Carbon Cloth Gas Diffusion Layer Research Articles

Study of Mechanical Properties of Membrane-Electrode Assemblies for Proton Exchange Membrane Fuel Cells By the Small-Punch Technique

Mechanical properties of membrane-electrode assemblies (MEAs) for proton exchange membrane fuel cells (PEMFCs) have been studied with the small-punch technique. MEAs were operated according to a non-accelerated aging protocol, with changes in humidification and cell temperature. After operation, degradation of the MEA is reflected by a decrease in power density, increase in the internal resistance, and decrease in open circuit voltage. Small disks (8 mm diam.) were taken from different locations of the degraded MEA to test the mechanical properties by small-punch. The results show an increase in plastic deflection and a decrease in maximum load of degraded MEAs, that result in lower apparent Young modulus. The degradation is larger in the area close to oxygen outlet. SEM microscopy shows that degradation affects majorly to the fibers of the carbon cloth gas diffusion layer (GDL). Combined small-punch and SEM correlate the change in mechanical properties on aged MEA with carbon fibers degradation. Such GDL degradation must also be principal responsible for the increase in the internal resistance.

(Digital Presentation) Temperature-Dependent Study for Electrochemical Surface Area on a Catalyst Layer Used in a PEFC

Electrochemical Surface Area (ECSA) is the portion of the catalyst layer electrochemically available to participate in electrode reactions inside a Polymer Electrolyte Fuel Cell (PEFC). ECSA is an important metric that diagnoses the performance/degradation of a catalytic layer and allows the development of new electrodes. This study aims to describe the behavior of the ECSA as a function of cell operating temperatures based on a self-designed experimental based on two approaches. The first is through an analytical-theoretical analysis, and the second is through empirical correlations that fit the experimental curves obtained. The primary variable to study in the present work is temperature. Based on previous studies, the temperature is one of the most influential variables that affect fuel cell efficiency.A single PEFC with an active area of 25cm2, a woven carbon cloth gas diffusion layer, a microporous layer, catalytic layer support with 0.5 mg cm-2 of Platinum on carbon, and Nafion 212 ® membrane were used to perform this study. The temperature range was varied in the range of 40 - 80 oC in steps of 10 °C, and a 100% of Relative Humidity on inlet gases, i.e., hydrogen and nitrogen, were settled at 100%. The characterization of the ECSA was performed with an in-situ cyclic voltammetry test, and the voltage sweep was conducted from 0 to 0.8 V. Finally, the volumetric flow was fixed at 0.1 L/min for both gases hydrogen on the anode side and nitrogen on the cathode side. The results show that the electrochemical surface area (ECSA) of the cathode catalyst decreases drastically as the operating temperature of the cell increases; this is due to the decrease of active sites by the fewer hydrogen ion pathways reaching higher temperatures. Consequently, the empirical correlation proposed for the electrochemically active area in this study helps the catalyst layer's diagnosis and predicts the level of performance for different temperatures. Figure 1

Pore‐scale analysis of a PEM fuel cell cathode including carbon cloth gas diffusion layer by lattice Boltzmann method

AbstractOne of the parameters which can greatly affect (i.e., enhances or diminishes) the performance of a PEM fuel cell cathode is the GDL microstructure. In the present study, the role of carbon cloth GDL microstructure on the performance of a PEM fuel cell cathode is investigated, for the first time. In this regard, three carbon cloth GDLs with three different microstructures are reconstructed via merging bundles of carbon fibers with sinusoidal form; afterward, the passing reactive air through these GDLs is simulated at pore‐scale using lattice Boltzmann approach. The distributions of oxygen and water vapor through the carbon cloths and the distribution of current density on the catalyst layers are presented and analyzed. The results show that when the carbon cloth bundles consist of more fibers (i.e., having larger cross‐sections), the penetration of oxygen toward the catalyst layer is more difficult, and consequently, the average current density is smaller. Besides, the results reveal that the through‐plane compactness of carbon cloth microstructure does not bring a significant effect on the distribution of current density and species.

