2003_01

Journal of Membrane Science 212 (2003) 213–223

Nafion/PTFE composite membranes for fuel cell applications

Fuqiang Liu1, Baolian Yi∗, Danmin Xing, Jingrong Yu, Huamin ZhangFuel Cell R&D Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China

Received 21 March 2002; received in revised form 2 October 2002; accepted 11 October 2002

Abstract

Porous polytetrafluoroethylene (PTFE) membranes were used as support material for Nafion®/PTFE composite membranes.The composite membranes were synthesized by impregnating porous PTFE membranes with a self-made Nafion solution. Theresulting composite membranes were mechanically durable and quite thin relative to traditional perfluorosulfonated ionomermembranes (PFSI); we expect the composite membranes to be of low resistance and cost. In this study, we used three kindsof porous PTFE films to prepare Nafion/PTFE composite membranes of different thickness. Scanning electron micrographsand oxygen permeabilities showed that Nafion resin is distributed uniformly in the composite membrane and completelyplug the micropores, there is a continuous thin Nafion film present on the PTFE surface. The variation in water content ofthe composite and Nafion 115 membranes with temperature was determined. At the same temperature, water content of thecomposite membranes was smaller than that of the Nafion 115. In both dry and wet conditions, maximum strength and breakstrength of C-325# and C-345# were larger than those of Nafion 112 due to the reinforcing effect of the porous PTFE films.And the PEMFC performances and the lifetime of the composite membranes were also tested on the self-made apparatus.Results showed that the bigger the porosity of the substrate PTFE films, the better the fuel cell performance; the fuel cellperformances of the thin composite membranes were superior to that of Nafion 115 membrane; and after 180 h stability testat 500 mA/cm2, the cell voltage showed no obvious drop.© 2002 Published by Elsevier Science B.V.

Keywords:Fuel cell; PTFE; Nafion; Composite membrane; Performance

1. Introduction

It is generally accepted that proton exchange mem-brane (PEM) fuel cells present an attractive alternativeto traditional power sources, but the cost of the cellcomponents precludes their immediate implementa-tion for most stationary and vehicular applications. Aprimary contributor to the PEM fuel cell’s high costis the perfluorosulfonated ionomer membrane (PFSI)

∗ Corresponding author. Tel.:+86-411-4379097.E-mail addresses:[email protected] (F. Liu),[email protected] (B. Yi).

1 Tel.: +86-411-4671991-614; fax:+86-411-4665057.

membrane. Fluorine-free or partially fluorinated mem-branes would be good candidates to substitute for thePFSI membranes, as long as their mechanical, chemi-cal, and electrochemical properties are comparable tothose of PFSI membranes. There are many reports inthe literature about PFSI/porous polytetrafluoroethy-lene (PTFE) composite membranes[1–9]. From acost perspective, it is important to note that the com-posite PFSI/PTFE membranes contain much less ofthe expensive PFSI polymer than traditional PFSImembranes such as Nafion® 117.

It is evident that composite membranes have manyadvantages: increase of both the mechanical strengthin the dry state and the dimensional stability in the

0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V.PII: S0376-7388(02)00503-3

214 F. Liu et al. / Journal of Membrane Science 212 (2003) 213–223

wet state, ease of handling, availability of very thinmembranes.

Now the available PFSI/PTFE composite mem-branes are manufactured by Gore and AssociatesInc. under the Gore Select trademark. The Gore Se-lect membranes have a translucent structure with novisible evidence of any micro-reinforcement[10].This reinforcement does not, of course, change theintrinsic chemical properties of the membrane (theconductance change is due to the thickness). Only themechanical properties (shrinkage upon dehydrationand membrane tensile strengths) and the hydraulicpermeation dramatically change[10].

To obtain the composite PFSI/PTFE compositemembranes, it is necessary to impregnate PFSI resinto the pore of the porous PTFE substrate membranes.Alcohol PFSI solutions are used typically for film de-position; the dissolution of PFSI polymers in alcoholsolutions at elevated temperatures and pressures hasbeen studied by a number of researchers[11–13].

