The long-term goal of a large body of relevant scientific research is to find a solution to the problem of providing industrial and domestic human needs with renewable and environmentally friendly energy ^[1][2][1,2]. The main fields of sustainable energy concern both the search for renewable energy sources ^[3][4][5][3,4,5] and methods for the production of ecological types of energy ^[6][7][8][9][6,7,8,9], which differ from traditional types based on hydrocarbon fuel ^[10][11][12][10,11,12]. The tasks relating to sustainable energy also include the development of technologies for the use of non-renewable energy sources: efficient waste-processing ^[13][14][15][13,14,15], the construction of nuclear mini-reactors [16], and the creation of energy devices based on the direct conversion of various types of energy into electrical and thermal energy ^[17][18][19][17,18,19]. A well-known device for directly converting the chemical energy of fuels into electrical energy is a fuel cell ^[19][20][21][19,20,21]. If the electrolyte in the fuel cell is a ceramic material that is permeable to oxygen ions, it is referred to as a solid oxide fuel cell (SOFC) ^{[21][22][23][24][25]}[21,22,23,24,25].

The advantages of SOFCs are the absence of noble metals in their composition and the flexibility of fuel types ^[24][26][27][24,26,27], while the disadvantages include high operating temperatures, which lead to chemical interactions between the parts of the SOFCs ^[28][29][28,29] and fast degradation ^[30][31][32][30,31,32]. The high temperatures required to operate SOFCs with conventional electrolytes on the basis of yttria-stabilized zirconia (YSZ) lead to the formation of metastable phases, sealing, and thermal and chemical incompatibility with electrode materials ^[33][34][35][33,34,35].

One of the ways to solve the described problem is to decrease the operating temperature of SOFCs and develop fuel cells operating at medium- ^[36][37][38][36,37,38] and low-temperature ranges ^[39][40][39,40]. This has resulted in investigations into new classes of electrolytes ^{[41][42][43][44]}[41,42,43,44] and the development of SOFCs enhanced with nanostructured materials ^[45][46][45,46]. The utilization of nanotechnologies, energy production and energy storage devices is extremely prospective due to their durability, sustainability, long lifetime, and low cost [47]. Among the alternative electrolytes used in low- and intermediate-temperature SOFCs, complex oxides with an ABO₃-type perovskite structure have attracted specific attention due to their high efficiency in energy conversion ^{[48][49][50][51]}[48,49,50,51,52]. For the first time, economical electrolyte materials based on doped lanthanum aluminate LaAlO₃ were reported by Fung and Chen in 2011 ^[52][53].

It is worth noting that previous generalizing works on lanthanum aluminate were aimed at the synthesis and characterization of LaAlO₃ phosphors (published by Kaur et al. in 2013 ^[53][54]) and at some properties and applications of LaAlO₃ not concerned with SOFCs (observed by Rizwan et al., in 2019) ^[54][55]. The prestudidesent overview is dedicated to recent progress in the design, characterization and application of electrolyte materials for SOFCs based on the doped LaAlO₃ complex oxides with a perovskite structure. The doped LaGaO₃ and LaAlO₃ phases constitute a family of oxygen-conducting electrolytes mainly, while other La-based perovskites (LaScO₃, LaInO₃, LaYO₃, LaYbO₃) exhibit protonic conductivity as well [49].

