Date of Completion

1-27-2014

Embargo Period

1-24-2014

Major Advisor

Prabhakar Singh

Associate Advisor

Harris Marcus

Associate Advisor

Alexander Agrios

Field of Study

Materials Science

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Solid state electrochemical systems such as solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) are attractive for power generation, carbon capture, hydrogen and syngas production. YSZ and LSM have been extensively studied as electrolyte and electrode materials respectively for solid oxide cells operating in 800-1000°C temperature range. During the fabrication and operation of these systems, significant degradation in performance has been reported due to the formation of electrically insulating phases La2Zr2O7 and SrZrO3 at the LSM-YSZ interface.

The contradictory dada in literature, the limited information about effect of sintering atmosphere and the porosity urged a necessary systematic research of the LSM-YSZ composite system. This thesis includes a systematic study of reaction between LSM and YSZ, the role of oxygen pressure, porosity, raw power, sintering temperature and time on the air electrode densification, microstructural evolution and interaction of LSM-YSZ composite. In addition, the effect of zirconate phase on the thermal expansion coefficient (TEC) of the composite cathode was studied. Also, the reversibility of the reaction was studied, namely the La2Zr2O7 formed in lower PO2 can be reversed back into LSM and YSZ when resintered in higher PO2. The prevention of the detrimental LSM-YSZ reaction was discussed thermodynamically and successfully demonstrated by Mn-doped YSZ powder.

Long term degradation remains an issue for SOEC development. Anode delamination has been frequently observed and considered as one of the largest contributor to the cell performance degradation. Previous study found the resistive compound La2Zr2O7 formed at the air electrode side, and La2Zr2O7 formation degrades the cell performance. So the understanding from the study of the LSM-YSZ composite was applied in a symmetric solid electrolysis cell to alleviate the LSM-YSZ reaction. The SOEC electrochemical performance was improved by introducing a Mn-doped YSZ interlayer between LSM and YSZ. The delamination of air electrode was mitigated by a porous sol-gel Mn-doped YSZ interlayer. The sol-gel structure with high surface area and porosity is believed to be the possible reason for the prevention of delamination by release the oxygen pressure at the air electrode side.

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