Energy Conversion and Management 76 (2013) 301–306

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Energy Conversion and Management

journal homepage: www.elsevier .com/ locate /enconman

Study on hydrophobicity degradation of gas diffusion layerin proton exchange membrane fuel cells

0196-8904/$ - see front matter � 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.enconman.2013.07.034

⇑ Corresponding author. Tel.: +86 411 84379123; fax: +86 411 84379185.E-mail addresses: xjli@dicp.ac.cn (X. Li), zhgshao@dicp.ac.cn (Z. Shao).

1 Co-corresponding author. Tel.: +86 411 84379153; fax: +86 411 84379185.

Shuchun Yu a,b, Xiaojin Li a,⇑, Jin Li a,b, Sa Liu a,b, Wangting Lu a,b, Zhigang Shao a,1, Baolian Yi a

a Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR Chinab University of Chinese Academy of Sciences, Beijing 100039, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 April 2013Accepted 20 July 2013

Keywords:Proton exchange membrane fuel cellGas diffusion layerHydrophobicity degradationPerformance

As one of the essential components of proton exchange membrane fuel cell (PEMFC), gas diffusion layer(GDL) is of importance on water management, as well on the performance and durability of PEMFC. Inthis paper, the hydrophobicity degradation of GDL was investigated by immersing it in the 1.0 mol L�1

H2SO4 solution saturated by air for 1200 h. From the measurements of contact angle and water perme-ability, the hydrophobic characteristics of the pristine and immersed GDLs were compared. To investigatethe causes for hydrophobicity degradation, the GDLs were analyzed by scanning electron microscopy, X-ray photoelectron spectroscopy and thermogravimetry. Further, the chemical compositions of H2SO4

solutions before and after immersion test were analyzed with infrared spectroscopy. Results showed thatthe hydrophobicity of immersed GDL decreased distinctly, which was caused by the damage of physicalstructure and surface characteristics. Moreover, the immersed GDL showed a worse fuel cell performancethan the pristine GDL, especially under a low humidity condition.

� 2013 Published by Elsevier Ltd.

1. Introduction

As a promising solution to replace conventional fossil energy,proton exchange membrane fuel cells (PEMFCs) are extensivelyused in various applications due to their high energy efficiency,low emission of greenhouse gases and promptness in start-up[1,2]. As one of the essential components of PEMFC, gas diffusionlayer (GDL) is useful to facilitate the transport of reactant gasesand the removal of product water [3,4] and thus of great impor-tance in improving the water management, a critical issue relatedto the performance and durability of PEMFC [5,6]. The porous GDLtypically consists of a microporous layer (MPL) and a gas diffusionbacking (GDB). The MPL composed of carbon powder and polytet-rafluoroethylene (PTFE) is applied on GDB by brushing, spraying,screen printing, etc. Many studies show that the MPL with properpore structure and hydrophobicity is beneficial in reducing thecontact resistance and in removing produced water in the catalystlayer [7]. With respect to GDB, it is also treated with hydrophobicPTFE to allow for an effective transport of liquid water and reactantgases [8]. Therefore, the hydrophobicity of GDL plays an importantrole in determining the water management characteristics duringthe process of PEMFC operations [9,10]. Moreover, the hydropho-

bicity of GDL must be watchfully controlled to achieve optimumperformance of the fuel cell without flooding or dehydration ofmembrane [11]. Simulation results also show that PTFE contentand the distributions of hydrophobic and hydrophilic regions with-in the GDL play a dominant role in influencing the effective trans-port of reactant gases and liquid water inside the GDL [12].

Besides, the degradation of GDL has a great effect on the dura-bility of PEMFC [13,14]. To date, few studies have been focused onthe durability and degradation of GDL, even though more and moreevidence has indicated that further investigation is necessary. Chenet al. [15] studied the durability of GDL under simulated PEMFCconditions and found the damage of physical structure and a de-crease of surface hydrophobicity due to carbon corrosion. Leeand Merida studied the durability of GDL under steady state andfreezing conditions [16]. However, reasons for the hydrophobicityloss during the degradation of GDL were not discussed in depth.Considering GDL loses its hydrophobicity under PEMFC environ-ment including air, water and high temperature, and high acidity,a better understanding of its hydrophobicity degradation is urgentand essential [17].

This paper aims to study the influence of water and gas environ-ment on the hydrophobicity degradation of GDL. For this purpose, asimple ex situ method is used to simulate PEMFCs environment. Itis found that the hydrophobicity degradation occurred under testconditions due to the changes of surface characteristics and dam-age of physical structure.

