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.     

The effect of relative humidity of the cathode on the performance and the uniformity of PEM fuel cells

The effect of relative humidity of the cathode (RHC) on proton exchange membrane (PEM) fuel cells has been studied focusing on automotive operation. Computational fluid dynamics (CFD) simulations were performed on a 300-cm² serpentine flow-field configuration at various RHC levels. The dependency of current density, membrane water contents, net water flux on the performance and the uniformity was investigated. The uniformity of current density and temperature was evaluated by employing standard deviation. The water balance inside a fuel cell was examined by describing electro-osmotic drag and back diffusion. It was concluded that the RHC has strong effect on the cell performance and uniformity. The dry RHC showed low cell voltage and non-uniform distributions of current density and temperature, whereas high RHC presented increased cell performance and uniform distributions of current density and temperature with well-hydrated membrane electrode assembly (MEA). Also the local current density distribution was strongly dependent on the local membrane water contents distribution that has complex phenomena of water transport. The elimination of external humidifier is desirable for the automotive operation, but it could degrade cell performance and durability due to dehydration of the MEA. Therefore a proper humidification of the reactant is necessary.     

Direct measurement of lateral current density distribution in a PEM fuel cell with a serpentine flow field

In a proton exchange membrane (PEM) fuel cell, local current density can vary drastically in the lateral direction across the land and channel areas. It is essential to know the lateral current density variations in order to optimize flow field design and fuel cell performance. Thus the objective of this work is to directly measure the lateral current density variations in a PEM fuel cell with a serpentine flow field. Five 1 mm-width partially-catalyzed membrane electrode assemblies (MEA), each corresponding to a different location from the center of the gas channel to the center of the land area are used in the experiments. Current densities for fuel cells with each of the partially-catalyzed MEAs are measured and the results provide the lateral current density distribution. The measurement results show that in the high cell voltage region, local current density is the highest under the center of the land area and decreases toward the center of the channel area; while in the low cell voltage region local current density is the highest under the center of the channel area and decreases toward the center of the land area. Besides, the effects of cathode flow rates on the lateral current density distribution have also been studied. Furthermore, comparisons have also been made by using air and oxygen in the cathode and it is found that when oxygen is used the local current density under the land is significantly enhanced, especially in the low cell voltage region.     

Dynamic characteristics of local current densities and temperatures in proton exchange membrane fuel cells during reactant starvations

Reactant starvation during proton exchange membrane fuel cell (PEMFC) operation can cause serious irreversible damages. In order to study the detailed local characteristics of starvations, simultaneous measurements of the dynamic variation of local current densities and temperatures in an experimental PEMFC with single serpentine flow field have been performed during both air and hydrogen starvations. These studies have been performed under both current controlled and cell voltage controlled operations. It is found that under current controlled operations cell voltage can decrease very quickly during reactant starvation. Besides, even though the average current is kept constant, local current densities as well as local temperatures can change dramatically. Furthermore, the variation characteristics of local current density and temperature strongly depend on the locations along the flow channel. Local current densities and temperatures near the channel inlet can become very high, especially during hydrogen starvation, posing serious threats for the membrane and catalyst layers near the inlet. When operating in a constant voltage mode, no obvious damaging phenomena were observed except very low and unstable current densities and unstable temperatures near the channel outlet during hydrogen starvation. It is demonstrated that measuring local temperatures can be effective in exploring local dynamic performance of PEMFC and the thermal failure mechanism of MEA during reactants starvations.     

Effects of hydrophilic/hydrophobic properties on the water behavior in the micro-channels of a proton exchange membrane fuel cell

The mobility of water droplets and water films inside a straight micro-channel of a proton exchange membrane fuel cell was simulated to study the effects of the hydrophilic/hydrophobic properties on water behavior. The volume-of-fluid model in the FLUENT package was used to keep track of the deformation of the liquid–gas interface. The results show that the water moved faster on a hydrophobic surface. But a hydrophobic channel side-wall was a disadvantage for the gas diffusion when the MEA had a hydrophilic surface. A hydrophilic channel side-wall with a hydrophobic MEA surface could avoid water accumulation on the MEA surface. The water and gas distribution under this condition was advantageous for water discharge and gas diffusion.     

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.     

NUMERICAL PREDICTION OF TEMPERATURE DISTRIBUTION IN PEM FUEL CELLS

A numerical three-dimensional model is developed that includes the energy equation to predict the temperature distribution inside a straight channel proton exchange membrane (PEM) fuel cell and the effect of heat produced by the electrochemical reactions on fuel cell performance. A control volume approach is used and source terms for transport equations, heat generation, and a phase change model are presented to facilitate their incorporation in commercial flow solvers. Predictions show that the fuel cell performance depends not merely on the inlet humidity condition, cell voltage, and membrane thickness but also on the temperature rise inside fuel cells.     