Effect of PEM fuel cell porous media compression on in-plane transport phenomena

Abstract Liquid-gas two-phase flow in the gas diffusion layer (GDL) of proton exchange membrane fuel cells is investigated using an ex-situ experimental setup. The mass transport phenomena is investigated in carbon paper and carbon cloth GDLs and at different compressions. Water percolation within the plane of the GDL is visualized with a CCD camera while its injection pressure is measured. Similarly, air percolation within the plane of GDL samples which were initially saturated with water is investigated. Experiments are conducted for the three flow regimes of stable displacement, capillary fingering, and viscous fingering. Images are analyzed to obtain the normalized wetted area. It is observed that while the GDL compression directly affects normalized wetted area for stable displacement and viscous fingering flow regimes, it has no impact on this parameter for capillary fingering flow regime. For stable displacement flow regime and for both carbon paper and carbon cloth samples, water percolation pressure increases with GDL compression. However, the water percolation pressure data obtained for capillary fingering flow regime does not suggest any discernible trend as a function of GDL compression. The findings in this study can be used to validate percolation models proposed by different schemes such as pore-network models.

Stochastic 3D Carbon Cloth GDL Reconstruction and Transport Prediction

This paper presents the 3D carbon cloth gas diffusion layer (GDL) to predict transport behaviors of anisotropic structure properties. A statistical characterization and stochastic reconstruction method is established to construct the 3D micro-structure using the data from the true materials. Statistics of the many microstructure characteristics, such as porosity, pore size distribution, and shape of the void, are all quantified by image-based characterization. Furthermore, the stochastic reconstruction algorithm is proposed to generate random and anisotropic 3D microstructure models. The proposed method is demonstrated by some classical simulation prediction and to give the evaluation of the transport properties. Various reconstructed GDLs are also generated to demonstrate the capability of the proposed method. In the end, the adapted structure properties are offered to optimize the carbon cloth GDLs.

The Effect of CO2 Bubble Distribution on Power Generation Performance of a Direct Formic Acid Fuel Cell

Introduction Recently, formic acid is expected as one of the energy career for renewable energy. Especially, direct formic acid fuel cells (DFAFCs) which use formic acid as fuel, have received considerable attention since they can generate higher power density than the other direct liquid fuel cells [1]. However, CO2 gas is generated in the anode reaction when DFAFCs are operating. If CO2 bubbles stay in the anode gas diffusion layer (GDL) the mass transport resistance in GDL increases and the power generation performance of DFAFCs decreases with the lapse of time. In this study, we have investigated the effect of CO2 bubble distribution on power generation performance by visualizing the CO2 bubble distribution in DFAFCs during power generation using X-ray CT. Experimental A single cell with 1 cm2 of active area was used. We used NR-212 as proton exchange membrane, PEM. Pd/C was used for anode catalyst and Pt/C was used for cathode catalyst. Moreover, two types of GDL with different structures of carbon paper and carbon cloth were used. The voltage sweep was performed at a sweep rate of -5 mV / sec until the cell voltage turned from the open circuit voltage to 0 V, and the cell voltage and current density during the sweep were recorded. A three-dimensional measurement X-ray CT apparatus (TDM-1000H-Ⅱ (2K), Yamato Scientific) was used for visualizing CO2 bubble distribution in DFAFCs. It took 15 minutes to visualize and the spatial resolution was 2.7 μm. Results and discussion From the measurement of the power generation characteristics, it was found that the maximum current density is higher with carbon cloth GDL than with carbon paper GDL, despite the lower open circuit voltage. In addition, using carbon paper GDL, the gradient of polarization curve became steep in the high current density region. These phenomena are generally considered to be caused by mass transport, and in this experiment it is thought that this is probably due to the effect of bubbles in the anode GDL. Therefore, in order to investigate the influence of bubbles in the anode GDL, visualization was performed. Fig. 1 shows the CO2 bubble distribution in the anode GDL at high current density operation using each GDL. These figures are three-dimensional representation of the bubble distribution in the anode GDL averaged over 15 minutes of visualization time. Furthermore, these are top views seen from the vertically upward direction of the GDL plane, the white part indicates that bubbles exist. As shown in Fig. 1, using carbon paper GDL, bubbles were present under most of the rib and also present under the channel, which was thought to cause the power generation performance degradation due to the mass transport. On the other hand, with the carbon cloth GDL, there were almost no bubbles under the channel, and bubbles and voids were concentrated under the rib. It is considered that in the carbon cloth, the bubbles were concentrated in part and the discharge path of the bubbles was formed, so that the bubbles were efficiently discharged and the power generation performance did not decrease even in the high current density region. Reference 1) Y. Zhu, Z. Khan, R.I. Masel, Journal of Power Sources, 139, 15-20 (2005) Figure 1

