Numerical study of cell performance and local transport phenomena in PEM fuel cells with various flow channel area ratios

Three-dimensional models of proton exchange membrane fuel cells (PEMFCs) with parallel and interdigitated flow channel designs were developed including the effects of liquid water formation on the reactant gas transport. The models were used to investigate the effects of the flow channel area ratio and the cathode flow rate on the cell performance and local transport characteristics. The results reveal that at high operating voltages, the cell performance is independent of the flow channel designs and operating parameters, while at low operating voltages, both significantly affect cell performance. For the parallel flow channel design, as the flow channel area ratio increases the cell performance improves because fuel is transported into the diffusion layer and the catalyst layer mainly by diffusion. A larger flow channel area ratio increases the contact area between the fuel and the diffusion layer, which allows more fuel to directly diffuse into the porous layers to participate in the electrochemical reaction which enhances the reaction rates. For the interdigitated flow channel design, the baffle forces more fuel to enter the cell and participate in the electrochemical reaction, so the flow channel area ratio has less effect. Forced convection not only increases the fuel transport rates but also enhances the liquid water removal, thus interdigitated flow channel design has higher performance than the parallel flow channel design. The optimal performance for the interdigitated flow channel design occurs for a flow channel area ratio of 0.4. The cell performance also improves as the cathode flow rate increases. The effects of the flow channel area ratio and the cathode flow rate on cell performance are analyzed based on the local current densities, oxygen flow rates and liquid water concentrations inside the cell.     

Numerical analysis on performances of polymer electrolyte membrane fuel cells with various cathode flow channel geometries

This paper investigated numerically the effect of cathode channel shapes on the local transport characteristics and cell performance by using a three-dimensional, two-phase, and non-isothermal polymer electrolyte membrane (PEM) fuel cell model. The cells with triangle, trapezoid, and semicircle channels were examined using that with rectangular channel as comparison basis. At high operating voltages, the cells with various channel shapes would have similar performance. However, at low operating voltages, the fuel cell performance would follow: triangle > semicircle > trapezoid > rectangular channel. Analyses of the local transport phenomena in the cell indicate that triangle, trapezoid, and semicircle channel designs increase remarkably flow velocity of reactant, enhancing liquid water removal and oxygen utilization. Thus, these designs increase the limiting current density and improve the cell performance relative to rectangular channel design.     

Numerical analysis of the influence of the channel cross-section aspect ratio on the performance of a PEM fuel cell with serpentine flow field design

This work numerically investigates the influence of the channel cross-section aspect ratio (defined as the ratio height/width) on the performance of a PEM fuel cell with serpentine flow field (SFF) design. The local current densities, velocity distributions, liquid water concentration in the membrane, hydrogen and oxygen concentrations and temperature were analyzed in the PEM fuel cell for 10 different aspect ratios, varying between 0.07 and 15, to understand the channel cross-section aspect ratio effect. The area of the channel cross section (1.06 mm²) and the total effective reactive area of the PEM fuel cell (256 mm²) were maintained constant in all cases. The obtained results show that at low operating voltages the cell performance is independent of the channel cross-section aspect ratio. At high operating voltages, where the influence of mass transporting velocity is predominant, as the channel cross-section aspect ratio increases the cell performance is improved. The models with high aspect ratio show, in general, more uniform current distributions, with the higher maximum and minimum intensity values, temperature distributions with smaller gradients and a superior contain of water in the membrane, which allows to obtain a higher performance. From these models the 10/06 and 12/05 aspect ratio present the best combination of variables, as shown by their polarization curves.     

Different flow fields,operation modes and designs for proton exchange membrane fuel cells with dead-ended anode

Water and nitrogen can accumulate in the anode channel in proton exchange membrane fuel cells (PEMFCs) with dead-ended anode (DEA) and can affect cell performance significantly. In this paper, the cell performance characteristics in DEA PEMFCs with three different anode flow fields under two operating modes are studied through measuring the cell voltages and local current densities. The effect of the anode exit reservoir is also studied for the three different anode flow fields. The experimental results show that the interdigitated flow field has the most stable cell performance under both constant pressure and pressure swing supply modes. Parallel and serpentine flow fields lead to very non-uniform local current distributions under constant pressure supply mode and experience severe fluctuations and spikes in local current densities under pressure swing supply mode. The results also show that anode pressure swing supply mode can achieve more stable cell performance than anode constant pressure supply mode for parallel and serpentine anode flow fields. The anode exit reservoir can significantly improve cell performance stability for parallel and serpentine flow fields, but has no significant effect on interdigitated flow fields. Besides, the results further show that PEMFCs with DEA can maintain very stable operation with anode serpentine flow field and an anode exit reservoir under pressure swing operation.     

