Evaluating Breakthrough Pressure in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer(GDL) of a proton exchange membrane fuel cell(PEMFC).An ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera.The breakthrough pressure,at which the liquid water penetrates the GDL and starts to form a droplet,was measured.The breakthrough pressure was found to be different for the GDLs with...     

Evaluating breakthrough pressure in gas diffusion layers of proton exchange membrane fuel cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer （GDL） of a pro- ton exchange membrane fuel cell （PEMFC）. An ex-situ testing was conducted on a transparent test cell to visu- alize the water droplet formation and detachment on the surface of different types of GDLs through a CCD cam- era. The breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. The breakthrough pressure was found to be different for the GDLs with different porosities and thick- nesses. The equilibrium pressure, which is defined as the minimum pressure required maintaining a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses. The breakthrough pressure increases for the thick GDL with smaller micro-pore size.     

Numerical investigation of liquid water dynamics in wave‐like gas channels of PEMFCs

For the air feed in proton exchange membrane fuel cells (PEMFCs), the wave‐like gas channel (GC) shows obvious advantages over the straight GC because the former enhances collision of secondary flow and diffusion in the gas diffusion layer (GDL). However, it is prone to water flooding, which brings greater pressure drop, larger pressure oscillation, and blocking of reaction area. In the present study, numerical models of the water dynamic processes, including water droplets emerging from micropores on the GDL surface and removing through the GC, are established based on the volume of fluid (VOF) method. Water coverage ratio and pressure drop are calculated to evaluate the water flooding. The effects of the dimensional parameters of wave‐like GC and contact angle of channel walls on the water accumulation are studied. The emergence and removal of liquid water is a quasiperiodic and oscillating process. Multicycle simulations show that channel pressure drop increases linearly with greater growth rate than channel length. The equilibrium position of water droplet is strongly dependent on the relative wettability of the GDL and bipolar plate (BPP) surfaces. And the geometric parameters of GC have a significant impact on the pressure, water removal behavior and detachment time. Smaller bent angle brings bigger pressure drop, and larger cycle length is helpful for relieving the oscillation of pressure.     

The role of the gas diffusion layer on slug formation in gas flow channels of fuel cells

Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 μm liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells.     

Visualization of water transport in GDL and flow channel of PEMFC

Liquid water penetrating the gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) was studied. The gas diffuse layer (GDL) has a great impact on PEMFC's performanc3e, and is an important component of a PEMFC. An ex‐situ test was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera. The breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. The breakthrough pressure was found to be different for GDLs with different porosities and thicknesses. The equilibrium pressure, which is defined as the minimum pressure required for maintaining a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. Also the drain performance using three kinds of different bipolar plates were compared in this paper. According to the result of experiment, the average diameters of porous GDLs were found determining the penetrating pressure. Serpentine flow channel proved the best pattern for drainage. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20330     

Numerical simulation of two‐phase cross flow in the gas diffusion layer microstructure of proton exchange membrane fuel cells

The cross flow in the under‐land gas diffusion layer (GDL) between 2 adjacent channels plays an important role on water transport in proton exchange membrane fuel cell. A 3‐dimensional (3D) two‐phase model that is based on volume of fluid is developed to study the liquid water‐air cross flow within the GDL between 2 adjacent channels. By considering the detailed GDL microstructures, various types of air‐water cross flows are investigated by 3D numerical simulation. Liquid water at 4 locations is studied, including droplets at the GDL surface and liquid at the GDL‐catalyst layer interface. It is found that the water droplet at the higher‐pressure channel corner is easier to be removed by cross flow compared with droplets at other locations. Large pressure difference Δp facilitates the faster water removal from the higher‐pressure channel. The contact angle of the GDL fiber is the key parameter that determines the cross flow of the droplet in the higher‐pressure channel. It is observed that the droplet in the higher‐pressure channel is difficult to flow through the hydrophobic GDL. Numerical simulations are also performed to investigate the water emerging process from different pores of the GDL bottom. It is found that the amount of liquid water removed by cross flow mainly depends on the pore's location, and the water under the land is removed entirely into the lower‐pressure channel by cross flow.     

