Cathode Gas Diffusion Layer Research Articles

Numerical simulation of two-phase flow in gas diffusion layer and gas channel of proton exchange membrane fuel cells

Liquid water within the cathode Gas Diffusion Layer (GDL) and Gas Channel (GC) of Proton Exchange Membrane Fuel Cells (PEMFCs) is strongly coupled to gas transport properties, thereby affecting the electrochemical conversion rates. In this study, the GDL and GC regions are utilized as the simulation domain, which differs from previous studies that only focused on any one of them. A Volume of Fluid (VOF) method is adopted to numerically investigate the two-phase flow (gas and liquid) behavior, e.g., water transport pattern evolution, water coverage ratio as well as local and total water saturation. To obtain GDL geometries, an in-house geometry-based method is developed for GDL reconstruction. Furthermore, to study the effect of GDL carbon fiber diameter, the same procedure is used to reconstruct three GDL structures by varying the carbon fiber diameter but keeping the porosity and geometric dimensions constant. The wall wettability is introduced with static contact angles at carbon fiber surfaces and channel walls. The results show that the GDL fiber microstructure has a significant impact on the two-phase flow patterns in the cathode field. Different stages of two-phase flow pattern evolution in both cathode domains are observed. The liquid water in the GDL experiences water invasion, spreading, and rising, followed by the droplet breakthrough in the GDL/GC interface. In the GC, the water droplets randomly experience accumulation, combination, attachment, and detachment. Due to the difference in surface wettability, the water coverage of the GDL/GC interface is smaller than that of the channel side and top walls. It is also found that the water saturation inside the GDL stabilizes after the water breakthrough, while local water saturation at the interface keeps irregular oscillations. Last but not the least, a water saturation balance requirement between the GDL and GC is observed. In terms of varying fiber diameter, a larger fiber diameter would result in less water saturation in the GDL but more water in the GC, in addition to faster water movement throughout the total domain.

Investigation of Water and Heat Transfer Mechanism in PEMFCs Based on a Two-Phase Non-Isothermal Model

In a proton exchange membrane fuel cell (PEMFC) system, proper management of water and heat transport is essential to improve its overall performance and durability. To comprehensively investigate the internal processes of PEMFCs, an improved two-phase non-isothermal model based on heat and water transfer mechanisms inside the fuel cell is developed. The results show that the model proposed in this work can predict the fuel cell’s performance accurately and is capable of exploring water and heat transfer phenomena inside fuel cells. Additionally, the water and heat transfer of cathodes and anodes under different relative humidity and temperatures are studied. It can be concluded that when the PEMFC operates under a constant voltage, the anode water content gradually increases, while the cathode water content gradually decreases. The maximum water content occurs at the interface between cathode catalyst layer and cathode gas diffusion layer, while the minimum value is attained at the interface between anode catalyst layer and anode gas diffusion layer. When the fuel cell operates at 0.75 V, although the water content of CCL is the highest, no back-diffusion of dissolved water occurs.

EFFECT OF NON-UNIFORMLY DISTRIBUTED POROSITY OF GAS DIFFUSION LAYERS ON MASS TRANSFER IN PROTON EXCHANGE MEMBRANE FUEL CELLS

Gas diffusion layer porosity can affect mass and charge transfer inside proton exchange membrane fuel cells and influence cell performance. Moreover, the electrochemical reaction rate distribution in various regions inside the cell is not uniform. Proper porosity distribution is very important to improve cell performance. In this paper, a three-dimensional fuel cell model with three steps porosity along the gas flow direction is established. Four cases of non-uniformly distributed porosity, both in anode and cathode gas diffusion layers are simulated, which are compared with uniform porosity distribution to study the effect of these structures on mass transfer inside the cell. Then, an optimization calculation is carried out to obtain the optimal porosity distribution along gas flow direction at 0.2 V and 0.6 V. The numerical results indicate that non-uniformly distributed porosity can change cell performance, and porosity increasing along the gas flow direction makes cell performance be better. Porosity increasing along the gas flow direction can improve the uniformity of current density distribution at low voltage. Diffusive mass flux plays a dominant role in reactant mass transfer. Higher porosity near the outlet region increases total mass flux at the interface, and the proportion of diffusion mass flux in total mass flux also increases.

