Forming of large scale bipolar plates for high power fuel cell stacks

Developing high-power (e.g. megawatt-scale) single fuel cell stacks is of significance to extending the application of hydrogen fuel cells in high-energy-consumption fields such as aerospace, maritime, and rail transportation. Bipolar plate is one of the core components of hydrogen fuel cell stacks. Currently, the mainstream hydrogen fuel cell stacks achieve a maximum power of about 200 kW with a bipolar plate area of approximately 600 cm2. While the megawatt-scale hydrogen fuel cell stacks requires large scale bipolar plates with an area of e.g. >2000 cm2 and higher geometric complexity of flow channel. However, the structural design and manufacturing process for such large scale bipolar plates remain unexplored. Based on the concept of “partitioned modular manufacturing”, the large scale bipolar plate is divided into multiple smaller scale bipolar plate modules in this work, and then integrated into a single component, which is then formed by applying multi-step stamping process to each module. Therefore, a so-called “partitioned multi-step stamping process” is proposed to form large scale bipolar plates with fine flow channels. Experimental validation was conducted using 0.1 mm thick titanium sheets and austenitic stainless steel sheets, demonstrating a prospective solution to manufacture large scale bipolar plates for high power hydrogen fuel cell stacks.


Introduction
Hydrogen possesses advantages such as zero emissions, high energy density, renewability, and abundant reserves.It has found extensive applications in various sectors, including transportation, energy storage, industrial production, and aerospace.Fuel cells can convert the chemical energy in hydrogen into electrical energy through electrochemical reactions.Among numerous types of fuel cells, Proton Exchange Membrane Fuel Cell (PEMFC) offers high power density and energy conversion efficiency, which enables it a crucial terminal application for hydrogen energy [1,2].
Bipolar plate is one of the core components of PEMFC, serving several essential functions including: (1) providing channels for reactants/products; (2) collecting reaction currents; (3) offering support and enhancing overall structural strength [3].Currently, materials used in the manufacturing of bipolar plates include graphite, metals and their alloys, composite materials, etc. Metal materials such as stainless steel, titanium, and their alloys are among the primary directions for the development of bipolar plate materials due to their characteristics of being lightweight, cost-effective, and excellent electrical and IOP Publishing doi:10.1088/1757-899X/1307/1/012030 2 thermal conductivity [4].The design and manufacturing processes of bipolar plate components are closely linked to the performance, lifespan, and cost of the fuel cell stack.The structure of the bipolar plate is illustrated in Fig. 1.With the continuous promotion of the "green transportation" concept, the application of hydrogen fuel cells in high-energy consumption sectors is becoming increasingly prominent.However, the current mainstream PEMFC operates in a power range of 100 to 200 kW, which is challenging to meet the highpower demands under specific conditions.The output power of PEMFC is closely related to the area of the bipolar plate, increasing the effective area of the reaction zone can enhance the contact area between fuel and oxidant, thereby increasing the power output of a single fuel cell stack.For instance, a PEMFC with an output power of 200 kW usually requires a bipolar plate area of 600 cm 2 , while megawatt-level PEMFC demand a bipolar plate area exceeding 2000 cm 2 .Therefore, one direct and effective method to enhance the output power of a fuel cell stack is to increase the area of the bipolar plate.
To meet the power density requirements of the next generation of high-performance fuel cells, bipolar plates must possess more complex flow channel structures and higher forming precision.According to the forming characteristics of ultra-thin metal sheets, traditional cold stamping processes for bipolar plates are prone to serious defects such as springback, local thinning, and cracking [5].To overcome these limitations, many scholars have proposed advanced forming processes, such as multistep stamping [6], electromagnetic bulging [7], rubber forming [8], warm stamping [9], etc.Compared the above process, parts formed using the multi-step stamping process exhibit improved aspect ratio, draft angles, and smaller corner radii.Moreover, this process has advantages such as high forming precision, low cost, and high efficiency.
As the area of bipolar plates increases to megawatt level, several challenging bottlenecks arise: (1) The larger forming area results in a linear increase in the pressure required for stamping process, and the commonly used stamping machine are unable to meet the forming requirements.(2) As the area of bipolar plate increases, the difference in channel forming depth between the border and central regions gradually arises.This leads to poor consistency in the overall bipolar plate dimensions, causing differences in local contact pressure between the membrane electrode assembly and the bipolar plate.Consequently, this variation can adversely impact the efficiency of power generation in the fuel cell.[10][11][12][13].(3) Tools suitable for manufacturing large-scale bipolar plates are expensive, and their processing is more challenging.Therefore, innovative approaches to the design methods and manufacturing processes for large-scale bipolar plates are still urgently needed to address these issues.
In this study, an innovative design method for large-format bipolar plates is introduced.Based on modular design concept, a large-scale bipolar plate is divided into a holistic structure composed of multiple small-scale bipolar plate modules.Correspondingly, a partitioned multi-step stamping process is proposed.This method involves multiple iterations of modular stamping on metal sheets to achieve high-precision manufacturing of large-scale bipolar plates.Finally, stamping tools for modular largescale bipolar plates were developed, and experimental validation was conducted using austenitic stainless steel (SUS316L) and commercial pure titanium (CP-Ti) ultra-thin sheets, which proved the advancement of the design and manufacturing method proposed in this study.