Droplet and slug formation in polymer electrolyte membrane fuel cell flow channels: The role of interfacial forces

A microfluidic device is employed to emulate water droplet emergence from a porous electrode and slug formation in the gas flow channel of a PEM fuel cell. Liquid water emerges from a 50 μm pore forming a droplet; the droplet grows to span the entire cross-section of a microchannel and transitions into a slug which detaches and is swept downstream. Droplet growth, slug formation, detachment, and motion are analyzed using high-speed video images and pressure-time traces. Slug volume is controlled primarily by channel geometry, interfacial forces, and gravity. As water slugs move downstream, they leave residual micro-droplets that act as nucleation sites for the next droplet-to-slug transition. Residual liquid in the form of micro-droplets results in a significant decrease in slug volume between the very first slug formed in an initially dry channel and the ultimate “steady-state” slug. A physics-based model is presented to predict slug volumes and pressure drops for slug detachment and motion.     

Lattice Boltzmann simulations of liquid droplet dynamic behavior on a hydrophobic surface of a gas flow channel

Using the multiphase free-energy lattice Boltzmann method (LBM), the formation of a water droplet emerging through a micro-pore on the hydrophobic gas diffusion layer (GDL) surface in a proton exchange membrane fuel cell (PEMFC) and its subsequent movement on the GDL surface under the action of gas shear are simulated. The dynamic behavior of the water droplet emergence, growth, detachment and movement in the gas flow channel is presented. The size of the detached droplet and the time of the droplet removing out of the channel under the influence of gas flow velocity and GDL surface wettability are investigated. The results show that water droplet removal is facilitated by a high gas flow velocity on a more hydrophobic GDL surface. A highly hydrophobic surface is shown to be capable of lifting the water droplet from the GDL surface, resulting in more GDL surface available for gas reactant transport. Furthermore, an analytical model based on force balance is presented to predict the droplet detachment size, and the predicted results are in good agreement with the simulation results. It is shown that the LBM approach is an effective tool to investigate water transport phenomena in the gas flow channel of PEMFCs with surface wettability taken into consideration.     

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.     

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.     

Interface resolving two-phase flow simulations in gas channels relevant for polymer electrolyte fuel cells using the volume of fluid approach

With the increased concern about energy security, air pollution and global warming, the possibility of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy systems has achieved considerable momentum. A computational fluid dynamic model describing a straight channel, relevant for water removal inside a PEFC, is devised. A volume of fluid (VOF) approach is employed to investigate the interface resolved two-phase flow behavior inside the gas channel including the gas diffusion layer (GDL) surface. From this study, it is clear that the impact on the two-phase flow pattern for different hydrophobic/hydrophilic characteristics, i.e., contact angles, at the walls and at the GDL surface is significant, compared to a situation where the walls and the interface are neither hydrophobic nor hydrophilic (i.e., 90° contact angle at the walls and also at the GDL surface). A location of the GDL surface liquid inlet in the middle of the gas channel gives droplet formation, while a location at the side of the channel gives corner flow with a convex surface shape (having hydrophilic walls and a hydrophobic GDL interface). Droplet formation only observed when the GDL surface liquid inlet is located in the middle of the channel. The droplet detachment location (along the main flow direction) and the shape of the droplet until detachment are strongly dependent on the size of the liquid inlet at the GDL surface. A smaller liquid inlet at the GDL surface (keeping the mass flow rates constant) gives smaller droplets.     

Three-dimensional numerical simulations of water droplet dynamics in a PEMFC gas channel

The dynamic behavior of liquid water emerging from the gas diffusion layer (GDL) into the gas flow channel of a polymer electrolyte membrane fuel cell (PEMFC) is modeled by considering a 1000 μm long air flow microchannel with a 250 μm × 250 μm square cross section and having a pore on the GDL surface through which water emerges with prescribed flow rates. The transient three-dimensional two-phase flow is solved using Computational fluid dynamics in conjunction with a volume of fluid method. Simulations of the processes of water droplet emergence, growth, deformation and detachment are performed to explicitly track the evolution of the liquid–gas interface, and to characterize the dynamics of a water droplet subjected to air flow in the bulk of the gas channel in terms of departure diameter, flow resistance coefficient, water saturation, and water coverage ratio. Parametric simulations including the effects of air flow velocity, water injection velocity, and dimensions of the pore are performed with a particular focus on the effect of the hydrophobicity of the GDL surface while the static contact angles of the other channel walls are set to 45°. The wettability of the microchannel surface is shown to have a major impact on the dynamics of the water droplet, with a droplet splitting more readily and convecting rapidly on a hydrophobic surface, while for a hydrophilic surface there is a tendency for spreading and film flow formation. The hydrophilic side walls of the microchannel appear to provide some benefit by lifting the attached water from the GDL surface, thus freeing the GDL-flow channel interface for improved mass transfer of the reactant. Higher air inlet velocities are shown to reduce water coverage of the GDL surface. Lower water injection velocities as well as smaller pore sizes result in earlier departure of water droplets and lower water volume fraction in the microchannel.     

Experimental validation of a lumped model of single droplet deformation,oscillation and detachment on the GDL surface of a PEM fuel cell

Proton Exchange Membrane Fuel Cell (PEMFC) performance significantly depends on electrodes water content. Liquid water emerging from the Gas Diffusion Layer (GDL) micro-channels can form droplets, films or slugs in the Gas Flow Channel (GFC). In the regime of droplets formation, the interaction with the gas flow leads to an oscillating mechanisms that is fundamental to study the detachment from the GDL surface. In this work, a numerical model of a droplet growing on the GDL surface is developed to describe the interaction between droplet and gas flow. Therefore, a lumped force balance is enforced to determine the center of mass motion law. Oscillation frequencies during growth and at detachment are found as a function of droplet size. The model is also exploited to find the relationship between droplet critical detachment size and gas velocity. The numerical results are compared with the experimental data previously published by the authors as well as with other experimental results available in the literature. The matching between the numerical and experimental data is very good. The low computational burden and the conciseness of the proposed approach make the model suitable for applications such as control and optimization strategies development to enhance PEMFC performance. Additionally, the model can be exploited to implement monitoring and diagnostic algorithm as well.     

Dynamic behaviour of liquid water emerging from a GDL pore into a PEMFC gas flow channel

A numerical investigation of the dynamic behaviour of liquid water entering a polymer electrolyte membrane fuel cell (PEMFC) channel through a GDL pore is reported. Two-dimensional, transient simulations employing the volume of fluid (VOF) method are performed to explicitly track the liquid–gas interface, and to gain understanding into the dynamics of a water droplet subjected to air flow in the bulk of the gas channel. The modeled domain consists of a straight channel with air flowing from one side and water entering the domain from a pore at the bottom wall of the channel. The channel dimensions, flow conditions and surface properties are chosen to be representative of typical conditions in a PEMFC. A series of parametric studies, including the effects of channel size, pore size, and the coalescence of droplets are performed with a particular focus on the effect of geometrical structure. The simulation results and analysis of the time evolution of flow patterns show that the height of the channel as well as the width of the pore have significant impacts on the deformation and detachment of the water droplet. Simulations performed for droplets emerging from two pores with the same size into the channel show that coalescence of two water droplets can accelerate the deformation rate and motion of the droplets in the microchannel. Accounting for the initial connection of a droplet to a pore was found to yield critical air inlet velocities for droplet detachment that are significantly different from previous studies that considered an initially stagnant droplet sitting on the surface. The predicted critical air velocity is found to be sensitive to the geometry of the pore, with higher values obtained when the curvature associated with the GDL fibres is taken into account. The critical velocity is also found to decrease with increasing droplet size and decreasing GDL pore diameter.     

Experimental investigation of water droplet–air flow interaction in a non-reacting PEM fuel cell channel

It has been well documented that water production in PEM fuel cells occurs in discrete locations, resulting in the formation and growth of discrete droplets on the gas diffusion layer (GDL) surface within the gas flow channels (GFCs). This research uses a simulated fuel cell GFC with three transparent walls in conjunction with a high speed fluorescence photometry system to capture videos of dynamically deforming droplets. Such videos clearly show that the droplets undergo oscillatory deformation patterns. Although many authors have previously investigated the air flow induced droplet detachment, none of them have studied these oscillatory modes. The novelty of this work is to process and analyze the recorded videos to gather information on the droplets induced oscillation. Plots are formulated to indicate the dominant horizontal and vertical deformation frequency components over the range of sizes of droplets from formation to detachment. The system is also used to characterize droplet detachment size at a variety of channel air velocities. A simplified model to explain the droplet oscillation mechanism is provided as well.     

