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.     

Fabrication of titanium bipolar plates for proton exchange membrane fuel cells by uniform pressure electromagnetic forming

Electromagnetic forming (EMF) is a type of noncontact and high-speed forming process that offers several advantages, such as high material formability, low springback, and low die cost. In this study, titanium bipolar plates (BPPs) for proton exchange membrane fuel cells are fabricated by EMF using a uniform pressure actuator that provided a uniform pressure distribution and high efficiency. A three-dimensional coupled electromagnetic-mechanical simulation model is established to optimize the dynamic forming process. Simulation results show that the channel depth is proportional to the impact velocity of the titanium workpiece. A forming window is established as a design guideline. Finally, a titanium BPP with sufficient channel depth (0.4 mm), high aspect ratio (0.67), low thinning rate (not exceeding 15.89%) and high corrosion resistance is successfully fabricated at a velocity of 286 m/s, thereby demonstrating that EMF is a feasible process for fabricating titanium BPPs.     

Formability improvement in multi-stage stamping of ultra-thin metallic bipolar plate for proton exchange membrane fuel cell

Multi-stage stamping process is the promising technology to fabricate the metallic bipolar plate (BPP) for proton exchange membrane (PEM) fuel cell. In the present study, a novel die design in the pre-forming stage is proposed and its effect on the formability of ultra-thin metallic BPP is verified by the finite element (FE) simulations of micro- and macro-scale BPP channels. It reveals that the multi-stage forming with the proposed die approach significantly improve the formability of ultra-thin BPP. As a result, the more uniform thickness distribution and considerable reduction of springback are beneficial to the fabrication of high quality metallic BPP. Furthermore, the relatively high reaction efficiency (～79.4%) of fuel cell stacks can be predicted, indicating the high fuel consumption. These findings demonstrate the feasibility and efficiency of the proposed die design in the fabrication of ultra-thin metallic BPP based on the perspectives of both the formability and energy efficiency.     

Flow channel shape optimum design for hydroformed metal bipolar plate in PEM fuel cell

Bipolar plate is one of the most important and costliest components of polymer electrolyte membrane (PEM) fuel cells. Micro-hydroforming is a promising process to reduce the manufacturing cost of PEM fuel cell bipolar plates made of metal sheets. As for hydroformed bipolar plates, the main defect is the rupture because of the thinning of metal sheet during the forming process. The flow channel section decides whether high quality hydroformed bipolar plates can be successively achieved or not. Meanwhile, it is also the key factor that is related with the reaction efficiency of the fuel cell stacks. In order to obtain the optimum flow channel section design prior the experimental campaign, some key geometric dimensions (channel depth, channel width, rib width and transition radius) of flow channel section, which are related with both reaction efficiency and formability, are extracted and parameterized as the design variables. By design of experiments (DOE) methods and an adoptive simulated annealing (ASA) optimization method, an optimization model of flow channel section design for hydroformed metal bipolar plate is proposed. Optimization results show that the optimum dimension values for channel depth, channel width, rib width and transition radius are 0.5, 1.0, 1. 6 and 0.5 mm, respectively with the highest reaction efficiency (79%) and the acceptable formability (1.0). Consequently, their use would lead to improved fuel cell efficiency for low cost hydroformed metal bipolar 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.     

Fabrication by vacuum die casting and simulation of aluminum bipolar plates with micro-channels on both sides for proton exchange membrane (PEM) fuel cells

Among manufacturing methods for bipolar plates, vacuum die casting is an ideal process because arbitrarily complicated shapes and mass production is possible with a high production rate. We report on the fabrication of bipolar plates with micro-channel arrays on both sides by vacuum die casting for use in proton exchange membrane (PEM) fuel cells. The formability, mechanical properties, and microstructures of samples fabricated under various experimental conditions—molten metal temperature, injection velocity, and vacuum assistance—are investigated, and the experimental and simulation results are compared. The die cavity, which is equal to the bipolar plate area, is 200 mm long, 200 mm wide, and 0.8 mm thick. The active area of the channel is 110 mm × 150 mm, and the total plate thickness is 1.1 mm (the width and depth of the channel are 1 and 0.3 mm, respectively). The cast material used in this study is Silafont-36 alloy (Al–Si–Mg–Mn). Good quality samples with very few casting defects are obtained under the following conditions: molten metal temperature of 700 °C; injection velocities for slow and fast shots of 0.3 and 2.5 m s⁻¹, respectively; and vacuum pressure of 30 kPa.     

