Detailed Explanation of the Four Major Water Electrolysis Hydrogen Production Technologies
With the proposal of the national "carbon peaking and carbon neutrality" goals, clean energy has been gradually popularized. Water electrolysis hydrogen production technology has stood out as one of the promising green hydrogen production processes with practical application value.
According to the differences in the operating environment of water electrolysis hydrogen production systems and the types of diaphragms used in electrolyzers, water electrolysis hydrogen production technologies are mainly divided into four categories: Alkaline Water Electrolysis (ALK), Proton Exchange Membrane Water Electrolysis (PEM), Anion Exchange Membrane Water Electrolysis (AEM) and Solid Oxide Electrolysis Cell (SOEC).
Alkaline Water Electrolysis for Hydrogen Production (ALK)
Alkaline water electrolysis is recognized as the most mature and cost-effective water electrolysis technology for hydrogen production. It typically uses potassium hydroxide (KOH) or sodium hydroxide (NaOH) as the electrolyte, with a concentration ranging from 20% to 30%. Porous polymer materials such as polyphenylene sulfide (PPS) and polysulfone are mostly adopted for the diaphragms. Its working principle is as follows: a direct current is applied between two electrodes, and the anode and cathode are separated by a diaphragm. At the cathode, water molecules are reduced to generate hydrogen gas and hydroxide ions. The generated hydroxide ions pass through the diaphragm to reach the anode, where they lose electrons and undergo oxygen evolution reaction, producing oxygen gas and water molecules. An alkaline water electrolysis system mainly consists of two parts: the alkaline electrolyzer main body and the balance of plant (BOP) auxiliary system. The electrolyzer main body is assembled from components including end pressure plates, gaskets, polar plates, electrodes, and diaphragms.
The primary drawbacks of alkaline water electrolysis technology lie in its low operating current density, low electrolyzer efficiency, and large floor space requirement. Particularly in winter, the equipment requires a long preheating period, with a start-up time of approximately 2 hours. Nevertheless, in China, the processing and preparation technologies for alkaline electrolyzers, diaphragms, and other related equipment and materials have been basically mature, and the industrial chain is relatively complete. Thus, it is currently the most suitable technical route for large-scale application in the country. According to research findings, the maximum hydrogen production capacity of a single domestic electrolyzer has reached 3000 Nm³/h, and the minimum direct current consumption of the electrolyzer can be as low as 4.2 kW·h/Nm³.
Proton Exchange Membrane Water Electrolysis for Hydrogen Production (PEM)
Proton exchange membrane water electrolysis technology uses a proton exchange membrane as the diaphragm without any liquid electrolyte. The diaphragm is generally made of perfluorosulfonic acid membrane, which has already been put into small-scale application. Compared with alkaline water electrolysis devices, proton exchange membrane electrolysis devices have advantages such as high flexibility, small floor space, and high output pressure, but they are faced with problems including short service life of electrolyzers and high equipment costs. Its working principle is as follows: at the anode, water molecules decompose to produce oxygen and hydrogen protons; the hydrogen protons migrate to the cathode and are then reduced to generate hydrogen gas.
The emergence of PEM water electrolysis technology is attributed to the development of proton exchange membranes, or solid polymer electrolytes. The application of PEM reduces the distance between the anode and cathode to several hundred microns or even tens of microns, which significantly cuts down the energy consumption caused by ion migration. The operating cell voltage of this water electrolysis method is around 2.0 V. Although the cell voltage is not significantly reduced, its operating current density is much higher than that of alkaline water electrolysis. Overall, it is more competitive in terms of energy consumption reduction.
Anion Exchange Membrane Water Electrolysis (AEM)
Based on the research concept of integrating the respective advantages of alkaline water electrolysis and PEM water electrolysis, the water electrolysis hydrogen production technology that uses solid polymer anion exchange membranes (AEM) to replace PEM proton exchange membranes has become a new research direction. Anion exchange membrane water electrolysis (AEM) refers to a hydrogen production process that adopts low-cost anion exchange membranes as diaphragms, low-concentration alkaline solutions or pure water as electrolytes, and non-precious metal catalysts as reaction catalysts.
AEM water electrolysis technology combines the advantages of ALK water electrolysis technology and PEM water electrolysis technology. Compared with PEM water electrolysis, AEM water electrolysis uses solid polymer anion exchange membranes as diaphragm materials, and has a wider range of options for membrane electrode catalysts and bipolar plate materials. In the future, breakthroughs in key materials such as anion exchange membranes and high-activity non-precious metal catalysts are expected to significantly reduce the manufacturing cost of electrolyzers.
Solid Oxide Electrolysis Cell for Hydrogen Production (SOEC)
Solid oxide water electrolysis refers to a process that converts electrical energy and thermal energy into chemical energy at high temperatures. Thanks to the high-temperature environment, the activity of the catalyst is significantly enhanced, which reduces the energy consumption of water splitting. The efficiency of high-temperature water electrolysis is also very high, reaching up to 90%. In addition to the advantage of low energy consumption, solid oxide water electrolysis has another prominent benefit: since it uses a solid electrolyte, it is less challenging to address corrosion issues. Similar to PEM water electrolysis, it has relatively low requirements for fluid distribution and control. However, the excessively high operating temperature leads to unsatisfactory chemical stability and mechanical stress resistance of the materials, which are prone to sintering and a decline in catalytic activity. At present, this technology is still in the laboratory testing stage.
As the earliest water electrolysis hydrogen production technology to achieve industrialization, alkaline water electrolysis (ALK) is already capable of large-scale hydrogen production applications, with megawatt-level hydrogen production systems now operational in China. From its initial adoption in the aerospace industry, to its subsequent use in supplying hydrogen for generator cooling in coal-fired power plants, and now to large-scale hydrogen production coupled with renewable energy sources, ALK technology has been the primary choice.
Proton exchange membrane (PEM) hydrogen production has developed rapidly over the past decade. Compared with the alkaline water electrolysis process, it features a smaller footprint and higher compatibility with renewable energy sources, and small-scale commercial operations have been realized in China. Anion exchange membrane (AEM) water electrolysis for hydrogen production is gradually transitioning from small-scale research applications to commercial demonstration phases. However, in terms of cost, energy efficiency, and service life, it still has a long way to go before large-scale commercial application can be achieved.
Overall, large-scale hydrogen production from renewable energy sources represents a beacon of hope for green and low-carbon energy transition as well as climate change mitigation, and it is also a crucial pathway for achieving economic structural transformation. With the rapid development of the hydrogen energy industry and policy support, large-scale deployment, cost reduction, and energy consumption reduction have become the industry’s consensus. The market demand and application prospects for water electrolysis hydrogen production will be broader.
Source: Wuxi Hydrogen Energy (Official WeChat Account)
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