Redox Cycles, Active Materials, and Reactors Applied to Water and Carbon Dioxide Splitting for Solar Thermochemical Fuel Production: A Review

Abstract

The solar thermochemical two-step splitting of H₂O and CO₂ based on metal oxide compounds is a promising path for clean and efficient generation of hydrogen and renewable synthetic fuels. The two-step process is based on the endothermic solar thermal reduction of a metal oxide releasing O₂ using a high-temperature concentrated solar heat source, followed by the exothermic oxidation of the reduced oxide with H₂O and/or CO₂ to generate pure H₂ and/or CO. This pathway relates to one of the emerging and most promising processes for solar thermochemical fuel production encompassing green H₂ and the recycling/valorization of anthropogenic greenhouse gas emissions. It represents an efficient route for solar energy conversion and storage into renewable and dispatchable fuels, by directly converting the whole solar spectrum using heat delivered by concentrating systems. This eliminates the need for photocatalysts or intermediate electricity production, thus bypassing the main limitations of the low-efficient photochemical and electrochemical routes currently seen as the main green methods for solar fuel production. In this context, among the relevant potential redox materials, thermochemical cycles based on volatile and non-volatile metal oxides are particularly attractive. Most redox pairs in two-step cycles proceed with a phase change (solid-to-gas or solid-to-liquid) during the reduction step, which can be avoided by using non-stoichiometric oxides (chiefly, spinel, fluorite, or perovskite-structured materials) through the creation of oxygen vacancies in the lattice. The oxygen sub-stoichiometry determines the oxygen exchange capacity, thus determining the fuel production output per mass of redox-active material. This paper provides an overview of the most advanced cycles involving ZnO/Zn, SnO₂/SnO, Fe₃O₄/FeO, ferrites, ceria, and perovskites redox systems by focusing on their ability to perform H₂O and CO₂ splitting during two-step thermochemical cycles with high fuel production yields, rapid reaction rates, and performance stability. Furthermore, the possible routes for redox-active material integration and processing in various solar reactor technologies are also described.

1. Introduction

New sustainable and alternative energy carriers are required to limit the CO₂ emissions and climate change arising from intensive fossil fuel utilization and combustion. The solar thermochemical production of synthetic fuels without greenhouse gas emission can be achieved via the splitting of H₂O and CO₂, using concentrated solar energy as an external high-temperature heat source for the chemical process [1,2,3]. The different solar fuels considered consist of hydrogen, syngas (with H₂ and CO as main components), and the derived fuels (including methanol or synthetic liquid fuels derived from Fischer–Tropsch synthesis). Hydrogen is an energy carrier enabling long-term storage and long-range transportation of solar energy and for being used as a fuel in direct combustion engines or in fuel cells. The utilization of CO₂ emissions captured, e.g., from industrial processes, is also a major challenge, notably for the CO₂ conversion into synthetic fuels driven by solar energy. Accordingly, this can help to contribute in the achievement of global net zero carbon emissions [4] and sustainable development goals (SDGs), in particular related to these actions aiming to ensure access to affordable and clean energy (SDG 7), and to combat climate change (SDG 13). H₂ production and CO₂ conversion/recycling into liquid synthetic fuels can be integrated in industrial processes for the development of an economic sector that would be based on three main industrial components:

(i) Resources and energy harvesting. Though the capture of atmospheric CO₂ by direct air capture (DAC) is not yet at a stage of industrial deployment, several industrial fields already have the potential to supply the required CO₂ for the start-up of this sector, which is rapidly expanding especially with respect to CO₂ capture and sequestration (CCS) [5,6]. H₂O can be extracted from seawater desalination. Finally, regarding the available primary energy sources, concentrated solar energy (CSE) has potential to supply the energy requirements of thermochemical processes.

(ii) H₂O and CO₂ splitting. Among the possible existing paths, photocatalysis and photo-electrolysis appear to be the most challenging to operate at industrial scale, based on the number of constraints related to the relevant materials composing the cells along with the limited solar-to-fuel efficiencies of such processes [7]. Water electrolysis is the most widely developed and commercially available owed to its similarity to hydrogen fuel cells, but more suitable and cost-effective materials are still required. As the major drawback, this pathway remains widely reliant on an inexpensive electricity source, and its coupling with a renewable intermittent energy source (solar or wind power) is considered in the framework of electrical storage of these renewables. Eventually, thermochemical redox cycles based on metal oxides are in an intermediate development stage [8,9]. Their benefits lie in a significantly higher energy efficiency potential because solar heat is directly converted into chemical bond energy, without the need for an intermediate conversion into electrical energy, as required for the electrolysis process.

(iii) Liquid fuel production is definitely the most advanced and developed process of this potentially emerging sector, as it can be based on the Fischer–Tropsch catalytic synthesis, which is an already existing industrial process.

In this review, the various pathways for thermochemical H₂O and CO₂ dissociation are described. Photochemical, photobiological, and electrochemical pathways [10,11] are thus not detailed. Reforming and gasification reactions coupled with solid oxidants (chemical looping processes [12,13]) are also not included because they require an input of hydrocarbon/carbonaceous materials (such as methane or solid carbon/biomass). Among the existing chemical routes, the dissociation (splitting) of H₂O and CO₂ using thermochemical cycles based on metal oxides appears to be the most favorable and energy-efficient pathway [14,15,16], because concentrated solar energy is directly used as the process heat source, without any intermediate energy losses associated with the production of electricity (case of electrolysis). Furthermore, this route offers a potentially higher energy conversion efficiency (solar-to-fuel) in comparison with direct thermolysis, thanks to a lower temperature, and thereby reduced radiative heat losses. Different types of relevant metal oxide redox cycles and the developed solar reactor technologies for the cycles’ implementation in solar processes are presented. In particular, the most advanced and adapted redox systems for H₂O and CO₂ splitting are reviewed. They include (i) volatile or non-volatile oxides with discrete transitions in the metal oxidation state (such as ZnO/Zn, SnO₂/SnO, or Fe₃O₄/FeO) accompanied by a phase change (solid-to-gas or solid-to-liquid) during the reduction step, as well as (ii) non-stoichiometric oxides (including ferrites, ceria, and perovskites redox systems) featuring a continuous redox activity over a variety of temperature and oxygen partial pressure, thanks to vacancies that facilitate oxide ion transport (no crystallographic phase change occurs during redox cycles). The first category involving simple oxides generally produces higher fuel amounts per mass of active material at the expense of recombination issues with O₂ during reduction. In contrast, the absence of phase change in the second category involving non-stoichiometric oxides eliminates the need for solid reactant transport between steps, but the fuel productivity remains limited due to the low oxygen exchange capacity. The current knowledge related to each of these systems is summarized, while emphasizing their existing advantages and constraints and the most promising reactor technologies applicable for the considered reactions.

