Interactions between Gas Diffusion Layer Structure and Membrane Durability in Fuel Cells

The membrane electrolyte assembly (MEA) is the key component of a proton exchange membrane fuel cell (PEMFC). The MEA usually consists of gas diffusion layers as outer layers to the inner catalyst coated membrane (CCM). There are various methods to prepare CCMs and decal transfer is currently a common method which was first introduced by Wilson and Gottesfeld [1] and further developed by other researchers [2]–[4]. The initial step of this method is that the catalyst ink is coated onto an intermediate substrate material creating a catalyst coated film (CCF). This step is followed by a transfer of the active layer onto the membrane by hot pressing the membrane and CCF between heated press platens maintained at a specific high temperature for an optimized time and pressure to ensure bonding.Extensive studies on the procedure of decal transfer, substrate material and its preparation, catalyst solvents, etc. have increased the transfer yield of decal transfer to more than 95% [5]. Additionally, in the last decade, industrial development has reduced its cost; however, the total cost for fuel cell manufacturing is still relatively high. One reason for this high production cost is the contamination sensitivity of the CCMs by external particles that can potentially be detrimental to fuel cell operation [6]. This requirement necessitates the use of cleanrooms and quality control equipment to prevent contaminants entering the manufactured components, thus increasing the overall costs. The purpose of the present research is to improve the understanding of the interactions between foreign contaminants and the manufacturing process in the context of MEA production quality. The specific objectives are to i) understand the impact of external particles on the catalyst layer decal transfer process and ii) improve the robustness of the decal transfer method in the presence of external particles.This research investigates the impact of incidental external particles that may be found on the membrane surface or the CCF prior to hot pressing. 60µm Silica microspheres (Si-M) were selected as a representative of solid particles and CCMs with purposely introduced Si-Ms were fabricated and several decal transfer methods and support materials were tested and imaged using the X-ray computed tomography (XCT) technique to analyze the impact on the fuel cell integrity and operation at the presence of the Si-M. While regular decal transfer protocols result in excessive membrane thinning in the presence of these particles, it was observed that by changing the rate of applied pressure and using alternative support materials when transferring the cathode catalyst onto a half-coated membrane, it was possible to reduce membrane thinning under the particles by more than 20% while maintaining transfer quality and cell performance (Fig. 1). In addition, it was observed that although anisotropic mechanical properties of the membrane can adversely affect the CCM topography after the decal transfer, a tuned protocol is able to prevent unwanted deformations and potential stress concentrations. Overall, it is envisioned that the outcomes of this work may enable relaxed quality control measures and manufacturing site cleanroom standards by reducing the potential effects of external particles on MEA production quality. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Ballard Power Systems, and W.L. Gore & Associates. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. Keywords: fuel cell; membrane durability; X-ray computed tomography; decal transfer; manufacturing References Wilson, M. S. & Gottesfeld, S. Thin-film catalyst layers for polymer electrolyte fuel cell electrodes. J. Appl. Electrochem. 22, 1–7 (1992).Shahgaldi, S., Alaefour, I. & Li, X. Impact of manufacturing processes on proton exchange membrane fuel cell performance. Appl. Energy 225, 1022–1032 (2018).Cho, H. J. et al. Development of a novel decal transfer process for fabrication of high-performance and reliable membrane electrode assemblies for PEMFCs. Int. J. Hydrogen Energy 36, 12465–12473 (2011).Thanasilp, S. & Hunsom, M. Effect of MEA fabrication techniques on the cell performance of Pt-Pd/C electrocatalyst for oxygen reduction in PEM fuel cell. Fuel 89, 3847–3852 (2010).Liang, X., Pan, G., Xu, L. & Wang, J. A modified decal method for preparing the membrane electrode assembly of proton exchange membrane fuel cells. Fuel 139, 393–400 (2015).James, B. D., Moton, J. M. & Colella, W. G. Mass Production Cost Estimation of Direct H2 PEM Fuel Cell Systems for Transportation Applications: 2018 Update. ASME 2014 12th Int. Conf. Fuel Cell Sci. Eng. Technol. collocated with ASME 2014 8th Int. Conf. Energy Sustain. V001T07A002–V001T07A002 (2018). Figure 1

