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Direct Solar Thermal Water Splitting Using Iron Electrodes at High Temperatures - A Review

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11 July 2024

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12 July 2024

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Abstract
Green hydrogen is a fuel that in the coming years will play a very important role in the energy transition process in developed countries, especially those seeking to reach a level of zero emis-sions, for which reason its demand is expected to increase significantly. This article analyzes the fundamentals to produce hydrogen, with a focus on the oxidation reaction of a thermochemical solar cycle for the dissociation of water vapor. Solar thermochemical cycles have been extensively investigated, but it is still in the development stage by groups of researchers who have focused on finding the right materials and conditions to improve the efficiency of the process, especially at high temperatures. The theoretical foundations are analyzed, compiled from exhaustive scientific investigations related to the oxidation of iron in water vapor, the relationship with the activation energy of the corrosive process, thermodynamic aspects and the kinetic model according to a heterogeneous reaction. In addition, some high temperature oxidation mechanisms, pH effects, reactors and materials (fluidized beds) used are presented. The scientific review indicates that the production of hydrogen through a thermochemical cycle is more efficient than electrochemical processes (electrolysis), if the limitations of the reduction stage of the cycle are overcome.
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Subject: Engineering  -   Chemical Engineering

1. Introduction

The greatest amounts of insolation are generally found at lower latitudes and in arid climates. The Atacama Desert in northern Chile has exceptionally ideal solar radiation conditions, and the potential for producing green hydrogen is widely acknowledged. As a result, this study aims to lay the groundwork for the synthesis of hydrogen using a thermo-chemical solar process while considering an environmentally favourable, inexpensive, and abundant catalyst material in the region, such as iron oxide. The goal of the research is to discover the theoretical foundations of hydrogen production by iron oxidation at high temperatures, considering the thermodynamic, thermochemical, and electrochemical foundations required to comprehend the heterogeneous reaction. The kinetic modeling of the fluidized bed material inside a thermochemical solar reactor requires this understanding.
The unique solar radiation conditions in the Atacama Desert offer significant potential for various solar technologies, including photovoltaic (PV) systems and concentrated solar power (CSP) plants. PV systems harness the sun's energy directly using solar panels, while CSP plants concentrate sunlight to generate heat and subsequently produce electricity. The desert's high levels of solar radiation allow for enhanced energy capture and conversion efficiency, making it an attractive site for solar power installations.
In addition, the region boasts a stable and predictable climate, with little rainfall and a lack of extreme temperature variations. This climatic consistency is crucial for the long-term performance and reliability of solar energy systems. Furthermore, 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.
Although thermochemical solar cycles using ceria have 25% annual efficiency compared to 14% for the electrolysis process (Alakaline eletrolysis) [1,2], they still present a low level of technological preparation to produce low emission hydrogen [3]. Regarding production costs, it is estimated that hydrogen from solar photovoltaic and wind energy is between 4.0 – 9.0 USD/Kg H2, and could be below 1.5 USD/Kg H2, with photovoltaic solar electricity costs of 14 USD/MWh for the year 2030 [3].
The essential thing about this review is that it provides the fundamental theoretical information to understand the difficulty that arises when using pure iron or iron oxides as a thermocatalyst material in a thermochemical cycle to produce hydrogen. For this reason, it is important to understand the reaction mechanisms of the material in the presence of water steam from thermodynamics, its identification as a non-catalytic surface heterogeneous reaction from electrochemistry and aspects of the oxidation mechanisms at high temperature considering water vapor as alkaline reactant and the effects of pH on the reaction. In addition, some solar reactors that have used fluidized bed materials are presented. This technical information is intended to provide the reader with data to replicate these reactors, especially for researchers who are just beginning the topic of thermolysis using iron. But at the same time, it shows the potential of its application in a high temperature thermolysis process considering only the oxidation stage.
The scarce information related to the oxidation of iron in the presence of water vapor, a product of the reaction with hydroxyl (OH), generates space for future research with iron and other materials such as copper slag that have iron oxides. Furthermore, because the scientific information collected is mainly based on studies of the reaction of iron with oxygen (O2).

