Summary:
The development of hydrogen energy can effectively solve the growing contradiction between economic development and ecological environment. Hydrogen fuel vehicles will be at the core of the hydrogen energy industry system. Accelerating the technology research and development of hydrogen fuel cell vehicles and improving the utilization rate of hydrogen energy on a large scale are of great significance to the formation of an industrial system characterized by low carbon emissions around the world. In the hydrogen energy industry chain, hydrogen storage technology is an indispensable link in the development of hydrogen energy. Various hydrogen storage technologies have been applied to vehicle-mounted hydrogen storage systems.
Development Status of On-board Hydrogen Storage Technology
According to the reaction of the hydrogen storage process, the on-board hydrogen storage technology can be divided into two categories: physical hydrogen storage and chemical hydrogen storage. Physical hydrogen storage includes high-pressure gaseous hydrogen storage and low-temperature liquid hydrogen storage. Chemical hydrogen storage includes organic liquid hydrogen storage and metal hydrogen storage. Hydrides store hydrogen.
The main parameters to measure the performance of hydrogen storage technology are hydrogen storage volume density, mass fraction, reversibility of hydrogen addition and desorption, hydrogen addition and desorption rate, and cycle life. Many research institutions at home and abroad have proposed new standards for hydrogen storage technology. When evaluating vehicle-mounted hydrogen storage technology, not only economics and periodicity, but also safety must be considered.
01High -pressure gaseous hydrogen storage technology
High-pressure gaseous hydrogen storage is currently the most mature vehicle-mounted hydrogen storage technology adopted by various manufacturers. The key is the hydrogen storage bottle. At present, the types of hydrogen storage bottles at home and abroad can be divided into the following five types: pure steel metal type (Type I), steel liner fiber winding type (II type), aluminum liner fiber winding type ( Type III), fiber-wound type with plastic liner (Type IV) and fiber-wound type without liner (Type V). The performance of the five types of hydrogen storage bottles is shown in Figure 1.
It can be seen from Figure 1 that the hydrogen storage capacity of type I and II hydrogen storage bottles is relatively low. With the increase of material strength, the metal is prone to hydrogen embrittlement, which cannot meet the requirements of on-board hydrogen storage. Type III and Type IV (as shown in Figure 2) hydrogen storage bottles are composed of an inner tank, a carbon fiber reinforced resin layer and a glass fiber reinforced resin layer, which reduces the mass of the hydrogen storage bottle itself, thereby increasing the hydrogen storage mass density. At present, Type III cylinders occupy the mainstream position in the domestic market. In October 2020, my country officially implemented the group standard "Compressed Hydrogen Plastic Liner Carbon Fiber Fully Wound Gas Cylinders for Vehicles", which means that Type IV cylinders will no longer be in the leading position in the domestic market. blank state.
Although Type IV hydrogen storage bottles have many advantages over other types of hydrogen storage bottles, key technical problems still need to be overcome in order to achieve mass production:
First, in the high-pressure gaseous hydrogen storage technology, due to the The bottle weight ratio coefficient is too low, which leads to the disadvantages of high transportation cost and high transportation risk during the transportation of hydrogen;
second, carbon fiber is a key material for hydrogen storage bottles, and the technical barriers are relatively high. Requirements for hydrogen materials, resources still need to be imported in large quantities from Japan, which increases the manufacturing cost of hydrogen storage bottles;
Third, type III hydrogen storage bottles are sealed by the sealing surface on the metal liner and the bottle valve. Different from the sealing structure design of type III hydrogen storage bottles, type IV hydrogen storage bottles need to consider the seal between metal and plastic.
The fiber composite material shell of the hydrogen storage bottle is different from the plastic lining material. The plastic will age with the prolongation of the working time, and the lining and the fiber winding layer will be separated. The hydrogen molecular mass is small, and it is easy to seep out of the molecular pores of the lining material. , as shown in the hydrogen leakage path ① in Figure 3; in addition, it is difficult to obtain strict sealing due to the different materials of the plastic lining and the metal bottle mouth, and hydrogen molecules are also easy to leak through the path ② in Figure 3. Hydrogen leakage in a confined space may cause an explosion accident, so the sealing of the hydrogen storage bottle and the selection of the material of the seal are very important.
At present, high-pressure gaseous hydrogen storage is the hydrogen storage technology with the highest degree of engineering, and the commonly used pressure values of high-pressure gaseous hydrogen storage bottles are 35MPa and 70MPa. It is worth noting that only by increasing the hydrogen storage pressure to increase the hydrogen storage density, the material, structure requirements and cost of the hydrogen storage equipment will also increase accordingly. While achieving high hydrogen storage density, light weight and low cost are also important development directions for high-pressure gaseous hydrogen storage technology.
