At present, the main body of China's energy structure is coal, oil, natural gas and other energy sources, and their utilization will inevitably produce products such as carbon dioxide. Therefore, it is imperative to develop green and clean energy, adjust energy power, and promote the clean and green transformation of energy structure under the premise of achieving the dual carbon goal.

Hydrogen energy has received widespread attention due to its completely green and clean characteristics. In recent years, with the gradual implementation of the dual carbon measures, the development of the entire upstream and downstream industry of hydrogen energy has become a top priority, promoting the application of hydrogen energy. At present, the most ideal way to produce hydrogen is through electrolysis of water. However, due to the need for improvement in technology and cost, there has not yet been a large-scale commercial application and promotion. However, in order to avoid the inevitable generation of carbon dioxide in the production process of gray hydrogen and blue hydrogen from deviating from the "dual carbon" goal, the development of electrolysis of water for hydrogen production technology is imperative.

Ion exchange membranes are required in hydrogen production by electrolysis of water, and ion exchange membranes are also one of their key structures. At present, proton exchange membrane technology (PEM) is widely used. PEM electrolysis hydrogen production technology can achieve fast start stop, which makes it very compatible with renewable energy sources with high volatility and intermittent characteristics. That is, it can be stored or directly used for electrolysis of water to produce hydrogen through renewable energy sources such as solar power and wind power, which is also the most ideal upstream hydrogen generation method. Unlike proton exchange membranes used in fuel cells, proton exchange membranes used in electrolysis of water are much thicker than those used in fuel cells. Taking Dongyue Future Hydrogen, a leading domestic proton exchange membrane enterprise, as an example, its proton exchange membrane used in fuel cells is about 15 microns, while the thickness of proton exchange membranes used in electrolysis of water exceeds 150 microns.

In previous articles, we have introduced that the mechanism of action of cation exchange membranes (proton exchange membranes) mainly involves the use of negatively charged sulfonic acid groups for cation transfer, such as proton exchange membranes that transfer H+. The negatively charged groups on the cation exchange membrane form a strong negative electric field, which attracts protons to pass through the membrane and repels anions to intercept them. The main reaction of electrolyzing water can be as follows: hydrogen ions receive electrons at the cathode and are reduced to hydrogen gas, while water molecules lose electrons at the anode and decompose into hydrogen ions and oxygen. Hydrogen ions produced at the anode spontaneously migrate to the cathode through proton exchange membranes due to the effect of an electric field, and are used for the generation of hydrogen gas at the cathode in a semi reaction.

Working principle of electrolytic water [1]

At present, the most successful and widely used proton exchange membrane for water electrolysis is still DuPont's Nafion membrane. In addition, there are other well-known companies producing membranes such as Flemion membrane, Acetex membrane, and Dow membrane, all of which belong to the perfluorosulfonic acid polymer membrane category. Perfluorosulfonic acid proton exchange membranes have excellent chemical stability, high mechanical strength, and outstanding ion conductivity at low temperatures and high humidity, which is beneficial for improving the efficiency of hydrogen production in water electrolysis. However, their disadvantage is that they are too expensive. The current mainstream perfluorosulfonic acid proton exchange membranes, such as Nafion membrane, Flemion membrane, Acetex membrane, and Dow membrane, differ mainly in the length of their fluorinated side chains. Generally speaking, the shorter the side chain, the greater the production difficulty. A higher sulfonic acid content can maintain the water content inside the membrane, resulting in better battery performance.

     To reduce the cost of perfluorinated sulfonic acid proton exchange membranes, there are currently many studies on the modification of perfluorinated sulfonic acid membranes. Through modification, the cost can be reduced to a certain extent while also meeting certain special performance requirements of the membrane. The current modification methods mainly include: firstly, modifying perfluorosulfonic acid membranes with polymers; Secondly, precious metal catalysts are deposited on the surface of the modified perfluorosulfonic acid membrane; The third is to modify the perfluorosulfonic acid film by surface treatment with etching [3]. There are studies on the modification of perfluorosulfonic acid membranes by chemically synthesizing hydrophobic aromatic polymers. Due to the obvious microphase separation structure between the components in the modified membranes, it can provide channels for proton transport, thereby improving the proton conductivity of the membrane [4]. By surface etching modification of the proton exchange membrane, the active area of the electrode and the binding force with the membrane can be increased, the wettability of the membrane can be improved, the membrane impedance can be reduced, and thus the water electrolysis performance can be improved. However, etching modification requires high parameter settings, otherwise it is not conducive to the water electrolysis process. This modification based on perfluorosulfonic acid membrane does not significantly reduce cost as Nafion membrane is still used as the raw material, but it is beneficial for improving performance.