In this study, we investigated composite proton ex-change membranes based on different kinds of poroussubstrate PTFE films made in China. The permeabil-ity, mechanical strength, membrane hydration weretested and compared with Nafion membranes. And thePEMFC performances and the lifetime of the com-posite membranes were also tested on the self-madeapparatus.

2. Experimental

2.1. Preparation of PFSI solution

Three grams of PFSI polymer, which was obtainedfrom Nafion 117 membranes chopped into smallpieces, was dissolved in 60 ml mixed solvent of waterand three kinds of alcohols. Dissolution took place at200◦C for 12 h in the self-made high pressure stain-less steel vessel. The undissolved residue was filteredout. The PFSI solution was prepared by mixing thesolution above with 6 ml DMF.

2.2. Membrane preparation

Composite membranes were made using three kindsof microporous PTFE films (manufactured in Beijingand Shanghai) with 0.3–0.5�m pores, denoted as 1#,

2#, 3#. The porous PTFE support films were firstcleaned by soaking in ethanol for 30 min at room tem-perature, and then dried in an oven at 50–60◦C for20 min. They were then extended over a flat glassplate, after that, the solution made above was pouredon the PTFE films. The glass plate was dried in ahot plate at 50–60◦C, and finally dried in an vacuumoven at 120–130◦C for 10 h. In the process, the grav-ity dragged Nafion solution into the pores of the PTFEfilms and after the evaporation of solvent, Nafion resincan completely plugged the micropores, thus makingthe composite membranes air-tight.

Annealing the air-dried composite was a criticalstep in preparing the membrane. In the PFSI mem-brane, phase separation between the hydrophobic flu-orocarbon backbone and the hydrophilic sulfonic acidgroups is thought to occur. The membrane polymerswells in contact with solvents such as lower aliphaticalcohols or water. At temperatures higher than roomtemperature the forces among the fluorocarbon chainsare weakened, resulting in phase inversion. But duringannealing the fluorocarbon chains in the polymer filmagain fuse together to form a durable and insolublelayer [12].

The thickness of the composite membranes is easilycontrolled by the amount of the Nafion solution. TheNafion uptake was calculated from the dry-weightdifference before and after impregnation. In order toprevent distortion to the PTFE porous support mem-branes during the cleaning, impregnation, and dryingoperations, the PTFE films was placed in a stainlesssteel holder. The 25-�m-thick composite membranebased on 3# PTFE substrate film was denoted asC-325#. Similarly, the 45-�m-thick composite mem-brane based on 1#, 2#, 3# PTFE substrate film weredenoted as C-145#, C-245# and C-345#, respectively.

2.3. Dimensional stability

Membranes specimens (size 5 cm× 6 cm) werestored in the atmosphere of 25◦C, 50% RH for 24 hand the distance between specified positions wasmeasured before (L1) and after (L2) the samples weresoaked in deionized water controlled at 80◦C for 8 h.Dimensional change (�L) was calculated by usingthe following equation:

�L (%) = L2 − L1

L1× 100 (1)

F. Liu et al. / Journal of Membrane Science 212 (2003) 213–223 215

2.4. Water content of membranes

Samples of the membranes were weighed (W1) af-ter immersion in deionized water for 8 h at controlledtemperature. Next, samples were weighed (W2) afterdrying in a vacuum oven at 120◦C for 12 h. Watercontent (�W) was calculated fromEq. (2).

�W (wt.%) = W1 − W2

W2× 100 (2)

2.5. Analysis by scanning electron microscopy

The porous PTFE surface and surface and cross-section of the composite membrane after impregna-tion were observed by means of a scanning electronmicroscope (SEM, JEM-1200EX). Cross cuts of thecomposite membrane were prepared by breaking themunder liquid nitrogen (77 K) with two small nippers.

