Perovskites in the Energy Grid and CO₂ Conversion: Current Context and Future Directions

Abstract

CO₂ emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO₂ levels in the atmosphere and convert CO₂ into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO₂ reduction has received great attention because of its potential to efficiently reduce the level of CO₂ in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO₂ conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO₂ conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO₂ transformation to value-added chemicals and chemical feedstocks.

1. Introduction

In the past decades, the increasing concentration of CO₂ in the atmosphere is attributed mainly to the utilization of fossil fuels and it has negatively impacted the climate [1,2,3,4,5,6,7,8]. The level of CO₂ in the atmosphere has increased from 322 ppm in 1967 to 407 ppm in 2017 and is predicted to reach to 600 ppm by 2100. Consequently, according to the International Panel for Climate Change (IPCC), the mean global temperature will increase by 1.9 °C by the year 2100. In 1992, many countries ratified the U.N. Framework Convention on Climate Change, which set out guidelines to reduce greenhouse gas levels in the atmosphere. In a comparative context, the average global emission of CO₂ due to the use of fossil fuels is in the order of 35 Gt, while the annual capture limit is 35 Mt. To help mitigate global warming, efficient technologies to reduce the levels of CO₂ and simultaneously produce value-added products, such as chemicals and fuels, by its conversion are a sustainable solution [9,10,11,12,13,14,15,16,17,18]. To this extent, a number of approaches have been investigated for the conversion of CO₂ into fuels [19,20,21,22,23,24,25,26,27,28]. One example of such technologies is high-temperature conversion (HT), which dissociates CO₂ molecules into hydrocarbons at temperatures higher than approximately 2000 °C in vacuum [29]. The process is associated with high energy consumption, and has issues of instability of the converted molecules under the applied harsh conditions. Another technology is the solar thermochemical conversion (STC), which can produce hydrocarbons at temperatures higher than 1000 °C, and where the CO₂ conversion rate and selectivity, e.g., to CO, are often found to be better than that of the photocatalytic technologies. The operating temperature of this process is also high and thus the chemicals produced often face stability issues as well. Among the high-temperature methods, the reverse water–gas shift chemical looping is a lower temperature CO₂ conversion process with operating temperatures in the range of 500–700 °C [29]. In this process, the conversion of CO₂ into CO takes place followed by hydrogenation of CO into fuels via Fisher–Tropsch synthesis (FTS).

Recently, among various nanostructured materials, semiconductor-based photocatalysts have shown great potential for CO₂ conversion into chemicals such as H₂, CH₄, CH₃OH, and NH₃, via complex photoreactions that harness solar photons as energy sources [21,22,23,24,25,26,27,28,29]. This process in general needs a high energy input to activate CO₂. In order to yield chemicals and fuels, the chemistry and thermodynamics of CO₂ should be well-known, as conversion of CO₂ is thermodynamically unfavorable due to the fact that the bond energy of C=O is 750 kJ/mol, making it difficult to break down. To facilitate the cleavage of the C=O bonds, catalytic approaches, namely, photo [1,30], photochemical [2,31], electrochemical [32,33], and thermochemical [34,35] reactions, are mainly being investigated. For instance, the high energy required to break C=O bonds for the formation of C–H bonds toward photocatalytic reduction of CO₂ with H₂O into hydrocarbon products is provided by incident light. On the other hand, the thermochemical process involves high temperature and pressure, and it is often challenging to control reaction selectivity [1,2,3]. Pretreatment at different temperatures has been found to affect reactivity and selectivity, while the overall process is often complicated and challenging to control, making it difficult to convert CO₂ to the desirable products with high efficiency. To this extent, the development of CO₂ conversion technologies at low temperatures involving novel catalysts, such as perovskite-based ones, becomes more attractive than classical methods, due to the inherent advantages that these materials can offer, which include sustainable catalytic efficiency, tunable selectivity, potential for scaling up to industrial applications, and the ability to improve conversion efficiency and selectivity via optimizing the catalyst morphology and structure [19,20,21,22,23,24,25,26,27,28,29].

2. Perovskite Structures and Properties

Recently, there has been an increasing interest toward the exploration of perovskites in the field of energy and environment for applications such as fuel cells, CO₂ conversion, vehicle exhaust gas purification, and water gas reactions, because of their low price, tunable oxidation state, structural properties, and thermal stability [24,26,28,36,37,38], while fundamental investigation of these compounds has the potential to shed light on the relationship between catalytic activities and solid-state properties. Perovskites are often found with the general formula ABO₃, where A stands for the alkali metal (Li, Na, K, and so on) and/or alkaline-earth metal (Mg, Ca, Sr, Ba, and so on) or rare earth metal cation, occupying the 12-fold coordinated cube-octahedral cages of oxygen sublattice, and B indicates the transition metal cation (such as Mn, Co, Fe, Ni, Cu, or Ti), which is surrounded by six oxygen atoms in an octahedral coordination and is also responsible for catalytic activity and electron transfer [23,24,26,27,29]. According to the given formula, many important cations can fit into the A and B positions for making the final crystalline structure, to effectively tune the material properties and help improve conversion efficiency and selectivity. The metals in the perovskites’ lattice are generally stable, as needed for the catalytic reactions at various conditions, while their cations in the A and B positions could be replaced as required via partial substitution [23,26,29].

Perovskite oxides mainly consist of a cubical crystalline structure where the cations have large ionic radii, which allow extensive coordination of oxygen molecules in the A-sites. Therefore, the larger A-site cations in the lattice (both in cubic and double-layered perovskites) are energetically favorable to be terminated at the AO surface. In fact, A-sites effectively block B-sites from coming into contact with reactants, such as CO₂, because less B-sites are exposed to the free surface to be available to participate in reactions, thus impacting the electrochemical transformations [26,27,29]. On the other hand, these cations have smaller ionic radii, thus less energy is utilized when they occupy the B-sites. When O and A join, cubic packs appear, while B occupies the octahedral voids of the packing. Thus, in an ideal compound structure, among the atom bonds, the B–O distance matches a/2 (where a is the unit cell length), while the A–O distance equals to a/$\sqrt{2}$ [10]. The following equation summarizes the ionic radii relation: ($r_A+r_O)=(\sqrt{2}(r_B+r_O$). Related studies have shown that the metal oxides are stable in their oxidative state and are thus used as catalysts in a variety of conditions with high efficiency and stability, while the ABO₃ structure and composition provide excellent tunability to control the final properties [10]. It has also been reported that the cubic form of perovskites is retained in the ABO₃ formula, although this structure is not always strictly obeyed, thus, for evaluation of the deviation from this structure, the tolerance factor (t) was introduced by Calio et al. [14]. Accordingly, a measure of the ionic distance of A and B can be provided as indicated in the following equation: t = ($r_A+r_O)/(\sqrt{2}(r_B+r_O$). For an ideal perovskite, t suggests unity, while lower values can also be obtained ranging from 0.75 to 1.0, and in such instances, the cubical structure of the compound is distorted, forming tetragonal shapes, lower symmetries, and rhombohedral forms [14,26,27]. Notably, in perovskites whose t values exceed 1, hexagonal structures are observed. This observation results from the large ionic radii of A ions and the smaller B ions. An example of such a compound is BaNiO₃ perovskite, which possesses a hexagonal shape (Figure 1).

Research on perovskite-based systems for environmental and energy applications has intensified in the past two decades as energy demands increase and environmental constrains become more stringent [39]. To this extent, the use of nanostructured perovskites for producing hydrogen using water and solar energy, as well as conversion of CO₂ to generate hydrocarbons and value-added chemicals, is being intensively investigated [19,21,22,23,24,25,26]. Materials, such as those based on Ti, are being investigated for their photocatalytic performance. In the crystalline structure of these materials, the different tilting rates of octahedra usually produce various crystal forms, as well as optical and electronic properties. In particular, the tilting degree affects the band structure, photoluminescence, dielectric tendencies, hole transport, and electron properties [16,28,40,41]. The tuning ability of the perovskites provides significant benefits compared to binary oxides, since the A-sites and B-sites offer considerable opportunity for the alteration and design of the band structure and final properties. For the double perovskites (Figure 1), the stoichiometric presence of double cations on the B-sites is beneficial during photocatalysis by visible light. Combination effects, for example, with ferroelectricity, could benefit the photocatalytic activities, while these materials have the potential of offering a wide range of overpotentials to introduce photoinduced reactions. When compared to metal oxides, these compounds have higher cathode conduction band (CB) energies, which are essential for the hydrogen evolution reaction (HER) [23,24,26,27,28,29,38].

