It is a type of electrolyzer that is characterized by having two electrodes operating in a liquid alkaline electrolyte solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH).
These electrodes are separated by a diaphragm, separating the product gases and transporting the hydroxide ions (OH−) from one electrode to the other. Currently, AEL systems are available in atmospheric (legacy technology) or pressurized versions
PEM electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE) that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes.
The PEM electrolyzer, which involves a proton-exchange membrane, was introduced to overcome the disadvantages of the alkaline electrolyzer technologies. PEM aims to solve the issues of partial load, low current density, and low-pressure operation.
A PEM system relieson rare materials, such as Titanium, Platinum or Iridium.
SOECs use steam instead of water for hydrogen production, a key difference from alkaline and PEM electrolyzers. Additionally, SOECs use ceramics as the electrolyte, resulting in low material costs. While they operate at high temperatures and with high electrical efficiencies of 79-84% (LHV), they require a heat source to produce steam.
SOEC electrolyzers can also be operated in reverse mode as fuel cells to convert hydrogen back into electricity, another feature that is distinct from alkaline and PEM electrolyzers (IEA, 2021).
An AEM electrolysis solution combines the benefits of PEM and alkaline systems by allowing the use of non-noble catalysts while achieving energy densities and efficiencies comparable to PEM technology.
AEM electrolysis isstill a developing technology; therefore, with a view to using it to eventually achieve commercially viable hydrogen production, AEM electrolysis requires further investigation and improvements, specifically regarding its power efficiency, membrane stability, robustness, ease of handling, and costreduction (Vincent & Bessarobov, 2018).
A microbial electrolysis cell is a technology related to Microbial fuel cells (MFC). Whilst MFCs produce an electric current from the microbial decomposition of organic compounds, MECs partially reverse the process to generate hydrogen or methane from organic material by applying an electric current.
Membraneless electrolyzers are developed to eleiminate the disadvantages of membrane based electrolysis technologies. Eliminating the membrane creates the opportunity to decrease capital costs by reducing device complexity, materials costs, and assembly costs. Furthermore,the risk of membrane fouling or degradation is eliminated.
Membraneless electrolyzers generally rely onflow- or buoyancy-induced separation of products whereby forced fluid flow (advection) and/or buoyancy forces are used to separate the O2 and H2 products before they can cross over to the opposing electrode (Esposito, 2017).