2. Synthesis, Structure and Morphology

For the synthesis of doped LaAlO₃ oxides, several well-developed techniques are usually used: solid-state reaction technology ^{[55][56][57][58]}[61,62,63,64], the mechanochemical route ^[59][65], co-precipitation ^[60][61][66,67] and organic-nitrate precursor pyrolysis ^{[62][63][64][65][66][67][68][69]}[68,69,70,71,72,73,74,75]. Employing conventional solid-state reaction technology, LaAlO₃ samples can be directly obtained from La₂O₃ and Al₂O₃. In ^[55][61], these initial reactants were ground down, homogenized in a water media, desiccated and pressed into pellets annealed at a temperature range of 780–1100 °C. Such a temperature regime allows for single-phase LaAlO₃ samples to be prepared. A similar technology was used in work ^[56][62] to synthesize LaAl_1−xZn_xO_3−δ (here, δ is the oxygen nonstoichiometry; δ = x/2 in the case of oxidation-state stable cations and one charge state difference between the host and impurity cations). As initial reagents, stoichiometric amounts of aluminium and zinc oxides were milled in ethanol. The heat treatment included five 24-h stages at a temperature range of 700–1100 °C. Single-phased LaAlO₃ and LaAl_0.95Zn_0.05O_3−δ were obtained at 1250 and 1200 °C, respectively. Fabian et al. ^[59][65] synthesised Ca-doped LaAlO₃ powders using the mechanochemical method. Oxide powders of La₂O₃, γ-Al₂O₃ and CaO in appropriate proportions were milled in a planetary mill at 600 rpm. The prepared powders were pressed into disks with polyethylene glycol as a plasticizer. The LaAlO₃ and La_1−xCa_xAlO_3−δ pellets were sintered at 1700 and 1450 °C, respectively, to achieve a desirable ceramic densification. LaAlO₃ complex oxides were prepared starting from water solutions of aluminium and lanthanum chlorides with a molar ratio for the metal components of 1:1 ^[60][66]. Solutions with high and low concentrations of starting reagents were mixed with an ammonium solution serving as a precipitation agent. The obtained gels were filtered, washed with distilled water and dried twice, at 25 °C for 24 h and at 100 °C for 2 h. The prepared powders were calcined at a temperature range of 600–900 °C for 1 h. The powder obtained from the high-concentration solution was annealed at 900 °C for 2 h in air, then ground in a rotary mill with zirconia balls in dry ethanol, pressed and calcined at 1300–1500 °C for 2 h. The most widely used technology for the preparation of LaAlO₃ and its doped derivatives is the pyrolysis of organic-nitrate compositions, known as the sol-gel ^[62][63][68][68,69,74] or autocombustion methods (or self-propagating high-temperature synthesis, and the Pechini method) ^{[64][65][66][67][69]}[70,71,72,73,75]. Utilizing different fuels during the pyrolysis process coupled with various annealing temperatures affects the crystallinity, powder dispersity, and ceramics density, determining the functional properties of the obtained LaAlO₃-based ceramic materials ^[68][70][71][74,76,77]. LaAlO₃ powders were prepared by Zhang et al. ^[62][68] from La(NO₃)₃·6H₂O and Al(NO₃)₃·9H₂O: they were dissolved in 2-methoxyethanol and then mixed with citric acid at a molar ratio of 1:1 to the total content of metal ions. The obtained solutions were heated and dried at 80 °C until gelatinous LaAlO₃ precursors were obtained, which were then calcined at 600–900 °C for 2 h. To obtain La_0.9Sr_0.1Al_0.97Mg_0.03O_3−δ powder, La(NO₃)₃·6H₂O, Al(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O, Sr(NO₃)₂, EDTA, C₂H₅NO₂ and NH₃·H₂O were used in ^[63][69]. The molar ratio of glycine and EDTA to overall metal-ion content was 1.2:1:1; the ratio of NH₃·H₂O to EDTA was adjusted to 1.15:1. The aqueous solution of metal nitrates was prepared and heated at 80 °C, and then the EDTA-ammonia solution and glycine were added. The colourless solution was dried, and the obtained brown resin was calcined at 350 °C; it was then ground down and calcined at 600–1000 °C for 3 h. The obtained powders were finally pressed into disks followed by sintering at 1600–1700 °C for 5 h. According to Adak and Pramanik ^[64][70], LaAlO₃ was prepared from a 10% aqueous polyvinyl alcohol precursor that was added to a solution obtained from La₂O₃ (99%) dissolved in nitric acid and Al(NO₃)₃·9H₂O. The organic-nitrate mixture was evaporated at 200 °C until dehydration; then, spontaneous decomposition and the formation of a voluminous black fluffy powder occurred. The obtained powders were ground down and annealed at 600–800 °C for 2 h to form a pure phase. Verma et al. ^[65][71] synthesized LaAlO₃ and La_0.9−xSr_0.1Ba_xAl_0.9Mg_0.1O_3−δ (x = 0.00, 0.01 and 0.03) samples from initial reagents composed of La(NO₃)₃·H₂O, Sr(NO₃)₂, Ba(NO₃)₂, Al(NO₃)₃·6H₂O and Mg(NO₃)₂·6H₂O initial reagents. C₆H₈O₇·H₂O was used as an organic fuel. The metal nitrates and citric acid were dissolved in distilled water, resulting in the formation of a transparent solution. The pH value required for proper combustion was achieved by the addition of ammonia solution. The literature shows that the annealing temperature of the precursor powders plays a significant role in complex oxide synthesis: this regulates the density of the final polycrystalline ceramic samples ^[72][78]. For practical applications, it is important to obtain LaAlO₃-based samples with a narrow distribution of fine-grained particles. These requirements were fulfilled in ^[60][66], where a fully converted LaAlO₃ phase was formed at relatively low temperatures. In more detail, the researcheuthors developed a co-precipitation technique enabling the formation of single-phase LaAlO₃ powders after its calcination in air at 900 °C for 2 h. A narrow particle size distribution for LaAlO₃ powder was achieved in ^[60][66], where milling in an ethanol medium was conducted. A Rietveld analysis of the XRD pattern confirmed the presence of a pure perovskite phase with a rhombohedral structure, referring to the R-3c space group. Reference ^[60][66] calculated unit cell parameters for the LaAlO₃ sample (a = 5.3556(1) Å and c = 13.1518(2) Å) agreed well with results from neutron powder diffraction ^[73][79]. The primitive LaAlO₃ cell consists of two formula units. The rotation of AlO₆ octahedra is caused by changes to the θ angle (Al–O–Al). Above 540 °C, a phase transition from the rhombohedral to cubic structure was observed for LaAlO₃ ^[73][79]. The cubic lattice of LaAlO₃ with a unit cell parameter of a = 3.8106(1) Å corresponds to the Pm3m space group ^[73][79]. Concluding the chapter about the synthesis methods of doped LaAlO₃ oxides, from the perspective of their use in SOFCs, the co-precipitation method should be noted as the most optimal synthetic method. The co-precipitation method with a subsequent sintering of samples at 900 °C is well-approved and allows for both single-phase powders with a narrow nano-size particle distribution and ceramic samples with high relative densities to be obtained.

3. Functional Properties

LaAlO₃, a basic (undoped) lanthanum aluminate, has very low electrical conductivity, equal to around 1 × 10⁻⁶ S cm⁻¹ at 900 °C ^[69][75]. La-site doping of LaAlO₃ with strontium enhances electrical conductivity because it improves the oxygen vacancy concentration responsible for oxygen-ion transport, ^[74][80]. Al-site modification of LaAlO₃ with acceptor dopants (for example, magnesium) can also increase the total and ionic conductivities: