He QiFollow

Semester

Fall

Date of Graduation

2019

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

Edward Sabolsky

Committee Member

Konstantinos Sierros

Committee Member

David Mebane

Committee Member

Harry Finklea

Abstract

The solid oxide fuel cell (SOFC) is an efficient high-temperature device that can directly convert chemical energy to electrical energy. Because of its high efficiency and environmental friendliness, extensive investigations have been made worldwide in past decades. Though broadly used as the anode material for Solid-Oxide Fuel Cells, Ni-YSZ is limited in some practical applications by severe carbon deposition on Ni in hydrocarbon fuels and a significant volume change of Ni-NiO among redox cycling. In order to overcome these problems and achieve excellent performance in the redox environment, a Ni-free ceramic material La0.5Sr1.5Fe1.5Mo0.5O6-δ (LSFM) is developed in this work.

LSFM possesses good phase stability in oxidizing and reducing atmospheres. Compared with Sr2Fe1.5Mo0.5O6-δ (SF1.5M) and other related compositions, LSFM displays a closer CTE to electrolytes, better reversibility (i.e., expansion reversibility and electrical conductivity reversibility) in redox cycling and higher power density in hydrogen. First, the CTE is reduced from 17.12×10-6 K-1 (SF1.5M) to 15.01×10-6 K-1 (LSFM), and the thermal expansion can be further reduced with a higher lanthanum doping. Second, the excellent reversibility of the chemical expansion during redox cycling is confirmed, which is necessary to perfect electrical conductivity reversibility. In three redox cycles, no conductivity loss is observed. The smaller thermal expansion and better reversibility result from a stronger metal-oxygen (M-O) bond, specifically, La-O bond, in the LSFM. The strong M-O bond is helpful to stabilize the crystal structure, limit the crystal cell's expansion and sometimes maintain the phase stability. The small polarization resistance (0.16 Ω·cm2) and the attractive maximum power density (1156 mW cm-2 at 800 oC in the humidified H2) make the LSFM as a promising anode candidate for SOFCs.

Besides the good redox stability, La also helps to accelerate the in-situ exsolution of nanoparticles (NPs) onto LSFM surface in H2. The exsolved material was characterized as uniformly distributed Fe NPs with a particle size of about 100 nm. Minor Fe exsolution occurs in a strong reducing atmosphere (wet or dry H2) at 800 oC in the first several hours. After that, the exsolution will stop automatically. Also, thorough and rapid dissolution of exsolved NPs is observed after re-oxidizing LSFM in the air at 800 oC for 1 hour. Most Fe NPs disappeared after the re-oxidizing and the phase evolution was recorded as the waved grain boundary and surface. Electrical conductivity relaxation (ECR) analysis demonstrated that the surface reaction kinetics on the LSFM anode is enhanced by in-situ exsolution. Based on electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis, the ionic conductivity was increased during exsolution. The higher surface catalytic activity and faster oxygen transportation lead to this enhanced electrochemical performance.

Based on SF1.5M and related perovskite materials, the positive effect of H2O on hydrogen oxidation reaction is proved and researched. Experimental results demonstrate that the electrochemical performance of perovskite material is significantly improved in humidified gas (low and high PH2 environment). This performance promotion mainly comes from the faster surface reaction process. In order to deeper understand this mechanism, the DFT calculation was performed. DFT modeling shows that the H2O plus surface oxygen vacancy formation step is the highest energy intermediate state on the surfaces and this state can be reduced with (1) a lower δ near the surfaces, (2) the formation of hydroxyl and hydride relative to the surfaces containing O vacancies. The promotion effect of humidity is further explained by the lower surface electron chemical potential with increase of O chemical potential, as well as enhanced interaction between the surface-bound H+ (surface hydroxyl) with the surface lattice oxygen in the 2HatO hydrated surfaces and interaction between surface adsorbed hydride H- with surface O vacancies in the HatM&HatO hydrated surfaces. The similar phenomenon is also observed from other perovskite materials, indicating the humidity promotion effect and the identified enhancing factors can be general for HOR on perovskite ceramics used in the SOFC anodes.

Embargo Reason

Publication Pending

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