CAs France Hydrogène reminds us on its site, the decomposition of water by electrolysis is written as allowing the separation of oxygen from hydrogen (H2O → H2 + ½ O2). This decomposition requires an input of electrical energy depending essentially on the enthalpy and the entropy of reaction. The classic values of the potentials of industrial cells are of the order of 1.7 to 2.1 V, which corresponds to electrolysis efficiencies of 70 to 85%. The electrical consumption of industrial electrolysers (including auxiliaries) is generally 4 to 5 kWh/Nm3 of hydrogen produced.
The minimum water supply to an electrolyser is 0.8 L/Nm3 of hydrogen produced. In practice, the actual value is close to 1 L/Nm3The water introduced must be as pure as possible because impurities remain in the equipment and accumulate during electrolysis, ultimately disrupting the electrolytic reactions by:
– The formation of sludge,
– The action of chlorides on electrodes.
An important specification for water is its ionic conductivity (which must be less than a few μS/cm). An electrolysis cell consists of two electrodes (anode and cathode, both electrically conductive) connected to a direct current generator, and separated by an electrolyte (ionic conductive medium).
This electrolyte can be:
– Either an acidic or basic aqueous solution,
– Either a proton exchange polymer membrane,
– Either a ceramic membrane conducting ions O2-.
There are many suppliers offering very diverse technologies, particularly in terms of the nature of the electrolyte and associated technology, ranging from possible upstream coupling with a renewable electricity supply (photovoltaic or wind), to the direct final supply of pressurized hydrogen.
Technologies
They are of two types and relate on the one hand to the type of structure (monopolar or bipolar) and on the other hand to the type of electrolyte: alkaline, PEM2 (or acid) or SOEC (SOFC technology). The first electrolysis devices had monopolar electrodes (i.e. each anode is connected to the positive pole and each cathode to the negative pole), the electrolysis cells then operate in parallel. The bipolar systems, developed subsequently, use intercalary plates acting as an anode on one side and a cathode on the other, the electrolysis cells then operate in series. Electrical conduction takes place inside the electrode through its thickness which has a low but non-zero ohmic drop. Bipolar assemblies offer the advantage of a higher current density and better compactness. This design, however, introduces an additional difficulty: the electrode has one side in an oxidizing medium (anode) and the other in a reducing medium (cathode). The vast majority of industrial systems are based on bipolar technology, while some suppliers of small capacity electrolysers still offer monopolar structures. The electrolysis cells must be sealed, electrically insulated and resistant to corrosion under sometimes high temperature and pressure conditions.
At the anode: 2 OH — → ½ O2 + H2O + 2 e-
A la cathode : 2 H2 + 2 e- → H2 + 2 OH-
(Source: France Hydrogène — Th. Alleau, revised February 2023)
Alkaline electrolysis
Alkaline electrolysis is the most widely used process in industry and is therefore mature. Electrolysers come in small or medium capacity modules (0.5–800 Nm3/h of hydrogen), using an aqueous solution of potassium hydroxide (or potash KOH) whose concentration varies according to the temperature (typically from 25% by mass at 80 °C up to 40% at 160 °C). Potash is preferred to soda, mainly for reasons of higher ionic conductivity at equivalent temperature levels, and better control of chloride and sulfate impurities. Disadvantage, KOH, delicate to handle, can present corrosion problems.
Modules typically include: a power supply, electrolysis cells, a water purification unit, a gas dehumidification unit, a hydrogen purification unit, a compressor and a control system. Some electrolyser technologies operate directly under pressure. Small capacity modules typically operate at 3 to 30 bar.
Note that several laboratories are conducting R&D work on alkaline fuel cells to replace the liquid electrolyte with solid membranes conducting OH anions.–If successful, which has not yet been achieved, these membranes could find an application in alkaline electrolysis.
Another alkaline technology: AEM with 1 MW power modules is under development. This process is a variant of the alkaline electrolyser with a bonded, soaked electrolyte but without an aqueous solution. More compact, less expensive in consumables, it could provide added value in terms of safety and cost.
SOEC : Solid Oxide Electrolysis Cell
PEM acid electrolysis (Proton Exchange Membrane)
Acid electrolysis is distinguished from the previous one by a solid electrolyte with a proton-conducting polymer membrane (see diagram). The advantages of this technology are the absence of liquid electrolyte, compactness, simplicity of design and operation, limitation of corrosion problems, significantly higher performance and less influence of variation in input conditions (interesting for intermittent renewable sources). However, the cost of the polymer membrane and the use of electrocatalysts based on noble metals lead to equipment that is now more expensive than alkaline electrolysers of the same capacity. Polymer membrane electrolysis is nevertheless considered by many to be a technology of the future, because it benefits from the many developments in fuel cells of comparable technology and the associated cost reduction.
These units can operate from atmospheric pressure to several tens of bars, or even a few hundred bars. This type of electrolyser is particularly suitable for coupling to a renewable energy source, because it supports, better than the alkaline electrolyser, the variations in available electrical power. The efficiencies of the two families of electrolysers are today very close to each other and approach, for the most powerful, a value close to 90%. Standards and codes on the design and/or installation of small capacity electrolysers are currently being developed, in particular within ISO TC 1974 dedicated to hydrogen technologies.
L’Small-capacity polymer membrane electrolysis is already a mature technology, used for several decades for underwater applications (oxygen plants on board American and British nuclear submarines) and space applications (for the generation of oxygen in the living compartments of satellites). French nuclear submarines are currently equipped with alkaline electrolysers, but should soon switch to PEM technology.
Please note: pure water being rarer than sea water, more and more systems only have sea water. There are then two solutions:
– Either treat seawater as is done in desalination plants (evaporation or reverse osmosis),
– Or, as was recently proposed by the Leiden Institute of Chemistry, to deposit at the anode, with the iridium, a manganese oxide which would prevent the formation of chlorine.
High temperature electrolysis (PCFC or SOEC)
This technology is a direct result of the development of the PCFC or SOFC fuel cell (see diagram), operating respectively in the ranges of 400 to 600 °C and 650 to 1000 °C. It is interesting if it is supplied with both electricity and heat to maintain the desired high temperature; the efficiency can then be greater than 80% if the heat of vaporization of the water is not taken into account. This technology separates water into H and O in the vapor state, which requires less energy. It is mainly intended to be coupled to a concentrated solar system or a high-temperature nuclear reactor. It is at the development stage in various laboratories such as CERAMATEC, Idaho National Engineering and Environmental Lab. in the USA and at CEA/LITEN in France.
A complementary technology is currently being developed: this involves coupling the two functions of electrolysis and fuel cell within the same device, since electrolysis and fuel cell are two technologies whose operations are reversible. Since the beginning of 2016, various public and industrial entities have begun to take an interest in this by applying SOEC/SOFC technology; this is the case of the French start-up Sylfen from the CEA and in the United States of the Boeing, Huntington Beach and Sunfire association. The first Sylfen demonstrator (Smart Energy Hub, called rSOC) was validated in May 2018.
