Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review

Abstract

Green hydrogen is poised to play a crucial role in the energy-transition process in developed countries over the coming years, particularly in those countries aiming to achieve net-zero emissions. Consequently, the for green hydrogen is expected to rise significantly. This article explores the fundamental methods of producing hydrogen, focusing on the oxidation reaction within a thermochemical solar cycle for the dissociation of steam. Solar thermochemical cycles have been extensively researched, yet they remain in the development stage as research groups strive to identify optimal materials and conditions to enhance process efficiency, especially at high temperatures. The article analyses theoretical foundations drawn from exhaustive scientific studies related to the oxidation of iron in steam, the relationship with the activation energy of the corrosive process, thermodynamic aspects, and the kinetic model of a heterogeneous reaction. Additionally, it presents various mechanisms of high-temperature oxidation, pH effects, reactors, and materials (including fluidized beds). This scientific review suggests that hydrogen production via a thermochemical cycle is more efficient than production via electrochemical processes (such as electrolysis), provided the limitations of the cycle’s reduction stage can be overcome.

1. Introduction

The highest levels of solar radiation are typically found in arid and hyperarid climates. The Atacama Desert in northern Chile offers ideal solar radiation, making it widely recognised for its potential usefulness in producing solar hydrogen (H₂). Additionally, the region boasts a stable and predictable climate, with minimal rainfall and a lack of extreme temperature variations. This climatic consistency is crucial for the long-term performance and reliability of solar energy systems. Moreover, the desert’s vast expanses of land provide ample space for large-scale solar installations, facilitating the deployment of cutting-edge solar technologies and enabling the generation of significant amounts of clean energy. The knowledge gained from studying solar resources in this unique environment not only contributes to advancements in solar technology but also serves as a valuable blueprint for harnessing solar power in other regions with similar radiation characteristics. There are numerous reasons that explain why the thermochemical process [1,2] plays an important role in producing hydrogen (H₂) in arid and hyperarid zones. Zones with high solar radiation have the following advantage: the high levels of solar radiation present in arid regions, such as the Atacama Desert, make them ideal for harnessing solar power.

Water electrolysis is a promising method for hydrogen production due to its ability to produce high-purity hydrogen, its integration with renewable energy sources, and its adaptability to different scales of operation. However, its high energy consumption and the challenge of reducing the use of exotic materials remain significant challenges for this type of technology, affecting energy-conversion efficiency and posing a major barrier to its large-scale adoption. Although thermochemical solar cycles using ceria have 25% annual efficiency compared to 14% for the electrolysis process [3,4], the cycles are still impeded by a low level of technological preparation for the production of low-emission hydrogen [5,6,7,8].

The utilization of abundant solar energy in thermochemical processes can result in the generation of hydrogen and has the following advantages. (i) The efficient splitting of water molecules into hydrogen and oxygen is facilitated using concentrated solar energy to achieve high temperatures in thermochemical water splitting. When high solar irradiance is available, this method can achieve higher efficiency than is achieved with other hydrogen-production methods. (ii) Hydrogen production is made sustainable and environmentally friendly by utilizing renewable solar energy in the process. This approach contributes to cleaner energy production by reducing dependence on fossil fuels and decreasing greenhouse gas emissions. (iii) Iron or iron oxide can be used as abundant and low-cost materials for the thermochemical process, which makes it economically viable. Cost-effective production is guaranteed in regions with high solar potential thanks to the high availability of these materials globally. (iv) Arid zones tend to have predictable and stable weather patterns with little rainfall and small variations in temperature. Solar thermochemical systems’ performance and reliability are enhanced by the consistent climatic conditions, which ensure reliable and continuous operation, and (v) technological advancements can be achieved by researching and developing solar thermochemical hydrogen production in high-irradiation zones with high solar potential. Through the application of this knowledge, global hydrogen-production efforts can benefit from improved efficiency, cost reduction, and the development of new materials. The thermochemical solar process (TSP) for recovering solar hydrogen is considered environmentally favourable and inexpensive. The electrodes used in this process are generally made from iron or iron oxide, materials that are abundant worldwide. The global size of the market for iron oxide is estimated to be valued at around USD 2629.4 million in 2023, with projections reaching USD 3.6 billion by the end of 2030 [9]. The thermocatalytic material used in the TSP is ceria [3,4]. This technology today yields low production of solar hydrogen due to technological problems [8]. Regarding production costs, it is estimated that hydrogen from solar photovoltaic energy and wind energy is between 4.0–9.0 USD/Kg H₂ and could be below 1.5 USD/Kg H₂, with the costs of photovoltaic solar electricity at 14 USD/MWh in 2030 [8].

Regarding thermal water-splitting using iron and iron oxides at high temperatures, the number of articles published has shown significant growth between the years 2007 and 2023, as shown in Figure 1, which illustrates the numbers of publications that contain the specific words, “thermal water splitting using iron and oxides at high temperatures” in Web of Science.

This review is essential, as it provides the fundamental theoretical information necessary to understand the challenges associated with using pure iron or iron oxides as thermocatalytic materials in thermochemical cycles for hydrogen production. It is crucial to comprehend the mechanisms of these materials’ reactions in the presence of water vapour from a thermodynamic perspective, identify the heterogeneous reactions that occur on non-catalytic surfaces through electrochemistry, and consider the oxidation mechanisms associated with reactions at high temperatures with steam.

2. Theoretical Background

Under specific thermodynamic conditions, the interaction between steam (H₂O_(g)) and pure iron metal (Fe) can produce hydrogen (H₂) through the oxidation of Fe at temperatures ranging from 200 to 1000 °C. This oxidation reaction occurs in an environment rich in H₂O_(g) and can be expressed as follows [10]:

$$3Fe+4H_{2}O \to F{e}_3{O}_4+4H_2$$

(1)

In 1926, Dunn et al. [11] proposed that high-temperature oxidation is governed solely by the diffusion of O₂ through the oxide layer. The presence of H₂O_(g) accelerates the oxidation process of mild steel to twice the rate associated with Fe pure [12,13].

