Gas diffusion layer deformation and its effect on the transport characteristics and performance of proton exchange membrane fuel cell

Cell/stack assembly force can strongly affect the transport characteristics and performance of a proton exchange membrane fuel cell (PEMFC) through causing the structural deformation. In this study, a mathematical model has been developed to investigate the effect of the assembly force for different gas diffusion layers (GDL) and membranes. The results indicate that the predominant deformation of the cell structure occurs in the porous GDL due to its weak mechanical strength. Thicker GDLs result into lower water content in the GDL structure, and can sustain a larger assembly force without the risk of “electrode flooding”; while thinner GDLs have higher water content, can maintain the hydration required for the membrane, and yield a better cell performance with less sensitivity to the variations in the assembly force. Thinner membranes yield better cell performance, but the cell performance is more sensitive to the changes in the assembly force. A combination of thin GDL and membrane is beneficial for better cell performance with reasonable sensitivity to the assembly force. For thinner GDLs, an optimal assembly force exists beyond which the cell performance is reduced; and practical cell assembly force will limit the GDL thickness.     

Enhancement of proton exchange membrane fuel cell performance by titanium-coated anode gas diffusion layer

Titanium was coated onto an anode gas diffusion layer (GDL) by direct current sputtering to improve the performance and durability of a proton exchange membrane fuel cell (PEMFC). Scanning electron microscopy (SEM) images showed that the GDLs were thoroughly coated with titanium, which showed angular protrusion. Single-cell performance of the PEMFCs with titanium-coated GDLs as anodes was investigated at operating temperatures of 25 °C, 45 °C, and 65 °C. Cell performances of all membrane electrode assemblies (MEAs) with titanium-coated GDLs were superior to that of the MEA without titanium coating. The MEA with titanium-coated GDL, with 10 min sputtering time, demonstrated the best performance at 25 °C, 45 °C, and 65 °C with corresponding power densities 58.26%, 32.10%, and 37.45% higher than that of MEA without titanium coating.     

Effect of water distribution in gas diffusion layer on proton exchange membrane fuel cell performance

Water management of proton exchange membrane fuel cells remains a prominent issue in research concerning fuel cells. In this study, the gas diffusion layer (GDL) of a fuel cell is partially treated with a hydrophobic agent, and the effect of GDL hydrophobicity on the water distribution in the fuel cell is examined. First, the effect of the position of the cathode GDL hydrophobic area relative to the channel on the fuel cell performance is investigated. Then, the water distribution in the fuel cell cathode GDL is observed using X-ray imaging. The experimental results indicate that when the hybrid GDL's hydrophobic area lies on the channel, water tends to accumulate under the rib, and the water content in the channel is low; this improves the fuel cell performance. When the hydrophobic area is under the rib, the water distribution is more uniform, but the performance deteriorates.     

Carbon film coating on gas diffusion layer for proton exchange membrane fuel cells

This study discusses a novel process to increase the performance of proton exchange membrane fuel cells (PEMFC). In order to improve the electrical conductivity and reduce the surface indentation of the carbon fibers, we modified the carbon fibers with pitch-based carbon materials (mesophase pitch and coal tar pitch). Compared with the gas diffusion backing (GDB), GDB-A240 and GDB-MP have 32% and 33% higher current densities at 0.5 V, respectively. Self-made carbon paper with the addition of a micro-porous layer (MPL) (GDL-A240 and GDL-MP) show improved performance compared with GDB-A240 and GDB-MP. The current densities of GDL-A240 and GDL-MP at 0.5 V increased by 37% and 31% compared with GDL, respectively. This study combines these two effects (carbon film and MPL coating) to promote high current density in a PEMFC.     

Study on the performance of a proton exchange membrane fuel cell related to the structure design of a gas diffusion layer substrate

Although characteristics of the gas diffusion layer (GDL) affect the performance of a proton exchange membrane fuel cell (PEMFC), mass transfer mechanisms inside the GDL and the performance of the PEMFC have not been directly correlated. To determine the design parameters of the GDL, the effects of substrate design of the GDL on performance of a PEMFC are investigated. By adding an active carbon fiber (ACF), which has a high surface area, the substrate is designed to have a different pore size structure. The results show that steady-state and transient responses are determined by capillary pressure gradient characteristics of the GDL made by pore size distribution of the substrate. The small macro-pore functions as water-retaining passage and the large macro-pore functions as water-removal passage. It is concluded that both small and large macro-pore must be present on the substrate to facilitate its function in a wide range of operating conditions.     

