The Role of Compressive Stress on Gas Diffusion Media

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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The Role of Compressive Stress on Gas Diffusion Media Morphology and Fuel Cell Performance Robert W. Atkinson, III,† Yannick Garsany,‡ Benjamin D. Gould,§ Karen E. Swider-Lyons,*,§ and Iryna V. Zenyuk*,∥ †

ASEE Postdoctoral Program, U.S. Naval Research Laboratory, Washington, District of Columbia 20375, United States EXCET Inc., Springfield, Virginia 22151, United States § U.S. Naval Research Laboratory, Washington, District of Columbia 20375, United States ∥ Department of Mechanical Engineering, Tufts University, 200 Boston Avenue 2600, Medford, Massachusetts 02155, United States ‡

S Supporting Information *

ABSTRACT: Understanding the respective morphology changes with compression of the gas diffusion layer (GDL) and microporous layer (MPL) in unitized gas diffusion media (GDM) is critical for polymer electrolyte fuel cell (PEFC) high-power performance, as the compression affects the ohmic resistance and the porosity that influences masstransport resistance. We present a comprehensive study of morphology of two types of GDM (paper-type SGL 29BC and felt-type Freudenberg H2315 C2) under varied levels of compression using X-ray computed tomography (CT) to link GDM microstructure to fuel cell performance. The SGL 29BC morphology evolves more significantly with compression in ways that we expect to occlude oxygen diffusivity, while transitions in the Freudenberg H2315 C2 are more gradual. Upon compression by 0− 34% its initial thickness, the 29BC pore-size distribution (PSD) shifts from bimodal (12.6 and 34.9 μm average pore radii) to unimodal (9.67 μm), extensive MPL surface cracks decrease in surface area and depth (5−2.2% crack surface area), and void volume fraction decreases from 0.45 to 0.18. Freudenberg H2315 C2 GDM maintains a unimodal PSD (10.5 to 8.33 μm average pore radii), has minimal surface cracking in its discrete MPL layer, and maintains a larger void volume (0.54 to 0.35) upon compression from 0 to 28% its initial thickness. As a result, PEFCs operated in hot and humid conditions (80 °C, 100% RH) with SGL 29BC applied as cathode GDM lose performance beyond 14% compression; the current density at 0.6 V decreases from 827.8 to 795.9 mA cm−2 as 29BC compression increases from 14 to 28% the uncompressed GDM thickness. Alternatively, PEFCs with Freudenberg H2315 C2 GDM at the cathode increase in current density at 0.6 V as compression increases from 14 to 28% (1007 to 1098 mA cm−2). KEYWORDS: fuel cells, gas diffusion layer, microporous layer, compression, X-ray computed tomography fibers together chemically in paper-type GDM, whereas mechanical binding is used for woven or felt GDM. SGL Group GDM (SIGRACET), which are nonwoven carbon papers, are fabricated by filling the fibers with carbonaceous binder and, after graphitization, immersing the GDLs into an aqueous dispersion of PTFE. Alternatively, Freudenberg GDM are felt-like. These GDM are fabricated using a dry-laid method, where PAN fibers are deposited into a thin fiber fleece mat and a hydroentangling process binds the fibers mechanically. In this process, 80−150 μm water jets, with close spacing of 15−50 jets cm−1, impinge onto the moving fiber mat, mechanically bonding the fibers. Subsequently, the fiber mat is carbonized by thermal annealing. The product GDLs, constructed of carbon

1. INTRODUCTION Gas diffusion media (GDM) are porous carbon layers used in polymer electrolyte fuel cells (PEFCs) as nonreactive, transport layers to promote uniform reactant distribution, facilitate electron transport, and improve heat and water removal from the catalyst layers (CLs) where oxygen reduction to water occurs at the cathode and hydrogen oxidation occurs at the anode. GDM consist of two distinct regions: a relatively thick, fibrous gas diffusion layer (GDL) and a teflonized microporous layer (MPL) that coats the side of the GDL in contact with the CL. Understanding the role of GDM in the performance of PEFCs is imperative for power-dense stacks which require effective oxygen transport and water clearing from the electrodes. GDM are traditionally fabricated from poly(acrylonitrile) (PAN), which is carbonized in a temperature range of 1200− 1350 °C in nitrogen.1 Carbonaceous binder is used to bind © XXXX American Chemical Society

Received: October 27, 2017 Accepted: December 5, 2017 Published: December 5, 2017 A