2. Theoretical Background

In specific thermodynamic conditions, the interaction between water vapor (H2O(g)) and Iron metal (Fe) can lead to the production of Hydrogen (H2) through the oxidation of Fe in a range of 200 to 1000 °C. This oxidation reaction occurs in an environment abundant in H2O(g) and is expressed as follows [4]:
3 F e + 4 H 2 O   → F e 3 O 4 + 4 H 2  
In 1926, Dunn et al. [5] proposed that the oxidation process at high temperatures is controlled only by the diffusion of O2 through the oxide layer. The presence of H2O(g) accelerates the oxidation process of the mild Fe and two grades of pure iron [6].
The H2 production from the use of Magnetite (Fe3O4) was proposed by Nakamura et al. in 1977 [7,8]. However, the production of H2 by oxidation with H2O(g) used the concentrated solar radiation was proposed by Lede et al. [9] in 1983. Recent studies have presented 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 fuel. [8,10]. The exothermic reaction from H2O(g) to produce H2 is expressed as follows [7,8].
3 F e O + H 2 O   → F e 3 O 4 + H 2
The Fe3O4 produced in this reaction is transformed in a solar oven to FeO through an endothermic sub-process [8,10]. However, a significant barrier arises in this sub-process as a result of the premature fusion of FeO occurring at a lower temperature than the required threshold for the complete thermodynamic cycle. This inherent challenge presents difficulties in effectively employing a thermochemical solar reactor for the production of H2, limiting its efficiency and feasibility. The reaction is expressed as follows [7,11].
F e 3 O 4 l → 3 F e O   l + 1 2 O 2   ;   a   T > 1875   K  
High-temperature corrosion is influenced by (i) temperature, (ii) gas composition, (iii) exposure time, and (iv) system pressure, and can be characterized through the thickness reduction (penetration) and the thickness growth rate. The oxidation rate increases significantly during increasing temperature [12], providing a homogeneous medium and improving the thermodynamic equilibrium [13]. The Gibbs free energy is expressed as follows.
Δ G o = − R T   ln K
When a metal is exposed to corrosion at high temperatures in the presence of water vapor, it yields a value of ΔGo, as determined by equation 5, based on the reaction xM + yH2O → MxOy + yH2 [12]. The expression for the Gibbs free energy is as follows:
Δ G o = − y R T ln P H 2 P H 2 O
Meredig & Wolverton et al. [14] indicate that during the corrosion process, the contact of a metal (M) in contact with steam can produce H2 by the following expression [14].
M O x − 1 + H 2 O → M O x + H 2
Where the expression for the Gibbs free energy is as follows
Δ G o = Δ H M O x − Δ H M O x − 1 − Δ H H 2 O − T S H 2 − S H 2 O   ≤ 0
Where MOx is the oxidized metal and MOx-1 is the reduced oxide.
The water-splitting reaction using FeO as electrode is favorable only at temperature under 800 °C, when ΔG° < 0. The theoretical FeO conversion decreases with increasing temperature [15] and Kodama & Gokon et al. [16] indicate that the oxidation of FeO in the presence of water vapor exhibited a spontaneous reaction (ΔG<0) at temperatures below 1000 K.
Young et al. [17] suggest that various metal oxides can form volatile compounds by direct reaction with water vapor and that hydroxides and oxy-hydroxides can be produced by hydration [18]. On the other hand, Belton & Richardson et al. [19] demonstrate 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 H2 production capacity [20].
Table 1 shows the thermodynamic parameters (∆Go, ∆Ho, ∆So, and T) used to determine the activation energy of the reaction of water vapor with iron, the products of the vapor-dissociation into molecules, ions, and iron oxidation products (oxides and hydroxides). Table 2 shows the typical thermodynamic parameters generated during the reactions oxidation/reduction process of metals in the presence of H2O(g) at high temperatures.

