## 3 Hydrogen Production Technologies
### Water Electrolysis Technologies
#### 3.2.2 Proton-Exchange Membrane Electrolysis  
The first PEMEL was idealized by [PERSON] in the early 1950s, having been later developed by General Electric Co. (Boston, MA, USA) in 1966 to overcome the drawbacks of AEL [58; 59; 60]. Figure 11 displays a schematic illustration of a proton-exchange membrane water electrolyzer, showing how the working principle of this technology is very similar to that of PEM fuel cells--actually being the exact reverse--with solid polysulfated membranes used as an electrolyte (proton conductor), through which the ionic agents move during the process [61; 62; 63].  
Finally, the protons and electrons re-combine at the cathode side to produce hydrogen, as shown in the following half-reactions:  
\[\text{Anode:}\qquad\qquad\text{H}_{2}\text{O}\;\;\longrightarrow\;2\,\text{H} ^{+}+\frac{1}{2}\,\text{O}_{2}+2\,\text{e}^{-} \tag{9}\]  
\[\text{Cathode:}\qquad 2\,\text{H}^{+}+2\,\text{e}^{-}\;\longrightarrow\;\text{H} _{2} \tag{10}\]  
These proton-exchange membranes have many important characteristics, such as low gas permeability, high proton conductivity, small thickness, and the potential to operate at high pressures and ambient temperatures. In terms of sustainability and environmental impact, PEMEL is also found to be one of the most favorable methods for conversion of renewable energy to highly pure hydrogen; this is mainly due to other promising advantages like its compact design, high current density (meaning higher efficiencies), fast response, and small footprint [63; 64; 65; 66]. Additionally, PEMEL plants are very simple,  
Figure 11: Schematic illustration of a proton-exchange membrane water electrolyzer.  
which is more attractive for industrial applications; these applications might include offshore wind parks [67], grid-independent/grid-assisted solar hydrogen generation and grid-independent integrated solar hydrogen energy systems [68].  
Electrocatalysts used in this method are usually noble metals such as platinum or palladium for the cathode [65; 69] and iridium/ruthenium oxide for the anode [70; 71; 72; 73], which makes the whole process more expensive than, say, alkaline water electrolysis. Here water is accrued by being pumped on the anode side, where it is electrochemically split into oxygen, hydrogen protons, and single electrons; these protons then travel via the proton-exchange membrane to the cathode side, while the electrons exit from the anode through the external power circuit, which provides the driving force to the chemical reaction.  
So, one of the main challenges of proton-exchange membrane water electrolysis is to reduce production cost while maintaining high efficiency. Substantial research has been devoted to this matter, namely, to tackle issues like relative electrolyzer sizing, operation intermittence, output pressure, oxygen generation, and water consumption. If such barriers are overcome, together with a strong investment in R&D, PEMEL capital costs could see a substantial reduction from around EUR 2000/kW\({}_{el}\) in 2020 to around EUR 900/kW\({}_{el}\) in 2030 [74]--with operational costs following the same path. The levelized cost of hydrogen (LCOH) is also expected to decreased, especially over the increase of PEMEL plant scales; a growth from 1 MW to 40 MW could represent a drop in LCOH values from EUR 7.37/kg to EUR 4.49/kg [75].  
Several authors have proposed a large number of different methods to increase the efficiency of PEM water electrolysis [76], and, as a result, this technology is ever approaching sustainable commercial market establishment [61]. Moreover, a new approach to this method is currently under development, which promises to combine AEL's low cost with PEMEL's high efficiency: anion-exchange membranes, made of polymers with anionic conductivity, which are set to replace the asbestos diaphragm and help improve overall electrolysis yield rates [77; 78].