Electrode reaction at Pt, O 2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model

Abstract

This study aims to make clear the reaction kinetics at Pt, O 2(g)/zirconia electrodes in the oxygen partial pressure, P O 2 , of ∼ 10 −4 − 1 atm at ∼ 400–∼800°C. By a critical review on the preceding studies, problems were pointed out in the application of the Langmuir adsorption isotherm to the P O 2 dependence of electrode conductance, in the assumption of electric double layer at the electrode interface, and of the inconsistency between the recent reaction model of surface diffusion controlled kinetics and the absolute rate theory. It was shown that the charge transfer kinetics cannot be the rate determining step (RDS) of the electrode reaction. The possible RDS was concluded to be either (i) dissociative adsorption of oxygen molecules on the Pt surface or (ii) surface diffusion of O ad atoms on the Pt surface to the Pt/zirconia contact. The diffusion of O ad atoms on the Pt surface was considered to be proportional to θ(1−θ)(∂μ O/∂ x), where θ is the occupancy of O ad atoms on Pt and μ O is the oxygen chemical potential on the Pt surface. The rate equation, current-potential relationship, and the electrode conductivity, σ E, were calculated for the cases the RDS is (i) and (ii), respectively. By comparing the calculated σ E versus log P O 2 relationship with the reported ones, it was shown that the RDS is (i) for T ≲ 500°C and is (ii) for T ≳ 600°C. In the former case, σ E is essentially constant irrespective to P O 2 , and in the latter case, σ E maximum appears on the σ E versus log P O 2 relations.