A PEM fuel cell’s durability and lifetime are affected by various degradation phenomena, among which chemical degradation of PEM material exerts the most damage. Chemical degradation of PEM occurs when reactive radicals such as hydroxyl (·OH) and hydroperoxyl (·OOH) are generated from the decomposition of hydrogen peroxide. The radicals chemically attack the main and side chains of the perfluorosulfonic acid-based PEM and change its internal chemical structure. This leads to excessive membrane thinning and pinhole formation that causes higher gas crossover and electrode shorting inside the PEM fuel cell1. The generation of harmful radicals accelerates in the presence of Fenton’s metal, such as iron (Fe)2. Evaluation of PEM fuel cell chemical degradation durability is performed using long-term pure chemical accelerated stress test (AST) under high temperature (T), low relative humidity (RH), and open circuit voltage (OCV) conditions3. Physical characterization of a membrane sample used in a fuel cell can help to visualize and assess damage incurred during an AST. These results, however, fail to encompass the formation rate of radicals and intermediate degradation products generated during the AST3. To overcome these issues, computational fluid dynamics models are developed that simulate the evolution of degraded products and predict the lifetime of PEM fuel cells4,5.The present work aims to develop an in-situ one-dimensional (1-D) transient chemical degradation model that describes reaction-transport phenomena within the membrane electrode assembly (MEA) in the presence of Fe and Ce cations. This work considers mechanically reinforced GORE-SELECT® 15 µm baseline (MemA) and chemically and mechanically reinforced 12 µm (MemB-Ce) membranes. The developed in-situ model simulates pure chemical AST operating conditions with variable Fe ion loading and follows the computational algorithms originally presented by Wong and Kjeang4. The model successfully predicted that side chain scission is followed by main chain unzipping, as reported in the literature6. The evaluation of the change in ionomer structure by chemical degradation was done by computing the temporal concentration profile of intermediate degraded ionomer species, hydrogen peroxide, and membrane thickness reduction. The 1-D model also encapsulates the concentration profile of Fe2+ and Fe3+ ions across the PEM and predicts their temporal distribution throughout the simulated AST duration under OCV conditions. It was observed that the MemA thickness decreased rapidly in the presence of Fe ions, as shown in Figure 1. The computational parameters and results were calibrated and validated using in-house AST results for MEAs with embedded Fe metallic particles, which were shown to completely dissolve during conditioning and hence treated as uniformly distributed Fe cations with an initial concentration of 900 ppm at the beginning-of-life7. The model identified a limiting Fe ion concentration limit of 100 ppm that accentuates the chemical attack. Experimentally, a low Fe concentration of 10 ppm was determined in the baseline MemA, which also reduced the membrane thickness within 1125 h of operation, as shown in Figure 1. It is known that cerium (Ce) ions can scavenge harmful radicals and increase the lifetime of PEM fuel cells8. The model, therefore, simulates the effectiveness of Ce ions in the MemB-Ce membrane and the temporal distribution of Ce3+/Ce4+ ions and shows a lower rate of membrane thickness reduction, as shown in Figure 1. The modelling framework was, therefore, able to predict PEM fuel cell lifetime in conjunction with the transport behaviour of both harmful and beneficial cations as well as their interactions during the chemical degradation process at OCV conditions. Keywords: PEM fuel cell, fuel cell durability, chemical degradation model, in-situ modelling, iron, cerium Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Ballard Power Systems, and W.L. Gore & Associates. It was also undertaken in part thanks to funding from the Canada Research Chairs program. References J. Healy et al., Fuel Cells, 5, 302–308 (2005).S. Kundu, L. C. Simon, and M. W. Fowler, Polym. Degrad. Stab., 93, 214–224 (2008).Y. Singh, F. P. Orfino, M. Dutta, and E. Kjeang, J. Electrochem. Soc., 164, F1331–F1341 (2017).K. H. Wong and E. Kjeang, J. Electrochem. Soc., 161, F823–F832 (2014).R. B. Ferreira, D. S. Falcão, and A. M. F. R. Pinto, Int. J. Hydrogen Energy, 46, 1106–1120 (2021).L. Ghassemzadeh, K. D. Kreuer, J. Maier, and K. Müller, J. Phys. Chem. C, 114, 14635–14645 (2010).N. Kumar et al., ECS Meet. Abstr., MA2023-02, 1919–1919 (2023).S. M. Stewart, D. Spernjak, R. Borup, A. Datye, and F. Garzon, ECS Electrochem. Lett., 3, F19–F22 (2014). Figure 1

Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells

We have heard it over and over again – internal combustion engines (ICE) operated with fossil fuels can cause severe damage to our health and environment.1 Nevertheless, they are still very widespread and will not disappear until a compelling alternative is found. One possible alternative is the low temperature polymer electrolyte fuel cell (LT-PEFC). They are commonly operated with hydrogen and air and only emit water. However, before LT-PEFCs can replace ICEs on a large scale mainly two barriers need to be overcome. Those barriers are cost and lifetime. Hence, reducing the costs of a fuel cell and increasing its lifetime are the two most relevant research fields regarding fuel cell commercialization. Whereas the costs are mainly dictated by the raw material prices (e.g. platinum) and the manufacturing process the reasons for limited lifetime are numerous. PEFCs in general are affected by various degradation effects at different locations of the fuel cell.2,3 The membrane suffers for example from pinhole formation and membrane thinning. The hydrophobicity of the gas diffusion layer decreases over operation time which decreases its water transport ability. The bipolar plates are prone to surface corrosion which reduces their conductivity. The catalyst and catalyst layer also represent a major degradation area. Some specific degradation mechanisms are carbon corrosion, catalyst detachment and catalyst particle growth. All degradation effects are complex functions of the specific operational parameters, making it challenging to identify them individually. This in turn is necessary to gain insights into limitations in fuel cell lifetime and is the key to successful development of mitigation strategies to reach a desired lifetime of >5,000 hours for automotive drive cycle operation. There are different techniques available to identify and characterize the individual degradation mechanisms. Some common ones are polarization curve or electrochemical active surface area measurements which are based on electrochemistry. Furthermore, imaging techniques like secondary electron microscopy (SEM) are frequently used for degradation analysis. Another established imaging technique is X-ray tomography. The main advantages of X-ray tomography over SEM imaging are its ability to capture the fuel cell area of interest as a whole and that the tomography scan can be recorded nondestructively and in-operando.4 For some time, fuel cell X-ray tomography was mostly carried out at large scale synchrotron beamline facilities. Unfortunately, they tend to have only very limited access and therefore only few experiments can be conducted in a short time period. In recent development though, lab sized X-ray microscopes have gained interest in the community.5 Such microscopes have excellent availability and therefore enable the possibility to carry out long term experiments. The present contribution will utilize this particular advantage. Our unique approach is to retrace fuel cell degradation with X-ray tomography imaging (cf. Figure 1) over time and correlate it with individual failure mechanisms. In particular, electrode degradation will be discussed. MEAs, with e.g. different carbon/platinum ratios, were exposed to accelerated stress tests and imaged at beginning and end of life. With the aid of X-ray tomography, we are able to segment the MEA and subsequently locate and analyze the degradation effects at the electrode. In combination with electrochemical measurements, exclusive in depth insights of fuel cell degradation are provided. In addition, a novel approach for post X-ray tomography processing is discussed. In summary, the combined findings lead to an improved understanding of LT-PEFC degradation which is a necessary first step in order to investigate new mitigation strategies to increase the fuel cell lifetime. Figure 1. A membrane electrode assembly visualized by X-ray computed tomography recorded at beginning of life. 1. S. M. Platt et al., ‘Two-stroke scooters are a dominant source of air pollution in many cities’, Nat. Commun., 5, p. 3749, 2014.2. J. Wu et al., ‘A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies’, J. Power Sources, 184, pp. 104–119, 2008.3. T. Engl, L. Gubler, and T. J. Schmidt, ‘Think Different! Carbon Corrosion Mitigation Strategy in High Temperature PEFC: A Rapid Aging Study’, J. Electrochem. Soc., 162, pp. F291–F297, 2015.4. J. Eller et al., ‘Progress in In Situ X-Ray Tomographic Microscopy of Liquid Water in Gas Diffusion Layers of PEFC, J. Electrochem. Soc., 158, B963 (2011).5. M. Andisheh-Tadbir, F. P. Orfino, and E. Kjeang, ‘Three-dimensional phase segregation of micro-porous layers for fuel cells by nano-scale X-ray computed tomography’, J. Power Sources, 310, pp. 61–69 (2016) Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. Figure 1

The overpotential at the cathode is significantly large in the proton exchange membrane fuel cells; therefore, the oxygen concentration in the oxygen reduction reaction (ORR) area is vital. Condensed water in the flow field and the gas diffusion layer (GDL) reduces oxygen transport to the ORR area. The enhancement of the mass transport of liquid water and oxygen is critical to improve fuel cell performance particularly at the high current density operations. It is required to reduce the total active area in the fuel cell stack and also it is leading to the significant cost reduction. Experimental investigation of oxygen transport has been limited by an inability to resolve water saturation-dependent properties. The two-phase thermos-fluid dynamics model of GDL was developed with Lattice Boltzmann Method (LBM) and it was integrated with computational fluid dynamics (CFD) based proton exchange membrane fuel cell (PEMFC) model to simulate the fuel cell performance with given design parameters and operational conditions [1, 2]. This work, utilizing the developed model, investigates the correlations between fuel cell performance and GDL structures and material properties. Some empirical studies were pursued and structure and property of the micro porous layer (MPL) in the GDL is correlated to the fuel cell performance particularly high current densities [3-5]. The model simulates the fuel cell performance with various porosity and permeability of MPL in the cathode GDL to investigate those correlations. Also the two-phase model can show the presence of residual liquid water in the cathode GDL which effects on the fuel cell performance. It can distinguish the residual liquid water in the GLD underneath of the flow field channel ribs and under the channels. Figure 1 gives an concept of integrated CFD based PEMFC model with micro-scale GDL model with LBM. Figure 2 shows residual liquid water in the cathode GDL with various operating conditions. In this fuel cell design, residual liquid water is correlated with cathode conditions, relative humidity, rather than the current density. It shows the potential capability of a model-based investigation of mass-transport to find solution of design and operational conditions in the PEMFC.

