Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: Part II. In operando synchrotron imaging for microscale liquid water transport characterization

Journal of Power Sources xxx (2017) 1e10

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Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: Part II. In operando synchrotron imaging for microscale liquid water transport characterization S. Chevalier a, N. Ge a, J. Lee a, M.G. George a, H. Liu a, P. Shrestha a, D. Muirhead a, N. Lavielle b, B.D. Hatton b, A. Bazylak a, * a

Thermofluids for Energy and Advanced Materials (TEAM) Laboratory, Department of Mechanical & Industrial Engineering, University of Toronto Institute for Sustainable Energy, Faculty of Applied Science & Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, M5S 3G8, Canada Department of Materials Science & Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, M5S 3G8, Canada

b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Novel hydrophobic eGDL are fabricated and tested in situ for in operando imaging.
The membrane liquid water content is measured via low frequency impedance.
eGDL micro-pores enhance liquid water removal, even under the rib.
eGDL fiber alignment prevents MEA deformation under the rib.
eGDLs facilitate membrane dry-out, even under highly humidified operating conditions.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 November 2016 Received in revised form 20 January 2017 Accepted 26 January 2017 Available online xxx

This is the second paper in a two-part series in which we investigate the impact of the gas diffusion layer structure on the liquid water distribution in an operating polymer electrolyte membrane (PEM) fuel cell through the procedures of design, fabrication, and testing of novel hydrophobic electrospun gas diffusion layers (eGDLs). In this work, fibre diameters and alignment in eGDLs are precisely controlled, and concurrent synchrotron X-ray radiography and electrochemical impedance spectroscopy (EIS) are used to evaluate the influence of the controlled eGDL parameters on the liquid water distribution and on membrane liquid water content. For eGDLs with small fibre diameters (150e200 nm) and correspondingly smaller pore sizes, reduced liquid water accumulation under the flow field ribs is observed. However, more liquid water is pinned onto the eGDL e at the interface with flow field channels. Orienting fibre alignment perpendicular to the flow field channel direction leads to improved eGDL-catalyst layer contact and prevents rib-channel membrane deformation. On the other hand, eGDLs facilitate significant membrane dry-out, even under highly humidified operating conditions at high current densities. © 2017 Elsevier B.V. All rights reserved.

Keywords: PEM fuel cell Electrospun gas diffusion layer (eGDL) Electrospinning X-ray radiography Membrane dry-out Liquid water management

1. Introduction * Corresponding author. E-mail address: [email protected] (A. Bazylak).

Polymer

electrolyte

membrane

(PEM)

fuel

cells

http://dx.doi.org/10.1016/j.jpowsour.2017.01.114 0378-7753/© 2017 Elsevier B.V. All rights reserved.

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electrochemical energy conversion devices which use hydrogen and oxygen to produce electricity with heat and water as the only local by-products. The gas diffusion layer (GDL) is a highly porous material which is primarily composed of electrically and thermally conductive carbon fibres, which provide transport pathways for reactants and products. It has been demonstrated [1] that the three-dimensional (3D) structure of the GDL strongly influences the nature of liquid water transport from the catalyst layer (CL) to the flow field channels, as well as the overall performance of the fuel cell. While water is required to maintain ionic conductivity of the membrane, excessive water tends to accumulate in the GDL and flow channels. Excess water accumulation in the fuel cell leads to increases in the mass transport resistance for reactant gases as well as overall performance degradations. Significant progress has been made towards investigating the relationship between the liquid water transport and GDL structure via advanced numerical simulation tools [2e5]. By employing numerical models, simulated GDL structural parameters such as pore size distribution and fibre diameter can be controlled in order to build a realistic GDL structure [6]. However, accurately modelling fuel cell performance under realistic operating conditions remains a challenge. In contrast, in situ visualizations of liquid water [7,8] have enabled the study of liquid water transport and the impact of GDL structural parameters under actual fuel cell operating conditions. Until the GDL structure and performance is better understood and optimized e through the use of in operando imaging and numerical models e fuel cell performance will be constrained by the design of conventional carbon fibre GDL structures that are commercially available. Recently, the fabrication of the GDL using electrospinning techniques has been recognized as a powerful tool for accurately controlling the 3D GDL structure [9e11]. Electrospinning is a scalable and versatile technique for the preparation of carbon nanofibres through the carbonization of electrospun polyacrylonitrile (PAN) [12]. In this technique, the fibre morphology and dimensions can be precisely controlled [13,14]. For example, the prescribed structure of nanofibrous membranes were obtained using a rotating drum to achieve effective fibre alignment [15]. Details of the fabrication methodology employed to manufacture these nanofibrous materials were reported in the first paper of this twopart series [16], wherein the relationship between fuel cell performance and the GDL structure was investigated. It was found that hydrophobic electrospun GDLs (eGDLs) with nano-fibres (200 nm in diameter) aligned perpendicularly to the channels led to improved membrane hydration and may have improved the CLeGDL contact relative to the other eGDL materials tested. Smaller fibres (150e200 nm diameter) were also shown to increase eGDL electrical conductivity compared to larger fibres (1 mm diameter). Synchrotron X-ray radiography is a powerful technique for visualizing liquid water in the GDL and the flow field channels at high spatial resolutions (spatial resolutions below 1 mm were reported by Manke et al. [17]) and at temporal resolutions as high as several milliseconds [18]. High imaging temporal resolution is critical for capturing the dynamic liquid/gas transport phenomena that occur during PEM fuel cell operation [19]. Examples of fast dynamic behaviour include the build-up of liquid water in a fuel cell operating in dead-ended anode mode [20], the observation of dynamic liquid water transport in the GDL [21], and eruptive water transport behaviour [22e24]. Synchrotron X-ray radiography was also used to investigate the impact of the GDL structure on the €tter et al. [25] used this technique transport of liquid water; Marko to identify preferential liquid water transport pathways through the cracks of the microporous layer (MPL). It was also reported that fibre alignment influences liquid water transport from the GDL regions under the lands (ribs) to the flow field channels, with

