The lanthanum strontium manganite La_1−xSr_xMnO_3−δ (LSM), as a representative of complex oxides with a perovskite ABO₃ structure with rhombohedral distortions in the compositional range of 0.2 ≤ x < 0.4 [56,57,58], is known to be used as a material for the fabrication of air electrodes for electrochemical devices operating at low- [59] and intermediate temperatures [30]. On the one hand, this is due to the high level of electronic conductivity σ_e for LSM (corresponding to 200 S cm⁻¹, 250 S cm⁻¹, and 320 S cm⁻¹ for La_0.8Sr_0.2MnO_3−δ (LSM20), La_0.7Sr_0.3MnO_3−δ (LSM30), and La_0.6Sr_0.4MnO_3−δ (LSM40), respectively, at 800 °C and at Po₂ = 1 bar [60]). Secondly, due to the closest values of the coefficient of linear thermal expansion (CTE) of the LSM (e.g., for LSM20 — 11.4 × 10⁻⁶ K⁻¹ in the range of 50–1000 °C, [61]; for LSM30 — 12.2 × 10⁻⁶ K⁻¹ and 13.2 × 10⁻⁶ K⁻¹ in the ranges of 200–650 °C and 650–900 °C, respectively, [62]; LSM40 — 12.7 × 10⁻⁶ K⁻¹ in the range of 50–1000 °C, [61];) to those for the solid electrolytes perspective for operating in IT-SOFCs and low-temperature SOFCs (LT-SOFCs) [15,22,24,63,64,65]. The CTE values can be mentioned for doped ceria (e.g., for Ce_0.8Sm_0.2O_2−δ (SDC) — 12.0 × 10⁻⁶ K⁻¹ in the range of 25–1000 °C, [66]; for Ce_0.8Gd_0.2O_2−δ (GDC) — 12.2 × 10⁻⁶ K⁻¹ in the range of 50–900 °C, [67]), (Sr,Mg)-doped LaGaO₃ (in general LSGM, e.g., for La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ — 12.1 × 10⁻⁶ K⁻¹ in the range of 25–1000 °C, [68]), Sc₂O₃-stabilized ZrO₂ (ScSZ) (for 8ScSZ — 10.4 × 10⁻⁶ K⁻¹ in the range of 30–1000 °C, [69]). In addition, LSM electrodes offer such an important advantage as the improved stability during operation under SOFC and solid oxide electrolysis cell (SOEC) conditions compared to Fe- [48] and Co- [70,71] containing electrodes.

It should be noted that the oxygen diffusion and interfacial heteroexchange parameters for LSM, which is predominantly an electron conductor, are lower than those of materials possessing high mixed oxygen ion and electron conductivity (MIECs), such as cobalt-based perovskites. For example, the oxygen self-diffusion coefficient (D*) and the oxygen surface exchange coefficient (k) for LSM20 at 800 °C were found to be 4.00 × 10⁻¹⁵ cm² s⁻¹ and 5.62 × 10⁻⁹ cm s⁻¹, respectively, compared to D* = 9.87 × 10⁻¹⁰ cm² s⁻¹ and k = 6.31 × 10⁻⁷ cm s⁻¹ for the La_0.8Sr_0.2CoO_3−δ MIEC material [72]. The values of the ionic conductivity, σ_i, for LSM are in a range of 10⁻⁴–10⁻⁷ S cm⁻¹ at 800–1000 °C and decrease significantly in the intermediate temperature range of 600–800 °C [30].

Recently Jiang summarized the literature data regarding the characterization of the oxide materials from the lanthanum strontium cobaltite ferrite (La,Sr)(Co,Fe)O_3−δ series, and showed that the composition La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (the acronym LSCF will be used from now on), as a representative of MIECs, is the most prominent cathode material used in IT-SOFCs [33]. LSCF, which has a perovskite structure with rhombohedral distortions [73,74], exhibits excellent electrical properties properties with ionic and electron partial conductivity values reaching approximately 1 × 10⁻² S cm⁻¹ and 1 × 10² S cm⁻¹, respectively, at 800 °C [75]. It demonstrates a moderate CTE value equal to 17.5 × 10⁻⁶ K⁻¹ in the range of 30–1000 °C [76]. The oxygen self-diffusion and surface exchange coefficients for LSFC amount 5 × 10⁻⁷ cm² S⁻¹ [77] and 6 × 10⁻⁶ cm S⁻¹ [78] at 800 °C. It should be noted that due to MIEC conductivity nature and the superior oxygen diffusion properties of LSCF compared to LSM, this material is preferred for use in electrochemical devices operated at lower temperatures, whereas LSM materials are still in high demand for high-temperature applications due to their CTE values being more compatible with electrolyte materials and higher total conductivity values.

The main drawbacks of conventional perovskite electrodes, which lead to the degradation of solid oxide cells during long-term operation, are the segregation of Sr at the electrode surface with the formation of the SrO layer for the LSM- [35,52] and LSCF-based [36,79,80,81,82] cells, and the high interaction of LSM [41,52,83,84,85,86] and LSCF [41,87,88,89,90] with Zr-containing electrolytes to form the SrZrO₃ (SZO) and La₂Zr₂O₇ (LZO) phases. In addition, due to the presence of CO₂ in the air, the SrCO₃ carbonate phase was formed on the surfaces of the LSM- [91,92,93] and LSCF- [94,95,96] electrodes. The formation of the same insulating phases limits the oxygen exchange at the electrode–electrolyte interface and reduces the electrocatalytic activity of the LSM [97,98,99,100] and LSCF [79,100,101,102,103,104,105] air electrodes for the ORR, followed by an increase in both the electrode ohmic and polarization resistances. Similar long-term operation tests have shown that the degradation of cells based on LSM [81] or LSCF [106,107] operating in electrolysis mode was higher than that in fuel cell mode, and when comparing two perovskite electrodes, the long-term durability of LSCF was one step ahead of that of LSM [106,108].

Automated methods developed for the detection of Sr nucleation seeds on the electrode surface [109,110], for the identification of the interlayer width between the perovskite and YSZ layers [111,112], and for the computational design and numerical simulation of the perovskite-based composite electrodes [113,114,115,116] allowed for the suggestion of possible degradation mechanisms and an understanding of the electrode behavior at both the atomic and macroscopic scales. The deposition of protective and non-catalytic layers on the electrode surface has been proposed as a solution to the problems of segregation [117,118,119], contaminant poisoning [120,121,122,123], and sluggish oxygen kinetics [124,125,126,127,128]. The organization of ceria-based buffer layers at the perovskite electrode/YSZ electrolyte interface and the replacement of the zirconium electrolyte in the composite electrode with other ionic conductors, methods widely used for other perovskite electrodes [129,130,131,132,133,134,135], may also help to reduce interactions and increase the activity and long-term stability of LSM [136,137,138] and LSCF [138,139,140,141,142,143,144,145,146,147] electrodes. The high reactivity of the (La, Sr)-containing electrodes with the conventional YSZ electrolyte facilitated extensive investigations of the LSM-based cathodes formed on the SDC [148,149,150,151], GDC [152], SSZ [151,153], LSGM [149], BaZr_0.8Y_0.2O_3−δ (BZY20) [154], La_9.5Si₆O_26.25 [155], and La_27.44W_4.56O_55.68 (LWO56) [156] electrolytes as an alternative. The LSCF-based cathodes were formed on the SSZ [157], SDC [158,159], Ce_0.9Gd_0.1O_2−δ (GDC10) [160,161,162], GDC [126,163,164,165,166,167] Y_0.1Ce_0.9O_1.95 [168], Nd_0.2Ce_0.8O_3−δ [169], SSZ [170], LSGM [171,172], GDC with LSGM [173], SDC with LSGM [174], BZY20 [175], BaZr_0.9Y_0.1O_3−δ [176], BaZr_0.8Yb_0.2O_3−δ (BZYb) [177], BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (BCZYYb) [178,179,180,181,182], BaCe_0.7Zr_0.1Y_0.2O_3−δ (BCZY) [183,184,185], and BaCe_0.7Zr_0.15Y_0.15O_3−δ (BCZY15) [186] electrolytes.

The chemical compatibility of the LSM and LSCF electrodes with oxygen-ion and proton-conducting electrolytes and the interdiffusion across the cathode/interlayer/electrolyte interfaces have been extensively observed in the recent reviews of Zhang et al. [41], Khan et al. [42] and Hanif et al. [187]. To briefly summarize the data presented in [41], it could be mentioned that LSM20, in contrast to LSCF, showed good chemical compatibility with La₂₇W₄NbO_55−δ up to 1400 °C [188] and with La₁₀Si₅₅Al_0.5O_26.75 at 1300 °C [189], whereas the La-deficient LSM40 did not react with La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM9182) at 1300 °C [190]. Secondary phases were found to form after sintering of LSM20 with SDC and BaCe_0.9Y_0.1O_3−δ at 1150 °C [191], La-deficient LSM20 with BZY20 and BaCe_0.8Y_0.2O_3−δ (BCY20) at 1100 °C [192], LSCF with BZY20 and BCY20 at 1100 °C [192]. The conclusions in [41] justified that LSCF had better chemical compatibility with LSGM and CeO₂-based electrolytes due to the formation of small amounts of impurity phases with LSGM9182 at 1300 °C and negligible interdiffusion with SDC at 1150 °C.

It should be noted that the use of LSM electrodes in contact with the BaCeO₃-based electrolytes is limited due to their active chemical interaction [193]. However, due to its high electronic conductivity and CTE compatibility, LSM can be successfully used as a collector for perspective layer electrodes for proton-conducting fuel cells operating in the IT range [194,195,196,197].