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.     

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 study on air-side performance of an integrated fin and micro-channel heat exchanger

A new type of aluminum heat exchanger with integrated fin and micro-channel has been proposed. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are systematically analyzed by a 3D numerical simulation. The effect of flow depth, fin height, fin pitch and fin thickness at different Reynolds number is evaluated by calculating Colburn factor j and Fanning friction factor f. A parametric study method is used to analyze the fin designed parameters affecting the performance of the heat exchanger. The results show that the contribution ratio of the fin geometries in descending order is flow depth, fin pitch, fin height and fin thickness. The air-side performance of the integrated fin and micro-channel heat exchanger is compared with that of the multi-louver fin micro-channel heat exchanger and the wavy fin micro-channel heat exchanger.     

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.     

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.     

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.     

Micro-roll forming of stainless steel bipolar plates for fuel cells

Stainless steel bipolar plates for use in proton exchange membrane (PEM) fuel cells have been identified as a lighter and cheaper alternative to graphite plates. Current manufacturing of metal bipolar plates by hydroforming or micro-stamping leads to excessive stretching of the material and therefore limits the channel depths that can be formed. Low channel depths for the bipolar plates will result in low overall fuel cell efficiency. In comparison, the bending-dominated deformation mode present in roll forming provides the potential to form metal bipolar plates with less thinning and to greater channel depths. In this work, the roll forming process is employed for the first time to form thin stainless steel sheets to micro-scale channel sections of the kind required for bipolar plates. This paper describes the process and machine design as well as the establishment of the forming methodology. Experimental trials are performed and the final part quality is evaluated in terms of material thinning, longitudinal bow and cross-sectional shape. The process was numerically analysed to understand the causes of the forming problems and shape defects observed in the experimental trials. The results of this work show that roll forming of micro-scale corrugated bipolar sheets is feasible. Furthermore, the findings provide a summary of both the practical difficulties and the possible advantages of using micro-roll forming to manufacture improved thin metal micro-corrugations for bipolar plates.     

Fabrication of micro-channel arrays on thin metallic sheet using internal fluid pressure: Investigations on size effects and development of design guidelines

Micro-feature (channel, protrusion, cavity, etc.) arrays on large area-thin metallic sheet alloys are increasingly needed for compact and integrated heat/mass transfer applications (such as fuel cells and fuel processors) that require high temperature resistance, corrosion resistance, good electrical/thermal conductivity, etc. The performance of these micro-feature arrays mainly affects the volume flow velocity of the reactants inside the arrays which directly controls the rate of convection mass/heat transport. The key factors that affect the flow velocity include channel size and shape, flow field pattern, flow path length, fluid pressure, etc. In this study, we investigated these micro-feature arrays from the manufacturability perspective since it is also an important factor to be considered in the design process. Internal fluid pressure (hydroforming) technique is investigated in this study with the specific goals to, first, understand if the so-called “size effects” (grain vs. feature size) are effective on the manufacturability of thin metallic sheet into micro-channels, and second, to establish design guidelines for the micro-channel hydroforming technique for robust mass production conditions. Thin stainless steel 304 blanks of 0.051 mm thick with three different grain sizes of 9.3, 10.6, and 17.0 μm were used in hydroforming experiments to form micro-channels with the dimensions between 0.46–1.33 and 0.15–0.98 mm in width and height, respectively. Based on the experimental results, the effect of the grain size on the channel formability was found to be insignificant for the grain size range used in this study. On the other hand, the effect of the channel (feature) size was shown to dominate the overall formability. In addition, FE models of the process were developed and validated with the experimental results, then used to conduct a parametric study to establish micro-channel design guidelines. The results from the parametric study suggested that in order to obtain the maximum aspect ratio (height-to-width ratio) a small channel width should be used. Even though a large channel width would result in a higher channel height, the height-to-width ratio was found to be lower in this case. In addition, higher aspect ratio could be obtained by using larger corner radius (R_d), wider distance between adjacent channels (W_int), or less number of channels. On the other hand, the variation in draft angle (α) between 5° and 20° in combinations with the other channel geometries was found to be insignificant on the channel formability/height. All in all, these channel parameters (W, R_d, W_int, α, channel number, etc.) should be taken into account simultaneously in the design process in order to obtain such a design of the micro-feature arrays that would meet both performance and manufacturing requirements and constraints.     