DOI: 10.1021/acsaem.7b00077 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials fibers with diameters of 6−8 μm, have unimodal or bimodal pore-size distributions (PSDs) with mean radii in the range of tens of micrometers.1,2 The properties of the GDL are highly dependent on the presence of binder, layer thickness, and fabrication method. The diffusion layers in commercial GDM are typically anisotropic due to the fiber preferential in-plane orientation during the roll-to-roll process. The GDL may be treated either with poly(tetrafluoroethylene) (PTFE) or by direct fluorination3 to form regions with mixed wettability to facilitate PEFC water management. A MPL is added to the GDL to reduce electrical and thermal contact resistances, reduce mechanical stresses at the GDL|CL interface, and improve water management from its hydrophobic nature.1,4,5 The MPL consists of microporous, teflonized carbon black material and can vary in thickness from 20 to 100 μm depending on the manufacturing process and application to the GDL. Additionally, GDL properties such as the PSD will affect MPL infiltration into the fiber region.1 The method chosen to apply the MPL to the GDL is one of many factors that may determine the composite GDM morphology and the impregnation of the MPL into the fibrous GDL. Examples of these methods include doctor blade, screen printing, spraying, and rod coating.6−9 Since GDM structure depends on the manufacturing process, so does its mechanical response to stress. The mechanical properties of the diffusion layer are considered to be governed by the structure of the carbon fibers and the effectiveness of the fiber network binding.10 In the absence of a MPL, carbon felts (such as the GDL in Freudenberg GDM) observe less compressive strain and very low residual strain during compression and after a compression cycle compared to carbon papers (such as the GDL in SGL GDM) and carbon cloths.11 In a carbon fiber paper, the relatively large residual strain following a compression cycle, or plastic deformation of the GDL, is reported to occur when the more delicate binder in the paper is damaged and fails to permanently support the more rigid carbon fibers.11 A felt GDL without binder is considered to be more stable and resistant to stress because it is mechanically supported by the network of carbon fibers without the need for the weak, brittle, polymeric binder material, rendering felt GDLs more resistant to elastic and plastic deformation.11 When a GDL is compressed, its pore volume decreases and the number of fiber contact points increases as strain increases.12 This has been supported by observations of a Freudenberg H2315 GDL without MPL using micro X-ray computed tomography (CT) that show that as the material is compressed, the pore volume decreases as fibers are mainly translated in the direction of the stress with the loss in pore volume vastly at the expense of pores with radii greater than 10 μm.13 The addition of PTFE to a GDL to improve water management has been observed to increase the mechanical stiffness of the GDM, reportedly from reducing the porosity in the diffusion layer and increasing the amount of binder holding the fiber network together.14,15 The addition of a MPL has also been found to increase the compressive modulus of the entire GDM compared to the GDL alone.11,15,16 Micro X-ray CT is a nonintrusive imaging technique that allows resolving the interior structure of materials on the microscale under a controlled environment.17−27 The technique is capable of differentiating between the MPL and GDL due to the different X-ray attenuation coefficients of these materials. Recent advances in laboratory-scale CT equipment

has enabled access to CT facilities, resulting in large numbers of publications over the course of the past 2 years. With a resolution of 1 μm, micro X-ray CT resolves porosity and poresize distribution in three dimensions within the GDL. Additionally, X-ray CT detects bulk MPL properties, such as distribution through the GDL thickness, as well as larger cracks and voids within the MPL. To resolve micropores within the MPL that are on the order of nanometers, higher resolution imaging techniques such as nano X-ray CT and scanning electron microscopy (SEM)28 are more appropriate. Previous studies used X-ray CT to measure GDL properties, such as spatially resolved porosity,2,13,23,25,29,30 effects of compression,2,13,26,27,31,32 and water distribution22,25,26,33−35 and evaporation.36 These studies primarily focused on either MPL-free GDLs or only on GDL fiber regions of the GDM, with limited attention to the MPL. Segmenting the MPL region is challenging unless image quality is sufficiently high because of the weak X-ray attenuation of the MPL. Prass et al. used micro X-ray CT to study the effects of compression on the CL|MPL interfacial inhomogeneity37 of SGL MPL and various CLs. Bock et al.38 and Burheim et al.39 investigated thermal conductivity of composite GDL|MPL material under varied levels of compression and showed X-ray CT tomographs for these composite GDLs. X-ray CT has been used to visualize the presence of liquid water in paper and felt GDLs in operando as a function of fuel cell current density.22 Our objective is to use X-ray CT to better correlate the internal morphology of compressed GDM to fuel cell performance, expanding upon our previous work on the influence of GDM compressive stress on fuel cell performance.28 In the prior work, we used ex situ SEM and N2 physisorption to characterize plastic deformations in the postmortem GDL and MPL and found that more significant MPL microstructure collapse in SGL 25BC with increasing compression corresponded to the onset of significant masstransport resistances and accompanying reductions in fuel cell voltage.28 In this work, we use X-ray CT to characterize the morphological properties of the GDL and MPL regions for two distinct types of GDM, SGL 29BC and Freudenberg H2315 C2, while they are actively under a range of compressive stresses. These two materials are representative of two different manufacturing conditions for applying a MPL to a fibrous GDL. The distinct uncompressed structures demonstrate unique morphological evolutions when subjected to compressive stress, which are captured using X-ray CT. We report spatially resolved solid fractions and volumes, macroscopic void volumes, pore-size distributions, and thickness reductions of the GDL and MPL regions in this study to describe GDM morphology evolution as a function of compressive stress. In addition, we explore the MPL crack deformation under compressive stress. The observed evolutions in GDM morphology are compared to the performance of single-cell PEMFCs operated under similar compressive stresses.

2. MATERIALS AND METHODS 2.1. Experimental Setup. The X-ray computed tomography experiments were conducted at micro-CT Beamline 8.3.2 at the Advanced Light Source (ALS). Monochromatic X-rays with 14 keV energy were selected with a double-multilayer monochromator. A 0.5 mm LuAG scintillator was used as a detector. With a sCMOS PCO.Edge camera and PCO.Dimax camera, 5× lenses produced 1.33 μm and 2.2 μm pixel dimensions, respectively. Horizontal fields of view (FOVs) of 3.3 mm and 4.2 mm were obtained with two cameras, respectively. Exposure times of 300 ms and 40 ms were used with B


