Doping effects on the physicochemical and electrochemical properties of lanthanum manganite

Abstract

The structural, thermal, electrical, and electrochemical properties of La_0.7Ca_0.3Mn_1-x(Co,Ni)_xO₃ and La_0.6Sr_0.4Mn_1-x(Co,Ni)_xO₃ (x = 0, 0.02, 0.05, 0.1) electrode materials were studied. Doping of the cubic perovskites with Co or Ni increases the fraction of Mn4+ ions, up to 49% in La_0.7Ca_0.1O₃ and 57% in La_0.6Sr_0.4Mn_0.95Co_0.05O₃. The 300-K conductivity of La_0.7Ca_0.3Mn_1-xNi_xO₃ passes through a maximum atx = 0.05, while that of La_0.6Sr_0.4Mn_1-xCo_xO₃ decreases steadily with increasingx. In the range 300–1100 K, the conductivity of the Ca-containing manganites exhibits semiconducting behavior, whereas that of the Sr-containing materials shows metallic behavior. No phase transformations were detected in this temperature range. In the four systems, the thermal-expansion coefficients are virtually independent ofx. In both undoped and doped electrode materials, the resistance parameterp/d of electrode layers on solid-electrolyte substrates shows semiconducting behavior at temperatures from 300 to 1100 K and oxygen partial pressures from 102 to 105 Pa. With increasing oxygen partial pressure or electrode-layer thickness (d = 15–100 mg/cm2),p/d decreases. The optimal electrode-layer thickness is found to be about 50 mg/cm2. The introduction of Co or Ni into the electrode materials decreases the polarization resistance of the electrode layer in gas/electrode/electrolyte systems. Compositions ensuring the lowest polarization resistance were found.