3. Kinetic Model

The mechanisms in which a pure metal or alloy is oxidized at high temperatures require a series of successive steps [18,31]. The steps are:
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.
However, the reaction between a solid (Fe) and a gas (H2O(g)) corresponds to a heterogeneous reaction where the kinetics is totally or partially controlled for the mass transport process. the product species generate by convection and diffusion process and can be gaseous, or insoluble solid [32]. Diffusion and heat transfer are phenomena that intertwine with the chemical kinetics of non-catalytic heterogeneous reactions, with their respective diffusion equations extending to fluidized solids [33]. The reduction of iron oxide with hydrogen or the oxidation of iron with steam serve as two typical examples of this type of reaction. However, these reactions occur under the conditions of a highly compacted unreacted solid with limited porosity, where the chemical reaction proceeds rapidly while diffusion proceeds relatively slowly [34].
Wen et al. [34] consider that a superficial non-catalytic heterogeneous reaction consists of the following steps (Figure 1):
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, solid media, and through the fluid film surrounding the solid.
The study involved to Surman et al. [35] accomplishing the oxidation of pure Fe at 500°C using hydrogen, water vapor or various mixtures of hydrogen-vapor. 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 played a crucial role as the rate-controlling step. This diffusion occurred from the metal-oxide interface through the porous oxide of the inner layer, ultimately depositing Fe3O4 on the crystals of the outer layer, which acted as sinks.
The oxidation kinetics was approximated to the parabolic law, and the vapor phase transport hypothesis led to the expression of the parabolic rate constant (Kw), represented by the following expression.
K w = 4.8 ∗ 10 4 ∗ F p ∗ T 1 / 2 ∗ P H 2 O P H 2 ∗ e − 38900 / R T  
Where, Kw is the parabolic rate constant in (g2/m4s) and Fp is the porosity factor of the inner layer, PH2O, and PH2 are the partial pressures of H2O and H2 respectively.
Park et al. [36] studied the hydrogen production used iron oxide and inert silica with H2O(g) under temperatures between 460-700°C and vapor partial pressure between 0.002-0.02 MPa. Whence, the Fe is oxidized to Fe3O4 (first step), then Fe3O4 is gradually oxidized to Fe2O3 in the second step, researchers have if the H2O(g) is in the equilibrium state of adsorption and that the reaction rate-determining step is the desorption of hydrogen from the active sites.
The proposed initial oxidation rate when the partial pressure of water is fixed and is represented by the following expression.
d X B d t t = 0 = b k 1 + K e _ a d s P H 2 O P H 2 O − b k 1 + K e _ a d s P H 2 O K e P H 2
Where, b is the stoichiometric coefficient of the oxidation reaction of Fe to Fe2O3, k is the overall rate constant based on unit solid volume, Ke is the chemical equilibrium constant, Ke_ads is the equilibrium adsorption constant, PH2O, and PH2 are the partial pressures of water and hydrogen respectively.
The heterogeneous kinetic models are explained in detail by Donovan & Berra et al. [37], Levenspiel et al. [38], and Klaewkla et al. [39]. However, Wen & Wang et al. [40] propose a kinetic model of the non-catalytic heterogeneous reaction in a solid-gas reaction system considering the heat and mass transfer as combined effects.
Valipour & Saboohi et al. [41] analyzed multiple mathematical models and developed a comprehensive model for a multi-reactant system utilizing porous granules within a sample of a mobile packed bed. Our model takes into account various factors, including external mass transfer, internal diffusion through the pores, chemical reactions, heat generation or consumption due to reactions, and heat transfer via effective conduction throughout the solid matrix.
The three main types of growth laws that have been established experimentally, linear, parabolic and the logaritmic law are illustrated in Table 3, this equations represent the oxidation reaction rate of a metal at high temperatures published in popular science magazines.
The Fe oxidation kinetic follows a linear-parabolic behavior in the presence of H2O(g), suggesting a transition process, where the diffusion of the hydroxyl ion is a determining factor in the kinetic during the oxidation process [42]. Fujii & Meussner et al. [46] indicated that the kinetic oxidation of iron at 1100°C can adjusted to a parabolic law during the first 3 hours and later a lienar behavior is established, where increase the weight per unit area of the all specimes (rate of weight gain was 6,2 mg/cm2 hr.). The effect of temperature on the reaction rate is obtained by applying the Arrhenius and is represented by the following expression [44,45].
K = K o e − Δ G o R T
Where K is the equilibrium constant, Ko is the reaction constant, ΔGo is the activation energy, R is the gas constant, and T is the absolute temperature, respectively.
If the reaction rate is governed by diffusion in the solid state through the oxide layer. During the diffusion process, the oxide thickness (rust) increase, but at the same time the kinetic decreases with time [31]. In the cathodic subprocess occur the reaction 2H2O(g) + 2e- → 2OH- + H2 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 that involved in the formation of intermediate products generated during ORR such as (i) H2O2, and (ii) HO2- [47].

