1. Introduction

1.1. General introduction to perovskites

The mineral CaTiO₃ was discovered by Geologist Gustav Rose in the Ural Mountains in 1839, and it was named perovskite in recognition beholden to Count Lev Alexevich von Perovski, an eminent Russian mineralogist [1–5]. The name perovskite represented any compound that has ABC₃ formula where an octahedron of C ions surrounded the B ion. The Earth’s crust contains various types of perovskites and the most abundant ones are MgSiO₃ and FeSiO₃. Perovskite family includes several types of oxides like transition metal oxides with the formula ABO₃. Some examples of ABO₃ perovskites and their corresponding properties are summarized in Table 1 [1, 2, 5].

Perovskite oxides exhibit an array of electrical properties and a variety of solid-state phenomena from insulating, semiconducting, metallic, and superconducting characters; therefore, they are very fascinating to be studied and applied in a large scale. Many of ABO₃ perovskites are cubic or nearly cubic in structure in their ideal form; however, one or more phase transitions may be achieved particularly at low temperature. In addition, many of them showed magnetic ordering and as a result, large variety of magnetic structures can be found. Some perovskites contained localized electrons, some contained delocalized energy-band states, and the behavior of other perovskites was a transition between these two types. The perovskite structures can incorporate ions of various size and charge showing great flexibility of composition. Moreover, substitutions of ions into the A- and/or B-sites or deviation from ideal stoichiometry resulted in altering the electronic properties of the perovskites. Perovskites exhibit atomic arrangement in the form of 3-dimensional array of corner sharing octahedra. On the other hand, layered perovskites included 2-dimensional layers of corner sharing octahedral separated by cations layers. As a result, the electronic energy bands of perovskites and layered perovskites are very unusual and their structure is unique in properties [1, 2]. Perovskites displayed diversity of electric, optical, and magnetic properties because of the fact that 90% of the elements in the periodic table can be stable in the perovskite structure and the feasibility of partial substitutions of cations in A- and B-sites forming A_1-xA′_xB_1-yB′_yO₃ [6]. Perovskites showed great interest in several applications due to their wide various and useful properties in photochromic, electrochromic, image storage, switching, filtering, and surface acoustic wave signal processing devices. They were utilized as catalytically active catalyst for several reactions like carbon monoxide and hydrocarbons oxidation, hydrogen evolution reaction and nitrogen oxides, and oxygen reduction reactions. They also have a good impact in many electrochemical applications like sensing, biosensing, photoelectrolysis of water-producing hydrogen, and fuel cells [1, 2].

Insulating	Metallic	Magnetic	Superconducting
			SrTiO₃ (n-type)
WO₃	ReO₃	PbCrO₃	Na_xWO₃ (t)
NaTaO₃	NaWO₃	LaCrO₃	K_xWO₃ (t)
SrTiO₃	KMoO₃	CaMnO₃	K_xWO₃ (h)
BaTiO₃	SrNbO₃	LaMnO₃	Rb_xWO₃(h)
KTaO₃	LaTiO₃	LaCoO₃	Cs_xWO₃ (h)
LiNbO₃	LaWO₃	LaFeO₃	Li_xWO₃ (h)

Table 1.

Some perovskites and corresponding properties [1].

t; tetragonal, h; hexagonal

1.2. Crystallography of the perovskite structure

In the ABO₃ form, B is a transition metal ion with small radius, larger A ion is an alkali earth metals or lanthanides with larger radius, and O is the oxygen ion with the ratio of 1:1:3. In the cubic unit cell of ABO₃ perovskite, atom A is located at the body center, atom B is located at the cube corner position, and oxygen atoms are located at face-centered positions (Figure 1). The 6-fold coordination of B cation (octahedron) and the 12-fold coordination of the A cation resulted in the stabilization of the perovskite structure. The perfect perovskite structure was described by Hines et al. as corner linked BO₆ octahedra with interstitial A cations [1–10]. Some distortions may exist in the ideal cubic form of perovskite resulted in orthorhombic, rhombohedral, hexagonal, and tetragonal forms (Figure 1) [3–7]. Figure 2 represented the distortion from cubic perovskite to orthorhombic one. In general, all perovskite distortions maintaining the A- and the B-site oxygen coordination was achieved by the tilting of the BO₆ octahedra and an associated displacement of the A cation [4].

V.M. Goldschmidt presented much of the early work on the synthetic perovskites and developed the principle of the tolerance factor t, which is applicable to the empirical ionic radii at room temperature [2–9]:

t= (rA+rO) / [21/2(rB+rO)],BB1

where r_A is the radius of the A-site cation, r_B is the radius of the B-site cation, and r_O is the radius of oxygen ion O^2–. The tolerance factor can be used to estimate the suitability of the combination of cations for the perovskite structure [2]. It is a real measure of the degree of the distortion of perovskite from the ideal cubic structure so the value of t tends to unity as the structure approaches the cubic form [4]. From the equation, the tolerance factor will decrease when r_A decreases and/or r_B increases. Based on the analysis of tolerance factor value, Hines et al. solely suggested that the perovskite structure can be estimated. For 1.00 < t < 1.13, 0.9 < t < 1.0, and 0.75 < t <0.9, the perovskite structure is hexagonal, cubic, and orthorhombic, respectively. For t < 0.75, the structure was adopted to hexagonal ilmenite structure (FeTiO₃) [4].

Generally, two requirements should be fulfilled for perovskite formation:

Electroneutrality; the perovskite formula must have neutral balanced charge therefore the product of the addition of the charges of A and B ions should be equivalent to the whole charge of the oxygen ions. An appropriate charge distribution should be attained in the forms of A¹⁺B⁵⁺O₃, A⁴⁺B²⁺O₃ or A³⁺B³⁺O₃.
Ionic radii requirements; r_A > 0.090 nm and r_B > 0.051 nm, and the tolerance factor must have values within the range 0.8 < t < 1.0 [2–8].

Figure 1.
Different perovskite unit cells. Blue spheres represent the A cations, yellow spheres represent the B cations and red spheres represent oxygen anions forming an octahedra [4].

Figure 2.
Perovskite distortion from (a) cubic to (b) orthorhombic [4].

1.3. Typical properties of perovskites

Perovskite exhibited a variety of fascinating properties like ferroelectricity as in case of BaTiO₃ and superconductivity as in case of Ba₂YCu₃O₇. They exhibited good electrical conductivity close to metals, ionic conductivity and mixed ionic and electronic conductivity. In addition, several perovskites exhibited high catalytic activity toward various reactions. Table 2 contains a summary of typical properties of perovskites. Several typical properties will be discussed in this section like ferroelectricity, magnetism, superconductivity, and catalytic activity [9].

Typical property	Typical compound
Ferromagnetic property	BaTiO₃, PdTiO₃
Piezoelectricity	Pb(Zr, Ti)O₃, (Bi, Na)TiO₃
Electrical conductivity	ReO₃, SrFeO₃, LaCoO₃, LaNiO₃, LaCrO₃
Superconductivity	La_0.9Sr_0.1CuO₃, YBa₂Cu₃O₇, HgBa₂Ca₂Cu₂O₈
Ion conductivity	La(Ca)AlO₃, CaTiO₃, La(Sr)Ga(Mg)O₃, BaZrO₃, SrZrO₃, BaCeO₃
Magnetic property	LaMnO₃, LaFeO₃, La₂NiMnO₆
Catalytic property	LaCoO₃, LaMnO₃, BaCuO₃
Electrode	La_0.6Sr_0.4CoO₃, La_0.8Ca_0.2MnO₃

Table 2.

Typical properties of perovskite oxides [9].

1.3.1. Dielectric properties

There are some properties inherent to dielectric materials like ferroelectricity, piezoelectricity, electrostriction, and pyroelectricity. One of the important characteristic of perovskites is ferroelectric behavior, which is obvious in BaTiO₃, PdZrO₃, and their doped compounds. The ferroelectric behavior of BaTiO₃ was strongly related to its crystal structure. BaTiO₃ was subjected to three phase transitions; as the temperature increases, it was converted from monoclinic to tetragonal then to cubic. At temperature higher than 303 K, BaTiO₃ does not show any ferroelectric behavior as it crystallizes into cubic structure. BaTiO₃ showed high dielectric constant based on the anisotropy of its crystal structure resulting in large dipole moment generation in BaTiO₃ [9].

