a review on methanol crossover in direct methanol fuel cells: challenges and achievements

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2011; 35:1213–1228

Published online 20 July 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1889

REVIEW PAPER

A review on methanol crossover in direct methanol fuelcells: challenges and achievementsMahmoud Ahmed1,*,† and Ibrahim Dincer2

1Department of Mechanical Engineering, Assiut University, Assiut 71516, Egypt2Faculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe Street North, Oshawa,Ontario L1H 7K4, Canada

SUMMARY

Direct methanol fuel cells have the potential to power future microelectronic and portable electronic devices because oftheir high energy density. One of the major obstacles that currently prevent the widespread applications of direct methanolfuel cells is the methanol crossover through the polymer-electrolyte membrane. Methanol crossover is closely related toseveral factors including membrane structure and morphology, membrane thickness, and fuel cell operating conditions suchas temperature, pressure, and methanol feed concentration. This work presents a comprehensive overview of the state-of-the-art technology for the most important factors, affecting methanol crossover in direct methanol fuel cells. In addition, thecurrent and future directions of the research and development activities, aiming to reduce the methanol crossover arereviewed and discussed in order to improve the performance of direct methanol fuel cells. Copyright # 2011 John Wiley& Sons, Ltd.

KEY WORDS

direct methanol fuel cells; methanol crossover; polymer-electrolyte membranes; methanol permeability

Correspondence

*Mahmoud Ahmed, Department of Mechanical Engineering, Assiut University, Assiut 71516, Egypt.†E-mail: [email protected]

Received 2 March 2011; Revised 28 May 2011; Accepted 28 May 2011

1. INTRODUCTION

Fuel cells are nowadays one of the most promising clean en-ergy technologies. Direct methanol fuel cells (DMFCs) havethe potential to power future microelectronic and portableelectronic devices. The direct methanol fuel cells offer someadvantages of effective operation at low temperatures, simpledesign, and environmentally-benign nature. Moreover, meth-anol is easier to handle because of its liquid nature at roomtemperature. It is of low cost and availability at industrial scaleas well as easy to store, and safe in use and delivery. Moreimportantly, unlike hydrogen polymer exchange membranefuel cells, DMFCs based on methanol aqueous solution donot need humidifying system and special thermal manage-ment ancillary devices. In addition, the power and energydensities are superior, even when compared with indirect fuelcell and newly developed lithium-ion batteries. Possibleapplications of the DMFC include video cameras, electricwheelchairs, portable-powered briefcases, notebook compu-ters, intelligent transportation systems (road signs, trafficlights, etc.), and military communications [1–3].

In general, a fuel cell works by converting chemical en-ergy of a fuel into electrical energy. DMFC, as shown in

Copyright # 2011 John Wiley & Sons, Ltd.

Figure 1, consists of a flow field (including collector ribsand channels), a diffusion layer (DL), and a catalyst layer(CL) on the anode and the cathode sides as well as apolymer-electrolyte membrane (PEM). Reactants enter thecell through the flow channels. The DL is typically madeof carbon cloth or paper, which not only tends to uniformlydistribute the reactants over the surface of the catalyst layers,but provides an electrical connection between the CL and thecurrent collector as well. Electrochemical reactions occur inthe catalyst layers, which are attached to both sides of themembrane. The catalyst layers must be designed in such amanner to facilitate the transport of protons, electrons, andreactants [4].

Typically, the liquid feed DMFC is operated at a tem-perature lower than 100�C. The methanol feed solution isforced to flow along the length of the flow channel andto penetrate the DL to arrive at the anode CL, where themethanol is electrochemically oxidized to form protons,electrons, and CO2. The produced CO2 in the CL, thenmoves backward through the DL to the flow channel, andit is swept by the stream of liquid solution towards the exitof the flow channel. The protons, formed at the anode CL,are transported through ion-conducting polymer within the

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Figure 1. Schematic representation of the direct methanol fuelcell (DMFC).

Methanol crossover in direct methanol fuel cellsM. Ahmed and I. Dincer

catalyst layers and the membrane to the cathode, wherethey react with oxygen and electrons transported via theexternal circuit to form water. The anode, cathode, andoverall cell reactions can be written as follows [5]:

Anode : CH3OHþ H2O ! 6Hþ þ 6e� þ CO2

Cathode : 1:5 O2 þ 6Hþ þ 6e� ! 3H2OOverall : CH3OHþ 1:5O2 ! 2H2Oþ CO2

The most pressing problem associated with DMFC is thatof fuel crossover because of permeate methanol together withwater from the anode to the cathode side. Methanol crossoverduring DMFC operation results in low power output becauseof chemical oxidation of methanol at the cathode with the helpof the cathode catalyst, causing (1) electrode depolarization,(2) mixed potential, resulting in the open-circuit voltage(OCV) of the DMFC below 0.8V (3) consumption of O2,(4) cathode catalyst poisoning by CO (an intermediate ofmethanol oxidation), and (5) serious water accumulation onthe cathode (water being produced by methanol oxidation),which limits O2 access to cathode catalyst sites. Moreover,the overall fuel utilization efficiency of the fuel cell is low-ered when there is excessive methanol crossover [6–8].

The objective of this review article is to present a compre-hensive overview of the state-of-the-art technology for themost important factors affecting methanol crossover inDMFCs. In addition, the current and future directions ofthe research and development activities that seek the

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reduction in the methanol crossover to improve the DMFCsperformance are reviewed and discussed comprehensively,and some potential solutions are also discussed.

2. CHALLENGES

Although considerable improvements in DMFCs design andcomponents have been made over the past years, numerouschallenging technical issues remain to be addressed beforethe widespread commercialization of this type of fuel cell.As previously reported, methanol crossover not only affectsthe performance of DMFCs, but influences the volumetricenergy density of DMFCs as well. The transport of protonstakes place with water molecules, and for high-proton conduc-tivity, PEM should be swollen enough with water, whereas itshould not for low methanol crossover. Methanol crossoveroccurs by the absorption of methanol bymembranes, diffusionthrough membranes, and desorption from membranes. How-ever, PEM, which satisfies both high-proton conductivityand low-methanol crossover for DMFC, is not easily devel-oped. Both proton conductivity and methanol permeabilityincrease with temperature. Concentration and rate of suppliedmethanol affect methanol crossover. Increasing membranethickness reduces methanol crossover and increases the flowresistance. Concentration and rate of supplied fuel/oxidantaffect methanol crossover [6,9]. These challenges and therespective efforts carried out to reduce methanol crossoverin DMFCs are discussed in the current manuscript.

3. METHANOL CROSSOVER

When methanol comes into contact with the membrane, itdiffuses through the membrane from anode to cathode. Itis dragged along with the hydrated protons under the influ-ence of electric current flowing across the cell. Hydraulicpressure can also result in methanol transport. Therefore,the methanol crossover through the membrane can bedriven by three transport mechanisms [10]: electro-osmoticdrag by proton transport, diffusion by methanol concentra-tion gradient, and convection by the hydraulic pressuregradient between the anode and the cathode. It wasreported that the diffusive mode of methanol transportdominates when the cell is idle, whereas the electro-osmotic drag dominates when the cell is operating [1].

When the convective and electro-osmosis drag modes areneglected, methanol crossover occurs only because of diffu-sion by methanol concentration gradient. In this case, themethanol crossover can be defined by using the simple formof Fick's law for diffusion across a membrane as follows:

j ¼ �DdC

dz(1)

where j is equal to the methanol flux, D is the diffusion coef-ficient, C is the concentration of methanol, and z is the positionwithin the membrane. This equation can be integrated to yield.

nt. J. Energy Res. 2011; 35:1213–1228 # 2011 John Wiley & Sons, Ltd.DOI: 10.1002/er

Methanol crossover in direct methanol fuel cells M. Ahmed and I. Dincer

j ¼ DH

LΔC (2)

Here, H is the partition coefficient (or the ratio of con-centration in the membrane to that in solution). L is thethickness of the membrane, and ΔC is the concentrationdifference between the solutions in contact with the mem-brane. Methanol permeability (DH) is defined as the prod-uct of the diffusion coefficient and partition coefficient andtakes into account both solubility and diffusion [11].

Direct methanol fuel cells are typically run under ambi-ent conditions with a little or no pressure difference acrossthe cell so that the hydraulic permeation can be ignored. Inthis case, the effect of convective transport through electro-osmotic drag is considered in estimating methanol cross-over. Equation (1) can be modified to incorporate the elec-tro-osmotic drag [1] as follows:

j ¼ �Ddc

dzþ c V (3)

where V is the convective velocity introduced by the pas-sage of current and the effects of electro-osmotic drag. Thisequation takes into account both diffusive and convectivecontributions. The value of V can be defined as follows:

V ¼ EDi M

θF

Here, ED is the electro-osmotic drag coefficient, i is thecurrent density, M is the methanol molar fraction at the in-terface between the anode CL, and the membrane, F isFaraday's constant, and θ is the volume fraction of metha-nol within the membrane.

When all methanol crossover mechanisms are consid-ered [10], the methanol crossover per unit area can be writ-ten as

j ¼ DHΔCL

þ C2KΔPmL

þ EDMi

F(4)

where ΔC represents the difference between methanol con-centration at the interface between the anode CL and themembrane and between the membrane and the cathodeCL, respectively; ΔP is the difference in the liquid pressureacross the membrane. In addition, there are other severalmathematical formulas that are used in estimating themethanol crossover [12–16].