X-ray Tomographic Analysis of Porosity Distributions in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

Introduction Research and development of polymer electrolyte fuel cells (PEFCs) has been conducted to increase their power density. The gas diffusion layer (GDL) and microporous layer (MPL) used in PEFCs are much thicker than the electrocatalyst layer and electrolyte membrane. Down-thinning GDL and MPL is expected to reduce gas diffusion resistance and increase the power density per volume of the cell stack. However, while the electrocatalyst layer and electrolyte membrane have been thinned to about 10 µm each, the MPL/GDL is still thick at about 150 µm. Thinner layers have been shown to reduce the molecular diffusion resistance of oxygen, resulting in a lower concentration overvoltage1. The application of highly conductive MPL can also reduce ohmic overvoltage by reducing contact resistance and electrical resistance. In addition, MPLs with appropriate hydrophilic and hydrophobic properties can prevent flooding in a high current density range2. Previous studies have investigated the possibilities and technical challenges of using porous metals as GDLs, which can be a thin layer with high strength3. In addition, a composite MPL with both hydrophobic and hydrophilic channels was developed and its effect on molecular diffusion resistance was investigated4. Based on these findings, in this study, we try to reduce contact resistance and electrical resistance by printing MPL layer onto carbon mesh, which is relatively stable and thinner than carbon paper, an existing material, as GDL, and to reduce molecular diffusion resistance by thinning the carbon mesh. We also add polytetrafluoroethylene, a hydrophobic material, and tin oxide, a hydrophilic material, to control hydrophobicity and hydrophilicity of MPLs. The objective of this study is to develop a self-supporting thin-layer MPL/GDL for improved power density. Experimental In this study, the MPL/GDL was fabricated by printing a MPL onto a carbon mesh (50 µm thick, Cosmotech, Japan) as a GDL using a screen-printing method. Cells were fabricated by using these materials. Current-voltage characteristics measurements, microstructure observation, and gas diffusion resistance measurements were conducted. The MPLs were initially composed of carbon black, carbon nanotubes (CNT), polytetrafluoroethylene (PTFE), hydrophilic tin oxide (SnO2), and solvent (a mixture of polyethylene glycol 600, and water). The MPL/GDL preparation was completed by heat treatment at 300°C for 30 min to remove solvent. A standard Pt/C electrocatalyst (TEC10E50E, Tanaka Kikinzoku, Japan) was used for preparing membrane-electrode assemblies (MEAs). As the electrochemical characterization in this study, current-voltage characteristics were evaluated using an electrochemical impedance analyzer (SAS SP-240, Bio-Logic Science Instruments, France), various overvoltages were separated, and molecular resistance of oxygen was measured. Microstructural observations were conducted using a focused ion beam scanning electron microscope (FIB-SEM). Results and discussion Figure 1 (a) shows the current-voltage characteristics of the cell using various MPL/GDLs. The thickness of the MPL/GDLs fabricated was about 80 µm. For comparison, the characteristics of a cell using a standard, commercially-available MPL/GDL (22BB, SGL Carbon, Germany) are also shown. These results show that the use of MPL significantly improves cell performance even with thin GDLs. Using a highly conductive material for the MPL and applying it to carbon-based fibrous materials reduced electrical resistance, and down-thinning MPL/GDLs layers reduced the molecular diffusion resistance of oxygen (Figure 1 (b)). However, the electrical resistance of the GDL substrate is relatively high, and contact resistance is still an issue due to the roughness of the MPL surface in contact with the electrocatalyst layer. If these technical issues can be solved, further improvement in cell performance could be achieved. MPLs with SnO2 showed high performance at a low humidity. This may be due to improved dry-up resistance by the addition of hydrophilic material, whereas flooding was noticeable at high humidity. It is suggested that an appropriate mixture of hydrophobic and hydrophilic channels can enhance the dry-up and flooding resistance of MPLs, resulting in improved cell performance. Acknowledgements This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). An educational part for young scientists was supported by the Japan Science and Technology Agency (JST) as a part of Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE), Grant Number JPMJAP2307. References N. Nonoyama, S. Okazaki, A.Z. Weber, Y. Ikogi, T. Yoshida, J. Electrochem. Soc. 158,B416 (2011).T. Kitahara, T. Konomi, H. Nakajima, and J. Shiraishi, Kikai B, 76 (761), 101 (2010).K. Yamamoto, M. Yasutake, Z. Noda, S. M. Lyth, J. Matsuda, M. Nishihara, A. Hayashi, and K. Sasaki, ECS Trans., 109 (9),265 (2022).P. Wang, H. Nakajima, and T. Kitahara, J. Electrochem, Soc., 171, 014501 (2024). Figure 1

