YBaCo₄O_7+δ-based materials have attracted much attention since last decade because of their potential applications as cathode materials in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The ideal matching of the relatively low thermal expansion coefficient (TEC) of YBaCo₄O_7+δ (7.5 × 10^-6 K^-1) with those of common oxide-ion electrolytes (10 – 12.5 × 10^-6 K^-1) in the temperature range of 300 – 900 °C alleviates the thermal stress during long-term operation in IT-SOFCs.¹ However, the practical applications of this class of materials is impeded by the irreversible oxygen absorption and phase decomposition at 600 – 900 °C. The decomposition products, such as BaCoO₃, Y₂O₃, and Co₃O₄, are easily found after heating YBaCo₄O_7+δ to the decomposition temperature region.²

With an aim to enhance the high-temperature phase stability, oxygen-storage capability, and electrocatalytic activity in IT-SOFCs, we present here the RBaCo4O_7+δ-based samples with a co-substitution in the R site with Y, In, and Ca and Ga substitution in the Co site.³ All twenty-four examined compositions, includingYBaCo_4-yGa_yO_7+δ, Y_1-xIn_xBaCo_3.3Ga_0.7O_7+δ, Y_1-xCa_xBaCo_3.3Ga_0.7O_7+δ, In_1-xCa_xBaCo_3.3Ga_0.7O_7+δ, and Y_0.5In_0.5BaCo_4-yGa_yO_7+δ series of materials, have been synthesized by a solid-state reaction. The phase stabilities have been assessed by long-term testing at 600 – 800 °C for 120 h. The phase of pristine and high-temperature annealed samples was characterized by X-ray diffraction (XRD). The thermal behavior which includes oxygen absorption and desorption was evaluated by thermogravimetric analysis (TGA) from room temperature to 1000 °C in air. For the electrochemical performance assessment, composites consisting of a mixture of Ce_0.8Gd_0.2O_2-δ (GDC) and the stabilized Ga-substituted RBaCo₄O_7+δ were applied as cathodes. The electrocatalytic activity for the oxygen reduction reaction in air was evaluated with a symmetric cell, which was fabricated by screen printing the composite cathodes onto two sides of a dense GDC pellet and co-sintering at 950 °C. The fuel cell performance of the composite cathodes was determined by an anode-supported single cell, which was made by a co-pressing method with NiO + GDC and GDC, respectively, as the anode and the electrolyte.

Figure 1 indicates the stable compositions in which no impurity peaks appear in the XRD pattern after 120 h long-term tests at 600, 700, and 800 °C. The results reveal that the synergistic effect of In and Y co-dopants could enhance the stability and mitigates the phase decomposition at a certain Ga content. While decreasing the Ga content, Y_0.5In_0.5BaCo_3.5Ga_0.5O_7+δ still remains single phase at all tested temperatures. However, Ca hinders the phase stability regardless of the other dopant on the R site, and there is no synergistic effect of In-Ca or Y-Ca co-dopants. The thermal behavior illustrates the reason for high-temperature decomposition with incomplete oxygen release at ~350 °C and continuous oxygen absorption above 400 °C.

The TECs of the high-temperature stable materials are all in the range of 7.0 – 9.2 × 10^-6 K^-1 at 200 – 900 °C. In addition, these materials display adequate electrical conductivity (3 – 20 S cm^-1) at 500 – 900 °C. Among the studied compositions, the symmetric cell with Y_0.9In_0.1BaCo_3.3Ga_0.7O_7+δ + GDC composite cathode exhibits the lowest area-specific resistance of 0.07 Ω cm² at 700 °C. The single cell and long-term performance with Y_0.9In_0.1BaCo_3.3Ga_0.7O_7+δ + GDC composite cathode are shown in Figure 2. The maximum power density reaches 0.33, 0.56, and 0.91 W cm^-2 at, respectively, 600, 650, and 700 °C. The voltage remains stable without a significant degradation with a constant current density of 0.7 A cm^-2 at 650 °C in the 140 h long-term test. In summary, the compatible TECs, high maximum power performance, and stable long-term performance demonstrate the advantages of Y_0.9In_0.1BaCo_3.3Ga_0.7O_7+δ as a potential cathode material for IT-SOFCs.

REFERENCEs

1. W. H. Kan, K.-Y. Lai, A. Huq, and A. Manthiram, J. Power Sources, 307, 454–461 (2016).

2. J.-H. Kim and A. Manthiram, Chem. Mater., 22, 822–831 (2010).

3. K.-Y. Lai and A. Manthiram, Chem. Mater. (2016) DOI: 10.1021/acs.chemmater.6b04122.