+ All Categories
Home > Documents > High Performance Vanadium Redox Flow Batteries with ...ecpower.utk.edu/Publications/docs/High...

High Performance Vanadium Redox Flow Batteries with ...ecpower.utk.edu/Publications/docs/High...

Date post: 30-Apr-2018
Author: lebao
View: 216 times
Download: 2 times
Share this document with a friend
Embed Size (px)
of 8 /8
doi: 10.1149/2.051208jes 2012, Volume 159, Issue 8, Pages A1246-A1252. J. Electrochem. Soc. Mench Q. H. Liu, G. M. Grim, A. B. Papandrew, A. Turhan, T. A. Zawodzinski and M. M. Selection Optimized Electrode Configuration and Membrane High Performance Vanadium Redox Flow Batteries with service Email alerting click here in the box at the top right corner of the article or Receive free email alerts when new articles cite this article - sign up http://jes.ecsdl.org/subscriptions go to: Journal of The Electrochemical Society To subscribe to © 2012 The Electrochemical Society
  • doi: 10.1149/2.051208jes2012, Volume 159, Issue 8, Pages A1246-A1252.J. Electrochem. Soc.

    MenchQ. H. Liu, G. M. Grim, A. B. Papandrew, A. Turhan, T. A. Zawodzinski and M. M. SelectionOptimized Electrode Configuration and Membrane High Performance Vanadium Redox Flow Batteries with

    serviceEmail alerting

    click herein the box at the top right corner of the article or Receive free email alerts when new articles cite this article - sign up

    http://jes.ecsdl.org/subscriptions go to: Journal of The Electrochemical SocietyTo subscribe to

    2012 The Electrochemical Society


  • A1246 Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012)0013-4651/2012/159(8)/A1246/7/$28.00 The Electrochemical Society

    High Performance Vanadium Redox Flow Batteries withOptimized Electrode Configuration and Membrane SelectionQ. H. Liu,a, G. M. Grim,a A. B. Papandrew,a A. Turhan,a, T. A. Zawodzinski,a,b,and M. M. Mencha,b,,z

    aBRANE Lab, Departments of Chemical and Biomolecular Engineering and Mechanical, Aerospace and BiomedicalEngineering, The University of Tennessee, Knoxville, Knoxville, Tennessee 37996, USAbOak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    The performance of a vanadium flow battery with no-gap architecture was significantly improved via several techniques. Specifically,gains arising from variation of the overall electrode thickness, membrane thickness, and electrode thermal treatment were studied.There is a trade-off between apparent kinetic losses, mass transfer losses, and ionic resistance as the electrode thickness is variedat the anode and cathode. Oxidative thermal pretreatment of the carbon paper electrode increased the peak power density by 16%.Results of the pretreatment in air showed greater improvement in peak power density compared to that obtained with pretreatmentin an argon environment. The highest peak power density in a VRB yet published to the authors knowledge was achieved at a valueof 767 mW cm2 with optimized membrane and electrode engineering. 2012 The Electrochemical Society. [DOI: 10.1149/2.051208jes] All rights reserved.

    Manuscript submitted April 3, 2012; revised manuscript received May 15, 2012. Published July 20, 2012. This was Paper 681presented at the Boston, Massachusetts, Meeting of the Society, October 914, 2011.

    Redox flow batteries (RFBs) have the potential to assume a crit-ical role in the future of environmentally-conscious energy use andconservation due to their marked design flexibility, uncoupled en-ergy and power capacities, and potential low cost. Their use in con-junction with intermittent power generation schemes, such as thosebased on wind and solar energy, improves the stability and overallefficiency of the grid with renewable resources.14 Among the myriadchemistries employed in redox flow battery systems, the all-vanadiumredox (VRB) pair has gained much attention due to its relative safetyand lower risk of cross-contamination by crossover between electrodecompartments.5,6 In order for redox flow battery systems to find a placein the commercial market, it is paramount that the costs and benefitsassociated with purchasing and maintaining a system are as attractiveas possible. Recent research done by Moore et al.7 has shown thatmany of the key costs associated with redox flow battery systems aredirectly related to the initial costs of procuring the materials used toconstruct the system. Without developing new materials for use inRFB systems, the most cost-effective method of reducing the initialexpenditures in the stack is to improve the overall cell performance.This method enables a reduction in stack size and associated costs tomeet similar power needs.

