Study of the the mechanism of cathodic reaction in SOFC

In Solid Oxide Fuel Cell the kinetic of the cathodic process is still under study. In fact, the correct interpretation of its mechanism could help to optimise the overall F.C. effectiveness. Moreover, the study and the use of appropriate materials should make it possible to lower the temperature of cell operation developing medium or low temperature SOFCs. These are important target for the technology of SOFC which is very close to the real application. Lanthanum Strontium manganite (LSM) based cathodes seems to be the most reliable with a good electrocatalytic activity among the materials studied so far. For these reasons we carried out some work dealing with LSM cathode in order to better understand some peculiarities of the oxygen reduction that we find controversial in literature. The experimental setup is constituted by a three electrode semi cell arrangement placed in a tubular furnace with a good temperature control. The cathode composed of a mixture of LSM and YSZ with a volume ratio equal to 1, was studied in air at temperatures between 1000 and 400°C. Good agreement of the results obtained by impedance and polarisation measurements has been obtained. At high temperature we noticed that care need to be taken at the cell arrangement and instrument performance in order to get reliable results. An apparent activation energy of 120 kJ/mol has been deduced. The kinetic parameters suggest two temperature region in which different phenomena control the process. At T lower than 700°C impedance and polarisation data show that the process is in activation control by the dissociative oxygen adsorption. At T higher than 700°C the kinetics of the process increase remarkably but oxygen ion transport through the electrolyte and gas diffusion at the electrode play a relevant role under polarisation condition. However, impedance measurement performed at open circuit potential do not suffer of these latter limitations.