Study of the effects of flow channel with non-uniform cross-sectional area on PEMFC species and heat transfer

In this study it is investigated the performance of a proton exchange membrane fuel cell. The results show that an inclination of 0.75° in the flow channel can effectively increase the current density generated by almost 9.5% and the maximum power density by 8%. With the use of more tapered channels the distribution of the reactants in the porous media leads to a better effective oxygen distribution, affecting directly the heat transfer inside the cell. In contrast, the pressure drop in the flow channel increase by factors of approximately 2 and 3.5 for angles of 0.5° and 0.75°, respectively.     

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

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

Influence of geometric parameters of the flow fields on the performance of a PEM fuel cell. A review

The proton Exchange membrane fuel cell (PEMFC) performance depends not only on many factors including the operation conditions, transport phenomena inside the cell and kinetics of the electrochemical reactions, but also in its physical components; membrane electrode assembling (MEA) and bipolar plates (BPs). Among the PEM stack components, bipolar plates are considered one of the crucial ones, as they provide one of the most important issues regarding the performance of a stack, the homogeneous distribution of the reactive gases all over the catalyst surface and bipolar plate areas through, the so call, flow channels; physical flow patterns or paths fabricated on the BPs surfaces to guide the gases all along the BPs for its correct distribution. The failure in flow distribution among different unit cells may severely influence the fuel cell stack performance. Thus, to overcome such possible failures, the design of more efficient flow channels has received considerable attention in the research community for the last decade.     

Simultaneous thermal and visual imaging of liquid water of the PEM fuel cell flow channels

Water flooding and membrane dry-out are two major issues that could be very detrimental to the performance and/or durability of the proton exchange membrane (PEM) fuel cells. The above two phenomena are well-related to the distributions of and the interaction between the water saturation and temperature within the membrane electrode assembly (MEA). To obtain further insights into the relation between water saturation and temperature, the distributions of liquid water and temperature within a transparent PEM fuel cell have been imaged using high-resolution digital and thermal cameras. A parametric study, in which the air flow rate has been incrementally changed, has been conducted to explore the viability of the proposed experimental procedure to correlate the relation between the distribution of liquid water and temperature along the MEA of the fuel cell. The results have shown that, for the investigated fuel cell, more liquid water and more uniform temperature distribution along MEA at the cathode side are obtained as the air flow rate decreases. Further, the fuel cell performance was found to increase with decreasing air flow rate. All the above results have been discussed.     

Study of the characteristics of temperature rise and coolant flow rate control during malfunction of PEM fuel cells

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm^?2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.     

Numerical analysis of high-temperature proton exchange membrane fuel cells during start-up by inlet gas heating and applied voltage

This paper investigates the start-up or warm-up process of a high-temperature proton exchange membrane fuel cell (HT-PEMFC) from room temperature to a desired temperature of ～180 °C. The heating strategy considered in this study involves an initial heating of the HT-PEMFC by a process referred to as inlet gas heating to a temperature above 100 °C. After the fuel cell reaches above 100 °C, a voltage is applied, where electrochemical reaction heating is expected to contribute to the heating process. Thus, a numerical transient non-isothermal three-dimensional model is derived to mimic the start-up process. Operational parameters such as anode inlet temperature, cathode inlet temperature, applied voltage and voltage application temperature are varied and their effects on the maximum temperature in the membrane electrode assembly (MEA) and temperature difference in the MEA are studied. Firstly, the distribution of temperature along the channel length indicates an increase of temperature during gas heating and as the voltage is applied at the voltage application temperature, the temperature increases at the centre of the MEA due to exothermic reactions. The two-dimensional temperature distribution indicates a temperature difference between the centre of the MEA and the regions below the bipolar plate where the temperature is relatively lower. Considering the whole start-up process with respect to time, the temperature difference exists throughout the process. This will be the key focus in the parametric study. The parametric study indicates that the inlet gas temperatures, applied voltage and the voltage application temperature affect the maximum temperature in the MEA and most importantly, the temperature difference in the MEA. This can cause thermal stresses to build-up if the increase rate of temperature difference is excessive. Setting the applied voltage high (thus, lower current density) is necessary to reduce the increase rate of temperature difference.     

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.     

Use of flexible micro-temperature sensor to determine temperature in situ and to simulate a proton exchange membrane fuel cell

In this work, a micro-temperature sensor on a 40 μm flexible stainless-steel substrate was fabricated using micro-electro-mechanical systems (MEMS). Embedding a micro-temperature sensor in a proton exchange membrane fuel cell (PEMFC) to monitor temperature will not damage the sensor during the experimental process. This investigation is the first to develop a micro-temperature sensor that can be placed anywhere between the membrane electrode assembly (MEA) and the flow-channel plate inside a PEMFC. The simulated temperature is consistent with the experimentally determined temperature. The performance curve is also consistent with experimental results, revealing the accuracy of the simulation and the effectiveness of monitoring temperature inside a PEMFC.     