Proper water management in cathode porous electrode of polymer electrolyte fuel cells (PEFCs) is essential for achieving better performance. Especially at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed in catalyst layer (CL) and gas diffusion layer (GDL). When the open pores in cathode CL and the interface between CL and GDL are filled with liquid water, oxygen cannot be supplied to the reaction sites. This phenomenon, called “water flooding”, is a critical barrier for high efficiency and high power density operation. To alleviate this issue, it is necessary to understand and control liquid water transport inside cathode porous electrode. It is well known that microporous layer (MPL) plays a critical role in suppressing liquid water flooding in PEFCs [1,2]. MPL is a much thinner and finer porous layer, which is typically composed of carbon black (CB) particles and PTFE, and inserted between cathode CL and GDL. Nam et al. [2] experimentally and numerically investigated the vapor condensation and liquid water breakthrough in porous layers, and showed that inserting MPL between CL and GDL reduces the size and saturation level of interfacial water droplets on CL surface. However, there is a risk that MPL addition on cathode side increases oxygen transport resistance through porous layers. In general, MPL is formed by coating the slurry of CB powder and PTFE onto GDL substrate and then heat-treated. Its pore structure is complicatedly affected by various fabrication parameters such as CB loading, PTFE/CB ratio, coating and dry processes. To control interfacial water transport between cathode CL and GDL and minimize water flooding, it is essential to optimize the fabrication parameters of MPL determining its microstructure. In this study, we focused on CB loading as one of the major fabrication parameters of MPL, and analyzed the pore structure of MPLs with different CB loadings using SEM-EDX and mercury porosimetry. Furthermore, the operation test and the optical visualization of liquid droplet behavior in the cathode electrode were conducted using a transparent cell, and the influence of CB loading of MPL on its microstructure, the interfacial water transport and the performance characteristics of PEFC was discussed. In the experiment, the MPLs were coated on the hydrophobized carbon paper (Toray, TGP-H-060) by using the doctor-blade technique. The SEM-EDX observation revealed that the average thickness of two MPLs with 1.0 and 4.0 mg/cm2 CB are 33.7 and 123.6 um, respectively. When the CB loading is increased from 1.0 to 4.0 mg/cm2, the thickness of MPL is enlarged approximately 3.7 times. Subsequently, the operation test of a PEFC was carried out at 70 oC under high-humidity conditions, and the effect of CB loading of MPL on the I-V characteristics was investigated. The MPL addition on the cathode side drastically improves the limiting current density, because it suppresses the formation of liquid water film at the CL/GDL interface and reduces the concentration overpotential. In addition, the cell voltage in the high current density range increases with an increase in CB loading of MPL. Since the thick MPL raises the temperature near the cathode reaction sites, the rate of condensation of product water at the CL decreases in the case of 4.0 mg/cm2CB. Therefore, the thick MPL with high CB loading beneficially reduces water flooding at the cathode CL/MPL interface and improves the cell performance.

The influence of gas diffusion layer (GDL) design parameters on the performance of a polymer electrolyte fuel cell (PEFC) was evaluated using a test apparatus in which the clamp pressure on the cell could be accurately controlled. The PEFC performance varies significantly depending on the humidity of the supplied gas. Under moderate humidity, the PEFC performance is enhanced when using an untreated GDL with high air permeability. Under high humidity, improved performance is achieved using a GDL with PTFE hydrophobic treatment. A microporous layer (MPL) coated GDL significantly reduces flooding on the electrode, resulting in a decrease in the concentration overpotential. This enhances the PEFC performance at high current densities. A substrate with high air permeability for the MPL coated GDL enhances the ability of the MPL to prevent flooding. Under low humidity, the MPL is also effective to prevent drying-up of the membrane electrode assembly (MEA), resulting in enhanced performance. A substrate with low air permeability for the MPL coated GDL is effective for the prevention of drying-up.

