Numerical modeling and experimental study of the influence of GDL properties on performance in a PEMFC

This work is to study the effect of properties of gas diffusion layer (GDL) on performance in a polymer electrolyte membrane fuel cell (PEMFC) by both numerical simulation and experiments. The 1-dimension numerical simulation using the mixture-phase model is developed to calculate polarization curve. We are able to estimate optimum GDL properties for cell performance from numerical simulation results. Various GDLs which have different properties are prepared to verify accuracy of the simulation results. The contact angle and gas permeability of GDLs are controlled by polytetrafluoroethylene (PTFE) content in micro-porous layers (MPLs). MPL slurry is prepared by homogeneous blending of carbon powder, PTFE suspension, isopropyl alcohol and glycerol. Then the slurry is coated on gas diffusion mediums (GDMs) surface with controlled thickness by blade coating method. Non-woven carbon papers which have different thicknesses of 200 μm and 380 μm are used as GDMs. The prepared GDLs are measured by surface morphology, contact angle, gas permeability and through-plane electrical resistance. Moreover, the GDLs are tested in a 25 cm² single cell at 70 °C in humidified H₂/air condition. The contact angle of GDL increases with increasing PTFE content in MPL. However, the gas permeability and through-plane electrical conductivity decrease with increasing PTFE content and thickness of GDM. These changes in properties of GDL greatly influence the cell performance. As a result, the best performance is obtained by GDL consists of 200 μm thick non-woven carbon paper as GDM and MPL contained 20 wt.% PTFE content.     

Effect of PTFE loading of gas diffusion layers on the performance of proton exchange membrane fuel cells running at high‐efficiency operating conditions

In this paper, the effects of microporous layer (MPL) addition and polytetrafluoroethylene (PTFE) loading of gas diffusion layers (GDLs) on the overall performance of the proton exchange membrane fuel cell have been investigated. The focus was on fuel cells that operate at relatively low current densities where the power demand is low, but the efficiency is of concern. The results show that, in the activation loss‐controlled region, the performance of the fuel cell operating with moderately PTFE‐treated carbon substrates is superior over that operating with coated GDLs. This is due to the addition of the MPL which lengthens the diffusion paths and significantly reduces the mass transport properties. Conversely, in the ohmic loss‐controlled region, the fuel cell with coated GDLs performs better than those with carbon substrates. This is explained by the enhanced contact of the GDL with the adjacent components after the MPL addition, which outweighs the negative effects associated with the activation loss‐controlled region. Also, it was found that the fuel cell performance becomes lower if the GDL is treated with a relatively high PTFE loading in either the carbon substrate (due to the decrease in the porosity of the GDL) or the MPL. Copyright © 2012 John Wiley & Sons, Ltd.     

Anode water removal and cathode gas diffusion layer flooding in a proton exchange membrane fuel cell

Anode water removal (AWR) is studied as a diagnostic tool to assess cathode gas diffusion layer (GDL) flooding in PEM fuel cells. This method uses a dry hydrogen stream to remove product water from the cathode, showing ideal fuel cell performance in the absence of GDL mass transfer limitations related to water. When cathode GDL flooding is limiting, the cell voltage increases as the hydrogen stoichiometry is increased. Several cathode GDLs were studied to determine the effect of microporous layer (MPL) and PTFE coating. The largest voltage gains occur with the use of cathode GDLs without an MPL since these GDLs are prone to higher liquid water saturation. Multiple GDLs are studied on the cathode side to exacerbate GDL flooding conditions to further confirm the mechanism of the AWR process. Increased temperature and lower cathode RH allow for greater overall water removal so the voltage improvement occurs faster, though this leads to quicker membrane dehydration.     

Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells

Polymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In a URFC, titanium (Ti)-felt is used as a gas diffusion layer (GDL) of the oxygen electrode, whereas typical carbon paper is used as a GDL of the hydrogen electrode. Different samples of Ti-felt with different structural properties (porosity and fiber diameter) and PTFE content were prepared for use as GDLs of the oxygen electrode, and the relation between the properties of the GDL and the fuel cell performance was examined for both fuel cell and electrolysis operation modes. Experimental results showed that the cell with a Ti-felt GDL of 80 μm fiber diameter had the highest round-trip efficiency due to excellent fuel cell operation under relatively high-humidity conditions despite degradation in performance in the electrolysis mode.     

Optimization of polytetrafluoroethylene content in cathode gas diffusion layer by the evaluation of compression effect on the performance of a proton exchange membrane fuel cell

This research investigates the optimal polytetrafluoroethylene (PTFE) content in the cathode gas diffusion layer (GDL) by evaluating the effect of compression on the performance of a proton exchange membrane (PEM) fuel cell. A special test fixture is designed to control the compression ratio, and thus the effect of compression on cell performance can be measured in situ. GDLs with and without a microporous layer (MPL) coating are considered. Electrochemical impedance spectroscopy (EIS) is used to diagnose the variations in ohmic resistance, charge transfer resistance and mass transport resistance with compression ratio. The results show that the optimal PTFE content, at which the maximum peak power density occurs, is about 5 wt% with a compression ratio of 30% for a GDL without an MPL coating. For a GDL with an MPL coating, the optimal PTFE content in the MPL is found to be 30% at a compression ratio of 30%.     

Estimation of through-plane and in-plane gas permeability across gas diffusion layers (GDLs): Comparison with equivalent permeability in bipolar plates and relation to fuel cell performance

The present work focusses on measuring the permeability across gas diffusion layers (GDLs) first in a dedicated cell and later in PEM fuel cell configuration with varying bi-polar plate designs. Eight carbon paper-based GDLs with and without the microporous layer (MPL), have been tested. An in-house designed dedicated cell allowed measuring pressure drop depending on flow rate, for i) through-plane and ii) in-plane direction. Further, transport measurements were conducted in 25 cm² bi-polar plates (BPs) in fuel cell configuration having single or multiple serpentine channels, by stacking the GDL inside. The results show that gas permeability in the dedicated cell for through-plane and in-plane can be estimated by using Darcy's law. However, for BPs, the flow is affected additionally by inertial contribution (Darcy-Forchheimer). Finally, the efficiency allowed by selected GDLs installed in a fuel cell under operation shows a relationship between the equivalent permeability and the fuel cell performance.     

Carbon nano-chain and carbon nano-fibers based gas diffusion layers for proton exchange membrane fuel cells

Gas diffusion layers (GDL) for proton exchange membrane fuel cell have been developed using a partially ordered graphitized nano-carbon chain (Pureblack^® carbon) and carbon nano-fibers. The GDL samples’ characteristics such as, surface morphology, surface energy, bubble-point pressure and pore size distribution were characterized using electron microscope, inverse gas chromatograph, gas permeability and mercury porosimetry, respectively. Fuel cell performance of the GDLs was evaluated using single cell with hydrogen/air at ambient pressure, 70 °C and 100% RH. The GDLs with combination of vapor grown carbon nano-fibers with Pureblack carbon showed significant improvement in mechanical robustness as well as fuel cell performance. The micro-porous layer of the GDLs as seen under scanning electron microscope showed excellent surface morphology showing the reinforcement with nano-fibers and the surface homogeneity without any cracks.     

Effects of the carbon black properties in gas diffusion layer on the performance of proton exchange membrane fuel cells

The microporous layer (MPL) as a part of diffusion medium has an important impact on mass transfer of proton exchange membrane fuel cell (PEMFC). In this study, MPLs of gas diffusion layers (GDLs) are prepared with different carbon blacks, and the properties of carbon blacks and their effects as MPLs on cell performance are systematically investigated. The results show that the GDL prepared by Acetylene Black (ACET) exhibits the best performance with a maximum power density up to 2.05 W cm⁻². Moreover, it still maintains extremely high performance with increasing current density even at humidity condition of 100% relative humidity, which means its excellent water/gas transportation capacity. This study contributes to deeply understanding the correlations between the properties of MPL material itself and their corresponding performance exhibited in cell. It also provides an important reference for enhancing cell performance and further advancing the practical applications of MPLs in PEMFC field.     