The production of hydrogen (H₂) using magnetite (Fe₃O₄) was proposed by Nakamura et al. in 1977 [14,15]. However, the production of H₂ through the oxidation of H₂O_(g) using concentrated solar radiation was proposed by Lede 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 H₂ from H₂O_(g). By harnessing the immense potential of solar energy, this innovative approach offers a sustainable and highly efficient method for generating H₂. The exothermic reaction using H₂O_(g) to produce H₂ is expressed as follows [14,15]:

$$3FeO+H_{2}O \to F{e}_3{O}_4+H_2$$

(2)

The Fe₃O₄ 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 H₂ production, limiting its efficiency and feasibility. The reaction is expressed as follows [14,18]:

$$F{e}_3{O}_4\left(l\right)\to 3FeO \left(l\right)+{\displaystyle \frac{1}{2}}{O}_2 ; T>1875 K$$

(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 significantly with rising temperatures [19], creating a homogeneous medium and enhancing thermodynamic equilibrium [20]. The Gibbs free energy is expressed as follows:

$${∆G}^o=-RT \mathrm{l}\mathrm{n}\left(K\right)$$

(4)

When a metal is exposed to corrosion at high temperatures (corroding metal, oxidation or thermal degradation), it yields a value of ΔG°, as determined by Equation (5), which is based on the reaction xM + yH₂O → M_xO_y + yH₂ [19]. The expression for the Gibbs free energy is as follows:

$${∆G}^o=-yRT\ln \left({\displaystyle \frac{P_{H_2}}{P_{H_{2}O}}}\right)$$

(5)

Meredig & Wolverton et al. [21] indicated that during the corrosion process, the contact of a metal (M) with steam can produce H₂ by the following reaction [21]:

$$M{O}_{(x-1)}+H_{2}O\to M{O}_x+H_2$$

(6)

where the expression for the Gibbs free energy is as follows:

$${∆G}^o={∆H}^{{MO}_x}-∆H^{{MO}_{(x-1)}}-∆H^{H_{2}O}-T\left(S^{H_2}-S^{H_{2}O}\right) \le 0$$

(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 °C, when ΔG° < 0. The theoretical conversion of FeO decreases with increasing temperature [22]. Kodama and Gokon et al. [23] indicated that the oxidation of FeO in the presence of water vapour is a spontaneous reaction (ΔG < 0) at temperatures below 1000 K.

In the use of iron (Fe) and its oxides at high temperatures for hydrogen (H₂) production, there are various mechanisms that would allow material agglomeration to occur. The adhesion or clumping of particles notably affects the efficiency and stability of the thermochemical process. Agglomeration is influenced by three factors: (i) at high temperatures, it is normal for Fe and Fe_nO_m particles to fuse together, forming larger particles; (ii) changes in the crystalline structure of Fe and Fe_nO_m at high temperatures cause physical changes that lead to agglomeration; and (iii) the thermal expansion of these materials causes mechanical stresses that facilitate agglomeration. During the chemical loop process, the morphology and support materials of the iron oxides can influence the stability and effectiveness of the process, as oxygen carriers play a crucial role during reactions, causing changes in their physical structures across several high-temperature redox cycles. Moreover, factors such as the reducing atmosphere and the behaviour of specific iron oxides such as hematite and magnetite under certain high-temperature conditions can contribute to different agglomeration behaviours, which affect the mechanism of hydrogen evolution. [24,25]

Young et al. [26] suggested that various metal oxides can form volatile compounds through direct reactions with water vapour and that hydroxides and oxy-hydroxides can be produced by hydration [27]. On the other hand, Belton & Richardson et al. [28] demonstrated the existence of a volatile iron hydroxide at temperatures exceeding 1300 °C. Compared to non-volatile metal oxides, volatile metal oxides exhibit a higher oxygen-exchange capacity, which directly correlates with their hydrogen-production capacity [29].

Iron hydroxides modify their reactive properties under specific conditions at high temperatures. The volatile iron hydroxides are (i) Fe(OH)₂ and (ii) Fe(OH)₃. These hydroxides can form under reducing conditions. Fe(OH)₂ is less stable than Fe(OH)₃ and can easily dehydrate or decompose at high temperatures, potentially forming iron oxides and releasing water vapour. On the other hand, Fe(OH)₃ can also undergo dehydration or decomposition when it is subjected to high temperatures. The stability, reactivity, and transformations of these hydroxides under elevated thermal conditions are key to optimizing the efficiency of hydrogen production in thermochemical cycles [30].

Table 1. Thermodynamic properties of water, oxides, and hydroxides.

Chemical Species	Gibbs Free Energy ΔG (kJ/mol)	Enthalpy ΔH (kJ/mol)	Entropy S (kJ/mol)	Temp. K (1 atm)	Refs.
H2O(l)	−237.1	−285.8	70.0	298	[31,32]
H2O(g)	−228.6	−241.8	188.8	298	[31,32]
	−214.1	−244.7	213.8	600	[32]
	−198.2	−247.1	228.6	900
H2(g)	0	0	130.6	298	[31,32]
	0	0	151.0	600	[32]
	0	0	163.1	900
OH− (aqueous ion)	−157.3	−230.0	−10.7	298	[31]
OH−(g)	34.7	38.9	183.7	298	[32]
	29.5	38.9	204.4	600
	24.9	38.4	216.5	900
Fe0	0	0	27.09	298	[31]
Fe2+	−90.0	−91.1	−107.1	298	[31]
Fe3+	−16.7	−49.9	−280.0	298	[31]
Fe.947O	−244.9	−266.3	56.6	298	[31]
	−245.1	−266.3	57.5	298	[32]
	−224.8	−263.7	93.0	600
	−205.8	−262.7	115.1	900
FeO	−251.4	−272.0	60.6	298	[31,32]
	−231.7	−269.4	97.3	600	[32]
	−213.1	−268.4	120.2	900
$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$	−774.4	−826.2	87.4	298	[31,32]
	−742.3	−824.2	87.4	298	[32]
	−661.4	−817.6	173.3	600
	−585.3	−808.4	235.4	900
$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$	−731.4	−811.6	93.0	298	[33]
	−1012.7	−1115.7	146.1	298	[31]
	−1015.2	−1118.4	146.1	298	[32]
	−914.4	−1107.4	272.1	600
	−821.8	−1088.6	368.3	900
FeO(OH)	−491.8	−562.6	60.4	298	[31]
Fe(OH)2	−486.9	−568.9	87.9	298	[32]
	−405.7	−564.1	160.4	600
	−327.6	−558.9	207.7	900

displays the thermodynamic parameters (∆G°, ∆H°, ∆S°, and T) used to determine the activation energy for the reaction of water vapour with iron and the products of vapour dissociation into molecules, ions, and iron-oxidation products (oxides and hydroxides).

