Role of Porous Transport Layers in Water Electrolyzers
Porous transport layers (PTLs), often referred to as gas diffusion layers or liquid/gas diffusion layers, are crucial components in water electrolyzer stacks. As shown in the figure below, PTLs facilitate the delivery of liquid reactants from the flow plate channels to the catalyst layer (①) while also allowing for the efficient removal of gaseous products—oxygen at the anode and hydrogen at the cathode (②). This process creates a countercurrent two-phase flow through the PTL. If the product gasses are not effectively removed from the system, H2 and O2 can clog the pores of the PTL. This blockage not only decreases the availability of liquid reactants for the reaction but also leads to membrane dehydration, which negatively impacts its ionic conductivity. This is why the porosity of PTLs is among the considerations, as high porosity of PTLs ensures sufficient pathways for liquid reactant transport and efficient gas removal. However, we will see that high porosity is not always the perfect solution, as it can sometimes lead to reduced mechanical strength or other performance issues. But that is a topic for another discussion—see Understanding Porosity in Gas Diffusion Layers for Fuel Cells and Electrolyzers for more details.
PTLs also facilitate electron transport between the catalyst layer and the bipolar plate (③). To ensure efficient charge transfer, the PTL must establish a strong electrical connection between these two components. This connection enables electrons to flow to the cathode, where they are used to produce H2. Lastly, PTLs aid in heat dissipation during operation, which is critical for preventing hotspot formation. These hot spots can damage or even destroy the membrane, particularly when operating at high current densities or elevated temperatures.
To achieve optimal performance, a PTL with properties that balance gas/water transport and electrical conduction must be used.
Another key factor to consider for PTLs is their stability. This depends on two main aspects: first, whether the PTL will be used on the cathode or anode side of the electrolyzer, and second, the operating pH of the stack—acidic for proton exchange membranes or basic for alkaline and alkaline anion exchange membrane electrolyzers.
Why does the side at which the PTL is used matter?
The placement of the PTL is important because the anode and cathode sides operate under different conditions and environments. Each side experiences distinct chemical reactions that do not only affect the performance but also the stability of the PTL.
By convention, the cathode is the site where reduction reactions occur, and in water electrolyzers, this is where the hydrogen evolution reaction (HER) takes place, generating H₂ gas. As a result, the cathode side operates under a reducing environment, which minimizes the likelihood of oxidation reactions. Materials on the cathode side are less prone to corrosion because they are not exposed to harsh oxidative conditions. Since oxidation is not a major concern, carbon materials are considered as the benchmark PTL for the cathode side.
The situation is different on the anode side, where oxidation reactions take place. As a consequence, materials used at the anode must withstand aggressive oxidative environments. To make matters worse, in water electrolyzers, the oxygen evolution reaction (OER) occurs at the anode, generating O₂ gas. Regardless of the reaction medium, OER is a sluggish reaction that requires very high potentials to proceed at a practical scale. As a result, high potentials of at least 2 V are often necessary applied on the anode side. For these reasons, carbon PTLs are not a viable choice at the anode side because carbon degrades and oxidizes at high potentials.
The best materials for anode PTLs are those that can resist corrosion and withstand high potentials. Metals like titanium, nickel, and stainless steel stand out as the benchmark choices because of their high corrosion resistance.
Why does the operating pH matter?
We’ve established that carbon is the preferred material for the cathode, while metal-based PTLs are ideal for the anode in water electrolyzers. Another important factor when choosing the right PTL is the operating pH of the electrolyzer. Why is this critical? Because certain materials are more stable in acidic environments, while others perform better in alkaline conditions. This is particularly crucial for the anode side, where metal PTLs are used.
A handy tool for determining metal stability across different voltages and pH levels is the Pourbaix diagram, also known as a potential–pH diagram. These diagrams show whether a metal will corrode, stay passive, or remain stable under specific electrochemical conditions. What does corrode stay passive, or remain stable mean in the context of Pourbaix diagrams?
Corrosion Region: In this region, the metal is oxidized and dissolves into the electrolyte, meaning it is losing material and degrading. In this region of the Pourbaix diagram, the metal is thermodynamically unstable.
