A micro-scale model for predicting contact resistance between bipolar plate and gas diffusion layer in PEM fuel cells

Contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) in a proton exchange membrane (PEM) fuel cell constitutes a significant portion of the overall fuel cell electrical resistance under the normal operation conditions. Most current methods for contact resistance estimation are experimental and there is a lack of well developed theoretical methods. A micro-scale numerical model is developed to predict the electrical contact resistance between BPP and GDL by simulating the BPP surface topology and GDL structure and numerically determining the status for each contact spot. The total resistance and pressure are obtained by considering all contact spots as resistances in parallel and summing the results together. This model shows good agreements with experimental results. Influences of BPP surface roughness parameters on contact resistance are also studied. This model is beneficial in understanding the contact behavior between BPP and GDL and can be integrated with other fuel cell simulations to predict the overall performance of PEM fuel cells.     

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.     

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.     

Numerical investigation of water droplet removal characteristics in novel block channels of PEMFC using dynamic wettability model

Liquid water transport and removal is one of the critical issues in the proton exchange membrane fuel cell (PEMFC) for achieving good performance and durability. In this study, two novel channels with different blocks are designed to study their effects on water removal using the volume of fluid (VOF) model considering the dynamic contact angle effect. It is found that compared with the conventional straight channel, both the one-block and two-block channels can promote liquid water removal. The one-block channel leads to faster water movement and removal on the gas diffusion layer (GDL) surface, but results in a much higher pressure drop. The separated two-block channel directly drags water away from the GDL surface by the capillary wicking effect of the block surface, achieving both faster water removal and smaller pressure drop. Effects of the droplet size, air velocity and static contact angle of GDL surface on water removal are investigated comprehensively in both the novel channels, as well as the conventional straight channel, with particular attention on the variations of water removal time, water coverage ratio and pressure drop.     

Numerical study of the effect of the GDL structure on water crossover in a direct methanol fuel cell

A two-dimensional two-phase non-isothermal mass transport model is developed to numerically investigate the behavior of water transport through the membrane electrode assembly (MEA) of a direct methanol fuel cell. The model enables the visualization of the distribution of the liquid saturation through the MEA and the analysis of the distinct effects of the three water transport mechanisms: diffusion, convection and electro-osmotic drag, on the water-crossover flux through the membrane. A parametric study is then performed to examine the effects of the structure design of the gas diffusion layer (GDL) on water crossover. The results indicate that the flow-channel rib coverage on the GDL surface and the deformation of the GDL can cause an uneven distribution of the water-crossover flux along the in-plane direction, especially at higher current densities. It is also found that both the contact angle and the permeability of the cathode GDL can significantly influence the water-crossover flux. The water-crossover flux can be reduced by improving the hydrophobicity of the cathode GDL.     

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.     

A fractal model for predicting permeability and liquid water relative permeability in the gas diffusion layer (GDL) of PEMFCs

In this study, a fractal model is developed to predict the permeability and liquid water relative permeability of the GDL (TGP-H-120 carbon paper) in proton exchange membrane fuel cells (PEMFCs), based on the micrographs (by SEM, i.e. scanning electron microscope) of the TGP-H-120. Pore size distribution (PSD), maximum pore size, porosity, diameter of the carbon fiber, pore tortuosity, area dimension, hydrophilicity or hydrophobicity, the thickness of GDL and saturation are involved in this model. The model was validated by comparison between the predicted results and experimental data. The results indicate that the water relative permeability in the hydrophobicity case is much higher than in the hydrophilicity case. So, a hydrophobic carbon paper is preferred for efficient removal of liquid water from the cathode of PEMFCs.     

A two-fluid model for two-phase flow in PEMFCs

In this study, a two-fluid (TF) model is developed for two-phase flows in proton exchange membrane fuel cells (PEMFCs). The drag force and lift force between gas and liquid phase are considered in N-S equations. In addition, a simplified model is introduced to obtain the liquid water droplet detachment diameter on the gas diffusion layer (GDL)/channel interface which involves the properties of the GDL/channel interface (contact angle and surface tension). The TF model and the simplified model for the prediction of water droplet detachment diameter on GDL/channel interface are validated by the comparison between the experimental data and the model results, respectively. The effect of the properties of GDL/channel interface (contact angle and surface tension) on two-phase behavior in PEMFCs is investigated, The results show that a high contact angle and a low surface tension are advantageous for liquid water removal in the gas channel and the GDL even though a low surface tension will lead to a low capillary force in the GDL.     