Table 2. Thermodynamic properties of the reaction of iron or oxides in water Vapour.

Reaction	Gibbs Free Energy ΔG (kJ/mol)	Enthalpy ΔH (kJ/mol)	Temp. K (pres. atm)	Refs.
$0.75 Fe+H_{2}O \to 0.25F{e}_3{O}_4+{4H}_2$	−14.52	−32.09	600	[34]
$Fe+2H_{2}O \to FeO+H_2$	−199.3	-	1000	[35]
$Fe+2H_{2}O \to Fe{\left(OH\right)}_{2\left(g\right)}+H_2$	38.9 + 5 × 80T (cal)	-	-	[28]
$2Fe+3H_{2}O \to F{e}_2{O}_3+{3H}_2$	48.9	-	973–1123	[36]
$3Fe+4H_{2}O \to F{e}_3{O}_4+{4H}_2$	11.0	-	973–1123	[36,37]
$3FeO+H_{2}O \to F{e}_3{O}_4+H_2$	-	−33.6	873	[22,23,38,39]
$F{e}_2{O}_3+H_{2}O \to 2FeO\left(OH\right)$	10.39	−47.54	400	[34]
$F{e}_3{O}_4+{2H}_{2}O \to Fe{\left(OH\right)}_2+2FeO\left(OH\right)$	41.02	−79.23	400	[34]
$F{e}_3{O}_4+{4H}_{2}O \to Fe{\left(OH\right)}_2+2Fe{\left(OH\right)}_3$	228.22	−120.48	600	[34]
$2F{e}_3{O}_4+H_{2}O \to 3F{e}_2{O}_3+H_2$	40.5	-	-	[40]
$2F{e}_3{O}_4+{4H}_{2}O \to 6FeOOH+H_2$	44.4	-	-	[40]

presents the typical thermodynamic parameters generated during the oxidation/reduction processes of metals in the presence of H₂O_(g) at high temperatures.

Kinetic Model

The mechanisms by which a pure metal or alloy is oxidized at high temperatures require a series of successive steps [27,41]. The steps are as follows:

• Step (a) adsorption of a gaseous component,
• Step (b) dissociation of the gaseous molecule and transfer of electrons,
• Step (c) nucleation and growth of crystals,
• Step (d) diffusion and transport of cations, anions, and electrons through the oxide layer.

Nevertheless, the reaction between a solid (Fe) and a gas (H₂O_(g)) corresponds to a heterogeneous reaction wherein the kinetics are totally or partially controlled for the mass-transport process. The product species, generated by convection and diffusion, can be gaseous or insoluble solids [42]. Diffusion and heat transfer intertwine with the chemical kinetics of non-catalytic heterogeneous reactions, with their respective diffusion equations extending to fluidized solids [43]. The reduction of iron oxide with hydrogen or the oxidation of iron with steam are two typical examples of such reactions. These reactions occur under conditions of a highly compacted unreacted solid with limited porosity, where the chemical reaction progresses rapidly while diffusion proceeds relatively slowly [44].

Wen et al. [44] consider that a superficial non-catalytic heterogeneous reaction consists of the following steps (Figure 2):

• Step (a) diffusion of fluid reactants through the fluid film surrounding the solid.
• Step (b) diffusion of the fluid reagents through the porous solid layer,
• Step (c) adsorption of the fluid reagents on the surface of the solid reagent,
• Step (d) chemical reaction with the solid surface,
• Step (e) desorption of the fluid products from the solid reaction surface,
• Step (f) diffusion of the product far from the reaction surface through the porous surface, the solid media, and the fluid film surrounding the solid.

A study by Surman et al. [45]. involved the oxidation of pure Fe at 500 °C using hydrogen, water vapour, or various mixtures of the two. The oxidation kinetics were compared with models of chemical diffusion and solid-state reactions. The diffusion of volatile iron in the form of Fe(OH)₂ was identified as the rate-controlling step. This diffusion occurred from the metal-oxide interface through the porous oxide of the inner layer and ended with the deposition of Fe₃O₄ on the crystals of the outer layer, which acted as sinks.

The oxidation kinetics occur according to the parabolic law. The expression of the parabolic rate constant (K_w) is represented by the following expression:

$$K_w=4.8·{10}^4·F_p·T^{1/2}·\left({\displaystyle \frac{P_{H_{2}O}}{P_{H_2}}}\right)·e^{-\mathrm{38,900}/RT}$$

(8)

where K_w is the parabolic rate constant in (g²/m⁴s), F_p is the porosity factor of the inner layer, and P_H2O and P_H2 are the partial pressures of H₂O and H₂, respectively.

Park et al. [46] studied the production of hydrogen using iron oxide and inert silica with H₂O_(g) at temperatures ranging from 460 to 700 °C and vapour partial pressures between 0.002 and 0.02 MPa. Initially, Fe is oxidized to Fe₃O₄; this is followed by the gradual oxidation of Fe₃O₄ to Fe₂O₃ in the second step. The researchers determined the equilibrium state of H₂O_(g) adsorption and found that the rate-determining step of the reaction is the desorption of hydrogen from the active sites.

The proposed initial oxidation rate when the partial pressure of water is fixed is represented by the following expression:

$${\left({\displaystyle \frac{{dX}_B}{dt}}\right)}_{t=0}=\left({\displaystyle \frac{bk}{1+K_{e\_ads}{·P}_{H_{2}O}}}\right)P_{H_{2}O}-{\displaystyle \frac{\left(\frac{bk}{1+K_{e\_ads}{·P}_{H_{2}O}}\right)}{K_e}}{P}_{H_2}$$

(9)

where b is the stoichiometric coefficient of the oxidation reaction from Fe to Fe₂O₃, k is the overall rate constant based on unit solid volume, K_e is the chemical equilibrium constant, K_{e_ads} is the equilibrium adsorption constant, and P_H2O and P_H2 are the partial pressures of water and hydrogen, respectively.