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ACS Applied Energy Materials PCO.Edge and PCO.Dimax, respectively, yielding on average 8000 counts on the 16-bit camera. For each X-ray CT scan, 1025 projections were acquired over a 180° rotation range with average scan time of about 7 min. The sample holder used for the compression experiments consisted of a stage made of aluminum and a Vespel cup. A flat stamp was used on top of a 3.2 mm diameter circular GDL to press the sample against the stage. Ultrafine pitch thread was used to control compression. The setup schematic and detailed description were provided in earlier works2,26 and are shown in the Supporting Information, Figure S1. The setup is designed to simulate GDM compression under land locations in the fuel cell. 2.2. Materials. The GDM investigated in this work were SGL 29BC (SIGRACET, SGL Carbon Inc.) carbon paper, which consisted of 5% PTFE loading in the GDL and 23% PTFE loading in the MPL, and H2315 C2 (Freudenberg FCCT SE & Co., also referred to as H23C2) carbon felt, which was not PTFE-treated in the GDL and had 40% PTFE loading in the MPL. The amount of GDM compression, the compressed thickness, and the compressive stress are reported in Table 1 for GDM compressions relevant in X-ray CT analysis and fuel

(which is also the direction for the GDM through-thickness) plane normal points outward from the screen. ImageJ/Fiji “Z-project” with summation function was used to sum the images in the z-direction, resulting in one 32-bit composite image. Each gray scale point on the resulting MPL height map corresponds to a MPL thickness in that location in default units. A MPL height map was imported into Matlab for color representation of the gray scale values. From this image, MPL height data were extracted by averaging all the height values, while the standard deviation was also determined. Porosity was computed with Fiji/ImageJ using the binary histogram for the segmented image. Through-thickness porosity was calculated by an in-house macro for Fiji/ImageJ. PSDs were obtained with a “Local Thickness” Fiji/ImageJ plugin.41 In this method, inscribed spherical kernels were used to evaluate PSDs. The diameter of the largest sphere that can fit within the object of interest is defined as the local thickness; the more detailed mathematical treatment was reported previously.2 Power density function (PDF) was used to characterize PSD. The characteristic of this function is that the integral of the PDF is equal to 1. The fits to the PDFs were generated with Matlab “cftool” plug-in using a log-normal bimodal PSD:

Table 1. Amount of GDM Compression, Compressed GDM Thickness, and Approximated Compressive Stress from Manufacturer Thickness−Compressive Load Data for SGL 29BC and Freudenberg H2315 C2 GDM compressed by (%) 0 14 28 34 41 0 7 14 28 40

thickness (μm)

PDF(r ) =

(1)

where PDF(r) is the normalized volume fraction of pores having radius r, f r,k is the fraction of pores that have the distribution k, and r0,k and σk are the characteristic radius and spread of distribution k, respectively. 2.5. Fabrication of 10 cm2 Membrane Electrode Assemblies. To prepare uniform CCMs, anode and cathode CLs were directly formed onto the Nafion XL membrane (25 μm thickness, Ion Power Inc.) via an automated, ultrasonic spray instrument in a layer-by-layer deposition method. The detailed procedure and a video of the deposition process can be found in a previous publication.42 Typically four passes and eight passes of the ultrasonic sprayer were sufficient to obtain the desired Pt loading on active geometric surface areas of 10 cm2 (31.6 mm × 31.6 mm) for the anode and cathode catalyst layers, respectively. The resultant CCMs were dried in ambient conditions overnight before being tested. Anode CLs were prepared with a carbon-supported Pt catalyst (Pt/ C; 50 wt % Pt, Ion Power Inc., manufacturer undisclosed) and an aqueous Nafion ionomer solution in H+ form (Liquion solution LQ1115, 1100EW, 15 wt %, Ion Power Inc.). The cathode CLs were prepared with carbon black (CB) supported Pt catalyst (Pt/CB; TEC10E50E, 46.7 wt % Pt, Tanaka Kikinzoku Kogyo K.K.) and one of two aqueous Nafion ionomer solutions in H+ form (Liquion solution LQ-1005, 1000EW, 5 wt %, Ion Power Inc. for the cells with Freudenberg H2315 C2 cathode GDM or LQ1115, 1100EW, 15 wt %, Ion Power Inc. for the cells with SGL 29BC cathode GDM). The anode and cathode catalyst layer inks were prepared for ultrasonic spraying by mixing the respective catalyst powder with 18 MΩ water and isopropyl alcohol solvent (IPA, ACS grade, Sigma-Aldrich) before adding the respective Nafion ionomer solution.42 The mass ratios of the ionomer binder to carbon black (I/C) were adjusted to 0.95 (i.e., 32 wt % ionomer in the dry catalyst layer) and 0.70 (i.e., 27 wt % ionomer in the dry catalyst layer) in the anode and cathode inks, respectively. The final anode CL Pt loading was 0.15 mgPt cm−2 (geometric), while the cathode CL Pt loading was 0.32 mgPt cm−2. The details of the catalyst layers are summarized in Table 2.

compressive stress (MPa)

SGL 29BC 227 195 163 151 134 Freudenberg H2315 C2 298 277 256 214 179

⎧ ⎡ (ln r − ln r )2 ⎤⎫ ⎪ ⎪ 1 0, k ⎢− ⎥⎬ exp fr , k ⎨ 2 ⎪ ⎪ ⎢⎣ ⎥⎦⎭ 2σk ⎩ rσk 2π k = 1,2