perpendicular fibre alignment facilitating improved water removal [26]. Antonacci et al. [27] employed synchrotron X-ray radiography to understand the role of MPL thickness on liquid water transport and found that reduced MPL thickness enhanced liquid water diffusion toward the anode. In their work, Chevalier et al. [28] combined synchrotron X-ray radiography with fuel cell impedance measurements to quantify the relationship between GDL oxygen diffusivity and liquid water saturation. These works [26e29] provided significant insights towards understanding the role of the GDL structure on in operando liquid water transport behaviour; however, to the authors' best knowledge, the impact of several aspects of GDL structure, such as fibre diameter and alignment, on the in operando distribution of liquid water remains poorly understood. In addition to liquid water management in the GDL and flow fields of the fuel cell, it is also critical to understand water management of the polymer electrolyte membrane. However, quantifying the water content in the membrane of an operating fuel cell can be challenging using X-ray imaging techniques due to the highly attenuating Nafion-based membrane at X-ray energy levels typically used for in operando imaging of fuel cells (15e24 keV [17,30]). As an alternative to direct visualization, electrochemical impedance spectroscopy (EIS) can be used to measure membrane hydration through the ohmic resistance of the fuel cell (measured at high frequencies, e.g. 1 kHz) and the low frequency impedance [31e35], e.g. below 1 Hz. Schneider et al. [36] reported the appearance of a capacitive arc at low frequencies in the Nyquist diagram for fuel cell impedance, and these results were attributed to low diffusion rates of water transport through the membrane. Rezaei Niya et al. [37,38] analyzed low frequency impedance using an equivalent electrical circuit from which they computed the characteristic time constant for water diffusion in the membrane. The characteristic time constant was identified to be a function of the diffusion coefficient of water through the membrane and the membrane thickness. Therefore, the value of this characteristic time can be used as an indicator of membrane hydration and shrinking/swelling. In this way, the impact of the GDL structure on the liquid water distribution in the GDL, the flow field channels, and on the membrane hydration can be investigated using a combination of concurrent synchrotron X-ray radiography and EIS, respectively. The feasibility of this approach was also demonstrated by Antonacci et al. [39]. This work is the second part in a series of two papers investigating the relationship between GDL structure and the distribution of liquid water in the channels, the GDL, and the membrane of an operating PEM fuel cell. Four eGDLs rendered hydrophobic with varying fibre diameters and alignment were investigated in this work. Concurrently performed synchrotron X-ray radiography and EIS were used to quantify the in operando distribution of liquid water in the GDLs and flow field channels and to evaluate membrane hydration. Specifically, the relationship between GDL structure and liquid water transport behaviour, such as liquid water pinning at the top surface of the GDL in the channels, accumulation of liquid water under the flow field lands, mechanical deformation of the membrane electrode assembly (MEA), and membrane dehydration were investigated. 2. Methodology 2.1. Fabrication and selection of eGDLs The fabrication process of the eGDLs is fully detailed in the first paper of this study [16], and a brief summary is provided in this section. Initially, a solution of PAN in N,N-dimethylformamide (DMF) was prepared and electrospun onto a rotating drum using

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a static voltage of 25 kV. In this fabrication process, the fibre diameter is controlled by the concentration of the PAN solution and the degree of fibre alignment is controlled by increasing the speed of the rotating drum. The concentration of PAN was set to 8 and 14 wt %; subsequently producing PAN-based fibrous materials with average fibre diameters of 0.28 mm and 2.11 mm, respectively. The rotational speed of the drum was set to 3000 rpm in order to produce aligned fibres, and to 20 rpm for the generation of randomly oriented fibres. Carbon fibres were produced from the fibrous PAN structure through a thermal treatment process. The fibrous PAN materials were initially stabilized at 240  C for 2 h while exposed to air (heating rate 1  C/min) and subsequently carbonized at 1100  C for 1 h with nitrogen exposure (heating rate 5  C/min). A wettability treatment was applied on the carbon fibre materials using a direct fluorination technique by molecular grafting to ensure minimal alteration of the eGDL structure. The molecular grafting was achieved by first creating free hydroxyl radicals on the fibre surface by exposure to air plasma. Secondly, the materials were placed in a vacuum chamber with 40 mL of trichloro(1H,1H,2H-perfluorooctyl)silane for 8 h. This molecule was chemically bonded at the surface of all the fibres of the eGDLs which created a hydrophobic material. It was assumed that the wettability treatment was distributed homogeneously throughout the bulk of the material. Although this assumption was not verified using characterization methods, diffusion of the small fluorosilane molecules in the porous electrospun network (inter-fibre distance of several microns) is likely. In the case that hydroxyl radicals are not formed through the entire thickness of the material via the air plasma treatment, fluorosilanes, even if not chemically bonded, should adsorb at the surfaces of the fibres. Additionally, the in situ performance of the treated eGDLs (measured in Part I of this study) supports our assumption, and the hydrophobic treatment appeared to be durable even after several hours of in situ tests. All eGDLs fabricated in this study underwent molecular grafting. The impacts of three main structural properties of the eGDLs on the in operando liquid water distribution were investigated in this study. These structural properties include the fibre diameter, fibre alignment, and presence of the macro-fibrous substrate TGP-H-060 carbon paper (named T60 in this paper). In the test case, which included TGP-H-060, the macro-fibrous substrate was positioned between the eGDL and the fuel cell flow field. The naming convention of the four eGDLs, used and their respective properties are summarized in Table 1. The structure of each eGDL was visualised using a scanning electron microscope (SEM), and the fibre diameters were measured using the image processing software (Fiji®). More details about the image processing procedure can be found in the first paper of this study [16]. 2.2. Fuel cell hardware & operating parameters A miniature cell with an active area of 0.68 cm2 (0.80 cm by 0.85 cm) was specifically designed for in-plane liquid water visualizations and the quantification of through-plane liquid water distributions. Both the anode and cathode hydrophilic flow fields

