Perovskite Oxides for Energy Applications

: Industrial and population growth is constantly increasing energy consumption. Since 81.3% of consumed energy is produced from fossil fuels, the high pollution and emission rates are leading to climate changes resulting in extreme weather conditions. The development of new materials and technology is necessary to achieve sustainable societal growth and development. In the field of new materials, perovskites are the main subject of much scientific research conducted in the last decade. Perovskites are usually divided into two groups: halides and oxides. The application of perovskite halides in photovoltaic and solar cells is already well-known and investigated. On the other hand, perovskite oxides do not possess optical and light-absorption properties comparable to perovskite halides, but they have promising electrical and magnetic properties and high reducibility rates. This review is dedicated to the perovskite oxides, preparation methods of this material and their application for energy transformation and storage applications. The perovskite oxides have high potential as working materials in solid oxide fuel cells (SOFC), Ca batteries, thermochemical energy storages (TCES) and ambient temperature magnetic refrigerators (MR).


INTRODUCTION
Industrial development and population growth are constantly increasing the energy demand.According to the statistical data [1], the biggest individual driver of primary energy growth was China followed by India and Indonesia as the next largest contributors.81.3% of consumed energy comes from fossil fuels (oil, gas, coal) which leads to the increase of greenhouse gases emissions causing climate changes.Therefore, it is very important to develop new technologies and materials for efficient energy conversion and storage.In the development of new materials, materials engineering has the main role.During the last decade, a group of ceramic called perovskites was the subject of scientific research due to the promising electrical, magnetic, photovoltaic and redox properties with possible energy applications [2].Perovskites can be divided into two groups: perovskite halides and perovskite oxides.The light absorption properties of perovskite halides as well as their application in photovoltaic and solar cells are already well-known and investigated [3].However, perovskite oxides are less investigated although they also possess promising energy application properties as reported by Goodenough [4] in 1955.This review gives an overview of the perovskite oxides structure, synthesis methods and potential applications for energy purposes (SOFCs, batteries, solar energy storages and magnetic refrigerators).

Perovskite Structure
The mineral form of calcium titanate (CaTiO 3 ) was discovered in the Ural mountains by German scientist Gustav Rose in 1839 and named 'perovskite' in honour of Count Lev Alekseyevich von Perovski, a Russian statesman and mineralogist [5].Nowadays, perovskites are a broad group of materials described with the general formula ABX 3 .In this formula A represents an alkaline earth metal ion (Ca, Sr, Ba) or a rare-earth metal (La, Nd, etc.) ion.The B-site is occupied by a transition metal (Mn, Fe, Cr, Ti, Pb) ion which is octahedrally coordinated by halogen (Cl, Br, I, F) or oxygen ions, generally marked with X.The ideal perovskite structure is cubic with the space group Pm3m shown in Fig. 1.
Figure 1 The ideal cubic perovskite structure [6] Due to the chemical flexibility of perovskites and the possibility to substitute A and B-site metal ions, cell distortion can occur.The distortion leads to the structural stabilization and is described using the Goldschmidt tolerance factor t defined as follows [7]: where R A , R B and R X represent the ionic radius of A-site cation, B-site cation and X-site anion.The perovskites with 1.00 < t < 1.13 will exhibit hexagonal symmetry (Fig. 2a), while compositions with t < 1.00 are associated with cubic (Fig. 1) and orthorhombic (Fig. 2b) symmetry.According to the research conducted by Hines et al. [8], the perovskite structure will be cubic for 0.90 < t < 1.00, and orthorhombic for 0.75 < t < 0.90.For t < 0.75, the hexagonal ilmenite structure characteristic for FeTiO 3 will be obtained [8].
The structure distortion and defects of perovskite oxides play the main role in their transport properties required for energy applications.Oxygen ion transport in perovskite oxides is mediated via a vacancy mechanism.If trivalent A-site ion is substituted with divalent ion, the oxygen vacancies will form to preserve the electroneutrality and enhance oxygen ionic conductivity [9].Furthermore, the oxygen nonstoichiometry leads to the mixed-valence state of B-site ions [10].The presence of multiple valences enables Zerner double exchange mechanism [11] described as follows: The Zerner double exchange mechanism is a type of magnetic exchange that occurs between ions in different oxidation states spanned by oxygen anion.When O gives up its spin-up electron to B (n+1)+ , its vacant orbital can be filled by an electron from B n+ .In one double exchange process, an electron moves between neighbouring cations retaining its spin [10,11].