High energy intake by incident light is needed to overcome reaction barriers and break the C=O bonds for the formation of C–H bonds in photocatalytic reduction of CO₂ into hydrocarbon products [2]. Progress towards commercialization is hindered by the availability of efficient, selective, and cost-effective catalysts. To this end, research efforts on the development of novel catalytic materials with tailored structural and morphological features for optimized performance, reactivity, selectivity, and stability are necessary [19,38,42,43]. The judicious choice of catalyst materials can tune CO₂ conversion reactions to achieve high selectivity and accelerated reaction rates. Among the available catalytic materials, perovskites are particularly attractive as mild operating conditions are used and wide control over the catalytic properties is achievable. Perovskites provide broad-spectrum activity, where applications can be anticipated utilizing visible and UV light, in particular, focusing on conversion into value-added chemicals for increasing chemical feedstocks and reducing CO₂ emissions in the atmosphere [1,2,5,38]. In this context, perovskite oxides have shown high stability, compositional flexibility, efficient catalytic capability, and long charge diffusion periods [23]. As a result, these materials can lead to accurate band gap and edge tuning, which are important in selective CO₂ reduction as well as in solar cell applications, due to their simple fabrication, high efficiency, and low costs [23,25,26,28,29,44,45]. Emerging CO₂ conversion technologies need to achieve reduction at a sustainable manner using mild operating temperatures, high conversion rates, and selectivity [46]. For example, earth-abundant and -derived perovskite materials are used in the gas–water shift reverse chemical looping to produce chemicals. The selection of technology is often based on manufacturing complexity, availability, productivity, and ability to tune the production of particular chemicals [47], as well as environmental assessment factors [48,49,50,51,52,53,54,55,56].

The structure of perovskite catalysts can play an important role in the conversion of CO₂ into value-added chemicals [19,20,21,23,24,25,26]. In particular, emphasis is given to achieve (i) high efficiency by enhancing mass transfer, (ii) the ability for separation of products, (iii) low-pressure drops, (iv) absence of clogging, (v) safety, and (vi) cost-and energy-effective production [36,57,58]. To this extent, the preparation method is important for controlling the properties and final structure of perovskites. Valuable information about tuning the material properties of perovskites can be extracted from their utilization in various applications including NO_x, oxidation of CO, combustion of hydrocarbons, and hydrogen production [58,59,60]. To affirm the application of these materials, their performance metrics have to be evaluated to determine the presence of active components [61,62,63]. For instance, much attention has been devoted to the development of structured perovskites that can be used in three-way converters. A challenge is testing the performance of the catalysts used [64,65,66]. In particular, the optimum operating conditions are difficult to determine, and therefore, the obtained results often deviate. Nevertheless, recent studies have shown significant improvement in the development of structured perovskite catalysts as demonstrated in Figure 2 [64]. Upon incorporation of metals to perovskites, the active phase constitutes a critical part of the structured form [67] contributing toward overcoming challenges related to catalytic performance, degradation, and diffusion limitations [19,42]. In particular, perovskite monoliths using various perovskite compositions, i.e., A = La, Ce, Y, Dy, or Sr; B = Mn, Cu, Fe, or Co, can be generated and tuned using steps such as extrusion of plastic pastes with catalytic powders, calcination, and drying procedures [64,65].

3. Chemical Looping

CO₂ treatment systems combining conversion with capture in a cost-efficient manner are particularly attractive toward commercialization. On the capture side, reduction of the cost of CO₂ separation processes could be implemented by focusing on (i) improving the efficiency of the capture materials [6,7,68,69,70] and (ii) optimizing process design and reducing the cost of equipment [71,72,73,74,75]. In addition, considerable time and financial investments are needed in order to develop the relevant cost-effective technologies to enhance the production of chemicals, while reducing the associated costs of the used systems. In this regard, chemical looping is one of the promising technologies with the ability to separate CO₂ from fuel feeds, while it involves a high-energy combustion process that utilizes redox reactions. Figure 3 demonstrates the relation between cost reduction benefits vs. anticipated time to commercialization for various related technologies [74,76]. Evidently, reduction of the cost of separation can offer high potential for commercial CO₂ set-ups from capture to production of the value-added chemicals and energy.

Studies have shown that chemical looping has evolved to accommodate various applications based on the recycle-based reaction design [51]. The process involves conversion, followed by regeneration of the involved solid materials [77]. According to Figure 4, significant developments on group A-1 process have been made. The process is split into two half-cycles. The reduction half-cycle reduces metal oxides, and the hydrocarbon fuels are oxidized to H₂O and CO₂ [78,79]. Fuels, such as biomass and coal, can be directly combusted in chemical looping via the oxygen uncoupling process [78,79,80]. A schematic representation of a mechanism for enhanced metal utilization in chemical looping reactions using reducible supports is also shown in Figure 5 [81].

Despite the high cost and increased toxicity concerns, Ni-based structured perovskites have received great attention because of their enhanced catalytic activity [52,82]. For instance, an almost complete CH₄ conversion has been reported using chemical looping and Ni-based oxygen carriers. Though, results have shown that catalytic performance is often compromised due to partial oxidation and cycle repetitions [82]. Similarly, Ni-based structures are responsive to deactivation by coke deposition and sulfur poisoning. In this regard, material optimization is required to solve these issues. On the other hand, Fe-based components serve as reliable oxygen carriers because of their affordability, mechanical strength, and the fact that they are environmentally safe. They also tend to suffer less from sulfur and carbon depositions [83,84]. Significantly, these materials have high oxygen retention at various operating temperatures (600–1800 °C). As a result, most of the current chemical looping technologies perform dry reforming using Fe-based oxygen carriers.

4. Engineering of Oxygen Carriers

The shortcomings in industrial chemical looping require improvement of oxygen carriers. In particular, there are three critical pathways to consider: (i) standardization and activation of the oxygen carriers, (ii) introducing promoters, and (iii) mixing metal oxides to leverage multiple benefits [53,85]. Currently, significant attention is being devoted toward the improvement of Fe- and Ni-based oxygen carriers and notable improvements have been achieved on material structures, promoters, and the types of chemical looping processes in which these materials are used [86,87,88]. Both the support and promoters can be chemically and physically active, the latter creating physical barriers to enhance thermal stability and the former providing redox attributes to improve reactivity.

In the case of Ni-based oxygen carriers, various heat-resistant compounds (such as TiO₂, ZrO₂, and Al₂O₃) and spinels (such as CaAl₂O₄, NiAl₂O₄, and MgAl₂O₄) are considered as effective promoters. Among these oxides, NiO/Al₂O₃ demonstrates significant reactivity, strong resistance, and thermal stability, while preventing carbon formation upon chemical looping [89,90,91]. Ipsakis et al. [92] revealed metal–support interactions for the NiO/Al₂O₃ oxygen carrier, and a reduction reaction for the NiO/TiO₂ oxygen carrier. Tijani et al. [93] observed significant oxygen capacity for Ni-Co/ZrO₂, Ni-Cu/ZrO₂, and Ni-Fe/ZrO₂, along with minimal interaction with the Al₂O₃ support. A conversion rate of 37% was observed for Ni-Cu/ZrO₂ in the air reactor, with negligible effect in the fuel reactor. Park et al. [94] observed a strong influence of NiO type on the oxidation and reduction rates of Mg-incorporated NiO/Al₂O₃ oxygen carriers in conjunction to Mg content, and both the rates were observed to increase with reaction temperature as well as with gas concentration [94].

