The membrane-electrode assembly, also known as an MEA (Membrane Electrode Assembly), is the electrochemical heart of PEM fuel cells and many types of electrolysers. It consists of a proton-conducting membrane coated on both sides with catalyst layers and gas diffusion layers.
The MEA plays a key role in determining the performance, efficiency, and service life of PEM fuel cells and electrolysers. Its function depends on the specific application: In electrolysers, it enables the splitting of water into oxygen and hydrogen; in fuel cells, the MEA converts hydrogen into electricity and heat.
Who uses MEA?
Although MEAs are a defining feature of PEM (proton exchange membrane) technologies, they are also used in other types of electrolysers and fuel cells. Here is an overview of the most important variants:
- PEM electrolysers and PEM fuel cells: MEAs are best known and most widely used in these technologies.
- Anion-exchange membrane (AEM) electrolysers and fuel cells: These also use MEAs, but with a membrane that conducts hydroxide ions instead of protons.
- Solid-oxide electrolyzers (SOEC) and solid-oxide fuel cells (SOFC): In this context, the term “MEA” is often used, even though the structure is somewhat different. The “membrane” here is a solid electrolyte, usually made of ceramic.
- Direct methanol fuel cells (DMFC): These use MEAs that are specifically optimized for the use of methanol as a fuel.
Alkaline electrolyzers (AELs), which are among the oldest and most widely used electrolyzer technologies, traditionally do not use MEAs in the strict sense of the term. Instead, they use a liquid electrolyte and separate electrodes. However, developments are also underway in this area toward MEA-like structures in order to improve efficiency.
Structure
The exact structure and materials of MEAs can vary depending on the technology. However, the fundamental purpose—the spatial organization of electrochemical reactions—remains the same in all cases. This diversity allows manufacturers to select the best configuration for each specific application, whether for cost-effective solutions with low power requirements, standard applications with a balanced performance-to-cost ratio, or high-performance applications that require maximum efficiency and longevity.
- Proton-conducting membrane (PEM): A thin polymer film in the center of the MEA that separates the anode from the cathode. It allows protons to pass through but blocks electrons and gases. It is usually made ofNafion® or other polymers.
- Catalyst layers: Finely dispersed precious metal particles (often platinum or iridium alloys) on both sides of the membrane. As the anode and cathode, respectively, they form the active part of the electrode. This is where the electrochemical reaction takes place.
- Subgaskets: Serve as a seal and provide structural stability.
- Gas Diffusion Layers (GDL): Porous, electrically conductive layers, often made of carbon cloth or carbon fiber paper. The GDL forms the “backbone” of the electrode and serves several important functions, such as uniform gas distribution and transport, water drainage, and electrical conductivity. Ensures uniform gas distribution and water drainage
How It Works
In the MEA of an electrolyzer, water is split into hydrogen and oxygen. When an electric current flows through the MEA, water is split into oxygen, protons (hydrogen ions), and electrons on the anode side. The protons migrate through the membrane to the cathode side, while the electrons flow through an external circuit. At the cathode, the protons combine with the electrons to form hydrogen gas. The catalyst on both sides of the membrane accelerates these reactions. The result is pure oxygen on one side of the MEA and pure hydrogen on the other.
- Anode: 2 H₂O → O₂ + 4 H⁺ + 4 e⁻
- Cathode: 4 H⁺ + 4 e⁻ → 2 H₂
- Overall reaction: 2 H₂O → 2 H₂ + O₂
- Anode: 2 H₂ → 4 H⁺ + 4 e⁻
- Cathode: O₂ + 4 H⁺ + 4 e⁻ → 2 H₂O
- Overall reaction: 2 H₂ + O₂ → 2 H₂O
MEA production at Hydrogenea in Bochum (© Hydrogenea)
Production
The production of an MEA is a highly complex process that requires the utmost precision. Generally speaking, a distinction can be made between manual sheet-to-sheet production and automated roll-to-roll (R2R) production. Here is an overview of the most important steps:
- Manufacturing the PEM from a polymer.
- Coating with a catalyst using various methods (slot-die coating, screen printing, inkjet printing)
- Drying of the applied catalyst layers using convection dryers, infrared dryers, or lasers.
- Applying the subgaskets by hot pressing.
- Add the GDL, which is also applied by pressing or gluing.
- Cutting and sizing the MEA to the correct dimensions, usually using laser or mechanical cutting methods.
Depending on the manufacturing method, the catalyst layer can be applied directly to the membrane (CCM—Catalyst-Coated Membrane) or to the GDL (CCG—Catalyst-Coated GDL). In both cases, the combination of the catalyst layer and the GDL forms the complete electrode.
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