Cerium-Doped Strontium Ferrate Perovskite Oxides: Sustainable Materials to Face Energy and Environmental Challenges

Abstract

Facing energy and environmental issues is recognized globally as one of the major challenges for sustainable development, to which sustainable chemistry can make significant contributions. Strontium ferrate-based materials belong to a little-known class of perovskite-type compounds in which iron is primarily stabilized in the unusual 4+ oxidation state, although some Fe³⁺ is often present, depending on the synthesis and processing conditions and the type and amount of dopant. When doped with cerium at the Sr site, the SrFeO_3−δ cubic structure is stabilized, more oxygen vacancies form and the Fe⁴⁺/Fe³⁺ redox couple plays a key role in its functional properties. Alone or combined with other materials, Ce-doped strontium ferrates can be successfully applied to wastewater treatment. Specific doping at the Fe site enhances their electronic conductivity for use as electrodes in solid oxide fuel cells and electrolyzers. Their oxygen storage capacity and oxygen mobility are also exploited in chemical looping reactions. The main limitations of these materials are SrCO₃ formation, especially at the surface; their low surface area and porosity; and cation leaching at acidic pH values. However, these limitations can be partially addressed through careful selection of synthesis, processing and testing conditions. This review highlights the high versatility and efficiency of cerium-doped strontium ferrates for energy and environmental applications, both at low and high temperatures. The main literature on these compounds is reviewed to highlight the impact of their key properties and synthesis and processing parameters on their applicability as sustainable thermocatalysts, electrocatalysts, oxygen carriers and sensors.

1. Introduction

Energy and environmental-related issues have attracted increasing attention in the last decade. Indeed, pollution, water scarcity, waste management, climate change and, consequently, the green transition towards renewable energies are among the most pressing challenges for humankind [1,2]. In this context, sustainable materials have a predominant role, from the synthesis and properties to the final application. For example, sustainable materials have been developed to improve the efficiency and performance of renewable energy technologies such as solar cells, wind turbines, fuel cells and rechargeable batteries [3]. Moreover, a range of sustainable materials has been applied for environmental purposes, including photocatalysis for water depollution and detoxication, membrane filtration and carbon dioxide sequestration [4,5,6,7]. Sustainable chemistry can substantially contribute to sustainable material production through (i) the discovery of new materials or improvement of existing ones [8,9,10], (ii) the use of waste precursors [11,12,13], (iii) efficient and versatile synthesis and processes [14] and (iv) basic knowledge of synthesis–structure–property relationships [15].

Perovskite oxides have been recognized for more than 30 years as versatile and efficient materials for several energy- and environment-related applications [5,16,17,18,19,20,21]. The perovskite structure was discovered in 1839 by the German scientist Gustave Rose, although the name was given by the Russian mineralogist Count Lev Alekseyevich von Perovski [22]. The first discovered perovskite material was CaTiO₃ with cubic symmetry [22]. In Figure 1, the structure of doped SrFeO₃ is shown as an example of a doped cubic perovskite oxide. A-sites (yellow) are occupied by the larger cations and surrounded by 12 oxygen anions (red). B-sites (black) are occupied by smaller cations and are at the center of the structure, surrounded by six oxygen anions (red). B-sites are connected to A-sites through oxygen bridges. A-sites, B-sites and O-sites can be partially replaced by dopants. Furthermore, multiple doping elements can occur at the same site. This structure is highly versatile and can accommodate oxygen vacancies. The original structure is cubic, but less symmetrical structures are very common, influenced primarily by variations in the ionic radii of the constituent cations/anions [23].

Indeed, to describe the stability of the perovskite structure and the distortion degree from the ideal cubic structure, Goldschmidt proposed the tolerance factor “t” [24]. This geometrical parameter was defined for these compounds in terms of the ionic radii as follows (Equation (1)):

r_A + r_O = t · 2^1/2 · (r_B + r_O)

(1)

where r_A, r_B and r_O are the ionic radii of A and B cations and the O anion, respectively. A t value close to 1 is found for ideal cubic or slightly distorted perovskite-type structures, whereas lower/higher t values are typical of strongly distorted ones; t values too far from 1 are indicative of perovskite-type structures with very low stability. Although perovskite oxides have been constantly studied since the nineteenth century, other perovskites, like hybrid perovskites (for example, CH₃NH₃PbBr₃, with both inorganic cations/anions and organic groups) or metal halide perovskites (CsPbX₃, where X is a halogen group anion), have been intensively studied only in the last ten years for photovoltaic applications, and this research is relatively recent [25,26].

Figure 1. The structure of Ce-doped SrFeO₃ as an example of a doped perovskite oxide structure (realized using VESTA Software version 3 [27]).

Figure 1. The structure of Ce-doped SrFeO₃ as an example of a doped perovskite oxide structure (realized using VESTA Software version 3 [27]).

Among the perovskite materials, doped manganites (SrMnO₃ [28,29], LaMnO₃ [30,31]), ferrites (LaFeO₃ [32,33]), cobaltites (LaCoO₃ [34,35], SrCoO₃ [36,37] and BaCoO₃ [38,39]), plus some titanates and nickelates [40,41,42,43,44], are the most studied in the literature for applications in energy, sensing and environment-related applications.

Concerning iron perovskites, the most common oxidation state is Fe³⁺ [45], according to which iron perovskite oxides are usually called “ferrites”, for example, in the case of LaFeO₃. Among less common iron perovskite oxides, doped SrFeO₃ perovskite oxides have been relatively less investigated, although they have peculiar properties, mainly due to the presence of Fe⁴⁺ at the B-site, according to which SrFeO₃-based compounds are called “ferrates” [46,47] to distinguish them from other Sr- and Fe-containing mixed oxides where iron is mainly present as Fe³⁺ (SrFe₁₂O₁₉ [48] or SrFe₂O₄ [49], for example). Another more specific term for SrFeO₃ (and for BaFeO₃) perovskite oxides is “meta ferrates” [50]. Other Sr- and Fe-containing mixed oxides known as “ferrates” or, more specifically, as “ortho-ferrates” are Sr₂FeO₄ mixed oxides with a K₂NiO₄-type structure [50,51]. Compounds containing Fe(VI) species, usually associated with alkali metals, are called “ferrates” as well, although they are not very stable and tend to transform into Fe(IV) or Fe(III) species [52,53,54]. It is worth noting that Fe(IV) is an unusual oxidation state [21,55] that can be found only in a few compounds as the main oxidation state. Other well-known Fe(IV)-containing perovskite oxides are BaFeO₃ [56] and CaFeO₃ [57,58]. These compounds have several structural features in common, such as the change in crystal structure with the oxygen vacancy content [57] and most catalytic [59] and electrical properties [60]. These similarities are due to the presence of Fe⁴⁺, demonstrating the central role of the iron oxidation state in the perovskite behavior. On the other hand, any difference between CaFeO₃/BaFeO₃ and SrFeO₃ is mainly related to the different ionic radii of the A-site cations.

SrFeO₃-based perovskite oxides have been studied as heterogeneous catalysts for thermochemical water splitting [61], clean energy production [62,63,64,65,66], CO₂ electroreduction [65,67] and water cleaning [68,69,70]. They have also been successfully applied as oxygen carriers for chemical looping reactions [71,72]. Moreover, especially in combination with other compounds, they can be applied as photocatalysts [73] or as hydroxylation catalysts for organic reactions. The type of dopant is able to considerably influence the functional properties of doped-SrFeO₃ perovskite oxides. For example, the effect of B-site doping by Sc was investigated by Yang et al. [65], who prepared, characterized and tested doped SrFeO₃ with the chemical composition Sr_0.9Fe_0.8Sc_0.1Co_0.1O_3−δ as an active and stable electrocatalyst for direct CO₂ electrolysis and power generation in symmetric solid oxide cells, reaching a maximum current density of 1.96 A·cm⁻² at 850 °C. In that work, it was reported that Sc had a positive effect on SrFeO₃ structure stabilization, promoting the transformation of the Co-doped SrFeO₃ from tetragonal to cubic [65]. Another example comes from Su et al. [66], who highlighted a stronger effect of W doping on the stability than on the activity of the electrocatalyst. In that wok, as another important aspect, the effect of the amount of doping was highlighted. If 10 mol% W was able to increment the activity of the electrocatalyst, a 20 mol% W was able to increase its stability, although at the expense of conductivity and activity [66]. The dopant may have also had an indirect effect on the material’s performance, as its introduction forces other cations, for example, iron, to exsolve as a metallic phase during reduction. This is the case described by Zheng et al. [74] for W-doped SrFeO₃. Sr segregation during redox processes can be well blocked by doping, as found by Li et al. [75] for Mo-doped strontium ferrates with SrFe_0.93Mo_0.07O_3−δ chemical composition. Li et al. ascribed the effect of Mo to its strong metal–oxygen bond and to the acidity of the corresponding oxide, MoO₃ [75].

Ce-doped SrFeO₃ compounds are relatively less known than other doped SrFeO₃ compounds, although their scientific and technological importance is rapidly growing, especially for sustainable applications. When strontium ferrates are doped with cerium, structure stability and other relevant properties are improved, and for this reason, these oxides can be used to face energy and environmental challenges [70,76,77,78,79].

In this review article, the literature on cerium-doped strontium ferrates is reviewed, with the aim of highlighting the effect of cerium doping on SrFeO₃ perovskite oxides and the resulting structural versatility of the perovskite oxide’s structure. The main properties and applications of Ce-doped SrFeO₃ are discussed. The methodologies and synthetic procedures used to prepare these compounds are also compared, highlighting the synthesis–structure–property–application relationships for the production of sustainable materials for energy and the environment. Finally, some limitations and challenges of Ce-doped SrFeO₃ are critically examined.

2. Background

2.1. Chemical–Physical Properties of Undoped Strontium Ferrates

SrFeO₃ is a compound with an ABO₃ perovskite-type structure, where A-sites are occupied by Sr²⁺ cations and B-sites by iron in its unusual 4+ oxidation state. Its peculiar properties were recently discussed by Tummino et al. in a review article [46]. Being a perovskite, this compound can keep a consistent number of oxygen vacancies without a structural collapse. The structure of SrFeO₃ in its fully oxygenated status is cubic (Pm-3m) with unit cell parameters of 3.851 Å (ICDD PDF n. 04-007-9970). However, stabilization of the fully oxygenated SrFeO₃ requires high oxygen pressures and special processing conditions [80]. An example of such oxygen sensitivity, taken from the work of Ikeda et al. [81], is shown in Figure 2a–d.