∑

0 0.65 2 2.8 3.6 0 0.0072 0.027 1.1 4.8

cell experiments. The compressive stress is calculated using thickness− compressive load data from the respective manufacturers. In order to achieve some of the compressions observed in this study, GDM were compressed to thicknesses not listed on the manufacturers’ thickness− compressive load curves. In these cases, the curve fits were extrapolated to the desired GDM compressed thickness value. 2.3. Image Reconstruction and Segmentation. Open source software Fiji/ImageJ was used for raw data preprocessing, which was imported into Octopus 8.6 to retrieve the phase contrast with the modified Bronnikov algorithm (MBA)40 and for tomographic reconstructions. Reconstructed images were cropped and segmented using Fiji/ImageJ. “StackReg” command was used in Fiji/ImageJ to align the image stacks. During segmentation, the void, GDL (fiber), and MPL regions were identified and segmented manually (the threshold value was validated with Otsu algorithm). The details of segmentations and gray-level histograms were reported in earlier work2 and are also shown in the Supporting Information, Figures S2 and S3. With high photon fluxes at the synchrotron facility, image quality was high and required a minimal amount of preprocessing. Volume rendering of the three regions was done with Avizo Fire 8.1. 2.4. Morphological Properties. The GDL thicknesses were measured in 10−15 locations by inspecting the image stack, and the standard deviation was determined manually. The MPL thicknesses were more variable, so more thickness data were collected to determine their average thickness and surface roughness. First, the segmented MPL image stack was converted to a binary image, such that locations within the MPL were labeled as 1 and all other locations as 0. The image stack was resliced in such a way that the z-direction

Table 2. Summary of Catalyst Layer Materials for in-House Prepared 10 cm2 Catalyst Coated Membranes

catalyst I/C ratio Pt loading C

anode catalyst layer

cathode catalyst layer

50 wt % Pt/C (Ion Power) 0.95 (32 wt % dry) 0.15 mgPt cm−2

46.7 wt % Pt/HSC (TKK) 0.70 (27 wt % dry) 0.32 mgPt cm−2


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Table 3. Average Thicknesses of CCM across the Active Area, Sum of Anode and Cathode GDM Thicknesses, MEA Total Thickness, and the Average Gasket Thicknesses Applied to Anode and Cathode To Achieve the Desired Compressiona SGL 29BC anode, SGL 29BC cathode

SGL 29BC anode, Freudenberg H2315 C2 cathode

36 ± 2 452 ± 10 488 ± 1

35 ± 4 453 ± 10 488 ± 1

203|203 178|178 152|178

203|203 178|178

av CCM thickness (μm) av sum of GDM thicknesses (μm) av MEA total thickness (μm) gasket thicknesses: anode|cathode (μm) 14% compression 28% compression 34% compression 40% compression 41% compression

127|152 127|152

a

The errors for the CCM thickness are for the variation in the nine thickness measurements used for the average. The errors for the sum of GDM thickness and MEA total thickness are the variation between the tested MEAs.

Figure 1. (a−c) In-plane tomographs of SGL 29BC GDM at identical MPL location (yellow linear guides) at (a, d) 0%, (b, e) 34%, and (c, f) 41% compression. (d−f) Through-plane tomographs of GDM corresponding to the in-plane tomographs shown in panels a−c for these three compressions with average MPL thickness listed to the right. The scale bar in panels a−c is 500 μm. field, gaskets, GDM, and the CCM were sealed together with eight bolts torqued to 10 N m of torque per bolt in a star pattern. Once assembled, the performance of single cells was tested using Scribner 850e Fuel Cell test systems from Scribner Associates, Inc. (Southern Pines, NC, USA). All experiments were conducted at 80 °C and ambient pressure (1 atm). Humidifiers were filled with 18.2 MΩ cm water from a Barnstead Nanopure System. The inlet dew point for both anode and cathode gases was set at 79 °C, corresponding to 100% inlet relative humidity (RH). Ultrapure gases (Alphagaz 2, Air Liquide) were supplied to the anode and cathode under stoichiometric flow conditions of 2|2 for H2|air, respectively. Below a current density of 120 mA cm−2, a minimum flow rate of 0.2 L min−1 was used. All experiments started by conditioning the membrane with the following sequence: the cell voltage was first held at 0.6 V in H2|air for 2 h, followed by 30 cycles alternating between 0.7 and 0.4 V with each voltage held for 10 min. Polarization data were measured potentiostatically (scan rate of 0.025 V per point from 0.95 to 0.40 V, 1 min hold). Six plots (three forward and three backward sweeps) were obtained for the aforementioned conditions. Cell ohmic resistance was measured at current densities above 100 mA cm−2 using the current−interrupt technique with the load box and the Fuel Cell V.3.2 software (Scribner Associates).

2.6. MEA Assembling and Testing. For assembly and compression of the membrane electrode assembly (MEA), the thicknesses of the CCM and GDM were measured with a digimatic micrometer (Mitutoyo, Model MDC-1″ PX) at nine evenly spaced locations over the component area. MEA compression was set for the laboratory cells through the use of rigid gaskets. These measurements of component thickness were averaged and used to calculate the required gasket thickness, PTFE Skived Tape (Enflo LLC), to achieve the desired GDM compression according to eq 2:

gasket thickness/μm = MEA total thickness − (MEA total thickness × %comp) (2) where the MEA total thickness is the sum of the CCM thickness and the anode and cathode GDM thicknesses (μm). Table 3 summarizes the average CCM thickness, sum of thickness of both GDM, average total MEA thickness and the gasket thicknesses in mil required to achieve the desired compression.28,42 For all MEAs, SGL 29BC was used as the anode GDM. Performance of the CCMs as a function of cathode GDM and degree of compression was evaluated using 10 cm2 single-cell test fixtures (single serpentine flow field, Fuel Cell Technologies). Cartridge heated end plates, current collectors, Poco graphite flow D


