Many scientists believe that hydrogen energy may become a pivotal secondary energy source on the world energy stage in the 21st century. In nature, hydrogen is easily combined with oxygen to form water, and hydrogen must be separated from water by electrolysis. The basic way of high-efficiency hydrogen production is to use solar energy. If solar energy can be used to produce hydrogen, it is equivalent to transforming endless, scattered solar energy into highly concentrated clean energy, which is of great significance.
At present, the methods of using solar energy to split water to produce hydrogen include solar thermal water splitting to produce hydrogen, solar power generation to electrolyze water to produce hydrogen, sunlight to catalyze photo-splitting of water to produce hydrogen, and solar biological hydrogen production, etc. The use of solar energy to produce hydrogen has great practical significance, but it is a very difficult research topic, and there are a lot of theoretical and engineering problems to be solved. And progress has been made in many aspects.
At present, the main methods of industrial-scale hydrogen production are as follows:
(1) Hydrogen production from hydrocarbon-containing fossil fuels. This was and is the most used method. It uses fossil fuels such as coal, oil or natural gas as raw materials to produce hydrogen. The basic reaction process of using steam and coal as raw materials to produce hydrogen is: C+H2O→CO+H2; the chemical reaction of hydrogen production using steam and natural gas as raw materials is: CH4+H2O→CO+3H2. The above reactions are all endothermic reactions, and the heat required in the reaction process can be obtained from the partial combustion of coal or natural gas, or an external heat source can be used. Since the large-scale exploitation of natural gas, 96% of hydrogen production is now based on natural gas. Natural gas and coal are both valuable fuels and chemical raw materials, their reserves are limited, and the hydrogen production process will pollute the environment. Using them to produce hydrogen obviously cannot get rid of people's dependence on conventional energy and damage to the natural environment.
(2) Hydrogen production by electrolysis of water. This method is based on the following reversible hydrogen-oxygen reaction: 2H2O→2H2+O2. The energy required to split water is provided by external electrical energy. In order to improve the efficiency of hydrogen production, electrolysis is usually carried out under high pressure, and the pressure used is mostly 3.0-5.0MPa. The current electrolysis efficiency is about 50% to 70%. Due to the low efficiency of water electrolysis and the consumption of a large amount of electricity, it is obviously uneconomical to use electricity produced by conventional energy sources to produce hydrogen by electrolysis of water on a large scale.
(3) Thermochemical hydrogen production. In this method, hydrogen is obtained by chemically decomposing water by adding high temperature heat. So far, there are various thermochemical hydrogen production methods, but the overall efficiency is not high, only 20%-50%, and there are still many process problems to be solved. Relying on this method for large-scale hydrogen production remains to be further studied.
(4) Biomass hydrogen production. Biomass hydrogen production is artificially imitating plant photosynthesis to decompose water to produce hydrogen. At present, the United States and the United Kingdom can produce 1 liter of hydrogen per hour with 1 gram of chlorophyll, and its conversion efficiency is as high as 75%.
According to the research of scientists, besides water, the raw material for hydrogen production can also use microorganisms to produce hydrogen. The initial exploration in this area was probably around 1942. Scientists first discovered that some algae's intact cells can use sunlight to generate a flow of hydrogen. Seven years later, another scientist proved through experiments that some fungi with photosynthesis can also produce hydrogen. Since then, many scientists have carried out research on the use of microorganisms to generate hydrogen from different angles. In recent years, 16 species of green algae and 3 species of red algae have been identified as capable of producing hydrogen. Algae produce hydrogen mainly through their own dehydrogenase, using inexhaustible water and gratuitous solar energy. It can be said that this is a form of solar energy conversion and utilization under the action of microorganisms. This hydrogen production process can be carried out at a lower temperature of 15-40 °C.
Biomass hydrogen production methods can be divided into three categories: biomass thermochemical gasification; biomass liquefaction and then conversion to hydrogen production; microbial chemical decomposition methods, including microbial anaerobic digestion, fermentation and metabolism methods. Due to its advantages of low energy consumption and environmental protection, biological hydrogen production technology has become a research hotspot in China and abroad, and will become the main development direction of hydrogen energy production technology in the future.
How hydrogen is stored
Hydrogen exists in gaseous form under normal conditions, which brings great difficulties to storage and transportation. There are three methods of hydrogen storage: high pressure gaseous storage, cryogenic liquid hydrogen storage and metal hydride storage.
1. High pressure gas storage
Gaseous hydrogen can be stored in underground warehouses or in cylinders. In order to reduce the storage volume, the hydrogen must be compressed first, which requires more compression work. In order to improve the hydrogen storage capacity, a hydrogen storage device with a microporous structure is currently being researched. in micropores. Microspheres can be made of plastic, glass and ceramic or metal.
2. Low temperature liquid hydrogen storage
The hydrogen gas is cooled to -253°C to become a liquid and then stored in a high vacuum insulated container. The liquid hydrogen storage process was first used in aerospace, its storage cost is relatively expensive, and its safety technology is also relatively complex.
3. Metal Hydride Storage
The hydrogenation metals used for hydrogen storage are mostly alloys composed of various elements. At present, a variety of hydrogen storage alloys have been successfully studied in the world, and they can be roughly divided into four categories: one is rare earth lanthanum nickel, etc., each kilogram of lanthanum nickel alloy can store 153L of hydrogen. The second is iron-titanium series, which is currently the most used hydrogen storage material. Its hydrogen storage capacity is 4 times that of the former, and it is low in price and high in activity. It can also release hydrogen at room temperature and pressure, which brings great advantages to the use. Great convenience. The third is magnesium, which is the metal element with the largest amount of hydrogen absorption, but it needs to be released at 287°C. The fourth is vanadium, niobium, zirconium and other multi-element systems. These metals are rare and precious metals, so they are only suitable for some special occasions.
Solid-state hydrogen storage methods mainly have the following potential advantages: smaller volume, lower pressure (higher energy efficiency) and more high-purity hydrogen production. Compressed gas and liquid storage is commercially viable today, but fully cost-effective storage systems have yet to be developed.
On-board hydrogen storage methods studied in the United States in 2005 include advanced metal hydride carbon-based materials and other high surface area adsorbent materials, chemical hydrogen storage materials, low-cost applicable tanks, compressed/low temperature hydrogen tanks, and new materials or processing methods such as cage-like Inclusion compounds and conductive polymers. Compressed/cryogenic tanks, metal hydrides, high-surface-area adsorbent materials, and carbon-based materials constitute on-board reversible hydrogen storage systems, as hydrogen regeneration and hydrogen absorption can occur onboard the vehicle. For chemical hydrogen storage methods and some metal hydrides, it is not possible for hydrogen regeneration to take place on the vehicle, so off-vehicle regeneration is necessary for these systems.
The R&D activities of the Korea Hydrogen Energy R&D Center are focused on three main areas: hydrides, nanomaterials, and compressed gas storage tanks. The current projects of the Korea Hydrogen Energy R&D Center engaged in the research and development of hydrogen storage materials mainly focus on the following aspects: the development of high-voltage storage systems for fuel cell vehicles, the development of high-performance hydrogen storage metal hydride materials, carbon nanomaterials for hydrogen storage, Carbon nano-hydrogen storage and chemical hydride storage and release.