Corrosion resistance characteristics of stamped and hydroformed proton exchange membrane fuel cell metallic bipolar plates

Metallic bipolar plates have several advantages over bipolar plates made from graphite and composites due to their high conductivity, low material and production costs. Moreover, thin bipolar plates are possible with metallic alloys, and hence low fuel cell stack volume and mass are. Among existing fabrication methods for metallic bipolar plates, stamping and hydroforming are seen as prominent approaches for mass production scales. In this study, the effects of important process parameters of these manufacturing processes on the corrosion resistance of metallic bipolar plates made of SS304 were investigated. Specifically, the effects of punch speed, pressure rate, stamping force and hydroforming pressure were studied as they were considered to inevitably affect the bipolar plate micro-channel dimensions, surface topography, and hence the corrosion resistance. Corrosion resistance under real fuel cell conditions was examined using both potentiodynamic and potentiostatic experiments. The majority of the results exhibited a reduction in the corrosion resistance for both stamped and hydroformed plates when compared with non-deformed blank plates of SS304. In addition, it was observed that there exist an optimal process window for punch speed in stamping and the pressure rate in hydroforming to achieve improved corrosion resistance at a faster production rate.     

Investigation of deformation mechanics and forming limit of thin-walled metallic bipolar plates

The present study is conducted on forming of the metallic bipolar plates made of 316 stainless steel sheet with a parallel serpentine flow field. The plastic deformation of straight and curved microchannels, forming limit criteria, and deformation mechanics during the process are investigated partially to present a reliable model for estimating fracture initiation. For this purpose, experimental stamping tests are employed to fabricate metallic bipolar plates and the process is simulated by finite element software. The validity of simulation results is examined by comparing thickness distribution and force-displacement curves reflecting 4.76% and 3.85% error rates, respectively. According to experimental observations, fracture starts at a channel depth of 0.610 mm. Hence, for determining the forming limit and predicting the fracture during the process, the deformation mechanic is studied at different points of the microchannels. Results of stress states analysis indicate that the stress state of plane-strain tension up to biaxial tension governs this process. Despite the presence of different loading paths during the process, the critical element in each channel is deformed under plane-strain tension. Therefore, a fracture model is developed based on thinning percentage and equivalent strain to predict the instability of metallic bipolar plates. According to the results, both the equivalent strain and thinning percentage criteria with critical limits of 0.56 and 33.45%, respectively, are considered as an allowable range of plastic deformation during the conventional stamping process of bipolar plates. Results indicate that maximum thinning in all directions is lower than 33.45% by using the modified stamping process.     

Studies of the deformation styles of the rubber-pad forming process used for manufacturing metallic bipolar plates

Rubber pad forming as a novel stamping technique has been widely used in the deep drawing, bulging, blanking, and flanging processes. It is a feasible method for manufacturing metallic bipolar plates, the surface of which has multi-array flow channels. For a single channel's fabrication, there are two different deformation styles: concave and convex. In this paper, the deformation characteristics of the two deformation styles are analyzed in detail with numerical simulation and experimental methods. The proper application conditions of the concave and convex deformation styles used to fabricate a certain metallic bipolar plate have been determined. If the ratio of the channel width to the rib width w/s>1, the concave style is more appropriate; otherwise, the convex style is preferred. Based on this theory, a bipolar plate sample (w/s<1) is successfully manufactured by the convex rubber-pad forming process.     

Integration of design of experiment and finite element method for the study of geometrical parameters in metallic bipolar plates for PEMFCs

Bipolar plates are among the most important components of polymer electrolyte membrane fuel cells that are responsible for a high percentage of their weights and costs. Metals are a suitable replacement for thick graphite plates that require high machining costs. The present study investigates the manufacturing process of metallic bipolar plates with a thickness of 0.1 mm using the stamping process. One of the problems with the formation of metallic bipolar plates is their rupture during plastic deformation. This study is aimed to investigate the effects of the geometrical parameters of bipolar plates, including channel width, rib width, channel depth, draft angle, and corner radius, on the formation of sheets. By designing some experiments, the effect of each parameter on the thinning percentage and filling depth of the bipolar plates were evaluated. The required data is obtained using the commercial finite element code. The results show that the channel depth, draft angle, and corner radius have the most effect on the plate thinning. In addition, the corner radius and the draft angle have the highest effect on the maximum channel depth. Finally, the thinning percentage and the filling depth for other geometric parameters can be predicted by a mathematical equation.     