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3. RESULTS AND DISCUSSION The morphological evolutions of paper-type SGL 29BC and felt-type Freudenberg H2315 C2 with MPL vary significantly as they are compressed. Figure 1 shows in-plane and throughplane gray scale tomographs for SGL 29BC at 0, 34, and 41% compression. For the in-plane tomographs (Figure 1a−c), an identical location is selected within the MPL that, depending on compression, corresponds to different z-locations, as shown by Figure 1d−f. Fibers in the GDL region are used as markers for alignment. It is evident that as compression increases to 34 and 41% (Figure 1b,c) from the uncompressed state (Figure 1a), the cracks within the MPL decrease in size and the MPL surface becomes smoother. For example, the circled crack visible when the GDM is uncompressed (Figure 1d, lower right) is not visible at 41% compression. From the cross-section tomographs it is apparent that the MPL is highly nonuniform in the zdirection with a large amount of MPL embedded within the fibrous GDL region, previously observed for paper-type39,43 SGL GDM and for felt-type SGL GDM.43 The MPL does not exist as a discrete layer. Instead, there is a nonuniform transition from MPL to GDL that forms a composite region demonstrated to have higher thermal conductivity than its constituent regions.39 The MPL thickness inhomogeneity is represented in Figure S4 in the Supporting Info, where a MPL height map gives a top-down perspective. The fibrous GDL regions of SGL GDM generally have large voids with bimodal distribution and mean radii of 15 and 40 μm, as shown by Zenyuk et al.2 Upon MPL deposition, the MPL penetrates into the large voids within the GDL region, creating a composite region containing both fibers and MPL. From the tomographs, we observe that compressive stress in the z-direction results in compressive strain in the z-direction of the GDL and composite regions, with commensurate reduction of porosity (Figure 1d− f). The average MPL thickness reduces from 103 to 84 μm as the total GDM thickness decreases from 227 to 134 μm with compression by 0−41%. Furthermore, the MPL roughness (which is defined here as a standard deviation of the thickness measurements) decreases from 33.2 to 22.4 μm as compression increases from 0 to 41%, indicating smoothening of the MPL surface upon compression. To further quantify MPL crack volume and area reduction in SGL 29BC GDM, a MPL volume is cut out from the GDM and larger cracks are segmented and volume rendered, with 0 and 41% compression shown in Figure 2a. A morphological procedure for openings of 4 μm is performed to volume render only connected cracks that are above 4 μm in size. At 0% compression the average crack depth (in the z-direction) is 100 μm, but after compressing GDM by 41% of its uncompressed thickness, the average crack depth is reduced to 25 μm, as shown in Figure 2c. Furthermore, the depth of the cracks has a broad distribution (24 μm) for uncompressed GDM that narrows significantly (7.8 μm) at 41% compression. Figure 2b shows that the crack area of the MPL surface (computed here for the in-plane tomographs shown in Figure 1a−c) decreases from 5 to 2% as the GDM is compressed by 0−41%. This is consistent with the ∼2% crack area values observed by Prass et al.37 for compressed SGL 29BC CL|MPL interfaces. In order to compare the effect of compression on the morphology of GDM from different manufacturers, we observe Freudenberg H2315 C2 GDM at 0, 14, and 28% compression. Figure 3 shows in-plane and through-plane gray scale

Figure 2. (a) Volume-rendered distribution of major cracks in SGL 29BC for 0% (gold) and 41% (gray) compression. (b) Percentage of MPL area occupied by cracks as a function of GDM compression and (c) average depth of cracks as a function of MPL thickness: 84 μm (41% compression), 90 μm (34% compression), and 103 μm (0% compression). The z-axis is oriented in the GDM through-plane direction; the y-axis is in-plane of the MPL.

tomographs for Freudenberg H2315 C2, where the locations within the MPL are shown by Figure 3a−c. Minor cracks are observed for 0% compression that are filled by surrounding MPL at higher compressions of 14 and 28%. Unlike for the uncompressed SGL 29BC MPL in through-plane view in Figure 1d, the uncompressed Freudenberg H2315 C2MPL in Figure 3d is observed to be almost freestanding on the underside of the GDM with minimal interpenetration into the well-defined GDL region. This is also confirmed with the MPL thickness map in Figure S5 in the Supporting Information. The uncompressed MPL roughness for Freudenberg H2315 C2 (15.6 μm) is half that of SGL 29BC (33.2 μm) and decreases to 11 μm as the compression increases from 0 to 28%. The MPLfree H2315 GDLs have an average radius of PSD smaller than 10 μm, which may be one of the reasons the MPL does not penetrate as deeply into the GDL region as the MPL does in SGL 29BC GDM, as shown in Figure 1. The thickness of the MPL of the Freudenberg H2315 C2 is 73, 57, and 46 μm for the 0, 14, and 28% compressions, which is thinner than the MPL in SGL 29BC GDM despite the fact that SGL 29BC is imaged at higher compressions. The GDL region in H2315 C2, characterized as a carbon felt, is also stiffer than that in SGL 29BC due to smaller pores and lower macroporosity within the GDL region (0.7 for H2315 C2 and 0.83 for 29BC), consistent with previous reports of lower compressive strain observed in carbon felts compared to carbon papers.11,44 The solid fractions of the MPL region and the fibers in the GDL region are plotted through the normalized thickness of the GDM for the compressions studied for SGL 29BC and Freudenberg H2315 C2 as shown by Figure 4a. The voids that we refer to here are macroscopic voids, such as pores in the GDL and large cracks in the MPL, and do not include micropores that are present within the MPL. For SGL 29BC at higher compressions, the MPL is found to span the entire thickness of GDM in some areas, which is also confirmed by visual inspection of Figure 1. On the other hand, for Freudenberg H2315 C2, the MPL region remains confined to one side of the GDL with less significant penetration into the GDL fiber region, even at higher compressions (28%). Both SGL and Freudenberg GDM show an increase in fiber solid fraction with compression, indicating a decrease in void space, in agreement with previously reported observations of GDL E


