Two-phase flow pressure drop hysteresis in an operating proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in an operating PEM fuel cell. The variables studied include air stoichiometry (1.5, 2, 3, 4), temperature (50, 75, 90 °C), and the inclusion of a microporous layer. The cathode channel pressure drops can differ in PEM fuel cells when the current density is increased along a path and then decreased along the same path (pressure drop hysteresis). Generally, the descending pressure drop is greater than the ascending pressure drop at low current densities (<200 mA cm⁻²), and the effect is worse at low stoichiometries and low temperatures. The results show that the hysteresis occurs with or without the inclusion of a microporous layer. Initial results show a modified Lockhart-Martinelli approach seems to be able to predict the two-phase flow pressure drop during the ascending path. The results compare well with photographs taken from the cathode flow field channel of a visualization cell.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

Three dimensional numerical simulation of gas-liquid two-phase flow patterns in a polymer-electrolyte membrane fuel cells gas flow channel

Water management in polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on fuel cell performance and durability. To investigate the two-phase flow patterns in PEMFC gas flow channels, the volume of fluid (VOF) method was employed to simulate the air-water flow in a 3D cuboid channel with a 1.0 mm × 1.0 mm square cross section and a 100 mm in length. The microstructure of gas diffusion layers (GDLs) was simplified by a number of representative opening pores on the 2D GDL surface. Water was injected from those pores to simulate water generation by the electrochemical reaction at the cathode side. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. The water injection rate was also amplified 10 times, 100 times and 1000 times to study the flow pattern formation and transition in the channel. Simulation results show that, as the flow develops, the flow pattern evolves from corner droplet flow to top wall film flow, then annular flow, and finally slug flow. The total pressure drop increases exponentially with the increase in water volume fraction, which suggests that water accumulation should be avoided to reduce parasitic energy loss. The effect of material wettability was also studied by changing the contact angle of the GDL surface and channel walls, separately. It is shown that using a more hydrophobic GDL surface is helpful to expel water from the GDL surface, but increases the pressure drop. Using a more hydrophilic channel wall reduces the pressure drop, but increases the water residence time and water coverage of the GDL surface.     

Numerical investigation of the impact of two-phase flow maldistribution on PEM fuel cell performance

Flow maldistribution usually happens in PEM fuel cells when using common inlet and exit headers to supply reactant gases to multiple channels. As a result, some channels are flooded with more water and have less air flow while other channels are filled with less water but have excessive air flow. To investigate the impact of two-phase flow maldistribution on PEM fuel cell performance, a Volume of Fluid (VOF) model coupled with a 1D MEA model was employed to simulate two parallel channels. The slug flow pattern is mainly observed in the flow channels under different flow maldistribution conditions, and it significantly increases the gas diffusion layer (GDL) surface water coverage over the whole range of simulated current densities, which directly leads to poor fuel cell performance. Therefore, it is recommended that liquid and gas flow maldistribution in parallel channels should be avoided if possible over the whole range of operation. Increasing the gas stoichiometric flow ratio is not an effective method to mitigate the gas flow maldistribution, but adding a gas inlet resistance to the flow channel is effective in mitigating maldistribution. With a carefully selected value of the flow resistance coefficient, both the fuel cell performance and the gas flow distribution can be significantly improved without causing too much extra pressure drop.     

Gas–liquid two-phase flow patterns in parallel channels for fuel cells

Two-phase flow in horizontal parallel channels has been experimentally investigated under fuel cell related operating conditions. Pronounced hysteresis is observed in the pressure drop versus flow characteristic curve when starting from either flooded or dry conditions. When gas is introduced into channels initially filled with water (flooded initial condition), both gas and liquid tend to flow predominantly in one channel at low gas or liquid flow velocities. As the gas flow velocity increases, even distribution of gas and liquid flow in both channels is observed, accompanied with a sudden decrease in the pressure drop. On the other hand, even gas and liquid flow distribution between both channels is found at comparatively lower gas flow velocities when starting with dry-gas flow conditions with liquid introduced into channels filled with gas (stratified flow regime). The flow regimes of this system are visualized in plots of the pressure drop against gas and liquid flow velocities. However, this phenomenon tends to vanish at high gas and liquid flow velocities, suggesting that high gas and liquid flow velocities are required to ensure even flow distribution in parallel channels. The hysteresis points appear at the same level of the pressure drop, reflecting intrinsic characteristics of the parallel channels used in this study. These results have important implications for PEM fuel cell operational strategies. In order to avoid reactant mal-distribution in parallel flow channels in the flow field in the two-phase flow regime, fuel cells should be operated at sufficiently high gas flow velocities.     

