Abstract
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, La0.6Sr0.4CoO3−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.
Highlights
The demand for stable and high-power density power sources has increased substantially over the last decade
Thermally induced transformations occurring at the nanometer scale and below must be taken into account as they can be detrimental to the catalytic activity, including the segregation of inactive phases toward the surfaces,[4] particle formation and coarsening,[5] phase transitions, and decomposition at high temperatures.[6]
The formation of cracks was observed in the HT-grown film (Figures 1g and S1c). They occurred after deposition on cooling (15 °C min−1 rate) and resulted from the differences in thermal expansion coefficients (TEC) between the film and the substrates (TEC; Si = 2.5 × 10−6 K−1,29 LSC64 = 20.5 × 10−6 K−1,30 and Gd-doped ceria (GDC) = 14.4 × 10−6 K−131)
Summary
The demand for stable and high-power density power sources has increased substantially over the last decade. The search for new ion- and electron-conducting metal-oxide materials, as well as innovative concepts and designs for energy conversion and storage devices has accelerated.[1] Notably, considerable attention was given to thin-film deposition techniques, such as pulsed laser deposition (PLD), as many exciting properties including conductivity and ion exchange/ diffusion kinetics have been shown to be tuned by the defect structure. Defect features such as lattice strain, structural defects, space charge at the interface and local chemical composition are all proposed to affect mass transport significantly.[2]. The preparation technique,[7] coating conditions [substrate temperature (Tsubstrate), background oxygen pressure (pO2) etc.],8 crystallization behavior,[9] and thermal history[10] of the samples have been shown to influence the electrochemical activity but often remain unreported in the literature
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