Degradation mitigation effects of pressure swing in proton exchange membrane fuel cells with dead-ended anode

High cost remains to be one of the primary obstacles for the commercialization of proton exchange membrane fuel cells (PEMFCs). To simplify the fuel cell system and reduce cost, dead-ended anode (DEA) is widely used. However, water and nitrogen can accumulate in the dead-ended anode, resulting in cell performance decrease and severe cell degradation. Anode pressure swing supply is a new technology which has been shown to be effective in reducing local water and nitrogen accumulation in the anode channel. In this work, the effects of pressure swing supply on fuel cell degradation have been experimentally studied. Two sets of experiments on the same fuel cell are conducted, one under conventional constant pressure operation and the other under pressure swing operation. Polarization curves show that pressure swing supply can significantly mitigate cell degradation during DEA operations. Electrochemical characterizations are performed to study the mechanisms of mitigations in cell degradation. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results show that pressure swing supply can significantly reduce electrolyte membrane degradation, but has no significant mitigation effect on the cathode catalyst degradation during DEA operation. Further examinations of the membrane-electrode-assembly (MEA) by scanning electron microscope (SEM) confirm the significant difference in membrane degradations since there is a very large difference in average thickness of the membranes after the degradation tests.     

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.     

Dynamic characteristics and mitigations of hydrogen starvations in proton exchange membrane fuel cells during start-ups

Hydrogen starvation during a start-up process in proton exchange membrane (PEM) fuel cells could result in drastic local current density variations, reverse cell voltage and irreversible cell damages. In this work, variations of local current densities and temperatures are measured in situ under both potentiostatic and galvanostatic modes. Experimental results show that when the cell starts up under potentiostatic mode with hydrogen starvation, current density undershoots occur in the downstream; while under the galvanostatic mode, local current density in the downstream almost drops to zero, but the current density near the outlet remains almost constant. The phenomenon of near constant current density near the outlet leads to a novel approach to alleviate hydrogen starvations - a hydrogen reservoir is added at the anode outlet. Experimental results show that the exit hydrogen reservoir can significantly reduce the zero current region and alleviate hydrogen starvations. A non-dimensional current-density variation coefficient is proposed to measure the magnitude of local current density changes during starvations. Experimental results show that the exit hydrogen reservoir can significantly reduce the current-density variations coefficient over the entire flow channel, indicating that adding an exit reservoir is an effective approach in mitigating hydrogen starvations.     

In situ comparison of water content and dynamics in parallel, single-serpentine, and interdigitated flow fields of polymer electrolyte membrane fuel cells

Water content and dynamics were characterized and compared in situ by simultaneous neutron and optical imaging for three PEM fuel cell flow fields: parallel, serpentine, and interdigitated. Two independent sets of images were obtained simultaneously: liquid water dynamics in the flow field (channels and manifolds) were recorded by a digital camera through an optical window, while the through-thickness integrated water content was measured across the cell area by neutron imaging. Complementary data from the concurrent images allowed distinguishing between the water dynamics on the cathode and the anode side. The transient water content within the cell measured using neutron imaging is correlated with optical data as well as with temporal variations in the cell output and pressure differentials across the flow fields. Water dynamics on both the cathode and anode side were visualized and discussed.The serpentine cell showed stable output across the current range and the highest limiting current. Parallel and interdigitated cells exhibited substantially higher water contents and lower pressure differentials than the serpentine. Anode flooding significantly impeded their performance at high current. At moderate current, cell output correlated with the changes in water distribution in the cathode flow field rather than with the variations in the overall water content. Performance of the interdigitated cell was similar to the serpentine one in spite of the vastly different water contents.The cell's water-content response to a step-change in current revealed three distinct stages of water accumulation. Flow field configuration greatly affected both the amount of water accumulated in the cell and the duration of each stage.     

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.     

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.     