Fractal model for prediction of effective thermal conductivity of gas diffusion layer in proton exchange membrane fuel cell

The process of heat transfer within porous media is usually considered as a transport through large numbers of straight channels with uniform pore sizes. For the prediction of effective thermal conductivity of gas diffusion layer (GDL), morphological properties such as the tortuosity of channels and pore-size distribution of this porous layer should be considered. Thus in this article, novel parallel and series-parallel prediction models of effective thermal conductivity for the GDL in proton exchange membrane fuel cell (PEMFC) have been derived by fractal theoretical characterization of the real microstructure of GDL. The prediction of fractal parallel model for carbon paper, a basal material of the GDL, is in good agreement with the reference value supplied by Toray Inc. The prediction results from the proposed models are also reasonable because they are distributed between the upper and lower bounds. Parametric effect has been investigated by using the presented models in dimensionless formalism. It can be concluded that dimensionless effective thermal conductivity (k^′eff${k^{\prime }}_{\text{eff}}$) has a positive correlation with effective porosity (?) or the pore-area fractal dimension (D_p) when k_s/k_g < 1; whereas it has a negative correlation with ? or D_p when k_s/k_g > 1 and with tortuous fractal dimension (D_t) whether k_s/k_g < 1 or not. Furthermore, these fractal models have been modified by considering the effect of polytetrafluoroethylene (PTFE) incorporated into the pore spaces of carbon paper, and the corresponding model prediction shows that there is an increase in the effective thermal conductivity due to the filling of PTFE that has high thermal conductivity.     

Liquid water breakthrough pressure through gas diffusion layer of proton exchange membrane fuel cell

The dynamic behavior of liquid water transport through the gas diffusion layer (GDL) of the proton exchange membrane fuel cell is studied with an ex-situ approach. The liquid water breakthrough pressure is measured in the region between the capillary fingering and the stable displacement on the drainage phase diagram. The variables studied are GDL thickness, PTFE/Nafion content within the GDL, GDL compression, the inclusion of a micro-porous layer (MPL), and different water flow rates through the GDL. The liquid water breakthrough pressure is observed to increase with GDL thickness, GDL compression, and inclusion of the MPL. Furthermore, it has been observed that applying some amount of PTFE to an untreated GDL increases the breakthrough pressure but increasing the amount of PTFE content within the GDL shows minimal impact on the breakthrough pressure. For instance, the mean breakthrough pressures that have been measured for TGP-060 and for untreated (0 wt.% PTFE), 10 wt.% PTFE, and 27 wt.% PTFE were 3589 Pa, 5108 Pa, and 5284 Pa, respectively.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.     

Gas diffusion layer for proton exchange membrane fuel cells—A review

Gas diffusion layer (GDL) is one of the critical components acting both as the functional as well as the support structure for membrane-electrode assembly in the proton exchange membrane fuel cell (PEMFC). The role of the GDL is very significant in the H₂/air PEM fuel cell to make it commercially viable. A bibliometric analysis of the publications on the GDLs since 1992 shows a total of 400+ publications (>140 papers in the Journal of Power Sources alone) and reveals an exponential growth due to reasons that PEMFC promises a lot of potential as the future energy source for varied applications and hence its vital component GDL requires due innovative analysis and research. This paper is an attempt to pool together the published work on the GDLs and also to review the essential properties of the GDLs, the method of achieving each one of them, their characterization and the current status and future directions. The optimization of the functional properties of the GDLs is possible only by understanding the role of its key parameters such as structure, porosity, hydrophobicity, hydrophilicity, gas permeability, transport properties, water management and the surface morphology. This paper discusses them in detail to provide an insight into the structural parts that make the GDLs and also the processes that occur in the GDLs under service conditions and the characteristic properties. The required balance in the properties of the GDLs to facilitate the counter current flow of the gas and water is highlighted through its characteristics.     

Novel single-layer gas diffusion layer based on PTFE/carbon black composite for proton exchange membrane fuel cell

A series of poly(tetrafluoroethylene)/carbon black composite-based single-layer gas diffusion layers (PTFE/CB-GDLs) for proton exchange membrane fuel cell (PEMFC) was successfully prepared from carbon black and un-sintered PTFE, which included powder resin and colloidal dispersion, by a simple inexpensive method. The scanning electron micrographs of PTFE/CB-GDLs indicated that the PTFE resins were homogeneously dispersed in the carbon black matrix and showed a microporous layer (MPL)-like structure. The as-prepared PTFE/CB-GDLs exhibited good mechanical property, high gas permeability, and sufficient water repellency. The best current density obtained from the PEMFC with the single-layer PTFE/CB-GDL was 1.27 and 0.42 A cm⁻² for H₂/O₂ and H₂/air system, respectively.     