Self-healing of super hydrophobic and hierarchical surfaces for gas diffusion layer

Surface wettability of gas diffusion media (GDM) is one of the key issues related to the water management in fuel cells. In this study, a facile coating approach of combining carbon black and polydimethylsiloxane (PDMS) is developed to fabricate the gas diffusion layer (GDL) with super hydrophobic and hierarchical surfaces. Due to the Wenzel and Cassie's effect, the fabricated GDL shows the average contact angle as high as 158° and the roll angle less than 5°. Its super durability could be identified by the constant potential oxidation with the oxidization peak current approaching to 0.1 mA cm⁻², an order of magnitude smaller than that of conventional GDL coated with polytetrafluoroethylene (PTFE) and carbon black (10/90 wt/wt). Furthermore, these hierarchical hydrophobic surfaces exhibit a recovery of hydrophobicity from 107° to 133° by heat treatment. The mechanism of the exceptional self-healing capability is investigated by microscopic and spectroscopic analysis. It is indicated that ring siloxanes with lower surface tension formed on GDL surface during heat treatment process. This paper provides a fundamental research on the hierarchical superhydrophobic surfaces of GDL and a promising solution to develop long-live super hydrophobic GDL.     

Numerical simulation of two-phase cross flow in microstructure of gas diffusion layer with variable contact angle

For a gas diffusion layer (GDL) with hydrophobic treatment, its hydrophobicity (contact angle) may change along the through-plane direction, and lead to different two-phase transport characteristics. In this study, such variable contact angle is implemented in a three-dimensional unsteady two-phase model based on the microstructure of GDL to study the liquid water transport characteristics along the in-plane direction caused by cross flow. It is found that during a liquid water intrusion process, the liquid water first moves through some of the pores that are easy to penetrate, forming a “fingering transport” mode; and after that, with more liquid water accumulated, the rest of the pores can also be filled, forming a “steady transport” mode. Increasing the differential pressure or decreasing the contact angle of GDL accelerates the liquid water intrusion, and this effect is weakened at higher differential pressures and contact angles. For a GDL with variable contact angle, the water transport characteristics in different cross sections normal to the through-plane direction with different contact angles are similar to the corresponding fixed contact angle cases in these cross sections, and the overall process of water intrusion with variable contact angle is similar to its corresponding average fixed contact angle case.     

Effect of surface dynamic wettability in proton exchange membrane fuel cells

It is known that the static contact angle reflecting the “contact area” between liquid and solid is insufficient to represent the dynamic wettability of a solid surface, and another parameter called the sliding angle is needed to describe the relative easiness of liquid moving on a solid surface. However, sliding angle has been largely neglected in the previous studies for proton exchange membrane fuel cell (PEMFC). In this study, three-dimensional multiphase simulations are carried out for a PEMFC with single straight flow channels considering both the static contact angles and sliding angles of gas diffusion layer (GDL) and catalyst layer (CL). The results show that the liquid water volume fraction in cathode CL (CCL) and GDL (CGDL) can be increased by several times when the sliding angle is increased while the static contact angle is kept constant. This could have significant implication on the water management strategy due to the considerable changes in the water transport and removal processes. Since GDL is much thicker than CL, changing the surface dynamic wettability of GDL has more significant effect on liquid water transport than changing the surface dynamic wettability of CL.     

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.     

