Electrolysis of water to produce H2 and O2
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1. Research progress on hydrogen production by electrolysis of water Hydrogen production by electrolysis of water is an important means to achieve industrial and low-cost preparation of H2, and can produce products with a purity of 99% to 99.9%. Every year, my country's electricity consumption for hydrogen production by electrolysis of water reaches more than (1.5×107) kW·h. When current passes between the electrodes, hydrogen is produced at the cathode, oxygen is produced at the anode, and water is electrolyzed [2]. The core part of the water electrolysis hydrogen production equipment is the electrolytic cell, and the electrode material is the key to the electrolytic cell. The quality of the electrode performance largely determines the cell voltage and energy consumption of water electrolysis, and directly affects the cost. The efficiency of providing electricity to decompose water to produce hydrogen is generally 75% to 85%. The process is simple and pollution-free, but the power consumption is large, so its application is subject to certain restrictions. The electrolysis of water is carried out in an electrolytic cell, which is filled with electrolyte and divided into an anode chamber and a cathode chamber by a diaphragm. Electrodes are placed in each chamber. Since water has very low conductivity, an aqueous solution (concentration of about 15%) with electrolyte is used. When current passes between the electrodes at a certain voltage, hydrogen is produced at the cathode and oxygen is produced at the anode, thereby achieving water electrolysis. Theoretically, platinum metals are the most ideal metals for water electrolysis electrodes, but in practice, nickel-plated iron electrodes are often used to reduce equipment and production costs. When water is electrolyzed, the electrode reaction formula is as follows [3]. In acidic solution, cathode reaction: 4H++4e=2H2∏=0V Anode reaction: 2H2O =4H++O2+4e∏=1.23V In alkaline solution, cathode reaction: 4H2O +4e=2H2+4OH∏=-0.828V Anode reaction: 4OH-=2H2O+O2+4e∏=0.401V As can be seen from the above formula, the overall reaction of water electrolysis is as follows, whether in acidic or alkaline solution. 2H2O=2H2+O2 The theoretical decomposition voltage of water has nothing to do with the pH value, so acidic or alkaline solutions can be used as electrolytes. However, from the perspective of electrolytic cell structure and material selection, the use of acidic solutions is prone to various faults. Therefore, alkaline solutions are now used in industry.
(1) Traditional alkaline electrolysis technology Alkaline water electrolysis is currently a common and mature method for preparing hydrogen. This method does not require high equipment, and the investment is mainly concentrated in the equipment; the hydrogen produced is of high purity, but the efficiency is not very high. The process is also relatively environmentally friendly and pollution-free, but it consumes a lot of electricity and is therefore subject to certain limitations. The pressure of water electrolysis in industry is generally between 1.65 and 2.2 V. The service life of the electrode material and the energy consumption of water electrolysis are key factors in evaluating the quality of alkaline water electrolysis electrode materials. When the current density is not large, the main influencing factor is the overpotential; when the current density increases, the overpotential and resistance voltage drop become the main factors of energy consumption. In practical applications, industrial electrodes should have the following features [3]: (1) high surface area; (2) high conductivity; (3) good electrocatalytic activity; (4) long-term mechanical and chemical stability; (5) small bubble precipitation; (6) high selectivity; (7) easy to obtain and low cost; (8) safety. Water electrolysis often requires a larger current density (above 4000 A/m2), so points 2 and 4 are more important. Because high conductivity can reduce the energy loss caused by ohmic polarization, high stability ensures the long life of electrode materials. 1 and 3 are the requirements for reducing the overpotential of hydrogen and oxygen evolution, and are also important indicators for evaluating electrode performance.
(2) Solid polymer electrolyte SPE water electrolysis technology Since the electrolyzer with liquid as electrolyte has low efficiency, is inconvenient to move, and often requires maintenance, people are actively looking for new electrolytes, which has prompted the development and application research of solid polymer electrolyte (SPE), also known as proton exchange membrane (PEM). At present, the electrolyzer uses solid Nafion perfluorosulfonic acid membrane as electrolyte. The electrode uses precious metals or their oxides with high catalytic performance, which are made into powder form with large specific surface area, and are bonded and pressed on both sides of the Nafion membrane using Teflon to form a stable combination of membrane and electrode.