3

consisted of 0.5 mm-wide parallel channels and lands (ribs). The membrane electrode assembly (MEA) was composed of a 23 mmthick reinforced Nafion® membrane HP (Ion Power) with coated CLs (10 mm). The platinum loading was 0.3 mg/cm2 on both the anode and cathode electrodes and a fresh MEA was used for each fuel cell assembly. Three fuel cells were assembled using only eGDL materials (A, B and B-a) at both the anode and cathode and placed between the CL and the flow field. In the fourth experimental fuel cell (eGDL B-s) eGDL B samples were placed between the MEA and a TGP-H-060 substrate in both the anode and cathode half cells. As a result in the fourth experimental fuel cell, the eGDL material was tested as a replacement for a conventional MPL between the CL and the typical carbon fibre substrate. The compression ratio of each eGDL was controlled using stacked spacers (polyethylene naphthalate (PEN) sheets) at the anode and cathode. For eGDLs A, B and B-a 75 mm-thick PEN sheets were used, and for eGDL B-s a 263 mmthick PEN sheet was used during fuel cell assembly. The use of spacers ensured the controlled compressed thicknesses of our GDLs; however, a uniform compression force on each GDL cannot be guaranteed along the ribs and channels. During testing, the fuel cell operating conditions were controlled with a Scribner 850e fuel cell test station (Scribner Associate Inc., Southern Pines, NC). Each fuel cell was fed with air and pure hydrogen at 2 bar (absolute), 100% relative humidity, and using flow rates of 1 L/min at the anode and cathode. A water-bath cooling system was used to maintain the cell temperature at 60  C. These highly humid conditions were intentionally chosen to enhance liquid water accumulation within the fuel cell. Each fuel cell was conditioned for 6 h with cyclic loading until maximum performance was reached prior to in operando liquid water visualizations. At the end of the conditioning step, the polarisation curve of each cell was measured. These curves are available in the supplementary information. 2.3. Synchrotron X-ray radiography Synchrotron X-ray radiography was used to perform in operando liquid water visualizations at the Biomedical Imaging and Therapy Bending Magnet (05B1-1) beamline at the Canadian Light Source Inc. (CLS) in Saskatoon, Canada [40]. A schematic of the experimental setup is presented in Fig. 1(a). The monochromatic Xrays at an energy level of 24 keV which were transmitted through the fuel cell were detected by an AA-40 scintillator. The scintillator converted these transmitted X-rays in the visible spectrum (with a spatial resolution of 10 mm). Images were captured with a chargecoupled device (CCD) camera (Hamamatsu ORCA Flash4.0) with a pixel size of 6.5 mm and an experimentally controlled temporal resolution of 3 s per frame. A typical unprocessed grey-scale image of the fuel cell in-plane view with the eGDL B-a is presented in Fig. 1(b). The X-ray images recorded during fuel cell operation were processed by accounting for the variation of X-ray absorption. Initially, a reference set of images, termed the dry-state images, was obtained while the fuel cell was operating at the open circuit voltage (OCV) with fully humidified reactant gases. In the dry-state

Table 1 Naming convention for eGDLs. The acronym T60 designates the TGP-H-060 carbon paper from Toray. Name

eGDL A

eGDL B

eGDL B-a

eGDL B-s

PAN concentration (wt%) Fibre alignment Substrate Fibre diameter (after thermal treatment in mm) Compressed thickness (mm)

14 no no 1.14 þ/ 0.07 75

8 no no 0.15 þ/ 0.01 75

8 yes no 0.20 þ/ 0.02 75

8 no yes (T60) 0.15 þ/ 0.01 263

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S. Chevalier et al. / Journal of Power Sources xxx (2017) 1e10

also accounted for in the last step of the image processing procedure. The obtained processed images (arrays of pixel intensity, P) were assumed to contain negligible amounts of background noise. The liquid water attenuation coefficient, mw , was measured using an in-house calibration device and methodology reported by Ge at al [30]. 2.4. Fuel cell impedance measurements

Fig. 1. In operando liquid water visualizations using synchrotron X-ray radiography. (a), Schematic of the experimental setup. (b), Raw grayscale radiograph of the fuel cell. The length of the scale bar in (b) is 1 mm.

image, it was assumed that liquid water was not present in the fuel cell. During operation, particularly with increasing current density, liquid water accumulates within the cell and increasingly attenuates the X-ray beam. This reduction in beam intensity passing through the cell is governed by the Beer-Lambert law:

Id ¼ I0 $expð  m1 $XÞ;

(1)

The fuel cell impedance was measured at two current densities, 0.5 A/cm2 and 1.0 A/cm2, using the operating conditions described in Section 2.2. The fuel cell current was slowly increased from OCV to 0.5 A/cm2 or 1.0 A/cm2 and kept constant for 15 min prior to performing EIS measurements. Fuel cell wet-state images were recorded throughout the experiments. The fuel cell impedance was analyzed using the equivalent electrical circuit presented in Fig. 2. This circuit was used to model the electrochemical reactions at the anode and cathode, the ohmic resistance due to the membrane, eGDL and flow field electronic resistances, and the liquid water transport in the membrane. The use of thin eGDLs (75 mm) and high mass flow rates led to high oxygen diffusion in the eGDL, and the mass transport impedance was observed to be relatively small compared to the activation and membrane impedances. Therefore, the mass transport impedance, usually modeled using the Warburg impedance [39], was not taken into account in the model proposed in Fig. 2. The impedance of the electrochemical reaction at the anode, Za , was computed as


Za ¼

1 þ j$u$Ca Ra

1

;

(4)

where I0 and Id are the X-ray beam intensities before and after passing though the fuel cell (see Fig. 1(a)), m1 [cm1] is the X-ray attenuation coefficient of the fuel cell materials, and X [cm] is the thickness of all the materials traversed by the beam. A second set of images, termed the wet-state images, was recorded when the fuel cell was operating and generating liquid water. The X-ray intensity in the wet-state images is also given by the Beer-Lambert law as

where Ra is the anode activation resistance, Ca is the anode double pffiffiffiffiffiffi ffi layer capacity, u is the angular frequency, and j ¼ 1 is the unit imaginary number. At the cathode, the electrochemical impedance, Zc , was obtained as

Iw ¼ I0 $expð  m2 $X  mw $Xw Þ;

where Rc is the cathode activation resistance, and ZCPE is the constant-phase element (CPE) used to model the electrochemical reaction at the cathode catalyst layer as a non-homogeneous reaction. The constant-phase element is comprised of a planar capacitor, Cc , and a constant phase angle, a, which accounts for losses that occur in porous electrodes. The CPE impedance is defined as