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Figure 3. (a−c) In-plane tomographs of Freudenberg H2315 C2 GDM at identical MPL location (yellow linear guides) at (a, d) 0%, (b, e) 14%, and (c, f) 28% compression. (d−f) Through-plane tomographs of GDM corresponding to the in-plane tomographs shown in panels a−c for these three compressions with average MPL thickness listed to the right. The scale bar in panels a−c is 500 μm.

void space loss and an increase in fiber contact points with strain.11,12 The GDL and MPL region-specific strain with compressive stress varies by GDM type. Figure 4b shows the volumes of MPL, fibers in the GDL region, and macroscopic voids in the GDL region as a function of compression. For the SGL 29BC, the void volume decreases by 78% with GDM compression from 0 to 41%, whereas the change in MPL volume is negligible. This is indicative that the larger voids within the GDL region and larger pores and cracks within the MPL region are compressed first before the MPL region is compressed. This behavior is distinct from that of Freudenberg H2315 C2; both the MPL volume and the void volume decrease as H2315 C2 is compressed. This indicates that not just larger voidsbut also the MPL, which is delicate and compressible when freestanding45 similar to the catalyst layer46are compressed. Though the uncompressed PSDs for the different GDM vary, both GDM exhibit unimodal PSD under compression. Figure 4c shows PSDs for various levels of compression, and Table 4 reports the average radii for unimodal or bimodal distributions (eq 1). Prior to compression, SGL 29BC has a bimodal PSD with average radii of 12.6 and 34.9 μm. The fraction of distribution that belongs to the smaller radii is 0.52, reported in the fourth column of Table 4. Upon compression, the PSD of SGL 29BC shifts closer to unimodal with mean radii of 11.4 and 9.67 μm, for 34 and 41% compression, respectively. Alternatively, Freudenberg H2315 C2 maintains a unimodal distribution prior to and during compression, with mean radii reduction from 10.5 to 8.33 μm from its uncompressed state to 28% compression. The morphological response to compressive stress in the zdirection is visually distinct for these two classes of GDM. These transitions are illustrated in the volume renderings of the

two GDM materials with false color segmentation of the regions of interest (GDL vs MPL) during compression, presented in Figure 5 (SGL 29BC) and Figure 6 (Freudenberg H2315 C2). As SGL GDM are compressed by 34%, the strain in the z-direction primarily occurs in the GDL region as the voids between the fibers collapse. This phenomena is most clearly seen by comparing the changes in volume renderings in Figure 5a,b. In the uncompressed state (Figure 5a) there is a large fibrous/void GDL region in the center of the GDM sandwiched between two MPL regions. During compression to 34%, this volume collapses more favorably relative to the MPL region and accounts for much of the GDM strain in the zdirection (Figure 5b,c). This behavior contrasts with that of Freudenberg GDM in Figure 6. As Freudenberg GDM are compressed, the strain in the z-direction primarily occurs in both the MPL and GDL regions. These two distinct morphological responses to compression are condensed to one figure of merit in Figure 7, which depicts the relative thickness ratio of each region, GDL and MPL, in the two GDM. SGL GDM can be characterized as having a MPL/GDL thickness ratio that increases during compression because the majority of the strain occurs preferentially in the GDL. Freudenberg GDM can be characterized as having a MPL/ GDL thickness ratio that decreases during compression because a slight majority of strain occurs in the MPL. The error bars depict standard deviations representative of MPL thickness variation and not experimental error. Fuel cell polarization and ohmic resistance data for a cell under different levels of compression with SGL 29BC and Freudenberg H2315 C2 at the cathode are shown in Figure 8 at 80 °C cell temperature and 100% inlet relative humidity. The polarization curves are also represented in 3D with variable compression in Figure S6 in the Supporting Information. For F


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Figure 5. Volume-rendered SGL 29BC GDM showing MPL, GDL, and void with separate renderings of individual components at (a) 0%, (b) 34%, and (c) 41% compression. The z-axis indicates the GDM through-plane direction; the y-axis corresponds to the in-plane direction.

Figure 4. (a) Solid fractions of MPL and GDL in through-plane direction. (b) Volumes of MPL, GDL fibers, and voids in the GDL region as a function of compression. (c) PSDs for the two GDLs at various compressions. SGL 29BC GDM data are presented in the left column at 0, 34, and 41% compression while Freudenberg H2315 C2 GDM data are presented in the right column at 0, 7.4, 14, and 28% compression.

