1. Introduction

2. New Non-Cubic Structural Types of Oxygen Ion and Proton Conductors

2.1. REE Molybdates and Tungstates

Systems La₂O₃–MO₃ (M = Mo, W) have been known by the fact that highly-conducting oxide ionic conductor La₂Mo₂O₉ (LAMOX) [83] has been firstly discovered, and then solid solutions La_6–xWO_12–δ (x = 0.3–0.7) with sufficiently high conductivity to be used as proton-conducting SOFC electrolytes [84,85,86] and proton-conducting membranes for hydrogen separation [87,88,89] have been revealed.

It is noteworthy that the specified compounds possess a rather unusual structure characterized by the large size unit cells with a complex arrangement. La₂Mo₂O₉ demonstrates first-order phase transitions at 580 °C [90]: low-temperature monoclinic α-phase (P21) undergoes a transformation into the high-temperature cubic β-phase (P213) with increasing the conductivity by two orders of magnitude (0.06 S cm^–1 at 800 °C). It is known that, e.g., the low-temperature monoclinic α-phase of LAMOX contains 312 atoms within the single unit cell. However, at room temperature after cooling, metastable modification of the β-phase, designated as the β՜-phase, partially preserves along with the α-modification which is attributed to the difficulty in arranging too large number of oxygen atoms within the unit cell of the low-temperature α-phase [91]. It has been demonstrated that the splitting of one of the three main positions of oxygen is present in the β՜-structure, which characterizes it as partially ordered [92]. Therefore, a mixture of α+β՜-phases is present at room temperature. It is evident that the connection between the highly conductive cubic phases and their low-temperature polymorphs significantly impacts the practical applications of highly conductive cubic electrolytes.

A search for highly conductive materials amongst non-cubic molybdates Ln₂MoO₆ (Ln = La, Pr, Nd, Sm, Gd, Dy) has been carried out during last years [93,94]. The oxygen pressure-dependent conductivity isotherms of Ln₂MoO₆ (Ln = La, Pr, and Nd) at 1 × 10⁻¹⁸ – 0.21 atm pO₂ indicate the presence of a mixed ionic and electronic conductivity within the temperature range of 700−900 °C [93]. The oxymolybdates with large cations (La, Pr, and Nd) possess a tetragonal symmetry. The highest ionic component of the conductivity, 10⁻³ S cm^–1 at 800 °C, has been discovered for Pr₂MoO₆ possessing a space group I4̅c2. All three layered light REE oxymolybdates Ln₂MoO₆ (Ln = La, Pr, and Nd) exhibit the proton conductivity in a wet atmosphere.

The conductivity of the monoclinic polymorphs of Ln₂MoO₆ (Ln = Sm, Gd, Dy) oxymolybdates has been investigated by theoretical and experimental methods [94]. All single-phase compounds are isostructural with a monoclinic symmetry. It is noteworthy that the ionic component of conductivity demonstrated an increase with decreasing the ionic radius in the series of molybdates: ~10^–5 S cm^–1 for Sm₂MoO₆, ~10^–4 S cm^–1 for Gd₂MoO₆ and ~10^–3 S cm^–1 for Dy₂MoO₆ at 800 °C according to impedance spectroscopy data. The oxygen pressure isotherms in Ln₂MoO₆ (Ln = Sm, Gd, Dy) samples indicated the electronic conductivity contribution in the temperature range of 500–800 °C at high partial oxygen pressures. Hence, the increase in the ionic component of the conductivity with decreasing lanthanide ionic radius in the series of the monoclinic oxymolybdates Ln₂MoO₆ (Ln = Sm, Gd, Dy) has been observed (Figure 2a) [94]. The oxygen migration barrier for Ln₂MoO₆, as calculated by means of the Density-Functional Theory (DFT) using Nudged Elastic Band (NEB) method, exhibits a decrease in the Ln row with a decrease in the ionicity degree of the Ln–O bond, as follows: La > Pr > Nd > Sm > Gd > Dy. The lowest energy barriers of oxygen migration in Ln₂MoO₆ (Ln = Sm, Gd, Dy) were observed for the O1–O1, O1–O2, O6–O6 and O4–O5 transitions. The DFT – NEB oxygen diffusion maps acquired according to these calculations are similar for Ln₂MoO₆ (Ln = Sm, Gd, Dy). For illustrative purposes, a map for Ln = Dy is provided in Figure 2b [94].

The objective of the work co-authored by Qin Li, and Venkataraman Thangadurai [95] was to develop new mixed conductors that would possess the chemical-physical properties of Ni–YSZ while avoiding the issues inherent to the conventional cermet anodes. Modern anode disadvantages that lead to a decline in SOFC operation include a decrease in porosity, the destruction of the cermet microstructure due to carbon deposition, and the formation of nickel sulfide during sulfur adsorption [96]. Furthermore, metal-based anodes tend to suffer from the particle agglomerations during the operation, which results in shortening the effective triple-phase boundary (TPB). TPB is an electrochemically active region where the contact of a solid electrolyte, a fuel (in the case of the anode) or oxygen (in the case of the cathode) molecule and an electrode occurs. In the aforementioned work, the authors have explored monoclinic structure of Sm_2‒xA_xM_1–yB_yO_6−δ (A = Ca, Sr; M = Mo, W; B = Ce, Ni) solid solutions as precursors for the development of solid oxide fuel cells anodes. Among the samples studied, Sm_1.8Ca_0.2MoO_6−δ (SMO) displays a total conductivity of 0.12 S cm^–1 at 550 °C in wet H₂ with an activation energy of 0.06 eV. Ca-doped SMO appears to be chemically stable against reaction with YSZ electrolyte at 800 °C for 24 h in wet H₂.

For the solid solutions based on lanthanum tungstate La_6–xWO_12–δ (x = 0.2–1) Magrasó et al. [86] proposed the formula La_28–xW_4+xO_54+1.5x(${\mathrm{V}}_{\mathrm{O}}^{••}$)_2–1.5x (x is W on La positions), with an increased unit cell, which ensured better agreement between the measured and X-ray densities than did the previously used formula La_6–xWO_12–δ (x = 0.2–1). Note that the tungstates also have a large unit cell with a complex structure, like α-La₂Mo₂O₉. Hence, large and complex unit cells are not rare in the La₂O₃–MO₃ (M = Mo, W) systems. This feature is possibly due to the considerable ion size mismatch between La and Mo(W).

Along with the best La_6–xWO_12–δ (x = 0.3–0.7) proton conductors, the La₂WO₆ compound [97,98,99] and other compounds of this system – La₁₈W₁₀O₅₇ [100], La₆W₂O₁₅ [101], and La₁₀W₂O₂₁ [102] – attracted researchers’ attention at the same time (Figure 3). A number of studies were concerned with the structure and ionic conductivity of these compounds [103,104,105,106,107]. γ-La₆W₂O₁₅ [106] and β-La₂WO₆ [104,105] were shown to have proton conductivity, however, it was two orders of magnitude lower than that of La_6–xWO_12–δ (x = 0.3–0.7).

