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Hydrogen technology is seen as a new hope for the energy transition in the context of increasing energy demand and scarcity of natural resources. Hydrogen is a key raw material for the chemical industry and can be used as a carbon-free fuel for industrial and transportation applications. "Green hydrogen production" refers to the use of renewable energy generation, through water electrolysis to produce hydrogen energy. This is a green and clean way of energy production that can "decarbonize" energy production. However, the cost of green hydrogen production is high compared with the steam reforming of carbon-based energy sources, such as methane, used for most industrial hydrogen production. Although the cost of generating electricity from renewable sources is low, the investment cost of electrolyzers is relatively high.
In 1789, the electrolysis of water to produce gas was first discovered, which started the development of hydrogen production from water electrolysis for more than two centuries. In 1800, water electrolysis was successfully achieved by voltaic reactor, and its gas products were determined as H2 and ○ 2. Then in the 1830s, Faraday's law put forward the scientific definition of water electrolysis, and the research of various electrolytic cell devices promoted the continuous development of hydrogen production technology from water electrolysis. Although people's cognition and demand for hydrogen energy are not complete, the development of electrolysis water hydrogen production technology is not smooth, but it does not interrupt the development of technology. With the adjustment of energy structure, the important position of hydrogen energy gradually appears, and the technology of electrolysis of water to produce hydrogen also develops rapidly. The basic principle of water electrolysis for hydrogen production is that water is used as raw material, and external voltage is applied to form a complete electric circuit. The injection of electric energy breaks the internal balance of water molecules, and cleavage occurs. Hydrogen and oxygen atoms are reconstructed, and H2 and O2 are finally released. Water electrolysis is the process of splitting water into hydrogen and oxygen by direct current through aqueous solution of KOH or NaOH. The chemical reactions are as follows:
Cathode: 2 h2o + 2 e - H2 + 2 oh -
Anode: oh - 2 1/2 o2 and H2O + e
Total equation: H2O= H2+1/2O2
2.1 Anion exchange membrane technology
In contrast to AEL or proton exchange membrane processes, AEM electrolysis combines the advantages of both. The structure of the AEM electrolyzer is similar to that of the PEM: a membrane, also called an ionomer, made of an ionically conducting plastic separates the electrodes on either side of the membrane. The electrodes are also made of ionomers and doped with catalyst particles. Unlike PEM, AEM electrolyzers can rely on non-noble metal catalysts such as nickel base, thus effectively reducing the material cost. Like the AEL process, the AEM reaction will take place in an alkaline environment. The water is electrolysed at the cathode to produce hydrogen. Other features of AEM electrolysis include high current density, high efficiency and flexibility. Although initial progress has been made, the focus of the laboratory remains on optimizing the membrane formulation. An important factor affecting its efficiency is the contact resistance between the membrane and the electrode. To make the resistance as small as possible, we need a good ion connection between the membrane and the electrode. Therefore, we not only need to continue to optimize the polymer formulation of the membrane, but also need to customize and develop electrode adhesives for the membrane. In addition, the team is also in the process of further optimization, such as coating, to achieve mass production of membrane materials.
2.2 Hydrogen production by electrolysis of alkaline water
Hydrogen production from alkaline water is one of the most mature hydrogen production technologies and the most commercialized hydrogen production technologies. As early as the beginning of the 20th century, large-scale hydrogen production equipment of 1MW level has been developed. The equipment for hydrogen production from alkaline water is mainly composed of electrolyte, anode, cathode and diaphragm. Alkaline electrolyzers usually use potassium hydroxide solution (20%~30%) as electrolyte. In the process of hydrogen production from water electrolysis, lye is not consumed and only plays the role of ion transport. Hydrogen production from alkaline water electrolysis is the earliest research and development technology and the most mature technology. It has the advantages of simple operation and low cost, but the disadvantages are low efficiency of electrolysis, poor adaptability of power supply fluctuation, strong causticity of lye, etc. The operating temperature of alkaline electrolytic cell is usually 40~80℃, and the conductivity of electrolyte in the electrolytic cell increases with the increase of temperature. Since the electrolyte is at room temperature during cold start, its resistance value is large, and the voltage of the electrolyzer is constant, so the current is small and the hydrogen production is small. The temperature of the lye increases with the increase of the conduction time. With the increase of lye conductivity, hydrogen production will increase gradually. The alkaline water electrolytic cell can be operated under 20%~110% variable load, with cold start for 2~3h and hot start for 15min. In order to solve the problem of non-designed operation capacity of electrolytic alkali water hydrogen production equipment, the usual method is hot standby, which keeps the alkaline solution in the electrolytic cell at a certain temperature, so as to start hydrogen production in time.
