Electrolyzer
An electrolyser is a device in which a chemical reaction , i.e. a material conversion, is brought about with the help of an electric current : electrolysis takes place.
In line with the variety of different electrolyses, there is also a multitude of electrolysers: In chlor-alkali electrolysis, for example, those with membranes , diaphragms or mercury cells .
Electrolysers for water electrolysis
The water electrolysis , the decomposition of water into hydrogen and oxygen , takes place according to the following reaction equation :
The electrical energy requirement for the production of 1 cubic meter of hydrogen in the standard state defines the efficiency of an electrolyzer: In a modern high-pressure electrolyzer, this energy requirement under a pressure of 12 bar is around 4.8 kWh per m 3 in the standard state; the efficiency is thus 62.5% (based on the lower calorific value of hydrogen).
Several system manufacturers (e.g. H-Tec, Electrolyser Corp., Brown Boveri, Lurgi, De Nora, Epoch Energy Technology Corp.) offer large electrolysis devices with an efficiency of over 80%.
A distinction is made between the following types of electrolysers for water electrolysis:
Alkaline electrolyzer
In the case of an alkaline electrolyzer, with a direct voltage of at least 1.5 volts, hydrogen is formed at the cathode and oxygen at the anode . The alkaline reaction equation is:
Potash lye ( potassium hydroxide solution, KOH) with a concentration of 20–40% is used as the electrolyte . A gas-tight membrane, the so-called diaphragm , allows OH - ions to be transported, but at the same time prevents the resulting product gases from mixing.
As electrodes , so-called " DSA-electrodes " ( D imensions s tabile A noden, usually titanium electrodes with a ruthenium oxide coating) are used. These are expanded metals , with a noble metal - catalyst oxide - z. B. ruthenium or iridium oxide - are coated. However, there are also systems with Raney nickel catalysts in a gas diffusion electrode . Alkaline electrolysers are used on a large scale around the world.
Acid or PEM electrolyzer
In the proton exchange membrane electrolyser, distilled water is split into hydrogen and oxygen by an electric current. It consists of a proton-permeable polymer membrane (" proton exchange membrane " or " polymer electrolyte membrane ", " PEM " for short ). This is coated on the cathode side with a porous electrode made of platinum supported on carbon and on the anode side with metallic or noble metals (mostly iridium and ruthenium ) present as oxides . An external voltage is applied to these electrodes. Water is supplied to the anode side of the electrolyzer (both half-cells can also be flooded with water, or only the cathode side, this depends on the intended use).
The catalytic effect of the noble metal electrode leads to the decomposition of the water on the anode side: Oxygen, free electrons and positively charged H + ions are created. The hydrogen ions diffuse through the proton-conducting membrane to the cathode side, where they combine with the electrons to form hydrogen.
Reversible fuel cells based on PEM can work both as fuel cells and as electrolysers and can thus serve as energy storage in combination with a gas storage device.
High temperature electrolyser
High temperature electrolysers work at working temperatures of around 900 ° C. Part of the necessary enthalpy of reaction is coupled in as heat. This leads to a decrease in the power requirement for electrolysis, which increases the efficiency compared to aqueous, alkaline electrolysis. With high-temperature electrolysers, efficiencies of up to approx. 90% based on the calorific value can be achieved.
High temperature co-electrolysis
In high-temperature co-electrolysis, water and carbon dioxide are reduced together to form synthesis gas . From 2010 onwards, more and more scientific publications and studies were published on the procedure. The Solid Oxide Electrolyser Cell ( SOEC ) was registered for a patent in 1980, from 2008 various patents on the properties of the electrodes were submitted and later granted.
Molybdenum sulfide as a catalyst
In 2011, researchers at the ETH Lausanne accidentally discovered in an experiment that molybdenum sulfide can be used as an efficient catalyst instead of platinum. Molybdenum sulfide is much cheaper than platinum, so the investment for an electrolyser is reduced.
Nickel-iron electrolyser
In 2017, an electrolyser based on a nickel-iron accumulator was presented, which combines the properties of a conventional accumulator and an electrolyser. The system, named Battolyseur by the researchers, can initially be charged and discharged like a conventional accumulator . If the accumulator reaches its capacity limit and electricity continues to be supplied, hydrogen is produced instead. Due to its technical properties, this design is very well suited for storing energy as part of the energy transition .
Traditional electrolysers
The Hofmann water decomposition apparatus was developed in the 19th century. In addition to traditional water electrolysis , other processes such as melt flow electrolysis have developed.
application areas
Electrolysers can be used as the basis of so-called power-to-gas technology for storing electrical energy. If there is excess energy in times of high solar power or wind power generation , so-called renewable gas would be generated. Electrolysers initially supply hydrogen , which can then be used in a Sabatier process together with carbon dioxide to produce methane . The electrolysers required for this can be used as a controllable load, among other things for network stabilization .
See also
Individual evidence
- ↑ R. Holze: Guide to electrochemistry. P. 206, Vieweg + Teubner Verlag, 1998, ISBN 978-3-519-03547-3
- ↑ Fraunhofer ISE : Reversible fuel cells - long-term storage for electrical energy. ( Memento of the original from March 8, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Büchi et al., Towards re-electrification of hydrogen obtained from the power-to-gas process by highly efficient H2 / O2 polymer electrolyte fuel cells . In: RSC Advances 4, (2014), 56139-56146, doi : 10.1039 / c4ra11868e .
- ↑ https://elib.uni-stuttgart.de/bitstream/11682/9060/1/2016-Dissertation-DHKoenig.pdf
- ↑ http://www.iam.kit.edu/wet/plainhtml/studien-%20und%20diplomarbeiten/Bachelor_3_Coelektrolyse_Krompic.pdf
- ↑ https://backend.orbit.dtu.dk/ws/files/103646063/Thesis_Youkun_Tao.pdf
- ↑ https://www.osti.gov/etdeweb/biblio/22090505
- ↑ https://patents.google.com/patent/US4395468A/en
- ↑ Developed by chance simplified hydrogen production (source: Der Standard as of April 14, 2011)
- ^ FM Mulder et al .: Efficient electricity storage with the battolyser, an integrated Ni-Fe-battery and electrolyser . In: Energy and Environmental Science . tape 10 , no. 3 , 2017, p. 756-764 , doi : 10.1039 / C6EE02923J .
- ↑ Ulrich Eberle, Rittmar of Helmholt: Sustainable transportation based on electric vehicle concepts: a letter overview . In: Energy and Environmental Science . tape 3 , no. 6 , 2010, p. 689-699 , doi : 10.1039 / C001674H .