Heat and mass transfer at the interface between cathode catalyst layer and gas diffusion layer of a proton exchange membrane fuel cell

Heat and mass transfer at the interface between cathode catalyst layer and gas diffusion layer of a proton exchange membrane fuel cell

Investigation of Gradient Platinum Loading and Porosity Distribution for Anion Exchange Membrane Fuel Cells

Abstract Anion exchange membrane fuel cells (AEMFCs) are in development as a low-cost alternative to proton exchange membrane fuel cells (PEMFCs). AEMFCs produce water at the anode side and consume it at the cathode side, resulting in no cathode water flooding like in PEMFCs. However, it brings complexity to water transportation behavior and requires appropriate water balance to avoid membrane drying out. In this study, a two-dimensional two-phase multi-physics model has been developed to investigate the impacts of three key electrode parameters (porosity, catalyst loading, and ionomer content) that are responsible for water production and transport as well as the performance of an AEMFC. A piecewise constant function along the x-direction (reactant diffusion direction) is used to apply the gradient on the porosity and platinum loading. The present results show that a larger porosity gradient near the cathode gas diffusion layer (GDL)/flow channel interface and lower near the GDL/microporous layer (MPL) interface can enhance mass transport and water removal, which is benefited the AEMFC performance. However, anode GDL porosity gradients show a lower AEMFC performance compared to the cathode porosity gradients. Moreover, it was confirmed that for both electrodes, the performance of AEMFC was significantly dependent on each electrode parameter.

High-resolution neutron imaging of salt precipitation and water transport in zero-gap CO2 electrolysis

The electrochemical reduction of CO2 is a pivotal technology for the defossilization of the chemical industry. Although pilot-scale electrolyzers exist, water management and salt precipitation remain a major hurdle to long-term operation. In this work, we present high-resolution neutron imaging (6 μm) of a zero-gap CO2 electrolyzer to uncover water distribution and salt precipitation under application-relevant operating conditions (200 mA cm−2 at a cell voltage of 2.8 V with a Faraday efficiency for CO of 99%). Precipitated salts penetrating the cathode gas diffusion layer can be observed, which are believed to block the CO2 gas transport and are therefore the major cause for the commonly observed decay in Faraday efficiency. Neutron imaging further shows higher salt accumulation under the cathode channel of the flow field compared to the land.

(Invited) Direct Ammonia Anion-Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) are a promising alternative to the more established proton exchange membrane fuel cell (PEMFC) technology, with advantages mainly related to the reduction in expense of materials used. While Hydrogen is the most popular fuel for AEMFCs, ammonia’s high energy density together with its established transport and distribution technology, make it an attractive candidate as an alternative fuel for AEMFCs.Although early attempts to utilize ammonia as a fuel in AEMFCs resulted in poor performance, recent studies of direct ammonia anion-exchange membrane fuel cells (in short - DAFCs), with advanced anion exchange membranes (AEMs) and electrocatalysts, have yielded new and promising performance measure values (see e.g. [1]) thus motivating further analyses and development of these systems. The two main challenges to be addressed when considering this technology are a slow ammonia oxidation reaction (AOR) and ammonia crossover.In this contribution, we will present a computational analysis of a DAFC, aimed at probing performance and performance stability of this system. A special emphasis will be placed on the effect of ammonia crossover and the resultant parasitic reaction (involving ammonia oxidation in the cathode) on system behavior. Following an appropriate introduction, we will describe our modeling approach, properties, and parameters, where our one-dimensional and transient model of a DAFC, including degradation kinetics capabilities, is conceptually based on our previous studies of hydrogen-fueled AEMFCs (e.g. [2-4]). The computational domain consists of a membrane, anode and cathode gas diffusion layers, and anode and cathode catalyst layers. The model captures species transport and significant electrochemical reactions taking place in the cell. After detailing our modeling approach, we will present calculations aimed at validating our model against experimental data of a DAFC operated with KOH-free anode feed at high temperatures (100 and 120 °C). Following this, we will show and discuss results concerning the effect of ammonia crossover on DAFC performance and performance stability. The initial (un-degraded) cell performance analysis reveals (among other things) a dependence of the parasitic reaction rate through the cathode on the value of current density. More interestingly, the simulated results demonstrate (under certain conditions) a surprising and nontrivial impact of ammonia crossover on cell longevity..

Which Properties Should Anion-Exchange Membranes Have to Achieve a Longer Fuel Cell Lifetime?

