trogadas electrochem solid state lett 2011-15-1 a5

Electrochemical and Solid-State Letters, 15 (1) A5-A8 (2012) A51099-0062/2012/15(1)/A5/4/$28.00 The Electrochemical Society

Composite, Solvent-Casted Naon Membranes for VanadiumRedox Flow BatteriesPanagiotis Trogadas,*,z Emmanuel Pinot,a and Thomas F. Fuller**

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, USA

The efcacy of metal oxide particles in mitigating vanadium ions crossover was investigated. 3 wt % SiO2 was incorporated withinrecast Naonmembrane fromwhichmembrane electrode assemblies were prepared by painting. The composite membranes exhibitedproton conductivities slightly lower (5455 mS cm1) than Naon. However, the permeability of vanadium ions was loweredby 8085%, while the performance was improved. These results demonstrated that loadings of SiO2 much lower than previouslyreported can mitigate vanadium-ion crossover. Additionally the direct application of the catalyst on the membrane was shown to bea viable MEA synthesis option. 2011 The Electrochemical Society. [DOI: 10.1149/2.004201esl] All rights reserved.

Manuscript submitted August 9, 2011; revised manuscript received September 19, 2011. Published November 18, 2011.

The vanadium redox ow battery (VRB)1 has received renewedattention because of its long cycle life, high reliability and storagecapacity, and fast response.24 It is considered to be a highly efcientand reliable system for large-scale energy storage applications suchas renewable energy technologies, remote area power systems, anduninterruptable power sources to supply energy to areas off the grid.2

VRB employs two vanadium redox couples for the positive(VO2+/VO2+) and negative (V3+/V2+) half cells. During charge-discharge processes, vanadium ions are oxidized/reduced to their cor-responding valence states,5 and pentavalent and bivalent vanadiumions are produced in the positive and negative electrolyte compart-ments respectively.

The half cells are separated by a proton exchangemembrane (PEM)that prevents the mixing of electrolytes and transfers protons betweenthe electrodes.2 The ideal characteristics of this membrane are lowpermeability to vanadium ions, high proton conductivity as well asmechanical and chemical stability.3

Peruorinated Naon membranes are commonly used inVRB since they meet the majority of the above mentionedcharacteristics.3,4, 610 Although, their high cost and the permeabilityof vanadium ions have limited the commercialization of VRBs. Thecrossover of vanadium ions through the membrane causes contam-ination of the electrolyte solutions leading to battery self-dischargeand hence a decrease of the efciency and energy capacity of thesystem.4,9, 1114

Alternative membranes with improved ion selectivity are soughtand can be categorized into: i) non-peruorinated polymers suchas ammoniated or sulfonated aromatic polymers,15,16 SPEEK17 andii) modied Naon membranes. In both cases, vanadium ion per-meability was decreased by the introduction of inorganic or organiccomponents into the membrane to block its hydrophilic clusters.3,1822

Our choice of SiO2 as an inhibitor of vanadium ions crossover inproton exchange membranes (PEMs) arises from its proven utility inthis role in VRB systems.18 However, in the present study, a simplefabrication method (solvent casting) is employed for the synthesisof composite membranes (instead of sol-gel method)23 while lowSiO2 loading (3 wt % instead of 910 wt %18) is used to avoid thedeterioration of the cluster network of Naon membrane.24 UsingNaon as a model PEM, the present study investigates i) the effect oflow SiO2 loading as well as ii) the direct application of carbon catalyston the PEM (instead of the commonly used graphite felt electrode) onmembrane properties and VRB performance. It has been reported thatthe oxygen content on the surface of carbon is greater than graphitefelt25 resulting in more facile electron transfer and hence enhancedelectrocatalytic activity.26,27

* Electrochemical Society Active Member.** Electrochemical Society Fellow.a Present address: Phelma, Institut Polytechnique de Grenoble, France.z E-mail: [email protected]

ExperimentalChemicals and reagents.Vanadyl sulfate, phenolphtaleine

(2 wt %), potassium chloride (KCl, 99%), sodium hydroxide (NaOH,97%), silica (SiO2) and titania (TiO2) particles were purchased fromVWR, whereas Naon solution 5% and 20% were obtained fromDupont.

Membrane synthesis and characterization.Composite Naon-SiO2 membranes were prepared by solvent casting. The precursorsolution was 5 and 20 wt % Naon, and an appropriate amountof silica was added to achieve a loading of 3 wt %. CommercialNaon 117, recast Naon and Naon - TiO2 membranes were pre-pared for comparison. Catalyst ink was prepared by mixing 0.2 gcarbon black (Vulcan XC 72), 1.7 g of 5 wt % Naon dispersion, and7 cm3 ofmethanol. The inkwas stirred for 24 h before use.MEAswereprepared by painting successive layers of this catalyst ink directly ontoeither side of the membrane. An infrared lamp was used to dry theMEA prior to the application of each layer. A polytetrauoroethylene(PTFE) mask was employed to maintain the active area of the MEA at5 cm2. After both the anode and cathode catalyst layers were applied,the MEA was hot pressed at 120C and 1.0 MPa. The carbon loadingof each electrode was estimated and gravimetrically maintained at10 0.05 mg cm2.

