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.     

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.     

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.     

Multi-pass serpentine flow-fields to enhance under-rib convection in polymer electrolyte membrane fuel cells: Design and geometrical characterization

The flow-field for reactant distribution is an important design factor that influences the performance of polymer electrolyte membrane fuel cells (PEMFCs). Under-rib convection between neighboring channels has been recognized to enhance the performance of PEMFCs with serpentine flow-fields. This study presents a simple design method to generate multi-pass serpentine flow-fields (MPSFFs) that can maximize under-rib convection in a given cell area. Geometrical characterization indicates that MPSFFs lead to significantly higher under-rib convection intensities and more uniform conditions, such as reactant concentrations, temperature, and liquid water saturation, compared with conventional serpentine flow-fields. The implications of the enhanced under-rib convection due to MPSFFs on the performance of PEMFCs are discussed.     

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.     

Enhancing under-rib mass transport in proton exchange membrane fuel cells using new serpentine flow field designs

New flow field configurations are developed to improve the performance of polymer electrolyte membrane fuel cells (PEMFCs). The developed designs aim to uniformly distribute the reactants over the reaction area of the catalyst layer surface, boost the under-rib convection mass transport through the gas diffusion layer, decrease the water flooding effect in the gas diffusion layer-catalyst layer interface, and maintain the membrane water content within the required range to augment protonic conductivity. To evaluate the performance parameters of a PEMFC, a comprehensive three-dimensional, two-phase mathematical model has been developed. The model includes the charge transport, electrochemical reactions, mass conservation, momentum, energy, and water transport equations. The results signify that the improved flow field patterns attain a considerable boosting of the output power, the under-rib convection mass transport, improvement of the reactant distribution over the catalyst layer surface and decline of the liquid water saturation in the gas diffusion layer-catalyst layer interface. The developed configurations achieve a higher power density of 0.82 W/cm² at a current density of 1.74 A/cm², compared to the standard serpentine configuration, which attains about 0.67 W/cm² at a current density of 1.486 A/cm².Accordingly, the develop configurations demonstrate a 22.6% enhancement in power density.     

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.     

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.     

质子交换膜燃料电池内部传热传质三维模拟

针对常规流场质子交换膜燃料电池提出了三维非等温数学模型。模型考虑了电化学反应动力学以及反应气体在流道和多孔介质内的流动和传递过程,详细研究了水在质子膜内的电渗和扩散作用。计算结果表明,反应气体传质的限制和质子膜内的水含量直接决定了电极局部电流密度的分布和电池输出性能;在电流密度大于0.3~0.4A/cm2时开始出现水从阳极到阴极侧的净迁移;高电流密度时膜厚度方向存在很大的温度梯度,这对膜内传递过程有较大影响。     

Three-dimensional numerical analysis of proton exchange membrane fuel cells (PEMFCs) with conventional and interdigitated flow fields

A three-dimensional, steady-state mathematical model is described to investigate the fluid flow, species transport and electrochemical reaction in the PEM fuel cells with conventional and interdigitated flow fields. The multidimensional characteristics of flow, species and current distributions are computed by method based on volume-control finite-discrete technique. After comparing the absolute value of convective and diffusion component of oxygen flux quantitatively, we found that forced convection transport mechanism is dominant for the interdigitated flow field design, however the diffusion transport is dominant for the conventional flow field design. The performance and the pressure loss of these two different designs are calculated and compared; results show that interdigitated flow field design have better performance for its advantage in mass-transport ability, however have larger pressure loss for its flow through the electrode. Finally, experimental results reported in the literature and predicted polarization curves are compared to evaluate the numerical model employed.     

Assessment of different bio-inspired flow fields for direct methanol fuel cells through 3D modeling and experimental studies

The performance impact of using bio-inspired interdigitated and non-interdigitated flow fields (I-FF and NI-FF, respectively) within a DMFC is investigated. These two flow fields, as well as a conventional serpentine flow field (S-FF, used as a reference), were examined as possible anode and cathode flow field candidates. To examine the performance of each of these candidates, each flow field was manufactured and experimentally tested under different anode and cathode flow rate combinations (1.3 mL/min [methanol] and 400 mL/min [oxygen], as well as 2 and 3 times these flow rates), and different methanol concentrations (0.50 M, 0.75 M, and 1.00 M). To help understand the experimental results and the underlying physics, a three dimensional numerical model was developed. Of the examined flow fields, the S-FF and the I-FF yielded the best performance on the anode and cathode, respectively. This finding was mainly due to the enhanced under-rib convection of both of these flow fields. Although the I-FF provided a higher mean methanol concentration on the anode catalyst layer surface, its distribution was less uniform than that of the S-FF. This caused the rate of methanol permeation to the cathode to increase (for the anode I-FF configuration), along with the anode and cathode activation polarizations, deteriorating the fuel cell performance. The NI-FF provided the lowest pressure drops of the examined configurations. However, the hydrodynamics within the flow field made the reactants susceptible to traveling directly from inlet to outlet, leading to several low concentration pockets. This significantly decreased the reactant uniformity across its respective catalyst layer, and caused this FFs performance to be the lowest of the examined configurations.     

