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

The in-plane permeability has been experimentally estimated for a number of carbon substrates and microporous layer (MPL)-coated gas diffusion layers (GDLs) as used in proton exchange membrane (PEM) fuel cells. The results show that the in-plane permeability of the tested carbon substrates decreases with increasing polytetrafluoroethylene (PTFE) loading and, in contrast, the greater is the PTFE loading in the MPL, the greater is the permeability. It has been shown that the in-plane permeability of the carbon substrates is reduced by an order of magnitude if they are coated with MPLs. Further, the permeability is different from one in-plane principal direction to another by a factor of about two. Finally, ignoring the inertial terms (for the reported flow rates) and the compressibility of the flowing air results in significant errors in the obtained values of the permeability.     

Effective permeability of gas diffusion layer in proton exchange membrane fuel cells

In gas diffusion layers (GDLs) of proton exchange membrane fuel cells (PEMFCs), effective permeability is a key parameter to be determined and engineered. In this study, through-plane (TP) and in-plane (IP) flow behaviors of GDLs are investigated analytically based on a scaling estimate method. The TP permeability and IP permeability of unidirectional fibers are determined first, based on that the minimum distance and the inscribed radius between fibers are adopted as the characteristic lengths for normal and parallel flows, respectively. The permeabilities of two-dimensional (2D) and three-dimensional (3D) GDLs are estimated by a proper mixture of the local TP and IP permeabilities of fiber alignments. The mechanistic model agrees closely with experimental and numerical results over a wide porosity range. With the new model, the influences of porosity and fiber orientation on flow behaviors are analyzed.     

Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 2. Absolute permeability

This is the second in a series of papers in which we present methods demonstrated in our group for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Here we describe a method for determining separately the in-plane (x, y-directions) and through-plane (z-direction), viscous and inertial permeability coefficients of macro-porous substrates and micro-porous layers by controlling the direction of the gas flow through the porous sample. The method is applied initially to the macro-porous substrate of the GDL alone and subsequently to the macro-porous substrate with different micro-porous layers applied on it. The permeability coefficients of the micro-porous layer are calculated from the two measurements. The permeability coefficients are calculated from the Darcy–Forchheimer equation by application of the method of least squares. The method was applied to GDLs having different contents of polytetrafluoroethylene (PTFE) and carbon types. The samples with a higher PTFE content have in-plane and through-plane viscous permeability coefficients higher than those of the samples with lower PTFE content. The in-plane and through-plane viscous permeability coefficients also depend on the carbon type.     

In-plane gas permeability of proton exchange membrane fuel cell gas diffusion layers

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter. A blending technique is also successfully adopted to report a closed-form relationship for in-plane permeability of three-directional fibrous materials.     

Neutron images of the through-plane water distribution of an operating PEM fuel cell

Neutron imaging has proven an invaluable tool for water metrology in operating proton exchange membrane fuel cells. Due to limitations in scintillator-based detector resolution, neutron imaging has been applied only to assessing the in-plane water distribution, without being able to distinguish water in the anode from the cathode. A new detector technology, based on micro-channel plates, enables a near order of magnitude improvement in the image resolution. This new detector technology will enable direct measurement of the through-plane water distribution in the gas diffusion layer, and enable the determination of the relative water content on the anode and cathode sides of a proton exchange membrane fuel cell. We report on the initial measurements with this new detector and discuss future measurement possibilities.     

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.     

Optimization of GDLs for high-performance PEMFC employing stainless steel bipolar plates

The effects of polytetraflouroethylene (PTFE) content in the gas diffusion layer (GDL) on the performance of PEMFCs with stainless-steel bipolar plates are studied under various operation conditions, including relative humidity, cell temperature, and gas pressure. The optimal PTFE content in the GDL strongly depends on the cell temperature and gas pressure. Under unpressurized conditions, the best cell performance was obtained by the GDL without PTFE, at a cell temperature of 65 °C and relative humidity (RH) of 100%. However, under the conditions of high cell temperature (80 °C), low RH (25%) and no applied gas pressure, which is more desirable for fuel cell vehicle (FCV) applications, the GDL with 30 wt.% PTFE shows the best performance. The GDL with 30 wt.% PTFE impedes the removal of produced water and increases the actual humidity within the membrane electrode assembly (MEA). A gas pressure of 1 bar in the cell using the GDL with 30 wt.% PTFE greatly improves the performance, especially at low RH, resulting in performance that exceeds that of the cell under no gas pressure and high RH of 100%.     

