Oxygen Permeation Through Cobalt-Containing Perovskites: Surface Oxygen Exchange vs. Lattice Oxygen Diffusion

Abstract

The oxygen permeation fluxes from p O 2 Ј to p O 2 Љ (p O 2 Ј Ͼ p O 2 Љ ) across cobalt-containing perovskite ceramic membranes La 1Ϫx Sr x CoO 3Ϫ␦ and SrCo 0.8 Fe 0.2 O 3Ϫ␦ were measured by gas chromatography as functions of oxygen chemical potential gradient, temperature, thickness, and catalytic activity on the surface. Power indexes 0.5 Ͼ n Ͼ 0 for uncatalyzed La 1Ϫx Sr x CoO 3Ϫ␦ and 1 Ͼ n Ͼ 0.5 for SrCo 0.8 Fe 0.2 O 3Ϫ␦ were obtained when J O 2 vs. p O 2 Ј n Ϫ p O 2 Љ n was plotted as a straight line. The results clearly indicate an overall permeation process controlled by both surface oxygen exchange and bulk oxygen diffusion for uncatalyzed La 1Ϫx Sr x CoO 3Ϫ␦ and SrCo 0.8 Fe 0.2 O 3Ϫ␦ . Application of a thin layer of catalytically active SrCo 0.8 Fe 0.2 O 3Ϫ␦ on the feeding-gas surface of La 0.5 Sr 0.5 CoO 3Ϫ␦ under the condition of a fixed p O 2 Ј ϭ0.21 atm and a varied p O 2 Љ not only increases remarkably the overall oxygen flux, but also changes a mixed control to a bulk diffusion control. This enables evaluation of the bulk transport properties of the mixed conductors. A coat of SrCo 0.8 Fe 0.2 O 3Ϫ␦ on the permeate side has little catalytic effect, especially at low p O 2 Љrange, due to the formation of a poorly conducting brownmillerite phase. The results explicitly show a higher activation energy for the surface exchange kinetics than for the ambipolar transport in the mixed conductors. The mechanism of the surface exchange is discussed, and an analytic expression that agrees well with the experimental results is obtained.

Oxygen permeation studies on La0.8Sr0.2Ga0.75Mg0.15Co0.10O3±δ

“…A similar tendency was also observed by Gharbage et al, Kharton and Marques, Murygin, Dou et al, and Huang and Goodenough [11,[18][19][20][21]. For instance, considerable divergences from linear theoretical dependence were found in the case of LSG for the composition La 0.9 Sr 0.1 Ga 0.8 -Fe 0.2 O 3−δ and proposed to be due to a diffusion overpotential caused by the slow oxygen surface exchange [11].…”

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO3 as a Model Material

“…A comprehensive and detailed discussion of oxygen incorporation into "electron-rich" perovskites in general is beyond the scope of this review. These materials were intensively studied in recent years with respect to chemical, electrochemical, and isotope exchange, as well as concerning common mechanistic aspects (see, for example, references [10,15,16,53,54,56,[159][160][161][162][163][164][165]). …”

Continuum and Quantum-Chemical Modeling of Oxygen Reduction on the Cathode in a Solid Oxide Fuel Cell

“…Techniques for measuring surface exchange and ionic diffusivity in LSM and LSCF include electron blocking, current interrupt, and isotope exchange/secondary ion mass spectroscopy (SIMS) [39][40][41][42][43][44][45][46][47][48][49][50]. But because the overall rate law for the surface reaction is likely nonlinear, and the rate laws for elementary steps depend exponentially on the surface potential, this limits the utility of average measurements to small perturbations from equilibrium.…”