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2. Experimental

2.1. Immersion test

To examine the influence of water and gas environment withinthe PEMFCs on the hydrophobicity degradation of GDL, the com-mercial GDL (Sunrise Power Co., Ltd., China) was put into a beakerfilled with 1.0 mol L�1 H2SO4 solution for 72 h in vacuum oven sothat the porous structure was flooded with sulfuric acid solution.Then the beaker was kept in a water bath and heated up to 70 �Cfor 1200 h. Moreover, the H2SO4 solution was purged with air dur-ing the immersion test. Herein, the 1.0 mol L�1 H2SO4 solution andair purge were selected to simulate the aggressive operation envi-ronment of PEMFCs and accelerate the degradation of GDL. Afterthe test, the GDL was washed several times with deionized waterand dried in a vacuum oven at 60 �C for further analysis.

2.2. Characterization of GDLs

The surface contact angles of GDL were measured by a contactangle system (Drop Shape Analyzer 100, Kruss, Germany) and thewater permeability was characterized using a home-made equip-ment as reported in the literature [18]. The test pressure was0.035 MPa and the water was collected when the water flow ratethrough GDL became constant.

To analyze the reason for hydrophobicity degradation of GDL,the surface and cross-sectional morphologies of GDLs were ob-served with a field emission scanning electron microscopy (FESEM;Hitachi, S-4800). Additionally, X-ray photoelectron spectroscopy(XPS, ESCALAB 250Xi) was used to characterize the chemical com-ponent, electronic structure and binding energy of the samples.Further, the chemical compositions of H2SO4 solutions before andafter the immersion test were studied with liquid infraredspectroscopy (IR, NEAT 6700). To reveal the hydrophobicity

Fig. 1. FESEM images of (a) pristine MPL surface, (b) pristine GDB surface, (c) cross-sectiosection of immersed GDL.

degradation quantitatively, the mass variations of the pristineand immersed GDLs were evaluated by thermal gravitational anal-ysis (TGA, TA DSC-Q1000, USA). The TGA was measured in therange of room temperature to 1000 �C in a nitrogen environmentwith a heating rate of 10 �C min�1.

2.3. Evaluation of single cells

After physical and chemical characteristics, the pristine GDLand the immersed GDL were used as the cathode GDLs for fuel celltests. The fuel cells with different cathode GDLs were operated at65 �C with H2/O2 at the gauge pressures of 0.05 MPa. Pure hydro-gen and oxygen were humidified before entering the cell and theflow rates were 50 ml min�1 and 100 ml min�1, respectively. Thebubbling humidifier temperature of both the cathode and anodewere kept at 50 �C under a low humidity condition (about RH50%) and 65 �C under a high humidity condition (about RH100%). The hydrogen and oxygen were fed countercurrent.

3. Results and discussion

3.1. Contact angle and water permeability measurements

After immersion of 1200 h, the contact angle decreased from159.30� to 148.00� on the MPL surface and from 156.30� to137.20� on the GDB surface, which indicated a decrease of surfacehydrophobicity of GDL. However, the contact angel was only con-sidered as a measure of the surface wettability. To further examinethe internal wettability of GDL, the water permeability was mea-sured. It was found that the water permeability of immersed GDLwas 418.07 g s�1 m�2, 4 times higher than that of the pristineGDL (81.92 g s�1 m�2). This can be well explained by the hydro-phobicity degradation of GDL [19].

n of pristine GDL, (d) immersed MPL surface, (e) immersed GDB surface and (f) cross-

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3.2. SEM analysis

Fig. 1 illustrated the FESEM images of MPL and GDB surfaces andcross-sections of GDLs before and after immersion. Compared to thepristine MPL (Fig. 1a), it was clearly seen that more large holes ap-peared on the immersed MPL surface and it became uneven asshown in Fig. 1d, which could increase the contact resistance withthe catalyst layer and affect the surface hydrophobicity [20]. Mean-while, compared to the pristine GDB (Fig. 1b), there was a damageof PTFE coating on the immersed GDB surface. As shown in the area

Fig. 2. XPS survey spectra of (a) the pristine GDB, (b) the imme

Table 1The atomic concentrations of surface elements on MPLs and GDBs with XPS.

Immersion time (h) MPL surface

Atomic % Ca Atomic % Fb Atom

0 52.76 47.02 0.221200 67.27 30.60 2.13

a C = carbon powder.b F = fluorine.c O = oxygen.

of red line in Fig. 1e, PTFE detached from the carbon fibers afterimmersion. The irreversiblebreakage of PTFE coating on the surfaceof GDB would cause a distinct effect on surface characteristics. AsPTFE coating separated from carbon fibers decreasing the bondingfunction on the fibers, the numbers of fibers that were not coatedwith PTFE increased, producing a greater proportion of the hydro-philic surface area on GDB. The hydrophobicity of GDL would de-crease with the increase of hydrophilic surface area [20].

Furthermore, the immersed GDL showed remarkable changes inits cross-section compared with the pristine GDL. As shown in

rsed GDB, (c) the pristine MPL and (d) the immersed MPL.

GDB surface

ic % Oc Atomic % Ca Atomic % Fb Atomic % Oc

40.41 59.31 0.2845.26 52.84 1.90

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Fig. 1f, there existed many large gaps in the center of the immersedGDL and its physical structure became looser, while the pristineGDL (Fig. 1c) showed a dense structure. As for GDL, the hydrophilicpores were for the transport of water and the hydrophobic poresfor the transport of gases. To maintain normal mass transfer andavoid flooding, the proportion of hydrophobic to hydrophilic sur-face area should remain relatively stable [20]. However, as the in-ner structure of immersed GDL became looser, the stableproportion of hydrophobic to hydrophilic surface area would bedamaged influencing the smooth transport of water and gases. Inaddition, large gaps in the inner structure of GDL could decreasethe flow resistance of liquid water in hydrophobic regions becauseof the smaller resistance force of large pores, which would affectthe effective gas diffusion from flow channel to catalyst layer. Asthe irreversible and marked changes of inner structure occurred,part of GDL would be filled with liquid water and water floodingwould be easy to happen [21].

3.3. XPS analysis

Fig. 2 showed the XPS survey spectra of MPL and GDB surfacesof GDL before and after immersion. These spectra were calibratedagainst the major carbon peak at the binding energy of 284.6 eV.As for the immersed GDB shown in Fig. 2b, the intensity of theF1s peak at 688.9 eV decreased compared to the pristine GDB(Fig. 2a), which meant the loss of fluorine element. In addition, asmall peak was observed in the vicinity of 532.7 eV, which coin-cided with the electronic binding energy of O1s, indicating thepresence of oxygen element on the surface of the immersed GDB.Similar phenomena occurred on the immersed MPL surface. Asshown in Fig. 2d, there existed O1s peak at 532 eV and a markeddecrease of the F1s peak at 688.9 eV compared to the pristineMPL (Fig. 2c).

The element contents on MPL and GDB surfaces of the im-mersed GDL can be obtained based on Fig. 2 and the results wereshown in Table 1. Note that the atomic concentration of oxygenelement increased from 0.22 to 2.13 on MPL surface, and from0.28 to 1.90 on GDB surface. Meanwhile, the atomic concentrationof fluorin element decreased from 47.02 to 30.60 on MPL surface,

Fig. 3. XPS C1s peaks of (a) the immers

and from 59.31 to 52.84 on the GDB surface. Considering thehydrophobicity of PTFE and the hydrophilicity of oxygen functionalgroups, the increase of oxygen concentration and the decrease offluorin concentration suggested the changes in the surface charac-teristics of GDL, destroying its surface hydrophobic properties andmaking it more hydrophilic [22].

In order to identify the oxygen functional groups, the C1s spec-tra of the immersed GDB and MPL were analyzed. As shown inFig. 3a, the carbon in C1s XPS could be divided into several parts,of which the main peak at 284.6 eV was attributed to graphitizedcarbon (CAC and C@C), the one at 285.3 eV could be ascribed tothe carbon in ether or hydroxyl (CAO), and the one at 286.4 eV cor-responded to the carbon in the double bonded carbon oxygengroups (C@O). The CAO groups mainly resulted from the oxidationof the breakage of C@C bonds on the surface of carbon materialsand the CAO groups could be further oxidized to the C@O groupsunder the erosion of H2SO4 solution and air [23]. In addition, itcan be found that the oxygen functional group on the immersedMPL was single CAO bond with the binding energy at 285.3 eVas shown in Fig. 3b.

3.4. IR analysis

Fig. 4 showed the typical transmittance infrared spectra ofH2SO4 solutions before and after immersion test. It was obviousthat some new peaks were observed from the spectra of the latterH2SO4 solution. The strong and sharp absorption peaks at1216.54 cm�1 and 1170.95 cm�1 were CF2 asymmetric stretchingvibration and symmetry stretching vibration, respectively. Addi-tionally, the strong absorption peaks at 1295.54 cm�1 and1021.54 cm�1 were assigned to the CAF stretching vibrationabsorptions. Meanwhile, the peak at 610.27 cm�1 was CAF defor-mation vibration absorption [24]. All the above results showedthe existence of chemical composition of PTFE in the solution, indi-cating that PTFE fell off from GDL. Considering that the concentra-tion of PTFE was related to the hydrophobicity of GDL, thedesquamation of PTFE could decrease the hydrophobic characterof GDL [17].

ed GDB and (b) the immersed MPL.

Fig. 4. Infrared spectra of H2SO4 solutions before and after immersion test.

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3.5. TGA analysis

Fig. 5 gave the TGA results of the pristine and immersed GDLs.From the pyrolysis characteristics of pristine GDL, we can see thatPTFE started to be pyrolyzed at the temperature of approximately500 �C and there was only one weight loss step in the entire pro-cess of the pyrolysis. Due to the fact that PTFE was entirely pyro-lyzed at 620 �C and that the carbon materials were not pyrolyzedbelow 620 �C, the relative content of PTFE in GDL can be calculatedaccording to the mass loss of GDL between 500 �C and 620 �C [25].From 500 �C to 620 �C, the mass losses of the pristine and the im-mersed GDLs were 8.87% and 7.73%, respectively. This indicatedthe loss of PTFE during the immersion test, which was consistentwith the results of IR analysis. As binder and hydrophobic agentin GDL, the decrease in the content of PTFE would reduce thehydrophobicity of GDL and destroy its structure. In addition, itwas evident that there existed a weight loss around 250 �C in theTGA curve of the immersed GDL due to the residual H2SO4 in GDL.

3.6. Single fuel cell performance

To study the effects of the GDLs before and after immersion teston fuel cell performance, a single cell test was carried out using a

Fig. 5. TGA results for GDLs.

5 cm2 active area test cell in humidified H2/O2. The polarizationcurves of single cells with different GDLs under the low and highhumidity conditions were shown in Fig. 6a and b, respectively.As shown in Fig. 6a, it was evidently seen that the immersedGDL showed a worse performance than the pristine GDL underthe low humidity condition. At the current density of1600 mA cm�2, the performance of the fuel cell with the pristineGDL achieved 0.405 V while that with immersed GDL was0.305 V. As well known, the membrane can be easily dehydratedunder the low humidity condition, leading to a decrease in fuel cellperformance. However, with the overall decrease of the hydropho-bicity of immersed GDL, more water generated at the cathode cat-alyst layer would enter into the GDL instead of membrane, whichwould aggravate the dehydration of membrane [11]. This mightbe the primary reason for the worse performance of immersedGDL under the low humidity condition. Besides, as shown inFig. 6b, the maximum power density of the fuel cell with the pris-tine GDL was 916.4 mW cm�2 at 2200 mA cm�2, which was118 mW cm�2 larger than that of the immersed GDL under thehigh humidity condition. The worse fuel cell performance withthe immersed GDL at the high humidity condition could be as-cribed to the decrease of its hydrophobicity. As the hydrophobicity

Fig. 6. The polarization curves of H2/O2 fuel cells operated at 65 �C with GDLsbefore and after immersion test under (a) a low humidity condition and (b) a highhumidity condition.

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of immersed GDL decreased, part of GDL could be filled with liquidwater, which would hinder the transport of oxygen to the catalystlayer and further decrease the fuel cell performance.

4. Conclusion

The hydrophobicity degradation of GDL was investigated byimmersing it in the 1.0 mol L�1 H2SO4 solution with air purge. Afterimmersion of 1200 h, the surface contact angles of GDL decreasedand the water permeability reached five times the pristine GDL.SEM images showed that the surface morphology and cross-sec-tion of the immersed GDL destroyed. XPS results indicated a de-crease of fluorine element and an increase of oxygen functionalgroups (CAO and C@O) on the surfaces of GDL. Further, liquidinfrared spectroscopy results showed that PTFE fell off from GDLto the acid solution and the relative content of PTFE in GDL de-creased from 8.87% to 7.73% by TGA analysis. In addition, the im-mersed GDL showed worse fuel cell performances than thepristine GDL under both high and low humidity conditions, espe-cially under the low humidity condition.

Acknowledgments

The authors acknowledge financial support from the NationalNatural Science Foundation of China (21076210 and 20936008).

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