Inactive spinels, such as NiAl₂O₄, are formed due to the strong interaction between Al₂O₃ and NiO, thus resulting in partial loss of chemical activity. In this regard, studies have suggested that the chemical reaction should be supplied with excess Ni to retain sufficient content of the NiO oxygen carrier [95,96]. According to Chen et al. [97], superior performance in terms of plant efficiency was noted for the NiO oxygen carrier compared to CuO- and Fe₂O₃-based systems. On the other hand, chemical passivism can be deployed to enhance the inert form of the Al₂O₃ promoter [98,99]. Yu et al. [100] studied the effectiveness of the NiAl₂O₄ spinel for conversion applications. It was noticed that an enhanced catalytic performance, together with an anticoking benefit, was achieved by the contribution of improved textural properties of the catalyst. Beierlein et al. [101] investigated the structural influence of NiAl₂O₃ on the CO₂ methanation reaction. The yield was observed to be higher for the catalyst comprising high surface area Ni. A strong influence of spatial distribution of Ni as well as support stability was also reflected on catalyst stability. In another study, Baek et al. [102] reported an accelerated rate of oxygen transport for Mg-Al layered double hydroxide-supported NiO oxygen carriers compared to Al₂O₃- or MgO-supported versions, on account of enhanced physicochemical features.

The addition of MgO/CaO when preparing Al₂O₃/NiO oxygen carriers has led to the creation of stable spinels. The use of reducible oxides, such as CeO₂, also increased the potential of Ni-based compounds as oxygen carriers, while additional important properties are realized due to the synergetic effects of CeO₂ and Ni [103,104,105,106]. After the reduction cycle, Ni particles are composed of two main phases, i.e., the metallic form and NiO. CH₄ is decomposed on the metallic surface of Ni forming a carbonaceous species, which is then gasified using oxygen from CeO₂ [106]. During this process, the lost oxygen is replenished at the core of Ni particles. Therefore, a continuous reduction is assured for the Ni particles. Even though Ni/CeO₂ oxygen carriers provide a high reactivity to CH₄, the high utilization and exposure to carbon formation reduce the surface area during chemical looping. CeO₂ sintering becomes more dominant after repeated cycles, an issue that needs to be fixed before realization in industrial applications [107,108,109]. Hossain et al. [110] reported a considerable reduction in nickel aluminate generation for NiO/Ce-doped Al₂O₃ oxygen carrier through the reduced interaction between metal and support. Notably, the availability of approximately 70% of incorporated NiO to proceed with reduction reactions was evidenced. The cyclic stability was also evidenced from the repeated redox reactions. Zhao et al. [111] investigated the effectiveness of Ni/Mg/Ce_0.6Zr_0.4O₂/Al₂O₃ for CO₂ conversion applications. CO₂ conversion rates of 4% and 61% were observed at 1 and 6 bar, respectively, and at 860 °C.

Fe-based oxygen carriers have also attracted considerable attention in chemical looping applications [112,113]. Bhui et al. [114] investigated the chemical looping process supported by Fe₂O₃ oxygen carriers. Interestingly, a significantly improved CO₂ yield of 88.7% was discerned, though only 17.5% was observed in the absence of Fe₂O₃. Hu et al. [115] developed an oxygen carrier based on Fe₂O₃/Al₂O₃ exhibiting superior crystal cell shrink resistance and significantly enhanced specific surface area. As a result, an increase of 13% and 37% in reduction and oxidation rates, respectively, was noticed, along with an appreciable regenerability after five redox cycles. In another study, Diego et al. [116] reported the effects of Fe content and elevated temperature in achieving effective chemical looping for Fe-Al oxygen carriers. Further studies have shown that promoters, such as MgO, Al₂O₃ and TiO₂, easily induce solid-to-solid transformations with the Fe oxides during continuous cycling. Thus, based on the promoter being used, compounds such as MgFeO₄, FeTiO₃, Ca₂Fe₂O₄, Fe₂SiO₄, and AlFeO₄ can be formed. These compounds are mainly formed during redox reactions, where the metal ferrites become physical barriers. Therefore, they prevent iron oxide sintering, while the oxygen-carrying capacity reduces since the ferrites are strong reducing agents [117,118]. In that regard, their oxygen storage capacity reduces considerably and it becomes difficult for the unreactive compounds to decompose. Significantly, considering that the spinel compounds are formed easily, the addition of the right amounts of Mg to the chemical looping reaction can stop the formation of FeAl₂O₄ [119]. On the other hand, MgAl₂O₄ contributes also through its high chemical and thermal stability, and specific heat capacity. Hafizi et al. [120] compared the performance of Fe₂O₃/Al₂O₃ and Fe₂O₃/MgAl₂O₄ oxygen carriers. It was observed that Mg modification of alumina could improve the performance of the oxygen carrier in chemical looping and sorption-enhanced chemical looping reforming processes. This was attributed to the prevention of FeAl₂O₄ spinel generation and resultant improvement in the activity of oxygen carrier under the influence of Mg by the generation of MgAl₂O₄. Liu et al. [121] observed an appreciable performance for an oxygen carrier based on Fe₂O₃/TiO₂ during 30 redox cycles. The combined incorporation of La and Ce was favorable to improve sintering as well as carbon deposit resistance of Fe₂O₃/Al₂O₃, as demonstrated by the work of Tang et al. [122]. An Fe₂O₃/ZrO₂-ZrO₂ core-shell structured oxygen carrier with enhanced performance in redox activity, sintering resistance, and structural as well as thermal stability was developed by Hu et al. [123] for CO₂ conversion applications.

Perez-Vega et al. [124] developed an oxygen carrier using MnFe₂O₃, which was efficient in terms of reactivity and mechanical properties. Furthermore, the addition of kaolin during the calcination step was beneficial to enhance the reactivity and strength, yet at the expanse of oxygen transport capacity. The applicability of TiO₂-doped MnFeO₃ was studied as well. Enhanced oxygen uncoupling as well as magnetic features for this oxygen carrier under the conditions of both oxidation and reduction were evidenced [125].

CeO₂ promotes iron oxides in chemical looping because of its high reactivity to methane using lattice oxygen [126]. Redox reactions facilitate the transfer of oxygen and its release from the lattice. When Ce⁴⁺ ions are replaced by cations of lower valence, space for oxygen is created at the active sites. After the consumption of surface oxygen, the additional gas is transferred to the surface [127]. In that regard, the presence of CeO₂ increases the reduction of Fe₂O₃. The effectiveness of Fe₂O₃-CeO₂/Al₂O₃ oxygen carrier was studied by Kang et al. [128], and a significant redox activity was evidenced. Also, the generation of syngas with high purity was achieved, together with a molar ratio of 2 for H₂/CO. Hafizi et al. [129] reported sustainable activity for Fe/Ca/Al₂O₃ oxygen carrier upon 15 cycles of oxidation and reduction.

The applicability of the NiFeAlO₄ spinel as a self-sustained oxygen carrier has been investigated by Kuo et al. [130] for the conversion of CO₂ to H₂. A significantly enhanced agglomeration resistance was observed for this oxygen carrier in comparison to other studied systems, such as NiO, Fe₂O₃, and NiFe₂O₄. Notably, the reduction reaction in a fixed bed reactor confirmed enhanced H₂ generation and CO₂ conversion. Zaabout et al. [131] made a performance comparison for (FeNiAl)₃O₄ with Fe₂O₃, and observed that the (FeNiAl)₃O₄ spinel was more efficient with respect to conversion rate and cyclic stability. In another work, Hu et al. [132] reported the use of the NiO/Fe₂O₃/MgAl₂O₄ oxygen carrier for CO₂ conversion via the chemical looping process. Under the conditions of reduction capacity >2 and temperature >873 K, a high extent of Fe₂O₃ reduction was discerned. Regarding the cyclic operations, superior stability was attained for five cycles.

Some attempts using Cu-based oxygen carrier systems have also been made. Huang et al. [133] introduced CuO/Al₂O₃ oxygen carrier exhibiting superior carbon deposition and deactivation resistance for multicycle redox operations, on account of better distribution of CuO on Al₂O₃. Furthermore, an oxygen carrier consisting of CuO, Fe₂O₃, and MgAl₂O₄ has been explored for chemical looping combustion applications [134]. Adanez-Rubio et al. [135] discussed the applicability of CuO/Mn₃O₄ as an oxygen carrier for chemical looping. The lifetime of this oxygen carrier was found to be significant for a large oxygen carrier/fuel ratio.

An oxygen carrier based on hierarchical Co/MgO rich in oxygen sites, activity, thermal reliability, and Co dispersion was reported by Li et al. [136]. Moldenhauer et al. [137] developed the CaMnO₃ oxygen carrier with high potential of cost-effectiveness. In addition, this oxygen carrier had the capability to enable reactivity through effective release of oxygen to the gaseous state, together with superior attrition resistance. Sulfur poisoning was a drawback of this material. Zhu et al. [138] found that Ce doping was more effective to enhance the reactivity, multicycle stability, syngas yield, and CO₂ activation of hexaaluminate oxygen carriers in comparison to doping with other metals, such as Fe, Si, and Zr. In another study, Huang et al. [139] reported appreciable stability for Fe-doped hexaaluminate oxygen carriers after 50 redox cycles.

5. CO₂-Derived Fuels

The conditions of the available CO₂ sources upon capture play an important role in the subsequent separation and conversion technology. In the post-combustion process, gases are obtained when fossil fuels burn where the composition of CO₂ is 3%–20%. In pre-combustion, higher CO₂ levels are typically obtained, reaching 15%–40% at higher temperatures. A shift reaction occurs during the production of synthesis gas using a water–gas combination, where fuel oxy-combustion is established [140].

5.1. Electrolysis

CO₂ can be reduced electrolytically into fuels in analogy to water electrolysis, where oxygen forms at the anode, while the reduction of the gas occurs at the cathode [141]. In this case, the electrolytic process is dependent on the catalyst used in the reaction and can yield products that include methanol, formic acid, ethylene, formaldehyde, and methane. Challenges such as catalyst deactivation, high overpotentials in cells, and limited current densities need to be overcome. Given that the thermodynamic potential of CO₂ is analogous to that of splitting water, the reduction of the gas results in a low faradaic efficiency [142]. Copper, for instance, reduces CO₂ having substantial current densities, workable faradaic efficiency, and cell overpotentials. As a result, hydrocarbons such as ethylene and methane are formed.

Sathre et al. [143] examined CO₂ reduction at high temperatures using solid oxide electrolytic cells (SOEC). The feasibility of reduction at high temperatures is to lower the electrical charge required, while increasing the rate of the electrode reaction [144]. In this case, efficient reduction of CO₂ can occur at approximately 800 °C using perovskites, such as La_0.8Sr_0.2Cr_0.5Mn_0.5O₃ (LSCM), to accelerate electrode kinetics. Pb-ceria composites are also used in the reduction reaction as co-catalysts [144,145]. Interestingly, at high temperatures, the co-electrolysis of water and CO₂ leads to the production of synthetic gas and oxygen at the cathode and anode, respectively. This process can operate with an efficiency of up to 70% [146].

5.2. Thermochemical and Photoelectrochemical Conversions

Thermochemical and photoelectrochemical conversions have received rather less attention compared to SOEC because they are still at an earlier research stage, thus there is still a long way to go to reach large-scale applications [147]. Similar to water splitting, the main challenge of the photochemical approach is the development of suitable photocathodes to facilitate the reduction process [148]. The issue of CO₂ availability in aqueous form also exists because of poor solubility. Nevertheless, investigations focus on non-aqueous solvents and three-phase interfaces (solid/liquid/gas) [149]. In this arrangement, the metallic electrodes should be partially dipped in CO₂ solutions, while the gas is supplied in the vapor stage.

On the other hand, thermochemical reduction is also under investigation using metal oxides [150,151]. For instance, at 1600 °C, ZnO is reduced to Zn using solar energy. Subsequently, at 360 °C, Zn is oxidized by CO₂ forming ZnO and CO. Although the thermochemical approach is viable for the conversion of CO₂ into value-added chemicals, it can be enhanced through the co-reaction between H₂O and CO₂, e.g., using CeO₂, to produce O₂, CO, and H₂ [152]. This reaction starts with the reduction of CeO₂ at temperatures of approximately 1420–1640 °C. Then oxidation using CO₂ and H₂O follows at approximately 900 °C. While the predicted solar-to-fuel conversion ranges between 16%–19%, only about 8% is realized experimentally [153]. Heat loss is also a significant issue in the reaction. Solar-assisted conversion using hydrogen is also under investigation [154]. For instance, synthetic gas is made by the reaction of hydrogen and CO, while the hydrogenation of CO₂ creates other fuels that are easy to distribute, store, and use.

6. Photon Capture

The formation of solar fuels starts with the identification and development of the proper materials for photon capture [154,155]. Suitable materials as solar absorbers used in electrochemical reactions include oxynitrides, carbon, nitrides, oxides, and carbon nitrides [156,157]. For photoelectrochemical fuel production, the absorbers should have the proper band gap and structure to use most of the available solar radiation, while they should also exhibit sufficient charge transport efficiency and stability. Furthermore, the optimum materials should be non-toxic, inexpensive, and abundant. In one case, metal deposition was conducted on thin print spots to target n-type porous Fe-based binary oxides and p-type Bi-based semiconductors. It was determined that FeVO₄ and CuBi₂O₄ were suitable visible-light semiconductors, while Bi-Cu and Bi-Ag-Cu composite systems also showed significant photocurrent capabilities.

Further assessments of suitable materials for water splitting have evaluated sulfides, fluorides, metal oxides, and their combinations. MnV₂O₆, in particular, has been experimentally proven to bear desirable water splitting properties [158,159,160]. Recent evidence also suggests that organometal halide perovskites have the potential of being efficient photo-absorbers, while they are formed from naturally occurring materials. As research continues with the development of suitable photo-absorbers and investigation of their structure–property relationships, an additional challenge that needs to be addressed is the stability under photoelectrochemical conditions.

7. Artificial Photosynthesis

In recent years, the production of solar fuels using artificial photosynthesis is under intense investigation, where CO₂ is reduced and the energy produced is stored in a chemical form [161]. The objective has been the scalable production of systems that can reduce CO₂ using water and solar energy to generate electrons [162]. To this end, ZnO/ZnTe/CdTe triple-layered gold-plated core photocathode and a perovskite solar cell containing CH₃NH₃PbI₃ have been employed. It was determined that light harvesting achieves high energy photons (>2.14 eV) at the photocathode. The utilization of perovskite halides in photovoltaic processes has attracted attention because these materials are abundant, have high climbing efficiencies, and have a relatively low cost of synthesis.

Photochemical CO₂ conversion to value-added chemicals is realized using homogeneous and heterogeneous systems, where CO₂ is reduced at photovoltaic junctions [163]. In that regard, thermodynamics forms the fundamentals of reduction using photovoltaic cells. The challenge in such utilization of photovoltaics is the development of abundant earth materials, which will also be non-toxic and affordable, and they will constitute sunlight-stable photovoltaic compounds [163,164,165]. In a prototype based on an imidazolium salt derivative developed by Toshiba, the conversion of CO₂ to ethylene glycol was realized at high efficiency and without harmful byproducts [164]. In this regard, molecular interactions between the catalysts coated on a metal surface, where CO₂ molecules are reduced, played a dominant role in the efficiency. Most of the currently available photomolecular technologies use two-electron reduction-conversion processes. Toshiba’s new molecular catalysts, however, employ multiple conversion where ethylene glycol is produced during reduction. It was determined that the efficiency of the catalysts can reach up to 80% [23]. Toshiba is further investigating the development of molecular catalysts to improve their application in artificial photosynthesis.