A 3D numerical study of heat transfer in a single-phase micro-channel heat sink using graphene,aluminum and silicon as substrates

The study describes numerical simulations conducted on micro-channel heat sinks. Three different shapes related to the micro-channel depth and width is chosen for examination. Silicon, aluminum, and graphene are used as substrate materials for this study. The overall heat sink consisted of an array of rectangular micro-channels. Three different surface heat fluxes and three different volumetric flow rates are used for three cases. Water with non-temperature-dependent thermal properties is used as a coolant for steady-state, fully developed laminar flow in the micro-channels. From a heat transfer (thermal performance) perspective, it is found that graphene most effectively reduce the thermal resistance. Based on these results, graphene was further studied as a substrate material for a micro-channel heat sink.     

Fabrication of high aspect ratio ceramic micro-channel in diamond wire sawing as catalyst support used in micro-reactor for hydrogen production

Ceramic is an ideal material for preparing micro-channel catalyst supports with their characteristics of high temperature resistance, corrosion resistance and mechanical strength. High aspect ratio micro-channel structure has the advantages of large specific surface area, strong mass and heat transfer performance and high material utilization. However, ceramic materials are hard and brittle, and it is difficult to fabricate micro-channel structures with aspect ratio more than 1.5:1 by traditional processing methods. In this paper, a cutting method of large diameter diamond wire sawing was proposed. The micro-channels with width of 520 μm and aspect ratio of more than 4:1 was successfully fabricated by this method. Furthermore, the integrity of the micro-channel structure processed by diamond wire sawing was analyzed. And than the effect of surface morphology in different processing parameters on the catalyst loading performance were studied. The catalyst loading strength of ceramic slices with different surface morphology was tested. Finally, the ceramic micro-channel array was used as the catalyst support in micro-reactor for hydrogen production via methanol steam reforming (MSR). The methanol conversion rate and H₂ production rate could reach 87.8% and 74.6 mmol/h, respectively under GHSV 12600 ml/g·h at 300 °C. The experimental results show that the large-diameter diamond wire sawing technology can be used to process ceramic microchannels with high aspect ratio; using ceramic microchannel arrays as catalyst supports in hydrogen production can obtain better reaction performance; the feasibility of ceramic materials were broadened as microchannel catalyst supports.     

Feasibility investigations on a novel micro-manufacturing process for fabrication of fuel cell bipolar plates: Internal pressure-assisted embossing of micro-channels with in-die mechanical bonding

In this paper, we present the results of our studies on conceptual design and feasibility experiments towards development of a novel hybrid manufacturing process to fabricate fuel cell bipolar plates that consists of multi-array micro-channels on a large surface area. The premises of this hybrid micro-manufacturing process stem from the use of an internal pressure-assisted embossing process (cold or warm) combined with mechanical bonding of double bipolar plates in a single-die and single-step operation. Such combined use of hydraulic and mechanical forming forces and in-process bonding will (a) enable integrated forming of micro-channels on both surfaces (as anode and cathode flow fields) and at the middle (as cooling channels), (b) reduce the process steps, (c) reduce variation in dimensional tolerances and surface finish, (d) increase the product quality, (e) increase the performance of fuel cell by optimizing flow-field designs and ensuring consistent contact resistance, and (f) reduce the overall stack cost. This paper explains two experimental investigations that were performed to characterize and evaluate the feasibility of the conceptualized manufacturing process. The first investigation involved hydroforming of micro-channels using thin sheet metals of SS304 with a thickness of 51 μm. The width of the channels ranged from 0.46 to 1.33 mm and the height range was between 0.15 and 0.98 mm. Our feasibility experiments resulted in that different aspect ratios of micro-channels could be fabricated using internal pressure in a controllable manner although there is a limit to very sharp channel shapes (i.e., high aspect ratios with narrow channels). The second investigation was on the feasibility of mechanical bonding of thin sheet metal blanks. The effects of different process and material variables on the bond quality were studied. Successful bonding of various metal blanks (Ni201, Al3003, and SS304) was obtained. The experimental results from both investigations demonstrated the feasibility of the proposed manufacturing technique for making of the fuel cell bipolar plates.     