2.6. Gas permeability of wet membranes(gas phase method)

Crossover of hydrogen and oxygen through the pro-ton exchange membrane is considered to be one ofthe most important phenomena in PEMFC. Here, weused the gas chromatograph method to measure theoxygen permeability of the membranes. The gas chro-matograph method is described elsewhere[14].

2.7. Mechanical strength of membranes

Samples of the membranes were dried in a vacuumoven at 80◦C for 10 h. Wet samples (membranesswollen by water) were immersed in pure water for10 h at room temperature. Mechanical properties(maximum strength and break strength) were mea-sured with a tension tester AG-2000A (Shimadzu,AUTOgraph) at room temperature. Tensile conditionswere based on Chinese Standard QB-13022-91 andsamples were measured using a programmed elonga-tion rate of 50 mm/min.

2.8. Electrode and MEA preparation

The 20 wt.% carbon-supported-platinum (Pt/C)catalysts were prepared by reducing H2PtCl6 withHCHO. Pt/C catalyst, carbon paper, PTFE suspension

and a Nafion solution (Du Pont) were used for elec-trode preparation. Carbon/PTFE slurry was brushedonto the two sides of hydrophobically treated carbonpapers, then the carbon papers were dried and calcined.Afterwards, the slurry of carbon-supported-platinumcatalyst dispersion was homogeneously brushed ontothe surface of the smoothed carbon paper, the carbonpaper was then dried and calcined in a flowing inertgas. The hydrophobically treated electrodes with theloading of Pt/C catalyst 0.3 mgPt/cm2 were obtained.5% Nafion solution (Du Pont) was sprayed onto thesurface of catalyst layer of electrode up to the dryloading of Nafion 0.6–1.2 mg/cm2. The solvent wasallowed to evaporate and the electrodes were placedin an oven at 80◦C for 0.5–1 h.

The membrane electrode assembly (MEA) wasassembled using a hot pressing process. Two elec-trodes with effective area 5 cm2 were hot-pressed toone piece of membrane at the temperature 140◦Cand 15–17 kg cm2 for 1–2 min to form MEA, thenthe MEA was positioned in a single cell fixture,and finally the single cell was installed in a fuelcell test station. The single cell was operated underthe conditions—humidifier temperature:TH2/TO2 =75/75◦C; cell temperature: 80± 2 ◦C; gas pressure:PH2/PO2 = 0.20/0.20 MPa; gas purity: 99.9% for H2and 99% for O2.

2.9. Fuel cell evaluation and life tests

The cell was maintained under open circuit condi-tion until an acceptable value, then was operated atlower current densities. During this period, the humid-ifiers and cell were gradually heated to the desiredtemperatures, then the current density was increasedto the highest operating value and kept unchangedfor 8–10 h. Thereafter the cell potential versus currentdensity data was recorded. The polarization curvesof cell potential versus current density were plottedand the electrode kinetic parameters of operating cellswere calculated.

Although no quantitative theory on the relation be-tween degradation rate and operation condition has notdeveloped, it is estimated that it is effective to operatea cell at high current densities for the rapid evalua-tion of membrane stability. In this study, life test wasperformed at 500 mA/cm2 by using single cell. Cellvoltage was measured continually.


3. Results and discussion

3.1. Characteristics of the substrate PTFE films

Fig. 1shows the SEM photomicrographs of the sur-face of the three kinds of porous PTFE films. Thethickness and porosity are listed inTable 1. The meandiameter of pores in the three kinds of PTFE sub-strate films observed by SEM was about 0.3�m. For1# PTFE film, many of the pores did not interconnectwith each other, leaving large areas of dense surfaceof the PTFE; in 2# PTFE film, most of the pores in-terconnected with each other and only a small frac-tion of the surface was occupied by the dense PTFE;while 3# PTFE film had a rough surface and there weremany small pores interconnected with each other. The

Fig. 1. SEM photomicrographs of different PTFE membranes.

