Determination of the optimal active area for proton exchange membrane fuel cells with parallel,interdigitated or serpentine designs

Effects of active area size on steady-state characteristics of a working PEM fuel cell, including local current densities, local oxygen transport rates, and liquid water transport were studied by applying a three-dimensional, two-phase PEM fuel cell model. The PEM fuel cells were with parallel, interdigitated, and serpentine flow channel design. At high operating voltages, the size effects on cell performance are not noticeable owing to the occurrence of oxygen supply limit. The electrochemical reaction rates are high at low operating voltages, producing large quantity of water, whose removal capability is significantly affected by flow channel design. The cells with long parallel flow field experience easy water accumulation, thereby presenting low oxygen transport rate and low current density. The cells with interdigitated and serpentine flow fields generate forced convection stream to improve reactant transport and liquid water removal, thereby leading to enhanced cell performance and different size effect from the parallel flow cells. Increase in active area significantly improves performance for serpentine cells, but only has limited effect on that of interdigitated cells. Size effects of pressure drop over the PEM cells were also discussed.     

A numerical study on thermal analysis and cooling flow fields effect on PEMFC performance

Thermal management and water management are two important interconnected topics in the design and increase the efficiency of PEM fuel cells. Suitable cooling flow field design with proper performance is an important factor in increasing the lifetime of PEM fuel cell, because non-uniformity of temperature reduces the stability and durability of PEM fuel cell. Different cooling strategies are considered for removing of heat generation by PEM fuel cell, because the fuel cell temperature remains in a tolerable range and homogeneous as possible. In the first step, determine the value and location of heat sources in fuel cell, is important and appropriate cooling strategy can be applied. In this study, a PEM fuel cell with serpentine gas flow field is simulated with six different cooling flow fields simultaneously, e.g. conventional serpentine (Model 1), typical of MPSFFs (model 2), typical of serpentine (Model 3), parallel-serpentine (Model 4), conventional spiral (Model 5) and conventional parallel (Model 6). This simulation showes a correct perception of temperature distribution in PEMFC. The results indicate that Model 5 has a good temperature and performance based on the minimum and maximum temperature gradient, Index of uniform temperature (IUT), however has a more pressure drop. For second choice when the pressure drop is important, the Model 3 has a better performance than other models. Also, thermal analysis in these cases shows that ohmic, entropic and reaction heating included 20%, 35%, 45% of the total heat generation by PEMFC, approximately.     

Liquid water transport in parallel serpentine channels with manifolds on cathode side of a PEM fuel cell stack

Water management in a proton exchange membrane (PEM) fuel cell stack has been a challenging issue on the road to commercialization. This paper presents a numerical investigation of air–water flow in parallel serpentine channels on cathode side of a PEM fuel cell stack by use of the commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different air–water flow behaviours inside the serpentine flow channels with inlet and outlet manifolds were discussed. The results showed that there were significant variations of water distribution and pressure drop in different cells at different times. The “collecting-and-separating effect” due to the serpentine shape of the gas flow channels, the pressure drop change due to the water distribution inside the inlet and outlet manifolds were observed. Several gas flow problems of this type of parallel serpentine channels were identified and useful suggestions were given through investigating the flow patterns inside the channels and manifolds.     

Effect of GDL compression on pressure drop and pressure distribution in PEMFC flow field

Improving reactant distribution is an important technological challenge in the design of a PEMFC. Flow field and the Gas Diffusion Layer (GDL) distribute the reactant over the catalyst area in a cell. Hence it is necessary to consider flow field and GDL together to improve their combined effectiveness. This paper describes a simple and unique off-cell experimental setup developed to determine pressure as a function of position in the active area, due to reactant flow in a fuel cell flow field. By virtue of the experimental setup being off-cell, reactant consumption, heat production, and water generation, are not accounted as experienced in a real fuel cell. A parallel channel flow field and a single serpentine flow field have been tested as flow distributors in the experimental setup developed. In addition, the interaction of gas diffusion layer with the flow distributor has also been studied. The gas diffusion layer was compressed to two different thicknesses and the impact of GDL compression on overall pressure drop and pressure distribution over the active area was obtained using the developed experimental setup. The results indicate that interaction of GDL with the flow field and the effect of GDL compression on overall pressure drop and pressure distribution is more significant for a serpentine flow field relative to a parallel channel flow field.     

