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Electrochemistry Communications 9 (2007) 2340–2345

A microfluidic approach for measuring capillary pressurein PEMFC gas diffusion layers

Joseph D. Fairweather a, Perry Cheung a, Jean St-Pierre b, Daniel T. Schwartz a,*

a University of Washington, Seattle, WA 98195-1750, USAb University of South Carolina, Columbia, SC 29208, USA

Received 27 June 2007; accepted 29 June 2007Available online 12 July 2007

Abstract

A dearth of experimental capillary pressure data limits our understanding and optimization of liquid water transport in PEMFC gasdiffusion layers (GDLs). A microfluidic device and method is described for measuring the capillary pressure as a function of liquid watersaturation for these thin porous materials with complex, heterogeneous wetting properties. A sample sandwich (hydrophilic membrane–GDL–hydrophobic membrane) is key for probing the entire hydrophilic and hydrophobic pore volume of the GDL during sequentialliquid intrusion and gas intrusion experiments. The capillary pressure curves for an as-purchased Toray 090 and two differentially-pro-cessed Avcarb P75T GDLs were evaluated; each material displayed highly repeatable, but quantitatively different, room temperaturecapillary pressure curves that matched qualitative differences in their macroscopic wettability. The measurements show that hysteresisbetween the liquid intrusion and gas intrusion curves is significant. For example, both the Toray and fully wet-proofed Avcarb GDLsappear hydrophobic during most of the liquid intrusion curve and hydrophilic during most of the gas intrusion curve. The implications ofthis work for water management, and future device designs and experiments are described.� 2007 Elsevier B.V. All rights reserved.

Keywords: Capillary pressure; Multiphase; Fuel cell; Gas diffusion layer; Hysteresis

1. Introduction

Gas diffusion layers (GDLs) are engineered porousmedia with a critical role in proton exchange membranefuel cells (PEMFCs). The GDL is responsible for support-ing the reaction sites in the thin catalyst layer, distributingreactant gases to the reaction sites, conducting the electri-cal current produced, and removing the product water.The removal of liquid water is critical, because liquid canblock reactant transport and limit the performance of aPEMFC [1]. The most common porous GDL materialsare 200–500 lm thick and composed of carbon papersheets or woven carbon fibers [2]. In order to increase thewicking of water away from the reaction sites, Teflon isincorporated into the carbon substrate to make the sur-

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.06.042

* Corresponding author. Tel.: +1 206 543 8388; fax: +1 206 685 3451.E-mail address: dts@u.washington.edu (D.T. Schwartz).

faces hydrophobic [2]. A comprehensive review of the exist-ing research in this field is provided by Wang [3].Enhancing the performance of PEMFC’s throughimproved liquid transport is an ongoing subject of research[4–6]. Electrochemical measurements of cells and stacks areused to indicate when liquid water leads to performancedegradation [5–8], but interest has been increasing for amore refined understanding of liquid water transport andthe role of GDL manufacturing on performance [9–14].

One of the critical constitutive relationships for describ-ing capillary flow in a porous material is the capillary pres-sure vs. saturation (Pc vs. Sl). For this work, we define thecapillary pressure as

P c ¼ P l � P g; ð1Þwhere Pl refers to the liquid water phase pressure and Pg

refers to the gas phase pressure. Capillary pressure is athermodynamic quantity governed by the liquid–gas

Table 1Physical properties for the 24 mm diameter GDL samples

Toray TGP090

Unsintered AvcarbP75T

Sintered AvcarbP75T

Thickness(lm)

280 240 240

Porosity, e 0.78 0.83 0.83Vpore (lL) 99 91 91Wettability Hydrophobic Hydrophilic Hydrophobic

J.D. Fairweather et al. / Electrochemistry Communications 9 (2007) 2340–2345 2341

interface and the properties of the confining porous med-ium (geometry and surface energetics), which can be de-scribed by the modified Young–Laplace equation [15]

P c ¼ �r cos hð Þ

Reff

ð2Þ

where r is the surface tension between the liquid water–airinterface, h is the contact angle of the liquid–air interfaceon the porous surface (taken with respect to the liquidphase), and Reff is the effective pore radius. The ‘‘wettabil-ity’’ is indicated by the contact angle on a smooth surface:a hydrophilic surface will have h < 90�, and a hydrophobicsurface will have h > 90�. GDL materials typically havemixed wettability from the combination of hydrophobicTeflon and mildly hydrophilic carbon fibers [16]. Thor-oughly describing the Pc vs. Sl relationship of a GDL re-quires probing both the hydrophilic and hydrophobicpore spaces (negative and positive capillary pressures,respectively).