Properties of Perovskite Oxides
Perovskite materials exhibit many interesting properties (Tab. 1) due to their tendency to oxygen nonstoichiometry and cell distortion.In this review, the focus will be on ferroelectric, magnetic and redox properties important for energy applications.

Ferroelectric Properties
Ferroelectricity is the phenomenon of spontaneous electric polarization of the material when an external electric field is applied.The ferroelectric materials possess a dielectric constant which is two magnitudes larger compared to those in an ordinary dielectric.Materials with ferroelectric properties can be applied in infrared cameras, vibration sensors, memory devices and capacitors [12].

Magnetic Properties
Magnetic properties are the main characteristic of the rare-earth manganites described with the general formula Re 1-x A x MnO 3 in which Re represents a trivalent rare-earth metal ion (La, Nd, Pr, etc.) and A is a divalent alkaline earth metal ion (most often Sr).These materials possess the property of colossal magnetoresistance (CMR) which allows them to change their electric resistance in the presence of a magnetic field [13].When a material possesses both electric and magnetic properties it is called multiferroic material.Those kinds of materials are highly applicable in spintronics, a new technology for data storage.Such devices utilise the spin degree of freedom of electrons and/or holes and the interaction with their orbital moments to store two times more data on the device of the same dimensions [14].Furthermore, perovskite oxides with magnetic properties are nowadays investigated as working materials in magnetic refrigeration systems [12].

Redox Properties
The oxygen nonstoichiometric perovskite oxides containing multiple valence ions are highly valuable for applications as redox materials.The thermochemical cycle is a combination of a high-temperature step, at which the oxide is partially reduced under the release of oxygen, and a low-temperature oxidation step.These thermochemical cycles can be applied for combustion of organic fuels, H 2 O or CO 2 splitting as well as in solar energy storage [15].

PREPARATION OF PEROVSKITE OXIDES 2.1 Solid-State Reactions
In solid-state reactions, perovskites are synthesized by mixing oxides or carbonates of the A and B-site metal ions.The mixing can be conducted in agate mortar with a pestle or in a planetary ball mill.The disadvantages of the solidstate reactions are inhomogeneity of the reaction mixture, incorporation of different impurities during repeated grinding and heating process and big particle size unsuitable for coatings.Neagu et al. [16] have reported that by administering the composition and nonstoichiometry, the growing nanoparticles from perovskite oxides can be accomplished through solid-state synthesis.To further improve synthesis procedures, methods from the solution were developed [14].

Synthesis Procedures from the Solution 2.2.1 Sol-Gel Process
The sol-gel alkoxide route is the very common synthesis procedure in the field of ceramics.It can be described as the formation of an oxide network through polycondensation reactions of a molecular precursor in a liquid.In this process, several stages are identified and the first one is the formation of sol from the precursors' solution via the condensation process.The advancing condensation process leads to gel formation followed by drying and calcination to obtain ceramics [12,17].
This process is broadly applied to aluminate and titanate and generally nanosized particles preparation.The use of the sol-gel process is limited due to the stability of the precursor and it is not suitable for the preparation of manganites, which are the most important magnetic perovskites [13].Furthermore, the control of the air humidity, concentration and pH value of the solution is necessary for synthesis optimization.

Pechini Method
The Pechini method is a variation of the typical sol-gel procedure which compensates previously stated disadvantages.This method is also called the polymeric precursor or mixed liquids method.The principle of this method is the formation of a chelate between A and B-site cations and carboxylic acid.The most often used carboxylic acid is citric acid.After the homogenous solution of metal-citrate chelates is obtained, the ethylene glycol is added to achieve the formation of a covalent network with uniformly distributed metal cations and the resulting solution is heated (150 -250 °C) leading to the condensation between COOH and OH groups.The process is finished when the resin mass is formed.The last step is calcination in order to achieve phase transformation of the resin precursor into a perovskite [12,17].

Citrate-Nitrate Autocombustion (CNA) Method
The CNA method is analogous to the Pechini method with the exception that no ethylene-glycol is added.In this method, citric acid has two roles; the first as a chelating agent of cations and the second as the fuel in the combustion process.The formation of a gel is achieved by heating citrate-nitrate chelates solution at temperatures above 300 °C.Further gel heating leads to gel drying and its combustion into a precursor powder which is then calcined to obtain ceramics [17].