Table 1Porosity and thickness of different PTFE substrate films

PTFE substrate film

1# 2# 3#

Thickness (�m) 30 30 15Porosity (%) 64 76 84

porosity of the three PTFE films accorded with theSEM photomicrographs. The order of the porosity is1# < 2# < 3#.

3.2. Nafion content of composite membranes

The Nafion content of composite membranes ofC-325# and C-345# is 85.6 and 90.7%, respectively.


Fig. 2. SEM photomicrographs: (a) surface of Nafion/PTFE composite membrane; (b) cross-section of the composite membrane.

C-345# (45�m) composite membrane was thickerthan C-325# (25�m) composite membrane, the Nafioncontent increases with the thickness of the mem-branes. C-345# composite membrane has a Nafioncontent about 91%. High Nafion content and thin com-posite membranes can lead to good fuel cell perfor-mance as indicated later.

3.3. SEM analysis

Fig. 2 presents SEM photomicrographs of the sur-face and the cross-section of C-345# membrane af-ter impregnation. The native PTFE membrane has arough, porous surface, but the Nafion/PTFE compositemembrane has a smooth surface with no visible pores.From the SEM of the cross-section of the compositemembrane, we can see that the pores of the porousmembrane are fully impregnated by the Nafion resin.Thus, we conclude that Nafion resin is distributed uni-formly in the composite membrane and completelyplug the micropores, there is a continuous thin Nafionfilm present on the PTFE surface. This thin film is veryimportant for the preparing the MEA, because whenhot-pressed it is the thin Nafion film present on the sur-face that adhere to the anode or the cathode electrodes.

3.4. Dimensional stability and water content ofmembranes

Table 2 shows dimensional change of C-325#,C-345# and Nafion membranes after soaked in hot wa-

ter for 8 h. It was found that Nafion/PTFE compositemembranes could decrease dimensional change com-pared with the standard Nafion membranes. For thecomposite membrane, the thinner membrane showedgood dimensional stability due to the lower Nafioncontent compared with that of the thicker compositemembrane.

It is also found that dimensional change of compos-ite membranes upon hydration was isotropic, while di-mensional change of Nafion 115 was anisotropy. Thisis because these membranes had the different manu-facturing process resulting the different structure ofthe membranes.

The variation in water content of C-325#, C-345#

and Nafion 115 membranes with temperature is shownin Fig. 3. Water content of all the membranes increasedas the temperature increased. At the same temper-ature, water content of Nafion 115 was larger thanthat of the composite membranes. For the compositemembranes, the thicker the membrane, the bigger thewater content. As we know, pretreatment procedure

Table 2Dimensional stability of Nafion/PTFE and Nafion® membranes

Type of membrane Dimensional change (%)

Length Breadth Thickness

C-325# 6.3 5.1 12.0C-345# 9.1 7.9 15.3Nafion® 112 17.0 6.0 25.5Nafion® 1135 11.5 4.2 37.7


Fig. 3. Temperature dependence of equilibrium water content of Nafion 115 and composite membranes.

may influence the water uptake of these membranes,so different procedure may lead to different results.

Composite membranes had a larger content of PTFEthan Nafion membranes, making the membranes morehydrophobic. Another factor affecting the water con-tent of the composite membranes is that the porousPTFE film in the composite membrane suppressed theswelling of Nafion as clearly indicated inTable 2.

3.5. Oxygen permeability

Fig. 4shows the temperature dependence of oxygenpermeability coefficients of Nafion 115 and C-325#

membranes. At the same temperature, oxygen per-meability coefficients of the composite membranewere slightly larger than those of Nafion membrane.Oxygen permeability coefficients of the both mem-branes had the same dependency on temperature: itincreased as the temperature increased. Yoshitakeet al. [14] supposed that oxygen in the membranediffuses through the part of the main chain polymer,namely, hydrophobic part in the membrane whilehydrogen diffuses through the part of ion exchangefunctional groups. Because composite membraneshave larger contents of PTFE than Nafion membranes,which makes them more hydrophobic, so it is not

surprising that oxygen permeability coefficients ofcomposite membranes is larger than those of Nafionmembrane. But we cannot deny that some defectsin the composite membranes could cause more oxy-gen permeate the composite membrane making theoxygen permeability coefficients larger.