Based on the above given equations (1–4), the rate ofmethanol crossover can be reduced by decreasing the dif-fusivity, increasing the membrane thickness or reducingthe methanol concentration at the anode CL. The methanolcrossover is closely related to the methanol permeability,and cell performance is directly related methanol cross-over. Although the exact relationship between permeabilityand methanol crossover is complex, several important fac-tors are controlling methanol crossover. These factors in-clude membrane material and morphology, membrane

Int. J. Energy Res. 2011; 35:1213–1228 # 2011 John Wiley & Sons, Ltd.DOI: 10.1002/er

thickness, and the cell operating parameters, such as tem-perature and methanol feed concentration. In the next sec-tion, the influences of these factors on methanol crossoverare reviewed based on the previous published data andtheir true interpretation.

4. FACTORSAFFECTINGMETHANOLCROSSOVER

4.1. Membrane material and morphology

Because the methanol crossover causes considerable cellvoltage losses in the DMFC, different approaches to mini-mize or eliminate methanol crossover have been carriedout such as: modification of Nafion membranes, usingnew alternative types of proton-conducting membranematerials, or exploring composite polymer materials.Table I presents the development polymer-electrolytemembranes for the DMFCs that have been reviewed anddiscussed in the present work. The reviewed polymer–electrolyte membranes are classified into four categoriessuch as modified Nafion membranes, copolymer membranes,blend membranes, and composite membranes.

Modified Nafion membranes. Nafion, a perfluorinatedsulfonic acid ionomer, has been the standard DMFC mem-brane. It has a highly phase separated morphology thatimpart excellent proton conductivity. On the contrary,using Nafion in DMFC applications causes a relativelyhigh methanol crossover. Therefore, many modificationsof Nafion have been attempted to reduce the methanolcrossover. These include: palladium (Pd) impregnated[17], laminating with poly(ethylene glycol)/silica (PEG/SiO2) [18], polymerization [19–22], deposit Pd films onthe surface by sputtering [23], incorporating nano-particles fillers into the polymers [24–26], casting[27–33], silica coating [34], intercalation [35], compositemembrane [36–40], blending membranes with tetraethoxyorthosilicate (TEOS) and a variety of organic silane[41,42], high temperature processing [43], partial substitu-tion of the sulfonic acid groups [44,45], and electrochem-ical modification [46,47]. Unfortunately, decreasingmethanol permeability was always accompanied by a lossin proton conductivity.

Copolymer membranes. Anumber of copolymer mem-branes have been synthesized to reduce methanol crossoverand improve proton conductivity for DMFCs via partiallysulfonated [48–50], direct copolymerization [51–61], sul-fonation [62–66], post sulfonation [67,68], the aromaticnucleophilic polycondensation [69–73], plasma polymeri-zation [74], and direct free radical polymerization [75].

Blend membranes. Blends research on polymer blendsincludes partially sulfonated polymer blend [79], poly(vinyl alcohol) and (sulfonated poly ether ether ketone)SPEEK [80–82], poly(vinyl alcohol) crosslinked with

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Table I. Developed polymer-electrolyte membranes for direct methanol fuel cells.

PEM Abbreviations Reference

Modified Nafion membranesPd nanophases in a Nafion polymer. Pd/ Nafion 17Organic–inorganic hybrid-laminated Nafion 115. Nafion115/Hy 18Nafion/polypyrrole composite. Nafion/P 19Nafion/sulfonated phenol formaldehyde resin composite. sPFR/Nafion 20Nafion/ crosslinked poly(2-Acrylamido-2-methyl-1-propanesulphonic acid). Nafion/AMPS 21Nafion/polymerized dopamine nano-layer. Nafion 22Pd-layered Nafion. Nafion117-Pd 23Nafion/(3-mercaptopropyl) methyldimethoxysilane nanocomposite. Nafion/MPMDMS 24Nafion/nano-size sulfated titanium dioxide solid superacid composite. Nafion/TiO2/SO4

2 25Nafion/2-acrylamido-2-methylpropanesulfonic acid -modified montmorillonite. AMPS-MMT/Nafion. 26Nafion–sulfonated organosilica nano-composite. Nafion/sDDS 27Composite silica/Nafion. SiO2/Nafion 28Polyimide/Nafion composite. PI/ Nafion 29Nafion/acid functionalized zeolite beta nanocrystals nanocomposite. Nafion/AFB 30Nafion/titanium dioxide Composite. Nafion/TiO2 31Nafion/Nano-sized Fe2O3-SO4

2 solid superacid composite. Fe2O3/SO42/Nafion 32

Porous layered oxide/Nafion nanocomposite. Nafion/APO 33Nano-silica layered Nafion composite. Nafion/silica 34Nafion/organo-clay nanocomposite. Nafion/clay 35Nafion/ternary composite materials. Nafion115/PI–PVA TSPS 36Nafion/sol-gel-derived alkaline phosphate glass. STP/ Nafion 37Nafion/zirconium meta-sulfonphenyl phosphonic acid-incorporated composite. Zr-msPPA/Nafion 38Nafion/polyphenylene oxide with phosphomolybdic acid (PMA) composite. Nafion/PPO-PMA 39Nafion/poly(diallyldimethylammonium)/PM multilayer films. PDDA/PM/ Nafion 40Blended Nafion/Sulfonated poly(ether ether ketone)s. Nafion/SPEEK 41Nafion/ORMOSIL hybrid. TEOS/DEDMS 42Heat-treated Nafion. HTN 43Nafion/sulfated b-cyclodextrin composite. Nafion/sb-CD 44Blended Nafion/sulfonated poly(ether ether ketone). Nafion/SPEEK 45Organic silica/Nafion composite. Nafion /SILCPM 46Nafion/polyaniline. Nafion /PA/PY 47

Copolymer membranesPartially sulfonated polystyrene block poly(ethylene -ran-butylene)block polystyrene copolymers.

SSEBS 48

Partially sulfonated block copolymers. SEBS 49Disulfonated poly(arylene ether benzonitrile) Copolymers. 6FCN-35/BPSH-40 50Sulfonated co-polyimide. Co-SPIs 51Sulfonated polyimides. SPI 52Sulfonated naphthalene dianhydride based polyimide copolymers. BAPS/ ODA 53sulfonated poly(phthalazinone ether sulfone). SPPES 54Sulfonated cardo poly(aryl ether sulfone). SPES-C 55Sulfonated naphthalenic polyimides. SPI-K 56Crosslinked sulfonated poly(arylene ether ketone) (SPAEKs). SPAEKs 57Epoxy-based cross-linked sulfonated poly(arylene ether ketone). Cr-SPAEK 58New comb-shaped poly(arylene ether sulfone) copolymers. S-PAES 59Crosslinked sulfonated poly(arylene ether sulfone). C-ABPSH40 60Sulfonated poly(arylene ether ketone) copolymers. C-SPAEK 61Sulfonated polyethersulfone Cardo. SPES-C 62A triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene). S-SIBS 63Sulfonated polyimide*. NTDA/DAPPS/ODA 64Sulfonated poly(styrene-b-butadiene-b-styrene) triblock copolymers. SBS (scSBS) 65Sulfonated poly(ether ether ketone). SPEEK 66Sulfonated poly(ether ether ketone) and crosslinked. SPEEK-C 67Aromatic Poly(ether ketone)s with Pendant Sulfonic Acid Phenyl. Me-SPEEKDK& Ph-SPEEKDK 68

(Continues)


1216 Int. J. Energy Res. 2011; 35:1213–1228 # 2011 John Wiley & Sons, Ltd.DOI: 10.1002/er

Table I. (Continued)


Polymerized polypyrrole on sulfonated poly(arylene ether ketone). SPAEK-C-(PPY/PWA)n 69Sulfonated poly(arylene ether sulfone) copolymer. BPSH 70Cross-linked sulfonated poly (arylene ether ketone). SPAEK-C/PVA 71polyaniline on sulfonated poly(arylene etherketone) SPAEK-C (PANI/PWA)n 72Cross-linked sulfonated poly(arylene ether ketone). C-SPAEK 73Sulfonated plasma polymerized. SPPM 74Zwitterionic crosslinked proton exchange. NaSS-4VP 75Sulfonated poly(styrene). SPS 76Sulfonated poly(arylene ether ether ketone ketone) copolymers. SPAEEKK 77Sulfopropylated ethylene-vinyl alcohol copolymers. s-EVOHs 78

Blend MembranesPartially sulfonated polymer blend. sPS/sPPO 79Poly(vinyl alcohol) and its ionic blends with sodium alginate and chitosan. PVA/SA/CS 80Poly(ether ether ketone)/Polyvinyl alcohol blend. SPEEK/PVA 81A polyvinyl alcohol/p-sulfonate phenolic resin PVA/SP 82Crosslinked poly(vinyl alcohol) containing sulfosuccinic acid. PVA/SSA 83Novel covalent-ionically cross-linked. C-SPEEK 84Polyelectrolyte complex (PEC). CS/(P(AA-AMPS) 85Poly(2,6-dimethyl-1,4-phenylene oxide)-based acid–base polymer blend. PPO 86Blends of ethylene-propylene-diene terpolymer (EPDM) and organophilized silicas. EPDM 87

Composite MembranesCrosslinked sulfonated poly(ether ether-ketone) /silica hybrid. SPEEK/SiO2 88Poly(vinyl alcohol) sulfosuccinic acid silica hybrid. PVA/SiO2 89Poly(vinyl alcohol) with embedded phosphotungstic acid. PVA/PWA 90Chitosan/titanate Nanotube Hybrid. CS/TNT 91a-zirconium phosphate and silicotungstic acid. PVA/ZrP/SWA 92Inorganic–organic hybrid–poly siloxane/poly (mal eicimide-co-styrene) network. AESA-Na 93Poly(vinyl alcohol)/sulfated b-cyclodextrin. PVA/sulfated -cyclodextrin 94Zeolite beta-filled chitosan. CS/Beta/SO3H 95Poly(vinyl alcohol) with polyimide/ 8-trime thoxy- silyl pro pyl glycerine ether-1,3,6-pyrenetrisulfonic acid composite.

