Fuel Cell Gas Diffusion Layers Research Articles

Freeze-dried solid foams prepared from carbon nanotube aqueous suspension: Application to gas diffusion layers of a proton exchange membrane fuel cell

Freeze-dried macroporous solid foams were prepared from the multi-walled carbon nanotube (MWCNT) aqueous suspensions dispersed by chitosan. Thin film shaped CNT solid foams were prepared, and applied to the gas diffusion layers (GDLs) of a laboratory scale proton exchange membrane fuel cell (PEMFC). It was demonstrated that the prepared carbon foams in this study were useful to a fuel cell GDL material. The prepared cell performances were fairly comparable to the cell prepared with conventional carbon paper for GDL material. The microstructures of the prepared carbon foams were found to affect on the PEMFC performances. It was suggested that the interconnected carbon networks formed during the freezing step closely link to the cell performances. Hence, the defection of the interconnected microstructure lead degradation of the GDL quality. The impedance measurement made clear that the prepared foam materials were also advantageous for reducing the ohmic resistance in PEMFC assembly. The kinetic resistance values and the thermal conductive characteristics suggested that the freezing process would also control the degree of overlaps among single CNTs in a freeze-dried bulk that influenced on the electrochemical properties.

Atomic Force Microscopy Investigation of Polymer Fuel Cell Gas Diffusion Layers before and after Operation

The ageing of microporous layers (MPL) of fuel cell gas diffusion layers has been analyzed using a special atomic force microscopy technique. The mean local adhesion force on the surface as well as the energy dissipation on the same area has been used as a measure for changes of surface properties corresponding to ageing of the MPL. The samples have been studied before and after fuel cell operation. In all cases, the changes due to operation are stronger on the cathode compared to the anode. The results from measurements under comparably dry and wet conditions are consistent with an increased loss of polytetrafluoroethylene (PTFE) at the cathode which leads to a hydrophobicity loss.

Estimation of water distribution and degradation mechanisms in polymer electrolyte membrane fuel cell gas diffusion layers using a 3D Monte Carlo model

Abstract Understanding of both water management in PEM fuel cells and degradation mechanisms of the gas diffusion layer (GDL) and their mutual impact is still at least incomplete. Different modelling approaches contribute to gain deeper insight into the processes occurring during fuel cell operation. Considering the GDL, the models can help to obtain information about the distribution of liquid water within the material. Especially, flooded regions can be identified, and the water distribution can be linked to the system geometry. Employed for material development, this information can help to increase the life time of the GDL as a fuel cell component and the fuel cell as the entire system. The Monte Carlo (MC) model presented here helps to simulate and analyse the water household in PEM fuel cell GDLs. This model comprises a three-dimensional, voxel-based representation of the GDL substrate, a section of the flowfield channel and the corresponding rib. Information on the water distribution within the substrate part of the GDL can be estimated.

Droplet pinning by PEM fuel cell GDL surfaces

In this work, side view images of liquid–gas–solid interfaces are observed during the evaporation of liquid water droplets on various commercially available untreated gas diffusion layers (GDLs). The change in contact diameter as a function of evaporative volume loss is measured to quantify the unpinning rates of micro-sized droplets. This contact diameter pinning behaviour during evaporation is correlated to the material topography, which is quantified through profilometry measurements. The carbon fibre paper with the smallest average roughness (15 μm) exhibits the strongest degree of pinning (unpinning at a rate of 0.13 mm/μL). Higher average surface roughnesses for felt (30 μm) and cloth yarn (32 μm) result in higher unpinning rates, 0.21 mm/μL and 0.19 mm/μL, respectively. These results indicate that common GDL materials exhibit Cassie–Baxter wetting behaviour, and reduced GDL roughness promotes droplet pinning. The material-specific droplet contact diameter progression should be considered during GDL selection for polymer electrolyte membrane (PEM) fuel cells. This work provides insight into the effect of GDL material properties on gas channel water management, as water droplets are expected to experience similar pinning to that observed in this work within the cathode gas channels of a PEM fuel cell.

