Published: April 19, 2011

r 2011 American Chemical Society 2399 dx.doi.org/10.1021/ef200354b | Energy Fuels 2011, 25, 2399–2404

ARTICLE

pubs.acs.org/EF

Study of the Ce3þ/Ce4þ Redox Couple in Mixed-Acid Media (CH3SO3Hand H2SO4) for Redox Flow Battery ApplicationZhipeng 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 inredox flow battery (RFB) technology. Experimental work is performed with the aim of evaluating the Ce3þ/Ce4þ redox couple inmixed-acid electrolyte for use in RFB technology. The mixed-acid media have their special advantages: (i) the Ce3þ/Ce4þ electrodereaction appears to be more reversible in mixed-acid solution; the peak splittings in mixed acid are significantly less than in sulfuricacid; the diffusion coeffcient in mixed acid is larger than in single acid (CH3SO3H or H2SO4); the exchange current density andstandard rate constant of the Ce3þ/Ce4þ redox reaction in mixed-acid media on graphite electrode are 8.05 � 10�3 A cm�2 and4.17 � 10�4 cm s�1; (ii) the solubility of cerium salt in mixed acid is larger than in sulfuric acid; a solution of 1 mol dm�3 ceriumcontaining 2mol dm�3MSA and 0.5mol dm�3 H2SO4 is sufficiently stable for more than 1month 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 CH3SO3Helectrolyte; the average Coulombic and energy efficiencies of the cell using mixed acid are 87.1 and 73.5%, respectively, which arecomparable to that of the all-vanadium RFB.

’ INTRODUCTION

Extensive fossil fuel consumption in our activities led toenvironmental problems. We should replace fossil fuel usage asmuch as possible with renewable energy sources to prevent thesituation from deteriorating even further. Redox flow battery(RFB) technology1,2 has received wide attention in the applica-tion for renewable energy storage systems. Much research hasbeen focused on all-vanadiumRFB3�5 for its excellent performance,where there is no decrease in capacity caused by the cross-mixing ofthe positive electrolyte and negative electrolyte. However, thestandard electrode potential of the V4þ/V5þ redox couple isrelatively low (about 1.0 V versus NHE). The Ce3þ/Ce4þ redoxcouple has a high positive potential (about 1.7 V versus NHE),which helps result in a higher cell voltage and a greater energystorage capacity. Therefore, it is attractive for use inRFB technology.

A Ce/Zn system with a cell using organic acid as positiveelectrolyte media was patented by Clarke et al.6�8 The electro-chemical behaviors of Ce3þ/Ce4þ in sulfuric acid,9�11 metha-nesulfuric acid (MSA),12 and nitric acid13 and its application14

were reported by different researchers. Hydrochloric acid15 andnitric acid16 are not suitable as the supporting electrolytes for usein a cerium RFB because of the oxidation of Cl� and the reduc-tion of NO3

�. The relatively low solubility of cerium salts insulfuric acid9,13 makes it difficult to prepare Ce3þ solution with aconcentration higher than 0.1 mol dm�3 in the sulfuric acidmedia with a concentration higher than 3 mol dm�3. On theother hand, working with a lower acid concentration brings theproblem of Ce4þ hydrolysis9 to solid HCe(OH)3(SO4)3. There-fore, sulfuric acid may also be unsuitable as the supportingelectrolyte for application in cerium RFB technology.

The above-mentioned investigations are all limited to single-acid media. To this day, there is no literature describing mixed-acid

media (CH3SO3H and H2SO4) used in RFB technology. Themixed-acid electrolyte has many advantages. The Ce3þ/Ce4þ

electrode reaction exhibits better reversibility in mixed-acidsolution. The diffusion coeffcient, exchange current density,and standard rate constant of the Ce3þ/Ce4þ redox reaction inmixed-acid media are all larger than in MSA. The solubility ofcerium salt in mixed acid is significantly larger than in sulfuricacid. Coulombic and energy efficiencies of the test cell usingmixed acid are all higher than using single MSA, and they arecomparable to that of all-vanadium RFB. The present researchconsists of the following parts: measurement of the kineticparameter, the stability of cerium mixed-acid electrolyte, andcharge�discharge experiment.

