In the 1960s, metals and alloys that could store hydrogen appeared in the kingdom of materials, collectively known as hydrogen storage metals. These metals or alloys have a strong ability to capture hydrogen, which can work at certain temperatures and pressure Under the conditions, hydrogen molecules first decompose into single atoms in the alloy (or metal), and these hydrogen atoms enter the gap between the alloy atoms "seeing a pin" and chemically react with the alloy to form a metal hydride (metal hydrides), the external manifestation is a large amount of "absorption" hydrogen, while releasing a lot of heat. When these metal hydrides are heated, they will undergo a decomposition reaction, and hydrogen atoms can be combined into hydrogen molecules to be released, and it is accompanied by a significant endothermic effect.
Hydrogen storage method
1. High-pressure gaseous hydrogen storage
Under normal pressure, the density of hydrogen is only 0.899g / L, and the volumetric energy density is very low. Therefore, high pressure must be applied to increase the volumetric energy density. High-pressure gaseous hydrogen storage is to pressurize hydrogen up to 70MPa and store it in a pressure-resistant gas tank. However, this method requires the use of ultra-high pressure resistant composite materials as hydrogen storage containers. However, the weight of the stored hydrogen is less than 1% of the gas tank. At the same time, there are hidden dangers of explosion during transportation and use.
2. Low temperature liquid hydrogen storage
Under normal pressure, hydrogen must be reduced to a boiling point of -252.7 ° C to become a liquid, and it needs to be protected by storage containers and transportation pipelines with excellent thermal insulation conditions. However, the vaporization of liquid hydrogen causes the continuous loss of hydrogen, and the maintenance of low temperature also requires the consumption of 30% of the energy of liquid hydrogen. Therefore, this method is expensive and the process is relatively complicated.
3. Hydrogen storage alloy
The so-called hydrogen storage alloy is a metal compound that can reversibly absorb, store and release hydrogen in large quantities under a certain temperature and hydrogen pressure. Hydrogen storage alloys can "absorb" a large amount of hydrogen at a certain temperature and a pressure higher than the equilibrium decomposition pressure, and if the metal hydride is heated, it will decompose and divide the stored hydrogen in the form of high-purity hydrogen. Release under low pressure. After calculation, certain metals equivalent to 30% of the weight of the hydrogen cylinder can "absorb" hydrogen equal to the hydrogen storage capacity of the cylinder, but its volume is less than 1/10 of the volume of the hydrogen cylinder.
Hydrogen storage alloys are one of the most widely studied and mature new high-performance large-scale hydrogen storage materials. They have high hydrogen storage density, good safety, and are suitable for large-scale hydrogen storage and transportation. The most important feature is the ability to absorb reversibly, Amplify the amount of hydrogen.
Classification of hydrogen storage alloys
Hydrogen storage alloys can be classified according to their chemical formulas, such as AB5, AB2, AB3, AB, and A2B, or they can be classified according to the main components of the alloy. At present, there are five types of hydrogen storage alloys that are more in-depth research.
Rare Earth
LaNi5 is a typical representative of rare-earth AB5-type hydrogen storage alloys. As early as 1969, Philips in the Netherlands discovered that LaNi5 has the advantages of fast hydrogen absorption, easy activation, and moderate equilibrium pressure, which triggered people to store hydrogen in the rare earth system. Material research boom. At that time, LaNi5 was used as the negative electrode material for MH / Ni batteries, but its capacity decayed too quickly and was expensive, so it failed to develop for a long time. However, in recent years, through element alloying, chemical treatment, non-stoichiometric ratio, different preparation and heat treatment processes, LaNi5 type rare earth hydrogen storage alloy has been successfully used as a negative electrode material for commercial MH / Ni batteries.
At present, the rare earth-magnesium-nickel-based hydrogen storage alloy has become a hot topic in the research of rare earth hydrogen storage materials at home and abroad. It is an alloy system in which Mg element is added on the basis of rare earth hydrogen storage alloy. The hydrogen storage capacity, electrochemical discharge capacity, and electrochemical kinetics of this system alloy are higher than those of the commercially available AB5 alloy, but the electrochemical cycle stability is not yet ideal. Scholars at home and abroad have done a lot of research work on the mechanism of electrochemical capacity attenuation and how to improve the electrochemical stability. For improving the electrochemical stability, the main methods include improving the alloy preparation process, annealing heat treatment, magnetization treatment, making a single-phase structure, making a composite phase structure alloy, determining the composition of important elements (such as Mg), surface treatment, element alloy…
Regarding the research and development of rare earth hydrogen storage alloys, we should focus on further adjusting and optimizing the chemical composition of the alloy, not only to optimize the chemical composition of the hydrogen absorption side of the alloy, but also to optimize the chemical composition of the non-hydrogen absorption side B, and Further optimize the structure of the alloy, the surface of the alloy, etc., so that the overall performance of the alloy is further improved.