For a complex porous medium like a GDL, h and Reff

are both functions of the liquid saturation Sl (the fractionof total pore volume occupied by liquid). Moreover, hdepends on whether liquid is being imbibed into or drainedfrom the media [15]. This makes capillary pressure a mem-ber of the interesting class of hysteretic thermodynamicproperties [15], and it means both liquid intrusion andgas intrusion curves must be probed to fully characterizethe Pc vs. Sl relationship. Both h and Reff are determinedby the GDL pores where the liquid/gas interfaces resideat a given saturation, so the Pc vs. Sl relationship providesa fingerprint for a given GDL material. The relationshipallows a window into how GDL manufacturing and fuelcell operation affects the interaction of the material withmultiphase water.

Experimental data for GDLs was first obtained byGostick et al. [9], who measured the Pc vs. Sl relationshipfor an octane–air and water–air system using the methodof standard porosimetry (MSP) [17]. They measured Pc

vs. Sl for a portion of the pore volume during gas intrusion,but MSP cannot be used to determine liquid intrusioncurves or probe pore volume that does not spontaneouslywet. Koido et al. [10] and Nguyen et al. [11] applied a pres-sure difference to force saturation changes in GDL sam-ples. Nguyen has measured Pc vs. Sl curves for bothimbibing and draining, while Koido has only examinedimbibing. Both used methods that are limited by the natu-ral breakthrough pressures of the GDL materials, so theyonly probed a relatively small pressure range. An alterna-tive approach has been to estimate the Pc vs. Sl relationshipfrom a material’s wetting and structure, either using a sim-ple model geometry [12] or through pore network simula-tions [13,14].

Here we discuss the construction and use of a microflu-idic device that is capable of measuring the Pc vs. Sl rela-tionship for thin porous materials such as GDLmaterials. An earlier version of this device has beendescribed previously [18]. The design is inspired by the

ASTM porous plate apparatus [19], but it has been adaptedfor both liquid intrusion and gas intrusion experiments car-ried out over the full range of saturation.

2. Experimental methods

2.1. Materials

2.1.1. GDL samples

Three commercially available GDL materials wereinvestigated. The first was an as-received Toray TGP-090sample (Toray CFA, Flower Mound, TX). The secondwas an unsintered Avcarb P75T sample (Ballard MaterialsProducts, Lowell, MA) that has been infiltrated withhydrophilic surfactant dispersion containing Teflon. Thethird was the treated Avcarb P75T after sintering at400 �C for 15 min to remove the surfactant. Table 1 liststhe physical properties of each sample. The unsinteredAvcarb is listed as hydrophilic because a water dropletplaced onto the unsintered Avcarb was spontaneouslyimbibed into the GDL, and the others are listed as hydro-phobic because they repelled the droplet.

2.1.2. Fluidic system

The microfluidic device, photographed in Fig. 1A, isdesigned to measure the pressure and control the satura-tion in the thin GDL samples we use. The body and trans-ducer have a small total volume (<250 lL) that iscomparable to the pore volume of the GDL samples (Table1). The syringe has continuous hydraulic contact to thesample through the rigid body; this continuous contactallows the syringe pump to directly control the sample sat-uration level.

The exploded schematic of the microfluidic device isshown in Fig. 1B. The acrylic flow cell (layers b–f) directsliquid water from the syringe (layer a) to the GDL samplehousing (h) and pressure transducer (g). The acrylic flowcell is built using a laminar construction technique alternat-ing between layers of acrylic (grey layers) and 150 lm dou-ble-sided Macbond adhesive (white layers, West CoastPaper Company, Kent, WA). All the acrylic and adhesivelayers are precisely cut using an M-300 CO2 laser platform(Universal Laser Systems, Scottsdale, AZ) and are press-fittogether using a jig.

