Wetting properties of gas diffusion layers: Application of the Cassie–Baxter and Wenzel equations

Applied Surface Science 258 (2012) 5619–5627

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Wetting properties of gas diffusion layers: Application of the Cassie–Baxter and Wenzel equations Valérie Parry ∗ , Grégory Berthomé, Jean-Charles Joud Grenoble Institute of Technologies – SIMaP, 1130 rue de la Piscine, 38402 Saint-Martin d’Hères, France

a r t i c l e

i n f o

Article history: Received 1 December 2011 Received in revised form 10 February 2012 Accepted 12 February 2012 Available online 17 February 2012 Keywords: Contact angle Wilhelmy plate Hysteresis Gas diffusion layer PEMFC ESEM

a b s t r a c t In this paper, the wetting behaviours of as received and aged commercial 10% PTFE loaded gas diffusion layer were studied using the Wilhelmy plate method with liquid water temperature ranging from 5 to 60 ◦ C. Comparison were made with an untreated sample and a PTFE smooth plate. These experimental results, supported by chemical and morphological surface characterizations, were discussed in the frame of the Wenzel and Cassie–Baxter regimes. For each wetting regime, surface fraction of solid, PTFE and carbon fibres and/or roughness coefficient were estimated by solving a system of Cassie–Baxter and/or Wenzel equations. The transition to one wetting regime to the other is also commented. Finally, the effects of ageing and of water temperature were studied. Ageing was found to alter the wetting behaviour of the GDL through its chemical degradation. An erosion and the crazing of the PTFE coating and an oxidation of the carbon fibres were pointed out. The decrease of the water surface tension linked to an increase of its temperature is also shown to lead to a better wetting and to an increase of the solid surface fraction value. This effect is reinforced by GDL ageing. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The wetting behaviour of an ideal flat material is fixed by its chemical composition and can be characterized by a contact angle via the Youngs relation. The surface is commonly said to be hydrophilic when the contact angle is lower than 90◦ and hydrophobic when higher than 90◦ . However for contact angle values near 90◦ , the appearance a true hydrophilic or hydrophobic behaviour is not as clear and solids often exhibit mixed and complex wetting tendencies. Moreover working with real solids imply that the roughness and the heterogeneity of the solid surface modify the wetting properties, altering the true contact angle  to an apparent contact angle  * , and produce contact angle hysteresis [1]. Measurements based on static contact angle experiments are not sufficient to characterize the wettability of real solids with intermediate wetting behaviour and dynamic contact angle (DCA) experiments should be preferred. The Wilhelmy plate method allows to measure two contact angles. An advancing and a receding contact angles are obtained respectively when the triple line is in controlled movement by wetting the dry area of the solid by the liquid or by withdrawing the liquid from a formerly wetted

∗ Corresponding author. Tel.: +33 476826535. E-mail address: [email protected] (V. Parry). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.038

region of the surface. The difference between advancing and receding contact angles is the contact angle hysteresis, characteristic of the non-ideal solid. Proton exchange membrane fuel cells (PEMFC) undergo intense development as a high-efficiency and low-emission source of energy. Large-scale deployment of this technology is facing today two major challenges: decrease of the cost and increase of the lifetime [2–4]. Water management in the gas diffusion layers (GDLs) has been identified as an important performance degradation phenomenon [5,6]. These GDLs are anisotropic porous materials made of carbon graphite fibres. They allow distribution of reactant gases through the porous matrix and collection of current through the fibres. Engineering of GDLs is a key parameter to carry out a better water management. Poor water management results in either membrane dehydration leading to a decrease of its ionic conductivity or flooding as water condensation takes place leading to deleterious effects on mass transport resistance. Several design features are employed to handle the liquid water that forms in the cell. The most commonly used method is to treat the hydrophilic carbon graphite fibres with hydrophobic polymer, such as polytetrafluoroethylene (PTFE), to prevent capillary effect throughout the GDL. At the fibre scale, these two different materials will produce mixed wetting behaviour [7]. Wood et al. [8] have studied the wetting properties of GDL using composite sessile-drop contact angle modeling with Cassie equation. They have also characterized GDL ageing with XPS

