Membrane electrodes are components that undergo electrochemical reactions and generate electrical energy. In addition to being related to the constituent materials (proton exchange membrane, catalyst, gas diffusion layer and binder), their performance also depends on the technical level of preparation. This article mainly introduces the structure and reaction principle of membrane electrodes, and briefly describes the future development direction of membrane electrode preparation technology based on existing preparation technologies and problems.

The membrane electrode is composed of a 5-layer structure and is the chemical reaction site of the fuel cell.

The structure of the membrane electrode includes a proton exchange membrane, cathode and anode catalysts, cathode and anode gas diffusion layers, (see Figure 1). Generally, a proton exchange membrane coated with cathode and anode catalysts on both sides is called a "three-in-one" membrane electrode, and a membrane electrode including a cathode and anode catalytic layer and a gas diffusion layer on both sides of the proton exchange membrane is called a "three-in-one" membrane electrode. It is a "five-in-one" membrane electrode.

During the operation of the fuel cell, chemical reactions are mainly concentrated at the membrane electrode. Hydrogen reaches the anode through the bipolar plate. It is oxidized under the action of the anode catalyst and releases electrons. Hydrogen ions pass through the proton exchange membrane and reach the cathode. The reaction relationship is: : H2=2H++2e-; At the other end of the battery, oxygen reaches the cathode through the bipolar plate. The oxygen reacts with the hydrogen ions and electrons passing through the proton exchange membrane under the action of the cathode catalyst to generate water. At the same time, The electrons form a current in the external circuit and can output electrical energy to the load. The reaction relationship is: 2H++1/2O2+2e-=H2O.

The second generation membrane electrode technology enhances the linkage efficiency of the catalytic layer and the proton exchange membrane

The first-generation membrane electrode preparation process mainly uses the hot pressing method (see Figure 2). Specifically, the catalyst slurry is coated on the gas diffusion layer to form the anode and cathode catalytic layers, and then it and the proton exchange membrane are heated and pressed. Combined together, the membrane electrode formed is called a "GDE" structure membrane electrode.

The advantage of this technology is that the membrane electrode has good ventilation performance, and the proton exchange membrane is not easily deformed during the preparation process; the disadvantage is that the catalyst is coated on the gas diffusion layer and is easily embedded into the gas diffusion layer through the pores, resulting in a decrease in the utilization rate of the catalyst and heat The adhesive force between the pressure-bonded catalyst layer and the proton exchange membrane is poor, resulting in low overall performance of the membrane electrode.

The second generation of membrane electrode preparation technology is catalyst coated membrane technology (catalystcoatedmembrane, referred to as CCM three-in-one technology) (see Figure 3). Specifically, the catalytic layer is directly coated on both sides of the proton exchange membrane, and then heated and pressed. It is combined with the gas diffusion layer to form a "CCM" structure membrane electrode. This technology improves the utilization rate of the catalyst, and because the core material of the proton exchange membrane is used as a binder, the resistance between the catalytic layer and the proton exchange membrane is reduced, improving the diffusion and movement of hydrogen ions in the catalyst layer, thereby improving Performance is the current mainstream application technology.

The structure is unstable and the process develops towards the integrity and ordering of the catalytic layer

In recent years, with the development of the fuel cell vehicle industry, the industry has put forward increasingly higher requirements for the performance of membrane electrodes. The second-generation membrane electrode preparation method also has the problem of unstable catalytic layer structure during the reaction process, and Pt particles are easy to fall off. problems that affect the service life of membrane electrodes. In response to this phenomenon, major research institutions have combined polymer material technology and nanomaterial technology to develop in the direction of ordering the catalytic layer. The ordered membrane electrode produced has excellent multiphase mass transfer channels, which greatly reduces the cost of the membrane electrode. Medium catalyst Pt loading, and improves the performance and service life of the membrane electrode.

The ordered membrane electrode combined with polymer material technology mainly builds a three-dimensional, ordered and porous inverse opal-like structure in the catalytic layer. This structure has a stronger and more complete catalytic layer than the second generation membrane electrode technology (see figure 4), which can reduce the loss of the number of Pt nanoparticles detached from the matrix during the reaction.

Ordered membrane electrodes combined with nanomaterial technology are mainly divided into TiO₂ nanotube membrane electrodes and carbon nanotube membrane electrodes. The former mainly uses the TiO₂ nanotube array as the carrier of the catalytic layer, which can evenly distribute Pt in the TiO₂ nanotube array and fix more Pt atoms, which has strong stability; the latter is on the cathode of the membrane electrode Carbon nanotubes are used as carriers in the catalytic layer to form an ordered and porous structure of the cathode catalytic layer, which improves the transmission rate of reaction gases, protons, electrons and water. The ordered structure can ensure the continuity of the pore structure and prevent Pt The agglomeration phenomenon of nanoparticles also maintains good electron transfer contact between the catalytic layer and the micropores of the gas diffusion layer, enhances its mass transfer capacity, and greatly improves the performance of the membrane electrode.

In addition to the performance of the three component materials of the proton exchange membrane, catalyst and gas diffusion layer, the performance of the membrane electrode is also related to the technical level of the preparation. It is also one of the main influencing factors. The second-generation membrane electrode process improves the bonding and heat sealing sequence of the three main materials, increases the utilization efficiency of the catalyst, strengthens the linkage between the catalytic layer and the proton exchange membrane, and improves the overall performance. However, there are still some shortcomings in this process. The structure is unstable and the catalytic layer is easy to fall off. Combining polymer material technology or nanomaterial technology to construct an ordered catalytic layer framework is one of the directions for improving membrane electrode preparation technology.

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