Water management in a novel proton exchange membrane fuel cell stack with moisture coil cooling

Water flooding causes severe degradation of the performance and lifetime of proton exchange membrane fuel cell (PEMFC). In this study, a novel PEMFC stack with in-built moisture coil cooling was designed and the effects of moisture coil cooling on water management in the new PEMFC stack under various operating conditions were investigated. The result showed that the performance of the PEMFC stack was significantly improved due to the moisture condensation under high current density, high operating temperature, high relative humidity and high operating pressure. The output power was increases by 21.62% (525.71 W) at 1600·mA cm⁻² while the increased parasitic power was no more than 35W. Moreover, degradation of the cathode catalyst layer after 100 h operation was also reduced by using moisture coil cooling. Compared with the situation without moisture condensation, the maximum decay rate of the cathode catalyst layer thickness after 100 h operation was reduced by 13.01%. Accordingly, the novel design is valuable and can be widely used in the future design of PEMFC.     

Performance investigation on a novel 3D wave flow channel design for PEMFC

Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel.     

Current distribution in polymer electrolyte membrane fuel cell with active water management

Air delivery is typically the greatest parasitic power loss in polymer electrolyte membrane fuel cell (PEMFC) systems. We here present a detailed study of an active water management system for PEMFCs, which uses a hydrophilic, porous cathode flow field, and an electroosmotic (EO) pump for water removal. This active pumping of liquid water allows for stable operation with relatively low air flow rates and low air pressure and parallel cathode channel architectures. We characterize in-plane transport issues and power distributions using a three by three segmented PEMFC design. Our transient and steady state data provide insight into the dynamics and spatial distribution of flooding and flood-recovery processes. Segment-specific polarization curves reveal that the combination of a wick and an EO pump can effect a steady state, uniform current distribution for a parallel channel cathode flow field, even at low air stoichiometries (α_air = 1.5). The segmented cell measurements also reveal the mechanisms and dynamics associated with EO pump based recovery from catastrophic flooding. For most operating regimes, the EO pump requires less than 1% of the fuel cell power to recover from near-catastrophic flooding, prevent flooding, and extend the current density operation range.     

Liquid water flooding process in proton exchange membrane fuel cell cathode with straight parallel channels and porous layer

Liquid water management plays a significant role in proton exchange membrane fuel cell (PEMFC) performance, especially when the PEMFC is operating with high current density. Therefore, understanding of liquid water behavior and flooding process is a critical challenge that must be addressed. To overcome PEMFC durability problems, a liquid water flooding process is studied in the cathode side of a PEMFC with straight parallel channels and a porous layer using FLUENT^® v6.3.26 software with a volume-of-fluid (VOF) algorithm and user-defined-function (UDF). The general process of liquid water flooding within this type of PEMFC cathode is investigated by analyzing the behavior of liquid water in porous layer and gas flow channels. Two important phenomena, the “first channel phenomenon” and the “last channel phenomenon”, and their effects on the flow distribution along different parallel channels are discussed.     

Effect of cathode channel dimensions on the performance of an air-breathing PEM fuel cell

A three dimensional, steady state, non-isothermal, single phase model was developed and simulations were carried out in order to find the effect of cathode channel dimensions (width, depth and height) on the performance of an air-breathing fuel cell. The model was solved using commercial CFD package Fluent (version 6.3). Separate user defined functions were written to solve the electrochemical equations and the water transport through the membrane along with the other governing equations. Analyses were carried out for three different channel widths (2, 4 and 6 mm), for three different channel depths (2, 6 and 10 mm) and for three different cell heights (15, 30 and 45 mm). Cell characteristics like current distribution, species distribution, oxygen mass transfer coefficient, cell temperature, cathode channel velocities and net water transport coefficients are reported. The results show that the cell performance improves with increase in cathode channel width, channel depth and with decrease in cell height. Maximum power density obtained was 240 mW/cm² for a channel width of 4 mm and channel depth of 6 mm. When the channel depth was 2 mm the performance was limited mainly due to the resistance offered by the channel for the buoyancy induced flow. For channel depths higher than 2 mm, the diffusion resistance of the porous GDL also contributed significantly to limit the performance to low current densities. At low current densities the fuel cell is prone to flooding whereas at high current densities ohmic overpotential due to dehydration of the membrane significantly contributes to the overall voltage loss.     