Detailed analysis of polymer electrolyte membrane fuel cell with enhanced cross‐flow split serpentine flow field design

Most generally used flow channel designs in polymer electrolyte membrane fuel cells (PEMFCs) are serpentine flow designs as single channels or as multiple channels due to their advantages over parallel flow field designs. But these flow fields have inherent problems of high pressure drop, improper reactant distribution, and poor water management, especially near the U‐bends. The problem of inadequate water evacuation and improper reactant distribution become more severe and these designs become worse at higher current loads (low voltages). In the current work, a detailed performance study of enhanced cross‐flow split serpentine flow field (ECSSFF) design for PEMFC has been conducted using a three‐dimensional (3‐D) multiphase computational fluid dynamic (CFD) model. ECSSFF design is used for cathode part of the cell and parallel flow field on anode part of the cell. The performance of PEMFC with ECSSFF has been compared with the performance of triple serpentine flow design on cathode side by keeping all other parameters and anode side flow field design similar. The performance is evaluated in terms of their polarization curves. A parametric study is carried out by varying operating conditions, viz, cell temperature and inlet humidity on air and fuel side. The ECSSFF has shown superior performance over the triple serpentine design under all these conditions.     

Dynamic characteristics of internal current during startups/shutdowns in proton exchange membrane fuel cells

Studying dynamic characteristics of proton exchange membrane fuel cells (PEMFCs) during startups/shutdowns is of great importance to proposing strategies to improve fuel cell performance and durability. In this study, internal current during startup and shutdown processes in PEMFC is investigated, and effects of gas supply/shutoff sequences and backpressure are analyzed by measuring local current densities and the cell voltage in situ. The experimental results show that when reactants were fed/shut off, internal current occurs and variation patterns of local current densities along the flow channel are different. During startups, local current densities in the downstream drop to negative values and internal current can be eliminated when air is first supplied into the cell. While during shutdowns, the results show that negative currents occur in the upstream, and if hydrogen is shut off first, all local current densities remain constant at zero, indicating the effectiveness of gas shutoff sequence in eliminating/mitigating internal current in PEM fuel cells. Further experimental results show that the magnitude of internal current increases with the pressure difference between the anode and the cathode.     

Numerical modeling of anode-bleeding PEM fuel cells: Effects of operating conditions and flow field design

Operating the PEM fuel cell in the dead-ended anode mode reduces the overall cost and complexity of the system but causes a voltage loss and carbon corrosion in the cathode catalyst layer due to hydrogen starvation in the anode. Whereas allowing an ultra-low flowrate at the anode outlet offers a very high utilization of hydrogen and achieves a stable voltage transient. Here, a time-dependent pseudo-three-dimensional, two-phase, and non-isothermal model is developed to study the optimum bleeding rate, which maximizes the hydrogen utilization, achieves a stable cell voltage and avoids carbon corrosion, which is commonly observed when the bleed rate is set to zero, i.e. the dead-ended mode. The model is validated against the experimental data by comparing the polarization curves and cell voltage transients during the dead-ended anode operation of small experimental cells with serpentine and straight anode flow channels. Moreover, the effects of operating conditions on cell performance during the anode bleeding operation mode are investigated. Results demonstrate that the hydrogen utilization exceeds 99% in the anode-bleeding mode without hydrogen starvation, and the cell performance improves significantly for higher anode pressure, lower cell temperature, and lower relative humidity at the cathode inlet. Lastly, it is found that serpentine channels in the anode improve the uniformity of the distribution of hydrogen compared to straight and interdigitated channels in the anode-bleeding mode while the cathode flow field consists of serpentine channels.     

Local transport phenomena and cell performance of PEM fuel cells with various serpentine flow field designs

The flow field design in bipolar plates is very important for improving reactant utilization and liquid water removal in proton exchange membrane fuel cells (PEMFCs). A three-dimensional model was used to analyze the effect of the design parameters in the bipolar plates, including the number of flow channel bends, number of serpentine flow channels and the flow channel width ratio, on the cell performance of miniature PEMFCs with the serpentine flow field. The effect of the liquid water formation on the porosities of the porous layers was also taken into account in the model while the complex two-phase flow was neglected. The predictions show that (1) for the single serpentine flow field, the cell performance improves as the number of flow channel bends increases; (2) the single serpentine flow field has better performance than the double and triple serpentine flow fields; (3) the cell performance only improves slowly as the flow channel width increases. The effects of these design parameters on the cell performance were evaluated based on the local oxygen mass flow rates and liquid water distributions in the cells. Analysis of the pressure drops showed that for these miniature PEMFCs, the energy losses due to the pressure drops can be neglected because they are far less than the cell output power.     