Fabrication of titanium bipolar plates by rubber forming and performance of single cell using TiN-coated titanium bipolar plates

In this study, a rubber forming method is used to fabricate titanium bipolar plates for proton exchange membrane fuel cells. A titanium blank with a thickness of 0.1 mm is compressed using a stamping mold equipped with a 200-ton hydraulic press to fabricate bipolar plates. A forming experiment is carried out by changing different parameters such as the punch velocity, punch pressure, rubber thickness, rubber hardness, and draft angle of the channel. The optimum forming conditions are found to be a rubber thickness of 10 mm, rubber hardness equivalent to that of Shore A 20, punch velocity of 30 mm s⁻¹, punch pressure of 55 MPa, and punch draft angle of 30°. The fabricated titanium bipolar plate is coated with a TiN layer. A single cell with a TiN-coated titanium bipolar plate is examined and compared to those with uncoated titanium and graphite bipolar plates. The initial performances (in terms of current densities) of the single cells with the uncoated titanium, TiN-coated titanium, and graphite bipolar plates are 396, 799, and 1160 mA cm⁻², respectively, at a cell voltage of 0.6 V.     

Study of thickness distribution and dimensional accuracy of stamped metallic bipolar plates

The aim of the present paper is to fabricate metallic bipolar plates (BPPs) with active area of $100{\text{cm}}^2$ modeled on an industrial gas flow field scheme. In order to produce these plates, a stamping process has been employed to reduce cost and increase production speed. 316L stainless steel sheets with the thickness of 0.1 mm have been used. The effect of forming forces on channel filling depth, channel width, rib width, the uniformity in the depth of formed channels and the thickness distribution of channels have also been studied. The stress and strain distributions of formed bipolar plate are investigated using finite element simulation. The results reveal that the stamping process does have the ability to produce a perfect bipolar plate by one-step forming die for desired dimensions. Moreover, increasing the stamping force to an optimum level has insignificant effect on the depth of the channels. However, stamping force increment results in an increase in width of the channels and ribs.     

Formability and flow channel design for thin metallic bipolar plates in PEM fuel cells: Modeling

Metallic bipolar plates (BPPs) are regarded as the most promising substitute of traditional carbon‐based BPPs due to their good mechanical properties, electrical conductivity, high productivity, and low cost in mass production. However, conventional design guidelines on flow channels of carbon‐based BPPs are not perfectly valid to metallic BPPs due to the formability of thin metallic sheets, which are at risk of material rupture especially when flow channels decrease. The objective of this work is to develop a forming limit model to establish the relationship between the channel height and channel geometric dimensions by the micro stamping process, and the maximum channel height can be predicted to guide channel design of metallic BPPs from the formability perspective. Firstly, an instability criterion for micro‐scale forming was proposed to estimate when the rupture occurs during the forming process. Forming height as the function of channel features is established, and the limit of forming depth can be predicted. Series of micro stamping experiments with various dimensions are conducted to validate the accuracy of the model. Influences of main parameters on the channel forming limit and evaluation of channel design are discussed based on the model. The model in this study is an effective supplement to the channel design principle for metallic BPPs. Based on the model, design, fabrication, and testing of metallic BPPs will be done in the following research, Part II: Experiments.     

Fracture prediction in the stamping of titanium bipolar plate for PEM fuel cells

In this paper, the stamping process was employed to fabricate metallic bipolar plates (MBPs). An account of low formability of the commercially pure titanium (CP–Ti), the fracture is the most common defect during its plastic deformation. Consequently, prediction of the fracture onset during the stamping was studied using three ductile fracture criteria including Rice-Tracey, Brozzo, Ayada, and a developed forming limit criteria based on consideration of the material size effect. The damage value in the lateral and central channel was evaluated to determine the critical channel and element. According to the results, the most accurate fracture prediction during stamping of titanium bipolar plates could be obtained via Brozzo ductile fracture criteria with an error rate of 3.68% compared to experiments. Moreover, the strain-based criteria represent higher fracture prediction errors compared to damage criteria. The stress state analysis showed the variation of stress triaxiality during the process leading to less accuracy of the strain-based criteria. According to the results, the damage function of the ductile damage criteria was more reliable for the semi-proportional loading path during the stamping of the titanium bipolar plates which makes them more suitable for accurate fracture prediction during the process.     