Threshold Fine-Tuning and 3D Characterisation of Porous Media Using X-ray Nanotomography

A common challenge in the X-ray nanotomography of porous media, such as fuel cell gas diffusion layers (GDLs), is to binarize nanotomography greyscale images in order to differentiate between solids and voids for structural characterisation and numerical flow analysis. In the process threshold determination is critical. This paper presents a study on determination of and fine-tuning threshold value based on comparison of material porosity and average fibre diameter obtained from nanotomography images with porosity data from density experiments and average fibre diameter achieved from scanning electron microscopy images respectively. The more accurate 3D reconstructed model is then used to calculate pore size distribution and average pore size, while the gas permeability of the representative 3D binary images are calculated using a single phase Lattice Boltzmann (LB) model in the D3Q19 regime. Keywords: X-ray nanotomography, gas diffusion layer (GDL), fuel cell, porous material, threshold, Lattice Boltzmann (LB)

AFM Investigation of PEM Fuel Cell Membranes and Gas Diffusion Layers

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Influence of threshold variation on determining the properties of a polymer electrolyte fuel cell gas diffusion layer in X-ray nano-tomography

Morphological parameters of a 3D binary image of a porous carbon gas diffusion layer (GDL) for polymer electrolyte fuel cells (PEFC) reconstructed using X-ray nano-tomography scanning have been obtained, and influence of small alterations in the threshold value on the simulated flow properties of the reconstructed GDL has been determined. A range of threshold values with 0.4% increments on the greyscale map have been applied and the gas permeability of the binary images have been calculated using a single-phase lattice Botlzmann model (LBM), which is based on the treatment of nineteen velocities in the three dimensional domain (D3Q19). The porosity, degrees of anisotropy and the mean pore radius have been calculated directly from segmented voxel representation. A strong relationship between these parameters and threshold variation has been established. These findings suggest that threshold selection can significantly affect some of the flow properties and may strongly influence the computational simulation of micro and nano-scale flows in a porous structure.

Characterisation of wettability in gas diffusion layer in proton exchange membrane fuel cells

This paper presents a detailed study of the wetting properties of three fuel cell gas diffusion layers (GDLs) having different morphologies and different contents of hydrophobic agent. An internal contact angle to water at temperature representative of PEM fuel cell operating conditions was directly obtained using the Washburn method with 66 ° C water as test liquid. These results show that the surface of the carbon fibres is hydrophilic under fuel cell operating conditions. Surface energy values of the GDL fibres, determined by a combination of the Washburn method and the Owens–Wendt two parameters theory, were found to be low, indicating that GDLs are wet very poorly by most liquids. About 40% of the total surface free energy values was related to a polar component allowing dipole–dipole and hydrogen bonding interactions with water. The origin of this polar character is discussed.

Numerical determination of the effective thermal conductivity of fibrous materials. Application to proton exchange membrane fuel cell gas diffusion layers

The knowledge of physical properties, such as the thermal conductivity, plays an important role in the management of the heat transfer in fibrous materials like PEFC gas diffusion layers. Measurement of thermal conductivity by experimental means is not easy (due to the anisotropy and the high porosity), therefore the availability of experimental data is rather limited. In this paper, the numerical determination of the effective thermal conductivity of fibrous materials is investigated using a three-dimensional approach. Two different fiber geometries were studied with randomly generated fiber structures with overlapping and non-overlapping fibers. The corresponding anisotropic thermal conductivities are computed through the solution of the energy transport equation. The results were validated through a comparison with existing experimental data and the influence of different parameters such as fiber orientation, fiber diameter and binding material were investigated.

The effects of wetproofing on the capillary properties of proton exchange membrane fuel cell gas diffusion layers