(2)

where Iw is the wet-state intensity after the X-ray beam passed through the operating fuel cell, m2 [cm1] is the X-ray attenuation coefficient of the fuel cell materials, mw [cm1]is the X-ray attenuation coefficient of liquid water, and Xw [cm] is the liquid water thickness. The variation in X-ray attenuation between the dry- and wet-state images was obtained from the ratio of Equations (1) and (2) as


 1 I m  m1 P¼ ln d ¼ Xw þ 2 $X mw Iw mw

(3)

where P represents the variation of X-ray intensity between the dry- and wet-state images. In ideal conditions, the X-ray attenuation from the fuel cell materials is identical in the dry- and wetstate images, therefore m1 ¼ m2 , and P is directly equal to the in operando liquid water thickness Xw in cm. Nevertheless, in operando fuel cell material displacements due to the membrane swelling or shrinking can lead to a scenario where m2 sm1 , and the values of P can become significantly negative which indicates nonnegligible material displacement in the fuel cell. Image processing entailed a normalization procedure which accounts for dark field images that are used to quantify the background noise. A correction was also applied to account for the linear decrease in beam intensity over time. Bulk fuel cell movement was


Zc ¼

1 1 þ Rc ZCPE

ZCPE ¼

1

;

(5)

1 : Cc $ðj$uÞa

(6)

According to Rezaei-Neya et al. [37], the impedance of the timedependent membrane-water-transport, Zl , can be modeled as follows:


Zl ¼

1 þ j$u$Cl Rl

1

;

(7)

where Rl is the liquid water resistance in the membrane, and Cl is a capacity which represents the liquid water transport rate in the membrane. From the values of Rl and Cl , the characteristic time of the water diffusion in the membrane, tm , can be computed as

tm ¼ Rl $Cl ;

(8)

This characteristic time is a function of multiple fuel cell parameters [37], but it depends, in particular, on the membrane

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Fig. 2. Electrical equivalent circuit used to analyze the fuel cell impedance.

thickness, the membrane water content, and the membrane diffusion coefficient. It was shown that tm decreases with membrane shrinkage and increases with increasing membrane hydration [37]. The fuel cell impedance, Z, was obtained by summing Equations (4), (5), and (7) and the fuel cell ohmic resistance, RU , as follows:

Z ¼ Za þ Zc þ RU þ Zl :

(9)

Equation (9) was fitted to the fuel cell impedance measured via EIS in order to obtain values of the following parameters: Ca , Ra , Rc , Cc , a, Rl , and Cl . The non-linear Generalized Reduced Gradient (GRG) solver was used to perform the curve fitting. The value of RU was assumed to be equal to the real part of the high frequency impedance at 5 kHz.

3. Results and discussion 3.1. Effect of the GDL structure on liquid water transport SEM images of the eGDLs are presented in Fig. 3. Fibre diameters of the eGDLs were measured to be 1.14 ± 0.07 mm for eGDL A, 0.15 ± 0.01 mm for eGDL B and 0.20 ± 0.02 mm for the eGDL B-a. The eGDL of type B had a fibre diameter smaller than the eGDL A due to a lower concentration of PAN used during the electrospinning process (see Table 1). In addition, the fibre to fibre distance was also observed to be larger in eGDL A compared to the eGDLs of type B, and it was reasonably assumed that the pore size of eGDL A was larger than the eGDL of type B. For comparison, the T60 substrate used in eGDL B-s is known to have a fibre diameter of 7 mm [2] and a mean pore size of 2.6 mm [41], which is significantly larger than the pore sizes of the eGDLs fabricated in this study. According to Cho et al. [42], a mean pore size of 0.17 mm was measured using the bubble point method for PAN materials fabricated in conditions similar to this study for a fibre diameter of 250 nm. The processed images of the fuel cell with eGDLs A, B, B-a and B-

s are presented in Fig. 4(a)e(d), respectively. Each image was obtained by averaging 76 processed images recorded during the last 5 min of the steady current operating point at 1.0 A/cm2, which was conducted just prior to the EIS measurement. The pixel intensities in the processed images range from 0.30 to 0.30 cm due to significant material displacement (negative values) and the accumulation of liquid water in the fuel cell (positive values). In the proceeding paragraphs, the impact of the GDL structure on liquid water distributions is analyzed. The origins of the material displacement observed in this work are discussed in the following subsections. The accumulation of liquid water was observed primarily in the flow field channels (black arrows in Fig. 4), under the flow field ribs (white arrows), or in the substrate of the fuel cell with eGDL B-s (grey arrows); however, liquid water accumulation was not observed in the bulk of the eGDLs. The spatial resolution of our experimental setup was 10 mm [19,30] which would have ensured the detection of liquid water in the eGDL (where the thickness was 75 mm). Consequently, the absence of liquid water observed in the bulk of the eGDLs cannot be attributed to any experimental setup limitations. According to the SEM images presented in Fig. 3, it was assumed that the eGDLs exhibit smaller pore sizes than the T60. The relatively smaller eGDL pores are expected to lead to lower liquid water saturations due to the reduced availability for liquid water to accumulate in small pores compared to the larger pores of macroscopic fibrous substrates, such as T60. This observation is in agreement with the results obtained from the performance analysis presented in the first paper of this study [16] as well as the in situ visualizations reported by Lee et al. [43]. In Fig. 4(b) and (c), similar liquid water distributions were observed in the fuel cells with eGDLs B and B-a (see black arrows). Significant liquid water pinning was observed with these eGDLs, where liquid water entering the fuel cell channels remained attached to the surface of the GDL. In eGDLs A and B-s, liquid water was found to preferentially accumulate at the bottom and the

Fig. 3. SEM images of the eGDLs investigated. (a), eGDL A. (b), eGDL B. (c), eGDL B-a. The scale bars indicate 10 mm.