Development of bipolar plates for fuel cells from graphite filled wet-lay material and a compatible thermoplastic laminate skin layer

In this paper a method with the potential to lead to the rapid production of thermoplastic polymer composite bipolar plates with improved mechanical properties, formability, and half-cell resistance is described. In our previous work it was reported that laminate structure composite bipolar plates made with a polyphenylene sulfide (PPS) based wet-lay material as the core and a polyvinylidene fluoride (PVDF)/graphite mixture as the laminate exhibited improved formability, through-plane conductivity, and half-cell resistance over that of wet-lay based bipolar plates. However, the mechanical strength of the laminate plates needed improvement. In this work laminate polymer composite plates consisting of a PPS/graphite-based laminate mixture and a PPS based wet-lay core are manufactured in an effort to improve mechanical strength. Additionally, our existing channel design has been altered to reduce the channel depth from 0.8 to 0.5 mm in an effort to improve the half-cell resistance by reducing the total plate thickness. The plates are characterized by their half-cell resistance and mechanical properties at ambient and elevated temperatures. The PPS based laminate plates exhibited half-cell resistances as low as 0.018 Ω cm², tensile strength of up to 37 MPa, and flexural strength of up to 60 MPa at ambient temperature. The laminate bipolar plates can be manufactured in several ways with two of them being discussed in detail in the paper.     

Effect of different graphite materials on the electrical conductivity and flexural strength of bipolar plates fabricated using selective laser sintering

Selective Laser Sintering provides a way to fabricate graphite composite bipolar plates for use in fuel cells. This significantly reduces time and cost at the research and development stage of bipolar plates, as compared with the conventional fabrication methods such as compression molding and injection molding. Different graphite materials, including natural graphite, synthetic graphite, carbon black, and carbon fiber, were investigated using the selective laser sintering process to fabricate bipolar plates. The effect of each material on the electrical conductivity and flexural strength of the bipolar plates was studied experimentally. With a proper combination of these materials, bipolar plates with electrical conductivity ranging from 120 to 380 S/cm and flexural strength ranging from 30 to 50 MPa have been obtained, which satisfy the requirements set by the Department of Energy and also are comparable with those developed by compression molding and injection molding. A modified percolation model was proposed to predict the electrical conductivity of the fabricated bipolar plates with different compositions. The analytical results calculated from the proposed model agree well with the experimental results. Finally, a single PEM (Proton Exchange Membrane) fuel cell unit was assembled using the fabricated bipolar plates, and its in-situ performance was studied.     

Methane steam reforming: Kinetics and modeling over coating catalyst in micro-channel reactor

Kinetics of methane steam reforming for hydrogen production has been studied through experiment in a micro-channel reactor over coating catalyst. The catalyst coating prepared by cold spray on stainless steel substrate is based on a mixture of Ni–Al oxides which is normally employed in industry for methane primary steam reforming. Two kinetic laws namely parallel as well as inverse models have been derived at atmospheric pressure, and power law type kinetics have been established using non-linear least squares optimization. With the above kinetics, simulation study has been carried out to find out temperature distribution in the micro-channel over coating catalyst at two different types of boundary conditions. The results show a quite different “cold spot” character and reactants, products distribution character in the reaction channel due to its own distinct heat and mass transfer features. The kinetics and simulation study results can be applied in aid of micro-channel reactor design, and suggestion has been proposed for catalytic coating preparation and optimization.