The polymer electrolyte membrane (PEM) fuel cell is a promising alternative power source for electric vehicles, forklifts, and stationary backup power due to its high power density, low operating temperature, and clean emissions. The gas diffusion layer (GDL) facilitates the transport of heat, reactant gases, water vapor, and liquid water in the PEM fuel cell. The GDL is a highly porous material composed of dense arrays of carbon fibers, resins, and polytetrafluoroethylene (PTFE). Additionally, it is typically coated with a micro porous layer (MPL), which results in the reduction of contact resistance at the catalyst layer interface and the overall improvement in water management. Effective water management must be achieved within the GDL over the entire operating lifetime in order to improve the commercial success of PEM fuel cells. Investigations into the effects of ageing on water management within the GDLs can be useful tools to guide this success. Water is required to maintain polymer electrolyte membrane hydration and ionic conductivity; however, excess water tends to accumulate in the catalyst layer, GDL, and flow channels. This accumulation limits mass transport and leads to performance losses. Therefore, effective water management in the GDLs is essential for improving cell performance, durability, and stability. Most GDL research has been focused on the impact of GDL materials and microstructure design on overall performance, rather than durability. Decreased GDL hydrophobicity has been found after only a few hundred hours of operation (1, 2). Therefore, a clear understanding of the influence of GDL degradation on the liquid water behavior is essential. As a tool to visualize this behaviour, synchrotron X-ray radiography is a powerful technique for visualizing liquid water transport behavior within an operating PEM fuel cell. High intensity X-rays generated at the BMIT-BM beamline of the Canadian Light Source were used for identifying liquid water inside the GDLs and capturing the multiphase flow behavior in an operating PEM fuel cell. Figure 1 is an example synchrotron X-ray radiograph of the fuel cell in operando, illustrating the spatial distribution of liquid water. The observed pixel brightness was correlated to the presence of liquid water, and the saturation profile across the GDL thickness was obtained. In this study, as-received fresh GDLs were immersed in a 35wt % solution of H2O2 at 90o C for 12 hours in order to perform an accelerated artificial ageing process. As-received fresh and artificially aged GDLs were then used to assemble specialized fuel cells for synchrotron imaging. Both the electrochemical performance and liquid water transport behavior were observed for a range of current densities. Additionally, the effect of changes in relative humidity on performance was compared to investigate the effect of artificial ageing. Notable differences in cell performance and liquid water transport behavior were observed at steady state operating conditions. The cell with aged GDLs showed lower cell voltage and higher quantities of liquid water than those of as-received fresh GDLs. The impacts of aging on the liquid water distributions within the GDL and on cell performance will be presented in this work. The objective of current work is to highlight the potential degrading effect on liquid water management in long term fuel cell operation.

Polymer electrolyte fuel cell (PEFC) is promising as a next generation power source for vehicles due to the high efficiency. To make the PEFC commercially competitive, the reduction of the cost is necessary. For the reduction of the cost of PEFC, it is required to increase the maximum current density. Under the high current density operation, the water produced in a catalyst layer (CL) accumulates in a gas diffusion layer (GDL) and the supply of oxygen is suppressed. Therefore, the water transport in the GDL is important to increase the maximum current density of PEFC. To improve the water transport in the GDL, the wettability distribution is important factor. Utaka el al. showed that better cell performance is obtained by forming the hydrophilic region in the hydrophobic GDL because the water transport in the GDL is controlled by the hydrophilic region (1). Similarly, Forner-Cuenca et al. showed that the cell performance is improved by the GDL with the hydrophilic region (2). There are some reports which showed the water transport is improved by the control of the wettability of the GDL. However, these reports are focused on the planer distribution in the GDL and the effect of the wettability distribution in the thickness direction is not clear. For the optimization of GDL structure, it is necessary to clarify the effect of the wettability distribution in the thickness direction. Therefore, in this study, the effect of the wettability distribution in the thickness direction was investigated. To evaluate the water transport in the GDL with the wettability distribution in the thickness direction, a lattice Boltzmann method (LBM) was used (3). Figure 1 shows the simulation domain and boundaries. The size of GDL is 80μm × 80μm × 80μm and the porosity of GDL is 70%. The bottom of the simulation domain is assumed the micro-porous layer (MPL) and it is treated as a solid wall. MPL crack as inlet boundary is set in the MPL. The top of the simulation domain is outlet boundary. The four sides of the simulation domain are solid walls. The contact angle of GDL, MPL and side walls is set 130º. The density of the two-phase in the simulation is set identical to water, ρ = 978kg/m3. The interfacial tension between the two-phase is σ = 6.40×10-2N/m, and liquid and gas viscosities are μL = 4.04×10-4Pa∙s and μG = 2.30×10-5Pa∙s respectively. Figure 2 shows simulation results of water transport in GDL. Figure 2 (a) shows the result for the GDL without the wettability distribution. The contact angle of the GDL is uniform (130 º). Figure 2 (b) shows the result for the GDL with the wettability distribution in the thickness direction. The contact angle from the GDL surface to the depth of 16 μm is 50º and the other regions of the GDL is 130º. In these figures, the water is shown in red. With the water transport in the GDL without the wettability distribution (Fig. 2 (a)), the water was inflowed to the large pore in the center of the GDL (0.32ms). The water was spread in the large pore (0.96ms). After the much water was accumulated in the GDL, the water was reached at the GDL surface (1.9ms). With the water transport in the GDL with the wettability distribution in the thickness direction (Fig. 2 (b)), the water was inflowed to the large pore in the center of the GDL and the water accumulated in the GDL as well as the GDL without the wettability distribution (0.32ms, 0.96ms). When the water was reached at the hydrophilic region in the top of GDL, the water in the GDL was pulled up to the GDL surface and the amount of the water in the center of the GDL was reduced (1.2ms). Compared with the GDL without wettability distribution, the time when the water reached the GDL surface was earlier in the GDL with wettability distribution. Therefore, the water transport in the thickness direction is improved by the wettability distribution in the thickness direction. In the final full paper, the large-scale simulation including the rib and channel will be carried out and the effect of the wettability in the thickness direction will be discussed. Reference (1) Y. Utaka, R. Koresawa, J. Power Sources, 363, 227-233 (2017). (2) A. Forner-Cuenca, J. Biesdorf, V. Manzi-Orezzoli, L. Gubler, T. J. Schmidt, P. Boillat, J. Electrochem. Soc., 163 (13), F1389 (2016). (3) S. Sakaida, Y. Tabe, and T. Chikahisa, J. Power Sources, 361, 133-143 (2017). Figure 1

