Lattice Boltzmann simulations of water transport in gas diffusion layer of a polymer electrolyte membrane fuel cell

The effect of wettability on water transport dynamics in gas diffusion layer (GDL) is investigated by simulating water invasion in an initially gas-filled GDL using the multiphase free-energy lattice Boltzmann method (LBM). The results show that wettability plays a significant role on water saturation distribution in two-phase flow in the uniform wetting GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially. The resulting water saturation distributions show that the saturation in the GDL has little change after water breaks through the GDL, and further confirm that the selective introduction of hydrophilic passages in the GDL would facilitate the removal of liquid water more effectively, thus alleviating the flooding in catalyst layer (CL) and GDL. The LBM approach presented in this study provides an effective tool to investigate water transport phenomenon in the GDL at pore-scale level with wettability distribution taken into consideration.     

PEMFC碳纸气体扩散层内气液两相流格子Boltzmann模拟

为了提高质子交换膜燃料电池（PEMFC）水管理,本文借助多相流格子Boltzmann模型（LBM）模拟分析了PEMFC碳纸气体扩散层（GDL）内的气液两相输运过程,主要研究了GDL疏水性对气液两相流的影响。结果表明：液态水流路径不仅受到GDL结构形态的影响,而且受到材料疏水性影响。液态水在疏水性弱的GDL中不仅容易沁入,而且容易在孔隙中达到饱和;相反,在疏水性较强的GDL中,液态水很难突破沁入小尺寸孔隙,而从孔径较大的孔隙流通,从而形成毛细力主导的指进流动。     

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.     

Water management studies in PEM fuel cells,part III: Dynamic breakthrough and intermittent drainage characteristics from GDLs with and without MPLs

The transport of liquid water and gaseous reactants through a gas diffusion layer (GDL) is one of the most important water management issues in a proton exchange membrane fuel cell (PEMFC). In this work, the liquid water breakthrough dynamics, characterized by the capillary pressure and water saturation, across GDLs with and without a microporous layer (MPL) are studied in an ex-situ setup which closely simulates a real fuel cell configuration and operating conditions. The results reveal that recurrent breakthroughs are observed for all of the GDL samples tested, indicating the presence of an intermittent water drainage mechanism in the GDL. This is accounted for by the breakdown and redevelopment of the continuous water paths during water drainage as demonstrated by Haines jumps. For GDL samples without MPL, a dynamic change of breakthrough locations is observed, which originates from the rearrangement of the water configuration in the GDL following the drainage. For GDL samples with MPL, no dynamic change of breakthrough location can be found and the water saturation is significantly lower than the samples without MPL. These results suggest that the MPL not only limits the number of water entry locations into the GDL (such that the water saturation is drastically reduced), but also stabilizes the water paths (or morphology). The effect of MPL on the two-phase flow dynamics in gas channels is also studied with multi-channel flow experiments. The most important result is that GDL without MPL promotes film flow and shifts the slug-to-film flow transition to lower air flow rates, compared with the case of GDL with MPL. This is closely related to the larger number of water breakthrough locations through GDL without MPL, which promotes the formation of water film.     

Pore-network simulations of two-phase flow in a thin porous layer of mixed wettability: Application to water transport in gas diffusion layers of proton exchange membrane fuel cells

Pore network simulations are performed to study water transport in a model gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells (PEMFCs) in relation with the change in hydrophobicity that might be due to aging or temperature effect. The change in hydrophobicity is taken into account by changing randomly the fraction of hydrophilic elements, pores or throats, in the network. The transport and equilibrium properties of the model GDL are computed as a function of liquid saturation as well as at breakthrough varying the fraction of hydrophilic elements. The results indicate that the hydrophilic element percolation threshold marks the transition between two domains. The system is found to be weakly dependent on the fraction of hydrophilic elements as long as this fraction is below the percolation threshold whereas an increase in wettability above the percolation threshold favours a greater blockage of the pore space by the water and therefore a diminished access of gas to the catalyst layer. This model may help assess the effect of a change in wettability on the fuel cell performance and may also help suggest better GDL designs in relation with the water management problem in PEMFCs.     

