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One of the major obstacles encountered when using solid oxide fuel cells with hydrocarbon fuels is sulfur poisoning. The current anode material used is Ni/YSZ and Ni is not sulfur tolerant; therefore, the performance of the cell will degrade over time due to the formation of NiS. Perovskites have demonstrated superior sulfur tolerance but lack the high conductivity and catalytic activity of Ni/YSZ cermets. One of the objectives of this effort is to explore the substitution of the A-site in an A2MgMoO6 perovskite with Sr and Ba, to create Sr2MgMoO6 (SMMO) and Ba2MgMoO6 (BMMO), respectively, to improve the sulfur tolerance of solid oxide fuel cells (SOFCs). Sr2MgMoO6, a double perovskite, has been previously studied and is suggested as a material of interest because of its relatively high conductivity and catalytic potential. Barium has not been previously studied and was selected as the dopant because the ionic radii (1.61 Å) resulted in a calculated tolerance factor of 1.036 for BMMO when compared to SMMO, which has an ionic radii of 1.44 Å and a calculated tolerance factor of 0.978. The tolerance factor for BaSrMgMoO6, a bi-substituted material synthesized for comparison as an intermediate formulation, was calculated to be 1.00. Another objective is to synthesize and characterize a series of lanthanum (La) doped Sr2MgMoO6 (SMMO) or La doped Sr2MgNbO6 (SMNO) anode materials, which can be used in combination with electrolytes containing lanthanum to mitigate the effects of lanthanum poisoning in SOFCs. Currently, a La0.4Ce0.6O1.8 (LDC) transition layer is used with many perovskite-based anode materials to prevent La diffusion into the anode from the La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) electrolyte, which can create a resistive La species that impedes electrochemical performance. To accomplish this, a new class of anode materials was synthesized with the goal of balancing La chemical potential between these neighboring materials. It was hypothesized that by incorporating La into the anode, the gradient of chemical activity between the anode and electrolyte would be decreased, which would prevent La diffusion. These anode materials were synthesized via a sol-gel methodology and characterized with X-Ray diffraction to assess phase purity. The conductivity of these materials was analyzed in the presences of both H2 and 100 ppm H2S/H2 to determine the stability and performance. Electrochemical performance of the specimens was measured by pasting the material onto an LSGM disk and stability was measured in sulfurous H2 (100 ppm H2S). Both SMMO and BMMO demonstrated superior stability when exposed to 100 ppm of H2S as compared to Ni/YSZ, but exhibited performance degradations due to sulfur accumulation in the transition layer. The stability experiments demonstrated that 40% mol La doped SMNO is stable in all pertinent environments while preventing La diffusion between the anode and the LSGM electrolyte. |
