Kinetics of oxidation of fuel cell cathode materials lanthanum strontium Manganates(III)(IV) at actual working conditions:: In situ powder diffraction studies

Time resolved synchrotron X-ray powder diffraction methods have been used to study the kinetics of oxidation of lanthanum strontium manganates(III, IV) in a Bow of oxygen. The process studied is La1-xSrxMnO3.00+ delta/2O(2) <-> La1-xSrxMnO3.00+delta (for x=0.00, 0.10, 0.15), A contraction of the unit cell is observed on oxidation of the materials. The unit cell volume changes linearly with the mean oxidation state of the manganese. Rate constants for the process can consequently be determined from time resolved X-ray diffractograms, The reactions follow first-order kinetics. Arrhenius plots based on rate constants determined at 4 temperatures in the range 700-900 degrees C are linear as an indication that only one rate determining step is involved. The activation energy for oxidation is 136 kJ mol(-1) for LaMnO3.00, 179 kJ mol(-1) for La0.90Sr0.10Mn1.01 O-3.01, and 160 kJ mol(-1) for La0.84Sr0.15Mn1.01O3.00. (C) 1998 Academic Press.