Oxygen exchange reaction on mixed conducting oxides: (Ba,Sr)(Co,Fe)O3-δ and (Bi,Sr)(Co,Fe)O3-δ SOFC cathode materials

A. Wedig, L. Wang, R. Merkle

Perovskites with electronic and ionic conductivity (oxygen vacancies) are used as cathode materials in solid oxide fuel cells. To understand more about the mechanism of the surface reaction (comprising steps such as adsorption, dissociation, incorporation), effective rate constants k are measured (e.g. by microcontact impedance spectroscopy) as function of pO2, T, DC-bias. Their variation with the perovskite's cation composition and related bulk properties (conductivity - see fig 1(a) and (b), defect concentrations and mobilities) gives further evidence about point defects involved in the rate-determining step. In particular the correlation with ionic conductivity (Fig. 1(b)) indicates that oxygen vacancies and their migration are involved in the rate determining step. DFT calculations on intermediates and transition states also help to elucidate the reaction mechanism.

Fig. 1: (a) no overall correlation of k with electronic conductivity, (b) increased k occurs in parallel to ionic conductivity (oxygen vacancies).

see also Poster  and annual report 2011  www.fkf.mpg.de/88262/IV_03_02.pdf

Publications:

L. Wang, R. Merkle, and J. Maier
Surface Kinetics and Mechanism of Oxygen Incorporation Into Ba1-xSrxCoyFe1-yO3-δ SOFC Microelectrodes
J. Electrochem. Soc. 157(12), B1802–B1808 (2010) DOI: 10.1149/1.3494224

L. Wang, R. Merkle, J. Maier, T. Acartürk, and U. Starke
Oxygen tracer diffusion in dense Ba0.5Sr0.5Co0.8Fe0.2O3-δ films
Appl. Phys. Lett. 94(7), 071908 (1–3) (2009) DOI: 10.1063/1.3085969

L. Wang, R. Merkle, Y. A. Mastrikov, E. A. Kotomin, and J. Maier
Oxygen exchange kinetics on solid oxide fuel cell cathode materials-general trends and their mechanistic interpretation
Journal of Materials Research 27(15), 2000-2008 (2012) DOI: 10.1557/jmr.2012.186

A. Wedig, R. Merkle, B. Stuhlhofer, H.-U. Habermeier, J. Maier, E. Heifets
Fast oxygen exchange kinetics of pore-free Bi1-xSrxFeO3-δ thin films
Phys. Chem. Chem. Phys. 13 (2011) 16530 DOI: 10.1039/C1CP21684H

A. Wedig, R. Merkle, J. Maier
Oxygen Exchange Kinetics of (Bi,Sr)(Co,Fe)O3-d Thin-Film Microelectrodes
J. Electrochem. Soc. (2013) accepted DOI: 10.1149/2.017401jes

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