PI-PVA-TSGEPS 96

Organic-inorganic hybrid alkaline by epoxide ring. AEMs s 97Polyvinyl alcohol pervaporation. PVA 98Sulfonated poly(styrene-ran-ethylene) and porous polyimide matrix. PI/SPES 99Self-cross-linkable sulfonated poly(ether ether ketone). SPEEK 100Phosphorylated titanate nanotubes embedded nanocomposite. CS/PTNT 101Solid superacid embedded chitosan hybrid. CS/STiO2 102Organic–inorganic hybrid. CS/silica 103Sulfonated poly(ether ether ketone) /tung stopho sphoric acid/ mesoporousmaterialcomposite.

SPEEK/TPA/MCM-41 104

sulfonated mesoporous benzene-silica incorporated poly(ether ether ketone)composite.

sPEEK-OMB 105

Poly(vinyl alcohol)-based hybrid Nanocomposite. PVA/SSA/sPBS 106Sulfonated poly(ether ether ketone) sub-layer and polyvinyl alcohol sub-layer. SPEEK/ PVA 107Functionalized carbon nanotube-poly(arylene sulfone) composite. PtRu/CNT-sPAS 108Sulfonated polystyrene/polytetrafluoroethylene composite. SPS/PTFE 109Dodecylbenzene sulfonic acid-doped/ polyethylene glycol/SiO hybrid. DBSA-PEG / SiO2 110Chitosan-Poly (Vinyl Alcohol) and Calcium Oxide Composite. CPV 111Macroporous polyimide composite. PAMPS 112A cross-linked hybrid. SPAEK 113Sulfonated and hydrogenated styrene/butadiene blocks copolymerand plasma-treated microporous polyethylene.

PE-SHSBS 114

Benzimidazole grafted poly(ether ether ketone and sulfonated poly(etherether ketone) composite.

SPEEK-PEEK-BI 115

SPEEK/PVDF/PWA 116

(Continues)


1217Int. J. Energy Res. 2011; 35:1213–1228 # 2011 John Wiley & Sons, Ltd.DOI: 10.1002/er

Table I. (Continued)


Sulfonated poly(ether ether ketone), poly(vinylidene fluoride) andphosphotungstic acid.

Nitrated sulfonated poly(ether ether ketone). Nitrated SPEEK 117Sulphonated tetramethyl poly(ether ether ketone)/epoxy/sulphonatedphenol novolac semi-IPN.

STMPEEK/TMBP/PNBS 118

*polymerPEM, polymer-electrolyte membrane.


sulfosuccinic acid [83], and crosslinked (sulfonated polyether ether ketone) [84].

Composite membranes. A number of composite mem-branes have been synthesized to reduce methanol crossoverand improve proton conductivity for DMFCs such as inor-ganic–organic material [88–97], composite with organic[98–100], composite with inorganic [101–104], nanocom-posite [105,106], and multilayer composite [107].

4. 2. Transport properties of membranes

An electrolyte with high proton conductivity and lowmethanol crossover is the key element of direct methanolfuel cells. To gain understanding of the methanol crossoverprocess in PEMs, transport phenomena in membranes mustbe considered. Therefore, in this section transport proper-ties related to methanol crossover such as transport of pro-ton (proton conductivity) and transport of methanol(methanol permeability) are discussed. As previouslyreported, the reviewed membranes are classified into fourcategories such as modified Nafion membranes, copolymermembranes, blend membranes, and composite membranes.

4. 2. 1. Modified Nafion membranesFigure2(a). shows the results of proton conductivity and

methanol permeability of modified Nafion membranes forDMFCs as listed in Table I. Themost favorably industrial re-gionlies to thetopleftofFigure2(a) (i.e., high proton conduc-tivity and low methanol permeability). Based on the figure,a significant achievement was observed in both proton con-ductivity and methanol permeability. The protonconductivityimproved from 0.0032Scm-1 (point #1) for Pd nanophasesin a Nafion polymer [28] to 0.117Scm-1 (point #30) forNafion/porous layered oxide nanocomposite [33], that is,increasing by about 36 times, which is remarkably higherthan that of 0.026Scm-1 for Nafion 117. In addition, themeth-anolpermeabilitydecreasesfrom7.26�10-6cm2s-1 (point #7)for Nafion/nano-size sulfated titanium dioxide solidsuperacid composite [25] to 0.0217�10-6cm2s-1 (point #2)for organic–inorganic hybrid-laminated Nafion 115 [18],that is, decreasingbyabout333 times.AworkonNafion/Zirco-niummeta-sulfonphenyl phosphonic acid-incorporated com-posite membrane with different concentration percentage of

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zirconium meta-sulfonphenyl phosphonic acid (Zr-msPPA)[38], and Nafion-infiltrated composite membrane [29]reports conductivities of the same order of magnitude asNafion 117 and significant reduction of methanol perme-ability. These works are shown as (points #17, and #27)in Figure 2(a). The reduction in methanol permeability isattributed to the nano crystalline layer-developedinterior of the Nafion 117 membrane [38]. The precedingdiscussion highlights the range of transport properties ofmodified Nafion membranes reported in previous literatures.

Figure 2(b) shows the variation of selectivity versusproton conductivity of modified Nafion membranes forDMFCs as listed in Table I. The most favorably industrialregion lies to the top right of Figure 2(b) (i.e., high protonconductivity and selectivity). The selectivity parameterdetermines the ratio of proton conductivity to methanolpermeability of a membrane and the higher selectivityvalue leads to a better membrane performance. Based onthe figure, the maximum selectivity value of 170�104

Sscm�3 was achieved by both Nafion/zirconium meta-sulfonphenyl phosphonic acid-incorporated [38], andNafion-infiltrated composite membranes [29] which is sig-nificantly (43 folds) higher than that of 4.0�104Sscm-3 forNafion117. It is followed by Nafion/2-acrylamido-2-methylpropanesulfonic acid-modified mont-morillonitemembrane (AMPS-MMT/N) [26]. It was found that selec-tivity value of about 90�104Sscm-3 is achieved at 3.0wt.%of AMPS-MMT. In fact, such a remarkably improved se-lectivity parameter of Nafion matrices is owing to the ef-fective role of AMPS-modified nanolayers in restrictingmethanol permeation while maintaining essential protonconduction properties.

4. 2. 2. Copolymer membranesFigure 3(a) shows the results of proton conductivity and

methanol permeability of copolymer membranes forDMFCs as documented in Table I. Based on the figure, asignificant achievement was observed in both proton con-ductivity and methanol permeability for different copolymermembranes. It was found that the proton conductivityimproved from 0.00025Scm-1 (point #4) for sulfonatedpolyethersulfone Cardo [62] to 0.181Scm-1 (point #13)for sulfonated plasma polymerized [74], that is, increasingby about 724 times, which is significantly higher than thatof 0.026Scm-1 for Nafion117. In addition, the methanol


Figure 2. (a) Variations of the measured proton conductivity ver-sus methanol permeability for modified Nafion membranes. (b)Variation of selectivity (conductivity/permeability) versus protonconductivity for modified Nafion membranes from the litera-tures: ( ) Pd/Nafion [17], ( ) Nafion115/Hy [18], ( ) Nafion/P [19], ( ) Nafion117-Pd [23], ( ) Nafion/sDDS [27], ( ) N/MPMDMS [24], ( ) TiO2/SO4

2[25], (X) SiO2/N [28], (+) sPFR/Nafion [20], (�) Nafion/silica [34], ( ) Nafion/PPO-PMA [39],( ) Nafion/AMPS [21], ( ) Nafion/clay [35], ( ) N/SPEEK[41], ( ) TEOS/DEDMS [42], ( ) HTN [43], ( ) PI/Nafion [29],(*) N/SILCPM [46], (^) Nafion/AFB [30], (&) Nafion [22], ( )Nafion/TiO2 [31], ( ) Nafion/sb-CD [44], ( ) AMPS/MMT/N[26], ( ) PDDA/PM/Nafion [40], ( ) STP/Nafion [37], ( )Fe2O3/SO4

2/Nafion [32], ( ) Zr-msPPA/Nafion [38], (#) Nafion /PA/PY[47], ($) N/SPEEK [45], (C) N/APO [33], (@) N115/PI–PVA–

TSPS [36].

