Bipolar plates for the next generation of proton exchange membrane fuel cells (PEMFCs): A review of the latest processing methods for unconventional flow channels

This review addresses the gap in literature regarding advanced manufacturing methods, such as additive manufacturing, for producing unconventional, intricate bipolar plate geometries in PEMFCs, evaluating their scalability and industrial readiness to enable the transition from laboratory concepts to affordable, efficient industrial production.

The Gist

Imagine a Proton Exchange Membrane Fuel Cell (PEMFC) as a high-tech, eco-friendly engine that runs on hydrogen instead of gasoline. It's the clean energy solution we need to replace fossil fuels. But inside this engine, there is a critical, unsung hero: the Bipolar Plate.

Think of the Bipolar Plate as the road system inside a bustling city.

• The Job: It has to do three things simultaneously:
  1. Deliver Traffic: It must guide hydrogen and oxygen (the fuel) smoothly to the "factories" (catalyst layers) where electricity is made.
  2. Clear the Debris: It must sweep away the waste (water) so the roads don't flood.
  3. Carry the Power: It must conduct the electricity generated by the factories out to the grid.

The Problem: The Old Roads Are Too Simple

For a long time, engineers built these "roads" using conventional manufacturing (like stamping metal or carving graphite). They used simple, straight, or winding patterns (like a snake).

• The Limitation: These methods are like using a hammer and chisel. You can't carve a tiny, intricate, 3D maze without breaking the material or spending a fortune.
• The Result: The old designs are okay, but they aren't perfect. They cause traffic jams (pressure drops), create puddles (flooding), and don't distribute fuel evenly. To get the next level of performance (like a car that goes 400 miles on a tank), we need unconventional, complex road maps—think of a fractal tree, a human lung, or a spiderweb. But you can't stamp those out of metal or carve them out of graphite easily.

The Solution: 3D Printing (Additive Manufacturing)

Enter Additive Manufacturing (AM), or 3D Printing. Instead of carving away material or stamping it, 3D printing builds the plate layer by layer, like stacking LEGO bricks or extruding frosting. This allows engineers to print the most complex, "impossible" shapes imaginable.

The paper reviews the different "3D printing styles" available for these fuel cell plates:

1. The "Frosting" Method (Fused Filament Fabrication - FFF)

• How it works: A plastic filament is melted and squeezed out of a nozzle, layer by layer.
• The Analogy: Like a hot glue gun or a cake decorator.
• Pros: Cheap and easy to use. You can make weird shapes quickly.
• Cons: The "roads" are rough (like a bumpy cake). Plastic doesn't conduct electricity well, so you have to paint it with metal later. It's also prone to leaks because the layers don't always stick perfectly.
• Verdict: Good for prototypes, but needs a lot of polishing to be a real engine part.

2. The "Laser Sculpting" Method (Vat Photopolymerization - SLA/DLP)

• How it works: A laser or light beam cures (hardens) liquid resin, turning it solid instantly.
• The Analogy: Like a high-tech 3D printer that uses a flashlight to "draw" solid plastic in a vat of goo.
• Pros: Incredible detail! It can print tiny, smooth channels (like the veins in a leaf) that look perfect.
• Cons: It only works with specific plastics, not metals. These plastics are often too weak or brittle for a real car engine. They also need to be coated in metal to conduct electricity.
• Verdict: Great for making the shape perfect, but the material needs a makeover to survive the harsh fuel cell environment.

3. The "Sandcastle" Method (Powder Bed Fusion - SLM/EBM)

• How it works: A laser or electron beam melts tiny metal powder particles together, fusing them into a solid block.
• The Analogy: Imagine a robot spreading a thin layer of sand, then using a super-hot laser to melt specific spots into rock, layer by layer.
• Pros: This is the heavy hitter. It can print real metal (like stainless steel or titanium) with incredible precision. The parts are strong, dense, and can handle the heat and pressure of a real car. It can create the complex "lung-like" structures that maximize efficiency.
• Cons: It's expensive, slow, and the surface can be a bit rough (like a sandcastle that hasn't been smoothed yet). It often needs a "polishing" step to make the roads smooth enough for traffic.
• Verdict: The most promising method for the future of real, high-performance fuel cells.

