Influence of architecture, materials, and processing on low temperature solid oxide fuel cell (LT-SOFC) performance

• Main Author: Zhang, Xinge
• Language: English
• Published: University of British Columbia 2009
• Online Access: http://hdl.handle.net/2429/11262

The goal of this dissertation is to develop low temperature solid oxide fuel cells (SOFCs) through the understanding of cell material and component fabrication technology. A typical anode supported thin electrolyte cell structure has been adopted, fabricated by wet ceramic processing and co-firing. Sm₀.₂Ce₀.₈O₁.₉ (SDC) electrolyte cells supported by Ni-Y₀.₁₀Zr₀.₈₄O₁₉₂ (YSZ) cermet substrates, with Sm₀.₅Sr₀.₅CoO₃ cathode and Ni-SDC anode, demonstrate a high performance of 0.89 W cm⁻² at 600°C. A designed experiment quantitatively reveals the internal shorting problem due to the mixed ionic and electronic conductivity of the SDC electrolyte. The internal shorting current density of the thin SDC cell reaches 0.85 A cm⁻² at 600°C under open circuit voltage (OCV) conditions, which limits the fuel utilization to less than 65% and electrical efficiency to below 25%. In order to eliminate the internal shorting problem, a unique bi-layered electrolyte structure has been developed by adding a thin zirconia based electrolyte layer as an electronic blocking layer. A YSZ/SDC bi-layered electrolyte cell prepared by wet ceramic processing and co-firing generated 0.34 W cm⁻² peak power density at 650°C, with an open circuit voltage (OCV) of over 1.0V. Further improvement of the cell performance was achieved by using a Sc₀.₂Ce₀.₀₁Zr₀.₇₉O₁.₉ (SSZ)/SDC bi-layered electrolyte. The cell reached 0.50W cm⁻² at 650°C. Electrochemical impedance analysis reveals that the ionic resistance of the bi-layered electrolyte prepared by co-firing is one order of magnitude higher than the theoretical value, indicating that interaction between the two electrolytes during the co-firing is a main limit. In order to eliminate the bi-layered electrolyte interaction, pulsed laser deposition (PLD) technology is applied for the bi-layered electrolyte cell fabrication. The cell fabricated by PLD reaches power densities of 0.95 W cm⁻² at 600°C, and 1.37 W cm⁻² at 650°C with open circuit voltage (OCV) values larger than 1.02 V, the highest performance ever reported in the literature. Nonetheless, the bi-layered electrolyte cells exhibit relatively high degradation rates. A study on the degradation of bi-layered electrolyte cells indicates that the cathode degradation is the main contributor. Therefore, an optimization of cathode compositions and fabrication conditions is important to improve the cell stability.