Figure 3. (a) Variations of the measured proton conductivity ver-sus methanol permeability for copolymer membranes. (b)Variation of selectivity (conductivity/permeability) versus protonconductivity for copolymer membranes from the literatures:( ) SSEBS [48], ( ) Co-SPIs [51], ( ) SPI [52], ( ) SPES-C[62], ( ) S-SIBS [63], ( ) SPS [76], ( ) SEBS [49], ( ) BAPS/ODA [53], ( ) 6FCN-35/BPSH-40 [50], ( ) NTDA/DAPPS/ODA[64], ( ) SPAEEKK [77], ( ) SPAEK-C-(PPY/PWA)n [69], ( )SPPM [74], ( ) SBS (scSBS) [65], ( ) SPEEK [66], ( ) BPSH[70], ( ) SPPES [54], ( ) SPEEK-C [67], ( ) NaSS-4VP [75],( ) SPES-C [55], ( ) SPI-K [56], (a) SPAEKs [57], (C) SPAEK-C/PVA [71], (@) SPAEK-C (PANI/PWA)n [72], (#) Cr-SPAEK [58],($) C-SPAEK [73], (+)s-EVOHs [78], (*) S-PAES [59], (&)Me-SPEEKDK & Ph-SPEEKDK [68], (^)C-ABPS H40 [60], (X)

C-SPAEK [61].


permeability decreases from 6.46�10-6cm2s-1 (point #19)for zwitterionic crosslinked proton exchange [75] to0.0008�10-6cm2s-1 (point #3) for sulfonated polyimides[52], that is, decreasing by about 8100 times. A work onpolyaniline on sulfonated poly(arylene ether ketone) with


thin polyaniline (PANI) and phosphotungstic acid (PWA)multilayers coated [72], Polymerized polypyrrole on sulfo-nated poly(arylene ether ketone) by deposit polypyrrole(PPY) and phosphotungstic acid (PWA) layer by layer[69], and Sulfonated Poly(arylene ether ether ketone ketone)

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Figure 4. (a) Variations of the measured proton conductivity ver-sus methanol permeability for blends membranes. (b) Variationof selectivity (conductivity/permeability) versus proton conduc-tivity for blends membranes from the literatures: ( ) PVA/SSA[83], ( ) sPS/sPPO [79], ( ) PVA/SA/CS [80], ( ) CS/(P(AA-AMPS) [85], ( ) PPO [86], ( ) C-SPEEK [84], ( ) SPEEK/PVA

[81], ( ) EPDM [87], ( ) PVA/SP [82].


copolymers with phthalazinone [77] reports conductivitiesof the same order of magnitude as Nafion and a significantreduction of methanol permeability. These works areshown as (points #24, #12, and #11) in Figure 3(a). The re-duction in methanol permeability is attributed to the thinpolyaniline, and phosphotungstic acid multilayers coated[72], and deposit polypyrrole and phosphotungstic acidlayer by layer [69]. The above discussion presents theavailable range of transport properties of copolymer mem-branes reported in previous literatures.

Figure 3(b) shows the variation of selectivity versusproton conductivity of copolymer membranes for DMFCsas listed in Table I. Based on the figure, several copolymermembranes achieved higher values of selectivity, the max-imum selectivity value of 311�104Sscm�3 was achievedby Sulfonated polyimides with sulfonation level 16mol %(SPI). However, the proton conductivity is one order of mag-nitude less than that of Nafion 117 [52]. It is followed byPolymerized polypyrrole on sulfonated poly (arylene etherketone) [69] with selectivity of 247�104Sscm�3, and sul-fonated plasma polymerized [74] with selectivity of 243�104Sscm�3. In addition, it was found that selectivity valueof about 202�104Sscm�3 is achieved by polyaniline onsulfonated poly(arylene ether ketone) [72].

4. 2. 3. Blends membranesFigure 4(a). shows the results of proton conductivity

and methanol permeability of blends and composite blendsmembranes for DMFCs as listed in Table I. Based on thefigure, a significant achievement was observed in both pro-ton conductivity and methanol permeability. The protonconductivity improved from 0.002Scm-1 (point#7) forpoly(ether ether ketone)/polyvinyl alcohol blend [81] to0.11Scm-1 (point #1) for crosslinked poly(vinyl alcohol)containing sulfosuccinic acid [83], that is, increasing byabout 55 times, which is remarkably higher than that of0.026Scm-1 for Nafion117. In addition, the methanol per-meability decreases from 11.78�10-6cm2s-1 (point # 7)for poly(ether ether ketone)/polyvinyl alcohol blend [81]to 0.058�10-6cm2s-1 (point # 6) for Novel covalent-ionically cross-linked [84], that is, decreasing by about203 times. A work on Poly(2,6-dimethyl-1,4-phenyleneoxide)-based acid–base polymer blend [86], and polyelec-trolyte complex (PEC) [85], reports conductivities of thesame order of magnitude as Nafion and a significant reduc-tion of methanol permeability. These works are shown as(points #5, and #4) in Figure 3(a). The reduction in meth-anol crossover is attributed to the solution ofbromomethylated-poly (2,6-dimethyl-1,4-phenylene oxide)(BrPPO) [86]. The above reported discussion emphasizesthe range of transport properties of modified Nafion mem-branes reported in previous literatures.

Figure 4(b). shows the variation of selectivity versusproton conductivity of blends and composite blends mem-branes for DMFCs as listed in Table I. The selectivity pa-rameter determines the ratio of proton conductivity tomethanol permeability of a membrane and the higher selec-tivity value leads to a better membrane performance. Based

1220 I

on the figure, the maximum selectivity value of 81�104

Sscm�3 was achieved by Poly (2,6-dimethyl-1,4-pheny-lene oxide)-based acid–base polymer blend [86], which issignificantly higher than that of 4�104Sscm�3 forNafion117. In fact, such a remarkably improved selectivityparameter is owing to the effective role of (BrPPO) inrestricting methanol permeation while maintaining essen-tial proton conduction properties.

4. 2. 4. Composite membranesFigure 5(a) shows the results of proton conductivity and

methanol permeability of composite membranes forDMFCs as reported in Table I. Based on the figure, a sig-nificant achievement was noticed in both proton conductiv-ity and methanol permeability. The proton conductivity


Figure 5. (a) Variations of the measured proton conductivity ver-sus methanol permeability for composite membranes. (b) Varia-tion of selectivity (conductivity/permeability) versus protonconductivity for composite membranes from the literatures:( ) SPEEK/SiO2 [88], ( ) PVA/SiO2 [89], ( ) CS/(P(AA-AMPS)[108], ( ) SPS/PTFE [109], ( ) DBSA-PEG / SiO2 [110], ( )PVA/PWA[90], ( ) PVA [98], (X) CS/PTNT [101], ( ) CS/STiO2

[102], ( ) CS/silica [103], ( ) PI/SPES [99],( ) CS/TNT [91],( ) sPEEK-OMB [105], ( ) PVA/SSA/sPBS [106], ( ) SPEEK/PVA [107], (+) CPV [111], () SPEEK/TPA/MCM-41[104], () PAMPS[112], () PVA/ZrP/SWA [92], ( ) AESA-Na [93],( ) SPEEK [100],( ) SPAEK [113], (@) PVA/sulfated b-cyclodextrin [94] ($) PE-SHSBS [114], (a) SPEEK-PEEK-BI [115],(C) SPEEK/PVDF/PWA[116],(#) CS/Beta/SO3H [95], (&) Nitrated SPEEK [117], (^)STMPEEK/TMBP/PNBS [118], (*) PI-PVA-TSGEPS [110], (�)

AEMss [97].


improved from 0.00013Scm-1 (point #7) for polyvinylalcohol pervaporation [98] to 0.25Scm-1 (point #19) formacroporous polyimide composite [112], that is, increas-ing by about 1920 times, which is significantly higher thanthat of 0.026 Scm-1 for Nafion117. In addition, the metha-nol permeability decreases from 12�10-6cm2s-1 (point # 15)


for sulfonated poly (ether ether ketone) sub-layer and poly-vinyl alcohol sub-layer [107] to 0.009�10-6cm2s-1 (point#7) for Polyvinyl alcohol pervaporation [98], that is, de-creasing by about 1333 times. A work on Sulfonated poly-styrene/polytetrafluoroethylene composite (SPS/PTFE)[109], Sulfonated poly(styrene-ran-ethylene) and porouspolyimide matrix (PI/SPES) [99], and Sulfonated and hy-drogenated styrene/butadiene block copolymer andplasma-treated microporous (PE-SHSBS) [114] reportsconductivities of the same order of magnitude as Nafionand a significant reduction of methanol permeability.These works are shown as (points #4, points #11, and#24) in Figure 5(a). The reduction in methanol permeabil-ity in (SPS/PTFE) is due to increasing DVB (a crosslinker)content. Increasing crosslinking density results in the in-crease of the diffusion resistance of methanol over water,finally, resulting in the suppression of methanol perme-ation or methanol crossover [109].

Figure 5(b) shows the variation of selectivity versusproton conductivity of composite membranes for DMFCsas listed in Table I. The selectivity parameter determinesthe ratio of proton conductivity to methanol permeabilityof a membrane. Based on the figure, the maximum selec-tivity value of 73�104Sscm�3 was achieved by sulfonatedpoly(styrene-ran-ethylene) and porous polyimide matrix[99], which is significantly higher than that of 4�104

Sscm�3 for Nafion 117. It is followed by sulfonatedpolystyrene/polytetrafluoroethylene composite membrane[109], where the selectivity value is about 62�104

Sscm�3. On the other hand, the ratio of styrene/divinylben-zene (styrene/DVB) of 85/15 attains the lowest methanolpermeability. In fact, such a remarkably improved of selec-tivity parameter is due to the sharp decrease in methanolpermeability with increasing crosslinking density. It wasfound that crosslinking is restricting methanol permeationwhile maintaining essential proton conduction properties[109].

To conclude based on the above discussion, there is asignificant potential for the development of membranearchitectures that can allow more independent control overthe proton conductivity, and methanol permeability charac-teristics. Consequently, achieving of highly selectivePEMs is very encouraging because PEMs with greater se-lectivity have potential for improved DMFC performance.It is worth mentioning that just because selectivity of aPEM is higher than Nafion, does not mean improvedDMFC performance can be achieved. One limitation of se-lectivity as a gauge of DMFC performance is due to thefact that a minimum conductivity is still required forDMFC operation, regardless of how low methanol perme-ability is.

