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 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.     

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.     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation of water droplet kinetics and optimization of channel geometry for PEM fuel cell cathodes

The kinetics and transport mechanisms of water droplets in model flow field channels of a low temperature polymer electrolyte fuel cell were investigated. The pressure drop at different air flows was measured for different channel geometries in a graphite plate as employed for fuel cell bipolar plates. The minimum air flow required for the movement of a water droplet in the flow channel was identified. From the experimental findings, recommendations for the development of flow fields with high condensate removal capabilities combined with low pressure differences were drawn to allow for an efficient operation of PEM fuel cells.     

Comparative analysis of two-phase flow in sinusoidal channel of different geometric configurations with application to PEMFC

The droplet dynamics inside a sinusoidal channel for PEMFC (polymer electrolyte membrane fuel cell) are investigated numerically using the VOF (volume of fluid) method. This study is done for three geometrically different channels corresponding to various non-dimensional sinusoidal distances (50, 25, 12.5, 16.7 and 8.3). The effects of key parameters like sinusoidal distance (pitch-amplitude ratio), radius of curvature and wall contact angle on the droplet removal in the flow channel are investigated. The performance of the sinusoidal as compared to the conventional channel is studied based on droplet removal rate and GDL (gas diffusion layer) surface water coverage. It is found that the droplet removal rate increases with increasing sinusoidal distance and wall contact angle. In addition, decrease in the sinusoidal distance results in a significant reduction in the average droplet speed and gas diffusion layer surface water coverage. It was also observed that broken bits of the droplet stuck on the wall corners accrued with a reduction in the wall contact angle. The curvy nature of the side walls generally induces a secondary flow effect which would be most beneficial in enhanced reactant diffusion and cell performance. It is suggested that the sinusoidal distance and wall contact angle effect on two-phase flow in a channel is highly significant. As such, needs to be considered for water management in sinusoidal channels.     

Droplet dynamics in a proton exchange membrane fuel cell flow field design with 3D geometry

Water management is crucial to achieve both high-performance and durability of proton exchange membrane fuel cell (PEMFC). Therefore, it is necessary to investigate the dynamic behavior of droplets in PEMFC channel for water management. In this paper, we explore the kinetics of droplets in a 3D flow field by experimental and theoretical analysis. More specifically, we examine the following four perspectives: 1) the movement and falling of droplets, and their force and deformation, 2) the superiority of 3D flow field drainage, 3) the pressure and viscous force under different scenarios including varying droplet sizes and velocities, and 4) the expression describing the shape change of droplets. The results show that the 3D flow field has a greater driving force on droplets and that their deformation affects the discharge of liquid water. Throughout the study, we provide better understanding of droplet dynamic in PEMFC gas channels. It enables to optimize the design and working conditions of these channels.     

Numerical simulation of two-phase flow in a multi-gas channel of a proton exchange membrane fuel cell

Understanding the two-phase distribution characteristics within the multi-gas channel of a fuel cell is important for improving fuel cell performance. In the paper, the volume of fluid model is used to predict the dynamic behaviour of water in the multi-gas channel, analyze the pressure drop, velocity distribution, and flow resistance coefficient between different channels, and investigate the influence of operating conditions, surface wettability and channel structure on the two-phase distribution characteristics in the channel. The results show that water undergoes the processes of growth, separation, single droplet transport, wall impact, droplet collision, liquid film formation, and liquid film transport in the multi-gas channel. Inlet velocity and surface wettability significantly affect the pressure drop, water saturation, and surface water coverage. As the inlet velocity and gas diffusion layer surface wettability increase, the flow resistance coefficient and unevenness of the distribution decrease, indicating that the in-channel flow distribution homogeneity is enhanced. The rectangular channel has better water removal and flow distribution uniformity than the tapered channel, and the unevenness of distribution decreases significantly with decreasing rectangular width, from 0.15715 to 0.00315. The research work is a guide to understanding water transport in multi-gas channels, accelerating water removal, and improving inter-channel flow distribution uniformity.     

Numerical simulation of liquid water emerging and transport in the flow channel of PEMFC using the volume of fluid method

Three-dimensional numerical simulation of liquid water emerging from the gas diffusion layer (GDL) surface to the gas flow channel in the proton exchange membrane (PEM) fuel cell (PEMFC) is carried out using the volume of fluid (VOF) method. The effects of the water velocity in the GDL hole, the airflow velocity and the wettability of the channel surfaces on the water emerging process and transport in the flow channel are investigated. It is found that at low water velocity, the water detaches from the water hole, forming discrete water droplets on the GDL surface, and is transported downstream on the GDL surface until removed from the GDL surface by the U-turn part of the flow channel; whereas at high water velocity, the continuous water column impinges the hydrophilic channel surface counter to the GDL surface, being directly removed from the GDL surface. The airflow velocity affects water detachment and impact process in the channel corner, and water droplet breakup is observed under high airflow velocity. The channel surface wettability influences water droplet shape and its transport in the channel. Rather than forming corner water films at the U-turn for hydrophilic channel surface, water maintains the droplet shape and smoothly passes through the U-turn for hydrophobic channel surface. The importance of the U-turn to the water removal is also discussed. The U-turn promotes water removal from the GDL surface at low water velocity and water breakup at high airflow velocity.     