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Fig. 4. In operando fuel cell images obtained after the image processing. (a), eGDL A. (b), eGDL B. (c), eGDL B-a. (d), eGDL B-s. Images obtained at 1.0 A/cm2. The scale bar indicates 1 mm. Liquid water under the rib, onto the GDL surface, and at the eGDL-substrate interface is identified by white, black, and grey arrows, respectively.

channel sides (Fig. 4(a) and (d)). Therefore, the observed liquid water pinning on the surface of the GDL at the channel interface was assumed to be caused by the small fibre diameter and pore size of eGDL B and eGDL B-a. The larger fibres and pore sizes of eGDL A and T60 (present in eGDL B-s) appear to prevent liquid water pinning. A similar relationship between the GDL structure and liquid water pinning was observed in previous ex situ studies [44,45]. This result illustrates how the GDL structure can significantly influence the liquid water transport behaviour in fuel cell channels. In fuel cells with eGDL A and B-s (Fig. 4(a) and (d)), liquid water accumulated under the rib of the flow field (see white arrows); however, this behaviour was not observed in the fuel cells with eGDL B and eGDL B-a (Figures (b) and (c)). It can be assumed that the increased pore size in eGDL A and eGDL B-s can create microscale cavities under the rib, where water is likely to condense and accumulate. In addition, the electrical conductivities of eGDL B and eGDL B-a measured in the first part of this work were found to be greater than the electrical conductivity of eGDL A. We assumed that the thermal conductivity characteristics of these materials followed the same trend, which would have also impacted the thermal management of the fuel cell. Therefore, although eGDLs A, B, and Ba led to reduced liquid water content compared to T60, only eGDLs B and B-a, which had the smallest pore sizes in this study and probably the highest thermal conductivity, led to improved liquid water mitigation under the rib. 3.2. Effect of the GDL structure on membrane deformation Material displacement led to negative water thickness values (seen in Fig. 4), and this is explained in this section. In Fig. 5(a)e(d), the dry-state images recorded during each fuel cell OCV (obtained as 16-bit images during the experiment) are presented in an 8-bit greyscale format. At the center of each image, the black horizontal

layer is the MEA, and the two bright layers sandwiched between the MEA and flow field are the eGDLs. In the fuel cells with eGDLs A and B (Fig. 5(a) and (b)), the MEA was observed to be particularly deformed (white arrows) due to the alternating rib and channel pattern of the flow fields. However, significantly reduced deformation was visible in the fuel cells with eGDL B-a and B-s (Fig. 5(c) and (d)). The use of a macro-fibrous substrate in dual-layered GDLs is known to prevent MEA deformation induced by the shape of the flow field. Therefore, the eGDL B-a with fibres aligned perpendicularly to the channel direction played a role similar to a macrofibrous substrate by preventing the deformation of the MEA. These findings are in agreement with that of Baik et al. [46] and Seo et al. [47], who used direct microscopic visualizations of commercial GDLs. The deformation of conventional GDLs via contact with the flow fields has been reported to promote liquid water accumulation under the rib [46,47]; however, this effect was not observed in the fuel cell with eGDL B (Fig. 4(b)). With the assumption that our eGDL materials deformed in a similar manner under rib and channel compression, we concluded that the eGDL deformation did not provide a significant contribution to the liquid water accumulation under the ribs of the fuel cells with eGDL A and eGDL B-s (Fig. 4(a) and (d)), whereas the water accumulation under the ribs was attributed to the larger pores of GDL A and B-s (as it was established in subsection 3.1). In Fig. 5(a) and (b), it can be noted that the locations of MEA deformation correspond to the locations of negative pixel values in Fig. 4(a) and (b). In Equation (3), the calculated pixel value, P, can be negative if m2 < m1 , whereby the fuel cell materials in the wet-state images attenuate fewer X-rays than the materials in the dry-state images. In this case, it was assumed that the porosity of the material in the wet state-images increased relative to the porosity of the material in the dry-state images. A scenario explaining this phenomenon is proposed next in Section 3.3 where the liquid

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Fig. 5. Dry-state images. (a), eGDL A. (b), eGDL B. (c), eGDL B-a; (d), eGDL. The length of the scale bar is 1 mm. The white arrows point out the MEA deformation. The color bar shows the pixel intensity.

water management in the membrane is analyzed. 3.3. Effect of the GDL structure on the membrane hydration The water transport in the membrane was investigated using

Fig. 6. Nyquist diagram of the fuel cell impedance at 0.5 and 1.0 A/cm2. (a), eGDL A. (b), eGDL B. (c), eGDL B-a. (d), eGDL B-s. The insert in (b) focuses on the membrane impedance measured at low frequency.

EIS. The fuel cell impedances measured at 0.5 A/cm2 and 1.0 A/cm2 for all fuel cells are presented in Fig. 6(a)e(d). It can be noted that the fuel cell impedance spectra exhibit low frequency arcs below 1 Hz (fuel cells with eGDLs A, B and B-s) and 0.5 Hz (the fuel cell with eGDL B-a). These low frequency arcs were attributed to the low diffusion rate of the water in the membrane [36,37], and according to Springer et al. [48], a small water diffusion coefficient results from a low content of water in the membrane. The channel impedance can also create a similar low frequency arc, but the reactant flow rates used in this work were kept sufficiently high to avoid the occurrence of such impedance [32]. Thus, from the fuel cell impedance measurement in Fig. 6, it can be concluded that the membranes experienced significant dehydration during fuel cell operation. The model presented in Fig. 2 was used to quantitatively analyze the liquid water management in the membrane. Good agreement was obtained between the fuel cell impedance model and the experimental data in Fig. 6. The values of the fuel cell model parameters obtained during curve fitting are given in the supplementary information. In Fig. 6(c), the low frequency arc was not observed in the impedance spectrum at 0.5 A/cm2, therefore the components related to the water transport in the membrane (see Fig. 2) were neglected and the value of Rl and Cl were not identified for this case. The water transport analysis in the membrane is based on the average value of pixel intensity, P, the characteristic time of water diffusion in the membrane, tm , and the value of the ohmic resistance, RU in Fig. 7(a)e(c), respectively. The pixel intensity, P, was computed by averaging all of the pixel values in the MEAþeGDLs regions of each fuel cell (see the labels in Fig. 4) for the two current densities studied. The negative values of P are considered indicators of material displacement in the fuel cell. The value of tm is an indicator of the membrane water content, and it decreases with decreasing membrane dehydration and/or membrane shrinkage [37]. The ohmic resistance, RU , is a function of the electrical contact