(3) High-temperature steam electrolysis process Another method of producing hydrogen by water electrolysis is high-temperature steam electrolysis. This is a method derived from solid oxide fuel cells. The electrolysis chamber generally uses Y2O3-stabilized ZrO2 as the electrolyte. The higher the temperature, the lower the resistance. However, from the perspective of the heat resistance of the material, the upper temperature limit is preferably 1000℃. Usually, a mixed sintered body of nickel and ceramic is used as the cathode, and a conductive calcium titanium composite oxide is used as the anode.
2. Development of biological hydrogen production The topic of using microorganisms to produce hydrogen has been studied for decades. In the 1930s, the first report of bacterial dark fermentation to produce hydrogen was reported. Subsequently, in 1942, Gaffron and Rubin reported that green algae used light energy to produce hydrogen, and in 1949, Gest and Kamen discovered phototrophic hydrogen-producing bacteria. Spruit confirmed in 1958 that algae can produce hydrogen through direct photolysis without the need for the fixation of carbon dioxide. Healy (1970)'s research showed that when the light intensity is too high, the hydrogen production process of Chlamydomonas moewsuii will be inhibited due to the production of oxygen. During the energy crisis in the 1970s, a lot of research was done on biohydrogen production around the world. Thauer pointed out in 1976 that dark fermentation was difficult to apply in actual production because it could only produce 4 mol of hydrogen and 2 mol of acetic acid from 1 mol of glucose at most. Phototrophic bacteria can completely convert substrates such as organic acids into hydrogen, so since then, research on biohydrogen production has basically focused on photofermentation. In the early 1980s, the support for renewable energy in research and development programs (R&D) around the world gradually decreased. By the early 1990s, environmental problems were becoming increasingly serious, and people's attention was focused on alternative energy. With the support of biohydrogen production R&D in Germany, Japan, and the United States, the field of algae using light energy to produce hydrogen from water has been widely studied. However, the overall solar energy conversion efficiency in this process is still very low. On the other hand, dark fermentation and phototrophic bacteria can produce hydrogen from low-cost substrates or organic waste. Since it can both produce clean energy and treat organic waste, the US and Japanese governments have supported several long-term research programs. It is expected that the practical application of biohydrogen production technology will be realized in the middle of the 21st century. It has been more than half a century since the discovery of microbial hydrogen production, but biohydrogen production has not been applied in practice. Many technical problems, such as the screening of microorganisms, the design of reactors, and the optimization of operating conditions, remain to be solved, and the cost of this technology has also received attention. Economically speaking, biohydrogen production technology cannot compete with traditional chemical hydrogen production technology in the near future. However, from the perspective of environmental protection, the prospects for biohydrogen production will be very broad. Biohydrogen production includes: photosynthetic biohydrogen production system (also known as direct biophotolysis hydrogen production system); photolysis biohydrogen production system (also known as indirect biophotolysis hydrogen production system); photosynthetic heterotrophic bacteria water gas conversion reaction hydrogen production system; photofermentation biohydrogen production system; anaerobic fermentation biohydrogen production system (also known as dark fermentation biohydrogen production system); photosynthesis-fermentation hybrid biohydrogen production system; in vitro hydrogenase biohydrogen production system, etc. Hydrogen energy is a clean and high calorific value energy source. Using renewable water resources in nature to produce hydrogen is undoubtedly the preferred method for mankind in the future.
After more than half a century of research, although water electrolysis hydrogen production and bio-hydrogen production technology have made great progress, they are still basically in the development stage and have not yet been put into practical use. Various restrictive factors such as low solar energy conversion efficiency, high energy consumption of water electrolysis hydrogen production, product inhibition, operating conditions, etc. make the hydrogen production rate of existing hydrogen production systems not high enough or not economical, and many other bottlenecks need to be further broken through. In order to further reduce production costs and expand production efficiency, we will prepare for future commercial operations.
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