## 4 State-of-the-Art Review  
Green hydrogen is one of the most promising clean and sustainable energy carriers, emitting only water as a byproduct of its production and no carbon emissions [100]. Having many attractive properties as an energy carrier, namely, a high energy density (which is more than double that of typical solid fuels [99]), hydrogen is mostly used today in industrial applications such as fertilizers [101], petroleum refining processes [102], chemical and petrochemical industries [103; 104], and fuel cells.  
Several authors have previously studied and described the various forms in which hydrogen can be produced, from renewable and non-renewable energy resources.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Characteristics** & **AEL 1** & **PEMEL 2** & **SOEL 3** & **Ref.** \\ \hline
**Operation Parameters** & & & & \\ Temperature (\({}^{\circ}\)C) & 40–90 & 20–100 & 650–1000 & [79; 90] \\ Pressure (bar) & \textless{}30 & \textless{}200 & \textless{}20 & [93; 94] \\ Current density (A/cm\({}^{2}\)) & 0.20–0.40 & 0.60–2.00 & 0.30–2.00 & [79; 93] \\ Voltage (V) & 1.80–2.40 & 1.80–2.20 & 0.70–1.50 & [90; 93] \\ \hline
**Nominal Features** & & & & \\ Cell area (m\({}^{2}\)) & \textless{}4 & \textless{}0.13 & \textless{}0.06 & [93; 94] \\ Production rate (m\({}^{3}\)/h) & \textless{}1400 & \textless{}400 & \textless{}10 & [93] \\ Gas purity (\%) & \textgreater{}99.50 & \textgreater{}99.99 & \textgreater{}99.90 & [77; 94] \\ \hline
**System Details** & & & & \\ Energy consumption & \textless{}5.55 & \textless{}5.40 & \textless{}3.80 & [93; 95] \\ (kWh/m\({}^{3}\)) & & & & \\ Efficiency (\%) & 51–60 & 46–60 & 76–81 & [93] \\ Stack lifetime (kh) & 60–120 & 60–100 & 8–20 & [79; 95] \\ Degradation (\%/y) & 0.25–1.50 & 0.50–2.50 & 3–50 & [93] \\ \hline
**Economic Parameters** & & & & \\ Capital cost & & & \\ (EUR/kWh) & 740–1390 & 1300–2140 & \textgreater{}2000 & [93; 95] \\ Maintenance cost & & & & \\ (\% of investment/year) & 2–3 & 3–5 & n.a. & [93] \\ \hline
**Miscellaneous** & & & \\ Advantages & Low capital cost; cheap catalysts; high durability; stable operation & Highest purity; compact & High efficiency; low energy & \\  & & design; high production & consumption; no need & [83; 96] \\  & & rate & for noble metal catalyst & \\ Disadvantages & Corrosive system; lowest purity; high energy consumption & High cost of rare & Large design; \\  & & components; & low durability; & \\  & & acidic environment; & small cell area; & [97; 98] \\ Maturity & Commercial & Near commercial & Demonstration & [74; 99] \\ \hline \hline \end{tabular}  
* Alkalye electrolysis; 2 proton-exchange membrane electrolysis; 3 solid oxide electrolysis. Note: The characteristics reported in this table are not inherent to each technology but merely what has been demonstrated (among the cited research). Often, the most mature technologies are the ones most developed, thus seeming like they have better inherent properties; that might not be the case—for instance, cell area is mainly dependent on development state, so it is safe to assume that it will naturally increase with time for PEMEL and SOEL.  
\end{table}
Table 2: Characteristics of different water electrolysis technologies. Adapted from [76].