2. H₂O and CO₂ Splitting by Direct Thermolysis

Single-step thermolysis corresponds to the dissociation of chemical bonds with oxygen atoms at a high temperature, based on the following reactions:

$${\mathrm{CO}}_2\to \mathrm{CO}+\frac{1}{2}{\mathrm{O}}_2$$

(1)

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\frac{1}{2}{\mathrm{O}}_2$$

(2)

The enthalpy and Gibbs free energy variations as a function of the temperature are displayed in Figure 1 (at atmospheric pressure). The ranges of temperature related to the different considered pathways are also indicated.

Thermolysis of CO₂ is highly endothermic, and it cannot proceed in a spontaneous way below ~3000 °C. This threshold of temperature is even higher for thermolysis of H₂O. Various simple aspects tend to indicate that operating at such elevated temperatures is not appropriate:

• The design of solar reactors able to withstand repeated thermal constraints is challenging, given that the materials that can potentially be thermally-resistant at such high temperatures are costly and not widely available.
• The reverse reaction (recombination of products) may be avoided, either by the means of H₂ (or CO) and O₂ separation at a very high temperature (thereby avoiding a potentially explosive mixture), or by carrying out a fast quenching of this gaseous species mixture. At present, there does not exist any available technology that would be able to address these points.
• Because radiative heat losses vary with the fourth power of temperature, it is not realistic to achieve high energy conversion efficiencies for this kind of high-temperature process.
• Concentrated solar energy is the only renewable heat source that could be employed to reach such high levels of temperature. However, directly heating a gas (such as H₂O or CO₂) with solar radiation is difficult although these gas species absorb at given wavelengths a fraction of the incident solar spectrum. Therefore, the process complexity has to be increased, for instance, with the addition of particles for promoting absorption and diffusion of solar radiation.

In spite of these issues, Jensen et al. [17] filed a patent on this process relying on the variation of CO₂ absorption spectrum with temperature. Indeed, according to the experimental results, by preheating the gas at 1800 K, solar radiation can be efficiently absorbed in order to drive a CO₂ photolysis into CO, O₂, and O species. The gas mixture can be further cooled down via bulk addition of CO₂ at room temperature in order to generate additional CO by reaction with O. The reported efficiency of CO₂ conversion to CO is ~6%. Such a process cannot be considered as a pure CO₂ thermolysis because concentrated photons trigger the mechanism instead of the temperature effect alone, even though high temperature is required for carrying out the reaction [18,19].

3. H₂O and CO₂ Splitting by Thermochemical Redox Cycles

3.1. Context

A suitable route to overcome the drawbacks associated with direct single-step thermolysis is to decompose this reaction in different steps by using thermochemical multi-step cycles. Proposed for the first time by Funk and Reinstrom [20], this method led to intensive research from laboratories during the first oil crisis in 1973, with the aim to produce hydrogen from water-splitting using the high-temperature heat released by nuclear reactors (~950 °C). The interest in such cycles then declined, but starting with the year 2000, it was actively renewed with the aim to valorize the potential of concentrated solar energy and to provide process heat at high temperatures [21,22].

The cycles most widely studied for high-temperature application (T > 1000 °C) are those composed on reduction and oxidation reactions of metal oxides [23,24]. The two steps involved are: (1) the metal oxide reduction (MO_x) at high temperature, yielding a reduced oxide material and gaseous oxygen (Equation (3)); and (2) the re-oxidation of this compound in the presence of either CO₂ or H₂O, to regenerate the initial metal oxide and produce either CO or H₂ (Equation (4)):

$${\mathrm{MO}}_{\mathrm{x}}\to {\mathrm{MO}}_{\mathrm{x}-\mathrm{y}}+\frac{\mathrm{y}}{2}{\mathrm{O}}_2 \mathrm{T} > 1000 °\mathrm{C}, \mathsf{\Delta }\mathrm{H} >> 0$$

(3)

$${\mathrm{MO}}_{\mathrm{x}-\mathrm{y}}+{\mathrm{yCO}}_2\left({\mathrm{H}}_2\mathrm{O}\right)\to {\mathrm{MO}}_{\mathrm{x}}+\mathrm{yCO}\left({\mathrm{H}}_2\right) \mathrm{T} < 1000 °\mathrm{C}, \mathsf{\Delta }\mathrm{H} < 0$$

(4)

The cost estimation for the H₂ produced with this kind of cycle was in the range 46–66 $/GJ (i.e., 5.5–7.9 $/kg), thus comparable or slightly higher than the cost of electrolysis using nuclear power [25], but with much more favorable prospects of being decreased, owed to the expected drop in solar infrastructure investment costs, as opposed to electricity production costs that will dramatically increase. Depending on assumptions and process scales, different cost ranges were reported for metal oxide-based cycles in various techno-economic analyses (e.g., H₂ production costs in the range 3.5–12.8 €/kg [26], 7.17–19.26 $/kg [27], or 6.68–13.06 €/kg [28], and potentially decreasing to 3.92 $/kg [29] or 2.5 $/kg [30]). Economic prospects are thus favorable for the solar process deployment at industrial scale. It was established that the choice and size of the solar concentrating system, co-generation, and heat recovery are critical to the system efficiency and economic viability of a solar-driven cycle for hydrogen production.

The separate or simultaneous generation of H₂ and CO species enables the on-demand production of a synthetic gas with a controlled H₂:CO mole ratio. This syngas can serve as the building block for the synthesis of methanol [31] or liquid fuels (synthetic hydrocarbon fuels). This synthesis is an essential stage for the development of this path, since it warrants a smooth transition from the current fossil fuel-based economy toward a future economy based on hydrogen and fuel cells, as well as a means of storage of H₂ in the form of a dispatchable fuel containing high energy density. Indeed, H₂ and CO offer a high energy density by weight, but their energy density by volume is low (as shown in Figure 2 [32], diesel being similar to gasoline and dimethylfuran (DMF) or third generation biofuel).

Once the syngas mixture is produced, the most directly available route to generate liquid hydrocarbon fuels relies on the catalytic Fischer–Tropsch synthesis. This synthesis method has been commercially employed since the 1920s for generating liquid fuels from fossil fuel-derived syngas produced from carbon/coal gasification or from steam natural gas reforming. The Fischer–Tropsch synthesis can be expressed using the following general reaction [33]:

2H₂(g) + CO(g) → -CH₂- (l) + H₂O(g)    ΔH = −165 kJ/mol

(5)

where -CH₂- is a link in the hydrocarbon chain. The carbon chain length that determines the fuel quality depends on several factors, including pressure, temperature, and composition of the syngas feedstock, and on the type of catalyst [34,35]. Furthermore, the separate production of the two syngas components makes the precise adjustment of its global composition easier and influences the reaction selectivity to one of the products of the Fischer–Tropsch synthesis. Finally, the poisoning of the catalyst by sulfur is avoided.