    Recently, a VRB with a peak power density of 557 mW cm2,8

    which is over five-fold higher than that of other conventional publishedsystems,912 has been demonstrated and reported by our group. Thishigh power density was obtained with a no-gap-serpentine architec-ture similar to fuel cells with carbon paper electrodes. The no-gapstructure and thin carbon paper electrodes enable lower ohmic resis-tances in the cell due to better contact between all components, andlower charge transfer distances across the cell. In addition, the appli-cation of a serpentine flow channel greatly enhances the mass transferin the porous electrode by distribution of the electrolyte across theentire membrane surface area. Nonetheless, additional performanceenhancement of this novel VRB is still possible through advanced ar-chitecture and material engineering. Sun and Skyllas-Kazacos13 suc-cessfully used a thermal pretreatment method to increase the elec-trochemical activity of a graphite felt electrode material traditionallyused in VRB systems. The enhanced activity of electrode materialswas attributed to the formation of surface active carboxylic acid func-tional groups thought to catalyze the redox reaction (VO+2 /VO

    2+) atthe cathode by producing preferentially active sites for the reaction.To date, these results have not been confirmed on the carbon papermaterial which was recently demonstrated to achieve dramatic poweroutput increase over conventionally used felt.8

    Electrochemical Society Active Member.Electrochemical Society Fellow.

    zE-mail: [email protected]

    In addition to the effort to improve the property of carbon paperelectrodes for better performance, various proton exchange mem-branes, which greatly contribute to the ohmic losses in a VRB, werealso investigated. Perfluorosulfonic Acid (PFSA) membranes are gen-erally used in the lab-scale VRB systems reported in literature due toreasonable electrochemical properties and chemical stability.14 In ourprevious work, only Nafion 117 membrane was used, and to date, theeffect of membrane properties on the power output is unknown.

    The motivations for this work are to further enhance the perfor-mance of a lab-scale 5 cm2 VRB single cell with no-gap-serpentinearchitecture and to clarify the effects of electrode configuration, ther-mal pretreatment of carbon paper electrodes, and membrane thicknessin such a way that the understanding gained from our research willhelp improve the development process for future materials and tech-nologies used in flow battery systems. The lab-scale performance (a)provides a useful benchmark of upper limit of performance, (b) allowsmaterial tests and (c) has been shown in other fields to yield resultsthat can be reproduced in larger cells.


    Single cell structure. A no-gap-serpentine single cell comprisedof a membrane, electrodes, electrolyte distributors, and current collec-tors was used in all experiments, as shown in Figure 1.8 The electrodesused were either carbon felt (with 3 mm uncompressed thickness) orcarbon paper (with 400 m uncompressed thickness-SGL 10 AA,SGL Group) with a geometric surface area of 5 cm2. One layer ofcarbon felt was used both at anode and cathode. Various layers ofcarbon paper were used at both sides. The thickness of the electrodewas adjusted by stacking 1 to 9 pieces of carbon paper. The other cellmaterials were graphite plates 76 76 13 mm (Fuel Cell Technolo-gies, Inc.) engraved with serpentine flow fields over a 5 cm2 activearea as electrolyte distributor, as well as a Nafion-series proton ex-change membrane (Nafion 115, 117, 211, and 212, Ion-Power, Inc.).The graphite plates were mounted by sandwiching between currentcollectors. The membranes were pretreated in distilled water at 80Cand a 1 mol l1 H2SO4 solution at 80C for 30 minutes each. Thecell was assembled using gaskets suitable to compress the electrodematerial to roughly 80% 6% of its original thickness.

    Thermal pretreatment of carbon felt and paper. The procedureused for thermal pretreatment of the carbon materials was based onreference 15. The carbon paper and felt were placed in muffle (aircase, Thermo Scientific) and tube furnaces (argon case) and heated to400C for 30 hours. Then, the carbon materials were allowed to cooldown in the furnace before beginning the experiments.

    http://dx.doi.org/10.1149/2.051208jesmailto:[email protected]

  • Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012) A1247

    Figure 1. Schematic of a no-gap-serpentine architecture vanadium redox flowbattery.

