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.     

Novel convergent-divergent serpentine flow fields effect on PEM fuel cell performance

A new design of convergent and divergent flow fields are being developed in single serpentine flow field pattern for proton exchange membrane (PEM) fuel cell. The channel depth is varied by means of inclination from inlet to outlet of the bipolar plate. By the varying inclined channel depth, it created convergent/divergent flow effect along the channel length in the single serpentine. Four different convergent flow fields are manufactured by the varying inclined channel depth from inlet to outlet as 1.5 mm–0.5 mm, 2.5 mm–1.5 mm, 3 mm～1 mm and 3.5 mm–0.5 mm, which are divergent flow fields as well by interchanging between inlet and outlet section. These convergent and divergent flow fields are compared with two conventional single serpentine having 1 mm and 2 mm constant channel depth for an active area of 4.7 cm². The experimental results showed that both convergent and divergent flow fields outperforms the conventional serpentine flow fields where maximum performance was achieved from convergent flow field C1 (1.5 mm–0.5 mm) improving 19–27% power than two conventional serpentine flow fields. Therefore this novel convergent serpentine flow field effect can improve PEM fuel cell performance by its suitable bipolar plate design.     

Development of a novel radial cathode flow field for PEMFC

This paper presents an innovative radial flow field design for PEMFC cathode flow plates. This new design, which is in the form of a radial flow field, replaces the standard rectangular flow channels in exchange for a set of flow control rings. The control rings allow for better flow distribution and use of the active area. The radial field constructed of aluminum and plated with gold for superior surface and conductive properties. This material was selected based on the results obtained from the performance of the standard flow channels of serpentine and parallel designs constructed of hydrophilic gold and typical hydrophobic graphite materials. It is shown that the new flow field design performs significantly better compared to the current standard parallel channels in a dry-air-flow environment. The polarization curves for a dry flow, however, show excessive membrane drying with the radial design. Humidifying the air flow improves the membrane hydration, and in the meantime, the fuel cell with the innovative radial flow field produces higher current compared to other channel designs, even the serpentine flow field. The water removal and mass transport capacity of the radial flow field was proven to be better than parallel and serpentine designs. This performance increase was achieved while maintaining the pressure drop nearly half of the pressure drop measured in the serpentine flow field.     

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.     

Effects of geometrical dimensions of flow channels of a large-active-area PEM fuel cell: A CFD study

Various flow field designs have been numerically investigated to evaluate the effect of pattern and the cross-sectional dimensions of the channel on the performance of a large active area PEM fuel cell. Three types of multiple-serpentine channels (7-channels, 11-channels and 14-channels) have been chosen for the 200 cm² fuel cell investigated and numerically analysed by varying the width and the land of the channel. The CFD simulations showed that as the channel width decreases, as in the 14-channels serpentine case, the performance improves, especially at high current densities where the concentration losses are dominant. The optimum configuration, i.e. the 14-channels serpentine, has been manufactured and tested experimentally and a very good agreement between the experimental and modelling data was achieved. 4 channel depths have been considered (0.25, 0.4, 0.6 and 0.8 mm) in the CFD study to determine the effects on the pressure drop and water content. Up to 7% increase in the maximum reported current density has been achieved for the smallest depth and this due to the better removal of excess liquid water and better humidification of the membrane. Also, the influence of the air flow rate has been evaluated; the current density at 0.6 V increased by around 25% when air flow rate was increased 4 times; this is attributed to better removal of excess liquid water.     

An inverse geometry design problem for optimization of single serpentine flow field of PEM fuel cell

The cathode flow-field design of a proton exchange membrane fuel cell (PEMFC) determines its reactant transport rates to the catalyst layer and removal rates of liquid water from the cell. This study optimizes the cathode flow field for a single serpentine PEM fuel cell with 5 channels using the heights of channels 2–5 as search parameters. This work describes an optimization approach that integrates the simplified conjugated-gradient scheme and a three-dimensional, two-phase, non-isothermal fuel cell model. The proposed optimal serpentine design, which is composed of three tapered channels (channels 2–4) and a final diverging channel (channel 5), increases cell output power by 11.9% over that of a cell with straight channels. These tapered channels enhance main channel flow and sub-rib convection, both increasing the local oxygen transport rate and, hence, local electrical current density. A diverging, final channel is preferred, conversely, to minimize reactant leakage to the outlet. The proposed combined approach is effective in optimizing the cathode flow-field design for a single serpentine PEMFC. The role of sub-rib convection on cell performance is demonstrated.     

