Surface Restructuring of Thin-Film Electrodes Based on Thermal History and Its Significance for the Catalytic Activity and Stability at the Gas/Solid and Solid/Solid Interfaces

Electrodes in solid-state energy devices are subjected to a variety of thermal treatments, from film processing to device operation at high temperatures. All these treatments influence the chemical activity and stability of the films, as the thermally induced chemical restructuring shapes the microstructure and the morphology. Here, we investigate the correlation between the oxygen reduction reaction (ORR) activity and thermal history in complex transition metal oxides, in particular, La_0.6Sr_0.4CoO_3-δ (LSC64) thin films deposited by pulsed laser deposition. To this end, three ∼200 nm thick LSC64 films with different processing and thermal histories were studied. A variety of surface-sensitive elemental characterization techniques (i.e., low-energy ion scattering, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry) were employed to thoroughly investigate the cationic distribution from the outermost surface to the film/substrate interface. Moreover, electrochemical impedance spectroscopy was used to study the activity and the stability of the films. Our investigations revealed that, despite the initial comparable ORR activity at 600 °C, the degradation rates of the films differed by twofold in the long-term stability tests at 500 °C. Here, we emphasize the importance of processing and thermal history in the elemental surface distribution, especially for the stability of LSC64 electrodes and propose that they should be considered as among the main pillars in the design of active surfaces.