Anion exchange membrane water electrolysis (AEMWE) is emerging as a promising technology for producing green hydrogen with reduced dependence on expensive platinum-group metals (PGMs). One of the key components that determines the efficiency, durability, and operating stability of an AEM electrolyzer is the anode.
The AEM electrolyzer anode is where the oxygen evolution reaction (OER) takes place. Unlike the cathode, where hydrogen is generated, the anode must operate under strongly oxidizing conditions and therefore places demanding requirements on catalyst activity, electrode structure, ion transport, and long-term chemical stability.
This article provides an overview of AEM anodes, including their function, catalyst materials, electrode architecture, fabrication methods, and performance characteristics.
1. What Is an AEM Electrolyzer Anode?
In an AEM electrolyzer, hydroxide ions are oxidized to produce oxygen and water, releasing electrons into the external circuit.
The overall anodic reaction is:
4OH⁻ → O₂ + 2H₂O + 4e⁻
The generated electrons travel through the external electrical circuit to the cathode, where hydrogen evolution takes place:
4H₂O + 4e⁻ → 2H₂ + 4OH⁻
The hydroxide ions generated at the cathode migrate through the anion exchange membrane toward the anode.
The overall reaction remains:
2H₂O → 2H₂ + O₂
The anode therefore has several simultaneous functions:
- Provide active sites for the oxygen evolution reaction
- Transport hydroxide ions to the catalyst surface
- Transport electrons away from the catalyst
- Facilitate oxygen and water transport
- Maintain electrical contact with the current collector
- Provide sufficient mechanical and chemical stability during long-term operation
A well-designed AEM anode must balance all of these functions.
2. OER Catalysts for AEM Electrolysis
One of the major advantages of AEM electrolysis is to use non-PGM catalysts, especially as compared to iridium used in PEMWE.
Common catalyst families include nickel-, iron-, cobalt-, and manganese-based materials, such as NiFeOx, CoOx, NiFeCoOx etc. All of these catalysts use earth abundant materials that are much less costly and unlikely to cause price run-up as AEMWE scales up.
Iron incorporation into nickel-based catalysts can substantially improve OER activity. NiFeOx materials are therefore particularly attractive for AEM electrolyzers.
3. What Is an AEM Anode Made Of?
Depending on how anode is made, there are roughly two types of anode electrodes, CCM and CCS.
CCM: the catalyst layer is deposited directly onto the membrane, creating a catalyst-coated membrane (CCM) architecture. This is typically done by coating membrane with catalyst ink (a mixture of catalyst, binder and solvent) using well-established coating methods, such as spray coating, doctor blade coating, Meyer bar coating or slot-die coating.
CCS: the catalyst is deposited onto a substrate to form an electrode that is subsequently assembled with the membrane. There are typically two ways of making a CCS anode. The first one is by coating a substrate with catalyst ink (a mixture of catalyst, binder and solvent) using well-established coating methods, such as spray coating, doctor blade coating, Meyer bar coating or slot-die coating. The second way is to grow catalysts on the substrate using wet chemistry, such as hydrothermal methods. CCS anode is also called GDE (gas diffusion electrode) since the substrate also acts as gas diffusion layer.
For AEMWE, the electrode substrate is mostly nickel-based, such as Ni foam, Ni fiber felt, Ni microporous layer (MPL) or well-engineered combination of those features, such as Ni foam with MPL, Ni felt with MPL. The main difference between them is surface roughness and porosity. MPL layer has the lowest surface roughness and preferably used to be in contact with membrane so that the interfacial resistance between electrode and membrane is lower, resulting to a better performance and longer durability. Porosity of all the substrate can be tuned to some extent. Interestingly, porosity can be engineered in a way that a porosity gradient may be created to enhance the performance of AEMWE, such as Ni foam with MPL and Ni felt with MPL that ElectroHy offers.
4. Binder in anode electrode
Typically, AEM water electrolyzers (AEMWEs) operate with a KOH solution circulating through the anode side. Depending on the system design, the cathode side may also have KOH solution circulating, or it may operate with water or another electrolyte configuration.