’EXPERIMENTAL SECTION

A graphite sheet (0.20 cm2), a platinum sheet (0.15 cm2), and arotating platinum disk electrode (0.12 cm2) were used as workingelectrodes. 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: aftergrinding with emery paper (1000 grade), the graphite electrode waswashed by ultrasonic cleaning in doubly distilled water for 10 min. ThePt electrodes was cycled in 2 mol dm�3 H2SO4 solution between �0.6and 2 V versus SCE for 25 min at a scan rate of 50 mV s�1.

Reagents of analytical reagent (AR) grade and doubly distilled waterwere used throughout. The Ce3þ solutions in mixed acid were preparedby neutralizing Ce2(CO3)3 (Alfa Corporation, Tianjin, China) in waterby adding concentrated MSA (Alfa Corporation, Tianjin, China) and

Received: November 24, 2010Revised: April 17, 2011

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sulfuric acid. Desired concentrations of mixed acid were obtained byadding the required amount of MSA and H2SO4 to this solution. TheCe4þ solution was obtained by the electro-oxidation of Ce3þ at carbonfelt at a constant current. The concentration of Ce4þ in the electrolytewas determined by redox potential titration using Fe2þ in 1 mol dm�3

HNO3 as the reductant. The Fe2þ solution concentration was deter-mined using potassium permanganate standard solution. The totalconcentration of cerium in the electrolyte was determined by inductivelycoupled plasma (ICP) spectroscopy.10

The cerium solutions of different states of charge were placed inhermetic 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 aslight precipitate to emerge was recorded. At the end of the 1 month testperiod, 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 partsof 3 � 5 � 1.5 cm3 by a Nafion 115 membrane (DuPont, Wilmington,DE). The electrolytes were stored in two external reservoirs (each25 mL). The electrolyte was pumped (flow rate of 11.5 cm min�1)through the electrode where the electrochemical reactions occurred.The electrodes were immersed in solution approximately 2 cm. Porouscarbon 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. Atitanium sheet was used as a current collector. A zinc sheet (5 cm in

width and 1 mm in thickness) negative electrode was polished andwashed 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 thebottom limit of discharging. An eight-channel BTS-5 V3A (Neware Ltd.,China) was employed for the charge�discharge experiments. Thepositive electrolyte was 18 mL 0.3 mol dm�3 Ce3þ solution. Thenegative electrolyte was 18 mL 0.3 mol dm�3 ZnSO4 solution.

’RESULTS AND DISCUSSION

Cyclic Voltammograms. A study of the Ce3þ/Ce4þ redoxcouple at carbon glassy electrode using cyclic voltammetry byPaulenova 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 dm�3 H2SO4 to almost 1200 mV in 1 moldm�3 H2SO4 at 293 K) were observed by them. They suggestedthat the especially slow kinetics of the Ce3þ/Ce4þ redox reactionwas the result of an especially large structural change associatedwith a conversion between Ce3þ and Ce4þ in sulfuric acid media.A similar study by Liu et al.16 also confirmed the slow kinetics ofthe Ce3þ/Ce4þ redox reaction in sulfuric acid media. Theyobserved large peak splittings, i.e., more than 462 mV for 0.5�2 mol dm�3 H2SO4 solutions containing 0.3 mol dm

�3 Ce4þ onplatinum and glassy carbon electrodes at a scan rate of 50 mV s�1.The cyclic voltammograms for a mixed-acid solution contain-

ing 1 mol dm�3 each of MSA and H2SO4 as well as 0.02 mol dm�3

Ce(CH3SO3)3 on the platinum electrode at various scan rates at293 K are shown in Figure 2. The cathodic peak corresponds tothe reduction of Ce4þ to Ce3þ, and the anodic peak correspondsto the oxidation of Ce3þ to Ce4þ. As seen in Figure 2, the anodicand cathodic peak potentials change slightly with the scan rates.The peak potential difference is 103 mV at a scan rate of 50 mVs�1. The peak splittings in Figure 2 are significantly less thanthose depicted in the literature.9,16 The ratio of the cathodic peakcurrent to the anodic peak current (ipc/ipa) is found to be about0.80 at 293 K. Fang et al.11 reported that the ratio of the cathodicpeak current to the anodic peak current is about 0.45 in sulfuricacid at a glassy carbon electrode at 298 K for the Ce3þ/Ce4þ