The impact of sample size on transport properties of carbon-paper and carbon-cloth GDLs: Direct simulation using the lattice Boltzmann model

The transport properties of gas diffusion layer (GDL) in proton exchange membrane fuel cell are important parameters in fuel cell modelling, and one method to measure them is to simulate the transport of each species at spatial resolutions of a few microns in a microstructure of the GDL. One issue in this method is the size of the microstructure as using an unnecessarily big sample could substantially increase the computational cost. Both carbon-paper and carbon-cloth GDLs are investigated and their microstructures are obtained using numerical generation and X-ray micro-tomography respectively at a resolution of 1.733 µm. For each reconstructed GDL, we use the orthorhombic lattice Boltzmann model to simulate fluid flow through some subsamples taken from it in both the in-plane and the through-plane directions; the results show that the permeability of all subsamples is anisotropic in that their permeability in the in-plane direction is higher than in the through-plane direction. For each GDL, we compare its permeability calculated using samples of different sizes and find its representative elementary volume (REV) – a volume above which the calculated permeability represents the average ability of the GDL to conduct fluids.

The effect of the gas diffusion layer on the performance of fuel cell catalyst layers in ethanol sensors

The influence of the gas diffusion layer (GDL) on sensing performance of two commercially available fuel cell catalysts and membranes were investigated in an ethanol sensor. The catalyst used was either platinum black (Pt) or 20% platinum on carbon (Pt/C). These catalysts were coated onto a carbon paper, carbon paper with a microporous layer and carbon cloth GDL. Nafion and porous polyvinyl chloride (PVC) containing sulfuric acid were used as membranes. A fourth sample set with no GDL present was also tested. It was found the GDL’s effect on performance depended on the membrane electrode assembly (MEA) employed. For a Pt-Nafion MEA, the carbon cloth GDL showed the best performance compared to the other two GDLs and no GDL. The removal of the GDL significantly impacts the structural integrity of the catalyst layer, effecting performance. MEAs prepared with a carbon paper GDL were prone to localized flooding at higher humidity’s. Pt/C-Nafion MEAs showed similar performance, regardless of GDL. Removal of the GDL with a Pt/C catalyst greatly affected the structural integrity of the MEA, decreasing performance. PVC membranes are more flexible in the choice of GDL and catalyst layer. Overall, the carbon paper-MPL-Pt/C-PVC showed the best performance due to having the greatest stability in performance at the two humidities tested.

Thermal Contact Resistance Measurements of Compressed PEFC Gas Diffusion Media

Localized temperature gradients in a polymer electrolyte fuel cell (PEFC) are known to decrease the durability of the polymer membrane. The most important factor in controlling these temperature gradients is the thermal contact resistance at the interface of the gas diffusion layer (GDL) and the bipolar plate. Here, we present thermal contact resistance measurements of carbon paper and carbon cloth GDLs over a pressure range of 0.7–14.5 MPa. Contact resistances are highly dependent upon the clamping pressure applied to a fuel cell, and in the present work, contact resistances vary from 3.5 × 10−4 to 2.0 × 10−5 m2 K/W, decreasing nonlinearly over the pressure range for each material tested. The contact resistances of carbon cloth GDLs are two to four times higher than contact resistances of carbon paper GDLs throughout the range of pressures tested. The data presented here also show that the thermal resistance of the sample is negligible in comparison to the thermal contact resistance. Controlling temperature gradients in a fuel cell is desirable, and the measurements presented here can be used to more accurately predict temperature distribution in a polymer electrolyte fuel cell.

