2. Synthesis, Structure and Morphology
For the synthesis of doped LaAlO
3 oxides, several well-developed techniques are usually used: solid-state reaction technology [
61,
62,
63,
64], the mechanochemical route [
65], co-precipitation [
66,
67] and organic-nitrate precursor pyrolysis [
68,
69,
70,
71,
72,
73,
74,
75].
Employing conventional solid-state reaction technology, LaAlO
3 samples can be directly obtained from La
2O
3 and Al
2O
3. In [
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 LaAlO
3 samples to be prepared. A similar technology was used in work [
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 LaAlO
3 and LaAl
0.95Zn
0.05O
3−δ were obtained at 1250 and 1200 °C, respectively.
Fabian et al. [
65] synthesised Ca-doped LaAlO
3 powders using the mechanochemical method. Oxide powders of La
2O
3, γ-Al
2O
3 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 LaAlO
3 and La
1−xCa
xAlO
3−δ pellets were sintered at 1700 and 1450 °C, respectively, to achieve a desirable ceramic densification.
LaAlO
3 complex oxides were prepared starting from water solutions of aluminium and lanthanum chlorides with a molar ratio for the metal components of 1:1 [
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 LaAlO
3 and its doped derivatives is the pyrolysis of organic-nitrate compositions, known as the sol-gel [
68,
69,
74] or autocombustion methods (or self-propagating high-temperature synthesis, and the Pechini method) [
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 LaAlO
3-based ceramic materials [
74,
76,
77].
LaAlO
3 powders were prepared by Zhang et al. [
68] from La(NO
3)
3·6H
2O and Al(NO
3)
3·9H
2O: 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 LaAlO
3 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(NO
3)
3·6H
2O, Al(NO
3)
3·9H
2O, Mg(NO
3)
2·6H
2O, Sr(NO
3)
2, EDTA, C
2H
5NO
2 and NH
3·H
2O were used in [
69]. The molar ratio of glycine and EDTA to overall metal-ion content was 1.2:1:1; the ratio of NH
3·H
2O 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 [
70], LaAlO
3 was prepared from a 10% aqueous polyvinyl alcohol precursor that was added to a solution obtained from La
2O
3 (99%) dissolved in nitric acid and Al(NO
3)
3·9H
2O. 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. [
71] synthesized LaAlO
3 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(NO
3)
3·H
2O, Sr(NO
3)
2, Ba(NO
3)
2, Al(NO
3)
3·6H
2O and Mg(NO
3)
2·6H
2O initial reagents. C
6H
8O
7·H
2O 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 [
78]. For practical applications, it is important to obtain LaAlO
3-based samples with a narrow distribution of fine-grained particles. These requirements were fulfilled in [
66], where a fully converted LaAlO
3 phase was formed at relatively low temperatures. In more detail, the authors developed a co-precipitation technique enabling the formation of single-phase LaAlO
3 powders after its calcination in air at 900 °C for 2 h. A narrow particle size distribution for LaAlO
3 powder was achieved in [
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 [
66] calculated unit cell parameters for the LaAlO
3 sample (
a = 5.3556(1) Å and
c = 13.1518(2) Å) agreed well with results from neutron powder diffraction [
79]. The primitive LaAlO
3 cell consists of two formula units. The rotation of AlO
6 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 LaAlO
3 [
79]. The cubic lattice of LaAlO
3 with a unit cell parameter of
a = 3.8106(1) Å corresponds to the
Pm3m space group [
79].
Concluding the chapter about the synthesis methods of doped LaAlO
3 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
LaAlO
3, a basic (undoped) lanthanum aluminate, has very low electrical conductivity, equal to around 1 × 10
−6 S cm
−1 at 900 °C [
75]. La-site doping of LaAlO
3 with strontium enhances electrical conductivity because it improves the oxygen vacancy concentration responsible for oxygen-ion transport
(Equation (2), [
80]
). Al-site modification of LaAlO
3 with acceptor dopants (for example, magnesium) can also increase the total and ionic conductivities
.:
The possibility of forming good oxygen-ionic conductivity by doping LaAlO
3 oxides has promoted studies on their potential application in SOFCs [
53,
65,
71,
82,
83,
84,
85,
86,
87,
88,
89,
90]. The co-doping strategy is a beneficial way to further increase ionic conductivity [
80,
82,
83,
87]; this is due to the fact that, along with
Ethe above-mentioned equation
(2), an additional quantity of oxygen vacancies can be formed according to the following mechanism [
80]:
According to the results of [
53], the simultaneous doping of LaAlO
3 with barium and yttrium drastically enhanced ionic transport. For example, the total conductivity of La
0.9Ba
0.1Al
0.9Y
0.1O
3−δ at 800 °C was close to that of YSZ (2 × 10
−2 S cm
−1). There are various ways to tailor the transport properties of LaAlO
3-based materials. For example, the doping of (La,Sr)AlO
3 with manganese resulted in total conductivity rising due to the substitution of Mn
3+ ions, which were transformed into Mn
2+ and Mn
4+ ions at the Al
3+ position, enhancing an electronic contribution [
75,
84]. Therefore, co-doped (La,Sr)(Al,Mn)O
3 is attributed to mixed ionic-electronic conductors (MIEC). The Pr-doping of (La,Sr)AlO
3 had a positive influence on transport properties due to the suppression of grain boundary resistivity [
85], and the isovalent substitution of La
3+-ions with Sm
3+-ions in (La,Sr)AlO
3−δ resulted in the formation of a pronounced mixed ion-electron conduction [
88] due to the generation of more electrons than in the case of the aliovalent substitution of La
3+ ions with Ba
2+ ions.