Dynamic modeling of the effect of water management on polymer electrolyte fuel cells performance

Water management in Polymer Electrolyte Fuel Cell (PEFC) is a key factor in fuel cell performance, and it is an important contributor to the proton exchange membrane durability. Water droplet accumulation in the channel causes non-uniform distribution of gas pressure and spatial inhomogeneity of the local current density in potentiostatic mode. These spatial and temporal fluctuations in the operating conditions imply unequal use of the membrane surface and the catalyst layer, producing uneven degradation and aging of the Membrane Electrode Assembly (MEA). In order to study the dynamic and spatial performance of the fuel cell, a three-level model has been developed. The model is composed of a two-phase, where steam and liquid water drops movement are considered in the channel model; liquid water and gas diffusion are considered in Gas Diffusion Layers (GDLs) model; and finally, the electrochemical reactions are represented in the electrochemical model. The complete model provides a wider understanding of the effect of water on PEFCs and allows to analyze the local current density and the water distribution in response to experimental set-up parameters such as anode and cathode gas flows, total current or channel geometries. The model has been validated using neutron images and segmented cells technique to evaluate the spatial distribution of liquid water and current density in the cell. The developed model and the simulation procedure proposed in this paper allow obtaining long-term dynamic simulations with low computational effort.     

Effect of wettability on water removal from the gas diffusion layer surface in a novel proton exchange membrane fuel cell flow channel

Effective water removal from the proton exchange membrane fuel cell (PEMFC) surface exposed to the flow channel is critical to the operation and water management in PEMFCs. In this study, the water removal process is investigated numerically for a novel flow channel formed by inserting a hydrophilic needle in the conventional PEMFC flow channel, and the effect of the surface wettability of the membrane electrode assembly (MEA) and the inserted needle on the water removal process is studied. The results show that the liquid water can be more effectively removed from the MEA surface for larger MEA surface contact angles and smaller needle surface contact angles. The pressure drop for the flow in the channel is also examined and it is seen to be indicative of the liquid water flow and transport in the flow channel, suggesting that pressure drop is a useful parameter for the investigation of water transport and dynamics in the flow channel.     

Investigation of hydrogen consumption in proton exchange membrane fuel cell with depth‐reduced channels

The failure of membrane electrode assembly (MEA) relates to the fuel consumption for proton exchange membrane fuel cell(PEMFC). In this study, a complete 3‐D computational model, including nine layers of the integrated fuel cell system, was developed to investigate the influences of depth‐reduced channels on hydrogen consumption. Simulation results showed that the different combinations of depth caused dissimilar pressure profiles, pressure drop, and distribution of hydrogen concentration in the depth‐reduced channels, and thus resulted in the variations of the current density and the hydrogen consumption. The maximum hydrogen mass flux in MEA was located at the area of channel entrance for all channels. This implied that the serious degradation of MEA always occurred around the channel entrance. In addition, a depth‐reduced channel with a shorter and deeper front section was able to reduce the maximum hydrogen mass flux by about 3–4% with losing current density about 2%. These results suggest that a depth‐reduced channel with a shorter and deeper front section can delay the serious degradation of MEA to prolong the lifetime. Copyright © 2007 John Wiley & Sons, Ltd.     

Thermal analysis of air-cooled PEM fuel cells

Air-cooled proton exchange membrane fuel cells (PEMFCs), having combined air cooling and oxidant supply channels, offer significantly reduced bill of materials and system complexity compared to conventional, water-cooled fuel cells. Thermal management of air-cooled fuel cells is however a major challenge. In the present study, a 3D numerical thermal model is presented to analyze the heat transfer and predict the temperature distribution in air-cooled PEMFCs. Conservation equations of mass, momentum, species, and energy are solved in the oxidant channel, while energy equation is solved in the entire domain, including the membrane electrode assembly (MEA) and bipolar plates. The model is validated with experiments and can reasonably predict the maximum temperature and main temperature gradients in the stack. Large temperature variations are found between the cool incoming air flow and the hot bipolar plates and MEA, and in contrast to water-cooled fuel cells, significant temperature gradients are detected in the flow direction. Furthermore, the air velocity and in-plane thermal conductivity of the plate are found to play an important role in the thermal performance of the stack.     

The effects of excess phosphoric acid in a Polybenzimidazole-based high temperature proton exchange membrane fuel cell

A series of experiments are conducted in order to investigate the performance of a proton exchange membrane (PEM) fuel cell using a commercially available polybenzimidazole (PBI)-based high temperature membrane. During the study a drastic degradation in performance is observed over time and a significant amount of solid material built-up is found in the flow field plate and the membrane-electrode assembly (MEA). The built-up material is examined by the use of a Scanning Electron Microscope (SEM). Further elemental analysis using Energy Dispersive X-ray Spectroscopy (EDS) finds that the built-up material contains large amount of phosphorus, thus relating it with the excess phosphoric acid found in the MEA. Additional experimental studies show that the built-up material is caused by the excess acid solution in the MEA, and when the excess phosphoric acid is removed from the MEA the fuel cell performance improves significantly and becomes very stable.