Magnesium
Magnesium alloys have the characteristics of low cost (rich resources, low price), light weight and high hydrogen storage capacity (the hydrogen storage capacity of hydrogen storage alloys is the highest, such as MgH2 hydrogen storage capacity of 7.6%). Therefore, magnesium-based alloys are considered to be the most promising alloy materials, and have become a hot spot in the field of hydrogen storage alloys in recent years. According to incomplete statistics, so far, more than 1,000 related magnesium-based hydrogen storage alloys have been studied at home and abroad, including almost all stable metal elements in the periodic table and some hydrogen storage materials composed of radioactive elements and magnesium. The shortcomings of this alloy are high hydrogen release temperature (generally 250 ~ 300 ℃), poor hydrogen release kinetics and poor corrosion resistance.
At present, from the perspective of the composition of the magnesium alloys studied, there are mainly magnesium-based hydrogen storage alloys and magnesium-based composite hydrogen storage materials. The typical representative of magnesium-based hydrogen storage materials is Mg2Ni. The series of alloys for electrochemical hydrogen storage are currently being studied more frequently. The main problem they face is the poor electrochemical cycling stability of alloy electrodes. Domestic and foreign scholars have made some progress mainly in the preparation process of alloy electrodes, alloying and substitution of elements, heat treatment and other methods to solve the electrochemical cycle stability. Magnesium-based composite hydrogen storage materials are a new development direction of magnesium-based hydrogen storage alloys in recent years. Composite hydrogen storage materials can take advantage of their respective materials and interact to optimize the electrode performance of the alloy. The materials compounded with the magnesium-based hydrogen storage alloy mainly include carbonaceous hydrogen storage materials, metal elements, and compounds. For nanocrystalline and amorphous Mg2Ni-based alloys, the electrode cycle declines rapidly. After compounding with graphite, the graphite layer on the surface of the alloy can effectively reduce the electrode decay rate and effectively increase the discharge capacity of Mg2Ni-type materials.
At the same time, a new thin-film metal hydride hydrogen storage technology has emerged in recent years, including pure Mg film, Mg-Pb film, Mg-Ni film, Mg-Nb film, Mg-V film, Mg-Al film, Mg- LaNi5 film. The thin film of hydrogen storage alloy has the characteristics of fast absorption, high hydrogen release speed, strong anti-pulverization ability, high thermal conductivity, etc., and can relatively easily surface the thin film. In addition, spraying a protective layer on the surface of the thin-film metal hydride can play the role of activating the thin-film metal hydride and protecting the hydride from impurities. However, the currently prepared magnesium thin films generally require Pb as a catalytic component to improve the hydrogen absorption performance of Mg, the cost is too high, and the hydrogen absorption performance is still not ideal. Therefore, there is an urgent need to find a cheap metal element to replace the more expensive Pb and V, or to use other methods such as hydrogen storage alloy compounding to obtain high-performance alloy materials with excellent dynamic properties.
For the research and development of magnesium-based hydrogen storage alloys, in addition to further adjusting and optimizing the chemical composition of the alloy, as well as further optimizing the structure of the alloy and the surface of the alloy, in the future, it can also be improved by means of surface coating alloy powder, mechanical ball milling, etc. Strive to further improve the overall performance of the alloy.
Titanium
A typical representative of titanium-based alloys is TiFe. Titanium-based alloys can absorb and release a large amount of hydrogen at room temperature, have good hydrogen storage performance (hydrogen storage is 1.8% ~ 4%), the decomposition pressure of hydride is very low (0.3MPa at room temperature), and constitute Elements are abundant in nature and cheap in price, and have been applied to a certain extent in industry. However, it is not easy to activate, easy to poison (especially vulnerable to CO gas poisoning), and the equilibrium pressure at room temperature is too low, making the hydride unstable. For this reason, many scholars use metals such as Ni to partially replace Fe to form a ternary alloy to achieve activation at room temperature, making it more practically valuable. For example, the Japan Institute of Metal Materials Technology has successfully developed a titanium-iron-oxide hydrogen storage system with the advantages of large hydrogen absorption, fast hydrogenation speed, and easy activation. In recent years, the research and development of Ti-V-Mn hydrogen storage alloys has been very active, and the hydrogen absorption of nano-structured hydrogen storage alloys formed by metastable decomposition can reach more than 2%.