4. High-Temperature Oxidation Mechanisms

Despite the extensive scientific information on the oxidation of iron at envioromental temperature and at high temperature due to the presence of the oxygen ion and in order to reduce or avoid iron corrosion, it is still not clear enough what the mechanism of electrochemistry reaction that explains the presence of oxides such as FeO, Fe3O4 and Fe2O3, especially when iron interacts with the hydroxyl ion (OH-).
Shrinivasan et al. [48] 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 stands in contrast to alternative methods that necessitated temperatures exceeding 2570 K [49].
Lede et al. [9] propose a kinetic mechanism of thermal dissociation of H2O between 2000 – 3000K represented by the following expressions.
H 2 O + G ↔ H + O H + G
H 2 O + H ↔ H 2 + O H
O H + H ↔ H 2 + O
O H + O ↔ O 2 + H
Where, G is the extinguishing gas (argon at room temperature or steam at 400 – 450K and between 2.5 - 9.4 bar, H is the H+ and OH is the OH- ions, respectively.
Fe corrosion is an electrochemical dissolution process that produces an electron transfer to an intermediate species formed by the interaction between ferrous ions and water. In the reduction of Fe, the intermediate species is found on the iron surface and must continue to react in a second reaction to form Fe [50].
The initial rate of oxidation of a newly exposed surface of Fe or steel recently exposed to H2O(g) is always lower, if we compare it to the rate of corrosion generated by O2. Since the trend is for H2 to dissolve in solid metals in the atomic form, rather than the molecular form, according to researchers Tuck et al. [6,42] it would seem unlikely that H2 would dissolve in an oxide network, as it is too large to diffuse, so they suggest that the cathodic reaction could occur within the scales.
Equation 15 shows the general reaction for the production of H2 gas.
2 H 2 O + 2 e −   → 2 O H − + H 2
The mechanism of evolution of H2 (HER) would occur on the surface of the pores and micropores where the hydroxyls ( OH − ) formed by the dissolution of the vapor are discharged through a cathodic reaction according to Figure 2.
Rahmel & Tobolski et al. [51] proposed that the oxide layer is still thin and malleable in the first step the of oxidation process. In the presence of H2O(g), the transport mechanisms are specifically through cavity or pore formation is probably to occur. The mechanism is represented by Figure 3.
Schuetze et al. [27] proposes that the H2O(g) is involved in the transport processes that lead to scale growth in the oxide layer. The researchers indicate, the Fe cations (Fe2+/Fe3+) can simultaneously diffuse towards the outer surface in this process.
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), thus releasing H2 that can move back in the pore towards the outer part of the scale, where it reduces the oxide thus forming a H2O(g) molecule again (Figure 3 and Table 4).
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Figure 4. Schematic diagram of the transport processes in the growth of oxide scales on iron for oxygen atmospheres containing H2O (adapted from [27]).
Figure 4. Schematic diagram of the transport processes in the growth of oxide scales on iron for oxygen atmospheres containing H2O (adapted from [27]).
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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 are shown in Figure 5 [52], Figure 6, and Table 5 [42].
The first reaction that occurs is from the formation of OH- ions, which would increase cation vacancies (eq.16) and would also be the main diffusion species [5,6,42].
F e + 2 H 2 O   → F e 2 + + 2 O H − + H 2
Yuan et al. [42] suggest the formation of ferrous oxide (FeO) (eq.17, Figure 7 and Figure 8), that is, iron in the presence of water vapor at high temperatures will precipitate FeO and Fe3O4 [42] or Fe2O3 [26].
F e 2 + + O H − → F e O + H +
However, FeO in contact with Fe(OH)2 [54] is generated at temperatures above 1300°C [19] and can form Fe2O3 in conjunction with the release of H2 gas [42].
Rahmel & Tobolski et al. in 1965 [51] proposed the existence of pores at the Fe/FeO interface during the oxidation process, forming oxide bridges from the metal to the scale, which allows 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 FenOm type will normally contain a variety of defects. These defects are responsible for the transport of material through the oxide layer and therefore play a critical role in the oxidation process [6,55].
Yuan et al. [42] indicate in their research that during the initial period (before 5 hours), the existence of a single layer of columnar grains may allow relatively fast transport of OH- ions across grain boundaries. Therefore, it is valid to assume that the hydroxyl ions interact with the surface Hematite and Magnetite, forming FeO.
However, some aspects remain to be investigated, such as that proposed by Stehle et al. [56], who indicate that at typical reaction temperatures (T> 327 °C), the mobility of oxygen and metal ions is expected to be very high, so diffusion limitations are not considered significant, except for the oxides that exceed the thickness of several microns, where the potential of iron III must be considered (eq.18) [42,56,57].
4 O 2 − + 2 F e 3 + + F e 2 +   → F e 3 O 4
Another aspect that should be considered is what was stated by Saunders et al. [18], who indicate that the oxide growth, including the adsorption, dissociation, and diffusion of the reagents, are altered in the presence of water vapor, for which reason it will imply considering thermodynamics, the development of microstructures and the processes of transport. Table 6 shows the standard reduction potentials that have been considered in the electrochemical analyzes of the oxidation of iron and iron oxides in steam.
From these reactions, electrochemical reaction kinetics can be understood due to the importance it has for the application of metal oxide redox reactions in energy conversion systems such as chemical loop systems and H2 storage [59].
In an activation regime, the speed of the electrochemical process is due to the transfer of electrons as the only variable that controls the speed of the global process [31], so the corrosion reaction can be expressed by the ionization of a metal. But the possibility that this reaction occurs spontaneously under real conditions forces us to study the energy changes associated with the reaction.
However, some H+ ions migrate into the metal-forming H2, therefore, the presence of H+ ions can promote stress corrosion cracking, through the process of hydrogen embrittlement [60].
FeO nucleation and growth are enhanced by increasing oxygen pressure [55,61]. Kogan et al. [62] studied the dissociation of water at temperatures of 2000, 2200, 2500, and 2800 K, and a pressure of 0.05 (bar), and determined that 25% (at 2500K) and 55% (at 2800 K) of the Water vapor dissociates at constant pressure, increasing at high temperatures.
Kodama et al. [63] indicates that the separation of water by a thermochemical process at a pressure of 0.01 bar and 2000K is barely perceptible, and by increasing the temperature to 2500K, the yield of H2 exceeds 15% at the same pressure. Young et al. [17] proposed that in pure steam or mixtures of water vapor and inert gas, the equilibrium value of pO2 (oxygen pressure) is determined by the degree of dissociation of H2O, and that, in the case of pure steam, the dissociation of one mole of water produces x moles of H2 and x/2 moles of O2, and x is calculated from the equilibrium expression shown in Equation 19.
K 1 2 = x 3 2 1 − x 2 1 + x 2 P T
Where PT is the total pressure and since K1 is small (x << 1), therefore the before expression approaches:
x = 2 K 1 2 P T 1 3
p O 2 = x P T
Ehlers et al. [64] propose that Fe(OH)2 formed within oxide scales at low oxygen pressure (PO2), migrates to the outer surface, where due to higher pressure hematite (Fe2O3) is formed. Khanna et al. [52] propose that iron in the presence of oxygen when oxidized forms a mixed scale of three oxides (FeO, Fe2O3, Fe3O4), whose composition varies with the temperature and with the partial pressure of O2 according to the iron phase diagram (Figure 9).
Ketteler et al. [65] in their research, consider a Fe-H2O system, to determine the stability of iron oxide as a function of the partial pressure of water, indicating as the phase limit for water condensation the range 125 K (1x10-11 mbar) up to 373K (1 bar). The calculated phase diagram is presented in Figure 10, as a function of oxygen pressure and temperature, according to the equilibrium constant of dissociation of water (2H2O → 2H2 + O2), according to equation 22. The researchers conclude that water in its gaseous state acts as an oxidizing compound and that the stability of iron oxides is determined by the partial pressure of the decomposition of water into hydrogen and oxygen.
k 1 = p O 2 p H 2 2 p H 2 O 2

