(212e) SrTiO3 Based Anode Materials for Solid Oxide Fuel Cells: A Computational Attempt to Understanding and Improving Performance

In recent years, there has been a significant interest in alternative anode materials for solid oxide fuel cells (SOFCs) that are active and stable under hydrocarbon fuel conditions. The requirements for promising SOFC anode materials are good chemical and physical stability under operating conditions, high ionic and electronic conductivity, and good catalytic activity for the anode reaction. Furthermore, the material should not be susceptible to deactivation by both carbon formation and sulfur poisoning. Several alternative anode materials have been proposed that possess fluorite, rutile, tungsten bronze, pyrochlore, perovskite, and spinel structures. Among these anode materials, perovskite and double perovskite oxides are the most promising SOFC anode materials since they exhibit high ionic/electronic conductivity and high catalytic activity for various electrochemical reactions. It is the hypothesis of this research that modern computational tools permit the systematic investigation of the effect of dopants in the perovskite structure on various anode properties including electronic and ionic conductivity, catalytic activity, and resistance to coking and sulfur poisoning. In particular, we are interested here in understanding recently reported conductivity measurements of Ga and Nb doped A-site deficient SrTiO₃ [1].

Various configurations of Ga doped Sr_0.9Ti_0.8Nb_0.2O_3-x at various concentrations of Ga were investigated to gain a better understanding of the electronic conductivity behavior of the material under SOFC operating conditions. Experimental results suggest that Ga doped Sr_0.9Ti_0.8Nb_0.2O₃ displays metallic behavior with a conductivity maximum at 10% Ga doping. Calculated densities of states of these materials suggest that 10% Ga doped Sr_0.9Ti_0.8Nb_0.2O₃ exhibits p-type doping behavior which would reduce the electronic conductivity of Nb (n-type) doped Sr_0.9Ti_0.8Nb_0.2O₃.  On the other hand, in its reduced form Sr_0.9Ti_0.7Nb_0.2Ga_0.1O_2.9 displays increased n-type doping behavior due to the presence of an oxygen vacancy. This effect disappears whenever two Ga atoms are in close proximity.

Ab initio thermodynamic calculations were performed to determine the relative thermodynamic stability of various doped SrTiO₃ systems under experimental anode conditions (P_O2= 10^-20 bar).  10% Ga doped systems tend to form oxygen vacancies at elevated temperatures. As a result, the most stable structure of 10% Ga doped Sr_0.9Ti_0.8Nb_0.2O₃ is in the reduced form (Sr_0.9Ti_0.7Nb_0.2Ga_0.1O_2.9) and displays n-type doping behavior.  To summarize, the experimentally observed conductivity maximum of A-site deficient 10% Ga-doped Sr_0.9Ti_0.7Nb_0.2Ga_0.1O_3-x can be explained by small amounts of Ga promoting the reduction of the material which increases conductivity. Introducing larger amounts of Ga results in (reduced) Ga-Ga sites that do not increase n-type doping behavior and conductivity.

[1] G. Xiao, et al., Synthesis and Characterization of A-site Deficient Perovskite Sr_0.9Ti_0.8-xGa_xNb_0.2O₃, Mater.Res.Bull. (2010), doi:10.1016/j.materresbull.2010.09.044