Dynamics of liquid water in the anode flow channels of PEM fuel cells: A numerical parametric study

A 3D volume of fluid (VOF) model for an anode channel in a PEM fuel cell has been built. The effects of the initial position of the water droplet, its size as well as the wettability of the gas diffusion layer (GDL) are investigated under different operating conditions. It is found that the initial position of the relatively small water droplet in the channel has almost no effect on the pressure drop and the time taken for the liquid water to move out from the channel; however, such effects become more profound as the size of the water droplet increases. Also, when the droplet is placed at the side wall of the channel, then it develops into pockets of water that are mainly located at the upper corners of the channel, thus causing a smaller pressure drop compared to the cases in which the water droplet is placed either on the surface of the GDL or on the top wall of the channel. Furthermore, the hydrogen velocity is found to have a negligible effect on the dynamics of liquid water; however, the pressure drop and removal time are significantly influenced by the hydrogen velocity. Moreover, as the size of the water droplet increases, the pressure drop increases and the time required for the liquid water to move out of the channel decreases. Finally, the pressure drop in the channel decreases and the removal time of the liquid water increases as the contact angle of the GDL decreases.     

Three-dimensional numerical simulation of water droplet emerging from a gas diffusion layer surface in micro-channels

The dynamic formation of water droplets emerging from a gas diffusion layer (GDL) surface in micro-channels was simulated using the volume of fluid (VOF) method. The influence of GDL surface microstructure was investigated by changing the pore diameter and the number of pore openings on the GDL surface. Simulation results show that the microstructure of the GDL surface has a significant impact on the two-phase flow patterns in gas flow channels. For a non-uniform GDL surface, three stages were identified, namely emergence and merging on the GDL surface, accumulation on the channel sidewalls and detachment from the top wall. It was also found that if the pore size is small enough, the flow pattern in the channel does not change with further reduction in the pore diameter. However, the two-phase flow patterns change significantly with the wettability of the GDL surface and sidewalls, but remain the same when the liquid flow rate is reduced by two orders of magnitude from the reference case.     

Microporous layer for water morphology control in PEMFC

We have used environmental scanning electron microscope to observe vapor condensation and liquid water morphology and breakthrough in porous layers of polymer electrolyte membrane fuel cell. These suggest presence of large droplets and high liquid saturation at interface of the catalyst layer (CL) and gas diffusion layer (GDL), due to jump in pore size. We develop a model for morphology of liquid phase across multiple porous layers by use of both continuum and breakthrough (percolation) treatments. Using the results of this model we show the liquid morphologies deteriorate the efficiency of electrochemical reactions in CL and increase the water saturation in GDL. Then we show that inserting a microporous layer between CL and GDL reduces both the droplet size and liquid saturation and improves the cell performance.     

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.     

3D lattice Boltzmann modeling of droplet motion in PEM fuel cell channel with realistic GDL microstructure and fluid properties

A 3D multi-component multi-phase lattice Boltzmann model is developed to study the droplet motion in the flow channel of proton exchange membrane fuel cell. The model is capable of reaching realistic density and viscosity ratio, tunable surface tension with low spurious velocity and is also validated by various benchmark tests in both static and dynamic states. For the first time, the effect of realistic microstructure of gas diffusion layer (GDL) on droplet dynamic behavior is comprehensively studied in terms of comparison with smooth channel, contact angle and droplet size with the motion processes clearly illustrated. The simulation results show the GDL microstructure can amplify the material wettability, affect the motion direction and impede the droplet motion. More hydrophobic GDL can effectively accelerate the transport. However, it is observed the droplet may reach the sidewall due to the presence of GDL and the motion is therefore severely impeded regardless of the GDL contact angle or droplet size, which is hard to avoid but deadly for the water management. For this problem, a novel water management strategy is proposed and the results show the hydrophilic side & top wall can effectively remove the liquid water from the GDL surface, decrease pressure drop and prevent reactant maldistribution.