4. The "Spray Paint" Method (Directed Energy Deposition - DED)

• How it works: A laser melts metal powder or wire as it is sprayed onto a surface.
• The Analogy: Like a very precise, high-speed spray painter that builds up a wall.
• Pros: Fast and good for fixing broken parts or adding thick coatings.
• Cons: The details aren't as sharp as the "Sandcastle" method. The roads might be too bumpy for a high-speed engine.
• Verdict: Better for repairs or coatings than building the whole plate from scratch.

Why This Matters

The paper argues that to make fuel cell cars as cheap and common as gas cars, we need to stop using "old school" manufacturing. We need to embrace 3D printing to create these unconventional flow channels.

• Efficiency: Complex shapes (like leaf veins) distribute fuel better, meaning the engine runs cooler and more efficiently.
• Cost: While 3D printing is currently expensive, it eliminates the need for expensive metal molds and reduces waste. As the technology matures, it could actually become cheaper for mass production.
• The Future: The goal is to print a fuel cell plate that is as thin as a credit card, as strong as steel, and has a road map as complex as a human lung, all in one piece.

In a nutshell: We are moving from carving simple roads out of stone to 3D printing intricate, high-speed highways out of metal. This shift is the key to unlocking the full potential of hydrogen fuel cells and getting us off fossil fuels faster.

Technical Summary

1. Problem Statement

The transition to sustainable energy requires the widespread adoption of Proton Exchange Membrane Fuel Cells (PEMFCs), particularly for transportation. However, the commercial viability of PEMFCs is currently hindered by the limitations of Bipolar Plates (BPs), which constitute 70–90% of the stack mass and 20–30% of the total cost.

• Design Limitations: Conventional BPs rely on standard flow channel geometries (serpentine, parallel, interdigitated) developed through heuristic intuition. These designs often fail to optimize reactant distribution, water management, and pressure drop simultaneously. Next-generation targets (e.g., 6.0 kW/L power density) require "unconventional" flow fields (e.g., fractal, biomimetic, microlattices) that are geometrically complex and non-intuitive.
• Manufacturing Bottlenecks: Conventional manufacturing methods (stamping, machining, molding) lack the resolution and precision to fabricate these intricate micro-features. They often require multi-step post-processing, limit design adaptability, and are economically unviable for low-to-medium volume production of complex geometries.
• Material Trade-offs: Graphite offers corrosion resistance but is brittle and difficult to machine into thin, high-density plates. Metals are strong and impermeable but prone to corrosion and require protective coatings. Polymers are lightweight but lack conductivity and chemical stability without modification.

2. Methodology

The paper conducts a comprehensive literature review and critical analysis of Additive Manufacturing (AM) technologies as a solution to the design and fabrication bottlenecks of PEMFC bipolar plates. The authors:

• Categorized AM Processes: Analyzed major AM families applicable to PEMFCs:
  • Vat Photopolymerization: Stereolithography (SLA) and Digital Light Processing (DLP).
  • Powder Bed Fusion (PBF): Selective Laser Sintering/Melting (SLS/SLM/DMLS) and Electron Beam Melting (EBM).
  • Material Extrusion: Fused Filament Fabrication (FFF).
  • Directed Energy Deposition (DED): Laser Metal Deposition (LMD) and Wire Arc AM (WAAM).
  • Jetting: Binder Jetting (BJ) and Material Jetting (MJ).
• Evaluated Performance Metrics: Assessed each technology against critical PEMFC requirements:
  • Geometric Fidelity: Ability to reproduce micro-channels (down to 20–50 µm) and sharp corners.
  • Surface Quality: Impact of as-built roughness on interfacial contact resistance (ICR) and flow dynamics.
  • Material Properties: Electrical conductivity, corrosion resistance, gas impermeability, and mechanical strength.
  • Scalability: Production speed, cost, and industrial readiness.
• Synthesized Case Studies: Reviewed specific experimental studies where AM-fabricated BPs were tested in situ or ex situ, comparing their electrochemical performance (power density, voltage, efficiency) against conventionally manufactured counterparts.