Because the anode is continuously supplied with a liquid electrolyte, an anion-conducting ionomer is not necessarily required to provide ionic transport within the electrode. For example, anodes fabricated using hydrothermal wet-chemistry methods are prepared without an ionomer or polymeric binder.
For anodes fabricated by first preparing a catalyst ink and subsequently coating it onto a substrate, however, a binder is commonly incorporated into the ink. Fluoropolymers such as PTFE and PVDF are frequently used for this purpose. Unlike an ionomer, the primary function of these binders is not to provide anion conductivity, but to mechanically bind the catalyst particles together and improve the integrity and adhesion of the catalyst layer.
The binder helps maintain the structural integrity of the catalyst layer under continuous electrolyte flooding and gas evolution. It can also improve adhesion between the catalyst layer and the underlying substrate, such as a porous transport layer (PTL), electrode substrate, or current collector. At the same time, the catalyst layer must maintain sufficient electronic conductivity and effective contact with both the membrane and the PTL/current collector to enable efficient electrochemical operation.
Therefore, the amount and type of binder must be carefully optimized. Excessive binder can improve mechanical stability but may block catalyst active sites, reduce electrical conductivity, impede electrolyte transport, or increase mass-transfer resistance. Insufficient binder, on the other hand, can result in poor adhesion and catalyst-layer instability during operation.
5. Key Performance Parameters
When evaluating an AEM anode, several parameters should be considered.
Overpotential
Overpotential describes the additional potential required beyond the thermodynamic potential to drive OER at a given current density.
Lower overpotential generally indicates better OER kinetics.
Current Density
AEM electrolyzers are increasingly being developed toward higher current-density operation.
An electrode that performs well at low current density may not maintain the same performance at commercially relevant current densities because water and gas transport become increasingly important.
Durability
Durability is particularly important for commercial electrolyzers.
A practical AEM anode must maintain:
- Catalyst activity
- Mechanical integrity
- Low interfacial resistance
during extended operation.
6. AEM Anode vs. PEM Anode
One of the most significant differences between AEM and PEM electrolysis is the catalyst system. PEM electrolyzers commonly rely on highly active and corrosion-resistant PGM-based anode catalysts such as iridium oxides because of the strongly oxidizing acidic environment. AEM electrolyzers operate in an alkaline environment, enabling the use of more abundant transition-metal catalysts.
| Parameter | AEM Anode | PEM Anode |
|---|---|---|
| Electrolyte environment | Alkaline | Acidic |
| Typical OER catalysts | NiFe-based and other transition-metal catalysts | IrO₂ / Ir-based catalysts |
| Common PTL materials | Nickel-based structures | Titanium-based structures with PGM coating |
| Key challenge | Validated durability in commercial stacks | Iridium utilization, cost, and durability |
The different chemical environments fundamentally change the materials requirements for the anode.
7. ElectroHy AEM Anode Solutions
ElectroHy provides materials and components for AEM water electrolysis development, including AEM anode catalyst materials and electrode components designed for research, development, and electrolyzer testing.
Our AEM electrolysis materials can support different stages of development—from catalyst screening and electrode fabrication to single-cell and stack testing.
Key areas include:
- OER nanopowder catalysts, such as NiFeOx, CoOx, NiFeCoOx, NiFeCeOx
- NiFeOx GDE anode of various substrates
- Anion exchange membrane of various thickness with or without reinforcement
- A variety of Nickel porous transport layers
- AEM 5-in-1MEAs
- AEM electrolyzer stacks and test hardware
- Custom AEM membrane
- Custom anode electrode
- Custom stacks
By combining catalyst materials, membranes, electrodes, and electrolyzer components, ElectroHy aims to provide researchers and engineers with practical materials for accelerating AEM electrolyzer development.
8. Conclusion
NiFe-based catalysts and other transition-metal OER materials provide an attractive pathway toward PGM-low or PGM-free AEM electrolysis.
As AEM electrolyzers move toward higher current densities and longer operating lifetimes, the future development of AEM anodes will therefore depend not only on discovering more active catalysts, but also on designing durable, manufacturable electrode structures.
Explore ElectroHy's AEM electrolysis materials and components to support your next electrolyzer research and development project.
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