Date of Completion

3-26-2014

Embargo Period

3-25-2014

Keywords

solid oxide electrolysis cells, hydrogen, performance degradation, materials, strontium-doped lanthanum manganite, silver, impurities, nickel-YSZ

Major Advisor

Prabhakar Singh

Associate Advisor

Alexander Agrios

Associate Advisor

Ugur Pasaogullari

Associate Advisor

Radenka Maric

Associate Advisor

Steven Suib

Field of Study

Chemical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Solid oxide electrolysis cells (SOECs) are high temperature electrochemical devices with the potential for large-scale, highly efficient production of hydrogen or syngas with negligible net greenhouse gas emissions. Long-term electrical performance degradation relating to materials interactions at elevated temperatures is one of the primary technical limitations to SOEC commercialization. In this work, solid oxide cells were fabricated and tested using various materials and conditions in order to further the understanding of SOEC degradation mechanisms. This knowledge can be used to develop new materials and fabrication methods that improve performance and long-term stability of SOECs.

SOEC anode delamination from the electrolyte is a major known contributor to performance degradation. Delamination was observed in the current study between strontium-doped lanthanum manganite (LSM) air electrodes (anodes) and yttria-stabilized zirconia (YSZ) electrolytes. Small particles of lanthanum zirconate (La2Zr2O7) form at the anode-side LSM-YSZ interface due to pressure buildup as oxide ions from the YSZ are oxidized to oxygen gas. The resulting interfacial fracturing and variance in thermal expansion weaken the LSM-YSZ interface, causing gradual delamination.

Silver-based electrodes were investigated for use in reduced operating temperatures (750 °C or below). Silver-palladium/gadolinia-doped ceria (Ag-Pd-GDC) composite electrodes were electrically stable with 0.8 V applied for 100 h at 750 °C. However, the Ag‑Pd was morphologically unstable due to agglomeration and electromigration, requiring further optimization of the composite structure, particle size, and composition. Oxide impurities in the Ag-Pd migrated to the electrode-electrolyte interfaces and caused local undulations in the YSZ electrolyte surface. These results demonstrate that material purity plays a crucial role in interfacial stability when using silver-based electrodes under applied voltage.

Performance and degradation of Ni-YSZ fuel electrodes (SOEC cathodes) were studied as functions of materials, preparation techniques, and operating conditions. Carbon deposition, nickel evaporation, and impurity poisoning were not observed to negatively affect performance. However, nickel particle size was too large for optimum performance, and agglomeration decreased nickel interconnectivity and contact with the current collector. Structural and compositional optimization is therefore required to make Ni-YSZ electrodes suitable for the electrolysis of water or carbon dioxide.

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