The effect of materials on proton exchange membrane fuel cell electrode performance

This paper describes the optimisation in the fabrication materials and techniques used in proton exchange membrane fuel cell (PEMFC) electrodes. The effect on the performance of membrane electrode assemblies (MEAs) from the solvents used in producing catalyst inks is reported. Comparison in MEA performances between various gas diffusion layers (GDLs) and the importance of microporous layers (MPLs) in gas diffusion electrodes (GDEs) are also shown. It was found that the best performances were achieved for GDEs using tetrahydrofuran (THF) as the solvent in the catalyst ink formulation and Sigracet 10BC as the GDL. The results also showed that our in-house painted GDEs were comparable to commercial ones (using Johnson Matthey HiSpec™ and E-TEK catalysts).     

Water transport characteristics in the gas diffusion media of proton exchange membrane fuel cell - Role of the microporous layer

Water transport through the gas diffusion media of a proton exchange membrane fuel cell (PEMFC) was investigated with a focus on the role of the microporous layer (MPL) coated on the cathode gas diffusion layer (GDL). The capillary pressure of the MPL and GDL, which plays a significant role in water transport, is derived as a function of liquid saturation using a pore size distribution (PSD) model. PSD functions are derived with parameters that are determined by fitting to the measured total PSD data. Computed relations between capillary pressure and liquid saturation for a GDL and a double-layered GDL (GDL + MPL) show good agreement with the experimental data and proposed empirical functions. To investigate the role of the MPL, the relationship between the water withdrawal pressure and liquid saturation are derived for a double-layered GDL. Water transport rates and cell voltages were obtained for various feed gas humidity using a two-dimensional cell model, and are compared with the experimental results. The calculated results for the net drag with application of the capillary pressure derived from the PSD model show good agreement with the experimental values. Furthermore, the results show that the effect of the MPL on the cell output voltage is significant in the range of high humidity operation.     

Effect of the micro porous layer design on the dynamic performance of a proton exchange membrane fuel cell

The mass transport characteristics of a gas diffusion layer (GDL) predominantly affect the performance of a proton exchange membrane (PEM) fuel cell. However, studies examining the transient response related to the GDL are insufficient, although the dynamic behavior of a PEM fuel cell is an important issue. In this study, the effects of the design of a micro porous layer (MPL) on the transient response of a PEM fuel cell are investigated. The MPL slurry density and multiple functional layers are treated as the variable design parameter. The results show that the transient response is determined by the capillary pressure gradient through the GDL. The trade-off relation for the PEM fuel cell performance under low and high humidity conditions due to the hydrophobic GDL is mitigated by designing a reverse capillary pressure gradient in the MPL.     

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%.     

Preparation of microporous layer for proton exchange membrane fuel cell by using polyvinylpyrrolidone aqueous solution

A new method of preparing microporous layer (MPL) for proton exchange membrane fuel cell (PEMFC) was presented in this paper. Considering the bad dispersion of PTFE aqueous suspension in the carbon slurry based on ethanol, polyvinylpyrrolidone (PVP) aqueous solution was used to prepare carbon slurry for microporous layer. The prepared gas diffusion layers (GDLs) were characterized by scanning electron microscopy, contact angle system and pore size distribution analyzer. It was found that the GDL prepared with PVP aqueous solution had higher gas permeability, as well as more homogeneous hydrophobicity. Moreover, the prepared GDLs were used in the cathode of fuel cell and evaluated with fuel cell performance and EIS analysis, and the GDL prepared with PVP aqueous solution indicated better fuel cell performance and lower ohmic resistance and mass transfer resistance.     

An analytical model for the transverse permeability of gas diffusion layer with electrical double layer effects in proton exchange membrane fuel cells

An analytical model is presented for the transverse permeability of gas diffusion layer (GDL) based on an ordered array of parallel charged circular cylinders at the steady state. The formula of calculating the permeability of the transverse direction is given by solving the fluid momentum equation in a unit cell. In the present approach, the proposed model is explicitly related to the porosity and fiber radius of fibrous porous media, the zeta potential, and the physical properties of the electrolyte solution. Besides, the effects of these parameters (the porosity, unit cell aspect ratio, fiber radius, and molar concentration) on the transverse permeability are discussed detailedly. The model predictions are compared with the previous studies in the available literature, and good agreement is found.     