Extensive studies are being conducted on polar materials to understand their potential as semiconductors to be used in photovoltaic applications [165,166]. In particular, the movement of dipoles in these materials is activated by sunlight, and as a result, charge carriers are generated. These materials comprise ferroelectrics, which are determined to exhibit pyro- and piezo-electricity. Research on these materials was enhanced with the discovery of photocurrent generation using para-electric BaTiO₃. The detection of the band gap in cadmium telluride including zinc sulfide was also a contributing factor. Nevertheless, most of the research has concentrated on the photovoltaic effect in non-centrosymmetric crystals, also called the galvanic effect [167]. The ferroelectric phenomena refer to the production of steady streams of current in short circuits.

The initial model for the bulk photoelectric effect utilized asymmetric materials; however, recent developments have seen the adaptation of theories on shift currents [168]. To this end, the photoelectric concept could be explained from a microscopic perspective using the shift and ballistic mechanism models. Firstly, the ballistic model for anisotropic and isotropic materials is centrosymmetric utilizing the p–n junctions in solar cells [169]. The model projects itself when the photoelectric materials are exposed to sufficient thermal illumination after which the valence bands get excited. As a result, a photocurrent is generated through the conduction band. During this process, the carriers lose energy and therefore move to the conduction band’s bottom via band-to-band transitions. This movement is meant to create equilibrium and thus continue the production of the bulk photocurrent [170]. Notably, the current shift model is characterized by a quantum mechanical nature, which implies that coherent excitations cause the thermal carriers.

Considering hydrogen production, artificial photosynthesis provides the means for efficiently converting solar energy to hydrogen [171,172]. To this end, powered water electrolysis using photovoltaics is a promising approach, yet the technique often suffers from low solar-to-fuel conversion efficiencies. Solar thermochemical conversion, on the other hand, offers a relatively high theoretical efficiency and can support large-scale production due to utilization of the vast solar spectrum.

To this extent, versatility in the preparation of various perovskite configurations is important. For example, useful information can be extracted from the development of perovskites based on metal halides, which have gained attention for the production of solar cells [173]. These materials have high potential, for instance, emitting bright light in diodes with considerable efficiency, having therefore the capability to impact the electronics industry, where both inorganic and organic materials can be combined to a composite [174,175]. The high conduction ability of perovskites can also be leveraged and used in making polycrystalline films. The ability to efficiently fabricate such structures can allow the implementation of solution-processing techniques to create high-performance systems [174]. Yet, most of the methods used cannot be easily consolidated for large-scale production [24]. Recent studies have explored vapor-based methods deeming them as technologies that can be adopted in the semiconductor and coatings industry for large-scale implementation.

8. Solar Fuels

Developments in green electricity production and electric transportation can play a vital role in reducing the emission levels of greenhouse gases [176]. It is projected that by 2050, the levels of greenhouse emissions will be reduced by approximately 80% following the adoption of alternative fuels. Indicatively, the White House aims to promote clean energy to drastically cut greenhouse gas pollution by 2035 [177]. Therefore, given the fact that the global consumption of electricity is being increased, and taking into account the lack of efficient means of storage, the development of alternative energy is imperative and important in order to ensure sustainability. In addition, more energy is required to propel industries, such as manufacturing and transportation, within the continuous digitalization transformation [178]. Solar energy is classified as a carbon-free source and it has the highest potential in replacing the consumption of fossil fuels. Various challenges have to be resolved to utilize it competitively. Firstly, ample energy storage solutions should be developed considering the intermittent nature of the resource. Secondly, plans on how to achieve the production of solar energy at a large scale to be used in transportation and other sectors should be identified. Solar energy currently accounts for a small fraction of the energy supply to grid systems [179,180,181]. Unfortunately, a setback is its intermittent supply. Production, however, can be supplemented by wind in places with less sunlight. According to the USA Energy Information Administration, it is expected that the production of solar energy will increase by 5%–7% yearly [182,183,184]. Yet, a challenge is to develop, in parallel, efficient energy storage systems. Most importantly, such systems should be carbon-neutral and robust to allow grid stability under the future demands [185]. Indicatively, if a 75% efficiency of electrolytic cells is merged with that of a cycle steam and a gas turbine generator using hydrogen at 60%, the result would be 45% efficiency [186,187]. Yet, the storage capacity for hydrogen and batteries cells should be considerably optimized.

Hydrocarbons, on the other hand, have high energy densities and can be synthesized from CO₂ (Figure 6) [188]. Natural gas synthesis can be feasible on a large scale using CO₂ and hydrogen with an efficiency in the order of 80%. The product can have an energy density that is three times higher that of the naturally occurring product. Therefore, considering the pivotal role of chemical fuels in electricity generation, solar energy conversion can be leveraged to create a desirable distribution [189,190]. Synthetic gas can be stored and supplied to the consumers to power heating and vehicle systems. Indicatively, in Germany, the storage capacity of natural gas exceeds 200 TWh. The country can supply this amount for several months. In addition, with the production of synthetic gas, the applications of the resource could extend, for instance, in fertilizer production, transportation, and pharmaceuticals [188,191,192].

Over the years, experts have tested the potential of solar fuels in the production of clean energy. Hydrogen, as an elemental fuel, has desirable attributes necessary for the production of clean energy; yet, it lacks a high volumetric density, and as a result, it cannot be easily stored or distributed [140,193,194,195,196]. On the other side, conversion of CO₂ to CH₄ attracts significant attention, while the reaction between CO₂ and H₂ also produces fuel that is easy to distribute, store, and use in existing supply infrastructures (Figure 6) [188]. On that note, the concept of converting CO₂ using solar energy is viable, thus generating solar fuels from green sources exhibiting sufficient energy densities [107]. Photocatalytic and photothermal applications have the potential of achieving considerable energy conversion efficiencies and commercial viability [140]. At present, the contribution of artificially generated solar fuels remains small, while efforts on reducing production costs need to be intensified, while challenges in the CO₂ capture and sourcing front as well as in enhancing the efficiency of solar-assisted conversion of CO₂ should be considered. The objective is to reduce production costs, extend the lifetime of the systems, and invest in infrastructural developments. These efforts will help bridge the gap between deployable technologies and current laboratory work.

Table 1 summarizes literature that reported perovskite catalysts for solar thermochemical CO₂ conversion applications. Sr doping is a suitable method to enrich the CO yield of the LaMnO_3-δ perovskite catalyst as highlighted by the work of Yamazaki et al. [197]. Demont et al. [198] indicated a two-step cycling for the redox activity of non-stoichiometric La_1−xSr_xMnO_3−δ upon solar-to-fuel conversion of CO₂. Nair et al. [199] supported the superior CO production efficiency of La_xSr_1−xMnO₃ perovskite catalysts, along with interesting reliability, by pointing out the benefit of Mn influence. An appreciable CO yield of 133.9 μmol/g was observed for the La_0.5Sr_0.5MnO₃ perovskite at 1673 K, as suggested by the study of Dey et al. [200]. At the same time, 1.8-fold higher CO yield was achieved by the Y_0.5Sr_0.5MnO₃ perovskite compared to La_0.5Sr_0.5MnO₃.

Upon CO₂ splitting, Nair et al. [201] observed superior reactivity for the La_xSr_1−xMnO₃ perovskite prepared using the Pechini approach in comparison to solid-state, glucose-assisted, and glycine combustion approaches. Riaz et al. [202] developed a lanthanum strontium manganite perovskite-based catalyst using electrospinning. Improved redox kinetics for this catalyst, together with an appreciable CO yield of 854.20 μmol/g per redox cycle consisting of reduction at 1400 °C and oxidation at 1000 °C, was reported. Furthermore, the electrospun catalyst was rich in structural stability.