4.3. Membrane thickness

It was reported that the thickness of the Nafion membraneis one of the most essential factors in determining themethanol crossover rate. The methanol crossover decreaseswith the increase of thickness, whereas increasing the

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Figure 6. Variation of Methanol permeability versus Methanolconcentration (M) for different types of membranes reported inthe literatures: ( ) SPEEK/SPVA [123] ,( ) SMMT/SPEEK[124], ( ) SPEEK/ PVA [107], ( ) PAMPS[112], ( ) Nafion/hydroxyapatite composite [125], ( ) CS/Silica [103], ( ) Nafion[22], ( ) PVA/sulfated b-cyclodextrin [94] ,( ) CS/Beta/SO3H[95],( ) Nafion 115 [94], ( ) Nafion117 [95], ( ) Co-SPIs[51], ( ) NTDA–BAPBDS/DABI [126], ( ) PVA/PSSA–MA [127].


thickness reduces the power density because of it highohmic loss [119]. However, this trend changes at the highestcurrent densities because membrane resistance to transportphenomena became dominant. This fact is related to the re-duction of resistance of charge transfer from the anode sideto the cathode side, and the reduction of concentration polar-ization in the polymer with the decreasing of membranethickness. Thus, the electrical performance of fuel cell withmembranes of various thicknesses shows that the perfor-mance trend is probably determined by the combined effectsof proton conductivity and fuel crossover. Something similaroccurs with the influence of the equivalent weight in theperformance of the cell. But at highest density currents, theelectrical performance of a DMFC may be decrease whenthe equivalent weight of the membrane decreases [120].

It was found for composite Nafion-based membranesthat both the conductance and the methanol permeationrate show a significant decrease with increasing thickness.The optimization of these parameters regarding favorableproperties is difficult. Moreover, for thinner membranes,the permeation rate changes by a factor of four to five witha temperature change from 25 to 65�C, whereas in thethicker membranes this factor is about three. At lowertemperatures, the variation of the methanol flux throughthe membranes versus thickness is small compared withthat at 65�C. Also, a strong decrease of permeation withthickness is observed [121]. It was found that the methanolcrossover limiting currents significantly decreases with themembrane thickness for disulfonated polyarylene etherbenzonitrile copolymers and Nafion at 80�C [122].

4.4. Methanol concentration

In fact, a higher methanol concentration is desirable inDMFCs because of higher energy density. Therefore,many researchers are investigated the influence of metha-nol concentration on methanol crossover to the cathodeside. Figure 6 shows the variation of methanol permeabil-ity versus methanol concentration (M) for different typesof membranes as reported in previous literatures. It wasfound that the methanol permeability of sulfonated polyether ether ketone [123], sulfonated poly ether ether ketone(SPEEK55) [107], Nafion [22], Nafion 115 [94], andNafion117 [95] increases with the increase of methanolconcentration. However, the methanol permeability of sul-fated poly (vinyl alcohol) decreased as the concentration ofmethanol increases. Also, adding sulfated poly (vinyl alco-hol) to sulfonated poly(ether ether ketone) membranes cansuppress the methanol crossover especially at high metha-nol concentration [123,107]. The methanol permeabilitydecreases with increasing methanol concentration of theacid treated sulfonated montmorillonite / sulfonated poly(ether ether ketone) nanocomposite membranes [124].

A work on three-dimensionally ordered macroporous(3DOM) polyimide matrix composite [112] indicated thatthe methanol permeability decreased with increasing meth-anol concentration. It was found that by applying the 3DOMmatrix, the methanol permeability of 2-acrylamido-2-

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methylpropanesulfonic acid polymer (PAMPS) was reducedbecause of its effect on the size and distribution of hydro-philic channels responsible for methanol permeation. Also,the permeability of Nafion/hydroxylapatite composite mem-brane [125], and chitosan/silica [103] decrease by the in-crease of methanol concentration. Work on poly (vinylalcohol)/sulfated b-cyclodextrin [94], and chitosan/Beta/SO3H [95] indicated that a reduction of methanol cross-over was observed, as methanol concentration increased.This is attributed to the excellent methanol resistance ofhydroxyls in poly(vinyl alcohol) [94]. The incorporatedzeolite beta particles can extend the diffusion path lengthof methanol in the membrane, rigidify the chitosan chains,and compress the volumes among chitosan chains. Thesereasons caused the decrease in methanol crossover ofchitosan/Beta hybrid membranes [95].

A work on 1,4,5,8-naphthalenetetracarboxylic dianhy-dride (NTDA), based sulfonated polyimides (SPIs) [126]shows the methanol permeability decreases with increasingmethanol concentration. Also, the methanol permeabilityof poly(vinyl alcohol)/poly(styrene sulfonic acid-co-maleicacid) (PVA/PSSA–MA) membrane [127] is at least seventimes lower than those of Nafion 117 in all methanolconcentration ranges. In addition, the increase of methanolconcentration leads to a slight decrease of methanol perme-ability through PVA/PSSA–MA membrane. On the otherhand, in the work on sulfonated co-polyimides (co-SPIs),the methanol permeability is hardly depended on methanolconcentration [51].

In summary, the methanol permeability increases with in-creasing the methanol concentration for SPEEK [123], and



Nafion [22,94,95] membranes. However, a significantdecrease in methanol crossover was reported by increasingthe methanol concentration for SPEEK/SPVA [123],SMMT/SPEEK [124], SPEEK/ PVA [107], PAMPS[112], Nafion/ hydroxyapatite composite membrane[125], CS/Silica [103], PVA/sulfated b-cyclodextrin [94],CS/Beta/SO3H [95], NTDA–BAPBDS/DABI [126], andPVA/PSSA–MA [127]. No significant change of methanolpermeability by increasing methanol concentration forCo-SPIs [51] was observed.

Figure 7 shows the variation of methanol crossoverlimiting current density (mAcm-2) versus methanol con-centration (M) for different types of membranes. For thePd/Nafion 117 membranes, the methanol crossover limit-ing current results at 20�C (point # 1), and 60�C (point#2) with 1, 2, and 5M methanol solution are shown inthe figure . It was found that methanol crossover increasedwith the increase of methanol concentration because ofhigher concentration gradient. In addition, the value ofmethanol crossover limiting current density increased withincreasing cell temperature. This is caused by a severemembrane swelling with the higher temperature methanolsolution. It was noticed that methanol crossover was sup-pressed by incorporating of Pd into the membrane, andlower crossover was seen with a higher Pd loading mem-brane [128]. More importantly, the methanol crossover ofNafion 117 membrane with 1 and 5Mmethanol was signif-icantly reduced by impregnating Pd.

A work on a pore-filling (PF) polymer electrolyte mem-brane (point # 3) shows that the methanol crossoverthrough Nafion117 membrane increased with methanolconcentration, whereas methanol crossover through PFmembrane saturated at high methanol concentration. This

Figure 7. Variation of Methanol crossover current versus Meth-anol concentration (M) for different types of membranesreported in the literatures: ( – ) Pd/Nafion [128], ( ) pore-filling membranes [129], ( ) Pd-Cu[130], ( ) Nafion [131],( – ) Nafion/Silica Hybrid [132], ( - ) PBI/PVPA [133].


is because the swelling of the filling polymer electrolyteof the PF membrane was prevented, and the solvent con-tent of the PF membrane was kept lower than that ofNafion117 membrane [129]. A work on Pd and Pd–Cualloy deposited Nafion membranes indicated that themethanol crossover current is significantly greater forunmodified membrane (point #4), and increases with in-creasing methanol concentration. The difference inmethanol crossover current density is more pronouncedbetween the Pd-sputtered and bare Nafion membraneswith increasing concentration. This is indicating theeffective suppression of methanol crossover by the Pd-sputtered membrane for higher methanol concentrations.The possible mechanism leading to the lower methanolcrossover rate in the Pd and Pd–Cu alloy sputteredmembranes is attributed to the effect of the coated Pdor Pd–Cu alloy, which is physically blocked by the pas-sage of methanol molecules [130].

For Nafion 112 and 1135 membranes (point #5) at60�C, the crossover current density is approximately line-arly proportional to the methanol concentration, with theNafion 112 membrane featuring higher crossover rate.The difference in crossover current density between thetwo membranes diminishes with methanol concentration;for example, the difference decreases from 32mAcm-2 at2M to11mAcm-2 at 4M. Thicker membranes have highercell internal resistance but lower methanol crossover.Therefore, the cell using Nafion 1135 has the best electro-chemical performance, and its power density is insensitiveto the anode stoichiometry [131].

The methanol crossover currents density at 25�C (point#6), and 80�C (point #7), measured with Nafion/silicahybrid membranes as a function of methanol concentra-tion are shown in the figure. It was found that the methanolcrossover current increases with increasing methanol con-centration [132]. The methanol crossover rate for celtec-Vwas measured at different methanol feed concentrationsand cell temperatures at 60�C (point #8), and 90�C (point#9). Methanol permeation increases in an approximatelylinear correlation with the methanol concentration. Also,methanol crossover increases with the increase of thetemperature. This is of utmost importance from a perfor-mance point of view. Furthermore, a lower crossover rateis associated with higher fuel efficiency for the same con-centration or a gain in volumetric energy density of thefuel. Comparing the methanol crossover with Nafion,Nafion shows significantly large values [133].