It was obtained fromFig. 4 that oxygen permeabil-ity coefficients of C-325# membrane is about 15%larger than that of the Nafion 115 membrane, whichmeans that using thinner composite membrane canaccelerate the crossover of H2 and O2 through thethin PEMs. From the oxygen permeability coefficientsof Nafion 115 and C-325# membranes, we can getthe oxygen permeation rate (qO2)of different mem-branes at 80◦C: theqO2 values of C-325# is 1.86×10−4 cm3/(cm2 s) and this rate is converted to cur-rent density, 3.1 mA/cm2, assuming that the oxygenpermeating through the membrane reacts at the op-posite electrode. While theqO2 value of Nafion 115is 3.25× 10−5 cm3/(cm2 s) and current density is es-timated 0.6 mA/cm2. Although we did not measuredH2 permeability in our experiment, as we know thatthe hydrogen permeation rate in Nafion membrane isabout one order of magnitude higher than that of oxy-gen, so the composite membrane must have a largerH2 permeability.


Fig. 4. Temperature dependence of oxygen permeability of Nafion/PTFE composite and Nafion membrane.

3.6. Mechanical strength of membranes

Mechanical strength of membrane affects manufac-turing conditions of MEA and durability of PEMFC.Table 3shows maximum strength and break strengthof dry and wet composite and Nafion membranes.In both dry and wet conditions, maximum strengthand break strength of C-325# and C-345# were largerthan those of Nafion 112 due to the reinforcing ef-fect of the porous PTFE films. For all membranes,membranes in wet conditions had lower mechanicalstrength than those in dry conditions. It is supposedthat water in the membranes function as a plasticizer.

Table 3Mechanical property of the composite membranes and Nafion®

membranes

Type ofmembrane

Condition Maximumstrength (MPa)

Break strength(MPa)

C-325# Dry 41.4 36.5Wet 38.2 32.7

C-345# Dry 27.0 25.5Wet 25.6 25.0

Nafion®112 Dry 26.6 23.5Wet 19.1 16.8

But upon hydration, the Nafion 112 lost a large part ofits strength while the loss in the hydrated compositemembranes was quite modest. These results are notrather surprising because of the hydrophobicity of thePTFE to water. For the composite membranes, withthe decrease of membrane thickness, both maximumstrength and break strength increased. The mechanicalstrength in both dry and wet conditions of the C-325#

was much larger than that of C-345#. The thicknessof the C-325# composite membrane is almost equal tothe thickness of the reinforcing PTFE film, so C-325#

could display the reinforced effect to a larger extent.

3.7. Fuel cell performance and stability test

Performances of fuel cells using composite mem-branes based on different substrate PTFE films areshown inFig. 5. The initial drop of the polarizationcurve is due to an electrochemical activation process,which is caused by the sluggish kinetics of oxygenreduction at the cathode surface. The following lineardecrease of the polarization curve with increasing cur-rent density is due to Ohmic overpotential, which isattributed to the ion flow through the electrolyte mem-brane and the electron flow through the electrode ma-terials. Membrane thickness can dramatically affect


Fig. 5. Potential–current polarization curves of H2/O2 fuel cells using 45-�m-thick composite membranes based on different PTFE substratefilms.

the cell performance, but in the present situation, thecomposite membranes have the same thickness, sothese results only indicate the effect of the porosityof PTFE films on the cell performance.Fig. 5 showsthat fuel cell with C-145# membrane had poorI–Vcurve. Compared with that, it is found that by usinghigh porosity PTFE films the fuel cell performancesare dramatically improved in the cell voltages andthe current densities. The results demonstrate that thecell performance is significantly enhanced when theporosity of the substrate PTFE films is increased from64 to 85%. As seen fromFig. 5, porosity of substratePTFE films plays an important role in cell perfor-mance, the bigger the porosity, the better the perfor-mance. Bigger porosity means that there are morepores in the substrate PTFE films into which Nafionresin can impregnate, so there are more proton pas-sageways in the composite membranes from anode tocathode.