Numerical study on heat transfer enhancement of a proton exchange membrane fuel cell with the dimpled cooling channel

As the power density of a proton exchange membrane fuel cell (PEMFC) increases, the problems of internal heat accumulation and non-uniform temperature distribution are becoming significant. In this paper, a novel cooling channel with dimple structures is designed and a three-dimensional PEMFC numerical model is established. When comparing to the conventional channels, the heat transfer performance of dimpled channel is 10% higher than the smooth one, and the pressure loss is almost 13% lower than that of wavy channel. In addition, the optimization of dimple structure parameters is investigated based on the index of uniformity temperature (IUT) and performance evaluation criteria (PEC) of heat transfer. It is found that a diameter-to-depth ratio of 4 is recommended when the dimple diameter is less than 0.80 mm. Furthermore, the clock-wise vortex observed inside the dimple is considered to be the main reason affecting heat exchange. This study will contribute to the design of cooling channels for high-power density PEMFCs in the future.     

Performance investigation of proton exchange membrane fuel cells with curved membrane electrode assemblies caused by pressure differences between cathode and anode

The membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEMFC) would deform when there is a pressure difference between the cathode and anode. In this study, a three-dimensional PEMFC model that incorporates a two-dimensional MEA deformation model is employed to investigate the effects of the curved MEA on oxygen transport, liquid water removal, pressure drops across cells, and resulting overall cell performances. The results show that the MEA deformation generates a gap between the rib and gas diffusion layer (GDL), thereby enhancing the oxygen transport into and liquid water removal from the cathode porous electrode. At an operating voltage of 0.4 V, the output current density is enhanced by 4.83%, 6.67%, and 8.61%, respectively, for the pressure differences of 10, 20, and 30 kPa between the cathode and anode, as compared with that without a pressure difference. An efficiency evaluation criterion (EEC) that represents a tradeoff between mass transfer enhancement and pressure drop penalty is introduced to evaluate the overall cell performance enhancement. The result shows that the EEC value increases with increasing the pressure difference between the cathode and anode, confirming the performance enhancement by the deformed MEA. In addition, the effects of MEA deformation are also examined at two sets of channel heights (H) and widths (W), i.e., WH=0.5 mm and WH=1.5 mm. At an operating voltage of 0.4 V, the output current density is enhanced by 20.18% at a pressure difference of 30 kPa for the cell with WH=0.5 mm, whereas it is improved only by 2.91% for the cell with WH=1.5 mm. Therefore, increasing the pressure difference between the cathode and anode is a more effective approach to performance enhancement for the cell with a small channel height and width.     

Transient response of a unit proton-exchange membrane fuel cell under various operating conditions

The transient response of proton-exchange membrane fuel cells (PEMFCs) is an important criterion in their application to automotive systems. Nevertheless, few papers have attempted to study experimentally this dynamic behaviour and its causes. Using a large-effective-area (330 cm²) unit PEMFC and a transparent unit PEMFC (25 cm²), systematic transient response and cathode flooding during load changes are investigated. The cell voltage is acquired according to the current density change under a variety of stoichiometry, temperature and humidity conditions, as well as different flooding intensities. In the case of the transparent fuel cell, the cathode gas channel images are obtained simultaneously with a CCD imaging system. The different levels of undershoot occur at the moment of load change under different cathode stoichiometry, both cathode and anode side humidity and flooding intensity conditions. It is shown that undershoot behaviour consists of two stages with different time delays: one is of the order of 1 s and the other is of the order of 10 s. It takes about 1 s for the product water to come up on to the flow channel surface so that oxygen supply is temporarily blocked, which causes voltage loss in that “undershoot”. The correlation of dynamic behaviour with stoichiometry and cathode flooding is analyzed from the results of these experiments.     

Application of a thermally conductive pyrolytic graphite sheet to thermal management of a PEM fuel cell

This work experimentally investigates the thermal performance of a pyrolytic graphite sheet (PGS) in a single proton exchange membrane fuel cell (PEMFC). This PGS with high thermal conductivity serves as a heat spreader, reduces the volume and weight of cooling systems, and reduces and homogenizes the temperature in the reaction area of the fuel cells. A transparent PEMFC is constructed with PGS of thickness 0.1 mm cut into the shape of a flow channel and bound with the cathode gas channel plate. Eleven thermocouples are embedded at different positions on the cathode gas channel plate to measure the temperature distribution. The water and water flooding inside the cathode gas channels, with and without PGS, were successfully visualized. The locations of liquid water are correlated with the temperature measurement. PGS reduces the maximum cell temperature and improves cell performance at high cathode flow rates. The temperature distribution is also more uniform in the cell with PGS than in the one without PGS. Results of this study demonstrate the promising application of PGS to the thermal management of a fuel cell system.     

