The effects of Sr doping on the crystal structures, thermal expansion behaviors, defect chemistries , electrical and electrochemical properties of the perovskite materials, La 0.5 Gd 0.5 Ba 1− x Sr x Co 2 O 5+ δ (LGBCOSr- x, x =0–0.5), were systematically analyzed and discussed. The X-ray diffraction (XRD) investigation revealed that LGBCOSr- x undergoes a phase transformation from tetragonal ( x =0.1) to cubic ( x =0.5), and the materials exhibit good thermochemical compatibility with Ce 0.9 Gd 0.1 O 1.95 (GDC) under 1100 °C. Partial replacement of Ba with Sr is found helpful in decreasing the thermal expansivity of LGBCOSr- x and increasing the ability of oxygen vacancy formation in the materials. The latter could be attributed to the enhancement in the overall electrical conductivity and oxygen transport process in terms of low cathodic polarization resistance in LGBCOSr- x materials with high Sr doping content. Further analysis on the AC impedance spectroscopy assisted with the DRT (distribution of relaxation times) methodology suggested that the charge transfer sub-process is a predominant step in the oxygen transport processes across the LGBCOSr- x materials. These findings indicate that La 0.5 Gd 0.5 Ba 1− x Sr x Co 2 O 5+ δ is a promising cathode material for intermediate temperature solid oxide fuel cells.
Aiming to lower the operating temperature of solid oxide fuel cells down to 500–700 °C, a Co-containing composite cathode La 0.6 Sr 0.4 CoO 3-δ -Ce 0.85 Sm 0.075 Nd 0.075 O 2-δ (LSC-SNDC) is prepared by the solid-liquid method. The crystallinity, grain size and microstructure of LSC powders are analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The LSC-SNDC cathodes with different mass ratios (LSC: SNDC = 100:0, 90:10, 70:30, 60:40 and 50:50) are implemented into symmetrical cells and full cells using screen printing. The effect of cathode sintering temperatures (900, 950, 1000, 1050 °C) is investigated at the symmetrical cell level. The lowest cathode polarization resistance ( R p ) is obtained with the 50:50 sample sintered at 950 °C, demonstrating a R p (symmetrical cell) of 0.0248, 0.0319, 0.0597, 0.1107 and 0.2304 Ω cm 2 and a maximum power density (full cell) of 1963, 1535, 1013, 585, and 302 mW cm −2 at 800, 750, 700, 650 and 600 °C, respectively. The LSC-SNDC (50:50) cathode exhibits a low thermal expansion coefficient of 12.3 × 10 −6 K −1 and good compatibility with other cell components, as confirmed from SEM observations. The current work demonstrates that LSC-SNDC is a promising cathode material for intermediate temperature solid oxide fuel cells.
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