couple. It indicates that the Ce3þ/Ce4þ electrode reactionappears to be more reversible in mixed-acid media.Current-Overpotential Curve. To determine the kinetic

parameters of theCe3þ/Ce4þ redox reaction, linear sweep voltam-metry (LSV) was employed to obtain the current-overpotential

Figure 1. (A) Front view of the unit Zn/Ce redox flow cell. (B) Verticalview of the unit Zn/Ce redox flow cell. (a) Zinc negative electrode.(b) Carbon felt positive electrode. (c) Nafion 115 ion-exchangemembrane. Volume of each tank, 25 mL; flow rate, 11.5 cm min�1.

Figure 2. Effect of the scan rate on the cyclic voltammogram of theCe3þ/Ce4þ redox couple on the platinum electrode in mixed-acidmedia. Scan rates (mV s�1): (a) 10, (b) 20, (c) 30, (d) 40, and(e) 50. Concentration: [Ce3þ] = 0.02 mol dm�3 and [MSA] =[H2SO4] = 1 mol dm�3. Temperature = 293 K.

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curve. At very small overpotential η, the current-overpotentialrelation17 is virtually linear (see Figure 3). It can be expressed as

i ¼ � i0Fη=RT ð1ÞBecause

Rct ¼ � η=i ð2Þ

j0 ¼ i0=A ð3Þwe have

j0 ¼ RT=ARctF ð4Þwhere 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, andRct is the charge-transfer resistance, which is the negativereciprocal slope of the current versus overpotential curve (�η/i).The value of Rct can be obtained using the CHI electrochemicalanalyzer software. When the bulk concentration (C*) of oxidizedspecies is equal to that of reduced species, the standard rateconstant, k0, is given by

k0 ¼ j0=FC� ð5ÞSome kinetic parameters of the Ce3þ/Ce4þ redox reactioncalculated using expressions 4 and 5 are shown in Table 1. Asseen in Table 1, the exchange current and standard rate constantin mixed-acid media are significantly larger than that in MSAmedia but very close to that in sulfuric acid.19 In addition, at thegraphite electrode, the kinetic parameters are calculated to be8.05 � 10�3 A cm�2 and 4.17 � 10�4 cm s�1, which areobviously larger than those at the platinum electrode. Matsudaand Ayabe17 suggested the following zone boundaries for LSV:

reversible Λ g 15; k0 g 0:3υ1=2 cm=s

quasi-reversible 15 g Λ g 10�2ð1 þ RÞ;

0:3υ1=2 g k0 g 2� 10�5υ1=2 cm=s

totally irreversible Λ e 10�2ð1 þ RÞ;

k0 e 2� 10�5υ1=2 cm=s

where Λ is the equivalent conductivity, R is the transfer coeffi-cient, υ is the scan rate, and k0 is the rate constant. Therefore, itcan be concluded that the Ce3þ/Ce4þ electrode reaction inmixed-acid media is quasi-reversible for its k0 value, satisfying thequasi-reversible condition.Stability of the Electrolyte. As mentioned in the Introduc-

tion, the solubility of cerium sulfate is low in terms of use in RFB

Figure 3. Current-overpotential curve for the Ce3þ/Ce4þ redox coupleinmixed-acid media on the graphite electrode at 293 K. Scan rate = 1mVs�1. Concentration: [Ce3þ] = [Ce4þ] = 0.2 mol dm�3 and [MSA] =[H2SO4] = 1 mol dm�3. Open-circuit potential = 1.162 V versus SCE.