Fractal model for prediction of effective hydrogen diffusivity of gas diffusion layer in proton exchange membrane fuel cell

This paper presents a novel prediction model of the effective hydrogen diffusivity for the gas diffusion layer (GDL) in proton exchange membrane fuel cell (PEMFC) by using fractal theory to characterize microstructure. With the consideration of pore-size distribution and Knudsen diffusion effect, a relationship between micro-structural parameters and effective hydrogen diffusivity of GDL is deduced. The prediction of effective hydrogen diffusivities of two samples shows that Knudsen diffusion effect makes the effective diffusivity value decrease, and after being treated with polytetrafluoroethylene (PTFE), carbon paper, a basal material of the GDL, exhibits a lower effective diffusivity value due to the decrease in the pore space and porosity. From the parametric effect study, it can be concluded that effective diffusivity has a positive correlation with pore area fractal dimension D_p or porosity ?, whereas it has a negative correlation with tortuosity fractal dimension D_t.     

Fractal model for predicting the effective binary oxygen diffusivity of the gas diffusion layer in proton exchange membrane fuel cells

We propose an analytical model to predict the effective binary oxygen diffusivity of the porous gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). In this study, we consider the fractal characteristics of the porous GDL as well as its general microstructure, and we adopt the Bosanquet equation to derive effective diffusivity. The fractal characterization of GDL enables us to model effective diffusivity in a continuous manner while taking into account the effect of pore size distribution. Comparison to two other theoretical models that are generally accepted in the simulation of PEMFCs shows similar trends in all three models, indicating that our proposed model is well founded. Furthermore, the predicted effective binary oxygen diffusivities of two samples show that after treatment with polytetrafluoroethylene (PTFE), the effective binary diffusivity of the GDL decreases. Based on the parametric effect analysis, we conclude that effective binary diffusivity is negatively correlated with tortuosity fractal dimension but positively correlated with the fractal dimension of pore area, porosity, or mean pore diameter. The proposed model facilitates fast prediction of effective diffusivity as well as multi-scale modeling of PEMFCs and thus facilitates the design of the GDLs and of PEMFCs.     

Optimization of cathode microporous layer materials for proton exchange membrane fuel cell

The microporous layer (MPL) of diffusion medium has an important impact on the water management ability of proton exchange membrane fuel cells. In this study, six kinds of carbon black were used to prepare the cathode MPL. The thickness, conductivity, pore structure, hydrophobicity, and surface microstructure of MPL were characterized. The single cell was prepared and electrochemical tests were performed. The results showed that the single cell prepared by Acetylene black (ACET) and Vulcan XC-72R has a considerable power generation performance. In addition, polyvinylidene fluoride hexafluoropropylene copolymer P(VDF-HFP) was used to replace Polytetrafluoroethylene (PTFE) as hydrophobic binder. MPL with different P(VDF-HFP) contents were prepared, and the single cell performance was investigated. The results showed that all the single cells prepared by P(VDF-HFP) were worse than that of PTFE. This study provides an important reference for further improving the performance of fuel cells from the perspective of material optimization with MPL.     

Effect of gas diffusion layer modulus and land-groove geometry on membrane stresses in proton exchange membrane fuel cells

The electrical functionality of PEM fuel cells is facilitated by minimizing the contact resistances between different materials in the fuel cell, which is achieved via compressive clamping. The effect of the gas diffusion layer (GDL) modulus on the in-plane stress in the membrane after clamping is studied via numerical simulations, including both isotropic and anisotropic GDL properties. Furthermore, the effect of cell width and land-groove width ratio on the in-plane stress in the membrane subjected to a single hygro-thermal cycle is investigated for aligned and alternating gas channel geometries. The results from varying the GDL properties suggest that the in-plane stress in the membrane after clamping is due to a non-linear and coupled interaction of GDL and membrane deformation. The results of the geometric studies indicate that when the gas channels are aligned, the cell width and land-groove width ratio affect the in-plane stress distribution, but do not significantly affect the stress magnitudes. However, when the gas channels are alternating, the cell width and land-groove width ratio have significant effect on the membrane in-plane stresses. The effect of land-groove geometry is qualitatively verified by a series of experimental compression tests.