Promotional Effect of Molten Carbonates on Proton Conductivity and Oxygen Reduction Reaction – An Experimental and Computational Study

Author

Xiaolei Xiong, University of South Carolina

Date of Award

2017

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Kevin Huang

Abstract

Recent research of Solid oxide fuel cells (SOFCs) is aimed to lower the operating temperature to an intermediate temperature (IT) range of 500 to 700^oC, while maintaining a proper performance. This Ph.D. research project investigates the promotional effects of alkaline carbonate eutectics on the proton conductivity of proton conducting electrolytes and cathodic ORR reactivity in SOFCs by both experimental and computational methods. The ionic conductivity of the MC-BZY composite above 500^oC increases with the higher loading of MC. The sample exhibited nearly a factor of two higher conductivity in H₂-containing atmosphere than in air. First-principles DFT modeling further investigated proton transfer at the interface of BaZrO₃ and molten carbonate. With the presence of carbonate ion, the energy barrier for proton migration becomes as low as 0.332 eV. The modeling indicates the reduction of energy barrier is resulted from the change of ratedetermining step from proton transfer between oxygen atoms to proton rotation around oxygen atom.

Infiltration of MC into porous cathode can reduce the polarization of resistance (R_p), i.e., enhance the oxygen reduction reaction (ORR) activity. The EIS analysis shows that MC has a beneficial effect on reducing R_p for different cathodes including Au,La_0.8Sr_0.2MnO_3-δ(LSM), La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ(LSCF) and La₂NiO_4+δ (LNO). Specifically, the study on MC loading effect was carried out on LSCF cathode. It shows that a higher loading makes a greater reduction on R_p and the degree of reduction is the same from 500 to 600^oC. As the loading increases to 1.4 wt%, the degree of R_preduction tends to reach a limit. First-principles DFT modeling was further used to investigate the incorporation of oxygen into MC. The formation of CO₅^2- in molten carbonate was considered as a chemisorption of gas oxygen on the surface of MC infiltrated cathodes. After the formation of CO₅^2-, it reacts with another CO₃^2- to form two CO₄^2-, which is a rate-limiting step on potential energy surface. After dissociation, oxygen atoms migrate in molten carbonate, which is energetically favor by intermolecular pathways. An O-O-O linkage is formed between carbonate ions, which facilitates the oxygen migration between carbonate ions.

Rights

© 2017, Xiaolei Xiong

Recommended Citation

Xiong, X.(2017). Promotional Effect of Molten Carbonates on Proton Conductivity and Oxygen Reduction Reaction – An Experimental and Computational Study. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4213