An inverse geometry design problem for optimization of single serpentine flow field of PEM fuel cell

The cathode flow-field design of a proton exchange membrane fuel cell (PEMFC) determines its reactant transport rates to the catalyst layer and removal rates of liquid water from the cell. This study optimizes the cathode flow field for a single serpentine PEM fuel cell with 5 channels using the heights of channels 2–5 as search parameters. This work describes an optimization approach that integrates the simplified conjugated-gradient scheme and a three-dimensional, two-phase, non-isothermal fuel cell model. The proposed optimal serpentine design, which is composed of three tapered channels (channels 2–4) and a final diverging channel (channel 5), increases cell output power by 11.9% over that of a cell with straight channels. These tapered channels enhance main channel flow and sub-rib convection, both increasing the local oxygen transport rate and, hence, local electrical current density. A diverging, final channel is preferred, conversely, to minimize reactant leakage to the outlet. The proposed combined approach is effective in optimizing the cathode flow-field design for a single serpentine PEMFC. The role of sub-rib convection on cell performance is demonstrated.     

Effect of humidity of reactants on the cell performance of PEM fuel cells with parallel and interdigitated flow field designs

This study investigates the effects of the relative humidity (RH) of the reactants on the cell performance and local transport phenomena in proton exchange membrane fuel cells with parallel and interdigitated flow fields. A three-dimensional model was developed taking into account the effect of the liquid water formation on the reactant transport. The results indicate that the reactant RH and the flow field design all significantly affect cell performance. For the same operating conditions and reactant RH, the interdigitated design has better cell performance than the parallel design. With a constant anode RH = 100%, for lower operating voltages, a lower cathode RH reduces cathode flooding and improves cell performance, while for higher operating voltages, a higher cathode RH maintains the membrane hydration to give better cell performance. With a constant cathode RH = 100%, for lower operating voltages, a lower anode RH not only provides more hydrogen to the catalyst layer to participate in the electrochemical reaction, but also increases the difference in the water concentrations between the anode and cathode, which enhances back-diffusion of water from the cathode to the anode, thus reducing cathode flooding to give better performance. However, for higher operating voltages, the cell performance is not dependent on the anode RH.     

The effect of flow distributors on the liquid water distribution and performance of a PEM fuel cell

Water management is one of the important factors which determine the performance of a Proton Exchange Membrane (PEM) fuel cell using hydrogen as fuel. For developing efficient water management systems, it is important to know the potential locations of formation and the nature of distribution of liquid water in the fuel cell. In the present study a PEM fuel cell with three different types of flow distributors are modeled and numerically simulated to find out the water formation and distribution characteristics. The model is validated by comparing the simulated polarization curve to experimental data. It is found that the type of flow distributor used plays a major role in determining the distribution of liquid water in the cell. A parallel flow distributor exhibits poor water removal capabilities whereas a serpentine flow distributor exhibits better water removal. A mixed flow distributor is found to give better water distribution characteristics compared to the parallel and serpentine distributors. Further the effect of liquid water formation and distribution on the species transport, temperature distribution and current generation are also investigated.     

Three-dimensional numerical study on cell performance and transport phenomena of PEM fuel cells with conventional flow fields

In this paper, a three-dimensional numerical model of the proton exchange membrane fuel cells (PEMFCs) with conventional flow field designs (parallel flow field, Z-type flow field, and serpentine flow field) has been established to investigate the performance and transport phenomena in the PEMFCs. The influences of the flow field designs on the fuel utilization, the water removal, and the cell performance of the PEMFC are studied. The distributions of velocity, oxygen mass fraction, current density, liquid water, and pressure with the convention flow fields are presented. For the conventional flow fields, the cell performance can be enhanced by adding the corner number, increasing the flow channel length, and decreasing the flow channel number. The cell performance of the serpentine flow field is the best, followed by the Z-type flow field and then the parallel flow field.     