Gas flow rate distributions in parallel minichannels for polymer electrolyte membrane fuel cells: Experiments and theoretical analysis

In the present work, instantaneous gas flow rates in each of two parallel channels of gas-liquid two-phase flow systems were investigated through measurements of the pressure drop across the entrance region. Liquid flow rates in two branches were pre-determined through liquid injection independently into each channel. Experiments were conducted in two different manners, i.e., the gas flow rate was varied in both ascending and descending paths. Flow hysteresis was observed in both gas flow rate distributions and the overall pressure drop of two-phase flow systems. Effects of liquid flow rates on gas flow distributions were examined experimentally. The presence of flow hysteresis was found to be associated with different flow patterns at different combinations of gas and liquid flow rates and flow instability conditions. A new and simple method was developed to predict gas flow distributions based on flow regime-specific pressure drop models for different experimental approaches and flow patterns. In particular, two different two-phase pressure drop models were used for slug flow and annular flow, separately. Good agreement was achieved between theoretical predictions and our experimental data. The developed new method can be potentially applied to predict gas flow distributions in parallel channels for fuel cells.     

Steady state and dynamic performance of proton exchange membrane fuel cells (PEMFCs) under various operating conditions and load changes

The steady-state performance and transient response for H₂/air polymer electrolyte membrane (PEM) fuel cells are investigated in both single fuel cell and stack configurations under a variety of loading cycles and operating conditions. Detailed experimental parameters are controlled and measured under widely varying operating conditions. In addition to polarization curves, feed gas flow rates, temperatures, pressure drop, and relative humidity are measured. Performance of fuel cells was studied using steady-state polarization curves, transient I–V response and electrochemical impedance spectroscopy (EIS) techniques. Different feed gas humidity, operating temperature, feed gas stoichiometry, air pressure, fuel cell size and gas flow patterns were found to affect both the steady state and dynamic response of the fuel cells. It was found that the humidity of cathode inlet gas had a significant effect on fuel cell performance. The experimental results showed that a decrease in the cathode humidity has a detrimental effect on fuel cell steady state and dynamic performance. Temperature was also found to have a significant effect on the fuel cell performance through its effect on membrane conductivity and water transport in the gas diffusion layer (GDL) and catalyst layer. The polarization curves of the fuel cell at different operating temperatures showed that fuel cell performance was improved with increasing temperature from 65 to 75 °C. The air stoichiometric flow rate also influenced the performance of the fuel cell directly by supplying oxygen and indirectly by influencing the humidity of the membrane and water flooding in cathode side. The fuel cell steady state and dynamic performance also improved as the operating pressure was increased from 1 to 4 atm. Based on the experimental results, both the steady state and dynamic response of the fuel cells (stack) were analyzed. These experimental data will provide a baseline for validation of fuel cell models.     

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.     

Along‐channel flooding prediction of polymer electrolyte membrane fuel cells

Non‐uniform current distribution in polymer electrolyte membrane (PEM) fuel cells results in local over‐heating, accelerated ageing, and lower power output than expected. This issue is quite critical when a fuel cell experiences water flooding. In this study, the performance of a PEM fuel cell is investigated under cathode flooding conditions. A two‐dimensional approach is proposed for a single PEM fuel cell based on conservation laws and electrochemical equations to provide useful insight into water transport mechanisms and their effect on the cell performance. The model results show that inlet stoichiometry and humidification, and cell operating pressure are important factors affecting cell performance and two‐phase transport characteristics. Numerical simulations have revealed that the liquid saturation in the cathode gas distribution layer (GDL) could be as high as 20%. The presence of liquid water in the GDL decreases oxygen transport and surface coverage of active catalyst, which in turn degrades the cell performance. The thermodynamic quality in the cathode flow channel is found to be greater than 99.7%, indicating that liquid water in the cathode gas channel exists in very small amounts and does not interfere with the gas phase transport. A detailed analysis of the operating conditions shows that cell performance should be optimized based on the maximum average current density achieved and the magnitude of its dispersion from its mean value. Copyright © 2010 John Wiley & Sons, Ltd.     

