| Solid oxide fuel cells (SOFCs) offer the advantage of using less expensive oxide catalysts and directly using hydrocarbon fuels without requiring external fuel reforming due to the higher operating temperatures of > 500°C compared to the proton exchange membrane fuel cells. However, the conventional operating temperature of ~ 1000 °C leads to undesired side reactions and thermal expansion mismatch among the cell components. These difficulties have generated considerable interest in intermediate temperature (500 - 800 °C) SOFC, but the lower operating temperature leads to poor oxygen reduction reaction kinetics with the conventional cathode material, La1-xSrx MnO3. In this regard, the cobalt-containing perovskite cathodes such as La1--xSrxCoO3--delta are appealing, but it suffers huge thermal expansion mismatch with the conventional electrolytes. To address this issue, this dissertation focuses on two series of new cathode materials.;First, the influence of the Ln3+ ions on the high temperature properties and performance in SOFC of the layered LnBaCo2O 5+delta (Ln = La, Nd, Sm, Gd, and Y) oxides is investigated systematically. The oxygen content (5+delta), thermal expansion coefficient (TEC), electrical conductivity, catalytic activity for the oxygen reduction reaction in SOFC, and oxygen permeability decrease with decreasing size of the Ln3+ ions from Ln = La to Y. These results suggest that lanthanide ions with an intermediate size may offer a tradeoff between catalytic activity and TEC. With an aim to tune the properties further, cationic substitutions are then pursued. For example, substitution of Sr for Ba is found to improve the chemical stability of GdBaCo2O5+delta with improved catalytic activity. Similarly, the substitution of Ni for Co is found to lower the TEC to 16.9 x 10-6 K-1 while still maintaining high catalytic activity at x = 0.4 in NdBaCo2-xNixO 5+delta.;Second, non-perovskite RBa(Co,M)4O7 (R = Y, Ca, and In and M = Zn, Fe, and Al) oxides having a hexagonal structure and corner-shared (Co,M)O4 tetrahedra are investigated. Among the various compositions investigated, YBaCo4-xZnxO7 (1 ≤ x ≤ 2) shows good long-term stability at high temperatures with an ideal matching of the TEC with those of standard electrolytes. The low TEC is attributed to the absence of spin state transitions with tetrahedral-site Co2+/3+ ions and relatively small amount of oxygen loss at higher temperatures. The YBaCo3ZnO7 + GDC composite cathodes exhibit low polarization resistance and performance in SOFC comparable to that of well-studied cobalt-based perovskite cathodes. |
