The specific power of the fuel cell stack is an important indicator of the technical level of the fuel cell stack. High specific power can reduce the number of stack hardware and greatly reduce the cost. In view of the fact that the current domestic stack specific power still lags behind the international advanced level, this article focuses on high-activity catalysts, enhanced composite proton exchange membranes, highly disturbed flow fields, conductive and corrosion-resistant thin metal bipolar plates, stack assembly and consistency, etc. In many aspects, technical approaches to improve the specific power of fuel cell stacks were discussed. Based on theoretical and practical accumulation, the correlation between activation polarization, ohmic polarization and mass transfer polarization of fuel cells and materials, components and assembly was analyzed. In order to further improve Fuel cell stack performance and specific power provide directional reference. 1. Introduction Transportation plays an important role in energy consumption and carbon emissions and is a key factor triggering the growth of oil consumption. The development of energy-saving and environmentally friendly new energy vehicles can not only reduce oil imports and reduce carbon emissions, but also achieve the transformation and upgrading of China's automobile industry. Fuel cell vehicles are one type of new energy vehicles. They have the characteristics of long driving range, short fuel refueling time, and compatibility with renewable energy. They are increasingly valued by governments, enterprises, and research institutions at all levels. Governments around the world have launched incentive and financial subsidy policies to promote the formation of the primary market for fuel cell vehicles. Fuel cell vehicles have gradually transitioned from large-scale demonstrations to commercial operations. The first commercialized fuel cell vehicles, represented by Toyota Mirai, Honda Clarity (parameter | picture), and Hyundai Nexo, have reached the level of traditional fuel vehicles in terms of performance and other aspects. The power level of fuel cells for vehicles is generally around 100kW, and the power output of fuel cells for commercial vehicles is between 30 and 200kW. The FCV80 car of SAIC Maxus Automobile Co., Ltd. is the first fuel cell vehicle to be sold in my country. Other car manufacturers have also launched fuel cell announcement products. From the perspective of models, most of them are concentrated in commercial vehicles. From the perspective of power level, domestic vehicle fuel Battery stacks are mainly 30~50kW, and the power level is generally lower than that of similar international fuel cell vehicles. On the surface, the reason is that companies are catering to the financial subsidy threshold, but on a deeper level, it is the difference between my country's high specific power technology and international advanced technology. There is still a certain distance to the level (whether it is the currently introduced electric pile or the local electric pile). Therefore, it is necessary to increase the power density, especially to load a fuel cell stack with a certain power in the limited space of a passenger car, which requires high power density. In addition, from the perspective of cost reduction, increasing power density can reduce the consumption of fuel cell materials, components and other hardware, which in turn can significantly reduce fuel cell costs. Improving the power density of fuel cells requires improving performance and reducing size. In terms of performance, from the analysis of the fuel cell polarization curve (see Figure 1), it can be seen that the performance of the fuel cell can be improved by reducing activation polarization, ohmic polarization, mass transfer polarization and other aspects. This requires improving the catalyst, membrane, double The performance of key materials such as plates needs to ensure the consistency of the stack; in terms of volume, it is necessary to reduce the thickness of hardware such as plates and improve integration. This article will start from theoretical analysis and engineering practical experience to explore effective ways to improve power density, providing reference for researchers and engineering technicians engaged in this field. 2. High activity and high stability catalysts and electrodes It can be seen from the fuel cell polarization curve that to improve fuel cell performance, we must first reduce activation polarization, and activation polarization is mainly closely related to catalyst activity. During the reaction process of the fuel cell, since the exchange current density of the oxygen reduction reaction (ORR) is much lower than that of the hydrogen oxidation reaction (HOR), the polarization loss generally comes from the cathode side (air side). Therefore, the research focus is to improve the activity of cathode-side catalysts. Currently, the commonly used commercial catalyst in proton exchange membrane fuel cells is platinum carbon catalyst (Pt/C), which is a supported catalyst composed of Pt nanoparticles dispersed on a carbon powder (such as XC-72) carrier. Actual use tests have found that This commercial catalyst has certain deficiencies in terms of activity and stability. The U.S. Department of Energy (DOE) catalyst indicators are shown in Table 1. Researchers have explored high performance through Pt crystal face control, Pt-M alloy catalysts, Pt-M core-shell catalysts, Pt surface modifications, Pt single atomic layer catalysts, etc. As a solution for active and highly stable catalysts, currently only Pt-M alloy catalysts can be practically applied in these studies. The Pt-M catalyst is an alloy catalyst formed by Pt and a transition metal. Through the electronic and geometric effects of the transition metal catalyst on Pt, the stability is improved and the mass specific activity is also improved. At the same time, the amount of precious metal is reduced, making the catalyst Costs have also been significantly reduced. Binary alloy catalysts such as Pt-Co/C, Pt-Fe/C, and Pt-Ni/C have demonstrated good activity and stability. Chen et al. used the structural changes of platinum-nickel alloy nanocrystals to prepare a highly active and highly stable Pt3Ni nanocage catalyst, which increased its mass