In recent years, polymers such as polybenzimidazole (PBI), polyether ether ketone (PEEK), and polysulfone (PS), which have no proton conductivity or low proton conductivity, have received widespread attention due to their excellent mechanical properties, chemical stability, thermal stability, and low cost. At present, research mainly focuses on proton acid doping, radiation grafting modification, etc. to enhance its proton conductivity and apply it to PEM water electrolysis technology [5]. This type of composite membrane or fluorine-free proton exchange membrane can greatly reduce costs and is relatively inexpensive. Its disadvantage is that its electrical performance still lags behind that of Nafion membrane, and more research and development are needed at present. Adding some hydrophilic inorganic nanoparticles or proton acids to polymers such as polybenzimidazole (PBI), polyether ether ketone (PEEK), and polysulfone (PS) can greatly improve their conductivity and antioxidant capacity. The main challenge is that these fluorine-free aromatic hydrocarbon polymer membranes have the problem of low proton conductivity at low sulfonation degrees and high swelling at high sulfonation degrees. Therefore, it is currently difficult to achieve high proton exchange capacity while maintaining low swelling degrees. At present, research on composite membranes and fluorine-free proton exchange membranes is still in its early stages, and it is difficult to achieve commercial application and replace traditional perfluorosulfonic acid proton exchange membranes in electrolysis of water.

For a long time, hydrogen production in China has mainly been based on gray hydrogen and blue hydrogen, mostly produced by petrochemical enterprises through by-products in the production process to meet the small amount of hydrogen demand in our factory. The number of domestic enterprises that focus on hydrogen production is limited and development is slow. As is well known, traditional coal hydrogen production, natural gas hydrogen production, and other chemical raw material hydrogen production technologies all have disadvantages such as high energy consumption, high pollution, long process flow, and low hydrogen purity. As a result, the hydrogen produced often needs to be purified before it can be used in certain specific scenarios. The green hydrogen obtained by electrolysis water hydrogen production technology has the advantages of zero emissions and high product purity, with the key being high technical difficulty and high cost. According to data from Gaogong Lithium Power Grid, research on PEM pure water electrolysis technology in China is limited to a small number of scientific research units or enterprises, such as China Shipbuilding Industry 718 Institute, China Electric Power Fengye, Dalian Institute of Chemical Physics of the Chinese Academy of Sciences, Android, and Shandong SEX Hydrogen Energy, with a low degree of industrialization. However, foreign companies Proton and Hydrogenics have occupied the global market for PEM electrolysis water products, which has also encouraged technological innovation and domestic substitution in key technologies such as electrolysis water hydrogen production in recent years to avoid technological bottlenecks.

The electrolysis process of water requires high acidity and high potential, which places high demands on the acid resistance and mechanical strength of proton exchange membranes. In its industry report on proton exchange membranes, China International Capital Corporation (CICC) mentioned that the production of mainstream perfluorosulfonic acid proton exchange membranes currently has high technical requirements. This is not only reflected in the strict patent protection and high technical difficulty of its PSVE monomer synthesis, the difficulty in transporting tetrafluoroethylene monomer, and the need for independent production capacity. The polymerization of perfluorosulfonyl resin (PFSR) also faces certain difficulties. It is also reflected in the high technical requirements of different methods in proton exchange membrane formation (complex post-treatment by melt extrusion method and insufficient continuous casting method), as well as the balance between high mechanical strength and strong ion exchange capacity in its performance. This also makes it rare for domestic companies to have a complete industry chain for proton exchange membrane production.

The proton exchange membrane (PEM) hydrogen production technology for water electrolysis has a higher working current density (up to 1-3 A/cm2) compared to other types of water electrolysis methods. It has high efficiency in water electrolysis, a pollution-free reaction process, light and compact structure, smaller volume at the same power, and a hydrogen purity of 99.999%. It is considered the most promising water electrolysis technology. In summary, whether it is proton exchange membranes for fuel cells or water electrolysis, their core structure and principles are similar. Therefore, enterprises with proton exchange membrane production capabilities often have layouts in the production of these products. In the face of the high cost of Nafion membranes, which greatly limits the development of the electrolysis water hydrogen production industry, the replacement of domestically produced proton exchange membranes has also been one of the goals of development in the past decade. With the continuous changes in national policy orientation and technological innovation, the localization of low-cost proton exchange membranes will promote the vigorous development of China's entire industry in electrolysis of water for hydrogen production and hydrogen energy.



Reference materials

[1] S. Shiva Kumar, V. Himabindu. Hydrogen production by PEM water electrolysis - A review. Material Sci Energy Technol, 2 (2019), pp. 442-454, 10.1016/j.mset.2019.03.002.

[2] Lin Caishun. Research status of proton exchange membrane water electrolysis technology [J]. Hydrometallurgy, 2010,29 (02): 75-78 DOI:10.13355/j.cnki.sfyj.2010.02.026.

[3] Chen Junliang, Yu Jun, Zhang Mengsha. Research progress on proton exchange membranes for polymer electrolyte membrane water electrolyzers [J]. Chemical Progress, 2017, 36 (10): 3743-3750 DOI:10.16085/j.issn.1000-6613.2017-0116.

[4] DANYLIV O, GUENEAU C, IOJOIU C, et al. Polyaromatic ionomers with a high-frequency backbone and perfluorosulfonic acids for PEMFC [J] Electrochimica Acta, 2016, 214:182-191.

[5] Chen Xiaoyong. Proton exchange membranes for fuel cells [J]. Chemical propellants and polymer materials, 2009,7 (3): 16-20

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