Surface reaction of nonstoichiometric oxide with oxygen gas
September 4-8, 2016
The cathode performance in solid oxide fuel cell (SOFC) is getting increased attention, due to higher total efficiency. The direction to lower operation temperature is of essential importance for the next generation SOFC. The cathode reaction is estimated as the slowest step for conventional SOFC using O2- ion conductor as a solid electrolyte, when the operation temperature is reduced. The rate-determining step (RDS) of highly reactive oxide cathodes known today, such as La1-xSrxCoO3 (LSC), La1-xSrxCo1-yFeyO3 (LSCF), and Ba1-xSrxCo1-yFeyO3 (BSCF), is the oxygen reduction reaction (ORR) at surface. Although the overall reaction is expressed as 1/2O2(g) + Vö -> Oo + 2h, its elementary RDS for ORR is still unclear to date. In the present report, the authors summarize our experimental approach using dense oxide cathode films of LC, LSC with very thin surface modification layer with variable La(+Sr)/Co ratios and, and Pr6O11 thin film cathode. In addition, the ab initio molecular dynamics (ai-MD) simulation for the oxygen gas splitting reaction on NiO surface will be discussed, in addition to experimental study using electron spectroscopy.
The elementary steps for ORR can be schematically illustrated as shown in Fig. 1. Several assumptions, such as consideration of only forward reactions, local equilibrium before and after RDS, and so on, leads to the dependence of rate constant of the elementary processes. The rate constant that includes electron activity, , is dependent on the defect chemistry, that is, the electron density as a function of oxygen partial pressure. Only the step (a) and (e) in Fig. 1 are the cases where the same dependency obtained for materials with different defect chemical characteristics like LC and LSC.
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Shu Yamaguchi, "Surface reaction of nonstoichiometric oxide with oxygen gas" in "Nonstoichiometric Compounds VI", ECI Symposium Series, (2016). http://dc.engconfintl.org/nonstoichiometric_vi/7