Table 1. Kinetic Parameters for Different SupportingElectrolytes

exchange current density, j0(�103 A cm�2)

standard rate constant, k0(�104 cm s�1)

electrolyte

(mol dm�3)

this

work

literature

values this work

literature

values

2 Ma 1.53b 0.79b

2 M þ 0.25 S 2.28b 1.18b

2 M þ 0.50 S 2.76b 1.43b

2 M þ 0.75 S 2.86b 1.48b

1 M þ 1 S 8.05c 4.17c

1 Sa 1.6d 19

4.2 S 11e 4

4.2 S 0.05f 4 0.01f 4

aM, MSA; S, H2SO4. The number before M and S is the value of theconcentration; for example, 2M= 2mol dm�3MSA and 1 S = 1mol dm�3

H2SO4.bThe values are obtained using a platinumworking electrode for

a solution containing 0.2 mol dm�3 each of Ce3þ and Ce4þ. cThe valuesare obtained using a graphite working electrode for a solution containing0.2 mol dm�3 each of Ce3þ and Ce4þ. dThe value is obtained using aplatinum working electrode for a solution containing 0.1 mol dm�3

Ce4þ. eThe value is obtained using a glassy carbon as the workingelectrode for a solution containing 0.013 mol dm�3 V4þ and 0.017mol dm�3 V5þ. fThe values are obtained using a glassy carbon as theworking electrode for a solution containing 0.017 mol dm�3 V2þ and0.016 mol dm�3 V3þ.

Table 2. Effect of the MSA Concentration on the SaturationIon Product of Cerous Sulphate at 298 K

total SO42�/HSO4

�

(mol dm�3)a

MSA

(mol dm�3)

Ce3þ

(mol dm�3) no ppt slight ppt Ksipb

0 0.5 1.51c 1.54d 0.86

1 0.5 2.02c 2.05d 2.06

2 0.5 1.98c 2.02d 1.94

3 0.5 2.23c 2.27d 2.77

4 0.5 2.88c 2.92d 5.97

5 0.5 2.52c 2.55d 4.00aThe total SO4

2� and HSO4� concentration was calculated according

to the amount of sulfuric acid consumed. bThe value of the saturationion product (Ksip) was calculated according to the “no ppt” values;therefore, it also had a negative deviation from the true value. cThevalues were calculated according to the amount of sulfuric acid con-sumed before a precipitation emerged (but very close to the saturationstate), which had a negative deviation from the saturation value. dThevalues were calculated according to the amount of sulfuric acid con-sumed until a slight precipitation emerged, which had a positivedeviation from the saturation value.

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technology. For a saturation Ce2(SO4)3 solution, the followingdissolving equilibrium can occur:

Ce2ðSO4Þ3ðsÞ h 2Ce3þ þ 3SO42� ð6Þ

To calculate the solubility product of Ce2(SO4)3, it is necessaryto determine the equilibrium concentrations of Ce3þ and SO4

2�.However, it is very difficult to distinguish between SO4

2� andHSO4

� in the solution. Therefore, in the following discussion,the solubility product (Ksp) is replaced by the “saturation ionicproduct (Ksip)” defined as

Ksip ¼ ½Ce3þ�2ð½SO42�� þ ½HSO4

��Þ3 ð7ÞThe parameter Ksip was first used to explain the solubility ofVOSO4 in sulfuric acid by Rahman and Skyllas-Kazacos.18 Thesaturation ion product involves the following assumptions: (a)the first dissociation constant of H2SO4 is infinite, which meansthat there are no neutral H2SO4 species, and (b) the Ce3þ andSO4

2�/HSO4� ion complexes are unstable and result in un-

associated Ce3þ ions.To evaluate the effect of the MSA concentration on the

solubility of Ce2(SO4)3 in mixed-acid solution, experiments werecarried out as follows. First, Ce2(SO4)3 solution was prepared byneutralizing 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 solutionobtained at the second step until a slight precipitate was observed.In the calculation of Ksip(Ce2(SO4)3), the total SO4

2�/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 dm�3 before

a slight precipitation appears in a 0.5 mol dm�3 Ce3þ solution inthe absence of MSA at 298 K. That is to say, the approximatesaturation ion product is 0.86 evaluated from expression 7.However, a mixed-acid solution involving 4 mol dm�3 MSA,2.88 mol dm�3 SO4