02Cryogenic liquid hydrogen storage technology
Cryogenic liquefied hydrogen storage is an emerging hydrogen storage technology that stores liquefied hydrogen gas in a heat-insulated vacuum container at a low temperature of 20K. Its hydrogen storage density is as high as 71kg/m3, which is more than twice that of high-pressure gaseous hydrogen storage at 80MPa. From the perspective of hydrogen storage density alone, liquid hydrogen storage is an ideal hydrogen storage method.
Liquid hydrogen is generally stored in hydrogen storage tanks, and its shape is mainly divided into two types: spherical and cylindrical. Since the manufacture and processing of larger spherical liquid hydrogen storage tanks is difficult and the cost is relatively high, the currently commonly used liquid hydrogen storage tanks are cylindrical. Due to the temperature difference in various parts of the storage tank, the phenomenon of "stratification" and "heat overflow" will appear in the tank. Usually, a plate with good thermal conductivity is installed vertically inside the hydrogen storage tank to eliminate the temperature difference between the upper and lower sides of the tank, or to directly export the heat out of the tank to solve the above problems. The boiling point of liquid hydrogen is low (–252.78°C), the latent heat of vaporization is small, and a very small amount of heat leakage will cause the medium to evaporate, so the liquid hydrogen storage tank is required to have good thermal insulation performance. There are two main types of insulation materials used in hydrogen storage devices: load-bearing materials and non-load-bearing multilayer materials. The former is easy to install, and the latter can effectively prevent heat leakage.
The main application technology of liquid hydrogen in the automotive field is the hydrogen internal combustion engine. In 2006, BMW launched the world's first hydrogen-powered car H7, which uses an internal combustion engine system that can burn both liquid hydrogen and gasoline. Car. At present, the low-temperature liquid hydrogen storage technology is still immature, and there are still the following problems:
the energy consumed in liquefying hydrogen accounts for 30% of the total energy. In addition, the daily gasification loss of liquid hydrogen is about 1% to 2%, which undoubtedly increases The cost of hydrogen storage;
the liquefaction process is complicated, and the requirements for hydrogen storage materials are very high. How to use and improve the low-temperature insulation technology in the design of liquid hydrogen storage tanks is a difficult problem;
The leakage of liquid hydrogen is serious, and there is a great potential safety hazard during transportation.
Low-temperature liquid hydrogen storage technology has been applied to vehicle-mounted hydrogen storage systems. In 2000, General Motors of the United States applied liquid storage tanks to cars. It has a total mass of 90kg and can store 4.6kg of hydrogen. The mass hydrogen storage density and volume hydrogen storage The densities are 5.1% and 36.6kg/m3 respectively. However, low-temperature liquid hydrogen storage technology has problems such as high liquefaction energy consumption and serious vaporization. In order to further commercialize and promote, technological breakthroughs should be made to reduce liquefaction energy consumption and hydrogen leakage rate.
03Organic liquid hydrogen storage technology
Organic liquid hydrogen storage technology is to achieve hydrogen storage by means of reversible hydrogenation and dehydrogenation reactions between unsaturated liquid organic matter and hydrogen. The technology is divided into three stages, namely hydrogenation reaction, storage and transportation, and organic liquid dehydrogenation process. The hydrogen storage medium can undergo hydrogenation reaction again under the action of the catalyst after the dehydrogenation reaction, realizing the recycling of the organic hydrogen storage material. As shown in Figure 4, different organic liquid hydrogen storage materials have different properties and hydrogen storage capacity, so they need to be selected according to specific conditions. The nature of the hydrogen storage medium is similar to that of gasoline, and it can be stored and transported at normal temperature and pressure, which is convenient and safe, and is suitable for long-distance transportation in large quantities.
During hydrogenation and dehydrogenation, catalysts can not only reduce the reaction temperature, but also improve the reaction rate of chemical hydrogen storage technology. Hydrogenation catalysts mainly include nickel-based catalysts, palladium and platinum-based catalysts, ruthenium-based catalysts, and rhodium-based catalysts. Conventional hydrogenation catalysts are nickel metal catalysts supported by aluminum. For deep aromatic hydrocarbon catalysis, noble metal catalysts are the first choice. Dehydrogenation catalysts are mainly noble metal catalysts, non-noble metal catalysts and mixed catalysts. The high activity of noble metal catalysts can improve the dehydrogenation efficiency of organic liquid hydrogen storage materials. Due to its high price, researchers have made non-noble metals have better dehydrogenation performance by improving the carrier or metal modification.