The structure of SrFeO_2.98, the chemical formula of which is usually written as Sr₈Fe₈O₂₃, is tetragonal (I4/mmm ICDD database PDF n 04-011-5465), whereas SrFeO_2.74, the chemical formula of which is usually written as Sr₄Fe₄O₁₁, is a disordered orthorhombic (Cmmm, ICDD database PDF n 04-011-5466), and finally SrFeO_2.5, the chemical formula of which is usually written as Sr₂Fe₂O₅, is an ordered orthorhombic (Ibmm ICDD database PDF n 01-089-8670), also known as “brownmillerite”. From the fully oxygenated structure to the most deficient one, the iron oxidation state decreases gradually from 4+ to 3+/2+. All these structures are stable at ambient conditions and can be obtained after a simple thermal treatment at different temperatures and oxygen partial pressures [80,81]. To this end, in 1992, Misuzaki et al. created an equilibrium phase diagram of the SrFeO_2.5–SrFeO₃ system [82] (Figure 2e) in which the correlations between p(O₂), temperature, δ values and phases are shown. The P/T diagram of Figure 2e also highlights the competition between the cubic phase and the brownmillerite phase. The formation of SrFeO₃ is also sensitive to the SrO-to-Fe₂O₃ ratio, as shown by the temperature-dependent phase diagram in Figure 2f. This diagram was created for the first time by Batti [Ann. Chim. (Rome) Pietro Batti vol. 52, pp. 941–961 (1962) Diagramma d’equilibrio del sistema SrO-Fe₂O₃] and then re-interpreted by other authors [83]. From all these data, it is evident that a distinctive feature of strontium ferrate, with respect to other inorganic perovskites, is the high sensitivity to the oxygen pressure/temperatures used in its synthesis/processing, which can transform it into different perovskite-type structures thanks to the versatility of oxygen atom arrangements around the iron at the B-site (with a corresponding change in the iron oxidation state).

A note on the ordered brownmillerite structure regards the fact that its oxygen transport is less favored than that in disordered structures [84,85]. This may be due to its greater thermodynamic stability, in agreement with Heifets et al. [86], who reported that only SrFeO_2.5, corresponding to the structure with the minimum oxygen content, was actually stable according to thermodynamical calculations. Fierro et al. [87] observed that, among three SrFeO₃ samples with different oxygen contents, the most deficient SrFeO_2.74 sample had an enhanced oxygen desorption rate and also the maximum catalytic activity for methane combustion (Figure 3). Besides oxygen, SrFeO₃ has a high tendency to adsorb CO₂ on the surface, forming SrCO₃ (Figure 3), although it does not seem to be strictly correlated with the oxygen vacancy content. Also, the reduction properties of SrFeO₃ are directly related to the oxidation state of iron, which, in turn, depends on the oxygen content in the perovskite (Figure 3).

Electronic properties of SrFeO₃ were studied by various authors, since this compound is different from other iron perovskite oxides like LaFeO₃ due to the presence of Fe⁴⁺ as the main cation at the B-site [58,88,89].

According to Jia et al. [https://pubs.rsc.org/en/content/articlelanding/2017/ra/c7ra06542f, accessed on 26 July 2025], SrFeO₃ has a half-metallic electronic character, as evidenced by the band structures and partial density of states shown in Figure 4a. Fe and O ions mostly contribute to the orbitals at Fermi energy. A strong hybridization of the Fe 3d and O-2p orbitals occurs according to the occupied configuration of d⁵L for Fe⁴⁺ to the negative charge transfer regime of SrFeO₃, typical of compounds with transition metal ions in relatively high oxidation states [88]. Sr-4d orbitals occupy the conduction bands in the range of 7 to 11 eV, whereas the spin Fe-3d states of SrFeO₃ remain unoccupied, suggesting the half-metallic features of SrFeO₃. The charge transfers from O-2p to Fe-3d suggest a covalent contribution to the bonding in SrFeO₃. However, Heifets et al. [86] found that the metallic or insulating character of SrFeO₃ depends on the oxygen content, which induces a structural change in the compound.

The optical parameters, namely, the refractive index n and extinction coefficient k, of undoped SrFeO₃ compound are shown in Figure 4b as a function of photon wavelength as reported by Jia et al. [88]. In the short-wavelength range they oscillate, whereas in the long-wavelength range, from about 700 nm, they remain almost constant at 3 and 1, although an increase in both parameters might be predicted for wavelengths higher than 1200 nm [88].

2.2. Chemical–Physical Properties of Cerium-Doped Strontium Ferrates

When cerium is added to the SrFeO₃, most of the material’s general features are maintained, although Fe⁴⁺ is partially converted to Fe³⁺, causing an evident change in the structure and in the properties. It has been reported that Fe²⁺ can also form, especially at Ce doping higher than 10 mol% [90,91,92]. Cerium possesses two oxidation states: Ce⁴⁺, which is the most common and has an ionic radius of 1.14 Å, and Ce³⁺, which can be formed under low oxygen partial pressures [93] and has an ionic radius of 1.34 Å [94]. To partially replace Sr at the A-site, Ce should have a radius that is as close as possible to that of Sr, which is 1.44 Å. However, Ce³⁺ is not the most stable cation, and thus, Ce⁴⁺ is the most suitable cation for this replacement. On the other hand, replacing iron at the B-site would create an unfavorable condition, since the difference between the radii of Fe⁴⁺ (0.585 Å) and Ce⁴⁺ (1.14 Å) is too large, and the ABO₃ perovskite would be strongly distorted [95], although it has been reported that cerium can enter either the A-site [96] or the B-site of the BaFeO₃ perovskite structure [97]. Ce⁴⁺ introduction at the Sr²⁺ site implies a disequilibrium of charge at the A-site, and this affects the B-site as well through the oxygen bridges of the perovskite structure, forcing part of the iron to reduce its oxidation state to 3+ to compensate for the perturbation of the A-site, as also discussed by Markov et al. [92]. It must be emphasized that if there are no other dopants at site B, the maximum solubility of Ce at site A is 14 mol%. But if, for example, iron is partially replaced by cobalt (or any other B-site dopant that stabilizes Fe⁴⁺), the average ionic radius at the B-site decreases, and more cerium can enter the A-sites. For example, Choi et al. [98] prepared a series of electrocatalysts by doping the A-site of a cobalt-doped SrFeO₃ with increasing amounts of cerium (up to 20 mol%) and succeeded because they had also added cobalt. On the contrary, when the B-site of the Ce-doped SrFeO₃ is co-doped with both cobalt and copper, ceria leaves the floor to copper and prefers to segregate, even in the presence of cobalt [99]. Figure 5a shows a diagram on the variation in the oxygen content of 10 mol% doped SrFeO₃ with pressure and temperature, as reported by Markov et al. [92]. It is worth noting that a deviation from the ideal defect model (dotted lines) is evident at high oxygen pressures (oxidizing environment). According to Markov et al. [92], a better model for this compound is described by a complex equation that takes into account the partial pressure of oxygen, the oxygen content in oxides and some thermodynamic parameters (solid lines) and implies that some number of anion sites are unavailable for oxygen ions (Figure 5a). From Figure 5b, it is evident that Fe³⁺ dominates the other oxidation states, whereas Fe²⁺ and Fe⁴⁺ cations depend almost linearly on the oxygen partial pressure, with Fe⁴⁺ in higher concentrations at higher oxygen pressures. Ce⁴⁺ cations dominate Ce³⁺ ones at high oxygen pressures, whereas Ce³⁺ increases at decreasing oxygen pressures (Figure 5c). According to Markov et al. [92], by doping Sr with 10 mol% Ce, the electron concentration on cerium increases by about an order of magnitude as compared to that of iron, and this may have important implications in the application of Ce-doped SrFeO₃, where charge transport plays a primary role. Similar considerations have been reported by Wang et al. for other doped SrFeO₃ perovskite oxides [72], evidencing that the oxygen p-band center and the net electronic charge both have an important influence on the lattice oxygen transport properties of this class of compounds.

Deganello et al. [77] studied the effect of cerium content on the structural and redox properties of Ce-doped SrFeO₃, and the most relevant literature results are reported in Figure 6a–d. The Ce-doped SrFeO₃ is cubic for most of the cerium doping contents. In Figure 6a, a graphical Rietveld refinement of 6 mol% doped SrFeO₃ is shown. However, for low cerium contents in the perovskite oxide structure, symmetry tends to be distorted. The undoped SrFeO₃ is slightly oxygen deficient and appears as tetragonal with a Sr₈Fe₈O₂₃ structure (see Figure 2a and Figure 6b). At low Ce contents (below 6 mol%), the perovskite structure does not lose its tetragonal distortion, although it tends toward a cubic structure, adopting a pseudo-tetragonal symmetry (P4/mmm ICDD database PDF n 00-059-0570f) (Figure 6b). At 6 mol%, it becomes cubic (Pm-3m, ICDD database PDF n 00-059-0573) (Figure 6a,b) and remains cubic at higher cerium contents. Since Fe³⁺ has a larger ionic radius, the cubic/pseudo-cubic cell volume of Ce-doped SrFeO₃ increases linearly up to the maximum doping level (Figure 6e), which has been identified as 14 mol% Ce [77]. At cerium contents higher than 14 mol%, CeO₂ segregates, and some tetragonal distortion occurs, which is more pronounced with increasing excess cerium content due to the in situ growth of perovskite- and fluorite-structured nanocrystals [100].

Contrary to the undoped SrFeO₃, the structure of Ce-doped SrFeO₃ remains cubic at all the oxygen partial pressures, although the cell parameter increases from 3.85 to 3.90 Å [77]. Oxygen storage capacity, derived from the integration of the TPD peak at low temperatures (Figure 6c) is higher for a moderate amount of cerium doping (Figure 6d), when oxygen vacancies can still “move” across the material [77]. It has also been reported that Ce-doped SrFeO₃ has good conductivity, even under reducing conditions, and quite high oxygen ion conductivity [92]. According to Trofimenko et al. [101], the direct current electrical conductivity (σ) of the monophase material (thus, if no cerium oxide phase is segregated) can reach a maximum at 500 °C at a Ce concentration of =0.05 mol (80 S cm⁻¹, E_a = 0.088 eV) (Figure 6f). More details on the electrochemical features of Ce-doped SrFeO₃ can be found in Section 3.2.

It is worth noticing that cerium doping at the A-site has a similar effect on the analogous compound BaFeO₃ with a hexagonal structure [96], stabilizing the cubic structure, although the solubility limit is much lower. It has also been reported that slight cerium doping at the B-site is possible for BaFeO₃ [97,102], whereas Ce doping at the B-site of SrFeO₃ has never been reported. This difference between BaFeO₃ and SrFeO₃ can be due to the larger ionic radius of Ba²⁺ with respect to Sr²⁺, which allows Ce⁴⁺ to enter the B-site instead of the A-site of the perovskite, although it has been postulated that Ce⁴⁺ might enter interstitial positions of the Fe site instead of occupying the center of the octahedral configuration of oxygens [95].

2.3. Preparation Procedures of Doped SrFeO₃ Perovskite Oxides

Given the high sensitivity of SrFeO₃-based materials to processing conditions, pre-treatments, preparation methodologies and experimental conditions themselves, it is worth discussing the main synthesis procedures that can be used for the preparation of doped SrFeO₃, including Ce-doped SrFeO₃ perovskite oxides, and the effect of these procedures on the material properties. Moreover, according to the recently published guidelines of the European Commission on sustainable materials synthesis [103], a careful design of synthetic steps can contribute to the sustainability of the final materials: for example, waste and scratches can replace traditional synthesis precursors or additives, and energy–time–cost consumption and waste production can be limited while increasing the efficiency of the materials in terms of performance. A graphical overview of the synthesis methodologies and procedures is presented in Figure 7.