High performance cathode based on carbon fiber felt for magnesium-air fuel cells

A series of carbon fiber felt/PTFE based gas diffusion layers (GDL) for Mg-air fuel cells were prepared by a simple method of immersing carbon fiber felt in PTFE suspension. Critical properties of the as-prepared GDL, including the surface morphology, electronic resistivity, porosity and gas permeability, have been characterized to investigate the effect of PTFE suspension concentration and PTFE content on the properties of the GDL. The micrographs indicated that the PTFE was homogenously dispersed on the carbon fiber felts and showed structure with a microporous layer. The as-prepared GDL exhibited good mechanical property, high electronic conductivity, sufficient water repellency and high gas permeability. Compared with the Mg-air fuel cell with a traditional carbon powder based cathode, the performance and the stability of Mg-air fuel cell with the carbon fiber felt based GDL are improved significantly.     

Review of gas diffusion layer for proton exchange membrane-based technologies with a focus on unitised regenerative fuel cells

Proton exchange membrane (PEM) based technologies (fuel cells and electrolysers) offer promising sustainable power generation and storage solutions for a diverse range of stationary and mobile applications. Unitised regenerative fuel cell (URFC) is an electrochemical cell that can operate both as a fuel cell (FC) and an electrolyser (E). However, for a widespread commercialisation, further improvements are required that address the durability, performance, and cost limitations. One of the main challenging components in developing URFCs is the gas diffusion layer (GDL) as it plays different vital roles, some of which are paradoxical in FC and E-modes. Therefore, in this paper, the published research on GDL of PEM-URFCs as well as relevant studies on PEM fuel cells and electrolysers are critically reviewed. The materials and novel methods to address the corrosion in E-mode are discussed. This is followed by presenting and discussing different properties of GDLs affecting the performance in FC and E-modes: i.e. porosity, thickness, pore size, transport properties, thermal and electrical conductivity, and the GDL compressibility. Finally, the main modifications of the GDLs, such as hydrophobisation and microporous layer application, to improve the performance of a URFC are analysed and discussed.     

In situ accelerated degradation of gas diffusion layer in proton exchange membrane fuel cell: Part I: Effect of elevated temperature and flow rate

Past studies have shown that both the substrate and microporous layer of the gas diffusion layer (GDL) significantly affect water balance and performance of a proton exchange membrane (PEM) fuel cell. However, little effort has been made to investigate the importance of GDL properties on the durability of PEM fuel cells. In this study, the in situ degradation behaviour of a commercial GDL carbon fiber paper with MPL was investigated under a combination of elevated temperature and elevated flow rate conditions. To avoid the possible impact of the catalyst layer during degradation test, different barriers without catalyst were utilized individually to isolate the anode and cathode GDLs. Three different barriers were evaluated on their ability to isolate GDL degradation and their similarity to a fuel cell environment, and finally a novel Nafion/MPL/polyimide barrier was chosen. Characterization of the degraded GDL samples was conducted through the use of various diagnostic methods, including through-plane electrical resistivity measurements, mercury porosimetry, relative humidity sensitivity, and single-cell performance curves. Noticeable decreases in electrical resistivity and the hydrophobic properties were detected for the degraded GDL samples. The experimental results suggested that material loss plays an important role in GDL degradation mechanisms, while excessive mechanical stress prior to degradation weakens the GDL structure and changes its physical property, which consequently accelerates the material loss of the GDL during aging.     

Highly efficient single-layer gas diffusion layers for the proton exchange membrane fuel cell

In the present study, a series of highly efficient single-layer gas diffusion layers (SL-GDLs) was successfully prepared by the addition of a vapor grown carbon nanofiber (VGCF) in the carbon black/poly(tetrafluoroethylene) composite-based SL-GDL through a simple and inexpensive process. The scanning electron micrographs of the as-prepared VGCF-containing SL-GDLs (SL-GDL-CFs) showed that the GDLs had a microporous layer (MPL)-like structure, while the wire-like VGCFs were well dispersed and crossed among the carbon black particles in the SL-GDL matrix. Utilization of the SL-GDL-CFs for MEA fabrication was also done by direct coating with the catalyst layer. Due to the presence of VGCFs, the properties of the SL-GDL-CFs, including electronic resistivity, mechanical characteristic, gas permeability, and water repellency, varied with the VGCF content, with the overall effect beneficial to the performance of the proton exchange membrane fuel cell (PEMFC). The best performances obtained from the PEMFC with VGCFs at 15 wt.% was approximately 63% higher than those without VGCFs, and about 85% as efficient as ELAT GDL, a commercial dual-layer GDL, for both the H₂/O₂ and H₂/air systems.     

A review of gas diffusion layer in PEM fuel cells: Materials and designs

The gas diffusion layer (GDL) plays a key role on reactant gas diffusion and water management in proton exchange membrane (PEM) fuel cells. This paper reviews recent developments of single- and dual-layer GDLs for PEM fuel cells and various materials and approaches used for development of novel GDL. A variety of carbon- and metal-based macroporous substrates are presented. Hydrophobic treatments using different fluorinated polymers are addressed. Engineering parameters which control the performance of microporous layer such as carbon treatment, wettability, thickness, and microstructure are also reviewed. In addition, future prospects for development of new GDL development are discussed.     

Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells

The purpose of this study is to investigate the effect of ploytetrafluoroethylene (PTFE)-treatment and microporous layer (MPL)-coating on the electrical conductivity of gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells (PEMFCs). The results show that, for PTFE-treated GDLs, the electrical conductivity in orthogonal in-plane directions is almost invariant with the PTFE loading. On the other hand, the in-plane conductivity of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be higher than that of the counterpart SGL 10BC (25% PTFE) and this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Further, the conductivity of each GDL sample was measured in two perpendicular in-plane directions in order to investigate the in-plane anisotropy. The results show that the electrical conductivity of the GDL sample in one direction is different to that in the other direction by a factor of about two. The contact resistance, the main factor affecting the through-plane conductivity, of PTFE-treated GDLs shows a different trend to the corresponding in-plane conductivity, namely it increases as the PTFE loading increases. On the other hand, the contact resistance of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be lower than that of the counterpart SGL 10BC (25% PTFE) and again this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Also, it was noted that the MPL coating appears to have a positive effect in reducing the contact resistance between the GDL and the bipolar plate. This is most likely due to the compressibility of the MPL layers that allows them to fill in the ‘gaps’ that exist in the surface of the bipolar plates and therefore establishes a good contact between the latter plates and the GDLs. Finally, good curve fitting of the contact resistance as a function of the clamping pressure has been achieved.     

Measurement of effective oxygen diffusivity in microporous media containing moisture

The mass transfer characteristics of the gas diffusion layer (GDL) are closely related to the cell performance of a polymer electrolyte fuel cell (PEFC). The oxygen diffusivity of paper type porous media, which is generally used as a GDL, was measured with respect to its liquid water content using experimental apparatus consisting of an oxygen sensor based on the galvanic cell. A numerical method was established to obtain the effective oxygen diffusivity of microporous test materials by calculating the oxygen concentration distribution on both sides of the test material. Experimental results indicate that the relative oxygen diffusivity of paper type GDLs increases nonlinearly as the water saturation decreases. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20295     

Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer

Water management in a PEM fuel cell significantly affects the fuel cell performance and durability. The gas diffusion layer (GDL) of a PEM fuel cell plays a critical role in the water management process. In this short communication, we report a simple method to measure the water transport rate across the GDL. Water rejection rates across a GDL at different cathode air-flow rates were measured. Based on the measurement results, the fuel cell operating conditions, such as current density, temperature, air stoichiometry and relative humidity, corresponding to membrane drying and flooding conditions were identified for the particular GDL used. This method can help researchers develop GDLs for a particular fuel cell design with specific operating conditions and optimize the operation conditions for the given PEM fuel cell components.     

