Hydrogen production by electrolysis of water, especially using renewable energy sources, is currently the lowest carbon emission process among the many hydrogen sources, also known as "green hydrogen". At present, four technologies are mainly used for hydrogen production by electrolytic water: alkaline water (AKL) electrolysis technology, proton exchange membrane (PEM) electrolysis technology, solid oxide (SOEC) electrolysis technology and basic anion exchange membrane (AEM) electrolysis technology. Among them, SOEC technology and AEM technology is still in the laboratory stage, there are only a few pilot projects on the market, it is undeniable that these two technologies have their respective technical advantages, but there is still a long way to go from the laboratory to large-scale production applications.
At present, the really large-scale application of the project electrolyzer is alkaline electrolyzer and PEM electrolyzer, in China, alkaline electrolyzer accounts for more than 90%, and in foreign countries, PEM electrolyzer is the mainstream. In fact, PEM electrolyzers are more technologically advanced than alkaline electrolyzers.
First of all, the technology of alkaline electrolyzer has been very mature, and the production capacity of the equipment includes 1-2000Nm³/h, which is low in production difficulty and convenient in use. The more advanced technology of PEM electrolyzer precisely means that PEM electrolyzer has a greater design, production and use difficulty, and the production capacity of the largest PEM electrolyzer is only 200Nm³/h, which is difficult to meet the increasing demand for green hydrogen in the future before the large-scale problem is solved. Secondly, from a cost point of view, the cathode of PEM electrolyzer needs to use the precious metal iridium, and the anode needs the precious metal platinum. In terms of iridium alone, according to the current cathode iridium load of 2-5mg/cm2, 1 ton of iridium metal can only supply 1GW of PEM electrolytic cell installed capacity, and the annual output of iridium is only 5 tons. Converted to a single device, a 200Nm³/h PEM electrolyzer requires 1Kg of iridium, according to the current market price of $203/g, the cost of iridium alone is $200,000. The electrode of the alkaline electrolyzer is generally nickel metal, the cost is very low, and the overall cost of the alkaline electrolyzer is only a quarter of the PEM electrolyzer.
In terms of performance, alkaline electrolyzer and PEM electrolyzer have little difference in hydrogen production power consumption, system energy consumption and efficiency, and there is a significant difference in current density and start-up time. The current density of the traditional alkaline electrolyzer is generally only 2000-3000A/m2, while the current density of the PEM electrolyzer is generally more than 10000A/m2. A higher current density means that the volume of the PEM electrolyzer is smaller and occupies a smaller area under the same hydrogen production quantity. The heat production of the electrolyzer is faster after the current density of the PEM electrolyzer is increased. With less time to reach the operating temperature, cold start time is shorter.
However, with the development of technology, the current density and start-up time of alkaline electrolyzers have been further improved.