Fig. 6 shows the potential–current polarizationcurves of 5 cm2 electrode membrane assemblies(MEA) obtained using a self-made apparatus. TheMEAs were based on Nafion 115, C-325# and C-345#

composite membranes. In the experiment, we usedNafion 115 membrane as a standard comparison in

the performance test because the cell based on Nafion115 membrane and Nafion/PTFE composite mem-branes have comparable performances. So, it maybe easy to compare the performances and electrodekinetic parameters between them. As may be seen inFig. 6, the polarization curves of the fuel cells basedon composite membranes exhibited better characteris-tics than that based on Nafion 115. The Nafion/PTFEcomposite membranes are more hydrophobic thanNafion 115 membrane because of higher PTFE con-tent, but the composite membranes are much thinnerthan Nafion 115 membrane. The lower membranethickness is sufficient to compensate for its low con-ductivity so that the areal resistance of the compositemembrane is lower than that of Nafion 115. Then weconclude that the application of thin composite mem-branes to PEMFC system is effective to obtain highperformance.

In Fig. 6, up to the end of the linear region, itis shown that the cell potential (E) versus currentdensity (i) data can be fitted by using the followingsemi-empirical equations[15–17]:

E = E0 − b log i − Ri (3)

E0 = Er + b log i0 (4)


Fig. 6. Potential–current polarization curves of H2/O2 fuel cells using Nafion 115, C-325# and C-345# composite membranes.

where Er is the reversible potential for the cell,i0the exchange current density andb the Tafel slope ofthe oxygen reduction reaction in the Pt/C catalyst,E0the cell potential at the current density of 1 mA/cm2,andR represents the cell resistance, such as ionic re-sistance of polymer membrane resistance, electronicresistance of electrodes and bipolar plates, contactresistance both between membrane and electrodesand between electrodes and bipolar plates, and masstransport resistance,b log i represents the activationpolarization of the electrodes, andRi represents theOhmic over-potential of the cell. The fitting of theabove equation to the experimental data is made by thenon-linear least-squares method, and the derived elec-trode kinetic parametersb andR are listed inTable 4,

Table 4Electrode kinetic parameters and calculated H2 crossoverJ(H2)for PEMFCs with Nafion® 115, C-325# and C-345# compositemembranes operating at 0.20/0.20 MPa of H2/O2 and 80◦C

Type ofmembrane

OCV(V)

Tafel parameters dm

(�m)J(H2)

(mA cm−2)b (mV) R (� cm2)

C-325# 0.956 37.2 0.26 25 0.74C-345# 0.975 46.1 0.29 45 0.60Nafion® 115 1.040 58 0.30 125 0.13

together with open circuit voltage (OCV), membranesthickness and calculated H2 crossoverJ(H2).

In Table 4, the Tafel slop for Nafion 115 membranein this experiment is 58 mV, which is consistent withthe results reported by Chu and Jiang[15] and Duet al.[18]. The Tafel slope for Nafion/PTFE compositemembranes increases with increasing thickness, whilethe values are much lower than that of Nafion 115.The oxygen reduction reaction that occurs in fuel cellprocess can be summarized as the follows[18]:

O2 + H+ + e− → intermediate species (5)

where the protonation process of the oxygen moleculeis the rate-determining step.