1.3.2. Electrical conductivity and superconductivity

One of the obvious properties of perovskites is superconductivity. Cu-based perovskites act as high-temperature superconductors, and La-Ba-Cu-O perovskite was first reported. The presence of Cu in B-site is essential for the superconductivity and various superconducting oxides can be manufactured with different A-site ions. Some examples of high temperature superconductors are YBa₂Cu₃O₇, Bi₂Sr₂Ca₂Cu₃O₁₀, and HgBa₂Ca₂Cu₃O_8+δ with critical temperature of superconducting transition (Tc) of 130–155 K. The superconductivity is associated with the layers of Cu-O in Cu-based perovskites, and the Tc value is associated with the Cu-O layers number in the crystal lattice. The synthesis of 5 or more Cu-O-layered perovskites did not achieve successfully due to the low chemical stability. One of the highly significant superconductors with great Tc value is YBa₂Cu₃O₇. In addition, the oxygen nonstiochiometry is one of the most significant reasons for the high value of Tc. In YBa₂Cu₃O_7–δ, when the value of δ < 0.5, it crystallizes into orthorhombic structure, which is superconductive. For δ > 0.5, it showed a tetragonal structure that does not show any superconductivity. The crystal structure affected greatly the superconductivity in high Tc oxides, and as a result, high Tc values can be achieved by improving the chemical stability of the perovskite crystal structure. Furthermore, some perovskites exhibited great electronic conductivity similar to that of metals like Cu. LaCoO₃ and LaMnO₃ are examples of perovskites exhibiting high electronic conductivity, and therefore they are utilized as cathodes in solid oxide fuel cells displaying superior hole conductivity of 100 S/cm. The electronic conductivity of the perovskites can be enhanced by doping the A-site with another cation, which resulted in increasing the quantity of the mobile charge carriers created by the reparations of charge [9].

1.3.3. Catalytic activity

Perovskites showed excellent catalytic activity and high chemical stability; therefore, they were studied in a wide range in the catalysis of different reactions. Perovskites can be described as a model of active sites and as an oxidation or oxygen-activated catalyst. The stability of the perovskite structure allowed the compounds preparation from elements with unusual valence states or a high extent of oxygen deficiency. Perovskites exhibited high catalytic activity, which is partially associated with the high surface activity to oxygen reduction ratio or oxygen activation that resulted from the large number of oxygen vacancies. Perovskites can act as automobile exhaust gas catalyst, intelligent automobile catalyst and cleaning catalyst, etc., for various catalytic environmental reactions. It was reported in the literature that perovskites containing Cu, Co, Mn, or Fe showed excellent catalytic activity toward the direct decomposition of NO at high temperature, which is considered one of the difficult reactions in the catalysis (2NO → N₂+O₂). Perovskites showed superior activity for this reaction at high temperatures because of the presence of oxygen deficiency and the simple elimination of the surface oxygen in the form of a reaction product. NO decomposition activity was enhanced upon doping. Also, under an atmosphere that is rich with oxygen up to 5%, Ba(La)Mn(Mg)O₃ perovskite exhibited superior activity toward the decomposition of NO [9].

Perovskite showed a great impact as an automobile catalyst; intelligent catalyst. Pd-Rh-Pt catalysts was utilized as an effective catalyst for the removal of NO, CO and uncombusted hydrocarbons. There is another catalyst that consists of fine particles, with high surface-to-volume ratio, and can be utilized to reduce the amount of precious metals used. However, these fine particles exhibited very bad stability under the operation conditions leading to catalyst deactivation. Therefore, perovskite oxides can be used showing redox properties to preserve a great dispersion state. Upon oxidation, Pd is oxidized in the form of LaFe_0.57Co_0.38Pd_0.05O₃ and upon reduction; fine metallic particles of Pd were produced with radius of 1–3 nm. This cycle resulted in partial replacement of Pd into and sedimentation from the framework of the perovskite under oxidizing and reducing conditions, respectively, displaying a great dispersion state of Pd. Also, this cycle improved the excellent long-term stability of Pd during the pollutants removal from the exhaust gas. Exposing the catalyst to oxidizing and reducing atmosphere resulted in the recovery of the high dispersion state of Pd. This catalyst is known as intelligent catalyst because of the great dispersion state of Pd and the excellent stability of the perovskite structure [9].

2. Methods of perovskite synthesis

2.1. Solid-state reactions

In solid-state reactions, the raw materials and the final products are in the solid-state therefore nitrates, carbonates, oxides, and others can be mixed with the stoichiometric ratios. Perovskites can be synthesized via solid-state reactions by mixing carbonates or oxides of the A- and B-site metal ions corresponding to the perovskite formula ABO₃ in the required proportion to obtain the final product with the desired composition. They are ball milling effectively in an appropriate milling media of acetone or isopropanol [11, 12]. Then the obtained product is dried at 100 °C and calcined in air at 600 °C for 4–8 h under heating/cooling rates of 2 °C/min. After that, the calcined samples are ground well and sieved. Then it was calcined again at 1300–1600 °C for 5–15 h under the heating/cooling rate of 2 °C/min to confirm the formation of single phase of perovskite. Again grinding and sieving was carried out for the calcined samples [11, 13, 14]. The synthesis of BaCeO₃-based proton conductor perovskites [13] and BaCe_0.95Yb_0.05O_3−δ [11] was achieved through the previous methodology using BaCO₃, CeO₂, and Yb₂O₃ as the starting materials and isopropanol as the milling media [10].

2.2. Gas phase preparations

Gas phase reaction or transport can be used for the deposition of perovskite films with a specific thickness and composition. Laser ablation [15], molecular beam epitaxy [16], dc sputtering [17], magnetron sputtering [18], electron beam evaporation [19], and thermal evaporation [20] techniques were developed for gas phase deposition. Gas phase deposition can be categorized into three types: (i) deposition at a low substrate temperature then postannealing at high temperature, (ii) deposition at an intermediate temperature of 873 to 1,073 K then postannealing treatment, and (iii) deposition at the crystallization temperature under suitable atmosphere. YBa₂Cu₃O₇ films can be synthesized by the coevaporation of Y, Cu, and BaF₂ then annealing at high temperatures in O₂ atmosphere wet with water vapor to reduce the annealing time and substrate interaction [20].

2.3. Wet chemical methods (solution preparation)

These methods included the sol-gel preparation, coprecipitation of metal ions using precipitating agents like cyanide, oxalate, carbonate, citrate, hydroxide ions, etc., and thermal treatment [21], which resulted in a single-phase material with large surface area and high homogeneity. These methods presented good advantages such as lower temperature compared to the solid-state reactions, better homogeneity, greater flexibility in forming thin films, improved reactivity and new compositions and better control of stoichiometry, particle size, and purity. Therefore, they opened new directions for molecular architecture in the synthesis of perovskites. Solution methods were classified based on the means used for solvent removal. Two classes were identified: (i) precipitation followed by filtration, centrifugation, etc., for the separation of the solid and liquid phases and (ii) thermal treatment such as evaporation, sublimation, combustion, etc., for solvent removal. There are several factors must be taken in consideration in solution methods like solubility, solvent compatibility, cost, purity, toxicity, and choice of presumably inert anions [10].

2.3.1. Precipitation

2.3.1.1. Oxalate-based preparation

This method is built on the assimilation of oxalic acid with carbonates, hydroxides, or oxides producing metal oxalates, water, and carbon dioxide as products [22]. The solubility problem is minimized as the pH of the resulting solution is close to 7. An oxidizing atmosphere like oxygen was used during calcination to avoid the formation of carbide and carbon residues [23]. Clabaugh et al. utilized an aqueous chloride solution with oxalic acid to obtain unique and novel complex compound of BaTiO(C₂O₄)₂ 4H₂O as a precursor for the preparation of finely divided and stoichiometric BaTiO₃ [24].

2.3.1.2. Hydroxide-based preparation

This method is often used due to its low solubility and the possible variety of precipitation schemes. The sol-gel process can be used to produce a wide range of new materials and improve their properties. It presented some advantages over the other traditional methods like chemical homogeneity, low calcination temperature, room temperature deposition, and controlled hydrolysis for thin film formation. BaZrO₃ powders in its pure crystalline form can be prepared by the precipitation in aqueous solution of high basicity [25]. LaCoO₃ was prepared by the simultaneous oxidation and coprecipitation of a mixture containing equimolar amounts of La(III) and Co(II) nitrates producing a gel containing hydroxide then calcination at 600 °C [26].

2.3.1.3. Acetate-based preparation

Different perovskites were prepared by mixing acetate ions alone or together with nitrate ions with the metal ions salts. La_1-xSr_xCoO₃ with x = 0, 0.2, 0.4, 0.6 [27] was prepared using acetate precursors then calcination at 1,123 K in air for 5 h. La_1-xSr_xCo_1-yFe_yO₃ [28] was prepared using iron nitrate and strontium, cobalt, and lanthanum acetates then calcination at 1,123 K in air between 5 and 10 h.