2.3.1. Solid-State Methods

Most of the doped SrFeO₃ compounds in the literature have been prepared by solid-state synthesis [91,104,105,106]. The Sr, Fe and dopant precursors, usually SrCO₃ and Fe₂O₃ and oxide or carbonate of the dopant cations (in this case CeO₂), are mixed in the form of powders to increase the contact between the reagent particles and create as homogeneous a mixture as possible, and then the mixture is thermally treated at high temperatures to initiate the solid-state reaction between the different reagents [91] (Figure 7A). Solid-state cation diffusion is promoted by repeated cycles of high-temperature treatment and mechanical grinding. It is also possible to press the precursor powder into pellets before the thermal treatment steps [107]. A solvent can also be added to facilitate the mixing of the precursor powders [105], followed by a drying step before the final thermal treatment. Ball milling, in dry mode or with a solvent, is often associated with solid-state synthesis to facilitate the solid-state diffusion among particles [104,108,109], making the synthesis more efficient, although possible contamination of the milling balls and high energy consumption may decrease the sustainability of this synthesis procedure. The risk of this very simple methodology is obtaining poorly reproducible products that are not very homogeneous due to inadequate mixing among the metal cation precursors. In its basic form, solid-state synthesis is a sustainable methodology, although repeated high-temperature treatments and/or high-energy ball milling might substantially increase the electrical energy needed for synthesis, reducing the sustainability level of this methodology. Moreover, the use of any solvent besides water is not recommended if a sustainable synthesis is sought. Markov et al. [92] prepared Ce-doped SrFeO₃ by solid-state synthesis and suggested using a slight Sr excess to avoid the formation of SrFe₁₂O₁₉ as a secondary phase.

2.3.2. Co-Precipitation

Co-precipitation has also been used for doped SrFeO₃, such as Ca-doped SrFeO₃ compounds [59]. The co-precipitation method (Figure 7B) requires supersaturation conditions, mixing solutions with soluble metal cations with another solution called the “precipitation agent” (i.e., potassium or sodium hydroxide). Successively, after filtration, homogeneously dispersed and washed particles undergo a heat treatment to obtain the final powder. Parameters to be considered to reach desired physical properties are temperature, mixing rate, pH and concentration. An advantage of the co-precipitation method is the easy preparation of perovskite oxides with good control of particle size [110]. On the other hand, the chemical homogeneity of the precipitate can be severely affected by the different relative solubility constants of the components and of the solid precipitate formed after co-precipitation of the metal precursors, and for this reason, usually, this methodology is not very efficient for multicomponent oxides.

2.3.3. Strategies Based on Sol–Gel: Pechini Method and Solution Combustion Synthesis

Pechini-like sol–gel methodologies have been frequently used for the synthesis of doped SrFeO₃ powders [63,111,112]. In the traditional Pechini-like procedure, metal cation precursors are usually metal nitrates, since they are highly water soluble. A pictorial scheme of the general procedure is shown in Figure 7C. After dissolving the metal precursors in water, citric acid is added to the solution as a chelating agent for the metal cations (Figure 7D). The regulation of pH values, water evaporation and addition of ethylene glycol in the right proportions facilitate hydrolysis and condensation of the metal citrate complexes, forming a wet gel. The wet gel is then dried in an oven and thermally treated at the desired temperatures to eliminate the organics and form the perovskite structure. A pH of 7, a citric acid-to-metal cation ratio of 2–2.5 and a citric acid-to-ethylene glycol molar ratio of 1.5–2 are commonly used, although slight variations have also been proposed [63,113]. Huan et al. [63] pre-fired the dried gel at 250 °C before calcination. Dos Santos-Gomez et al. [114] froze the wet gel in liquid nitrogen and dehydrated it for 2 days before calcination. This supplementary step, however, is time-consuming and introduces further energy consumption and additional reagents (liquid nitrogen), which can only be justified if the benefits of this additional step overcome the increase in energy and time demand. Other sol–gel methods include the use of polyvinylpyrrolidone (PVP) as a dispersant and N,N-dimethylformamide (DMF) and ethanol as solvents [115].

Solution combustion synthesis (SCS) is a valuable methodology for the synthesis of Ce-doped SrFeO₃ [62,116]. In this technique, a thermally induced redox reaction occurs between a fuel (reducing agent) and an oxidizing agent in the presence of metal cations (Figure 7E) [117,118,119,120]. Initiation of self-combustion is usually promoted by hot plates or muffle furnaces. Alternatively, Xiao et al. used microwaves as a convenient energy source to activate the combustion synthesis of Mo-doped SrFeO₃ electrocatalysts [121]. In microwave-assisted solution combustion synthesis, microwave radiation can penetrate the gel and deliver extra energy for the initiation of the combustion process and formation of the mixed oxide [122].

Mainly citric acid [116], glycine [62], sucrose [76] and urea [123] have been used as fuels for the preparation of doped SrFeO₃, whereas metal nitrates or carbonates are typically used as oxidants and metal sources (Figure 7F). It is required that the fuel has a complexing ability towards metal cations. The steps that are expected for this synthesis method are (i) mixing fuel, metal-based oxidants (i.e., Sr(NO₃)₂, Ce(NO₃)₃, Fe(NO₃)₃) and any additive (i.e., ammonia and/or NH₄NO₃ and/or any additional microstructural template) in water; (ii) heating the mixture until a sticky gel is obtained; and (iii) increasing the temperature to trigger combustion. Variations with respect to classical reagents have been proposed: Irshad et al. directly used lemon or orange juice as fuel, chelating agents and soft microstructural templates [124].

The SCS design begins by selecting the fuel type(s), F/M (fuel-to-metal cation molar ratio) ratio, Φ (reducer-to-oxidizer ratio) and pH regulation, if necessary, to reach the desired conditions that will affect the final materials properties and functionality. In particular, the combustion fuel may affect the texture, morphology and reduction properties of the final powders [76,125], while the pH regulator plays a fundamental role when the fuel functional groups need to be fully dissociated for better interaction with metal cations. Other additives that can be employed are soft or hard templates, ranging from organic to inorganic and even insoluble, to impart specific textural properties to the final material, especially in terms of porosity and specific surface area [126,127].

The optimal F/M ratio depends on (i) the chemical composition of the combustion mixture, i.e., type of metal cations, type of fuel and their mutual interaction, and (ii) the pH. In fact, different organic ligands have different molecular structures and numbers of functional groups.

In the case of citric acid—where an oligomerization occurs through a cross-bridging with the metal cations or through a condensation—the presence of alkaline earth metals in the combustion mixture requires at least a F/M of 2, since the formation of chelates by citrate functional groups is less favored than that of transition metals [128]. However, the F/M ratio can be decreased to 1 and even less if the fuel is a polyfunctional organic molecule like, for example, sucrose, the main functional groups of which are reactive hydroxyl groups instead of carboxylic ones, with more functional groups per molecular unit. It may be convenient to use a mixed fuel, where two organic compounds with different chelating groups interact with each other and cooperate in the chelation process, taking advantage of the mixed fuel polyfunctionality, especially in the case of multidoped SrFeO₃ [76]. Regulation of F/M for pH-sensitive fuels is strongly related to pH regulation, allowing for full dissociation of the acid groups. Moreover, conditions should be recalibrated in the presence of an insoluble reactant, metal precursor or hard template, especially if it contains surface functional groups that can chelate the other soluble components of the combustion mixture. It may happen that, for example, when a silica hard template is used, its functional groups on the surface are able to adsorb citric acid and dissociate it, acting as a base, without further pH regulation. The F/M influence on morphology and phase purity cannot be ruled out, although it has never been confirmed by systematic studies.

Another crucial parameter is Φ, defined as follows (Equation (2)):

Φ = Σred/[(−1)·Σox]

(2)

Φ is not only a simple molar ratio between moles of fuel and oxidants but also contains the summations of reducing species (expressed in moles) multiplied by their reducing valence (Σred) and oxidant species (expressed in mol) multiplied by their oxidation valence (Σox). According to propellant chemistry, reducing and oxidizing valences correspond to the number of exchanged electrons during the combustion reaction. The combustion ratio’s influence can be explained based on the principle of the combustion triangle: the oxidizer and the combustible (fuel) react in the presence of a combustion trigger and generate heat, which—in stationary conditions—sustains the process. The event can be initiated if the energy supply is enough to overcome the activation barrier and if the Φ value—which considers the number of exchanged electrons and is the index of real oxidizing and reducing capabilities—is not too high or too low. For instance, Φ = 1 means the initial mixture does not require atmospheric oxygen for complete fuel oxidation. NH₄NO₃ is a common additive/reactant to regulate Φ, and its amount should be maintained at its minimum due to its possible negative effects, such as its derivatives (toxic gases). It has also been reported that, when in excess, its endothermic decomposition can partially inhibit the combustion process [129].

Beyond these general considerations, Deganello et al. [116] and Palma et al. [130] discussed the effect of several solution combustion synthesis parameters on the properties and/or performance of Ce-doped SrFeO₃. For example, it was found that a gel formed prior to combustion should be fully dried and the reducer-to-oxidizer ratio should be over-stochiometric (1.6) to obtain reproducible and performant products for the abatement of contaminants of emerging concern (CECs) and dyes in wastewater [130]. Although scale-up is not straightforward due to safety issues and the difficulty in controlling some parameters (adiabaticity, evolution of gases, loss of material and residual carbon), SCS still represents a versatile methodology for preparing doped strontium ferrates [76,116,131]. Indeed, it is a low-cost, simple, fast and rather reproducible method, and it allows for the regulation of a variety of parameters for fine-tuned materials, which are thus suitable for several applications [117], as is possible to observe in Section 3 and Section 4.

2.3.4. Other Synthetic Methods and Post-Treatments

Although the most common methodologies for preparing doped SrFeO₃ perovskites are discussed in Section 2.3.1–Section 2.3.3, it is worth noting that other synthesis methodologies have also been adopted.

For instance, a Pr-doped SrFeO₃ was prepared by freeze-drying from the nitrate precursors and ethylenediaminetetraacetic acid (EDTA) in liquid nitrogen and further treating the mixture at 300 °C for the pyrolysis of the organic precursors, followed by treatment at 800 °C for the formation of the final perovskite oxide phase [132].

In the freeze-drying-based methodologies, the initial precursor solution is frozen dropwise into liquid nitrogen, followed by sublimation of the solvent in the absence of the liquid phase. Interestingly, in on study, Sr was maintained under stoichiometric conditions to minimize any Sr segregation, whereas other authors have used an excess of Sr to avoid SrFe₁₂O₁₉ segregation [92]. This aspect is controversial, and it needs to be investigated further in the future.

Zapata-Ramírez et al. directly deposited Sb-doped SrFeO₃ electrocatalysts by spray pyrolysis onto a ceria-based electrolyte starting from the same precursors used for the Pechini synthesis [133]. This methodology is very convenient when the powder needs to be deposited onto a solid support.

Using electrospinning with PVP and DMF, it was possible to prepare La-doped SrFeO₃ for methylene blue degradation [134]. This technique has the advantage of allowing for very good morphological control of the powder at the nano level by changing, for example, the injector-tip-to-collector distance.

Irrespective of the type of method chosen, usually, high-temperature thermal treatment (calcination) concludes the synthetic process to favor the crystallization of the perovskite structure and the removal of organic residuals [118]. However, other treatments have been proposed to obtain strontium ferrate powders with the desired characteristics. For example, Krzystowczyk et al. [111] sieved the strontium ferrate calcined powder to select only the 150–250 micrometer-sized particles. In the same vein, other doped strontium ferrates underwent a complex process of grinding, granulation by a wet modality and sieving [135]. Tummino et al. washed the calcined Ce-doped SrFeO₃ powders with MilliQ^® water for two hours under constant stirring, then filtered and dried the powder with the aim of dissolving any residual unburnt organics [68]. Recently, Sr_0.85Ce_0.15FeO_3−δ incorporated into an alumina membrane was post-treated with a heating step in an argon atmosphere, which was optimized to favor the sintering process and limit the unwanted Sr-Al phase formation [136].