Numerical-experimental investigation of using rubber blank holder on wrinkling of metallic bipolar plates formed by stamping process

Stamping is the regarded as the best among manufacturing method of metallic bipolar plates considering its less production time and higher mass production capability. The present research studied the manufacturing of metallic bipolar plates with a parallel serpentine flow field made of a 304 stainless steel plate of 0.1 mm thickness via stamping. The wrinkling is a defect created in stamping these plates. It prevents the correct assembly of the plates and, hence, reduces the efficiency of the assembly. Accordingly, this paper first investigates the effects of using rubber blank holders on wrinkling reduction around the bipolar plates. Then, the influence of the geometric parameters of the rubber blank holder, such as the thickness, width, hardness, and compression percentage, are studied by the response surface methodology. Finally, the most appropriate path for the assembly of the plates in terms of wrinkling is selected. According to the results, a rubber blank holder reduces the wrinkles around the bipolar plate. Among the studied parameters, the compression percentage and hardness of the blank holder contributed most to the reduction in the wrinkling around these plates for better assembly. Furthermore, wrinkling decreases with an increase in the compression percentage, hardness, width, and thickness of the rubber blank holder throughout the selected paths at 3 mm and 6 mm distances. Among the distances selected on the plate edges, a distance of 6 mm from the edges is the best path for assembling the plates.     

Fabrication of micro channels for titanium PEMFC bipolar plates by multistage forming process

Bipolar plates made of ultra-thin titanium sheet metals feature high corrosion resistance, electrical conductivity and strength-weight ratio. They are considered as a satisfying candidate for the proton exchange membrane fuel cell (PEMFC) used in vehicles. Bipolar plates have fine and complex sub-millimeter channel features which are employed to guide the flow of hydrogen, oxygen and cooling medium, to support the reaction space, to provide sealing structure, etc. Stamping process is an efficient method to fabricate those micro features with high precision and efficiency. However, the aspect ratio of those micro features is limited by the low forming limit of titanium sheet metals. Improving the aspect ratio of metallic bipolar plates could facilitate the uniform heat and mass transfer and further improve the efficiency and performance of PEMFC. Hence a challenging issue for the stamping process of bipolar plates is how to further improve the limit aspect ratio after forming. The multistage forming process is an efficient method to increase the ultimate forming depth and the accuracy by reducing the deformation localization tendency. This work aims to investigate the applicability of that process in fabricating high-aspect-ratio micro-channels on titanium sheet metals. Single stage forming experiments of the micro channels featured with fillet radius of 0.15 mm are first conducted. It is found that the ultimate forming depth of the channels parallel to the rolling direction is 12.67% higher than in the transverse direction. Furthermore, a series of punches and dies with three different fillet radii are employed in the multistage forming process. The ultimate depth of formed channels is revealed to increase from 438.1 to 621.0 μm, i.e. the aspect ratio from 0.46 to 0.67, comparing to the single stage forming process. Hence titanium bipolar plates with a higher aspect ratio could be achieved by utilizing the presented method. The contact resistances of the samples fabricated via single and three-stage forming were further tested to find out that the samples with a higher aspect ratio formed in a three-stage manner have an evident lower contact resistance.     

Investigation of formability of metallic bipolar plates via stamping for light-weight PEM fuel cells

Bipolar plates (BPs) are one of the main members which constitute a significant percentage of a fuel cell system in terms of cost, weight and structural strength. Although frequently used graphite BPs have low density, high conductivity and corrosion resistance, machining the desired flow channels on the graphite plates is an important issue. On the other hand, metallic BPs can be considered a reasonable alternative material to graphite in the view of the material cost, fabrication of flow channels and some post-processes in which the large-scale manufacturing of graphite BPs is more complex compared to cutting and stamping processes for metal ones. This study offers a comparison of the formability of four different metals with four flow channel depths as bipolar plates formed by stamping. 304 Stainless Steel (SS 304), pure Titanium - Grade2 (CP–Ti) and Aliminium (Al 6016 and Al 3104) are chosen as the BP materials. A serpentine type flow channel with two different channel widths are formed on to 0.1 mm thick sheets. The channel width is chosen as 1.2 mm and 1.8 mm for the channel depths of 0.36 mm–0.55 mm, and 0.54 mm–0.82 mm, respectively. The stamping processes of the BPs materials are simulated via commercially available eta/Dynaform v5.9.4. software and formability characteristics are obtained for sixteen various cases. As a result, it is determined that SS 304 is the more appropriate material in the view of the formability for such a complex form.     