3. Key Contributions

The paper provides a structured assessment of how different AM technologies enable "unconventional" flow channel designs that were previously impossible to manufacture.

• Vat Photopolymerization (SLA/DLP):
  • Strengths: Highest resolution (down to 20 µm), smooth surfaces, and ability to create complex biomimetic (e.g., lung-inspired) and fractal flow fields with minimal post-processing.
  • Limitations: Limited to photopolymer resins (insulating); requires conductive coatings (e.g., silver, gold) or infiltration to function as BPs. Not yet compatible with high-performance thermoplastics like PEEK.
• Powder Bed Fusion (SLM/DMLS/EBM):
  • Strengths: Capable of producing dense, near-net-shape metallic BPs with high precision. Enables complex internal structures (e.g., graded porosity, microlattices) and fine features (0.3 mm channels).
  • Limitations: High surface roughness (as-built) increases contact resistance; requires extensive post-processing (polishing, coating) to meet flatness and corrosion standards. High energy consumption and residual stress issues.
• Material Extrusion (FFF):
  • Strengths: Low cost and accessible for rapid prototyping of polymer-based BPs.
  • Limitations: Poor resolution (~100 µm), visible layer lines, and high anisotropy. Requires conductive fillers or metal lamination, which often results in suboptimal electrical performance and gas leakage risks due to interlayer voids.
• Directed Energy Deposition (DED) & Jetting:
  • DED: Primarily useful for coating or repairing existing plates rather than fabricating intricate BPs due to coarse microstructures and distortion.
  • Binder/Material Jetting: High precision for complex geometries but currently limited by the need for sintering (causing shrinkage) or the lack of conductive materials in the raw feedstock.

4. Results and Findings

• Performance Gains: Studies cited in the review demonstrate that AM-fabricated unconventional flow fields (e.g., fractal, baffled, mesh) can significantly outperform conventional designs.
  • Example: Trogadas et al. reported a 20–25% increase in peak power density using lung-inspired fractal flow fields compared to serpentine designs.
  • Example: Jin et al. achieved a 31.4% higher power density with 300 µm wide channels printed via SLM compared to 500 µm channels, due to improved mass transport.
  • Example: Wu et al. (referenced as Zhang et al. in text) showed that stepped parallel channels via SLM increased power density by 26% over standard parallel layouts.
• Surface Roughness & Contact Resistance: A critical finding is that the "as-printed" surface roughness of metallic BPs (often >1 µm) creates high interfacial contact resistance (ICR). While post-processing (polishing, coating) reduces ICR, it adds cost and complexity. SLA/DLP offers smoother surfaces but lacks intrinsic conductivity.
• Scalability vs. Complexity: AM is currently most viable for low-to-medium volume production where the cost of tooling for conventional methods is prohibitive. For mass production, conventional stamping remains cheaper, but AM offers superior material utilization and design freedom that can offset costs through improved catalyst efficiency and reduced stack volume.

5. Significance and Future Outlook

This review fills a critical gap in the literature by linking advanced flow channel designs directly to manufacturing feasibility. It argues that the stagnation in PEMFC performance is not just a material science issue but a manufacturing limitation.

• Paradigm Shift: The paper advocates for a shift from "designing for manufacturing" (conventional constraints) to "manufacturing-enabled design" (AM capabilities), allowing for optimized, non-intuitive geometries that maximize catalyst utilization and minimize pressure drops.
• Future Directions:
  1. Hybrid Manufacturing: Combining AM for complex flow fields with conventional methods for bulk material properties.
  2. In-situ Monitoring: Implementing real-time defect detection to reduce the need for post-processing.
  3. Material Development: Creating conductive, corrosion-resistant polymer composites or metal powders specifically tailored for AM to eliminate the need for secondary coatings.
  4. Process Optimization: Refining parameters to minimize residual stress and surface roughness in metallic BPs to reduce post-treatment costs.

Conclusion: Additive manufacturing is the key enabler for the next generation of high-performance, compact, and affordable PEMFCs. While challenges in cost, surface finish, and material conductivity remain, the ability to fabricate optimized, unconventional flow channels makes AM indispensable for advancing fuel cell technology beyond current industrial limits.