Hydrogen storage technology: knocking on the hydrogen economy gate

In the future energy structure, a group of new energy represented by hydrogen energy will occupy an increasingly important position. As one of the important technologies in the field of energy storage, hydrogen storage is a key node that must be overcome for hydrogen energy applications. Experts predict that once the hydrogen storage technology matures, it will not only change the current energy structure, but also drive the rise of a number of new materials industries and provide strong support for the new synthesis routes such as the production of methanol from carbon dioxide.

The key to the use of hydrogen energy is hydrogen storage. On November 13, 2006, the main scientists in the international hydrogen community submitted a “Hundred Years Memorandum” of hydrogen energy to the leaders of the Group of Eight. In the memorandum, scientists pointed out that at the beginning of the 21st century, human beings are facing two major crises of climate change and traditional petrochemical energy, and hydrogen energy is optimally utilized in the solution to the aforementioned crisis. However, the application of hydrogen energy must overcome the issue of hydrogen storage.

Tsinghua University professor and domestic famous hydrogen energy expert Mao Zongqiang believes that after the petrochemical era, hydrogen can only come from water, and the primary energy of dissociated water is none other than non-renewable energy. Solar power generation and hydrogen production from electrolyzed water will be the most reliable source of hydrogen in the future. However, hydrogen storage involves the heat transfer and mass transfer in the porous interface micro-region of the material, as well as the dynamic excitation of hydrogen molecules and hydrogen atoms and their energy level migration. The situation is more complicated.

Taking a hydrogen-fueled vehicle as an example, a five-seat hydrogen fuel cell-powered car requires about 4 kg of hydrogen to travel 500 km, and the volume of the fuel tank is 50 to 60 liters. Therefore, the volumetric hydrogen storage density must reach 67 to 80 kg/cubic. Meter. The U.S. Department of Energy has proposed the minimum hydrogen storage requirement for a unit mass of hydrogen storage density of 6.5% and a unit volume of hydrogen storage density of 62 kg/m3. However, currently there are almost no hydrogen storage methods that can meet the minimum requirements of the US Energy Agency.

Therefore, hydrogen storage technology is the key to the application of hydrogen energy. Once the hydrogen storage technology is mature, the enthalpy that restricts the application of hydrogen energy will be broken, and hydrogen energy will have a lot to do in the fields of new energy vehicles and new types of fuel cells.

Hydrogen storage materials and technology are being explored by Mao Zongqiang. High-pressure hydrogen storage is the most widely used hydrogen storage method at present. Its advantages are obvious. It can provide enough hydrogen in an instant to ensure that hydrogen fuel vehicles run at high speed, and can also close valves in an instant and stop. Supply air. High-pressure hydrogen storage generally uses gas high-pressure hydrogen storage containers. These containers are made of a new lightweight composite material. The inner tube of the hydrogen cylinder is an aluminum alloy and is surrounded by high-strength carbon fibers impregnated with resin.

The principle of metal hydrogen storage is chemical hydrogen storage. Mao Zongqiang said that certain metals have a strong ability to capture hydrogen, and under a certain temperature and pressure, they can “absorb” hydrogen in large amounts, react to produce metal hydrides, and release heat at the same time. Afterwards, these metal hydrides are heated and they decompose again, releasing the hydrogen stored therein. These metals, which "take up" hydrogen, are called hydrogen storage alloys. The commonly used hydrogen storage alloys include rare earth, titanium, zirconium, and magnesium.

“Since the metal hydrogen storage requires high temperature working conditions and other factors, the current metal hydrogen storage remains in the laboratory stage.” Mao Zongqiang said that once the metal hydrogen storage technology matures, it will inevitably spawn rare earth, titanium, zirconium and magnesium. Waiting for a series of alloy materials.

Hydrogen storage of organic matter is also a promising hydrogen storage method. According to Mao Zongqiang, the hydrogen storage agent for organic liquid compounds is mainly benzene and quinoline. Its principle is to use benzene or ** to react with hydrogen to produce cycloethane or methylcyclohexane. The carrier is in a liquid state at 0.1 MPa at room temperature, and its storage and transportation are simple and easy, and hydrogen is generated by catalytic dehydrogenation reaction for use. The hydrogen storage technology has the characteristics of large hydrogen storage capacity, high energy density, and simple storage equipment, and has become a promising hydrogen storage technology.

In addition, carbonaceous hydrogen storage materials have also been of concern. Carbonaceous hydrogen storage materials are mainly high surface area activated carbon, graphite nanofibers and carbon nanotubes. After special processing of high specific surface area of ​​activated carbon, in 2 ~ 4MPa and ultra-low temperature, the quality of hydrogen storage density of up to 5.3% ~ 7.4%. Currently reported hydrogen storage carbon materials include carbon nanofibers, carbon nanotubes, and other high carbon atom cluster materials.

In addition to these traditional methods, scientists are currently actively exploring new hydrogen storage methods such as hydrogen storage in glass microspheres, high-pressure and liquid-hydrogen composite technologies, hydrogen storage alloys and high-pressure composite technologies, and hydrogen storage in underground caverns.

Among many hydrogen storage methods, one of the hydrogen storage methods that is expected to be industrialized in the near future is the hydrogen storage of inorganic substances. Mao Zongqiang said that many ionized hydrides, such as complex metal hydrides, can be separated to liberate hydrogen, and their theoretical mass storage hydrogen densities are as high as 19.6% and 10.7%, respectively. The current research mainly focuses on the release of hydrogen catalysts, the control of hydrogen absorption and release rates, and the reuse of hydrides. Once these technologies are mature, industrialization is not a problem.

Pave the way for the hydrogenation of carbon dioxide to make methanol With the increasing depletion of traditional petrochemical resources and the ever-increasing intensity of global carbon dioxide emission reduction, a group of green synthetic routes that can turn carbon dioxide into waste will become development hot spots. Carbon dioxide hydrogenation to methanol is such a new route. If carbon dioxide is used to make methanol, industrialization will lead to changes in the source of raw materials in the petrochemical industry. It is understood that carbon dioxide from methanol production once sparked a global debate on the “methanol economy”. Nobel Prize-winning scientist and renowned organic chemist George A. Ola once proposed that a cyclical model of hydrogen production from renewable energy sources and the use of CO2 hydrogenation to synthesize methanol can be used as a way to cope with energy shortages after the oil and gas era. Nobel Prize-winning physicist Carlo Lubia has also publicly proposed to substitute carbon dioxide for methanol to replace the now-popular carbon capture and storage, and to reduce emissions while providing raw materials for industry. The hydrogen production process and cost control are the key to the commercial application of carbon dioxide to methanol technology.

Our common ammonia gas is also an effective hydrogen carrier. Mao Zongqiang said that after the ammonia gas is decomposed and reformed, a large amount of hydrogen can be obtained, which is expected to become an important hydrogen storage method in the future. Once this hydrogen storage technology is successfully developed, it will change the hydrogen production process and greatly reduce costs. By then, the carbon dioxide to methanol route will have greater advantages.