    Electrochemical tests and material characterization. A solutionof 1 mol l1 VOSO4 xH2O (20.87 wt% of vanadium, 99.9% purity,Alfa Aesar, U.S.) dissolved in 5 mol l1 H2SO4 was used as an elec-trolyte. The initial electrolyte for charging used at both sides wasthe same aqueous solution only containing VO2+, SO2+4 and support-ing electrolyte. The anolyte and catholyte volumes were 50 mL and100 mL, respectively. A constant current method with the currentdensity of 100 mA cm2 followed by constant voltage method of 1.8V was used to charge the cell to a theoretical state of charge (SoC)of 100%, viz. the VO2+ ions were fully converted to V2+ ion and(VO2)+ ion in the anolyte and catholyte, respectively. The cutoff volt-age for the constant current charging is 1.8 V and the cutoff current forthe constant voltage charging is 5 mA. Two peristaltic pumps (ColeParmer) were used to circulate the electrolytes with the flow rate of20mL min1 during charging and from 20 to 90 mL min1 during dis-charging. Based on the channel cross section, the mean linear velocityis 120 m min1 at the flow rate of 90 mL min1.

    N2 purging in the anolyte reservoir was used to prevent the oxida-tion of V2+ during testing.

    Polarization curves and high frequency resistance (HFR) measure-ments were performed with a Scribner 857 potentiostat. The polar-ization curve tests consisted of a series of structured discharge stepsof a fully charged electrolyte using evenly spaced current incrementsof 20 mA cm2 from open circuit voltage (OCV) for each particularconstruction of the single cell. Each step in the polarization curveslasted for 30 seconds. The experimental procedure was repeated atleast five times for each test case to ensure the repeatability of theresults. The errors we can attribute to build-to-build variation (e.g.misalignment of components, differences in compression, potentialmembrane degradation and other such factors are

  • A1248 Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012)

    course of each experiment. The decrease of state of charge is between11% and 76%,19 depending on the configuration and properties of thecomponents of the cell. The peak power densities shown in Figure 2bcorrespond to a state of charge of 60%. In order to confirm that thepeak power density is available when the cell is under normal oper-ating conditions, various current densities from 200 to 800 mA cm2

    were used to discharge the cell. The results are shown in Figure 2c.There is one plateau region that corresponds to the operating range ofthe cell in all discharging curves. The SoC at the end of the plateauregion is in the range of 11%38%, which is much lower than 60%.We conclude that these high peak power densities can be achievedduring normal steady state operation.

    Comparison of carbon felt and carbon paper electrodes. As analternative to the widely used porous carbon felt (porosity 95%) inmost of the previous studies,20,21 a thinner but more porous (poros-ity 76.4% at 20% compression) carbon paper material was chosenas the electrode. Figure 3a depicts the raw and IR-free polarizationcurves of single cells operating with electrodes composed of almostequal thicknesses of thermally treated carbon felt and layered carbonpaper. Both have a similar open circuit voltage (OCV) and overvolt-age at 20 mA cm2 (177 and 164 mV for carbon felt and carbonpaper respectively). However, it is clear that the cell voltage from asingle cell with carbon felt electrodes is much lower than one usingcarbon paper electrodes. This is due in part to the larger area specificresistance (ASR) of the cells employing carbon felt (14001800 mcm2), which is two times greater than those using carbon paper (650880 m cm2), as shown in Figure 3b. The contact resistances withdifferent architecture and materials were also measured. Typically,the contact resistance is 0.2 and 20.2 m for carbon paper and car-bon felt materials, respectively.19 The cell with carbon felt electrodesalso shows low limiting current density of 620 mA cm2, comparedto a value of 850 mA cm2 for those that use carbon paper elec-trodes. Clearly, carbon paper is a promising material for flow batteryelectrodes.

    Performance of a VRB with layered carbon paper electrodes.Switching to the layered carbon paper electrode, which results in

    performance improvement when compared to the felt, has been previ-ously shown.8 However, the effect of paper electrode thickness at bothsides on performance is unknown. To investigate this, the thickness ofthe electrodes was changed by changing the number of pieces of car-bon. The pressure drop across the cell with one layer of carbon paperused at both sides increases from 4,826 2,758 Pa (0.7 0.4 psi) to50,331 8,274 Pa (7.3 1.2 psi) with increasing flow rate from 20to 90 mL min1. With three layers of carbon paper, the pressure dropis 2,068 1,379 Pa to 24,131 4,136 Pa (0.3 0.2 to 3.5 0.6psi) under 2090 mL min1 flow rates. Figure 4 depicts the results ofthis symmetrical layering in terms of polarization, ASR, and powerdensity curves. Table I summarizes all the results. Figure 4a showsthat increasing the electrode thickness initially improves the overallperformance. In this region, the main benefit of the additional materialmay stem from the added reaction surface area the material conveys,and improved flow distribution through the electrode that results in ahigher peak power density and limiting current. However, with fur-ther increase in the number of layers to 5, 7 and 9, the improvement

    Figure 3. Comparison of cells using carbon felt and paper electrodes witha no-gap serpentine architecture, () carbon paper, () carbon felt withthermal treatment at 400C for 30 hours. The IR-free curves of () carbonpaper and () treated carbon felt are also shown in panel (a). Nafion 117membrane was used. Flow rate was 50 mL min1.

    in reaction surface area is diminished by the increase in ASR. Thisresults in lower peak power density values, as seen in Figure 4b and4(c). The peak power density values for each electrode configura-tion is also plotted in Figure 4e, to better show the initial improve-ment in peak power followed by a reduction as electrode thicknessis increased. This decrease in peak power can be explained throughtwo possible scenarios, depending on the main reaction zone in thelayered electrode region. If the main reaction zone is near the currentcollector, as the thickness is increased, performance and peak powerwill asymptote due to the restriction in ion transport, whereas if themain reaction zone is near the membrane, as more layers are added

    Table I. Properties of a no-gap-serpentine architecture single cell constructed with one to nine layers of carbon paper electrodes.

    Layers ofcarbon paper OCV / V

    Area specificresistance / m cm2

    Limiting currentdensity at the flow

    rate of 90 mL min1/mA cm2

    Peak power density /mW cm2

    Theoretical SoC atpeak powerdensity /%

    1 1.68 606687 898 455 713 1.69 515580 920 557 615 1.72 598700 937 544 637 1.71 641777 960 482 679 1.74 741868 956 482 69

  • Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012) A1249

    Figure 4. Comparison of VRB single cell constructed with different layers of carbon paper electrode. (a) Polarization curves, (b) ASR curves, (c) power densitycurves, (d) Limiting current densities vs. thickness of electrodes at both sides at flow rates of (*) 20 mL min1 and (x) 90 mL min1. (e) Peak power densitiesvs. thickness of carbon paper electrode.

    there active (i.e. vanadium) species transport will be the limiting fac-tor. The IR-free curves show that 3 layers and 7 layers behave almostidentically and 5 layers behave slightly better in both kinetic and masstransport regions when IR losses are compensated. Furthermore, thelimiting current density slightly improves as more layers are added, asshown in Figure 4d. Both of these trends suggest that the electrolyteflow and species transport is enhanced inside the electrode layers asthickness is increased, and consequently the main limitation is dueto the ohmic losses. Therefore, it is possible to suggest that the mainreaction zone in the multiple electrode layer structure may take placecloser to the current collector, which is also consistent with somemodeling studies in the literature.22

    Performance of thermally treated carbon papers in air andargon. Another approach to further enhance the performance ofVRB single cells is to modify the surface properties of carbon paper

    materials through thermal pretreatment. The performance compari-son of thermally pretreated and untreated carbon papers is shown inFigure 5, and the results are summarized in Table II. Power densityis increased by 16% to 638 mW cm2 with thermal pretreatment ofcarbon paper in air. The OCP of treated carbon paper is 1.73 V, 0.04V higher than that of untreated carbon paper (1.69 V). Overvoltage atlow currents is significantly reduced through thermal pretreatment ofcarbon paper electrode in air atmosphere. The ASRs for treated anduntreated carbon paper are in the range of 515580 and 515660 mcm2, respectively.

