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

Abstract: Gas diffusion layers (GDLs) play a crucial role for the performance of proton exchange membrane fuel cells (PEMFCs). The utilization of composite foam-rib flow fields (CFRFFs) can alter the reactant gas transfer pattern, hence improving the efficiency of under-rib reactant gas transfer and water drainage. The impact of the cathode and anode GDL thicknesses (h_{c,GDL} and h_{a,GDL}) on the performance of CFRFF design is investigated by three-dimensional multiphase non-isothermal numerical simulation in this study. The results indicate that for the conventional rib flow field (CRFF) design, there is an optimal h_{c,GDL} for optimal cell performance, while for the CFRFF design, as h_{c,GDL} becomes thinner, the cell performance increases, and the trend is dominated by the variation of the oxygen concentration. Under a thin GDL, the rib width of the CRFF design should be as small as possible to minimize concentration polarization loss, while the rib width of the CFRFF design can be slightly larger. Furthermore, by decreasing the thickness of h_{a,GDL} in both the CRFF and CFRFF designs, there is an increase in the dissolved water content in the ionomer of the cathode CL and a subsequent decrease in the Ohmic polarization loss.

• The paper demonstrates that minimizing cathode GDL thickness in CFRFF leads to enhanced performance, contrasting with the non-monotonic response in CRFF.
• The paper shows that composite foam-rib flow fields mitigate under-rib flooding and enable uniform oxygen distribution, improving limiting current density.
• The paper reveals that optimized GDL and rib configurations reduce Ohmic losses and enhance water management for robust PEMFC performance.

Analysis of Gas Diffusion Layer Thickness Effects in PEM Fuel Cells with Composite Foam-Rib Flow Fields

Introduction

The gas diffusion layer (GDL) is a fundamental component in proton exchange membrane fuel cells (PEMFCs), mediating reactant transport, water management, and electron conduction within the membrane electrode assembly. Emerging flow field architectures, particularly the composite foam-rib flow field (CFRFF), seek to optimize mass and water transport—critical factors for cell performance—by substituting traditional solid ribs with partially porous metal foam ribs. This work presents a thorough numerical investigation into how cathode and anode GDL thicknesses affect PEMFC performance in both conventional rib flow field (CRFF) and CFRFF architectures, emphasizing multiphase, non-isothermal two-phase transport physics using a validated 3D FEM framework.

Model Framework and Methodology

Three-dimensional, steady-state, multiphase, non-isothermal simulations were conducted on PEMFC unit cells featuring either a CRFF or CFRFF cathode architecture. The CFRFF included a hybridization of conventional solid ribs and nickel foam ribs (MFRs), whereas the CRFF maintained a fully solid rib structure. Key parametric variations included the cathode GDL thickness ($h_{c,\mathrm{GDL}}$), the anode GDL thickness ($h_{a,\mathrm{GDL}}$), and the cathode rib width ratio ($\phi_\mathrm{rib}$), all within experimentally realistic domains.

The model captured coupled charge, species, and energy transport, including phase change and EOD effects. The mesh was demonstrated to yield grid-independent cell current/power performance metrics, and simulation agreement with experimental polarization data was explicitly validated, ensuring predictive reliability.

Cathode GDL Thickness: Comparative Effects in CRFF and CFRFF

A primary finding is that the functional dependence of cell performance on cathode GDL thickness diverges significantly between CRFF and CFRFF architectures:

• In CRFFs: Cell performance exhibits a non-monotonic response to GDL thinning. There exists an optimal $h_{c,\mathrm{GDL}}$ (here, 130 μm) that maximizes power output. Thinning below this value exacerbates under-rib flooding due to the solid rib barrier, increasing mass transport resistance and causing localized oxygen deprivation in catalyst sites. Overly thick GDLs, conversely, lengthen reactant and water removal pathways, degrading both oxygen availability and current density homogeneity.
• In CFRFFs: Performance is positively and strictly correlated with decreasing $h_{c,\mathrm{GDL}}$ across the entire examined range. GDL thinning improves oxygen access and water removal uniformly, with the MFR structure mitigating under-rib water accumulation. This enables higher limiting current densities and more uniform oxygen and current distribution, and the performance gain is driven primarily by reduced concentration polarization at low cell voltages.

Cathode Rib Width Effects at Thin GDLs

Rib width plays a critical role under thin GDL conditions ($h_{c,\mathrm{GDL}}=60$ μm):

• In CRFFs: Increasing $\phi_\mathrm{rib}$ (wider ribs) directly reduces performance by sharply increasing under-rib polarization loss. The solid rib acts as a barrier to both oxygen and water; wider ribs expand these dead zones, fostering flooding and inhibition of electrochemical activity.
• In CFRFFs: The effect of rib width is inverted due to the favorable mass transfer characteristics of the MFR. Moderate increases in rib width do not result in substantial oxygen starvation or localized flooding, and, more importantly, increase the dissolved water content in the cathode ionomer. This reduces Ohmic losses, slightly improving power output (a peak power increase of 1.9% for $\phi_\mathrm{rib}=0.58$ vs. $0.33$). The CFRFF hence enables structural robustness and easier assembly without needing ultra-narrow ribs.

Anode GDL Thickness Effects

Reductions in anode GDL thickness ($h_{a,\mathrm{GDL}}$) yield modest but consistent gains for both CRFF and CFRFF, owing largely to lower Ohmic polarization via enhanced proton conductivity and increased dissolved water in the cathode ionomer. However, effects on concentration polarization and oxygen transport are negligible, since the cathode dominates the limiting current regime.

Effects of Cathode Relative Humidity

Performance sensitivity to cathode relative humidity (RHC) displays the expected competition between Ohmic improvement (due to better membrane hydration) at high voltages and worsened mass transfer/flooding (due to excessive liquid water generation) at low voltages. Across all RHC, CFRFF retains superior power density and limiting current output, reinforced by robust mass and water management.

Summary and Implications

This systematic parametric study leads to several robust design recommendations:

• CFRFF architectures inherently outperform CRFFs over the tested GDL thickness, rib width, and humidity regimes, with performance improvements mainly accruable through GDL thinning.
• For CFRFFs, the cathode GDL should be as thin as practical, as both oxygen transport and water management are monotonically improved and not limited by under-rib flooding as in CRFFs.
• Rib width optimization is architecture-dependent: For CRFFs, ribs must be made as narrow as possible; in CFRFFs, moderate rib thicknesses are not detrimental due to the metal foam’s facilitation of two-phase transport.
• Anode GDL thinning offers incremental Ohmic benefits but is non-critical for mass transfer.
• High RHC operation enhances peak performance but may penalize limiting current, necessitating careful control of water management strategies.

The results provide actionable guidance for PEMFC developers, particularly for automotive and stationary applications where operational robustness, manufacturability, and high power density are required. The evidence for strict monotonic performance gains with GDL thinning in CFRFFs, in stark contrast to the non-monotonic trend in CRFFs, constitutes a key finding. This indicates pathway towards ultra-thin, high-conductivity GDL-CFRFF assemblies as a future architecture baseline.

Conclusion

The comprehensive multiphysics modeling confirms that the CFRFF architecture, combined with minimized cathode GDL thickness, leads to superior PEMFC performance by alleviating mass transport limitations and under-rib flooding endemic to CRFFs. The decoupling of optimal rib width and GDL thickness constraints in CFRFFs allows for both performance maximization and practical design flexibility. These insights are expected to influence the next generation of PEMFC stack design, facilitating higher power densities and more efficient water management in real-world applications.