Ion exchange capacity (IEC) of the membranes was measured bytitration.28 Membrane resistance was measured with a milliohmmeter(Agilent 4338B) at 25C and 100% relative humidity. Proton conduc-tivity was then calculated based on membrane thickness and electrodearea.

A diffusion cell (Permegear side-bi-side cell) was used for themeasurement of the permeability of vanadium ions (3+ and 4+).Two glass cells (5 mL volume each) were separated by the sam-ple membrane (5 cm2 exposed area). The V(IV) solution wasprepared by dissolving VOSO4 5H2O in 1 M H2SO4. V(III) solutionwas prepared by the electrochemical reduction of V(IV) solution. Theleft cell was lled with 0.1 M V4+ and/or V3+ solution in 0.5 M sulfu-ric acid (H2SO4) while the right cell was lled with 0.1 M MgSO4 in0.5 MH2SO4 in order to equalize the ionic strengths of the solutions29and minimize the difference in osmotic pressure.29 The concentrationdifference across the membrane established the ux of vanadium ions.The solution in the right cell was sampled and the concentration ofvanadium ions was measured by UV-Vis spectroscopy (Genesis 10S).The peaks associated with V3+ and V4+ ions could be clearly seen atapproximately 600 nm and 760 nm respectively.30

The vanadium redox cell used in the experiments was constructedin-house. The end plates were made from chemically resistant PVCand graphite with serpentine ow elds onto which gold current col-lectors were attached. 50 mL of 0.1 M V4+ in 0.5 M H2SO4 solutionwas fed (ow rate 0.2 mL sec1) into the positive side and 50 mLof 0.1 M V3+ in 0.5 M H2SO4 was fed into the negative side. The

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A6 Electrochemical and Solid-State Letters, 15 (1) A5-A8 (2012)

Table I. Conductivity at room temperature and IEC values ofNaon and composite Naon membranes.

Conductivity Ion exchangeMembrane (S m1) capacity (meq g1)

Naon 5.87 0.1 0.90 5 103Recast Naon (5 wt %) 5.94 0.07 0.90 3 103Recast Naon (20 wt %) 6 0.07 0.92 2 103Naon (5 wt %)-TiO2 5.46 0.05 0.88 5 103Naon (20 wt %)-TiO2 5.64 0.2 0.88 3 103Naon (5 wt %)-SiO2 5.36 0.2 0.88 2 103Naon (20 wt %)-SiO2 5.52 0.2 0.88 4 103

cell was charged/discharged (0.8 to 1.5 V vs. SHE) at constant currentdensity (60 mA cm2).

Results and DiscussionThe composite membranes were transparent and had a uniform

distribution of additives. Table I presents the conductivity and IEC;several membranes of each type were tested to ensure reproducibility.Composite Naon - (SiO2, TiO2) membranes have lower conductivityand IEC values than commercial and recast Naon due to the llingof the polar clusters of Naon by SiO2/TiO2 nanoparticles.31

To evaluate the effect of SiO2/TiO2 additives on the permeabilityof vanadium ions for composite membranes, the crossover of V4+ andV3+ ions wasmeasured using a diffusion cell. The permeability of V2+ions was not measured since V2+ is rapidly oxidized in air.10 Severalmembranes of each type were tested to ensure that the results obtainedand trends seen were reproducible. The evolution of V3+ and V4+ions concentration with time for all membranes tested is presented inFigure 1. It can be clearly seen that V3+, V4+ concentrations in theright cell increase signicantly faster when Naon is used as theseparator. The composite Naon - SiO2 or TiO2 membranes showan 8590% and 6065% reduction of vanadium ions concentra-tion respectively. The larger decrease in permeability of vanadiumions for the composite Naon - SiO2 membrane is attributed to thelarger particle size of silica compared to titania (50 and 30 nmrespectively).

The permeability of vanadium ions can be calculated from Eq. 1based on the assumptions: i) permeability is independent of vanadium-ion concentration; ii) the concentration of vanadium ions in the left cellis constant; and iii) pseudo-steady state exists in the membrane.32,33

VRdcR(t)

dt= A P

L(cL cR(t)) [1]

where cL, cR(t) the vanadium ion concentration (M) in the left andright cell respectively, A and L the area (cm2) and thickness (cm) ofthe membrane, P the permeability of vanadium ions (cm2 min1), andVR the volume (cm3) of the right cell.