A numerical study on the performance of PEMFC with wedge-shaped fins in the cathode channel

The gas flow field design has a significant influence on the overall performance of a proton exchange membrane fuel cell (PEMFC). A single-channel PEMFC with wedge-shaped fins in the cathode channel was proposed, and the effects of fin parameters such as volume (0.5 mm³, 1.0 mm³, and 1.5 mm³), number (3, 5, and 9), and porosity of the gas diffusion layer (GDL) (0.2, 0.4, 0.6, and 0.8) on the performance of PEMFC were numerically examined based on the growth rate of power density (GRPD) and polarization curve. It was shown that wedge-shaped fins could effectively improve the PEMFC performance. With an increase in fin volume, the distributions of oxygen mass fraction in the outlet area of the cathode channel were lower, the drainage effect of the PEMFC improved, and GRPD also increased accordingly. Similar results were obtained as the number of fins increased. The GDL porosity had a greater effect than the wedge-shaped fins on the improvement in PEMFC performance, but the influence of GDL porosity weakened and the GRPD of porosity decreased as the porosity increased. This study provides an effective guideline for the optimization of the cathode channel in a PEMFC.     

Numerical simulation of water droplet transport characteristics in cathode channel of proton exchange membrane fuel cell with tapered slope structures

In proton exchange membrane fuel cells (PEMFC), the design of the cathode flow field is very important, because an excellent flow channel design can not only accelerate the transmission rate of liquid water, but also affect the distribution of electrode reactants and electrode products which influence the electrochemical performance of the fuel cell. This study presents three new channels (models 1,2 and 3), which were created using two unilateral slopes and a bilateral slope structure with tapered tube lengths of 0.4, 1.2 and 0.8 mm, respectively. The dynamic behavior of liquid water under the three design schemes is numerically studied based on the volume of fluid method. And the influence on the performance of fuel cell was analyzed synthetically. The results indicate that the introduction of a tapered and sloping structure can improve the transmission efficiency of the droplets in the flow channel, and the maximum droplet removal time of the new channel can be reduced by 24.4%compare with standard conventional flow channel. The slope structure guides the flow path of water droplet and reduces the occurrence of droplet spatter. Influenced by the slope and tapered structures, the turbulence of airflow near the bottom surface (gas diffusion layer)of the flow channel is enhanced and Oxygen concentration in the cathode is raised, which improves the mass transfer capacity and average current density of reactive surface. In conclusion, the new type of channel with a tapered and sloping structure has a potential to improve the performance of water management in the cathode channel of PEMFC.     

Optimization of block structure parameters of PEMFC novel block channels using artificial neural network

Flow channel design is critically important to the performance of proton exchange membrane fuel cell (PEMFC) due to its great influence on liquid water removal and mass and heat transfer. Block flow channel shows good prospect to improve liquid water removal and mass transport, benefiting the PEMFC performance. In this study, the block structures, namely the length, width and height of the block, are optimized for a novel block channel using data-driven surrogate model based on the artificial neural network (ANN). The training/test datasets are obtained from a three-dimensional multi-phase model based on the volume of fluid (VOF) method, with the water removal time (T) and the maximum channel pressure drop (ΔP) taken as the output and optimization objectives. The results show that the ANN prediction agrees well with the physical model results, with the coefficient of determination (R²) of T and ΔP are 0.99598 and 0.99677, respectively. The block parameters are further optimized using the comprehensive scoring method considering both T and ΔP. The block parameters with the length of 0.8 mm, width of 0.375 mm and height of 0.75 mm are found to be the optimum in terms of the highest score. The optimum parameters obtained from the data-driven surrogate model are verified by the physical model, indicating that the ANN model is an effective and fast method to optimize block structure of block flow channel from the perspective of liquid water removal and channel pressure drop.     

Detection of liquid water in the flow channels of PEM fuel cell using an optical sensor

An optical sensor was developed with the capability of detecting liquid water in the flow channels of a proton exchange membrane fuel cell (PEMFC) as well as simultaneously measuring temperature. This work is an extension of previous research in which an optical temperature sensor was developed for measuring the in situ temperature of PEM fuel cells based on the principles of phosphor thermometry. The optical sensor was installed in the cathode flow channel of a 5 cm² proton exchange membrane fuel cell. The fuel cell was tested under both dry and humid conditions. Liquid water formation in the flow channels was quantitatively measured from the experimental data. An observed time fraction value was estimated for characterizing flow channel flooding. The observed time fraction of liquid water in the flow channel was found to be closely related to the relative humidity of reactants and the operating current of the fuel cell.     