Single-walled carbon nanotube interlayer modified gas diffusion layers to boost the cell performance of self-humidifying proton exchange membrane fuel cells

A tradeoff between the low humidity and the high performance remains a key challenge for the proton exchange membrane fuel cell (PEMFC). In this work, a novel self-humidifying gas diffusion layer (GDL) with a single-walled carbon nanotube (SWCNT) nonwoven layer between the gas diffusion substrate and the hydrophobic microporous layer is controllably prepared to elevate the cell performance under dry conditions. The membrane electrode assembly (MEA) with 0.25 mg cm⁻² SWCNT loading exhibits a current density of 0.69 A cm⁻² at 0.6 V, which is 392.8% higher than that of the counterpart without the SWCNT interlayer at the same relative humidity. Moreover, the SWCNT interlayer with rational pore structure and proper wettability dramatically improves the water retention capacity of MEA, thus enhancing the low-humidity performance of MEA. The structure design of GDL provides an effective strategy for self-humidifying PEMFC control optimization.     

Effect of the anisotropic thermal conductivity of GDL on the performance of PEM fuel cells

Gas diffusion layers (GDLs) are one of the main components in proton exchange membrane (PEM) fuel cells. In this paper, the effect of anisotropic thermal conductivity of the GDL is numerically investigated under different operating temperatures. Furthermore, the sensitivity of the PEM fuel cell performance to the thermal conductivity of the GDL is investigated for both in-plane and through-plane directions and the temperature distributions between the different GDL thermal conductivities are compared. The results show that increasing the in-plane and through-plane thermal conductivity of the GDL increases the power density of PEM fuel cells significantly. Moreover, the temperature gradients show a greater sensitivity to the in-plane thermal conductivity of the GDL as opposed to the through-plane thermal conductivity.     

Stability and performance improvement of a polymer electrolyte membrane fuel cell stack by laser perforation of gas diffusion layers

The performance and stability of a hydrogen-driven polymer electrolyte membrane fuel cell stack (6-cell PEFC stack) are investigated with regard to pore flooding within the gas diffusion layers (GDLs). Two short stacks with various GDLs (Toray TGP-H-060 untreated and laser-perforated) were characterized at different operating conditions by several characterization techniques such as constant current load, polarization curve, chronoamperometry and chronovoltammetry. The experimental results reveal that the perforation of the cathode GDLs improves the water transport in the porous media and thus the performance as well as the stability of the operating stack in medium and high current density range. A reduced pore flooding is verified when using the customized laser-perforated GDLs. The GDL perforation has a huge potential to balance the inhomogeneous in-plane saturation conditions between the inlet and outlet area of the cell and to compensate to a certain degree the effects of temperature distribution within a stack regarding the water management.     

Study on ice-melting performance of gradient gas diffusion layer in proton exchange membrane fuel cell

In this study, a three-dimensional model was established using the lattice Boltzmann method (LBM) to study the internal ice melting process of the gas diffusion layer (GDL) of the proton exchange membrane fuel cell (PEMFC). The single-point second-order curved boundary condition was adopted. The effects of GDL carbon fiber number, growth slope of the number of carbon fibers and carbon fiber diameter on ice melting were studied. The results were revealed that the temperature in the middle and lower part of the gradient distribution GDL is significantly higher than that of the no-gradient GDL. With the increase of the growth slope of the number of carbon fiber, the temperature and melting rate gradually increase, and the position of the solid-liquid interface gradually decreases. The decrease in the number of carbon fibers has a similar effect as the increase in the growth slope of the number of carbon fibers. In addition, as the diameter of the carbon fiber increases, the position of the solid-liquid interface gradually decreases first and then increases.     

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.     

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.     

Water permeation analysis on gas diffusion layers of proton exchange membrane fuel cells for Teflon-coating annotation

This study presents an analysis of water permeation of a polytetrafluoroethylene (PTFE)-coated gas diffusion layer (GDL) to determine the influence of hydrophobic treatment on the GDL for diagnosis of water flooding. It is found that the behaviour of water drainage is controlled by the pore configuration instead of the hydrophobicity in GDL. Better water drainage is achieved by the action of the Teflon coating in modulating the GDL pore configuration to give both a larger average pore size and a wider distribution of pore size. The results show that water penetration through the GDL must overcome a threshold surface tension defined by the largest pore range. A 30 wt.% PTFE coating of a GDL is shown to generate a satisfactory pore configuration, explaining the improved cell polarization performance with a lower driven pressure (∼1.91 kPa) and a higher rate of water drainage.     

