2024-02-07 23:00:00
The spectacular rise in energy prices at the start of the 2020s, coupled with the need for decarbonization of industry, propelled the hydrogen sector towards rapid development. This energy vector for mobility can store 123 MJ · kg–1 is, in fact, used in particular for the production of ammonia, but also in the steel industry, as well as as a reagent in the processes of refining crude oil into petroleum products, fuels and biofuels. It can also be used to temporarily store excess energy produced by renewable energy sources, in order to compensate for their intermittency during their development.
However, despite rising energy prices, the production of renewable hydrogen (emitting less than 3.38 kg of CO2 per kg of H2(g) produced from a renewable energy source), as opposed to carbon hydrogen from steam reforming, remains very expensive. Hydrogen is also qualified as low carbon when its production emits less than 3.38 kg of CO2 per kg of H2(g) product, but the energy source used is not qualified as renewable. Hydrogen from steam reforming emits around 11 kg of CO2 per kg of H2(g) product. The price of the kg of hydrogen obtained should, for its part, reach less than €3, while its production by water electrolysis still costs between €10 and €20.
Water electrolysis currently represents barely 4% of hydrogen production, but should progress rapidly by 2030 to replace steam reforming. Several processes are used to make it. The oldest, and most mature, is alkaline water electrolysis with diaphragm. The second, more recent, still being optimized, is PEMWE electrolysis (Proton Exchange Membrane Water Electrolysis). In an acidic environment, it uses a proton exchange membrane. Still emerging, the alkaline water electrolysis process with anion exchange membrane (AEMWE), is one of the most recent. Other processes (electrolysis powered by capillary, electrolysers without membranes, high temperature electrolyser) have been the subject of recent international publications.
If the acidic environment allows good yields (80-90%), high current densities (600-2,000 mA cm–2), and a purer gas at the output (99.99%), it requires the use of noble and rare metals, such as Pt or Ru, and has a low durability (40,000 h). The alkaline environment, conversely, reaches lower current densities (200-400 mA cm–2), with lower efficiency (60-70%), and lower outlet gas purity (99.5%), but it does not use precious metals from the Pt group, and it has better durability (80,000 hours).
By virtue of these data, and the rarity and price of metals used in an acidic environment, one of the most promising avenues is the use of an alkaline environment for the electrolysis of water. However, it appears necessary, on the scale of the hydrogen sector, to improve the performance of the electrode materials used in this environment, based on Ni and transition metals, to achieve the objectives of the European Union in horizons 2024 and 2030 (49 then 48 kWh · kg–1 hydrogen). New technologies, such as SLM metal 3D printing, can also enable the emergence of new electrolyzers, in order to no longer use anion exchange membrane.
The objective of this article is to take stock, in the context of alkaline electrolyzers, of work concerning the development of electrodes (composition, current and emerging manufacturing techniques, performances, evaluation methods and monitoring protocols). ‘tests). It focuses on electrodes, a priority optimization factor for modern electrolyzers, while addressing the general context of mainly alkaline electrolyzers, but without detailing it.
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