Carbon cloth based on PAN carbon fiber practicability for PEMFC applications

Carbon fiber cloth based on PAN (polyacrylonitrile) material was woven and fabricated into the gas diffusion layer (GDL) for PEMFC applications. This paper describes the newly developed carbon cloth as GDL and proves its feasibility for PEMFC. Such carbon cloth based GDLs have performance equal to that of conventional carbon papers verified using the standard test instrument. The mechanical tests show that as a supporting base, carbon cloth is more practical than carbon paper because of its superior compressibility, elasticity, and flexibility performance, making it more appropriate for ongoing manufacturing and assembly processes. Furthermore, even though carbon paper is structurally flatter and smoother than carbon cloth, the discharge curves of both substrates coated with a MPL (micro-porous layer) showed similar current density (around 750 mA/cm²) at 0.6 V. This indicates that the developed carbon cloth with MPL has achieved the required performance and provides an alternative selection from carbon paper as GDL.     

Investigation on the effect of microstructure of proton exchange membrane fuel cell porous layers on liquid water behavior by soft X-ray radiography

In order to investigate the effect of microstructure of PEMFC porous layers on the liquid water transport, liquid water accumulation and discharge behavior in the operating PEMFC was visualized by laboratory-based soft X-ray radiography. The utilization of low energy X-ray made it possible to visualize the liquid water behavior in the PEMFC with the spatial resolution of 0.8 μm and the temporal resolution of 2.0 s frame⁻¹, and the cross-sectional imaging can resolve the each components of the PEMFC. The visualization results showed that adding the MPL prevents the accumulation of liquid water in the substrate layer from contacting and forming the liquid water film on the catalyst layer. Furthermore, it was found that the liquid water distribution in the carbon paper and the carbon cloth GDL was completely different. The liquid water in the carbon cloth GDL concentrates at the weaves of fiber bundle and was effectively discharged to the channel. These visualization results suggested that the microstructure of the PEMFC porous layers strongly affect the liquid water behavior in the PEMFC, and the detailed understanding of the pore structures and the network of liquid water is essential for keeping the oxygen transport path to the catalyst site.     

Effect of GDL compression on pressure drop and pressure distribution in PEMFC flow field

Improving reactant distribution is an important technological challenge in the design of a PEMFC. Flow field and the Gas Diffusion Layer (GDL) distribute the reactant over the catalyst area in a cell. Hence it is necessary to consider flow field and GDL together to improve their combined effectiveness. This paper describes a simple and unique off-cell experimental setup developed to determine pressure as a function of position in the active area, due to reactant flow in a fuel cell flow field. By virtue of the experimental setup being off-cell, reactant consumption, heat production, and water generation, are not accounted as experienced in a real fuel cell. A parallel channel flow field and a single serpentine flow field have been tested as flow distributors in the experimental setup developed. In addition, the interaction of gas diffusion layer with the flow distributor has also been studied. The gas diffusion layer was compressed to two different thicknesses and the impact of GDL compression on overall pressure drop and pressure distribution over the active area was obtained using the developed experimental setup. The results indicate that interaction of GDL with the flow field and the effect of GDL compression on overall pressure drop and pressure distribution is more significant for a serpentine flow field relative to a parallel channel flow field.     

Numerical study for in-plane gradient effects of cathode gas diffusion layer on PEMFC under low humidity condition

A numerical study about in-plane porosity and contact angle gradient effects of cathode gas diffusion layer (GDL) on polymer electrolyte membrane fuel cell (PEMFC) under low humidity condition below 50% relative humidity is performed in this work. Firstly, a numerical model for a fuel cell is developed, which considers mass transfer, electrochemical reaction, and water saturation in cathode GDL. For water saturation in cathode GDL, porosity and contact angle of GDL are also considered in developing the model. Secondly, current density distribution in PEMFC with uniform cathode GDL is scrutinized to design the gradient cathode GDL. Finally, current density distributions in PEMFC with gradient cathode GDL and uniform cathode GDL are compared. At the gas inlet side, the current density is higher in GDL with a gradient than GDL with high porosity and large contact angle. At the outlet side, the current density is higher in GDL with a gradient than GDL with low porosity and small contact angle. As a result, gradient cathode GDL increases the maximum power by 9% than GDL with low porosity and small contact angle. Moreover, gradient cathode GDL uniformizes the current density distribution by 4% than GDL with high porosity and large contact angle.     

Implications of non-uniform porosity distribution in gas diffusion layer on the performance of a high temperature PEM fuel cell

The present study discusses a detailed investigation on the implications of non-uniform porosity distribution in the gas diffusion layer (GDL) on the performance of proton exchange membrane fuel cell (PEMFC). A three-dimensional, single-phase, isothermal model of high-temperature PEMFC is employed to study the effect of non-uniform porosity distribution in GDL. The different porosity configurations with stepwise, sinusoidal, and logarithmic variation in porosity along the streamwise direction of GDL are considered. The numerical experiments are performed, keeping average porosity as constant in the GDL. The electrochemical characteristics such as the oxygen molar concentration, power density, current density, total power dissipation density, average diffusion coefficient, vorticity magnitude, and overpotential are studied for a range of porosity distributions. Furthermore, the variations of oxygen concentration, average diffusion coefficient, and vorticity magnitude are also discussed to showcase the influence of non-uniform porosity distribution. Our study reveals that the PEM fuel cell performance is the best when the porosity of the GDL decreases logarithmically in the streamwise direction. On the contrary, the performance deteriorates when the GDL porosity decreases sinusoidally. Also, it has been observed that the effects of non-uniform porosity distribution are more pronounced, especially at higher current densities. The outcomes of present investigation have potential utility in GDL fabrication and membrane assembly's sintering process for manufacturing high valued PEMFC products.     

Study of the mechanical behavior of paper-type GDL in PEMFC based on microstructure morphology

As the softest part in a proton exchange membrane fuel cell (PEMFC), the gas diffusion layer (GDL) could have a large deformation under assembly pressure imposed by bipolar plate, which would have an impact on the cell performance. So, there is an urgent need to clearly reveal the mechanical behavior of GDL under certain pressure. In this paper, the mechanical behavior of paper-type GDL of PEMFC is studied, considering the complex contact environment in the fibrous layered structure. The microstructure of GDL is reconstructed stochastically, then the stress-strain relationship of GDL is explored from the perspective of solid mechanics by using the finite element method. Based on microstructure morphology, it is found that contact pairs and pore space of microstructure are two key factors determining the nonlinearity of the compressive curve. The equivalent Young's modulus increases with the decrease of porosity and carbon fiber diameter but it is not very sensitive to the carbon paper thickness. The results indicate that with the increase in acting pressure, the average porosity of the carbon paper decreases, and the nonuniformity of porosity along the through-plane direction increases. Furthermore, a reasonable explanation for the increase of concentration loss and the decrease of ohmic loss is given from the microstructure findings of the present study.     

Synchrotron X-ray tomography for investigations of water distribution in polymer electrolyte membrane fuel cells

Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models.     

Enhanced mechanical and electrochemical durability of multistage PTFE treated gas diffusion layers for proton exchange membrane fuel cells

Gas Diffusion Layers (GDLs) of Proton Exchange Membrane Fuel Cells (PEMFCs) are usually subject to polytetrafluoroethylene (PTFE) treatment in a single stage. The impact of multistage PTFE treatment on the mechanical and electrochemical durability of GDLs used in PEMFCs, is reported here. With the same total amount of PTFE in the GDL, substrates treated with PTFE in multiple stages are seen to possess distinctly improved mechanical and electrochemical durability compared to GDLs treated with PTFE in a single stage. The difference in structure and hydrophobicity of the GDLs, when they are subjected to the two different PTFE treatment routes, are examined to understand the reasons for the improvements. The results indicate that there is a change in surface morphology, pore size distribution, and hydrophobicity of the GDL samples depending on the treatment route adopted. It is observed that it is possible to establish a gradient in PTFE profile in the GDL by adopting multistage treatment. Such gradients counteract loss in hydrophobicity resulting from compression cycles during cell assembly and carbon corrosion due to electrochemical aging. The results reveal that GDLs subject to multistage PTFE treatment, can have increased lifetime as opposed to conventional single stage PTFE treated GDL.     