Several categories of multi-step cycles can be distinguished, mainly based on the reactions and technology to be implemented for chemical processing [21,27,36,37,38,39,40]. Thermochemical cycles with the number of steps above two offer the advantage of lower operating temperatures, which are compatible with a nuclear heat source, but they require more complicated management of the material flow and transport of reactants and products, additional separation steps, and corrosion issues, which in turn represent strong process limitations [41]. Regarding the two-step cycles, a distinction can be made between the cycles for which the reduction step leads to a partial reduction of the oxide (non-stoichiometric cycles) or a different species (stoichiometric cycle). This species can then be obtained in a gaseous form (cycles with volatile oxides) or not (cycles with non-volatile oxides). Non-stoichiometric cycles have a lower productivity in H₂/CO per unit mass of oxide. As for the cycles with volatile oxides, they generate a gaseous product that easily recombines (with O₂) and must be recovered by condensation and filtration, but whose specific surface area is significant (several dozens of m²/g) and regenerated at each cycle. Besides these technological criteria, the thermodynamic properties related to these cycles are highly important, as they will determine the maximal efficiencies obtained through theoretical energy/exergy analyses [21,42]. The different sources and wells are represented in Figure 3 showing the mass and energy flow diagrams.

Carbon-neutral fuels produced from redox cycles driven by concentrated solar energy have the potential to decarbonize parts of the transportation sector and industrial processes. For providing a sustainable alternative to fossil-derived energy carriers, both the environmental impacts and economic aspects of their production are important. For this purpose, the solar thermochemical fuel production pathways were also characterized based on techno-economic and life-cycle analyses to demonstrate environmental and cost benefits [27,43,44,45].

3.2. Cycles Based on Volatile Oxides

3.2.1. ZnO/Zn Redox Pair

The two-step ZnO/Zn cycle was widely investigated for its significantly elevated theoretical energy conversion efficiency (conversion efficiency of solar energy into chemical energy ranging from 35% to 50% depending on the extent of heat recovery [46,47]).

The reduction kinetics of ZnO was investigated by using a solar-driven thermogravimetric system [48,49,50], yielding an activation energy of 361 ± 53 kJ/mol for a temperature of 1800–2100 K and a zero-order reaction. Other solutions were considered [51], especially the reduction in the form of aerosol, which enhances the pre-exponential factor by several orders of magnitude [52,53]. The global efficiency of this step widely depends on the solar heat transfer to the solid reacting particles (direct irradiation is thus preferable) and on the Zn(g) and O₂ separation to avoid their recombination [54]. A rapid gaseous quenching is commonly used as the relevant solution (Zn vaporization starts at a temperature of 907 °C followed by its recrystallization based on a heterogeneous mechanism from nucleation sites), though the energy thus released from products can hardly be recovered in the process.

Zinc re-oxidation takes place following two stages encompassing a 1st rapid stage corresponding to the available zinc surface oxidation, followed by a 2nd longer stage limited by diffusion inside zinc particles (resulting in particle surface passivation). The particle size thus directly impacts the reaction rate, as fine particles will limit the effect of the diffusion mechanism [55,56,57]. In the case of an oxidation step with only H₂O [58,59] or CO₂ [60,61], the gas oxidant concentration shows a weak influence on the reaction kinetics, in contrast to temperature. The main different characteristics between the oxidation reactions with H₂O and CO₂ are related to the kinetics: the activation energies of the oxidation step were determined to be respectively 87.7 ± 7 kJ/mol [55] and 162 ± 25.3 kJ/mol [62] for similar particles. Despite this strong difference, the co-splitting of H₂O and CO₂ was investigated [63]. A close connection between the inlet gas composition and the generated syngas quality was evidenced. The oxidation of gaseous Zn by either CO₂ or H₂O was studied [64], showing a 10 times increase in the kinetics in comparison with solid Zn particles. The heterogeneous oxidation of Zn(g) provides rapid conversion of Zn to ZnO [65,66]. In spite of this performance, the application of this method is unlikely, because its implementation is difficult. Though it is produced in the gaseous phase during the reduction step, Zn vapor must be condensed to avoid its recombination and enable its separation from O₂ (thus only solid Zn is available for the oxidation step), while the released energy during its condensation cannot be recovered and is thus lost via the gas quenching.

The main challenges of this cycle highlighted from previous works are:

• The reduction step temperature is elevated (above 2000 K).
• The quenching of gases from >2000 K to >1200 K is a key step during the reduction: its efficiency is decisive for the zinc oxidation step (depending on both size of zinc particles and purity) and global process energy efficiency.
• The re-oxidation step is limited by a passivation phenomenon arising from the formation of a ZnO layer at the particle surface.

The carbon species addition to zinc oxide can be considered in order to decrease the temperature of the reduction step (carbothermal reduction). Indeed, carbon is a reducing agent that helps to facilitate the reaction with the generation of CO in the first step [67]. The first step of the cycle thus becomes:

$$\mathrm{ZnO}\left(\mathrm{s}\right)+\mathrm{C}\to \mathrm{Zn}\left(\mathrm{g}\right)+\mathrm{CO}, \mathsf{\Delta }\mathrm{G} < 0 \mathrm{for} \mathrm{T} > 1230 \mathrm{K}$$

(6)

The studied carbonaceous sources were methane [68,69,70,71] and carbon/coals [72,73,74] (graphite, activated carbon, charcoal, petroleum coke, etc.) or biomass [75,76]. The type of carbon is essential for the reduction step, and charcoal and activated carbon were shown to be good reducing agents. The retained reaction mechanism consists of two successive solid–gas reactions [77]:

$$\mathrm{ZnO}\left(\mathrm{s}\right)+\mathrm{CO}\to \mathrm{Zn}\left(\mathrm{g}\right)+{\mathrm{CO}}_2$$

(7)

$${\mathrm{CO}}_2+\mathrm{C}\left(\mathrm{s}\right)\leftrightarrow 2\mathrm{CO}$$

(8)

3.2.2. SnO₂/SnO Redox Pair

Since elemental tin can be in the form of different oxidation states, +II or +IV, several redox pairs can be considered including SnO₂/SnO, SnO₂/Sn, and SnO/Sn. A first cycle scheme composed of three steps was patented by D. Souriau in 1971, with the aim to valorize the heat released by a nuclear reactor as an energy source. The cycle involves the high-temperature SnO₂ reduction into SnO (Equation (9)), which disproportionates at 600 °C into liquid metallic Sn and SnO₂ (Equation (10)). Then, liquid Sn reacts with H₂O to produce H₂ and regenerate the tin dioxide (Equation (11)):

$${\mathrm{SnO}}_2\to \mathrm{SnO}+\frac{1}{2}{\mathrm{O}}_2$$

(9)

$$2\mathrm{SnO}\to \mathrm{Sn}\left(\mathrm{l}\right)+{\mathrm{SnO}}_2$$

(10)

$$\mathrm{Sn}\left(\mathrm{l}\right)+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{SnO}}_2+2{\mathrm{H}}_2$$