The relationship between limiting current density andmethanol concentration can be estimated from thefollowing equation JI ¼ 6FD

L

� �Cin . Where JI is a steady

state limiting current density; Cin is the methanol concen-tration at the feed edge. This equation was developed byassuming that the methanol crossover is only because ofdiffusion [10]. Based on the above equation, the variationof limiting current density is linearly increased with themethanol concentration, if the diffusion coefficient is con-stant. However, increasing the temperature leads to an

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increase of diffusion coefficient. This will result in increasingthe limiting current density as the temperature increases.

4.5. Fuel cell temperature

Figure 8 shows the methanol permeability as a function oftemperature for different types of membranes reported inthe literatures: Nafion 117 and Phosphonated polypho-sphazene [134], sulfonated poly(arylene ether sulfone) co-polymer (BPSH) [70], sulfonated polyethersulfone Cardo(SPES-C) [62], Sulfonated poly(ether ether ketone)(SPEEK) [66], sulfonated poly(styrene) (SPS) [76], sulfo-nated poly(styrene-isobutylene-styrene) (S-SIBS) [63],sulfonated co-polyimide (Co-SPIs) [51], Nafion112 [51],partially fluorinated ionomeric [135], sulfonated poly(ary-lene ether sulfone)–silica nanocomposite [136], sulfonatedpoly(phthalazinone ether sulfone) (SPPES) [54], and sulfo-nated poly(ether ether ketone)/poly(vinylidene fluoride)/phosphotungstic acid (SPEEK/PVDF/PWA) [116]. Basedon the figure, the methanol permeability of all membranesincreases with the increase of temperature. In addition, anArrhenius-type dependency of methanol permeability ontemperature exists for all these membranes. It is worth not-ing that the methanol permeability of the SPES-C [62],SPS[76], S-SIBS[63], membranes are considerably smallerthan that of both Nafion 117, and Nafion 112 membraneover the temperature range measured. We also observedthe similar results of methanol permeability in SPEEK[66], SPPES [54], and SPEEK/PVDF/PWA membranes[116]. It is important to note that the methanol permeabilityof these membranes is considerably smaller than that of

Figure 8. Variation of methanol permeability versus cell temper-ature for different types of membranes reported in the litera-tures: ( ) Nafion 117 [134] ,( ) Phosphonatedpolyphosphazene [134], ( ) BPSH [70], ( ) SPES-C [62],( ) SPEEK [66], ( ) SPS [76], ( ) S-SIBS [63], ( ) Co-SPIs[51], ( ) Nafion 112 [51], ( ) Partially Fluorinated Ionomeric[135], ( ) Sulfonated oly(arylene ether sulfone)–silica nanocom-posite [136], ( ) SPPES[54], ( ) SPEEK/PVDF/PWA [116].

1224 I

Nafion 117, and Nafion 112 membranes over the tempera-ture range 25–80�C. The different methanol permeation ofthese membranes and Nafion membranes can be explainedby the difference in their microstructures.

To conclude, the methanol permeability increases withthe increase of temperature, and an Arrhenius-type depen-dency of methanol permeability on temperature exists forall these membranes. The methanol permeability of SPES-C, SPS, S-SIBS, SPEEK, SPPES, and SPEEK/ PVDF /PWA is much less than that of Nafion.

5. ACHIEVEMENTS

Although great improvements in fuel cell design and com-ponents have been made over the past years, several issuesremain to be addressed before PEM fuel cells can becomecompetitive enough to be used commercially. Polymer-electrolyte membranes in DMFC should transport protonsas an electrolyte and prevent the methanol crossover. Toreduce methanol crossover, different approaches are fol-lowed. The most important approaches considered in thismanuscript are (1) to modify the Nafion membrane, (2) todevelop alternative membranes, or (3) exploring compositepolymer materials.

1. A work on modified Nafion membranes such as Zir-conium meta-sulfonphenyl phosphonic acid (Zr-msPPA)/Nafion composite membranes showed aconsiderable reduction of methanol permeabilitywith increasing Zr-msPPA content. This is becauseof the Zr-msPPA nano conductors acted as crystal-line barriers to methanol permeation [137]. However,the proton conductivity also decreased with increas-ing Zr-msPPA content, but its effect was not as con-siderable as with methanol permeation because of theinherent, high conductivity of Zr-msPPA. The meth-anol permeability reduced by two orders of magni-tude (48 times) compared with that of Nafion117,whereas the proton conductivity is at the same orderof magnitude as Nafion117 [38]. Moreover, the poly-imide/Nafion (PI/Nafion) composite membrane has avery low methanol crossover compared with aNafion 112 membrane because of the fact that thecomposite membrane mechanically prevents Nafionfrom swelling and consequently reducing the metha-nol crossover. The methanol permeability is eightytimes lower than that of Nafion 112. In addition,the proton conductivity is the same as Nafion 112.It was reported that there is a significant improve ofthe performance of a DMFC with PI/Nafion compos-ite membrane because of the use of a very thin mem-brane and a high methanol concentration [97].

2. A work on alternative membranes such as polymer-ized polypyrrole on sulfonated poly (arylene etherketone) SPAEK-C-(PPY/PWA)n [20] shows a con-siderable decrease of methanol permeability with in-creasing the number of bilayers compared with the


Table II. Transport properties and selectivity of the modifiedpolymer-electrolyte membranes for direct methanol fuel cells.

PEM Relativeconductivity

Relativepermeability

Relativeselectivity

Reference#

(Zr-msPPA)/Nafion

0.984–0.45 0.08–0.02 12.3–22.5 38

(PI/Nafion)* 0.95 0.012 78 97SPAEK-C-(PPY/PWA)2–10

0.91–0.24 0.0313–0.0073

28.8–32.6 69

SPPM 1.17 0.0358 32.5 74SPAEK-C-(PANI/PWA)5

1.224 0.046 26.6 72

PI/SPES** 1.47 0.05 29 99PPO 0.77 0.047 16.3 86

*Reference values of Nafion 112.**Reference values of Nafion 1135.PEM, polymer-electrolyte membrane.For the list of the definitions for the aforementioned items, pleaserefer to Table 1.


Nafion 117. It is reported that the SPAEK-C mem-branes having two regions hydrophilic one, and hy-drophobic one. Methanol diffuses primarily throughthe hydrophilic water-rich regions. When the surfaceof SPAEK-C is coated by the polypyrrole (PPY) andphosphotungstic acid (PWA) films, the electrostaticinteractions between the polyacid/poly base pairsblock themethanol transport pathways [20]. However,increasing the number of bilayers of (PPY/PWA)shows a slightly negative effect on proton conductiv-ity. The methanol permeability reduced by threeorders of magnitude compared with that of Nafion117, whereas, the proton conductivity is the same or-der of magnitude as the Nafion 117. Similarly, awork on polyaniline on sulfonated poly(arylene etherketone) SPAEK-C (PANI/PWA)n[ 72] shows a lowmethanol permeability because of close pores byforming of polyelectrolyte multilayered films on thesubstrate surface. The methanol permeability is twoorders of magnitude less than that of Nafion 117, andproton conductivity is higher than that of Nafion 117.

In addition, a work on sulfonated plasma polymerized(SPPM)[74] shows lower permeability of plasma mem-branes because of highly crosslinked structure of theplasma polymerized membranes and their water bindingcharacteristics, which could effectively restrict methanoldiffusion. The methanol permeability is about 28 times lessthan that of Nafion 117, and the proton conductivity isslightly higher than that of Nafion 117. A work on Sulfo-nated poly(styrene-ran-ethylene) and porous polyimidematrix (PI/SPES) composite shows a significant reductionof methanol permeability because of pore-filling of theporous PI matrix with SPSE polymer. It results in sup-pressed water absorption, and less dimensional changemay indicate less methanol crossover though the mem-brane [99]. A work on poly(2, 6-dimethyl-1,4-phenyleneoxide)-based acid–base polymer blend shows a consider-able reduction of methanol permeability because of the ef-fective role of (BrPPO) in restricting methanol permeationwhile maintaining essential proton conduction properties.The methanol permeability is 21 times less than that ofNafion 117 and the proton conductivity is in the same or-der of magnitude as Nafion 117 [86].

In summary, the relative transport properties withrespect to Nafion 117 such as conductivity, methanol per-meability, and selectivity of the best modified membranesare shown in Table II. The modified Nafion membranesare (Zr-msPPA)/Nafion, and (PI/Nafion). The developedalternative membranes are SPAEK-C-(PPY/PWA)n,SPPM, SPAEK-C-(PANI/PWA)n, PI/SPES, and PPO.

6. CONCLUSIONS

In this review paper, the development of new PEMs for theDMFC has been reviewed and the following importantpoints are extracted.


• The up to date modified membranes with the highestselectivity are as follows: (i) the modified Nafionmembranes such as (Zr-msPPA)/Nafion, and (PI/Nafion); (ii) the developed alternative membranessuch as SPAEK-C-(PPY/PWA)n, SPPM, SPAEK-C-(PANI/PWA)n, PI/SPES and PPO.

• The methanol permeability increases with increasingthe methanol concentration for SPEEK and Nafionmembranes. However, a significant decrease in meth-anol crossover was reported by increasing the metha-nol concentration for SPEEK/SPVA, SMMT/SPEEK,SPEEK/ PVA, PAMPS, Nafion/HA, CS/Silica, PVA/sulfated b-cyclodextrin, CS/Beta/SO3H, NTDA–BAPBDS/DABI, and PVA/PSSA–MA. No signifi-cant change of methanol permeability was observedby increasing methanol concentration for Co-SPIs.

