Degradation of gas diffusion layers through repetitive freezing

This work investigates the degradation of an individual gas diffusion layer (GDL) by repeated freezing cycles. The pore size distribution, gas permeability, surface structure, and contact angle on the surface of the GDL were measured in four different types of GDL: SGL paper with a microporous layer (MPL); SGL paper with 5 wt% of polytetrafluoroethylene (PTFE) loading; Toray paper without PTFE loading; and Toray paper with 20 wt% of PTFE loading. After repeated freezing cycles, the porosity of the GDL without PTFE was reduced by 27.2% due to the volumetric expansion of the GDL. The peak of the log differential intrusion moved toward a smaller pore diameter slightly because of the repeated freezing process. The crack of the MPL increased in its width and length after repeated freezing cycles. The through-plane gas permeability of the GDL with the MPL doubled after repeated freezing cycles due to the growth of the crack in the MPL, but was very small for the GDLs with Toray paper. Besides, the GDLs with PTFE loading showed a relatively larger decrease in the contact angle on the surface than the GDL without PTFE loading due to the separation of PTFE from the carbon fiber during the repeated freezing process.     

Lattice Boltzmann simulations of anisotropic permeabilities in carbon paper gas diffusion layers

The multiple-relaxation-time (MRT) lattice Boltzmann method (LBM) with multi-reflection solid boundary conditions is used to study anisotropic permeabilities of a carbon paper gas diffusion layer (GDL) in a fuel cell. The carbon paper is reconstructed using the stochastic method, in which various porosities and microstructures are achieved to simulate different samples. The simulated permeability and tortuosity show anisotropic characteristics of the reconstructed carbon papers with in-plane permeability higher than through-plane, and in-plane tortuosity lower than through-plane. The calculated permeabilities are in good agreement with existing measurements. The relationship between the permeability and the porosity is fitted with empirical relations and some fitting constants are determined. Furthermore, the obtained relationship of tortuosity and porosity is used in a fractal model for permeabilities. The results indicate that the fractal model and the Kozeny–Carman equation provide similar predictions on the through-plane permeability of the carbon paper GDL.     

Effect of weave tightness and structure on the in-plane and through-plane air permeability of woven carbon fibers for gas diffusion layers

In this study, woven gas diffusion layers (GDLs) with varying weave type and tightness are investigated. Plain and twill weave patterns were manufactured in-house. The in-plane and through-plane air permeability of the woven samples were tested, and mercury intrusion porosimetry (MIP) tests were performed to study the pore structure. It was found that the twill weave has a higher permeability than the plain weave, which is consistent with literature. Like non-woven carbon papers, woven GDLs have higher in-plane permeability than through-plane permeability; however it has been shown that it is possible to manufacture a GDL with higher through-plane permeability than in-plane permeability. It was also concluded that the percentage of macropores in the weave is the driving factor in determining the through-plane air permeability. This work lays the groundwork for future studies to attempt to characterize the relationship between the weave structure and the air permeability in woven GDLs.     

Numerical study on permeability of gas diffusion layer with porosity gradient using lattice Boltzmann method

In this paper, the lattice Boltzmann method (LBM) has been employed to explore the permeability and internal fluid flow behavior of the gas diffusion layer (GDL). Three different non-uniform porosity distributions are designed as linear type, stepped type, and transitional type and compared with constant porosity samples. Results show that the linear porosity gradient distribution leads to higher permeability values compared with the other two types. For samples with total porosity of 0.65 and 0.75, optimal porosity gradient distributions bring about an enhancement of permeability have been found. The impact of porosity gradient distribution on the velocity field is presented. Dependencies of permeability with porosity and tortuosity are demonstrated through several fitted equations.     

Analytic determination of the effective thermal conductivity of PEM fuel cell gas diffusion layers

Accurate information on the temperature field and associated heat transfer rates are particularly important in devising appropriate heat and water management strategies in proton exchange membrane (PEM) fuel cells. An important parameter in fuel cell performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL). Estimation of the effective thermal conductivity is complicated because of the random nature of the GDL micro structure. In the present study, a compact analytical model for evaluating the effective thermal conductivity of fibrous GDLs is developed. The model accounts for conduction in both the solid fibrous matrix and in the gas phase; the spreading resistance associated with the contact area between overlapping fibers; gas rarefaction effects in microgaps; and salient geometric and mechanical features including fiber orientation and compressive forces due to cell/stack clamping. The model predictions are in good agreement with existing experimental data over a wide range of porosities. Parametric studies are performed using the proposed model to investigate the effect of bipolar plate pressure, aspect ratio, fiber diameter, fiber angle, and operating temperature.     

Feasibility of periodic surface models to develop gas diffusion layers: A gas permeability study

A geometric modeling scheme called periodic surface model (PS) is used to construct three dimensional (3D) models of a gas diffusion layer's (GDL) microstructure, which allows for rapid model construction and modification of representative volume elements (RVE) with embedded periodic boundary conditions. The reconstructed PS models are optimized with the help of the genetic algorithm embedded in MATLAB to generate models with refined mesh for computational fluid dynamics (CFD) analysis. The GDL geometry is built in ANSYS/ICEM CFD, automatically, using a customized code that couples MATLAB and ICEM. To verify the validity of the suggested modeling approach the microstructures of the GDLs with different porosity and fiber orientation are generated and the in-plane and through-plane permeability and tortuosity are calculated using ANSYS/FLUENT software. The numerically predicted values of in-plane and through-plane permeability are compared to experimental measurements. Using the genetic algorithm significantly decreases the fibers intersection volume in the RVE, especially as porosity decreases. It has been found that the tortuosity of the GDL is a function of the spatial orientation of the fibers in the RVE, when the fibers are at a small angle, the in-plane tortuosity can be higher than the through-plane tortuosity.     

Utilization of 3D printed carbon gas diffusion layers in polymer electrolyte membrane fuel cells

3D printing and carbonisation is used to produce designed gas diffusion layer materials for polymer electrolyte membrane fuel cells (PEMFC). Using a desktop UV 3D printer, designed porous microstructures are printed with micro and macro-scale features. Successful improvement of the pyrolysis process maintains the structural accuracy during carbonisation, reducing the material to electrically conductive carbon. The size of the material allows for testing in a lab scale fuel cell with 1.5 × 1.5 cm electrode size, which shows lower but interesting electrochemical performance (power density of 205 mW cm^?2). Challenges associated with integration of a 3D printed structure into a membrane electrode assembly are highlighted, including the low open circuit voltage caused by large amounts of membrane deformation and subsequent hydrogen crossover. This study shows that it is possible to design and manufacture a gas diffusion layer for fuel cells. Numerical simulation on the new GDL structure shows that advective-diffusive transport of oxygen in the 3D printed design is superior to conventional carbon paper. This study serves as the first attempt to implement 3D printed microstructures as GDL into PEMFC.     

Modeling of composite fibrous porous diffusion media

To engineer the desired properties of fibrous porous media, a parametric modeling approach is needed to support the rational design of the materials before the fabrication. In this study, we propose a methodology that enables the accurate representation of three-dimensional (3D) microstructures of fibrous porous media and prediction of their transport properties. Toray TGP-H-060 gas diffusion layer (GDL) is selected as an example to demonstrate the feasibility of the suggested design methodology. The detailed microstructure of the GDL with the inclusion of locally distributed binder is constructed using an extended periodic surface (PS) modeling technique. A 3D morphological approach is taken to create the binder distribution within the fibrous microstructure. Transport properties including permeability, relative diffusivity, and tortuosity and local structure characteristics of the generated microstructure, under different binder loading are calculated. It is shown that the detailed model of the fiber-binder composite has a strong influence on the predicted properties.     

Diffusion parameter correlations for PEFC gas diffusion layers considering the presence of a water-droplet

A polymer electrolyte fuel cell (PEFC) produces electrical energy according to the electrochemical reactions carried out inside the cell. During the energy conversion, water molecules are also produced at the cathode side, which affects the gas diffusion layer (GDL) diffusion parameters. The generated water-drops from the reaction may give partial or total blockage of the reactant gases and also material oxidation. The mentioned phenomena influence the performance of the PEFCs. This paper aims to describe and quantify the impact on diffusion parameters of GDLs when the size of the formed water-drops inside the layer is varying. This study considers digitally generated GDLs, in which the porosity, gas-phase tortuosity and diffusibility are studied. The fluid flow behavior through the three-dimensional porous domain representing the GDL is obtained with the lattice Boltzmann method (LBM). Depending on the water-drop size, the impact of the mentioned parameters can be computed. For the current study a spherical water-drop whose radius varies between the 15 and 35% of the size of the domain was considered. The studied parameters showed a dependency of the water-drop radius, each changing independently and several correlations to predict the behavior of the mentioned diffusion transport parameters are proposed.     