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

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

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 operating temperatures on performance and pressure drops for a 256 cm proton exchange membrane fuel cell: An experimental study

Cell performance and pressure drop were experimentally investigated for two commercial size 16 cm × 16 cm serpentine flow field proton exchange membrane fuel cells with Core 5621 and Core 57 membrane electrode assemblies at various cell temperatures and humidification temperatures. At cell temperature lower than the humidification temperature, the cell performance improved as the cell temperature increased, while reversely at cell temperature higher than the humidification temperature. At a specified cell temperature, increasing the cathode and/or anode humidification temperature improved the cell performance, and their effects weakened as cell temperature decreased. The effects of the cell and the humidification temperature on the pressure drops were closely related to the reactant feed mode. For the constant stoichiometric flow rate mode, both cathode and anode pressure drops increased as humidification temperature and average current density increased. For the constant mass flow rate mode, both cathode and anode pressure drops increased as humidification temperature increased, while anode pressure drops decreased and cathode pressure drops increased as average current density increased. The optimal cell performance occurred at cell temperature of 65 °C and humidification temperature of 70 °C. The effects of these operating parameters on the cell performance and pressure drop were analyzed based on the catalytic activity, membrane hydration, and cathode flooding.     

Experimental analysis of an ejector for anode recirculation in a 10 kW polymer electrolyte membrane fuel cell system

For analyzing ejector's performance in the system, an ejector for a 10 kW polymer electrolyte membrane fuel cell (PEMFC) system was first designed, manufactured, and a 10 kW PEMFC system bench was built up. A proportional valve and PI pressure feedback control method were adopted to control the hydrogen supply and anode inlet pressure. During the test, performances between dead-ended anode (DEA) mode and ejector mode were compared. Ejector's performances in the system, i.e., volume flow recirculated ratio, difference pressure, dynamic responses of primary pressure, anode inlet pressure, and recirculated gas flow rate during the purge process and current variation condition, were investigated. The results show that pressure adjustment is accurate, continuous, and fast using the proportional valve and PI pressure feedback control method. The hydrogen consumption rate in the ejector mode can reduce from 5% to 10% compared with the rate in the DEA mode except for the stack current 5 A and 10 A conditions. For better water removal out of the anode channel in ejector mode, the maximum stack power increases from 5.11 kW (DEA mode) to 9.56 kW (ejector mode). Anode pressure surge caused by the purge valve switching enhances the ejector's recirculated performance significantly.     

Study of novel flow channels influence on the performance of direct methanol fuel cell

The existing flow channels like parallel and gird channels have been modified for better fuel distribution in order to boost the performance of direct methanol fuel cell. The main objective of the work is to achieve minimized pressure drop in the flow channel, uniform distribution of methanol, reduced water accumulation, and better oxygen supply. A 3D mathematical model with serpentine channel is simulated for the cell temperature of 80 °C, 0.5 M methanol concentration. The study resulted in 40 mW/cm² of power density and 190 mA/cm² of current density at the operating voltage of 0.25 V. Further, the numerical study is carried out for modified flow channels to discuss their merits and demerits on anode and cathode side. The anode serpentine channel is unmatched by the modified zigzag and pin channels by ensuring the better methanol distribution under the ribs and increased the fuel consumption. But the cathode serpentine channel is lacking in water management. The modified channels at anode offered reduced pressure drop, still uniform reactant distribution is found impossible. The modified channels at cathode outperform the serpentine channel by reducing the effect of water accumulation, and uniform oxygen supply. So the serpentine channel is retained for methanol supply, and modified channel is chosen for cathode reactant supply. In comparison to cell with only serpentine channel, the serpentine anode channel combined with cathode zigzag and pin channel enhanced power density by 17.8% and 10.2% respectively. The results revealed that the zigzag and pin channel are very effective in mitigating water accumulation and ensuring better oxygen supply at the cathode.     