Using multi-path spiral flow fields to enhance under-rib mass transport in direct methanol fuel cells

New multi-path spiral flow field designs are developed to improve the under-rib convection mass transport, and consequently the performance of direct methanol fuel cells. The new designs are based on the approach of maximizing the number of the flow paths and flow path patterning. Three new designs are proposed, one design with two flow paths and two designs with three flow paths. To assess the effect of the proposed designs on fuel cell performance, a three-dimensional, isothermal, and single-phase mathematical model for the DMFC is developed and validated using the experimental data available in the literature. Results clearly indicate a significant increase in fuel cell performance with the enhancement of convection mass transport. It is found that fuel cell power increases by 104% and 74% at inlet methanol concentrations of 0.25 M and 0.5 M, respectively, with the use of convection-enhanced spiral flow fields. Furthermore, comparing the predicted results at 0.25 M and 0.5 M inlet methanol concentrations reveals that the power obtained with the newly developed design at 0.25 M inlet methanol concentration is approximately the same as that obtained using a conventional spiral flow field at 0.5 M inlet methanol concentration. Therefore, the approach of using convection-enhanced flow fields enables a reduction in the required inlet methanol concentration, which in turn tackles the methanol crossover problem without affecting the output power of the cell.     

Experimental investigation on scaling and stacking up of proton exchange membrane fuel cells

The performance of a proton exchange membrane fuel cell (PEMFC) with various flow channel design (serpentine and interdigitated) with different landing to channel ratios (L:C = 1:1; 2:2) for an active area of 25 cm² and 70 cm², for single cell and two cells stack is studied and compared. The effect of back pressure on the PEMFC performance is also investigated. This study establishes a strong relation between back pressure and power output from a PEMFC. It was concluded that the interdigitated flow channel gives better results than the serpentine flow channel configuration for various landing to channel ratios. It was also found that power outputs do not proportionally increase with active area of the membrane electrode assembly (MEA). Similarly, stacking up studies with single cell and two cell stack shows that the two cell stack has reduced power densities when compared to that of a single cell. The effect of cooling channels with natural and forced convection by using induced draught fan on the performance of a PEMFC stack is also studied. Fuel distribution and temperature management are found to be the significant factors which determine the performance of a PEMFC stack.     

Numerical investigation into transient response of proton exchange membrane fuel cell with serpentine flow field

The transient response of a proton exchange membrane fuel cell (PEMFC) with a serpentine flow field design is investigated using a three‐dimensional numerical model. The simulations consider three different flow field designs with 7, 11, and 15 bends, respectively. For the flow field design with 11 bends, three different channel width ratios are considered, namely 25%, 50%, and 75%. The channel width ratio is defined as the ratio of the channel width to the total channel/rib width. The simulation results show that for all of the flow field designs, an overshoot in the local current density occurs when the voltage is reduced instantaneously from 0.7 to 0.5 V because of the high and uniform oxygen mass fraction. Conversely, a significant undershoot occurs when the voltage is increased instantaneously from 0.5 to 0.7 V because of the low and nonuniform oxygen mass fraction. The overshoot and undershoot phenomena are particularly evident in the PEMFC with a 15‐bend flow field. For the flow field design with 11 bends, the channel width ratio has little effect on the current density at an operating voltage of 0.7 V. However, at an operating voltage of 0.5 V, the oxygen concentration into the catalyst and diffusion layers increases with the increasing channel width ratio, which leads to higher current density. As a result, a more significant overshoot phenomenon is observed in the flow field with a width ratio of 75%. Copyright © 2012 John Wiley & Sons, Ltd.     

Investigation of bio-inspired flow channel designs for bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. The objective of this paper is to present two new flow channel patterns: a leaf design and a lung design. These bio-inspired designs combine the advantages of the existing serpentine and interdigitated patterns with inspiration from patterns found in nature. Both numerical simulation and experimental testing have been conducted to investigate the effects of two new flow channel patterns on fuel cell performance. From the numerical simulation, it was found that there is a lower pressure drop from the inlet to outlet in the leaf or lung design than the existing serpentine or interdigitated flow patterns. The flow diffusion to the gas diffusion layer was found be to more uniform for the new flow channel patterns. A 25 cm² fuel cell was assembled and tested for four different flow channels: leaf, lung, serpentine and interdigitated. The polarization curve has been obtained under different operating conditions. It was found that the fuel cell with either leaf or lung design performs better than the convectional flow channel design under the same operating conditions. Both the leaf and lung design show improvements over previous designs by up to 30% in peak power density.     