(11)

In a study performed by Charvin et al. [78], the formation of the two phases was not observed although dismutation occurred, which hinders the recovery of Sn(l) in liquid phase, and thus the remaining steps of the cycle become caducous. Since then, the SnO₂/SnO cycle was rather considered and developed [79,80,81]: the 1st step corresponds to the same reaction of Souriau cycle, while the 2nd one bypasses the disproportionation step and directly employs SnO as the reducing agent:

$${\mathrm{SnO}}_2\to \mathrm{SnO}\left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2$$

(12)

$$\mathrm{SnO}+{\mathsf{\alpha }\mathrm{CO}}_2+\left(1-\mathsf{\alpha }\right){\mathrm{H}}_2\mathrm{O}\to {\mathrm{SnO}}_2+\mathsf{\alpha }\mathrm{CO}+\left(1-\mathsf{\alpha }\right){\mathrm{H}}_2,0<\mathsf{\alpha }<1$$

(13)

Metallic tin was indeed shown to not be necessarily more reactive than SnO for water splitting [78]. This cycle was patented by PROMES for the production of H₂ (WO 2008113944).

Similar to ZnO/Zn cycle, products recombination is limiting for the reduction step efficiency [54]. Decreasing the total pressure in the reaction chamber facilitates the gaseous species production, according to Le Chatelier’s principle, and enables a decrease in the temperature required for the reaction (1900 K at 20 kPa versus a theoretical temperature of 2342 K at atmospheric pressure). The gas species dilution by using a neutral gas can also be considered in order to decrease the O₂ partial pressure and therefore to limit products recombination. Similarly, the temperature required for the reduction reaction is also decreased. Finally, a rapid gas quenching allows the SnO condensation and the decrease in the gas temperature below the reaction temperature. The condensation mechanism follows a heterogeneous process from the nucleation sites. As for ZnO, the use of carbon as reducer lowers the reduction temperature of SnO₂ [82].

During the re-oxidation step, several parameters should be considered. The composition of the reduced oxide powder (content in reduced tin species) may vary according to the operating conditions (SnO or Sn/SnO₂). Indeed, partial SnO disproportionation may occur during the temperature increase in the oxidation step or during the cooling stage in the first reduction step [81]. Similarly, the powder morphology plays a major role (particle size, specific surface area) because the re-oxidation reaction is a solid–gas reaction. From this viewpoint, the SnO recovery by vapor condensation favors the formation of powders composed of grains with size of the order of several dozen nanometers, which results in a significantly high specific surface area [80]. Finally, the CO₂ or H₂O splitting is not following the same kinetic rates [80]. H₂O starts to be reduced from 525 °C with an activation energy of 51 ± 7 kJ/mol, while CO₂ is only reduced from 700 °C, with an activation energy of 88 ± 7 kJ/mol.

SnO₂/SnO cycle offers several advantages compared to the ZnO/Zn cycle [79]:

• The reduction step temperature is below the one of the ZnO/Zn cycle and could be further decreased by a total pressure decrease inside the reaction chamber. In addition, the decrease in the operating temperature directly results in a higher energy efficiency [83] and thus a more favorable economic viability of the process.
• The dissociation rate of the oxide is significant and is less impacted by the gas quenching stage at the reactor outlet in comparison with zinc oxide (a product with 72% purity is obtained, against 48% for ZnO [84]). Indeed, the zinc boiling point is 907 °C, while that of SnO is 1425 °C; then, prompt condensation of the latter is facilitated.
• Eventually, SnO is a reactive compound for steam reduction.

3.3. Cycles Based on Non-Volatile Oxides

The cycles based on non-volatile oxides offer the advantage of avoiding the formation of gaseous species during the reduction step. This eliminates the need for any transportation of the solid between cycle steps, and both reduction and hydrolysis reactions can be conducted in the same reactor while only alternating the temperature, provided that the phase change (melting) does not occur. The first proposed non-volatile oxide cycle was the iron oxides-based cycle (involving magnetite/wüstite phase transition). The magnetite phase (Fe₃O₄) can be reduced at a temperature of 2100 K and is thus in a liquid phase (melting point at 1811 K). The hydrolysis step starts at a minimum temperature of 450 K [85]. Experimental studies have demonstrated a complete reduction under inert gas, while the reactivity of the hydrolysis step is high, although it is still limited due to passivation caused by the formation of a surface oxide layer [86,87,88]. Furthermore, the experimental studies revealed the beneficial effect of quenching after magnetite reduction to avoid the wüstite dismutation into metallic iron and magnetite below a temperature of 570 °C. The theoretical energy efficiency for this cycle is estimated at 35% (based on the higher heating value HHV of H₂).

Fe₃O₄ → 3FeO + ½O₂

(14)

3FeO + H₂O → Fe₃O₄ + H₂

(15)

In order to decrease the cycle temperature, mixed oxide compounds (based on M₃O₄/MO redox pair) were developed and investigated, involving the formation of a solid solution of iron oxide, offering both a higher reducibility at lower temperature and a better thermal stability of the material that remains in solid phase throughout the cycle [89]. The M_xFe_3−xO₄/M_xFe_1−xO redox pair thus obtained can indeed be reduced at a lower temperature than pure Fe₃O₄, and the material remains in the solid phase during the high-temperature step. The metallic dopant is incorporated in the magnetite spinel structure, thus forming a ferrite material. The reduction step is further facilitated by such a doping because it creates a structural deformation in the structure of the spinel, thus favoring oxygen mobility. A variety of dopants were investigated, including Mg, Mn, Co, Ni, Cu, and Zn. Iron oxides dissolved within YSZ matrix also showed improved reactivity [90].

Another cycle relies on the CeO₂/Ce₂O₃ system [91]. The reactions of the two-step cycle are comprising the thermal reduction step of Ce⁴⁺ into Ce³⁺ and the hydrolysis step of Ce³⁺ with steam (or CO₂).

2CeO₂ → Ce₂O₃ + ½O₂

(16)

Ce₂O₃ + H₂O (CO₂) → 2CeO₂ + H₂ (CO)

(17)

When heating above about 1950 °C under reduced pressure, a complete reduction extent to Ce³⁺ can be achieved in liquid phase. Then, in the next oxidation step (hydrolysis), Ce₂O₃ features both a high reactivity (with complete oxidation conversion) and a fast kinetic rate from ~400 °C (e.g., complete Ce₂O₃ re-oxidation to CeO₂ in about 5 min in comparison with about 30% conversion in 2 h for FeO oxidation), and the particle’s size does not have a significant influence due to the facilitated oxygen diffusion in the ceria crystalline lattice. However, the reduction step temperature of the cycle remains elevated, and a partial CeO₂ sublimation occurs during heating; the theoretical energy efficiency of this cycle is ~40%. A variant three-step cycle based on Ce⁴⁺/Ce³⁺ redox pair was also proposed involving mixed cerium oxides (e.g., with pyrochlore structure Ce₂Si₂O₇ or Ce₂Ti₂O₇ [92]). As described in the next section related to non-stoichiometric (and non-volatile) oxides, doping strategies of ceria with another secondary metal (Ce_xM_1−xO₂) can help to improve the ceria reduction rate by creating oxygen vacancies (oxygen sub-stoichiometry), thus enabling the O₂-releasing step to be conducted at a lower temperature [93,94,95].

3.4. Cycles Based on Non-Stoichiometric Oxides

3.4.1. Cycles Based on Ferrites

The first research investigations on the topic of non-stoichiometric oxides were related to ferrite systems [89], which are among the most advanced systems to have been tested to the stage of a 100 kW (thermal) pilot-scale demonstrator [96]. The Fe₃O₄/FeO two-step cycle is complicated to be implemented as such. On the one hand, the required temperature for thermal reduction of Fe₃O₄ is very high and above the oxide melting point, hence the reduction reaction proceeds in liquid phase (FeO melting point is 1377 °C). The FeO product then requires a milling to obtain a fine powder in order to warrant a sufficient specific surface area for the oxidation step. Furthermore, the produced FeO species disproportionates into Fe₃O₄ and metallic Fe above 570 °C, and therefore the quenching of the product below this temperature is required. A proposed approach to overcome these issues is the substitution of some iron atoms in the crystalline lattice with other metals. As such, the idea of this method is to insert metals that are more readily reducible, although being less reactive to the oxidant (steam or CO₂) than metallic iron (case of Co, Ni, Mn, etc.). Thus, the resulting ferrites with inserted dopants (with (Fe_1−xM_x)₃O₄ formula, where M = Mg, Mn, Co, Ni, Zn, etc.) still remain reactive to the oxidizing agents such as water, while they can be partially reduced to a (Fe_1−xM_x)₃O_4−δ non-stoichiometric form at much lower temperatures than Fe₃O₄.

On the basis of the numerous research studies performed on this topic, nickel-based ferrites appear to offer the best performances in terms of fuel production yield and thermochemical stability with a reduction step temperature maintained below ~1400–1500 °C [97,98,99]. Nevertheless, ferrites usually do not have a sufficient cycling stability because of unavoidable sintering and induced losses of redox activity and fuel production capacity. To avoid this phenomenon, the coating of the active phase on a ceramic inert support was widely considered [100,101]. The studies revealed that the best-performing substrates include yttrium-stabilized zirconia (YSZ) [102] and monoclinic zirconia (m-ZrO₂) [103], but the reduction rates remain below 30%. Given that the substrate may account for 90 wt% of the mass of the reacting material, the mass-specific H₂ productivity (mole of H₂ per mass of involved material) for this kind of solution remains weak.

3.4.2. Cycles Based on Ceria

Cerium dioxide or ceria (CeO₂) is used widely in the automobile industry as a catalytic material for oxidizing the exhaust gases to permit abatement of gaseous pollutants. This application relies on the facilitated solid-state diffusion and exchange of oxygen ion in the ceria crystalline lattice, which is also advantageous for the reduction and oxidation steps of thermochemical cycles [104,105]. The first study on ceria redox cycle for fuel production was conducted in 2006 by Abanades and Flamant [91]. When decreasing the reduction step temperature of ceria, only a partial reduction of Ce⁴⁺ in ceria occurs (oxygen vacancies are thus created via a non-stoichiometric reduction), which relates to the CeO₂/CeO_2−δ redox pair system [106,107,108,109,110]. In brief, for pure ceria, the reduction extent remains low, and it is controlled chiefly by the heating rate used to reach the reduction temperature (the maximum oxygen non-stoichiometry δ at equilibrium is determined by the operating temperature under a fixed oxygen partial pressure), which limits the overall fuel productivity per cycle, but the rapid oxidation kinetics make it possible to consider a large number of redox cycles during sunny periods.

The reduction capacity of the ceria-based material can be further enhanced if ceria is doped with aliovalent or tetravalent cationic elements [111,112,113,114,115,116,117,118,119]. Various studies focused on the proposal of new formulations for doped ceria, including their chemical synthesis and testing implementation and have, for instance, demonstrated the benefit of doping with Zr⁴⁺ (Ce_xZr_1−xO₂) [120,121,122]. By comparison, the previously investigated Ni-doped ferrites yielded a maximum fuel production of 250 µmol/g_oxide, against 333 µmol/g_oxide for ceria-zirconia mixed oxide, and the latter was more efficient in terms of kinetics of H₂ production and cycling stability.

Various ceria doping and shaping strategies were described [94] and showed their strong effects on the ceria redox cycle performances. The structure and morphology of the material is of prime importance, as it may affect the high-temperature material stability [94,123,124,125,126]. These two important properties are dependent on the methods involved during the material synthesis (e.g., as a powder) and its a posteriori shaping and structuring. An attractive concept for material shaping has recently been assessed, consisting in using ceria in the form of porous structures (e.g., reticulated open-cell foams with a dual-scale porosity) [127,128,129]. The porosity at the macroscopic scale is useful for the efficient volumetric heating of the whole reacting material, which can be itself micro-structured to enhance the specific surface area inside the struts (solid–gas interface area) [130,131]. The elaboration of three-dimensionally ordered macroporous structures (3DOM) using biomimetic approaches has also been considered (e.g., cork-templated or wood-templated ceria) [132,133,134,135].

Despite various recent advances in the field, several aspects can still be developed in order to plainly take profit of the advantages of ceria-based redox cycles:

• The ceria formulations can further be optimized (incorporation of several new dopants, utilization of composite materials [136], etc.).
• When the material is used in the form of a powder, the considered synthesis methods warranting high-temperature material resistance must be preferred.
• When the reacting material is structured (porous architecture), thermally-resistant supports (substrates) and efficient coating methods are needed. Alternatively, original methods need to be developed for the elaboration of architectured porous structures (e.g., biomimetics, 3D printing, etc.) [137,138,139,140,141,142].

3.4.3. Cycles Based on Perovskites

The perovskite structure is generally related to the ABO₃ compounds, where A and B are cations (B cation size is smaller than that of A). In addition, A and B cations are bonded to twelve and six oxygen atoms, respectively (Figure 4a). The main advantage of such a structure is that the substitution of some A and B sites with other cations is possible and this may alter the structural stability by creating defects. Hence, these substitutions make it possible to generate compounds of $A_{1-x}{A}_x^{\prime }{B}_{1-y}{B}_y^{\prime }{O}_3$ type (0 < x,y < 1) and to finely adjust their thermochemical properties, as required in redox cycles [9,143,144,145,146].