Gas diffusion through differently structured gas diffusion layers of PEM fuel cells

Proton exchange membrane fuel cell (PEMFC) gas diffusion layers (GDLs) play important parts in diffusing gas, discharging liquid water, and conducting electricity, etc. When liquid water is discharged through GDL to gas channel, there will be some pores of GDLs occupied by liquid water. In this study, based on a one-dimensional model, the distribution of liquid water phase saturation is analyzed for different GDL structures including GDL with uniform porosity, GDL with sudden change porosity (GDL with microporous layer (MPL)) and GDL with gradient porosity distribution. The effect on gas diffusion of the changes of porosity and liquid saturation due to water remaining in GDL pores is calculated. The conclusions are that for uniform porosity GDL, the gas diffusion increases with the increase of porosity and contact angle and increases with the decrease of the thickness of GDL; for GDL with MPL, the larger the MPL porosity and the thinner the MPL thickness are, the stronger the gas diffusion is; for gradient change porosity GDL with the same average equivalent porosity, the larger the porosity gradient is, the more easily the gas diffuses. The optimization for GDL gradient structure shows that the GDL with a linear porosity distribution of 0.4x+0.4$0.4x+0.4$ is the best of the computed cases.     

Modeling the original and cyclic compression behavior of non-woven gas diffusion layers for fuel cells

It is established that the compression behavior of gas diffusion layers (GDL) is dependent on the level of the mechanical stress it experienced during its lifetime. As a matter of fact, every cycle of compression induces damages in the GDL, including fibers breakage and/or their spatial reorganization. As observed in the experimental work, the first cycle of compression of GDLs as received from the suppliers is already altered by a previous compression that is applied during the manufacturing process. This paper then presents a model able to predict the cyclic behavior of GDL, considering the existence of this compressive stress applied during the manufacturing process. The experimental mechanical properties of the three main types of non-woven GDL (rolls, sheets and felts) were first measured and then predicted using the proposed model, thereby allowing to separate the influences of the manufacturing process, the type of fibers, the presence of a micro-porous layer and a hydrophobic treatment on the GDL.     

Thermal and electrochemical performance analysis of a proton exchange membrane fuel cell under assembly pressure on gas diffusion layer

In this study, the effect of clamping pressure on the performance of a proton exchange membrane fuel cell (PEMFC) is investigated for three different widths of channel. The deformation of gas diffusion layer (GDL) due to clamping pressure is modeled using a finite element method, and the results are applied as inputs to a CFD model. The CFD analysis is based on finite volume method in non-isothermal condition. Also, a comparison is made between three cases to identify the geometry that has the best performance. The distribution of temperature, current density and mole fraction of oxygen are investigated for the geometry with best performance. The results reveal that by decreasing the width of channel, the performance of PEMFC improves due to increase of flow velocity. Also, it is found that intrusion of GDL into the gas flow channel due to assembly pressure deteriorates the PEMFC performance, while decrease of GDL thickness and GDL porosity have smaller effects. It is shown that assembly pressure has a minor effect on temperature profile in the membrane-catalyst interface at cathode side. Also, assembly pressure has a significant effect on ohmic and concentration losses of PEMFC at high current densities.     

Effects of compression on mechanical integrity,gas permeability and thermal stability of gas diffusion layers with/without sealing gaskets

The compressibility, the gas permeability and the thermal stability of 4 commercially available uncoated GDLs (or carbon substrates) and MPL-coated GDLs are, with/without Teflon and silicone sealing gaskets, investigated before and after performing ex-situ compression tests mimicking the compressive stresses within the fuel cell. The results show that the gas permeability of the tested GDLs are impacted more with Teflon gaskets than with the silicone gaskets and this is due to the lower stiffness of the former gaskets. Likewise, the GDLs are more deformed with the Teflon gaskets than with silicone gaskets and this is due to the same reason mentioned above. The thermogravimetric analysis (TGA) data suggests that the bare carbon substrates, unlike the MPL-coated GDLs, lose up to 40% of the PTFE material after the compression test either with or without sealing gaskets.     

Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 1. Wettability (internal contact angle to water and surface energy of GDL fibers)

This is the first in a series of papers in which we present state-of-the-art methods demonstrated at Case for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Most of the methods used today for measuring wettability properties of GDLs are related to the external contact angle to water. The external contact angle however does not describe adequately capillary forces acting on the water inside the GDL pores. We show as well that the direct method of estimation of the internal contact angle using goniometry on micrographs is impractical. We propose and describe in this paper a method for estimating the internal contact angle to water and the surface energy of hydrophobic and hydrophilic gas diffusion media. The method was applied to GDLs having different contents of hydrophobic agent and carbon types. The method can be applied separately to different components of the GDL including macro-porous substrates and micro-porous layers. The uncertainty estimates using this method are usually within 3% of the measured value.