Further studies of the compounds in the system Ln₂O₃–WO₃ (Ln = Nd – Yb) enabled the authors of the work [108] to identify a novel class of oxide ionic conductors Ln₁₄W₄O₃₃ with pseudorhombohedral structure. Reliable description of this structure has still not been accomplished up to now.

Tungstates Ln₁₄W₄O₃₃ were divided by McCarthy [109] into three groups. The structure of heavy REE tungstates Ln₁₄W₄O₃₃ (where Ln = Er–Lu) is the most understandable, corresponding to a simple rhombohedral cell. The structure of Ln₁₄W₄O₃₃ (Ln = Ho, Y) is particularly complicated due to the splitting of the strong reflections (214), (431), (422) of a simple rhombohedral cell and has not been completely solved. Ln₁₄W₄O₃₃ (Ln = Nd–Dy) is even more problematic, as its structure is proposed to be pseudorhombohedral, with only 4 non-splitted reflections. The study of the structure continued later, and in 1996 the authors [110] proposed a body-centered monoclinic unit cell for Gd₁₄W₄O₃₃, but further study of this compound by electron diffraction and refinement of the data by the X-ray diffraction method (with Rietveld refinement) led the authors to conclude that the Gd₁₄W₄O₃₃ unit cell has a larger size and is not fully described within the framework of the proposed model. Thus, for the tungstates Ln₁₄W₄O₃₃ (Ln = Nd–Dy) and Ln₁₄W₄O₃₃ (Ln = Ho, Y) the structure has not been completely resolved even today. Probably, in this case a specific structure forms as well which possesses similar to lanthanum tungstates and molybdates giant cell up to ∼8800 ų in size [86,111,112,113], which is structurally closer to short-chain proteins than to ordinary inorganic materials [111].

Ln₁₄W₄O₃₃ (Ln = Nd, Sm, Gd) tungstates have a wide electrolytic range (10^–15–1 atm, Figure 4) above 700 °C. It should be noted that under oxidizing conditions the contribution of hole conductivity is absent in these compounds. Among the pseudorhombohedral phases of Ln₁₄W₄O₃₃ (Ln = Nd, Sm, Gd), Nd₁₄W₄O₃₃ exhibits the highest oxygen-ion conductivity (4 × 10^–4 S cm^–1 at 700 °C). The electrolytic domain of the second half of the REE Ln₁₄W₄O₃₃ (Ln = Dy, Ho, Er, Tm, Yb) series becomes narrower (10^–10–10^–5 atm) and at the same time has a slight slope indicating the presence of impurity electronic charge carriers. Investigations in a reducing atmosphere confirmed that the reduction tendency increases with decreasing lanthanide ionic radius in the Ln₁₄W₄O₃₃ tungsten series. The variations of the total conductivity in the series of pseudorhombohedral phases Ln₁₄W₄O₃₃ (Ln = Dy, Ho, Er, Tm, Yb) are associated with local changes in the short-range order structure, as evidenced by Raman spectroscopy (Figure 5).

In the work [114], a search for the synthesis conditions for various polymorph modifications of the Nd₂WO₆ ceramics was carried out including application of the technique of long-term isothermal calcinations. The phase formation of neodymium tungstate Nd₂WO₆ from mechanically activated oxides in a wide temperature range of 25–1600 °С was studied [115]. The conditions of forming various polymorph modifications were determined: low-temperature rhombic (β-Nd₂WO₆ and δ- Nd₂WO₆ (P2₁2₁2₁ (No. 19)) and high-temperature monoclinic Nd₂WO₆ (sp. gr. C12/c1 (15)) ones. Although β-Nd₂WO₆ and δ-Nd₂WO₆ stabilized in ceramics for a first time belong to the same space group (P2₁2₁2₁ (No. 19), their structures and, thus, XRD patterns differ. It was successful in stabilizing δ-Nd₂WO₆ in a pure form by using long-term isothermal calcination during 100 h at 900 °C. Moreover, due to mechanical activation, the same polymorph was successfully obtained by short-term calcination at 1600 °C for 1 h. For both modifications of neodymium tungstate, rhombic δ-Nd₂WO₆ and monoclinic Nd₂WO₆, the protonic conductivity with the effective activation energy of ~1.05 eV was discovered. However, for Ca-containing solid solutions (Nd_1–xCa_x)₂WO_6-δ (x = 0.01), which total conductivity increases compared to the undoped monoclinic Nd₂WO₆, hole conductivity predominates in air. Similar to the tetragonal Nd₂MoO₆ [93], the total conductivity of monoclinic modification of Nd₂WO₆ is as low as ~ 1 × 10^–6 S cm^–1 at 550 °С in dry air.

Figure 5. Raman spectra of a series of tungstates Ln₁₄W₄O₃₃ (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) [108]. Reprinted from Ceramics International, Vol. 50, Anna Shlyakhtina et al, Impact of Ln cation on the oxygen ion conductivity of Ln₁₄W₄O₃₃ (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) tungstates, Pages No. 704-713, Copyright (2024), with permission from Elsevier.

Figure 5. Raman spectra of a series of tungstates Ln₁₄W₄O₃₃ (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) [108]. Reprinted from Ceramics International, Vol. 50, Anna Shlyakhtina et al, Impact of Ln cation on the oxygen ion conductivity of Ln₁₄W₄O₃₃ (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) tungstates, Pages No. 704-713, Copyright (2024), with permission from Elsevier.

In ternary systems Ln₂O₃ – Bi₂O₃ – MO₃ (Ln = La, Pr, Nd; M = Mo, W), a rich phase polymorphism was observed as well [116,117,118,119]. The formation of various cubic, tetragonal, pseudotetragonal and monoclinic phases, or their mixture, takes place depending on the molar ratio of cations. Such materials include various compounds, including those with monoclinic Bi_3.24La₂W_0.76O_10.14-like, tetragonal β- and β’-Bi₂O₃-like, Bi₁₄WO₂₄-like and Bi₁₄W₂O₂₇-like, Aurivillius-like Bi₂MO₆, cubic δ-Bi₂O₃-like, and numerous others structures, some of which remain unresolved. Such compounds demonstrated a high total conductivity (up to ~10^–4 – 10^–1 S cm^–1 at 700 °C). The oxide ionic conductivity contribution is believed to be high, and the protonic conductivity component apparently emerges in some cases [116,117,118,119].

Various REE tungstates and molybdates can exhibit a good catalytic performance when used as catalysts themselves or as catalyst supports for metal-based catalysts in various oxidation reactions generally due to a high oxygen mobility (as discussed above), reactivity of Mo/W active sites and ability of Mo/W and sometimes REE cations to change their charge and form low-valent species cooperatively working with other active sites (including metallic ones in the case of tungstate/molybdate supports). These reactions include dry reforming [120] and oxidative coupling of methane [121], partial and total oxidation of propane [122], selective oxidation of propene and isobutene into acrolein and methacrolein, respectively [121,123], and others [121,124].