In the analysis of proton exchange membrane (PEM) fuel cell components, the capillary pressure vs. saturation ( P C ( S L )) curve is an increasingly popular tool for understanding the interaction of liquid water with the porous gas diffusion layer (GDL) material. In this study, hysteretic water/air P C ( S L ) measurements were combined with mercury intrusion porosimetry (MIP) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging to quantify the effects of fluoropolymer loading on GDL samples. Commercially wetproofed carbon fiber papers with 0–40 wt.% Teflon loading were investigated. MIP showed a slight reduction in characteristic pore radii and a significant loss of pore volume at the highest Teflon loading. Water/air P C ( S L ) measurements showed a significant reduction in water wetting between samples with 0 and 5 wt.% Teflon loading, but negligible additional wetproofing at loadings from 10 to 40 wt.%. ToF-SIMS imaging, a technique that is sensitive to monolayer surfaces coverages, found that GDL materials with 5 wt.% Teflon loading displayed nearly complete fluoropolymer coverage on the carbon substrate, confirming P C ( S L ) measurements showing that all of the wetproofing occurs in a narrow range of Teflon loadings. Results for P C ( S L ) measurements were fitted using a bundle-of-capillaries model. The apparent water intrusion contact angles fell between 130° and 133° in the rough Teflonated pore space (regardless of loading), whereas the apparent gas intrusion contact angles fell between 66° and 70° for the same materials.

Effect of hydrophobic gas diffusion layers on the performance of the polymer exchange membrane fuel cell

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different concentrations of hydrophobic agents in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the hydrophobic agent content of the carbon fiber cloth and fuel cell performance. The paper examines the effect of hydrophobic agent content on GDL thickness, contact angle, air permeability, and surface and through-plane resistivity. Carbon fiber cloth is impregnated with hydrophobic agent concentrations of 0, 3, 5, 10, 30, and 50 wt%, and the resulting GDLs are subjected to performance tests. When the test piece area is 25 cm 2, the test temperature 80 °C, the gasket thickness 0.36 mm, and the hydrophobic agent content 5 wt%, a fuel cell using the GDL has a current density of 1430 mA cm −2 at 0.3 V.

Visualization of unstable water flow in a fuel cell gas diffusion layer

Modeling two-phase flow in proton exchange membrane (PEM) fuel cells is hampered by a lack of conceptual understanding of flow patterns in the gas diffusion layer (GDL). In this paper, pore-scale visualizations of water in different types of GDLs were used to improve current understanding of flow and transport phenomena in PEM fuel cells. Confocal microscopy was used to capture the real-time transport of water, and pressure micro-transducers were installed to measure water breakthrough pressures. Three types of fuel cell GDLs were examined: TO series (Toray Corp., Tokyo, Japan), SGL series (SGL Carbon Group, Wiesbaden, Germany), and MRC series (Mitsubishi Rayon Corp., Otake City, Japan). The visualizations and pressure measurements revealed that despite difference in “pore” structures in the three types of GDLs, water followed distinct flow paths spanning several pores with characteristics similar to the “column flow” phenomena observed previously in hydrophobic or coarse-grained hydrophilic soils. The results obtained from this study can aid in the construction of theories and models for optimizing water management in fuel cells.

Effect of carbon fiber cloth with different structure on the performance of low temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different structure in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the structure of the carbon fiber cloth and fuel cell performance.The paper discusses the relationship between fuel cell performance and structure of the carbon fiber cloth, and also examines the effect of the carbon fiber cloth’s thickness, air permeability, surface resistivity, XRD and elemental analysis. Carbon fiber cloth is carbonized at rates of 190, 220, 250, 280, and 310 °C min −1 respectively, and the resulting carbon fiber cloth is tested in cells. When the test piece area is 25 cm 2, the test temperature 40 °C, the gasket thickness 0.36 mm, and the carbonization rate 280 °C min −1, a fuel cell using the carbon fiber cloth achieves a current density of 1968 mA cm −2 and a maximum power density of 633 mW cm −2 at 0.3 V.

Lattice-Boltzmann Simulations of Multiphase Flows in PEM Fuel Cell GDLs and Micro-channels

The lattice Boltzmann (LB) method is employed to simulate single and multiphase flow through porous gas-diffusion-layer (GDL) of a Proton Exchange Membrane Fuel Cell (PEMFC). The porous layer is composed of layers of fibers. The structure is reconstructed numerically and validated by other published data on porosity and pore-size distributions. The in-plane gas-permeability is greater than the through-plane values and has greater sensitivity to fiber orientations. The permeability decreases as the GDL is compressed. When liquid is introduced, multiple droplets emerge from the other side of the GDL through surface pores. On hydrophobic GDL, the liquid advancing inside the GDL is in the form of several interconnected streams leading to surface openings. As the hydrophobicity is increased the liquid break-through occurs at lower saturation.