2.3PEM water electrolyzer
The proton exchange membrane is used to conduct protons in the water electrolyzer to insulate the gas on both sides of the electrode and avoid the disadvantage of using strong alkaline liquid electrolyte in AWE. Proton exchange membrane hydrolyser uses proton exchange membrane as electrolyte and pure water as reactant. In addition, PEM has low hydrogen permeability and produces high purity hydrogen, so only water vapor needs to be removed. With zero spacing structure and low ohmic resistance, the overall efficiency of electrolytic process is significantly improved and the volume is more compact. Wide pressure control range, hydrogen output pressure up to several mpa, suitable for rapidly changing renewable power input. Therefore, proton exchange membrane electrolysis for hydrogen production is a promising green hydrogen production technology. It should also be noted that the bottleneck of PEM hydroelectrolysis hydrogen production is cost and life. The cost of bipolar plate electrolyzer and membrane electrode is about 48% and 10% respectively. At present, the international advanced level of PEM is as follows: single cell performance of 2A cm -- 2@2V, total platinum catalyst load of 2~3mg/cm2, stable operation time of 6×104~8×104h, hydrogen production cost of about $3.7 per kg hydrogen. The research of reducing the cost of proton exchange membrane electrolyzer mainly focuses on the core components such as membrane electrode, gas diffusion layer and bipolar plate based on catalyst and proton exchange membrane.
3. Common abnormalities and improvement measures of hydrohydrogen electrolytic cell
Symptom 1: The voltage of individual cells in the electrolyzer rises during regular monitoring, sometimes exceeding 3.0 V. Disposal method: close and release the pressure, open the stop valve under the pressure increasing plate of the electrolyzer, and unload the electrolyte containing impurities. Rinse the electrolyzer with distilled water or nitrogen. Then, after filtering the precipitate or repreparing the electrolyte, the electrolyte is placed in the tank. Continue to monitor unit voltage after startup. If not, the battery voltage in the adjacent pole frame can be shorted to more than 3.0V, so no gas is produced. After doing this, the errors are usually eliminated.
Disturbance phenomenon 2: alkali leakage or gas leakage on the insulation seal. Troubleshooting method: First clean lye leakage with distilled water, tighten tensile bolts and nuts correctly after drying (apply uniform force), start the machine, detect bubbles with soapy water, if no bubbles, then troubleshoot.
Fault phenomenon 3: The total voltage of the electrolyzer is too high, and the rectifier current cannot be set to the rated value. Solution: When the electrolyte concentration is normal and the rectifier device is in good condition, when the BAD temperature is higher than 80℃, the voltage is higher than 16 V, and the current is lower than 150 A, stop the machine to replace the electrolyte in the electrolyte. The electrolyte cannot be used again after replacement. Prepare a new electrolyte and add it to the electrolyzer. Fault phenomenon 4: the foundation of the positive plate of the electrolyzer is smoking and igniting. Elimination method: immediately cut off the power supply, remove the insulation pipe and insulation bottom plate, remove the small parts on it, rinse with water and dry. If any burn marks are found, they must be replaced and installed before use.
Symptom 5: In the process of hydrogen production, gas units are found to be less and less. Elimination method: When the alkali leaching is normal and the control pressure difference is normal, the fault is caused by the fracture of asbestos membrane tissue in the tank. Stop the machine at this time, otherwise it will be dangerous. Replace the asbestos film cloth and reassemble.