The substantial advancements and availability of new cost-effective materials have drawn significant attention to the technology of anion-exchange membrane fuel cells (AEMFCs). The anion exchange membrane (AEM) is a core component in AEMFCs, essential for conducting hydroxide ions and, more importantly, controlling water transport between the fuel cell electrodes. In this contribution, we apply the computational methodology outlined in Refs. [1,2] to explore the effects of various membrane properties on AEMFC performance and its stability. We specifically consider the following parameters: (A) water diffusivity, (B) membrane thickness, (C) ion exchange capacity (IEC), (D) maximum hydration level (λmax) corresponding to ionomer/water contact (maximum number of water molecules per functional group), (E) hydroxide conductivity, and (F) degradation rate constant of the AEM functional group.First, we describe our modeling approach, which includes a one-dimensional isothermal and time-dependent model of AEMFC operations. The model considers the chemical degradation process of the ionomeric materials in the cell, transport phenomena through the anode and cathode gas diffusion layers, anode and cathode catalyst layers, and the membrane. Finally, the electrochemical reactions, hydrogen oxidation in the anode and oxygen reduction in the cathode, are described by Butler-Volmer kinetics. We present our main findings, demonstrating that membrane water diffusivity and λmax have the most significant impact on AEMFC lifetime, followed by membrane thickness and IEC, attributed to enhanced water distribution across the cell. We also demonstrate the significance of improving functional group stability to performance stability, while AEM hydroxide conductivity shows a negligible effect on AEMFC lifetime. Finally, we perform dimensional analysis and provide an analytical, useful correlation to estimate the AEMFC lifetime of selected membranes from the literature and compare it to the measured operation time. In conclusion, this work highlights important AEM parameters aimed at improving AEM designs in order to enhance the performance stability of AEMFCs.[1] D.R. Dekel, I.G. Rasin, S. Brandon, Predicting performance stability of anion exchange membrane fuel cells, J. Power Sources. 420 (2019) 118–123. https://doi.org/10.1016/j.jpowsour.2019.02.069.[2] K. Yassin, I.G. Rasin, S. Brandon, D.R. Dekel, Elucidating the role of anion-exchange ionomer conductivity within the cathode catalytic layer of anion-exchange membrane fuel cells, J. Power Sources. 524 (2022) 231083. https://doi.org/10.1016/j.jpowsour.2022.231083.

Pore-Scale Saturation and Liquid Water Pathways in PEFCs: Insights from Correlative 2D and 3D X-Ray Imaging

Improving the efficiency of polymer electrolyte fuel cells (PEFCs) through high power density operation necessitates liquid water management in the cathode gas diffusion layer (GDL), where liquid water saturation inhibits oxygen diffusivity. To this end, there have been numerous efforts aimed at understanding liquid water transport behaviour to guide GDL design improvements. X-ray imaging of representative in-operando miniaturized PEFC samples has been instrumental to much of recent progress [1–4] in understanding liquid water distribution patterns. Radiographic two-dimensional (2D) images have enabled visualization of liquid water dynamics [3] while three-dimensional (3D) computed tomography images have provided pore-scale information of liquid water saturation states in the GDL [2] and of its transport mechanism [4]. However, how liquid water pathways are formed and the likelihood for preferential pathways remain unclear.In this work, we investigate factors that influence liquid water distribution and pathway definition using correlative rapid 2D and long duration 3D operando X-ray datasets which enable a visualization of liquid water breakthrough dynamics and the corresponding pore-scale interactions in the GDL. The correlated images together with GDL pore structure visualization are used to identify three characteristic pore-scale saturation behaviour with sample regions highlighted in Fig. 1. These regions include locations with restricted liquid water breakthrough, locations with apparent large breakthrough porous pathways where breakthrough liquid water is observable at least in the 2D images, and similarly porous regions where no liquid water breakthrough is observed. It is found that the behaviour shown in the identified regions are strongly linked to local pore-scale structural characteristics and capillary pressure distribution in the GDL. Investigating the 3D virtual GDL from around the microporous layer through to the channel interface, it is shown that liquid water pathways get increasingly defined with a decrease in spatial uniformity towards the flow channels. Pore-scale analysis and observed flow mechanisms show that the defined pathways are locally accessible paths of least capillary resistance dictated by pore radius and associated hydrophobicity. These findings highlight the important role that pore scale topology plays in addition to pore size distribution for future GDL design aimed at improving water management. Keywords— operando, fuel cell, water, pore structure, X-ray imaging Acknowledgments Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, and Canada Research Chairs.

Multiscale Study of PEMFC in Marine Environment: Impact of a NaCl Spray on Durability

In the wake of the growing environmental challenges, hydrogen produced from renewable energy is a good candidate as a substitute for conventional fuels. Indeed, fuel cells can produce electricity thanks to air and hydrogen, and only reject water. The use of this technology is currently emerging in various sectors, such as the maritime industry. However, sea salt aerosols, mostly composed of sodium chloride (NaCl), can damage fuel cells. So far, only a few studies have been dedicated to the impact of NaCl on Proton Exchange Membrane Fuel Cells (PEMFCs) though showing that salty air can induce a significant loss of performance without clearly identifying the underlying physicochemical mechanisms [1][2][3].Thus, the present experimental work investigates the effects of a salt spray injected into the airflow of single PEMFC cells and stacks at various time scales and contamination levels, with the final aim of identifying the appropriate PEMFC stack protection needed for maritime applications. A dedicated experimental setup generating salt spray in the cathode airflow of a single cell and 5-cells stack has been designed, realized and characterized. The NaCl concentration range used is from 10 mg/m3 to 5 g/m3, to reproduce different types of salty environments. Cells and stacks are operated in constant current density mode.The study is conducted under the conditions of accelerated stress tests. When polluted at high concentrations, single cells degradation becomes significant after only a few minutes of operation (Figure 1) and after a few hours for stacks. Even under low concentrated environments, the single cell voltage decreases faster than the baseline (ie reference voltage with pure water in air stream), leading to PEMFC lifetime shortening. It has been observed that cathode rinsing with nitrogen flow or operation under pure water results in partial cell performance recovery. Moreover, starts-up and shuts-down operated during tests can have a positive or negative influence on cell performance, depending on the break duration.Even when the tests revealed only weak decrease in power, Membrane Electrode Assemblies (MEA) and bipolar plates (BP) post-mortem characterizations highlight degradations. Scanning Electron Microscopy (SEM) studies coupled with Energy Dispersive X-ray Spectroscopy (EDS) have contributed to the identification of components in MEA and BP. As expected, chlorine reaches not only the cathode gas diffusion layer (GDL) and catalyst layer (CL) but also the anode CL by crossing the membrane [4]. According to the literature, chloride adsorption on Pt surface or Pt dissolution reduces the active area [3] whereas sodium ion contaminates the membrane [1]. Moreover, corrosion products from metallic components have been found. Corrosion marks identified as stainless steel from BP are present in the stack effluents and MEAs. As for single cell, compounds from current collectors (gold, nickel and copper) are present in the GDL and CL. In the near future, for single cell tests, non-metallic current collectors will be used in order to discriminate the effects of the direct degradation of salt spray on MEA and indirect degradation coming from corrosion products of metallic cell components.Overall, the present study reports that NaCl effects on PEMFCs not only depend on the concentration of the NaCl mist but also on the MEA type, current collectors/BP material, and startup/shutdown procedure. In the full paper and oral presentation, tests conducted with less concentrated NaCl droplets will also be presented, with the aim of determining adegradation threshold for single cells.

Investigating the Influence of Humidity on Liquid Water Transport Mechanisms in Fuel Cell Gas Diffusion Layers Using Operando X-Ray Computed Tomography

The intermittent nature of many renewable energy sources, such as solar and wind, presents the need to acquire on-demand renewable energy either through energy storage or energy production methods. The polymer electrolyte membrane (PEM) fuel cell is quickly becoming an attractive solution as it can produce high power densities under a rapid change in load while producing zero local carbon emissions [1] and is ideal for large-scale applications, such as automotive [2]. However, barriers to their widespread implementation are attributed to the high costs associated with mass transport losses at high current density operation due to inefficiencies in liquid water management in the gas diffusion layer (GDL) [3][4]. Understanding the relationship between liquid water distributions in the cathode GDL and transport properties of PEM fuel cells under varying operating conditions can lead to improved GDL configurations for improved performance. Previously, studies have been conducted to characterize the effect of operating temperature on liquid water pathways in GDLs by using 3D imaging techniques on operando PEM fuel cells [5]. High-speed, high-resolution 3D imaging techniques, such as computed tomography (CT), enable the visualization of dynamic pore-scale effects and are advantageous in elucidating transport mechanisms in the GDL.In this work, the effect of operating relative humidity on the formation of liquid water pathways in the GDL will be explored by imaging an operando PEM fuel cell with synchrotron X-ray CT. To date, a custom PEM fuel cell has been developed in-house which features a novel design with two rotary unions on either side of the cell. The rotary unions enable continuous rotation of the cell during imaging, and thus are central to achieving high temporal resolution for visualizing the development of preferential water pathways. High spatial resolution (pixel resolution of 1.44 microns per pixel [6]) is also obtained with the synchrotron beam, which allows for the visualization of liquid water in individual pores at the microscale in the GDL. Visualizing water in the micropores of the GDL can help us further understand water transport mechanisms within the complex structure of the GDL to ultimately develop methods to expel excess water efficiently. In the next steps of this research, we will use the results from electrochemical testing and the reconstructed images of the fuel cell to draw important conclusions about the relationship between liquid water distributions in the GDL and fuel cell performance. As well, the effect that operating relative humidity has on this relationship will be determined. The aim of this work is to inform optimal design for GDL materials for improved PEM fuel cell performance, which will ultimately accelerate their utilization on a global scale as a reliable and sustainable energy source.[1] P. Shrestha, CH. Lee, K. F. Fahy, M. Balakrishnan, N. Ge, and A. Bazylak, “Formation of Liquid Water Pathways in PEM Fuel Cells: A 3-D Pore-Scale Perspective,” Journal of The Electrochemical Society, vol. 167, no. 5, p. 054516, Jan. 2020, doi: 10.1149/1945-7111/ab7a0b.[2] S. Park, J. W. Lee, and B. N. Popov, “A review of gas diffusion layer in PEM fuel cells: Materials and designs,” International Journal of Hydrogen Energy, vol. 37, no. 7, pp. 5850– 5865, Apr. 2012, doi: 10.1016/J.IJHYDENE.2011.12.148.[3] Y. Nagai et al., “Improving water management in fuel cells through microporous layer modifications: Fast operando tomographic imaging of liquid water,” Journal of Power Sources, vol. 435, p. 226809, Sep. 2019, doi: 10.1016/J.JPOWSOUR.2019.226809.[4] U. U. Ince et al., “3D classification of polymer electrolyte membrane fuel cell materials from in situ X-ray tomographic datasets,” International Journal of Hydrogen Energy, vol. 45, no. 21, pp. 12161–12169, Apr. 2020, doi: 10.1016/J.IJHYDENE.2020.02.136.[5] Hong Xu, Shinya Nagashima, Hai P. Nguyen, Keisuke Kishita, Federica Marone, Felix N. Büchi, Jens Eller, Temperature dependent water transport mechanism in gas diffusion layers revealed by subsecond operando X-ray tomographic microscopy, Journal of Power Sources, Volume 490, 2021, 229492, ISSN 0378-7753, https://doi.org/10.1016/j.jpowsour.2021.229492.[6] Canadian Light Source, “Detectors,” BMIT. [Online]. Available: https://bmit.lightsource.ca/tech-info/detectors/. [Accessed: 05-Apr-2022].

(Digital Presentation) Effect of Compression Pressure on Water Removal and Power Generation Performance of PEFC with Modified Electrode/Channel Structure

Water flooding in cathode electrodes of polymer electrolyte fuel cells (PEFCs) is a critical issue that must be solved for achieving high efficiency and high power density operations. To alleviate this problem, it is necessary to design the optimum electrode/channel structure for actively removing liquid water from the porous media to the flow channel. In the previous studies, Okuhata et al. [1] revealed that the introduction of penetration holes to gas diffusion electrodes (GDEs) effectively promotes the water discharge and improves the limiting current density. Nishida et al. [2] investigated the effect of channel hydrophilization on the water behavior in the cathode of an operating cell using the cross-sectional visualization technique. It was found that the hydrophilization of cathode channels encourages the water suction from the gas diffusion layer (GDL) to the channel and enhances the oxygen diffusibility to the catalyst layer (CL). Our research group has proposed a novel hybrid electrode/channel structure combining the electrode perforation and channel hydrophilization and has investigated its effect on the improvements of water drainage and cell performance. In this study, we directly visualized the liquid water distribution in the cathode GDL of a customized PEFC with the hybrid electrode/channel structure using X-ray imaging and evaluated the transient response of cell voltage under the constant-current operations. Furthermore, the influence of compression pressure on the water behavior in the perforated GDL and the performance characteristics of the customized cell was also examined.For the X-ray investigation, we fabricated a specialized miniature fuel cell with the effective electrode area of 2.88 cm2. The membrane electrode assembly (MEA) was sandwiched between two carbon separators with a single-serpentine flow field (number of channels: 7, channel width: 1.0 mm, depth: 1.0 mm, total length: 88.5 mm). Fig. 1 shows a schematic cross-sectional view on the cathode side of the experimental cell. Since the cathode GDL is irradiated with X-ray beam in the in-plane direction, the through-plane distributions of water can be observed in the GDL under the 4th channel and under the land between the 3rd and 4th channel. To promote the water discharge from the fine porous media, a penetration groove (width: 300 μm, length: 11 mm) was installed into the GDL, MPL and CL and positioned along the right sidewall of the 4th channel. The channel walls on the cathode side were coated with the silica-based hydrophilic solvent. The contact angle of water on the hydrophilized channel is 9.8o. In the hybrid electrode/channel structure, liquid water accumulated in the CL and MPL is firstly discharged into the large groove in the in-plane direction. Subsequently, water droplets gathered in the groove are grown up and attached to the hydrophilized channel walls. The droplets are spread out on the hydrophilic sidewalls and the liquid films are moved upward. Fig. 2 presents the distributions of water saturation in the perforated GDL of the customized cell assembled under two different compression pressures of 0.5 and 1.0 MPa. The current density was increased stepwise from 0.14 to 1.02 A/cm2 for 8 min under the room temperature and non-humidification condition. All images were captured at t=8 min after starting the operation. Dry hydrogen (stoichiometry: 1.5@2.0A/cm2) and oxygen (stoichiometry: 3.0@2.0A/cm2) were supplied to the anode and cathode in the counter-flow arrangement. It was found that the introduction of the hybrid structure effectively decreases the water saturation around the groove due to the in-plane water discharge. The reduction of saturation under the land is much less than under the channel because the strong mechanical compression crushes the pore structure under the land regions and suppresses the water transport. In addition, the low-saturation region of 28% or less around the groove is enlarged with a decrease in compression pressure. The low compression pressure maintains the relatively large porosities in diffusion media and facilitates the water removal.