Numerical study on the compression effect of gas diffusion layer on PEMFC performance

A numerical method is developed to study the effect of the compression deformation of the gas diffusion layer (GDL) on the performance of the proton exchange membrane fuel cell (PEMFC). The GDL compression deformation, caused by the clamping force, plays an important role in controlling the performance of PEMFC since the compression deformation affects the contact resistance, the GDL porosity distribution, and the cross-section area of the gas channel. In the present paper, finite element method (FEM) is used to first analyze the ohmic contact resistance between the bipolar plate and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and products. We investigate the effects of the GDL compression deformation, the ohmic contact resistivity, the air relative humidity, and the thickness of the catalyst layer (CL) on the performance of the PEMFC. The numerical results show that the fuel cell performance decreases with increasing compression deformation if the contact resistance is negligible, but an optimal compression deformation exists if the contact resistance is considerable.     

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/${\text{cm}}^2$ demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.     

Optimum design of the slotted-interdigitated channels flow field for proton exchange membrane fuel cells with consideration of the gas diffusion layer intrusion

The design of the flow field greatly affects the flow distribution and the final performance of the proton exchange membrane fuel cell (PEMFC) system. The clamping force between the gas diffusion layer (GDL) and the flow field plate (FFP) will cause the inhomogeneous compression of the GDL. Then the GDL will be intruded into the reactant gas channels and eventually change the flow distribution. This paper presents a study on the effect of the intrusion of the GDL on the flow field in a PEMFC, and tries to explain the reason for poor performance of the previous design of the flow field other than those have been studied in other papers such as GDL roughness, porosity, etc. First of all, a linear analytical model is used to analyze the sensitivities of the flow field to the GDL intrusion, and then used to estimate the effect of the GDL intrusion on the flow field distribution. Secondly, a multi-objective optimization model is proposed to eliminate the nonuniform distribution in the flow field with the GDL intrusion taken into consideration. Subsequently, three different designs are analyzed and compared with each other as a demonstration to show the effect of the GDL intrusion. From the analysis results, it is recommended that the effect of the GDL intrusion on the multi-depth flow field should be taken into consideration and the flow field should be insensitive to the GDL intrusion to obtain high robust performance. The results obtained in the study provide the designer some useful guidelines in the concept design of flow field configurations.     

Effects of ratio variation in substrate and micro porous layer penetration on polymer exchange membrane fuel cell performance

A gas diffusion layer (GDL) facilitates the diffusion of reactant gas and the discharge of the generated water. The GDL performs various functions, such as conducting heat and electrons generated by electrochemical reactions and providing mechanical support for the catalyst layer. In this study, the effects of ratio variation in the substrate and microporous layer (MPL) penetration region on the proton exchange membrane fuel cell (PEMFC) performance were investigated. Furthermore, the reasons for these performance tendencies are explained based on the thermogravimetric analysis, contact angle, scanning electron microscopy, mercury porosimetry, electrical resistance, electrochemical impedance spectroscopy, and capillary pressure gradient. The experimental results indicate that the MPL penetration ratio within 15–20% of the total GDL thickness and the combined ratio of the MPL and MPL penetration within 35–40% is the best for the overall PEMFC performance. In addition, when the substrate ratio is excessively low, water flooding substantially occurs in the substrate, and this accumulated water functions as a back pressure, causing severe capillary condensation in the MPL penetration region and thus depriving the supply of the reactant gas.     

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.     

An integrated model of the water transport in nonuniform compressed gas diffusion layers for PEMFC

Water transport in gas diffusion layer (GDL) is a very important issue for high power density Proton Exchange Membrane Fuel Cell (PEMFC). During the GDL and bipolar plate (BPP) assembly process, the water transport behavior is greatly influenced by the nonuniform compression on the GDL, which leads to uneven distribution of the internal mass transport pores. In this study, an integrated model is developed to predict the water transport in nonuniform compressed GDL. Firstly, a GDL compression deformation model is built to obtain the relationship between the GDL deformation and assembly clamping force based on energy method. Then, a water transport model is established by considering the probability density function (PDF) of the pore size for the compressed GDL. The accuracy of the integrated model has been verified by comparing with the finite element method (FEM) and the computational fluid dynamics (CFD) simulation results. The influence of assembly clamping force, GDL thickness and channel geometry are analyzed based on the integrated model. Drainage pressure increases monotonically with the assembly clamping force and is divided into three stages. For the baseline case, 0.2 mm of GDL thickness and small rib-channel ratio is conducive to improving drainage capacity. It provides the guidance for matching of GDL/BPP assembly condition and performance prediction of PEMFC.     

