Configuration effects of air,fuel, and coolant inlets on the performance of a proton exchange membrane fuel cell for automotive applications

The configuration of fuel, air, and cooling water paths is one of the major factors that influence the performance of a proton exchange membrane fuel cell (PEMFC). In order to investigate the effects of these factors, a quasi-three-dimensional dynamic model of a PEMFC has been developed. For validation, simulation results are compared with experimental data in one-flow configuration case and show good agreement with the experimental cell performance data. Five different flow configurations are then simulated to systematically investigate the effects of fuel, air, and cooling channel configuration on the local current and species distribution. Voltage and power vs. current density for five different configurations are compared. The type 1 configuration, which has a fuel–air counter flow and an air-coolant co-flow, has the highest performance in all ranges of current density because the membrane remains the most hydrated. When the operating current density increases, the effects of temperature on membrane hydration slightly decrease. It is confirmed that fuel cell performance improves with increased humidity until flooding conditions appear. An interesting result shows that it is possible to lower the fuel cell operating temperature to improve fuel cell hydration, which in turn improves cell performance. In addition, the different flow configurations are shown to have an effect on the pressure losses and local current density, membrane hydration, and species mole fractions. These results suggest that the model can be used to optimize the flow configuration of a PEMFC.     

Effect of cathode inlet manifold configuration on performance of 10-cell proton-exchange membrane fuel cell

A 10-cell proton-exchange membrane fuel cell (PEMFC) stack with 10 cathode flow channels is employed to investigate the effect of airflow inlet manifold configuration on the overall performance. Four different types of airflow inlet manifold with a 90° turn are considered. First, the flow patterns according to the manifold configuration are numerically sought. The computational result for the improved inlet manifold predicts about 8.5% increase in the uniformity of the airflow distribution. The experiments are carried out to confirm the numerical predictions by measuring actual airflow distributions through the fuel cell stack. The polarization curve and the power curve for the 10-cell PEMFC are also obtained to determine the effect of inlet manifold configuration on the actual performance. The maximum power output increases by up to 10.3% on using the improved airflow inlet manifold.     

Numerical studies on rib & channel dimension of flow-field on PEMFC performance

Distributions in reactant species concentration in a PEMFC cause distributions in local current density, temperature and water over the area of a PEMFC. These can lead to effects such as flooding or drying of the membrane and cause stresses in different regions of the fuel cell. Changing flow-field configuration, including channel path length, width, or height to distribute the gas more evenly, is one method of minimizing these stresses. This work numerically investigated how serpentine flow-fields with different channel/rib's cross section areas affect performance and species distributions for both automotive and stationary conditions. Further, the influence of flow direction to performance and its distribution was also reported. The prediction revealed that for stationary condition, narrower channel with wider rib spacing gives higher performance but opposite results when automotive condition is applied.     

Numerical simulation of thermal–hydraulic characteristics in a proton exchange membrane fuel cell

The thermal–hydraulic characteristics of a proton exchange membrane fuel cell (PEMFC) are numerically simulated by a simplified two‐phase, multi‐component flow model. This model consists of continuity, momentum, energy and concentration equations, and appropriate equations to consider the varying flow properties of the gas–liquid two‐phase region in a PEMFC. This gas–liquid two‐phase characteristic is not considered in most of the previous simulation works. The calculated thermal–hydraulic phenomena of a PEMFC are reasonably presented in this paper, which include the distributions of flow vector, temperature, oxygen concentration, liquid water saturation, and current density, etc. Coupled with the electrochemical reaction equations, current flow model can predict the cell voltage vs current density curves (i.e. performance curves), which are validated by the single‐cell tests. The predicted performance curves for a PEMFC agree well with the experimental data. In addition, the positive effect of temperature on the cell performance is also precisely captured by this model. The model presented herein is essentially developed from the thermal–hydraulic point of view and can be considered as a stepping‐stone towards a full complete PEMFC simulation model that can help the optima design for the PEMFC and the enhancement of cell efficiency. Copyright © 2003 John Wiley & Sons, Ltd.     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell

Using a specially designed current distribution measurement gasket in anode and thin thermocouples between the catalyst layer and gas diffusion layer (GDL) in cathode, in-plane current and temperature distributions in a proton exchange membrane fuel cell (PEMFC) have been simultaneously measured. Such simultaneous measurements are realized in a commercially available experimental PEMFC. Experiments have been conducted under different air flow rates, different hydrogen flow rates and different operating voltages, and measurement results show that there is a very good correlation between local temperature rise and local current density. Such correlations can be explained and agree well with basic thermodynamic analysis. Measurement results also show that significant difference exists between the temperatures at cathode catalyst layer/GDL interface and that in the center of cathode endplate, which is often taken as the cell operating temperature. Compared with separate measurement of local current density or temperature, simultaneous measurements of both can reveal additional information on reaction irreversibility and various transport phenomena in fuel cells.     

Numerical analysis on the effects of manifold design on flow uniformity in a large proton exchange membrane fuel cell stack

The size and configuration of manifold can affect the flow characteristics and uniformity in proton exchange membrane fuel cell (PEMFC) stack; then its efficiency and service life. Based on the simulation results of a single fuel cell considering electrochemical reaction, a stack model with 300 porous media is established to numerically investigate the performances of a large commercial PEMFC stack. The effects of manifold width and configuration type on the pressure drop and species concentration are studied by computational fluid dynamics (CFD). The results show that the uniformity for most cases of U-type configuration is better than those of Z-type configuration. For U-type configuration, a very good uniformity can be obtained by selecting anode inlet manifold width of 20 mm and anode outlet manifold in range from 25 to 30 mm; the uniformity is bad for all cathode inlet manifold width, relatively better uniformity can be achieved by adjusting cathode outlet manifold width. For Z-type configuration, bad uniformity is found for cathode inlet and outlet manifold with all width; a relatively good uniformity can be obtained with suitable anode manifold width of 35 mm. The research can provide some references to improve gas distribution uniformity in large PEMFC stacks.     

Numerical investigation on the effect of flow field and landing to channel ratio on the performance of PEMFC

Three-dimensional numerical investigation of PEMFC with landing to channel ratio (L:C) of 2:2 for 25-cm² serpentine-parallel channel has been simulated, and the obtained results have been validated with the polarization curve obtained through experiments. It is found that the maximum error in the polarization curve is less than 4%, and thus a very good deal exists between the simulation study and experimentation. Upon validation, the study has been extended for various flow path designs with different L:C ratio numerically. The prediction reveals that the L:C ratio of 2:2 exhibits the better performance for all the flow channels considered, and it is found that the straight-zigzag flow field with L:C ratio of 2:2 attributes the maximum power density of 0.3250 W/cm² for an optimum open circuit voltage of 0.4 Volts with minimal pressure drop. Oxygen consumption in the cathode flow channels of serpentine-parallel, serpentine-zigzag, and straight-parallel are 77.08%, 10.41%, and 42.70% lesser than that of straight-zigzag PEMFC, respectively. The pressure drop in the flow channel of serpentine-parallel, serpentine-zigzag, and straight-parallel with landing to channel ratio 2:2 are 78.18%, 95.81%, and 48.33% higher than that of straight-zigzag flow field, respectively. The polarization curve, hydrogen (H₂), oxygen (O₂), water content along the flow channel and the proton conductivity, H₂O content across the membrane electrolyte, and current density contour at the GDL/catalyst interface of the anode side for all flow channel configurations have been presented and discussed.     

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.     