• For all reviewed membranes, increasing the methanolconcentration results in an increasing of the methanolcrossover current density.

• The methanol permeability increases with the increaseof temperature, and an Arrhenius-type dependency ofmethanol permeability on temperature exists for all ofthese membranes. The methanol permeability ofSPES-C, SPS, S-SIBS, SPEEK, SPPES, and SPEEK/PVDF /PWA are much less than that of Nafion.

• The methanol crossover decreases with the increase ofmembrane thickness.

REFERENCES

1. Zhao TS, Kreuer KD, Nguyen TV. Advances in FuelCells, Vol. 1. Elsevier Ltd, 2007.

2. Srinivasan S. Fuel Cells: From Fundamentals toApplications. Springer, 2006.

1225


3. Kamarudin SK, Daud WRW, Ho SL, Hasran UA.Journal of Power Sources 2007; 163:743–754.

4. Deluca N, Elabd YA. Journal of Polymer Science:Part B: Polymer Physics 2006; 44:2201–2225.

5. Ahmad H, Kamarudin SK, Hasran UA, Daud WRW.International Journal of Hydrogen Energy 2010;35:2160–2175.

6. Zaidi SM, Matsuura T. Polymer Membranes for FuelCells. Springer Science: NY, USA, 2009.

7. Li N, Fane A, Ho WS, Matsuura T. Advanced Mem-brane Technology and Applications. John Wiley &Sons, Inc.: Hoboken, New Jersey, USA, 2008.

8. Zhao TS, Xu C, Chen R, Yang WW. Progress inEnergy and Combustion Science 2009; 35:275–292.

9. Kim H, Cho J, Yoon J, Chang H. 2001 ECS Confer-ence 2001.

10. Han J, Liu H. Journal of Power Sources 2007;164:166–173.

11. Cussler EL. Diffusion Mass Transfer in Fluid Sys-tems (3rd Edition edn). Cambridge University Press:New York, U.S.A, 2007.

12. Wang ZH, Wang CY. Journal of the ElectrochemicalSociety 2003; 150–4:A508–A519.

13. Liu W, Wang CY. Journal of the ElectrochemicalSociety 2007; 154–3:B352–B361.

14. Chiu YJ. International Journal of Hydrogen Energy2010; 35:6418–6430.

15. Ye D, Zhu X, Liao Q, Li J, Fu Q. Journal of PowerSources 2009; 192:502–514.

16. Eccarius S, Garcia B, Hebling C, Weidner JW. Jour-nal of Power Sources 2008; 179:723–733.

17. Kim YM, Park KW, Choi JH, Park IS, Sung YE.Electrochemistry Communications 2003; 5:571–574.

18. Lin CW, Fan KC, Thangamuthu R. Journal of Mem-brane Science 2006; 278:437–446.

19. Park HS, Kim YJ, Hong WH, Lee HK. Journal ofMembrane Science 2006; 272:28–36.

20. Wu Z, Sun G, Jin W, Wang Q, Hou H, Chan KY, XinQ. Journal of Power Sources 2007; 167:309–314.

21. Choa KY, Jung HY, Shin SS, Choi NS, Sung SJ,Park JK, Choi JH, Park KW, Sung YE. Electrochi-mica Acta 2004; 50:589–593.

22. Wang J, Xiao L, Zhao Y, Wu H, Jiang Z, Hou W.Journal of Power Sources 2009; 192:336–343.

23. Yoon SR, Hwang GH, Cho WI, Hong S-A, Ha HY.Journal of Power Sources 2002; 106:215–223.

24. Ladewig BP, Knott RB, Martin DJ, Costa JD, Lu GQ.Electrochemistry Communications 2007; 9:781–786.

25. Wu Z, Sun G, Jin W, Hou H, Wang S, Xin Q. Jour-nal of Membrane Science 2008; 313:336–343.

26. Hasani-Sadrabadi MM, Dashtimoghadam E, MajediFS, Kabiri K, Solati-Hashjinb M, Moaddel H. Jour-nal of Membrane Science 2010; 365:286–293.

1226 I

27. Lia C, Sun G, Ren S, Liu J, Wang Q, Wu Z, Sun H,Jin W. Journal of Membrane Science 2006; 272:50–57.

28. Jiang R, Kunz HR, Fenton JM. Journal of MembraneScience 2006; 272:116–124.

29. Nguyen T, Wang X. Journal of Power Sources 2010;195:1024–1030.

30. Holmberg BA, Wang X, Yan Y. Journal of Mem-brane Science 2008; 320:86–92.

31. Barbora L, Acharya S, Verma A. MacromolecularSymposia 2009; 277:177–189.

32. Sun L, Wang S, Jin W, Hou H, Jiang L, Sun G. Inter-national Journal of Hydrogen Energy 2010;35:12461–12468.

33. Hudiono Y, Choi S, Shu S, Koros WJ, Tsapatsis M,Nair S. Microporous and Mesoporous Materials2009; 118:427–434.

34. Kim D, Scibioh MA, Kwak S, Oh I-H, Ha HY.Electrochemistry Communications 2004; 6:1069–1074.

35. Song M-K, Park S-B, Kim Y-T, Rhee H-W.Molecular Crystals and Liquid Crystals 2003; 407:15/[411]–23/[419].

36. Fang Y, Wang T, Miao R, Tang L, Wang X. Electro-chimica Acta 2010; 55:2404–2408.

37. Wang Y-J, Colby RH, Kim D. Journal of MembraneScience 2011; 366:421–426.

38. Hwang M, Ha H-Y, Kim. Journal of Membrane Sci-ence 2008; 325:647–652.

39. Sauk J, Byun J, Kim H. Journal of Power Sources2005; 143:136–141.

40. Xiang Y, Zhang J, Liu Y, Guo Z, Lu S. Journal ofMembrane Science 2011; 367:325–331.

41. Tsai J-C, Cheng H-P, Kuo J-F, Huang Y-H,Chen C-Y. Journal of Power Sources 2009; 189:958–965.

42. Kim YJ, Choi WC, Woo SI, Hong WH. Journal ofMembrane Science 2004; 238:213–222.

43. Ramya K, Dhathathreyan KS. Journal of MembraneScience 2008; 311:121–127.

44. Jeon J-D, Kwak S-Y. Journal of Power Sources2008; 185:49–54.

45. Tsai J-C, Cheng H-P, Kuo J-F, Huang Y-H, ChenC-Y. Journal of Power Sources 2009; 189:958–965.

46. Li T, Yang Y. Journal of Power Sources 2009;187:332–340.

47. Huang QM, Zhang QL, Huang HL, Li WS, HuangYJ, Luo JL. Journal of Power Sources 2008;184:338–343.

48. Kim J, Kim B, Jung B. Journal of Membrane Science2002; 207:129–137.

49. Kim B, Kim J, Jung B. Journal of Membrane Science2005; 250:175–182.



50. Kim YS, Sumner MJ, Harrison WL, Riffle JS,McGrath JE, Pivovar BS. Journal of the Electro-chemical Society 2004; 151(12):A2150–A2156.

51. Okamoto K-I, Yin Y, Yamada O, Islama MN, HondaT, Mishima T, Suto Y, Tanaka K, Kita H. Journal ofMembrane Science 2005; 258:115–122.

52. Woo Y, Oh SY, Kang YS, Jung B. Journal of Mem-brane Science 2003; 220:31–45.

53. Einsla BR, Kim YS, Hickner MA, Hong Y-T, HillML, Pivovar BS, McGrath JE. Journal of MembraneScience 2005; 255:141–148.

54. Fang J, Shen PK, Liu QL. Journal of Membrane Sci-ence 2007; 293:94–99.

55. Zhang Q, Gong F, Zhang S, Li S. Journal of Mem-brane Science 2011; 367:166–173.

56. Liu C, Li L, Liu Z, Guo M, Jing L, Liu B, Jiang Z,Matsumoto T, Guiverc MD. Journal of MembraneScience 2011; 366:73–81.

57. Chen X, Chen P, An Z, Chen K, Okamoto K. Journalof Power Sources 2011; 196:1694–1703.

58. Zhang Y, Fei X, Zhang G, Li H, Shao K, Zhu J, ZhaoC, Liu Z, Han M, Na H. International Journal of Hy-drogen Energy 2010; 35:6409–6417.