Parameter sensitivity examination for a complete three-dimensional,two-phase,non-isothermal model of polymer electrolyte membrane fuel cell

Parameter sensitivity analysis is carried out for a complete three-dimensional, two-phase, non-isothermal model of polymer electrolyte membrane (PEM) fuel cell with a parallel flow field design. The model couples the two-phase flow of the multi-component reactants and liquid water, species transport, electrochemical reactions, proton and electron transport, and the electro-osmosis transport, back diffusion of water in the membrane, and energy transport. Twenty nine parameters, which are classified into the structural or transport parameters of porous layers (tortuosity, porosity, permeability, proton conductivity, electron conductivity, and thermal conductivity) as well as the electrochemical parameters (anodic and cathodic exchange current densities, anodic and cathodic transfer coefficients for anode and cathode reactions), are used to implement individual parameter investigation. The results show the parameters can be divided in to strongly sensitive, conditional sensitive and weak sensitive parameters according to its effect on the cell polarization curve. The optimization of parameters of cathode gas diffusion layer (GDL) and catalyst layer (CL) is more important to improve cell performance than that of anode GDL and CL because liquid water transport and removal affect significantly membrane hydration and reactant transport. Electrochemical parameters determine the activation potential and the slope of ohmic polarization hence these parameters can be used to fit experimental polarization curve more effectively than the other parameters.     

The effects of buoyancy on the performance of a PEM fuel cell with a wave-like gas flow channel design by numerical investigation

This study performs numerical simulations to investigate the effects of buoyancy on the gas flow characteristics, temperature distribution, electrochemical reaction efficiency and electrical performance of a proton exchange membrane fuel cell (PEMFC) with a novel wave-like gas flow channel design. In general, the simulation results show that compared to the straight geometry of a conventional gas flow channel, the wave-like configuration enhances the transport through the porous layer and improves the temperature distribution within the channel. As a result, the PEMFC has an improved fuel utilization efficiency and an enhanced heat transfer performance. It is found that the buoyancy effect increases the velocity of the reactant fuel gases in both the vertical and the horizontal directions. This increases the rate at which the oxygen gas is consumed in the fuel cell but improves the electrical performance of the PEMFC. The results show that compared to the conventional straight gas flow channel, the wave-like gas flow channel increases the output voltage and improves the maximum power density by approximately 39.5%.     

On the relative influence of convection in serpentine flow fields of PEM fuel cells

In polymer electrolyte membrane fuel cells, power losses associated with slow reaction kinetics and mass-transport limits can be strongly influenced by convective flow characteristics. Specifically, convection in the form of channel bypass may be utilized to simultaneously increase reactant concentration and reduce product concentration in the catalyst layer, thus reducing the activation and mass-transport overpotentials. An analytical model is developed here to predict the flow pattern and pressure field in general single-serpentine flow field geometries. The model predicts that a significant portion of the total flow through the fuel cell can occur as in-plane convective flow through the gas diffusion layer under realistic operating conditions. Further, by comparing the in-plane rates of diffusive and convective transport it is found that the dominant mechanism depends on the geometric and material parameters of the flow field. In particular, it is found that the relative influence of convection depends highly on in-plane permeability of the gas diffusion layer and channel length, and is relatively independent of gas diffusion layer thickness. By designing fuel cells to utilize enhanced in-plane convection, it is suggested that losses associated with low oxygen content as well as liquid water buildup in the catalyst layer can be reduced.     

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.     