    In the case of the carbon felt electrodes, the thermal pretreatmenthad a discernible effect on the fiber surface, as observed by electronmicroscopy shown in Figure 6. Specifically, a distinct increase insurface roughness was observed as a result of the treatment. This maybe due to the partial removal of a non-carbon coating that adheresto the fibers of the electrode as a side effect of the manufacturing

  • A1250 Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012)

    Figure 5. Comparison of (a) polarization and (b) power densities vs. currentdensities curves of a single cell used () untreated and () thermallytreated carbon papers. ASR value of () untreated and () thermally treatedcarbon papers are also shown in panel (a). Nafion 117 membrane was used.

    process. As shown in Figure 6, the thermal pretreatment of the carbonpaper electrodes induces relatively little change when compared tothe untreated case. This implies that the change in cell performanceas a result of the heat-treatment is not simply a function of increasedsurface roughness or removal of a coating on the carbon fibers withinthe paper electrode material.13 To further explore the effects of thethermal pretreatment process, an alternate pretreatment procedure wasdevised, in which the electrode material was heated in an inert argonenvironment using the same time and temperature settings outlined inthe original procedure. In Figure 7, the results of this pretreatment arecompared to the original treatment by comparing performance in cells.It is clearly shown that the performance of the air treated electrodeis superior to the argon treated electrode through the whole currentrange. This change in treatment method suggests that the surfacemorphology changes of the carbon electrode after the treatment is notthe only source of improvement, but the oxygen concentration in theenvironment also positively effects the performance. The increasedoxygen concentration might alter the surface functionalization during

    Figure 6. Scanning electron microscope images of (a) untreated carbon feltand (b) thermally treated carbon felt in air, and (c) untreated carbon paper and(d) thermally treated carbon paper in air.

    thermal treatment and, therefore, improve the performance to a higherextent than found in an argon environment.

    Comparison of different perfulorinated sulfonic acidmembranes. Although performance enhancement was ob-tained through a three-step electrode optimizing processes (electrodeselection, electrode construction optimization, and electrode thermalpretreatment), the single cell performance is still restricted by theionic conductivity of the membrane separator. Therefore, four Nafionseries membranes, specifically Nafion 115, 117, 211, and 212with thethickness of 127, 183, 25, and 51 m, respectively, were selected fortesting. The comparison of performance of the membranes is shownin Figure 8. Three layers of carbon paper were used at both anode andcathode for all cases. As shown in Table III, the peak power density ofNafion 212 is 767 mW cm2, followed in order by 211, 115 and 117.Decreasing the thickness of the membrane increases power densitybecause of the reduction of ionic resistance across the membrane.However,, the short-circuit current, which represents the minimumcutoff current due to vanadium ion crossover during constant voltagecharging, increases with decreasing membrane thickness .As a result,Nafion 211 membrane suffers from severe ion crossover during thepolarization curve test, which corresponds to a short-circuit currentdensity of 100 mA cm2.This is the reason why the peak powerdensity of Nafion 211 is lower than that of Nafion 212. This indicatesthe trade-off between ohmic resistance from the membrane, whichcan be reduced through decreasing the membrane thickness, and ioncrossover through the membrane, which decreases with membranethickness increasing. Based on this trade-off, Nafion 212 was selectedas the optimal membrane for charging-discharging tests. Of course,this rate of crossover reduces the overall cycling efficiency of thebattery.

    Charging-discharging performance. Based on the electrode op-timization and membrane selection processes described above, threelayers of thermally treated carbon paper electrode were used for further

    Table II. Performance comparison of a no-gap-serpentine architecture single cell constructed with untreated and thermally treated carbon papersin different atmosphere.