Effects of the humidity and the land ratio of channel and rib in the serpentine three‐dimensional PEMFC model

The construction of a reliable numerical model and the clarification of its operational conditions are necessary for maximizing fuel cell operation. Numerous operating factors, such as mole fractions of species, pressure distribution, overpotential, and inlet relative humidity, affect the performance of proton exchange membrane fuel cells (PEMFCs). Among these operational parameters, geometrical shape and relative humidity are investigated in this paper. Specifically, the land ratio of the gas channel and rib is an important parameter affecting PEMFC performance because current density distribution is influenced by this geometrical characteristic. Three main variables determine the current density distribution, namely, species concentration, pressure, and overpotential distributions. These distributions are considered simultaneously in assessing fuel cell performance with a given PEMFC cell‐operating voltage. In this paper, three different land ratio models are considered to obtain better PEMFC performance. Similarly, three different inlet relative humidity variations are studied to achieve an enhanced operating condition. A three‐dimensional numerical PEMFC model is developed to illustrate the current density distribution as the determining factor for PEMFC performance. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of channel-rib width ratio and relative humidity on performance of a single serpentine PEMFC based on electrochemical impedance spectroscopy

The geometry configuration of proton exchange membrane fuel cell (PEMFC) which is considered as a promising energy conversion device has great influence on PEMFC performance. In this paper, effect of channel-to-rib width ratio and relative humidity of reactant gas on the performance are compared based on two single PEMFCs. The EIS testing results below 50 A are given and analyzed. The results obtained from polarization curves, power density curves and EIS fitting results prove that: 1. Compared with cell 3:4, the anode high humidification has a greater addition to the performance of cell 1:1; 2. PEMFCs with different geometry configurations of flow field have their own suitable working condition ranges; 3. The charge transfer resistance is the dominating factor when current loading is below 2.0 A cm^?2.     

Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.     

Numerical simulation analysis of the performance on the PEMFC with a new flow field designed based on constructal-theory

The design of the flow field structure has an important impact on the performance of PEMFC. An excellent design of the flow field will optimize the gas-liquid distribution inside the fuel cell, and enhance the diffusion of the reactant gases while reducing problems such as water flooding or uneven mass transfer of reactants, thus improving the overall performance of the cell. A new form of flow field based on the design ideas of Constructal-theory and Murray's Law was proposed in this paper. In this study, the PEMFC with the new and conventional flow fields were compared under the same conditions, and it is proved that the cell with the new flow field has a more balanced performance on output power and global pressure drop in contrast with conventional flow fields. In this study, the output power density of the PEMFC with the new flow field increased by an average of 1.35% compared to the PEMFC with Parallel flow field and Single Channel Serpentine flow field, and the pressure drop was reduced by 47.67% and 90.06% respectively compared to the PEMFC with the Single Channel Serpentine flow field and Double Channel Serpentine flow field. Meanwhile, the distribution of current density characteristics in a PEMFC with the new flow field was investigated and optimization of its structure size is analyzed. The reason for its non-uniform distribution of current density was revealed in this study, and an improvement scheme was proposed to improve the uniformity of current density, and the results of structural optimization research will have a certain guiding effect on practical applications.     

Experimental study and optimization of flow field structures in proton exchange membrane fuel cell under different anode modes

As one of the critical components in the proton exchange membrane fuel cell (PEMFC), the flow field is crucial to the improvement of cell performance. However, the current research on flow field structure lacks consideration of the influence of different anode modes, which makes the existing flow field structure rules have limitations in the practical application of PEMFC. In this paper, the PEMFC characteristics of parallel flow field, S-shaped flow field, multi-serpentine flow field and single-serpentine flow field at the cathode side are compared experimentally in the dead-end anode (DEA) mode and hydrogen circulation anode (HCA) mode, respectively. Especially, the spatial current density distribution and parasitic power of different flow field structures are measured. The results show that the performance trends of different flow field structures change with the DEA and HCA anode modes. In DEA mode, the PEMFC is prone to flooding, and the flow field with high gas velocity in the channel has better drainage ability, which can obtain higher cell performance. The HCA mode is helpful for the discharge of water in the PEMFC, which effectively alleviates the adverse impact of water accumulation on the overall performance, and the mass transport ability of the flow field structure plays a leading role in the cell performance improvement. In addition, although the high gas flow velocity has better drainage ability in the flow field, it may lead to a decrease in the current density distribution uniformity and PEMFC net output power density. Based on the comprehensive consideration of the experimental results, the multi-serpentine flow field is more suitable for DEA mode, and the S-shaped flow field is more suitable for HCA mode.     

