Xingbo Liu
Mechanical & Aerospace Engineering
Benjamin M. Statler College of Engineering & Mineral Resources

Roles of Oxygen Lattice Defects on the Oxygen Reduction Reaction Kinetics in Solid Oxide Fuel Cell Cathodes

Location: EB1 Room 1011

Friday, August 19th 2016 - 11:00 am

We report our research on the roles of oxygen lattice defects on the kinetics of Oxygen reduction reaction (ORR) in the mixed ionic & electronic conducting (MIEC) SOFC cathode. For the perovskites with ABO3 lattice structure, oxygen vacancies are the primary defects that controlling the ORR. We developed a multi-domain 1-D physical model incorporating multi-step charge transfer to examine the competitive behaviors between the paralleled 3PB and 2PB kinetic pathway during MIEC electrode activation. Analyses by V-I polarization curve, Tafel estimation, and local 3PB current constitution have identified the limitation of surface oxygen ion diffusion as the mechanism for 3PB-to-2PB kinetic transition. The model also demonstrated surface reactions are driven predominantly by electrochemical forces at the 3PB, while being controlled by oxygen vacancy concentration variation at regions away from 3PB. For the Ruddlesden-Popper (R-P) phases with An-1A'2BnX3n+1 structures, primary defects are oxygen interstitials. The governing factors of the ORR are identified as oxygen adsorption and incorporation based on the findings in reaction orders from electrochemical impedance spectroscopy (EIS), stoichiometry related chemical capacitance and intrinsic anisotropic properties. The incorporation rate is proven to drastically depend on the amount of interstitial oxygen. Since the unfilled interstitial sites serve to accommodate the adsorbed oxygen during incorporation, like vacancies in the perovskite structure, more Oxygen interstitials would seem to suppress the kinetics of this process. We proposed a physical model to reconcile the discrepancy between the experimental results and intuitive reasoning. Based on supporting evidence, this model illustrates a possibility of how oxygen interstitials works to regulate the exchange rate, and how the contradiction between oxygen vacancies and oxygen interstitials is harmonized so that the latter in the R-P structure also positively promotes the incorporation rate in the ORR.

Dr. Xingbo Liu received his Ph.D. on Materials Science from University of Science and Technology Beijing in 1999, and he subsequently went to West Virginia University as a postdoc. Currently, he is the professor & associate chair for research in Mechanical & Aerospace Engineering Department at West Virginia University. Dr, Liu has developed a national recognized research program on materials for next generation energy conversion and storage, with the focus on solid oxide fuel cells and batteries. Dr. Liu has been serving leading roles in TMS, ACerS, and ECS, and he has received numerous awards, including one R&D 100 Award (2011) for his development of SOFC interconnect coating, TMS Early Career Faculty Fellow Award (2010), West Virginia Innovator of the Year (2013), WVU CEMR Researcher of the Year (2015, 2011), Outstanding Researcher Awards (2015, 2011, 2009, 2008), and several others. Most recently, Dr. Liu was elected as the Fellow of ASM International (2015), and received TMS Brimacomb Medal (2016).

North Carolina State University