Oxide Ion Conduction and Surface Exchange Reactions of Mixed Conductive La_0.65Ca_0.35FeO_3−δ Based on Oxygen Permeation Study

Abstract

La0.65Ca0.35FeO3−δ (LCF6535) displayed higher oxygen permeation flux (J O2 ) compared to La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and could potentially replace the typical cathodes employed in solid oxide fuel cells. However, the oxide ion conduction and surface exchange reaction of the LCF6535 have not been studied thus far. This study investigates the ion conductivity and surface exchange reaction of LCF6535 by simultaneous investigation of the oxygen permeation flux and oxygen chemical potential. The ion conductivity (σi) of LCF6535 was determined to be 0.29 S cm–1 at 1000 °C, and this high σi is the key contributor to the high J O2 of LCF6535. Furthermore, the J O2 of LCF 6535 at 800 and 850 °C was found to be limited by surface exchange reaction on the low oxygen partial pressure (P l) side. The rate-determining step for the surface exchange reaction was determined to be the charge transfer relating to O2 molecule desorption, which results from the lower hole concentration on the P l side under the reduction condition. The increase in hole concentration achieved on the P l side is a possible solution for improving the surface exchange reaction and oxygen permeability.

Evaluating the Rate-determining Steps of Oxygen Permeation in Ba_0.5La_0.5FeO₃₋<i>_δ</i> Perovskite

“…Using a similar experimental setup, our research group previously described the OCP for the LCaF membrane. 17 The EMF measured by the YSZ sensor on the oxidized and reduced surfaces are denoted by E s(rich) and E s(lean) , respectively. These two EMFs were generated due to dioxygen activity on the corresponding sides of sample surfaces and can be explained following the Nernst equation below 17,18…”

“…Where The bulk EMF (E bulk ) corresponds to the oxygen activity through the volume of a sample can be estimated by the equation 17,18…”

Surface Exchange Reaction of Mixed Conductive La0.65Ca0.35FeO3−δ during Oxygen Evolution and Incorporation as Traced by Operando X-ray Photoelectron Spectroscopy

“…15,16 To understand the surface exchange mechanism, a previous study examined influences of oxygen partial gradient on the surface oxygen chemical potential drop and J Od 2 in LCF. 17 reaction is restricted by surface density of electron−holes. Thus, it is highly necessary to examine the microscopic view by directly observing the chemical state at the surface during oxygen evolution and incorporation, hence the need for an operando experiment.…”

“…Applications of mixed conductive oxides are also commonly required to overcome the issue of slow surface exchange reactions at moderate temperature (<800 °C). , To understand the surface exchange mechanism, a previous study examined influences of oxygen partial gradient on the surface oxygen chemical potential drop and J O 2 in LCF . This study found that the concentration of electrons and holes on the surface of an LCF film affects the evolution of O 2 molecules.…”

Oxygen Release and Incorporation Behaviors Influenced by A-Site Cation Order/Disorder in LaCa2Fe3O9 with Unusually High Valence Fe3.67+

“…Oxides including mixed and high valence states of transition-metal cations are an important class of materials in energy-related technology fields. They are used, for example, as cathodes in solid oxide fuel cells and as ceramic membranes separating oxygen from air. − The perovskite structure oxides like La 1 –x A x CoO 3−δ and La 1– x A x FeO 3−δ , where A is Ca, Sr, or Ba, are typical such compounds and are intensively studied. − Controlling the valence state of the constituent transition-metal cations is a key for tuning the compound’s properties. We here focus on La 1– x Ca x FeO 3−δ -related compounds and investigate the mixed and high valence states of Fe.…”