2�/HSO4�, and 0.5 mol dm�3 Ce3þ shows

no signs of precipitation. In other words, the total SO42� and

HSO4� concentration can reach 2.88 mol dm�3 for a 0.5 mol

dm�3 Ce3þ mixed-acid solution in the presence of 4 mol dm�3

MSA. The value of the saturation ion product (approximately5.97) 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 ioncan promote the transformation of SO4

2� 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

product of Ce2(SO4)3 decreased with the further increase of theMSA concentration from 4 to 5 mol dm�3, suggesting intricateinterplay among Ce3þ, SO4

2�, and CH3SO3�.

Cerium mixed-acid solutions (25 mL) of different states ofcharge were allowed to stand for 30 days at 293, 313, and 333 K.The test solution involved 1 mol dm�3 total cerium, 2 mol dm�3

MSA, and 0.5 mol dm�3 H2SO4. The time taken for a slightprecipitate to emerge was recorded (see Table 3). At a lowerCe4þ concentration, a longer time is required before a slightprecipitate appears in the solution. Therefore, the precipitationfrom the charged electrolyte could be a problem in certainapplications where the system is likely to keep fully charged forextended periods and high temperatures are experienced. Apreliminary test shows that a 1 mol dm�3 Ce mixed-acid solutioncontaining 2 mol dm�3 MSA and 0.5 mol dm�3 H2SO4 at any

Table 3. Stability of 1 mol dm�3 Cerium in a Mixed-Acid Solution Containing 2 mol dm�3 MSA and 0.5 mol dm�3 H2SO4 atDifferent Temperatures and States of Chargea

precipitation time (days)

electrolyte state of charge (%) 293 K 313 K 333 K

100% Ce3þ 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 80 no ppt after 30 days no ppt after 30 days 18

10% Ce3þ þ 90% Ce4þ 90 no ppt after 30 days no ppt after 30 days 6

100% Ce4þ 100 no ppt after 30 days no ppt after 30 days 3a Stability refers to time taken (in days) for a slight precipitate to appear in solution. Themixed acid initially involved 2mol dm�3MSA and 0.5mol dm�3

H2SO4 and was not analyzed after charging.

Figure 4. (A) Current versus time curves and (B) i versus t�1/2 plots.Concentration: [Ce3þ] = [Ce4þ] = 0.2 mol dm�3 and (a) 2 mol dm�3

MSA, (b) 2 mol dm�3 MSAþ 0.75 mol dm�3 H2SO4, (c) 2 mol dm�3

MSAþ 0.5mol dm�3H2SO4, and (d) 2mol dm�3MSAþ 0.25mol dm�3

H2SO4. Temperature = 293 K. Working electrode = platinum sheet.

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state of charge is a perfectly stable system at temperatures upto 313 K.Chronoamperometry. At a planar electrode, the diffusion-

limited current following the application of a potential stepfollows the familiar Cottrell equation.17

iðtÞ ¼ nFAD1=2C�π1=2t1=2

ð8Þ

For electrodes with pure linear diffusion, the diffusion coefficientcan be calculated from a plot of i versus t�1/2. The slope of thebest-fit line is then nFAD1/2C*/π1/2, and D can be found from

D ¼ ðslopeÞ2πðnFAC�Þ2 ð9Þ

where n is the number of electrons transferred in the reaction andC* is the bulk concentration of the diffusing species. The othersymbols have their usual meaning.Chronoamperometric curves and i versus t �1/2 plots for