2.3.1.4. Citrate-based preparation

Citrate precursors can be used and undergo several decomposition steps in the synthesis of perovskite [29]. These steps included the decomposition of citrate complexes and removal of CO₃^2– and NO₃^¯ ions. LaCo_0.4Fe_0.6O₃ can be prepared by this method, and the mechanism was investigated by thermogravimetry, XRD, and IR spectroscopy.

2.3.1.5. Cyanide-based preparation

Rare earth orthoferrites (REFeO₃) and cobalt compounds (RECoO₃) were prepared using cyanides complexes via thermal decomposition of the rare earth ferricyanide and cobalticyanide compounds [30]. LaFe(CN)₆ 6H₂O, LaCo(CN)₆ 5H₂O, and even ferrocyanides such as NH₄LaFe(CN)₆ 5H₂O are precipitated from the aqueous solution. This method presented some advantages like control of stoichiometry and low calcination temperature. The same method was used for the preparation of europium and other rare earth hexacyanoferrate compounds [31].

2.3.2. Thermal treatment

2.3.2.1. Freeze-drying

The freeze-drying method can be achieved through the following steps: (i) dissolution of the starting salts in the suitable solvent, water in most cases; (ii) freezing the solution very fast to keep its chemical homogeneity; (iii) freeze-drying the frozen solution to get the dehydrated salts without passing through the liquid phase; and (iv) decomposition of the dehydrated salts to give the desired perovskite powder. The rate of heat loss from the solution is the most important characteristic for the freezing step. This rate should be as high as possible to decrease the segregation of ice-salt. Also, in case of multicomponent solutions, the heat loss rate should be high to prevent the large-scale segregation of the cation components [10, 12, 21].

2.3.2.2. Plasma spray-drying

This method was applicable to various precursors, including gaseous, liquid, and solid materials. It was applied for the preparation of various ceramic, electronic, and catalytic materials. It presented many advantages in terms of economy, purity, particle size distribution, and reactivity. This method was achieved through two steps: (i) injection of the reactants and (ii) generation and interaction of the molten droplets (with substrate or with the previously generated droplets). The thick film of YBa₂Cu₃O_x covering large areas was prepared via this approach, and the optimum superconducting oxide phase was obtained by varying the preparation conditions like plasma parameters, substrate temperatures, and film postdeposition treatment [32].

2.3.2.3. Combustion

A redox reaction, which is thermally induced, occurs between the oxidant and fuel. A homogenous, highly reactive, and nanosized powder was prepared by this method. When compared with the other traditional methods, a single-phase perovskite powder can be obtained at lower calcination temperatures or shorter reaction times. One of the most popular solution combustion methods is citrate/nitrate combustion, where citric acid is the fuel and metal nitrates are used as the source of metal and oxidant. It is similar to the Pechini process “sol-gel combustion method” to a large extent, but in citrate/nitrate combustion, ethylene glycol or other polyhydroxy alcohols are not used. In addition, in citrate/nitrate combustion, the nitrates are not eliminated in the form of NO_x, but they remain in the mixture with the metal-citrate complex facilitating the auto-combustion. Iron, cobalt, and cerium-perovskite can be prepared via citrate/nitrate combustion synthesis [12, 33]. In addition, uniform nanopowder of La_0.6Sr_0.4CoO_3−δ was prepared by the combined citrate–EDTA method, where the precursor solution was made of metal nitrates, citric acid, and EDTA under controlled pH with ammonia [34]. La_0.8Sr_0.2Co_0.2Fe_0.8O_3−δ [35] and Sr- or Ce-doped La_1−xM_xCrO₃ catalysts [36] were prepared by citrate/nitrate combustion method. Furthermore, the Pechini “citrate gel” process includes two stages: (i) a complex was formed between the metal ions and citric acid, then (ii) the produced complex was polyesterified with ethylene glycol to maintain the metal salt solution in a gel in a homogenous state. This approach presented some advantages like high purity, minimized segregation, and good monitoring of the resulting perovskite composition. LaMnO₃ [37, 38], LaCoO₃ [39–41], and LaNiO₃ [42] were prepared by citric acid gel process producing nanophasic thin films [10].

2.3.2.4. Microwave synthesis

The microwave irradiation process (MIP), evolving from microwave sintering, was applied widely in food drying, inorganic/organic synthesis, plasma chemistry, and microwave-induced catalysis. MIP showed fascinating advantages: (i) fast reaction rate, (ii) regular heating, and (iii) efficient and clean energy. The microwave preparations were achieved in domestic microwave oven at frequency of 2.45 GHz with 1 kW as the maximum output power. Dielectric materials absorbed microwave energy converted directly into heat energy through the polarization and dielectric loss in the interior of materials [43]. The energy efficiency reached 80–90% which is much higher than the conventional routes. MIP was recently utilized to prepare perovskites nanomaterials reducing both the high temperature of calcination (higher than 700 °C) and long time (greater than 3 h) required for pretreatment or sintering [10]. GaAlO₃ and LaCrO₃ perovskites with ferroelectric, superconductive, high-temperature ionic conductive and magnetic ordering properties, faster lattice diffusion, and grain size with smaller size were prepared in MIP [44–47]. CaTiO₃ powders prepared in MIP presented a fast structural ordering than powders dealt in ordinary furnace [48]. Hydrothermal conventional and dielectric heating were utilized to prepare La–Ce–Mn–O catalysts. Hydrothermal MIP leads to formation of La_1−xCe_xMnO_3+εCeO₂ (x + ε = 0.2) with enhanced catalytic activity [49] while using the conventional heating methods lead to formation of LaMnO₃ + CeO₂. Moreover, nanosized single-phase perovskite-type LaFeO₃ [50], SmFeO₃, NdFeO₃, GdFeO₃, barium iron niobate powders [51], KNbO₃ [52], PbWO₄ [53], CaMoO₄ [54] and MWO₄ (M: Ca, Ni) [55], strontium hexaferrite [56], and SrRuO₃ [57] were prepared in MIP showing finer particles, higher specific surface areas and shorter time for synthesis of single crystalline powders.

3. Doping of perovskites

The different properties of perovskites and their catalytic activity are highly affected by the method of synthesis, conditions of calcination (time, atmosphere, fuel, temperature, etc.), and A- and/or B-site substitutions. The catalytic activity of the perovskite is highly affected by partial or total substitutions on A- and/or B-site cations because of the oxidation state modification, the variation of the chemical state of the elements at A- and/or B-site, the generation of oxygen vacancies, the mobility of oxygen lattice, and the formation of structural defects [58–60]. The powerful bond between the B-site metal ions and the oxygen ions can be used to determine the basic characters of perovskites, and as a result, the B-site cation is responsible for the perovskite catalytic activity [61, 62]. Therefore, partial substitution of B-site cation with other metals M in AB_1-yM_yO₃ will display the properties of both metals: the main metal B and the dopant one M [62]. On the other hand, the cation A can stabilize the unusual oxidation states of B-site cations by the controlled formation of crystal lattice vacancies, which lead to different catalytic performances [61].

Upon doping A- and/or B-sites in ABO₃ perovskite oxides, the catalytic activity, ionic and electronic conductivity, and flexible physical and chemical properties can be altered for utilization in various applications [63–66]. Different cations with different sizes and charges can be hosted in the A- and B-sites of these perovskites; thus, many studies can be performed to utilize doped perovskites in various applications. Multiple cationic substitutions can be accepted in the stable perovskite lattice provided that Goldschmidt tolerance factor ranged between 0.75 and 1 and electroneutrality are preserved [59, 67, 68]. Therefore, variable amounts of different structural and electronic lattice defects can be accommodated in the perovskite structure as a result of their nonstoichiometry. This will further affect the activity of the perovskite and stabilize the unusual valence states of different metal ions [61, 67]. Some physical characteristics of perovskite-type oxides seriously associated with structural characters were affected greatly by the structural deformations from the ideal cubic structure of the perovskite [69].

