Why is electrolysis important
Lexicon> Letter E> Electrolysis
Definition: an electrochemical process in which chemical reactions are driven with the help of electrical energy
More specific terms: low temperature electrolysis, high temperature electrolysis, alkaline electrolysis, water electrolysis, co-electrolysis
Categories: electrical energy, basic terms
Author: Dr. Rüdiger Paschotta
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Original creation: November 19, 2010; last change: 02/27/2021
The electrolysis is an electrochemical process. Here chemical reactions are driven with the help of electrical energy. In the context of energy technology, electrolysis is used to produce chemical energy carriers, often hydrogen. So electrical energy is converted into chemical energy. (The reverse process takes place in rechargeable batteries and fuel cells.) In other cases it is about the production of substances for non-energetic uses, for example aluminum, chlorine or caustic soda. Electrolysis is also used to purify (refine) certain metals.
Typically, an electrolyzer (electrolysis apparatus) contains two electrodes that are brought into contact with an electrically conductive liquid (an electrolyte). An electrical voltage is applied between the electrodes so that an electrical current flows. The electrical current strength directly determines the amount of substance converted per second. The negative electrode is called cathode referred to, the positive as anode. New chemical substances can form on the electrodes, and it is also possible for electrode material to dissolve into the solution. To avoid this, electrodes can be made of certain precious metals (e.g. gold or platinum) or graphite.
The anode and cathode compartments are often separated from one another by a porous wall (a diaphragm, e.g. made of a resin, or a special membrane). This can be used, for example, to prevent the gases produced from mixing with one another. However, such a membrane also more or less hinders the flow of current and in this way can reduce the power converted and / or make a somewhat increased electrical voltage necessary, which reduces the efficiency.
In water electrolysis for the production of hydrogen and oxygen, the conductivity of the water is increased by adding certain acids or bases (alkalis). (Alkaline water electrolysis based on the Claude process is widespread.) Hydrogen is produced at the cathode, and oxygen at the anode, which must remain separate from the hydrogen and can also be used. In order to avoid oxidation of the anode, it must be made of a noble metal or nickel, for example.
The technology of electrolysers has been used for decades, especially on an industrial scale, and is already very mature. For small systems in the lower kilowatt range, the specific system costs are relatively high (several thousand euros per kilowatt), while large systems in the megawatt range can be built for a few hundred euros / kW. Considering the maturity of the technology, significant cost reductions through further developments are not to be expected.
There are reversible cells that can be used for electrolysis on the one hand, but also vice versa as fuel cells on the other.
Energy efficiency of electrolysis
The energy conversion in electrolysis can be done quite energy efficiently, i. H. with a high degree of efficiency. For this it is particularly important that the electrical voltage is hardly chosen higher than physically necessary. However, high current densities, as required by a high production rate, and the formation of gases at the electrodes often force a somewhat higher voltage. In the case of water electrolysis, an efficiency of a little over 70% is typically achieved, sometimes even more than 80%. The efficiency can be significantly higher in partial load operation.An electrolyser that is only supposed to use surplus electricity will hardly be able to achieve maximum energy efficiency in practice.
If an electrolyser were only used for the use of excess electricity, i.e. with a few full load hours per year, then the balancing of efficiency and costs would lead to lower energy efficiency, e.g. B. at about 60 to 65%. This problem could only be solved by a significant reduction in the specific investment costs. However, now that electrolysers have been optimized for decades, there does not seem to be any great potential for this.
If hydrogen electrolysis is to be used to store electrical energy, the energy losses when converting hydrogen into electricity are of course also significant; the cycle efficiency is then, depending on the technology, closer to 40% or even lower.
Further research and development could lead to improved electrolysis technologies, which above all would also enable a higher degree of efficiency. For example, approaches for high-temperature electrolysis (steam electrolysis, e.g. in solid oxide electrolysis cells at 800 to 1000 ° C), in which part of the energy required can be supplied in the form of high-temperature heat instead of electricity, are of interest. This can be understood in such a way that initially the evaporation of the water can take place solely with the supply of heat and thus correspondingly less electrical energy is required. High-temperature electrolysers can also be implemented without expensive precious metals. However, such procedures are still at an early stage of development. They would be particularly interesting in connection with solar thermal power plants.
A possible source of the required high temperature heat would be a methanation plant, i. H. for the production of methane from the generated hydrogen.
Some electrolyzers can operate at high pressure; H. Hydrogen already compressed z. B. to 10 bar, while others only work at normal pressure, so that additional compression may be necessary (with additional energy expenditure).
The role of electrolysis in energy technology
So far, electrolysis has not played an important role in energy technology. It could become more important in the future as part of a hydrogen economy. Here, hydrogen would be obtained from electrical energy by means of electrolysis, and vice versa, electrical energy could also be generated from hydrogen with the help of fuel cells. Large amounts of energy could be z. B. with the help of hydrogen pipelines transport relatively low loss over long distances. Hydrogen would also enable energy to be stored over longer periods of time, which would be of interest, for example, in connection with the use of stochastically occurring renewable energies such as wind energy and solar energy. However, considerable energy losses occur during the conversions - so far at least approx. 50% when converting electricity to hydrogen and back. In addition, storing the hydrogen is complex.
Another variant is Power to Gas with additional methanation: If methane is obtained from the hydrogen, this can be fed into the natural gas network. In this way, existing natural gas networks and natural gas storage facilities can be used as a very efficient transport and storage infrastructure.
Non-energetic applications of electrolysis (mainly in metallurgy and the chemical industry) are already significant today. This requires around 5% of total electricity generation in the USA - significantly less in Europe because hydrogen is mainly obtained from natural gas.
Electrolysis with sea water
There are considerations to operate electrolysis on a large scale in systems in the sea, with the operating energy being generated either by floating photovoltaic systems or by wind turbines. Sea water would then have to be used here. Unfortunately, this cannot usually be used directly for electrolysis because the chloride content leads to the undesired formation of chlorine (with various side effects) and because calcium compounds can form deposits on the electrodes that reduce energy efficiency. Therefore, a seawater desalination would first have to be carried out here, z. B. with reverse osmosis. This would cause a certain additional electricity requirement, which is, however, almost negligible compared to that of the electrolysis. In this respect, there would be no decisive obstacle to the use of seawater for electrolysis.
A special case of electrolysis is the so-called co-electrolysis. Here two or more different electrolysis processes are carried out simultaneously in one device. In particular, in recent years an electrolysis process has been developed in high-temperature solid-state cells in which hydrogen is generated from water and carbon monoxide is generated from carbon dioxide at the same time. This means that synthesis gas can be generated directly, not only with simpler equipment, but also with significantly higher energy efficiency. Such developments could enable a more efficient and cost-effective large-scale production of electricity-based synthetic fuels in the future.
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See also: electrical energy, chemical energy, chemical energy storage, hydrogen, hydrogen economy, RE gas, power to gas, power to X
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