GDE-type MEA, also known as gas diffusion electrode, it’s a core component in PEMFC structure which converts chemical energy into electrical energy. The electrode has porous structure to fully connect the catalyst, reaction gas & liquid in three-phase region, remove remove water and reactants generated by the reaction timely, and ensure good proton transmission. The porosity, hydrophobicity, hydrophilicity, electron conductivity, catalyst utilization, and other factors of MEA, which have a significant impact on the final performance of proton exchange membrane fuel cells. In addition, by changing the specifications of electrodes and the types of catalytic coatings, GDE membrane electrodes are also widely used in zinc-air batteries, nickel hydrogen batteries, and the chlor alkali industry, etc.

Preparation method of GDE membrane electrode assembly

GDE-type MEA is the first-generation of membrane electrode preparation technology, directly coat the catalytic layer on GDL which is applied a microporous layer on surface to reduce roughness and porosity, then place the PEM between cathode diffusion layer and anode diffusion layer coated with the catalytic layer, and press both sides of proton exchange membrane via hot pressing process to complete the preparation of membrane electrode assembly, that’s why it’s also known as hot pressing MEA. This production process first appeared on phosphoric acid fuel cells (PAFCs) and was further developed and improved on this basis. The detailed steps are as follows:

First, mix poly tetra fluoroethylene(PTFE) emulsion or Nafion solution, alcohol solvent such as isopropanol, Pt/c catalyst in a certain proportion to form catalyst slurry. Then, the electrocatalyst slurry is evenly coated on the surface of gas diffusion layer by methods of rolling press, brush coating, spraying, screen printing, etc, and porous electrodes are obtained through high- temperature oven drying, sintering or other processes. Finally, the cathode/anode gas diffusion layer covered with catalytic layer is pressed onto both sides of PEM by hot pressing technology, and the GDE membrane electrode is finally prepared by hot pressing.

Features of gas diffusion electrode for PEMFC

Advantages: The preparation process of GDE gas diffusion electrodes is already very mature, which can meet the commercial customization requirements with large quantities and multiple sizes. CL coating on gas diffusion electrode helps to form higher porosity and improve reaction speed, and ensure that proton exchange membrane will not deform. Generally, teflon is used as a binder, which has good hydrophobic properties and can quickly discharge reactants to ensure smooth current transport.

Disadvantages: However, during the production process of GDE-MEA, catalytic slurry can easily penetrate into the gas diffusion layer from the pores, result in catalyst loss and low utilization efficiency. and a thicker catalyst layer also increases the resistance of proton conduction. On the other hand, in order to ensure that no damage occus on PEM during the hot pressing process, a thicker PEM is usually chosen, which also increases the internal resistance of the whole membrane electrode. There is also a structural defect in this manufacturing process. Due to differences in expansion coefficients, stratification between CL and membrane is more likely to occur, which not only increases contact resistance but also shortens cell life.

Process technology of GDE hot pressing MEA

In the process of manufacturing hot-pressing membrane electrode assembly, there are many factors that affect performance including pressure, time and temperature. The temperature will affect the proton transfer channel between CL and PEM, and the proton exchange membrane will become a glassy state at a certain hot-pressing temperature, and tightly bond with the softened catalytic layer, furture to reduce contact resistance and ensure smooth proton transfer. On the contrary, when the hot pressing temperature is too high, it will destroy the active groups in Nation membrane, which is not conducive to electron conduction. Choosing an appropriate hot-pressing time is also important for the performance of MEA. Too short time will cause loose combination between CL and PEM, and too long time will also damage the structure of the catalytic layer and the active groups in PEM. The impact of hot-pressing pressure is mainly reflected in two aspects: for one thing, insufficient pressure leads to weak contact pressure between layers in MEA, resulting in excessive impedance that is not conducive to electron transmission. On the other hand, as a place for solid-liquid-gas triple-phase reactions, the catalytic layer must ensure a high porosity to facilitate the transport of substances such as gas and water. Excessive pressure may cause structural damage and reduce performance. For the gas diffusion layer, fragile materials such as carbon fibers cannot withstand too much pressure, which may cause massive fiber breakage and hinder proton conduction.

In addition, a stable and uniform coating technology for catalysts is also crucial, for instance, use ultrasonic spraying technology to improve availability and reduce costs. Generally speaking, it is necessary to design differentiated GDE structure for specific application, and precise argumentation and optimization are required for the main parameters of electrode such as porosity, hydrophilicity, hydrophobicity, etc. The subdivision for catalysts based on different application fields is also a essential condition for preparing high-quality GDE-type MEAs.