Table 4. Parameters for Bimodal or Unimodal log-Normal PSDs for SGL 29BC and Freudenberg H2315 C2 29BC

H2315 C2

compression (%)

r1, r2 (μm)

σ1, σ2 (μm)

f

0 34 41 0 7.4 14 28

12.6, 34.9 11.4 9.67 10.5 9.42 8.67 8.33

0.63, 0.31 0.36 0.48 0.46 0.43 0.40 0.43

0.52

Figure 6. Volume-rendered Freudenberg H2315 C2 showing MPL, GDL, and void with separate renderings of individual components at (a) 0%, (b) 14%, and (c) 28% compression. The z-axis indicates the GDM through-plane direction; the y-axis corresponds to the in-plane direction.

MEAs with either cathode GDM, the ohmic resistance decreases with GDM compression as fiber contact points increase and electrical and thermal contact resistances decrease. However, this favorable effect of compression must be balanced with water management and oxygen transport through the GDM for reductions in ohmic resistance to translate to higher current densities. Though in these particular operating conditions PEMFCs with Freudenberg H2315 C2 GDM at the cathode outperform those with the SGL 29BC, this may not be true at all operating conditions and different GDM may be preferred for different applications. As SGL 29BC GDM are compressed, the morphology evolves in ways that hinder oxygen diffusion to the active sites and water egress. The extensive and deep cracks in the MPL surface of 29BC (Figure 1) lose area and depth with increasing

Figure 7. Thickness ratio of MPL to GDL for SGL 29BC and Freudenberg H2315 C2 as a function of compression. Bars represent the variability of MPL thickness in the respective GDM, not experimental error.

compressive stress as the MPL is redistributed in the plane perpendicular to the stress (Figure 2b). These cracks in the MPL have previously been shown to facilitate water removal G


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in a 142 mA cm−2 decrease in current density from that measured at the optimal 14% compression at 0.6 V. This greater decline in current density indicates a significant oxygen transport resistance that may mark the onset of critical transitions in the GDM structure when compressed by 41% the uncompressed thickness. At these levels of compression in 29BC, macroscopic voids account for a void volume fraction of only 0.12 of the total GDM volume compared to 0.46 when the GDM is uncompressed. Additionally, the MPL region gets redistributed perpendicular to the direction of the stress to fill the large MPL surface cracks (Figure 2), the MPL surface roughness continues to decrease (Figure S4), and the thickness of the MPL region decreases (Figure 1). Though not able to be observed directly by micro X-ray CT, we expect that these microscale MPL structural changes correspond with MPL microstructure loss, which we observed previously with N2 porosimetry by compressing SGL 25BC by 25% its initial thickness and greater.28 We expect the combination of these effects to greatly reduce oxygen diffusivity through SGL 29BC at the highest level of compression. This is manifested in the mass-transport region of the polarization curve at 0.4 V where the largest discrepancy in current density occurs, as current density falls from 1139 to 882.1 mA cm−2 by increasing compression from 14 to 41%. This increase in mass-transport resistance with compression is also illustrated in the iRcorrected polarization curves (Figure 8b) as the strong trend of decreasing current densities with increasing compression persists. Unlike for MEAs with SGL 29BC at the cathode wherein performance decreases when compression increases beyond 14%, MEAs with Freudenberg H2315 C2 as cathode GDM are less sensitive to compression until the GDM is compressed by more than 28% its uncompressed thickness. At 80 °C and 100% RH (Figure 8c), the marginal increase in current density from 14 to 28% compression is explained by the reduction in cell ohmic resistance at the higher compression as the iR-corrected cell voltages (Figure 8d) are nearly indistinguishable. This behavior matches the minor morphological changes illustrated in Figure 4 and Figure 7 for Freudenberg H2315 C2 in this range of compression relative to SGL 29BC and the ability of the Freudenberg GDM to maintain double the volume fraction of void (0.34 void fraction at 28% compression) compared to 29BC (0.17 void fraction at 34% compression) at similar levels of GDM compression. We suggest that the relatively high volume fraction of void in H2315 C2 is one of the most significant differences resulting in its remarkably higher PEFC performance at these operating conditions. We also consider that the difference in cathode catalyst layer ionomer between the MEAs with Freudenberg H2315 C2 (1000EW) and SGL 29BC (1100EW) may favor higher performance in the former. Additionally, MPL properties of the Freudenberg H2315 C2 may better suit this GDM for humid PEFC operation. Specifically, the MPL in H2315 C2 is thinner than that in 29BC and also has much higher PTFE content (40 vs 23 wt % PTFE). Decreasing MPL thickness reduces the barrier for water removal from the fuel cell and increasing PTFE content in the MPL increases the through-plane permeability of GDM.48,49 Both of these effects favor improved oxygen transport. Further increasing the H2315 C2 compression to 40% results in considerable mass-transport losses, as evidenced by a significant reduction in current density at cell voltages below 0.65 V. The current density at 0.4 V (Figure 9b) falls from 1640 to 1186 mA cm−2 when the compression is increased from 28

Figure 8. Fuel cell polarization and ohmic resistance (left column) and iR-corrected polarization curves (right column) as a function of GDM compression at 80 °C cell and 100% inlet RH for MEAs containing (a, b) SGL 29BC at the cathode and (c, d) Freudenberg H2315 C2 at the cathode. Hydrogen and air at the anode and cathode, respectively, flow at a stoichiometric excess of 2.