2.3.5. Synthesis of Doped SrFeO₃ Perovskite Oxides: Important Aspects and General Guidelines

Doped SrFeO₃ perovskite oxides can be prepared by several methodologies, and the most commonly used are described in Figure 7 and/or discussed in Section 2.3.1–Section 2.3.4. When using Pechini-like sol–gel methodologies and solution combustion synthesis, dopants can be better incorporated in the perovskite oxide thanks to the formation of a gel network among the reactants. In addition, solution combustion synthesis allows for using waste-derived and/or insoluble precursors, provided that a preliminary investigation on the synthesis–structure–property relationships is performed to optimize the doped SrFeO₃ structure and functional properties, which depend considerably on the synthesis method and processing conditions. If a multi-phase composition is obtained—i.e., segregation of CeO₂ or other oxides or Sr₃Fe₂O₇-type phases—the effect of minor phase formation together with the main perovskite oxide phase should be further explored according to the selected application to check for any possible synergic effect between the two phases in the functional properties and performance. Post-treatments might be also considered, depending on the type of application/technology selected. It is recommended to use highly detailed synthesis protocols and test their reproducibility and scalability due to the high sensitivity of doped SrFeO₃ to synthesis and processing conditions. A morphological control of the doped SrFeO₃ powders can be better obtained with synthesis methodologies that make use of complexing agents and hard templates. Most importantly, a safe and sustainable design approach should be adopted, taking into account all the aspects of the material synthesis, from the selection of the precursors to the investigation of the material’s reusability and durability (end of use) [103].

3. Applications of Ce-Doped SrFeO₃ Perovskite Materials

All the applications of Ce-doped SrFeO₃ in energy and environment-related sectors stem from three key characteristics of these compounds: (i) high oxygen storage capacity and exceptional oxygen mobility in the crystal structure, (ii) the presence of Fe⁴⁺ and the redox couple Fe⁴⁺/Fe³⁺ and (iii) mixed ionic–electronic conduction properties. This section addresses the main applications of these materials as thermocatalysts for wastewater cleaning, electrocatalysts for energy production and oxygen carriers for chemical looping technologies. Moreover, other applications such as sensors, supercapacitors and antibacterial agents are discussed.

3.1. Thermocatalysts for Wastewater Cleaning

Water scarcity is a global problem that affects many countries and is becoming more severe due to a combination of factors, including population growth, urbanization, climate change and unsustainable practices in water use and disposal [137]. As demand for water increases, water resources are being depleted faster than they can be replenished, leading to water shortages, droughts and water stress in many regions. Moreover, natural water systems are polluted by an increasing number of contaminants of emerging concern (CECs), which are a group of chemicals that are not commonly monitored or regulated but have the potential to negatively impact human health and the environment [138]. These contaminants include a wide range of substances such as pharmaceuticals, personal care products, hormones, pesticides and industrial chemicals. Such molecules are recalcitrant to traditional biological treatments and, therefore, can enter the environment through a variety of sources, including wastewater treatment plants, agricultural runoff and landfills. Once in the water supply, these contaminants can persist for a long time and can accumulate in the food chain, potentially posing risks to human health and the environment. In this context, a range of technologies has been developed and deployed for CEC removal and abatement from water systems [139]. These technologies include adsorption (e.g., on activated carbon), membrane filtration (e.g., nanofiltration and reverse osmosis) and advanced oxidation processes (AOPs) such as ozonation, Fenton-based reactions, electrochemical processes and photocatalysis. Integrated chemo-catalytic ceramic membranes have also been proposed in the literature [7].

At first, SrFeO₃ perovskites were considered as photocatalytic materials for the abatement of organic pollutants in water remediation under either UV or solar light. Indeed, SrFeO₃ materials have band structures and band gaps (between 1.80 and 3.75 eV) that are suited for this application [140,141]. In the photocatalytic process, a semiconductor material (SrFeO₃ in this case) absorbs light to generate an electron (e⁻)–hole (h⁺) pair. The electron and the hole are free to move within the material to reach the surface where they can generate reactive oxygen species (ROS), such as hydroxyl (OH^∙), superoxide anions (O₂^∙−), hydroperoxyl (HOO^∙) radicals, hydrogen peroxide (H₂O₂) and singlet oxygen (¹O₂), which can then promote the chemical degradation of water pollutants. Under light irradiation, SrFeO₃ perovskites have been shown to be effective in the abatement of various model pollutants such as dyes, e.g., methyl orange [142] and methylene blue [140]; phenols [143]; and nitrobenzene [141]. The comparison of the reported materials is complicated by the different testing conditions described in the literature. However, no SrFeO₃ perovskites have been reported so far to outperform benchmark materials such as P25 (TiO₂) in the photodegradation of water pollutants.

Interestingly, Leiw et al. [144] reported the ability of the perovskite SrFeO_3−δ to degrade acid orange 8 (AO8) and bisphenol A (BPA) when dispersed in water under dark ambient conditions, which is a remarkable property, since the abatement process does not require exposure to light as in photocatalysis or additional chemicals as in Fenton reactions. For instance, BPA (starting concentration: 440 μM) was completely removed by treatment with SrFeO_3−δ (21 mM) within 24 h at room temperature, with a 17% residual TOC, indicating the persistence of some degradation by-products. Following this direction, azo dye degradation was also tested in dark conditions using A- and B-site-deficient perovskite series with the general formulae Sr_xFeO₃ (with x = 0.95, 0.90, 0.85 or 0.80) and SrFe_yO₃ (with y = 0.95, 0.90, 0.85, 0.80 or 0.70) [145]. Such cation deficiencies in the perovskite structure noticeably enhanced the catalytic degradation of azo dyes via oxygen–dark activation. Recently, in a similar approach, the group of da Silva Júnior effectively combined darkness and UV light catalytic processes to degrade ciprofloxacin using SrFeO_3−δ [146].

Regarding Ce-doped materials, Tummino et al. [68] performed a comparative study of the ability of Sr_0.85Ce_0.15FeO_3−δ to catalyze the bleaching of orange II and rhodamine B under the commonly adopted light exposure and, more interestingly, in a dark and moderately heated environment. As a result, it was revealed that the Sr_0.85Ce_0.15FeO_3−δ exhibited thermocatalytic activity in the absence of light, thus unlocking a range of new applications for this material. Figure 8a shows a model water solution contaminated with methyl orange (acidified with HCl at pH = 1.5) before and after catalytic bleaching with a cerium-doped strontium ferrate perovskite under dark conditions. The thermocatalytic degradation of organics by strontium ferrates is often studied according to pseudo-first-order kinetics, as shown in Figure 8b, for the abatement of BPA at different temperatures. Figure 8c reports the Arrhenius plot for the kinetic constants measured for the abatement of different pollutants by SrFeO_3−δ perovskites tested under dark conditions in the temperature range of 20–80 °C.

The reactivity of SrFeO₃ is often ascribed to the redox capacity of Fe³⁺/Fe⁴⁺ species in association with the perovskite structure and the easy formation of oxygen vacancies [78]. Ce doping can be used to enhance the thermocatalytic activity of the material by the interaction between the oxygen-bridged redox couples Fe³⁺/Fe⁴⁺ and Ce³⁺/Ce⁴⁺ and Fe⁴⁺–Fe³⁺, which can facilitate a reaction with oxygen species during the degradation process. Tummino et al. [68] used electron paramagnetic resonance (EPR) spectroscopy to investigate the reactive oxygen species (ROS) generated by Sr_0.85Ce_0.15FeO_3−δ. In this type of analysis, spin-trap compounds, i.e., molecules that react with free radicals to form stable adducts that have distinctive EPR signals (Figure 8d), were added to dispersions of perovskites in contaminated water, which were either kept in dark conditions or exposed to simulated solar light. Among the spin-trap compounds, 5,5-dimethyl- 1-pyrroline-N-oxide (DMPO), 2,2,6,6-tetramethyl-4-piperidone hydrochloride (4-oxo-TMP) and 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO) were employed to specifically detect hydroxyl radicals, singlet oxygen and superoxide anion radicals, respectively [148]. The spin-trapping EPR experiments revealed that the Sr_0.85Ce_0.15FeO_3−δ powder was able to generate ROS even in the absence of light (Figure 8e), with a prevalence of hydroxyl radicals (DMPO-OH adduct) over the other investigated species such as superoxide anions (DEPMPO-O₂⁻ adduct). Chen et al. [147] recently reviewed thermocatalytic materials for water depollution. Based on the literature, they proposed different mechanisms and combinations of them. Two mechanisms involving hydroxyl radical generation are reported in Figure 8f. As the well-known mechanisms in the photo-induced degradation of organics, in the hopping conduction (HC) mechanism, the energy received by the semiconductor material is used to excite electrons in the valence band into the conduction band, leaving positive holes (h⁺) in the valence band. Highly oxidative h⁺ can directly oxidize H₂O into hydroxyl radicals, while e⁻ can reduce the adsorbed oxygen, forming different ROS. In the surface electron transfer (SET) mechanism, the degradation process initiates with the direct transfer of electrons from the adsorbed pollutant molecules to the surface redox couples of the thermocatalytic material. The trapped electrons can then move through the material and will be consumed by adsorbed oxygen to generate ROS, which can further degrade organic molecules.

The practical implementation of Ce-doped SrFeO_3−δ perovskites for the depollution of real wastewater systems strongly relies on the development of new engineering solutions for their integration into the existing industrial facilities. In this respect, integration with membrane filtration technologies offers several advantages, which include catalyst recovery and concentration of the water pollutants before or during degradation. Figure 7 gives graphical representations of the systems for the integration of a Ce-doped SrFeO_3−δ thermocatalyst within a membrane unit, as reported in the literature. Nanofiltration (NF) is a pressure-driven filtration technology that operates with membranes that typically have pore sizes smaller than 5 nm, which makes them effective at retaining dissolved organic molecules and, thanks to the surface charge (Donnan exclusion [149]), multivalent ions from water. Therefore, NF is commonly used in water depollution and softening, although it has been increasingly applied to the food and beverage sector for the concentration and purification of juices and milk and to recover and purify products from the pharmaceutical and biotechnology industries [150]. NF has been synergistically integrated with water depollution by Ce-doped SrFeO_3−δ according to the two configurations depicted in Figure 7A,B [151]. In the first approach (Figure 7A), filtration and thermocatalytic abatement of pollutants are performed simultaneously. The NF membrane concentrates the contaminants in the wastewater while recovering the perovskite powder, thus producing a clean permeate. At the same time, the Ce-doped SrFeO_3−δ powder degrades the organic contaminants in the membrane retentate, which is therefore depolluted. Hence, the integrated NF-perovskite system converts the wastewater into two streams: a permeate, which is purified thanks to the ability of the membrane to retain dissolved pollutants, and a concentrate, defined as “depolluted water” (Figure 9), resulting from the degradation of the organic pollutants by the perovskite powder. Despite both streams being clean, they have different compositions because NF membranes present different permeability towards inorganic ions and non-toxic organic matter dissolved in wastewater. Therefore, depending on the intended application, the NF membrane and the filtration parameters can be selected to obtain a permeate and a concentrate with characteristics (e.g., conductivity, dissolved organic carbon, transparency) suitable for their reuse. Moreover, the degradation of the pollutants on the retentate side of the NF membrane is beneficial for the quality of the permeate. For instance, the configuration in Figure 9a was tested with an NF membrane presenting ~98% rejection towards BPA [151]; i.e., a membrane feed with a BPA concentration of 50 ppm resulted in a permeate contaminated with 1 ppm BPA. Thus, the addition of Ce-doped SrFeO_3−δ at the membrane feed and the consequent thermocatalytic abatement of the BPA concentration allowed for a reduction in the BPA traces in the permeate. On the other hand, the wastewater samples needed to be heated at 50 °C before filtration to achieve appreciable degradation rates, which implies additional running costs for the process. Therefore, NF can also be used as a pre-concentration step before the abatement of the water pollutants over a Ce-doped SrFeO_3−δ catalytic bed reactor (Figure 9b), thus reducing by several folds the volume of wastewater that needs to be heated and the related costs. This configuration also permits increasing the contact time between the thermocatalyst and the contaminants, thus improving the reaction efficiency.