Within the last decades, fuel cells have become the center of attention of research community, industry, and the general public. This is due to the emphasis on the sustainability of the society and the emerging concept of a hydrogen economy. Proton exchange membrane fuel cells (PEM FCs) represent a mature and established fuel cell technology. The flexibility of operation, the high degree of fuel utilization, and the output power density make them suitable for mobile applications such as hydrogen-fueled vehicles or on-site energy generation. Membrane electrode assembly (MEA) is the mainstay of the PEM FCs. The MEA consists of the proton exchange membrane, catalytic layers, and gas diffusion layers (GDLs). Single MEAs are interconnected by bipolar plates with flow field channels homogenously distributing reactants and realizing easy removal of the generated water. Individual cells are compressed together, forming the fuel cell stack. The GDL plays a three-fold role in the performance of PEM fuel cells. It has to offer good electrical conductivity, ensure good reactant distribution along the reaction zone, and facilitate the removal of the water produced by the fuel cell operation. The optimal combination of these three aspects of GDL allows the FC to achieve high performance. Furthermore, the homogeneous distribution of the reactants is crucial for the durability and lifespan of the FC. The inhomogeneous reactants distribution leads to a local starvation of the electrode, leading to high local overvoltage and thus shortening the lifespan of overloaded parts. Although GDLs play a crucial role in the operation of PEM fuel cells, so far, they have received little attention. Hence, the objective of our study is to compare commercially available GDLs with respect to their properties, namely permeability under conditions close to the operating fuel cell.The approach of combining advanced mathematical modeling with extensive experimental characterization was used to achieve this task. Testing cells allowing precise determination of the in-plane permeability of various GDLs were designed based on the computational fluid dynamics modeling. According to the contemporary literature, two different carbon based GDLs were examined, namely carbon-cloth and carbon-paper, which principally differ in the carbon-fiber orientation in their internal structure. The investigated properties of GDLs thus include thickness, porosity, internal structure, and the presence or absence of microporous layer.The achieved experimental results show that at given compression, thicker carbon paper GDLs are superior to carbon cloth samples in terms of permeability and deformation resistance. The obtained permeability values of GLDs at different degrees of compression were compared with predictions based on the Carman-Kozeny equation to assess the accuracy of this semi-empirical prediction. Finally, the importance of the optimal flow field and GDL combination in terms of reactant distribution in the fuel cell stack of industrial conditions is documented via mathematical modelling based on the obtained experimental data.This work was supported by the European Regional Development Fund-Project “Fuel Cells with Low Platinum Content” (No. CZ.02.1.01/0.0/0.0/16_025/0007414).

