Effects of gas diffusion layer thickness on PEM fuel cells with composite foam-rib flow fields

This study utilizes 3D multiphase non-isothermal simulations to demonstrate that while conventional rib flow fields require an optimal cathode GDL thickness, composite foam-rib flow fields achieve superior performance with thinner cathode GDLs driven by enhanced oxygen concentration, and that reducing anode GDL thickness in both designs lowers Ohmic polarization by increasing water content in the cathode ionomer.

The Gist

Imagine a Proton Exchange Membrane Fuel Cell (PEMFC) as a tiny, high-tech city where electricity is generated. In this city, hydrogen and oxygen are the citizens, and they need to meet at a specific "town square" (the catalyst layer) to shake hands and create energy.

However, getting these citizens to the town square is tricky. They have to travel through a porous sponge called the Gas Diffusion Layer (GDL). Think of the GDL as a crowded hallway connecting the main road (the flow channel) to the town square.

This paper investigates how the thickness of that hallway and the design of the walls affect how well the city runs. The researchers compared two types of city layouts:

1. The Traditional City (CRFF): Uses solid, flat walls (ribs) to separate the roads.
2. The Modern City (CFRFF): Uses a mix of solid walls and metal foam (a sponge-like material) for the walls.

Here is what they discovered, explained simply:

1. The Thickness of the Hallway (GDL Thickness)

The researchers asked: Is a thicker or thinner hallway better for the city?

• In the Traditional City (Solid Walls):
  Imagine a hallway with solid walls. If the hallway is too thick, the citizens (oxygen) have a long, tiring walk to get to the town square. If it's too thin, the "rain" (liquid water produced by the reaction) gets stuck under the solid walls, creating a flood that blocks the citizens from entering.

  • The Result: There is a "Goldilocks" zone. The hallway needs to be just right (about 130 micrometers thick) to balance the walking distance and the risk of flooding.

• In the Modern City (Metal Foam Walls):
  Now, imagine the walls are made of metal foam. This foam acts like a sponge that helps suck up water and lets air flow through it easily, even under the walls.

  • The Result: Because the foam handles the water so well, the hallway doesn't get flooded even if it's very thin. In fact, the thinner the hallway, the better! A thinner path means oxygen gets to the town square faster, and the city runs more efficiently.

2. The Width of the Walls (Rib Width)

Next, they looked at how wide the solid walls should be, specifically when the hallway is very thin.

• In the Traditional City:
  If you make the solid walls wider, you create a bigger "flood zone" underneath them where water gets trapped. This chokes off the oxygen supply.

  • The Fix: Keep the walls as narrow as possible to prevent flooding.

• In the Modern City:
  Because the metal foam walls are so good at managing water, making them slightly wider doesn't cause a flood. In fact, slightly wider walls help keep the "ionomer" (a special glue inside the city) wet, which helps electricity flow better.

  • The Fix: You can afford to make the walls slightly wider without hurting performance.

3. The Other Side of the City (Anode GDL)

They also checked the hallway on the hydrogen side (the anode).

• The Discovery: Making this hallway thinner helps a little bit, but not as much as the oxygen side. It mostly helps by keeping the "glue" (ionomer) wet, which reduces electrical resistance. It's like greasing the gears—it helps, but it's not the main engine of performance.

4. The Weather (Humidity)

Finally, they looked at how humidity affects the city.

• High Humidity: Good for the "glue" (keeps it conductive) but bad for the "hallway" (too much water clogs the path).
• The Modern City Wins: The metal foam design handles high humidity much better than the traditional solid walls. It can keep the air flowing even when it's "raining" inside the cell.

The Big Takeaway

The paper proves that changing the design of the walls (using metal foam) completely changes the rules.

• Old Rule: You need a medium-thick hallway and narrow walls to avoid floods.
• New Rule: With metal foam walls, you can use very thin hallways and slightly wider walls to get maximum power.

In a nutshell: By swapping solid walls for a sponge-like metal foam, engineers can build fuel cells that are thinner, more efficient, and better at handling water, leading to cleaner and more powerful energy for the future.

Technical Summary

1. Problem Statement

Proton Exchange Membrane Fuel Cells (PEMFCs) rely heavily on the Gas Diffusion Layer (GDL) for reactant transport, water management, and electron conduction. While the thickness of the GDL ($h_{GDL}$) is a critical design parameter, its optimal value depends on the flow field architecture.

• Conventional Rib Flow Fields (CRFF): Solid ribs often cause liquid water accumulation (flooding) in the "under-rib" region, blocking oxygen transport to the catalyst layer (CL).
• Composite Foam-Rib Flow Fields (CFRFF): This design replaces part of the solid rib with porous metal foam to enhance under-rib gas transfer and water removal.
• The Gap: While the impact of GDL thickness on CRFF designs is well-studied (typically showing an optimal thickness), the interaction between GDL thickness and the unique transport mechanisms of CFRFF designs remains unclear. Specifically, it is unknown if the "optimal thickness" rule applies to CFRFF or if thinner layers are universally better due to improved transport pathways.