Coprecipitation Method
The coprecipitation method is the least explored method for the perovskite oxides preparation.The principle of the method is the dissolution of A and B-side metal cation sources in a convenient solvent (e.g.aqueous nitric acid).The simultaneous precipitation (coprecipitation) of different cations in the solution occurs after the addition of a chemical reagent that decreases the solubility of the present compounds.To achieve coprecipitation of all cations in the solution and to obtain homogeneous products, the control of the temperature, concentration, pH value and homogeneity of the solution is essential.The ammonium carbonate is the most popular precipitation agent since the carbonates degrade during the calcination process and leave the system in the form of CO 2 .The pH value of the solution is most often controlled by the addition of liquid ammonia or urea and the homogeneity of the solution is achieved by stirring [12].

Hydrothermal Synthesis
The main principle of hydrothermal synthesis is the conduction of the reaction in the aqueous solution or suspension of the precursor at high temperature and pressure.The difference of this method compared to the previously listed methods is that crystalline powders can be obtained without the calcination step.Furthermore, the advantage of this method is the possibility to change particle size and shape by controlling reaction temperature, pH, time and concentration of the reactants [12,18].

Synthesis of Nanostructured Perovskite Oxides
Although the nanostructured perovskite oxides can be synthesised by previously mentioned sol-geland hydrothermal methods [17,18], they are often prepared by the molten salt method [19].In this method, sources of metal ions (oxides) are mixed with salt (NaCl and KCl) and then sintered at a temperature above the melting point (950 °C) of the salt.At this temperature, the oxides particles rearrange and then diffuse rapidly in a liquid state of the salt.With further sintering, particles of the perovskite phase are formed through the nucleation and growth processes.In the field of nanostructured perovskite oxides, barium (BaTiO 3 ) and lead (PbTiO 3 ) titanates are the most investigated for their application in capacitors, sensors and acoustic transducers [19].

APPLICATIONS OF PEROVSKITE OXIDES 3.1 Perovskites in Solid Oxide Fuel Cell (SOFC)
SOFCs are nowadays introduced as an efficient and environmentally friendly source of electric power.They operate at high temperatures (about 1000 °C) and allow the transport of oxygen ions through the ceramic components of the cell.To lower the working temperature to the intermediate temperature range (650 -850 °C), ceramics with higher electronic and ionic conductivity are necessary and perovskite oxides are ideal candidates for that purpose.Further requirements for perovskite oxides application in SOFCs are their chemical stability, porosity, oxygen deficiency and low cost [20].The schematic of SOFC is shown in Fig. 3.The fuel (steam-reformed natural gas) enters the cell at the anode side, while air enters the fuel cell at the cathode side.Oxygen from the air combines with free electrons to form oxide ions at the cathode and these oxides travel from the cathode to the anode through the solid electrolyte.At the anode, oxide ions react with hydrogen to form water steam and with CO to form CO 2 , releasing free electrons.These free electrons travel to the cathode through the external circuit, producing electricity [10].

Perovskites as Cathodes
Perovskite oxides possess electric conductivity due to the Zerner double exchange mechanism.The electric conductivity increases with temperature until the value when the material starts to melt or degrade (above 1300 °C).To start the electrochemical reaction, oxygen molecules absorbed on the cathode need to be transformed into the oxygen anions which then diffuse through the cathode and electrolyte [20].Taking that into account, the structure of the perovskite cathode needs to be porous.Moreover, the electric conductivity of perovskites depends on symmetry.The research has shown that the cubic perovskites have the highest conductivity which decreases with the increase of the crystal structure distortion.On the other hand, ionic conductivity can be improved by the Asite cation substitution which causes the formation of oxygen vacancies to neutralize the charge of the system.To sum up, the convenient cathode material needs to have ionic conductivity 10 -3 -10 -5 S/cm, and electronic conductivity higher than 100 S/cm [10,20].In the first high-temperature SOFCs, strontium doped lanthanum manganites (La 1-x Sr x MnO 3 , LSMO) were investigated as cathode materials due to their compatibility with yttriastabilized zirconia (YSZ) electrolyte.Unfortunately, the disadvantage was material poisoning caused by Cr from the Cr 2 O 3 interconnectors.Furthermore, lanthanum cobaltite (LaCoO 3 ) was investigated as a potential cathode material.Since it degrades in a chemical reaction with YSZ, it is convenient in SOFCs with doped cerium oxides as electrolytes.Strontium doped LaCoO 3 with chemical formula La 1-x Sr x CoO 3 (LSCO) possess ionic conductivity and significantly higher electric conductivity of 1000 S/cm.The main disadvantage of LSCO is low stability.To increase stability, a part of Co is substituted with Fe resulting in a stable perovskite phase with a chemical formula La 1-x Sr x Co 1-y Fe y O 3 (LSCFO).Even though this substitution decreases electric conductivity, for x = 0.4 and y = 0.2, its value of 300 S/cm is enough for the SOFCs application [10,21].

