Development of a novel hydrophobic/hydrophilic double micro porous layer for use in a cathode gas diffusion layer in PEMFC

In this study, a gas diffusion layer (GDL) was modified to improve the water management ability of a proton exchange membrane fuel cell (PEMFC). We developed a novel hydrophobic/hydrophilic double micro porous layer (MPL) that was coated on a gas diffusion backing layer (GDBL). The water management properties, vapor and water permeability, of the GDL were measured and the performance of single cells was evaluated under two different humidification conditions, R.H. 100% and 50%. The modified GDL, which contained a hydrophilic MPL in the middle of the GDL and a hydrophobic MPL on the surface, performed better than the conventional GDL, which contained only a single hydrophobic MPL, regardless of humidity, where the performance of the single cell was significantly improved under the low humidification condition. The hydrophilic MPL, which was in the middle of the modified GDL, was shown to act as an internal humidifier due to its water absorption ability as assessed by measuring the vapor and water permeability of this layer.     

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.     

Through-plane thermal conductivity of the microporous layer in a polymer electrolyte membrane fuel cell

Understanding the thermal properties of the microporous layer (MPL) is critical for accurate thermal analysis and improving the performance of proton exchange membrane (PEM) fuel cells operating at high current densities. In this study, the effective through-plane thermal conductivity and contact resistance of the MPL have been investigated. Gas diffusion layer (GDL) samples, coated with 5%-wt. PTFE, with and without an MPL are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Effective thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15 bar at 0.30 W/m°K and 55 μm, respectively. The effective thermal conductivity of the GDL substrate containing 5%-wt. PTFE varied from 0.30 to 0.56 W/m°K as compression was increased from 4 to 15 bar. As a result, GDL containing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL.     

Visualization of the water distribution in perforated gas diffusion layers by means of synchrotron X-ray radiography

Perforated gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs) were investigated by means of in-situ synchrotron X-ray radiography during operation. We found a strong influence of perforations on the water distribution and transport in the investigated Toray TGP-H-090 GDL. The water occurs mainly around the perforations, while the holes themselves show varying water distributions. Some remain dry, while most of them fill up with liquid water after a certain period or might serve as drainage volume for effective water transport.     

A numerical investigation of the effects of GDL compression and intrusion in polymer electrolyte fuel cells (PEFCs)

The purpose of this work is to numerically investigate the effects of non-uniform compression of the gas diffusion layer (GDL) and GDL intrusion into a channel due to the channel/rib structure of the flow-field plate. The focus is placed on accurately predicting two-phase transport between the compressed GDL near the ribs and uncompressed GDL near the channels, and its associated effects on cell performance. In this paper, a GDL compression model is newly developed and incorporated into a comprehensive three-dimensional, two-phase PEFC model developed earlier. To assess solely the effects of GDL compression and intrusion, the new fuel cell model is applied to a simple single-straight channel fuel cell geometry. Numerical simulations with different levels of GDL compression and intrusion are carried out and simulation results reveal that the effects of GDL compression and intrusion considerably increase the non-uniformity, particularly, the in-plane gradient in liquid saturation, oxygen concentration, membrane water content, and current density profiles that in turn results in significant ohmic and concentration polarizations. The present three-dimensional GDL compression model yields realistic species profiles and cell performance that help to identify the optimal MEA, gasket, and flow channel designs in PEFCs.     

A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique

A novel ultrasonic-spray method for preparing gas diffusion electrodes (GDEs) for proton exchange membrane fuel cell (PEMFC) is described. Platinum (Pt) loaded on Nafion^®-bonded GDEs were prepared by the ultrasonic-spray method on various commercial woven and non-woven gas diffusion layers (GDLs) at several Pt loadings in the range of 0.40-0.05 mg cm⁻². The ultrasonic-sprayed GDEs were tested and compared to commercial and hand-painted GDEs. It was found that the GDEs prepared by the ultrasonic-spray method exhibited better performances compared to those prepared by the hand-painting technique, especially at low Pt loadings. GDEs fabricated by the ultrasonic-spray method with a platinum loading of 0.05 mg cm⁻² exhibited a peak power rating of 10.9 W mg⁻¹ compared to 9.8 W mg⁻¹ for hand-painted GDEs. For all experiments using various GDLs, Sigracet SGL 10BC exhibited the best performance with a peak power of 0.695 W cm⁻².     

Controlling gas diffusion layer oxidation by homogeneous hydrophobic coating for polymer electrolyte fuel cells

Reduced production costs and enhanced durability are necessary for practical application of polymer electrolyte fuel cells. There has been a great deal of concern about degradation of the gas diffusion layer located outside the membrane electrode assembly. However, very few studies have been carried out on the degradation process, and no suitable methods for improving the durability of the cell have been found.In this work, the influence on the cell performance and factors involved in the degradation of the gas diffusion layer has been clarified through power generation tests.Long-term power generation tests on single cells for 6000 h were carried out under high humidity conditions with homogeneous and inhomogeneous hydrophobic coating gas diffusion layers. The results showed that the increase in the diffusion overvoltage from the gas diffusion layer could be controlled by the use of a homogeneous coating. Post-analyses indicated that this occurred by controlling oxidation of the carbon fiber.     

Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells

The gas diffusion layer (GDL) is important for maintaining the performance of polymer electrolyte membrane (PEM) fuel cells, as its main function is to provide the cells with a path for fuel and water. In this study, the mechanical degradation process of the GDL was investigated using a leaching test to observe the effect of water dissolution. The amount of GDL degradation was measured using various methods, such as static contact angle measurements and scanning electron microscopy. After 2000 h of testing, the GDL showed structural damage and a loss of hydrophobicity. The carbon-paper-type GDL showed weaker characteristics than the carbon-felt-type GDL after dissolution because of the structural differences, and the fuel cell performance of the leached GDL showed a greater voltage drop than that of the fresh GDL. Contrary to what is generally believed, the hydrophobicity loss of GDL was not caused by the decomposition of polytetrafluoroethylene (PTFE).     

Lifetime prediction of a polymer electrolyte membrane fuel cell via an accelerated startup–shutdown cycle test

To expand commercial applications of polymer electrolyte membrane fuel cells (PEMFCs), the evaluation time for their durability must be shortened. This article provides a straightforward accelerated degradation testing (ADT) procedure for PEMFC for easy and quick implementation of the procedure. The ADT procedure includes statistical modeling of degradation patterns of membrane electrode assemblies (MEAs) in PEMFCs under startup–shutdown cycling conditions. For this purpose, we propose a nonparametric degradation model to describe the nonlinear performance degradation paths of PEMFC MEAs. The analysis results indicate that the nonparametric approach provides more accurate estimates of the observed degradation data than other parametric approaches. Based on the nonparametric degradation model, we suggest a method to predict failure-times under normal operating conditions by estimating the time-scale factor under accelerated operating conditions.     

PEM fuel cell electrodes

The design of electrodes for polymer electrolyte membrane fuel cells (PEMFC) is a delicate balancing of transport media. Conductance of gas, electrons, and protons must be optimized to provide efficient transport to and from the electrochemical reactions. This is accomplished through careful consideration of the volume of conducting media required by each phase and the distribution of the respective conducting network. In addition, the issue of electrode flooding cannot be neglected in the electrode design process. This review is a survey of recent literature with the objective to identify common components, designs and assembly methods for PEMFC electrodes. We provide an overview of fabrication methods that have been shown to produce effective electrodes and those that we have deemed to have high future potential. The relative performances of the electrodes are characterized to facilitate comparison between design methodologies.     

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.     

Coupled mechanical stress and multi-dimensional CFD analysis for high temperature proton exchange membrane fuel cells (HT-PEMFCs)

We use a combined finite element method (FEM)/computational fluid dynamics (CFD) methodology to numerically investigate the effects of gas diffusion layer (GDL) compression/intrusion on the performance of a phosphoric acid-doped polybenzimidazole (PBI) membrane-based high temperature proton exchange membrane fuel cell (HT-PEMFC). Three-dimensional (3-D) FEM simulations are conducted under various displacement clamping conditions to analyze cell deformation characteristics. Then, a multi-dimensional HT-PEMFC CFD model is applied to the deformed cell geometries to study transport and electrochemical processes during HT-PEMFC operations. Our numerical simulation results reveal that the maximum stresses in the deformed GDLs always occur near the edge of the ribs. The combined effects of GDL compression/intrusion considerably increase spatial non-uniformity in the species and current density distributions, and reduce cell performance.     

Simulation and experimental analysis of the clamping pressure distribution in a PEM fuel cell stack

High performance and efficiency are often reported in single-cell polymer electrolyte membrane (PEM) fuel cell (FC) experiments. This however, can reduce substantially when moving from single-cell experiments to multiple cells. Fuel cell performance is degraded for many reasons when adding cells, but; possibly the most important, is contact resistance between the bipolar plate and gas diffusion layer (GDL). Contact resistance is in direct relation to the clamping configuration and clamping pressure applied to a FC stack. Simulation of a single cell and 16-cell FC was performed at various clamping pressures resulting in detailed 3D plots of stress and deformation. The stress on the GDL, for any value of clamping pressure simulated in this study, is around 1.5 MPa for the 16-cell stack and around 4 MPa in single cell simulations. Experimental testing of clamping pressure effects was performed on a 16-cell stack by placing a thin pressure-sensitive film between GDL and bipolar plate. Clamping pressure was applied using various loads, durations, and two types of GDLs. The results from experimental testing show that pressure on the GDL is in the range of 0–2.5 MPa. When using rectangular cells, experimental results show nearly zero pressure in the center of each cell and the center cells of the stack, regardless of clamping method.     

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.