Optimization of the Cathode Gas Diffusion Layer Also Matters for Water Electrolyzers

Polymer electrolyte water electrolyzers (PEMWEs) are a promising technology for the storage of energy from intermittent renewable sources such as wind and solar. PEMWEs split water into hydrogen and oxygen electrochemically. Under typical operating conditions, the hydrogen evolution reaction (HER) in the cathode is not limited by reactant transport, since it is supplied by the rapid transport of protons from the polymer electrolyte and electrons from the external circuit. There are very limited studies on the role of the cathode gas diffusion layer (GDL), typically a carbon-paper based layer. In this study, we investigated the effect of thickness, presence of a microporous layer, and wettability of the cathode GDL. Results show that cathode GDL properties have a significant effect on the performance of the PEMWE cells. Figure 1a shows the importance of the GDL thickness with the optimized GDL compression. The thickest GDL, MGL370 (i.e. 370µm thick ) has the best performance compared to MGL280 (280µm thick) and MGL190 (190µm thick). Compression of the GDL also affects the performance: when the compression is increased from 12% to 25% of the thickness, a performance loss is observed possibly due to damaged carbon fiber network and collapse the pores of the GDL (Figure 1b). Acknowledgment: Financial support from the US Department of Energy through the Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cells Technology Office is gratefully acknowledged. Figure 1

Effects of MPL Pore Size Distribution on Components of Oxygen Transport Resistance Increased Due to Water Accumulation in a PEFC

To increase the power output of PEFC, management of generated water is extremely important. A gas diffusion layer (GDL) with a microporous layer (MPL) on the cathode side is thought to play important roles in drainage of product water and optimizing oxygen transport, but the details of the mechanism have not been elucidated. In this study, to reveal the water and oxygen transport phenomena, the components of oxygen transport resistance were evaluated using the limiting current analysis (1) for two types of MPL with different pore size distributions. Further, the water distributions in the cathode side MPL and catalyst layer (CL) were observed by a freezing method and a cryo-SEM (2). The active area of the cell used in this study was 1.8 cm2 (0.9 cm × 2.0 cm). The anode and cathode flow fields were parallel straight type with 1.0 mm wide ribs and channels. The anode side was supplied with hydrogen, and the cathode side was supplied with a mixed gas of oxygen and nitrogen. The gas flow rates were 500 sccm for the anode and 4000 sccm for the cathode. The relative humidity of the supplied gases was 90%. Experiments were conducted under two different cell temperature, 30°C (low temperature condition) and 80°C (high temperature condition). The catalyst coated membrane was produced by Japan Gore (PRIMEA). We used two types of MPL with different pore size distributions for the cathode GDL: MPLs with one peak (conventional type) and with two peaks. The MPLs are called MPL-1 and MPL-2, respectively. For the anode side, a GDL with an MPL (SGL, 28 BC) was used. This study used the limiting current analysis developed by the authors (1). The developed method extended the conventional method to the separation under flooded conditions, considering vapor transport and water distribution. Inside the CL, it was assumed that the water production rate dominates the water accumulation, which is proportional to current density. Outside the CL, it was assumed that the product of total pressure and current density determines the water accumulation (1). The experiments were conducted with different back pressures (110 kPa, 140 kPa, 180 kPa) and different oxygen concentrations (1%, 2%, 4%, 6%, 8%, 12%, 15%, 18%, 21%) for the cathode side. Figure 1 shows the total oxygen transport resistances at 110 kPa. There is a significant difference in the tendencies of the total oxygen transport resistance between the MPL-1 and the MPL-2 under the low temperature condition, while the tendencies are similar under the high temperature condition. This suggests that the effects of water accumulated in the cell are larger at lower temperature. Figure 2 shows the results separating the total oxygen transport resistance into two resistances. The I Lim is the limiting current density. The R P and R NP are the pressure-dependent and -independent oxygen transport resistances. These correspond to the oxygen transport resistances outside and inside the CL. Under the low temperature condition, the pressure-dependent oxygen transport resistances (corresponding to those outside the CL) are different between the MPL-1 and the MPL-2. The pressure-independent resistances (corresponding to those inside the CL) are similar. These mean that the smaller total oxygen transport resistances with the MPL-2 in Figure 1 are caused mainly by the smaller resistance outside the CL. Before the separation, we predicted that the increase in the total oxygen transport resistance is suppressed by the MPL-2 due to enhanced drainage of product water from the CL. However, Figures 2(a) and (c) revealed that the R P of the MPL-2 becomes smaller than that of the MPL-1 rather than the R NP. This may be considered because the larger pores in the MPL-2 form drainage pathway for product water at the low temperature condition, which significantly contributes to increasing the oxygen transport pathway through the smaller pores. As expected from Figure 1, there is no significant difference in the R P and the R NP between two types of the MPL under the high temperature condition (Figures 2(b) and (d)). The water distribution in the vicinity of MPL-2 was observed after discontinuing operation at 2.0 A/cm2 just before the limiting current condition. The experiment was conducted with 30°C, 180 kPa back pressure, and 21% oxygen concentration. Cryo-SEM images showed that there was ice at the MPL/CL interfaces. Additionally, ice crystals were observed at the larger pores in the MPL-2, supporting the above consideration, the drainage pathway for product water through the larger pores.Reference (1) Y. Iizuka, et al., ECS Transactions, 104(8), 83-92 (2021). (2) Y. Aoyama, et al., Electrochem. Commun., 41, 72-75 (2014). Figure 1

(Digital Presentation) Effect of Compression Pressure on Water Removal and Power Generation Performance of PEFC with Modified Electrode/Channel Structure

To prevent the water flooding in polymer electrolyte fuel cells (PEFCs), it is essential to investigate the water transport in cathode diffusion media of operating cells and design the optimum electrode/channel structure for encouraging the water discharge. Our research group has proposed a hybrid structure with the electrode perforation and channel hydrophilization. This study directly visualized the water distribution in the cathode gas diffusion layer (GDL) of a customized PEFC using X-ray radiography and characterized the cell performance under the constant-current operations. Furthermore, the influence of compression pressure on the water behavior in the perforated GDL and the performance characteristics of the customized cell was examined. It was found that the modified electrode/channel structure enhances the water discharge from the diffusion media and effectively reduces the water saturation around the penetration groove. The excessive mechanical compression of the customized cell suppresses the water removal through the diffusion media.

Multiscale Study of PEMFC in Marine Environment: Impact of a NaCl Spray on Durability

The present work investigates the effects of a salt spray injected into the airflow of Proton Exchange Membrane Fuel Cells (PEMFCs) at various time scales and contamination levels, with the final aim of identifying the appropriate protection for maritime applications. A dedicated experimental setup generates a sodium chloride (NaCl) mist in the cathode airflow to contaminate single cells and stacks. Constant current density tests led to lifetime shortening with partial recovery after cathode rinsing. Then, Membrane Electrode Assemblies (MEAs) post-mortem characterizations revealed that NaCl was present in the cathode gas diffusion layer, and chlorine reached the catalyst layer for high concentrations. Furthermore, chlorides enhanced current collectors and bipolar plates degradation as corrosion products were identified in MEAs and stack effluents. Overall, this study reports that NaCl effects on PEMFCs not only depend on the concentration but also on the MEA type, plate material, and startup/shutdown procedure.