Effects of operating parameters on transport phenomena and cell performance of PEM fuel cells with conventional and contracted flow field designs

A contracted parallel flow field design was developed to improve fuel cell performance compared with the conventional parallel flow field design. A three-dimensional model was used to compare the cell performance for both designs. The effects of the cathode reactant inlet velocity and cathode reactant inlet relative humidity on the cell performance for both designs were also investigated. For operating voltages greater than 0.7 V because the electrochemical reaction rates are lower with less oxygen consumption and less liquid water production, the cell performance is independent of the flow field designs and operating parameters. However, for lower operating voltages where the electrochemical reaction rates gradually increase, the oxygen transport and the liquid water removal efficiency differ for the various flow field designs and operating parameters; therefore, the cell performance is strongly dependent on both the design and operating parameters. For lower operating voltages, the cell performance for the contracted design is better than for the conventional design because the reactant flow velocities in the contracted region significantly increase, which enhances liquid water removal and reduces the oxygen transport resistance. For lower operating voltages, as the cathode reactant inlet velocity increases and the cathode reactant inlet relative humidity decreases, the cell performance for both designs improves.     

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.     

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.     

Novel serpentine-baffle flow field design for proton exchange membrane fuel cells

An appropriate flow field in the bipolar plates of a fuel cell can effectively enhance the reactant transport rates and liquid water removal efficiency, improving cell performance. This paper proposes a novel serpentine-baffle flow field (SBFF) design to improve the cell performance compared to that for a conventional serpentine flow field (SFF). A three-dimensional model is used to analyze the reactant and product transport and the electrochemical reactions in the cell. The results show that at high operating voltages, the conventional design and the baffled design have the same performance, because the electrochemical rate is low and only a small amount of oxygen is consumed, so the oxygen transport rates for both designs are sufficient to maintain the reaction rates. However, at low operating voltages, the baffled design shows better performance than the conventional design. Analyses of the local transport phenomena in the cell indicate that the baffled design induces larger pressure differences between adjacent flow channels over the entire electrode surface than does the conventional design, enhancing under-rib convection through the electrode porous layer. The under-rib convection increases the mass transport rates of the reactants and products to and from the catalyst layer and reduces the amount of liquid water trapped in the porous electrode. The baffled design increases the limiting current density and improves the cell performance relative to conventional design.     

Investigation of bio-inspired flow channel designs for bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. The objective of this paper is to present two new flow channel patterns: a leaf design and a lung design. These bio-inspired designs combine the advantages of the existing serpentine and interdigitated patterns with inspiration from patterns found in nature. Both numerical simulation and experimental testing have been conducted to investigate the effects of two new flow channel patterns on fuel cell performance. From the numerical simulation, it was found that there is a lower pressure drop from the inlet to outlet in the leaf or lung design than the existing serpentine or interdigitated flow patterns. The flow diffusion to the gas diffusion layer was found be to more uniform for the new flow channel patterns. A 25 cm² fuel cell was assembled and tested for four different flow channels: leaf, lung, serpentine and interdigitated. The polarization curve has been obtained under different operating conditions. It was found that the fuel cell with either leaf or lung design performs better than the convectional flow channel design under the same operating conditions. Both the leaf and lung design show improvements over previous designs by up to 30% in peak power density.     

Three dimensional,two phase flow mathematical model for PEM fuel cell: Part II. Analysis and discussion of the internal transport mechanisms

The internal transport mechanisms, which were acquired from the modeling results in Part I of this series, are discussed and compared for PEM fuel cells with a conventional flow field and an interdigitated flow field. The modeling results show that the oxygen concentration fraction in an interdigitated flow field is higher than that in a conventional flow field to increase the reaction rate, and the liquid water saturation in the former flow field is much less than that in the latter one at the cathode side to reduce the concentration overpotential largely. However, if the cathode inlet air in a PEM fuel cell with interdigitated flow field is not humidified, the performance of this fuel cell is inferior to that of a PEM fuel cell with conventional flow field because of a larger ohmic overpotential. As a result, the humidification is important for an interdigitated flow field to acquire a much better performance than a conventional flow field.     

