# Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review
## 2 Theoretical Background  
Under specific thermodynamic conditions, the interaction between steam (H\({}_{2}\)O\({}_{(g)}\)) and pure iron metal (Fe) can produce hydrogen (H\({}_{2}\)) through the oxidation of Fe at temperatures ranging from 200 to 1000 \({}^{\circ}\)C. This oxidation reaction occurs in an environment rich in H\({}_{2}\)O\({}_{(g)}\) and can be expressed as follows [10]:  
\[3 Fe+4H_{2}O\ \to Fe_{3}O_{4}+4H_{2} \tag{1}\]  
In 1926, [PERSON] et al. [11] proposed that high-temperature oxidation is governed solely by the diffusion of O\({}_{2}\) through the oxide layer. The presence of H\({}_{2}\)O\({}_{(g)}\) accelerates the oxidation process of mild steel to twice the rate associated with Fe pure [12; 13]. The production of hydrogen (H\({}_{2}\)) using magnetite (Fe\({}_{3}\)O\({}_{4}\)) was proposed by [PERSON] et al. in 1977 [14; 15]. However, the production of H\({}_{2}\) through the oxidation of H\({}_{2}\)O\({}_{(g)}\) using concentrated solar radiation was proposed by [PERSON] et al. [16] in 1983. Recent studies have introduced notable innovations in the application of solar thermochemical reactors for converting concentrated solar energy into chemical fuels. These advancements involve a two-step thermochemical process specifically designed to produce H\({}_{2}\) from H\({}_{2}\)O\({}_{(g)}\). By harnessing the immense potential of solar energy, this innovative approach offers a sustainable and highly efficient method for generating H\({}_{2}\). The exothermic reaction using H\({}_{2}\)O\({}_{(g)}\) to produce H\({}_{2}\) is expressed as follows [14; 15]:  
\[3 FeO+H_{2}O\ \to Fe_{3}O_{4}+H_{2} \tag{2}\]  
The Fe\({}_{3}\)O\({}_{4}\) produced in this reaction is transformed into FeO in a solar oven through an endothermic subprocess [15; 17]. However, a significant barrier arises in this subprocess due to the premature fusion of FeO, which occurs at a temperature lower than the threshold temperature required for the complete thermodynamic cycle. This inherent challenge presents difficulties in effectively employing a thermochemical solar reactor for H\({}_{2}\) production, limiting its efficiency and feasibility. The reaction is expressed as follows [14; 18]:  
\[Fe_{3}O_{4}(l)\to 3 FeO\ (l)+\frac{1}{2}O_{2}\ ;\ T>1875\ K \tag{3}\]  
High-temperature corrosion is influenced by (i) temperature, (ii) gas composition, (iii) exposure time and (iv) system pressure and can be characterised by the reduction in thickness (penetration) and the rate of thickness growth. The oxidation rate increases  
Figure 1: Number of publications per year with the keywords “thermal water splitting using iron and oxides at high temperatures” retrieved from Web of Science. significantly with rising temperatures [19], creating a homogeneous medium and enhancing thermodynamic equilibrium [20]. The Gibbs free energy is expressed as follows:  
\[\Delta G^{o}=-RT\ln(K) \tag{4}\]  
When a metal is exposed to corrosion at high temperatures (corroding metal, oxidation or thermal degradation), it yields a value of \(\Delta G^{\circ}\), as determined by Equation (5), which is based on the reaction xM + yH\({}_{2}\)O \(\rightarrow\) M\({}_{x}\)O\({}_{y}\) + yH\({}_{2}\)[19]. The expression for the Gibbs free energy is as follows:  
\[\Delta G^{o}=-yRT\ln\left(\frac{P_{H_{2}}}{P_{H_{2}O}}\right) \tag{5}\]  
[PERSON] et al. [21] indicated that during the corrosion process, the contact of a metal (M) with steam can produce H\({}_{2}\) by the following reaction [21]:  
\[MO_{(x-1)}+H_{2}O\to MO_{x}+H_{2} \tag{6}\]  
where the expression for the Gibbs free energy is as follows:  
\[\Delta G^{o}=\Delta H^{MO_{x}}-\Delta H^{MO_{(x-1)}}-\Delta H^{H_{2}O}-T\left( S^{H_{2}}-S^{H_{2}O}\right)\ \leq 0 \tag{7}\]  
In the above equation, MO\({}_{x}\) is the oxidized metal and MO\({}_{x-1}\) is the reduced oxide. The water-splitting reaction using FeO as an electrode is favourable only at temperatures below 800 \({}^{\circ}\)C, when \(\Delta G^{\circ}<0\). The theoretical conversion of FeO decreases with increasing temperature [22].