Identification of the reaction mechanism of the Pt, O2(g)|yttria-stabilized zirconia system -: Part I:: General framework, modelling, and structural investigation

This study presents a general framework for modelling heterogeneous electrode reaction mechanisms and reports about their structural properties. The procedure proposed is derived from classical systems and control theory. Relationships between abstract expressions and measurable electrochemical quantities are established. The procedure is illustrated by means of the O(2)(g) reduction at the interface between Pt and yttria-stabilized zirconia (YSZ) solid electrolytes. Dissociative adsorption of oxygen on the Pt surface, surface diffusion of atomic oxygen, and a two-electron transfer step near the triple phase boundary are considered. The. surface concentration of adsorbed atomic oxygen is taken as time dependent state-variable. The reaction mechanism is formulated in state-space form and structural investigation of the resulting set of equations is performed on the corresponding transfer function. The concept of structural identifiability is demonstrated. Expressions describing the steady-state current-voltage behaviour and the frequency response behaviour are derived. It is shown that all unknown parameters can be uniquely estimated from steady-state and impedance measurements. (C) 1999 Elsevier Science B.V. All rights reserved.