By precisely selecting the cations filling the A and B sites and the doping elements, structural deformations can possibly be obtained, as well as modifications of the crystal electronic configuration, which may promote the migration/diffusion of the oxygen and thus facilitate its release from the material. The combination of this attractive property with the electronic stability of the structure is useful to achieve significant oxygen sub-stoichiometry [147,148,149,150,151,152,153,154]. Perovskites with tunable reduction capacities and oxygen ion diffusion rates that are superior to those of ferrites and ceria-based systems can thus be proposed [155,156,157,158,159,160,161,162,163,164,165].

The effect of different dopants on the oxygen evolution and carbon dioxide splitting abilities of lanthanum manganite perovskites was discussed [166,167]. Perovskite formulations such as La_xSr_1−xMnO₃ (LSM) and variants with A and B site doping were studied [168,169,170,171,172,173,174,175,176] (production of H₂ or CO higher than ~200 µmol/g, for a reduction step temperature of 1400 °C, as illustrated in Figure 4b), confirming a fuel production capacity potentially higher than the one measured for ceria [177,178,179]. These performances remain close to those measured for substituted ceria (e.g., ceria-zirconia), but they show promise for a significant potential of material development.

Research works focused on the optimization of the composition and reactivity of materials (perovskite series with A or B site substitutions having a formulation Ln_1−xAe_xBO_3−δ) that were synthesized and characterized for the splitting of H₂O or CO₂. For instance, Ca-, Sr-, and Al-doped SmMnO₃ [149,150], Ca- and Ga-doped LaMnO₃ [151], Ce-substituted LaSrMnO₃ [152], Ce_xSr_2−xMnO₄ [160], Ca_0.5Ce_0.5MnO₃ [159], double-site Ce-substituted (Ba,Sr)MnO₃ [180], La_0.5Sr_0.5Mn_1−xA_xO₃ (A = Al, Ga, Sc) [163], SrTi_0.5Mn_0.5O_3−δ [164], CaTi_0.5Mn_0.5O_3−δ [165], La_0.6Sr_0.4Mn_1−yCr_yO_3−δ [169,181], Sr- and Mn-doped LaAlO_3−δ [182], La_0.4Sr_0.6Mn_1−xAl_xO₃ [183], and La_1−x(Ca,Sr)_xMn_1−yAl_yO₃ [184] are some examples of the numerous materials claimed as potentially attractive candidates for this application. However, the reported performance data have to be considered cautiously because the operating conditions may vary significantly from one material study to another (temperatures, oxygen partial pressures, oxidant concentrations, heating rates, etc.), and performance cannot be directly compared to assess the real capability of the investigated materials. Moreover, apart from the global fuel production output per mass of material, the fuel production rate must also be high enough to warrant both reasonable global cycle duration and high number of cycles performed each sunny day. Unfortunately, the fuel production rates observed in the oxidation step do not compare favorably with pure ceria for most of perovskite formulations investigated to date. However, theoretical calculations showed that perovskites have the potential for improved solar-to-fuel efficiencies during isothermal or near-isothermal redox cycling beyond those achievable by ceria [185].

Kinetic studies were performed to identify the most suitable models of solid–gas reactions for the reduction and oxidation steps [186]. Correlations were also established between geometric parameters of the crystalline structure of perovskites and the redox performances obtained during cycling [187]. In summary, based on the numerous perovskite formulations studied so far, it appears generally that the oxidation kinetics are much lower than for pure ceria, but conversely, the oxygen exchange capacity of considered perovskites (thus the achievable global fuel productivity per mass of material) is higher.

4. Solar Reactor Technologies Developed for Redox Cycles

Two main solar reactor concepts can be proposed to implement redox cycles: the reactors in which both cycle steps can proceed in a single reactor chamber and the reactors in which the functions of reduction and re-oxidation are decoupled in different systems allocated to each step. Concentrated solar heat integration is of prime importance for achieving an optimal solar reactor efficiency [188,189]. The most relevant solar reactor technologies developed to date and applied specifically to two-step cycles are presented.

4.1. Single-Chamber Solar Reactors

This kind of solar reactor gathers the different available technologies for implementing the non-volatile oxide cycles. The reacting material can be either in the form of divided particles (grains or powders of pure material or coated as thin films) or structured with a porous architecture (via the coating of a substrate or by shaping) [190]. In both cases, the target is to offer a large specific surface area for reactions to promote transport phenomena at the solid–gas interfaces.

The possible reactor technological concepts include the fluidized beds with internal particles’ circulation, namely circulating fluidized beds (Figure 5). Due to the particles’ motion and mixing, a homogenized temperature of the solid can be obtained, the reduction/re-oxidation steps can be performed in alternance, and the particles’ sintering can be alleviated. The gas flow is injected in the central part of the cylindrical reactor for the particles’ fluidization and for transporting them in the upper region of the bed, so that the upper part of the particle bed is located at the focal point of the solar concentrating system, enabling the reduction reaction. The gas flow-rate is dependent on the particles’ size and density that determine the conditions needed for their proper fluidization. When reaching the solar-irradiated zone, the particles return down at the bottom part of the bed via the annular zone at the periphery of the draft tube, thereby transferring a part of the absorbed energy from the top irradiated zone to the bottom zone. An effective thermal homogenization on the vertical axis can be achieved, thus ensuring a more uniform temperature in comparison with packed-beds, and the inert gas flow-rate required for particles’ fluidization is restricted to the central region. Once the thermal reduction step is finished, solar irradiation is then stopped, and a fraction of the inlet flow of inert gas is replaced by the oxidizing gas. This reactor concept was, for instance, implemented and operated by using ZrO₂ supporting particles coated with Ni-ferrite (NiFe₂O₄) [191].

Another interesting reactor concept consisting of a monolithic reactor was developed, devoted to the solar-driven production of hydrogen fuel, in the framework of a 100 kW (thermal) pilot prototype (Figure 6) [96]. The reacting oxide material (e.g., doped ferrite) is coated on a porous ceramic structure such as honeycomb monolithic, and the whole system acts as both a volumetric radiative absorber and a thermochemical reactor. To allow continuous fuel production during solar operation, two similar parallel reactors are settled in the focal area. While the first one is subjected to concentrated solar radiation for the thermal reduction step, the second one operates the material re-oxidation (ferrite hydrolysis), and vice versa for cycle steps’ alternance. Such reactors thus involve a fixed reactant, and the temperature alternance between each step ensures continuous operation via the heliostats’ monitoring for control of incident flux.