Perovskites as Anodes
The most often used anodes in fuel cells are Ni cermets, composite materials prepared from ceramics and metal.These materials have excellent performance when pure H 2 is used as fuel.When fuel contains impurities as S, H 2 S or CO, the corrosion and degradation of Ni electrodes will occur.Perovskites are a good alternative due to their high ionic and electronic conductivity, stability in reducing atmospheres and possibility to change chemical composition via substitution.As anodes, titanates and chromites are widely investigated, especially SrTiO 3 and yttria doped SrTiO 3 .Despite that, the main challenge is to develop perovskite oxide with a catalytic activity comparable to Ni [10,22].

Perovskites as Electrolytes and Interconnectors
Ionic conductivity is the main requirement for the application of perovskite oxide as an electrolyte in SOFCs.Furthermore, chemical compatibility with electrode materials and stability for different oxygen partial pressures are very important.Moreover, mechanical stability is an important criterion for the SOFCs geometry.Strontium doped lanthanum gallates (La 1-x Sr x GaO 3 , LSGO) possess good ionic and electronic conductivity that could be additionally increased by partial substitution of Ga with Mg (La 1-x Sr x Ga 1-y Mg y O 3 , LSGMO).Although the decrease in electrolyte thickness contributes to the resistance decrease, the electrolyte should not be too thin in order to maintain the cell potential.In the case of the LSGMO electrolyte, the optimal electrolyte thickness is 100 μm [23].As interconnectors in SOFCs, LaCrO 3 compounds are used.Their role is to separate the fuel and gaseous oxidant.Besides that, interconnectors need to possess high electric conductivity at temperatures about 1000 °C.Furthermore, the good interconnector material needs to be impermeable for gases, chemically stable for fuel and oxidant and thermochemically compatible with other cell components.To satisfy listed requirements, La and Cr in LaCrO 3 are often substituted with Ca 2+ , Mg 2+ and Sr 2+ [23].

Perovskite as Cathodes in Batteries
The widely used lithium-ion batteries (LIB) enabled the development of portable electronic devices and they are nowadays present in every mobile phone.However, these batteries have reached their maximum in the aspect of energy density, i.e. the amount of energy that can be stored in a given mass of a substance or system.The promising alternatives are calcium-based batteries, which can provide 2 electrons per atom, achieving double energy density compared to lithium.With a mass ratio of 3.56%, Ca is the fifth most abundant element in the Earth's crust, it is nontoxic and its standard reduction potential is -2.87 V.Moreover, the theoretical energy density of Ca batteries is 2.06 Ah/cm 3 , which is twice as much as LIB [24].Ca battery cells are analogous with LIBs on the cathode side.During the battery discharge, the carrier ion (Ca 2+ ) migrates from the anode to the cathode through the electrolyte and electrons are flowing through an external circuit (Fig. 4).During the battery charging, these processes are reversed [24].Although the research of Ca batteries started in 1935, the main problem was to find materials suitable for reversible intercalation of Ca 2+ ions, as well as an electrolyte that enables battery charging at room temperatures.Due to that, the development of Ca batteries was stopped and it resumed in the last decades due to the development of new materials-perovskites [24].For the research of suitable cathode materials, the density-functional theory (DFT) is widely used.DFT is based on the computational quantum mechanical modelling method to investigate the electronic structure of particular atoms, molecules and condensed phases.According to the DFT, the most suitable cathode materials for Ca batteries are perovskite oxides with a general formula CaMO 3 (M = Mo, Cr, Mn, Fe, Co, Ni) [24][25][26][27].
Arroyo et al. [26] have investigated the application of CaMoO 3 as cathode material in Ca batteries with 0.4 M Ca(BF 4 ) 2 in propylene and ethylene carbonate as electrolytes.The DFT study marked CaMoO 3 as potential cathode material due to its high electric conductivity, no crystal defects and activity in the voltage range 2 -3 V. Despite this, deintercalation of Ca 2+ ions was not achieved since the diffusion of Ca 2+ inside the perovskite structure was too low.Pathreeker et al. [27] have prepared CaMnO 3 as cathode material, while 0.5 M Ca(BF 4 ) 2 in acetonitrile was used as electrolyte.In this case, the reversible intercalation and deintercalation of Ca was achieved, which was confirmed by X-ray diffractometry (XRD) showing the formation of new phases-cation deficient Ca 1-x MnO 3 and MnO 2 .The disadvantage is that phase transformation occurred after only one working cycle which could be a significant problem for multi-cycling investigations.Even though the perovskite structure is stable, transitional metals show a tendency to crystallize forming a related spinel structure.DFT studies have shown that CaCo 2 O 4 is a promising cathode material in Ca batteries due to the high specific capacity and rate of ionic diffusion.This was investigated by Cabello et al. [28] who have prepared CaCo 2 O 4 based cathodes and observed reversible capacity of 30 -100 mAh/g 1 after 30 cycles.

