Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.     

Effect of air flow on liquid water transport through a hydrophobic gas diffusion layer of a polymer electrolyte membrane fuel cell

The transport of liquid water through an idealized 2-D reconstructed gas diffusion layer (GDL) of a polymer electrolyte membrane (PEM) fuel cell is computed subject to hydrophobic boundary condition at the fibre–fluid interface. The effect of air flow, as would occur in parallel/serpentine/interdigitated type of flow fields, on the liquid water transport through the GDL, ejection into the channel in the form of water droplets and subsequent removal of the droplets has been simulated. Results show that typically water flow through the fibrous GDL occurs through a fingering and channelling type of mechanism. The presence of cross-flow of air has an effect both on the path created within the GDL and on the ejection of water into the channel in the form of droplets. A faster rate of liquid water evacuation through the GDL (i.e., more frequent ejection of water droplets) as well as less flooding of the void space results from the presence of cross-flow. These results agree qualitatively with experimental observations reported in the literature.     

Gas diffusion layer design focusing on the structure of the contact face with catalyst layer against water flooding in polymer electrolyte fuel cell

High humidity must be maintained inside polymer electrolyte fuel cells to achieve high ion conductivity. However, water condensation blocks the diffusion of the reaction gas in the gas diffusion layer under water saturation conditions which are produced by the product water. This effect is known as flooding and causes a sudden drop in the cell voltage. Therefore, advanced water management is required in such fuel cells. Internal water management is generally carried out by making adjustments to the gas diffusion layer. This study reports that the extremely highly flood-resistant gas diffusion layer has been developed, based on simple carbon paper. It was experimentally revealed that flooding is controlled by a gas diffusion layer with a smaller pore-structure facing the catalyst layer and it is one of the governing factors for flooding in the gas diffusion layer.     

Apparent contact angles of liquid water droplet breaking through a gas diffusion layer of polymer electrolyte membrane fuel cell

The lattice Boltzmann method is used to simulate the three-dimensional dynamic process of liquid water breaking through the gas diffusion layer (GDL) in the polymer electrolyte membrane fuel cell. An accurate method is introduced to analyze asymmetric droplet shape. Ten micro-structures of Toray GDL were built based on a stochastic geometry model. It was found that asymmetric droplets are produced on the GDL surfaces. Their local apparent contact angles vary with different view angles and geometries. They are different to the idealized contact angles by symmetric simplification. It was concluded that the apparent contact angles are influenced by GDL structures and view angles. This information can help to bridge the gap between mesoscale and cell-scale simulations in the field of fuel cell simulation.     

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).     

Prevention of the water flooding by micronizing the pore structure of gas diffusion layer for polymer electrolyte fuel cell

In polymer electrolyte fuel cells, high humidity must be established to maintain high proton conductivity in the polymer electrolyte. However, the water that is produced electrochemically at the cathode catalyst layer can condense in the cell and cause an obstruction to the diffusion of reaction gas in the gas diffusion layer and the gas channel. This leads to a sudden decrease of the cell voltage. To combat this, strict water management techniques are required, which usually focus on the gas diffusion layer. In this study, the use of specially treated carbon paper as a flood-proof gas diffusion layer under extremely high humidity conditions was investigated experimentally. The results indicated that flooding originates at the interface between the gas diffusion layer and the catalyst layer, and that such flooding could be eliminated by control of the pore size in the gas diffusion layer at this interface.     

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.     

Experimental study on carbon corrosion of the gas diffusion layer in polymer electrolyte membrane fuel cells

Generally, the GDL of a PEM fuel cell experiences three external attacks: dissolution of water, erosion of gas flow, and corrosion of electric potential. Of these degradation factors, this study focuses on the carbon corrosion of electric potential and investigates its impact through the accelerated carbon corrosion test. This study confirms that carbon corrosion occurs at the GDL, which decreases the operating fuel cell’s performance. To discover the effects of carbon corrosion, the GDL property changes are measured through various devices, including a scanning electron microscopy, a thermo gravimetric analyzer, and a tensile stress test. Carbon corrosion causes not only loss of weight and thickness but also degradation of mechanical strength in the GDL. In addition, the GDL shows serious damage in its center.     

Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells

Flooding of the membrane electrode assembly (MEA) and dehydrating of the polymer electrolyte membrane have been the key problems to be solved for polymer electrolyte membrane fuel cells (PEMFCs). So far, almost no papers published have focused on studies of the liquid water flux through differently structured gas diffusion layers (GDLs). For gas diffusion layers including structures of uniform porosity, changes in porosity (GDL with microporous layer (MPL)) and gradient change porosity, using a one-dimensional model, the liquid saturation distribution is analyzed based on the assumption of a fixed liquid water flux through the GDL. And then the liquid water flux through the GDL is calculated based on the assumption of a fixed liquid saturation difference between the interfaces of the catalyst layer/GDL and the GDL/gas channel. Our results show that under steady-state conditions, the liquid water flux through the GDL increases as contact angle and porosity increase and as the GDL thickness decreases. When a MPL is placed between the catalyst layer and the GDL, the liquid saturation is redistributed across the MPL and GDL. This improves the liquid water draining performance. The liquid water flux through the GDL increases as the MPL porosity increases and the MPL thickness decreases. When the total thickness of the GDL and MPL is kept constant and when the MPL is thinned to 3 μm, the liquid water flux increases considerably, i.e. flooding of MEA is difficult. A GDL with a gradient of porosity is more favorable for liquid water discharge from catalyst layer into the gas channel; for the GDLs with the same equivalent porosity, the larger the gradient is, the more easily the liquid water is discharged. Of the computed cases, a GDL with a linear porosity 0.4x + 0.4 is the best.     

The impact of rib structure on the water transport behavior in gas diffusion layer of polymer electrolyte membrane fuel cells

Using the multiphase lattice Boltzmann method (LBM), the liquid water transport dynamics is simulated in a gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells (PEMFCs). The effect of rib structure on the water invasion process in the micro-porous GDL is explored by comparing the two cases, i.e., with rib and without rib structures. The liquid water distribution and water saturation profile are presented to determine the wetting mechanism in the GDL. The results show that the liquid water transport in the GDL is strongly governed by capillary force and the rib structure plays a significant role on water distribution and water transport behavior in the GDL. Comparison of two cases confirms that the rib structure influences on the location of water breakthrough. The liquid water distribution and water saturation profile indicate that the high resistance force underneath the rib suppresses the growth of water cluster, resulting in the change of flow path. After water breakthrough, the liquid water distribution under the channel has little variation, whereas that under the rib continues to change. The predicted value of effective permeability is in good agreement with Carman-Kozeny correlation and experimental results in the literature. The results suggest that the LBM approach is an effective tool to investigate the water transport behavior in the GDL.     

Effect of liquid water on transport properties of the gas diffusion layer of polymer electrolyte membrane fuel cells

Water management in polymer electrolyte membrane (PEM) fuel cells is of importance due to its impact on the performance, durability and ultimately the cost of the cell. In the gas diffusion layer (GDL), liquid water has a direct effect on species and heat transport. The amount of liquid water in the GDL affects the relative permeability and capillary pressure, which govern the convective and diffusive transport of liquid water. Liquid water acts as a barrier to the diffusion of gases through the void region and facilitates in heat transfer. In this study, the full morphology model was used in order to investigate the effects of liquid water presence on the transport properties of the carbon paper GDL and examine the applicability of using various laws to estimate the transport properties in the presence of liquid water. The numerical results were compared against published experimental data. Further, the method of standard porosimetry was used to experimentally measure the effect of Teflon treatment on the capillary pressure of carbon paper. It was found that the addition of PTFE to the GDL results in the increase of capillary pressure; however, further increases to the PTFE loading did not result in additional changes to the capillary pressure.     

Effect of material properties on evaporative water removal from polymer electrolyte fuel cell diffusion media

This work is devoted to delineating the fundamentals of evaporative water removal from diffusion media (DM) to achieve highly efficient and durable gas purge. Multiphase water transport from DM during gas purge is characterized by a balance of internal capillary liquid water flow and water vapor diffusion. In this study, DM with polytetrafluoroethylene (PTFE) content ranging from 0 to 20 wt%, and DM with three different geometric pore structures are utilized to understand this material property effect. It is found that overall evaporative water removal rate increases as PTFE content decreases and as the geometric pore structure changes from a two- to a more three-dimensional structure. This is due to the increase of wettability and porous space favorable for the water transport. The effect of phase-change-induced (PCI) flow and capillary flow on water removal is compared, and it is found that PCI flow is dominant at lower saturation of DM, whereas capillary flow is dominant at higher saturation. The results of this study build upon a previous study by the authors (Cho and Mench [17]), and are useful to understand the competing phenomena of water removal in PEFC DM. The ultimate goal of this work is to guide material design to achieve purge that preserves membrane durability with reduced shutdown power requirements.     

Study on water transport in hydrophilic gas diffusion layers for improving the flooding performance of polymer electrolyte fuel cells

For hydrogen-based polymer electrolyte fuel cells (PEFCs), water transport control in gas diffusion layers (GDLs) by wettability distribution is useful to suppress the flooding problem. In this study, the water transport of a novel GDL with hydrophilic-hydrophobic patterns was investigated. First, we clarified that the water motion in the hydrophilic GDL with microstructures could be reproduced by the enlarged scale model. The scale model experiment also showed that the same water behavior in hydrophilic GDL can be obtained from Capillary numbers (Ca) in a range of Ca ~ 10^?5 to 10^?3. As the computational load is inversely proportional to Ca, the computational load could be reduced by 1/100th by using Ca ~ 10^?3, which is 100 times higher than PEFC operation (Ca ~ 10^?5). Finally, the simulation with Ca ~ 10^?3 was performed, and we showed that the GDL with straight region of contact angle 50° minimized the water accumulation.     

