Abstract:
Nickel-based anodes are the most commonly used anodes for solid oxide fuel cell (SOFC). Unfortunately, with nickel based anodes, internal reforming of hydrocarbon fuels is often accompanied by carbon deposition, which covers the active nickel sites, resulting in degradation of cell performance. Ni0.375Cu0.375Mg0.25Al2O4 spinel catalyst was investigated for its use as a potential internal reforming layer for SOFC anodes.
To determine if any reactions with NiO, Y0.08Zr0.92O2 (YSZ), or Gd0.2Ce0.8O2 (GDC) occur during fuel cell fabrication, Ni0.375Cu0.375Mg0.25Al2O4 was thoroughly mixed with NiO-YSZ and NiO-GDC and fired at temperatures ranging from 1300°C - 1500°C. Room temperature X-ray powder diffraction of the samples revealed spinel peak shifts, indicating compositional changes towards a higher nickel concentration spinel.
Rietveld refinements of high temperature X-ray diffraction patterns were used to calculate the coefficient of thermal expansion of the spinel. With a calculated CTE of 10 x 10⁶ K⁻¹ from 25°C to 850°C, the CTE of the oxide spinel is at least 5% lower than Ni-YSZ, YSZ, and La0.8Sr0.2MnO3 (LSM).
When 0.015g of spinel was added to the fuel cell anode, no change in the peak power density was observed while operating under hydrogen. However, 0.015g of spinel addition while operating under a non-coking methane fuel gas mixture caused an increase in peak power density from 74 mW⋅cm⁻² to 98 mW⋅cm⁻² (32% increase) at 850°C and 40 mW⋅cm⁻² to 52 mW⋅cm⁻² (31% increase) at 750°C.
After 50 hours operating on a coking methane fuel gas mixture at 850°C, a fuel cell with no spinel additions had a peak power density drop of approximately 16%. With 0.015g of added spinel, the peak power density degraded by a lesser amount, 10%. Finally, with 0.05g of spinel added, the peak power density decreased by only 6%. After endurance testing, no carbon fibers were observed on the anode during SEM imaging. Raman shift shows the fingerprint graphitic carbon bands on the anode with no spinel additions, but not on the fuel cells with spinel additions.
Due to the coefficient of thermal expansion mismatch, Ni0.375Cu0.375Mg0.25Al2O4 is not well suited for use as a reforming catalyst for high temperature SOFCs. However, it can be used in intermediate temperature fuel cells (IT-SOFCs). In addition, with optimization of the processing and application to the anode, Ni0.375Cu0.375Mg0.25Al2O4 could eliminate the need for upstream fuel reforming in IT-SOFCs.
Description:
Thesis completed in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