Effect of manufacturing processes on formability and surface topography of proton exchange membrane fuel cell metallic bipolar plates

Metallic bipolar plates in PEM fuel cells offer low-volume, low-mass and low-cost stack fabrication in addition to superior durability when compared to composite bipolar plates, which suffer due to their much higher thickness and less durability. This study aims to address the formability and surface topography issues of metallic bipolar plates fabricated by stamping and hydroforming technologies. Particular emphasis was given to process repeatability, surface topology, and dimensional quality of bipolar plates that would greatly affect the corrosion and contact resistance characteristics. Thin metal sheets of several alloys (i.e., SS304, SS316L, SS430, Ni270, Ti grades 1 and 2) were used in the fabrication experiments. SS304 and SS316L were shown to possess better formability when compared to other alloys that were used in this study, while SS430 and Ti grade 2 demonstrated the worst among all. Channel formability was observed to be greatly affected by the hydroforming pressure, while it does not differ much above certain level of stamping force. The confocal microscopy analyses showed that surface roughness values of the formed samples were altered significantly when compared to the initial flat blanks. In general, increasing hydroforming pressure and stamping force yielded higher surface roughness values at channel peaks. In addition, the surface topography was shown to be influenced mainly by the pressure level rather than the pressure rate in hydroforming process.     

Effect of manufacturing processes on contact resistance characteristics of metallic bipolar plates in PEM fuel cells

In this study, metallic bipolar plate (BPP) samples manufactured with stamping and hydroforming under different process conditions were tested for their electrical contact resistance characteristics to reveal the effect of manufacturing type and conditions. Punch speed and force in stamping, and pressure and pressure rate in hydroforming were selected as variable process parameters. In addition, two different channel sizes were tested to expose the effect of BPP micro-channel geometry and its consequences on the contact resistance. As a general conclusion, stamped BPPs showed higher contact conductivity than the hydroformed BPPs. Moreover, pressure in hydroforming and geometry had significant effects on the contact resistance behavior of BPPs. Short term corrosion exposure was found to decrease the contact resistance of bipolar plates. Results also indicated that contact resistance values of uncoated stainless steel BPPs are significantly higher than the respective target set by U.S. Department of Energy. Conforming to literature, proper coating or surface treatments are necessary to satisfy the requirements.     

Effect of manufacturing conditions on the corrosion resistance behavior of metallic bipolar plates in proton exchange membrane fuel cells

Metallic bipolar plates are one of the promising alternatives to the graphite bipolar plates in proton exchange membrane fuel cell (PEMFC) systems. In this study, stainless steel (SS304, SS316L, and SS430), nickel (Ni 270), and titanium (Grade 2 Ti) plates with an initial thickness of 51 μm were experimented as bipolar plate substrate materials in corrosion resistance tests. In addition to unformed blanks, SS316L plates were formed with stamping and hydroforming processes to obtain bipolar plates under different process conditions (stamping force, hydroforming pressure, stamping speed, hydroforming pressure rate). These bipolar plates, then, were subjected to corrosion tests, and the results were presented and discussed in detail. Potentiodynamic polarizations were performed to observe corrosion resistance of metallic bipolar plates by simulating the anodic and cathodic environments in the PEMFC. In order to determine the statistical significance of the corrosion resistance differences between different manufacturing conditions, analysis of variance (ANOVA) technique was used on the corrosion current density (I_corr, μA cm⁻²) values obtained from experiments. ANOVA for the unformed substrate materials indicated that SS430 and Ni have less corrosion resistance than the other substrate materials tested. There was a significant difference between blank (unformed) and stamped SS316L plates only in the anodic environment. Although there was no noteworthy difference between unformed and hydroformed specimens for SS316L material, neither of these materials meet the Department of Energy‘s (DOE) target corrosion rate of ≤1 μA cm⁻² by 2015 without coating. Finally, stamping parameters (i.e. speed and force levels) and hydroforming parameters (i.e. the pressure and pressure rate) significantly affected the corrosion behavior of bipolar plates.     

Fabrication of metallic bipolar plate for proton exchange membrane fuel cells by rubber pad forming

In this paper, the rubber pad forming process is used to fabricate the metallic bipolar plate for a proton exchange membrane (PEM) fuel cell, which has multi-array micro-scale flow channels on its surface. The rubber pad forming process has the following advantages: high surface quality and dimensional accuracy of the formed parts, low cost of the die because only one rigid die is required, and high efficiency. The process control parameters (rubber hardness, internal and outer radii, draft angle) of the rubber pad forming are analyzed by the finite element method using the commercial software Abaqus. After that, the rubber pad forming process is used to manufacture a metallic bipolar plate of SS304 stainless steel with perfect flow micro-channels. The results of this effort indicated that the rubber pad forming process is a feasible technique for fabricating the bipolar plates of PEM fuel cells.     

A robust design of the forming process parameters of the metallic bipolar plate for proton exchange membrane fuel cells

Metallic bipolar plates are important components of the proton exchange membrane fuel cell. To achieve higher performance of fuel cells, dozens of fine flow channels with the width of approximately 1 mm and a high aspect ratio of over 0.3 are required for bipolar plates. Those functional demands push the forming process of bipolar plates to the limit of material and tools, making the rupture sensitive to even a tiny deviation of the tools' geometric dimensions and material properties. To address that, a robust design method is established in this work to analyze the rupture probability by introducing the stochastic variation of material properties and tools’ geometric dimensions. Utilizing the method, the rupture probability is decreased from 19.14% to 0.18% during the practical forming of metallic bipolar plates by parameters control. The yield rate is also verified to be over 99.5%.     

Two-stage forming approach for manufacturing ferritic stainless steel bipolar plates in PEM fuel cell: Experiments and numerical simulations

Multi-stage micro-channel forming by stamping, as a method for cost effective and efficient for mass production, was performed for ultra-thin ferritic stainless steel sheets with thicknesses of 0.1 and 0.075 mm, as a good substitute for traditional graphite bipolar plates of proton exchange membrane fuel cell. Attention was directed to enhance the final forming depth and minimize localized thinning, extremely important aspects of the micro-channel on bipolar plate, by the proposed forming process. A forming depth at the first forming stage was chosen as a process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. Finite element simulations for the two-stage forming process were conducted to optimize the punch radius and forming depth at the first stage for improving the formability. The comparative study between the simulations and the experimental results could validate improvements in the formability by the proposed approach. In particular, this study could support the existence of an optimum forming depth at the first forming stage. Based on the simulation results, a mathematical model was established to identify the dominant factor needed for formability improvement and to propose a methodology for the process optimization of the multi-stage forming.     

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.     

Investigation of intrusion effects of a gas diffusion layer into channel cross sections depending on channel parameters of metallic bipolar plates

The constructive design of a flow field layout and the channel cross section parameters from a metallic half- or bipolar plate can have a significant influence on the performance characteristics of a fuel cell. Especially the intrusion effect of Gas Diffusion Layers (GDL) under preload conditions leads to a narrowing of the effective flow channel cross section at the active area. In this work, it is assumed that the intrusion effect is dependent on the channel parameters from metallic channel plates. This aspect is investigated experimentally. First, the developed GDL test bench, as well as the test methodology, is explained. Further, a general definition of the channel parameters is shown. Two general equations will be derived, with this it is possible to calculate the channel parameters and channel cross section area analytically. Furthermore, an analytic-empirical intrusion model is derived and validated based on the measurement results. With the intrusion model, it is possible to calculate the effective cross-sectional area with a good approximation. These results can be used for further investigations, for example for flow simulations. Finally, the investigation results of two variation series are listed. At the first series the parameter channel width (CW) and at the second series the channel-to-rib-ratio (CRR) is varied. The considered two measurement parameters are the intrusion in the channel middle and the channel width reduction. It turns out that the investigated both channel parameters influences the GDL intrusion behavior. Particularly, between the channel width parameter and the intrusion in channel middle, a significant dependency can be determined. In the presented experimental series, a certain channel width can be detected, over that the intrusion does not increase any further. Moreover, in the experimental series the dependency between CRR and intrusion in the middle of the channel as well as between CRR and the channel width reduction can be demonstrated. As the CRR increases, both results tend to decrease. In a concluding error analysis, the test results are critically examined and evaluated. In principle, however, the assumption of the dependence between intrusion effect and channel parameters can be sufficiently proved by means of the developed test bench and the test method.     

Investigations on the variation of corrosion and contact resistance characteristics of metallic bipolar plates manufactured under long-run conditions

Tribological variations, surface conditions (roughness, hardness, coating) and surface interactions between micro-stamping dies and bipolar plate blanks play a critical role in determining the surface quality, channel formation and precision of bipolar plates. This study is aimed to understand the cause, mechanism and consequences of interactions between micro-stamping process conditions and bipolar plate quality. A total of 2000 repeated micro-stamping of 51 μm-thick uncoated and 1 μm-thick ZrN coated SS316L sheet blanks into an array of 750 μm micro-channels were performed using 175-220 kN force levels with constant stamping speed of 1 mm/s. Microscopic examinations were conducted periodically on both die and coated & uncoated plate surfaces to observe topographic variations. In addition, corrosion and contact resistance tests were carried out in the same intervals. Analysis of variance (ANOVA) technique was used to determine the significance of the process parameters on channel height, roughness, corrosion and contact resistance differences. The results revealed similar roughness trends for die and plate surfaces during 2000 micro-stampings. ZrN coating with 1 μm thickness dramatically improved corrosion and contact resistance behavior of plates.