In a proton exchange membrane (PEM) fuel cell, the gas diffusion layer (GDL) sits between the microporous layer and the bipolar plate — an apparently thin component that performs four critical simultaneous functions. It must transport reactant gases (hydrogen at the anode, oxygen at the cathode) uniformly to the catalyst layer. It must conduct electrons from the catalyst layer to the bipolar plate, and back again, with minimum ohmic resistance. It must remove liquid water produced at the cathode without flooding the catalyst layer. And it must transfer heat away from the active area to maintain stable operating temperatures. The GDL does all of this in a layer typically between 150 and 400 micrometres thick.
For engineers specifying GDL materials, this functional density means every structural property of the nonwoven substrate matters. Among these, fiber distribution uniformity is the most consequential — and the most frequently underspecified.
The Role of Fiber Distribution in GDL Performance
The fiber architecture of a carbon nonwoven GDL is not simply structural. It defines the local porosity field, which directly governs where reactant gas travels and where water accumulates. In a non-uniform substrate — one produced by dry-laid carding or air-laying processes — fiber density varies across the plane. Dense zones restrict gas permeability and create higher local resistance. Sparse zones allow gas to channel preferentially, bypassing large portions of the active catalyst area.
These local variations translate directly into current density non-uniformity across the membrane electrode assembly. Areas receiving less reactant operate at lower current density; areas receiving more can saturate. The resulting current distribution hotspots raise local temperatures, accelerate membrane dehydration, and impose higher mechanical stress on the membrane. Studies of degraded PEM stacks consistently trace premature membrane failure to regions of chronic GDL non-uniformity.
Wet-laid nonwoven production addresses this at the process level. In wet-laying, carbon fibers are dispersed in a water slurry at low concentration, then formed into a sheet by draining the suspension through a porous forming wire. The fiber-water system behaves hydrodynamically, and fibers settle into a statistically isotropic, uniform distribution — the same physics that give fine paper its homogeneity. Basis weight variation across a wet-laid carbon paper is typically below 3–5%, compared to 10–15% in comparable dry-laid products. This translates measurably into more uniform local current density and reduced in-situ degradation rates.
Key GDL Specifications
When evaluating a carbon fiber nonwoven for GDL applications, the following parameters define suitability for PEM fuel cell stacks:
- Electrical conductivity: In-plane sheet resistance of 2–10 Ohm/sq is the standard range for fuel cell GDLs. carbon365 materials cover this range with tunable fiber content and formation density.
- Areal weight (basis weight): 10–150 gsm. Fuel cell GDLs typically use 20–60 gsm substrates to minimise mass transport path length and ohmic resistance while maintaining mechanical integrity under bipolar plate clamping.
- Porosity and gas permeability: Open porosity must be sufficient for reactant gas flux at operating current density (typically 1–3 A/cm²) without excessive pressure drop. Pore size distribution is governed by fiber diameter (typically 5–8 µm for PAN-based carbon fiber) and formation density.
- Compression behaviour: Under bipolar plate clamping pressure (typically 0.5–2 MPa), the GDL compresses and pore structure changes. Materials must maintain adequate gas permeability and electrical contact at operating compression without irreversible structural collapse.
- Hydrophobic/hydrophilic balance: GDLs are typically PTFE-treated to achieve a hydrophobic character that prevents flooding while allowing liquid water removal. The base nonwoven must accept PTFE treatment uniformly — a function of fiber surface chemistry and substrate homogeneity.
- EM shielding: The electrical conductivity of carbon365 papers (kHz range EM shielding capability) is a secondary benefit relevant to system integration in stack enclosures.
