Solid State Ionics 136–137 (2000) 991–996 www.elsevier.com / locate / ssi A simple method for the determination of surface exchange and ionic transport kinetics in oxides * Mortaza Sahibzada , William Morton, Anne Hartley, Dionissios Mantzavinos, Ian S. Metcalfe School of Chemical Engineering, University of Edinburgh, King’ s Buildings, Edinburgh EH93JL, UK Abstract A method is proposed for obtaining the surface exchange coefficient ( k) and chemical diffusion coefficient ( D ) for oxide materials under reducing conditions. Transient oxygen stoichiometry re-equilibration data is obtained from powdered samples using solid electrolyte coulometry where oxygen dosing / titration is performed by means of electrochemical cells. The data has been fitted to a suitable diffusion model using a non-linear parameter optimisation method from which the parameters k and D have been extracted. This method has been applied to the perovskite material La Sr Co Fe O 0.6 0.4 0.2 0.8 32d 23 25 at intermediate temperatures and reduced oxygen pressures in the range 1 3 10 –6.3 3 10 atm. The results obtained in this study were found to be consistent with those obtained by means of other relaxation techniques.  2000 Elsevier Science B.V. All rights reserved. Keywords: Surface exchange coefficient; Chemical diffusion coefficient; Transient measurements; Solid electrolyte coulometry; LSCF perovskites; SOFC cathodes 1. Introduction diffusion coefficient ( D ). The parameters k and D are usually determined by means of a relaxation method Oxygen conducting solid oxide materials find based on either thermogravimetry, in which one applications as the electrolyte / electrodes in solid follows the mass change of a sample during an oxide fuel cells (SOFCs), catalysts for oxidation abrupt change of oxygen partial pressure ( P(O )), or 2 processes, component materials in solid state oxygen electrical conductivity measurements, where sample sensors, and membranes in ceramic oxygen resistance is detected as a function of time after generators. The performance of these materials is P(O ) has changed [1]. 2 dependent upon the kinetics of oxygen take up from In this paper a simple alternative method for the gas phase, reflected in the surface exchange determining surface and bulk kinetic parameters is coefficient ( k), and the kinetics of oxygen ion reported, where k and D are determined by means of transport in the solid, reflected in the chemical a coulometric relaxation method using a powdered La Sr Co Fe O (LSCF 6428 for the rest of 0.6 0.4 0.2 0.8 32d the text) sample material. LSCFs are a family of *Corresponding author. Tel.: 144-131-650-7254; fax: 144- doped perovskite oxides that exhibit mixed ionic and 131-650-6551. E-mail address: m.sahibzada@ed.ac.uk (M. Sahibzada). electronic conductivity due to oxygen non-stoichiom- 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(00)00517-8