Humidity of reactant fuel on the cell performance of PEM fuel cell with baffle-blocked flow field designs

The objective of this work is to examine the effects of humidity of reactant fuel at the inlet on the detailed gas transport and cell performance of the PEM fuel cell with baffle-blocked flow field designs. It is expected that, due to the water management problem, the effects of inlet humidity of reactant fuel gases on both anode and cathode sides on the cell performance are considerable. In addition, the effects of baffle numbers on the detailed transport phenomena of the PEM fuel cell with baffle-blocked flow field are examined. Due to the blockage effects in the presence of the baffles, more fuel gas in the flow channel can be forced into the gas diffuser layer (GDL) and catalyst layer (CL) to enhance the chemical reactions and then augment the performance of the PEMFC systems. Effect of liquid water formation on the reactant gas transport is taken into account in the numerical modeling. Predictions show that the local transport of the reactant gas, the local current density generation and the cell performance can be enhanced by the presence of the baffles. Physical interpretation for the difference in the inlet relative humidity (RH) effects at high and low operating voltages is presented. Results reveal that, at low voltage conditions, the liquid water effect is especially significant and should be considered in the modeling. The cell performance can be enhanced at a higher inlet relative humidity, by which the occurrence of the mass transport loss can be delayed with the limiting current density raised considerably.     

Water flooding and two-phase flow in cathode channels of proton exchange membrane fuel cells

The water flooding and two-phase flow of reactants and products in cathode flow channels (0.8 mm in width, 1.0 mm in depth) were studied by means of transparent proton exchange membrane fuel cells. Three transparent proton exchange membrane fuel cells with different flow fields including parallel flow field, interdigitated flow field and cascade flow field were used. The effects of flow field, cell temperature, cathode gas flow rate and operation time on water build-up and cell performance were studied, respectively. Experimental results indicate that the liquid water columns accumulating in the cathode flow channels can reduce the effective electrochemical reaction area; it makes mass transfer limitation resulting in the cell performance loss. The water in flow channels at high temperature is much less than that at low temperature. When the water flooding appears, increasing cathode flow rate can remove excess water and lead to good cell performance. The water and gas transfer can be enhanced and the water removal is easier in the interdigitated channels and cascade channels than in the parallel channels. The cell performances of the fuel cells that installed interdigitated flow field or cascade flow field are better than that installed with parallel flow field. The images of liquid water in the cathode channels at different operating time were recorded. The evolution of liquid water removing out of channels was also recorded by high-speed video.     

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.     

Channel aspect ratio effect for serpentine proton exchange membrane fuel cell: Role of sub-rib convection

A complete three-dimensional, two-phase, non-isothermal model for proton exchange membrane (PEM) fuel cells was used to investigate the effect of the sub-rib convection on the performances for the single and triple serpentine flow fields at various channel aspect ratios and different thermal constraints. The occurrence of sub-rib convection, which is affected by the serpentine flow field, significantly influences the cell performance if the oxygen supply or membrane moisture content was limited. For single serpentine flow field in which sub-rib convection presents under all ribs, changing channel aspect ratio has minimal effects on cell performance since the oxygen supply is sufficient. For triple serpentine flow field or for serpentine cell with poor external heat loss, owing to limited sub-rib convection or to low membrane moisture content, decrease in channel aspect ratio significantly enhances cell performance. Blocking up the sub-rib convection markedly reduces cell performance. Flow field design for PEM fuel cell should take into consideration the effects of sub-rib convection flow on cell performance.     

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.     

质子交换膜燃料电池双极板的设计与模拟分析

质子交换膜燃料电池是直接将化学能转换为电能的装置,双极板上的流道结构对燃料电池的工作性能具有较大的影响。根据应用要求设计了具有平行流道、蛇形流道及希尔伯特分形流道的双极板结构,模拟计算了氢气在不同类型的流道和气体扩散层中的分布状态,分析了燃料电池的输出电流密度和功率密度随电极间电压的变化特点,比较了不同的流道结构对燃料电池输出电流密度的影响,以及不同的工作温度及气体压强的情况下,燃料电池输出电流密度随温度及压强的变化规律。