Thus, for Ln-Bi-Ni-Co molybdates (Ln = La, Ce, Pr, Sm, Tb) in the partial oxidation of propane into propene and carbon dioxide, it was demonstrated that Ln-containing catalysts show better catalytic activity compared to Ln-free ones. The Ce- and Tb-containing catalysts provided highest propane conversion and, on the other hand, highest selectivity with respect to CO₂. The La- and Pr-containing catalysts demonstrated the highest selectivity with respect to propene. The Sm-containing catalyst possessed the lowest activity [122].

2.2. A₂M₃O₁₂ Family

The compounds with formula A₂M₃O₁₂ or A₂(MO₄)₃ (A = Y³⁺, Ln^3+/4+ and other 2+, 3+ or 4+ cations, M = Mo⁶⁺, W⁶⁺, V⁵⁺, P⁵⁺) is a large family of materials, being a part of rich polymorphism of oxides with various stoichiometry and various functional properties [52,125,126,127,128]. The oxides in A₂M₃O₁₂ family commonly found the application related to their negative or close-to-zero thermal expansion coefficient [50,129,130,131,132,133]. However, due to their high ionic conductivity, such oxides are recently considered for using as SOFC/SOE electrolytes [132,134,135,136,137], additives for composite cathodes [50,130], permselective membranes [135] and catalysts for various processes [138,139,140].

A₂M₃O₁₂ oxides generally possess monoclinic or orthorhombic structure depending on A and M cation nature. E.g., Y tungstate, Sc tungstate and molybdate crystallize in the orthorhombic structure [133,139]; the lighter rare earth tungstates crystallize in monoclinic structure (C2/c) whereas the heavy rare earth tungstates do it in the orthorhombic structure (Pcna) [133]. Such oxides possess open-framework structure with corner-shared AO₆ and WO₄ polyhedral units, and every oxide anion is bound to one A cation and one M cation [129,130,131,133]. Representations of the monoclinic and orthorhombic polymorphs are given in Figure 6.

A₂M₃O₁₂ oxides are predominately ionic conductors. Some of these oxides can possess mixed ionic-electronic conductivity due to the ability of A-site cations to vary their charge (such as Eu^3+/4+). Total ionic transference number is generally close to 1, however, for some compounds (e.g., Ln₂W₃O₁₂, Ln = La, Sm, Eu, Gd) can be 0.8 or lower at high temperatures [132,136,137]. According to the nowadays knowledge, there are two main charge carriers in A₂M₃O₁₂ oxides [136,137,141,142,143]:

• O^2– (typical transference number ~0.65–1) which migrates via vacancy and interstitial mechanisms;
• (MO₄)^2– (typical transference number ~0–0.35) which migrates via so-called “Rock and Roll” mechanism (can be considered as O^2– vehicle mechanism as well).

Figure 6. Structure of Dy₂(WO₄)₃ compound: (a) orthorhombic (Pnma) and (b) monoclinic (P2₁/c) polymorphs [130]. Reprinted from Journal of Alloys and Compounds, Vol. 990, Natalia Kireeva, Aslan Yu. Tsivadze, Oxide ceramics of A₂M₃O₁₂ family with negative and close-to-zero thermal expansion coefficients: Machine learning-based modeling of functional characteristics, Pages No. 174356, Copyright (2024), with permission from Elsevier.

Figure 6. Structure of Dy₂(WO₄)₃ compound: (a) orthorhombic (Pnma) and (b) monoclinic (P2₁/c) polymorphs [130]. Reprinted from Journal of Alloys and Compounds, Vol. 990, Natalia Kireeva, Aslan Yu. Tsivadze, Oxide ceramics of A₂M₃O₁₂ family with negative and close-to-zero thermal expansion coefficients: Machine learning-based modeling of functional characteristics, Pages No. 174356, Copyright (2024), with permission from Elsevier.

“Rock and Roll” mechanism of (MO₄)^2– anion migration proceeds in the following way. MO₄ tetrahedron in regular position reorients and rolls into the interstitial site. Then neighboring MO₄ tetrahedron occupies the position previously occupied by early mentioned MO₄ tetrahedron. (MoO₄)^2– anions are demonstrated to be more mobile than (WO₄)^2– ones facilitating both rotations and hops [133,141,142,143].

Such transport features provide a good ionic conductivity of A₂M₃O₁₂ oxides (up to ~10^–3 S cm^–1 at 700 °C, Figure 7) and oxygen self-diffusion coefficients (Figure 8) which can be estimated using Nernst – Einstein equation (up to ~10^–9 cm² s^–1) [132,135,136,137]. Further increase in ionic conductivity can be achieved by doping A- and M-sites with various cations. E.g., doping Sm tungstate with small amounts of Ca²⁺ and Zn²⁺ led to increase in the ionic conductivity by a factor of 1.5–2 [135]. Further developments of the materials belonging to this family in order to increase oxygen mobility without losing in stability would amplify their electrochemical (e.g. IT SOFC/SOE electrolytes) and catalytic (e.g. efficient and stable to coking catalysts for fuel transformation or their supports) applications.

As with the REE tungstates/molybdates outlined in the preceding section, numerous members of the A₂M₃O₁₂ family can function as catalysts or their supports in various reactions due to their transport properties, and the capacity of the cations to alter their charge as surface active site features including participation of the surface (Mo/W)–O and (Mo/W)–${\mathrm{V}}_{\mathrm{O}}^{••}$ species in redox processes. Such reactions include partial oxidation [144,145], dry and oxy-dry reforming [146] and oxidative coupling of methane [121], and other hydrocarbons [147,148,149]. selective oxidation of propene and isobutene into acrolein and methacrolein, respectively [121,123], reactions of alcohols’ oxidation [147,150] and others [121,124,147,148].

In partial oxidation of methane, the Fe₂Mo₃O₁₂ catalyst displays a decrease in selectivity with respect to methanol as the temperature rises. The selectivities with respect to CO, CO₂, and formaldehyde exhibit a range of temperature dependencies at various pressures (3 – 67 bar) [144]. For the A₂Mo₃O₁₂ (A = Al, Ga, In, Sc, Cr, Fe) catalysts in partial oxidation of methane at 750 °C and ambient pressure, different values of CH₄ conversion and the products’ selectivities were demonstrated depending on the A cation nature. Various metal additives have been demonstrated to enhance CH₄ conversion (Table 1) [145]. For dry and oxi-dry reforming of methane over the Ce₂W₃O₁₂ catalyst, better performance was demonstrated for the oxi-dry reforming reaction. Total oxidation into carbon dioxide is preferable at T < 800 °C, at higher temperatures, partial oxidation into syngas CO + H₂ occurs. The partial oxidation of methane into carbon monoxide can be attributed to oxygen species easily moving to the catalyst surface, thereby promoting the methane oxidation [146].

2.3. Composites Based on REE Molybdates and Tungstates

Some works devoted to composite materials in tungstate and molybdate systems with REE are of interest [151,152].

For the first time, an extensive study of composites (100–x) La₂Mo₂O₉ – x La₂Mo₃O₁₂ in a wide concentration range (x = 5, 10, 15, 20, 30, 100) was carried out [151]. Presented results show that (100–x) La₂Mo₂O₉ – x La₂Mo₃O₁₂ composites have a high ionic conductivity in the range of x =10–15% (Figure 9). The α-La₂Mo₂O₉−La₂Mo₃O₁₂ composite materials are a new class of highly-conductive materials demonstrating elevated oxide ionic conductivity. In this case, it is to be noted that composites contain low-temperature monoclinic α-phase La₂Mo₂O₉ along with monoclinic La₂Mo₃O₁₂ phase. Increase in conductivity, oxygen surface exchange constant and diffusion coefficient for the composites compared to the individual oxides is observed (Figure 10) which is due to a segregation of various ions on grains’ surface and formation of a new fluorite-like phase La₅Mo₃O₁₆ at the La₂Mo₂O₉-La₂Mo₃O₁₂ interphase. 3D-modeling of the composite microstructure was carried out based on the SEM image analysis data in order to estimate the conductivity of the interphase La₅Mo₃O₁₆ layer between La₂Mo₂O₉ and La₂Mo₃O₁₂ grains. The electrical conductivity values for the composite materials calculated from the 3D-modeling of the microstructure and the experimentally measured ones correlate and demonstrate composite effect.

Figure 9. Concentration dependencies of the bulk conductivity in air for (100–x) mol. % La₂Mo₂O₉ – x mol.% La₂Mo₃O₁₂ oxides, where * open red circles correspond to conductivity data calculated from the Nernst – Einstein equation using oxygen diffusion coefficient [151]. Reprinted with permission from Ref. [151]. Copyright (2022) American Chemical Society.

Figure 9. Concentration dependencies of the bulk conductivity in air for (100–x) mol. % La₂Mo₂O₉ – x mol.% La₂Mo₃O₁₂ oxides, where * open red circles correspond to conductivity data calculated from the Nernst – Einstein equation using oxygen diffusion coefficient [151]. Reprinted with permission from Ref. [151]. Copyright (2022) American Chemical Society.

Figure 10. Arrhenius plots for oxygen surface exchange constant (a) and tracer diffusion coefficient (b) for the La₂Mo₂O₉ (1), La₂Mo₃O₁₂ (2) and La₂Mo₂O₉ (85 mol. %) – La₂Mo₃O₁₂ (3) samples [151].

Figure 10. Arrhenius plots for oxygen surface exchange constant (a) and tracer diffusion coefficient (b) for the La₂Mo₂O₉ (1), La₂Mo₃O₁₂ (2) and La₂Mo₂O₉ (85 mol. %) – La₂Mo₃O₁₂ (3) samples [151].

The protonic conductivity of composites in the La₂O₃–WO₃ system was studied using as an example the protonic conductor La₁₄W₄O₃₃ (5 × 10^–5 S cm^–1 at 600 °C), which is two-phased material comprised of anion-deficient fluorite-like phase La₁₀W₂O₂₁ (18 wt.%) and rhombic phase γ-La₆W₂O₁₅ (82 wt.%) [152]. A high Nd content in the composite based on the γ-La₆W₂O₁₅ leads to forming the solid solutions based on pseudorhombohedral phase for La_14–xNd_xW₄O₃₃ for x = 12 and 14. Nd-containing composites based on the γ-La₆W₂O₁₅ possess the protonic conductivity which gradually decreases with decreasing Nd content, and transform into the pure pseudorhombohedral phases La_14–xNd_xW₄O₃₃ (x = 12, 14) with oxide-ionic conductivity (Figure 11a). Synthesis at 1500 °C of mechanically activated 5La₂O₃+2WO₃ oxide mixture resulted in forming the mirror composite including 81 wt.% of pure La₁₀W₂O₂₁ phase and 19 wt.% of γ-La₆W₂O₁₅. In this case, the maximum proton conductivity in wet air (1 × 10^–3 S cm^–1 at 700 °C) (Figure 11b) comparable to that of the known proton conductor La_6–xWO_12–δ (x = 0.6) was discovered [85]. Between two mirror composites with the γ-La₆W₂O₁₅/La₁₀W₂O₂₁ phases ratios of 82/18 and 19/81, the highest conductivity was demonstrated for the La₁₀W₂O₂₁ based composite, with the difference in the protonic conductivity being in an order of magnitude (Figure 11b).

Figure 11. (a) Arrhenius plots of conductivity in dry (filled data points) and wet (open data points) air for γ-La₆W₂O₁₅–based composites and pseudorhombohedral solid solutions, synthesized between 1450 and 1500 °C: (1) La₁₀W₂O₂₁/γ-La₆W₂O₁₅ composite (1450 °С); (2) La₁₀W₂O₂₁/γ-La₆W₂O₁₅/Nd6 composite (1500 °С); (3) La₁₀W₂O₂₁/γ-La₆W₂O₁₅/Nd8 composite (1500 °С); (4) La₁₀W₂O₂₁/γ-La₆W₂O₁₅/Nd10 composite (1500 °С); (5) La₂Nd₁₂W₄O₃₃ pseudorhombohedral solid solution (1500 °С); (6) Nd₁₄W₄O₃₃ (1500 °С); (b) Arrhenius plots of conductivity in dry (1) and wet (2) air for the La₁₀W₂O₂₁-based composite [152]. Reprinted from International Journal of Hydrogen Energy, Vol. 48, A.V. Shlyakhtina et al., Proton /oxygen ion conductivity ratio of Nd containing La₁₀W₂O₂₁/γ-La₆W₂O₁₅ tungstates, Pages No. 22671-22684, Copyright (2023), with permission from Elsevier.

Figure 11. (a) Arrhenius plots of conductivity in dry (filled data points) and wet (open data points) air for γ-La₆W₂O₁₅–based composites and pseudorhombohedral solid solutions, synthesized between 1450 and 1500 °C: (1) La₁₀W₂O₂₁/γ-La₆W₂O₁₅ composite (1450 °С); (2) La₁₀W₂O₂₁/γ-La₆W₂O₁₅/Nd6 composite (1500 °С); (3) La₁₀W₂O₂₁/γ-La₆W₂O₁₅/Nd8 composite (1500 °С); (4) La₁₀W₂O₂₁/γ-La₆W₂O₁₅/Nd10 composite (1500 °С); (5) La₂Nd₁₂W₄O₃₃ pseudorhombohedral solid solution (1500 °С); (6) Nd₁₄W₄O₃₃ (1500 °С); (b) Arrhenius plots of conductivity in dry (1) and wet (2) air for the La₁₀W₂O₂₁-based composite [152]. Reprinted from International Journal of Hydrogen Energy, Vol. 48, A.V. Shlyakhtina et al., Proton /oxygen ion conductivity ratio of Nd containing La₁₀W₂O₂₁/γ-La₆W₂O₁₅ tungstates, Pages No. 22671-22684, Copyright (2023), with permission from Elsevier.

