In Situ Impedance Analysis of Oxygen Exchange on Growing La_0.6Sr_0.4CoO_3−δ Thin Films

Abstract

The further development of solid oxide fuel and electrolysis cells (SOFC/SOEC) strongly relies on research activities dealing with electrode materials. Recent studies showed that under operating conditions many perovskite-type oxide electrodes are prone to changes of their surface composition, leading to severe changes of their electrochemical performance. This results in a large scatter of data in literature and complicates comparison of materials. Moreover, little information is available on the potentially excellent properties of surfaces immediately after preparation, that is, before any degradation by exposure to other gas compositions or temperature changes. Here, we introduce in situ impedance spectroscopy during pulsed laser deposition (IPLD) as a new method for electrochemical analysis of mixed ionic and electronic conducting (MIEC) thin films during growth. First, this approach can truly reveal the properties of as-prepared MIEC electrode materials, since it avoids any alterations of their surface between preparation and investigation. Second, the measurements during growth give information on the thickness dependence of film properties. This technique is applied to La0.6Sr0.4CoO3−δ (LSC), one of the most promising SOFC/SOEC oxygen electrode material. From the earliest stages of LSC film deposition on yttria-stabilized zirconia (YSZ) to a fully grown thin film of 100 nm thickness, data are gained on the oxygen exchange kinetics and the defect chemistry of LSC. A remarkable reproducibility is found in repeated film growth experiments, not only for the bulk related chemical capacitance but also for the surface related polarization resistance (±10%). Polarization resistances of as-prepared LSC films are extraordinarily low (2.0 Ω cm2 in 40 μbar O2 at 600 °C). LSC films on YSZ and on La0.95Sr0.05Ga0.95Mg0.05O3−δ (LSGM) single crystals exhibit significantly different electrochemical properties, possibly associated with the tensile strain of LSC on LSGM.

Oxygen exchange kinetics and nonstoichiometry of pristine La0.6Sr0.4CoO3−δ thin films unaltered by degradation

“…In an earlier study using IPLD it was already discussed that about 2-3 nm LSC are required to cover the Pt current collectors to an extent that true LSC properties are measured. 19 In accordance with this study, the rst measured spectrum aer 2 nm (100 pulses) includes three clearly distinguishable features ( Fig. 5).…”

“…Details of the method are described elsewhere. 19 At the beginning a corundum plate is placed directly on the platinum heating wires. To contact the counter electrode, a platinum sheet is placed on top and xed by a corundum frame with an opening.…”

“…The following interpretation of these arcs is based on the current paths discussed in ref. 19 and 24. The rst feature in the high frequency regime (100 kHz -1 kHz) is attributed to a current path which is limited by the sheet resistance of the LSC thin lm R WE, sheet and the small double layer capacitance at the interface between the current collector and the electrolyte C CC, dl .…”

“…14 Consequently, data for the electrical properties of LSC aer deposition calculated from measurements of different authors at different conditions vary within several orders of magnitude. 9,[15][16][17] In this study, a recently developed technique was employed to perform electrochemical impedance spectroscopy measurements on thin lms during and directly aer pulsed laser deposition 18,19 on LSC thin lms, grown on yttria stabilized zirconia (YSZ) single crystals. It was possible to track the electrochemical properties of the deposited material directly during growth and thus achieve an electrochemical characterization of pristine LSC thin lms yet unaltered by degradation processes.…”

Controlling the Oxygen Electrocatalysis on Perovskite and Layered Oxide Thin Films for Solid Oxide Fuel Cell Cathodes

“…The authors proposed that the increased ORR activity results from the increased oxygen vacancies in epitaxial LSC64 films due to the large tensile strain of high-quality films. Employing in situ impedance spectroscopy during thin film growth, Rupp et al [97] also showed an enhancement in the oxygen surface exchange kinetics of tensile-strained epitaxial LSC64 films on (001)-oriented La 0.95 Sr 0.05 Ga 0.95 Mg 0.05 O 3−δ substrates compared to unstrained films on yttria-stabilized zirconia (YSZ) substrates. Similar to the results by Lee et al [96], the authors also found the same trend of enhanced oxygen surface exchange kinetics as film thickness decreases in epitaxial LSC64 films but they interpreted that this trend may be attributed to the capability of in-situ measurement as the films never exposed to any thermal cycling such as cooling from PLD preparation temperature or heating for impedance measurements.…”