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measurements [8]. In following papers, they quantified GDL mass transport resistance due to GDL hydrophobicity loss [9] and they have characterized the surface properties of over 30 different GDL types with Washburn measurements and Owens–Wendt modeling [10]. In a previous paper [11] we have characterized the internal wettability of various commercial GDLs with different PTFE loadings. Surface free energies of the GDLs fibres were determined by a combination of the Washburn and the Owens–Wendt two parameters theory. The values were found to be low indicating that GDLs are wet very poorly by most liquids, in agreement with Wood et al. [10]. An internal contact angle to 66 ◦ C water was directly obtained using the Washburn method. These results showed that treated GDLs are not fully hydrophobic at temperature representative of PEMFC operating conditions, in agreement with Lim et al. [12]. Using optically meniscus height measurement method based on the Wilhelmy plate gravimetric technique, they measure the surface contact angle of GDLs with 60–80 ◦ C liquid water. Increasing temperature was found to decrease the measured surface contact angle to less than 90 ◦ C, indicating that the GDLs surface was hydrophilic under these conditions. The aim of this paper is to analyse the wetting behaviour of as received and aged commercial GDLs taking into account their roughness and mixed wettability. Dynamic contact angles are measured according to the Wilhelmy plate method with liquid water at temperatures ranging from 5 to 60 ◦ C. Comparison are made with contact angles measured on PTFE smooth plate at the same water temperature. These experimental results supported by chemical and morphological surface characterizations are discussed in the frame of the Wenzel and Cassie–Baxter regimes [13–15]. 2. Experimental 2.1. Materials Two commercial GDLs from Freudenberg, H2315 and H2315T10A, were studied. They are composed of non-woven graphite fibres of 10 ␮m in diameter. Hydrophobic post-treatment is the single difference between the two GDLs. H2315 was not treated with hydrophobic polymer whereas H2315T10A contains 10 wt% PTFE. These samples appear as rigid plate with a thickness of 230 ␮m. Wilhelmy plate experiments were performed with square shape samples with dimensions of 25 mm × 25 mm. Artificial ageing of the treated GDLs samples were performed by immersion in 50 ◦ C water for 100 h. Surface analysis was performed afterwards on dry aged samples. A smooth PTFE plate, from Krüss, with dimensions 20 mm × 30 mm × 1 mm was also studied. 2.2. Surface characterizations 2.2.1. Dynamic contact angle experiments Dynamic contact angle experiments were performed using a Krüss K100MK2 tensiometer tooled up with a thermostat vessel allowing the monitoring of the temperature of the liquid. Each sample was held by a metal clamp attached to the micro balance. The liquid water, placed in a beaker located in the thermostat vessel, was raised by a screw-type motor allowing the submersion or inversely the removal of the sample from liquid. The measurement speed was fixed to 3 mm min−1 . Maximum and minimum immersion depth were set to 12 mm and 1 mm respectively. Surface changes can be monitored at the time-scale of the minute, as the typical time needed for one cycle is about 7 min. Experiment repeatability is achieved by running measurements with at least three cycles.

The micro balance measures a resulting force corrected from the weight of the plate as a function of the immersion depth of the sample. The contact angle is calculated from the Wilhelmy equation: F = L cos  − 

(1)