Water droplet detachment characteristics on surfaces of gas diffusion layers in PEMFCs

Water management is one of the key issues affecting the performance and stability of proton exchange membrane fuel cells (PEMFCs). Water detachment on the gas diffusion layer (GDL) surface is critically important to water management in PEMFCs. In this study, water droplet detachment characteristics under various GDL surface contact angles and channel heights are investigated, by using a customized transparent model cell for direct ex-situ water visualization. The droplet height, chord, height/chord ratio, and contact angle hysteresis at the instant of droplet detachment are quantitatively analyzed. The droplet detachment is easier for higher gas Reynolds number (Re_g). The height and chord of the droplet both decrease with Re_g for both GDLs with and without PTFE but their decrement rates become smaller in higher Re_g regions for all the channel heights investigated. Compared with droplets on the untreated GDL, the droplet height/chord ratio on the PTFE-treated GDL with larger static contact angle is increased by 36.7%, 64.1% and 76.0% and the contact angle hysteresis is reduced by 17.1%, 16.3% and 12.6% for the channel height H of 1 mm, 2 mm and 3 mm, respectively, which indicates that the PTFE-treated GDL improves water detachment. It shows that the water detachment is improved by reducing the channel height due to the smaller contact angle hysteresis at the instant of droplet detachment.     

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.     

Numerical investigation of drop dynamics in presence of wettability gradient inside a serpentine channel of proton exchange membrane fuel cell

Water management significantly affects the performance of a proton exchange membrane fuel cell (PEMFC). Therefore, interest is felt to numerically investigate water droplet movement and slug formation inside the microchannel (gas) of PEMFC. Two important parameters—water coverage ratio and pressure drop have been studied in detail. A U-shaped geometry with a round corner is used for this purpose. 3D unsteady-state models are used to study the drop dynamics using commercial CFD software ANSYS FLUENT 18. For tracking of water drop dynamics, the volume of fluid model is used. Two different situations are simulated. In the first case, the investigation of hydrodynamics of the 0.4 mm drop adhered to the surface of the gas diffusion layer (GDL) has been done. In the second case, simulation of air-water slug flow has been done. GDL surfaces at upstream and downstream of bend are modified using user-defined functions, such that the GDL surface has a dynamic contact angle with respect to the direction of flow. This makes it a continuously hydrophilic surface at upstream and continuously hydrophobic surface at downstream with respect to the direction of flow. The impact of GDL wettability on water retention and removal has been discussed. It is noted that the presence of a gradient facilitates the removal of water drop adhered to the GDL surface. For the case of a suspended drop with an increase of 1°/mm in the magnitude of the gradient, a decrease of 30% is observed in water coverage ratio and pressure drop observed in the channel. Such modified surfaces aids in the conversion of slugs to film at the downstream of bend that reduces maldistribution. The pressure fluctuations and average pressure drop are reduced by 66% when subjected to the aforementioned hybrid gradient.     

Measurement of internal wettability of gas diffusion porous media of proton exchange membrane fuel cells

One of the major problems of current proton exchange membrane (PEM) fuel cells is water management. The gas diffusion layer (GDL) of the fuel cell plays an important role in water management since humidification and water removal are both achieved through the GDL. Various numerical models developed to illustrate the multiphase flow and transport in the fuel cell. The accuracy of these models depends on the accurate measurement of the GDL properties such as wettability, surface energy, and porosity. Most of the studies conducted for measuring the wettability of the GDL are based on the external contact angle measurements. However, the external contact angle does not describe adequately capillary forces acting on the water inside the GDL pores. In a recent study, the capillary penetration technique has been used to measure indirectly the wettability of the GDL based on the experimental weight increase due to penetration of the liquid into the porous sample. In essence, the mass penetration technique was used along with the Washburn's equation. The shortcoming of this method is that the external factors such as the mass of the meniscus formed outside the sample as well as evaporation occurring during the experiment were not considered. It was found that these factors affect the wettability measurements of the GDL, especially for a hydrophilic sample. In this paper, the experimental setup of the capillary penetration method has been modified to control the evaporation rate as the liquid is penetrated into the sample. Also, the capillary penetration technique which was initially used based on mass penetration has been modified to the height penetration method to eliminate the effect of the weight of the meniscus formed outside the sample. The experiments were performed for a time period of 10 s. For this time period, it was found that the Washburn's equation is not an accurate model since it does not include the frictional work effects that are significant at the first few seconds of the experiments. Therefore, the Washburn's equation was replaced by a more general form. Using the Levenberg-Marquardt optimization technique, the experimental data obtained from the height penetration technique is fitted to the theoretical curve to find the internal contact angles of a sample GDL. Finally, these contact angle results are used to determine the surface tension of the GDL using two approaches: the Owens-Wendt surface tension components and the equation-of-state models.     