Adoption of novel porous inserts in the flow channel of pem fuel cell for the mitigation of cathodic flooding

Accumulation of excess water in Proton exchange membrane fuel cell (PEMFC) is one of the significant technical challenges that needs great attention, since it makes the performance of the fuel cell highly unpredictable and unreliable. To address this formidable task, herein by inserting porous inserts in inline and staggered arrangements on the provisions of landing surface of serpentine flow field, we minimize water clogging in gas diffusion layer. Two types of porous inserts namely porous carbon inserts (PCI), porous sponge inserts (PSI) of sizes 2 mm × 2 mm x 2 mm (2 mm porous inserts) and 4 mm × 2 mm x 2 mm (4 mm porous inserts) are tested for water management of PEMFC, and their respective performances are analyzed. The results showed that power density produced by MSI flow field is 9.5% and 11.57% higher than serpentine flow field for 2 mm and 4 mm PCI respectively while the MSS flow field produced 31.81% and 42.56% higher performance in terms of power density compared with serpentine flow field for 2 mm and 4 mm PCI respectively. The MSS flow field with 4 mm PCI produced 27.77% higher power density compared with 2 mm PCI. Using porous sponge insert instead of porous carbon insert increases the power density by 23.33% for 2 mm porous insert and the power density increases 21.73% for 4 mm PSI in MSS flow field. Increasing the size of PSI from 2 mm to 4 mm increases the power density by 26.12% in MSS flow field.     

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.     

Direct measurement of current density under the land and channel in a PEM fuel cell with serpentine flow fields

One of the most common types of flow field designs used in proton exchange membrane (PEM) fuel cells is the serpentine flow field. It is used for its simplicity of design, its effectiveness in distributing reactants and its water removal capabilities. The knowledge about where current density is higher, under the land or the channel, is critical for flow field design and optimization. Yet, no direct measurement data are available for serpentine flow fields. In this study a fuel cell with a single channel serpentine flow field is used to separately measure the current density under the land and channel, which is either catalyzed or insulated on the cathode. In this manner, a systematic study is conducted under a wide variety of conditions and a series of comparisons are made between land and channel current density. The results show that under most operating conditions, current density is higher under the land than that under the channel. However, at low voltage, a rapid drop off in current density occurs under the land due to concentration losses. The mechanisms for the direct measurement results and general guidelines for serpentine flow field design and optimizations are provided.     

Optimization of PEM fuel cell flow field via local current density measurement

The serpentine flow field is the leading type of flow field used today in proton exchange membrane (PEM) fuel cells and for this reason optimization of serpentine flow field design is extremely important. In this study, a unique technique developed in house is utilized to separately measure current density under the land and channel on a variety of serpentine flow field geometries. Each flow field is tested under a wide variety of operating conditions thereby providing guidance for the optimum design geometry. Experimental results show that generally flow fields with both thinner lands and thinner channels provide better overall performance. However, the optimal flow field designs are highly dependent on fuel cell operating parameters.     

Performance of DMFC with SS 316 bipolar/end plates

This work mainly emphasizes the development of new materials and design for a bipolar/end plate in a direct methanol fuel cell (DMFC). According to the DOE requirements, preliminary studies show that SS 316 (Stainless Steel 316) is a suitable candidate. Several flow field designs were studied and a modified serpentine design was proposed. SS 316 end plates were fabricated with an intricate modified serpentine flow field design on it. The performance of a single stack DMFC with SS 316 end plates were studied with different operational parameters. A long-term test was carried out for 100 h with recycling the methanol and the contaminants in the MEA were characterized. The stack efficiency is found to be 51% and polarization losses are discussed. SS 316 with low permeability resulted in an increased pressure drop across the flow field, which increased the fuel cell performance. The use of SS 316 as bipolar plate material will reduce the machining cost as well as volume of the fuel cell stack.     

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.     

