Low Thermal Expansion RBa(Co,M)₄O₇ Cathode Materials Based on Tetrahedral-Site Cobalt Ions for Solid Oxide Fuel Cells

RBa(Co,M) 4 O 7 (R = Y, Ca, In and M = Zn, Fe, Al) oxides with a hexagonal structure and corner-shared (Co,M)O 4 tetrahedra have been synthesized and investigated as cathode materials for solid oxide fuel cells (SOFC). Among the various compositions investigated, YBaCo 4− x Zn x O 7 (1 ≤ x ≤ 2) shows long-term stability at high temperatures. Thermal expansion coefficient (TEC) values of the RBa(Co,M) 4 O 7 specimens were determined to be 6 × 10 −6 to 13 × 10 −6 o C −1 in the range of 80−900 °C, which provides good thermal expansion compatibility with the standard SOFC electrolyte materials. The low TEC is attributed to the absence of spin state transitions of tetrahedral-site Co 2+/3+ ions and relatively small amount of oxygen loss with increasing temperature. The YBaCo 3 ZnO 7 + GDC composite cathodes exhibit low polarization resistance and good fuel cell performance comparable to that of well-studied Co-based perovskite cathodes, but with the critical advantage of an ideal matching of the TEC value with those of standard electrolytes.