Error of Darcy's law for serpentine flow fields: An analytical approach

Serpentine flow fields and other flow fields with partial under-land cross-flow are commonly used in various energy devices, such as proton exchange membrane (PEM) fuel cells and redox flow batteries, due to their higher mass transfer rate to reaction sites and better product removal capability. Accurately predicting the under-land cross-flow rate and pressure drop in such flow fields is crucial in flow field design optimizations. Darcy's law is the most commonly used model in predicting the under-land cross-flow and pressure drop in such flow fields. However, since the Darcy's law neglects inertial effect, its validity in different designs and operating conditions needs to be carefully studied. In this work, mathematical models for a serpentine flow field are developed based on both the Darcy's law and a modified Darcy's law that includes the inertial effect. Both models are solved and analytical solutions are obtained. The predicted pressure drops and under-land cross-flow rates from the two models are compared with experimental data and the results show that under some conditions, both the Darcy's law and the modified Darcy's law can predict pressure drop and under-land cross-flow rate reasonably well. However, under other conditions the Darcy's law can result in significantly large errors in predicting both pressure drop and under-land cross-flow rates. Further studies provide the variations of errors from the Darcy's law with different parameters, including channel length, gas diffusion layer (GDL) thickness, land width, inlet flow rate, GDL permeability and GDL inertial coefficient.     

Impact of channel geometry on two-phase flow in fuel cell microchannels

An important function of the gas delivery channels in PEM fuel cells is the evacuation of water at the cathode. The resulting two-phase flow impedes reactant transport and causes parasitic losses. There is a need for research on two-phase flow in channels in which the phase fraction varies along the flow direction as in operating fuel cells. This work studies two-phase flow in 60 cm long channels with distributed water injection through a porous GDL wall to examine the physics of flows relevant to fuel cells. Flow regime maps based on local gas and liquid flow rates are constructed for experimental conditions corresponding to current densities between 0.5 and 2 A cm⁻² and stoichiometric coefficients from 1 to 4. Flow structures transition along the length of the channel. Stratified flow occurs at high liquid flow rates, while intermittent slug flow occurs at low liquid flow rates. The prevalence of stratified flow in these serpentine channels is discussed in relation to water removal mechanisms in the cathode channels of PEM fuel cells. Corners facilitate formation of liquid films in the channel, but may reduce the water-evacuation capability. This analysis informs design guidelines for gas delivery microchannels for fuel cells.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

An analytical model for hydrogen and nitrogen crossover rates in proton exchange membrane fuel cells

In this paper, the hydrogen and nitrogen crossover through the membrane in proton exchange membrane fuel cells, are investigated by developing a semi-empirical analytical model. Different factors that affect the gas crossover rates were considered including pressure drop in gas diffusion layer (GDL) and catalyst layer (CL), operating temperature, relative humidity (RH) of the reactants, GDL compression, and the current density effect on the membrane temperature. The model is validated by published experimental data. It is found that RH is the most important parameter, followed by temperature. The hydrogen pressure drop through GDL and CL greatly depends on the GDL substrate properties, microporous layer (MPL) and CL. When permeability is low, an increase in current density reduces gas crossover. GDL compression, when MPL is used, was found to have a low impact on gas crossover. Gas crossover is improved with current density due to an increase in membrane temperature.     

Numerical investigation of liquid water distribution in the cathode side of proton exchange membrane fuel cell and its effects on cell performance

A three-dimensional unsteady two-phase model for the cathode side of proton exchange membrane fuel cell (PEMFC) consisting of gas diffusion layer (GDL) with hybrid structural model is developed to investigate liquid water behaviors under different operating and geometrical conditions and to quantitatively evaluate effects of liquid water distribution on reactant transport and current density distribution. Simulation results reveal that liquid water transport processes and distributions are significantly affected by inlet air velocity, wall wettability and water inlet position, which in turn play a prominent role on local reactant transport and cause considerable disturbances of the current density. Liquid water film spreading on the gas channel (GC) top wall is identified as the most desirable flow pattern in the GC based on overall evaluations of current density magnitude, uniformity of current density distribution and pressure drop in the GC. Modification to GDL structure is proposed to promote the formation of the desirable flow pattern.     

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.     

Gas–liquid two-phase flow distributions in parallel channels for fuel cells

In the present study, gas–liquid two-phase flow in a parallel square minichannel system oriented horizontally and at an incline is studied under operating conditions relevant to fuel cell operations. Flow mal-distribution in parallel channels occurs at low gas and liquid flow rates. In general, high superficial gas velocities are required to ensure even flow distribution, and the minimum gas flow rates required to achieve even distribution depend on the liquid flow rates, channel orientation and experimental procedures. As the inclination angle is increased, a higher gas flow rate is required to ensure even gas–liquid flow distribution while flow channels inclined downward seems to help in improving the even flow distribution. The presence of flow hysteresis phenomena indicate that multiple flow distributions exist at the same given flow conditions when the gas flow rates are varied in ascending and descending manners. Flow mal-distribution and flow hysteresis are directly linked with flow stability. More specifically, the actual gas and liquid distribution in parallel channels is determined by the stability of mathematical solutions of mass and momentum balance equations and also the flow history. For the first time, the present work investigates flow distributions in fuel cell flow fields by accounting for two-phase flow conditions. In addition, a novel approach is introduced to ensure flow distributions and their stability through contour construction of isobars where unstable flow region can be identified, which can be used in the design of parallel channel flow fields, especially for fuel cells.     