59. Kim DS, Robertson GP, Guiver MD. Macromole-cules 2008; 41:2126–2134.

60. Feng S, Shang Y, Xie X, Wang Y, Xu J. Journal ofMembrane Science 2009; 335:13–20.

61. Zhang Y, Cui Z, Zhao C, Shao K, Li H, Fu T, Na H,Xing W. Journal of Power Sources 2009; 191:253–258.

62. Li L, Wang Y. Journal of Membrane Science 2005;246:167–172.

63. Elabd YA, Napadensky E, Sloan JM, Crawford DM,Walker CW. Journal of Membrane Science 2003;217:227–242.

64. Yin Y, Fang J, Cui Y, Tanaka K, Kita H, OkamotoK-I. Polymer 2003; 44:4509–4518.

65. Won J, Park HH, Kim YJ, Choi SW, Ha HY, Oh I-H,Kim HS, Kang YS, Ihn KJ. Macromolecules 2003;36:3228–3234.

66. Li L, Zhang J, Wang Y. Journal of Membrane Sci-ence 2003; 226:159–167.

67. Li H, Zhang G, Wu J, Zhao C, Zhang Y, Shao K, HanM, Lin H, Zhu J, Na H. Journal of Power Sources2010; 195:6443–6449.

68. Liu B, Robertson GP, Kim D-S, Guiver MD,Hu W, Jiang Z. Macromolecules 2007; 40(1934):1944.

69. Lin H, Zhao C, Ma W, Li H, Na H. InternationalJournal of Hydrogen Energy 2009; 34:9795–9801.

70. Kim YS, Hickner MA, Dongd L, Pivovar BS,McGrath JE. Journal of Membrane Science 2004;243:317–326.


71. Zhao C, Lin H, Na H. International Journal ofHydrogen Energy 2010; 35:2176–2182.

72. Zhao C, Lin H, Zhang Q, Na H. International Jour-nal of Hydrogen Energy 2010; 35:10482–10488.

73. Zhang Y, Wan Y, Zhang G, Shao K, Zhao C, Li H,Na H. Journal of Membrane Science 2010; 348:353–359.

74. Jiang Z, Jiang Z-J, Yu X, Meng Y. Plasma Process.Polym. 2010; 7:382–389.

75. Ye Y-S, Chen W-Y, Huang Y-J, Cheng M-Y, YenY-C, Cheng C-C, Chang F-C. Journal of MembraneScience 2010; 362:29–37.

76. Carretta N, Tricoli V, Picchioni F. Journal of Mem-brane Science 2000; 166:189–197.

77. Kim DS, Guiver MD. Journal of Polymer Science:Part A: Polymer Chemistry 2008; 46:989–1002.

78. Huang C-H, Wu H-M, Chen C-C, Wang C-W, KuoP-L. Journal of Membrane Science 2010; 353:1–9.

79. Jung B, Kim B, Yang JM. Journal of Membrane Sci-ence 2004; 245:61–69.

80. Smitha B, Sridhar S, Khan AA. Journal of AppliedPolymer Science 2005; 95:1154–1163.

81. Yang T. International Journal of Hydrogen Energy2008; 33:6772–6779.

82. Wu C-S, Lin F-Y, Chen C-Y, Chu PP. Journal ofPower Sources 2006; 160:1204–1210.

83. Rhim J-W, Park HB, Lee C-S, Jun J-H, Kim DS, LeeYM. Journal of Membrane Science 2004; 238:143–151.

84. Wang J, Zhao C, Zhang G, Zhang Y, Ni J, Ma W,Na H. Journal of Membrane Science 2010;363:112–119.

85. Jiang Z, Zheng X, Wu H, Wang J, Wang Y. Journalof Power Sources 2008; 180:143–153.

86. Fu R-Q, Julius D, Hong L, Lee J-Y. Journal of Mem-brane Science 2008; 322:331–338.

87. Escribano PG, Canovas MJ, Ojeda MC, Delri'o C,Sa'nchez F, Acosta JL. Journal of Polymer Science:Part B: Polymer Physics 2009; 47:1203–1210.

88. Feng S, Shang Y, Liua G, Dong W, Xie X, Xu J,Mathur VK. Journal of Power Sources 2010;195:6450–6458.

89. Kim DS, Park HB, Rhim JW, Lee YM. Journal ofMembrane Science 2004; 240:37–48.

90. Li L, Xu L, Wang Y. Materials Letters 2003;57:1406–1410.

91. Geng J, Jiang Z, Wang J, Shi Y, Yang D, Xiao L.Chemical Engineering and Technology 2010; 33,No. 2: 244–250.

92. Helen M, Viswanathan B, Murthy SS. Journal ofMembrane Science 2007; 292:98–105.

93. Kuo P-L, Jheng W-H, Liang W-J, Chen W-F. Jour-nal of Power Sources 2010; 195:6434–6442.

1227


94. Yang T. International Journal of Hydrogen Energy2009; 34:6917–6924.

95. Wang Y, Yang D, Zheng X, Jiang Z, Li J. Journal ofPower Sources 2008; 183:454–463.

96. Fang Y, Miao R, Wang T, Wang X. Pure andApplied Chemistry 2009; 81-12:2309–2316.

97. Tripathi BP, Kumar M, Shahi VK. Journal of Mem-brane Science 2010; 360:90–101.

98. Pivovar BS, Wang Y, Cussler EL. Journal of Mem-brane Science 1999; 154:155–162.

99. Nguyen TH, Wang C, Wang X. Journal of Mem-brane Science 2009; 342:208–214.

100. Li H, Zhang G, Wu J, Zhao C, Jia Q, Lew CM, ZhangL, Zhang Y, Han M, Zhu J, Shao K, Ni J, Na H. Jour-nal of Power Sources 2010; 195:8061–8066.

101. Wang J, Zhao Y, Hou W, Geng J, Xiao L, Wu H,Jiang Z. Journal of Power Sources 2010; 195:1015–1023.

102. Wang J, Zhang Y, Wu H, Xiao L, Jiang Z. Journal ofPower Sources 2010; 195:2526–2533.

103. Wang J, Zhang H, Jiang Z, Yang X, Xiao L. Journalof Power Sources 2009; 188:64–74.

104. Bello M, Javaid Zaidi SM, Rahman SU. Journal ofMembrane Science 2008; 322:218–224.

105. Cho E-B, Luu DX, Kim D. Journal of Membrane Sci-ence 2010; 351:58–64.

106. Cho E-B, Kim H, Kim D. The Journal of PhysicalChemistry. B 2009; 113:9770–9778.

107. Yang T, Xu Q, Wang Y, Lu B, Zhang P. Interna-tional Journal of Hydrogen Energy 2008; 33:6766–6771.

108. Joo SH, Pak C, Kim EA, Lee YH, Chang H, SeungD, Choi YS, Park J-B, Kim TK. Journal of PowerSources 2008; 180:63–70.

109. Shin J-P, Chang B-J, Kim J-H, Lee S-B, SuhDH. Journal of Membrane Science 2005; 251:247–254.

110. Thangamuthu R, Lin CW. Solid State Ionics 2005;176:531–538.

111. Mat NC, Liong A. Engineering Letters 2009; 17(4):1–4.

112. Munakata H, Yamamoto D, Kanamura K. Journal ofPower Sources 2008; 178:596–602.

113. Lin H, Zhao C, Jiang Y, Ma W, Na H. Journal ofPower Sources 2011; 196:1744–1749.

114. Navarro A, Ri C, Acosta J. Journal of Polymer Sci-ence: Part B: Polymer Physics 2008; 46:1684–1695.

115. Li H, Zhang G, Ma W, Zhao C, Zhang Y, Han M,Zhu J, Liu Z, Wu J, Na H. International Journal ofHydrogen Energy 2010; 35:11172–11179.

116. Xue S, Yin G, Cai K, Shao Y. Journal of MembraneScience 2007; 289:51–57.

1228 I

117. Lin C-K, Kuo J-F, Chen C-Y. Journal of PowerSources 2009; 187:341–347.

118. Fu TZ, Liu J, Cui ZM, Ni J, Zhang G, Yu HB, ZhaoCJ, Shi YH, Na H, Xing W. Fuel Cells 2009;9-5:570–578.

119. Lin C, Wang CY. J. Power Sources 2003; 113:145–150.

120. Bae B, Kho BK, Lim T, Oh I, Hong S, Ha HY. J.Power Sources 2006; 158:1256–1261.

121. Dimitrova P, Friedrich KA, Vogt B, Stimming U.Journal of Electroanalytical Chemistry 2002; 532:75–83.

122. Kim YS, Sumner MJ, Harrison WL, Riffle JS,McGrath JE, Pivovar BS. Journal of the Electro-chemical Society 2004; 151:A2150–A2156.

123. Yang T. Journal of Membrane Science 2009;342:221–226.

124. Gosalawit R, Chirachanchai S, Shishatskiy S, NunesSP. Journal of Membrane Science 2008; 323:337–346.

125. Park Y-S, Yamazaki Y. Polymer Bulletin 2005;53:181–192.

126. Chen K, Hu Z, Endo N, Fang J, Higa M, OkamotoK-I. Journal of Membrane Science 2010; 351:214–221.

127. Kang M-S, Kim JH, Won J, Moon S-H, Kang YS.Journal of Membrane Science 2005; 247:127–135.

128. Jiang R, Zhang Y, Swier S, Wei X, Erkey C, KunzHR, Fenton JM. Electrochemical and Solid-State Let-ters 2005; 8:A611–A615.

129. Yamauchi A, Ito T, Yamaguchi T. Journal of PowerSources 2007; 174:170–175.

130. Prabhuram J, Zhao TS, Liang ZX, Yang H, WongCW. Journal of the Electrochemical Society 2005;152:A1390–A1397.

131. Liu F, Lu G, Wang C-Y. Journal of the Electrochem-ical Society 2006; 153:A543–A553.

132. Miyake N, Wainright JS, Savinell RF. Journal of theElectrochemical Society 2001; 148:A905–A909.

133. Gubler L, Kramer D, Belack J, Ünsal Ö, Schmidt TJ,Scherer GG. Journal of the Electrochemical Society2007; 154:B981–B987.

134. Zhou X, Weston J, Chalkova E, Hofmann MA, Am-bler CM, Allcock HR, Lvov SN. Electrochimica Acta2003; 48:2173–2180.

135. Tricoli V, Carretta N, Bartolozzi M. Journal of theElectrochemical Society 2000; 147:1286–1290.

136. Lee CH, Min KA, Park HB, Hong YT, Jung BO, LeeYM. Journal of Membrane Science 2007; 303:258–266.

137. Boutry DT, Geyer AD, Guetaz L, Diat O, Gebel G.Macromolecules 2007; 40:8259–8264.


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