## 3 Hydrogen Production Technologies
### Water Electrolysis Technologies
#### 3.2.3 Solid Oxide Electrolysis  
[PERSON] and [PERSON] were the first to develop solid oxide electrolysis, in the 1980s [79; 80]. This method has attracted significant attention due to the conversion process of electrical into chemical energy, along with the high-efficiency production of pure hydrogen [61; 81]. Solid oxide electrolysis operates at high pressures and temperatures, being novel by using water in the form of steam--as seen in Figure 12, showing a schematic illustration of the process; it conventionally uses O\({}^{2-}\) as the ionic agent, which mostly come from yttria-stabilized zirconia [82]. SOEL's operating principle is very similar to AEL's, only slightly differing the half-reaction equations:  
\[\text{Cathode:}\qquad\text{H}_{2}\text{O}+2\,\text{e}^{-} \longrightarrow \text{H}_{2}+\text{O}^{2-} \tag{11}\] \[\text{Anode:}\qquad\qquad\text{O}^{2-} \longrightarrow \frac{1}{2}\,\text{O}_{2}+2\,\text{e}^{-} \tag{12}\]  
Nowadays, some proton ceramic conducting materials have been studied to replace regular ionic agents on solid oxide fuel cells, due to these showing higher thermodynamic efficiency and superior ionic conductivity at the operating temperatures [83]. Proton ceramic electrolysers could allegedly deliver pure dry hydrogen straight from steam, in this way averting costly processes downstream--like further gas separation and compression. Yet, the development of such technology has undergone some constraints linked to limited electrical efficiency, mainly due to poor electrode kinetics and electronic leakage [84]. Proton ceramic electrochemical cells, on the other hand, produce hydrogen at intermediate temperatures through solid oxide proton conductors. Some reliable and highly robust electrodes are needed to increase the electrochemical efficiency of this process, as well as to ease the conduction of stable lower-temperature electrolysis; these cells, coupled with custom catalysts and a specific ceramic architecture could operate reversibly with great performance [85]. Recent studies [86] have successfully achieved self-sustainable reversible hydrogen operations, having confirmedly credited the remarkable electrocatalytic activity to superior proton conduction. This lower-temperature operation grants a set of numerous benefits, namely, lower heat losses, the possibility of using lower-heat-grade materials, and reduced capital costs due to a decrease in surface-area needs [87]. Others confirmed this trend [88], showing how PCECs can perform with extremely high Faradaic efficiencies and low long-term degradation, while inherently providing CO\({}_{2}\) sequestration and H\({}_{2}\) with purity levels suited for natural gas use--presenting as a very positive alternative to conventional electrolysis. Besides, insufficient long-term stability leading to serious deterioration caused by electrolysis--which was considered to be irreversible before--has been found to be completely eliminated through reversible cycling between electrolysis and fuel-cell modes [89]. Solid oxide electrolysis thus presents as an advantageous method to produce hydrogen, although still having some issues preventing it to be commercialized on a large scale, namely related to a lack of stability, degradation, and very high temperatures requirements [90; 91; 92]. This is also why it is especially not adequate for coupling with intermittent power sources but more with nuclear or combined cycle power plants [67]. Currently, SOEL capital costs still fluctuate considerably and are quite uncertain, mainly due to its pre-commercial status; although being surely situated above EUR 3000/kW\({}_{el}\)[93], experts suggest that solid oxide systems could experience the strongest  
Figure 12: Schematic illustration of a solid oxide water electrolyzer. relative cost reduction by 2030, reaching values as low as EUR 750/kW\({}_{el}\) by 2030 with production scale-up [74].