Design strategy for a polymer electrolyte membrane fuel cell flow-field capable of switching between parallel and interdigitated configurations

Novel water management strategies are important to the development of next generation polymer electrolyte membrane fuel cell systems (PEMFCs). Parallel and interdigitated flow fields are two common types of PEMFC designs that have benefits and draw backs depending upon operating conditions. Parallel flow fields rely predominately on diffusion to deliver reactants and remove byproduct water. Interdigitated flow fields induce convective transport, known as cross flow, through the porous gas diffusion layer (GDL) and therefore are superior at water removal beneath land areas which can lead to higher cell performance. Unfortunately, forcing flow through the GDL results in higher pumping losses as the inlet pressure for interdigitated flow fields can be up to an order of magnitude greater than that for a parallel flow field. In this study a flow field capable of switching between parallel and interdigitated configurations was designed and tested. Results show, taking into account pumping losses, that using constant stoichiometry the parallel flow field results in a higher system power under low current density operation compared to the interdigitated configuration. The interdigitated flow-field configuration was observed to have lower overvoltage at elevated current densities resulting in a higher maximum power and a higher limiting current density. An optimal system power curve was produced by switching from parallel to interdigitated configuration based on which produces a higher system power at a given current density. This design method can be easily implemented with current PEMFC technology and requires minimal hardware. Some of the consequences this design has on system components are discussed.     

Hydrogen pressure drop characteristics in a fuel cell stack

Research on hydrogen pressure characteristics was performed for a fuel cell stack to supply a rule of hydrogen pressure drop for flooding diagnostic systems. Some experiments on the hydrogen pressure drop in various operating pressure, temperature, flowrate and stack current conditions were carried out, and hydrodynamic calculation was managed to compare with the experiment results. Results show that the hydrogen pressure drop is strongly affected by liquid water content in the flow channel of fuel cells, and it is not in normal relation with flowrate when the stoichiometric ratio is inconstant. The total pressure drop can be calculated by a frictional pressure loss formula accurately, relating with mixture viscosity, stack temperature, operating pressure, stoichiometric ratio and stack current. The pressure drop characteristics will be useful for predicting liquid water flooding in fuel cell stacks before flow channels have been jammed as a diagnostic tool in electric control systems.     

Performance superiority of an arc-shaped polymer electrolyte membrane fuel cell over a straight one

Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a promising electricity-producing technology but needs further improvement to become economically viable. Oxygen transfer to reaction zone is known as one of the main PEMFC performance-limiting factors. Accordingly, various recent studies have been focused on fuel cell design to improve oxygen transfer. The present study numerically investigates the influences of converting a straight (or planar) PEMFC to a bent (arc-shaped) one. The idea of PEMFC bending originates from the fact that it can create a velocity component perpendicular to gas diffusion layer (GDL) and exert centrifugal force on channel gas flow towards the GDL; thereby, enhancing oxygen transfer. The results indicate that PEMFC bending can enhance performance up to about 8.33% for the examined operating conditions. It is also observed that PEMFC bending impact factor generally increases with operating pressure, stoichiometry ratio and bending angle, but decreases with operating voltage.     

Current density distribution in an HT-PEM fuel cell with a poly (2, 5-benzimidazole) membrane

High-temperature proton exchange membrane (HT-PEM) fuel cells were more useable than traditional low-temperature proton exchange membrane fuel cells. To investigate the current density distribution in a single HT-PEM fuel cell with a poly (2, 5-benzimidazole) membrane, a modified current distribution measuring device was developed. This device included not only a current distribution measuring gasket to collect local current but also a segmented gas diffusion layer (GDL) to hinder electron transfer in the GDL along the gas flow direction. The effects of this device installation configuration and operating conditions on the current density distribution were analyzed. One of the important findings was that proton transfer along an in-plane direction in the membrane and electron transfer along an in-plane direction in the GDL really occur in HT-PEM fuel cells. These results were very helpful for the optimization of the flow field and operating parameters of the HT-PEM fuel cells.