Anion exchange membrane water electrolysis (AEMWE) is an emerging hydrogen production technology that combines some of the advantages of conventional alkaline electrolysis and proton exchange membrane (PEM) electrolysis. AEMWE can potentially operate with low-cost, non-platinum-group-metal catalysts while using a compact zero-gap cell architecture.
However, achieving high performance and long-term durability requires more than simply selecting an appropriate catalyst and membrane. The porous transport layer (PTL) is another critical component of the AEMWE membrane-electrode assembly (MEA).
The PTL provides electrical contact to the catalyst layer while facilitating water transport, hydrogen and oxygen removal, mechanical support, and heat transfer. Its structure and surface properties can therefore have a significant influence on AEMWE cell performance.
This article discusses the function of PTLs in AEM water electrolysis, commonly used materials, important design parameters, and the differences between nickel foam, nickel felt, and other PTL configurations.
1. What Is a PTL in AEM Water Electrolysis?
A porous transport layer is a porous, electrically conductive material positioned between the catalyst layer and the bipolar plate or current collector.
A simplified AEMWE cell can be represented as:
Bipolar Plate → PTL → Catalyst Layer → AEM → Catalyst Layer → PTL → Bipolar Plate
Depending on the cell design, the PTL may also be integrated with the catalyst layer to form a catalyst-coated porous electrode.
The PTL is sometimes referred to as a porous transport electrode (PTE), particularly when the catalyst layer is directly deposited onto the porous substrate.
The terminology can vary between research groups and manufacturers, but the underlying functions are similar.
2. Why Is the PTL Important?
The PTL performs several functions simultaneously.
2.1 Electrical conduction
The catalyst layer needs an efficient electronic pathway to the external circuit.
During electrolysis:
- Electrons are supplied to the cathode;
- Electrons are removed from the anode;
- The PTL provides an electrically conductive pathway between the catalyst layer and the current collector.
A PTL with high electrical resistance can therefore increase ohmic losses.
2.2 Water transport
At the anode, the oxygen evolution reaction (OER) takes place. Under alkaline conditions, the reaction can be represented as:
4OH⁻ → O₂ + 2H₂O + 4e⁻
The PTL must facilitate electrolyte transport to the catalyst layer and provide pathways for oxygen bubbles to escape toward the flow channels. The exact water and electrolyte transport requirements depend on the cell architecture and operating conditions.
2.3 Gas removal
Electrolysis produces gaseous hydrogen and oxygen.
The PTL must provide pathways for the generated gases to leave the catalyst layer and reach the flow channels.
Poor gas transport can result in:
- Gas accumulation;
- Increased mass-transfer resistance;
- Non-uniform current distribution;
- Increased cell voltage.
At higher current densities, gas transport becomes increasingly important.
2.4 Mechanical support
The PTL also helps support the catalyst layer and membrane during cell assembly and operation.
This is particularly important in a compressed zero-gap architecture.
The PTL must withstand:
- Assembly pressure;
- Differential pressure;
- Thermal cycling;
- Hydration/dehydration.
2.5 Heat transport
Electrolysis generates heat, and temperature gradients can negatively affect cell operation.
A conductive metallic PTL can contribute to heat transfer between the catalyst layer and the bipolar plate.
3. Typical PTL Materials for AEMWE
Unlike PEM electrolyzers, where titanium-based porous transport layers are commonly used on the anode because of the highly oxidizing environment, AEMWE offers more flexibility in material selection.
Common PTL materials include:
- Nickel foam;
- Nickel felt;
- Stainless steel-based materials;
- Titanium fiber/felt;
- Carbon-based materials for cathode side.
Among these, nickel-based PTLs are particularly attractive for AEMWE, especially when the system is designed around non-PGM catalysts.
4. Nickel Foam as an AEMWE PTL
Nickel foam is a three-dimensional porous metallic structure consisting of interconnected nickel ligaments. Its open structure provides a high surface area and interconnected pathways for fluid transport.
Advantages
Nickel foam can provide:
- High electrical conductivity;
- High porosity;
- Good water transport;
- Efficient gas removal;
- Relatively large accessible surface area;
- Good mechanical strength.
Nickel foam is particularly attractive when a catalyst is deposited directly onto the porous structure.
For example:
NiFeOx catalyst / Ni foam
can function as an integrated anode electrode.
This approach can reduce the number of interfaces between the catalyst layer and current collector. Depending on the foam structure, nickel foam can be compressed to different thicknesses, depending on the material properties and cell design, with corresponding changes in porosity.
5. Nickel Felt as an AEMWE PTL
Nickel felt is typically composed of interconnected nickel fibers.
Compared with conventional nickel foam, nickel felt can offer a different combination of:
- Fiber diameter;
- Pore size;
- Porosity;
- Compressibility;
- Thickness;
- Surface area.
Nickel felt can be particularly attractive for catalyst-coated electrodes because the fibrous structure provides numerous contact points and interconnected transport pathways.
For example:
NiFeOx catalyst / Ni felt
can be used as an AEMWE anode.
The flexibility and compressibility of nickel felt can also help accommodate variations in cell compression.
Note that nickel felt is generally much more expensive material than nickel foam.
6. What Is an MPL and Why Add It to a PTL?
A microporous layer (MPL) is a fine porous layer applied to the surface of a porous transport substrate.
For example:
Nickel felt → MPL → catalyst layer
or
Nickel foam → MPL → catalyst layer
The purpose of an MPL is to modify the interface between the relatively coarse PTL and the catalyst layer.
A conventional PTL may have relatively large pores. If the catalyst layer is deposited directly onto it, the catalyst ink can penetrate deeply into the porous structure.
This can lead to:
- Non-uniform catalyst distribution;
- Catalyst loss into the substrate;
- Poor catalyst utilization;
- Increased catalyst-layer thickness;
- Inconsistent electrode fabrication.
An MPL provides a finer interfacial structure that can help control catalyst penetration. Additionally, the lower surface roughness of an MPL can increase contact area and reduce the interfacial resistance, leading to a higher performance.
7. Catalyst-Coated PTLs
One important AEMWE electrode configuration is to directly coat the catalyst onto the PTL.
For example:
NiFeOx / Ni felt or NiFeOx / Ni foam or NiFeOx / Ni MPL
The resulting structure can serve as a catalyst-coated porous electrode.
This approach offers several advantages.
Reduced interface resistance: The catalyst is directly connected to the conductive PTL.
Simplified MEA fabrication: Instead of coating the membrane, the catalyst can be deposited onto the porous electrode.
Flexibility in catalyst selection: Different catalyst formulations can be screened without changing the membrane.
This is particularly useful for R&D applications.
8. PTL and Catalyst Layer Interaction
PTL performance cannot be considered independently from the catalyst layer.
For example, suppose a catalyst ink is deposited onto nickel felt.
The ink contains:
- Catalyst particles;
- Ionomer or binder;
- Solvent.
During drying, the ink can penetrate into the PTL. If penetration is excessive, some catalyst may become buried inside the PTL and become difficult to access electrochemically. If penetration is insufficient, the catalyst may form a layer with poor mechanical adhesion.
Therefore, the relationship between:
PTL pore structure ↔ MPL ↔ catalyst particle size ↔ ionomer content ↔ ink rheology
is important.
This is one reason why changing the PTL can significantly change the measured AEMWE performance even when the catalyst formulation remains unchanged.
9. ElectroHy AEM PTLs
ElectroHy provides materials and components for AEM water electrolysis research and development, supporting applications from catalyst screening to MEA, single-cell, and stack testing.
Our AEM PTLs include materials and components for both cathode and anode development, including:
- Nickel felt;
- Nickel felt with a microporous layer (MPL);
- Nickel foam;
- Nickel foam with an MPL;
- Nickel microporous layer (MPL);
- Titanium felt;
- Titanium felt with a microporous layer (MPL);
- Titanium microporous layer (MPL).
These options allow researchers and engineers to investigate different porous structures and interfacial configurations when developing and optimizing AEM electrolyzer cells.
10. Summary
The porous transport layer is a critical but sometimes overlooked component of an AEM water electrolyzer.
A properly designed PTL must simultaneously provide:
- Electrical conduction;
- Water transport;
- Gas removal;
- Catalyst support;
- Mechanical stability;
- Heat transport.
Nickel foam and nickel felt are particularly attractive for AEMWE because they combine good electrical conductivity with porous structures suitable for water and gas transport. Adding a microporous layer can further improve the interface between the PTL and catalyst layer by controlling catalyst penetration and providing a more uniform coating surface.
For demanding AEMWE applications, PTL design should not be treated as a standalone material-selection problem. The membrane, catalyst layer, ionomer, PTL, compression, flow field, and operating conditions form an integrated system.
Ultimately, the best PTL is the one that provides the appropriate balance of transport, electrical, mechanical, and electrochemical properties for the specific AEMWE architecture.
Explore ElectroHy's AEM electrolysis materials and components for your next hydrogen research and development project.
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