Two-phase transport in the cathode gas diffusion layer of PEM fuel cell with a gradient in porosity

Two-phase transport in the cathode gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) is studied with a porosity gradient in the GDL. The porosity gradient is formed by adding micro-porous layers (MPL) with different carbon loadings on the catalyst layer side and on the flow field side. The multiphase mixture model is employed and a direct numerical procedure is used to analyze the profiles of liquid water saturation and oxygen concentration across the GDL as well as the resulting activation and concentration losses. The results show that a gradient in porosity will benefit the removal rate of liquid water and also enhance the transport of oxygen through the cathode GDL. The present study provides a theoretical support for the suggestion that a GDL with porosity gradient will improve the cell performance.     

Investigation of sintered stainless steel fiber felt as gas diffusion layer in proton exchange membrane fuel cells

The feasibility of using sintered stainless steel fiber felt (SSSFF) as gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs) is evaluated in this study. The SSSFF is coated with an amorphous carbon (a-C) film by closed field unbalanced magnetron sputter ion plating (CFUBMSIP) to enhance the corrosion resistance and reduce the contact resistance. The characteristics of treated SSSFF, including microscopic morphology, mechanical properties, electrical conductivity, electrochemical behavior and wettablity characterization, are systematically investigated and summarized according to the requirements of GDL in PEMFC. A membrane electrode assembly (MEA) with a-C coated SSSFF-15 GDL is fabricated and assembled with a-C coated stainless steel bipolar plates in a single cell. The initial peak power density of the single cell is 877.8 mW cm⁻² at a current density of 2324.9 mA cm⁻². Lifetime test of the single cell over 200 h indicates that the a-C coating protects the SSSFF-15 GDL from corrosion and decreases the performance degradation from 30.6% to 6.3%. The results show that the SSSFF GDL, enjoying higher compressive modulus and ductility, is a promising solution to improve fluid permeability of GDL under compression and PEMFC durability.     

The influence of gas diffusion layer wettability on direct methanol fuel cell performance: A combined local current distribution and high resolution neutron radiography study

The influence of the anode and cathode GDL wettability on the current and media distribution was studied using combined in situ high resolution neutron radiography and locally resolved current distribution measurements. MEAs were prepared by vertically splitting either the anode or cathode carbon cloth into a less hydrophobic part (untreated carbon cloth ‘as received’) and a more hydrophobic part (carbon cloth impregnated by PTFE dispersion). Both parts were placed side by side to obtain a complete electrode and hot-pressed with a Nafion membrane. MEAs with partitioned anode carbon cloth revealed no difference between the untreated and the hydrophobised part of the cell concerning the fluid and current distribution. The power generation of both parts was almost equal and the cell performance was similar to that of an undivided MEA (110 mW cm⁻², 300 mA cm⁻², 70 °C). In contrast, MEAs with partitioned cathode carbon cloth showed a better performance for the hydrophobised part, which contributed to about 60% of the overall power generation. This is explained by facilitated oxygen transport especially in the hydrophobised part of the cathode gas diffusion layer. At an average current density of 300 mA cm⁻², a pronounced flooding of the cathode flow field channels adjacent to the untreated part of GDL led to a further loss of performance in this part of the cell. The low power density of the untreated part caused a significant loss of cell performance, which amounted to less than 40 mW cm⁻² (at 300 mA cm⁻²).     

Influence of compressive stress on the pore structure of carbon cloth based gas diffusion layer investigated by capillary flow porometry

Gas diffusion layer (GDL) is subjected to compressive stress at high temperature along with polymer electrolyte membrane in the fabrication process and in assembling the fuel cell stacks. Compressive stress decreases the thickness of GDL, electrical conductivity, permeability, and affects the pores. Carbon cloth based GDL withstands higher strain level when compared to carbon paper and the pore structure is also disrupted to a greater extent in cloth based GDL. In the present paper, we have addressed the effects of stress on pore structure of cloth based GDL. An optimum GDL must offer low mass transport resistance in an operating PEM fuel cell. The pore size analysis of pristine GDL and GDLs pressed at different pressure levels (200, 600 & 1000 kg cm⁻²) and their characteristics are evaluated using capillary flow porometry. The compressive stress affects the three types of pores in GDL called bubble point pore, mean flow pore and smallest pore. The change in electrical resistance, wetting behavior and surface morphology is also examined as a function of compressive stress. The fuel cell performances using these GDLs pressed at different compressive stresses are also evaluated and presented. The highest PEMFC performance is achieved at a compressive stress of 200 kg cm⁻², which could be attributed to the combined effect of reduced ohmic resistance and optimized pore structure. The order of increasing performance in terms of current density is observed to be j₂₀₀ > j_Pristine > j₆₀₀ > j₁₀₀₀ at 0.15 V. The thicknesses and pore sizes of custom made GDL for optimum fuel cell performance are recommended.     

Three dimensional numerical simulation of gas-liquid two-phase flow patterns in a polymer-electrolyte membrane fuel cells gas flow channel

Water management in polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on fuel cell performance and durability. To investigate the two-phase flow patterns in PEMFC gas flow channels, the volume of fluid (VOF) method was employed to simulate the air-water flow in a 3D cuboid channel with a 1.0 mm × 1.0 mm square cross section and a 100 mm in length. The microstructure of gas diffusion layers (GDLs) was simplified by a number of representative opening pores on the 2D GDL surface. Water was injected from those pores to simulate water generation by the electrochemical reaction at the cathode side. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. The water injection rate was also amplified 10 times, 100 times and 1000 times to study the flow pattern formation and transition in the channel. Simulation results show that, as the flow develops, the flow pattern evolves from corner droplet flow to top wall film flow, then annular flow, and finally slug flow. The total pressure drop increases exponentially with the increase in water volume fraction, which suggests that water accumulation should be avoided to reduce parasitic energy loss. The effect of material wettability was also studied by changing the contact angle of the GDL surface and channel walls, separately. It is shown that using a more hydrophobic GDL surface is helpful to expel water from the GDL surface, but increases the pressure drop. Using a more hydrophilic channel wall reduces the pressure drop, but increases the water residence time and water coverage of the GDL surface.     

Catalyst layers for proton exchange membrane fuel cells prepared by electrospray deposition on Nafion membrane

The electrospray deposition method has been used for preparation of catalyst layers for proton exchange membrane fuel cells (PEMFC) on Nafion membrane. Deposition of Pt/C + ionomer suspensions on Nafion 212 gives rise to layers with a globular morphology, in contrast with the dendritic growth observed for the same layers when deposited on the gas diffusion layer, GDL (microporous carbon black layer on carbon cloth) or on metallic Al foils. Such a change is discussed in the light of the influence of the Nafion substrate on the electrospray deposition process. Nafion, which is a proton conductor and electronic insulator, gives rise to the discharge of particles through proton release and transport towards the counter electrode, compared with the direct electron transfer that takes place when depositing on an electronic conductor. There is also a change in the electric field distribution in the needle to counter-electrode gap due to the presence of Nafion, which may alter conditions for the electrospray effect. If discharging of particles is slow enough, for instances with a low membrane protonic conductivity, the Nafion substrate may be charged positively yielding a change in the electric field profile and, with it, in the properties of the film. Single cell characterization is carried out with Nafion 212 membranes catalyzed by electrospray on the cathode side. It is shown that the internal resistance of the cell decreases with on-membrane deposited cathodic catalyst layers, with respect to the same layers deposited on GDL, giving rise to a considerable improvement in cell performance. The lower internal resistance is due to higher proton conductivity at the catalyst layer-membrane interface resulting from on-membrane deposition. On the other hand, electroactive area and catalyst utilization appear little modified by on-membrane deposition, compared with on-GDL deposition.