The type of the metal ion at the B-sites and their partial substitutions can be used to determine the catalytic activity of perovskites. The substitution of B-site metal ion with various metal ions M in the doped perovskite AB_1-yM_yO₃ showed a vast spectrum for the alteration of the catalytic and physical properties of the prepared perovskite [64, 67]. There is an effective synergism between the crystal lattice of the perovskite and the metal ions dissolved in the lattice upon doping. This synergism resulted in enhanced redox reaction and better catalytic activity of the prepared perovskite [70]. As well, a dramatic change in the transport and magnetic properties of the ABO₃ perovskite can be achieved upon doping the B-site due to an ionic valence effect and/or an ionic size effect [64]. Furthermore, upon doping the B-site in ABO₃ perovskites with transition metals especially noble metals, the stability of the perovskite was improved and the catalytic activity was enhanced greatly [71]. In addition, the incorporation of two different B ions with appropriate various charge and size may be altered the simple perovskite structure. If the two ions in B-site were used with equimolar amounts, the resulted perovskite can be represented as AB_0.5B′_0.5O₃ with unit cell appearing as doubled along the three axes. In addition, if B and B′ have different charges, there is a slight shift of the oxygen toward the highly charged ion in the ordered structure although the octahedral symmetry of B and B′ cations is maintained [5]. Different B-sites doped perovskites were mentioned in the literature showing enhanced catalytic properties like LaNi_xCo_1-xO₃ [72], LaB_0.9Pd_0.1O₃ [73], LaMn_1−xPdxO₃ [74], BaFe_1−xY_xO_3−δ [75], BaFe_0.85Cu_0.15O_3−δ [76], LaNi_1-xFe_xO₃ [77], LaFe_0.95-xCo_xPd_0.05O₃ [78] and LaCo_0.95Pd_0.05O₃ [79]. On the other hand, (La_1-xSr_x)_yMnO_3±δ [80], La_1-xCe_xGaO₃, La_1-xPr_xGaO₃ and La_1-xNd_xGaO₃ [81], La_1-xCa_xMnO₃ [82], La_1-xNa_xMnO_3+δ, La1_-xCa_xMnO_3+δ [83], (Ba_0.93Fe_0.07)TiO₃ [84], La_1-xSr_xNiO₃, and La_1-xSr_xMnO₃ [85] are examples of A-site doped perovskites.

4. Characterization of perovskites

X-ray powder diffraction (XRD) can be used to differentiate the different phases of the prepared perovskites. Single-crystal XRD is another analysis used to characterize the structure of the perovskite. Thermal stability of the prepared perovskites can be tested using thermal analysis techniques like TGA, DTA, and DSC. On the other hand, scanning (SEM) and transmission (TEM) electron microscopies can be utilized to identify the different morphological and surface characteristics of the prepared perovskites. Also, BET can be utilized for surface area measurement. In addition, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) can be used to completely identify the formed phases [10, 86–88].

4.1. XRD

XRD can be used for the phase identification and the relative percents of different phases of the prepared materials. Also, some structural parameters like particle size, lattice parameters (a, b, and c), lattice volume, and theoretical density can be calculated from the XRD data. Also, XRD can be used to optimize the preparation conditions of the different perovskites [3, 87–89]. Galal et al. prepared SrPdO₃ by citrate/nitrate combustion method at different pH values; 2, 7, and 10 at calcination temperature 750 °C for 3 h and the XRD patterns of these samples were shown in Figure 3A. The XRD data were compared with the ICCD card of SrPdO₃ (card number 00-025-0908). For pH 2, the experimental data and the theoretical one are well matched and supported the formation of the primary orthorhombic perovskite phase of SrPdO₃ (the major diffraction peak 110) and the appearance of secondary phase SrPd₃O₄ (210). Only SrPd₃O₄ phase appeared in case of samples prepared at pH 7 and 10 as (110) peak disappeared. Therefore, pH 2 was the optimal pH for SrPdO₃ preparation. Also, the type of fuel (citric acid, urea, and glycine) used in the preparation of SrPdO₃ can be optimized using XRD (Figure 3B). SrPdO₃ was the primary phase in all cases but with different percents of SrPdO₃ (110) with respect to SrPd₃O₄ (210). The high percent was in case of urea and the small one in case of citric acid. Some structural parameters were calculated and summarized in Table 3 with good matching with theoretical data.

Figure 3.
XRD patterns of SrPdO₃ prepared by combustion method at different pH values (A) and different fuels (B). Miller indices (h, l,k) are written in black line for SrPdO₃, red line for SrPd₃O₄ and the symbol (*) for SrCl₂. 6H₂O [89].

	Lattice Structure	Particle Size (nm)	Lattice Parameters (A˚)	Lattice Volume (˚A³)	Theoretical density (g/cm³)
Standard SrPdO₃ (ICCD card; 00-025-0908)	Orthorhombic		a = 3.977 b = 3.530 c = 12.82	179.98	4.47
citrate-nitrate method	Orthorhombic	34.0	a = 3.983 b = 3.541 c = 12.80	180.59	4.45
urea-nitrate method	Orthorhombic	45.4	a = 3.954 b = 3.563 c = 12.82	180.63	4.45
glycine-nitrate method	Orthorhombic	25.7	a = 3.972 b = 3.527 c = 12.83	179.69	4.47

Table 3.

Structural parameters calculated from XRD data [89].

4.2. SEM and TEM

SEM and TEM can be used to study the morphology and surface characteristics of the perovskite nanomaterials. The preparation conditions, synthesis method, type of A- and B-site metal ions, and doping A- and/or B-sites affected greatly the SEM of the prepared perovskites [88–94]. Galal et al. prepared LaNiO₃, LaCoO₃, LaFeO₃, and LaMnO₃ by the microwave-assisted citrate method at 720 W as operating power for 30 min under microwave irradiation. The SEM images for the different perovskites were shown in Figure 4 presenting different morphologies depending on the kind of metal ion at B-site, respectively. LaNiO₃ showed compact surface with high degree of ordering while LaCoO₃ and LaMnO₃ showed spherical grains agglomerations with smaller grain size in case of LaMnO₃. LaFeO₃ showed dissimilar morphology with a porous surface containing particles with bonelike shape. In addition, LaFeO₃ presented greater electrocatalytic activity toward hydrogen evolution reaction compared to other types of perovskites [90].

Furthermore, the high-resolution TEM (HRTEM) can be used to show the different morphologies and particle characteristics of the different perovskites [86, 88, 95]. HRTEM images for LaNiO₃, LaCoO₃, LaFeO₃, and LaMnO₃ by the microwave-assisted citrate method were shown in Figure 5, respectively. The HRTEM images clearly showed orthorhombic phase with high crystallinity in case of LaFeO₃, while HRTEM images of LaNiO₃, LaCoO₃, and LaMnO₃ showed hexagonal distorted rhombohedral phases. The diffraction patterns obtained via HRTEM for the different perovskites were comparable with the XRD data [86, 95].

Figure 4.
SEM micrographs of (A) LaNiO₃, (B) LaCoO₃, (C) LaFeO₃ and (D) LaMnO₃ prepared by the microwave-assisted citrate method at 720 W for 30 min, with a magnification of 35,000 times [90].

Figure 5.
HRTEM micrographs of (A) LaNiO₃, (B) LaCoO₃, (C) LaFeO₃ and (D) LaMnO₃ prepared by the microwave-assisted citrate method at 720Wfor 30 min [86].

4.3. BET

The electrochemical performance and electrocatalytic activity of the perovskites are greatly associated with the specific surface area of the materials; therefore, it is necessary to measure the specific surface area of the prepared materials. The surface area values of different perovskites can be measured by Brunauer–Emmett–Teller (BET) nitrogen adsorption. The preparation conditions, synthesis method, type of A- and B-site metals, and presence of different dopants can greatly affect the surface area of the prepared perovskites [86, 87, 95]. Biniwale et al. prepared LaFeO₃ via different methods, namely, sol-gel, combustion, and coprecipitation and measured the corresponding surface area and the average pore diameter of the prepared perovskites. The order of decreasing the surface area of the prepared LaFeO₃ was sol-gel (16.5 m² g^–1) > combustion (9.3 m² g^–1) > coprecipitation method (5.4 m² g^–1). The order of increasing the average pore diameter of the prepared LaFeO₃ was sol-gel (119 °A) < coprecipitation method (140 °A) < combustion (205 °A). Sol-gel and combustion methods resulted in porous surface with internal pores contributing to higher surface area, while coprecipitation method resulted in less internal pores and lower surface area due to longer calcination time [87].

4.4. Thermal analysis

Thermal analysis can be utilized to identify the thermal stability and the decomposition temperature of the prepared perovskites. The optimum calcination temperature of any perovskite can be identified using thermal analysis [88, 96, 97]. Galal et al. prepared SrPdO₃ for the first time, and its optimum calcination temperature was investigated using TGA and DTG of the citrate complex of Sr and Pd (Figure 6A). The breakdown of the citrate complex occurs at ~330 °C through a smooth weight loss step. SrPdO₃ formation was achieved at ~750 °C through a sharp weight loss step [96].