Effect of height/width-tapered flow fields on the cell performance of polymer electrolyte membrane fuel cells

Water flooding at the cathode is a critical issue in polymer electrolyte membrane fuel cells, since it will block the oxygen transport, resulting in a large concentration loss. To address this issue, two tapered flow fields with varying height or width are proposed in this study, as the tapered channels can gradually increase flow velocity which is beneficial in water removal. To confirm the proposed function, both experiments and simulation are performed. The results prove that the tapered flow fields with high velocity at the downstream region can significantly enhance water removal in flow channels, which avoids the mass transport limitation and improves the cell performance at high current densities. Additionally, the tapered design can also enhance the under-rib convection between adjacent channels, which helps to remove accumulated water in the gas diffusion layer.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 2 – Transient water accumulation in an interdigitated cathode flow field

An interdigitated cathode flow field has been tested in situ with neutron radiography to measure the water transport through the porous gas diffusion layer in a PEM fuel cell. Constant current density to open circuit cycles were tested and the resulting liquid water accumulation and dissipation rates with in-plane water distributions are correlated to measured pressure differential between inlet and outlet gas streams. The effect of varying the reactant gas relative humidity on liquid water accumulation is also demonstrated. These results provide evidence that the reactant gas establishes a consistent in-plane transport path through the diffusion layer, leaving stagnant regions where liquid water accumulates. A simplified permeability model is presented and used to correlate the relative permeability to varying gas diffusion layer liquid water saturation levels.     

Three-dimensional multi-phase simulation of PEM fuel cell considering the full morphology of metal foam flow field

It is well-known that flow field design is of primary importance to optimization of proton exchange membrane (PEM) fuel cell. Traditional channel-rib flow fields, e.g. parallel or serpentine channels, always lead to non-uniform distributions of reactant gas, liquid, current density and so on between the channel and rib regions. Metal foam materials with high porosity (>90%) have been proposed as alternative flow fields for PEM fuel cells. In this study, influences of metal foam flow field on the transport phenomena coupled with the electrochemical reactions in PEM fuel cell are investigated using a three-dimensional (3D) multi-phase non-isothermal model. Specifically, the full morphology of metal foam flow field is taken into account in the 3D simulation after validated against experimental permeability data. The full morphology inclusion enables capture of the detailed gas flow from the flow field into the gas diffusion layer (GDL) and the current collection at the metal foam/GDL interface. In addition, compared with the conventional channel-rib flow fields, the metal foam design greatly increases fuel cell performance in the high current density regime. In addition, the oxygen and current density distributions in PEM fuel cell with the metal foam flow field are more uniform than those in the conventional one. Though the current collection area at the GDL surface is much smaller in the metal foam flow field, the relevant Ohmic loss won't increase significantly due to the improved physical contact by the fine pore structure of metal foam over the GDL.     

Performance enhancement of planar solid oxide fuel cell using a novel interconnector design

This study presents a novel interconnector design, termed groove and rib-finned interconnector, to improve the performance of the planar solid oxide fuel cell (SOFC). We have conducted a detailed comparative study on the flow characteristics and electrical performance of conventional straight channel interconnectors and novel interconnectors through a three-dimensional model. Compared with the conventional straight channel interconnector, the result shows that the novel interconnector can provide higher fuel utilization, and the output power density at a low fuel flow is still higher than that of the conventional design at a high fuel flow. The novel interconnector increases the velocity and vorticity of the reactant gas, and promotes gas disturbance, and enhances the mass transport in the electrode. The novel interconnector eliminates the oxygen-free zone of the cathode under the rib and provides sufficient oxygen with uniform concentration distribution for the electrochemical reaction. Therefore, the novel interconnector significantly reduces the activation and concentration overpotentials and improves the electrical performance of the SOFC stack.     

Numerical investigation on the characteristics of mass transport and performance of PEMFC with baffle plates installed in the flow channel

A 3D numerical model of proton exchange membrane fuel cell (PEMFC) with the installation of baffle plates is developed. The majority of the conservation equations and physical parameters are implemented through the user defined functions (UDFs) in the FLUENT software. The characteristics of mass transport and performance of PEMFC are investigated. The results reveal that the baffle plate can enhance the mass transport efficiency and the performance of PEMFC. The baffle plate installed in the PEMFC flow channel increases the local gas velocity, which can promote the reactant gas transport and the liquid water removal in the porous electrode. As a result, the reactant gas concentration is larger in the porous electrode, which enhances the fuel cell performance for decreasing the over-potential of concentration. The fuel cell output power increases with the blockage ratio of the baffle plate. Considering the extra pumping power resulted from pressure loss caused by the baffle plate, the fuel cell with the blockage ratio of 0.8 is found to perform best in terms of the fuel cell net power generation. The fuel cell performance increases first with the baffle plate number, due to the better reactant distribution and water management, but decreases when the baffle plate number is too large, due to the excessive blockage for the reactant gas transport to the channel downstream. The PEMFC investigated with 5 baffle plates in the channel is found to be optimal. A channel design to achieve gradually increasing blockage ratios is also proposed, which exhibits better cell performance than the design with even blockage ratios.