Hydrogen has many colours and can be produced with a broad range of technologies. Green hydrogen is produced by water electrolysis with renewable electricity. Water electrolysis is the cleanest and most sustainable way to produce hydrogen.
Electrolysis is the electrochemical process splitting water into hydrogen and oxygen by supplying electrical (or thermal) energy given by the equation:
There are currently three main technologies for electrolysis:
Alkaline Electrolyser
Proton Exchange Membrane Electrolyser
Solid Oxide Electrolyser
The alkaline electrolyser (ALK) is a mature technology.
In an alkaline electrolyser, the electrolyte is usually a 25-30% aqueous KOH-solution and is operated at 60-90˚C. The electrodes are immersed in the liquid electrolyte, separated by a separator that only allows transport of ionic charges. Historically, the separator was made of asbestos, but is currently made of Zirfon PERL.
When a direct current is applied to the water, the water molecule is split into oxygen and hydrogen. The electrolyte let the ions be transported between the electrodes. The purity is 99,5-99,9% for the hydrogen.
The electrolysis reactions:
Proton exchange membrane (PEM) systems are based on the solid polymer electrolyte concept for water electrolysis introduced in the 1960s.
PEM electrolysers that are commercially available today, are more flexible and tend to have smaller footprint than the alkaline electrolysers.
A proton exchange membrane separates the two half-cells, and the electrodes are usually directly mounted on the membrane. The membrane only allows transportation of hydrogen ions. It is necessary to use noble metal catalysts like iridium for the anode and platinum for the cathode. Water is supplied at the anode. The cell temperature of a PEM cell is 50-80˚C.
The electrolysis reactions are as follow:
Electrolysis of water can be performed at high temperatures using steam.
A Solid Oxide Electrolyser (SOEL) is a high-temperature electrolyser that performs a solid oxide electrolysis and operates at temperatures of 700-900˚C. The technology is currently immature and has only been tested at laboratory scale. Hig-temperature operation results in higher electrical efficiencies than alkaline and PEM, but it has challenges in material stability and also depends on waste heat. The high temperature steam is either supplied by an external heat source or by an electrical heater, therefore the applicability of SOEL is limited to specific instances (read more in part 2).
SOEL uses a solid ion-conducting ceramic as the electrolyte and comprise of three layers. Yttria-stabilized zirconia is often used as electrolyte.
The half reduction equations:
Project Manager / Maritime Energy Advisor, Greensight
Tine is a Marine Engineer with specialization in offshore wind from NTNU. With a background as Package Lead in TechnipFMC, she has extensive knowledge on project execution, engineering and technology transfer, as well as being Greensight’s expert on the maritime market.
We are discussing different electrolysers technologies in three parts. Part 2 present hydrogen applications, while advantages and disadvantages are discussed in part 3.
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