Preparation technology of membrane electrode for PEM fuel cell

Preparation technology of membrane electrode for PEM fuel cell – Cheersonic

Membrane electrodes are the site of heterogeneous material transport and electrochemical reactions, which determine the performance, lifetime and cost of proton exchange membrane fuel cells. The membrane electrode and the bipolar plates on both sides constitute the basic unit of the fuel cell—the single fuel cell. In practical applications, multiple single cells can be combined into a fuel cell stack according to the needs of the design to meet the needs of different power outputs.

The design and optimization of the MEA structure, the selection of materials and the optimization of the fabrication process have always been the technical keys of PEMFC research. In the development process of PEMFC, membrane electrode technology has undergone several generations of innovation, which can be roughly divided into three types: hot pressing method, CCM method and ordered membrane electrode. The advantages and disadvantages of the three types of MEA and the latest research progress will be analyzed and introduced below.

1. GDE hot pressing membrane electrode

The first generation of MEA preparation technology is to use the hot pressing method to press the CL-coated cathode and anode GDL on both sides of the PEM to obtain the MEA, which is called the “GDE” structure.

The preparation process of GDE-type MEA is relatively simple. Since the catalyst is coated on the GDL, it is conducive to the formation of pores in the MEA, and at the same time, it can protect the PEM from deformation. However, the amount of catalyst coated on GDL in the preparation process of GDE-type MEA is not well controlled, and the catalyst slurry easily penetrates into GDL, which causes some catalysts to fail to function fully, and the utilization rate is even lower than 20%, which increases MEA. the cost of. In addition, since the expansion system of the catalyst-coated GDL is different from that of the PEM, during the long-term operation of the fuel cell, it is easy to cause the interface between the two to be partially peeled off, resulting in an increase in the internal contact resistance of the fuel cell, and the comprehensive performance of the MEA is not enough. ideal. At present, the preparation process of GDE structure MEA has been rarely used and has been basically eliminated.

2. CCM three-in-one membrane electrode

Using roll-to-roll direct coating, screen printing, spraying and other methods, the slurry composed of catalyst, Nafion and appropriate dispersant is directly coated on both sides of the proton exchange membrane to obtain MEA.

Compared with the GDE type MEA preparation method, the CCM type is better, not easy to peel off, and at the same time reduces the transfer resistance between the catalyst layer and the PEM, which is conducive to improving the diffusion and movement of protons in the catalyst layer, thereby promoting the catalytic layer and PEM. The contact and transfer of protons between them reduces the resistance of proton transfer, which greatly improves the performance of MEA. The research on MEA has shifted from GDE type to CCM type. In addition, the overall cost of the MEA is reduced because the Pt loading of the CCM type MEA is relatively low and the utilization rate is greatly improved. The disadvantage of CCM type MEA is that it is prone to “water flooding” during the operation of the fuel cell. The main reason is that there is no hydrophobic agent in the catalytic layer of the MEA, there are fewer gas channels, and the gas and water transmission resistance is large. Therefore, in order to reduce the gas and water transmission resistance, the thickness of the catalyst layer is generally not more than 10 μm.

Due to the good comprehensive performance of CCM-type MEA, it has been commercialized in the field of automotive fuel cells. For example, Toyota Mirai, Honda Clarity and so on. The CCM-type MEA developed by Wuhan University of Technology in China has been exported to Plug Power in the United States for use in fuel cell forklifts. The CCM-type MEA developed by Dalian Xinyuan Power has been applied to the truck, and the loading capacity of Pt-based precious metals is as low as 0.4mgPt/cm2. The power density reaches 0.96W/cm2. At the same time, Kunshan Sunlight, Wuhan Himalaya, Suzhou Qingdong, Shanghai Jiaotong University, Dalian Institute of Chemical Physics and other enterprises and colleges and universities are also developing high-performance CCM-type MEA. Foreign companies such as Chemours, Gore, and Ballard have realized the development and commercialization of CCM-type MEA in mass production.

3. Ordered membrane electrode

The catalytic layers of GDE type MEA and CCM type MEA are mixed with catalyst and electrolyte solution to form catalyst slurry and then coated. The efficiency is very low, and there is a large polarization phenomenon, which is not conducive to the large current discharge of the MEA. In addition, the platinum loading in the MEA is relatively high. The development of high-performance, long-life, and low-cost MEAs has become the focus of attention. The Pt utilization rate of the ordered MEA is very high, which effectively reduces the cost of the MEA, and at the same time realizes the efficient transportation of protons, electrons, gases, water and other substances, thereby improving the comprehensive performance of the PEMFC.

Ordered membrane electrodes include carbon nanotube-based ordered membrane electrodes, catalyst thin-film-based ordered membrane electrodes, and proton conductor-based ordered membrane electrodes.

Carbon Nanotube-Based Ordered Membrane Electrodes

The graphitic lattice properties of carbon nanotubes are durable to high potentials, the interaction with Pt particles and their elasticity improve the catalytic activity of Pt particles, and films based on vertically aligned carbon nanotubes (VACNTs) have been developed in the past decade or so. electrode. The vertical alignment mechanism enhances the gas diffusion layer, drainage capacity and utilization of Pt.

VACNTs can be divided into two types: one is VACNTs composed of curved, sparse carbon nanotubes; the other is VACNTs composed of straight, dense carbon nanotubes.

Ordered Membrane Electrodes Based on Catalyst Thin Films

The ordering of catalyst films mainly refers to Pt nano-ordered structures such as Pt nanotubes and Pt nanowires. Among them, the representative of catalyst-ordered membrane electrodes is NSTF, a commercial product of 3M Company. Compared with traditional Pt/C catalysts, NSTF has four main characteristics: the catalyst carrier is an ordered organic whisker; the catalyst forms a Pt-based alloy film on the whisker-like organism; there is no carbon carrier in the catalytic layer; NSTF catalyst The thickness of the layer is below 1um.

Ordered membrane electrodes based on proton conductors

The main function of the proton conductor ordered membrane electrode is to introduce nanowire-like polymer materials to promote the efficient transport of protons in the catalytic layer. Yu et al. prepared TiO2/Ti structures of TiO2 nanotube arrays (TNTs) on titanium sheets, followed by annealing in hydrogen atmosphere to obtain H-TNTs, and prepared Pt-Pd particles on the surface of H-TNTs by SnCl2 sensitization and replacement method, and obtained High power density fuel cells.

Based on the fast proton conduction function of Nafion nanowires, a new ordered catalyst layer was synthesized for the first time by the Institute of Nuclear Research and the Department of Automotive of Tsinghua University. It has the following characteristics: Nafion nanorods are prepared by in-situ growth on the proton exchange membrane, and the interface contact resistance is reduced to zero; Pt particle catalytic layer is deposited on Nafion nanorods, which has the functions of catalyst and electron conduction phase at the same time; Nafion nanorods have fast proton conduction.

4. Conclusion

Ordered membrane electrodes are undoubtedly the main attack direction of the next generation membrane electrode preparation technology. While reducing the loading of platinum group elements, five aspects need to be further considered: ordered membrane electrodes are very sensitive to impurities; through material optimization, Characterization and modeling to broaden the operating range of membrane electrodes; introduction of fast proton conductor nanostructures in the catalytic layer; low-cost mass production process development; in-depth study of the interaction between the membrane electrode’s proton exchange membrane, electrocatalysts, and gas diffusion layers and synergy.

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