As discussed before, oxygen diffuses through thehydrophobic parts in the Nafion membranes, thuswe considered that the electrode kinetics of the oxy-gen reduction will be faster in the fuel cells withNafion/PTFE composite membranes, which have alarger percent of PTFE.

The composite membranes are formed by impreg-nating Nafion solution into the pores of PTFE films,then recast composite membranes are formed. It hasbeen reported that recast films are more soluble andpermeable than commercial membranes. Besides, thecomposite membranes are much thinner than Nafion


Fig. 7. Life test of H2/O2 fuel cell of C-325# composite membrane.

115 membrane, all these will promote the proton trans-port. So, the difference of Tafel slopes of Nafion 115and Nafion/PTFE composite membranes is probablythe combined effect of oxygen concentration in theelectrode/membrane interface and Ohmic resistance ofthe membranes.

We know that the area specific Ohmic resistance ofhumidified Nafion 115 at 80◦C is about 0.125� cm2,while the cell resistance obtained in the experimentis much larger than this value (0.30� cm2). In ourexperiment, the MEA is mounted in single cell withstainless steel end plates and mesh current collec-tors. So, the contact resistance between steel stain-less current collectors, diffusion layers and othercomponents may be responsible for such a largeresistance.

In the table,Rvalues, which mainly cause the linearvariation of the cell potential with current density, de-crease in the order: Nafion 115> C-345# > C-325#.As discussed above, the low membrane thickness issufficient to compensate for its low conductivity sothat the areal resistance of the composite membraneis lower than that of Nafion 115.

The value of OCV is a good measure for the gascrossover. The cell OCV is a mixed potential, deter-mined by the balance of the current density for O2

reduction and that for crossover H2 oxidation[19,20].In the table,J(H2) value for Nafion 115 is largerthan those for the composite membranes, indicatingthat composite membranes have a much larger gascrossover rate. But the values obtained here are muchsmaller than those obtained in the oxygen permeabil-ity section. The difference among them might be at-tributed to the different membrane conditions in theoxygen permeability and fuel cell performance test[19].

In fact, the PTFE porous membranes and Nafionresin are held together by intimate interfacial con-tact through mutual diffusion of molecules so thatmechanical force can be transferred across the inter-face. The interfacial forces holding the two phases to-gether may arise from van der Waals forces. Thus, it isvery necessary to evaluate the forces between the twophases when using the composite membranes in fuelcell. The stability of 25-�m-thick composite mem-brane had been examined by operating a single cell at500 mA/cm2 for about 180 h as shownFig. 7. It wasfound that the cell voltage was kept constant for morethan 180 h. This shows that during operation the com-posite membrane is stable and the interfacial forcesbetween the PTFE porous membrane and Nafion resinare strong.


4. Conclusion

Three kinds of porous PTFE films were usedas support material for preparing the Nafion/PTFEcomposite membranes. For the obtained compositemembranes, Nafion resin is distributed uniformly inthe composite membrane and completely plug themicropores, there is a continuous thin Nafion filmpresent on the PTFE surface. Nafion/PTFE compos-ite membranes could decrease dimensional changecompared with the standard Nafion membranes. Inboth dry and wet conditions, maximum strength andbreak strength of C-325# and C-345# were larger thanthose of Nafion 112 due to the reinforcing effect ofthe porous PTFE films. The composite membranesare much thinner than Nafion 115 membrane. Thelower membrane thickness is sufficient to compensatefor its low conductivity so that the areal resistanceof the composite membrane is lower than that ofNafion 115. Thus, low areal resistance leads to lowercell resistance and better performance. Porosity ofsubstrate PTFE films plays an important role in cellperformance, the bigger the porosity, the better theperformance. The characteristics of PEM fuel cellsbased on C-325# and C-345# membranes are betterthan those obtained in PEM fuel cell based on Nafion115 membrane. After 180 h life test, the compositemembrane showed good stability. So, we concludethat it is very promising to use in fuel cell.

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