5. 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−), this reaction has an endothermic character, and its equilibrium shifts to the right side with increasing temperature.
Research indicates that at the temperature of 300°C (25 MPa), the concentrations of both H+ (stable in acidic solutions) and OH− (stable in basic solutions) [60] are approximately three orders of magnitude above those values of water at ambient temperature. Therefore, water has been considered both acidic and alkaline [66].
To produce green hydrogen from a thermochemical process (thermolysis), it is necessary to reach temperatures in the range of 800 – 1400 °C in the reactor [67], this temperature range has been reached in a solar thermal concentrator system of down beam.
Iron as a material for obtaining hydrogen has been considered for the thermochemical solar process because the conversion of Fe3O4 to FeO improves significantly with higher thermal reduction temperatures [42]. However, its application is complicated because in a thermochemical cycle (thermal reduction stage) the fusion of Fe3O4 occurs at temperatures above 2227°C (Figure 11) [7,68] and FeO melts at temperatures as low as 1370 [11] or 1400°C [16]. These factors complicate the design and operation of thermochemical reactors (STWS) based on Fe3O4/FeO [11]. But the thermal oxidation stage (hydrolysis) is carried out at temperatures between 200 – 1000°C [4,7,10,28,56]. According to the scientific literature, the temperature affects the process because the oxidation of iron by steam is thermodynamically favorable in the range of 127-527 °C [11,24] or in the temperature ranges of 650-750 °C. [42], 700-850 °C [26], but reaction temperatures in the range of 717 to 1127 °C [57] have also been investigated.

6. Material Used in Fluidized Bed

It is important to highlight that the selection of iron oxide is because it is an environmentally friendly base material, with a relatively low cost and that it is generally used in the redox reaction due to its high oxygen-by-weight ratio in mass with sufficient reaction time [44].
From the reviewed scientific literature, it was determined that a temperature above 1300°C can be obtained when using a thermochemical solar reactor and some metallic material (cerium oxide or zinc oxide) in a fluidized bed [67,69]. The thermal powers obtained in reactors that use water-steam and different types of material in a fluidized bed are presented in Table 7. The iron oxide materials investigated for the separation of water in the oxidation stage at the laboratory level are presented in Table 8.
Gokon et al. [29], observed that the hydrogen production rate after steam injection reached a maximum value of 12.3 Ncm3/min at 25 min, after which the hydrogen production rate decreased rapidly. This shows that the production of hydrogen through a fluidized bed is feasible.
Below is presented in Table 9, a list of thermochemical solar reactors, which have used a fluidized bed material and different gases for fluidization, considering the geometry of the reactor and the material used for its construction.

7. Discussion

Direct Solar Thermal Water-Steam Splitting Using Iron Electrodes at High Temperatures represents a groundbreaking and innovative approach to overcoming some of the most pressing challenges in the renewable energy and hydrogen production sectors, particularly in regions blessed with high solar radiation. This cutting-edge technology harnesses the boundless and eco-friendly power of the sun to produce hydrogen, a versatile and environmentally friendly fuel.
One of the main advantages of this approach lies in the use of a thermochemical iron (Fe) catalyst, which are not only relatively cost-effective but also widely available, mainly in copper slag, is an industrial waste, but it presents a variety of metal oxides that can be used. The search for information on applications of copper slag, no evidence was found of its use as a catalyst material to produce hydrogen, through thermolysis. However, it was determined that the copper slag in Chile has a significant concentration of fayalite, which is composed of 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 worth noting that the United States Geological Survey (USGS) estimates massive iron ore reserves, further underscoring the accessibility of this material. For instance, in 2017, gross iron ore reserves were estimated at a staggering 170,000 million metric tons, with an iron content of approximately 82,000 million metric tons. However, the 2016 production was a mere 2,106 million metric tons, highlighting the vast untapped potential.
Furthermore, the technology's ability to efficiently split water into hydrogen (H2) and oxygen (O2) at high temperatures is of paramount significance. It offers a compelling solution for storing excess solar energy as hydrogen, which can be effectively utilized in fuel cells or other applications during periods of reduced or no sunlight. This effectively addresses one of the most critical issues plaguing solar power – intermittency – and contributes to the creation of a more dependable and consistent energy supply. Nevertheless, it's essential to acknowledge that the high operating temperatures exert considerable stress on the materials, particularly the electrodes and electrolyzer. This necessitates the development of advanced engineering solutions to ensure the technology's longevity, durability, and cost-effectiveness over the long term. Furthermore, optimizing the system design and energy conversion efficiency remains a significant challenge to enhance the competitiveness of this process relative to other hydrogen production methods.
Finally, while this technology boasts immense potential, its practicality and impact are inherently linked to the solar resources and local conditions of the region in which it's deployed. Geographical areas blessed with abundant sunlight stand to reap the greatest benefits, making this innovation an ideal fit for regions with high solar potential. As we continue to refine and develop this technology, it has the potential to play a pivotal role in transitioning towards a more sustainable and environmentally friendly energy landscape.

