Bulk- and Powder Surface Exchange Coefficients of (La,Sr)CoO3-d

Abstract

1. Introduction La0.6Sr0.4CoO3-δ (LSC64) has been recognized as a promising cathode for SOFC, which has high mixed conductivity of electron hole and oxide ion. Its high mixed conductivity makes oxygen reduction reaction (ORR) possible to take place not only at triple-phase boundary but also on two-phase boundary of electrode surface and gas phase. This two-phase boundary reaction proceeds: oxygen gas diffusion to the electrode surface, adsorption, dissociation of the oxygen atom, incorporation into the electrode, then bulk diffusion. In the case of LSC64 thin film, the rate limiting step is the surface reaction [1]. It is necessary to understand the surface reaction kinetics to improve ORR on LSC64 cathode. Surface exchange coefficients of LSC64 bulk are reported in several papers including ref. 1. However ORR kinetics of LSC64 porous cathode can’t be explained by the reported values. In this study, therefore, surface exchange coefficients of LSC64 powder are determined, which is better to simulate surface reaction kinetics of porous electrode. 2. Experimental Surface exchange coefficients, k, of LSC64 powder were determined by a pulse isotope exchange (PIE) method [2]. The measurements were performed in oxygen partial pressure range from 10-3to 10-2 bar at 573-773 K. Sample powder was loaded at the center of a quartz tubular micro reactor (ϕ2 mm). Typical values of the powder mass and length of the powder bed were 0.03-0.05 g and 13-15 mm, respectively. A 16O2-N2 gas mixture was used as the carrier gas. A six-port valve was used for injection of the 18O2/He pulse gas (500 μL) into the 16O2/N2 carrier gas. The response to an 18O2-enriched pulse gas was analyzed by an on-line mass spectrometry (OmniStar TM GSD320, Pfeiffer-Vacuum). The pulse experiments are carried out under following conditions. (i) The number of 18O atoms is negligibly small compared to that of the exchangeable oxygens in the oxide, hence f(O18) ≈ 0. (ii)There is a large diffusion length of 18O in the non-porous oxide particle. Under these conditions, mass balance analysis yields a simple equation for surface exchange coefficients [2]: k = nV m/t r*ln(f(18g.i)/f(18g.e)) where f(g.i18) and f(g.e18) are the 18O isotope molar fractions in the pulse at the inlet and exit of the reactor, respectively; n is the total number of oxygen atoms in the free volume of the packed bed; V m is molecular volume; t r is the average residence time of the labeled gas in the reactor. 3. Results and discussion Temperature dependence of k for LSC64 powder are shown in Fig. 1. Surface exchange coeffiicients k of LSC64 powder showed temperature dependence at 300-400℃. In comparison to LSC64 bulk, surface exchange coefficients, k, has different tendency. This suggests that surface reaction kinetics of LSC powder (or porous LSC64) might be different.