Visualisation of water droplets during the operation of PEM fuel cells

A transparent proton exchange membrane fuel cell (PEMFC) has been designed to enable visualisation of water droplets during its operation. Images of the formation of droplets on the surface of the gas diffusion layer (GDL) on its cathode side, which result in water accumulation and blockage to the airflow channels, were recorded using a CCD camera. Measurement of the cell current and droplet characterisation have been carried out simultaneously and the effect of the airflow and external resistive load has been quantified. The droplet images show that water accumulation occurs first in the middle channels of a serpentine reactant-flow fuel cell design and that no droplets are formed at the bends of the flow channels. Water blockage to the airflow path was caused by the overlapping of two land-touching droplets developing on each side of the channel. Flooding was found to be more susceptible to the airflow than the other test operating conditions.     

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.     

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.     

Numerical investigation of water dynamics in a novel wettability gradient anode flow channel for proton exchange membrane fuel cells

The effective removal and transport of water in flow channels play an important role in the water management of proton exchange membrane fuel cells (PEMFCs). In this paper, a novel design of anode serpentine flow channel with the wettability gradient wall is discussed and numerically investigated by utilizing the volume-of-fluid (VOF) method. The effects of the contact angle and the wettability gradient of channel walls, as well as hydrogen flow velocity and water droplet size, on the droplet dynamic behavior are studied. The results indicate that compared with the conventional flow channel, the water droplet can be more effectively removed from the turning part in the wettability gradient flow channel. And the water removal ability in the turning part is improved with the increase of the wettability gradient. Moreover, the wettability gradient flow channel can also improve the water removal performance for the cases with different hydrogen flow velocities and water droplet sizes. This study provides ideas for guiding the design of flow channel to effectively enhance anode water management.     

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.     

Numerical simulation of water droplet transport characteristics in cathode channel of proton exchange membrane fuel cell with tapered slope structures

In proton exchange membrane fuel cells (PEMFC), the design of the cathode flow field is very important, because an excellent flow channel design can not only accelerate the transmission rate of liquid water, but also affect the distribution of electrode reactants and electrode products which influence the electrochemical performance of the fuel cell. This study presents three new channels (models 1,2 and 3), which were created using two unilateral slopes and a bilateral slope structure with tapered tube lengths of 0.4, 1.2 and 0.8 mm, respectively. The dynamic behavior of liquid water under the three design schemes is numerically studied based on the volume of fluid method. And the influence on the performance of fuel cell was analyzed synthetically. The results indicate that the introduction of a tapered and sloping structure can improve the transmission efficiency of the droplets in the flow channel, and the maximum droplet removal time of the new channel can be reduced by 24.4%compare with standard conventional flow channel. The slope structure guides the flow path of water droplet and reduces the occurrence of droplet spatter. Influenced by the slope and tapered structures, the turbulence of airflow near the bottom surface (gas diffusion layer)of the flow channel is enhanced and Oxygen concentration in the cathode is raised, which improves the mass transfer capacity and average current density of reactive surface. In conclusion, the new type of channel with a tapered and sloping structure has a potential to improve the performance of water management in the cathode channel of PEMFC.     

Simultaneous visualization of oxygen distribution and water blockages in an operating triple-serpentine polymer electrolyte fuel cell

Visualization inside polymer electrolyte fuel cells (PEFCs) is important for elucidating reaction distributions to improve the performance and durability of the cells. An O₂-sensitive porphyrin luminescent dye film was used to visualize oxygen partial pressures and water blockages simultaneously in triple-serpentine gas flow channels in an operating PEFC. Water droplets formed near the exit of a gas-flow channel lowered the oxygen partial pressure noticeably over the channel by blocking air flow near the entrance. Meanwhile, air was continuously supplied from the other channels through the gas diffusion layer, thus allowing power to be generated in the blocked channel. With water blockages, however, the catalyst layer under the channel became flooded by the water produced during the reaction, and the flooded state continued to exist in the catalyst and/or porous layers, even after blowing the water droplet out, so that the power generation was lowered along the channel.     

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.     

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.     

Direct observation of the two-phase flow in the air channel of a proton exchange membrane fuel cell and of the effects of a clogging/unclogging sequence on the current density distribution

A small single-channel fuel cell prototype was built with the objective of monitoring the appearance and transport of water droplets in the gas channels in usual operating conditions. It allows the simultaneous observation of droplets and of their local effects on current density. The first results show that the air flow rate seems to control the transition between two different water removal mechanisms: a plug flow when the air stoichiometry is low, with significant disturbances in the local current density, pressure drop and fuel cell performance, and a more conventional flow with steadier removal of smaller droplets when the stoichiometry is higher.     

Influence of corner structure of fuel cell serpentine channel on water removal

Proton exchange membrane fuel cell (PEMFC), as a representative of fuel cell technology, has attracted much attention for its huge advantages in transportation application. Water management has a significant impact on the lifetime and performance of PEMFC. This study numerically investigates the movement of a water droplet in a single serpentine flow channel of PEMFC with different U-turn designs. The transport characteristics of the water droplet are obtained under both low and high airflow speeds, respectively. It is found that the droplet can pass through the U-turn for large fillet radius, while it is stuck in the corners of the U-turn for small fillet radius. The water droplet is transported as a whole without breakup under low airflow speed, but it is split into small droplets under high airflow speed at the U-turn. Hydrophobic channel surface promotes water removal at the U-turn of the channel, but its effect is weakened under high airflow speed.