3. Cobaltite-Based Oxides with Layered Structure and Their Composites

3.1. Misfit Layered Oxides

Misfit layered oxides are characterized by a general formula AO–(MO)_m–AO–CoO₂ or (M_mA₂O_2+m)_qCoO₂, where m = 1 or 2, A = Ca, Sr, Ba, etc., M = Co, Bi, etc. The layered structure of these compounds is comprised of repeating distorted rock salt (RS) type M_mA₂O_2+m layers and CoO₂ layers of a hexagonal (H) CdI₂-like structure with incommensurate b-axes. The parameter q represents the incommensurability ratio (b_RS/b_H) that is typically close to 0.62 [79,80,157]. The example of misfit layered oxides is calcium cobaltite (CCO), which is composed of [Ca₂CoO_3±δ] layer sandwiched between two [CoO₂] layers (Figure 13) [79,80,157,158,159,160]. The compound can be expressed with a general formula of [CoO₂] [Ca₂CoO_3±δ]_q (q = 0.614 – 0.69, typically 0.62) or Ca₃Co_4–xO_9±δ (C349, CCO abbreviations). Both subsystems crystallize in a monoclinic crystal structure, where only the b lattice parameter varies.

Ca₃Co₄O₉ is generally considered to be a thermoelectric material [80,161,162]. However, due to moderate values of the thermal expansion coefficient (TEC = 10–11 × 10^–6 K^–1 [163,164] providing excellent thermomechanical compatibility with the majority of solid state electrolytes, based on ZrO₂, CeO₂, LaGaO₃ and BaCe(Zr)O₃), mixed-conducting (e^–/O^2–) and, in the case of proper doping, triple-conducting properties [165] and fast surface exchange kinetics (k^* = 1.6 × 10^–7 cm s^–1 at 700 °C [160]), the interest in such compound as IT SOFC/SOE air electrode [157,159,165,166,167] and electrocatalyst [159,165,168] is growing.

Mixed ionic and electronic conduction observed in this material has been reported to originate from ionic conduction in oxygen deficient Ca₂CoO_3–δ′ layers and electronic conduction in the CoO₂ layers [163]. The total conductivity of CCO, generally provided by the electronic component, ranges 10¹–10² S cm^–1 at 700 °C depending on the synthesis technique and heat treatment mode [161,167,169,170,171,172,173,174,175]. It is related to high structural anisotropy of CCO material and low stability limit equal to 926 °C under air atmosphere, as can be seen in the phase diagram represented in Figure 14a. Therefore, CCO ceramics produced by conventional solid state sintering at 900 °C during 24 h possess a relatively low density (~ 50%) which leads to the formation of plate-like grains during the crystallization process (Figure 14b) and, as a result, decreased conductivity, while for highly dense samples obtained using spark plasma sintering (Figure 14c), conductivity reached 100 S cm^–1.

Along with application of different synthesis techniques, a variety of approaches were employed to modify the electrical properties of CCO [176]. In the case of iso- and hetero-valence substitution on the Ca-site with alkaline earth [162,177,178] and alkali [176,179,180] metals, lanthanides [181,182], Y [183,184], Bi [185,186], Ag [178,187], Cu [188], the change in the conductivity was found to occur mainly due to modifying the carrier concentration and their mobility without fluctuating the band structure of the material. In the case of Bi-doping, a large increase in electrical conductivity was obtained due to the increased amount of Bi₂O₃ phase in the grain-boundary region [185]. Doping with basic monovalent Na+ ions in the Ca-site in CCO was found to generate a high density of extra charge carrier species with the increased Co oxidation state and facilitate both proton uptake and diffusion properties of CCO [180].

Figure 14. (a) Ca–Co–O phase diagram under air atmosphere [171]; a surface SEM image of CCO compact, fabricated using (b) a conventional solid-state sintering at 900 °C during 24 h [174] and (c) spark plasma sintering [175]. (a) Reprinted from Journal of the European Ceramic Society, Vol. 33, M.A. Madre et al., Preparation of high-performance Ca₃Co₄O₉ thermoelectric ceramics produced by a new two-step method, Pages No. 1747-1754, Copyright (2013), with permission from Elsevier. (b) Reprinted from Journal of the Ceramics International, Vol. 41, G. Constantinescu et al., Effect of Na doping on the Ca₃Co₄O₉ thermoelectric performance, Pages No. 10897-10903, Copyright (2015), with permission from Elsevier. (c) Reprinted from Journal of Solid State Chemistry, Vol. 313, Hohan Bae et al., Nonideal defect structure and high-temperature transport properties of misfit-layered cobalt oxide, Pages No. 123299, Copyright (2022), with permission from Elsevier.

Figure 14. (a) Ca–Co–O phase diagram under air atmosphere [171]; a surface SEM image of CCO compact, fabricated using (b) a conventional solid-state sintering at 900 °C during 24 h [174] and (c) spark plasma sintering [175]. (a) Reprinted from Journal of the European Ceramic Society, Vol. 33, M.A. Madre et al., Preparation of high-performance Ca₃Co₄O₉ thermoelectric ceramics produced by a new two-step method, Pages No. 1747-1754, Copyright (2013), with permission from Elsevier. (b) Reprinted from Journal of the Ceramics International, Vol. 41, G. Constantinescu et al., Effect of Na doping on the Ca₃Co₄O₉ thermoelectric performance, Pages No. 10897-10903, Copyright (2015), with permission from Elsevier. (c) Reprinted from Journal of Solid State Chemistry, Vol. 313, Hohan Bae et al., Nonideal defect structure and high-temperature transport properties of misfit-layered cobalt oxide, Pages No. 123299, Copyright (2022), with permission from Elsevier.

The doping on the Co-site with transition-metal elements [189,190,191,192,193], Ga [194,195], Mo, W [191,196], Ti [162,197] was found to result in significant alterations to the band structure, particularly when doping in CoO₂ layer. In examining the high-temperature transport properties of the Ca₃Co_4–xTi_xO₉ system, it was observed that when the Ti content reached 0.3, Ti ions appeared in both CoO₂ and Ca₂CoO₃ sublayers. The disturbance of the CoO₂ layer resulted in a tendency for holes to jump to more distant low-energy sites instead of neighboring ones. This led to a change in the transport mechanism from thermally activated hopping to two-dimensional variable range hopping (2D-VRH) [197]. In contrast, temperature dependence of electrical conductivity of Ca₃Co_4–xM_xO_9+δ (M = Ni, Fe, Mn, x = 0, 0.2, 0.4, 0.6), considered to be substituted in the CoO₂ layers, was found to demonstrate that the hole hopping conduction mechanism is dominant from room temperature up to 1000 K across the entire concentration range [192]. The exception was Ca₃Co_4–xCu_xO_9+δ (x = 0–0.3) samples, which exhibited a conductivity behavior similar to that of undoped CCO, displaying metallic-like behavior at low temperatures (below a transition temperature) and semiconducting behavior at high temperatures. It is noteworthy that the transition temperature increased with Cu doping. The authors proposed that Cu substitution occurred predominantly in the Ca₂CoO₃ layers, which resulted in an increase in CCO conductivity, while Ni, Fe, and Mn doping occurred in the CO₂ layers, leading to a decrease in conductivity [192]. However, in the recent studies [189,198], Fe-doping was shown to occur in the Ca₂CoO₃ layers – at the Co1 site at low doping, and at Ca1 site at high doping [198]. It was demonstrated that Fe doping reduces the effective charge of O atoms in CCO, which results in the weakening of the co-valence bond and the formation of oxygen vacancies in CCO. This, in turn, increases the electrochemical activity of the related electrodes [189]. Furthermore, Fe doping enhances the hydration behavior, protonation, and proton migration (see Table 2).