The effectiveness of a Ce-doped SrFeO_3−δ fixed-bed reactor was proven in the abatement of oil residues in water produced by the offshore extraction of hydrocarbons [69].

The heat used to activate Ce-doped SrFeO_3−δ for water depollution can also be exploited to drive water distillation through a membrane in the integrated process that is schematized in Figure 9c. Membrane distillation (MD) is a thermal separation process that relies on a porous hydrophobic membrane that repels liquid water but allows steam to pass through it [152]. As the hot wastewater is fed to the membrane, water molecules evaporate and pass through the membrane as vapor, leaving behind organic pollutants, dissolved salts and thermocatalytic powder. The vapor is then condensed on the other side of the membrane to form a purified permeate stream, while the organic pollutants are degraded by the thermocatalyst. MD was tested in combination with Ce-doped SrFeO_3−δ for the degradation of BPA. Compared to NF, MD provides full BPA rejection and much lower operating pressure. The integrated MD-thermocatalysis process allowed for the fast degradation of BPA and the recovery of the thermocatalytic powder without additional steps [70]. The integrated system was shown to be able to sustain at least four concentration cycles with minimal impact on the activity of the thermocatalytic powder. On the other hand, MD units cannot retain volatile pollutants [153], and for long operation times, scaling and high carbonate concentrations might be detrimental to the thermocatalytic activity of the Ce-doped SrFeO_3−δ powder.

In the fourth approach (Figure 9d), a perovskite-based active layer is applied on the surface of the membrane filter. When water flows through the membrane, the thermocatalyst oxidizes organic contaminants, thus producing a clean permeate while depolluting the retentate. For instance, Ce-doped SrFeO_3−δ was embedded in a graphene oxide (GO) matrix to obtain a thermocatalytic active layer with rejection in the nanofiltration range [154]. A perovskite loading of 98.6 wt% in the reactive layer was found to be the best compromise to achieve a membrane with high rejection towards water pollutants and good thermocatalytic activity. When wastewater was filtered at 50 °C, this membrane was able to retain and degrade BPA and mitigate fouling. A variant of the fourth approach is to incorporate the thermocatalytic perovskite directly in an alumina membrane through a mechanical/thermal treatment. The results demonstrated that the so-prepared thermocatalytic membranes can have great potential for continuous membrane filtration and simultaneous degradation of organic pollutants. When considering methyl orange, up to 100% removal is achieved at room temperature, whereas up to 93% of bisphenol A is removed at temperatures approaching 60 °C.

Real wastewater effluents typically contain large amounts (mg L⁻¹) of non-toxic organic matter (NOM), which can deposit on the membrane surface and clog membrane pores (fouling) at the detriment of the permeate flux, thus requiring frequent cleaning cycles. Ce-doped SrFeO_3−δ has been proven to mitigate fouling in NF membranes when dispersed as a powder in the feed [151] or immobilized on the membrane surface [154]. On the other hand, the efficiency of thermocatalytic perovskite in the abatement of water pollutants is strongly hindered by NOM, e.g., by passivating the active surface and ROS scavenging [151].

The recent literature shows that Ce-doped SrFeO_3−δ exhibits catalytic activity in the abatement of organic pollutants in water. Moreover, the integration of this technology with membrane filtration allows for the design of systems potentially applied to the treatment of real wastewater effluents. Nevertheless, some challenges concerning the synthesis, the activity and the stability of Ce-doped SrFeO_3−δ need to be addressed before the full-scale implementation of thermocatalytic water depollution systems (see Section 4).

3.2. Electrocatalysts for Energy Production

Strontium ferrates are more and more frequently considered in the field of energy production. One of the main roles of these compounds is their catalytic function as cathodic and anodic materials in cobalt- and precious-metal-free electrochemical devices [1]. For instance, solid oxide fuel cells (SOFCs) are all-solid-state energy conversion devices that convert chemical energy in fuels directly to electricity [155] and are required to be characterized by high energy efficiency, low emissions, high exhaust heat quality and excellent fuel flexibility [156]. SOFCs usually have an asymmetrical structure, where the cathode and the anode have different microstructures and are constituted by different materials. In these years, much attention has been paid to symmetrical SOFCs (SSOFCs), in which the two electrodes are composed of the same materials with the same microstructure. This configuration has many advantages, such as the simplification of the fabrication and regeneration processes and the minimization of compatibility problems [64]. In general, SOFCs need the presence of electrode materials with reductive-oxidizing properties that can act in an oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) (SSOFCs require bivalent activities), with sufficient long-term redox stability and high mixed ionic–electronic conductivity (MIEC) [64,155]. All these features can be found in SrFeO₃-based materials due to their electronic structure and peculiar ability to accommodate oxygen vacancies in stable crystalline structures, as reported in several papers, where the doping/co-doping effects were also reported as the keys to activity and stability improvement [47,64,121,155,156,157,158,159]. Variations in this theme are represented by the employment of SrFeO₃ in IT-SOFCs (intermediate-temperature solid oxide fuel cells, operating at 500–800 °C [76], proton-conducting fuel cells (PCFC) (which work at lower temperatures, around 600 °C, exploiting the fact that proton conductivity has lower activation energy than oxygen [160]) and solid oxide electrolysis cells (SOECs), where electrical energy is supplied to drive the reversed reactions with respect to SOFCs (i.e., obtaining H₂ from H₂O) [161].

In electrocatalytic applications, the doping modulation often occurs on the transition metal at the perovskite B-site, since it is considered the main factor responsible for reactions due to chemical and electronic states’ effects, surface electron transfer properties, metal–oxygen bond strength and, eventually, the capability to balance oxygen vacancies and to influence surface acidity [162,163]. Nevertheless, it has been demonstrated that ionic–electronic conductivity is also connected to the A-site features, especially in relation to the formation/suppression of oxygen deficiencies [164] and structural stabilization [165,166], which, in turn, leads to enhanced catalytic activity. Particularly, as already pointed out, A-site cerium doping can stabilize the perovskite cubic structure, improving the electronic conductivity with respect to less symmetric systems, as it maximizes the overlapping between the 2p orbitals of oxygen and d orbitals of the transition metals at the B-site [167]. In addition, by increasing Ce concentration, the oxygen deficiency δ, the average valence state of B-site cation and the thermal expansion coefficient (TEC) decrease [76]. This latter factor is crucial in electrochemical cells, since significant discrepancies among TEC values between electrodes and electrolytes can bring about a mismatch between the components and consequent thermomechanical instability (another common strategy to avoid this detriment is replacing B-site cobalt-based electrodes as much as possible [46,168,169].

In more detail, at the end of the 1990s, the above-mentioned group of Trofimenko et al. studied Sr-Ce-Fe-O systems, imputing high oxygen exchange and electrical conductivity (with a minimum activation energy) to the monophasic Ce-doped strontium ferrate with respect a two-phase mixture of 0.8 SrCeO₃–0.2 SrFeO₃ [101], thus suggesting these monophasic materials as suitable cathodes for high-temperature electrochemical cells. The study pointed out that the electrical conductivity characteristics are the consequence of the structure, type and concentration of the charge carriers. Indeed, it was found that the conductivity for Sr_0.9Ce_0.1FeO₃ was p-type in the region of higher oxygen partial pressures and n-type under reducing conditions. The reason for this behavior is mainly inherent to the nature of electrons in the iron [170] ions of SrFeO_3−δ: at high oxygen pressure, the interaction with the gas phase leads to the formation of electron holes (Fe⁴⁺) and, thus, a higher p-type electronic transport, whereas when the oxygen partial pressure decreases, the generation of electrons (Fe²⁺) [47,171] occurs (n-type). Without considering temperature effects, other parameters related to carrier mobility affect the conductivity of SrFeO_3−δ: it decreases by increasing the vacancy concentration and the Fe-O-Fe bond length [60]. Additionally, excessive oxygen deficiency is not desired because, beyond reducing the p-type conductivity, it induces the phase transformation to an inferior conducting brownmillerite structure. The paper of Trofimenko et al. was recalled by Deganello et al. [77], who prepared Sr_1−xCe_xFeO_3−δ (0 ≤ x < 0.15) powders as cathodic materials. It was found that a cerium doping dose of 0.06 induced an optimal oxygen vacancy concentration and the lowest area-specific resistance (ASR), which was analyzed by electrochemical impedance spectroscopy (EIS). The improved properties were correlated with the stabilization of the octahedral Fe³⁺ site and the cubic environment, which favored the interaction between the redox couples Ce⁴⁺–Ce³⁺ and Fe⁴⁺–Fe³⁺ connected in the solid solution by oxygen bridges [77]. In the same work, oxygen adsorption was identified as the rate-determining factor for the overall oxygen reduction process in Ce-doped strontium ferrates. Indeed, the role of a SOFC cathode is to reduce oxygen (ORR) to form oxide ions: these species travel through the electrolyte to the anode, where they are subjected to oxidation and produce electricity, plus water or other by-products. This mechanism implies that the performance of the cathode is limited by the surface oxygen exchange kinetics of the ORR and the bulk oxide ion mobility [98].

