The membrane electrode is the core component of fuel cells, which integrates the transport and electrochemical reactions of heterogeneous materials, directly determining the performance, lifespan, and cost of proton exchange membrane fuel cells. The membrane electrode and the bipolar plates on both sides together form a single fuel cell, and the combination of multiple single cells can form a fuel cell stack to meet various power output requirements. The design and optimization of MEA structure, material selection, and manufacturing process optimization have always been the focus of PEMFC research. In the development process of PEMFC, membrane electrode technology has undergone several generations of innovation, mainly divided into three types: GDE hot pressing method, CCM three in one membrane electrode, and ordered membrane electrode.
1. GDE Hot Pressed Film Electrode
The first generation MEA preparation technology used a hot pressing method to compress the cathode and anode GDLs coated with CL on both sides of PEM to obtain MEA, known as the "GDE" structure.
The preparation process of GDE type MEA is indeed relatively simple, thanks to the catalyst being uniformly coated on the GDL. This design not only facilitates the formation of pores in MEA, but also cleverly protects PEM from deformation. However, this process is not flawless. If the amount of catalyst coated on the GDL cannot be precisely controlled, the catalyst slurry may penetrate into the GDL, resulting in some catalysts not fully exerting their efficiency, and the utilization rate may even be as low as 20%, greatly increasing the manufacturing cost of MEA.
Due to the inconsistency between the catalyst coating on GDL and the expansion system of PEM, the interface between the two is prone to delamination during long-term operation. This not only leads to an increase in internal contact resistance of fuel cells, but also greatly reduces the overall performance of MEA, far from reaching the ideal level. The preparation process of MEA based on GDE structure has been basically eliminated, and few people have paid attention to it.
2. CCM Three In One Membrane Electrode
By using methods such as roll to roll direct coating, screen printing, and spray coating, a 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 has better performance, is not easy to peel off, and reduces the transfer resistance between the catalyst layer and PEM, which is beneficial for improving the diffusion and movement of protons in protons. Catalyst layer, thereby promoting the catalytic layer and PEM. The contact and transfer of protons between them reduce the resistance of proton transfer, thereby greatly improving the performance of MEA. The research on MEA has shifted from GDE type to CCM type. In addition, due to the relatively low Pt loading of CCM type MEA, the overall cost of MEA is reduced 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 fuel cells. The main reason is that there is no hydrophobic agent in the MEA catalytic layer, there are fewer gas channels, and the transmission resistance of gas and water is relatively high. Therefore, in order to reduce the transmission resistance of gas and water, the thickness of the catalyst layer is generally not greater than 10 μ m.
Due to its excellent comprehensive performance, CCM type MEA has been commercialized in the field of automotive fuel cells. For example, Toyota Mirai, Honda Clarity, etc. 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 trucks, with a platinum based precious metal loading capacity as low as 0.4mgPt/cm2. The power density reaches 0.96W/cm2. At the same time, companies and universities such as Kunshan Sunshine, Wuhan Himalaya, Suzhou Qingdong, Shanghai Jiao Tong University, and Dalian Institute of Chemical Physics are also developing high-performance CCM type MEAs. Foreign companies such as Komu, Gore
3. Ordered Membrane Electrode
The catalytic layer of GDE type MEA and CCM type MEA is mixed with catalyst and electrolyte solution to form a catalyst slurry, which is then coated. The efficiency is very low and there is a significant polarization phenomenon, which is not conducive to the high current discharge of MEA. In addition, the platinum loading in MEA is relatively high. The development of high-performance, long-life, and low-cost MEAs has become a focus of attention. The Pt utilization rate of ordered MEA is very high, effectively reducing the cost of MEA, while achieving efficient transport of protons, electrons, gases, water and other substances, thereby improving the comprehensive performance of PEMFC.
Ordered membrane electrodes include ordered membrane electrodes based on carbon nanotubes, ordered membrane electrodes based on catalyst thin films, and ordered membrane electrodes based on proton conductors.
Carbon Nanotube Based Ordered Membrane Electrode
The graphite lattice characteristics of carbon nanotubes are resistant to high potentials, and their interaction and elasticity with Pt particles enhance the catalytic activity of Pt particles. In the past decade or so, thin films based on vertically aligned carbon nanotubes (VACNTs) have been developed. Electrode. The vertical arrangement mechanism enhances the gas diffusion layer, drainage capacity, and Pt utilization efficiency.
VACNT can be divided into two types: one is VACNT composed of curved and sparse carbon nanotubes; Another type is hollow carbon nanotubes composed of straight and dense carbon nanotubes.
Ordered Membrane Electrode Based On Catalyst Thin Film
The ordering of catalyst thin films mainly refers to Pt nano ordered structures, such as Pt nanotubes, Pt nanowires, etc. Among them, the representative of catalyst ordered membrane electrode 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; Catalyst forms Pt based alloy thin film on whisker like organisms; There is no carbon carrier in the catalytic layer; The thickness of the NSTF catalyst layer is below 1um.
Ordered Membrane Electrode Based On Proton Conductor
The main function of proton conductor ordered membrane electrode is to introduce nanowire polymer materials to promote efficient proton transport in the catalytic layer. Yu and others. TiO2/Ti structures of TiO2 nanotube arrays (TNTs) were prepared on titanium sheets, followed by annealing in a hydrogen atmosphere to obtain H-TNTs. Pt Pd particles were prepared on the surface of H-TNTs using SnCl2 sensitization and displacement methods, resulting in a high-power density fuel cell.
The Institute of Nuclear Science and the Department of Automotive Engineering at Tsinghua University have synthesized a novel ordered catalyst layer for the first time based on the fast proton conduction function of Nafion nanowires. It has the following characteristics: Nafion nanorods are grown in situ on proton exchange membranes, and the interface contact resistance is reduced to zero; Deposition of Pt particle catalytic layer on Nafion nanorods, with both catalytic and electron conducting functions; Nafion nanorods have fast proton conductivity.
Ordered membrane electrodes are undoubtedly the main direction of 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 highly sensitive to impurities; Expand the working range of membrane electrodes through material optimization, characterization, and modeling; Introducing fast proton conductor nanostructures into the catalytic layer; Low cost mass production process development; In depth study of the interactions and synergistic effects between membrane electrode proton exchange membrane, electrocatalyst, and gas diffusion layer.
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Advantages of Membrane Electrode Preparation Technology and Ultrasonic Spraying Method:
(1) By optimizing parameters such as ultrasonic nozzle power and frequency, the atomized catalyst slurry can have small rebound and be less prone to overspray, thereby improving the utilization rate of the catalyst;
(2) The ultrasonic vibration rod disperses the catalyst particles highly, and the ultrasonic dispersion injector has a secondary stirring effect on the catalyst slurry, greatly reducing the probability of platinum chemical pollution and reduced reaction activity area;
(3) Easy to operate, highly automated, suitable for mass production of membrane electrodes.
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