Dual-doping has also been successfully employed to enhance the total electrical conductivity of CCO, including Na/Mo [164,199], Na/Cu and La/Cu [164,200], Gd/Cu [165], Y/Fe [201] systems.

Figure 15 summarizes temperature dependence of the total conductivity of some single and dual-doped systems. Among single-doped materials, the highest electrical conductivity values were obtained for Ca_2.9Bi_0.1Co₄O_9–δ (656 S cm^–1 at 880 °C [202]), Ca_0.85Na_0.15Co₄O₉ (549 S cm^–1 [180]) and Ca₃Co_3.9Cu_0.1O_9–δ (218.8 S cm^–1 at 800 °C [191]) (Figure 15). Promising results were demonstrated for Ca_2.9Ag_0.4Co₄O₉ and Ca_2.9Sr_0.1Co_3.92O₉ (~145 S cm^–1 at 700 °C [162,178]). However, it should be noted that Bi and Cu-doped systems demonstrate a sharp dependence of the total conductivity on temperature, which may be unfavorable from the application point. Stabilization of the conductivity across the entire range of the SOFC/SOEC application for the Cu-doped system was reached using dual-doping, for the Ca_2.7La_0.3Co_3.8Cu_0.2O₉ system the total conductivity at 600 °C reached 187 S cm^–1 [200].

An increase in the total conductivity of doped CCO resulted in an enhancement of the electrochemical performance of the related electrodes due to uniform current distribution across the electrode [165,180,182,184,188,202]. When employing undoped CCO as the electrode functional layer, it is advisable to utilize current collectors with higher conductivity due to insufficient conductivity of CCO with increasing electrode porosity. Current collectors can be fabricated from a noble metal (e.g., Au, Ag, with the exception of Pt because of its interaction with CCO) [159,203] or an oxide electronic conductor [158,159].

The oxygen diffusion in CCO is provided by oxygen vacancies localized in the middle of rock salt layer which can be generated since Co charge in this layer can be 2+, 3+ and 4+, and the formation of such vacancies is compensated by decreasing the content of electron holes in the CoO₂ layer [159,160,168,175,204]:

$${\mathrm{O}}_{\mathrm{O}}^{\times }+2{\mathrm{h}}^{•}\rightleftarrows 0.5{\mathrm{O}}_2+{\mathrm{V}}_{\mathrm{O}}^{••}+2 \mathrm{e} \prime .$$

(1)

The content of the vacancies depends on the oxygen partial pressure as well (Figure 16a). Moreover, the material’s stability also shows strong correlation with the temperature and oxygen partial pressure. Since in the SOFC mode, in the cells with an oxygen-ion conducting electrolyte, the oxygen partial pressure at the cathode reaches 10^–3–10^–4 atm, thus, the CCO electrode may perform without significant degradation in the temperature range of 600–700 °C (Figure 16b). Contrarily, the CCO electrode may successfully operates in a wide temperature range in the SOEC mode or in the SOFCs with a proton-conducting membrane, which was demonstrated in a number of studies [158,159,170,179,205,206].

Since Co cations’ charge ranges from 2+ to 4+, doping with 2+ cations such as, for instance, Cu²⁺ can increase the vacancy content, thus, along with an increase in the total conductivity, the ionic conductivity may also be increased [168,183]:

$$\mathrm{CuO}\to {\mathrm{C} \mathrm{u} ″}_{\mathrm{Co}}+{\mathrm{O}}_{\mathrm{O}}^{\times }+{\mathrm{V}}_{\mathrm{O}}^{••}.$$

(2)

Unfortunately, the oxygen diffusion coefficient of CCO is not high enough (D^* = 2.7 × 10^–10 cm² s^–1 at 700 °С, Figure 17) [160,207]. Hence, the promising approach is using composite SOFC/SOE air electrodes comprised CCO and an oxide-ionic conductor such as Ce_0.8Gd_0.2O_2–δ (CGO) [205] or Sm_0.075Nd_0.075Ce_0.85O_2–δ [208], as well as Ce_0.8Pr_0.2O_2–δ [209], PrBaCo₂O_6–δ [167] or BaCe_0.5Zr_0.3Y_0.1Yb_0.1O_3–δ [159], exhibiting partial electronic conductivity in air, or catalytically active additive PrO_x [210]. An interesting approach was proposed by Guo et al. [211] to use a two-phase composite comprised CCO and La_0.7Sr_0.3CoO₃ (LSC) with an acceptable level of the total conductivity (147 S cm^–1 at 700 °C) and TEC (15.3 × 10^–6 K^–1) for the potential electrode application. Recently, Li et al. [212] proposed to use an interlayer based on La_0.6Sr_0.4FeO_3–δ (LSF) of the CCO electrode and YSZ electrolyte interface, which allowed increasing twice hydrogen production efficiency in a reversible cell compared to that with CGO layer.

On the other hand, CCO possess a high oxygen surface reactivity (k^* = 1.5×10^–7 cm s^–1 at 700 °C, Figure 17) apparently provided by the presence of weakly bound oxygen species on the surface which make available a fine performance in oxygen reduction/evolution reaction and, hence, compensate relatively low oxygen diffusion coefficient values and provide a good SOFC/SOE air electrode operation [159,160,165,206]. CCO-based materials demonstrate good transport properties in the presence of chemical potential gradient according to electrical conductivity relaxation data (Table 3) [189,207]. In the case of applying such a potential gradient, oxygen vacancies can easily be generated or occupied without significant surface exchange limitations. In the case of CCO-based electrode, this this can promote oxygen diffusion by rapid migration of oxygen vacancies from the electrolyte across the electrode – electrolyte interface [204].

CCO-based compounds can be used as catalysts for hydrogen and syngas production via sorption-enhanced steam reforming of ethanol (SESRE) and glycerol (SESRG) or their supports. Due to their transport features described above, CCO acts as an oxygen carrier is such a reaction. Furthermore, under the reaction conditions it can partially undergo a reversible phase change [37,38,213]:

$${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9\stackrel{\mp 3{\mathrm{O}}_2}{\rightleftarrows }3\mathrm{CaO}+4\mathrm{Co},$$

(3)

where CaO acts as a CO₂ sorbent thus removing this product and shifting the reaction equilibrium, and metallic Co acts as a catalytically active component [37,38,213]. In particular, CCO may be involved in chemical looping methane combustion (CLMC, reaction (4), Figure 18) and then in the SESRG process (reaction (5)), and after that be regenerated (reaction (6)) [37,38,213]. According to the in situ XRD studies [213], the regeneration process (reaction (6)) proceeds step-wise via the formation of intermediate products such as CoO, Co₃O₄, CaCo₂O₄ and Ca₂Co₂O₅ depending on the temperature.

Ca₃Co₄O_{9 (s)} +1.5CH_{4 (g)} = 3CaO_(s) + 4Co_(s) + 1.5CO_{2 (g)} + 3H₂O_(g).

(4)

C₃H₈O_{3 (g)} + 3H₂O_(g) + 3CaO = 7H_{2 (g)} + 3CaCO_{3 (s)}

(5)

3CaCO_{3 (s)} + 4Co_(s) + 3O_{2 (s)} = Ca₃Co₄O_{9 (s)} + 3CO_{2 (g)}

(6)

There are three stages of SESRG process of CCO-based catalysts (Figure 19):

• Pre-breakthrough: During this stage, a significant portion of the carbon dioxide generated by steam reforming and side water gas shift reactions is sorbed by the catalyst, thereby enhancing the production of hydrogen.
• Breakthrough: At this stage, the sorption of carbon dioxide by CaO slows due to a decrease in the content of available sorption sites. Consequently, the sorption enhancement effect weakens.
• Post-breakthrough: At this stage, the sorbent becomes fully saturated by carbon dioxide. The concentrations of products then reach their equilibrium values [37,38,213].

Figure 19. Typical SESRG product concentration depending on time [38,213].

Figure 19. Typical SESRG product concentration depending on time [38,213].

The duration of these stages depends on the catalyst composition and the conditions.

Doping CCO with 1–2 wt. % of noble metals (such as Pt, Pd, Ru, Ir, Rh) increases glycerol conversion and hydrogen yield in pre- and post-breakthrough periods, and the best results have been demonstrated for 1 wt. % Pt-doped CCO (Figure 20) (glycerol conversion ~100 %, hydrogen yield ~90 %) [38]. In such catalysts, noble metals are exsolved as nanoparticles [37,38,213].

3.2. Ba Cobaltite Intergrowth Family

The other compounds based on layered cobaltite can be assumed while replacing the rock-salt layer in abovementioned misfit structure of layered cobaltites with a perovskite layer which results in another type of intergrowth structure. In such a structure, there are repeating n octahedral layers of perovskite [Ba_n+1Co_nO_3n+3], CdI₂-type layer 4[CoO₂] and additional cobalt cations (4Co) in the interfacial layer resulting in a general formula of [Ba_n+1Co_nO_3n+3] [Co₈O₈] for this intergrowth cobaltite family. The members of this family with n = 1, 2 and 3 are Ba₂Co₉O₁₄ (often abbreviated as BCO), Ba₃Co₁₀O₁₇ and Ba₄Co₁₁O₂₀, respectively. In this structure, the Co cations are typically charged 2+ and 3+, e.g., ${\mathrm{Ba}}_2{\mathrm{Co}}_2^{2+}{\mathrm{Co}}_6^{3+}{\mathrm{O}}_{14}$ and ${\mathrm{Ba}}_3{\mathrm{Co}}_2^{2+}{\mathrm{Co}}_8^{3+}{\mathrm{O}}_{17}$ for n = 1 and 2, respectively. The other cation is Ba²⁺ which can be partially substituted by other alkaline earth metals and/or lanthanides. Figure 21 demonstrates the structure of Ba₂Co₉O₁₄ [214,215,216].

Figure 20. Pre-breakthrough hydrogen yield in SESRG over reduced CCO-based catalysts: (a) CCO, (b) 1 wt. % Pt/CCO, (c) 2 wt. % Pt/CCO, (d) 2 wt. % Pd/CCO, (e) 2 wt. % Rh/CCO, (f) 2 wt. % Ru/CCO, (f) 2 wt. % Ir/CCO [38].

Figure 20. Pre-breakthrough hydrogen yield in SESRG over reduced CCO-based catalysts: (a) CCO, (b) 1 wt. % Pt/CCO, (c) 2 wt. % Pt/CCO, (d) 2 wt. % Pd/CCO, (e) 2 wt. % Rh/CCO, (f) 2 wt. % Ru/CCO, (f) 2 wt. % Ir/CCO [38].

Figure 21. Intergrowth structure of the layered BCO containing the CdI₂-based Co₈O₈ layers and perovskite Ba₂CoO₆ block [215]. Reprinted from Chemical Engineering Journal, Vol. 451, Minkyeong Jo et al., Layered barium cobaltite structure materials containing perovskite and CdI₂-based layers for reversible solid oxide cells with exceptionally high performance, Pages No. 138954, Copyright (2023), with permission from Elsevier.

Figure 21. Intergrowth structure of the layered BCO containing the CdI₂-based Co₈O₈ layers and perovskite Ba₂CoO₆ block [215]. Reprinted from Chemical Engineering Journal, Vol. 451, Minkyeong Jo et al., Layered barium cobaltite structure materials containing perovskite and CdI₂-based layers for reversible solid oxide cells with exceptionally high performance, Pages No. 138954, Copyright (2023), with permission from Elsevier.

Due to the mixed O^2–+e^– type of conductivity, high total conductivity values (100–240 S cm^–1 between 450 to 650 °C) and compatibility with major electrolytes types such as YSZ, GDC, LSGM, etc., reduced unwanted cation diffusion of the alkaline earth elements into the electrolyte, and reversible reducibility without structural destruction, BCO is considered as a promising material for SOFC/SOE oxygen electrodes (including protonic ceramic cells) [207,215,216,217,218] and electrocatalysts [215,219,220]. Moreover, BCO is capable of hydration accompanied by reduction of Co³⁺ cations and thus can be used as an electrocatalyst for hydrogen storage [220].

A further increase in the total conductivity can be achieved by doping BCO with various cations. E.g., for Nd_0.1Ca_0.1Ba_1.8Co₉O₁₄, an increase in total conductivity compared to undoped BCO has been observed which has been attributed to the synergistic effects of co-doping of Nd³⁺ and Ca²⁺ in the Ba-site probably due to a combined effect of strong reactivity of Ca²⁺ with oxygen and high electronegativity of Nd³⁺ with a large structural parameter [215].

However, there are two major drawbacks of BCO. First, it has been reported that Ba₂Co₉O₁₄ and Ba₃Co₁₀O₁₇ are metastable at high temperatures (1000 °C and higher) and transform into 2H-perovskite-related oxides in the series (Ba₈Co₆O₁₈)_α(Ba₈Co₈O₂₄)_β [214]. On the other hand, BCO with Ba-site doped with Nd³⁺ and Ca²⁺ have demonstrated a stable performance with no observable degradation during 400 h of operation in SOFC and SOE modes [215]. At second, BCO possesses a low oxygen mobility according to the work [207] (Figure 22). On the other hand, the same work [207] states that it is impossible to derive accurate transport parameters since an oxidation process at the BCO surface is still not understood. Nevertheless, the promising approach is the use of composites based on BCO and solid electrolytes with a high oxygen mobility [216,217].

4. Other Materials

4.1. Magnetoplumbites and Related Structures

Magnetoplumbite-structured oxides with general formula AM₁₂O₁₉ (A are typically large cations such as Ca, Sr, Ba, Bb, Ln, etc.; M are typically small cations such as Fe, Al, Ga, etc.) are members of the magnetoplumbite-group materials, which also include diaoyudaoites (AM₁₁O₁₇), hibonites (AM₁₂O₁₉), nežilovites (AM₁₂O₁₉), plumboferrites (A₂M₁₁O₁₉) and others [221,222,223,224,225,226,227]. Both A- and M-positions are often doped with various 2+, 3+ and 4+ cations. They are generally used as materials with specific magnetic properties, however, they are attracting attention as materials for SOFC/SOE electrodes [228] and catalysts for various processes such as methane partial oxidation, oxygen evolution reaction, etc. [229,230,231,232,233] due to their mixed ionic-electronic conductivity and moderate thermal expansion coefficient (~11 × 10^–6 K^–1 which is close to that of YSZ).

The AM₁₂O₁₉ magnetoplumbite structure is based on ten layers of approximately closest packing oxide anions perpendicular to the c-axis in the hexagonal unit cell. Two of these layers contain large A cations neighboring every fourth oxide anion. Small M cations are distributed over five interstitial sites: three with octahedral coordination, one with tetragonal one and one with trigonal-pyramidal one (coordination number 5). Among these layers, there are cubic blocks with spinel structure (S, [M₆O₈]²⁺) and hexagonal blocks (R, [AM₆O₁₁]^2–), and these blocks repeat which can be written as RSR’S’, where R’ and S’ are rotated 180° around c-axis with respect to R and S blocks, respectively [221,225,228]. Figure 23 and Figure 24 demonstrate visualizations of the AM₁₂O₁₉ structure for aluminates and ferrites, respectively.

The mixed ionic-electronic character of the electrical conductivity of magnetoplumbites includes the contribution of metal cations, oxide anions, and electron transport. It is reported that the electrical conductivity of magnetoplumbites is anisotropic, and the conductivity along the hexagonal axis is about order of magnitude lower compared to that within the basal plate, and their temperature dependencies are similar [234,235]. Electronic conductivity can be both p-type and n-type and is assumed to be related to the presence of M²⁺ and M³⁺ cations (such as Fe^2+/3+) that exchange with electrons [234,235,236,237]. Typical values of electronic conductivity vary with composition in the range of ~10^–3–10¹ S cm^–1 at 300 °C [238,239,240,241]. Aliovalent doping can lead to an increase in total conductivity due to the generation of additional charge carriers.

Metal cation diffusion also occurs in magnetoplumbites [237,239,240]. E.g., tracer diffusion coefficient values of Pb²⁺ are ~10^–8 cm² s^–1 at 700 °C for PbFe₁₂O₁₉ and PbFe₁₁O_17.5, D^* values of Sr²⁺ are ~10^–10 cm² s^–1 at 900 °C for SrFe₁₂O₁₉ [239].

There is a lack of information on oxygen diffusion for magnetoplumbites. In different works [229,241], the oxygen diffusivity is characterized as from low to excellent. The oxygen-related defects may include oxygen vacancies and interstitial oxygen species [233,237]. It is also known that the oxygen diffusion is anisotropic: it is suppressed along the c-axis and prefers to take place within the mirror plane (Figure 23) [230,231,232,242]. The values of the oxygen tracer diffusion coefficient normal to the c-axis and along the c-axis measured for Ba hexaaluminate are 9.35 × 10^–13 cm² s^–1 and 2.59 × 10^–14 cm² s^–1 at 1400 °C, respectively [232]. It has also been reported that the oxygen diffusivity in the surface layer is faster compared to that within the bulk [233,243].

Magnetoplumbite-like compounds, including various hexaaluminates, are regarded as prospective catalysts or supports for many reactions of syngas production, generally for methane transformation reactions (partial oxidation, steam, dry and oxy-dry reforming) [39,230,231,233,244,245,246,247,248]. Such materials serve as oxygen carriers providing mobile oxygen species for participation in catalytic reactions.

Furthermore, under reducing conditions, they are capable of exsolving metal nanoparticles such as Ni⁰ and Fe⁰ which are reactive to methane oxidation into syngas. In particular, for MAl_12–xNi_xO_19-δ (M = alkaline earth or rare earth cation), Ni⁰ nanoparticles can be generated after the partial oxygen removal from the mirror planes and the reduction of Ni cations situated between the spinel block and the mirror planes followed by Ni⁰ species migration into the surface defect sites. Such a reducibility depends on the nature of the large cations present in the mirror plane. Due to this, doping with Ni leads to an increase in syngas yield and a prevention of the methane combustion, however, this may also lead to an increase or decrease in carbon formation depending on the catalyst composition and its synthesis conditions [231,244].

In the case of Fe-containing hexaaluminates, such as MAl_12–xFe_xO_19-δ (where M represents an alkaline earth or rare earth cation), Fe cations play important role in methane oxidation reactions. In particular, for Fe-doped Ba and La hexaaluminates, it was reported that some of Fe³⁺ cations occupying Al³⁺ sites suppress methane combustion and are reactive in methane oxidation into syngas. On the other hand, O6-Fe³⁺ (Oh) cations in LaFe₃Al₉O₁₉ exhibit a high activity in methane combustion. Catalytically active Fe⁰ species generated on the surface contribute into increasing the syngas yield [130,233,245].

In the work authored by Z. Cheng et al. [233], the performance of CeO₂/BaFe₃Al₉O₁₉ catalysts in chemical looping dry reforming of methane was studied. Almost total conversion of methane and a high selectivity with respect to syngas were demonstrated. A proposed reaction mechanism is presented in Figure 25. Methane directly reacts with oxygen species in BaFe₃Al₉O₁₉ which serves the dual function of oxygen carrier and catalyst. This results in a redistribution of oxygen within BaFe₃Al₉O₁₉, which in turn affects the concentration of products. Initially, active surface oxygen species are consumed leading to the complete oxidation of methane:

CH_{4 (g)} + 4MO_(s) = 4M_(s) + CO_{2 (g)} + 2H₂O_(g).

(7)

Subsequently, the generation of oxygen vacancies on the surface facilitates the diffusion of less active bulk oxygen to the surface, thereby enabling its participation in the partial oxidation of methane into syngas:

CH_{4 (g)} + MO_(s) = M_(s) + CO _(g) + 2H_2(g).

(8)

During the reaction, both CeO₂ and BaFe₃Al₉O₁₉ are gradually reduced to form CeFe_1-xAl_xO₃ perovskite phase, which is conducive to the migration of lattice oxygen participating in partial oxidation of methane as well. The Ce³⁺ and Fe²⁺ cations with unsaturated coordination at the interface can function as active sites for methane adsorption and activation [233].

Figure 25. Reaction mechanism model for chemical looping dry reforming of methane over CeO₂/BaFe₃Al₉O₁₉ [233]. Reprinted from Fuel Processing Technology, Vol. 218, Zheng Cheng et al, Effect of calcination temperature on the performance of hexaaluminate supported CeO₂ for chemical looping dry reforming, Pages No. 106873, Copyright (2021), with permission from Elsevier.

Figure 25. Reaction mechanism model for chemical looping dry reforming of methane over CeO₂/BaFe₃Al₉O₁₉ [233]. Reprinted from Fuel Processing Technology, Vol. 218, Zheng Cheng et al, Effect of calcination temperature on the performance of hexaaluminate supported CeO₂ for chemical looping dry reforming, Pages No. 106873, Copyright (2021), with permission from Elsevier.

5. Conclusions and Perspectives

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