Application of Open Pore Cellular Foam for air breathing PEM fuel cell

Open Pore Cellular Foam (OPCF) has received increased attention for use in Proton Exchange Membrane (PEM) fuel cells as a flow plate due to some advantages offered by the material, including better gas flow, lower pressure drop and low electrical resistance.In the present study, a novel design for an air-breathing PEM (ABPEM) fuel cell, which allows air convection from the surrounding atmosphere, using OPCF as a flow distributor has been developed. The developed fuel cell has been compared with one that uses a normal serpentine flow plate, demonstrating better performance.A comparative analysis of the performance of an ABPEM and pressurised air PEM (PAPEM) fuel cell is conducted and poor water management behaviour was observed for the ABPEM design.Thereafter, a PTFE coating has been applied to the OPCF with contact angle and electrochemical polarisation tests conducted to assess the capability of the coating to enhance the hydrophobicity and corrosion protection of metallic OPCF in the PEM fuel cell environment. The results showed that the ABPEM fuel cell with PTFE coated OPCF had a better performance than that with uncoated OPCF.Finally, OPCF was employed to build an ABPEM fuel cell stack where the performance, advantages and limitations of this stack are discussed in this paper.     

Numerical study of a new cathode flow-field design with a sub-channel for a parallel flow-field polymer electrolyte membrane fuel cell

The cathode flow-field design of a polymer electrolyte membrane (PEM) fuel cell is crucial to its performance, because it determines the distribution of reactants and the removal of liquid water from the fuel cell. In this study, the cathode flow-field of a parallel flow-field PEM fuel cell was optimized using a sub-channel. The main-channel was fed with moist air, whereas the sub-channel was fed with dry air. The influences of the sub-channel flow rate (SFR, the amount of air from the sub-channel inlet as a percentage of the total cathode flow rate) and the inlet positions (SIP, where the sub-channel inlets were placed along the cathode channel) on fuel cell performance were numerically evaluated using a three-dimensional, two-phase fuel cell model. The results indicated that the SFR and SIP had significant impacts on the distribution of the feed air, removal of liquid water, and fuel cell performance. It was found that when the SIP was located at about 30% along the length of the channel from main-channel inlet and the SFR was about 70%, the PEM fuel cell exhibited much better performance than seen with a conventional design.     

Experimental research on water management in proton exchange membrane fuel cells

A simulated cathode flow channel experiment system was set up based on the gas flow rate and water flow rate in the PEM fuel cell. With the assistance of the visualization system, high-sensitivity double parallel conductance probes flow regime inspecting technique was adopted successfully in the experiment system to inspect the flow regime of the gas–liquid two-phase flow in the PEM fuel cell. The research results show that the double parallel conductance probes inspecting system and the flow regime image system for the gas–liquid two-phase flow in the PEM fuel cell simulated channel both can judge the slug flow and annular flow in it, and the double parallel conductance probes flow regime inspecting system can divide the annular flow into three subtypes. The main probes inspecting system and the assistant image system validate reciprocally, which enhances the experimental veracity. The typical flow regimes of the PEM fuel cell simulated channel include slug flow, annular flow with big water film wave, annular flow with small water film wave and annular flow without water film wave. With the increase of the liquid superficial velocity, the frequencies of liquid slug and wave of liquid film increase. The flow regime map in the flow channel of the PEM fuel cell was developed. The flow regime of the gas–liquid two-phase flow in a PEM fuel cell in different operating conditions can be forecasted with this map. With the PEM fuel cell operating condition in this study, the flow regimes of gas–liquid two-phase flow for different cases are all annular flow with small water film wave, and the liquid film waves more with bigger current density. With the location closer to the channel outlet, the liquid film waves are more for the same current density.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.     

Comparison of temperature distributions inside a PEM fuel cell with parallel and interdigitated gas distributors

A comparison of the temperature distributions in a proton exchange membrane (PEM) fuel cell between the parallel-flow gas distributors and the interdigitated gas distributor has been discussed in detail. An electrochemical–thermal coupled numerical model in a five-layer membrane-electrode assembly (MEA) is developed. The temperatures for the reactant fuels as well as the carbon fibers in the porous electrode are predicted by using a CFD technique. The overpotential across the MEA is varied to examine its effect on the temperature distributions of the PEM fuel cell. It is found that both the fuel temperature and the carbon fiber temperature are increased with increasing the total overpotential. In addition, the fuel and carbon-fiber temperature distributions are significantly affected by the flow pattern that cast on the gas distributor. Replacing the parallel-flow gas distributor by the interdigitated gas distributor will increase the local maximum temperature inside the PEM fuel cell.     