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.     

Experimental study on the performance of PEM fuel cells with interdigitated flow channels

In this work, the effects of interdigitated flow channel design on the cell performance of proton exchange membrane fuel cells (PEMFCs) are investigated experimentally. To compare the effectiveness of the interdigitated flow field, the performance of the PEM fuel cells with traditional flow channel design is also tested. Besides, the effects of the flow area ratio and the baffle-blocked position of the interdigitated flow field are examined in details. The experimental results indicate that the cell performance can be enhanced with an increase in the inlet flow rate and cathode humidification temperature. Either with oxygen or air as the cathode fuel, the cells with interdigitated flow fields have better performance than conventional ones. With air as the cathode fuel, the measurements show that the interdigitated flow field results in a larger limiting current density, and the power output is about 1.4 times that with the conventional flow field. The results also show that the cell performance of the interdigitated flow field with flow area ratio of 40.23% or 50.75% is better than that with 66.75%.     

Investigation of mass transport and cell performance on μDMFC with different anode flow fields

The study systematically analyzes the performance of micro direct methanol fuel cell (μDMFC) with different flow fields. A two‐phase three‐dimensional model is developed to evaluate the mass transport accurately. The transport of methanol and air, the pressure distribution, the anode saturation, and the methanol crossover are numerically observed, the under‐rib convection is also investigated numerically. The flow fields with an active area of 0.64 cm² are fabricated on silicon wafers by micro electromechanical system technology. Performance of μDMFCs with different flow fields is sorted as: double‐serpentine flow field (DSFF) > single‐serpentine flow field (SSFF) > triple‐serpentine flow field (TSFF), and the dynamic test results indicate the cell with DSFF takes the shortest time to reach a stable power output when compared with other cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical investigation of the performance of symmetric flow distributors as flow channels for PEM fuel cells

This work reports on the performance of a single PEM fuel cell using symmetric flow patterns as gas delivery channels. Three flow patterns, two symmetric and one serpentine, are taken from the literature on cooling of electronics and they are implemented in a computational model as gas flow channels in the anode and cathode side of a PEMFC. A commercial CFD code was used to solve the physics involved in a fuel cell namely: the flow field, the mass conservation, the energy conservation, the species transport, and the electric/ionic fields under the assumptions of steady state and single phase. An important feature of the current modeling efforts is the analysis of the main irreversibilities at different current densities showing the main energy dissipation phenomena in each cell design. Also, the hydraulic performance of the flow patterns was studied by evaluating the pressure drop and pumping power. The first part of this work reveals the advantages of using a serpentine pattern over the base symmetric distributors. The second part is an optimization of the symmetric patterns using the entropy minimization criteria. Such an optimization led to the creation of a flow structure that promotes an improved performance from the point of view of power generation, uniformity of current density, and low pumping power.     

Effect of channel length on interdigitated flow-field PEMFC performance: A computational and experimental study

This study examines the effect of increased channel length on the distribution of flow gases and cell performance in an interdigitated flow-field PEMFC. A numerical model was used to simulate the pressure distribution and gas flow in 5 cm and 25 cm channel length interdigitated flow fields that have eight channels with fixed channel and land widths of 1 mm. The results show the distribution of flow under land areas (cross flow) is subject to maldistribution in the longer cell while the shorter cell produces relatively homogenous cross flow along its length. An experimental test cell was designed to run with the same channel lengths, under similar conditions to those modeled. Inlet pressure data was recorded to account for parasitic pump losses, used to calculate net system power curves. Results show that a shorter channel interdigitated flow field may produce both higher maximum power and limiting current densities compared with longer cells.