Figure 6.
(A) TG spectrum of Sr and Pd mixed citrate complex, heating rate was 10 °C min⁻¹ [27] and (B) FTIR spectra for LaFeO₃ synthesized using (a) co-precipitation method, (b) combustion method and (c) sol–gel method [87].

4.5. FTIR

The chemical bonding and chemical structure of the prepared perovskites can be investigated via FTIR. The FTIR can give structural confirmation similar to that obtained via XRD [87, 97–100]. Biniwale et al. prepared LaFeO₃ via different methods: sol-gel, combustion, and coprecipitation, and the FTIR for them was shown in Figure 6B [87]. The FTIR of LaFeO₃ showed an absorption band at 558 cm^–1 related to the stretching vibration mode of Fe-O. Another band appeared at 430 cm^–1 was related to the deformation vibration mode of O-Fe-O. LaFeO₃ prepared via coprecipitation method showed a sharp band at 3609 cm^–1, which is related to La-O in lanthanum oxide. In case of the other two methods, the band at 3600 cm^–1 disappeared indicating the formation of relatively pure perovskite phase. Other bands appeared at 1360 and 1480 cm^–1, indicating other phases in case of coprecipitation method. As a result and as mentioned in literature, the absorption peak around 558 cm^–1 was related to the stretching modes of metallic oxygen bond [87, 97–100].

4.6. XPS

The surface compositions of the various components of the prepared perovskites can be identified via XPS [101–106]. Lee et al. prepared La_0.9FeO₃ and LaFeO₃ samples and identify their structural composition via XPS analysis [102]. Figure 7 showed the XPS spectra of La (3d), Fe (2p), and O (1s) in La_0.9FeO₃ and LaFeO₃ samples. The binding energy of La (3d_5/2) was 833.5 eV and 833.8 eV in case of LaFeO₃ and La_0.9FeO₃, respectively, corresponding to the La³⁺ ions in the form of oxide. By contrast, the binding energy of Fe (2p_3/2) was 710.2 eV for both samples corresponding to Fe³⁺ ions in the form of oxide. The Fe (2p) XPS signal cannot distinguish between Fe³⁺ and Fe⁴⁺. The XPS signal of O (1s) was divided into two peaks in case of La_0.9FeO₃ appearing at 529.9 and 532.1 eV. While for LaFeO₃, O (1s) XPS signal was divided into three peaks appearing at 529.4, 531.9, and 534.4 eV. The O (1s) binding energy values at 529.9 and 529.4 eV in both samples are attributed to lattice oxygen species. The peaks at 532.1 and 531.9 eV are ascribed to the chemisorbed oxygen species as OH¯ or O¯. The chemisorbed oxygen species appeared at binding energy higher than that of lattice oxygen species by 2.1–2.5 eV. The peak appeared at 534.4 eV in case of LaFeO₃ was ascribed to the adsorbed water species associated with the surface lanthanum oxide which is highly hygroscopic [102].

Figure 7.
XPS spectra of La(3d), Fe(2p) and O(1s) in La_0.9FeO₃ and LaFeO₃ samples [102].

5. Applications of perovskites

Inorganic perovskite-type oxides exhibited attractive physical and chemical characteristics such as electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability, and supermagnetic, photocatalytic, thermoelectric, and dielectric properties. They are fascinating nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing. The catalytic activity of these oxides is higher than that of many transition metals compounds and even some precious metal oxides.

Nanoperovskites are recently utilized in electrochemical sensing of alcohols, gases, amino acids, acetone, glucose, H₂O₂, and neurotransmitters exhibiting good selectivity, sensitivity, unique long-term stability, excellent reproducibility, and anti-interference ability. Moreover, they have been utilized as catalysts in oxygen reduction and hydrogen evolution reactions exhibiting high electrocatalytic activity, lower activation energy, and high electron transfer kinetics. In addition, some perovskites are promising candidates for the development of effective anodic catalysts for direct fuel cells showing high catalytic performance.

5.1. Sensors and biosensors

5.1.1. Gas sensors

There are a number of requirements that the materials utilized as gas sensors must satisfy, namely, good resemblance with the target gases, manufacturability, hydrothermal stability, convenient electronic structure, resistance to poisoning, and adaptation with existing technologies. There is a wide variety of materials that can be used as gas sensors like semiconductors, namely, SnO₂, In₂O₃, and WO₃, and perovskite oxides, namely, LaFeO₃ and SrTiO₃. Perovskite oxides are fascinating materials as gas sensors due to their perfect thermal stability, ideal band gap “3–4 eV,” and difference in size between the cations of A- and B-sites, allowing different dopants addition for controlling semiconducting properties and their catalytic properties. Perovskites including titanates, ferrites, and cobaltates were utilized as gas sensors for detecting CO, NO₂, methanol, ethanol, and hydrocarbons [107–110]. LaCoO₃ prepared via high-energy ball milling exhibited the highest amount of grain boundaries, the best CO gas sensing properties, and the smallest crystallite size of 11 nm compared to that prepared via solid-state and sol-gel reactions. The maximum response ratio increased to 26% in case of milling method with maximum response ratio temperature of 125 °C compared to 7% and 17% in case of solid-state reaction and sol-gel method. In addition, the specific surface area increased greatly from 4 m² g^–1 to 66 m² g^–1 by extra milling step, and the mobility of the oxygen was enhanced by growing the extra milling step and surface area [107]. A summary of various perovskite oxides for different gas sensing was given in Table 4.

Perovskite	Sensing for	Response ratio %	Oxide type	Reference
LaCoO₃	CO	75% for 100 ppm CO in dry air at 125 ◦C	p-type	[107]
La_0.9Ce_0.1CoO₃	CO	240 % with respect to 100ppm CO in air	p-type	[108]
NdFe_1–xCo_xO₃	CO	1215% at 170 ºC for 0.03% CO gas	When x<0.3; p-type conduction behavior, when x > 0.3; n-type.	[109]
La_0.8Pb_0.2Fe_0.8Cu_0.2O₃	CO	Not reported	p-type	[111]
La_0.8Pb_0.2FeO₃	methanol	146.6 at 230 ◦C for 400 ppm methanol	p-type	[112]
LaFeO₃	ethanol	Not reported	p-type	[113]
LaMg_0.1Fe_0.9O₃	ethanol	Not reported	p-type	[114]
SrFeO₃	ethanol	Not reported	p-type	[115]
SmFe_0.9Mg_0.1O₃	ethanol	Not reported	p-type	[116]
LaMnO₃	ethanol	Not reported	Not reported	[117]
La_0.875Ba_0.125FeO₃	ethanol	sensitivity to 500 ppm ethanol is 58 at 170 °C	p-type	[118]
Ca_xLa_1-xFeO₃	ethanol	Not reported	Not reported	[119]
LaCo_0.1Fe_0.9O₃	ethanol	Not reported	p-type	[120]
LaFeO₃	ethanol	Not reported	p-type	[121]
La_1−xPb_xFeO₃	ethanol	80% for 100 ppm in a wide temperature range of 140–240 ◦C	x = 0.30; p-type, x = 0.32–0.50; n-type	[122]
LaFeO₃ and SmFeO₃	NO₂, CO	Not reported	p-type	[123]
LaFeO₃	NO₂	Not reported	Not reported	[124]
LaFeO₃	NO_x	Not reported	Not reported	[125]
SrTi_1−xFe_xO_3−δ	hydrocarbons	Not reported	p-type	[126]
LnFeO₃ (Ln = La, Nd and Sm)	hydrocarbons	Not reported	p-type	[127]

Table 4.

A summary of different perovskites for gas sensing.