mixed-acid solutions containing 2 mol dm�3 MSA and sulfuricacid with different concentrations as well as 0.2 mol dm�3 each ofCe3þ and Ce4þ are demonstrated in Figure 4. Diffusion coeffi-cients of Ce4þ ion inmixed-acid solutions calculated according toexpression 9 are shown in Table 4. From Table 4, it can be seenthat all of the values of the Ce4þ diffusion coefficient in mixed-acid media are larger than that in 2 mol dm�3 MSA media,indicating that mixed-acid media are more suitable for applica-tion in Ce/Zn RFB than single-acid media. The Ce4þ diffusioncoefficient underwent a maximum in 2 mol dm�3 MSA þ 0.25mol dm�3 H2SO4 solution and then decreased with the increaseof the H2SO4 concentration. The result is in agreement with thatobtained from the rotating disk electrode (shown in Table 4).Modiba and Crouch19 reported a value of 2.4� 10�6 cm2 s�1 forCe4þ in H2SO4 solution. On the other hand, the charge-transferresistance diminished with the increase of the H2SO4 concentra-tion. The values of charge-transfer resistance shown in Table 4were obtained using the method depicted in the Current-Over-potential Curve section. Therefore, a mixed-acid solution con-taining 2 mol dm�3 MSA and 0.5 mol dm�3 H2SO4 was used inthe charge�discharge test as a compromise of the diffusioncoeffecient, charge-transfer resistance, and solubility of cerium salts.Charge�Discharge Curves. A Ce/Zn system with a cell

using MSA as positive electrolyte media was patented20 in2004. As mentioned previously, sulfuric acid is not well-suitableas the supporting electrolyte for use in ceriumRFB because of thelow solubility of cerium sulfate. Therefore, a comparison wasmade only between MSA and mixed acid in the following charge�discharge experiments. The composition of the positive half-cellelectrolyte was 0.3mol dm�3 Ce(CH3SO3)3 in (a) 2.5mol dm�3

MSA, (b) 3mol dm�3MSA, and (c) 2mol dm�3MSAþ 0.5moldm�3 H2SO4. The negative half-cell electrolyte was 0.3 moldm�3 ZnSO4 aqueous solution.The typical plot of cell voltage versus time for the first

charge�discharge cycle is given in Figure 5. The upper limit ofcharging is 2.8 V, and the bottom limit of discharging is 0.5 V. Asseen in Figure 5, the time taken for charging the cell using 2.5moldm�3 MSA as positive supporting electrolyte media is 19 min,corresponding to 65.6% extent of charge. It is 21 min for 3 moldm�3 MSA, corresponding to 72.5% extent of charge. For mixedacid containing 2 mol dm�3 MSA and 0.5 mol dm�3 H2SO4, it is22.5 min, reaching a 77.7% extent of charge. The high extent ofcharge indicates a high use rate of the electrolyte. It helps toimprove the practical capacity of the cell. In addition, from thisplot, the Coulombic and voltage efficiency values were calculatedas (a) 85.1 and 80.3% for 2.5 mol dm�3MSA, (b) 85.7 and 84.1%for 3 mol dm�3 MSA, and (c) 86.7 and 84.5% for 2 mol dm�3

MSA þ 0.5 mol dm�3 H2SO4. The Coulombic and energy effi-ciencies of the Ce/Zn cell using mixed acid as the supportingelectrolyte are higher than those using singleMSA. The change inCoulombic and energy efficiencies of the Ce/Zn cell using mixedacid as the positive supporting electrolyte in the first 10 cycles isgiven in Figure 6. As shown in Figure 6, the average Coulombicand energy efficiencies of this cell are 87.1 and 73.5%, respec-tively, which are comparable to those (about 80 and 62.6%) forthe all-vanadium RFB.21 This indicates that the self-dischargebecause of diffusion of the species of cerium through the mem-brane is small. Clarke et al.22 reported that such cerium�zincRFBmay be operated without a separator or with a separator thatallows for at least partial mixing of the negative and positive

Table 4. Ce4þ Diffusion Coefficients and the Ce4þ/Ce3þ Reaction Charge-Transfer Resistances at 298 Ka

D (cm2 s�1)

[Ce4þ] = [Ce3þ] (mol dm�3) MSA (mol dm�3) H2SO4 (mol dm�3) RDE CA Rct (Ω)

0.20 2.00 0 2.68� 10�6 2.56� 10�6 112

0.20 2.00 0.25 8.35� 10�6 8.27� 10�6 75

0.20 2.00 0.50 6.98� 10�6 6.92� 10�6 62

0.20 2.00 0.75 5.93� 10�6 5.87� 10�6 60aRDE, rotating platinum disc electrode (0.12 cm�2); CA, chronoamperometry. Values of CA and Rct are obtained using the platinum sheet workingelectrode (0.15 cm�2).