specific activity and area specific activity by 36 times and 22 times, respectively. In terms of the application of Pt alloy catalysts, Toyota Motor Corporation disclosed that the Pt-Co alloy catalyst was used in the commercial fuel cell vehicle Mirai (parameter | picture), which increased its catalyst activity by 1.8 times. The Pt3Pd/C catalyst developed by the Dalian Institute of Chemical Physics, Chinese Academy of Sciences (Dalian Institute of Chemical Physics) has been verified in fuel cell stacks, and its performance can completely replace commercial catalysts; in addition, the Dalian Institute of Chemical Physics has also developed an ultra-small PtCu alloy The mass specific activity of the catalyst is 3.8 times that of current Pt/C; the mass and area specific activities of the PtNi nanowire alloy catalyst are 2.5 times and 3.3 times that of Pt/C respectively (see Figure 2), showing good application prospects. Currently, for Pt-M catalysts, it is necessary to solve the problem of transition metal dissolution under fuel cell operating conditions. Metal dissolution not only reduces the catalyst activity, but also causes membrane degradation caused by metal ions. Therefore, further research is needed to improve the stability of Pt-M catalysts. To ensure the stability of Pt alloy catalysts, in addition to improving its own stability, it is also necessary to start from the system control strategy to reduce the attenuation conditions of the catalyst, which plays an important role in improving the stability of the catalyst. In addition to improving catalyst activity and reducing activation polarization, electrode structure is also very important for performance improvement. Electrodes are usually composed of diffusion layers and catalytic layers. A properly designed electrode structure is beneficial to reducing ohmic polarization and mass transfer polarization. The development trend of electrodes is to further reduce the thickness of the catalytic layer to improve reaction efficiency, increase the mass transfer flux of the gas diffusion layer, improve the mass transfer process, and then increase the ultimate current density of the electrode, increasing the operating current to 2.5~3A/cm2 or higher. Toyota Motor Corporation's Mirai fuel cell stack uses a thin, low-density diffusion layer, which significantly reduces ohmic polarization and mass transfer polarization, greatly increasing the operating current density. 3. Reinforced composite film It can be seen from Figure 1 that to improve performance, in addition to reducing activation polarization by increasing catalyst activity, as the current increases, the slope of the straight line section of the voltammetry curve is mainly determined by ohmic polarization, in which the ohmic polarization of the membrane occupies a major share. In order to improve performance, the membranes currently used for automotive proton exchange are gradually becoming thinner, from tens of microns to more than ten microns or less, in order to reduce the ohmic polarization of proton transfer and obtain higher performance. However, the film is more susceptible to mechanical damage and chemical degradation under vehicle operating conditions (such as dynamic changes in operating conditions such as operating pressure, dry humidity, and temperature). The composite membrane is modified from the homogeneous membrane. It uses the resin of the homogeneous membrane to combine with organic or inorganic substances to enhance certain functions than the homogeneous membrane. Therefore, reinforced composite films are the main solution for film applications. Reinforced composite membranes not only ensure the performance of the film but also enhance its mechanical strength and chemical durability. The first technical approach to achieve this is mechanical enhancement; the second is chemical enhancement (see Figure 3). For example, a mechanically reinforced membrane uses a porous film (such as porous PTFE) or fiber as a reinforced skeleton and is impregnated with perfluorosulfonic acid resin to make a composite reinforced membrane. The resin distributed between the porous membranes ensures proton conduction, and the porous base membrane makes the membrane The strength is improved, and the dimensional stability is also greatly improved, such as the composite membrane of Gore Company in the United States, the patented Nafion/PTFE composite reinforced membrane and carbon nanotube reinforced composite membrane of Dalian Institute of Chemical Physics in China. Chemical enhancement is to prevent chemical attenuation caused by free radicals during the electrochemical reaction. Adding a free radical quencher can decompose and eliminate free radicals during the reaction online and improve durability. The Dalian Institute of Chemical Physics prepared a composite membrane by adding 1wt.% CsxH3? Good proton conductivity also enhances the catalytic decomposition ability of H2O2. Nanjing University added the antioxidant vitamin E to the proton exchange membrane. Its main component, α-tocopherol, can not only capture free radicals and change them to an oxidized state, but can also be reduced again with the help of permeating hydrogen, thus improving the life of the fuel cell. 4. Bipolar plate flow field and materials Bipolar plate is an important component of the fuel cell. Its function is to support the membrane electrode and conduct electrons, distribute reaction gas and take away generated water. Therefore, in terms of fuel cell performance, bipolar plates not only affect ohmic polarization, but also affect mass transfer polarization. From the perspective of reducing ohmic polarization, the bipolar plate must have good electronic conductivity. Currently commonly used bipolar plates include graphite materials, graphite composite materials, and metal materials. All three bipolar plate materials have good conductivity, but there are some special considerations for different application scenarios. Pure graphite bipolar plates have good electrical conductivity, but they usually require mechanical carving of flow channels, resulting in low processing efficiency and high cost. They are the first generation of bipolar plate technology and have been gradually replaced. Graphite composite materials are usually made by mixing carbon powder and resin and