Impact of Contact Pressure Cycling on Non-Woven GDLs of HT-PEM Fuel Cells

The Gas Diffusion Layer (GDL) plays an important and critical role within High Temperature Polymer Electrolyte Membrane fuel cells (HT-PEMFC). GDLs usually consist of woven (cloth) or non-woven (e.g. felt or paper) arrays of carbon fibers. The main tasks of a GDL are:Gas transport and distribution of reactants and reaction productsElectrical conductivityHeat transportationWater managementSupport of Membrane-Electrode-Assemblies (MEA). The influence of the GDL on the performance of a fuel cell is an extensively discussed topic nowadays (1, 2). The impact on the durability also needs attention. Physical damages, like for example fiber intrusion through the catalyst layer into the membrane, due to high contact pressures have been confirmed prior to this (3, 4). For this reason, ex-situ measurements with an ad hoc designed compression device (5) have been performed. In this work we will examine in more detail the dependency on compression and the behavior of non-woven carbon cloth GDL material. Therefore the compression device, used before, had to be optimized and rescaled furthermore. This allows a better geometrical resolution while utilizing the tool in micro-computed tomography (µ-CT). Results with the new device will be shown within this work.In addition to the ex-situ investigations of the GDL defects based on compression forces, in-situ electrochemical measurements of MEAs like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) will be carried out. Therefore, MEAs were physically stressed with specific contact pressure cycling tests. One cycle starts at 0.2 MPa up to 1.5 MPa (steps: 0.2, 0.5, 1.0 and 1.5 MPa) and will be repeated 3 times with conditioning or relaxation time over night for each contact pressure and electrochemical characterization during day time.Supplementary, GDLs will be analyzed separately with the help of the new compression device and the µ-CT. Focus of this work lies on the investigation of the porosity of the non-woven GDL material, which is utilized in the investigated MEAs. Within bipolar plates (BPP) with their various flow fields, the porosity of a GDL typically shows a non-uniform behavior (6). Due to the channel and land areas of the serpentine flow fields utilized in this study, the compression on the carbon fibers is locally different. Under the land areas the GDL will be squeezed and within the channels a dilation of the GDL material will be observed (4, 7). To analyze reversible and irreversible defects, the in-situ contact pressure cycling will be imitated completely ex-situ with pure non-woven GDL material. With help of the reconstructed and binary µ-CT data, the porosity of the GDL will be determined and illustrated within 3-dimensional models for each contact pressure (Figure 1). Figure 1: 3D-Model of an uncompressed non-woven GDL

Impact of Compression on Effective Thermal Conductivity and Diffusion Coefficient of Woven Gas Diffusion Layers in Polymer Electrolyte Fuel Cells

AbstractThe contrasting effect of compression on the ability of gas diffusion layer (GDL) in polymer electrolyte membrane fuel cell to conduct fluid, heat and electron implies that there is an optimal clamping force for cell performance. For a given GDL, understanding its associated optimal compression needs to know how its conductive ability changes with compressive pressure. In this paper we investigated the impact of compression on the effective diffusion coefficient and thermal conductivity of a carbon‐cloth GDL. The interior microstructures of the GDL under different compressions were acquired using X‐ray tomography; microscopic models were then developed to simulate gas diffusion and heat transfer in the microstructures in both in‐plane and through‐plane directions. The effective diffusion coefficient and thermal conductivity were calculated by volumetrically averaging the simulated gas diffusive and thermal flux rates at micron scale. The results show that both effective diffusion coefficient and thermal conductivity were anisotropic and their values in the in‐plane direction were higher than in the through‐plane direction. With porosity decreasing under the compression, the effective diffusion coefficient decreased faster in the through‐plane direction than in the in‐plane direction; the formula derived by Nam and Kaviany was capable of describing the change of the effective diffusion coefficient with porosity in the in‐plane direction but not in the through‐plane direction. For heat transfer, as the porosity decreased, the thermal conductivity increased faster in the through‐plane direction than in the in‐plane direction, and the increase in both directions could fit to the formula of Das et al.

Bulk and contact resistances of gas diffusion layers in proton exchange membrane fuel cells

A multi-electrode probe is employed to distinguish the bulk and contact resistances of the catalyst layer (CL) and the gas diffusion layer (GDL) with the bipolar plate (BPP). Resistances are compared for Vulcan carbon catalyst layers (CL), carbon paper and carbon cloth GDL materials, and GDLs with microporous layers (MPL). The Vulcan carbon catalyst layer bulk resistance is 100 times greater than the bulk resistance of carbon paper GDL (Toray TG-H-120). Carbon cloth (CCWP) has bulk and contact resistances twice those of carbon paper. Compression of the GDL decreases the GDL contact resistance, but has little effect on the bulk resistance. Treatment of the GDL with polytetrafluoroethylene (PTFE) increases the contact resistance, but has little effect on the bulk resistance. A microporous layer (MPL) added to the GDL decreases the contact resistance, but has little effect on the bulk resistance. An equivalent circuit model shows that for channels less than 1 mm wide the contact resistance is the major source of electronic resistance and is about 10% of the total ohmic resistance associated with the membrane electrode assembly.