The pressure difference is recorded from the transducer(Omega Engineering, Stanford, CT) through custom Lab-view 8 control software (100 Hz sampling rate), that also

Fig. 1. A photograph (A) and exploded schematic (B) of the microfluidicdevice. The body (layers b–f) and sample housing (layer h) were built usinglaminar construction with acrylic layers (grey) bonded by adhesive layers(white). Individual layers (a–i, 1–7) are discussed in the text.

2342 J.D. Fairweather et al. / Electrochemistry Communications 9 (2007) 2340–2345

controls the syringe pump. A NE-500 syringe pump (NewEra Pump Systems, Farmingdale, NY) with a 250 lLgastight syringe (Hamilton Company, Reno, NV)controls fluid displacement. De-ionized water at room tem-perature (�25 �C) is used after being de-gassed at 85 �C for30 min.

2.1.3. Sample housing

Each GDL sample is permanently encased in a separateacrylic housing with its components numbered in Fig. 1B.The GDL sample (4) is sandwiched between two mem-branes (3 and 5) and the resulting GDL sandwich is sealedbetween two adhesive layers (2 and 6) and acrylic plates (1and 7) to complete the acrylic sample housing.

A 100 lm thick hydrophilic membrane (layer 5; nylon,1.2 lm characteristic pore size; GE Osmonics, Minne-tonka, MN) on the liquid side of the GDL provides inti-mate liquid contact between the GDL sample and theliquid water, and inhibits air from entering the body duringgas intrusion by increasing the bubbling pressure. A100 lm hydrophobic membrane (layer 3; treated nylon,0.45 lm pore size; GE Osmonics) on the gas side similarlyinhibits water from breaking through the top of the sampleduring liquid intrusion by increasing the breakthroughpressure. The use of paired membranes to access pores withdifferent wettabilities has previously been reported [20].The hydrophilic membrane is 100% saturated with liquidand the hydrophobic membrane is 0% saturated for thepressure range used in these experiments, so the mem-branes will not affect the measured Pc vs. Sl curve.

The acrylic plates on the liquid and gas side of the GDLsupport the sample and are vented to allow phase contactand redistribution. Because of air’s low viscosity and thesmall flow rates used, it is assumed that the gas gauge pres-sure in the GDL is always zero, so at equilibrium the gaugepressure of the liquid Pl equals Pc, according to Eq. (1).

During construction, the outer perimeter of the mem-branes and GDL sample are sealed with epoxy. The hous-ing then is assembled and compressed at approximately70 psi to permanently seal the adhesive layers. The com-pression of the GDL was not varied for this study, but thisis an obvious parameter to investigate in the future to seethe effect on the Pc vs. Sl relationship [21].

2.2. Operation

Before testing each GDL sample, the acrylic flow cell iscompletely filled with liquid water, to eliminate trapped airand ensure continuous hydraulic contact between the syr-inge and the GDL sample. The flow cell and GDL housingare wetted separately, then bolted together and sealed bythe neoprene gasket (layer i in Fig. 1B). This leads to anuncertainty in the initial saturation, which is corrected asexplained below in ‘‘Calculating Pc vs. Sl’’.

After cell assembly, the pressure response of the GDL istested as the pump moves liquid water into and out of thesample. Liquid water is repeatedly injected into anddrained from the GDL sample at a constant flow rate of5 lL/min, while the liquid gauge pressure is maintainedbetween ±30,000 Pa. The range is chosen to lie well withinthe bubbling and breakthrough pressure of the membranes.This cycling procedure is done to evenly wet the sampleand to establish the total volume change between the posi-tive and negative pressure limits.

Once the pressure response is repeatable for constant

flow liquid intrusion/gas intrusion cycles (usually after 2–3 cycles), a stepwise liquid intrusion/gas intrusion experi-ment is performed. The concept for each step is sketchedin Fig. 2.

Each step takes place in four stages: in part A, equilib-rium is initially present in the liquid pressure at a given syr-

Fig. 2. The four stages (A–D) for each stepwise liquid/gas intrusionexperiment, with the top showing the volume imbibed from the syringeand the bottom showing the corresponding local differential liquidpressure measured by the pressure sensor.

J.D. Fairweather et al. / Electrochemistry Communications 9 (2007) 2340–2345 2343

inge volume. At this stage a Pc,i vs. Vsyr,i point is known.During part B, liquid water is injected into the system ata constant rate for 30 s while pressure is measured. Theflow is stopped in Part C and the local liquid pressurerelaxes to the equilibrium value at (Pc,i+1,Vsyr,i+1). The pro-cess is repeated to construct the Pc vs. Vsyr curve.

