Progressive Stamping for Mass-Production of Bipolar Plates in Fuel Cells

April 7, 2024

The increasing prevalence of electric vehicles has given rise to several energy storage methods for transport applications, one of which is the fuel cell. Fuel cells can be used as a direct energy source for electric vehicle motors, or as range extenders to be used in combination with a conventional battery.

Bipolar plates are an integral part of fuel cells, making up 80% of the fuel cell’s weight, 50-65% of its volume, and roughly 40% of the total cost (tang et al,2021). Bipolar Plates serve as a separator between individual cells while forming a conductive pathway between cathodes and anodes of one membrane electrode assembly (MEA) to another. Each Bipolar Plate also controls the movement of reactant gases between cathodes and anodes and supports the even distribution of electrons between cells. Given that the reactant gases used for fuel cells are highly flammable, Bipolar Plate manufacturers must maintain high-quality control standards to guarantee safety.

Bipolar plates for fuel cells in transportation applications have varied requirements. BPPs must be lightweight and have minimal volume for vehicle and fuel cell performance while possessing high electrical conductivity and low gas permeability; bipolar plates must also be able to resist the corrosive environment within fuel cells. These distinct requirements restrict the materials and manufacturing methods used to produce BPPs. Currently, BPPs are either made of metallic materials or graphitic materials. The manufacturing methods for metallic BPPs include powder metal forging, stamping, and hydroforming, while graphitic BPPs are made using compression molding and injection molding.

BPP and MEA assembly of Fuel cell stack (Tang, 2021)

Conventional manufacturing methods for BPPs have several limitations such as a waiting period and manual labor costs associated with tooling, lowering the feasibility of mass manufacturing BPPs. Progressive stamping and additive manufacturing can mitigate many of the challenges facing conventional processes and provide a viable, cost-competitive method for mass-producing bipolar plates.

Challenges of Bipolar Plate Manufacturing

Material Selection

The two prevalent material choices for BPPs are metallic materials (alloy, coated, non-coated), and graphitic. The properties of each type of material present several advantages and disadvantages for BPP.

Graphite Bipolar Plates have high corrosion resistance and thermal resistance, and are lightweight; however, graphite and graphite-based materials have a high porosity requiring BPPs to be thicker and thus occupy more volume in the fuel cell. Graphite is also brittle and difficult to fabricate, making it time-consuming and expensive to machine the flow fields. Metal Bipolar Plates have a lower material cost compared to graphite and have a higher mechanical strength allowing BPPs to be thinner and occupy less volume within the fuel cell. However, metals corrode easily, and self-passivation could occur following corrosion, creating high contact resistance, and lowering the electrical conductivity of the BPP.

Developments in the fuel cell industry over time proposed new innovative materials to mitigate these challenges such as thermoplastic graphite composite or nylon-matrix stainless steel (Tang et al, 2021). One recent development has been a study on coated metals. Improved performance of coated materials on metallic bipolar plates can reduce the corrosion rate.

The US Department of Energy has released an ideal target rate of bipolar plates in Fuel cells for transportation in 2020, listed below.

CHARACTERISTIC

UNITS

2015 STATUS

2020 TARGETS

Cost

$/kWnet

Plate weight

kg/kWnet

<0.4

0.4

Plate H2 permeation coefficient

Std cm³ /(sec cm² Pa) @ 80°C, 30 atm, 100% RH

<1.3x10⁻¹⁴

Corrosion, anode

µA/cm²

no active peak

<1 and no active peak

Corrosion, cathode

µA/cm²

<0.1

Electrical conductivity

S/cm

>100

Areal specific resistance

ohm cm

0.006

<0.01

Flexural strength

MPa

>34 (carbon plate)

>25

Forming elongation

20–40

Design and Fabrication

The fabrication of Bipolar Plates requires the precise balancing of structural integrity and efficiency when determining the thickness of the plates. The design must be thick enough to prevent deformation during operation while minimizing thickness for plate performance and to reduce the volume occupied within the fuel cell. Another challenge of manufacturing Bipolar Plates is that they require considerable fabrication processes to produce the flow field patterns integral to BPP performance. Common patterns include serpentine, parallel, interdigitated, and mesh-type patterns.

Serpentine Flow Field Pattern, Image Courtesy of Journal of Energy Storage

Progressive Die Stamping for High Volume Production of Bipolar Plates

Progressive Die Stamping for coated metallic Bipolar Plates can be a viable solution for high-volume production of BPPs. Progressive die stamping is manufacturing where a continuous flat sheet of metal is moved down an assembly line where the metal sheet is stamped at multiple stations, each with distinct dies.

Progressive Die Stamping is a relatively fast manufacturing method capable of fabricating the complex flow field patterns of Bipolar Plates with a low production cycle time suitable for mass manufacturing. In addition, progressive die stamping can also produce plates with lower thicknesses than heavier graphitic BPPs or BPPs made of machined metal. One of the potential drawbacks of Progressive Die Stamping is that spring-back could occur, a process where a stamped sheet metal elastically shifts back to its original shape resulting in dimensions that deviate from the design specifications (Tang et al, 2021).

progressive stamping center plate — Center slot plate

LPR Global offers Progressive Die Stamping Dies and Additive Manufacturing Services suitable for Bipolar Plate production.

Learn more about our progressive die manufacturing and contract manufacturing services, or contact us to speak with a specialist.

Gundlapalli, R., & Jayanti, S. (2021). Dataset on performance of large-scale vanadium redox flow batteries with serpentine flow fields. In Data in Brief (Vol. 35, p. 106835). Elsevier BV. https://doi.org/10.1016/j.dib.2021.106835

Tang, A., Crisci, L., Bonville, L., & Jankovic, J. (2021). An overview of bipolar plates in proton exchange membrane fuel cells. In Journal of Renewable and Sustainable Energy (Vol. 13, Issue 2). AIP Publishing. https://doi.org/10.1063/5.0031447