Effect of air flow on liquid water transport through a hydrophobic gas diffusion layer of a polymer electrolyte membrane fuel cell

The transport of liquid water through an idealized 2-D reconstructed gas diffusion layer (GDL) of a polymer electrolyte membrane (PEM) fuel cell is computed subject to hydrophobic boundary condition at the fibre–fluid interface. The effect of air flow, as would occur in parallel/serpentine/interdigitated type of flow fields, on the liquid water transport through the GDL, ejection into the channel in the form of water droplets and subsequent removal of the droplets has been simulated. Results show that typically water flow through the fibrous GDL occurs through a fingering and channelling type of mechanism. The presence of cross-flow of air has an effect both on the path created within the GDL and on the ejection of water into the channel in the form of droplets. A faster rate of liquid water evacuation through the GDL (i.e., more frequent ejection of water droplets) as well as less flooding of the void space results from the presence of cross-flow. These results agree qualitatively with experimental observations reported in the literature.     

Innovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode

Liquid water transport is one of the key challenges regarding the water management in a proton exchange membrane (PEM) fuel cell. Conventional gas diffusion layers (GDLs) do not allow a well-organized liquid water flow from catalyst layer to gas flow channels. In this paper, three innovative GDLs with different micro-flow channels were proposed to solve liquid water flooding problems that conventional GDLs have. This paper also presents numerical investigations of air–water flow across the proposed innovative GDLs together with a serpentine gas flow channel on PEM fuel cell cathode by use of a commercial computational fluid dynamics (CFD) software package FLUENT. The results showed that different designs of GDLs will affect the liquid water flow patterns significantly, thus influencing the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown. Several gas flow problems for the proposed different kinds of innovative GDLs were observed, and some useful suggestions were given through investigating the flow patterns inside the proposed GDLs.     

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.     

Effects of serpentine flow field with outlet channel contraction on cell performance of proton exchange membrane fuel cells

A serpentine flow field with outlet channels having modified heights or lengths was designed to improve reactant utilization and liquid water removal in proton exchange membrane (PEM) fuel cells. A three-dimensional full-cell model was developed to analyze the effects of the contraction ratios of height and length on the cell performance. Liquid water formation, that influences the transport phenomena and cell performance, was included in the model. The predictions show that the reductions of the outlet channel flow areas increase the reactant velocities in these regions, which enhance reactant transport, reactant utilization and liquid water removal; therefore, the cell performance is improved compared with the conventional serpentine flow field. The predictions also show that the cell performance is improved by increments in the length of the reduced flow area, besides greater decrements in the outlet flow area. If the power losses due to pressure drops are not considered, the cell performance with the contracted outlet channel flow areas continues to improve as the outlet flow areas are reduced and the lengths of the reduced flow areas are increased. When the pressure losses are also taken into account, the optimal performance is obtained at a height contraction ratio of 0.4 and a length contraction ratio of 0.4 in the present design.     

Study of novel flow channels influence on the performance of direct methanol fuel cell

The existing flow channels like parallel and gird channels have been modified for better fuel distribution in order to boost the performance of direct methanol fuel cell. The main objective of the work is to achieve minimized pressure drop in the flow channel, uniform distribution of methanol, reduced water accumulation, and better oxygen supply. A 3D mathematical model with serpentine channel is simulated for the cell temperature of 80 °C, 0.5 M methanol concentration. The study resulted in 40 mW/cm² of power density and 190 mA/cm² of current density at the operating voltage of 0.25 V. Further, the numerical study is carried out for modified flow channels to discuss their merits and demerits on anode and cathode side. The anode serpentine channel is unmatched by the modified zigzag and pin channels by ensuring the better methanol distribution under the ribs and increased the fuel consumption. But the cathode serpentine channel is lacking in water management. The modified channels at anode offered reduced pressure drop, still uniform reactant distribution is found impossible. The modified channels at cathode outperform the serpentine channel by reducing the effect of water accumulation, and uniform oxygen supply. So the serpentine channel is retained for methanol supply, and modified channel is chosen for cathode reactant supply. In comparison to cell with only serpentine channel, the serpentine anode channel combined with cathode zigzag and pin channel enhanced power density by 17.8% and 10.2% respectively. The results revealed that the zigzag and pin channel are very effective in mitigating water accumulation and ensuring better oxygen supply at the cathode.     

Liquid water transport in straight micro-parallel-channels with manifolds for PEM fuel cell cathode

Water management in a proton exchange membrane (PEM) fuel cell stack has been a challenging issue on the road to commercialization. This paper presents a numerical investigation of air–water flow in micro-parallel-channels with PEM fuel cell stack inlet and outlet manifolds for the cathode, using a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different air–water flow behaviours inside the straight micro-parallel-channels with inlet and outlet manifolds were simulated and discussed. The results showed that excessive and unevenly distributed water in different single PEM fuel cells could cause blockage of airflow or uneven distribution of air along the different flow channels. It is found that for a design with straight-channels, water in the outflow manifold could be easily blocked by air/water streams from the gas flow channels; the airflow could be severely blocked even if there was only a small amount of water in the gas flow channels. Some important suggestions were made to achieve a better design.     