    Samples OCV / VArea specific

    resistance / m cm2

    Limiting current densityat the flow rate of 90 mL

    min1/ mA cm2Peak power density /

    mW cm2Theoretical SoC at peak

    power density /%

    Raw carbon papers 1.69 515580 920 557 61Thermally treated in air 1.73 515660 980 638 56

    Thermally treated in argon 1.74 600710 970 555 63

  • Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012) A1251

    Figure 7. Comparison of polarization curves (a) and power densities (b) of aVRB single cell with a no-gap serpentine architecture used thermally treatedcarbon in air () and argon (). Nafion 117 membrane was used.

    charging-discharging tests in a no-gap-serpentine architecture VRBsingle cell. Nafion 212 was also used because of its highest powerdensity output among the four kinds of Nafion membranes. Figure 9ashows the charging-discharging performance and SoC changes alongwith discharging. The charging and discharging currents and currentdensities are 0.5/ 0.5 A and 0.1 A/ 0.1 A cm2, respectively. Thereis a stable plateau region in both charging and discharging curves.The plateau occurs at 1.5 1.6 V for the charging step and at 1.51.3 V for the discharging step. The stable cell voltage and narrowpotential difference during charging and discharging results in lowpolarization losses, high power density and efficiency. The columbicand energy efficiencies are 82.6% and 75.3%, respectively. Because ofthe good activity of carbon paper and low resistance of the no-gapconfiguration, the voltage efficiency is as high as 91.2%. Comparedto the high voltage efficiency, the current efficiency is relatively low,possibly due to the crossover of vanadium active species throughthe relatively thin membrane Nafion 212. To test this hypothesis,

    Figure 8. Comparison of (a) polarization and (b) power densities vs. currentdensities curves of a VRB single cell with a no-gap serpentine architectureused different membranes () Nafion 115, () Nafion 117, () Nafion211, () Nafion 212. The ASR value is also shown in panel (a) with differentsymbols () Nafion 115, () Nafion 117, () Nafion 211, () Nafion 212.

    Nafion 117 was compared with Nafion 212 for charging-dischargingtests. As shown in Figure 9a, the Nafion 117 membrane shows thesame kind of charging-discharging performance as Nafion 212, butwith a higher charging voltage and lower discharging voltage. Thishigher over potential from the larger ohmic resistance of Nafion 117,which is confirmed by larger area specific resistances during charging-discharging tests, is shown in Figure 9b.Under the conditions appliedin this work, the VRB single cell with a Nafion 117 membrane can bedischarged to a lower apparent SoC (10%) compared to the Nafion212 membrane (20%), which results in higher current efficiency of98.7% and overall efficiency of 86.6%. However, in a typical batteryapplication the depth of discharge is usually limited to 20% SoC.The overall efficiencies of Nafion 117 and 212 in this practical rangeare calculated to be 79% and 75%, respectively. To clarify, in returnfor a sacrifice of 4% in overall efficiency, a gain of 20% in peak poweris obtained by using Nafion 212 instead of Nafion 117. These results

    Table III. Performance comparison of a no-gap-serpentine architecture single cell with different membranes.


    thickness / m OCV / VArea specific

    resistance / m cm2

    Limiting currentdensity at the flow

    rate of 90 mLmin1/ mA cm2

    Peak powerdensity / mW cm2

    Theoretical SoC atpeak powerdensity /%

    Nafion 115 127 1.66 399467 994 726 54Nafion 117 183 1.73 514658 978 638 56Nafion 211 25 1.56 141198 812 755 59Nafion 212 51 1.66 219254 955 767 54

  • A1252 Journal of The Electrochemical Society, 159 (8) A1246-A1252 (2012)

    Figure 9. (a) Charging-discharging performance of a VRB single cell of ano-gap serpentine architecture with three layers of thermally pretreated carbonpaper electrode. State of charge value is shown during discharging process.() Nafion 212 and () Nafion 117 membranes were used. (b) ASRs vs. timeduring charging and discharging processes.

    indicate that the use of a membrane with low ionic resistance (in ourcase Nafion 212) will be more suitable for applications where highpower density is required.