5.1.2. Glucose sensor

It is very important to analytically determine H₂O₂ and glucose in many fields like food, clinic, and pharmaceutical analyses. H₂O₂ is considered one of the most important oxidizing agents in chemical and food industries. Glucose is a fundamental metabolite for most of the living organisms and for the clinical examination of diabetes mellitus, a worldwide health problem. As a result, it is very important to construct biosensors for the sensitive determination of H₂O₂ and glucose [128–137]. Different types of enzymatic glucose sensors were constructed and used in the literature exhibiting the advantages of simplicity and sensitivity. However, enzymatic glucose sensors suffered from the lack of stability and the difficult procedures required for the effective immobilization of enzyme on the electrode surface. The lack of enzyme stability was attributed to its intrinsic nature because the enzyme activity was highly affected by poisonous chemicals, pH, temperature, humidity, etc. As a result, most attention was given for sensitive, simple, stable, and selective nonenzymatic glucose sensor. Different novel materials were proposed for the electrocatalytic oxidation of glucose like noble nanometals, nanoalloys, metal oxides, and inorganic perovskite oxides. Inorganic perovskite oxides as nanomaterials exhibited fascinating properties for glucose sensing like ferroelectricity, superconductivity, charge ordering, high thermopower, good biocompatibility, catalytic activity, and the ability of the perovskite structure to accommodate different metallic ions [128–137]. Zhen Zhang et al. utilized carbon paste electrode modified with Co_0.4Fe_0.6LaO₃ as a promising nonenzymatic H₂O₂ and glucose sensor. This sensor displayed perfect electrocatalytic activity toward H₂O₂ and glucose oxidation in alkaline medium due to the presence of large amount of active sites in the modifier. The linear dynamic range for H₂O₂ at this surface was 0.01 to 800 μM with low detection limit of 2.0 nM. For glucose, two ranges were obtained from 0.05 to 5 μM and from 5 to 500 μM with detection limit of 10 nM. The proposed sensor exhibited rapid response, excellent long-term stability, and anti-interference ability toward ascorbic acid, uric acid, and dopamine [132]. Furthermore, Atta et al. have recently modified SrPdO₃ perovskite with a film of gold nanoparticles to be utilized as a nonenzymatic voltammetric glucose sensor. The studied sensor exhibited high electrocatalytic activity toward glucose oxidation exploring the effective synergism between SrPdO₃ and gold nanoparticles. SrPdO₃ perovskite facilitated the charge transfer process and acted as an effective supporting substrate for gold nanoparticles. The catalytic activity of SrPdO₃ was attributed to the deficiency of the surface for oxygen which resulted in enhanced intrinsic reactivity toward glucose oxidation. Another reason for the catalytic activity of SrPdO₃ was the matrix effect induced by the stable crystal structure of the perovskite where there is a homogenous distribution of Pd⁴⁺ cations in the inert matrix of the perovskite during the reaction. This nanocomposite showed good performance toward glucose sensing in terms of highly reproducible response, high sensitivity, wide linearity, low detection limit, good selectivity, long-term stability, and applicability in real urine samples and blood serum [137]. A summary of different types of perovskites used for enzymatic and nonenzymatic H₂O₂ and glucose sensing was given in Table 5, exhibiting high sensitivity, wide linear range, low detection limit, anti-interference ability, applicability in real samples, and long-term stability.

Perovskite	Sensing for	Sensor type	Detection limit	Sensitivity	Reference
graphite/SrPdO₃/Au_nano	glucose	Non-enzymatic	10.1 μmol L^-1	422.30 μA/ mmolL^-1	[137]
LaTiO₃-Ag0.1	glucose	Non-enzymatic	2.50 × 10⁻⁹ M	7.80 × 10² µA mM⁻¹cm⁻²	[128]
LaNi_0.5Ti_0.5O₃/CoFe₂O₄	H₂O₂	Non-enzymatic	23 nM	3.21 µA µM⁻¹ cm⁻²	[129]
LaNi_0.6Co_0.4O₃	H₂O₂ and glucose	Non-enzymatic	1 nM H₂O₂ 8 nM glucose	1812.84 µA mM⁻¹ cm⁻² for H₂O₂ and 643.0 µA mM⁻¹ cm⁻² for glucose	[130]
CPE–La_0.66Sr_0.33MnO₃–GO_x (glucose oxidase)	glucose	Enzymatic	Not reported	158.1 µAmol⁻¹ L	[131]
Co_0.4Fe_0.6LaO₃	H₂O₂ and glucose	Non-enzymatic	2 nM H₂O₂ and 10 nM glucose	Not reported	[132]
LaTiO₃–Ag0.2	glucose	Non-enzymatic	2.1×10⁻⁷ M	784.14 µA mM⁻¹ cm⁻²	[133]
LaNi_0.5Ti_0.5O₃–NiFe₂O₄	glucose	Enzymatic	0.04 mM	Not reported	[134]
La_0.6Ca_0.4Ni_0.7Fe_0.3O₃	H₂O₂	Non-enzymatic	Not reported	Not reported	[135]
La_0.5Sr_0.5CoO_3-δ	H₂O₂	Non-enzymatic	Not reported	Not reported	[136]
La_0.7Sr_0.3NiO₃ /chitosan/GCE	H₂O₂	Enzymatic	9.0×10^-8 mol/L	Not reported	[138]
LaNiO₃	H₂O₂ and glucose	Non-enzymatic	33.9 nM H₂O₂	Not reported	[139]

Table 5.

A summary of different perovskites for H₂O₂ and glucose sensing.

5.1.3. Neurotransmitters sensor

Dopamine (DA) is an essential catecholamine neurotransmitter that exists in the mammalian central nervous system. The depletion of DA can lead to Parkinson’s disease; therefore, its determination is very crucial. The interference of ascorbic acid (AA) and uric acid (UA) with DA is the major problem in DA detection [89, 140–143]. Therefore, it is very important to present a modified surface which can be sensitively and selectively detect DA even in presence of high concentration of AA and UA. Atta et al. presented carbon paste electrode modified with SrPdO₃ (CpE/SrPdO₃) as a promising electrochemical DA sensor in biological fluids with unique long-term stability and low detection limit of 9.3 nmol L⁻¹. CpE/SrPdO₃ can simultaneously detect DA in the presence of high concentrations of AA and UA and can successfully detect DA in human urine samples with excellent recovery results in terms of selectivity, accuracy, precision, and detection limit. The proposed sensor showed high sensitivity, good selectivity, and anti-interference ability [89]. Moreover, higher response toward DA oxidation was achieved at CpE/SrPdO₃ compared to electrodeposited palladium nanoparticles modified CpE (CpE/Pd) with equivalent loading of Pd⁴⁺ salt. The higher catalytic activity at CpE/SrPdO₃ was explained in terms of the oxygen–surface interaction between the oxygen atoms of the hydroxyl groups and the transition element in the perovskite. One characteristic of perovskite is the deficiency of its surface for oxygen. As a result, the “dihydroxy”-oxygen adsorb onto perovskite surface with the formation of a “moderate” bond between oxygen atoms and the transition element in the oxide [89]. Furthermore, the descriptor that controls the catalytic process in perovskites is the type of transition metal in the perovskite, which is related to the number of occupied d orbital states of a specific symmetry, for example, of the active metal. This is associated with the surface ability to bond oxygen on the basis of the calculations of the density functional theory. Therefore, the oxygen adsorption energy represented a perfect descriptor for the oxidation of DA at CpE/SrPdO₃. A summary of different perovskites used for neurotransmitters sensing was given in Table 6.

Perovskite	Sensing for	Linear dynamic range	Detection limit (nM)	Reference
LaFeO₃ microspheres made up of nanospheres	DA	2 ×10^-8 – 1.6 × 10^-6 M	59	[140]
CpE/SrPdO₃	DA	7 – 70 µmol L⁻¹	9.3	[89]
Graphite/SrPdO₃	L-dopa EP, NE, DA, DOPAC and ST	0.6 – 9 μmol L⁻¹	1.63	[141]
LaFeO₃ nanostructure dendrites	DA	8.2 × 10⁻⁸ – 1.6 × 10⁻⁷ M	62	[142]
LaFeO₃ nanoparticles	DA	1.5×10⁻⁷ – 8.0×10⁻⁴ M	30	[143]

Table 6.

A summary of different perovskites for neurotransmitters sensing.

