least partial mixing of the negative and positive 2403
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The preliminary investigation shows that the cerium mixedacid system is attractive and electrochemically promising for use in RFB technology.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ86-0731-88836291. Fax: þ86-0731-8879850. E-mail:
[email protected].
Figure 6. Coulombic and energy efficiencies of the Zn/Ce cell using mixed acid as the supporting electrolyte in the first 10 cycles. Positive electrolyte (18 mL) = 0.3 mol dm3 Ce3þ in 2 mol dm3 MSA þ 0.5 mol dm3 H2SO4. Negative electrolyte (18 mL) = 0.3 mol dm3 ZnSO4 solution.
electrolyte. Thus, in the Ce/Zn RFB, membranes are suitable for use even if such membranes exhibit some leakage or permeability for catholyte and/or anolyte into the opposite compartment. As shown in Table 4, the Ce4þ diffusion coefficient in mixedacid media is larger than that in single MSA media. The masstransfer condition at the electrode surface can affect the value of the overpotential. The increase in the diffusion coefficient leads to smaller polarization and a decreased rate of side reactions. In addition, the charge-transfer resistance of the Ce3þ/Ce4þ electrode reaction in mixed acid is less than in MSA, which also results in smaller polarization and a decreased rate of side reactions. The elimination of side reactions can improve Coulombic and energy efficiencies. The preliminary test shows that mixedacid media are more suitable for applicaton in the Zn/Ce cell than single acid media.
’ CONCLUSION As a kind of supporting electrolyte, mixed-acid media (CH3SO3H and H2SO4) are first reported for application in Ce/Zn RFB technology. The mixed-acid media exhibit many advantages. Reversibility of the Ce3þ/Ce4þ Electrode Reaction. The peak potential difference (103 mV) in mixed acid are significantly less than in sulfuric acid (more than 400 mV).9,16 The ratio (ipc/ipa) is found to be about 0.80 at 293 K, which is larger than in sulfuric acid (about 0.45).11 The diffusion coeffcient in mixed acid is also larger than in single acid (CH3SO3H or H2SO419). The exchange current density and standard rate constant of the Ce3þ/Ce4þ reaction in mixed-acid media (about 2.86 103 A cm2 and 1.48 104 cm s1) are also larger than those in MSA (1.53 103 A cm2 and 7.9 105 cm s1) but very close to those in sulfuric acid (1.6 104 cm s1) reported in the literature.19 Stability of Cerium Salt. The solubility of cerium salt in mixed acid is obviously larger than in sulfuric acid. A solution of 1 mol dm3 cerium containing 2 mol dm3 MSA and 0.5 mol dm3 H2SO4 is sufficiently stable at temperatures up to 313 K for more than 1 month. Performance of the Cell. The extent of charge and energy efficiency of the cell using mixed-acid media are larger than those using single MSA media. The average Coulombic and energy efficiencies of the cell using mixed acid are 87.1 and 73.5%, respectively, which are comparable to that for the all-vanadium RFB.21 Its average discharge voltage can reach 2.166 V, which is significantly larger than that of all-vanadium RFB (about 1.5 V).
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