2. Methodology

The study employed a three-dimensional (3D), multiphase, non-isothermal numerical simulation using COMSOL Multiphysics.

• Geometries: Two models were compared:
  • CRFF: Traditional solid rib flow field.
  • CFRFF: A hybrid design where the solid rib is partially replaced by Nickel metal foam (Metal Foam Rib, MFR), with the foam height set to 50% of the total rib height.
• Variables:
  • Cathode GDL thickness ($h_{c,GDL}$): Varied from 60 to 290 $\mu$m.
  • Anode GDL thickness ($h_{a,GDL}$): Varied from 60 to 290 $\mu$m.
  • Rib Width Ratio ($\phi_{rib}$): The ratio of solid rib width to total channel width.
  • Operating Conditions: Different cell voltages (0.1–1.0 V) and cathode relative humidity ($RHC$).
• Physics: The model solved conservation equations for mass, momentum, energy, charge (electron/ion), and species transport, including phase change (evaporation/condensation) and electro-osmotic drag.
• Validation: The model was validated against experimental data from literature, showing good agreement in polarization curves.

3. Key Contributions

• Novel Design Analysis: First comprehensive investigation into how GDL thickness affects PEMFCs utilizing the Composite Foam-Rib Flow Field (CFRFF).
• Mechanism Elucidation: Detailed analysis of the distinct mechanisms governing mass transfer and water removal in CFRFF versus CRFF, specifically regarding under-rib oxygen transport.
• Design Optimization Guidelines: Established conflicting optimization rules for rib width and GDL thickness depending on whether the cell uses CRFF or CFRFF architectures.

4. Key Results

A. Impact of Cathode GDL Thickness ($h_{c,GDL}$)

• CRFF Design: Exhibits a non-monotonic relationship. Performance increases with thickness up to an optimum, then declines.
  • Optimum: 130 $\mu$m.
  • Reasoning: Thinner GDLs reduce transport distance but cause severe under-rib flooding (liquid water accumulation) due to solid ribs, blocking oxygen. Thicker GDLs increase diffusion resistance.
• CFRFF Design: Exhibits a monotonic relationship where performance increases as thickness decreases.
  • Reasoning: The metal foam structure prevents severe under-rib flooding. Thinner GDLs shorten the oxygen transport path and water removal path, reducing concentration polarization without the penalty of flooding seen in CRFF.
• Performance Gap: CFRFF consistently outperforms CRFF across all thicknesses, with the performance gap widening as $h_{c,GDL}$ decreases.

B. Impact of Rib Width ($\phi_{rib}$) at Thin GDLs (60 $\mu$m)

• CRFF: Increasing rib width decreases performance.
  • Mechanism: Wider ribs increase the area of the "dead zone" where oxygen transport is blocked by liquid water, significantly increasing concentration polarization.
• CFRFF: Increasing rib width increases performance (up to a point).
  • Mechanism: The metal foam mitigates the flooding issue. A slightly wider rib increases the solid contact area, which improves proton conductivity and increases dissolved water content in the ionomer (reducing Ohmic resistance), outweighing minor concentration losses.

C. Impact of Anode GDL Thickness ($h_{a,GDL}$)

• Reducing $h_{a,GDL}$ improves performance for both designs, but the effect is modest compared to the cathode.
• Mechanism: Thinner anode GDLs improve hydrogen transport, enhancing electro-osmotic drag of water from anode to cathode. This increases the water content in the cathode ionomer, reducing Ohmic polarization (proton resistance) without significantly affecting concentration polarization.

D. Effect of Relative Humidity ($RHC$)

• For CFRFF, the best performance is achieved with thinner GDLs combined with higher relative humidity. High humidity improves membrane conductivity, while the thin GDL ensures efficient gas transport.

5. Significance and Conclusions

This study fundamentally shifts the design paradigm for PEMFCs using advanced flow fields:

1. Divergent Optimization: Unlike CRFFs, which require a "Goldilocks" GDL thickness to balance diffusion and flooding, CFRFFs benefit from thinner GDLs to maximize performance.
2. Rib Width Strategy: For thin GDL applications, CRFFs require minimal rib widths to avoid flooding, whereas CFRFFs can utilize slightly larger rib widths to optimize ionic conductivity.
3. Water Management: The metal foam in CFRFF effectively decouples the trade-off between gas transport and water removal, allowing for more aggressive thinning of the GDL without the risk of catastrophic flooding.

Conclusion: The adoption of Composite Foam-Rib Flow Fields allows for the use of significantly thinner GDLs (e.g., 60 $\mu$m) to achieve superior cell performance, a configuration that would be detrimental in conventional flow field designs due to flooding. This provides a clear pathway for optimizing next-generation high-performance PEMFCs.