Pore network modeling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells

A pore network model of the gas diffusion layer (GDL) in a polymer electrolyte membrane fuel cell is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions. Geometric parameters of the pore network model are calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials and the model is subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, the relative permeability of water and gas and the effective gas diffusivity are computed as functions of water saturation using resistor-network theory. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. Alternative relationships are suggested that better match the pore network model results. The pore network model is also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but it may become a significant source of concentration polarization as the GDL becomes increasingly saturated with water.     

Effects of gas diffusion layer structure on the open circuit voltage and hydrogen crossover of polymer electrolyte membrane fuel cells

The clamping pressure of polymer electrolyte membrane fuel cells for vehicle applications should be typically high enough to minimize contact resistance. However, an excessive compression pressure may cause a durability problem. In this study, the effects of gas diffusion layer (GDL) structure on the open circuit voltage (OCV) and hydrogen crossover have been closely examined. Results show that the performances of fuel cells with GDL-1 (a carbon fiber felt substrate with MPL having rough surface) and GDL-3 (a carbon fiber paper substrate with MPL having smooth surface) are higher than that with GDL-2 (a carbon fiber felt substrate with MPL having smooth surface) under low clamping torque conditions, whereas when clamping torque is high, the GDL-1 sample shows the largest decrease in cell performance. Hydrogen crossover for all GDL samples increases with the increase of clamping torque, especially the degree of increase of GDL-1 is much greater than that of the other two GDL samples. The OCV reduction of GDL-1 is much greater than that of GDL-2 and GDL-3. It is concluded that the GDL-3 is better than the other two GDLs in terms of fuel cell durability, because the GDL-3 shows the minimum OCV reduction.     

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.     

Numerical investigations on liquid water removal from the porous gas diffusion layer by reactant flow

Water removal from the gas diffusion layer (GDL) is crucial for the efficient operation of proton exchange membrane (PEM) fuel cell. Static pressure gradient caused by the fast reactant flow in the flow channel is one of the main mechanisms of water removal from GDL. Reactant can leak or cross directly to the neighboring channel via the porous GDL in the cells with serpentine flow channel and many of its modifications. Such cross flow plays an important role for the removal of liquid water accumulated in the GDL especially under land area. To investigate the characteristics of liquid water behavior in the GDL under pressure gradient, the fibrous porous structure of the carbon paper is modeled by three dimensional impermeable cylinders randomly distributed in the in-plane directions and unsteady two-phase simulations are conducted. It is shown that the permeability from the numerical model matches well the experimental measurements of the common GDLs in the literature. The contact angle and pressure gradient are the key parameters that determine the initiation and the process of liquid water transport in the GDL which is initially wet with stagnant liquid water. It has been observed that the larger contact angle results in faster water removal from the GDL. Numerical simulations are performed for a wide range of pressure gradient with different contact angles to determine the minimum pressure gradient that initiates the liquid water transport in the GDL. It is found that the amount of pressure gradient caused by the cross flow is sufficient and effective to get rid of the liquid water accumulated in the GDL. The simulation results are also compared with experimental data in literature showing a good agreement. The characteristics of liquid water discharging from the gas diffusion layer are also described.     

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.     

Analysis of surface and interior degradation of gas diffusion layer with accelerated stress tests for polymer electrolyte membrane fuel cell

The effects of surface and interior degradation of the gas diffusion layer (GDL) on the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs) have been investigated using three freeze-thaw accelerated stress tests (ASTs). Three ASTs (ex-situ, in-situ, and new methods) are designed from freezing ?30 °C to thawing 80 °C by immersing, supplying, and bubbling, respectively. The ex-situ method is designed for surface degradation of the GDL. Change of surface morphology from hydrophobic to hydrophilic by surface degradation of GDL causes low capillary pressure which decreased PEMFC performance. The in-situ method is designed for the interior degradation of the GDL. A decrease in the ratio of the porosity to tortuosity by interior degradation of the GDL deteriorates PEMFC performance. Moreover, the new method showed combined effects for both surface and interior degradation of the GDL. It was identified that the main factor that deteriorated the fuel cell performance was the increase in mass transport resistance by interior degradation of GDL. In conclusion, this study aims to investigate the causes of degraded GDL on the PEMFC performance into the surface and interior degradation and provide the design guideline of high-durability GDL for the PEMFC.     

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.