Electrochemical Oxygen Reduction Catalyzed by the Perovskite La^1-xMxMnO³ (M: Pb, Sr, K) Systems

Abstract

Ternary manganese oxides of La_1-xSrxMnO₃, La_1-xKxMnO₃ and La_1-xPbxMnO₃ with perovskite structure were prepared, and their physicochemical properties and electrocatalysis for oxygen reduction were studied. The crystal structures of La_1-xPbxMnO₃ and the other oxides were cubic and trigonal, respectively, and the lattice constants decreased with the increase of x. This decrease is attributable to the increase of Mn⁴⁺ in the lattice, whose radius is smaller than that of Mn³⁺. The resistivities, measured by a four prove method, also decrease with the increase of x, since the formation of the σ* conduction band is brought about by the increase of Mn⁴⁺. The oxygen reduction currents were measured in two testing systems, i. e., the dispersion system and the system using the Teflon bonded electrodes. The current-potential curves of the oxygen reduction measured in the dispersion system were dependent on the kind of the working electrodes (Pt and Au). Morever, the variation of the catalytic activity with x, obtained in the Lai, La_1-xSrxMnO₃ system, was different from that with the Teflon bonded electrode. These differences indicate that the dispersion system is not appropriate for the testing of the electrocatalytic activity for the oxygen reduction, but suggest that this system is useful for the evalution of the promoter of the electrocatalyst. It is found that the order of the electrocatalytic activity was Lai, La_1-xSrxMnO₃>La_1-xKxMnO₃>La_1-xPbxMnO₃, and that these activities increase with the increase of x, in the measurement using the Teflon bonded electrodes. These variations in the activity are explained by the theory of the σ* band formation previously reported.