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.     

Evaluating breakthrough pressure in gas diffusion layers of proton exchange membrane fuel cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer （GDL） of a pro- ton exchange membrane fuel cell （PEMFC）. An ex-situ testing was conducted on a transparent test cell to visu- alize the water droplet formation and detachment on the surface of different types of GDLs through a CCD cam- era. The breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. The breakthrough pressure was found to be different for the GDLs with different porosities and thick- nesses. The equilibrium pressure, which is defined as the minimum pressure required maintaining a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses. The breakthrough pressure increases for the thick GDL with smaller micro-pore size.     

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.     

Microporous layer coated gas diffusion layers for enhanced performance of polymer electrolyte fuel cells

The influence of microporous layer (MPL) design parameters for gas diffusion layers (GDLs) on the performance of polymer electrolyte fuel cells (PEFCs) was clarified. Appropriate MPL design parameters vary depending on the humidification of the supplied gas. Under low humidification, decreasing both the MPL pore diameter and the content of polytetrafluoroethylene (PTFE) in the MPL is effective to prevent drying-up of the membrane electrode assembly (MEA) and enhance PEFC performance. Increasing the MPL thickness is also effective for maintaining the humidity of the MEA. However, when the MPL thickness becomes too large, oxygen transport to the electrode through the MPL is reduced, which lowers PEFC performance. Under high humidification, decreasing the MPL mean flow pore diameter to 3 μm is effective for the prevention of flooding and enhancement of PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Both reduction of the MPL thickness penetrated into the substrate and increase in the PTFE content to 20 mass% enhance the ability of the MPL to prevent flooding.     

Comparing through-plane diffusibility correlations in PEFC gas diffusion layers using the lattice Boltzmann method

One of the key elements in a polymer electrolyte fuel cell (PEFC) is the gas diffusion layer (GDL). The GDL offers mechanical support to the cell and provides the medium for diffusing the reactant gases from the flow plates to the electrolyte enabling the electrochemical reactions, and therefore the energy conversion. At the same time, it has the task of transporting the electrons from the active sites, near to the electrolyte, towards the flow plates.Describing the fluid flow and mass transport phenomena through the GDLs is not an easy task not only because of their complex geometries, but also because of these phenomena occur at microscale levels. Most of the PEFC models at cell scale make assumptions about certain microscale transport parameters, assumptions that can make a model less close to the reality. The purpose of this study is to analyze five different proposed correlations to estimate the through-plane (TP) diffusibility of digitally created GDLs and using lattice Boltzmann (LB) models. The correlations are ranked depending on their precision, accuracy and symmetry. The results show that the best estimation is given when the porosity and gas-phase tortuosity are taken into account in the correlation.     

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.     

Experimental investigation of current distributions in a direct methanol fuel cell with various gas diffusion layers and Nafion membranes

The influences of the gas diffusion layer (GDL) properties on the current distributions of a direct methanol fuel cell are investigated. Cathode GDLs with different hydrophobicity/hydrophilicity, air permeability, microporous layer (MPL), thickness, and texture properties are examined. Among the GDLs examined, a thin hydrophobic GDL with an MPL has the most homogeneous current distribution, which is primarily ascribed to the better water management capabilities of the cathode GDL properties. The differences in the current distribution among the different GDLs are more apparent when the air flow rate and loaded current are lower. The effect of the membrane thickness on the current distributions is also investigated. Among the membranes examined, Nafion^® 112 has different current distributions from the others, whereas there is no noticeable difference between the current distributions with Nafion^® 115 and Nafion^® 117. The current distribution with Nafion^® 112 is most affected by the enhanced methanol crossover and the high mixed potential.     

