In this thesis (La,Sr)CoO3-d thin films were prepared by PLD and a simple sol-gel route. The electrochemical performance towards oxygen reduction was investigated by impedance spectroscopy. Structural and morphological investigations were performed using XRD, SEM, AFM and TEM.
The surface composition was investigated by LEIS and XPS. From that data several structure-property relations were found.
The structure and electrochemical performance of (La,Sr)CoO3-d thin-film electrodes prepared by PLD on single crystalline YSZ strongly depends on the deposition temperature of the films. Surprisingly thin films deposited at rather low temperatures (340-510C) and exhibiting hardly any signal in Bragg-Brentano XRD patterns showed the best electrochemical performance. It was possible to prepare LSC film electrodes with polarization resistances as low as 0.1 Ohmcm at 600C.
Many electrodes showed a pronounced degradation when operating between 500 and 700C. Interestingly, the films exhibiting the lowest resistances showed the lowest degradation within a few days of operation. It was further possible to employ a sol-gel route to prepare electrodes with the same performance as the electrodes prepared by PLD. Again it seems that there is a correlation between the degree of crystallinity and the resistance. As the sol-gel thin films are smooth and dense and have no open porosity this extremely fast oxygen reduction cannot be attributed to a strongly increased surface area. It seems to be related to the lower crystallinity or a consequence of the lower crystallinity. The surface composition was investigated by LEIS and XPS. Different results concerning the surface composition of as prepared and heat treated samples deposited at 450C and 650C are achieved.