Materials
Stamping experiments were conducted using ultra-thin sheets (0.1mm) of SUS316L and CP-Ti.The mechanical properties of the two materials in the rolling direction (RD), diagonal direction (DD), and transverse direction (TD) were characterized through uniaxial tensile tests.Detailed experimental methods and specimens size refer to the ASTM-E8M standard.The tested mechanical properties of the two materials are summarized in Tables 1 and 2.

Large-scale bipolar plate design
To address the issue related to large-scale bipolar plates at the end of Section 1, this study proposes a novel design approach for large-scale bipolar plates.The concept of "modularity" is integrated into the design of large-scale bipolar plates for the first time, by dividing large-scale bipolar plates into multiple small-scale modules with identical characteristics.Through adjusting the number of modules, the size (length and width) of the large-scale bipolar plate can be customized.However, to minimize the size consistency fluctuations between modules during forming process, the number of modules composing the large-scale bipolar plate should be reduced.Since each module is independent, it must possess complete bipolar plate functionality, including gas inlets and outlets, distribution regions, flow channel regions, and other functional features.Based on the bipolar plate design method of "single plate with multiple chambers", the small-scale modules designed in this study consist of two flow channel regions.Each flow channel region is equipped with corresponding gas inlets and outlets, as well as distribution regions.Specifically, a common gas inlet and outlet are placed in the central region of the module, supplying gas simultaneously to the two side flow channel regions.This design enhances the effective utilization of the bipolar plate's area.Through the modular design method for large-scale bipolar plates described above, we evenly distribute the large characteristic dimensions (length and width) of an integrated large-scale bipolar plate onto each small-format module.This approach helps reduce the difference in channel depths between the border and central regions (along and perpendicular to the flow channel direction) during the forming process, optimizing the uniform distribution of reactants and products within the flow channel regions.A test bipolar plate is designed in this study as shown in Fig. 2a, where the blue dashed box represents a single small-scale module, and the entire plate is composed of 2 modules.Taking Module-1 as an example, the yellow dashed box represents the gas inlets and outlets, the white dashed box represents the distribution regions, and the green dashed box represents the flow channel regions.Theoretically, by increasing the quantity of modules, bipolar plate with an unlimited area can be manufactured.The red dashed box represents the stamping overlap region between the two modules, which will be discussed in subsequent sections.
The designed area of the test bipolar plate is 530 cm 2 .The general dimensions and cross-sectional dimensions of the flow channels are illustrated in Fig. 2a and Fig. 2b, respectively.The characteristic dimensions (length and width) of this test plate are half of the target large-format bipolar plate.After validating the feasibility of the large-scale bipolar plate design method proposed in this study through experiments, it can be further applied to large-scale bipolar plate design projects.

Partitioned multi-step stamping process
Based on the modular design scheme for large-scale bipolar plates introduced in Section 2.2, this section outlines the corresponding partitioned multi-step stamping process, as illustrated in Fig. 3. Traditional stamping processes involve the integral forming of an entire large-scale bipolar plate in a single step.However, the large-scale bipolar plate is partitioned into multiple independent small-scale modules within the modular design.Hence, this study proposes a novel forming process for large-scale bipolar plates.During this process, each module undergoes independent stamping.After forming one module at a station, the metal sheet is then moved to the next module position, aligning it with the stamping station, and the stamping process is repeated.This iterative procedure enables the partitioned stamping of large-scale bipolar plates.