8. Conclusions

Direct solar thermal water splitting using Fe electrodes at high temperatures is a promising advancement in the field of renewable energy and hydrogen production. By leveraging solar power and affordable materials, it offers a sustainable and scalable solution to the challenges of clean hydrogen production. As this technology matures and overcomes its current challenges, it has the potential to play a vital role in the transition to a more sustainable and greener energy future, reducing our reliance on fossil fuels and lowering carbon emissions across various industries.
According to data collected from the review of scientific literature, iron will precipitate in the form of ferrous oxide, hematite and magnetite during the oxidation of iron in steam at high temperature, generating hydrogen. Therefore, it is valid to assume that iron, iron oxide or 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 vapor are fundamental for the design of the solar reactor, as well as for an adequate selection of the catalyst metal.
Future recommendations will be aimed at determining whether mineral waste (copper slag) containing metal oxides (iron oxides) are feasible to be used as thermocatalytic materials for the production of hydrogen. For this reason, a morphological and thermochemical characterization of the copper slag produced in Chile will be carried out to determine its subsequent use in a 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) Material 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 to 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.

Author Contributions

Conceptualization, M.F. and F.M.G.M.; methodology, M.F. and F.M.G.M.; validation, A.S., E.F., D.P. A.Sa. and N.T.; investigation, M.F., N.T. D.P. and F.M.G.M.; writing—original draft preparation, M.F. and F.M.G.M.; writing—review and editing, M.F, A.Sa E.F., D.P, A.S., N.T. and F.M.G.M.; visualization M.F., A.S., and F.M.G.M.; supervision, F.M.G.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors would like to thank the Programa de Doctorado en Energía Solar of the Universidad de Antofagasta, Chile. The authors are grateful for the support of ANID-Chile through the research projects FONDECYT Iniciación 11230550 and ANID/ FONDAP 1522A0006 Solar Energy Research Center SERC-Chile.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Step diagram of the non-catalytic superficial heterogeneous reaction of the electrochemical dissolution of iron in water vapor.
Figure 1. Step diagram of the non-catalytic superficial heterogeneous reaction of the electrochemical dissolution of iron in water vapor.
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Figure 2. Diagram of the electrochemical dissolution of iron.
Figure 2. Diagram of the electrochemical dissolution of iron.
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Figure 3. The mechanism in O2-H20 vapor mixtures after long periods (adapted from [51].)
Figure 3. The mechanism in O2-H20 vapor mixtures after long periods (adapted from [51].)
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Figure 5. Iron oxidation mechanism showing diffusion of various ionic species (adapted from [52,53]).
Figure 5. Iron oxidation mechanism showing diffusion of various ionic species (adapted from [52,53]).
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Figure 6. Mechanism of Fe oxidation processes in steam (adapted from [42].)
Figure 6. Mechanism of Fe oxidation processes in steam (adapted from [42].)
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Figure 7. Cross-sectional SEM image of oxides formed on iron after 1 h of oxidation at 650 °C [42].
Figure 7. Cross-sectional SEM image of oxides formed on iron after 1 h of oxidation at 650 °C [42].
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Figure 8. Cross-sectional SEM image of oxide scale formed on iron after 10 h of steam oxidation at 650 °C, where the outermost white layer is nickel-plated [42].
Figure 8. Cross-sectional SEM image of oxide scale formed on iron after 10 h of steam oxidation at 650 °C, where the outermost white layer is nickel-plated [42].
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Figure 9. Phase diagram, depicting the stability of various iron oxides as a function of oxygen pressure and temperature [52,53].
Figure 9. Phase diagram, depicting the stability of various iron oxides as a function of oxygen pressure and temperature [52,53].