Perovskites in Solar Energy Storage
The concentrating solar power (CSP) technology is a promising concept for the energy supply and sustainable development.The thermal energy storage (TES) combined with CSP plants offers the opportunity to balance the supply and energy demand by reducing undesirable impacts of the intermittent availability of solar energy.The concept of the TES system relies on two-tank, molten-salt storage that enables the supply of solar energy during the off-sun hours.These two-tank molten-nitrate salt systems have a temperature limit of around 560 °C and a low specific storage capacity of < 450 kJ/kg.The system performance can be improved by increasing operating temperature above 600 °C or by the increase of the moltensalt quantities [29].Due to the listed system limitations, the current researches are directed to the reducible perovskite oxides for thermochemical energy storage (TCES) applications.The concept of the TCES system is given in Fig. 5. Particles in cold storage are lifted to the top of the concentrated solar radiation receiver where they are irradiated as they travel down.The receiver is operated under the vacuum or with a sweep of gas to provide low oxygen partial pressure necessary for particle reduction and heating necessary for energy storage.Afterwards, the hot reducing particles from the receiver are stored in a hot storage tank and are discharged into a bubbling fluidized bed reactor.This reactor is supplied with air to fluidize and exothermically oxidize the particles.The resulting heat is transferred to the power cycle working fluid in order to produce electrical energy.The O 2 necessary for the process can be supplied from the air, which simplifies the storage plant construction and reduces the cost.This concept allows energy storage and release over a broad temperature range [30].
Studies of TCES redox cycles of simple multivalent cation oxides such as Co 3 O 4 /CoO, Fe 3 O 4 /FeO and Mn 3 O 4 /Mn 2 O 3 found that the main disadvantages were low applicability for large-scale systems due to the high cost and slow re-oxidation kinetics.La-rich perovskites were identified as the most suitable material from the reversibility aspect, but their high cost is motivating the new researchers to look for low-cost perovskite materials.Oxygen deficient CaMnO 3-δ is that kind of material and has shown redox stability up to 1250 °C.Furthermore, the A and B-site doped CaMnO 3-δ , such as Ca 1-x Sr x MnO 3-δ and CaMn 1-y Cr y O 3-δ are identified as promising TCES materials.The research conducted by Imponenti et al. [30] has shown that Ca 1-x Sr x MnO 3-δ (x ≤ 0.1) compositions possess the highest specific energy storage with a thermodynamic limit of about 700 kJ/kg at 900 °C and partial oxygen pressure of 10 -4 bar.Moreover, > 50% of energy storage was attributed to chemical energy storage due to the reduction process.Babiniec et al. [31] have studied Al and Ti-doped CaMnO 3-δ which were showing lower specific energy capacity of 390 kJ/kg at temperatures above 1000 °C.The newest research conducted by Jin et al. [32] investigated the application of CaCo 0.05 Mn 0.95 O 3−δ in the TCES systems.The results have shown an energy capacity of about 424 kJ/kg without losing reversibility.Even the low Co doping (x = 0.05) can weaken the metaloxygen bonds and facilitate the lattice oxygen diffusion to the surface-active oxygen sites.Although the listed materials have great potential for TCES applications, future research needs to be focused on the preparation of low molecular weight materials with a high oxygen deficiency (δ ≥ 0.25) to achieve high reduction enthalpy and redox capacity.