carbon365 GDL range at a glance: Sheet resistance 2–10 Ohm/sq · Basis weight 10–150 gsm · Fiber types: virgin PAN-based, recycled carbon fiber · Non-flammable · Thermoformable
Wet-Laid vs. Dry-Laid Carbon Fiber Nonwovens for GDLs
| Property | Wet-Laid (carbon365) | Dry-Laid (carded / air-laid) |
|---|---|---|
| Fiber distribution uniformity | Isotropic, CV <5% basis weight | Anisotropic, CV typically 10–15% |
| Minimum achievable basis weight | 10 gsm and below feasible | Typically 50+ gsm for structural integrity |
| Recycled carbon fiber compatibility | Yes — rCF disperses well in water slurry | Limited — short/irregular fibers cause defects |
| Surface smoothness | High — low surface roughness aids microporous layer adhesion | Moderate — lofted surface can cause MPL delamination |
| Fiber length compatibility | Up to ~20 mm staple fiber | 6–60 mm typical; continuous possible |
| Scalability | Roll-to-roll, high throughput | Roll-to-roll, high throughput |
The surface homogeneity advantage of wet-laid substrates becomes particularly important when the GDL is coated with a microporous layer (MPL). An uneven substrate surface creates variable MPL thickness, introducing resistance non-uniformity at the catalyst interface. A flat, uniform wet-laid carbon paper provides a consistent foundation for MPL coating that translates into better and more reproducible stack performance.
From Fuel Cells to Electrolysers
The structural requirements of PEM electrolyser porous transport layers (PTLs) overlap significantly with fuel cell GDLs, though the operating environment differs substantially. At the cathode of a PEM electrolyser, hydrogen evolves under elevated pressure in a reducing environment — conditions where carbon-based PTLs perform well, analogous to the fuel cell anode. Phoenix Non Woven's carbon fiber papers are therefore evaluated for cathode PTL applications in PEM electrolysers and for both anode and cathode layers in anion exchange membrane (AEM) electrolysers, where the oxidising conditions are less aggressive than in PEM anodes.
As green hydrogen production scales — driven by policy targets in the EU, USA, and Asia — PTL supply chains are becoming a critical bottleneck. Wet-laid carbon papers offer a manufacturable, scalable path to meeting this demand, with the added advantage of recycled fiber integration to reduce embodied carbon in the electrolyser itself.
Specify carbon365 for Your GDL or PTL Application
Request technical data sheets, samples, and engineering support for fuel cell and electrolyser development projects.
Explore carbon365 →Frequently Asked Questions
A typical PEM fuel cell gas diffusion layer requires a through-plane electrical resistance low enough to avoid significant ohmic losses, alongside in-plane conductivity for current spreading. carbon365 wet-laid carbon fiber papers achieve sheet resistances in the range of 2–10 Ohm/sq, which corresponds to through-plane resistivities suitable for state-of-the-art PEM stacks operating at high current densities.
Non-uniform fiber distribution in a GDL creates localized regions of high and low gas permeability. Areas with fiber agglomerations restrict reactant access to the catalyst layer, causing local current density hotspots and elevated temperatures. Over time, these thermal and electrochemical stress concentrations accelerate membrane degradation and reduce overall stack lifetime. Wet-laid production creates an isotropic, highly homogeneous fiber mat that eliminates these hotspots and supports uniform current distribution across the active area.
Yes. Phoenix Non Woven's wet-laid process is compatible with both virgin and recycled carbon fibers (rCF). Recycled carbon fiber from aerospace and automotive waste streams can be processed into GDL-grade papers, provided fiber length, surface chemistry, and contamination levels meet specification. Using rCF reduces material cost and supports the circular economy goals increasingly required by OEM procurement policies.
In a PEM fuel cell, the gas diffusion layer (GDL) distributes hydrogen or oxygen gas to the catalyst layer while conducting electrons. In a PEM electrolyser, the equivalent component on the anode side is called a porous transport layer (PTL). The PTL must handle liquid water transport (in) and oxygen gas transport (out) simultaneously under elevated pressure and oxidising conditions. While fuel cell GDLs are typically carbon-based, electrolyser PTLs often use titanium sintered materials or carbon papers with enhanced corrosion resistance. Phoenix Non Woven's carbon fiber papers are relevant primarily for the cathode side of PEM electrolysers and for anion exchange membrane (AEM) systems.
carbon365 wet-laid carbon fiber papers are produced across an areal weight range of 10 to 150 gsm (grams per square metre). Thinner, lower-basis-weight grades in the 20–50 gsm range are typical for fuel cell GDLs, where minimising ohmic resistance and mass transport losses is critical. Higher basis weights provide greater mechanical robustness for applications such as electrolyser PTLs or composite reinforcements.