SOFC interface studies: Kinetic Considerations

As previously shown the morphology of the cathode-YSZ electrolyte interface changes from a smooth surface to a roughened structure during the passage of current (\ref{Lasse97}, \ref{Karin01). These structural changes are probably responsible for the current induced improvement of the electrode performance and inductive hysteresis phenomena often observed in SOFC kinetic studies (\ref{TJ01}). Fig.\,\ref{cath_laser} shows the YSZ surface developed below a Pt point electrode polarised at -0.10\, V at $1000^\circ$C for a period of 85 days. The structural as well as the compositional changes in this and the corresponding anode interface investigated in more detail in part I (\ref{Tine}) of this work. Although extrapolation to $1000^\circ$C of the cation mobility in YSZ yields values in the order of $\mathrm{cm^2s^{-1}}$, i.e.\ by far to low to account for any transfer of YSZ material, the fact that these changes are observed points to a significant contribution from surface and grain boundary transport. The consequence of a significant tranport number for cations is a transfer of YSZ material from the anode-YSZ interface to that of the cathode. This is supported by the observation of a porous YSZ structure below the anode (\ref{Tine}) as well as the very high capacity contribution at low freqencies \ref{Lasse97}. Considering the rough surface structure formed on the YSZ-cathode interface it is most likely that the reaction zone is not confined to the perimeter of the contact area. A significant contribution may come from channels formed along the interface, i.e. an elongation of the triple phase boundary. Based on the assumption of a formation of interfacial reaction zones (i.e. pores) which are only stable in the presence of a local current, a model describing the hysteresis observed is developed. Fig.\,\ref{sweeps} shows the qualitative agreement between model calculations and experimental potential sweeps. Also the inductive impedance observed at low frequencies is predicted.