Electrochemical Properties of Nanocrystalline La_0.5Sr_0.5CoO_3−x Thin Films

Abstract

As part of an investigation of new cathode materials for intermediate temperature solid oxide fuel cells, we have investigated several perovskite oxides with cobalt ions on the B sites in both bulk and thin film forms. Of particular interest is the composition La 0.5 Sr 0.5 CoO 3-x (LSCO) which has exceptional properties for oxygen reduction at intermediate temperatures in ceria based fuel cells. Thin films of LSCO were deposited on both sides of a dense polycrystalline gadolinia doped ceria substrate by pulsed laser deposition under conditions that lead to the formation of nanocrystalline films. The electrochemical properties for oxygen reduction were determined in a symmetric electrochemical cell by alternating current (AC) impedance spectroscopy. The results were analyzed using the Adler-Lane-Steele (ALS) model to obtain the diffusion and surface exchange coefficients and the thermodynamic factor. We show that the thermodynamic factor, a measure of how easy it is to create oxygen vacancies, is much higher than observed in conventional cathodes. As a result, the electrode composition changes little with temperature and oxygen partial pressure, the large chemical contribution to the thermal expansion is reduced, and the electrode has good stability. The use of a nanostructured electrode does not significantly affect the fundamental material parameters (surface exchange and diffusion coefficients), and the very low area specific resistance (0.09 ohm cm 2 at 600 °C) observed is because the synthesis method gives a very high surface area (80 μm -1 ).

Nanoscaled La0.6Sr0.4CoO3−δ as intermediate temperature solid oxide fuel cell cathode: Microstructure and electrochemical performance

“…1 This enhanced oxygen surface exchange may be linked to the occurrence of small cobalt rich precipitations in the surface layer of the thin films, as preliminary results of the chemical analysis performed by TEM revealed. Such surface near cobalt rich phases were also observed by Wang et al [21] on pulsed laser deposition (PLD) derived nanoscaled La 0.5 Sr 0.5 CoO 3−ı cathodes, however no enhanced surface exchange was observed. A more detailed and extensive analysis covering this topic is currently in progress.…”

“…In some cases, the material selection is motivated by the materials properties. In [21] Au is preferred to Pt, to avoid possible influences of the catalytic properties of Pt on the electrochemical reaction. However, how large are these influences in reality?…”

“…9. [21] a Applied by metal organic deposition, followed by a high temperature step. b Applied by spin coating, followed by a sintering step.…”

Oxygen Exchange Activity of La-Sr-Co-O PLD Films and Quantification of (La,Sr)CoO3-δ/(La,Sr)2CoO4+δ/O2Triple Phase Boundary Activity

“…The ORR activities of La 1-x Sr x CoO 3-δ P films (x = 0.2 and 0. , red = Co-stoichiometric, orange = most active Co-deficient P. References: Wang et al, 34 Yang et al, 35 Kawada et al, 36 Crumlin et al (15 nm R with y = 0.5 -assumed to be a dense layer -deposited on P with x = 0.2, 5 0.4 6 ), Sase et al, 3 Ma et al 8 (transformed from ASR to k q using Eq. 3), Lee et al 7,37 of these two-phase films are given.…”

SrCoO 3−δ thin films by atomic layer deposition

“…Therefore the cathode performance is usually the rate-limiting step in the IT-SOFCs [5]. Consequently, further improvement of the IT-SOFC performance requires either the development of novel high-performance cathode materials optimized for the intermediate-temperature operation, or increasing the active surface area of the existing cathode materials [6,7], as the electrode reaction rate has been observed to be limited by the oxygen surface exchange reaction rate, and thus the cathode surface area [8,9].…”