Kinetics and Mechanisms of Oxygen Surface Exchange on La[sub 0.6]Sr[sub 0.4]FeO[sub 3−δ] Thin Films

Abstract

The thermodynamic properties as well as oxygen exchange kinetics were examined on mixed ionic and electronic conducting ͑La 0.6 Sr 0.4 ͒ 0.99 FeO3 −␦ ͑LSF64͒ thin films deposited on MgO single crystals. It is found that thin films and bulk material have the same oxygen stoichiometry for a given temperature and oxygen partial pressure ͓i.e., the incorporation reaction has the same reaction enthalpy ͑⌬H 0 = −105 KJ/mol͒ and entropy ͑⌬S 0 = −75.5 J/mol/K͒ as found for bulk material͔. The thin film shows smaller apparent electrical conductivity than reported for bulk. This is due to imperfections in the film, which is not totally dense and contains closed porosity. Electrical conductivity relaxation was used to determine the surface exchange coefficient and its dependence on the temperature and oxygen partial pressure. Relaxation curves showed a good fit to a simple exponential decay. The vacancy surface exchange coefficient ͑k V ͒ determined from K chem shows a slope ͑log k V vs log P O 2 ͒ between 0.51 and 0.85. It is further found that k V is proportional to the product of the oxygen partial pressure and the vacancy concentration ͑k V ϰ P O 2 ␦ ϱ ͒. Different reaction mechanisms that can account for the observed P O 2 and ␦-dependence of k V are analyzed. It is proposed that the vacancies are the active sites of adsorption of molecular oxygen and that the rate determining step for the exchange reaction is splitting of the adsorbed oxygen. © 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3062941͔ All rights reserved.Manuscript submitted January 4, 2008; revised manuscript received October 8, 2008. Published February 12, 2009 Mixed ionic and electronic conductors ͑MIECs͒ have potential applications in solid oxide fuel cells ͑SOFCs͒, oxygen separation membranes, and thin-film sensors. One group of promising materials with high oxygen-ion conductivity, good catalytic activity for oxygen reduction, and high chemical stability in reducing atmosphere is strontium-doped lanthanum ferrite ͑LSF͒ or La 1−x Sr x FeO 3 . Presently, the search for superior SOFC cathode or oxygen membrane material is to a large degree based on a trial-and-error approach. An understanding of the detailed mechanism of the oxygen reduction process over model cathode ͑or membrane͒ materials, such as LSF, would enable researchers to make more qualified guesses on what materials to choose and how to optimize these by, e.g., partial substitution of the elements. In this study, different reaction mechanisms for the oxygen reduction on LSF are described and compared to experimental investigations carried out on a thin film.The oxygen transport in a MIEC is determined by the oxygen exchange over the gas-solid interface and diffusion of both oxide ions and electrons/holes in the bulk. The oxygen transport is generally limited by the surface exchange process or by the bulk diffusion, depending on membrane thickness because the electronic charge carrier density and mobility is large when compared to those of the oxide ion. The ratio of the oxygen chemica...

An Investigation of Oxygen Reduction Kinetics in LSF Electrodes

“…First, the ohmic component increased from 0.25 · cm 2 at p(O 2 ) = 1 atm to 0.39 · cm 2 at p(O 2 ) = 0.01 atm, implying that there was a decrease in the conductivity of the LSF-YSZ. 10,31 The increased ohmic resistance is observed here because the conductivity of LSF-YSZ electrodes prepared by infiltration are lower following calcination to 1373 K. 31 Second, there was a significant change in the shape of the non-ohmic part of the Nyquist plots with decreasing p(O 2 ). As noted above, there are at least two arcs in the spectrum for the 1373-K composite, corresponding to low (LF) and high (HF) frequency processes, with the LF feature gaining in prominence with decreasing p(O 2 ).…”

“…7,9,[16][17][18][19][20][21][22][23][24] Finally, a number of mathematical models have stressed the importance of maximizing the electrode active area 7,8,25,26 and point to the adsorption of molecular oxygen on a perovskite vacancy site as the most probable rate-limiting step. 7,8,10 Despite a large amount of work aimed at characterizing the oxygenreduction reaction on various materials, there is still much that is not known about the reaction or how to promote it. Characterization of oxygen reduction on working electrodes can be particularly difficult because high performance SOFC cathodes are usually composites of the conductive perovskite and the electrolyte material (e.g.…”

Comparison of Electrochemical Properties of La0.6Sr0.4FeO3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

“…[3][4][5] Therefore, generally other compositions such as (La,Sr)(Cr,Mn)O 3 , [6][7][8][9] (La,Sr)(Cr,V)O 3 , 10,11 (La,Sr)(Cr,Ru)O 3 , 12,13 or La and Fe doped SrTiO 3 14,15 are tested under reducing conditions. 16 Although Sr-doped LaFeO 3-δ (LSF) is a typical SOFC cathode material, it is also rather stable in reducing conditions, [17][18][19] with La 0.6 Sr 0.4 FeO 3-δ decomposing below 10 −27 bar pO 2 at 600…”

Porous Thick Film Lanthanum Strontium Ferrite Stress and Oxygen Surface Exchange Bilayer Curvature Relaxation Measurements

“…For instance, chemical oxygen surface exchange coefficient (k) discrepancies greater than 4 orders of magnitude are reported at identical temperature and oxygen partial pressure conditions for the MIEC material La 0.6 Sr 0.4 FeO 3-δ (LSF64). [10][11][12][13] Similarly, large k discrepancies have been observed for other MIEC materials such as lanthanum strontium cobalt iron oxide [14][15][16][17] and reduced cerium oxide. [18][19][20][21] Since recent studies have shown that strain can alter k, 20,21 it is likely that some of these observed k discrepancies are due to differences in MIEC stress state.…”