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The ability of hydrogen to react with oxygen was first noticed by Henry Cavendish in 1766. The first electrolyzer subsequently appeared in 1800, when Nicholson and Carlisle induced a static charge in water. From then until today, electrolyzers are used for a variety of applications.

The electrolyzer is an electrochemical device that separates water into its components - oxygen and hydrogen, by means of an electric charge. The process by which this reaction takes place is called electrolysis. The three main building blocks of the electrolyzer are a cathode (negative charge), a membrane, and an anode (positive charge). According to the type of the electrolyte, the electrolyzers could be the following:


Polymer electrolyte membrane (PEM) electrolyzer Source: Wikipedia

Polymer electrolyte membrane (PEM) electrolyzer

The electrolyzers that use a polymer electrolyte membrane (PEM) are currently one of the most promising types in the market. The electrolyte used in the PEM equipments is a thin, solid ion-conducting membrane. One of the advantages of these electrolyzers is that they use a bipolar design and can be made to operate on high differential pressures across the membrane. Other important advantages are the high-purity hydrogen and the easy cooling of the system due to the low temperatures of operation. Due to the latter, the PEM electrolyzers are able to meet the intermittency of renewable energy sources such as wind and solar. However, the biggest drawback of the PEM electrolyzers is their cost. They are slightly more expensive than the alternatives, due to the usage of precious metals as catalysts. 

Alkaline water electrolyzer (AEL) Source: Ahmad Kamaroddin, Mohd Fadhzir & Sabli, Nordin & Tuan Abdullah, Tuan Amran. (2018). Hydrogen Production by Membrane Water Splitting Technologies. 10.5772/intechopen.76727.

Alkaline water electrolyzer (AEL)

The alkaline water electrolyzers have been used for more than a century. They are regarded as one of the most promising technologies to decarbonize the industry. The main difference with the PEM electrolyzers is that the AELs are using a liquid electrolyte, whereas the PEMs are using a solid one. The main electrolytes used in the AELs are potassium hydroxide (KOH) or sodium hydroxide (NaOH). The AELs operate by transporting hydroxide ions (OH-) through the electrolyte from the cathode to the anode with hydrogen being generated on the cathode side. Alkaline electrolyzers work well at operating temperatures between 20 – 110 °C and pressures of 1 – 40 bar. The main advantage of the AELs over the PEMs is their lower price, however, most AEL electrolyzers are bulky and the hydrogen produced by them needs additional purification and they lack the flexibility of operation.

Solid oxide electrolyzer (SOEC) Source: Sohal, Manpreet & Virkar, Anil & Rashkeev, Sergey & Glazoff, Michael. (2021). Modeling Degradation in Solid Oxide Electrolysis Cells. 10.2172/993195.

Solid oxide electrolyzer (SOEC)

The solid oxide electrolyzers (SOEC) operate in very high temperatures between 500o and 850o degrees. These types of electrolyzers are relatively new and not yet commercially available. The process that occurs in the system and splits the water molecule into oxygen and hydrogen is called high-temperature electrolysis (HTE) or steam electrolysis. Electrons from the external circuit combine with water at the cathode to form hydrogen gas and negatively charged ions. Oxygen then passes through the sliding ceramic membrane and reacts at the anode to form oxygen gas and generate electrons for the external circuit. Due to the high temperature of the operation, they have the potential to be much more energy-efficient than the AEL and PEM electrolyzers.

Anion exchange membrane (AEM) electrolyzer Source: Ito, Hiroshi & Miyazaki, Naoki & Sugiyama, Shota & Ishida, Masayoshi & Nakamura, Yuka & Iwasaki, Shinya & Hasegawa, Yasuo & Nakano, A.. (2018). Investigations on electrode configurations for anion exchange membrane electrolysis. Journal of Applied Electrochemistry. 48. 10.1007/s10800-018-1159-5.

Anion exchange membrane (AEM) electrolyzer

An anion exchange membrane in the AEM electrolyzer acts as a semipermeable membrane through which anions permeate while oxygen or hydrogen are impermeable through the membrane in an electrochemical reaction. This exchange process can produce high purity hydrogen. The anion exchange membranes are an attractive alternative to the PEM membranes, due to their low cost of production and ease of manufacturing.