Carrillo et al. [203] reported the effectiveness of Cr infusion to significantly enhance CO production kinetics using the La_0.6Sr_0.4MnO₃ perovskite. Likewise, good cyclic stability was offered through the prevention of structural decay. Bork et al. [204] also pointed out enhanced efficiency and fuel yield of the La_0.6Sr_0.4Mn_0.2Cr_0.8O_3−δ perovskite catalyst for solar-to-fuel applications. Luciani et al. [205] highlighted the possibility of Fe infusion to boost the redox characteristics of the La_0.6Sr_0.4MnO₃ perovskite. Although, the CO yield was observed to reduce with Fe content. Ezbiri et al. [206] reported significant redox activity for an Al-doped form of the La_1−xSr_xMn_1−yAl_yO_3−δ perovskite catalyst.

Cooper et al. [207] revealed the feasibility of Ca or Sr (A-site) as well as Al (B-site) doping to significantly improve the reduction rate of lanthanum manganite perovskite catalysts. Compared to reference CeO₂, values up to 13 times higher were achieved for these doped lanthanum manganite perovskite forms, i.e., La_0.6Sr_0.4Mn_0.6Al_0.4O₃ and La_0.6Ca_0.4Mn_0.6Al_0.4O₃. Sastre et al. [208] reported great redox properties with acceptable cyclic performance for La_0.6Sr_0.4Mn_1−xAl_xO₃ perovskite. The extent of reduction was increased with Al content, while the oxidation extent was found optimal at a content of Al_x = Al_0.5.

Bork et al. [209] observed an almost 25-fold higher CO₂ splitting ability for the La_0.6Sr_0.4Cr_1−xCo_xO_3−δ perovskite compared to ceria. By the use of this catalyst, the added benefit of reduced solar-to-fuel reactor temperature of approximately 300 °C was reported. This catalyst also demonstrated good cyclic performance. Hare et al. [210] revealed the effectiveness of silica support to enhance CO production of the La_0.75Sr_0.25FeO₃ perovskite catalyst. Accordingly, an improvement of approximately 200% was observed at a relatively low temperature. This was attributed to silica-promoted surface area increase of the perovskite [211].

Hare et al. [212] prepared a La_xCa_1−xFe_yMn_1−yO₃ perovskite catalyst and disclosed that the combinational influence of Ca and Fe could enrich structural stability. Ramos et al. [213] examined the role of B-site transition metals, such as Co, Fe, and Mn, in LaCo_xFe_yMn_1−x−yO₃ for low temperature solar-to-CO conversion. The surface segregation of Co and Fe contributed to the activity of this catalyst.

Dey et al. [214] reported enhanced CO production for a catalyst based on Y_0.5Sr_0.5MnO₃ at comparatively low redox temperature conditions on account of highly disordered/distorted structural influence. Rao and Dey [215] acquired a CO yield of 757 µmol/g at 1173 K by the use of the Y_0.5Sr_0.5MnO₃ perovskite. At 1673 K, an O₂ release of 483 µmol/g was observed for this catalyst. Takalkar et al. [216] reported the beneficial effect of reduced Pr, as well as increased Sr content, to enhance the O₂ release extent and CO yield of the Pr_xSr_(1−x)MnO_3−δ perovskite catalyst for solar-to-fuel conversion of CO₂. Compared to CeO₂, a considerably higher CO generation efficiency was reported for the Pr_0.5Sr_0.5MnO_3−δ perovskite material [217].

Table 1. Perovskite catalysts in solar thermochemical CO₂ conversion.

Catalyst	Reactor	Reduction Temperature (°C)	Reducing Atmosphere	Oxidizing Atmosphere	CO Yield (μmol/g)	O2 Yield (μmol/g)	Reference
LaMn0.5Co0.5O3	TGA	1400	<2 ppm O2 premixed Ar	50% CO2/Ar	145	83	Nair et al. [199]
Ba0.5Sr0.5FeO3	TGA	1000	<2 ppm O2 premixed Ar	50% CO2/Ar	136	582	Nair et al. [199]
La0.5Sr0.5MnO3	TGA	1400	<2 ppm O2 premixed Ar	50% CO2/Ar	269	248	Nair et al. [199]
La0.5Sr0.5MnO3	TGA	1500	10 ppm O2 premixed N2	1 atm CO2	230	111.6	Dey et al. [200]
Y0.5Sr0.5MnO3	TGA	1400	10 ppm O2 premixed N2	1 atm CO2	196.4	108	Dey et al. [200]
La0.6Sr0.4Mn0.6Al0.4O3	TGA	1400	<2 ppm O2 premixed Ar	50% CO2/Ar	205	210	Nair et al. [201]
La0.25Sr0.75MnO3	TGA	1400	N2	CO2/air	786.32 ± 41.6	516.24 ± 19.55	Riaz et al. [202]
La0.6Sr0.4Cr0.75Mn0.25O3	TGA	1400	Ar	0.5 atm CO2	~7 *	~1.2 *	Carrillo et al. [203]
La0.6Sr0.4MnO3	TGA	1350	N2	CO2	469.1	348.8	Luciani et al. [205]
La0.6Sr0.4Mn0.8Fe0.2O3	TGA	1350	N2	CO2	329.9	286.0	Luciani et al. [205]
La0.6Ca0.4Mn0.6Al0.4O3	TGA	1375	3 × 10−5 bar O2 mixed Ar	50% CO2/Ar	ca. 420	-	Cooper et al. [207]
La0.6r0.4Mn0.6Al0.4O3	Fixed bed reactor	1250	Ar	5% CO2/He	114	266	Sastre et al. [208]
La0.6Sr0.4Cr0.8Co0.2O3−δ	TGA	1200	Ar	50% CO2/Ar	157	-	Bork et al. [209]
SiO2-supported La0.75Sr0.25FeO3	Quartz microreactor connected with Cirrus MKS MS	950	10% H2/He	10% CO2/He	1700	-	Hare et al. [211]
La0.75Ca0.25MnO3	Quartz microreactor connected with Cirrus MKS MS	950	10% H2/He	10% CO2/He	1680	-	Hare et al. [212]
La0.5Ca0.5Fe0.25Mn0.75O3	Quartz microreactor connected with Cirrus MKS MS	600	10% H2/He	10% CO2/He	1450	-	Hare et al. [212]
LaCo0.5Fe0.25Mn0.25O3	Quartz U-tube reactor connected with Cirrus MKS MS	550	10% H2/He	10% CO2/He	1780	-	Ramos et al. [213]
Y0.5Sr0.5MnO3	TGA	1400	Ar	40% CO2/Ar	757	483	Dey et al. [214]
Y0.5Ca0.5MnO3	TGA	1400	Ar	40% CO2/Ar	671	575	Dey et al. [214]
Pr0.18Sr0.80Mn0.99O2.951	TGA	1400	Ar	CO2/Ar	637.6	255.0	Takalkar et al. [216]
La0.5Sr0.5MnO3−δ	TGA	1400	Ar	50% CO2/Ar	338.7	684.3	Takalkar et al. [217]

Note: * means that the unit is in mL/g. TGA means thermogravimetric analyzer.

Table 1. Perovskite catalysts in solar thermochemical CO₂ conversion.

Catalyst	Reactor	Reduction Temperature (°C)	Reducing Atmosphere	Oxidizing Atmosphere	CO Yield (μmol/g)	O2 Yield (μmol/g)	Reference
LaMn0.5Co0.5O3	TGA	1400	<2 ppm O2 premixed Ar	50% CO2/Ar	145	83	Nair et al. [199]
Ba0.5Sr0.5FeO3	TGA	1000	<2 ppm O2 premixed Ar	50% CO2/Ar	136	582	Nair et al. [199]
La0.5Sr0.5MnO3	TGA	1400	<2 ppm O2 premixed Ar	50% CO2/Ar	269	248	Nair et al. [199]
La0.5Sr0.5MnO3	TGA	1500	10 ppm O2 premixed N2	1 atm CO2	230	111.6	Dey et al. [200]
Y0.5Sr0.5MnO3	TGA	1400	10 ppm O2 premixed N2	1 atm CO2	196.4	108	Dey et al. [200]
La0.6Sr0.4Mn0.6Al0.4O3	TGA	1400	<2 ppm O2 premixed Ar	50% CO2/Ar	205	210	Nair et al. [201]
La0.25Sr0.75MnO3	TGA	1400	N2	CO2/air	786.32 ± 41.6	516.24 ± 19.55	Riaz et al. [202]
La0.6Sr0.4Cr0.75Mn0.25O3	TGA	1400	Ar	0.5 atm CO2	~7 *	~1.2 *	Carrillo et al. [203]
La0.6Sr0.4MnO3	TGA	1350	N2	CO2	469.1	348.8	Luciani et al. [205]
La0.6Sr0.4Mn0.8Fe0.2O3	TGA	1350	N2	CO2	329.9	286.0	Luciani et al. [205]
La0.6Ca0.4Mn0.6Al0.4O3	TGA	1375	3 × 10−5 bar O2 mixed Ar	50% CO2/Ar	ca. 420	-	Cooper et al. [207]
La0.6r0.4Mn0.6Al0.4O3	Fixed bed reactor	1250	Ar	5% CO2/He	114	266	Sastre et al. [208]
La0.6Sr0.4Cr0.8Co0.2O3−δ	TGA	1200	Ar	50% CO2/Ar	157	-	Bork et al. [209]
SiO2-supported La0.75Sr0.25FeO3	Quartz microreactor connected with Cirrus MKS MS	950	10% H2/He	10% CO2/He	1700	-	Hare et al. [211]
La0.75Ca0.25MnO3	Quartz microreactor connected with Cirrus MKS MS	950	10% H2/He	10% CO2/He	1680	-	Hare et al. [212]
La0.5Ca0.5Fe0.25Mn0.75O3	Quartz microreactor connected with Cirrus MKS MS	600	10% H2/He	10% CO2/He	1450	-	Hare et al. [212]
LaCo0.5Fe0.25Mn0.25O3	Quartz U-tube reactor connected with Cirrus MKS MS	550	10% H2/He	10% CO2/He	1780	-	Ramos et al. [213]
Y0.5Sr0.5MnO3	TGA	1400	Ar	40% CO2/Ar	757	483	Dey et al. [214]
Y0.5Ca0.5MnO3	TGA	1400	Ar	40% CO2/Ar	671	575	Dey et al. [214]
Pr0.18Sr0.80Mn0.99O2.951	TGA	1400	Ar	CO2/Ar	637.6	255.0	Takalkar et al. [216]
La0.5Sr0.5MnO3−δ	TGA	1400	Ar	50% CO2/Ar	338.7	684.3	Takalkar et al. [217]

Note: * means that the unit is in mL/g. TGA means thermogravimetric analyzer.

Mulmi et al. [218] used a non-stoichiometric Ba₂Ca_0.66Nb_1.34−xFe_xO_6−δ perovskite for thermochemical CO generation. An increase of CO generation with Fe content was observed. A perovskite photocatalyst comprising mixed Fe oxides was developed by Zhang et al. [219]. Notably, almost complete CO₂ to CO conversion was reported in the temperature range of 900–980 °C.

A literature summary of various CO₂ conversion perovskite photocatalysts is presented in Table 2. Xu et al. [220] reported the applicability of the CsPbBr₃ perovskite quantum dot photocatalyst for solar fuel generation from CO₂. A CO₂ reduction rate of 23.7 μmol/g/h was achieved using this photocatalyst. Moreover, a selectivity of more than 99.3% was discerned. Interestingly, the incorporation of graphene oxide (GO) was beneficial to exceed the electron consumption rate by 25%. Hou et al. [221] also reported analogous observations for CsPbBr₃ perovskite quantum dots with a dimension of 3–12 nm. A photocatalyst developed using a mixed halide perovskite, CsPb(Br_0.5/Cl_0.5)₃, exhibited high performance in terms of efficiency and CO selectivity [222]. Zhou et al. [223] explored the possibility of the Cs₂AgBiBr₆ perovskite for photochemical conversion of CO₂ to solar fuels. For this catalyst, an electron consumption of approximately 105 µmol/g was noted down.

A composite photocatalyst comprising CsPbBr₃ perovskite nanocrystals and Pd nanosheets was developed by Xu et al. [224] for CO₂ conversion. With pure CsPbBr₃ perovskite nanocrystals, approximately 2.4 times higher electron utilization efficiency was achieved by the incorporation of Pd nanosheets. In another study, Wan et al. [225] developed a CsPbBr₃ perovskite quantum dots/UiO-66 (NH₂) nanocomposite photocatalyst exhibiting improved CO₂ reduction activity as well as CO selectivity. Wang et al. [226] prepared a solar-to-CO₂ reduction catalyst by decorating the PbBiO₂Br perovskite with carbonized polymer dots in the presence of a self-sacrificing ionic liquid reaction source. The resultant composite photocatalyst exhibited advantages in CO yield, reliability, and selectivity.

Shan et al. [227] used a boron-doped version of SrTiO₃ for photocatalytic conversion of CO₂. It was reported that the photocatalytic activity of boron-doped SrTiO₃ enhanced by almost 3-fold by the improvement of charge separation potential. Ag infusion was found beneficial to enhance CO generation of the H₂SrTa₂O₇ perovskite photocatalyst, according to the research conducted by Wang et al. [228], owing to reduced reaction overpotential. Accordingly, a CO selectivity of approximately 70% was noted after the infusion of 0.5 wt% Ag. A computational analysis conducted by Hafez et al. [229] supported the suitability of a series of oxynitride perovskites, such as LaTiO₂N, TaO₂N with Ba, Sr, or Ca, and NbO₂N with Ba or Sr.

A photocatalyst based on sodium tantalate (V_O-NaTaON) nanocubes exhibiting concurrently enhanced light absorption and charge separation ability was reported by Hou et al. [230]. Consequently, a broad spectrum of photoreduction of CO₂ to fuels was observed for this photocatalyst.

Jang et al. [162] developed a tandem solar cell consisting of the CH₃NH₃PbI₃ perovskite and ZnTe photocathode for the selective conversion of CO₂. Notably, a conversion efficiency of more than 0.35% and 0.43% was observed for solar-to-CO and solar-to-fuel conversions, respectively. Wang et al. [231] developed a photocatalyst possessing improved solar-to-fuel conversion performance by graphene oxide decoration of the CH₃NH₃PbBr₃ perovskite. As a result, a significant CO yield was obtained on account of enhanced electron extraction and transfer efficiency.

Table 2. Perovskite photocatalysts for CO₂ conversion.

Catalyst	Light Source	Solvent	Products	CO Yield (μmol/g)	CH4 Yield (μmol/g)	H2 Yield (μmol/g)	Reference
CsPbBr3 quantum dots (3.05–8.65 nm)	100 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4, H2	49.5	22.9	1.07	Xu et al. [220]
CsPbBr3 quantum dots/GO composite	100 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4, H2	58.7	29.6	1.58	Xu et al. [220]
CsPbBr3 quantum dots (3–12 nm)	300 W Xe lamp with an AM 1.5 G filter	Ethyl acetate/H2O	CO, CH4, H2	34.1 ± 0.1	12.2 ± 0.1	0.80 ± 0.03	Hou et al. [221]
CsPb(Br0.5/Cl0.5)3 nanocrystals	300 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4	ca. 750	ca. 125	-	Guo et al. [222]
Cs2AgBiBr6 nanocrystals	100 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4	14.1	9.6	-	Zhou et al. [223]
CsPbBr3 nanocrystals/Pd nanosheet composite	150 W Xe lamp with with a 420 nm optical filter	H2O	CO, CH4, H2	12.633	3.935	1.167	Xu et al. [224]
CsPbBr3 quantum dots/UiO-66 (NH2) composite	300 W Xe lamp with a 420 nm UV-cut filter	Ethyl acetate/H2O	CO, CH4	98.57	3.08	-	Wan et al. [225]
PbBiO2Br/carbonized polymer dots composite	300 W Xe lamp	H2O	CO	ca. 48	-	-	Wang et al. [226]
Boron-doped SrTiO3	300 W Xe arc lamp	H2O	CO, CH4, O2	21 *	14 *	-	Shan et al. [227]
Ag-loaded H2SrTa2O7	300 W Xe lamp with λ > 200 nm	H2O	CO, H2	0.39 *	-	0.25 *	Wang et al. [228]
Sodium tantalate nanocubes	300 W Xe lamp with λ > 400 nm	H2O	CO, CH4	75	26	-	Hou et al. [230]
N-doped graphene quantum dots-grafted sodium tantalate nanocubes	300 W Xe lamp with λ > 400 nm	H2O	CO, CH4	180	45	-	Hou et al. [230]

Note: * means that the unit is in μmol/h/g.