Effect of gas diffusion layer compression on PEM fuel cell performance

The gas diffusion layer (GDL) plays a very important role in the performance of Proton Exchange Membrane (PEM) fuel cells. The amount of compression on the GDL affects the contact resistance, the GDL porosity, and the fraction of the pores occupied by liquid water, which, in turn, affect the performance of a PEM fuel cell. In order to study the effects of GDL compression on fuel cell performance a unique fuel cell test fixture was designed and created such that, without disassembling the fuel cell, varying the compression of the GDL can be achieved both precisely and uniformly. Besides, the compression can be precisely measured and easily read out. Using this special fuel cell fixture, the effects of GDL compression on PEM fuel cell performance under various anode and cathode flow rates were studied. Two different GDL materials, carbon cloth double-sided ELAT and TORAY™ carbon fiber paper were used in these studies. The experimental results show that generally the fuel cell performance decreases with the increase in compression and over-compression probably exists in most fuel cells. In the low current density region, generally there exists an optimal compression ratio.     

Effects of gas diffusion layer structure on the open circuit voltage and hydrogen crossover of polymer electrolyte membrane fuel cells

The clamping pressure of polymer electrolyte membrane fuel cells for vehicle applications should be typically high enough to minimize contact resistance. However, an excessive compression pressure may cause a durability problem. In this study, the effects of gas diffusion layer (GDL) structure on the open circuit voltage (OCV) and hydrogen crossover have been closely examined. Results show that the performances of fuel cells with GDL-1 (a carbon fiber felt substrate with MPL having rough surface) and GDL-3 (a carbon fiber paper substrate with MPL having smooth surface) are higher than that with GDL-2 (a carbon fiber felt substrate with MPL having smooth surface) under low clamping torque conditions, whereas when clamping torque is high, the GDL-1 sample shows the largest decrease in cell performance. Hydrogen crossover for all GDL samples increases with the increase of clamping torque, especially the degree of increase of GDL-1 is much greater than that of the other two GDL samples. The OCV reduction of GDL-1 is much greater than that of GDL-2 and GDL-3. It is concluded that the GDL-3 is better than the other two GDLs in terms of fuel cell durability, because the GDL-3 shows the minimum OCV reduction.     

Analysis of transient response of a unit proton-exchange membrane fuel cell with a degraded gas diffusion layer

The transient response characteristics and durability problems of proton-exchange membrane fuel cells are important issues for the application of PEM fuel cells to automotive systems. The gas diffusion layer is the key component of the fuel cell because it directly influences the mass transport mechanism. In this study, the effects of GDL degradation on the transient response of the PEM fuel cell are systematically studied using transient response analysis under different stoichiometric ratios and humidity conditions. With GDLs aged by the accelerated stress test, the effects of hydrophobicity and structural changes due to carbon loss in the GDL on the transient response of PEM fuel cells are determined. The cell voltage is measured according to the sudden current density change. The degraded GDLs that had uneven hydrophobicity distributions cause local water flooding inside the GDL and induce lower and unstable voltage responses after load changes.     

Experimental characterization of in-plane permeability of gas diffusion layers

Recent studies indicate that PEM fuel cell performance may be strongly influenced by in-plane permeability of the gas diffusion layer (GDL). The current study employs a radial flow technique for obtaining in-plane permeability of GDLs, using either gas or liquid as the impregnating fluid. A model has been developed and experimentally verified to account for compressibility effects when permeability measurements are conducted using a gas. Permeability experiments are performed on samples of woven, non-woven, and carbon fiber-based GDL at various levels of compression using air as the impregnating fluid. Woven and non-woven samples are measured to have significantly higher in-plane permeability compared to carbon fiber paper at similar solid volume fractions.