The heterogeneous kinetic models have been detailed extensively by Donovan & Berra et al. [47], Levenspiel et al. [48], and Klaewkla et al. [49] However, Wen & Wang et al. [50] proposed a kinetic model for the non-catalytic heterogeneous reaction in a solid-gas system, considering both heat and mass transfer as combined effects.

Valipour & Saboohi et al. [51] analysed multiple mathematical models and developed a comprehensive model for a multi-reactant system using porous granules within a sample of a mobile packed bed. Our model accounts for various factors, including external mass transfer, internal diffusion through the pores, chemical reactions, heat generation or consumption due to reactions, and heat transfer through effective conduction within the solid matrix.

The three main types of growth laws that have been established experimentally, the linear, parabolic and logarithmic laws, are illustrated in

Table 3. Reaction-rate equations applicable to high-temperature oxidation.

Equation	Observation	Refs.
Logarithmic reaction rate:
$X=K_{reaction}\ln \left( t+t_o\right)+A$	It represents the initial oxidation states at low temperatures.	[41]
$X=K_{reaction}\ln \left( Bt+1\right)$	-	[41]
Linear reaction rate:
${\displaystyle \frac{dx}{dt}}=K_{Lineal}$	It represents a constant rate of oxide growth applicable at very high temperatures.	[41]
$X=K_{Lineal}t+C_{Lineal}$	-	[41]
${\displaystyle \raisebox{1ex}{$∆W$}\!\left/ \!\raisebox{-1ex}{$A$}\right.}= K_L t$	-	[52]
Parabolic reaction rate:
${\displaystyle \frac{dX}{dt}}={\displaystyle \frac{K_{Parab}}{x}}$	It adjusts to the processes controlled by the diffusion of species.	[41]
$X^2=K_{parab}t+C_{parab}$	-	[41,53]
${\left({\displaystyle \raisebox{1ex}{$∆W$}\!\left/ \!\raisebox{-1ex}{$A$}\right.}\right)}^2=K_{parab}\ast t$	-	[52]
${\displaystyle \frac{dX}{dt}}=k f(X)$	The function f(X) depends on the reaction mechanism for diffusion in one, two, or three dimensions.	[54]
${\displaystyle \frac{dX}{dt}}=k\left(T\right)\ast f(X)$ $k\left(T\right)=k_o\ast e^{\left({\displaystyle \frac{{-E}_a}{RT}}\right)}$	The reaction rate of a sample of iron slag in water vapour.	[55]

Where X is the thickness of oxide consumed per surface unit or the weight gain per area unit; ΔW/A is the mass gain per unit area (g/cm²); t is the time; k(T) is the reaction rate constant; f(X) is the function that represents the reaction mechanism; ko, K_reaction, K_Lineal, and K_Parab are reaction constants; E_a is the activation energy; R is the gas constant; T is the absolute temperature; A and B are constants; C_Lineal and C_Parab are constants of integration.

.

The Fe oxidation kinetics follow a linear−parabolic behaviour in the presence of H₂O_(g), suggesting a transitional process wherein the diffusion of the hydroxyl ion is a determining factor during the oxidation [52]. Fujii & Meussner et al. [56] indicated that the kinetic oxidation of iron at 1100 °C can be adjusted to a parabolic law during the first 3 h, after which a linear behaviour is established, increasing the weight per unit area of all specimens (the rate of weight gain was 6.2 mg/cm² per hour). The effect of temperature on the reaction rate is obtained by applying the Arrhenius equation, which is represented by the following expression [54,55]:

$$K=K_o{e}^{{\displaystyle \frac{-∆G^o}{RT}}}$$

(10)

where K is the equilibrium constant, K_o is the reaction constant, ΔG° is the activation energy, R is the gas constant, and T is the absolute temperature.

If the reaction rate is governed by diffusion through the oxide layer in the solid state, the oxide thickness (rust) increases during the diffusion process. However, at the same time, the kinetics slow down over time [41]. In the cathodic subprocess, the reaction 2H₂O_(g) + 2e⁻ → 2OH⁻ + H₂ occurs as a product of the high-temperature dissociation process. The scientific literature indicates that the formation of OH⁻ from H₂O_(g) is proportional to the concentration of H₂O_(g) and that the appearance of OH⁻ cannot be interpreted in terms of the simple dissociation of H₂O_(g). It is necessary to consider the pathway involved in the formation of intermediate products generated during ORR such as (i) H₂O₂ and (ii) HO₂⁻ [57].

3. Mechanisms of High-Temperature Oxidation

Despite extensive scientific knowledge about the oxidation of iron at both environmental and high temperatures due to the presence of oxygen ions and despite efforts to reduce or avoid iron corrosion, the electrochemical reaction mechanism that explains the presence of oxides such as FeO, Fe₃O₄, and Fe₂O₃ [58], especially when iron interacts with the hydroxyl ion (OH⁻), remains insufficiently understood.

Shrinivasan et al. [59] developed a multi-step optical system that enables water decomposition for OH⁻ ion detection, demonstrating that rate constants can be accurately measured at lower temperatures, such as 500 K. This approach contrasts with alternative methods that require temperatures exceeding 2570 K [60].

Lede et al. [16] proposed a kinetic mechanism of thermal dissociation of H₂O between 2000–3000 K, as represented by the following expressions:

$$H_{2}O+G\leftrightarrow H+OH+G$$

(11)

$$H_{2}O+H\leftrightarrow H_2+OH$$

(12)

$$OH+H\leftrightarrow H_2+O$$

(13)

$$OH+O\leftrightarrow O_2+H$$

(14)

where G is the extinguishing gas (argon at room temperature or steam at 400–450 K and between 2.5–9.4 bar), H is the H⁺ ions, and OH is the OH⁻ ions.

Fe corrosion is a process of electrochemical dissolution involving electron transfer to an intermediate species formed by the interaction between ferrous ions and water. In the reduction of Fe, this intermediate species is found on the iron surface and must undergo a subsequent reaction to form Fe [61].