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S. Chevalier et al. / Journal of Power Sources xxx (2017) 1e10

Fig. 7. Analysis of the water transport in the membrane at 0.5 A/cm2 and 1.0 A/cm2. (a), Average pixel intensity in the eGDL. (b), Characteristic time of the membrane. (c), Fuel cell ohmic resistance.

resistances in the fuel cell, the eGDL electrical conductivity, and membrane ionic conductivity. The material displacement, in Fig. 7(a), was found to increase with increasing current density. On the other hand, the values of tm decrease with increasing current density, and RU increases slightly with increasing current density for all of our fuel cells. From these observations, it can be concluded that at higher current densities all the fuel cells experienced more membrane dehydration causing membrane shrinkage and material displacement. This result shows that the use of eGDLs induced severe membrane drying even when highly humidified inlet gases were used (reactants with 100% RH at 60  C in this study). In addition, the membrane shrinkage, revealed from EIS analysis, explains the material displacement between the dry-state and wet-state images, see Fig. 8(a) and (b). At OCV, the membrane reached its maximum expansion due to swelling, and this resulted in the undulating pattern observed in Fig. 5 under the channel (see white arrows). During fuel cell operation at high current density the membrane shrank and wrinkled. As a consequence, membrane displacement facilitated the creation of void spaces into which the eGDL expanded. According to the average value of P (Fig. 7(a)), less material displacement was measured in the fuel cells with eGDL B-a (fibre alignment) and eGDL B-s (presence of T60) compared to the fuel

cells assembled with eGDL A and B. The values of tm were also higher for the fuel cells with eGDL B-a and B-s compared to the other materials, so we concluded that the membrane hydration was better with with eGDL B-a and eGDL B-s. The addition of T60 and fibre alignment in the eGDL prevented MEA deformation; therefore, it can be assumed that the CL-eGDL contact was also improved and facilitated enhanced heat transfer between the CL and the eGDL. This enhanced heat transfer would have, in turn, led to comparatively lower CL temperatures (with respect to eGDL that led to less effective heat transfer rates). In the cases of ineffective heat transfer, higher temperatures at the CL may have led to membrane dry-out and shrinkage. Therefore, the alignment of the fibres in the eGDL is as beneficial as the presence of a macro-fibrous substrate for ensuring good CL-eGDL contact, and provides the advantage of reduced ohmic resistances (see Fig. 7(c) eGDL B-a and B-s). This result is in agreement with the polarisation curve analysis from the first paper of this work [16], wherein it was assumed that the eGDL with aligned fibres ensured a better CL-eGDL contact compared to that of nonaligned fibre eGDLs. 4. Conclusions In this work, hydrophobic eGDLs were fabricated and assembled

Fig. 8. Illustration of the material displacement between the dry-state images at the OCV (a) and the wet-state image during fuel cell operations (b). The red arrow indicates the membrane displacement during the fuel cell operation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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S. Chevalier et al. / Journal of Power Sources xxx (2017) 1e10

in PEM fuel cells to investigate the relationship between the eGDL structure and the liquid water distribution in the eGDLs and channels as well as the water transport in the membrane. The liquid water distribution was imaged using synchrotron X-ray radiography, and the water transport in the membrane was investigated using EIS. It was found that the submicron porous structure of the eGDLs enabled efficient transport of liquid water. The small fibre diameters and fibre to fibre distances of eGDLs B and B-a led to reduced ohmic losses and enhanced liquid water removal under the rib, compared to eGDLs A and B-s. However, dominant liquid water pinning onto the eGDL surface in the fuel cell channels was observed, which showed that the structure of the eGDL also impacts the liquid water transport in the channel. As a drawback of the efficient liquid water removal induced by the eGDLs, severe membrane drying was observed from the processed X-ray images and the fuel cell impedance analysis. Poor thermal management was presented as the cause of membrane drying, since membrane shrinkage increased with increasing operating current. To prevent MEA deformation, fibre alignment perpendicular to the channels was found to be beneficial in reducing membrane dry-out. We propose the reason for this improvement was a reduced CL-eGDL thermal contact resistance that was realized with fibre alignment. The addition of a macrofibrous substrate (T60) adjacent to the eGDL was found to have a similar effect, but this addition came along with an increase in electrical resistance that ultimately reduced the fuel cell performance. Throughout the two papers of this study, it was shown that the fabrication of novel GDLs using electrospinning was a promising means to investigate the relationship between the GDL structure and fuel cell performance. Our approach provides a new methodology for optimizing GDL structural parameters such as fibre diameter, pore size, and fibre alignment, particularly for the purpose of improving water and thermal management in PEM fuel cells. Acknowledgements The Thermofluids for Energy and Advanced Materials (TEAM) laboratory at the University of Toronto is gratefully acknowledged for their support and assistance. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), the NSERC Collaborative Research and Training Experience Program (CREATE) Program in Distributed Generation for Remote Communities, the Canada Research Chairs Program, and the Ontario Ministry of Research and Innovation Early Researcher Award are gratefully acknowledged. Graduate scholarships received by H. Liu from Connaught Fund, by N. Ge from the Friends of Ara Mooradian and Mercedes-Benz Scholarships, by M. G. George from NSERC Canada Graduate Scholarship (CGS) and the Ontario Graduate Scholarship (OGS), by P. Shrestha from the Bert Wasmund Graduate Fellowship in Sustainable Energy Research, D. Muirhead from NSERC CGS and OGS, and by J. Lee from Hatch Ltd., and the David Sanborn Scott and Ron D. Venter awards are also gratefully acknowledged. Research described in this paper was performed at the BMIT facility at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. Authors acknowledge the receipt of support from the CLS Post-Doctoral and Graduate Student Travel Support Program. The authors would like to acknowledge Dr. George Belev, Dr. Adam Webb, Dr. Ning Zhu, Dr.

