The electrolysis of water is a chemical process consisting in separating the hydrogen and oxygen atoms from the water molecule using electricity. It is one of the main means of hydrogen production.
This article is a part of our dossier on hydrogen, innovation and ecology.
How about an unlimited fuel that could replace oil and only release… water? A beautiful promise, no? Wouldn’t it be even better if this miracle needed, to be produced… only water? (and electricity, of course.) It is that of hydrogen produced by electrolysis, or “green hydrogen“. This method of hydrogen production is both the most popular and the least used (4% of world production).
The principle of water electrolysis
Electrolysis is an electrochemical process by which we will come to create a reaction thanks to an electric current. What interests us here consists of transforming water into dihydrogen (and oxygen). The first was made on May 2, 1800 by William Nicholson and Sir Anthony Carlisle. The latter carried out a simple experiment: they immersed two electrodes producing an electric current in water, and observed that bubbles formed around each electrode (the gas collected at the level of the cathode will be dihydrogen and the gas collected around of the anode will be oxygen). (rq: according to some, the electrolysis of water was discovered in 1789)
There are mainly 3 elements:
- An electrolyte (the space that will transmit electrons from one electrode to another).
- Two electrodes: an anode and a cathode.
The main difference between the different types of electrolysis will be based on
- heat and pressure conditions
- the materials used
The chemical reactions
At the cathode a chemical reduction reaction will occur which will release dihydrogen:
2H2O => O2 + 4H+ +4e–
This semi-reaction is called “Water oxidation catalysis” (WOC).
At the anode, a chemical oxidation reaction will occur, which will produce pure oxygen:
2H2O+4e– => H2 + OH–
This semi-reaction is called “oxygen reduction reaction” (ORR)
The role of the electrolyte
The electrolyte will have the difficult task of allowing the electric current (the electrons) to pass without the gases produced at the level of the electrodes becoming contaminated. This is really where the heart of the differences between water electrolysis processes (PEM, alkaline, solid oxides) lies. For example, alkaline electrolysis uses an alkaline liquid, which makes it possible to have a relatively inexpensive system, but which is not very flexible, nor spatially (the liquid moves, so it does not like to be moved too much) , nor for the current (it does not tolerate variations well). Conversely, PEM electrolysis is flexible, but its electrodes and the electrolyte (a membrane, often composed of Naflon) are very expensive.
Calculate the efficiency of electrolysis
The efficiency of each process is evaluated according to the Faraday efficiency, or Faradic efficiency, which is equal to the actual efficiency divided by the theoretical efficiency. Indeed, in reality there are always multiple losses (in particular of energy, with the heat which escapes).
To calculate the mass produced during electrolysis, Faraday’s laws are used, which are written as follows:
m = (Q/F) x (M/z)
“m” is obviously the mass, Q is the electric charge, F is Faraday’s constant (96,485 coulombs per mole), M is the molar mass of the substance, and z is the valence of the substance.
The different kinds of water electrolysis
There are several types of electrolyzers.
Alkaline electrolysis uses a solution of potassium hydroxide (KOH, also called potash) as the electrolyte. It takes place at medium temperature (80°C to 160°C) and moderate pressure (3 to 30 bars).
This is the most used method, because it requires the least investment, its materials being ordinary.
It has a yield of 60 to 70% or 68 to 77%. Its problem is that it does not tolerate intermittency well, so in itself it is not suitable for storing wind and solar power (except, if I understood correctly, with Lhyfe’s solution).
For further details, you can read our article on alkaline electrolysis.
PEM electrolysis (Proton Exchange Membrane)
PEM (Proton Exchange Membrane) electrolysis uses a solid membrane as the electrolyte.
It has a yield of 62 to 77%
It requires larger investments (electrolysers are twice as expensive), using noble metals (mainly platinum) for the catalysts. They are used in particular to produce oxygen in certain nuclear submarines. These electrolysers are also more suitable for intermittent energy sources (solar, wind).
It is essentially the same as a fuel cell (the PEMFC, “Proton Exchange Membrane Fuel Cell“). In particular, this means that research efforts in the field are doubly useful.
This technology normally operates at a moderate temperature (70 to 80°C). Research is experimenting with this technology at 130-180°C.
For further details you can consult our page about proton exchange membrane electrolysis.
Anion exchange membrane electrolysis
The anion exchange membrane electrolysis (AEM) would combine the advantages of
- Alkaline electrolysis by using a basic environment, not very aggressive for materials, allowing the use of conventional metals as electrodes;
- PEM electrolysis using a membrane allowing a compact design.
The Gen-Hy and Enapter projects are notably based on this technology. However, the latter is still experimental.
To go further, you can read our article on anion exchange membrane electrolysis.
High temperature electrolysis
Technologies propose to dispense with the noble catalysts of PEM electrolysis by operating at high temperatures. Paradoxically, this would radically reduce investment and operating costs. This is for example the case of SOEC (Solid Oxide Electrolysis Cell) electrolysis. By increasing the temperature of the electrolysis to between 700 and 1000°C, we are able to radically improve the efficiency: the process would reach a yield of 90%.
“Technical-economic projections for very large units suggest a cost of around €2/kg, which is lower than other electrolysis technologies and which is close to the cost of the hydrogen currently produced by steam reforming of hydrocarbons … less CO2 emissions. »Vers la production massive d’hydrogène décarboné
The units would also be reversible, i.e. they could act as fuel cells and thus be ideal for storing intermittent energy.
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