The initial rate of oxidation of an Fe or steel surface newly exposed to H₂O_(g) is always lower compared to the rate of corrosion induced by O₂. Researchers Tuck et al. [12,52] suggested that while H₂ tends to dissolve in solid metals in atomic rather than molecular form, it is unlikely to dissolve in an oxide network due to its large size, which impedes diffusion. Therefore, they propose that the cathodic reaction could occur within the scales.

The general reaction for the production of H₂ is shown below:

$$2H_{2}O+2e^{-} \to 2O{H}^{-}+H_2$$

(15)

The mechanism of evolution of H₂ (HER) would occur on the surface of the pores and micropores, where the hydroxyls ($\mathrm{O}{\mathrm{H}}^{-})$ formed by the dissolution of the vapour are discharged through a cathodic reaction according to Figure 3.

Rahmel & Tobolski et al. [62] proposed that the oxide layer is still thin and flexible in the first step the of oxidation process. In the presence of H₂O_(g), the transport mechanisms are likely to occur specifically through cavity or pore formation [63]. The mechanism is represented by Figure 4.

Schuetze et al. [37] proposed that the H₂O_(g) is involved in the transport processes that lead to scale growth in the oxide layer. The researchers indicate that the Fe cations (Fe²⁺/Fe³⁺) can simultaneously diffuse towards the outer surface in this process. Based on the knowledge of the structure and diffusion of iron oxides, the oxidation mechanism of pure iron above 570 °C in an iron−oxygen systems can be better understood. In particular, in the large pores in the inner part of the scale of FeO, a circulation mechanism is assumed that consists of the oxidation of Fe in contact with H₂O_(g), releasing H₂ to can move back in the pore towards the outer part of the scale, where it reduces the oxide, thus forming an H₂O_(g) molecule again [64].

The first reaction that occurs is from the formation of OH⁻ ions, which would increase cation vacancies and would also be the main diffusion species [11,12,52]; this reaction is expressed as follows:

$$Fe+2H_{2}O \to F{e}^{2+}+2O{H}^{-}+H_2$$

(16)

Yuan et al. [52] suggested the formation of ferrous oxide (FeO) as shown in Figure 5; that is, iron in the presence of water vapour at high temperatures will precipitate FeO and Fe₃O₄ [52] or Fe₂O₃ [36], as follows:

$$F{e}^{2+}+O{H}^{-}\to FeO+H^{+}$$

(17)

However, FeO in contact with Fe(OH)₂ [65] is generated at temperatures above 1300 °C [28] and can form Fe₂O₃ in conjunction with the release of H₂ [52].

Rahmel & Tobolski et al., in 1965 [62], proposed the existence of pores at the Fe/FeO interface during the oxidation process, where they form oxide bridges from the metal to the scale. This structure allows for further oxidation of the metal without substantial inhibition. A mixture of H₂/H₂O is formed in these pores, and through an oxidation/reduction mechanism, O₂ is transported to the Fe surface.

An oxide of the Fe_nO_m type will typically contain a variety of defects. These defects are crucial for the transport of material through the oxide layer and thus play a critical role in the oxidation process [12,66].

Yuan et al. [52] suggested that during the initial period (before 5 h), the presence of a single layer of columnar grains may facilitate relatively fast transport of OH⁻ ions across grain boundaries. It is therefore reasonable to assume that the hydroxyl ions interact with the surface hematite and magnetite, forming FeO.

However, certain aspects remain under investigation. Stehle et al. [67] highlighted that at typical reaction temperatures (above 327 °C), the mobility of oxygen and metal ions is expected to be very high, so diffusion limitations are generally not considered significant. This exception applies to oxides that exceed a thickness of several microns, where the potential of Fe³⁺ must be taken into account [52,67,68].

$$4O^{2-}+2F{e}^{3+}+F{e}^{2+} \to F{e}_3{O}_4$$

(18)

Another aspect that should be considered, as stated by Saunders et al. [27], is that oxide growth is influenced by the simultaneous processes of adsorption, dissociation, and diffusion of reagents, which are altered in the presence of water vapour.

Table 4. Standard reduction potentials.

Half Reaction	Standard Reduction Potential, E° (V)	Ref.
$H_2 \to 2H^{+}+2e^{-}$	E° = 0.000 (V)	[69]
$Fe \to F{e}^{2+}+2e^{-}$	E° = −0.447 (V)	[69]
Reaction
$3Fe+4H_{2}O \to F{e}_3{O}_4+{4H}_2$	-	[55]
$2FeO+H_{2}O \to F{e}_2{O}_3+H_2$	E° = 0.356 (V)	[55]
$2F{e}_3{O}_4+H_{2}O \to 3F{e}_2{O}_3+H_2$	E° = 0.210 (V)	[40]
$2F{e}_3{O}_4+{4H}_{2}O \to 6FeOOH+H_2$	E° = 0.230 (V)	[40]

displays the standard reduction potentials that have been considered in the electrochemical analyses of the oxidation of iron and iron oxides in steam.

From these reactions, the kinetics of electrochemical reactions can be understood, highlighting their significance for the application of metal oxide redox reactions in energy-conversion systems such as chemical loop systems and hydrogen storage [70].

In an activation regime, the speed of the electrochemical process is dictated solely by electron transfer, which controls the pace of the overall process [41]. Hence, the corrosion reaction can be expressed by the ionization of a metal. However, the possibility that this reaction occurs spontaneously under real conditions necessitates the study of the energy changes associated with the reaction. Moreover, some H⁺ ions migrate into the metal, forming H₂. Consequently, the presence of H⁺ ions can promote stress-corrosion cracking through the process of hydrogen embrittlement [71].

FeO nucleation and growth are enhanced by increasing oxygen pressure [66,72]. Kogan et al. [73] studied the dissociation of water at temperatures of 2000, 2200, 2500, and 2800 K at a constant pressure of 0.05 bar. They determined that 25% of the water vapour dissociates at 2500 K and 55% at 2800 K, with the rate of dissociation increasing at higher temperatures.

Kodama et al. [74] reported that the separation of water through a thermochemical process at a pressure of 0.01 bar and a temperature of 2000 K was barely noticeable, but when the temperature was increased to 2500 K, the yield of H₂ exceeded 15% at the same pressure. Young et al. [26] proposed that in pure steam or mixtures of water vapour and inert gas, the equilibrium value of P_O2 (oxygen pressure) is determined by the degree of dissociation of H₂O. In the case of pure steam, the dissociation of one mole of water produces x mol of H₂ and x/2 mol of O₂, with x calculated from the equilibrium expression shown in Equation (19), as follows:

$$K_1^2={\displaystyle \frac{x^3}{2{\left(1-x\right)}^2\left(1+\frac{x}{2}\right)}}{P}_T$$

(19)

where P_T is the total pressure and K₁ is small (x << 1), such that the above expression approaches the following:

$$x={\left({\displaystyle \frac{2K_1^2}{P_T}}\right)}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}}$$

(20)

$$p_{O_2}=x{·P}_T$$

(21)

Ehlers et al. [75] proposed that Fe(OH)₂, formed within oxide scales at low oxygen pressure (P_O2), migrates to the outer surface where, due to higher pressure, hematite (Fe₂O₃) is formed. Khanna et al. [64] suggested that iron, in the presence of oxygen, forms a mixed scale of three oxides (FeO, Fe₂O₃, and Fe₃O₄), with the composition varying according to temperature and the P_O2.

Ketteler et al. [75], in their research, considered a Fe-H₂O system to determine the stability of iron oxide as a function of the partial pressure of water, identifying the phase limit for water condensation as ranging from 125 K (1 × 10⁻¹¹ mbar) to 373 K (1 bar). They focus on the functions of oxygen pressure and temperature, according to the equilibrium constant for the dissociation of water (2H₂O → 2H₂ + O₂) (see Equation (22)). The researchers concludes that water in its gaseous state acts as an oxidizing agent and that the stability of iron oxides is determined by the partial pressure from the decomposition of water into hydrogen and oxygen.

$$k_1={\displaystyle \frac{p_{O_2}{p}_{H_2}^2}{p_{H_{2}O}^2}}$$

(22)

4. Effect of pH on Oxidation and Temperature

The pH value is normally based on the equilibrium reaction of the dissociation of water (H₂O → H⁺ + OH⁻), which has an endothermic character, and the equilibrium of which shifts to the right with increasing temperature. Research indicates that at 300 °C (25 MPa), the concentrations of both H⁺ (stable in acidic solutions) [71] and OH⁻ (stable in basic solutions) are approximately three orders of magnitude higher than their concentrations in water at ambient temperature. Consequently, water can be considered both acidic and alkaline [76].

To produce green hydrogen via a thermochemical process (thermolysis), temperatures ranging from 800 to 1400 °C are necessary in the reactor [77]. This temperature range has been achieved using a solar thermal concentrator system with a down-beam configuration. Iron is considered for hydrogen production in the thermochemical solar process because the conversion of Fe₃O₄ to FeO improves significantly at higher thermal-reduction temperatures [52]. However, its application is challenging because in the thermal-reduction stage of the thermochemical cycle, Fe₃O₄ melts at temperatures above 2227 °C (Figure 6) [14,78] and FeO melts at temperatures as low as 1370 °C [18] or 1400 °C [23]. These factors complicate the design and operation of thermochemical reactors based on Fe₃O₄/FeO [18]. However, the thermal-oxidation stage (hydrolysis) is carried out at temperatures ranging from 200 to 1000 °C [10,14,17,38,67]. According to the scientific literature, the temperature affects the process because the oxidation of iron by steam is thermodynamically favourable within the temperature ranges of 127–527 °C [18,34], 650–750 °C [52], and 700–850 °C [36], although reaction temperatures ranging from 717 to 1127 °C [68] have also been investigated.

5. Fluidized Bed Reactors for Thermochemical Water Splitting and Their Materials

It is important to highlight that iron oxide is selected because it is an environmentally friendly base material that is relatively low-cost and is commonly used in redox reactions due to its high oxygen-by-weight ratio, which ensures sufficient reaction time [54].

According to the reviewed scientific literature, temperatures above 1300 °C can be achieved using a thermochemical solar reactor with metallic materials such as cerium oxide or zinc oxide in a fluidized bed [77,79]. The thermal outputs obtained in reactors using water steam and various materials in a fluidized bed are presented in

Table 5. Temperatures reached in a thermochemical solar reactor with a fluidized bed.

Thermal Power	Temperatures Reached or Required	Thermal Fluid	Fluidized Bed Material	Particle Size (µm)	Radiation or Concentration Ratio	Refs.
1 kWth	1300 °C	Water/Steam	Coal-coke	140 (200–300) (300–500) (500–710)	477 W/cm2	[74,80,81,82]
10 to 20 kWth	2200 °C	Water/Steam	ZnO reagent powder	1–5	-	[83]
3 MWth	1500 °C	Water/Steam	Cerium oxide material	<300 (100–300)	1.5 kW/m2	[38,77,84,85]
110 kWth	1400 °C	Gases	Non-stoichiometric cerium oxide particles	10–210	-	[79,86]
100 kWth	960–1100 °C	Air	Quartz sand particles	100–500	-	[87]
250 kWth	800–1000 °C	Air	A mixture of coal-coke and quartz sand	100-300 (coal) 100–700 (sand)	-	[88]
30 kWth	560 °C	Air	A mixture of sand and basalt		-	[89]
450 kWth	770 °C	Air	Isotropic materials		4.8 kWh/m2/900 suns	[90,91]
140.63 Wth (2 kWe) 231.32 Wth (4 kWe)	250 °C	Air	Sand, ceramic casting media (carbo Accucast ID50) y SiC		65 kW/m2 (2kWe) 115 kW/m2 (4 kWe)	[92]

. Table 5 presents the materials used in the production of H₂ via fluidized beds, along with pertinent details such as (i) thermal potential, (ii) temperature ranges within the reactors, (iii) type of fluidized material, and (iv) particle size. Additionally, it includes information on the concentration relationships referenced in the literature. The iron-oxide materials investigated for water separation in the oxidation stage at the laboratory level are detailed in

Table 6. Iron Oxide Materials.

Iron Oxides Material		Sample/Particle Size (µm)	Temp. (°C) (Time)	Partial Vapour Pressure	Thermal Fluid (Flow)	Refs.
Magnetite	Fe3O4	30–50 100–125	575 673	-	-	[23,74]
The partial substitution of iron in Fe3O4 by Ni, Co, and Zr, to form mixed metal oxides.	(Fe(1−x) Mx)3O4 NiFe2O4 NiFe2O4/m-ZrO2	Sample in a quartz tube	1000	-	-	[23,38,74]
Magnetite supported on zirconium, stabilized with cubic yttria	Fe3O4/c-YSZ	Ceramic foam	1100 (80 min)	75% of the steam pressure at 90 °C, 1 bar	H2O/N2 (10 Ncm3/min)	[86]
Commercial nickel ferrite supported on zirconium oxide	NiFe2O4/ ZrO2	Sample on Pt cup	1000 (60 min)	steam pressure at 80 °C, 1 bar	H2O/N2 (4 mL/min)	[93]
Unsupported commercial nickel ferrite	NiFe2O4	212–710	1.6–1.7 kWth (10–92 min)	51% (0.51 atm) of the steam pressure at 82 °C, 1 atm	H2O/N2 (0.24 Ndm3/min)	[39]
Monoclinic magnetite supported on zirconium substrates	Fe3O4/m-ZrO2	Polished Fe bar without oxidation	397–602 (180 min)	-	Nitrogen (100 cc/min) Argon (200 cc/min) Liquid water (12.5 µL/min)	[67]
Iron oxide with various support materials, ZrO2, CeO2, yttria-stabilized zirconia (YSZ), and gadolinia-doped ceria (GDC)	Fe2O3/ZrO2 Fe2O3/CeO2 Fe2O3/YSZ Fe2O4/GDC	150–300	550	-	H2O/Ar (prop. 5:95) (300 mL/min)	[70]

. Table 6 displays the materials used, focusing on iron-based compounds such as magnetite, which have been doped with materials including zircon, nickel, caesium, and gadolinium. It also details the sample type, partial pressure, environment type, and exposure time. Gokon et al. [39] observed that the rate of hydrogen production after steam injection reached a peak of 12.3 Ncm³/min at 25 min, after which point the rate of hydrogen production rapidly decreased. This demonstrates that hydrogen production through a fluidized bed is feasible.

Table 7. Technologies of thermochemical solar reactors with fluidized beds.

Reactor Geometry	Bed/Gas	Material	Power/Radiation	Reactor	Year/Refs.
Diameter = 5 cm Height = 32 cm	Charcoal/CO2	Silice glass tube	2 kW/ 400 W/cm2		1983 [94]
	ZrO2, SiC/air, N2, CO2	Clear quartz tube	6.5 kW/ 1000 W/m2		1983 [95]
0.90 m high, 0.78 m wide, with a radius of curvature of 0.78 m	Alumina particles/air	Refractory stainless steel (AISI 310)	30–45 kW/ 1 kW/m2		1988 [96]
Diameter = 2 cm	ZnO + Al2O3/ CH4 + Ar	Quartz tube	15 kW/ 4000 suns		1995 [97]
Diameter = 45 mm, thickness 2.5 mm	NiFe2O4—ZrO2/N2	Quartz tube	6 kW		2008 [98]
25 mm in diameter, thickness 1.5 mm, height 25 cm.	CaO or CaCO3/H2O, Ar, CO2	Quartz tube	75 kW/ 4250 suns		2009 [99]
420 mm in length, 62.3 mm in inner diameter, and 7 mm in thickness	Coke/CO2	Stainless steel with a quartz window	6 kW/ 477 W/cm2		2010 [80]
The inner diameter was 45 mm, and the thickness was 2.5 mm.	NiFe2O4/vapour	Stainless-steel tube with a quartz cap	3 × 6 kW/ 637 W/cm2		2011 [39]
Length 420 mm, inner diameter 62.3 mm, thickness 7 mm	Coke/Ar-vapour CeO2/N2	stainless-steel tube (SUS310S) with a quartz window	18 kW/ 637 W/cm2		2015 [81,84]
42 cm diameter at the top	Quartz sand particles/air	Inconel and stainless steel, with a quartz window	133 kW/ 950 kW/m2		2016 [100]
0.2 m long and 0.3 m internal diameter at the top and 0.007 m thick	Quartz particles/N2	Stainless steel tube with quartz window (5 mm thick)	21 kW/ 903 kW/m2		2018 [101]
35.6 cm long and 2.54 cm outer diameter	Iron aluminate (FeAl2O4)/N2	Silicon carbide cylindrical tubes (SiC)	10 kW		2019 [102]
Inside diameter of 7.62 cm and a height of 8 cm	Sand, carbon Accucast ID50 y SiC/air	Stainless-steel tube (304)	2kWe/ 65 kW/m2 4 kWe/ 115 kW/m2		2020 [92]
Tube with 40 mm inner diameter, 10 mm wall thickness, and 1780 mm length	Quartz sand/air	Pure iron metal tube, hot particle container made of stainless steel AISI 304	10–40 kWe		2022 [103]

below presents a list of thermochemical solar reactors that have utilized fluidized-bed materials and different gases for fluidization, considering the geometry of the reactor and the materials used in its construction. Additionally, the concentration ratio is provided

6. Discussion

The latest technological devices that have been designed and manufactured for the recovery of solar hydrogen using photovoltaic technology, such as solar concentration, direct water-splitting using solar thermal energy with Fe electrodes, and FenOm-type oxides operating at high temperatures are an example of the multiple types of sustainable alternatives to address the global energy crisis. This method directly harnesses solar energy, enhancing sustainability and scalability. By improving efficiency, solar thermal water-splitting presents a viable pathway for large-scale hydrogen production, contributing to the broader adoption of sustainable hydrogen technologies. Furthermore, using Fe or Fe_nO_m electrodes at high temperatures offers a complementary approach by potentially reducing overpotential through high-temperature operation; additionally, these materials are abundant and low-cost, whether they are obtained through direct extraction or recycling. The experimental setup for direct solar thermal water-splitting using Fe electrodes involves the use of concentrated solar energy to achieve and maintain the high temperatures necessary for the reaction. A solar concentrator focuses sunlight onto a reactor chamber, heating it to temperatures between 800 °C and 1000 °C. Within this high-temperature environment, the iron electrodes act as thermochemical catalysts, facilitating the water-splitting reaction by cycling between oxidation and reduction states. The abundance and low cost of iron make it an attractive choice for this process. High temperatures improve the catalytic activity and stability of iron, addressing some common challenges associated with non-precious-metal catalysts. This setup exemplifies a sustainable and efficient method for hydrogen production, leveraging solar energy and abundant materials to contribute to the development of renewable-energy technologies.