As mentioned above, a common strategy to enhance electrochemical performance is to tune the B-site. After their first work, the group of Trofimenko et al. obtained a series of Sr_1−xCe_xFe_1−yCo_yO_3−δ perovskites [91], given cobalt’s ability to increase electrical conductivity. Subsequently, Colomer et al. [172] deposited symmetrical electrodes of Sr_0.8Ce_0.1Fe_0.7Co_0.3O_3−δ on Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte ceramic pellets to investigate the electrochemical properties of the electrode–electrolyte interface at intermediate temperatures (500–766 °C). They suggested that the enhanced oxygen ion diffusion of the doped SrFeO_3−δ provided superior cathodic performances compared with that of La_0.6Sr_0.4Co_0.2FeO_3−δ (LSCF, traditionally employed as cathodes) at 700 °C. Another attempt to apply these A-site Ce-doped and B-site Co-doped SrFeO₃ compounds was performed by Choi et al. [98], where Sr_1−xCe_xFe_0.8Co_0.2O_3−δ (x = 0.10, 0.15, 0.20) samples were adopted as cathodic materials, tested in an SOFC button cell with scandia-stabilized zirconia as the electrolyte and NiO–YSZ as the anode. The best performance was seen at the intermediate Ce doping fraction (x = 0.15), which also led to a stronger ability of CO₂ dissociation and a higher intrafacial transport of oxygen. Other results were the maintenance of the perovskite cubic structure in the whole doping range without structural changes/distortions at elevated temperatures and the improvement of the thermal compatibility with the electrolyte by increasing the cerium dopant concentration. Nevertheless, the highest loading level studied (Ce fraction = 0.20) gave the poorest button cell performance as well as the highest ASR from EIS tests. This evidence was explained by the presence of the segregated CeO₂ phase, caused by the excess A-site dopant, which exhibited low electrical and/or ionic conductivity. Based on this background [76], a recent study reported the behavior of a series of samples with the fixed composition Sr_0.85Ce_0.15Fe_0.67Co_0.33O_3−δ, varying the synthesis conditions. The materials were prepared by solution combustion synthesis using mixtures of sucrose and polyethylene glycol (PEG) with different molecular weights as combustion fuels (sucrose, sucrose-PEG1000, sucrose-PEG20000). The aim was to find correlations among the adoption and variation of PEG as a secondary fuel and the perovskite structure, redox properties, morphology, texture and electrocatalytic activity in IT-SOFC cathodes (evaluated through EIS and overpotential measurements). The most significant conclusion was that the addition of high-molecular-weight PEG to the sucrose primary fuel led to an improvement in both oxygen reduction and evolution processes. This result was attributed to the formation of high-valence Fe at a low temperature; actually, PEG addition limited cerium solubility in the Co-doped SrFeO₃ through the stabilization of the highly oxidized B-site species Fe⁴⁺ and Co⁴⁺. Another advantageous effect of PEG20000 was that it guided the formation of nanostructured crystalline domains, which enhanced the oxygen exchange rate in the doped strontium ferrate electrocatalysts. An interesting point arose comparing this material with that synthesized with only sucrose, which showed the highest values of surface area, porosity and oxygen deficiency but a much lower stabilization efficiency towards highly oxidized cations that inhibited the achievement of the best electrochemical performance.

The combination of Ce in the A-site and Co in the B-site was proposed again for SrFeO_3−δ-based symmetric electrodes to be used in the CO₂ electrolysis process [173]. SOECs with the characteristics of high-temperature operation and high conversion efficiency are widely applied in the direct conversion of CO₂ into CO as the main product (further employable as a major reactant for Fischer–Tropsch synthesis), and notably, symmetric SOECs have attracted attention because of their lower preparation costs. The Ce/Co co-doped strontium ferrates were synthesized by the combustion method and were formed with co-exsoluted CeO₂ and Co–Fe alloy in 5% H₂ at 800 °C. The most promising material was selected as Sr_0.9Ce_0.1Fe_0.9Co_0.1O₃ (with respect to the material with only 5% Ce doping), showing an excellent electrolysis performance at 800 °C, with good CO₂ tolerance and oxygen activity. The efficiency of the composition with 10% Ce doping was attributed to the ease of CeO₂ exsolution and Co–Fe alloy formation on the surface of perovskite. On the one hand, the explanation given by the authors regarding the role of the Co–Fe alloy concerns the improvement in CO₂ adsorption and activation resulting from the metal/oxide interface. Moreover, the Co–Fe alloy could provide electrons to promote the CO₂ reduction reaction. On the other hand, ceria with high oxygen storage capacity was beneficial to carbon dioxide reduction kinetics due to the redox activity (Ce⁴⁺ ↔ Ce³⁺), also facilitating oxygen ionic transport at the cathode–electrolyte interface. A similar strategy was adopted in the work of Wang et al. on Co- and Sc-doped SrFeO₃ for symmetrical solid oxide cells, where an A-site vacancy was introduced in the structure to favor the exsolution of Co–Fe alloy nanoparticles during reduction, and Sc doping was used to stabilize the cubic structure and increase the activity and stability of the electrocatalyst [65].

The modulation of cobalt doping in Sr_0.85Ce_0.15Fe_xCo_yO₃ powders as electrocatalysts for IT-SOFCs was the focus of a recent paper [174]. The influence of both the Fe/Co ratio and B-site stoichiometry (Fe + Co) was evaluated, evidencing that structure, redox properties and electrochemical properties change in relation to the B-site composition. In particular, nonstoichiometry (Fe + Co = 0.86) was the most impactful factor in enhancing oxygen mobility and adsorption capacity: this evidence was explained by the formation of ceria and the Ruddlesden–Popper (RP) phase Sr₃Fe₂O_7−δ that were segregated during the synthesis from the unbalanced nonstoichiometric B-site. These extra phases are indeed very well-known for their interaction with oxygen. As a consequence, the nonstoichiometric sample reached the highest oxygen reduction ability at 600 °C (a typical IT-SOFC operating temperature) and the lowest activation energy (0.65 eV).

Other types of B-site-doped Ce-SrFeO_3−δ have been investigated, as in the study by Li et al. [158], partially replacing Fe⁴⁺ with Ru⁴⁺. Such materials were prepared by the combustion method and designed as electrodes in a symmetrical cell operating at an intermediate temperature, with the aim of exploiting cerium doping to increase the structural stability under fuel conditions and ruthenium doping to do the same in ambient air. The results showed that Ce doping effectively enhanced the stability of the perovskite in a reducing atmosphere, and the Ce/Ru coexistence in the perovskite structure induced the formation and segregation of nanoscale SrO, CeO₂ and Ru⁰. At 800 °C, a Sr_{0. 8}Ce_0.2Fe_0.95Ru_0.05O_3−δ-based SSOFC with a La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ electrolyte showed small polarization resistance values and excellent peak power densities not only in H₂ fuel but also in propane. The exsolution of Ru⁰ was important to the H₂ oxidation, though the superficial ceria was important to the absorption of C₃H₈ molecules. Moreover, the simultaneous superficial exsolution of SrO, compared with that in the sample with ceria only, was found to be important for coke resistance under carbonaceous fuel. All these data point out the possibility of favorably exploiting the formation of extra phases, formed in situ together with the main strontium ferrate phase and, therefore, in intimate contact.

The co-presence of Ce and Ru was also exploited by Fu et al. [175], who produced electrocatalysts in which Sr_0.9Ce_0.05Fe_0.95Ru_0.05O₃ oxides were hybridized with in situ grown RuO₂ nanoparticles (SCFR–RuO₂). These materials were shaped as nanofibers through the electrospinning technique, using Sr, Ce, Fe and Ru salts as precursors, solubilized in DMF and in the presence of PVP.

After the electrospinning procedure, the obtained fibers were calcined to remove the PVP polymer, and then the samples were subjected to a cycle of reduction and oxidation to provoke, respectively, Ru in situ exsolution and oxidation and achieve the final product SCFR–RuO₂. The presence of RuO₂ effectively enhanced the OER catalytic activity and could endow a low work function to the nanofibers, favoring the electron transfer during the catalytic reaction, as also confirmed by density functional theory (DFT) calculations. However, the work does not highlight cerium’s central role in the activity of the materials. The paper of She’s group [176] describes a Ce/Ni-doped strontium ferrate for generic use in OER, which represents a crucial electrochemical process for various energy conversion and fuel production technologies (see polarization curves for SrFeO₃, Sr_0.95Ce_0.05FeO_3−δ and Sr_0.95Ce_0.05Fe_0.9Ni_0.1O_3−δ in Figure 10a). The compound Sr_0.95Ce_0.05Fe_0.9Ni_0.1O_3−δ (SCFN) was proven to drive an enhancement in mass-specific activity, which amounted to more than an order of magnitude compared to the undoped parent oxide. The material also demonstrated an attractive small overpotential and high reaction rate during the OER cycles (Figure 10b), in addition to robust operational durability with negligible activity loss under alkaline OER conditions. It was again ascertained that the A/B-site doping induced the synergistic promotion of structural and electronic modulation as in Figure 10c, ascribing to Ce the formation of a 3D corner-sharing cubic structure and to Ni the generation of strong electronic interactions between the active sites and OER intermediates, thanks to the high metal–oxygen covalency and high-valence Fe⁴⁺ and Ni³⁺ ions, which are beneficial for promoting charge transfer.

A similar composition but a diverse approach was adopted by the group of Ni [158,177] in which an alcohol-fueled SSOFC was based on the activity of Sr_0.8Ce_0.2Fe_0.95Ni_0.05O_3−δ or Sr_0.8Ce_0.2Fe_0.95Ru_0.05O_3−δ. The exsoluted CeO₂ and FeNi₃ (precipitated in a reducing environment, as visible in Figure 9d) contributed to fuel reforming, C-C bond cleavage and coke consumption in the anode chamber. Sr_0.8Ce_0.2Fe_0.95Ru_0.05O_3−δ and Sr_0.8Ce_0.2Fe_0.95Ni_0.05O_3−δ for symmetrical cells (SSOFCs) have good electrical conductivity, coke resistance and dynamic stability thanks to the reversible cerium oxide and metal exsolution from the perovskite. In a recent study, Wu et al. found that Sr_0.9Ce_0.1Fe_0.9Co_0.1O₃ symmetric electrodes were very active in CO₂ electrolysis due to Ce exsolution and the formation of a Co–Fe alloy upon reduction treatment [173]. The composition with 10 mol% Ce was more effective than that with 5 mol% Ce, suggesting an important role of cerium dopant in the electrocatalytic process.

The combination of Ce at the A-site and Ni at the B-site was recently proposed again by Cui et al. [79], who synthesized a (Sr_0.8Ce_0.20)_0.95Fe_0.9Ni_0.1O_3−δ to be used in PCFC, although the role of cerium was mentioned only marginally. In that work, a better thermal matching between the electrode and the electrolyte and an increase in oxygen vacancies at the operating temperature, accelerating the cathodic ORR, were attributed to the presence of nickel. At the anode side, Ni also improved the stability of the symmetric electrode, reduced the impedance activation energy and globally lowered the polarization resistance of symmetrical cells [79].

It must be noted that the comparison of the cerium effect in electrocatalytic applications with other A-site SrFeO_3−δ dopants is not straightforward since these elements are often present within different compositions, varying in the B-site. Some examples reported in the literature concern La_0.5Sr_0.5Fe_0.9Nb_0.1O_3−δ [178], Ln_0.5Sr_0.5Fe_0.9Mo_0.1O_3−δ [179] (where Ln represents lanthanides as La, Pr and Nd), Sr_0.6Pr_0.4Fe_0.7Co_0.2Nb_0.1O_3−δ [180], Sm_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ [181,182] and Sr_0.5La_0.5Fe_0.45Co_0.45Nb_0.1O_3−δ [183]. The main results for the simplest systems are summarized in Table 1, which highlights the importance of a precise tuning of site A in the SrFeO₃ electrocatalysts, as it can seriously influence the physical–chemical characteristics (particularly acting on the oxygen deficiency and mobility), performance, stability, durability and cost of electrochemical devices.

Table 1. Main findings on the effects of cerium and other A-site dopants on the functional properties and electrochemical performance of SrFeO₃ perovskites.

Compound	Application	Main Results	References
Sr0.94Ce0.06FeO3−δ	SOFCs	Cubic structure; Ce solubility < 0.15; Ce optimal amount (0.06) improved structural and redox properties with the lowest ASR.	[77]
Sr0.9Ce0.1FeO3	Electrochemical cell	Structure: a mixture of tetragonal and orthorhombic phases; Ce solubility ≤ 0.15; Ce doping caused high oxygen exchange and electrical conductivity.	[91,101]
Sr0.5Sm0.5FeO3−δ	SSOFCs	Cubic structure (stable under reducing conditions up to 750 °C); Sm solubility(max) reported = 0.5; the material exhibited good structural stability and catalytic activity in a redox atmosphere at IT and a semiconductor behavior both in cathode and anode atmospheres.	[184]
Sr0.5Bi0.5FeO3−δ	IT-SOFCs	Cubic structure; Bi solubility(max) reported = 0.5; Bi-doping promoted TEC compatibility, oxygen ionic mobility and low ASR.	[185]
Sr0.7Ba0.3FeO3−δ	MIEC systems	Cubic structure (orthorhombic after reduction); Ba solubility = 0.5; Ba optimal amount (0.3) enhanced the oxygen permeability and reduction tolerance.	[186]
Sr10.8Gd0.2FeO3−δ	SSOFCs	20% Gd improved electrocatalytic activity compared with recognized perovskite oxides like La0.75Sr0.25Cr0.5Mn0.5O3−δ and Sr2Fe1.5Mo0.5O6−δ.	[113]
Sr0.35Pr0.65FeO3−δ	SSOFCs	(Sr1−xPrx)0.95FeO3−δ series had a progressive phase transition up to the orthorhombic phase for x ≥0.6. The total electrical conductivity and the polarization resistance in air were comparable across the series, while the highest conductivity and lowest polarization resistance under reducing conditions were for Pr0.6, which was identified as optimal, with moderate thermal expansion coefficients and improved electrical properties and stability.	[132]

Table 1. Main findings on the effects of cerium and other A-site dopants on the functional properties and electrochemical performance of SrFeO₃ perovskites.

Compound	Application	Main Results	References
Sr0.94Ce0.06FeO3−δ	SOFCs	Cubic structure; Ce solubility < 0.15; Ce optimal amount (0.06) improved structural and redox properties with the lowest ASR.	[77]
Sr0.9Ce0.1FeO3	Electrochemical cell	Structure: a mixture of tetragonal and orthorhombic phases; Ce solubility ≤ 0.15; Ce doping caused high oxygen exchange and electrical conductivity.	[91,101]
Sr0.5Sm0.5FeO3−δ	SSOFCs	Cubic structure (stable under reducing conditions up to 750 °C); Sm solubility(max) reported = 0.5; the material exhibited good structural stability and catalytic activity in a redox atmosphere at IT and a semiconductor behavior both in cathode and anode atmospheres.	[184]
Sr0.5Bi0.5FeO3−δ	IT-SOFCs	Cubic structure; Bi solubility(max) reported = 0.5; Bi-doping promoted TEC compatibility, oxygen ionic mobility and low ASR.	[185]
Sr0.7Ba0.3FeO3−δ	MIEC systems	Cubic structure (orthorhombic after reduction); Ba solubility = 0.5; Ba optimal amount (0.3) enhanced the oxygen permeability and reduction tolerance.	[186]
Sr10.8Gd0.2FeO3−δ	SSOFCs	20% Gd improved electrocatalytic activity compared with recognized perovskite oxides like La0.75Sr0.25Cr0.5Mn0.5O3−δ and Sr2Fe1.5Mo0.5O6−δ.	[113]
Sr0.35Pr0.65FeO3−δ	SSOFCs	(Sr1−xPrx)0.95FeO3−δ series had a progressive phase transition up to the orthorhombic phase for x ≥0.6. The total electrical conductivity and the polarization resistance in air were comparable across the series, while the highest conductivity and lowest polarization resistance under reducing conditions were for Pr0.6, which was identified as optimal, with moderate thermal expansion coefficients and improved electrical properties and stability.	[132]

3.3. Oxygen Carriers for Chemical Looping Technologies

Beyond electrochemical devices, doped strontium ferrates, including Ce-doped ones, have been successfully employed in chemical looping technology (CL) [107,187]. The general chemical looping concept implies that a metal oxide, acting as an oxygen carrier, releases its lattice oxygen and is reduced to a lower oxidation state of the metal. The reduced oxygen carrier is then withdrawn from the site of this reaction; separately re-oxidized (e.g., with air), closing the chemical loop; and recycled to repeat the cycle [187]. Such reversible reduction and oxidation can be applied in several processes, such as airless combustion, where the oxygen needed for fuel conversion comes from the solid material instead of air [188] (generating a significant improvement in safety conditions [187]). Besides combustion, chemical looping in the presence of reduced metal oxides has also been proposed to produce combustibles such as hydrogen and/or carbon monoxide starting from H₂O and/or CO₂ [188]. It is worth recalling that a mixture of H₂ and CO, forming syngas, can be further converted into hydrocarbon fuel [189,190]. An example of a chemical looping gasification process is reported in Figure 11.

The main characteristics that a suitable oxygen carrier for CL must possess are good oxygen capacity and mobility; favorable thermodynamics; the capability to be re-oxidized rapidly; resistance to attrition, melting, agglomeration and carbonation; the ability to withstand many redox cycles; reasonable cost and convenience of supplying the constitutive elements; and avoidance of rare (e.g., lanthanum) or toxic (e.g., cobalt) metals [187,188]. Thanks to their advantageous features considered throughout this review, strontium ferrates have been taken into account for oxygen storage and air separation processes (for producing purified oxygen and nitrogen): in particular, the low temperature at which these mixed oxides start losing oxygen, even at high pO₂, has been identified as the primary advantage, but these materials also showed potential applicability in hydrogen production and catalytic CH₄ combustion [111,188]. Moreover, in a paper reporting the epoxidation of ethylene to ethylene oxide using chemical looping, Ag was supported by the combination of a cubic SrFeO₃ and a layered Ruddlesden–Popper Sr₃Fe₂O₇ phase (at different ratios), using Ag as the catalyst while the perovskite was the oxygen carrier. From the results, which showed the maximum yield of ethylene oxide with a 1:1 SrFeO₃/Sr₃Fe₂O₇ ratio, the synergistic effect of the two phases, differing in crystal structure, chemical potential and solid-state oxygen transport, was verified [187].

In this vein, bare and A- or B-doped strontium ferrate sorbents have been demonstrated to possess suitable thermodynamic properties, excellent redox stability and oxygen-carrying capacity at relatively low temperatures [46,111,112,192,193]. Regarding Ce-containing samples, Marek et al. prepared different specimens either by mechanical mixing of CeO₂ with the precursors of SrFeO_3−δ and calcining together or by depositing CeO₂ on the SrFeO_3−δ particles by impregnation [194]. Both the material types were further impregnated with the Ag catalyst and again employed in the epoxidation of ethylene, exploiting the CL setup (see Figure 12a). In the scheme of ethylene epoxidation by the chemical looping operation mode, MeOx represents the oxygen carrier with a depleted reservoir of O_lattice, whereas MeO_y represents the regenerated oxygen carrier.

The outcomes demonstrated that the chosen systems reached performances close to the values obtained in the epoxidation process with oxygen in the gas phase (up to 60% selectivity and 10% C₂H₄ conversion). Higher conversion and selectivity were observed when the oxygen reservoir, namely the strontium ferrate, was enriched with ceria (5 mol%) on the surface but particularly in the bulk, confirming the strong dependence on the solid support as a direct donor of O_lattice to the silver catalyst. The presence of CeO₂ increased the chemical potential of oxygen (μO₂) in the supports, improving the rate of reduction and reoxidation. Finally, the theoretical benefits of the chemical looping epoxidation were mainly ascribed to the non-use of gaseous oxygen in reduction, allowing higher conversions in a single step and avoiding the cost of purified oxygen and the risk of creating explosive mixtures. Within the same group pursuing this research topic, the study of the solid mixture of CeO₂–SrFeO₃ was followed by the preparation of a doped perovskite with the nominal composition Sr_0.95Ce_0.05FeO₃ [194]. Although the study was explicitly focused on the influence of the preparation procedure and of the Ag catalyst particle size on the charge transfer and the overall CL epoxidation performance, some other considerations can be made. First, the Ce-doped perovskite was labeled Sr_1−xCe_xFeO₃ (x ≤ 0.05) since the authors could not exclude the presence of segregated ceria derived from the synthesis, even after a prolonged calcination process. Regarding the application, among the samples tested, the best performance in terms of selectivity and conversion was displayed in the presence of Sr_1−xCe_xFeO₃. Compared to bare SrFeO₃, the Ce-doped oxygen carrier was the most active in chemical looping epoxidation, attaining approximately 60% selectivity and 15% C₂H₄ conversion thanks to its superior oxygen diffusion ability. Such fast ionic conduction driven by the cerium addition, although in a small amount, allowed the metal oxide to donate more oxygen [194]. In a recent paper [194], Ag and Ce were combined again with strontium ferrate; in particular, the redox behavior of a nonstoichiometric perovskite oxide modified with Ag, CeO₂ and Ce was assessed for chemical looping air separation. The results, derived from thermogravimetric analysis and from the cyclic release/uptake of O₂ in a packed-bed reactor, demonstrated that the addition of ca. 15 wt.% of silver at the surface of SrFeO_3−δ lowered the temperature of oxygen release in N₂ by approximately 60 °C and more than tripled the amount of oxygen released per CL cycle at 500 °C (Figure 12b). If the impregnation of SrFeO_3−δ with Ag increased the concentration of oxygen vacancies at equilibrium, the addition of ceria on the surface or into the bulk of the perovskite resulted in more modest changes, with a decrease in temperature for O₂ release of about 10–20 °C and a moderate increase in oxygen yield per reduction cycle. In detail, the strontium ferrate structurally doped with Ce to form Sr_0.95Ce_0.05FeO_3−δ showed a higher T_onset at around 411 °C and a slower rate of oxidation in air but a faster rate of reduction in N₂ than unmodified SrFeO_3−δ.

Ce-modified SrFeO_3−δ was also investigated for oxidative dehydrogenation (from now on, ODH) of ethane coupled with CO₂ splitting (hereinafter, CS) through a chemical looping scheme (Figure 12c) [90]. The importance of joining ODH and CS reactions lies in some issues connected to the conversion of ethane to ethylene, which represents one of the most common raw materials as a base for other chemicals. Thermal steam cracking and direct dehydrogenation of ethane are conventional routes for ethylene production that require high energy consumption, remarkable coke deposition and significant NO_x and CO₂ emissions. Alternatively, ODH in the presence of oxygen can convert ethane at lower operating temperatures, but it can lead to over-oxidation, resulting in poor ethylene selectivity and a high CO₂ penalty. A new strategy was proposed based on the exploitation of carbon dioxide as a mild oxidant for the dehydrogenation of ethane with the CO co-production from CO₂ dissociation. Despite the various advantages, this latter process requires a relative excess of CO₂ to C₂H₆ in the feeding gas stream, leading to low CO₂ conversion and increasing the burden of downstream product separation. In this context, the CL method can overcome the presented problems by adopting this scheme: ethane is first converted into ethylene and water in the oxidative dehydrogenation reactor, where a selective oxygen carrier provides the oxygen source required. The subsequent step occurs in the CO₂ splitting reactor, where the reduced oxygen carrier regenerates itself by CO₂ exposure, and in the meantime, CO is co-produced. For this application, Tian et al. [90] selected different samples prepared by the sol–gel method: a SrFeO₃ calcined at 850 °C, a SrFeO₃ calcined at 1200 °C and two Ce-doped strontium ferrates with Sr substitution fractions of 0.2 and 0.4, labeled 0.2 Ce/SrFeO₃ and 0.4 Ce/SrFeO₃, respectively. In commenting on the preparation/characterization of the samples, the authors claimed that the introduction of Ce was found to facilitate the formation of the SrFeO_3−δ perovskite phase under milder calcination conditions as compared to the sample without doping. However, as is reasonable, given the significant fraction of A-site doping, characteristic peaks indexable to segregated CeO₂ were also observed in the X-ray diffractograms related to both Ce-modified samples. The results concerning the ODH-CS reactions showed that a proper amount of Ce in the A-site tended to improve the activity of the sample by altering the reducibility, the Fe²⁺/(Fe³⁺ + Fe⁴⁺) ratio and the number of active oxygen species on the near-surface of the particle. The 0.2 Ce/SrFeO₃ sample, during 39 cycles of redox testing, displayed up to 29% ethane conversion and 82% ethylene selectivity, and DFT calculations for this sample further revealed that the increased ODH activity was ascribable to the lower surface oxygen vacancy formation energy. Regarding the surface oxygen activity of the segregated CeO₂ cluster, it was estimated to be relatively poor in comparison with that of the strontium ferrate substrate, as indicated by the much higher oxygen vacancy formation energy of the ceria cluster. From this perspective, too much Ce doping would hinder the overall activity of the oxygen carrier, which can also be confirmed by the lower activity of 0.4 Ce/SrFeO₃ in the ODH reaction. With respect to the CS step, CO generation by 0.2 Ce/SrFeO₃ was maintained at a stable CO₂ conversion of about 69.4% over the cycles, also retaining perovskite structural stability. Finally, CO₂ regeneration was preferred to that by O₂ since oxygen led to a higher proportion of non-selective electrophilic oxygen species on the near-surface of the regenerated sample and an inferior ethylene selectivity.