After 20 steps of liquid intrusion, the flow direction isreversed, and gas is intruded (while liquid is drained) for20 steps. Each stepwise liquid intrusion/gas intrusion cycletook place over 100 min. The initial liquid pressure isalways set to about �30,000 Pa, while the flow rate is cho-sen such that the highest pressure is +30,000 Pa after 20liquid intrusion steps.

The capillary number can be used to predict phase dis-placement behavior, and ensure that the experiment isrun in the same regime as an operating fuel cell. The capil-lary number is a ratio of the viscous forces to capillaryforces, defined as [22]

Ca ¼ uh iili

r

where huii is the superficial velocity of the invading phase,li is the viscosity of the invading phase, and r is the surfacetension between the two phases. The flow rates are variedbetween 1.3 and 2.1 lL/cm2/min, which for water invadingat 25 �C yields a capillary number range of 2.6–4.3 · 10�6.This corresponds roughly to the capillary number of a fuelcell operating with a current of 1 A/cm2 at 80 �C (around5 · 10�6). For this capillary number range, capillary forcesare important, and significant fingering of the intrudingphase is expected, as opposed to a smooth displacing front[23].

2.3. Calculating Pc vs. Sl

Because the initial saturation of the GDL is unknown,correlating Vsyr with Sl requires comparison to a well-defined reference state. The reference state chosen is theirreducible liquid volume, Vir, of the GDL sandwich. Inporous media, the capillary pressure asymptotes towardnegative infinity when it approaches the irreducible liquidvolume of the sample [15]. Thus, by using the irreduciblevolume as a reference, any syringe volume (Vsyr,i) can beadjusted to a total volume (Vsyr,i + Vir).

To determine Vir, the average initial dry weight (Mdry) isfirst measured for a set of GDL material samples. The sam-ples are then wetted with water under vacuum. To drainthe samples down to their irreducible volume, several layersof highly hydrophilic tissue paper (VWR International,West Chester, PA) are firmly pressed to the samples for15 min in a humidified air chamber (to minimize evapora-tion). The samples are again weighed (Mir) and the irreduc-ible volume is determined through

V ir ¼M ir �Mdry

qwater

ð3Þ

where qwater is the density of water. This measurement hasindicated that the irreducible volume is less than 1 lL/cm2

for all samples (irreducible saturation is less than 5%), sowe assume that the saturation at our lowest liquid pressureis effectively zero. Our liquid saturation (Sl) is then simply

Sl ¼V syr

V pore

; ð4Þ

3. Results and discussion

Fig. 3 shows a typical stepwise liquid intrusion/gasintrusion experiment for a sintered Avcarb sample.Fig. 3a illustrates how the controlled variable, Vsyr, is chan-ged during an experiment. The volume of water imbibedinto the GDL sample is stepped up 20 times until the targetmaximum pressure (�30,000 Pa) is achieved, and after-wards the water is drained in a similar stepwise fashion.The corresponding pressure response due to the appliedvolume change is shown in Fig. 3b.

Negative liquid pressure corresponds to the liquidwater–air interface present in hydrophilic-like (wettable)pores whereas positive differential liquid pressure corre-sponds to hydrophobic-like (non-wetting) pores. The hys-teresis between the liquid intrusion and gas intrusionbranches of the liquid pressure curve is dramatic, as shownin Fig. 3b. During liquid intrusion, positive liquid pressuremust be applied to force liquid into the sintered AvcarbGDL. However, to drain the same pore volume, negativeliquid pressure must be applied. While the GDL appearshydrophobic (positive capillary pressure, implying air-wetting) when liquid water is being moved into the mate-rial, it acts as a hydrophilic material (negative capillarypressure, water-wetting) when water is being removed.

Fig. 3. Typical experimental data for sintered Avcarb. The syringe volume(a) controls the sequence of stepwise liquid/gas intrusion as the liquidpressure response (b) is measured. Insets show some expanded steps.

Fig. 4. A set of liquid intrusion (filled points) and gas intrusion (outline)Pc vs. Sl curves for the Toray 090 (triangles), treated Avcarb P75T(circles), and treated and sintered Avcarb P75T (squares) sample. Solidcurves have been added to guide the eye.

2344 J.D. Fairweather et al. / Electrochemistry Communications 9 (2007) 2340–2345

All measurements are taken after reproducible cyclicbehavior is observed, so the response can be replicatedrepeatedly. Hysteresis is common in the porous materialsliterature, and is largely due to ‘‘ink-bottle’’ effects and hys-teresis in the contact angle h [15,24,25]. Pc vs. Sl data arecalculated from dynamic data like that in Fig. 3, usingthe methods described in Section 2.3.

Fig. 4a–c shows a set of liquid intrusion and gas intru-sion Pc vs. Sl curves for the Toray 090, treated AvcarbP75T, and treated and sintered Avcarb P75T GDL sam-ples, respectively. Each Pc vs. Sl curve in Fig. 4 correspondsto three replicate experiments, showing good reproducibil-ity of the hysteretic effects between liquid intrusion and gasintrusion.

The Pc vs. Sl curves obtained for each sample matcheswell with qualitative observations; the GDLs that repelwater (Fig. 4a and c) have largely positive liquid intrusioncurves, whereas the sample that spontaneously imbibeswater has a largely negative liquid intrusion curve. The car-bon fibers used in the GDL support are of intermediatewettability, with an advancing contact angle of around90� [26], which means samples can easily appear hydropho-bic during liquid intrusion and hydrophilic during gasintrusion. Hydrophobic agents such as Teflon can onlydo so much to mitigate this effect, since the receding con-tact angle of these agents is also often close to 90�[24,27]. Thus, the hydrophilic character during gas intru-sion is not surprising even for GDLs with hydrophobicliquid intrusion curves.

Quantitatively, the primary change in saturation occursover �104 < Pc < + 104 Pa, which is similar to the rangeobserved by Gostick and Koido [9,10]. However, we notethat Nguyen has reported that the primary change in Sl

occurs over a much narrower range, �200 < Pc < + 200 Pa[11]. We are examining the effect of relaxation time on thesemeasurements, since Fig. 3b clearly shows the transient canextend longer than the 120 s we allowed after each step.

J.D. Fairweather et al. / Electrochemistry Communications 9 (2007) 2340–2345 2345

All the curves in Fig. 4 display nearly asymptotic behav-ior as the irreducible liquid saturation and total saturationare approached. As discussed above in Section 2.3, sepa-rate gravimetric measurements indicate that the irreducibleliquid saturation is less than 5%; that is, the liquid phaseremains continuous even at very low saturation, andalmost all of the liquid can be removed through flow tothe surface. It is not surprising that the liquid phase hascontinuous access to the surface in these very thin samples.

4. Conclusions and implications

Careful control of GDL design and manufacturingcould lead to marked improvements in liquid water trans-port. We have shown that a microfluidic approach is capa-ble of measuring the liquid intrusion and gas intrusion Pc

vs. Sl curves for common GDL materials with differencesin each material clearly displayed. It was also shown thatit is possible to probe both hydrophilic-like and hydropho-bic-like pore volumes as well as approach the irreduciblewater and irreducible air phase saturations. Because ofthe device’s ability to measure the pressure response duringboth liquid intrusion and gas intrusion, valuable insightcan be gained into the effect of hysteresis on the apparentwettability of the GDL materials. The membranes usedto sandwich the GDL samples allow a wider range of sat-urations to be probed than was previously possible in GDLexperiments. As changes to GDL design and manufactur-ing are tested, the use of a microfluidic tool like this canprovide Pc vs. Sl relationships that fingerprint the mate-rial’s fundamental capability to transport liquid water.

We are working on improved device designs, longeroperating windows (to allow complete equilibration), dif-ferent operating parameters (temperature, GDL compres-sion pressure, etc.), use of the dynamic pressure data tounderstand transport properties as well as capillary ther-modynamics of GDL materials, and providing an appro-priate empirical Pc vs. Sl relationship.

Acknowledgments

This study was funded by Ballard Power Systems, theBoeing-Sutter endowment, and the University of Washing-

ton Fuel Cell Infrastructure Equipment Grant. We thankColleen Legzdins of Ballard Power for helpful conversa-tions and feedback.

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