Passivation Region: Under these conditions, the metal forms an oxide or a hydroxide film or layer on its surface. This formed “passivation layer” prevents the metallic surface from being exposed to oxygen, protecting it from corrosion. Consequently, in this region, the metal does not degrade nor dissolve into the electrolyte as long as the passivation layer remains intact.
Immunity Region: In this region, there is no noticeable attack on the metal. The metal does not corrode nor form oxides. It remains unchanged even after prolonged exposure, maintaining its original properties. This indicates that the metal is at its most stable in this region.
For the next discussion, keep in mind that the operating pH for PEM water electrolyzers is 0 to 2, while that for anion exchange membrane electrolyzers ranges from 10 to 14. In both types, the anode operates at potentials between 1.3 and 2 V.
Looking at the Pourbaix diagram of titanium, you can see that at Point A (low pH = 2 and high potential = 1.8 V), titanium is in the passivation region, forming an oxide layer, TiO₂. This means that titanium would be stable as an anode PTL in PEM systems because this oxide layer helps stop further corrosion or damage.
As we move from Point A (pH = 2) to Point B (pH = 12) on the Pourbaix diagram, we see that titanium is no longer in the passivation region but instead in a corrosion region, where it forms TiO₃ + 2 H₂O. This indicates that titanium is more reactive and less stable under high pH conditions, like those in AEM water electrolyzers. As such, titanium is less suitable as an anode PTL for AEMWE systems.
So, what can we now use as an anode PTL for AEM water electrolyzers? Fortunately, there are alternative materials that perform better in the high pH conditions of AEM systems. Nickel and nickel-based alloys, for instance, are commonly used due to their excellent stability in alkaline environments and resistance to corrosion. As you can see in the nickel–H2O system Pourbaix diagram, nickel forms a protective oxide layer in these conditions, much like titanium does in acidic environments.
Can we use nickel as the anode PTL for PEM water electrolyzers? Short answer is, it is not recommended. Nickel is not stable in the acidic environments of PEMWE. As seen in the Pourbaix diagram, nickel corrodes at low pH (0–2), 1.8 V.
Summary of the Corrosion Behavior of Carbon, Titanium, and Nickel in different Environments
| Point | pH | Potential [V] | Carbon | Titanium | Nickel |
| A | 0 | 0 | I | P | C |
| B | 0 | 1 | C | P | C |
| C | 0 | 1.5 | C | P | C |
| D | 7 | –0.3 | I | P | C |
| E | 7 | 0.5 | C | P | C |
| F | 10 | 0.5 | C | P | P |
| G | 14 | –0.8 | I | P | I |
| H | 14 | 1.8 | C | C | P |
Porous Transport Layers available at CAPLINQ
At CAPLINQ, we offer a wide range of materials designed as porous transport layers in water electrolyzers:
LINQCELL Graphitized Carbon Fiber Papers| Ideal for use as the cathode in both PEM and AEM water electrolyzers
LINQCELL Sintered Titanium Fiber Felt and Powder| Benchmark Anode PTL for PEM water electrolyzers
LINQCELL Sintered Nickel Fiber Felt and Nickel Wire Mesh| Benchmark Anode PTL for AEM water electrolyzers
LINQCELL Sintered Stainless Steel Fiber Felt| Inexpensive alternative to nickel and titanium PTL
We are also proud to represent Ionomr’s membranes, which are known for their durability and high performance in water electrolyzers and fuel cells.
- AEMION+™-AF3-HWC9-70 (TDS)| For AEM Water Electrolyzers and Fuel Cells
70 micron thick membrane with woven PEEK reinforcement
Recommended Operating Condition: 1 M KOH, 70 °C - AEMION+™-AF3-HWK9-75 (TDS)| For AEM Water Electrolyzers and Fuel Cells
75 micron thick membrane with woven PEEK reinforcement
Recommended Operating Conditions: 0.1–2 M KOH, ≤90 °C
For guide on how to select the right ion exchange membrane for your water electrolyzers, see Choosing the Right Ion Exchange Membrane for Your Water Electrolyzer and Fuel Cell Applications.
For more information or assistance in selecting the right porous transport layer or membrane for your water electrolyzer application, feel free to contact us at CAPLINQ. Our team is here to help guide you through your options and ensure you get the perfect materials for your needs.