Figure 5. Typical charge�discharge curves of the Zn/Ce cell usingdifferent supporting electrolytes at 293 K. Positive electrolyte (18 mL) =0.3 mol dm�3 Ce3þ in the following acid solutions: (a) 2.5 mol dm�3

MSA, (b) 3 mol dm�3 MSA, and (c) 2 mol dm�3 MSAþ 0.5 mol dm�3

H2SO4. Negative electrolyte (18 mL) = 0.3 mol dm�3 ZnSO4 solution.

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electrolyte. Thus, in the Ce/Zn RFB, membranes are suitable foruse even if such membranes exhibit some leakage or permeabilityfor catholyte and/or anolyte into the opposite compartment.As shown in Table 4, the Ce4þ diffusion coefficient in mixed-

acid media is larger than that in single MSA media. The mass-transfer condition at the electrode surface can affect the value ofthe overpotential. The increase in the diffusion coefficient leadsto smaller polarization and a decreased rate of side reactions. Inaddition, the charge-transfer resistance of the Ce3þ/Ce4þ elec-trode reaction in mixed acid is less than in MSA, which alsoresults in smaller polarization and a decreased rate of side reac-tions. The elimination of side reactions can improve Coulombicand energy efficiencies. The preliminary test shows that mixed-acid media are more suitable for applicaton in the Zn/Ce cellthan single acid media.

’CONCLUSION

As a kind of supporting electrolyte, mixed-acid media (CH3-SO3H and H2SO4) are first reported for application in Ce/ZnRFB 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 significantlyless than in sulfuric acid (more than 400mV).9,16 The ratio (ipc/ipa)is found to be about 0.80 at 293 K, which is larger than in sulfuricacid (about 0.45).11 The diffusion coeffcient in mixed acid is alsolarger than in single acid (CH3SO3H or H2SO4

19). The exchangecurrent density and standard rate constant of the Ce3þ/Ce4þ

reaction in mixed-acid media (about 2.86 � 10�3 A cm�2 and1.48 � 10�4 cm s�1) are also larger than those in MSA (1.53 �10�3 A cm�2 and 7.9 � 10�5 cm s�1) but very close to those insulfuric acid (1.6 � 10�4 cm s�1) reported in the literature.19

Stability of CeriumSalt.The solubility of cerium salt inmixedacid is obviously larger than in sulfuric acid. A solution of 1 moldm�3 cerium containing 2 mol dm�3 MSA and 0.5 mol dm�3

H2SO4 is sufficiently stable at temperatures up to 313 K for morethan 1 month.Performance of the Cell. The extent of charge and energy

efficiency of the cell using mixed-acid media are larger than thoseusing single MSA media. The average Coulombic and energyefficiencies of the cell using mixed acid are 87.1 and 73.5%,respectively, which are comparable to that for the all-vanadiumRFB.21 Its average discharge voltage can reach 2.166 V, which issignificantly larger than that of all-vanadium RFB (about 1.5 V).

The preliminary investigation shows that the cerium mixed-acid system is attractive and electrochemically promising for usein RFB technology.

’AUTHOR INFORMATION

Corresponding Author*Telephone: þ86-0731-88836291. Fax: þ86-0731-8879850.E-mail: dbzhou_09@126.com.

’ACKNOWLEDGMENT

The authors acknowledge the support of literature providedby Faizur Rahman (Center for Refining and Petrochemicals,Research Institute, King Fahd University of Petroleum andMinerals, Dhahran, Saudi Arabia).

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Figure 6. Coulombic and energy efficiencies of the Zn/Ce cell usingmixed acid as the supporting electrolyte in the first 10 cycles. Positiveelectrolyte (18 mL) = 0.3 mol dm�3 Ce3þ in 2 mol dm�3 MSA þ0.5 mol dm�3 H2SO4. Negative electrolyte (18 mL) = 0.3 mol dm�3

ZnSO4 solution.