Non-precious transition metals such as cobalt and nickel are attractive for electrocatalysis due to their low cost. However, challenges related to conductivity, catalytic activity, maintenance of active surface sites, and geopolitical issues (with regard to cobalt) hamper their widespread application. Advances in materials science have focused on the improvement of iron-based alloys or compounds and on surface modifications with high corrosion resistance. These improvements can increase the catalytic properties, active-site density, and durability of the electrodes, overcoming problems of material degradation. High-temperature operation improves the conductivity and catalytic activity of Fe and Fe_nO_m, generating a higher density of active surface sites. The oxidation of iron in steam at high temperatures produces a significant amount of H₂, demonstrating the high efficiency and reactivity of Fe under these thermal conditions. The high-temperature environment accelerates the reaction rate and improves the purity of the produced H₂, making the process suitable for various applications. These results support the viability of using Fe and Fe_nO_m catalysts in solar thermochemical cycles.

To advance the technology of direct solar thermal water-splitting using Fe electrodes at high temperatures, several specific areas of future research have been recommended. Improvements in materials science focus on developing Fe-based alloys or composites, surface modifications, and corrosion-resistance techniques. These advancements can improve catalytic properties and increase active-site density and electrode durability, overcoming issues of material degradation. Economic-viability research involves cost-reduction strategies, life-cycle analysis, and scalability studies. These efforts aim to make the technology economically feasible by reducing manufacturing costs, understanding the full economic and environmental impacts, and exploring possibilities for large-scale production.

One of the main advantages of this method is the use of a thermochemical iron (Fe) catalyst, which is not only cost-effective but also widely available, primarily in copper slag—an industrial waste. This waste contains a variety of metal oxides that can be utilized. Despite the lack of evidence for copper slag’s use as a catalyst material for hydrogen production through thermolysis, it was found that copper slag in Chile contains a significant concentration of fayalite, comprising two moles of iron oxide (FeO) and one mole of silicon oxide (SiO₂).

This utilization not only reduces the overall cost of the process but also enhances its sustainability and feasibility for large-scale implementation. It’s noteworthy that the United States Geological Survey (USGS) estimates there are vast iron-ore reserves, further emphasizing the accessibility of this material. For instance, in 2017, gross iron ore reserves were estimated at an impressive 170,000 million metric tons, with an iron content of approximately 82,000 million metric tons. However, the 2016 production was only 2106 million metric tons, highlighting the vast untapped potential.

Moreover, the technology’s capability to efficiently split water into hydrogen (H₂) and oxygen (O₂) at high temperatures is crucial. It offers an effective solution for storing excess solar energy as hydrogen, which can be utilized in fuel cells or other applications during periods of reduced or no sunlight. This effectively tackles one of the key issues with solar power—intermittency—and contributes to a more reliable and consistent energy supply. Nonetheless, it is critical to recognize that the high operating temperatures place considerable stress on the materials, especially the electrodes and electrolyzers. This necessitates the development of advanced engineering solutions to ensure the technology’s longevity, durability, and cost-effectiveness over the long term. Additionally, optimizing the system design and energy conversion efficiency remains a significant challenge that must be overcome to enhance the competitiveness of this process relative to other hydrogen-production methods.

While this technology has immense potential, its practicality and impact are inherently linked to the solar resources and local conditions of the region in which it is deployed. Geographical areas with abundant sunlight stand to benefit the most, making this innovation particularly well-suited for regions with high solar potential. As we continue to refine and develop this technology, it has the potential to play a crucial role in transitioning towards a more sustainable and environmentally friendly energy landscape.

7. Conclusions

Direct solar thermal water-splitting using Fe (catalyst metal) at high temperatures is a promising development in the field of renewable energy and hydrogen production. By harnessing solar power and affordable materials, it offers a sustainable and scalable solution to the challenges of clean hydrogen production. As this technology matures and addresses its current challenges, it could come to play a vital role in transitioning to a more sustainable and greener energy future, reducing our reliance on fossil fuels and lowering carbon emissions across various industries.

A review of the scientific literature suggests that iron will precipitate in the forms of ferrous oxide, hematite, and magnetite during the oxidation of iron in steam at high temperatures, generating hydrogen. It is therefore reasonable to assume that iron, iron oxide, and other metal oxides present in slags, such as copper slag, will also precipitate rust and hydrogen. The theoretical thermodynamic and electrochemical aspects of the reaction of metal oxides with water vapour are fundamental to the design of the solar reactor, as well as to the adequate selection of the catalyst metal.

Future recommendations will focus on determining whether mineral waste (copper slag) containing metal oxides (iron oxides) is feasible to use as a thermocatalytic material for hydrogen production. For this purpose, a morphological and thermochemical characterization of the copper slag produced in Chile will be conducted to determine its potential use in water-splitting using a thermochemical solar reactor.

Future research holds significant promise for advancing this technology and addressing various challenges. Here are some key areas of research that can help further develop this innovative approach: (i) materials science and corrosion resistance, (ii) efficiency enhancement, (iii) thermochemical-cycle optimization, (iv) integrated energy storage, (v) economic viability, (vi) long-term durability and reliability, (vii) scalability and modular design, (viii) hydrogen purity and quality, and (ix) market integration and policy support. Incentives to encourage the adoption of solar thermal water/steam-splitting could make this technology more efficient, cost-effective, and environmentally friendly. By addressing these research areas, we can potentially overcome the current challenges and pave the way for its widespread adoption as a clean and sustainable method for hydrogen production.