where  is the liquid surface tension,  is the contact angle, L is the perimeter of the sample. The buoyancy force , depends linearly on the immersion depth [16]. Temperature corrections were applied to surface tension using data from [17]. An advancing contact angle is determined from force data obtained during the submersion and a receding contact angle is determined from force data related to the removal of the sample from the liquid. 2.2.2. Conventional scanning electron microscopy Surface morphology of the as received and aged treated samples was characterized by a ZEISS Ultra 55 field emission gun scanning electron microscope (SEM). 2.2.3. Environmental scanning electron microscopy Environmental scanning electron microscope (ESEM) FEI QUANTA 200 equipped with a Peltier cooling stage was used to condense water droplets on surface of as received 10 wt% PTFE loaded and untreated GDLs by adjusting the pressure of the water vapour in the specimen chamber and the temperature of the cooling stage. 2.2.4. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) analyses were carried out in a Vacuum Generator XR3E2 apparatus with a Mg K˛ X-ray source at 15 kV, 20 mA operating condition. Analyses were carried out at an angle of 90◦ between the sample surface and the hemispherical analyser. Pressure in the sample chamber was less than 10−8 Pa. Sample dimensions were 1 cm × 1 cm and the analysed average area is about 40 mm2 . Survey spectra, from 0 to 1100 eV binding energy, were collected with a data rate of 2 points per eV, and signal-averaged over three scans. C 1s, O 1s and F 1s high-resolution spectra were collected with a data rate of 10 points per eV and signal-averaged over 15 scans. The pass energy was set at 30 eV for both survey and high-resolution spectra resulting in an energy resolution of approximately 0.1 eV. The electron binding energy scale was calibrated by setting the C 1s peak of graphitized carbon at 285.0 eV. Spectra were analysed using a free-ware software package, Fitt 1.2 (GTK) (Photoelectron Spectroscopy Lab, Seoul National University) with Shirley background corrections. Estimates of the elemental composition of the samples were obtained from the intensities of the photoelectron lines, using a standard quantitative XPS analysis. The atomic concentrations (at.%) were derived from the areas of the characteristic photoelectron lines after subtraction of a Shirley background, divided by the sensitivity factor [18]. 3. Results 3.1. Dynamic contact angle experiments Fig. 1 presents plots of the cosine of the measured contact angle versus immersion depth for three samples: the untreated GDL, the as received 10 wt% PTFE loaded GDL and the PTFE smooth plate. Cycles are obtained when samples are immersed in and withdrawn from 40 ◦ C liquid water. The lower (full symbols) and the upper (empty symbols) curves represent the advancing and receding situations respectively. Experiments were performed on PTFE loaded GDLs sample and PTFE plate at temperatures varying from 5 to 60 ◦ C. Cosine of the measured advancing and receding contact angles are plotted according water temperature in Fig. 2. Wetting

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Table 1 Summary of the GDLs wetting regimes deduced from the Whilhelmy plate measurements. H2315T10A

H2315 First cycle

Second cycle

Advancing curve

Slope Regime

Even Cassie–Baxter hydrophobic

Even Cassie–Baxter hydrophobic

Uneven Wenzel hydrophilic

Receding curve

Slope Regime

Uneven Wenzel hydrophilic

Even Cassie–Baxter hydrophilic

Even Cassie–Baxter hydrophilic

Fig. 1. Wilhelmy cycles of the untreated GDL (green triangles), the as received 10 wt% PTFE loaded GDL (blue diamonds) and the PTFE smooth plate (red circles) measured in 40 ◦ C water. The lower (full symbols) and the upper (empty symbols) curves represent the advancing and receding situations respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

regimes deduced from the Whilhelmy plate measurements are summarized in Table 1. 3.1.1. PTFE smooth slide For the PTFE smooth slide, results are characterized by a low hysteresis value of about 17◦ and even cycle slopes (Fig. 1). In the range of 5–60 ◦ C, contact angle values decrease with increasing temperature, from 113◦ to 93◦ and from 86◦ to 81◦ for advancing and receding angles respectively (Fig. 2). These values are in good agreement with data reported in the literature [19].

3.1.2. GDL H2315T10A–10 wt% PTFE Concerning the GDL loaded with 10 wt% PTFE, high and roughly constant – in the range of the studied water temperature – hysteresis value of about 65◦ is measured. The even advancing curve in Fig. 1 is related to a high apparent contact angle, greater than PTFE one. On the contrary, the receding slope is uneven and related contact angle value is close or lower than PTFE one, depending on the temperature range. The evenness of the slopes is though to be related to the surface morphology. For advancing contact angles, the sample exhibits a Cassie–Baxter regime, with an apparent contact angle value  * higher than the true one . The rough texture of the GDL is filled with air leading to a levelling of its roughness and a smooth advancing slope [20]. During the receding phase, water experiences the contact with the GDL texture leading to an irregular receding slope related to a Wenzel regime [21]. According to Fig. 2, contact angle values diminish with increasing water temperature. However, for temperature greater than 30–40 ◦ C, the decrease is getting worse leading to contact angle values lower than 90◦ for the receding case.

3.1.3. GDL H2315–untreated For the untreated GDL, a reduction in hysteresis after the initial change from the dry to the wetted surface is observed in Fig. 1. Alterations appear in the advancing contact angle, changing to an hydrophilic behaviour, with the receding contact angle staying at a constant level [5]. The first advancing slope is smooth and superimpose with the advancing curve of the 10 wt% PTFE loaded GDL. This behaviour is related to a high apparent contact angle, characteristic of an air trapped situation, the hydrophobic case of the Cassie–Baxter regime. The receding slope is even, contrary to the case of the PTFE loaded GDL, and related to an hydrophilic behaviour and an apparent contact value of about 20◦ . This change can be attributed to the formation of a water film filling in the texture of the GDL, typical of the hydrophilic case of the Cassie–Baxter regime. The uneven second advancing slope is related to a Wenzel regime where water experiences the GDL roughness.

3.2. Conventional scanning electron microscopy

Fig. 2. Plots of the cosine of the measured advancing (full symbols) and receding (empty symbols) contact angle versus liquid water temperature for the as received 10 wt% PTFE loaded GDL (blue diamonds) and the PTFE smooth plate (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3.2.1. GDL H2315T10A–10 wt% PTFE SEM surface view of the GDL loaded with 10 wt% PTFE is displayed in Fig. 3(a). We can observed an inhomogeneous repartition of PTFE which is partially responsible for the mixed wetting behaviour of the GDL. Polymer deposition is mainly achieved at fibres intersections. According to the cross section observation in Fig. 3(c), PTFE distribution is not uniform throughout the sample and spread out to the outer surfaces [5]. From both Fig. 3(a) and (c), PTFE distribution can be estimate over 30 ␮m – or 3 fibres – depth. Selection of the top fibres from Fig. 3(a), followed by image binarization allow us to roughly evaluate the surface fraction of the GDL. Resulting image is presented in Fig. 3(b). Calculated surface fraction of the GDL (fibres and PTFE) is in the range of 30%.

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Fig. 5. Environmental scanning electron micrograph of the untreated GDL obtained at 7.7 ◦ C and 9.6 Torr allowing an apparent contact angle calculation of 70–75 ◦ . Fig. 3. (a) SEM surface view of the as received 10 wt% PTFE loaded GDL. (b) Image binarization allowing a rough evaluation of the surface fraction of the as received 10 wt% PTFE loaded GDL. (c) SEM cross section of the as received 10 wt% PTFE loaded GDL.

3.2.2. Artificially aged GDL H2315T10A–10 wt% PTFE SEM surface observation of the artificially aged GDL is presented in Fig. 4. Immersion in 50 ◦ C water for 100 h results in erosion and crazing of the PTFE coating. This may be linked to the decrease of the hydrophobic properties of the GDL in high temperature water. 3.3. Environmental scanning electron microscopy ESEM is used as a contact angle analysis tool to investigate wetting properties of GDLs sample. Decreasing the temperature of the cooling stage to a few degree allows water droplets to condense on GDLs fibres and the measure of an advancing contact angle [22]. ESEM observations on 10 wt% PTFE loaded and untreated GDL samples are displayed in Figs. 5 and 6 respectively. Images of water droplets show strong topographic contrast thus reliable observations can be made. Even if the droplets are not the same size, they show the wetting characteristics of the fibres. In the following, the roughness at the level of the fibre is neglected. According to Fig. 5, untreated GDL fairly present hydrophilic behaviour with droplets in the Wenzel regime since carbon fibres are nearly chemically homogeneous. Treated GDL presented in Fig. 6 is more hydrophobic as observed by Zhang et al. on Toray carbon papers [23]. Due to local mixed wetting properties, the droplets are in the Cassie–Baxter regime. Mathematical model able to correct surface tilt has been proposed

Fig. 4. SEM surface view of the artificially aged GDL loaded with 10 wt% PTFE.

to calculate the real contact angle from the ESEM images on flat samples [24]. In our case of non axi-symmetric droplets condensed on fibres, only an apparent contact angles can be measured. The inclination of the observing point does not allow any refinements between advancing and receding contact angles. The contact angles measured from ESEM images for water gave values in the range of 70–75◦ for the untreated GDL and 90–95◦ for the treated GDL. This last result is in good agreement with the value deduced from our previously published Washburn experiments [11]. 3.4. X-ray photoelectron spectroscopy 3.4.1. General XPS-based analysis Survey spectra of the as received and artificially aged 10 wt% PTFE loaded GDL are displayed in Fig. 7. Samples surface compositions are dominated by carbon, fluorine and oxygen. Appearance of Au 4f photoelectron line is due to the application of gold foil on the edges of the sample in order to reduce charge effects. The fitted high resolution spectra are displayed in Figs. 8–10 respectively. The measured binding energies and the related chemical bonding of each component are reported in Table 2 [25,26]. The C 1s XPS spectra, displayed in Fig. 8, is fitted by two dominant peaks at the binding energies of 285.0 eV and 292.6 eV. The first is assigned to C C bonds and is related to the graphite fibres.

Fig. 6. Environmental scanning electron micrograph of the treated GDL obtained at 6.0 ◦ C and 8.0 Torr allowing an apparent contact angle calculation of 90–95 ◦ .

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Table 2 Measured binding energies, related chemical bondings and elemental surface concentrations of as received and artificially aged 10 wt% PTFE loaded GDLs. Component

Chemical bonding

Binding energy (eV) (± 0.1 eV)

As received

Aged

Carbon fibers

C C C O, C O

285.0 286.4 289.1 532.5 538.0

38.6 6.1 1.3 2.3 0

26.5 12.5 2.4 7.3 0.6

292.6 688.9 292.6 + DCE* 288.9 + DCE**

10.6 23.3 5.8 12.0

8.4 16.8 8.3 17.2

C O, C O PTFE

C C C C

F F F F

Surface composition (at.%)

DCE: Differential charging effect. * DCE = 6.7 eV. ** DCE = 5.7 eV.

oxidized forms of carbon (C O type bonds at 286.4 eV and C O type bonds at 289.1 eV). The F 1s spectrum in Fig. 9 can be fitted by one component at 688.9 eV corresponding to C F bonds in the PTFE. An additional peak related to the C F bonds is observed at higher binding energy due to differential charging effect. The gap between normal and charged peaks is the same for the both C 1s and F 1s spectra and is 6.7 eV for the as received sample and 5.7 eV for the aged sample. The O 1s photoelectron line displayed in Fig. 10 is linked to the oxidized forms of carbon (C O and C O type bonds at 532.5 eV and 537.8 eV). Regarding the atomic concentration in Table 2, the ratio of concentrations between fluorine and fluorinated carbon in PTFE is close to the theoretical value of 2 for both normal and charged peaks. This allows us to be confident towards our XPS decompositions.

Fig. 7. Survey spectra of the as received (blue) and artificially aged (black) 10 wt% PTFE loaded GDL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

The second peak is related to the C F bonds in the PTFE. An additional component related to the C F bonds is observed at higher binding energy due to differential charging effect. Differential charging effect arises when different regions of an insulating sample experience different electrical potential [27]. The two smaller components between the two major peaks are related to the

Fig. 8. C 1s spectra of the as received (blue full circles) and artificially aged (black empty circles) 10 wt% PTFE loaded GDL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3.4.2. GDL H2315T10A–10 wt% PTFE Regarding the atomic concentrations of the as received sample in Table 2, oxidized forms of carbon represent 20 at.% of the surface of the graphite fibre and the PTFE surface concentration is 51.7 at.% or 59.6 wt%. Treated GDL is loaded at a global level of 10 wt% PTFE. From our XPS analysis, we can conclude to an inhomogeneous repartition of the PTFE over the sample. This result is in good agreement with SEM observations of the as received sample presented

Fig. 9. F 1s spectra of the as received (blue full circles) and artificially aged (black empty circles) 10 wt% PTFE loaded GDL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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mixed surface is supposed to be comprised of solid and water pockets, with contact angles  f and 0◦ respectively. Wenzel equation is applied to fibres contact angle to compensate the effects of roughness due to fibre ridges through a coefficient r [13,14,21]. With the subscript characters a for the advancing slope, a1 for the first advancing slope, r for the receding slope, application of the virtual work principle yields to the apparent contact angle through the following equations. For H2315T10A: ∗ = s r cos fp − (1 − s ) cos fp,a

(2)

cos fp = p cos p,a + (1 − p ) cosf

(3)

For H2315: ∗ = s r cos f − (1 − s ) cos f,a

(4)

∗ cos f,r

(5)

1

Fig. 10. O 1s spectra of the as received (blue full circles) and artificially aged (black empty circles) 10 wt% PTFE loaded GDL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

in Fig. 3(a) and (c). PTFE coating is often applied by saturating the GDL with a PTFE emulsion followed by a drying step to remove the liquid. Mathias et al. [5] have shown that a slow drying rate is critical to obtain a uniform distribution throughout the sample. When dried quickly, PTFE migrates to the outer surfaces. From SEM and XPS results a quite homogeneous PTFE distribution of 59.6 wt% over about 30 ␮m – or 3 fibres – depth is expected. This result is compatible with a PTFE loading of 10 wt% over the entire depth of the GDL. Considering a density of 1.70 for graphite fibres and 2.40 for PTFE, PTFE and carbon fibres relative surface fractions can be evaluated to 51.1% and 48.9% respectively. 3.4.3. Artificially aged GDL H2315T10A–10 wt% PTFE According to Table 2, after ageing the surface concentrations of PTFE stay almost constant at a value of 50.7 at%. As seen in Figs. 8 and 10 there is a clear increase of the oxidized forms of carbon. Their concentrations on the surface of the graphite fibres increase from 20 at.% for the as received sample to 46 at.% for the aged sample. The increase of oxygen compounds enhances the polar character of graphite fibres, allows interactions with a polar liquid like water then increases their hydrophilic character. 4. Discussion 4.1. Wetting behaviour 4.1.1. Cassie–Baxter regime 10 wt% PTFE loaded and untreated GDLs exhibit Cassie–Baxter regimes as detailed in Table 1. For the treated sample and for the first advancing slope of the untreated sample, the texture of the GDL is filled with air and the apparent contact angle value is higher than the real one. For the receding slope of the untreated sample, a water film fills in the roughness leading to an apparent contact angle value lower than the real one. Cassie–Baxter equation describe the wetting of heterogeneous surfaces made of different species characterized by their own contact angle [15,21]. In the following, the subscript characters fp, p and f name heterogeneous surface made of carbon fibre and PTFE, homogeneous surface made of PTFE or made of carbon fibres respectively. For the hydrophobic GDL, the mixed surface is supposed to be comprised of solid and air, with contact angles  fp and 180◦ respectively. The solid itself being composed of PTFE with contact angle  p and carbon fibres with contact angle  f . The surface fraction of solid is denoted s and the relative surface fraction of PTFE is denoted p . For the wetted untreated GDL, the

= s r cos f + (1 − s )

Contact angle values obtained from experimental results can be ∗ ,  ∗ ∗ used: fp,a p,a , f,a and f,r from dynamic contact angle experi1

ments,  fp and  f from ESEM experiments. In ESEM, the inclination of the observing point does not allow any refinements between advancing and receding contact angles and the roughness at the level of the fibre is neglected. Temperature corrections apply through temperature coefficient of −0.1◦ /◦ C [28–30]. Dependency of contact angles on temperature is reported to be very small if not zero. Large temperature dependence would involve a solid near its melting point which is not the case in our study. Contact angles values deduced from experiments are reported in Table 3. Surface fractions of empty spaces, PTFE and carbon fibres and roughness coefficient can be estimated by solving a system of Cassie–Baxter equations. Solutions of Eqs. (2) and (5) give a surface fraction value of 18 ± 5% and a roughness coefficient of 2.1 ± 1. From Eq. (3) surface fraction value of PTFE relative to carbon fibre is 56 ± 12%. From SEM results, surface fraction of the GDL is evaluated to 30% and from XPS results, assessments of the relative surface fraction of the PTFE and carbon fibres are 51.1% and 48.9% respectively. Relative surface fractions for carbon fibres and PTFE calculated with Cassie–Baxter equations are similar to the XPS experimental value. However, calculated solid surface fraction value is not in agreement with the one obtained by SEM. Looking at dynamic contact angle results in Fig. 1, the first advancing slope of the untreated sample can be related to the hydrophobic case of the Cassie–Baxter regime. However, this curve is superimpose with the advancing curve of the 10 wt% PTFE loaded GDL. This can be possible if the surface fraction of empty space is high, fading the weight of the s r cos  f term in Eq. (4). According to  f value from Table 3, calculated surface fraction and roughness coefficient, the solution of Eq. (4) gives a value of ∗ . This result is close to the experimental data 135 ± 12◦ for cos f,a reported in Table 1.

1

4.1.2. Wenzel regime As detailed in Table 1, 10 wt% loaded and untreated GDLs exhibit Wenzel regimes for the receding and the second advancing slope respectively. The rough surface of the PTFE treated sample yields ∗ higher than  , the one of a smooth sura contact angle value fp fp face of the same material due to the increased contact between the solid and the liquid for a given projected area created by the roughness. For the same reasons, the apparent contact angle value of the untreated sample f∗ is lower than the real one  f . Apparent contact angles are obtained through the following Wenzel equations: For H2315T10A: ∗ cos fp,r = R cos fp

(6)

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Table 3 Summary table of contact angle values deduced from experiments. Contact angle

∗ fp,a

∗ fp,r

∗ f,a

∗ f,a

∗ f,r

 fp

 p,a

f

Origin Temperature range Value at 20 ◦ C (±2◦ )

DCA 5–60 ◦ C 148

DCA 5–60 ◦ C 79

DCA 40 ◦ C 148

DCA 40 ◦ C 50

DCA 40 ◦ C 22

ESEMa 6 ◦C 93

DCA 5–60 ◦ C 108

ESEMa 8 ◦C 74

2

Inclination of the observing point does not allow any refinements between advancing and receding contact angles.

Fig. 11. Scheme of the wetting process of the treated GDL for the advancing situation.

Form this simple model, we can explain the two types of wetting regime observed in our experimental conditions.

For H2315: ∗ cos f,a = R cos f

(7)

2

where R is the roughness due to empty space between fibres (seen in Fig. 3(a) and (b)). The roughness at the fibre scale is neglected. Roughness coefficient R is estimated from Wenzel Eq. (7). Eq. (6) is not used because  fp value from ESEM is close to 90◦ and will increase the roughness uncertainty. Contact angle values used in Eq. (7) are reported in Table 3. Solutions of Eq. (7) give a roughness coefficient R value of 2.3 ± 0.5. 4.2. From a Cassie–Baxter to a Wenzel regime From experimental results, GDL loaded with 10 wt% PTFE exhibits a Cassie–Baxter regime for the advancing contact angle and a Wenzel regime for the receding one. The advancing situation is summarized in Fig. 11. PA , the pressure in a point of the triple line, is given by Eq. (8). PA = Patm + gzmax = Patm + 

1 + gz R1 (x)

1 + gz = Pz + gz = Patm −  R2 (x)

4.3. Effect of water temperature 4.3.1. Ageing According to SEM and XPS observations of the artificially aged PTFE treated GDL immersed in 50 ◦ C water for 100 h, ageing leads to a chemical degradation of the sample through an erosion and the crazing of the PTFE coating and an oxidation of the carbon fibres. In terms of wetting, these occurrences can be considered as a decrease of the PTFE surface fraction and an alteration of the fibre contact angles. Since the surface fraction value of fibres remains low, the effect of a decrease of the advancing contact angle on the advancing slope of the Wilhelmy cycle is faded. However, the result of a decrease of the receding contact angle with ageing is more pronounced. This effect is illustrated in Fig. 13. Dynamic contact angles experiments were performed on a treated GDL at different temperatures. Three successive tests were performed on the same sample. First, the GDL

(8)

With  is the liquid density,  is the liquid surface tension and R1 (x) is the radius of curvature at a surface point with abscissa x and azimuth z. PA is the pressure exerted by the liquid on the air trapped bubbles at the surface of the sample. Conversely for the receding situation described in Fig. 12, the following equation applies: Patm

Fig. 12. Scheme of the wetting process of the treated GDL for the receding situation.

0

cos θ *

a

1

-0,2

12 mm immersio n in 20°C wate r advancing receding

-0,4

14 mm Immersio n in 20°C wate r af te r 12 mm immersio n in 40°C wate r advancing receding

-0,6

(9)

-0,8

The pressure in B, a point on the triple line, is given by Eq. (10).

-1

1 PB = Patm −  = Patm − gzmax R2 (x = 0)

10wt% PTFElo aded GDLA

(10)

Finally, we have PA > PB . In the receding situation the pressure exerted by air trapped bubbles (≥PA ) is higher than the pressure PB . In this way, the trapped air pockets are drained off and water experiences the contact with the GDL roughness.

0

2

4

6

8

10

12

14

Immersion dept h (mm)

Fig. 13. Wilhelmy cycles of a 10 wt% PTFE loaded GDL. (blue triangles) 12 mm immersion in 20 ◦ C water. (pink diamonds) 15 mm immersion in 20 ◦ C water after a 12 mm immersion in 40 ◦ C water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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GDL exhibits the same wetting properties as h GDLPTFE wetted by 40–50 ◦ C water. In the advancing situation and for the highest water temperature, the calculated true contact angles of the treated GDL are hydrophilic. This is not consistent with measured hydrophobic apparent contact angle in the frame of the Cassie–Baxter model. For this water temperature range, the roughness coefficient and surface fraction values may not be accurate. The wettability of solid surfaces is governed by both the surface roughness and the surface energy. Liu at al. [32] showed that the surface roughness improve the wettability of a hydrophobic solid surface to hot water. When the temperature of water is increased, the surface tension of water decreases leading to a better wetting and an increase of the surface fraction value. This effect is accentuated since GDL materials are more hydrophilic than PTFE and more strongly reinforced by the degradation of PTFE coating on the GDL fibres and/or to the oxidation of graphite fibres. 5. Conclusion