The electrical conductivity values of LaAlO
3-based ceramic materials are summarized in
Table 1. Analysis of these data confirms that the simultaneous modification of both sublattices of LaAlO
3 results in improved conductivity compared to those reached using single doping approaches. However, it should be noted that the Sr- and Mg- co-doped LaAlO
3 materials exhibit mixed ionic-electronic conduction in air atmospheres over a wide temperature range (800–1400 °C), while predominant ionic transport occurs for more reduced atmospheres (for example, wet hydrogen). This is typical behaviour for various La-based perovskites [
49] as well as for other perovskite-related ion-conducting electrolytes [
91].
Thermal expansion coefficients (TECs) play an important role in material selection when seeking to avoid thermal incompatibilities between various parts of SOFCs. According to da Silva and de Miranda [
75], the average TEC values for LaAlO
3 and La
0.8Sr
0.2AlO
3 were equal to around 11.4 × 10
−6 and 9.9 × 10
−6 K
−1, respectively. These data confirm that the TEC values of LaAlO
3-based materials were close to those of the conventional YSZ electrolyte, i.e., 10.9 × 10
−6 K
−1 [
92].
The chemical compatibility of La
0.9Sr
0.1Al
0.97Mg
0.03O
3−δ as an electrolyte material with NiO-Ce
0.9Gd
0.1O
2−δ(GDC), Sr
0.88Y
0.08TiO
3−δ and La
0.75Sr
0.25Cr
0.5Mn
0.5O
3−δ as anode SOFC materials was thoroughly investigated in [
87] using XRD analysis and scanning electron microscopy with energy-dispersive X-ray spectroscopy. The obtained results demonstrated that Sr
0.88Y
0.08TiO
3−δ and La
0.75Sr
0.25Cr
0.5Mn
0.5O
3−δ interacted with La
0.9Sr
0.1Al
0.97Mg
0.03O
3−δ due to the interdiffusion of Sr
2+, Ti
4+, Mn
3+ and Cr
3+ cations into the La
0.9Sr
0.1Al
0.97Mg
0.03O
3−δ lattice. An interaction between La
0.9Sr
0.1Al
0.97Mg
0.03O
3−δ and NiO-GDC at 1300 °C was not detected, which means that joint utilization is possible.
The XRD patterns of two mixtures, La
0.8Sr
0.2Ga
0.85Mg
0.15O
3−δ/La
0.9Sr
0.1AlO
3−δ and NiO/La
0.9Sr
0.1AlO
3−δ (annealed at 1450 °C), confirmed that there were no chemical interactions between these components [
93]. The authors noted that doped LaAlO
3 materials can serve as additives to the composite electrolytes and the anode-protective layers [
93]. In addition, Mn-doped LaAlO
3 phases are considered a constituent part of the composite electrolytes, providing for the effective electrochemical oxidation of methane via ethylene and ethane [
94].
4. Applications in SOFCs
There are fragmentary data on the application of lanthanum aluminate electrolytes in SOFCs.
An SOFC was fabricated with 70% NiO–30% YSZ as an anode, a samarium doped ceria (SDC) as an interlayer, La
0.9Ba
0.1Al
0.9Y
0.1O
3−δ (LBAYO) as an electrolyte and lanthanum strontium manganite (LSM) as a cathode, and tested in [
53]. LBAYO films with thicknesses of 63 and 74 μm were electrophoretically deposited on the LSM pellets with a diameter of 25 mm and a thickness of 2 mm. The LSM substrates and the deposited LBAYO films were then annealed at 1450 °C for 2 h to achieve full electrolyte densification. A NiO-YSZ anode with a thickness of 40 μm was screen-printed on the LBAYO/LSM sample and then sintered at 1500 °C for 6 h. To avoid chemical interactions between the NiO and the LBAYO film, an SDC buffer layer with a thickness of 10 μm was additionally screen-printed on the LBAYO film between the electrolyte and the anode. Humidified hydrogen was used as a fuel, while air was used as an oxidant. The open-circuit voltage (OCV) values of the fabricated cells were 0.927 and 0.953 V, while the maximum power density values were 0.306 and 0.235 W cm
−2 for the LBAYO electrolyte layers with thicknesses of 63 and 74 μm, respectively. The long-term stability experiments demonstrated negligible degradation of the SOFC with LBAYO electrolyte over 10 days.
Another Ni-GDC/GDC/La
0.9Sr
0.1Al
0.97Mg
0.03O
3−δ/GDC/La
0.75Sr
0.25FeO
3−δ electrolyte-supported cell was tested in [
87]. For this single cell with a La
0.9Sr
0.1Al
0.97Mg
0.03O
3−δ electrolyte thickness of 550 μm, the OCV and
Pmax values at 800 °C were found to be equal to 0.925 V and 0.195 W cm
−2, respectively.
5. Conclusions
Concluding the short review about the applications of
doped LaAlO3 oxides as electrolyte materials in SOFCs, it is worth to notice that: on the one side, the power characteristics of SOFCs with doped LaAlOdoped LaAlO3 oxides as electrolyte materials in SOFCs, it is worth noting that: on the one side, the power characteristics of SOFCs with doped LaAlO3 electrolytes are approximately equal to the Pmax values of SOFCs with Sr, Mg-doped LaGaO3 electrolytes without buffer layers [1]; on the another side, the commercial production of the LaAlO electrolytes without buffer layers [52]; on the another side, the commercial production of the LaAlO3-based electrolytes is significantly cheaper than yhe production of the LaGaO3-based electrolytes. That is why, further investigations of the LaAlO3-based electrolytes would be continued.