The porous transport layer, or PTL, performs several critical functions inside a PEM water electrolyzer. Located between the catalyst-coated membrane (CCM) and the bipolar plate, the anode PTL must conduct electrons, deliver water to the catalyst layer, remove the generated oxygen, and support the membrane electrode assembly under compression.

These functions are closely connected, but they do not always require the same pore structure. Fine pores can improve contact with the catalyst layer, while larger pores generally provide more open pathways for water and oxygen transport. Designing an effective PTL therefore requires more than simply maximizing porosity.

YIANTENG composite PTL sample

The structural trade-off in a single-layer PTL

A PTL with small, uniformly distributed pores creates more contact points with the catalyst layer, improving electrical and mechanical coupling at the interface. This can:

• Reduce interfacial contact resistance

• Improve current distribution

• Increase catalyst utilization

• Distribute compression more evenly

• Reduce local mechanical stress on the membrane

However, making the structure too dense may restrict two-phase transport. At high current density, the PTL must continuously supply water while removing increasing amounts of oxygen. If the transport pathways are too narrow or tortuous, oxygen can accumulate within the PTL, obstruct water delivery to parts of the catalyst layer, and increase mass-transport losses.

A highly open PTL presents the opposite trade-off. Larger pores facilitate water and oxygen movement, but the reduced solid contact area may increase interfacial resistance or create a less uniform mechanical interface with the catalyst layer.

The engineering challenge is therefore not choosing between small and large pores. It is determining where each pore structure should be positioned.

How a composite PTL separates different functions

A composite PTL combines a fine porous titanium layer with a more open titanium mesh. In a typical arrangement, fine porous titanium is positioned close to the catalyst-coated membrane, while titanium mesh is placed between the porous titanium and the bipolar-plate flow field. This creates a graded transport structure rather than relying on one material to perform every function.

Figure 1. Conceptual composite PTL cross-section showing layer positions and transport pathways

Fine porous titanium near the catalyst layer

The layer facing the CCM requires careful control of surface condition, pore size and mechanical properties. Compared with coarse meshes or randomly oriented titanium fibers, fine porous titanium provides a relatively smooth and uniform contact surface, giving:

• More contact points with the catalyst layer

• More uniform electron transfer

• Lower interfacial contact resistance

• More consistent pressure distribution

• Reduced risk of local membrane or catalyst-layer damage

Its interconnected pores still allow water to reach the oxygen-evolution interface and oxygen to move away from the reaction sites. The objective is not to eliminate porosity, but to balance permeability with interface quality.

Titanium mesh near the bipolar plate

The side facing the bipolar plate has different requirements. An open titanium mesh provides larger pathways for water and oxygen, connects the porous titanium layer with the flow field, and spreads mechanical loads across a wider area. It supports:

• Lateral water distribution

• Oxygen transport toward the flow channels

• Mechanical support beneath bipolar-plate ribs

• More uniform compression of the porous titanium layer

Mesh geometry, including wire diameter, opening size, thickness and orientation, must be selected for the operating conditions and flow-field design.

Why transport beneath the bipolar-plate ribs matters

The bipolar plate contains alternating ribs and flow channels. Water can enter the PTL directly beneath the channels, but beneath a solid rib, transport depends on lateral pathways through the PTL.

The rib also compresses the PTL. Excessive or highly localized compression reduces pore volume and changes local permeability, and uneven compression can put non-uniform stress on the CCM. Without an intermediate support structure, several issues may occur beneath the rib:

• Local PTL densification

• Longer lateral water-transport paths

• More difficult oxygen removal

• Non-uniform compression of the catalyst layer

• Differences in local electrical contact

Placing a suitable titanium mesh between the bipolar plate and porous titanium helps distribute the rib load and maintain more open transport pathways.

Figure 2. Qualitative comparison of transport and compression beneath a bipolar-plate rib

The objective is not to prevent compression completely. Controlled compression is necessary for electrical contact and mechanical stability. The goal is to avoid excessive local deformation while maintaining effective water, gas and electron pathways.

Potential advantages of a composite PTL

These advantages depend on correct material selection and structural matching. Adding a mesh does not automatically improve every electrolyzer design.

Key parameters in composite PTL design

A composite PTL should be evaluated as part of the complete cell structure rather than as an isolated component. Important design considerations include:

The mesh should be open enough to support transport, but fine enough to distribute the load without creating new local stress concentrations. Likewise, the porous titanium layer must provide enough contact area without becoming so dense that it restricts water and oxygen movement.

Composite PTL development at YIANTENG

YIANTENG develops porous titanium and titanium-based components for PEM water electrolysis. Our engineering approach considers the relationship between porous structure, surface roughness, mechanical strength, electrical contact, mass transport and stack compression.

Porous titanium thickness, pore structure and dimensions can be adapted to different development and stack requirements, and titanium mesh specifications can be selected to suit the flow field and the intended transport function. Where required, surface treatments and conductive coatings can be applied to improve interface conductivity and long-term stability under PEM anode operating conditions.

Conclusion

The best PTL is not necessarily the one with the highest porosity or the smallest pores. A high-performance PTL must establish reliable contact with the catalyst layer, provide effective pathways for water, oxygen and electrons, and remain mechanically stable under stack compression.

Combining fine porous titanium near the CCM with open titanium mesh near the bipolar plate allows these functions to be handled by different structural layers.

Put the right pore structure in the right position.