## 4 Steam Electrolysis and Thermo(-Electro-)Chemical Cycles
### Hybrid Thermochemical Cycles  
In hybrid thermochemical cycles, water is split into hydrogen and oxygen using different types of energy sources, such as solar, electricity, biomass combustion, and so on.  
Likely, the most investigated hybrid method is the so-called Westinghouse cycle, which is a simplification of sulfur-iodine, where nothing changes regarding oxygen production, while hydrogen is formed by electrolysis of a SO\({}_{2}\) aqueous solution--SO\({}_{2}\) Depolarized Electrolysis (SDE)--where sulfuric acid is produced at the anode and hydrogen at the cathode. With respect to direct water electrolysis, the oxidation of aqueous sulfur dioxide is thermodynamically more favorable than O\({}_{2}\) oxidation, and, actually, the SDE process can be carried out, saving 40 to 75% of electric power with respect to alkaline H\({}_{2}\)O electrolysis [102, 103]. In this context, ENEA was a partner in the SOL2 HY2 (Solar-To-Hydrogen Hybrid Cycles) EU project, where it was mainly involved in developing a proper catalyst to be used in a solar reactor present at the DLR Centre of Julich [104]. An important aspect of this project was to study the feasibility of using SO\({}_{2}\) from industrial wastes to feed the cycle in periods with insufficient solar irradiation. In this regard, ENEA developed simulation tools to investigate the coupling of the process with TES systems based on molten nitrates and the possibility to use a contemporary central receiver system and a PTC plant in order to ensure the continuity of hydrogen production.  
Another, and a very peculiar, alternative hybrid route is represented by the sulfur-ammonia cycle, where the disproportionation of the ammonium sulfite intermediate, thermodynamically spontaneous under reaction conditions, is catalyzed with the energy of light photons and the presence of photocatalytic materials. ENEA participated in the validation of this cycle, in particular, regarding the thermal part of the process, in the framework of the national funding scheme PRIN2019 [105]. Both processes are summarized in Table 4.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Process** & **No. of Steps 1** & **Reactions 2** & **Pros (+) \& Cons (-)** & **Ref.** \\ \hline \multirow{8}{*}{**Westinghose process**} & & & + & No iodide and iodide & \\  & & & & compounds are present & \\  & & SO\({}_{2}\) + 2H\({}_{2}\)O \(\rightarrow\) H\({}_{2}\)SO\({}_{4}\) + H\({}_{2}\) & + & Easier separation & \\  & & **(eletrochemical, T < 100 \({}^{\circ}\)C)** & & procedures than the & \\  & & H\({}_{2}\)SO\({}_{4}\) \(\rightarrow\) H\({}_{2}\)O + SO\({}_{2}\) + \(\
icefrac{{1}}{{2}}\)O\({}_{2}\) & & “sulfur-iodine” process & [102, 103] \\  & & **(T \(\approx\) 850 \({}^{\circ}\)C)** & \(-\) & Issues regarding the & \\  & & & & efficiency of the & \\  & & & & electrolysis step & \\ \hline \multirow{8}{*}{**Sulphur-ammonia cycle**} & & (NH\({}_{4}\))\({}_{2}\)SO\({}_{3}\) + H\({}_{2}\)O \(\rightarrow\) (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) + H\({}_{2}\) & + & No iodide and iodide & \\  & & **(photocatalytic, T \(\leq\) 100 \({}^{\circ}\)C)** & & compounds are present & \\  & 3 + 1 & (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) \(\rightarrow\) 2 NH\({}_{3}\) + H\({}_{2}\)O + SO\({}_{3}\) & + & No necessity for & \\  & & (SO\({}_{2}\), NH\({}_{3}\)) & **(T \(\approx\) 400-500 \({}^{\circ}\)C)** & & electric power & [105, 106] \\  & & absorption in & SO\({}_{3}\) \(\rightarrow\) SO\({}_{2}\) + \(\
icefrac{{1}}{{2}}\)O\({}_{2}\) & \(-\) & Photocatalytic & \\  & & (T \(\approx\) 850 \({}^{\circ}\)C) & & hydrogen production is & \\  & & & SO\({}_{2}\) + H\({}_{2}\)O + 2 NH\({}_{3}\) \(\rightarrow\) (NH\({}_{4}\))\({}_{2}\)SO\({}_{3}\) & & quite slow & \\  & & **(T \(\leq\) 100 \({}^{\circ}\)C)** & & & \\ \hline \hline \end{tabular}  
* Excluding separation steps. 2 In all cases, summing up all steps, water splitting is obtained: H\({}_{2}\)O \(\rightarrow\) H\({}_{2}\) + \(\
icefrac{{1}}{{2}}\)O\({}_{2}\).  
\end{table}
Table 4: Main characteristics of hybrid thermochemical cycles for hydrogen production.