Water permeation through gas diffusion layers of proton exchange membrane fuel cells

Water transport through gas diffusion layer of proton exchange membrane fuels cells is investigated experimentally. A filtration cell is designed and the permeation threshold and the apparent water permeability of several carbon papers are investigated. Similar carbon paper with different thicknesses and different Teflon loadings are tested to study the effects of geometrical and surface properties on the water transport. Permeation threshold increases with both GDL thickness and Teflon loading. In addition, a hysteresis effect exists in GDLs and the permeation threshold reduces as the samples are retested. Moreover, several compressed GDLs are tested and the results show that compression does not affect the breakthrough pressure significantly. The measured values of apparent permeability indicate that the majority of pores in GDLs are not filled with water and the reactant access to the catalyst layer is not hindered.     

Activity and durability of electrocatalytic layers with low platinum loading prepared by magnetron sputtering onto gas diffusion electrodes

This article presents development of magnetron sputtering technology for catalytic Pt coating production. Low Pt-loaded gas diffusion electrodes were prepared by a single-step magnetron sputtering in different regimes. Pt and Pt-carbon films (95–97 wt% Pt) were deposited directly onto the gas diffusion layers (GDLs): carbon paper (Sigracet® 39 BC) and carbon cloths (GDL-CT, ELAT® LT 1400 W). A bipolar pulsed direct current (DC) magnetron sputtering with pulsed frequency 100 kHz was used as the method for catalysts deposition. Total Pt loading in the thin films varied in the range 0.04–0.13 mg*cm⁻². Obtained catalysts onto GDLs were investigated by scanning/transmission electron microscope (S/TEM), X-ray Absorption Near Edge Structure (XANES) and potentiodynamic methods. The deposited catalysts had a high electrochemical surface area (ECSA) and stability determined by the durability stress test (DST) method. The highest ECSA was obtained with Pt target in the pulsed (100 kHz) sputtering mode. This ECSA values were rather stable during 3000 cycles of stress-tests versus 500 cycles for Pt deposited by polyol method on the same gas diffusion layers.     

Carbon black / PTFE composite hydrophobic gas diffusion layers for a water-absorbing porous electrolyte electrolysis cell

The characteristics of a water-absorbing porous electrolyte electrolysis cell, in which pressurized water is injected directly into the electrolyte layer, are investigated. High water support force is required for the gas diffusion layer (GDL) in this novel cell design, and therefore here we report a new type of hydrophobic GDL comprising an acetylene black (AB) and poly(tetrafluoroethylene) (PTFE) composite film. The method of preparation of the AB/PTFE slurry, film formation methods, and the AB/PTFE weight ratio were investigated and optimized. The ball-milling and transfer method were suitable for preparing uniform AB/PTFE slurry and successfully covering AB/PTFE film without any cracks on micro-porous layer coated carbon paper, respectively. An investigation about different PTFE weight ratios against AB from 0.1 to 6 showed a serious trade-off character between electrical resistance R, gas permeability V′, and water support force P_lim. The 1/2.5 of AB/PTFE weight ratio was most optimal, which showed to have most equivalent R (2.5 Ω cm^?2), V′(136 mL atm^?1 cm^?2 min^?1), and P_lim (0.25 MPa). We also confirmed that fabricated GDL with optimal condition was worked as the blocking layer against water injected through electrolyte layer and pressurized by nitrogen gas, and as gas-permeation layer for generated hydrogen gas in water electrolysis test.     

Roughness effects of gas diffusion layers on droplet dynamics in PEMFC flow channels

Water management remains one of the major challenges in optimising the performance of PEMFCs, in which liquid accumulation and removal in gas diffusion layers (GDLs) and flow channels should be addressed. Here, effects of GDL surface roughness on the water droplet removal inside a PEMFC flow channel have been investigated using the Volume of Fluid method. Rough surfaces are generated according to realistic GDL properties by incorporating RMS roughness and roughness wavelength as the main characteristic parameters. Droplet dynamics including emergence, growth, detachment, and removal in flow channels with various airflow rates are simulated on rough substrates. The influences of airflow rate on droplet dynamics are also discussed by comparing the detachment time and droplet morphology. The liquid removal efficiency subject to different surface roughness parameters is evaluated by droplet detachment time and elongation, and regimes of detachment modes are identified based on the droplet breakup location and detachment ratio. The results suggest that rough surfaces with higher RMS roughness can facilitate the removal of liquid inside flow channel. Whilst surface roughness wavelength is found less significant to the liquid removal efficiency. The results here provide qualitative assessments on identifying the key surface characteristics controlling droplet motion in PEMFC channels.