Xinxin Zhang

Date of Graduation

2016

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Harry Abernathy

Committee Member

Ismail Celik

Committee Member

Edward Sabolsky

Committee Member

Xueyan Song

Abstract

New hetero-structured La2NiO4+delta (LNO)/ (La0.6Sr0.4)0.95Co0.2Fe0.8O3-delta (LSCF) SOFC cathodes have been successfully fabricated by the infiltration technique for long-term stability and oxygen reduction reaction (ORR) kinetic consideration. The current comparative study of LSCF and LNO-infiltrated LSCF shows a significant reduction in polarization resistance by one order of magnitude and a 67% increase in maximum power density by using LNO-infiltrated LSCF cathodes at 750°C in air. The electronic conductivity relaxation (ECR) results demonstrate that the remarkable improved oxygen exchange properties of LNO/LSCF hetero-structured cathodes are attributed to not only the increase of surface reaction area, but also the introduction of LNO surface and LNO-LSCF interface with rapid oxygen exchange behavior. A trace amount of Co, Sr and more oxygen defects stabilized in the LNO layer have been identified by TEM, EDXS, XPS, XRD and iodine titration experiments, giving rise to the enhancement of stability and surface catalytic activity. A low degradation rate of 0.39% at a constant current density of 250 mAdeltacm-2 can be still achieved for the fuel cell using LNO-infiltrated LSCF cathodes after long-term durability of about 500 h at 750°C, accompanied by nanoparticles growth/aggregation, delamination and slight lanthanum enrichment on LNO surfaces. The benefits from the presence of the LNO nanoparticles demonstrate that LNO-infiltrated LSCF materials can act as high active surface oxygen exchange cathodes with promising fast ORR behavior and stability.;To further enhance the long-term stability of infiltrated (La0.6Sr0.4)0.95Co0.2Fe0.8O3-delta (LSCF) cathode, two kinds of new hetero-structured cathodes, CeO2 & LNO co-infiltrated LSCF and La2-xNiO4+delta-infiltrated LSCF, have been designed. La2NiO4+delta (LNO), as an superior performance promoter, and CeO2, as a particle growth retardant due to the high melting point and good wettability, were co-infiltrated into the LSCF backbone to mitigate the delamination and growth/aggregation of in-situ formed nano-sized infiltrants. A more advantageous microstructure was confirmed in the co-infiltrated sample relative to those infiltrated with LNO alone. The original morphology was largely retained in the co-infiltrated sample after heat treatment, showing finer surface particles, better connection between the infiltrant network and the LSCF backbone. XPS and EIS results reveal that the substitution of stoichiometric La2NiO4+delta by A-site deficient La2-xNiO4+delta can effectively suppress lanthanum enrichment and enhance the stability after cathodic polarization treatment observed by measurement. The advantages of both modified hetero-structured cathodes suggest that CeO2 & La2-xNiO4+delta co-infiltrated LSCF is a promising hetero-structured cathode to mitigate the LSCF electrode degradation.

Download

Share

COinS