Three-dimensional two-phase flow model of proton exchange membrane fuel cell with parallel gas distributors

A non-isothermal, steady-state, three-dimensional (3D), two-phase, multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with parallel gas distributors. A key feature of this work is that a detailed membrane model is developed for the liquid water transport with a two-mode water transfer condition, accounting for the non-equilibrium humidification of membrane with the replacement of an equilibrium assumption. Another key feature is that water transport processes inside electrodes are coupled and the balance of water flux is insured between anode and cathode during the modeling. The model is validated by the comparison of predicted cell polarization curve with experimental data. The simulation is performed for water vapor concentration field of reactant gases, water content distribution in the membrane, liquid water velocity field and liquid water saturation distribution inside the cathode. The net water flux and net water transport coefficient values are obtained at different current densities in this work, which are seldom discussed in other modeling works. The temperature distribution inside the cell is also simulated by this model.     

An analytical analysis on the cross flow in a PEM fuel cell with serpentine flow channel

A serpentine flow channel can be considered as neighboring channels connected in series, and is one of the most common and practical channel layouts for polymer electrolyte membrane (PEM) fuel cells, as it ensures the removal of liquid water produced in a cell with good performance and acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross directly to the neighboring channel via the porous gas diffusion layer (GDL) due to the high‐pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area resulting in a substantially lower amount of pressure drop in an actual PEM fuel cell compared with the case without cross flow. In this study, an analytical solution is obtained for the cross flow in a PEM fuel cell with a serpentine flow channel based on the assumption that the velocity of cross flow is linearly distributed in the GDL between two successive U‐turns. The analytical solution predicts the amount of pressure drop and the average volume flow rate in the flow channel and the GDL. The solution is validated over a wide range of the thickness and permeability of the GDL by comparing the results with experimental measurements and 3‐D numerical simulations in literature. Excellent agreement is obtained for the permeability less than 10^?9 m², which covers the typical permeability values of the GDLs in actual PEM fuel cells. The solution presents an accurate and efficient estimation for cross flow providing a useful tool for the design and optimization of PEM fuel cells with serpentine flow channels. Copyright © 2010 John Wiley & Sons, Ltd.     

Local transport phenomena and cell performance of PEM fuel cells with various serpentine flow field designs

The flow field design in bipolar plates is very important for improving reactant utilization and liquid water removal in proton exchange membrane fuel cells (PEMFCs). A three-dimensional model was used to analyze the effect of the design parameters in the bipolar plates, including the number of flow channel bends, number of serpentine flow channels and the flow channel width ratio, on the cell performance of miniature PEMFCs with the serpentine flow field. The effect of the liquid water formation on the porosities of the porous layers was also taken into account in the model while the complex two-phase flow was neglected. The predictions show that (1) for the single serpentine flow field, the cell performance improves as the number of flow channel bends increases; (2) the single serpentine flow field has better performance than the double and triple serpentine flow fields; (3) the cell performance only improves slowly as the flow channel width increases. The effects of these design parameters on the cell performance were evaluated based on the local oxygen mass flow rates and liquid water distributions in the cells. Analysis of the pressure drops showed that for these miniature PEMFCs, the energy losses due to the pressure drops can be neglected because they are far less than the cell output power.     

Effects of geometrical dimensions of flow channels of a large-active-area PEM fuel cell: A CFD study

Various flow field designs have been numerically investigated to evaluate the effect of pattern and the cross-sectional dimensions of the channel on the performance of a large active area PEM fuel cell. Three types of multiple-serpentine channels (7-channels, 11-channels and 14-channels) have been chosen for the 200 cm² fuel cell investigated and numerically analysed by varying the width and the land of the channel. The CFD simulations showed that as the channel width decreases, as in the 14-channels serpentine case, the performance improves, especially at high current densities where the concentration losses are dominant. The optimum configuration, i.e. the 14-channels serpentine, has been manufactured and tested experimentally and a very good agreement between the experimental and modelling data was achieved. 4 channel depths have been considered (0.25, 0.4, 0.6 and 0.8 mm) in the CFD study to determine the effects on the pressure drop and water content. Up to 7% increase in the maximum reported current density has been achieved for the smallest depth and this due to the better removal of excess liquid water and better humidification of the membrane. Also, the influence of the air flow rate has been evaluated; the current density at 0.6 V increased by around 25% when air flow rate was increased 4 times; this is attributed to better removal of excess liquid water.