Numerical modeling of species transport and functional characteristics of a proton exchange membrane fuel cell using an agglomerate model with a multi-phase model

In this study, the species transport and functional characteristics of a Proton Exchange Membrane Fuel Cell (PEMFC) using the agglomerate model are carried out. The modeling is two-dimensional and a two-phase flow is provided to separate liquid water and gas inside the fuel cell. The water transfer modeling is carried out using three mechanisms of vapor, liquid water, and dissolved water through the membrane. The entropy generations due to viscous loss, heat transfer, Ohmic losses, mass transfer, and latent heat are obtained. The effects of agglomerate radius, active layer thickness, permeability, and porosity on the current density, species transport, power density, liquid water, temperature distribution, and entropy generation are investigated. The results show that permeability has a significant effect on water management formed in the cathode gas diffusion layer. Also, the total entropy generation is more evident at the low voltages. The findings show that using a smaller agglomerate radius and larger active layer thickness has a better effect on the current density and thus results in higher power density. At 0.7 V, as the agglomerate radius doubles, the current decreases by 41 %. At 0.6 V, however, a 34 % increase in power density is seen in the minimum agglomerate radius.

Optimal design of cathode gas diffusion layer with arrayed grooves for performance enhancement of a PEM fuel cell

A novel cathode gas diffusion layer (GDL) with arrayed grooves is proposed to enhance the performance of polymer electrolyte membrane (PEM) fuel cells. The influence of various geometric parameters regarding the novel GDL on internal physical quantities transport and cell performance is examined by a 3D multiphase fuel cell model. Results found that oxygen diffusion and water drainage are remarkably enhanced for the novel GDL, thereby leading to an augmented fuel cell performance. A reasonable design of structure parameters, such as the groove size and interval, the length of the GDL with arrayed grooves, and the nonuniform arrayed grooves, could further benefit current density homogeneity and performance for fuel cells. The results reveal that the novel GDL with a groove width, length and interval of 0.1 mm, 0.3 mm and 1.0 mm, respectively, a total arrayed groove length of 15 mm, and a nonuniform arrayed grooves exhibits a better cell performance than all the other designed GDLs and the traditional GDL tested in this simulation. The optimally designed GDL with arrayed grooves improves the maximum power density by approximately 5.6%, enhancing the current density within the regions of CCL near the outlet, thereby favoring the operational stability of fuel cells.

Effects of Cathode Gas Diffusion Layer Configuration on the Performance of Open Cathode Air-Cooled Polymer Electrolyte Membrane Fuel Cell

The design of a gas diffusion layer (GDL) is an effective way to manage water transport, thus improving the performance of air-cooled fuel cells. In the present study, three group designs of GDL with polytetrafluoroethylene (PTFE)—uniformly doped, in-planed sandwich doped and through-plane gradient doped—are proposed, and their effects on the performance of air-cooled fuel cells are explored by numerical simulation. The distribution of key physical quantities in the cathode catalyst layer (CCL), current density and the uniformity of current density distribution in the CCL were analyzed in detail. The results show that properly reducing the amount of PTFE in GDL is beneficial to promoting the water retaining capacity of air-cooled fuel cells, and then improving the performance of fuel cells. The performance of the in-plane sandwich GDL design cannot exceed the design with 10% PTFE uniformly doped, and this design will aggravate the uneven distribution of current density in CCL. Compared with the design of GDL with 40% PTFE uniformly doped, the current density can be improved by 22% when operating at 0.6 V by gradient increasing the PTFE content in GDL from the GDL/MPL interface to the gas channel. Furthermore, this design can maintain as good a current density uniformity as uniformly doping schemes.

The effects of GDL anisotropic transport properties on the PEFC performance

PurposeThis paper aims to numerically investigate the impact of gas diffusion layer (GDL) anisotropic transport properties on the overall and local performance of polymer electrolyte fuel cells (PEFCs).Design/methodology/approachA three-dimensional numerical model of a polymer electrolyte fuel cell with a single straight channel has been developed to investigate the sensitivity of the fuel cell performance to the GDL anisotropic transport properties – gas permeability, diffusivity, thermal conductivity and electrical conductivity. Realistic experimentally estimated GDL transport properties were incorporated into the developed PEFC model, and a parametric study was performed to show the effect of these properties on fuel cell performance and the distribution of the key variables of current density and oxygen concentration within the cathode GDL.FindingsThe results showed that the anisotropy of the GDL must be captured to avoid overestimation/underestimation of the performance of the modelled fuel cell. The results also showed that the fuel cell performance and the distributions of current density and oxygen mass fraction within the cathode GDL are highly sensitive to the through-plane electrical conductivity of the GDL and, to a lesser extent, the through-plane diffusivity, and the thermal conductivity of the GDL. The fuel cell performance is almost insensitive to the gas permeability of the GDL.Practical implicationsThis study improves the understanding of the importance of the GDL anisotropy in the modelling of fuel cells and provides useful insights on improving the efficiency of the fuel cells.Originality/valueRealistic experimentally estimated GDL transport properties have been incorporated into the PEFC model for the first time, allowing for more accurate prediction of the PEFC performance.