## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### Green Hydrogen Production and Consumption
#### 2.4.1 The State of the Art
During these developments, four types of water electrolysis technologies have been introduced based on the electrolyte used, operating conditions, and their ionic agents (OH\({}^{-}\), H\({}^{+}\), and O\({}_{5}\)\({}^{-}\)): (i) alkaline water electrolysis (A-WE); (ii) anion exchange membrane water electrolysis (AEM-WE); (iii) proton exchange membrane water electrolysis (PEM-WE); and (iv) solid oxide water electrolysis (SO-WE) [120]. The AE cell in alkaline water electrolysis (A-WE) consists of two electrodes (an anode and a cathode) immersed in a highly concentrated aqueous alkaline electrolyte consisting of 20 to 30 mass% KOH/NaOH. Alkaline electrolysis operates at lower temperatures, such as 30-80 \({}^{\circ}\)C. In traditional AE, the most commonly used anode and cathode materials are low-cost steel or nickel alloy-plated steel materials. The two electrodes are arranged in a zero-gap (or quasi-zero-gap) formation using a thin diaphragm, which enables the separation of the product gases. The diaphragm is permeable to hydroxide ions and water. The major challenges associated with AE are the handing of the corrosive electrolyte and the limited current densities due to moderate OH mobility. Furthermore, the diaphragm does not completely prevent the crossover of gases from one half-cell to the other. A developing approach in alkaline electrolysis is the use of anion exchange membranes (A-EM) made up of polymers with anionic conductivity instead of an asbestos diaphragm [121]. In PEM-WE, water is electrochemically split into hydrogen and oxygen at their respective electrodes: hydrogen at the cathode and oxygen at the anode. PEM-WE is carried out by pumping water to the anode, where it is split into oxygen (O\({}_{2}\)), protons (H\({}^{+}\)), and electrons (e\({}^{-}\)). These protons travel via a proton-conducting membrane to the cathode side. The electrons exit from the anode through the external power circuit, which provides the driving force (cell voltage) for the reaction. At the cathode side, the protons and electrons recombine to produce hydrogen [122]. PEM-WE uses a dense proton exchange membrane and a pure water feed. PEM-WE is considered to be an exceptional method for high-purity hydrogen production in future industrial applications due to its high current density, greater energy efficiency, smaller gas crossover, and wider operating temperatures (20-80 \({}^{\circ}\)C). However, the best-performing catalysts are carbon-supported platinum on the cathode and IrO\({}_{2}\) on the anode side, and the porous transport layers and bipolar plates are titanium-based to avoid corrosion [123]. This type of electrolysis is efficient but relies on electrocatalysts based on scarce elements such as iridium (to catalyze the anodic oxygen evolution reaction (OER)), which is one of the rarest metals on Earth, and platinum (to catalyze the cathodic hydrogen evolution reaction (HER)). In 2011, the EU identified both of these elements as critical raw materials (CRMs) due to their scarcity, cost, and supply risk [124]. In addition, PEM-WEs currently lean on fluorine-based polymers, which have a significant environmental impact due to the emission of fluorocarbon gases at the production stage of tetrafluoroethylene. The phasing-out of fluoropolymers for environmental reasons has been recently proposed, and the European Commission is currently active in restricting their use in the near future [125]. AEM-WE is an emerging technology that combines the advantages of the mentioned technologies--A-WE (low-cost, abundant materials and pure water) and P-EM (moderate temperature and membrane separation--and appears to be an excellent solution in comparison. The current state of the art in AEM-WE requires a future roadmap for the systematic development and commercialization of systems and components [126].