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Figure 10. The phase diagram in a water atmosphere represents the stability of iron oxides [65].
Figure 10. The phase diagram in a water atmosphere represents the stability of iron oxides [65].
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Figure 11. Operating temperature of typical thermochemical water splitting cycles in the reduction stage (adapted from Hiroki Miyaoka, 2016) [68].
Figure 11. Operating temperature of typical thermochemical water splitting cycles in the reduction stage (adapted from Hiroki Miyaoka, 2016) [68].
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Table 1. Thermodynamic parameters of Water, Oxides, and Hydroxides.
Table 1. Thermodynamic parameters 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)		 Ref.
H2O (l) -237,1 -285,8 70,0 298 [21,22]
H2O (g) -228,6 -241,8 188,8 298 [21,22]
-214,1 -244,7 213,8 600 [22]
-198,2 -247,1 228,6 900
H2 (g) 0 0 130,6 298 [21,22]
0 0 151,0 600 [22]
0 0 163,1 900
OH_ (aqueous ion) -157,3 -230,0 -10,7 298 [21]
OH (g) 34,7 38,9 183,7 298 [22]
29,5 38,9 204,4 600
24,9 38,4 216,5 900
Fe 0 0 27,09 298 [21]
Fe2+ -90,0 -91,1 -107,1 298 [21]
Fe3+ -16,7 -49,9 -280,0 298 [21]
Fe.947 O -244,9 -266,3 56,6 298 [21]
-245,1 -266,3 57,5 298 [22]
-224,8 -263,7 93,0 600
-205,8 -262,7 115,1 900
FeO -251,4 -272,0 60,6 298 [21,22]
-231,7 -269,4 97,3 600 [22]
-213,1 -268,4 120,2 900
F e 2 O 3 -774,4 -826,2 87,4 298 [21,22]
-742,3 -824,2 87,4 298 [22]
-661,4 -817,6 173,3 600
-585,3 -808,4 235,4 900
F e 3 O 4 -731,4 -811,6 93,0 298 [23]
-1012,7 -1115,7 146,1 298 [21]
-1015,2 -1118,4 146,1 298 [22]
-914,4 -1107,4 272,1 600
-821,8 -1088,6 368,3 900
FeO (OH) -491,8 -562,6 60,4 298 [21]
Fe (OH)2 -486,9 -568,9 87,9 298 [22]
-405,7 -564,1 160,4 600
-327,6 -558,9 207,7 900
Table 2. Thermodynamic Parameters of the Reaction of Iron or Oxides in Water Vapor.
Table 2. Thermodynamic Parameters of the Reaction of Iron or Oxides in Water Vapor.
Reaction Gibbs free energy
ΔG (kJ/mol)		 Enthalpy
ΔH(kJ/mol)		 Temp. K
(pres. atm)		 Ref.
0.75   F e + H 2 O   → 0.25 F e 3 O 4 + 4 H 2 -14,52 -32,09 600 [24]
F e + 2 H 2 O     → F e O + H 2 -199,3 - 1000 [25]
F e + 2 H 2 O   → F e O H 2 g + H 2 38,9 + 5*80T (cal) - - [19]
2 F e + 3 H 2 O   → F e 2 O 3 + 3 H 2 48,9 - 973 - 1123 [26]
3 F e + 4 H 2 O   → F e 3 O 4 + 4 H 2 11,0 - 973 - 1123 [26,27]
3 F e O + H 2 O   → F e 3 O 4 + H 2 - -33,6 873 [15,16,28,29]
F e 2 O 3 + H 2 O   → 2 F e O O H 10,39 -47,54 400 [24]
F e 3 O 4 + 2 H 2 O   → F e O H 2 + 2 F e O O H 41,02 -79,23 400 [24]
F e 3 O 4 + 4 H 2 O   → F e O H 2 + 2 F e O H 3 228,22 -120,48 600 [24]
2 F e 3 O 4 + H 2 O   → 3 F e 2 O 3 + H 2 40,5 - - [30]
2 F e 3 O 4 + 4 H 2 O   → 6 F e O O H + H 2 44,4 - - [30]
Table 3. Reaction rate equations, applicable to high-temperature oxidation.
Table 3. Reaction rate equations, applicable to high-temperature oxidation.
Equation Observation Ref.
Logarithmic reaction rate:
X = K r e a c t i o n ln   t + t o + A It represents the initial oxidation states at low temperatures. [31]
X = K r e a c t i o n ln   B t + 1 - [31]
Lineal reaction rate:
d x d t = K L i n e a l It represents a constant rate of oxide growth applicable at very high temperatures. [31]
X = K L i n e a l t + C L i n e a l - [31]
Δ W A =   K L   t - [42]
Parabolic reaction rate:
d X d t = K P a r a b x It adjusts to the processes controlled by the diffusion of species. [31]
X 2 = K p a r a b t + C p a r a b - [31,43]
Δ W A 2 = K p a r a b ∗ t - [42]
d X d t = k   f X The function f(X) depends on the reaction mechanism for diffusion in one, two, or three dimensions. [44]
d X d t = k T ∗ f X
k T = k o ∗ e − E a R T 		The reaction rate of a sample of iron slag in water vapor. [45]
Where:
X: the thickness of oxide consumed per surface unit or the weight gain per area unit.
ΔW/A: the mass gain per unit area (g/cm2)
t: is the time.
k(T): the reaction rate constant.
f(X): function that represents the reaction mechanism.
ko, Kreaction, KLineal, KParab: reaction constants.
Ea: is the activation energy.
R is the gas constant.
T is the absolute temperature.
A, B: constants.
CLineal, CParab: constants of integration.
Table 4. The nomenclature used in the transportation process was proposed by Schuetze [27].
Table 4. The nomenclature used in the transportation process was proposed by Schuetze [27].
Nomenclature Meaning
V F e ’ ’ ’ Fe3+ vacancy on a catión position
V F e ’ ’ Fe2+ vacancy on a catión position
h ∗ h+ electron-hole
V O ∗ ∗ O vacancy on an anion position