A conventional polymer electrolyte fuel cell (PEFC) incorporates a membrane electrode assembly (MEA) which is comprised of the anode catalyst layer-polymer electrolyte -cathode catalyst layer sandwiched between two gas diffusion layers (GDLs), and bipolar plates. Conventional GDL is typically comprised of highly porous carbon paper or cloth to help diffuse the reactant gases onto the electrode, which is coated with a thin micro-porous layer (MPL). At high current densities, mass transport limitations of fuel or oxidizer in the PEFC occur in porous structures of the GDL, particularly at the cathode, which result in a sharp drop in the output voltage. The key to minimizing mass-transport losses is effective water management in the cell. Liquid water in macro-pores of GDL decreases the fuel cell performance at high current density due to the lack of oxygen reaching the catalyst layer. A new MEA structure is recently introduced, where the carbon paper backing layer is eliminated and the entire gas diffusion layer consists of only the MPL [1]. We have further improved on this concept by directly depositing the MPL onto the CCM, resulting in an improvement in the interfacial contact between the MPL and the catalyst layer, as well as a simplified fabrication and assembly process. Spray deposition method was used for depositing this MPL onto a commercial CCM [2]. This concept was proven to work and perform better than a conventional cell with a micro-channel flow field used to provide the desired pathway for the reactant gases throughout the cell substituting for the macro porous carbon paper and the millimeter sized flow field. In this work, a porous foam is utilized as the flow field to distribute the reactant gases over the micro-porous layer instead of the micro-channel flow field. Various pore sizes of the foam, i.e. 60-100 PPI, are used to compare with the conventional micro channel flow field. Contact resistance, permeability, electrochemical impedance spectroscopy, and mechanical properties are used to further characterize the new MEA structure incorporating the porous foam flow field. This method can be an effective way of enabling very high power density operation by reducing the mass transport limitations, and provides an improvement in the interfacial contact between the MEA and the flow field as well as improved mechanical properties. [1] T. Kotaka, Y. Tabuchi, U. Pasaogullari, and C. Y. Wang, Electrochim. Acta, 146, 618 (2014). [2] J. Park, U. Pasaogullari, L. J. Bonville, ECS Transactions, 69, 1355-1362 (2015).

In-plane and through-plane electrical conductivities and contact resistances of a Mercedes-Benz catalyst-coated membrane, gas diffusion and micro-porous layers and a Ballard graphite bipolar plate: Impact of humidity, compressive load and polytetrafluoroethylene

Polymer electrolyte fuel cells (PEFCs) are attracting attention as energy device of low environmental impact because of their high generation efficiency and their emitting only water. The gas supplied to the PEFC is humidified, and water is produced at the cathode catalytic layer by the electrochemical reaction between oxygen and protons. The gas diffusion layer (GDL) and micro porous layer (MPL) in PEFC plays an important role in the transport of oxygen and water. When liquid water accumulates in the GDL, the supply of oxygen to the cathode is inhibited, resulting in poor generation performance. In order to maintain the performance of PEFCs, it is important to manage the water in the cell.Attempts have been made to visualize the liquid water in GDLs and MPLs for better GDL and MPL design; X-ray micro computed tomography (micro-CT) is one powerful tool for this. Visualization of liquid water in GDLs and MPLs using micro-CT has so far revealed the behavior of the produced water[1,2]. However, in previous studies, it has been difficult to visualize liquid water in the catalyst layer, which is the reaction field where the water is produced. This is because the spatial resolution of micro-CT is larger than a µm, which is insufficient for analyzing liquid water in the catalytic layer. X-ray CT measurements with a resolution of at least 150 nm are needed to measure the distribution of liquid water in the cross section of the catalyst layer. In this study, a measurement system to reveal the distribution of liquid water in the catalyst layer was developed using a 150 nm-spatial-resolution X-ray CT with a synchrotron radiation x-ray microscope called X-ray nano-CT.A GDL specimen prepared cut to a disk shape with 1 mm diameter. The GDL specimen mounted on a copper stage with high thermal conductivity connected Peltier element module. The micro-CT and nano-CT measurements were carried out in BL20XU beamline at SPring-8, Japan. An X-ray beam energy was 30 keV with a double crystal monochromator. The nano-CT measurements were performed using Zernike phase contrast with voxel size of 38.4 nm. We measured the micro-CT and nano-CT of the dry condition of the GDL specimen. Then, the GDL specimen injected liquid water was scanned. Injection of liquid water into the GDL specimen is not easy because of water-repellent of GDLs. Helium gas saturated water vapor at 40 °C was sprayed to the GDL at 30 ml/min, then the GDL cooled to 10 °C using a Peltier element module and the vapor was condensed inside of the GDL. The measurement setup is shown in Figure 1. This water injection in GDL method was based on the method reported by Kato et al [2].Figure 2 shows the optical system of the nano-CT used in this study. The optical system of this nano-CT provides a condenser zone plate for sample illumination, a Fresnel zone plate for enlarged image, and phase ring for Zernike phase contrast. These optics allow CT with a high spatial resolution of 150 nm to be measured even for materials with low X-ray absorption.A three-dimensional structure of GDL was revealed by CT measurements in each scale order. The distribution of liquid water in GDL were visualized with nanoscale spatial resolution (Figure 3). We have successfully visualized the three-dimension distribution of liquid water in the GDL with nm order scale using nano-CT. The newly established nano-CT with a spatial resolution of 150 nm order is extremely useful for elucidating liquid-water behavior in PEFCs containing catalyst layers, which was not clear before. Acknowledgements This work is based on results obtained from a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This study was partially supported by the Synchrotron radiation experiments performed at BL20XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal numbers 2021A2009, 2021B1015).