Performance of a proton exchange membrane fuel cell stack with thermally conductive pyrolytic graphite sheets for thermal management

This work experimentally investigates the effects of the pyrolytic graphite sheets (PGS) on the performance and thermal management of a proton exchange membrane fuel cell (PEMFC) stack. These PGS with the features of light weight and high thermal conductivity serve as heat spreaders in the fuel cell stack for the first time to reduce the volume and weight of cooling systems, and homogenizes the temperature in the reaction areas. A PEMFC stack with an active area of 100 cm² and 10 cells in series is constructed and used in this research. Five PGS of thickness 0.1 mm are cut into the shape of flow channels and bound to the central five cathode gas channel plates. Four thermocouples are embedded on the cathode gas channel plates to estimate the temperature variation in the stack. It is shown that the maximum power of the stack increase more than 15% with PGS attached. PGS improve the stack performance and alleviate the flooding problem at low cathode flow rates significantly. Results of this study demonstrate the feasibility of application of PGS to the thermal management of a small-to-medium-sized fuel cell stack.     

Thermal management of a proton exchange membrane fuel cell stack with pyrolytic graphite sheets and fans combined

This work characterizes the thermal management of a proton exchange membrane fuel cell (PEMFC) stack with combined passive and active cooling. A 10-cell PEMFC stack with an active area of 100 cm² for each cell is constructed. Six thermally conductive 0.1-mm-thick Pyrolytic Graphite Sheets (PGSs) are cut into the shape of flow channels and bound to the six central cathode gas channel plates. These PGSs, which are lightweight and have high thermal conductivity, function as heat spreaders and fins and provide passive cooling in the fuel cell stack, along with two small fans for forced convection. Three other cooling configurations with differently sized fans are also tested for comparisons (without PGSs). Although the maximum power generated by the stack with the configuration combining PGSs and fans was 183 W, not the highest among all configurations, it significantly reduced the volume, weight, and cooling power of the thermal management system. Net power, specific power, volumetric power density, and back work ratio of this novel thermal management method are 179 W, 18.54 W kg⁻¹, 38.9 kW m⁻³, and 2.1%, respectively, which are superior to those of the other three cooling configurations with fans.     

外界供给参数对PEMFC输出性能及水热平衡的影响

为了改善质子交换膜燃料电池(PEMFC)内部的水热平衡,从而进一步改善PEMFC的输出性能,文章建立了PEMFC的三维模型,通过改变PEMFC的外界供给参数(进气速度、加湿率以及冷却水流速),应用COMSOL模拟仿真得到了PEMFC的极化曲线和功率曲线、流道和气体扩散层(GDL)的水浓度分布情况,以及冷却水流速对PEMFC温度的影响。研究结果表明:随着进气速度和加湿率的逐渐增加,PEMFC的输出性能均逐渐提升,但是,过高的加湿率可能导致电极水淹;随着冷却水流速的增加,PEMFC温度加速下降,膜内温度分布变得更均匀。     

The impact of channel path length on PEMFC flow-field design

Distributions in reactant species concentration in a PEMFC due to local consumption of fuel and local transport of water through the membrane cause distributions in current density, temperature, and water concentration in three dimensions in a PEMFC. These distributions can lead to flooding or drying of the membrane that may shorten the life of an MEA. Changing the cell's flow-field pattern to distribute the gas more evenly is one method of minimizing these stresses. This paper investigates how 200 cm² serpentine flow-fields with different number of gas paths, and thus different gas path lengths, affect performance and species distribution. The results show how the local temperature, water content, and current density distributions become more uniform for serpentine flow-field designs with shorter path lengths or larger number of channels. These results may be used to develop universal heuristics and dimensionless number correlations in the design of flow-fields and stacks.     

An inverse geometry design problem in optimizing the shape of the gas channel for a proton exchange membrane fuel cell

The inverse design problem technique presented in this paper is intended for optimizing the shape of the gas channel at the cathode side in a proton exchange membrane fuel cell (PEMFC). The technique uses the desired current densities located on a carbon plate near the outlet of the channel at the cathode side as a starting point. The desired current density distributions can be obtained by modifying the current density distributions of the existing PEMFC with rectangular gas channels. The geometry of the redesigned gas channel is generated using a B-spline curve method, which enables the shape of the fuel channel to be completely specified using only a small number of control points, thus applying the technique of parameter estimation for the inverse design problem. Results show that by utilizing the redesigned optimal gas channel, the total current of the PEMFC can be increased, and at the same time the phenomenon of saturated water accumulation in the channel can be greatly reduced.     

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.