Table 2. Perovskite photocatalysts for CO₂ conversion.

Catalyst	Light Source	Solvent	Products	CO Yield (μmol/g)	CH4 Yield (μmol/g)	H2 Yield (μmol/g)	Reference
CsPbBr3 quantum dots (3.05–8.65 nm)	100 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4, H2	49.5	22.9	1.07	Xu et al. [220]
CsPbBr3 quantum dots/GO composite	100 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4, H2	58.7	29.6	1.58	Xu et al. [220]
CsPbBr3 quantum dots (3–12 nm)	300 W Xe lamp with an AM 1.5 G filter	Ethyl acetate/H2O	CO, CH4, H2	34.1 ± 0.1	12.2 ± 0.1	0.80 ± 0.03	Hou et al. [221]
CsPb(Br0.5/Cl0.5)3 nanocrystals	300 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4	ca. 750	ca. 125	-	Guo et al. [222]
Cs2AgBiBr6 nanocrystals	100 W Xe lamp with an AM 1.5 G filter	Ethyl acetate	CO, CH4	14.1	9.6	-	Zhou et al. [223]
CsPbBr3 nanocrystals/Pd nanosheet composite	150 W Xe lamp with with a 420 nm optical filter	H2O	CO, CH4, H2	12.633	3.935	1.167	Xu et al. [224]
CsPbBr3 quantum dots/UiO-66 (NH2) composite	300 W Xe lamp with a 420 nm UV-cut filter	Ethyl acetate/H2O	CO, CH4	98.57	3.08	-	Wan et al. [225]
PbBiO2Br/carbonized polymer dots composite	300 W Xe lamp	H2O	CO	ca. 48	-	-	Wang et al. [226]
Boron-doped SrTiO3	300 W Xe arc lamp	H2O	CO, CH4, O2	21 *	14 *	-	Shan et al. [227]
Ag-loaded H2SrTa2O7	300 W Xe lamp with λ > 200 nm	H2O	CO, H2	0.39 *	-	0.25 *	Wang et al. [228]
Sodium tantalate nanocubes	300 W Xe lamp with λ > 400 nm	H2O	CO, CH4	75	26	-	Hou et al. [230]
N-doped graphene quantum dots-grafted sodium tantalate nanocubes	300 W Xe lamp with λ > 400 nm	H2O	CO, CH4	180	45	-	Hou et al. [230]

Note: * means that the unit is in μmol/h/g.

9. Challenges and Future Directions

Perovskite materials show great promise for CO₂ conversion applications, yet several challenges need to be resolved for large-scale implementation. For example, perovskite cells currently make use of only a small portion of the materials’ active sites, thus more efforts on developing large surface area systems using various nanotechnology-based engineering approaches are needed. The design of perovskites providing sustainable performance has also been a challenge. Under thermal stress, in particular, it is imperative for these materials to maintain optimum performance for the desired duration. For instance, solar cells should operate efficiently at temperatures ranging from −40 to +85 °C for long periods of time. On that note, standardized testing protocols for perovskites are necessary. Currently, stability tests may offer valuable clues on the tolerance levels of materials under consideration [148], yet more accurate and reproducible results are required to venture into large-scale production.

New and optimized systems of perovskites with more compositions are also needed to overcome chemical stability and sustained performance issues at high temperatures in CO₂ and H₂O environments. Studies have shown that the ideal perovskite-based solar conversion systems should have fast kinetics, and the “right” thermodynamics to yield in high efficiency during water and CO₂ conversion, while maintaining their mechanical stability. Entropy considerations should be given particular attention, while further comprehension of the tenability degree of perovskites is required using approaches such as computational thermodynamics [154]. The objective is to gain insight about the effects of doping on entropy. Future studies on this matter need to also use approaches such as density functional theory, where the outcome is anticipated to be helpful in the prediction of thermodynamic attributes.

This review also discussed the potential of oxygen carriers in chemical looping, during which a two-step process is carried out, where the selected metal oxides undergo reduction and oxidation; thus, in this case, it is evident that oxides with a high affinity for oxygen are desirable. The objective is to optimize the perovskite materials so that they exhibit stable regeneration capability and achieve sustainable efficiency, while further investigations are required to determine reaction mechanisms and to show mechanical stability for use in industrial chemical looping combustion reactors.

Finally, the use of toxic components in perovskites has been a major concern. Investors require that the issue of material toxicity should be addressed, now that perovskite-based photovoltaic technologies are steadily penetrating various markets. With the growing demand for clean energy, a comprehensive assessment of perovskites is required to determine the most highly performing and least toxic ones. Similarly, the solvents used in the redox reactions should have low toxicity as well. In addition, metals such as silver and gold should be avoided if possible, in order to develop cost-effective systems. Instead, carbon electrodes should be explored as they have proven to be a reliable alternative [232,233,234].

10. Conclusions

The anthropogenic CO₂ emissions in the atmosphere due to the consumption of fossil fuels has been a critical concern associated with global warming. To this end, the introduction of novel conversion technologies for green production of fuels is expected to suppress the effects of CO₂ accumulation in the atmosphere and simultaneously help meet the globally increasing energy demand. This review presents progress on the use of perovskites for CO₂ conversion and how properties and structural variations can impact the related processes and final products. Current developments reveal promise for these materials, yet also motivate for further studies, particularly with focus on enhancing efficiency, reactivity, selectivity, and stability at high temperatures, reducing cost, increasing throughput, lowering toxicity, reducing energy consumption, establishing reproducibility, and ensuring longer lifetime.

The exploration of CO₂ conversion has led to the discovery or adaptation of useful technologies, such as chemical looping. This process has the potential of producing clean energy through the ability to capture and reduce CO₂; thus, upon large-scale implementation of the technology, economies can have the chance to realize sustainable energy. This realization is heavily based on material development and nano-engineering with particular efforts dedicated toward optimization of oxygen carriers to significantly enhance efficiency and sustainable performance. The optimization of photovoltaic materials is also underway for application in solar cells, where they catalytically produce photoelectric currents. Yet, the quantity of electricity produced needs to be considerably increased, so that the novel solar cells become competitive and head toward commercialization. To further expand the applicability of perovskite materials for energy and environmental applications, issues such as toxicity need to be addressed as well. Toxicity can be caused inherently by one or more of the components of the perovskite material, such as Pb, as well as the used solvents during operation.

Overall, aside from the challenges experienced in the utilization of perovskites, there are many opportunities to exploit. For instance, the production of hydrogen through water photolysis produces a clean and reliable form of energy, which can be scaled up using photoelectric generators in the solar cells, while the extra heat produced in solar cell operations can be tapped for electricity production. Although R&D continues to discover new applications of perovskites that make these materials more valuable, the biggest challenge is breaking the scalability limitations to fully realize their potential for the production of fuel and valuable chemicals that will significantly contribute toward meeting the future energy, environmental, and sustainability demands.