In-situ measurements of GDL effective permeability and under-land cross-flow in a PEM fuel cell

When reactant gases flow along a channel in serpentine flow field of a proton exchange membrane (PEM) fuel cell, there is a pressure difference between the adjacent channels and it produces an under-land cross-flow (or under-rib convection) from the higher pressure side to the lower pressure side through the gas diffusion layer (GDL). A unique experimental setup is developed for in-situ measurement of this cross-flow and the GDL effective permeability at the cathode side of a PEM fuel cell under dry and realistic humidified gas conditions. The non-Darcy effect, defined as a function of the Forchheimer number is studied and compared for both 1 mm and 2 mm land widths and both dry and humidified air conditions. Finally, a dimensional analysis is performed and the non-dimensional cross-flowrate is shown to increases linearly with the increase of the non-dimensional pressure difference.     

Study of new flow field geometries to enhance water redistribution and pressure head losses reduction within PEM fuel cell

Efficiency of fuel cell is dependent on reactant distribution, products evacuation, pressure losses and many of these factors is dependent on the design of flow field plate. With an effective design, reactant distribution, pressure drop, and water and heat management can be further improved. In this work, two new designs, as multi-serpentine set-up with additional slots and hybrid geometry, on stainless steel bipolar plates, are presented. Electrical performance, and pressure head losses are analyzed by electrochemical methods such as polarization curve and use of electrochemical noise as a diagnostic tool to further understand the impact of water management on performance. On the one hand, multi-serpentine design shows the best electrical performance with an increase of 0.2 V (66%) at 0.9 A/cm² in comparison of traditional serpentine design. On the other hand, hybrid design reveals the lowest pressure head losses, with a decrease of 2 mbar (about 50%) in comparison of traditional serpentine design, and a higher stability with time that can be useful to downsize compressor and provide lower impact on fuel cell stack durability.     

Comparison of PEMFC performance with parallel serpentine and parallel serpentine-baffled flow fields under various operating and geometrical conditions; a parametric study

Optimal flow channel design of a fuel cell is crucial to further improve the performance of polymer electrolyte membrane fuel cell (PEMFC). In this work, a comprehensive parametric study was conducted to analyze the performance of a PEMFC with conventional parallel serpentine flow fields (PSFF) and parallel serpentine-baffled flow fields (PSBFF). A three-dimensional two-phase computational fluid dynamics model was used to numerically simulate the fuel cell performance. The effects of operating parameters such as pressure, temperature, and stoichiometric ratio, as well as the geometric parameters of channel height to channel width ratio and rib width to channel width ratio for both flow fields on fuel cell performance were investigated. The results show that as pressure, temperature, and stoichiometric ratio increase, cell performance increases for both flow fields, with a more substantial rate of improvement for the PSBFF design. A 16.1% improvement in cell performance at an operating pressure of 1 atm, a 19.9% improvement at a cell temperature of 70 °C, and a 16.1% improvement at a stoichiometric ratio of 2 were obtained when PSBFF was used instead of PSFF. By increasing the channel height and rib width, the cell performance for PSBFF remains almost constant due to the improved forced convection of the gas mixture and the reduction in concentration loss, while the cell performance for PSFF decreases significantly. At the largest channel height to channel width ratio of 1.5 and rib width to channel width ratio of 1.315 studied in this work, an improvement in cell performance of 53.3% and 58.5%, respectively, was achieved when PSBFF was used instead of PSFF. In addition, PSBFF was more effective in removing water from the porous zones than PSFF under all conditions.     

An analytical analysis on the cross flow in a PEM fuel cell with serpentine flow channel

A serpentine flow channel can be considered as neighboring channels connected in series, and is one of the most common and practical channel layouts for polymer electrolyte membrane (PEM) fuel cells, as it ensures the removal of liquid water produced in a cell with good performance and acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross directly to the neighboring channel via the porous gas diffusion layer (GDL) due to the high‐pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area resulting in a substantially lower amount of pressure drop in an actual PEM fuel cell compared with the case without cross flow. In this study, an analytical solution is obtained for the cross flow in a PEM fuel cell with a serpentine flow channel based on the assumption that the velocity of cross flow is linearly distributed in the GDL between two successive U‐turns. The analytical solution predicts the amount of pressure drop and the average volume flow rate in the flow channel and the GDL. The solution is validated over a wide range of the thickness and permeability of the GDL by comparing the results with experimental measurements and 3‐D numerical simulations in literature. Excellent agreement is obtained for the permeability less than 10^?9 m², which covers the typical permeability values of the GDLs in actual PEM fuel cells. The solution presents an accurate and efficient estimation for cross flow providing a useful tool for the design and optimization of PEM fuel cells with serpentine flow channels. Copyright © 2010 John Wiley & Sons, Ltd.