For the research and development of titanium-based alloys, the most commonly used methods are still through further adjustment and optimization of the alloy's chemical composition (that is, by using transition metals, rare earth metals and other parts to replace Fe or Ti) and optimization of the alloy's structure and surface of the alloy; The second is to change the single traditional smelting method, such as the use of mechanical alloying method to produce alloys.
Zirconium
ZrMn2 is a typical representative of zirconium-based hydrogen storage alloys. The alloy has the characteristics of large amount of hydrogen absorption and discharge, large cycle life, and easy activation, because it can form a dense oxide film in alkaline electrolysis, which effectively prevents the further oxidation of the electrode; but there are difficulties in initial activation, hydride generation heat Disadvantages such as large and unclear discharge platform.
People have improved the overall performance of ZrMn2 through the over-place method. For example, Ti is used to replace part of Zr, and Fe, Co, Ni, etc. are used to replace part of Mn. Platform pressure and high hydrogen absorption capacity.
For the research and development of zirconium-based alloys, the most commonly used method is still to further adjust and optimize the chemical composition of the alloy, as well as optimize the alloy structure and surface of the alloy.
Vanadium
This type of alloy has the advantages of large reversible hydrogen storage capacity and fast diffusion rate of hydrogen in hydride. Its disadvantages are that the alloy has poor cycle stability in charge and discharge, and the cycle capacity decay rate is fast. Vanadium-based hydrogen storage alloys have been applied early in the fields of hydrogen storage, purification, compression, and hydrogen isotope separation. V3TiNi0.56Mx is the most studied vanadium-based solid solution hydrogen storage alloy, where x = 0.046 ~ 0.24; M is Al, Si, Fe, Cu, Zr and other elements, mainly used in the field of nickel-hydrogen batteries.
Therefore, for the research and development of vanadium-based solid solution hydrogen storage alloys, optimizing alloy composition and structure, adopting new alloy preparation techniques, and modifying the alloy surface are still the main research directions for further improving alloy performance.
Application of hydrogen storage alloy
1. Nickel-hydride secondary battery anode material
Nickel-hydrogen secondary batteries have begun to replace traditional nickel-cadmium batteries and are widely used in mobile communications, instrumentation, testing equipment, emergency power supplies, etc. due to their large capacity, large current discharge, and no memory effect. As the anode material of nickel-hydride battery, hydrogen storage alloy is not only the key material for battery preparation, but also the most mature field of application of hydrogen storage alloy. The hydrogen storage alloy used in the negative electrode of the battery should meet: high and stable electrochemical capacity, appropriate equilibrium hydrogen pressure (0.01 ~ 0.5MPa at room temperature), good catalysis for hydrogen anode polarization, and strong resistance to anodic oxidation , Anti-corrosion ability of alkaline solution, good thermal and electrical conductivity. At present, the most widely used at home and abroad is the rare earth-based AB5 type alloy. Its electrochemical specific capacity is generally 280 ~ 330mA · h / g, and its cycle life is more than 500 times. It is easy to activate; another type that has been practical is the AB2 type Ti- Zr-V-Ni-Cr series alloy has an electrochemical specific capacity of 360 ~ 400mA · h / g, but activation is difficult. The vanadium-based solid solution hydrogen storage alloy has attracted great attention at home and abroad because of its high electrochemical specific capacity, and has become the focus of research and development of high-capacity nickel-hydride batteries. The A2B magnesium hydrogen storage alloy is rich in resources and price. As the third generation electrode alloy, advantages such as lower cost have also become a research and development hotspot. The hydrogen storage alloy is used as the negative electrode of the battery, and the secondary battery made has important applications in the fields of mobile phones, laptop computers, electric vehicles and space technology.
2. Storage and transportation of hydrogen
Hydrogen storage and transportation is an important part of the hydrogen energy utilization system. The hydrogen energy storage and transportation device made of hydrogen storage alloys is actually a metal-hydrogen reactor, which is divided into stationary and mobile types. In addition to the basic performance of the hydrogen storage alloy, such as high hydrogen storage capacity, it also requires this device to have good heat exchange characteristics, so that the alloy can discharge and supply heat in a timely manner during the hydrogen absorption and release process, and secondly, the container containing the alloy must have good air tightness. , Pressure resistance, corrosion resistance, hydrogen embrittlement resistance. At present, such devices developed in tests include tube type, heat tube type, internal baffle type, cylindrical type, unit stacked type, etc. The tested hydrogen storage alloys include MmNi4.5Mn0.5, TiFe series alloy, Mg series alloy, etc. Magnesium-based alloys have special advantages in mobile hydrogen storage devices for automobiles due to their light weight and large hydrogen storage capacity. However, magnesium-based alloys are high-temperature hydrogen storage alloys, and a large amount of heat exchange during the hydrogen absorption and desorption process must be solved. problem.