    In this study we show that carbon paper generally performs betterthan carbon felt as an electrode material in vanadium redox flowbatteries with a no-gap design. Increasing the number of carbonpapers on each side from one to three layers showed a 23% increasein peak power density. From this three-layer configuration, a furtherincrease in the number of layers to 5, 7 and 9 resulted in 3%,13% and 14% decreases in peak power density, respectively. TheIR-free performance curves indicated that the ohmic losses were theprimary cause of the decrease in peak power. But then again, thecell limiting current density continuously improved as the numberof layers was increased. Both findings suggest that the reaction zonemay be concentrated near the current collector in the cell. The thermalpretreatments discussed also have a discernible and pronounced effect

    on the performance of systems using either paper or felt electrodes.From the analysis of area specific resistance data gathered in situ, aswell as high-magnification SEM images of the electrodes and alternatepretreatment methods, the improvement in performance by thermaltreatment is found to be not only dependent on the surface morphologychanges, but also closely related to the oxygen concentration in thepretreatment environment, which is suspected to alter the surfacefunctionalization. As a result of these deductions, future effort isfocused on optimizing these inherent properties during the electrodetreatment process to further improve the performance.

    Based on studies of the effects of PFSA membrane thickness,it was discovered that decreasing the thickness effectively increasespeak power density of a single cell, but thinner membranes suffer fromion crossover. Within the practical operation range of a battery, a gainof 20% in peak power is obtained with a sacrifice of 4% in overallefficiency when the optimal membrane thickness is selected. Thehighest peak power density to date of 767 mW cm2 was obtainedwith three layers of thermally treated carbon paper electrode and aNafion 212 membrane.


    This work was partially funded by the Experimental Program toStimulate Competitive Research (EPSCoR) under NSF grant EPS-1004083. Partial support of this work was also provided through theNSF Early Career Development Award # 0644811. Dr. Zawodzinskithanks the Department of Energy Office of Electricity for partiallysupporting this work.


    1. M. Skyllas-Kazacos, et al., Journal of the Electrochemical Society, 158(8), R55(2011).

    2. B. Dunn, H. Kamath, and J. M. Tarascon, Science, 334(6058), 928 (2011).3. A. Z. Weber, et al., Journal of Applied Electrochemistry, 41, 1 (2011).4. L. Li, et al., Advanced Energy Materials, 1, 394 (2011).5. E. Sum, M. Rychcik, and M. Skyllas-Kazacos, Journal of Power sources, 16(2), 85

    (1985).6. E. Sum and M. Skyllas-Kazacos, Journal of Power sources, 15(23), 179 (1985).7. M. Moore, et al., Sensitivity Analysis of Design Variables of an All-Vanadium Redox-

    Flow Battery. 2011: ECS.8. D. Aaron, et al., Journal of Power sources, 206, 450 (2012).9. E. Kjeang, et al., Journal of the American Chemical Society, 130(12), 4000 (2008).

    10. M. Kazacos and M. Skyllas-Kazacos, Journal of the Electrochemical Society, 136(9),2759 (1989).

    11. D. Chen, et al., Journal of Power sources, 195(7), 2089 (2010).12. M. Skyllas-Kazacos and C. Menictas. The vanadium redox battery for emergency

    back-up applications. 1997: IEEE.13. B. Sun and M. Skyllas-Kazacos, Electrochimica acta, 37(7), 1253 (1992).14. X. Li, et al., Energy Environ. Sci., 4, 1147 (2011).15. A. T. Yaching Tseng, Grim M. Grim, and Matthew M. Mench, in The 3rd Inter-

    national Forum on Multidisciplinary Education and Research Center for EnergyScience. 2010: Okinawa, Japan.

    16. D. Aurbach, Journal of Power sources, 89(2), 206 (2000).17. A. Z. Weber and J. Newman, Chemical Reviews, 104(10), 4679 (2004).18. D. Aaron, et al., Journal of Applied Electrochemistry, 41, 1 (2011).19. A. Turhan, A. B. Papandrew, G. Grim, T. A. Zawodzinski, and M. M. Mench, Analy-

    sis of Thermally-Treated Graphite Felt Electrodes for Vanadium Redox Flow Batter-ies, in 220th ECS Meeting and Electrochemical Energy Summit, T.E. Society, Editor.2011: Boston, U.S.

    20. S. Zhong, et al., Journal of Power sources, 45(1), 29 (1993).21. F. Mohammadi, et al., Journal of Power sources, 52(1), 61 (1994).22. D. You, H. Zhang, and J. Chen, Journal of Electroanalytical Chemistry, 625(2), 165