other components in a certain proportion. The flow field can be processed by molding method, which has good economic efficiency; however, the addition of non-conductive substances such as resin will affect the flow field to a certain extent. Electrical conductivity, especially at high current densities, is not conducive to increasing power density; therefore, graphite composite materials must improve the electrical conductivity as much as possible while ensuring the compactness and processability of bipolar plates. Metal is a good conductor of electricity and heat, and its use as a bipolar plate material is becoming more and more common, especially in vehicle space constraints (such as passenger cars), which require fuel cells to have higher power density. Thin metal bipolar plates have become the preferred solution for increasing the power density of fuel cells due to their ability to achieve thinner bipolar plates and their inherent excellent conductive properties. Currently, most major automobile companies use metal bipolar plate technology, such as Toyota Motor Corporation, Honda Co., Ltd., Hyundai Motor Co., Ltd., etc. The technical challenge of metal bipolar plates is that they are corrosion-resistant in fuel cell environments (acidic, electrical potential, moist heat) and are compatible and non-polluting to other fuel cell components and materials. Currently, the commonly used metal bipolar plate materials are stainless steel or titanium with surface coating. A large amount of academic research work has been conducted at home and abroad on the corrosion-resistant coating technology on the surface of fuel cell stainless steel bipolar plates. The coating materials must ensure both corrosion resistance and conductivity. Representative coating materials are shown in Table 2. Generally speaking, surface coating materials can be divided into three categories: metals, metal compounds and carbon coatings; metals include precious metals and metal compounds. Precious metal coatings, such as gold, silver, platinum, etc., despite their high cost, are still used in special fields due to their superior corrosion resistance and contact resistance similar to graphite. In order to reduce costs, the thickness of the treatment layer should be reduced as much as possible, but pinholes should be avoided. Metal compound coating is a surface treatment solution that has been widely studied at present, such as Ti-N, Cr-N, Cr-C, etc., which show high application value. In addition to metal coatings, there is also some exploration in carbon films for metal bipolar plates, such as graphite, conductive polymers (polyaniline, polypyrrole) and diamond-like films. Toyota Motor Corporation’s Ce2O3H2O patented technology (US2014356764) disclosed Bipolar plate surface treatment technology using SP2 hybrid orbital amorphous carbon with high conductivity. In addition to coating materials, coating preparation technology is also an important factor in improving its corrosion resistance and ensuring electrical conductivity. The coating must be free of pinholes and cracks; pinholes in the surface treatment layer of metal bipolar plates are a common problem in bipolar plate materials. Due to the particle deposition of the coating during the preparation process, a discontinuous phase is formed. This results in the existence of pinholes, causing electrochemical corrosion based on the base material to occur through the pinholes in the coating in the fuel cell operating environment. In addition, due to the different linear expansion coefficients of the coating metal and the substrate, the thermal cycle that occurs during working conditions will cause micro-cracks, which is also a problem worthy of attention. The problem can be alleviated by adding a transition layer. Dalian Institute of Chemical Physics and Dalian University of Technology collaborated on research on surface modification technology of metal bipolar plates, using pulse bias arc ion plating technology to prepare multi-layer film structures. The results show that multi-layer structure design can improve the conductivity and conductivity of bipolar plates. Corrosion resistance. Reasonable bipolar plate flow field design and layout can reduce mass transfer polarization, help improve performance under high current density, and further increase the power density of the stack. Toyota Motor Corporation has launched a new 3D flow field design concept in the Mirai fuel cell vehicle stack (see Figure 4), which changes the traditional snake-shaped and parallel groove-shaped 2D flow field configuration to make the fluid perpendicular to the ethanolamine (MEA) ) The components of the gas diffusion layer and the catalytic layer, the reactants and products do not simply rely on concentration difference diffusion to reach and leave the reaction interface, but have forced convection, which greatly improves the mass transfer driving force of the fuel cell and significantly improves the performance. In addition, this 3D flow field has a certain water storage function, which is conducive to humidity adjustment during fuel cell operation and can improve fuel cell performance under low humidification. Through simulation calculations, it can be further confirmed that the 3D flow field strengthens the drainage capacity of the flow channel and diffusion layer (see Figure 5, the 3D flow field has a zero saturation zone of water in the diffusion layer), and at the same time increases the forcing of oxygen in the catalytic layer. Convection, especially at high currents, has greatly improved fuel cell performance compared with 2D conventional parallel trench flow fields. 5. Stack assembly and consistency Stack assembly and consistency are crucial to improving stack performance. Assembly determines the degree of coordination between the components of the stack. Only a well-assembled stack can maximize the performance of the components. Consistency is an important indicator to measure the performance of the stack. A stack with good consistency can operate at high current density. It is beneficial to improve the power density of the stack. The stack assembly process is usually carried out on a press. Generally, the ME is placed according to a certain assembly sequence and positioning method.

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