Electrode materials for alkaline water splitting

The spectacular rise in energy prices in the early 2020s, coupled with the need to decarbonize industry, has propelled the hydrogen sector towards rapid development. This energy carrier for mobility, capable of storing 123 MJ · kg ^–1 , is used in the production of ammonia, in the steel industry and as a reagent in the refining of crude oil into petroleum products, fuels and biofuels. It can also be used to temporarily store surplus energy produced by renewable energy sources, to offset their intermittency as part of their development.

However, despite rising energy prices, the production of renewable hydrogen (emitting less than 3.38 kg of CO ₂ per kg of H _2(g) produced from a renewable energy source), as opposed to carbon-based hydrogen from steam reforming, remains very expensive. Hydrogen is, moreover, qualified as low-carbon when its production emits less than 3.38 kg of CO ₂ per kg of H _2(g) produced, but the energy source employed is not qualified as renewable. Hydrogen from steam reforming emits around 11 kg of CO ₂ per kg of H _2(g) produced. The price per kg of hydrogen obtained should, meanwhile, reach less than €3, whereas its production by water electrolysis still costs between €10 and €20.

Water electrolysis currently accounts for barely 4% of hydrogen production, but is set to grow rapidly by 2030, replacing steam reforming. Several processes are used to achieve this. The oldest, and most mature, is alkaline water electrolysis with a diaphragm. The second, more recent process, still undergoing optimization, is PEMWE (Proton Exchange Membrane Water Electrolysis). In an acidic environment, it uses a proton exchange membrane. Alkaline water electrolysis with anion exchange membrane (AEMWE), still in its infancy, is one of the most recent processes. Other processes (capillary-fed electrolysis, membrane-free electrolyzers, high-temperature electrolyzers) have been the subject of recent international publications.

While the acidic medium offers good yields (80-90%), high current densities (600-2,000 mA · cm ^–2 ), and a purer gas 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 medium, on the other hand, achieves lower current densities (200-400 mA · cm ^–2 ), with lower efficiency (60-70%), and lower output gas purity (99.5%), but does not use precious...