Impedance Studies of Oxygen Exchange on Dense Thin Film Electrodes of La[sub 0.5]Sr[sub 0.5]CoO[sub 3−δ]

Abstract

Solid-state electrochemical cells with dense oriented thin film electrodes of La 0.5 Sr 0.5 CoO 3Ϫ␦ (LSCO) were prepared on (100) surfaces of single-crystal disks of yttria-stabilized zirconia (YSZ) by the pulsed laser deposition technique. Oxygen exchange at the electrodes was studied with alternating current impedance spectroscopy under various temperature and oxygen partial pressure conditions. Three distinctive features were observed in the impedance spectra from high to low frequency corresponding to contributions from the ionic conduction of the YSZ electrolyte, ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. An equivalent circuit model of the electrode process is used to fit the impedance data. The time constant for the oxygen surface exchange was derived from the impedance simulation. The surface chemical exchange coefficients, k chem , were calculated from the time constants as a function of temperature and pO 2 . k chem is 7 ϫ 10 Ϫ4 cm/s at T ϭ 700ЊC and pO 2 ϭ 1 atm. The activation energy at pO 2 ϭ 1 atm is Ϸ1.1 eV. The interfacial conductivity data were also derived from the impedance simulations as a function of temperature and pO 2 . The activation energy for the interfacial transport at pO 2 ϭ 1 atm is Ϸ1.6 eV.

The Superior Properties of La0.6Ba0.4CoO3-δThin Film Electrodes for Oxygen Exchange in Comparison to La0.6Sr0.4CoO3-δ

“…4a (LSC ∼2500 μF/cm 2 , LBC ∼10000 μF/cm 2 for the two thin films at 510 • C). Yang et al 48 also measured similarly high capacitances in the medium frequency range on La 0.5 Sr 0.5 CoO 3-δ thin films. Assuming a capacitor thickness as small as the interatomic distance of the two cation layers in an elementary cell (d = 1.9 Å), a relative permittivity ε r can be calculated by…”

“…The resulting C chem values are plotted in Fig. 12 together with C chem values directly obtained from the impedance measurements in our study but also from measurements on thin films by Kawada 23 and Yang (La 0.5 Sr 0.5 CoO 3-δ ) 48 at 600…”

“…11 Yang et al assumed a similar reaction mechanism for the medium frequency range and obtained an activation energy of 1.6 eV for La 0.5 Sr 0.5 CoO 3-δ thin films. 48 A more detailed discussion of this minor part to the overall polarization resistance is beyond the scope of this paper. Fig.…”

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.…”

Advanced Measurement and Modeling Techniques for Improved SOFC Cathodes

“…In the case of the perovskite mixed conductors La 1-x Sr x Co y Fe 1-y O 3-δ , workers have successfully addressed the first of these problems by making dense thin film electrodes [13,[43][44][45][46][47] . These films are often sufficiently thin (300~1000 nm) that bulk transport remains equilibrated, and thus interfacial rates can be isolated.…”