## 4 Steam Electrolysis and Thermo(-Electro-)Chemical Cycles
### Water Splitting Thermochemical Cycles  
WSTCs are multi-step processes where chemical intermediates are cyclically consumed and regenerated, water is the only inlet species, and hydrogen and oxygen are the only products [90]. Each of the cycle steps occurs at a different temperature, with a maximum level between 800 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C. Processes with a maximum temperature below 1000 \({}^{\circ}\)C are of particular interest, given their feasibility to be coupled with CSP and IV generation nuclear reactors, like high-temperature gas reactors. The research on WSTCs focuses on processes compatible with the temperatures available from CSP plants and MS mixtures. ENEA was directly involved in the investigation of three thermochemical cycles (see Table 3), developing both lab-scale experimental facilities and simulation models. The sulfur-iodine and the modified sulfur-iodine processes were studied in the national project FISR-TEPSI (Tecnologie e Processi innovativi per affrontae la transizione e preparare il futuro del Sistema Idrogeno) and in the EU project HyCycles (materials and components for hydrogen production by sulfur based thermochemical cycles) [91; 92; 93], while ferrites were considered within INNOHYP (innovative high-temperature routes for hydrogen production) Project. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Process**} & **No. of** & \multirow{2}{*}{**Reactions**} & \multirow{2}{*}{**Pros (+) \& Cons (-)**} & **Ref. ** \\  & & & & \\  & & & & \\ \multirow{6}{*}{Sulfur–iodine} & \multirow{6}{*}{3} & 2H\({}_{2}\)O + I\({}_{2}\) + SO\({}_{2}\) \(\rightarrow\) H\({}_{2}\)SO\({}_{4}\) + 2 HI & + & \\  & & **(Bunsen reaction, 20–120 \({}^{\circ}\)C)** & - & Complex and numerous separation \\  & & 2 HI \(\rightarrow\) I\({}_{2}\) + H\({}_{2}\) & steps & \\ \multirow{6}{*}{Sulfur–iodine} & \multirow{6}{*}{3} & **(300–500 \({}^{\circ}\)C)** & - & High corrosive intermediates \\  & & H\({}_{2}\)SO\({}_{4}\)\(\rightarrow\) H\({}_{2}\)O + SO\({}_{2}\) + 1/2 O\({}_{2}\) & - & Presence of iodide and iodide-based \\ \multirow{6}{*}{Sulfur–iodine} & \multirow{6}{*}{3} & **(800–1000 \({}^{\circ}\)C)** & compounds & \\  & & & & \\ \cline{1-1}  & & & & \\The sulfur-iodine cycle has the advantage of being based on liquid or gaseous intermediates, which are readily transportable in a chemical reaction plant. The major drawbacks are complex separative processes and issues regarding corrosion. In particular, the Bunsen reaction needs to be performed with a large excess of iodine in order to separate sulfuric acid and hydridic acid (HI) solutions; the former is still purified and concentrated by heating [95], and the latter undergoes another separation process (typically, an energetically expensive distillation [96]) to remove iodine from HI. In the most common schemes, sulfuric acid results at a concentration of around 80 wt.% [91], while hydridic acid is obtained at the azeotropic composition (57 wt.%) [91]. Both acids are then thermally decomposed [97; 98; 99], and in both cases, a heterogeneous catalyst is needed in order to operate with acceptable temperatures and reaction rates. Another problem is represented by the limited thermodynamic conversion for HI cracking [91]. Some of the concerns presented by the sulfur-iodine process may be overcome by employing oxides, sulfates, and iodides as intermediates (modified sulfur iodine) [93; 100]. This alternative route was validated using nickel compounds, as described in Table 3. Hydrogen is quantitively produced, reacting to metallic nickel with sulfuric acid, avoiding the necessity of catalysis. The obtained sulfate is decomposed at around 900 \({}^{\circ}\)C into nickel oxide, sulfur dioxide (to be reused in the Busen section), and oxygen. Then, it is necessary to retrieve metallic nickel, and this can be achieved by forming nickel iodide (from NiO plus HI) and decomposing that salt into iodine (recycled into the Bunsen reactor) and elemental nickel.