Other types of monolithic solar reactors enabling the integration of massive porous structures made of ceria (monoliths, reticulated foams) have been recently developed (SUNFUEL solar reactor installed at CNRS-PROMES in Figure 7) [130,131,136]. In such reactors, the reactive material is usually cycled at two distinct temperatures for optimized process performance (temperature-swing cycles), although isothermal cycles have also been considered but with much lower fuel production output [114,192,193,194,195,196,197,198]. Reticulated porous ceria structures (foams) were prepared by hard templating with a suitable geometry and different pore size densities and then cycled under a wide range of operating conditions (temperatures, O₂ partial pressure, gas flow rates, oxidant mole fraction, H₂O or CO₂ oxidant) in order to determine the material’s redox activity. Such foams showed remarkable performances for the splitting of H₂O and CO₂ (fuel production rates up to ~10 mL/min/g) thanks to their dual scale porosity (open cell structure for homogeneous heating and interconnected porosity inside the struts for enhancing the reactivity with the gaseous oxidant species). The cycling stability of materials was assessed during a high number of successive cycles in the reactor under real concentrated solar irradiation conditions [130,131,199]. Ceria foams coated with a thin perovskite layer (La_0.5Sr_0.5Mn_0.9Mg_0.1O₃) were also elaborated, which allowed an improvement of the global fuel output at the expense of slower oxidation kinetics due to a diffusion barrier at the surface [136]. Moreover, ordered porous structures with a graded porosity (to favor volumetric solar radiation absorption) were designed and manufactured by 3D printing, and their cycling was studied in the solar reactor, which also proved to be effective for the splitting of H₂O and CO₂ [139]. Another shaping strategy was considered consisting of the robocasting of 3D printed and sintered ceria scaffold structures with hierarchical porosity for solar thermochemical fuel production from the splitting of CO₂ [200].

Other alternative shaping strategies were considered in order to integrate the materials in the reactor in a divided form (e.g., particulates). For example, 3-dimensional ordered macroporous (3DOM) ceria materials were synthesized via biomimetics (with cellular ordered microstructure prepared from a cork template, cell diameter ~25 µm). The materials were cycled in different solar reactors with direct or indirect heating [132,133,134]. Ceria porous microspheres were also elaborated from ion exchange resins and cycled in solar reactors [137], as well as fibrous sintered ceria pellets to demonstrate their suitability for solar fuel production application [140]. The obtained performances were similar to those of ceria foams (~200–300 µmol/g of H₂/CO, peak production rate: 9.5 mL/min/g) under comparable cycling conditions.

Alternatively, a membrane solar reactor was developed for continuous and isothermal splitting of CO₂ (and H₂O) under a gradient of O₂ partial pressure across the membrane [138,201,202,203,204]. As described in Figure 8, the original process was demonstrated in a solar reactor under high-flux solar irradiation using a dense ceramic membrane with tubular shape (mixed ionic-electronic conducting materials, MIEC) that separates a reduction zone at low p_O2 (sweep side) and an oxidation zone fed with reactive gas flow (feed side). The reactive part of the tubular redox membrane was located inside a well-insulated cavity receiver for homogeneous heating, which was fed with a carrier Ar flow on the sweep side to facilitate the transport and removal of the permeated oxygen [138]. The dynamic response of the solar fuel evolution upon varying the operating conditions in the membrane reactor (temperature, CO₂ mole fraction, and feed gas flow rate) was assessed by measuring the evolved gas production rates. Continuous CO₂ dissociation was achieved on the feed side inside the tubular membrane with in situ spatial separation of O₂ and CO across the membrane. The CO and O₂ production rates were sharply enhanced by increasing the operating temperature (up to 1550 °C). The increase in CO₂ concentration or oxidant gas flow rate also enhanced the process performance. Reliable solar membrane reactor operation, under real concentrated sunlight, was successfully demonstrated for the first time, with stable and unprecedented CO production rates up to 0.071 μmol/cm²/s at 1550 °C and CO/O₂ ratio of 2. An original composite membrane integrating two different perovskite coatings on each side of the ceria membrane, with a sandwich-like structure, was designed and tested under concentrated sunlight [201]. Thin perovskite layers (inner side: La_0.5Sr_0.5Mn_0.9Mg_0.1O₃ and outer side: Ca_0.5Sr_0.5MnO₃) were coated to enhance oxygen ion transfer. With such a membrane structure, a CO production increase (>0.13 μmol s⁻¹ cm⁻²) and simultaneous oxygen separation (with CO:O₂ ratio of 2) were observed, respectively, on the inner and outer sides of the oxygen transport membrane. These results outperform the production rates achieved with uncoated ceria membranes, which demonstrates the interest of using composite membranes made of a densified core material coated with redox-active perovskite layers.

4.2. Decoupled Reactors for Separated Reduction and Oxidation Steps

This type of reactor concept is adapted to the cycles in which the reduction step is carried out in a first solar receiver/reactor, and the oxidation step for H₂/CO production is performed separately at a lower temperature in a second oxidation reactor. The heating and cooling stages of the cavity receiver are thus not required, and the solar reactor is specifically designed and optimized for the reduction step. The global system comprising both reactors (solar and oxidation reactors) can potentially be operated in a continuous mode [205]. The most widespread concept relates to the solar reactors devoted to the thermal reduction of volatile oxides.

A prototype reactor with rotating cavity (rotary kiln type) was developed for ZnO dissociation (Figure 9) [206]. Thanks to the cylindrical cavity rotation, the ZnO particles, that are injected by the means of a screw feeding system in the axial region, are evenly distributed and spread on the reactor walls, thus avoiding the existence of hot regions ascribed to the non-uniform incident solar flux. A permanent inert gas flow is required, and its injection at the rear face of the quartz window allows its protection from Zn vapor deposition. The produced gas species are exiting via the outlet annular section along the screw feeding system, where their cooling/quenching is ensured by the injection of large amounts of inert gas at room temperature (such a quenching method thus consumes inert gas, inducing energy penalties for inert gas separation and recycling). The Zn particles (with ~50% Zn content purity) are finally separated and recovered by filters at the outlet. A different prototype of a rotating solar reactor was also developed at CNRS-PROMES for continuous operation with direct ZnO particles’ injection in the high-temperature cavity and Zn product recovery as a fine reactive powder (composed of nanosized particles) at the reactor outlet (Figure 10) [207,208].

The solar reactor prototype with the highest thermal power was developed during the European project Solzinc for the reduction of zinc oxide with carbon materials (Figure 11) [67]. This reactor is thus designed for the carbothermal-reduction of ZnO at lower temperatures (below 1000 °C), which noticeably alleviates the thermal constraints imposed on the reactor materials and the technological issues associated with the gas quenching required in the thermal dissociation process, at the expense of the carbon source requirement. The solar carbothermal reduction of ZnO was mainly studied in shrinking packed-bed reactors, thus based on a similar technology, at smaller scale [77].

The considered solar reactor is composed of two separated cavities. The upper cavity of the reactor, composed of SiC plates, acts as the solar receiver, as it receives and absorbs solar irradiation in order to re-emit the energy to the lower cavity that contains a packed-bed of ZnO particles, which are heated by radiations emitted by the separation heat transfer plate. This configuration enables a production of a significant amount of reduced Zn particles without the risk of particles’ deposition on the quartz window. The production of 50 kg/h of Zn with 95% purity was achieved, at an operating temperature range of 1300–1500 K, with a process efficiency of 30%.