3. Hydrogen recovery, separation and purification
Petrochemical and other industries often have a large amount of hydrogen-containing tail gas. For example, synthetic ammonia tail gas contains 50% to 60% H2. Pass the hydrogen-containing tail gas through a separation bed equipped with a hydrogen storage alloy. The hydrogen is absorbed by the alloy, and then the alloy can be heated. Get pure hydrogen. A recycling device made of LaNi5 alloy jointly developed by American Air Products and Chemical Products Company and MPD Company recovers synthetic ammonia tail gas with a hydrogen recovery rate of 75% -95% and a product hydrogen purity of 98.9%. The hydrogen storage alloy can also be used for the purification of hydrogen. For example, the use of Mm2Ni4.5Mn0.5 alloy can increase the purity of industrial hydrogen to 99.9999%. In the nuclear industry, the separation of hydrogen isotopes such as hydrogen H2, heavy hydrogen D2, neon T, etc., is achieved using the equilibrium pressure difference between the reaction of H2, D2, T and the alloy at the same temperature. The alloy used for hydrogen isotope separation is V0.9Cr0. 1. TiCr, etc. Hydrogen often contains O2, CO2, CO, SO2, H2O and other impurity gases, which are easy to poison the alloy. Therefore, hydrogen storage alloys used for hydrogen recovery, separation, and purification require good anti-toxic properties. The types and contents of impurity gases in hydrogen It has become one of the important benchmarks for selecting such hydrogen storage alloys. Studies have shown that the rare earth based hydrogen storage alloys have strong resistance to O2 and H2O poisoning, while the titanium based hydrogen storage alloys have strong resistance to CO2 and CO poisoning.
4. On-board hydrogen storage system
The hydrogen stored in the hydrogen storage alloy is used as a fuel instead of gasoline to drive a car. This car has no CO2 emissions and is a truly pollution-free car. In addition, it is not affected by the price of oil and there is no problem of exhaustion of resources. The biggest advantages of hydrogen storage alloys for hydrogen storage are the high volumetric hydrogen storage density and the high degree of safety. This is because hydrogen is stored atomically in the hydrogen storage alloy. There are many ways to store hydrogen, but from the figure below we can clearly see that storing hydrogen through hydrogen storage alloys greatly saves space and improves efficiency. Therefore, the application of hydrogen storage alloys in automobiles is regarded as the future development direction of automobiles.
Different materials to store the same volume of hydrogen
5. Application of thermal energy system and other fields
Hydrogen storage alloys can be used in heat storage devices, heat pumps (refrigeration, air conditioning), etc., using the thermal effects of hydrogen storage alloys in absorbing and releasing hydrogen. Hydrogen storage alloy heat storage devices can generally be used to recover industrial waste heat. The advantage of using hydrogen storage alloys to recover industrial waste heat is that the heat loss is small and heat energy at a higher temperature than the waste heat source can be obtained. The heat storage device experimentally developed by the Japan Institute of Chemical Technology is mainly composed of two interconnected heat storage tanks A and B. The heat storage tank is filled with Mg2Ni alloy, and the heat from the waste heat source heats the Mg2Ni alloy in the heat storage tank A to release The hydrogen flows to the heat storage tank B and is stored to achieve heat storage, and the hydrogen flows backward to release heat. Its heat storage capacity is about 4360kJ, which can effectively use industrial waste heat of 300 ~ 500 ℃. The key to the use of hydrogen storage alloys for heat storage is to select a suitable hydrogen storage alloy based on the temperature of the waste heat, the pressure of the alloy to absorb and release hydrogen, and the enthalpy. The working principle of the hydrogen storage alloy heat pump is: the hydrogen storage alloy splits the hydrogen at a certain temperature, pressurizes the hydrogen above its equilibrium pressure and then performs the hydrogenation reaction, thereby obtaining a temperature higher than the heat source, the same in the heat pump system There are two containers filled with hydrogen storage alloy, but the types of hydrogen storage alloy filled in the two containers are different. Use hydrogen storage alloy heat pump to cool or do air conditioning with high efficiency, low noise and no freon pollution. Hydrogen storage alloys can also be used to prepare metal powders, reaction catalysts, and hot-pressure sensors using the metal hydrogenation reaction pressure-temperature variation laws.