5.2. Solid oxide fuel cells

Fuel cells have come into view as efficient alternatives to combustion engines due to their potential to minimize the environmental influence of the use of fossil fuels. A fuel cell uses certain type of chemical fuel as its energy source, and there is a direct transformation of chemical energy into electrical energy like a battery. Fuel cells are attractive because of their great efficiency, modular and distributed nature, low emissions, zero noise pollution, and role in any future hydrogen fuel economy. There are several types of fuel cells depending on operating temperature, fuel type, electrolyte type, and mobile ions. Polymer electrolyte membrane fuel cells, molten carbonate, phosphoric acid or alkali fuel cells, and solid oxide fuel cell are the most common examples of fuel cells [144]. Table 7 contained some fuel cells types and some selected features [144]. Here we will concern on solid oxide fuel cells. Solid oxide fuel cells (SOFCs), based on conducting electrolyte in the form of an oxide-ion, can generate electricity and heat and they are considered as energy-saving, environment-friendly, and effective energy conversion devices. SOFCs exhibited several features compared to the other types of fuel cells like high-energy conversion efficiency, cheap materials, low sensitivity to the fuel impurities, low pollution emissions, environmental compatibility, and excellent fuel flexibility [145–160]. Figure 8 showed the working principle of a solid oxide fuel cell [157]. The high temperature of SOFC operation resulted in the difficult choice of the proper materials and the decreased cell durability. Thus, providing materials for SOFCs with good performance at intermediate temperatures (500–800 °C) is very essential so that the cell cost and the startup and shut down time can be reduced and its long-term stability can be improved [148, 155]. Perovskite oxides exhibited fascinating properties like good electrical conductivity similar to that of metals, high ionic conductivity, and perfect mixed ionic and electronic conductivity. Depending on the differences in the electrical conductive characteristics of perovskites, they are chosen as an effective component in SOFC [9]. In addition, mixed-conduction perovskite oxides possess beneficial electrochemical reaction; structural, thermal, and chemical stabilities; high electrical conductivity; high catalytic activity toward the oxygen reduction; and ideal mixed electronic and ionic conductivities to be used as effective component for intermediate temperatures SOFC (IT-SOFC) [147–160]. Shao and Haile utilized Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ as an effective cathode for intermediate SOFC with the fuel of humidified H₂ and the cathode gas of air. This cathode exhibited 1010 and 402 mW cm^–2 as the maximum power density at 600 and 500 °C, respectively [145]. On the other hand, Goodenough reported the use of double perovskite Sr₂MgMnMoO_6-δ as an anode material for SOFC with dry methane as the fuel and 438 mW cm^–2 as the maximum power density at 800 °C. This anode material showed long-term stability, stability in reducing atmosphere, tolerance to sulfur, and characteristic of oxygen deficiency [161]. Table 8 contained a summary of different perovskites used as anode and cathode for SOFC illustrating the fuel type, the operating temperature, and the maximum power density.

Figure 8.
Schematic diagram showing the working principle of a solid oxide fuel cell [157].

Type	Temperature °C	Fuel	Electrolyte	Mobile ion
PEM: polymer electrolyte membrane or proton exchange membrane	70–110	H₂, methane	Sulphonated polymer (Nafion)	(H₂O)_nH⁺
AFC: Alkali fuel cell	100–250	H₂	Aqueous KOH	OH¯
PAFC: Phosphoric acid fuel cell	150–250	H₂	H₃PO₄	H⁺
MCFC: Molten carbonate fuel cell	500–700	Hydrocarbons, CO	(Na, K)₂CO₃	CO₃ ^2-
SOFC: Solid oxide fuel cell	700–1000	Hydrocarbons, CO	(Zr, Y)O_2-δ	O^2-

Table 7.

Fuel cell types and selected features [144].

Perovskite	Anode/cathode in the cell	Fuel used	Operating temperature °C	Maximum power density mW cm^-2	Reference
Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ	Cathode	Humidified H₂ (~3% H₂O)	600 500	1010 402	[145]
Ba_0.5Sr_0.5Co_0.2Fe_0.8O_3-δ	Cathode	Humidified H₂	800	266	[147]
La_0.7Sr_0.3Co_0.5Fe_0.5O₃	Cathode	Not reported	Not reported	Not reported	[148]
La_0.6Sr_0.4Fe_0.8Co_0.2O₃	Cathode	glycerol	800	Not reported	[149]
LaBaCuFeO_5+x	Cathode	Humidified H₂	700 550	327 105	[150]
LaBaCuCoO_5+x	Cathode	Humidified H₂	700 550	432 171	[150]
Y_0.8Ca_0.2BaCoFeO_5+δ	Cathode	Humidified H₂	650	426	[151]
NdFeO₃	anode	sulfur vapor or SO₂	620 650	0.154 0.265	[153]
Sm_0.5Sr_0.5CoO_3-δ	cathode	Not reported	700	936	[158]
Sr₂Fe_1.4Mo_0.6O_6−ı	cathode	H₂ or methanol	Not reported	Not reported	[159]
La0.75Sr0.25Cr0.5Mn0.5O3	anode	methane	Not reported	Not reported	[162]
Sr₂MgMnMoO_6-δ	anode	dry methane	800	438	[161]
La_0.8Sr_0.2Cr_0.97V_0.03O₃	anode	dry methane	800	Not reported	[163]

Table 8.

A summary of different perovskites for SOFC.

5.3. Catalyst

Perovskite oxides can be widely used as catalyst in modern chemical industry, exhibiting appropriate solid-state, surface, and morphological properties [6]. Several perovskites exhibited enhanced catalytic activity toward different reactions like hydrogen evolution and oxygen evolution and reduction reactions [9].

5.3.1. Hydrogen evolution reaction

Because of the advantages of high heat of combustion, abundant sources, and no pollution, hydrogen is considered as an ideal fuel. Hydrogen evolution reaction (HER) is a fascinating reaction in the renewable energy field. This reaction is very important in (i) metal electrodeposition and corrosion in acids, (ii) storage of energy through production of hydrogen, and (iii) as the hydrogen oxidation reaction reverse in low-temperature fuel cells. One of the most studied reactions in electrochemistry is the electrocatalysis in HER. The material used for HER should have (i) intrinsic electrocatalytic activity, (ii) considerable active surface area per unit volume, and (iii) good stability. To reduce the cost of electrolytic HER, the overpotential required for the operation of the electrolyzer at considerable current densities should be reduced. The overpotential reduction can be achieved through the electrode active surface area enhancement or by the selection of electrode materials of high catalytic activity. The steps of the reaction in acidic solutions are as follows:

M + H3O++e–↔MHads+ H2OE1

MHads+ H3O++e–↔H2+ M + H2OE2

MHads+ MHads↔H2+ 2ME3

The first step in HER is the proton discharge (volume reaction, Eq. (1)), which is followed by electrodesorption step (Heyrovsky reaction, Eq. (2)) or proton recombination step, physical desorption, (Tafel reaction, Eq. (3)) [93]. Galal et al. confirmed the high catalytic activity of different perovskite oxides toward hydrogen evolution reaction [90–94, 96]. LnFeO₃ perovskites (Ln= Gd, La, Sm, and Nd) were prepared by the microwave assistant-citrate method, and single-phase perovskites were formed with uniform distribution of small average particle size. Tafel and electrochemical impedance measurements were used to study the catalytic activity of LnFeO₃ toward HER showing the effect of the type of the lanthanide ion on HER and the partial substitution effect at the La-site in La_1-ySm_yFeO₃. The order of decreasing the catalytic activity toward HER was NdFeO₃ > LaFeO₃ > SmFeO₃ > GdFeO₃ based on activation energies calculations and the strength of Fe-O bond, which is related to A-type metal ion. Furthermore, the order of decreasing the catalytic activity in case of doped samples was La_0.75Sm_0.25FeO₃ > La_0.5Sm_0.5FeO₃ > La_0.25Sm_0.75FeO₃ > LaFeO₃ > SmFeO₃ displaying greater catalytic activity of ternary perovskites compared to that of binary ones [94]. On the other hand, Galal et al. prepared SrPdO₃ by the citrate method for the first time showing enhanced catalytic activity toward HER up to 100 times with respect to the unmodified surface with 27.9 kJ mol⁻¹ as the calculated activation energy. The rate-determining step was the hydrogen adsorption on the catalyst and the order of the reaction at the catalyst surface was 0.86 [96]. Table 9 contained a summary of different perovskites used as catalysts for HER with the values of exchange current density at constant overpotential, activation energy, reaction order, and the rate-determining step.

Perovskite	Exchange current density j/ μA.cm^-2	Activation energy kJ mol^-1	Reaction order	Rate determining step	Reference
NdFeO₃	-107.82	24.68	0.72	adsorption of hydrogen on the catalyst	[94]
LaFeO₃	-367.37	41.15	0.37	adsorption of hydrogen on the catalyst	[94]
SmFeO₃	-147.23	57.89	0.41	adsorption of hydrogen on the catalyst	[94]
GdFeO₃	-451.86	81.37	0.72	adsorption of hydrogen on the catalyst	[94]
La_0.75Sm_0.25FeO₃	Not reported	Not reported	Not reported	adsorption of hydrogen on the catalyst	[94]
SrPdO₃	−946.3	27.9	0.86	adsorption of hydrogen on the catalyst	[96]
LaFeO₃	−105.6	51.61	0.37	adsorption of hydrogen	[90]
LaCoO₃	−348.9	45.37	0.74	adsorption of hydrogen	[90]
LaNiO₃	−158.5	41.15	0.94	adsorption of hydrogen	[90]
LaMnO₃	−41.9	55.05	0.87	adsorption of hydrogen	[90]
SrRuO₃ (microwave)	−5326.0	6.67	1.14	adsorption of hydrogen	[92]
SrRuO₃ (sol-combustion)	−425.5	49.45	0.98	adsorption of hydrogen	[92]
SrRuO₃ (coprecipitation)	−298.0	86.32	0.88	adsorption of hydrogen	[92]
CaRuO₃ (coprecipitation)	-1064.9	42.60	0.94	adsorption of hydrogen	[93]
BaRuO₃ (coprecipitation)	-3687.2	17.68	1.03	adsorption of hydrogen	[93]

Table 9.