Numerical study of two-phase flow patterns in the gas channel of PEM fuel cells with tapered flow field design

In this study the air–water two-phase flow in a tapered channel of a PEMFC was numerically simulated using the volume of fluid (VOF) method. In particular, a 3D mathematical model of the fuel cell flow channel was used to obtain a reliable evaluation of the fuel cell performance for different taper angles and different temperatures and to calculate the total amount of water produced. This information was then used as boundary conditions to simulate the two-phase flow in the cell channel through a 2D VOF model. Typical operating conditions were assigned and the numerical mesh was constructed to represent the real fuel cell configuration. The results show that tapering the channel downstream enhances the water removal due to increased airflow velocity. In the rectangular channel no film formation is noted with a marked predominance of slug flow. In contrast, as the taper angle is increased the predominant two-phase flow pattern is film flow. Finally many contact angles have been used to simulate the effect of the hydrophobicity of a GDL surface on the motion of the water. As the hydrophobicity of a GDL surface is decreased the presence of film is more evident even for less tapered channels.     

Numerical simulation of two-phase flow in gas diffusion layer and gas channel of proton exchange membrane fuel cells

Liquid water within the cathode Gas Diffusion Layer (GDL) and Gas Channel (GC) of Proton Exchange Membrane Fuel Cells (PEMFCs) is strongly coupled to gas transport properties, thereby affecting the electrochemical conversion rates. In this study, the GDL and GC regions are utilized as the simulation domain, which differs from previous studies that only focused on any one of them. A Volume of Fluid (VOF) method is adopted to numerically investigate the two-phase flow (gas and liquid) behavior, e.g., water transport pattern evolution, water coverage ratio as well as local and total water saturation. To obtain GDL geometries, an in-house geometry-based method is developed for GDL reconstruction. Furthermore, to study the effect of GDL carbon fiber diameter, the same procedure is used to reconstruct three GDL structures by varying the carbon fiber diameter but keeping the porosity and geometric dimensions constant. The wall wettability is introduced with static contact angles at carbon fiber surfaces and channel walls. The results show that the GDL fiber microstructure has a significant impact on the two-phase flow patterns in the cathode field. Different stages of two-phase flow pattern evolution in both cathode domains are observed. The liquid water in the GDL experiences water invasion, spreading, and rising, followed by the droplet breakthrough in the GDL/GC interface. In the GC, the water droplets randomly experience accumulation, combination, attachment, and detachment. Due to the difference in surface wettability, the water coverage of the GDL/GC interface is smaller than that of the channel side and top walls. It is also found that the water saturation inside the GDL stabilizes after the water breakthrough, while local water saturation at the interface keeps irregular oscillations. Last but not the least, a water saturation balance requirement between the GDL and GC is observed. In terms of varying fiber diameter, a larger fiber diameter would result in less water saturation in the GDL but more water in the GC, in addition to faster water movement throughout the total domain.     

Effect of fiber orientation on Liquid–Gas flow in the gas diffusion layer of a polymer electrolyte membrane fuel cell

In this study, the lattice Boltzmann method was used to simulate the three-dimensional intrusion process of liquid water in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). The GDL was reconstructed by the stochastic method and used to investigate fiber orientation's influence on liquid water transport in the GDL of a PEMFC. The fiber orientation can be described by the angle between a single fiber and the in-plane direction; three different samples were simulated for three different fiber orientation ranges. The simulated permeability correlated well with the anisotropic characteristics of reconstructed carbon papers. It was concluded that the fiber orientation had a significant effect on the liquid invasion pattern in the GDL by changing the pore shape and distribution of the GDL. The results indicated that the stochastically reconstructed GDL, taking into account the fiber orientation, better demonstrates the mass transport properties of the GDL.     

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).