Both the hydrogenation and dehydrogenation process conditions of organic liquids are extremely harsh. In the process of hydrogenation/dehydrogenation, the status of the catalyst cannot be ignored. While satisfying the hydrogenation/dehydrogenation mechanism of organic liquid hydrogen storage materials, it is also necessary to actively synthesize high-efficiency, low cost catalyst. Although some progress has been made in hydrogen storage in organic liquids, reducing the addition/dehydrogenation temperature and developing low-cost, high-activity catalysts are issues that must be addressed in future research.
04Metal hydride hydrogen storage technology
Metal hydride hydrogen storage is a technical means of using metals to absorb and release hydrogen under certain conditions. Under a certain temperature and pressure, hydrogen and metal react to form metal hydride, and hydrogen is stored in the interatomic space of the metal in the form of atoms; when the metal hydride is heated by the outside world, its own decomposition reaction occurs, and hydrogen atoms will combine to form Molecular hydrogen is released, accompanied by an endothermic effect. Common hydrogen storage metal elements include magnesium, titanium, vanadium and so on.
In industrial production, hydrogen storage materials are mostly alloys rather than simple metals. Hydrogen storage alloys are intermetallic compounds composed of metal elements A that are easy to form stable hydrides and transition metals B that have a low affinity for hydrogen. At present, the more common hydrogen storage alloys include magnesium-based A2B-type hydrogen-storage alloys, rare-earth-based AB5-type hydrogen-storage alloys, and titanium-based hydrogen-storage alloys.
Metal hydrides have high hydrogen storage energy density, and the hydrogen storage per unit volume is 1000 times that of the gaseous state at normal temperature and pressure; the alloy has stable chemical properties and good safety during storage and transportation. It is one of the hydrogen storage methods with good development prospects. At the beginning of 2001, Toyota Motor Corporation announced that it had successfully developed a new type of fuel cell vehicle powered by a hydrogen storage alloy with a range of more than 300km. Metal hydride hydrogen storage technology has the advantages of high hydrogen storage density and good safety, and has been applied in vehicles. The research and development of high-performance metal hydride materials can not only further promote the development of the hydrogen energy industry, but also promote the development of hydrogen energy in various industries.
Comparison of different hydrogen storage methods
Different hydrogen storage technologies have different hydrogen storage capacities and have different advantages and disadvantages, as shown in Figure 5.
High-pressure gaseous hydrogen storage is currently the most commercialized hydrogen storage technology. The set pressure of high-pressure hydrogen storage tanks is generally 35MPa and 70MPa. While improving the pressure-bearing capacity of hydrogen storage tanks, its reliability must also be guaranteed.
Low-temperature liquid hydrogen storage technology has high hydrogen storage density, and liquid hydrogen is more suitable for short-term and large-scale use scenarios. In order to further realize the industrialization of liquid hydrogen, it is necessary to continue to reduce liquefaction energy consumption and hydrogen leakage rate in storage and transportation.
Organic liquid hydrogen storage technology has high safety, but extremely harsh conditions are required in the hydrogenation and dehydrogenation processes, and there is a problem of how to develop high-conversion and high-stability catalysts.
Metal hydride hydrogen storage technology is simple, convenient and safe to charge hydrogen, but hydrogen storage materials are expensive, and the problem of how to reduce hydrogen storage costs in large-scale hydrogen storage has not been completely resolved, and it cannot be promoted for the time being.
Summary and Outlook
At present, various hydrogen storage technologies have been applied in automobiles. However, some domestic hydrogen storage technologies and materials still have a certain distance from the commercialization and scale of hydrogen energy. Under the background of low-carbon development and energy transformation, the hydrogen energy industry leads new development opportunities. my country is expected to build 200 hydrogen refueling stations in 2025, manufacture and produce 50,000 hydrogen fuel cell vehicles, complete 1,000 hydrogen refueling stations in 2050, and reach the goal of 5 million hydrogen fuel cell vehicles. The industry has put forward a large number of market demands. Considering the cost and safety of hydrogen storage, high-pressure gaseous hydrogen storage technology is currently the best choice for on-board hydrogen storage, and high-pressure gaseous hydrogen storage is still the most popular hydrogen storage technology in the short to medium term. In the long run, after capacity expansion and breakthroughs in key technologies, cryogenic liquid hydrogen storage technology and metal hydride hydrogen storage technology are expected to become mainstream hydrogen storage methods.