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. The next weeks we will discuss the different electrolyser technologies.
We will discuss the technologies in three parts:
- Part 1: Electrolyser technologies
- Part 2: Application (link)
- Part 3: Advantages and disadvantages of the different technologies(link)
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. The 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:
The resulting purity is higher than for alkaline and is typically greater than 99,99% H2. PEM has a compact module design because of the solid electrolyte and has a high current density operation compared to alkaline.
Solid Oxide Electrolyser
Electrolysis of water can be performed at high temperature using steam. A Solide Oxide Electrolyser (SOEL) is a high-temperature electrolyser that perform 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. High temperature operation results in higher electrical efficiencies than alkaline and PEM, but it has challenges in material stability and also depend 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 (more in part 2)
SOEL use 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: