Proton-conducting solid oxide fuel cells (H-SOFCs) require highly active oxygen electrodes to enhance their electrochemical performance and stability at intermediate and low temperatures. However, conventional oxygen electrode materials often suffer from high thermal expansion coefficients and insufficient catalytic activity at reduced temperatures, making them unsuitable for H-SOFC applications. Misfit-layered oxides (MLOs) can offer a promising solution due to their unique structural and catalytic properties. In this study, a series of Na_0.15X_2.85Co₄O_9-δ (X=Ca, Sr, Ba; abbreviated as NCC, NSC, and NBC) materials were synthesized by a glycine-nitrate combustion method. These materials possess a misfit-layered structure ([X₂CoO_3-δ] and [CoO_2-δ] layers), which reduces their thermal expansion coefficients and enhances the catalytic activity of oxygen electrodes at intermediate and low temperatures. Among the Na_0.15X_2.85Co₄O_9-δ materials, NSC exhibits the optimum performance (i.e., an electrical conductivity of 57.1 S·cm^-1 at 700 ℃ in 5% (in volume fraction) H₂O atmosphere, and an area-specific resistance (ASR) of 0.19 Ω·cm² at 650 ℃ in an NSC|BZCYYb (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ)|NSC symmetric cell, with only a ~2% increase in ASR after 100 h at 600 ℃). In a full-cell test with NSC|BZCYYb|Ni-BZCYYb, the oxygen electrode achieves power densities of 1273 mW·cm^-2 and 362 mW·cm^-2 at 650 ℃ and 500 ℃, respectively, and demonstrates a stable operation for 165 h at a constant current density of 0.54 A·cm^-2 at 600 ℃. This work demonstrates that the misfit-layered Na_0.15Sr_2.85Co₄O_9-δ material has a significant promising application in H-SOFCs.

In this study, three materials of the misfit-layered Na_0.15X_2.85Co₄O_9-δ (NXC, X=Ca, Sr, Ba; abbreviated as NCC, NSC, NBC) system were synthesized by a glycine-nitrate combustion method. These materials featured an alternating stacking structure composed of [X₂CoO_3-δ] and [CoO_2-δ] layers. The NXC series electrode materials were obtained via calcination in a muffle furnace at 800 ℃ for 2 h, followed by grinding. The intrinsic properties of the materials (i.e., electrical conductivity and thermal expansion coefficient) were characterized. Electrode slurries were prepared via grinding the materials, which were then screen-printed onto BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) electrolyte pellets to fabricate NXC|BZCYYb|NXC symmetric cells. These symmetric cells underwent temperature sensitivity, humidity sensitivity, and stability tests. The material with the optimal comprehensive performance was selected to construct NXC|BZCYYb|Ni-BZCYYb full cells. The full cells were characterized by current density-voltage measurement, electrochemical impedance spectroscopy (EIS) at 650-500 ℃, and relaxation time (DRT) analysis.

Among the three materials, NCC and NSC both exhibit well-formed phases, whereas NBC produces a significant amount of the secondary phase BaCoO_3-δ. The thermal expansion coefficients (TECs) of NCC and NSC are relatively low, while that of NBC is notably higher due to the high TEC of the secondary phase. In terms of electrical conductivity, NSC demonstrates the optimum performance among the three materials, achieving 40.8 S·cm^-1 at 500 ℃ in dry air, which is significantly superior to those of NCC and NBC. At 650 ℃, the polarization resistances (R_p) of NCC and NSC both are 0.190 Ω·cm², while NBC exhibits a higher R_p of 0.252 Ω·cm², indicating that Ca- and Sr-doped electrodes possess comparable charge transfer capabilities at elevated temperatures, whereas the Ba-doped material performs slightly worse. Also, NSC displays a superior performance in both humidity sensitivity and stability tests. Based on the analysis above, NSC in the NXC series with the maximum conductivity, catalytic activity, and stability is selected as an oxygen electrode for the NSC|BZCYYb|Ni-BZCYYb full cell for subsequent testing. NSC exhibits excellent performance and stability, delivering power densities of 1273, 1095, 704, 362, 170 mW·cm^-2 and 91 mW·cm^-2 at 650-400 ℃. Moreover, it demonstrates a stable operation for 165 h at an applied current density of 0.54 A·cm^-2, confirming its potential as an oxygen electrode for H-SOFCs.

This study synthesized a series of misfit-layered Na_0.15X_2.85Co₄O_9-δ (NXC, X=Ca, Sr, Ba; abbreviated as NCC, NSC, and NBC) oxygen electrode materials by a glycine-nitrate combustion method. The influences of elements Ca, Sr, and Ba on the crystal structure and electrochemical properties were systematically investigated. Among the synthesized materials, Na_0.15Sr_2.85Co₄O_9-δ (NSC) demonstrated optimal electrochemical performance and stability. It exhibited an electrical conductivity of 57.1 S·cm^-1 at 700 ℃ in a 5% H₂O+95% air atmosphere. The NSC symmetric cell showed the lowest area-specific polarization resistances (ASR) of 0.190, 0.323, 0.590 Ω·cm², and 0.911 Ω·cm² at 650-500 ℃, along with excellent long-term stability. These superior characteristics were further confirmed in NSC|BZCYYb|Ni-BZCYYb full-cell tests, achieving high power densities of 1273, 1095, 704, 362 mW·cm^-2 at 650-500 ℃, respectively. Moreover, the cell maintained stable operation for 165 h at an applied current density of 0.54 A·cm^-2, demonstrating a remarkable stability. These results indicated that the triple-conducting (H⁺/O^2-/e^-) misfit-layered NSC material could be a highly promising oxygen electrode candidate for intermediate-to-low temperature solid oxide cells.