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Tomasz Wysokinski, and the BMIT group of the Canadian Light Source Inc. for their generous assistance. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.114. References [1] L. Cindrella, A.M. Kannan, J.F. Lin, K. Saminathan, Y. Ho, C.W. Lin, J. Wertz, Gas diffusion layer for proton exchange membrane fuel cellsda review, J. Power Sources 194 (2009) 146e160. [2] J. Hinebaugh, Z. Fishman, A. Bazylak, Unstructured pore network modeling with heterogeneous PEMFC GDL porosity distributions, J. Electrochem. Soc. 157 (2010) B1651. [3] M. Fazeli, J. Hinebaugh, A. Bazylak, Investigating inlet condition effects on PEMFC GDL liquid water transport through pore network modeling, J. Electrochem. Soc. 162 (2015) F661eF668. [4] J.T. Gostick, Random pore network modeling of fibrous PEMFC gas diffusion media using voronoi and delaunay tessellations, J. Electrochem. Soc. 160 (2013) F731eF743. [5] R. Lenormand, E. Touboul, C. Zarcone, Numerical models and experiments on immiscible displacements in porous media, J. Fluid Mech. 189 (1988) 165. [6] A.Z. Weber, R.L. Borup, R.M. Darling, P.K. Das, T.J. Dursch, W. Gu, et al., A critical review of modeling transport phenomena in polymer-electrolyte fuel cells, J. Electrochem. Soc. 161 (2014) F1254eF1299. [7] S. Tsushima, S. Hirai, In situ diagnostics for water transport in proton exchange membrane fuel cells, Prog. Energy Combust. Sci. 37 (2011) 204e220. [8] A. Bazylak, Liquid water visualization in PEM fuel cells: a review, Int. J. Hydrog. Energy 34 (2009) 3845e3857. rida, Morphologically controlled fuel cell transport layers [9] D. Todd, W. Me enabled via electrospun carbon nonwovens, J. Power Sources 273 (2015) 312e316. rida, Synthesis of transport layers with controlled anisotropy [10] D. Todd, W. Me and application thereof to study proton exchange membrane fuel cell performance, J. Power Sources 311 (2016) 182e187. [11] M.D.R. Kok, J.T. Gostick, Transport properties of electrospun fibrous membranes with controlled anisotropy, J. Memb. Sci. 473 (2015) 237e244. [12] M. Inagaki, Y. Yang, F. Kang, Carbon nanofibers prepared via electrospinning, Adv. Mater 24 (2012) 2547e2566. braud, G. Schlatter, L. Tho €ny-Meyer, [13] N. Lavielle, A.-M. Popa, M. de Geus, A. He R.M. Rossi, Controlled formation of poly(ε-caprolactone) ultrathin electrospun nanofibers in a hydrolytic degradation-assisted process, Eur. Polym. J. 49 (2013) 1331e1336. [14] C.J. Luo, E. Stride, M. Edirisinghe, Mapping the influence of solubility and dielectric constant on electrospinning polycaprolactone solutions, Macromolecules 45 (2012) 4669e4680. [15] P. Katta, M. Alessandro, R.D. Ramsier, G.G. Chase, Continuous electrospinning of aligned polymer nanofibers onto a wire drum collector, Nano Lett. 4 (2004) 2215e2218. [16] S. Chevalier, N. Lavielle, B. Hatton, A. Bazylak, Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: I. Fabrication, morphological characterization, and in situ performance, J. Power Sources (2016) (submitted). [17] I. Manke, C. Hartnig, M. Grünerbel, J. Kaczerowski, W. Lehnert, N. Kardjilov, A. Hilger, J. Banhart, W. Treimer, M. Strobl, Quasiein situ neutron tomography on polymer electrolyte membrane fuel cell stacks, Appl. Phys. Lett. 90 (2007) 184101. [18] A. Lamibrac, J. Roth, M. Toulec, F. Marone, M. Stampanoni, F.N. Büchi, Characterization of liquid water saturation in gas diffusion layers by X-ray tomographic microscopy, J. Electrochem. Soc. 163 (2016) F202eF209. [19] S. Chevalier, N. Ge, M.G. George, J. Lee, R. Banerjee, H. Liu, P. Shrestha, D. Muirhead, J. Hinebaugh, Y. Tabuchi, T. Kotaka, A. Bazylak, Synchrotron Xray radiography as a highly precise and accurate method for measuring the spatial distribution of liquid water in operating polymer electrolyte membrane fuel cells, J. Electrochem. Soc. 164 (2017) F107eF114. [20] S. Chevalier, N. Ge, J. Lee, P. Antonacci, R. Yip, M.G. George, H. Liu, R. Banerjee, M. Fazeli, A. Bazylak, In situ analysis of voltage degradation in a polymer electrolyte membrane fuel cell with a dead-ended anode, Electrochem. Commun. 59 (2015) 16e19. [21] P. Deevanhxay, T. Sasabe, S. Tsushima, S. Hirai, Observation of dynamic liquid water transport in the microporous layer and gas diffusion layer of an operating PEM fuel cell by high-resolution soft X-ray radiography, J. Power Sources 230 (2013) 38e43. [22] C. Hartnig, I. Manke, R. Kuhn, N. Kardjilov, J. Banhart, W. Lehnert, Crosssectional insight in the water evolution and transport in polymer electrolyte fuel cells, Appl. Phys. Lett. 92 (2008) 134106. [23] C. Hartnig, I. Manke, R. Kuhn, S. Kleinau, J. Goebbels, J. Banhart, High-resolution in-plane investigation of the water evolution and transport in PEM fuel cells, J. Power Sources 188 (2009) 468e474. [24] G. Flipo, C. Josset, G. Giacoppo, G. Squadrito, B. Auvity, J. Bellettre, Eruptive

Please cite this article in press as: S. Chevalier, et al., Journal of Power Sources (2017), http://dx.doi.org/10.1016/j.jpowsour.2017.01.114