Figure. 3. A schematic diagram of partitioned multi-step stamping process
Due to the challenging forming requirements of flow channels in the test bipolar plate, single-step cold stamping is insufficient to meet the forming demands.Therefore, a multi-step stamping process is employed for each module's forming.Taking into account the improvement of the number of forming steps on the formability of ultra-thin metal sheets and considering manufacturing costs, this study determines that the multi-step forming process involves two steps (pre-forming and final forming), as shown in Fig. 3.It should be noted that during the pre-forming step, the formed flow channel structure does not merely about decreasing depth below the design value; instead, it involves changing the crosssectional shape of the channel.This modification induces plastic deformation of the ultra-thin metal sheets in the entire region, optimizing the plastic flow of materials during the final forming process.This approach aims to reduce local thinning and cracking of the bipolar plate, enhancing the formability of the ultra-thin metal sheets under complex geometric shapes [14].
The tools for the partitioned multi-step stamping process of large-scale bipolar plates were developed, including preforming and final forming (Fig. 4).In this study, the flow channel corner radii of the preform tool was enlarged from 0.1 mm from 0.25 mm, compared to the final form tool. Since this process targets only a single small-scale module for stamping within a single step, it can reduce the tonnage requirements on the stamping machine compared to traditional integral cold forming processes.Additionally, it can decrease the difficulty in manufacturing the tools and the associated machining costs.Furthermore, a "stamping overlap region" (refer to the feature inside the red dashed box in Fig. 2a) is positioned between two adjacent modules.This region is shared by the two modules and is repeatedly stamped in partitioned forming steps of the two modules.Setting up a "stamping overlap region" offers several advantages: (1) It enhances the effective utilization of the bipolar plate's area.( 2) During the partitioned stamping process, manual movement of the blank is required for forming adjacent modules.This can lead to positioning errors in the metal sheets, resulting in lower positional accuracy between the two modules after forming.By setting special geometric features in the stamping overlap region, it serves as a precise positioning reference for adjacent modules after the initial stamping, which can effectively improve the positional accuracy between modules.(3) Reinforcement features can be introduced in the stamping overlap region to enhance the overall rigidity of the large-scale bipolar plate.

Experimental methodology
Stamping experiments were conducted on ultra-thin SUS316L and CP-Ti sheets to validate the feasibility of the large-scale bipolar plate design and manufacturing method proposed in this study.A pressure test is conducted before experiments.Gradually increase the pressure and measure the forming depth of flow channel after each stamping.When there is no significant change in depth, it indicates that the pressure is sufficient, and this pressure is used for subsequent experiments.Firstly, single and multiple stamping process were both conducted using only the final forming tool as a reference.Then the ultra-thin metal sheets were subjected to multi-step stamping using both the pre-forming and final forming tools for a single module to confirm the formability improvement.
In response to the uneven channel forming depth issue caused by large characteristic dimensions, comparative experiments on multi-step stamping of ultra-thin CP-Ti sheets were conducted using both the partitioned multi-step stamping tools (Fig. 4) and the testing tools (Fig. 5), which also include preforming and final forming.Measurements of channel forming depth were then conducted on the bipolar plate specimens after forming.Ten channels were selected from the border to the central region, sequentially numbered from 1 to 10, and their forming depths were measured.For each waved channel, data collection was performed at intervals of one period along the direction of channel extension (Fig. 5a).The displayed data is the mean value of three repeated stamping specimens.

Mechanical properties of ultra-thin metal sheets
From the mechanical properties data in Section 2.1, it can be seen that the yield strength, tensile strength, and elongation parameters of ultra-thin SUS316L sheet are significantly higher than those of ultra-thin CP-Ti sheet.Additionally, the ultra-thin SUS316L sheet exhibits lower anisotropy (as indicated by the R value) compared to the pronounced anisotropy observed in the ultra-thin CP-Ti sheet.This suggests that the formability of ultra-thin SUS316L sheet is much greater than that of ultra-thin CP-Ti sheet.Consequently, during the forming process of intricate flow channel structures, ultra-thin CP-Ti sheet tends to experience premature occurrences of local thinning and cracking.The use of two materials with significantly different mechanical properties aims to validate the wide applicability of the proposed bipolar plate design and manufacturing method across different metal materials.