A summary of different perovskites for HER catalysis.

5.3.2. Oxygen reduction and oxygen evolution reactions

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are considered one of the most important electrode reactions in many industrial processes like fuel cells, metal electrowinning, water electrolysis, electro-organic synthesis, cathodic protection, and rechargeable metal air batteries [95, 164–166]. Platinum-based catalysts and precious metal oxides are the most common catalysts for ORR or OER, but they are expensive and scarce. Therefore, it is very important to develop other catalysts for ORR or OER. Mixed metal perovskite oxides of transition and rare earth metals are promising low-cost alternatives to precious metal catalysts for both ORR and OER [165]. Perovskite oxide exhibited unique electronic and magnetic properties, defective structure, and good cation ordering resulting in disorder-free channels of oxygen vacancies and enhanced mobility of oxygen ions [164]. Some perovskite oxides were reported as electrocatalysts for ORR and OER and summarized in Table 10 [95, 164–166]. Ruizhi Yang et al. prepared Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ by sol-gel method and utilized it as ORR or OER catalyst in basic medium of KOH. The proposed catalyst exhibited higher catalytic activity toward OER than the unmodified electrode [164]. Furthermore, Galal et al. prepared LaFeO₃ by microwave-assisted citrate method and studied its catalytic activity toward OER in acid medium of HClO₄. LaFeO₃ exhibited greater electrocatalytic activity toward OER by about 100-folds compared to the unmodified electrode. The current density at 1.5 V increased from 3.6 × 10⁻⁵ in case of unmodified electrode to 1.2 × 10⁻³ A/cm² in case of modified one. The calculated activation energy was 20 kJ/mol, which is much lower than that reported for other iron compounds and even some precious metal oxides like RuO₂. This was attributed to the matrix effect induced by the stable crystal structure of the perovskite [95]. In addition, La_0.6Ca_0.4CoO₃, prepared by the sol-gel method, showed high catalytic activity and relative stability toward oxygen electrochemistry in basic medium of KOH. La_0.6Ca_0.4CoO₃ exhibited single-phase perovskite structure, high conductivity, and large surface area [165, 166].

Perovskite	Medium	Exchange current density mA.cm^-2	Activation energy kJ mol^-1	Reaction order	Rate determining step	Reference
LaFeO₃	HClO₄	1.24×10⁻⁵	20.13	0.82	Difficult to be determined	[95]
Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃	KOH	Not reported	Not reported	Not reported	Not reported	[164]
La_0.6Ca_0.4CoO₃	KOH	Not reported	Not reported	Not reported	Not reported	[165]
La_1-xCa_xCoO₃ series (0 < x < 0.6)	KOH	Not reported	Not reported	Not reported	Not reported	[166]

Table 10.

A summary of different perovskites for ORR or OER catalysis.

5.4. Solar cells

Solar energy is a green source of energy that can be used instead of energy sources based on fossil fuels. Solar radiation can be directly converted into electrical energy in a suitable way creating various applications for solar energy. Solar energy can be efficiently converted into electricity using photovoltaic solar cells based on silicon. The obvious disadvantage of silicon-based solar cell is the high price of electricity generated from it so that there is a potential need to develop solar cell with low cost. Recently, attention was paid to solar cells based on organic/inorganic solid-state methylammonium lead halide (CH3NH3PbX3, X=Br, I) hybrid perovskite. This type of solar cells presented effective points such as a conversion efficiency of about 20%; its cost is lower than that of conventional silicon solar cells and the availability of the raw materials. These 3D organometal halide perovskite exhibited excellent intrinsic properties for photovoltaic applications like excellent stability, appropriate band gap (~1.55 eV), high absorption coefficient (1.5 × 10⁴ cm^–1 at 550 nm), long hole-electron diffusion length (~100 nm for CH₃NH₃PbI₃ and ~1 μm for CH₃NH₃PbI_3-xCl_x), high carrier mobility and transport, charge carriers with small effective mass, low temperature of processing, and easy processing steps [167–179]. Figure 9A showed the structure of ABX₃ perovskite (X = oxygen, carbon, nitrogen or halogen), where A and B cations are placed in a cubo-octahedral and an octahedral site, respectively. A is usually divalent and B is tetravalent when O^2– anion is used. On the other hand, Figure 9B showed the structure of CH₃NH₃PbI₃ halide perovskite where the A-site is occupied by CH₃NH₃⁺ (organic component) and B-site cation is occupied by Pb²⁺. As indicated by the example, halide perovskites contain monovalent and divalent cations in A- and B-sites, respectively, to maintain electrical neutrality [169, 171]. Like oxide perovskite, the tolerance factor of halide perovskite should be as close to one to maintain a stable and symmetrical crystal structure [169].

Figure 9.
(a) ABX₃ perovskite structure showing BX₆ octahedral and larger A cation occupied in cubo-octahedral site. (b) Unit cell of cubic CH₃NH₃PbI₃ perovskite [171].

The quality of the perovskite film is very crucial for solar cells. Several methods have been used to form perovskite films with high quality such as single step solution method, vapor assistant solution process, sequential deposition of inorganic and organic precursor, and coevaporation of the precursors [167]. CH₃NH₃PbI₃ perovskite film was prepared with high quality by adding small amounts of N-methyl-2-pyrrolidone and a mixture of g-butyrolactone and dimethylsulfoxide via a solution method. A power conversion efficiency of 11.77% with fill factor of 80.52% was obtained based on the structure of ITO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrenesulfonate)/perovskite/ PCBM (fullerene-derivative phenyl-C61-butyric acid methyl ester)/Ca/Al under one sun illumination (100 mW cm^–2) [167]. Table 11 contained a summary of different models based on perovskites used for solar cells applications with the values of power conversion efficiency, fill factor, method of perovskite formation, solar cell composition, cost, and stability.

Solar cell composition	Perovskite formation	Power conversion efficiency	Fill factor	Cost	Stability	Reference
ITO/PEDOT:PSS/ CH₃NH₃PbI₃ perovskite/ PCBM/Ca/Al	Solution method	11.77 %	80.52 %	low	highly stable	[167]
conducting polymer (poly-aniline) CH₃NH₃PbI₃ perovskite	Solution method	21 %	Not reported	low	Not reported	[168]
Mesoporous TiO₂/CH₃NH₃PbI₃/ graphite paste	Solution method	1.11 %	44.5 %	low	slightly stable	[174]
Ti foil/TiO₂ nanotubes (TNTs) with organic– inorganic halide perovskite absorber “CH₃NH₃PbI₃” and transparent carbon nanotube electrode	Solution method	8.31 %	64 %	low	highly stable	[175]
ITO/conductive-polymer/CH₃NH₃PbI₃/C₆₀/BCP/Ag	Solution method	16.5 %	75 %	low	highly stable	[177]
glass/ITO/RGO/CH₃NH₃PbI₃/PC₆₁BM/bathocuproine (BCP)/Ag	Solution method	10.8 %	71.6 %	low	highly stable	[179]

Table 11.

A summary of different models based perovskites for Solar cells applications

6. Conclusions

Inorganic perovskite-type oxides are excellent nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing, exhibiting attractive physical and chemical characteristics. They showed electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability and supermagnetic, photocatalytic, thermoelectric, and dielectric properties. Nanoperovskites have been utilized as catalysts in oxygen reduction and hydrogen evolution reactions exhibiting high electrocatalytic activity, lower activation energy and high electron transfer kinetics. In addition, some perovskites are promising candidates for the development of effective anodic catalysts for direct fuel cells showing high catalytic performance. Moreover, they are recently utilized in electrochemical sensing of alcohols, gases, glucose, H₂O₂, and neurotransmitters. They can enhance the catalytic performance in terms of unique long-term stability, sensitivity, excellent reproducibility, selectivity, and anti-interference ability. In addition, organometallic halide perovskites exhibited efficient intrinsic properties to be utilized as a photovoltaic solar cell with good stability and high efficiency.

Acknowledgments

The authors would like to acknowledge the financial support from Cairo University through the Vice President Office for Research Funds.

Kristian Kroschel