Effects of Fabricated Gas Diffusion Layers with Different Reinforce Materials in Proton Exchange Membrane Fuel Cell (PEMFC)

This study uses different carbon fiber reinforcing materials impregnated with different phenolic resin concentrations to produce composite materials for use as fuel cell gas diffusion layer (GDL) s...

Characterisation of mechanical behaviour and coupled electrical properties of polymer electrolyte membrane fuel cell gas diffusion layers

Local compression distribution in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC) and the associated effect on electrical material resistance are examined. For this purpose a macroscopic structural material model is developed based on the assumption of orthotropic mechanical material behaviour for the fibrous paper and non-woven GDLs. The required structural material parameters are measured using depicted measurement methods. The influence of GDL compression on electrical properties and contact effects is also determined using specially developed testing tools. All material properties are used for a coupled 2D finite element simulation approach, capturing structural as well as electrical simulation in combination. The ohmic voltage losses are evaluated assuming constant current density at the catalyst layer and results are compared to cell polarisation measurements for different materials. The results show that the largest part of the polarisation difference found between roll-good and batch type materials with wide channel flowfields is well captured by the simulation and is due to additional electrical losses in the locally low compressed GDL. Thus, for the first time a broader understanding of the significant performance impact of diffusion layer mechanical properties is generated. However, at higher loads an interaction of compression with electrical and additional heat and mass transport effects occurs, which will be included in the next part of the study. This part is limited to structural mechanics and coupled electrical transport effects.

On the dynamics and control of through-plane water distributions in PEM fuel cells

We provide a semi-analytic solution to simplify an experimentally validated numeric realization of a two-phase, reaction–diffusion, distributed parameter model of the through-plane water distributions as they evolve inside polymer electrolyte membrane (PEM) fuel cell gas diffusion layers. The semi-analytic solution is then analyzed for stability and to gain insight into the dynamics of the equilibrium (steady-state) water distributions. Candidate distributions for vapor and liquid water are then identified which allow maximum membrane hydration while simultaneously avoiding voltage degradation that results from anode liquid water accumulation (flooding). The desired anode water distributions could be maintained via control of the anode channel conditions (boundary value control) with the ultimate goal to maximize the hydrogen utilization and prolong fuel cell life.

Numerical and microfluidic pore networks: Towards designs for directed water transport in GDLs

Based on a pore network representation of porous media, numerical and experimental methods are employed to explore the design of gas diffusion media in order to control and direct water transport. Randomized pore networks with structured biasing in both diagonal and radial directions show a marked influence on liquid water transport through the two-dimensional media. Radial biasing is shown to have the beneficial effect of decreasing the network saturation, which is a desirable quality in fuel cell gas diffusion layers. Similar flow patterns are obtained from both numerical simulations and experiments, and it is found that relatively small radial biasing can yield a 43% decrease in average saturation compared to pore networks without a prescribed radial gradient.

Effect of carbon fiber paper made from carbon felt with different yard weights on the performance of low temperature proton exchange membrane fuel cells

This study employed fuel cell gas diffusion layers (GDLs) consisting of carbon fiber paper made from carbon fiber felt with different yard weights in proton exchange membrane fuel cells (PEMFCs), and investigated the relationship between the yard weight of the carbon fiber paper and the fuel cell performance and thickness of the gasket. In this paper we discuss the relationship between carbon fiber felt with different yard weights and fuel cell performance and also explore the effect of carbon fiber paper thickness, air permeability, surface resistivity, and structural study. We focused on the material used for the gas diffusion layer in this study. Carbon fiber paper made in-house in this study contained 10 wt% (all percentages are by weight unless otherwise noted) phenolic resin. When the tested area was 25 cm 2, the test temperature was 40 °C, the gasket thickness was 0.06 mm, and the yard weight 70 g m −2, fuel cell current density was 1968 mA cm −2 at a load 0.3 V. When the gasket thickness was 0.36 mm and yard weight was 190 g m −2, fuel current density was 1710 mA cm −2 at a load of 0.3 V.

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.