from GDM by providing channels where liquid water may collect and then bypass much of the thick MPL region to the cathode GDL region and, ultimately, the flow fields.9 Additionally, as the average pore size in the hydrophobic GDL region decreases with increasing 29BC compression (Table 4), the capillary pressure resistance increases and water transport from the catalyst layer to the flow fields requires more water pressure.47 The observation that both GDM void volume, observed in Figure 4b, as well as the volume of micropores in the MPLobserved with N2 physisorption and SEM in our previous work28decrease as compression increases, suggests that oxygen diffusion is compromised at higher compressions. Despite reductions in GDM thickness with compression, concomitant reductions in average pore radius, pore volume, and increases in tortuosity hinder oxygen transport as compressive stress on the GDM is increased. Combined, these effects can explain the reduction in current density from increasing 29BC compression greater than 14% during operation at 80 °C and 100% RH (Figure 8a). Increasing the GDM compression marginally from 28 to 34% results in only a slight reduction in current densities; however, increasing compression by a similar margin from 34 to 41% results in a considerable decrease in current density. The effect of compression is most apparent at lower cell voltages in the bar chart in Figure 9a. Compressing the GDM by 41% results

Figure 9. Current density at 0.7, 0.6, and 0.4 V as a function of GDM compression at 80 °C cell and 100% inlet RH for MEAs with (a) SGL 29BC GDM at the cathode and (b) Freudenberg H2315 C2 at the cathode. H


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The Freudenberg H2315 C2, by contrast, had a thinner MPL and much larger void volume at similar levels of compression. As such, H2315 C2 GDM benefited from reductions in contact resistance at higher compression without sacrificing oxygen diffusivity that is critical for high current density operation. Though micro X-ray CT enabled the observation of significant microscale changes in the morphology of both the MPL and GDL regions of compressed GDM, we can also infer from the studies that MPL porosity plays a major role in the loss of performance with compression. Future studies with nano-CT may better resolve changes in the MPL microporosity with compression and show how these changes are reflected in fuel cell performance.

to 40%, suggesting onset of a considerable mass-transport resistance. An increase in mass-transport resistance at the highest compression may be caused by the constriction in average pore size in the GDL region (Table 4), the loss of GDM void volume with compression (Figure 4), or possible damage to the microstructure of the MPL, since we observe the MPL to be compressed to a similar degree as the GDL region as compression increases (Figure 7). Definitive attribution is difficult because we could not access 40% compression in the micro X-ray CT experimental cell due to the stiffness of the Freudenberg H2315 C2. We expect these effects in compressed H2315 C2 GDM to act in concert to inhibit oxygen diffusion to the active sites and liquid water egress. We caution the reader that the observed structure− performance relationships are for a single set of operating conditions and that the same relationships may not hold true for other operating conditions such as hot and dry or cold and wet operation. This highlights the need for better operando studies and accompanying multiscale models over a large range of operating conditions to fully understand the underlying relationships between structure, operation, and performance of a PEFC.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00077. Additional figures containing gray scale histogram, schematic of the sample holder, gray scale tomographs, and segmented tomographs of SGL 29BC, MPL thickness maps for SGL 29BC and Freudenberg H2315 C2, and 3D polarization curves for the two GDM (PDF)

4. CONCLUSIONS We used X-ray CT to characterize the morphological properties of the GDL and MPL regions for two distinct types of GDM paper-type SGL 29BC and felt-type Freudenberg H2315 C2 while they are actively under a range of compressive stresses. The two GDM structures demonstrated both distinct microstructure in the uncompressed state and unique morphological responses when subjected to compressive stress. The SGL 29BC morphology evolved drastically with compression, while the Freudenberg H2315 C2 morphology transitioned more gradually. As the SGL 29BC GDM compression increased from 0 to 34% (2.8 MPa), (a) widespread and deep cracks in the MPL surface decreased in depth (97 to 32 μm) and surface area (5 to 2.2% crack surface area); (b) large voids in the GDL region preferentially collapsed before the MPL region, and the PSD transitioned from bimodal (12.6 and 34.9 μm average pore radii) to unimodal (9.67 μm); and (c) the void volume fraction decreased from 0.45 to 0.18. As the Freudenberg H2315 C2 GDM compression increased from 0 to 28% (1.1 MPa), (a) minimal surface cracking was observed in the thinner, segregated MPL region; (b) the unimodal PSD was maintained, and average pore radii decreased from 10.5 to 8.33 μm; and (c) void volume fraction in the GDM was considerably higher and decreased from 0.54 to 0.35. As a result of differing responses and sensitivities to compression, PEFC behavior in hot and humid conditions (80 °C, 100% inlet RH) follows distinct trends with compression depending on the GDM applied at the cathode. With the 29BC at the cathode, PEFC performance decreases with compression greater than 14%. When Freudenberg H2315 C2 GDM are applied, PEFC performance is optimized at a compression of 28%. We attribute these distinct performance trends to changes in oxygen diffusivity in compressed GDM. By losing considerable void volume and MPL surface cracks in the very thick MPL region, the compressed SGL 29BC transitioned to a more MPL-dominated structure that occluded oxygen diffusion, resulting in poorer PEFC performance at low cell voltages.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +1 (617) 627-7956. *E-mail: [email protected]. Tel.: +1 (202) 404-3314. ORCID

Iryna V. Zenyuk: 0000-0002-1612-0475 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.W.A. is an American Society for Engineering Education postdoctoral fellow. I.V.Z. acknowledges support from the National Science Foundation under CBET Award 1605159 and an Office of Naval Research summer faculty fellowship. We thank Dr. Dula Parkinson for beamline support. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We are also grateful to the Office of Naval Research for financial support.

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The Role of Compressive Stress on Gas Diffusion Media

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