F e ∗ ∗ ∗ Fe3+ ion on an interstital position
Table 5. Transport process nomenclature proposed by Yuan et al. [42].
Table 5. Transport process nomenclature proposed by Yuan et al. [42].
Nomenclature Meaning
V O ∗ ∗ Fully ionized oxygen vacancy
O H O ∗ Substitutional hydroxide ion
H i ∗ h+ electron-hole
e ′ Electron
OMS Original Metal Surface
Table 6. Standard Reduction Potential.
Table 6. Standard Reduction Potential.
Half reaction Standard Reduction Potential, E° (V) Ref.
H 2   → 2 H + + 2 e − E° = 0,000 (V) [58]
F e   → F e 2 + + 2 e − E° = - 0,447 (V) [58]
Reaction
3 F e + 4 H 2 O   → F e 3 O 4 + 4 H 2 - [45]
2 F e O + H 2 O   → F e 2 O 3 + H 2 E° = 0,356 (V) [45]
2 F e 3 O 4 + H 2 O   → 3 F e 2 O 3 + H 2     E° = 0,210 (V) [30]
2 F e 3 O 4 + 4 H 2 O   → 6 F e O O H + H 2     E° = 0,230 (V) [30]
Table 7. Temperatures reached in a Thermochemical Solar Reactor with Fluidized Bed.
Table 7. Temperatures reached in a Thermochemical Solar Reactor with Fluidized Bed.
Thermal Power Temperatures reached or required Thermal Fluid Fluidized Bed Material Particle Size (µm) Ref.
1 kWth 1300 °C Water/Steam Coal – coke 140
(200-300)
(300-500)
(500-710)		 [63,70,71,72]
10 a 20 kWth 2200 °C Water/Steam ZnO reagent powder 1 - 5 [73]
3 MWth 1500°C. Water/Steam Cerium Oxide Material < 300
(100 – 300)		 [28,67,74,75]
110 kWth 1400 °C Gases Non-stoichiometric cerium oxide particles 10 - 210 [69,76]
100 kWth 960 – 1100 °C Air Quartz sand particles 100 - 500 [77]
250 kWth 800 – 1000°C Air A mixture of coal-coke and quartz sand 100-300 (coal)
100 – 700 (sand)		 [78]
30 kWth 560 °C. Air A mixture of sand and basalt [79]
450 kWth 770 °C. Air Isotropic materials [80,81]
140,63 Wth
(2kWe)
231,32 Wth
(4 kWe)		 250 °C Air Sand, ceramic casting media (carbo Accucast ID50) y SiC [82]
Table 8. Iron Oxide Materials.
Table 8. Iron Oxide Materials.
Iron Oxides Material Sample/Particle Size (µm) Temp. (°C)
(time)		 Partial Vapor Pressure Thermal Fluid (flow) Ref.
Magnetite Fe3O4 30 -50
100 -125		 575
673		 - - [16,63]
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 - - [16,28,63]
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)		 [76]
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)		 [83]
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)		 [29]
Monoclinic magnetite supported on zirconium substrates Fe3O4/m-ZrO2 Polished Fe Bar without oxidation 397 – 602
(180 min)		 - Nitrogen
(100 cc/min)
Argon
(200cc/min)
Liquid water
(12,5 µL/min)		 [56]
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)		 [59]
Table 9. Technologies of thermochemical solar reactors with fluidized bed.
Table 9. Technologies of thermochemical solar reactors with fluidized bed.
Reactor Geometry Bed /gas Material
 Reactor Year/
Ref.
Diameter=5cm
Height= 32cm		 Charcoal /CO2 Silice glass tube Preprints 111933 i001 1983
[84]
ZrO2, SiC
/aire,N2,CO2 		clear quartz tube Preprints 111933 i002 1983
[85]
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) Preprints 111933 i003 1988
[86]
Diameter= 2cm ZnO+Al2O3/
CH4+Ar		 quartz tube Preprints 111933 i004 1995
[87]
Diameter= 45 mm, thickness 2.5 mm NiFe2O4 – ZrO2 /N2 quartz tube Preprints 111933 i005 2008
[88]
25mm in diameter, thickness 1.5mm, height 25cm. CaO or CaCO3 /H2O,Ar,CO2 quartz tube Preprints 111933 i006 2009
[89]
420mm in length, 62.3mm in inner diameter, and 7mm in thickness Coke/CO2 Stainless steel with a quartz window Preprints 111933 i007 2010
[70]
The inner diameter was 45 mm, and the thickness was 2.5 mm. NiFe2O4 /vapor Stainless steel tube with a quartz cap Preprints 111933 i008 2011
[29]
length 420 mm, inner diameter 62.3 mm, thickness 7 mm Coke/Ar-vapor
CeO2/N2 		Stainless steel tube (SUS310S) with a quartz window Preprints 111933 i009 2015
[71,74]
42 cm diameter at the top quartz sand particles/air Inconel and stainless steel, with a quartz window Preprints 111933 i010 2016
[90]
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) Preprints 111933 i011 2018
[91]
35.6 cm long and 2.54 cm outer diameter Iron Aluminate (FeAl2O4) / N2 Silicon Carbide Cylindrical Tubes (SiC) Preprints 111933 i012 2019
[92]
inside diameter of 7.62 cm and a height of 8 cm sand, carbo Accucast ID50 y SiC /air Stainless Steel tube
(304)		 Preprints 111933 i013 2020
[82]
Tube with 40 mm inner diameter, 10 mm wall thickness, and 1780 mm length quartz sands/ air Pure iron metal tube, hot particle container made of stainless steel AISI 304 Preprints 111933 i014 2022
[93]
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