10

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

S. Chevalier et al. / Journal of Power Sources xxx (2017) 1e10 water transport in PEMFC: a single-drop capillary model, Int. J. Hydrog. Energy 40 (2015) 14667e14675. € tter, I. Manke, P. Krüger, T. Arlt, J. Haussmann, M. Klages, H. Marko H. Riesemeier, C. Hartnig, J. Scholta, J. Banhart, Investigation of 3D water transport paths in gas diffusion layers by combined in-situ synchrotron X-ray radiography and tomography, Electrochem. Commun. 13 (2011) 1001e1004. €tter, K. Dittmann, J. Haußmann, R. Alink, D. Gerteisen, H. Riesemeier, H. Marko J. Scholta, J. Banhart, I. Manke, Influence of local carbon fibre orientation on the water transport in the gas diffusion layer of polymer electrolyte membrane fuel cells, Electrochem. Commun. 51 (2015) 133e136. P. Antonacci, S. Chevalier, J. Lee, N. Ge, J. Hinebaugh, R. Yip, Y. Tabuchi, T. Kotaka, A. Bazylak, Balancing mass transport resistance and membrane resistance when tailoring MPL thickness for PEM fuel cells, Electrochim. Acta 188 (2016) 888e897. S. Chevalier, J. Lee, N. Ge, R. Yip, P. Antonacci, Y. Tabuchi, T. Kotaka, A. Bazylak, In operando measurements of liquid water saturation distributions and effective diffusivities of polymer electrolyte membrane fuel cell gas diffusion layers, Electrochim. Acta 210 (2016) 792e803. €tter, J. Haußmann, R. Alink, C. To €tzke, T. Arlt, M. Klages, H. Marko H. Riesemeier, J. Scholta, D. Gerteisen, J. Banhart, I. Manke, Influence of cracks in the microporous layer on the water distribution in a PEM fuel cell investigated by synchrotron radiography, Electrochem. Commun. 34 (2013) 22e24. N. Ge, S. Chevalier, J. Hinebaugh, R. Yip, J. Lee, P. Antonacci, T. Kotaka, Y. Tabuchi, A. Bazylak, Calibrating the X-ray attenuation of liquid water and correcting sample movement artefacts during in operando synchrotron X-ray radiographic imaging of polymer electrolyte membrane fuel cells, J. Synchrotron Radiat. 23 (2016). S. Chevalier, D. Trichet, B. Auvity, J.C. Olivier, C. Josset, M. Machmoum, Multiphysics DC and AC models of a PEMFC for the detection of degraded cell parameters, Int. J. Hydrog. Energy 38 (2013) 11609e11618. S. Chevalier, C. Josset, A. Bazylak, B. Auvity, Measurements of air velocities in polymer electrolyte membrane fuel cell channels using electrochemical impedance spectroscopy, J. Electrochem. Soc. 163 (2016) F816eF823. S. Chevalier, B. Auvity, J.C. Olivier, C. Josset, D. Trichet, M. Machmoum, Detection of cells state-of-health in PEM fuel cell stack using EIS measurements coupled with multiphysics modeling, Fuel Cells 14 (2014) 416e429. B.P. Setzler, T.F. Fuller, A physics-based impedance model of proton exchange membrane fuel cells exhibiting low-frequency inductive loops, J. Electrochem. Soc. 162 (2015) F519eF530. M.N. Tsampas, S. Brosda, C.G. Vayenas, Electrochemical impedance spectroscopy of fully hydrated Nafion membranes at high and low hydrogen partial pressures, Electrochim. Acta 56 (2011) 10582e10592.

[36] I.A. Schneider, M.H. Bayer, A. Wokaun, G.G. Scherer, Impedance response of the proton exchange membrane in polymer electrolyte fuel cells, J. Electrochem. Soc. 155 (2008) B783. [37] S.M. Rezaei Niya, M. Hoorfar, Process modeling of the ohmic loss in proton exchange membrane fuel cells, Electrochim. Acta 120 (2014) 193e203. [38] S.M. Rezaei Niya, R.K. Phillips, M. Hoorfar, Process modeling of the impedance characteristics of proton exchange membrane fuel cells, Electrochim. Acta 191 (2016) 594e605. [39] P. Antonacci, S. Chevalier, J. Lee, R. Yip, N. Ge, A. Bazylak, Feasibility of combining electrochemical impedance spectroscopy and synchrotron X-ray radiography for determining the influence of liquid water on polymer electrolyte membrane fuel cell performance, Int. J. Hydrogen Energy 40 (2015) 16494e16502. [40] T.W. Wysokinski, D. Chapman, G. Adams, M. Renier, P. Suortti, W. Thomlinson, Beamlines of the biomedical imaging and therapy facility at the Canadian light sourcedPart 1, Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 582 (2007) 73e76. [41] A. El-kharouf, T.J. Mason, D.J.L. Brett, B.G. Pollet, Ex-situ characterisation of gas diffusion layers for proton exchange membrane fuel cells, J. Power Sources 218 (2012) 393e404. [42] T.H. Cho, T. Sakai, S. Tanase, K. Kimura, Y. Kondo, T. Tarao, M. Tanaka, Electrochemical performances of polyacrylonitrile nanofiber-based nonwoven separator for lithium-ion battery, Electrochem. Solid State Lett. 10 (2007) A159. [43] J. Lee, R. Yip, P. Antonacci, N. Ge, T. Kotaka, Y. Tabuchi, A. Bazylak, Synchrotron investigation of microporous layer thickness on liquid water distribution in a PEM fuel cell, J. Electrochem. Soc. 162 (2015) F669eF676. [44] J.Z. Fishman, H. Leung, A. Bazylak, Droplet pinning by PEM fuel cell GDL surfaces, Int. J. Hydrog. Energy 35 (2010) 9144e9150. [45] A.D. Santamaria, P.K. Das, J.C. MacDonald, A.Z. Weber, Liquid-water interactions with gas-diffusion-layer surfaces, J. Electrochem. Soc. 161 (2014) F1184eF1193. [46] K.D. Baik, B.K. Hong, K. Han, M.S. Kim, Correlation between anisotropic bending stiffness of GDL and land/channel width ratio of polymer electrolyte membrane fuel cells, Int. J. Hydrog. Energy 37 (2012) 11921e11933. [47] J.H. Seo, K.D. Baik, D.K. Kim, S. Kim, J.W. Choi, M. Kim, H.H. Song, M.S. Kim, Effects of anisotropic bending stiffness of gas diffusion layer on the MEA degradation of polymer electrolyte membrane fuel cells by wet/dry gas, Int. J. Hydrog. Energy 38 (2013) 16245e16252. [48] T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, Polymer electrolyte fuel cell model, J. Electrochem. Soc. 138 (1991) 2334.

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