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.     

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.     

Effect of compression on liquid water transport and microstructure of PEMFC gas diffusion layers

This work explores how the degradation of the gas diffusion layer (GDL) under compression contributes to the formation of preferential pathways for water transport. Fluorescence microscopy is used to provide ex situ visualization of liquid water transport through the GDL placed beneath an optically transparent clamping plate. Transient image data obtained with a CCD camera indicates that areas of compression in the GDL coincide with preferential pathways for water transport and break-through. Preferential flow of water through the smaller pores resulting from GDL compression is contrary to the expected behaviour in a hydrophobic medium, and this suggests a loss of hydrophobicity. Scanning electron microscopy (SEM) is used to investigate the effect of compression on the morphology of the GDL. These SEM images show that compressing the GDL causes the breakup of fibers and, indeed, deterioration of the hydrophobic coating.     

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.     

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.     

Effect of gas diffusion layer properties on the time of breakthrough

A visualization technique is employed to measure the time of breakthrough for GDL samples with different characteristics. It is shown that the time of breakthrough is reversely proportional to the hydrophobic content of the GDL sample. The effects of the GDL thickness and injection flow rate on the time of breakthrough are also studied. The results show that the capillary pressure of a thin porous medium is not independent from the time of breakthrough. Thus, it is not legitimate to calculate the saturation of the medium by multiplication of the flow rate and time of breakthrough. These findings help better understanding of flooding phenomena, which may lead to the development of more effective GDLs for PEM fuel cell.     

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

Steady saturation distribution in hydrophobic gas-diffusion layers of polymer electrolyte membrane fuel cells: A pore-network study

A pore-network model is developed to simulate liquid water transport in a hydrophobic gas-diffusion layer (GDL) during the operation of polymer electrolyte membrane fuel cells (PEMFCs). The steady saturation distribution in GDLs is determined through a numerical procedure using a pore-network model combined with invasion-percolation path-finding and subsequent viscous two-phase flow calculation. The simulation results indicate that liquid water transport in hydrophobic GDLs is a strongly capillary-driven process that almost reaches the pure invasion-percolation limit with zero capillary number. A uniform flux condition is found to better reflect the actual phenomenon occurring at the inlet boundary for liquid water entering a GDL than a uniform pressure condition. The simulation further clarifies the effect of the invaded pore fraction at a uniform-flux inlet boundary in modifying water transport in GDLs. Finally, the effect of the GDL thickness on the steady saturation distribution is investigated.     

Pore-scale numerical study of multiphase reactive transport processes in cathode catalyst layers of proton exchange membrane fuel cells

understanding interactions between multiphase flow and reactive transport processes in catalyst layers (CL) of proton exchange membrane fuel cells is crucial for obtaining better performance and lower cost. In this study, a pore-scale model is developed to simulate coupled processes occurring in CLs, including oxygen diffusion, electrochemical reaction, and air-liquid two phase flow. Simulation conducted in an idealized local CL structures shows that the pore-scale model successfully captures dynamic behaviors of liquid water including generation, growth and subsequent migration, as well as the interaction between multiphase flow and reactive transport. Pore-scale simulation is then conducted in hydrophobic CLs with complicated structures where carbon, platinum, ionomer and pores are resolved. It is found that filling modes of the liquid water in the CLs are different. Before forming the continuous flow paths in CLs, liquid water presents as tiny droplets in pores surrounding relative large pores. After the continuous flow paths are formed, liquid water dynamic behaviors follow the capillary fingering mechanism. The multiphase flow and reactive transport processes are closely coupled with each other, and as liquid water saturation increases the reaction rate decreases. Increasing the hydrophobicity can alleviate the water flooding, accelerate the water breakthrough, and facilitate the water evaporation.     

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.     

Effects of hydrophilic/hydrophobic properties of gas flow channels on liquid water transport in a serpentine polymer electrolyte membrane fuel cell

The droplet dynamics in the serpentine flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a serpentine channel with the aim of effectively preventing flooding. Three-dimensional two-phase flow simulations employing the volume of fluid (VOF) method have been performed. Liquid droplets emerging from four adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the serpentine flow channel. The effects of contact angle variation of the channel walls on liquid water removal have been tested in terms of liquid water saturation and coverage of liquid water on the gas diffusion layer (GDL) surface. The numerical results show that the hybrid case, which consists of hydrophilic channel walls at the straight part and hydrophobic walls at the turning part of the serpentine flow channels, enhances water removal compared with two other cases in which the channel wall is homogeneously hydrophilic or hydrophobic. The three-dimensional visualization of liquid water droplets reveals that the hydrophobic wall at the turning part reduces the water saturation in the channel and the hydrophilic wall at the straight part prevents the liquid water from covering the GDL surface.     

Liquid and oxygen transport through bilayer gas diffusion materials of proton exchange membrane fuel cells

In proton exchange membrane fuel cells (PEMFCs), a hydrophobic micro-porous layer (MPL) is usually placed between the catalyst layer (CL) and the conventional gas diffusion layer (GDL) to relieve the flooding. In this paper, a pore network model is developed to investigate how the MPL structure affects the liquid and oxygen transport properties of the bilayer gas diffusion material (GDM) consisting of fine MPL and coarse GDL. The regular three-dimensional pore network constructed to represent the bilayer GDM are composed of the cubic pores that are connected by the narrow throats of square cross section. Based on this model, the capillary pressure, liquid permeability, and oxygen effective diffusivity as a function of GDM liquid saturation are determined. Parameter studies are performed to elucidate the influences of MPL thickness and of MPL crack width. Also analyzed are the liquid distributions in different structural GDMs at the moment of breakthrough. The results reveal a liquid saturation jump at the MPL/GDL interface in the plain bilayer GDM, but a liquid saturation drop in the defective bilayer GDM.     

Scale effect and two-phase flow in a thin hydrophobic porous layer. Application to water transport in gas diffusion layers of proton exchange membrane fuel cells

Pore network simulations are performed to study water transport in gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The transport and equilibrium properties are shown to be scale dependent in a thin system like a GDL. A distinguishing feature of such a thin system is the lack of length scale separation between the system size and the size of the representative elementary volume (REV) over which are supposed to be defined the macroscopic properties within the framework of the continuum approach to porous media. Owing to the lack of length scale separation, two-phase flow traditional continuum models are expected to offer poor predictions of water distribution in a GDL. This is illustrated through comparisons with results from the pore network model. The influence of inlet boundary conditions on invasion patterns is studied and shown to affect greatly the saturation profiles. The effects of GDL differential compression and partial coverage of outlet surface are also investigated.     

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.     

Dynamic water transport and droplet emergence in PEMFC gas diffusion layers

The dynamics of liquid water transport through the gas diffusion layer (GDL) and into a gas flow channel are investigated with an ex situ experimental setup. Liquid water is injected through the bottom surface of the GDL, and the through-plane liquid pressure drop, droplet emergence and droplet detachment are studied. The dynamic behaviour of water transport in and on the surface of the GDL is observed through fluorescence microscopy, and the through-plane liquid pressure drop is measured with a pressure transducer. With an initially dry GDL, the initial breakthrough of liquid water in the GDL is preceded by a substantial growth of liquid water pressure. Post-breakthrough, droplets emerge with a high frequency, until a quasi-equilibrium liquid water pressure is achieved. The droplet emergence/detachment regime is followed by a transition into a slug formation regime. During the slug formation regime, droplets tend to pin near the breakthrough location, and the overall channel water content increases due to pinning and the formation of water slugs. Droplets emerge from the GDL at preferential breakthrough locations; however, these breakthrough locations change intermittently, suggesting a dynamic interconnection of water pathways within the GDL. The experiments are complemented by computational fluid dynamics (CFD) simulations using the volume of fluid method to illustrate the dynamic eruption mechanism.