Formability improvement of multi-step stamping process
For ultra-thin CP-Ti sheets, the results revealed cracking in flow channel region (Fig. 6a) for the single step stamping bipolar plate, while the multi-step stamping bipolar plate did not exhibit any cracking defects across the entire range (Fig. 6b).Therefore, it can be inferred that the multi-step stamping process, by incorporating a pre-forming step with different flow channel cross-sectional shapes, engages as much material as possible within the flow channel structure to participate in plastic deformation, thereby enhancing the formability of ultra-thin metal sheets.Changing only the number of forming steps without altering the flow channel cross-sectional shape of the tool, has a relatively small impact on the formability of ultra-thin metal sheets.Repeating the above experiments with ultra-thin SUS316L sheets yielded results similar to those obtained with ultra-thin CP-Ti sheets (Fig. 6c, 6d).

Channel depth consistency improvement of partitioned multi-step stamping process
The partitioned multi-step stamping process was applied to form large-scale bipolar plates using ultrathin SUS316L and CP-Ti sheets.The formed large-scale bipolar plate specimens are depicted in Fig. 7.
The results indicate that the partitioned multi-step stamping process allows multiple modular stamping on a single metal sheet, facilitating the formation of large-scale bipolar plates.For bipolar plates formed by testing tool, the measurements results (Fig. 8a) indicate a gradual decrease in channel forming depth from border region to central area.For a single channel, the forming depth in the middle is greater than at the two ends, consistent with trends observed in previous research [10,11].Across the entire measurement region of the specimen (distance in the channel width direction from channel 1 to 10 is 12mm, channel length is 105mm, with a total of 8 periods), the maximum observed difference in channel forming depth reaches 60μm.For the partitioned bipolar plate, the characteristic dimensions (length and width) of a single module are relatively smaller.The measurements result of a single active area (Fig. 8b) indicate that, across the entire measurement region of the specimen (distance in the channel width direction from 1 to 10 is 12mm, channel length is 45mm, with a total of 4 periods), the maximum observed difference in channel forming depth is less than 10μm.Simultaneously, a comparison was made between the forming depths of channels in the adjacent 4 modules of the large-scale bipolar plate specimen (Fig. 9a), sampling method is the same as before.The results (Fig. 9b) demonstrate a consistent trend in the distribution of channel forming depth data among multiple modules of the large-format plate specimen.Across the entire measurement region of the specimen (690mm in length and 380mm in width), the maximum difference in channel forming depth is less than 10μm.It is noteworthy that the partitioned multi-step stamping process of the large-scale bipolar plate underwent four times stamping processes on the same ultra-thin metal sheet, indicating the excellent consistency of the proposed partitioned multi-step stamping process.

Conclusions
In this study, a modular design approach for large-scale bipolar plates used in high-power PEMFC is proposed for the first time.Correspondingly, a partitioned multi-step stamping process is designed.Process test tools were developed, and stamping experiments were conducted using ultra-thin SUS316L and CP-Ti sheets to manufacture a test bipolar plate with an area of 530 cm 2 .The test bipolar plate were then subjected to dimensional measurements to validate the feasibility of the design and manufacturing method.The main conclusions are summarized as follows: (1) The partitioned multi-step stamping process has demonstrated success in manufacturing largescale bipolar plates and is applicable to various metal materials such as SUS316L and CP-Ti.The consistency of channel depths in the formed parts is satisfactory (deviation less than 10μm for the 530 cm² test bipolar plate), and the channel dimensions remain stable even after four times stamping processes.
(2) The modular design approach for large-scale bipolar plate can reduce the requirements on stamping machine and tools during stamping process.Besides, it addresses the consistency issues in channel depth arising from larger characteristic dimensions.Theoretically, by adjusting the size (length and width) as well as the quantity of the modules, it is possible to design and manufacture large-scale bipolar plate suitable for high-power PEMFC applications.

Fig. 1 .
Fig. 1.A schematic diagram of metal bipolar plate structure

Figure. 2 .
Figure. 2. a) Structural design of partitioned bipolar plate, b) cross-sectional dimensions of flow channels

Fig. 5 .
Fig. 5. a) Schematic diagram of testing tool, b) cross-sectional dimensions of flow channels

Fig. 6 .
Fig. 6.Single module BPP specimen for CP-Ti: a) and b); and SUS316L: c) and d) on single/multi-step stamping

Fig. 8 .
Fig. 8. Distribution diagram of channel forming depth for a) testing specimen and b) partitioned multi-step stamping specimen

Fig. 9 .
Fig. 9. Consistency test of partitioned multi-step stamping specimen: a) Sampling region, b) channel forming depth data