V3+ ions exhibit the highest and V4+ the lowest crossover ratesthrough the membrane. This is expected based on their reported dif-

Table III. Coulombic and energy efciency of Naon andcomposite Naon membrane based MEAs (10 cycles, 60 mA cm2charge-discharge current density).

Coulombic EnergyMembrane Efciency (%) Efciency (%)

Naon 90 67Recast Naon (5 wt %) 88.5 65.5Recast Naon (20 wt %) 88.7 65.7Naon (5 wt %)-TiO2 90.6 67.6Naon (20 wt %)-TiO2 90.9 67.9Naon (5 wt %)-SiO2 91.4 70.4Naon (20 wt %)-SiO2 91.4 70.4

fusion coefcients.18 Composite Naon - SiO2 membranes exhibitthe lowest V3+, V4+ permeability (Table II) demonstrating that theincorporation of large particles (even at such small quantities of3 wt %) into the polar clusters of a peruorosulfonic membrane canrestrain vanadium ions crossover.

The lower conductivity compared to Naon (Table I) raises thequestion of the suitability of the composite membranes for VRBs.The conductivity inuences the achievable power and therefore thecell area needed to meet the system power requirements. On the otherhand, the permeability affects the self-discharge and thus the capacityof the system. Hence, there is a design trade based on economics thatdetermines the optimum between the two properties. This argumentis beyond our scope, and we simply dene a ratio of electrical con-ductivity to permeability of vanadium ions. The ratio is a crude gaugeof the value of these composite membranes. Table II shows that thisratio for composite Naon - SiO2 membrane is highest for V3+, V4+(approximately 6 1012 and 8 1012 S sec m3 for V3+ and V4+ ions)suggesting that the battery performance will be improved.

Cyclic performance curves were obtained with Naon and com-posite Naon-(SiO2, TiO2) based MEAs (Table III). All the MEAshad the same catalyst loading and were tested at 60 mA cm2 charge-discharge current density and at room temperature. Table III comparesthe performance obtained using Naon and composite Naon - (SiO2,TiO2) MEAs. As expected, composite Naon - SiO2 based MEAshave higher coulombic (ratio of discharge to charge capacity)19 andenergy efciency than Naon MEAs due their lower vanadium ionscrossover (Table II).

Thus, these reproducible results (8085%/5060% reduction invanadium ions permeability and 7580%/4050% increase in ratiofor V3+, V4+ upon silica and titania addition respectively) illustratethe promise of using metal oxides even at low loadings to mitigatevanadium ions crossover. The cyclic performance results are similarto the results obtained when graphite felt was used as the catalyst10,18suggesting that direct application of the catalyst on themembranemaybe a viable MEA fabrication method. However, its catalytic propertiesand behavior have to be examined under extensive battery cycling.Finally, TiO2 is reported to become unstable in vanadium solutionafter extensive battery cycling19 and thus is not a suitable material for

Table II. Vanadium ion permeability and the ratio of conductivity to permeability for Naon and composite Naon membranes.

Permeability V3+ Ratio for V3+ Permeability V4+ Ratio for V4+Membrane (1011 m2 sec1) (1012 S sec m3) (1011 m2 sec1) (1012 S sec m3)

Naon 0.33 2.8 103 1.8 0.293 2.3 103 2Recast Naon (5 wt %) 0.34 3.1 103 1.73 0.30 2.5 103 1.95Recast Naon (20 wt %) 0.32 2.6 103 1.91 0.29 2.5 103 2.07Naon (5 wt %)-TiO2 0.18 8.3 104 3.08 0.15 6.7 104 3.60Naon (20 wt %)-TiO2 0.16 6.7 104 3.65 0.13 5 104 4.34Naon (5 wt %)-SiO2 0.095 9.5 103 5.65 0.068 1.3 104 7.76Naon (20 wt %)-SiO2 0.085 8.5 103 6.48 0.062 1.2 104 8.91


Electrochemical and Solid-State Letters, 15 (1) A5-A8 (2012) A7

Figure 1. (a) and (b) Vanadium ions (3+, 4+) concentration as a function of time.

VRB applications; TiO2 supported on another metal oxide may be aviable option to enhance its stability.

ConclusionsThe effect of incorporating metal oxides into solvent casted Naon

membranes on vanadium ions permeability rate as well as the use of

carbon black instead of graphite felt as the electrode in vanadiumredox battery was studied. The results obtained demonstrated thatlow loading of silica oxide enabled a signicant reduction (approxi-mately 8085%) in the macroscopic rate of vanadium ions permeationrate. Battery performance tests conrmed that the reduction in perme-ability was achieved without compromising either membrane protonconductivity or performance.


A8 Electrochemical and Solid-State Letters, 15 (1) A5-A8 (2012)

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