Research on two‐phase flow considering hydrogen crossover in the membrane for a polymer electrolyte membrane fuel cell

Hydrogen crossover has an important effect on the performance and durability of the polymer electrolyte membrane fuel cell (PEMFC). Severe hydrogen crossover can accelerate the degradation of membrane and thus increase the possibility of explosion. In this study, a two‐phase, two‐dimensional, and multiphysics field coupling model considering hydrogen crossover in the membrane for PEMFC is developed. The model describes the distributions of reactant gases, current density, water content in membrane, and liquid water saturation in cathode electrodes of PEMFC with intrinsic hydrogen permeability, which is usually neglected in most PEMFC models. The conversion processes of water between gas phase, liquid phase, and dissolved water in PEMFC are simulated. The effects of changes in hydrogen permeability on PEMFC output performance and distributions of reactant gases and water saturation are analyzed. Results showed that hydrogen permeability has a marked effect on PEMFC operating under low current density conditions, especially on the open circuit voltage (OCV) with the increase of hydrogen permeability. On the contrary, the effect of hydrogen permeability on PEMFC at high current density is negligible within the variation range of hydrogen permeability in this study. The nonlinear relations of OCV with hydrogen diffusion rate are regressed.     

Optimization of blocked flow field performance of proton exchange membrane fuel cell with auxiliary channels

The flow field optimization design is one of the important methods to improve the performance of proton exchange membrane fuel cell (PEMFC). In this study, a new structure with staggered blocks on the parallel flow channels of PEMFC and auxiliary flow channels under the ribs is proposed. Through numerical calculation method, the effect of blocks auxiliary flow field (BAFF) on pressure drop, reactant distribution and liquid water removal in the fuel cells are investigated. The results show that when the operating voltage is 0.5 V, the current density of BAFF is 21.74% higher than that of the straight parallel flow field (SPFF), and the power density reaches 0.65 W cm^?2. BAFF improves performance by equalizing the pressure drop across sub-channels, promoting the uniform distribution of reactant, and enhancing transport across the ribs. In addition, through parameter analysis, it is found that BAFF can discharge liquid water in time at the conditions of high humidification, high current density and low temperature, which ensures the output performance of the fuel cell and improves the durability of the fuel cell. This paper provides new ideas for the improvement of PEMFC flow field design, which is beneficial to the development of PEMFC with high current density.     

Comparison of current distributions in proton exchange membrane fuel cells with interdigitated and serpentine flow fields

Current distributions in a proton exchange membrane fuel cell (PEMFC) with interdigitated and serpentine flow fields under various operating conditions are measured and compared. The measurement results show that current distributions in PEMFC with interdigitated flow fields are more uniform than those observed in PEMFC with serpentine flow fields at low reactant gas flow rates. Current distributions in PEMFC with interdigitated flow fields are rather uniform under any operating conditions, even with very low gas flow rates, dry gas feeding or over-humidification of reactant gases. Measurement results also show that current distributions for both interdigitated and serpentine flow fields are significantly affected by reactant gas humidification, but their characteristics are different under various humidification conditions, and the results show that interdigitated flow fields have stronger water removal capability than serpentine flow fields. The optimum reactant gas humidification temperature for interdigitated flow fields is higher than that for serpentine flow fields. The performance for interdigitated flow fields is better with over-humidification of reactant gases but it is lower when air is dry or insufficiently humidified than that for serpentine flow fields.     

Polarization characteristics and property distributions of a proton exchange membrane fuel cell under cathode starvation conditions

Property distribution and polarization characteristics of a proton exchange membrane fuel cell (PEMFC) under cathode starvation conditions were investigated numerically and experimentally for a unit cell. The polarization curves of a lab‐scale PEMFC were measured with increasing current density for different cell temperatures (40°C, 50°C, and 60°C) at a relative humidity of 100%. To investigate the local temperature, water content and current density on the membrane, and gas velocity in the channel of the PEMFC, numerical studies using the es‐pemfc module of the commercial flow solver STAR‐CD, which were matched with experimental data, were conducted. Temperature, current density on the membrane, and water content in the MEA were examined to investigate the effect of cell temperature on performance under the cathode starvation condition. At cathode starvation conditions, the performance of a higher cell temperature condition might drop significantly and the mean temperature on the membrane increase abruptly with increasing cell temperature or current density. Copyright © 2009 John Wiley & Sons, Ltd.     

Internal behavior of segmented fuel cell during cold start

In order to improve cold start capability and survivability of proton exchange membrane fuel cell (PEMFC), a fundamental understanding of its internal behavior is required. In this study, the cold start processes of a PEMFC with different operating conditions have been investigated, and the characteristics of current density and temperature distributions are studied through in-situ experiments with a printed circuit board (PCB). It is found that the start ability of PEMFC is strong at −3 and −5 °C, but weak at −7 and −10 °C. Also the self-start ability can be enhanced by decreasing the initial current load. Polarization curves show almost no degradation after successful cold start at −3 and −5 °C, while the PEMFC degrades a lot after failed cold start at low temperature like −10 °C. Also electrochemical impedance spectroscopy (EIS) shows a big degradation after galvanostatic mode cold starts. Local current density of segmented cell results shows that the highest current density is initially near the inlet region and then quickly moves downstream, reaching to the region near the middle eventually during the successful cold start process. However, during the failed cold start process, the highest current density is initially near the inlet region of the flow channels and quickly moves down stream, reaching the upper left corner region (A1) before shut down eventually. For both successful and failed cold starts, the highest temperature can be observed near the middle of the cell after the reaching of the highest current density.     

Numerical study of cold start performance of proton exchange membrane fuel cell with coolant circulation

It has been well recognized that cold start is one of the key issues of proton exchange membrane fuel cell (PEMFC) used as the engine of vehicles. Coolant circulation is usually launched synchronously with the fuel cell during cold start to avoid sudden large temperature variation, which greatly increases the cell thermal mass, lowers the heating rate, and worsens the cell performance. Considering the flow and heat transfer of coolant circulation, a three-dimensional, transient, multi-disciplinary model for cold start is built up. The numerical results agree reasonably well with experimental data, indicating that the model can be used for the investigation of PEMFC cold start processes. The analysis of circulation parameter effects shows that increasing the coolant flow rate or coolant tank capacity has little influence on the cell voltage, but will increase the non-uniformity of temperature distribution along flow direction. At lower start-up temperature, this non-uniformity is more obvious. With higher coolant flow rate, although the distribution of current density becomes more evenly, the ice formation amount increases and its distribution and location are greatly affected.     

Influences of gas relative humidity on the temperature of membrane in PEMFC with interdigitated flow field

A fundamental understanding of the water balance of a fuel cell during operation is crucial for improving the cell performance and durability. The humidification in the anode or cathode has an important effect on the flow characteristics and cell efficiency. Three-dimensional steady mathematical model based on the electrochemical, current distribution, fluid motion continuity equation, momentum and energy equation, boundary layer theory has been developed to simulate PEMFC with interdigitated flow field using the computational fluid dynamics (CFD). Effects on the current density and temperature differences have been simulated and analyzed respectively, when the humidification in the anode or cathode is from 0% to 100% respectively. The numerical results show that the humidification strongly influences the current density and temperature difference so as to affect the cell efficiency. Under the same operation conditions and low humidification conditions, anode humidification can better enhance the performance of the battery and improve the extent of PEM humidification.