3.4. Other Applications

It has been reported that cerium doping in SrFeO₃ perovskite-type structures highly enhances their low-temperature gas sensing ability of SrFeO₃ towards hydrogen peroxide, promoting Fe⁴⁺-to-Fe³⁺ reduction [78]. An amperometric sensor based on Sr_0.85Ce_0.15FeO₃ perovskite shows good stability, reproducibility and cost-effectiveness and could be very attractive for the manufacture of non-enzymatic hydrogen peroxide sensors. Peak potential decreases and peak current increases until reaching a plateau, indicating that both depend on the cerium inside the perovskite and not on the segregated cerium that forms when the solubility limit is overcome. It is worth noting that sensing properties have also been detected in Ce-doped BaFeO₃, another perovskite with Fe in a 4+ oxidation state, where the addition of 5 mol% Ce at the A-site of BaFeO₃ increases the electrical conductivity of BaFeO₃ by one order of magnitude [96]. In this case, the sensing ability towards oxygen was observed in the temperature range of 500–700 °C.

Iron perovskite oxides, including doped SrFeO₃ materials, can be successfully used as supercapacitors for energy storage [196,197]. Although Ce-doped SrFeO₃ materials have never been investigated as supercapacitors, they might be potentially used in energy storage thanks to the stability of the redox couple Fe⁴⁺–Fe³⁺.

Ce-doped SrFeO₃ materials with Co-Cu co-doping at the B-site were used as multipurpose compounds, as recently reported by Tummino et al. [99]. Specifically, two oxide powders with the compositions Sr_0.85Ce_0.15Fe_0.67Co_0.23Cu_0.10O_3−δ and Sr_0.85Ce_0.15Fe_0.67Co_0.13Cu_0.20O_3−δ were synthesized by solution combustion synthesis. They were tested both as catalysts of soot oxidation after high-temperature activation and as antibacterial agents in ambient conditions or activated by both UV exposure and low-temperature excitation to induce the generation of reactive species (ROS). The outcomes showed that these compounds reacted differently to various stimuli, being significantly efficient under high-temperature and UV radiation, and that the increasing amount of copper, together with the presence of a proportionally segregated ceria phase, positively influenced the materials’ features and performances. The antibacterial properties were of particular interest, since, beyond the well-known antimicrobial properties of copper—especially as nanoparticles [198]—such activity has also been reported for bare [199] and doped strontium ferrate perovskite oxides themselves [200]. In general, the proposed mechanism involves the SrFeO₃ surface reactivity promoted by (i) the nano-sized structure that physically promotes perovskite–microorganism interactions, (ii) the ability of perovskite cations exposed charges to damage the pathogen cell membrane, (iii) convenient electronic state occupancy in the presence of cations with multiple oxidation states and (iv) oxygen vacancies.

4. Limitations and Challenges for Technological Applications of Ce-Doped SrFeO₃

In Figure 13, the four main limitations of Ce-doped SrFeO₃ perovskite oxides are displayed. Synthesis and processing conditions are of paramount importance to the properties and applications of Ce-doped SrFeO₃, which can be finely tuned and exploited thanks to the knowledge of the synthesis–structure–property relationships. In fact, layered perovskite can be formed along with the main perovskite phase (Figure 13a), and a good control of the synthesis conditions used is required to obtain single-phase perovskite oxides. Solution combustion synthesis, which was the preparation method chosen in most of the described works [68,69,70,76,130,151,154], is rather reproducible on a lab scale—provided that the parameters affecting the powder properties are all known and controlled—but it requires some further investigation to be applied for industrial production.

The long-term operation of depolluting units based on strontium ferrates is challenged by catalyst deactivation, which can occur due to the formation of surface carbonates [130,201] and metal leaching [130]. It has been reported that SrCO₃ forms on the surface [87,130] (Figure 13b) after prolonged storage in ambient conditions and especially during catalytic reactions, for instance, when Sr-containing perovskites are employed as thermocatalysts [201] and electrocatalysts [202]. A solution was found for this issue, since Sr from SrCO₃ can be re-incorporated at the A-site of the perovskite upon a fast, high-temperature regenerative treatment under air at 1000 °C [100]. When the problem affects the performance in operando, a preliminary strategy must be adopted. In this regard, Liu et al. synthesized, via a sol–gel method, different SrFe_0.9M_0.1O_3−δ compounds (where M stands for Fe, Al, Zr, Nb, W) as potential components of oxygen transport membranes and solid oxide fuel cell electrodes [203]. The design and modification of the B-site dopant served to understand the effect on stability and CO₂ tolerance. It was found that Zr⁴⁺, Nb⁵⁺ and W⁶⁺ ions inhibited carbon dioxide adsorption, improving the related tolerance. From another point of view, it is possible to observe that the interaction with carbon dioxide can be exploited in systems where CO₂ is the substrate for further reactions (Section 3.2 and Section 3.3).

Metal leaching can occur in solution, representing another issue when working in a water matrix. Figure 13c shows the metals that can be released from Ce-doped SrFeO₃ at different pHs, evidencing that strontium is the element most affected by this phenomenon [130]. However, operating at pH > 5, the perovskite presents good stability. A similar result was obtained for Sr_0.85Ce_0.15Fe_0.67Co_0.33−xCu_xO₃ materials (described in Section 3.4), thus excluding the role of homogeneous phenomena involving iron, copper and cobalt species that could have a role as solubilized ions, both as catalysts and/or antibacterial agents. Careful regulation of structural stability and synthesis methodology can limit the leaching extent [204,205].

Lastly, the low surface area is typical of perovskite-type oxides and does not exceed 30 m²/g [206]. SrFeO₃ perovskite oxides have an even lower surface area, below 10 m²/g, depending on the synthesis/processing conditions and on the cerium content [77,116,130]. As an example, the trapped unburned organic carbon of mixed oxides prepared by solution combustion synthesis can be a cause of a further decrease in surface area and porosity. Special procedures to obtain perovskite oxides with better surface area and porosity have been proposed [207,208], and the same strategies can be applied as well to Ce-doped SrFeO₃ materials. For example, Otaguro et al. obtained mesoporous SrFeO₃ from hydro-garnets after an acid-washing treatment to free the perovskite pores from the SrCO₃ [208]. Materials prepared by solution combustion synthesis and post-treated by a washing step underwent the release of synthesis residuals from pores, increasing surface area and pore volume (Figure 13d), as discussed at the end of Section 2.3.4. However, it should be remembered that increasing the surface area of perovskite oxides and simultaneously introducing and/or maintaining the Ce doping in SrFeO₃ is not always a trivial task. Therefore, a more convenient solution for the full exploitation of these materials for technological applications could be the adoption of strategies able to enhance their oxygen vacancy content/mobility [209,210], defectivity [211,212] and surface morphology [213].

5. Conclusions and Perspectives

Ce-doped SrFeO₃ perovskites can be considered sustainable materials to address energy and environmental issues for three main reasons, which are reflected in their applicability in diverse technological fields. Figure 14 provides an overview of the main characteristics, applications and limitations of Ce-doped strontium ferrate discussed in this work.

Indeed, (i) Ce-doped SrFeO₃ perovskite oxides exhibit improved properties compared to undoped SrFeO₃ perovskite oxides, allowing for their application as thermocatalysts, electrocatalysts, sensors and oxygen carriers for chemical looping reactions; (ii) they are versatile materials, as they can be successfully applied at both low and high temperatures and in oxidative or reductive environments; and (iii) the knowledge of their structures and physicochemical behavior as well as synthesis–structure–property relationships is very broad. The applicability of Ce-doped SrFeO₃ perovskite oxides can be exploited in various technological fields, as described in this review, from environmental remediation to bacterial abatement, from energy production/storage to oxygen transport-related fields.

Cerium doping improves the structural stability and properties of SrFeO₃. The primary effect of cerium is to stabilize the Fe⁴⁺/Fe³⁺ redox couple in a cubic structure, increasing oxygen mobility and enabling the perovskite to catalyze both reduction and oxidation reactions. The Fe⁴⁺/Fe³⁺ redox couple and the different geometric arrangements allowed the oxygens surrounding the iron to be key determinants of their properties and applications. Ce-doped SrFeO₃ perovskite oxides are a clear example of how a material can be shaped and tailored for a specific application and how a deep understanding of a material’s key properties extends its synthesis efficiency and applicability, making a substantial contribution to the sustainable development of materials for energy and the environment.

The main limitations related to the technological applicability of these materials—low surface area, carbonate formation, acidic leaching, and redox sensitivity—were discussed, and suggestions were provided to mitigate or overcome them. The high sensitivity of the iron oxidation state—and thus of the SrFeO₃ structure and properties—on the synthesis procedure can be a limitation if left unchecked, although it can also represent a valuable strategy for modifying the structure and properties according to the desired application. Similarly, the apparent disadvantage of exsolving elements from the parent perovskite structure by doping can be wisely exploited to create in situ formed nanocomposite materials with synergistic functionalities.

In the future, it will be essential to prepare these materials from waste-derived precursors to increase their sustainability. It is possible to target the metal precursors (i.e., rust as a source of iron or possibly other types of waste) and/or the synthesis mixture. For example, waste-derived organics can be used as fuel in solution combustion synthesis. Furthermore, to increase the TRL level of the research in the field, it is necessary to further investigate the reproducibility and scalability of the synthesis methods for industrial applications. In this regard, Deganello et al. proposed some solutions for the large-scale synthesis of perovskites [120]. Coupling with other materials or incorporation in industrial systems (i.e., graphene, nanofiltration units, etc.) are other viable strategies to make Ce-doped strontium ferrates increasingly versatile materials.

While this review is primarily intended as an informative article on Ce-doped SrFeO₃ materials, it also represents a useful source of information on the synthesis, structure and technological applications of perovskite oxides and contains important suggestions on limitations, challenges, and sustainability aspects of perovskite oxides, from their synthesis to their structure, properties and applications.