Perovskites in Magnetic Refrigerators
Ambient temperature magnetic refrigeration (MR) technology has the potential to replace classical gas compression technology.The main advantages are low power consumption, non-toxicity and eco-compatibility of working media.The MR technology is based on a magnetocaloric effect (MCE), which is defined as a tendency of certain materials to heat up when placed in a magnetic field and to cool down when the magnetic field is removed [33].The material suitable for MR needs to possess large magnetic entropy change (ΔS M ), large adiabatic temperature change and high relative cooling power.Gadolinium (Gd) is considered as the most active material (ΔS M = 3.25 J/kgK in a magnetic field of 1 T) for application as working media in ambient temperature MR.The main disadvantage of the Gd application is its high price, so it is necessary to investigate other lower-cost materials.Lanthanum manganites (LaMnO 3 ) and related doped compounds of a general formula La 1-x A x Mn 1-y M y O 3 have been studied since the 1950s due to their astonishing magnetic properties originated from the superexchange mechanism.The superexchange mechanism is also called Kramers-Anderson superexchange and is described as strong antiferromagnetic coupling between two nearest neighbour cations through non-magnetic anion [33].
It is well known that the effect of mixed valence and spin states is significant for the magnetic properties of the material [4].In pure LaMnO 3 , Mn is present in a +3 oxidation state.Since LaMnO 3 is stable in air in an oxygenexcess form, the mixed +3/+4 Mn valence will occur.The convenient way to achieve Mn mixed-valence is to substitute a part of La with an alkaline earth metal cation, usually Sr or Ba.Rostamnejadi et al. [34] have researched on the magnetic properties of La 0.67 Sr 0.33 MnO 3 comparing it to the other magnetic materials.The material has shown paramagnetic to ferromagnetic transition at about 100 °C making it suitable for the ambient temperature MR.Furthermore, the measured ΔS M was 5.15 J/kgK which was even higher than for Gd.Phan et al. [35] have investigated magnetic properties of a LaMnO 3 doped with Ca and Ba.The obtained results for La 0.7 Ca 0.18 Ba 0.12 MnO 3 and La 0.7 Ca 0.06 Ba 0.24 MnO 3 materials have shown significantly lower ΔS M of around 1.70 J/kgK in comparison to the pure Gd.The same research group has prepared Ca and Pb doped LaMnO 3 materials and the largest ΔS M was measured for La 0.7 Ca 0.1 Pb 0.2 MnO 3 (3.72J/kgK).La 0.6 Ca 0.2 Pb 0.1 MnO 3 had a lower ΔS M of about 2.50 J/kgK.Moreover, it was observed that the large magnetic entropy change of the prepared materials occurred around 25 °C, allowing the water to be used as a heat transfer fluid.According to the investigations, rare-earth manganites have the potential to replace Gd and to become promising working materials for magnetic refrigeration in household air conditioning systems and refrigerators.

CONCLUSIONS AND FUTURE REMARKS
The perovskite oxides (ABO 3 ) are the subject of much scientific research due to their promising electric, magnetic and redox properties necessary for energy applications.These materials can be prepared using solid-state reactions or procedures from the solution or melt.By synthesis parameters optimization, it is possible to prepare nanosized materials with controlled oxygen nonstoichiometry required for the transport properties.
Most publications in the field of perovskite oxides are dedicated to the group of rare-earth manganites, especially LaMnO 3 , which possess astonishing electric and magnetic properties due to the uncoupled electron in the 5d orbital of La.These properties can be easily improved by the partial substitution of La 3+ with an alkaline earth metal ion, most often Sr 2+ .Furthermore, the potential development of Ca batteries has brought CaMnO 3 materials into the spotlight.Additionally, it was observed that CaMnO 3 has promising properties for thermochemical energy storage application due to its high reducibility and redox stability at temperatures above 1000 °C.
To sum up, perovskite oxides are promising new materials in the development of new technologies necessary for sustainable social development.They are a low-cost alternative to noble-based materials with high functionality and stability.The main disadvantage of perovskite oxides is harsh preparation conditions requiring high temperatures (> 1000 °C).Furthermore, the largescale preparation of perovskite oxides is a great challenge, as most of the published work is limited to a laboratory scale.Therefore, for broad perovskite oxides application, it will be necessary to develop new low-cost and mildtemperature preparation procedures.However, the high number of publications in this field promises the future development of proposed perovskite oxides applications.

Figure 4
Figure 4Schematic of a Ca battery using a Ca metal anode and an intercalation perovskite cathode[24]

Figure 5
Figure5 Schematic of a concentrating solar power plant receiver with perovskite oxide thermochemical energy storage and fluidized bed re-oxidation and heat transfer[29]

Figure 6
Figure 6Schematic comparison of a classic compressor and innovative magnetic cooling process[33]