Fig. 14. Standard representation presenting the evolution of the apparent contact angle versus true contact angle for the treated GDL and a hypothetical GDL (h GDLPTFE ) exclusively made of PTFE.

was tested in 20 ◦ C water. The immersion depth was set to 12 mm. Then, the GDL was tested in 40 ◦ C water over the same immersion depth. Finally the GDL was once again tested in 20 ◦ C water but over a longer immersion depth set to 15 mm. The two immersions in 20◦ water are displayed in Fig. 13. The two advancing slope are superimposed. Since the surface fraction of empty space prevails, ageing is not detectable. However, the receding curve linked to the part of the GDL tested in high temperature water is singular. Immersion in high temperature water alters the surface chemistry and leads to a decrease of the receding contact angle. Ageing is obvious since apparent contact angle is linked to the real one simply through a roughness coefficient. 4.3.2. Summary curve To study the effect of water temperature on the wetting properties of the treated GDL, true contact angles  fp are calculated from our experimental results. In the advancing situation, a Cassie–Baxter regime described by Eq. (2) is assumed with a roughness coefficient of 2.1 ± 1 and a surface fraction value of 18 ± 5%. In the receding case, the GDL is supposed to follow a Wenzel regime described by Eq. (6) with a roughness coefficient of 2.3 ± 0.5. Comparisons are made with the PTFE sample. Apparent contact angle values of a hypothetical GDL (h GDLPTFE ) exclusively made of PTFE are calculated from the experimental contact angle values  p , assuming the same wetting regime, roughness coefficient and surface fraction as the 10 wt% PTFE loaded GDL. Results are presented in Fig. 14 using the standard representation proposed by de Gennes [21] and Shibuichi [31] showing the apparent contact angles cos  * versus the true contact angle cos . Low (5 ◦ C
The wetting behaviours of as received and aged commercial 10 wt% PTFE loaded GDL were studied using the Wilhelmy plate method with liquid water temperature ranging from 5 to 60 ◦ C. Comparison were made with an untreated sample and a PTFE smooth plate. These experimental results, supported by chemical and morphological surface characterizations, were discussed in the frame of the Wenzel and Cassie–Baxter regimes. For advancing contact angles, the rough texture of the treated GDL is filled with air and the sample exhibits a Cassie–Baxter regime, with an apparent contact angle value higher than the one of a smooth surface of the same material. For receding contact angles, water is believed to experience the contact with the GDL texture typical to a Wenzel regime. For each situation, surface fraction of empty spaces, PTFE and carbon fibres and/or roughness coefficient were estimated by solving a system of Cassie–Baxter and/or Wenzel equations. The transition to one wetting regime to the other was also explained. The effect of ageing on the wetting properties was also studied using SEM and XPS observations. Ageing lead to a chemical degradation of the GDL through erosion and crazing of the PTFE coating and an oxidation of the carbon fibres. Thus, the PTFE surface fraction decreases and the hydrophilic character of the fibre increases. Alteration of the wetting behaviour is more obvious on the receding contact angle since apparent contact angle is simply linked to the real one through a roughness coefficient. Finally, the effect of water temperature was studied via the representation of the apparent contact angles versus the true contact angle. In the receding situation and in the advancing situation and for the low water temperature range, the 10 wt% PTFE loaded GDL presents the same wetting behaviour as a hypothetical GDL made of PTFE. In the advancing situation and for the highest water temperature, the decrease of the surface tension of water leads to a better wetting and to an increase of the surface fraction value. This effect is believed to be reinforced by the degradation of PTFE coating on the GDL fibres and/or to the oxidation of graphite fibres. Acknowledgements This work was funded by the French Research National Agency (ANR) through the PAN-H program (project CHAMEAU ANR-06PANH-022) and by the Grenoble Institute of Technology (BQR 2008). References [1] R.E. Johnson, R.H. Dettre, Adv. Chem. Ser. 43 (1964) 112.

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