A different vertical-axis solar reactor concept was developed for the thermal ZnO dissociation (named Gravity Fed Solar Thermochemical Reactor, Figure 12) [209]. In this concept, particles are injected via fifteen distributors located at the periphery of the reactor to homogenize the particles’ distribution in the reactor cavity. Driven by gravity, the fed particles fall as a curtain on the surrounding plates made of a refractory material forming the cavity receiver, and the cavity slope and surface were designed for proper operation. The particles are reduced and vaporized zinc is produced during their progress downward in the conical region, while the evolved gases are carried by an inert gas flow to the reactor outlet at the conical cavity bottom. The unreacted particles are then recovered in the annular region located between the gas outlet and the lower part of the reactor.

The PROMES laboratory also designed and developed new technological concepts suitable for the continuous thermal reduction of volatile oxides. A first prototype featuring a rotating cavity was assembled, as already shown in Figure 10 [208]. One of the goals and originality of the designed system was the possibility to conduct reactions under reduced pressure to enhance the kinetics [50,51]. A second different reactor prototype with a vertical solar irradiation axis was then developed, also enabling operation at reduced pressure (Figure 13) [84]. The reacting material, in the form of compressed pellets, is continuously fed into the insulated cavity via a screw feeding piston in the lower cavity part, and it is simultaneously heated by both the direct concentrated solar radiation and the emitted IR radiation from the cavity walls. Another advantage of this kind of reactor is a low distance between the reduction zone in the cavity and the outlet evacuation zone connected to the particle filtration system, which alleviates the recombination reaction. The metallic fine powders (Zn or SnO) recovered in the filter at the outlet contain ~40% of reduced species for ZnO and ~70% for SnO₂, under an operating total pressure of about 20 kPa with a solar-to-chemical energy conversion efficiency of about 2–3%.

With the objective to integrate heat between both cycle steps and to warrant continuous operation, two different reactors with moving solid oxide reactant were developed, enabling an integrated flow and transport of the reacting solid from one reaction chamber to the other. The “CR5” concept (named Counter-Rotating-Ring Receiver/Reactor/Recuperator, Figure 14a) involves two chambers separated by a stack of rotating rings [210]. The rotation drives the ring surface coated with metal oxide (ferrites) from the reduction zone to the oxidation zone. The hot part that exits the reduction zone of one reheats the entering colder parts of its two neighbors, thus enabling heat recovery (30%). The challenge and limitation of this concept reside in this assembly of rings, which must also ensure an efficient gas-tight separation of the two chambers’ atmosphere. Though the concept was experimentally tested with a prototype involving cobalt ferrites, the available data are not sufficient to really assess its actual efficiency.

In the other concept (Figure 14b) [211], the particles are driven by a stationary screw towards the upper part of the reactor where they are subjected to concentrated radiation. Due to gravity, they go back down through the hollow axis of the screw, which acts as a heat exchanger between upward and downward particles, to the re-oxidation chamber. The authors highlight the possibility for it to continuously operate, unlike the fluidized bed, which allows a batch operation. This concept was, however, not proven, and achieving tightness due to stacked particles seems difficult, similar to the transportation of particles to the high temperature zone.

5. Conclusions

The production of green hydrogen from water-splitting and the utilization/valorization/recycling of CO₂ by thermochemical cycles coupled to a concentrated solar energy source is a relevant and attractive approach. Solar radiations are converted into heat with a reasonable efficiency, then this energy is directly used to supply the required heat to the thermochemical cycle. This allows a higher global energy efficiency for the conversion of the solar resource into fuel in comparison with the paths involving photons or electricity as intermediate sources of energy. An overview of the most suitable and advanced thermochemical redox cycles was provided and the associated solar reactor technologies applicable to H₂O and CO₂ splitting were described. Each system was discussed and critically analyzed, highlighting its advantages and drawbacks depending on the type of redox materials involved, with focus on limitations and possible methods to improve their performance (e.g., doping with other metals, control of morphology and microstructure, etc.).

• The volatile oxide cycles offer the advantages of relatively high specific fuel productivity due to discrete oxidation state transitions (leading to high oxygen amounts being released and recovered at each cycle), and of their specific surface area being renewed during each cycle thanks to the vaporization/condensation mechanism. However, it is difficult to prevent products’ recombination during gas cooling in the reduction step. ZnO/Zn and SnO₂/SnO thermochemical cycles are the most attractive (with potential maximum fuel productivity up to ~12.3 mmol/g for ZnO and 6.7 mmol/g for SnO₂).
• Non-volatile oxide cycles mainly relate to non-stoichiometric oxides. They allow a lower reduction step temperature, absence of phase change during cycling, and the possibility to carry out the two reactions of the cycle in the same reactor without any reactant transfer. However, they feature a lower mass-specific fuel productivity (in the range 0.1–0.4 mmol/g) than for the stoichiometric oxides (due to a limited amount of oxygen being exchanged during solid-state reactions, as represented by the lattice oxygen sub-stoichiometry δ), a possible loss of specific surface area during the cycling (due to sintering), and the need to optimize the formulations and the methods for the materials synthesis.

The first materials being investigated were iron-based, but due to the limitations related to their deactivation or their low mass-specific fuel productivity, research studies on ceria (CeO₂), either doped or not, are particularly active due to a high reactivity of the material with H₂O or CO₂ with high fuel production rates and thermochemical cycling stability. More recently, the ABO₃ perovskite materials are particularly attracting attention due to a significant potential for development related to their attractive redox properties (larger sub-stoichiometry accessible thanks to the oxygen mobility in comparison to pure ceria) and to many possibilities of various formulations (high flexibility offering the possibility to incorporate many dopants). The oxygen transport properties in cerium- and perovskite-based materials are particularly exploited in the H₂O and CO₂ splitting cycles. Research studies are being pursued in various fields pertaining to the formulation (selection of chemical/crystalline characteristics), materials synthesis, and shaping (elaboration of stable macroscopic porous structures during cycling) for their integration in solar reactors.

The typical solar reactors applicable for each reaction system were described, including either single chamber reactors, with both steps being performed alternately in the same reactor, or decoupled reactors, with solid reactant transfer between reaction steps. Extreme operating conditions (including very high temperatures and radiative fluxes, reducing/oxidizing atmospheres) and the requirement to manage thermal losses should be considered in the design and development of solar receivers/reactors with the ultimate aim to optimize their energy and thermochemical (solar-to-fuel) efficiencies.

Identification or discovery of new materials with large oxygen exchange capabilities, rapid chemical kinetics and stability, and their integration in efficient solar reactors are essential. Further advancements in redox material properties’ characterization and performance optimization, along with innovative reactors’ design, are still of critical importance to make this technology commercially viable. Finally, the thermochemical process scale-up and integration in concentrated solar plants must be taken into account for the synthesis of clean solar fuels relying exclusively on the water and carbon dioxide splitting.