Determination of the anisotropic permeability of a carbon cloth gas diffusion layer through X‐ray computer micro‐tomography and single‐phase lattice Boltzmann simulation

SUMMARYAn investigation of the anisotropic permeability of a carbon cloth gas diffusion layer (GDL) based on the integration of X‐ray micro‐tomography and lattice Boltzmann (LB) simulation is presented. The method involves the generation of a 3D digital model of a carbon cloth GDL as manufactured using X‐ray shadow images acquired through X‐ray micro‐tomography at a resolution of 1.74 µm. The resulting 3D model is then split into 21 volumes and integrated with a LB single‐phase numerical solver in order to predict three orthogonal permeability tensors when a pressure difference is prescribed in the through‐plane direction. The 21 regions exhibit porosity values in the range of 0.910–0.955, while the average fibre diameter is 4 µm. The results demonstrate that the simulated through‐plane permeability is about four times higher than the in‐plane permeability for the sample imaged and that the corresponding degrees of anisotropy for the two orthogonal off‐principal directions are 0.22 and 0.27. The results reveal that flow channelling can play an important role in gas transport through the GDL structure due to the non‐homogeneous porosity distribution through the material. The simulated results are also applied to generate a parametric coefficient for the Kozeny–Carman (KC) method of determining permeability. The current research reveals that by applying the X‐ray tomography and LB techniques in a complementary manner, there is a strong potential to gain a deeper understanding of the microscopic fluidic phenomenon in representative models of porous fuel cell structures and how this can influence macroscopic transport characteristics which govern fuel cell performance. Copyright © 2010 John Wiley & Sons, Ltd.

Pore scale 3D modelling of heat and mass transfer in the gas diffusion layer and cathode channel of a PEM fuel cell

Flooding of the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells can be a bottleneck to the system’s efficiency and even durability under certain operating conditions. Due to the small scale and complex geometry of the materials involved, detailed insight into the pore scale phenomena that take place are difficult to measure or simulate. In the present effort, a direct 3D microscale model of a portion of the PEM cathode channel and carbon cloth GDL is used to parametrically investigate local heat and fluid flow at the GDL’s pore scale and their effects on condensation of water vapour that leads to flooding. The 3D simulation through the microscale geometry is among the first appearing in the international literature. The Navier–Stokes, energy and water vapour transport equations are solved at steady state and in three-dimensional space for a range of inlet velocities and cloth fibre material properties, using a conjugate heat transfer approach to calculate the temperature field within the solid fibres. Psychrometric calculations are applied to provide indications of the conditions and areas most prone to condensation based on the calculated local temperatures and water vapour concentration.

A Numerical Study of Structural Change and Anisotropic Permeability in Compressed Carbon Cloth Polymer Electrolyte Fuel Cell Gas Diffusion Layers

AbstractThe effect of compression on the actual structure and transport properties of the carbon cloth gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) are studied here. Structural features of GDL samples compressed in the 0.0–100.0 MPa range are encapsulated using polydimethylsiloxane (PDMS) and by employing X‐ray micro‐tomography to reconstruct direct digital 3D models. Pore size distribution (PSD) and porosity data are acquired directly from these models while permeability, degree of anisotropy and tortuosity are determined through lattice Boltzmann (LB) numerical modelling. The structural models reveal that structural change proceeds through a three‐step process, while PSD data suggests a characteristic peak in the pore diameter of 10–14 μm and a decrease in the mean pore diameter from 33 to 12 μm over the range of tested pressures. A mathematical relationship between compression pressure and permeability is determined based on the Kozeny–Carman equation, revealing a one order of magnitude reduction in through‐plane permeability for a two order of magnitude increase in pressure. The results also reveal that the degree of anisotropy peaks in the range of 0.3–10.0 MPa, suggesting that in‐plane permeability can be maximised relative to through‐plane permeability within a material‐specific range of compression pressures.

High-Resolution Visualization of Water Accumulation Behavior in an Operating PEMFC by Soft X-ray Radiography

A carbon-cloth gas diffusion layer (GDL) displays better performance than a carbon-paper GDL under humidified condition. To investigate the relationship between GDL micro-structure and liquid water transport phenomena, water accumulation behavior in an operating Proton Exchange Membrane Fuel Cell (PEMFC) was visualized by soft X-ray radiography. Utilization of the soft X-ray range made it possible to visualize the liquid water in a PEMFC with the spatial resolution of 0.5 um and temporal resolution of 1.0 sec/frame. The water accumulated in a carbon-cloth GDL was locally distributed, and a larger amount of water accumulated in it than in a carbon-paper GDL. These results suggested that the broader pore distribution creates oxygen transport paths even if water accumulates in the GDL, and the pore structure has great importance. Furthermore, to investigate liquid water transport in more detail, stereoscopy technique has developed to get three-dimensional information with high temporal and spatial resolution.

EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF PORE-FORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

The performance of five layer membrane electrode assemblies having different characteristics of gas diffusion layers was determined at 70°C cell temperature and ambient pressure. The maximum power density at 0.6 V was 0.36 W/cm2 for the membrane electrode assembly prepared with the gas diffusion layer having minimum thickness (SGL BC 30 type). On the other hand, the maximum power density at 0.5 V was 0.44 W/cm2 for the membrane electrode assembly prepared with SGL BC 34 type gas diffusion layer. It was found that resistance of a membrane electrode assembly is strongly dependent on gas diffusion layer thickness. Moreover, membrane electrode assemblies prepared with carbon paper gas diffusion layers resulted in higher performance than the assembly prepared with carbon cloth gas diffusion layer. Addition of pore-forming agents, which were ammonium carbonate, ammonium bicarbonate, ammonium sulfonate, and ammonium oxalate, to the catalyst ink lowered the performance.

Measurement of Liquid Water Accumulation in a PEMFC with Dead-Ended Anode

The operation and accumulation of liquid water within the cell structure of a polymer electrolyte membrane fuel cell (PEMFC) with a dead-ended anode is observed using neutron imaging. The measurements are performed on a single cell with active area, Nafion 111-IP membrane, and carbon cloth gas diffusion layer. Even though dry hydrogen is supplied to the anode via pressure regulation, accumulation of liquid water in the anode gas distribution channels was observed in most tested conditions. Moreover, the accumulation of liquid water in the anode channels is followed by a significant voltage drop. Anode purges and cathode surges are also used as a diagnostic tool for differentiating between anode and cathode water flooding. The rate of accumulation of liquid water, and its impact on the rate of cell voltage drop is shown for a range of temperature, current density, cathode inlet RH, and air stoichiometric conditions. Operating the fuel cell under dead-ended anode conditions offers the opportunity to observe water dynamics and measured cell voltage during large and repeatable transients.

Characterization of gas diffusion layers for PEMFC

A carbon-filled gas diffusion layer (CFGDL), which is in the configuration similar to conventional carbon cloth gas diffusion layer (GDL) coated with carbon layer on both faces, was investigated and compared with conventional carbon paper-based single-layer and dual-layer GDLs. Like the carbon cloth GDL, CFGDL has presented superior performances over the single-layer or dual-layer GDL in all three polarization (activation, ohmic and concentration) controlled regions under electrochemical characterizations (steady-state polarization and electrochemical impedance spectra). The results from SEM showed that CFGDL has the same thickness of 0.11 mm as that of single-layer GDL, while dual-layer GDL has a thickness of 0.18 mm. The fully filled carbon paper with carbon/PTFE filler, as seen in the SEM image, displayed good support for the catalyst layer and electrolyte phase, allowing good electrical contact between the GDL/catalyst/membrane and GDL/flow field plate to be achieved. From porosimetry analysis, CFGDL presented a lower porosity of 67% and a much smaller average pore diameter of 4.7 μm compared to the single-layer GDL (porosity of 77% and pore diameter of 35.8 μm) and dual-layer GDL (porosity of 73% and pore diameter of 25.5 μm); however, it also gave the largest limiting current density, which reflects the improvement in mass transportation. This phenomenon is likely attributed to the fast removal of micro-water droplets formed in the CFGDL structure of the electrode.

Analysis of Gas Diffusion Layer and Flow-Field Design in a PEMFC Using Neutron Radiography

A carbon-cloth gas diffusion layer (GDL) displays better performance than a carbon-paper GDL under humidified conditions. A straight flow field displays better performance than a serpentine flow field. To investigate these phenomena, neutron radiography was used to compare the amount of liquid water that accumulated in test fuel cells. It was found that a larger amount of water accumulated in a carbon-cloth GDL than in a carbon-paper GDL. From the viewpoint of cell performance, the carbon-cloth GDL was less influenced by the accumulated water than the carbon-paper GDL. It is assumed that the broader pore distribution of a carbon-cloth GDL creates oxygen-diffusion paths even if water accumulates in the GDL. With a serpentine flow field, water accumulated in the corner and the gas bypassed the flow field. These phenomena are the main causes of performance deterioration with a serpentine flow field.