Ceria-Stabilized Molten Tin in SOFC Anodes
It was found that when tin oxide is present alone it is reduced to metal which, being in liquid form due to the relatively high temperature, segregates to the surface of the anode. This in turn negatively affects performance by causing a significant increase in polarization resistance. When cerium and tin are both present in SDC anodic scaffolds, on the other hand, a 10-fold increase in power output when compared to ceria alone, with a concomitant decrease in cell polarization resistance (Figure 1). The performance was found to be stable over a period of 20 hours at 600°C with the open circuit voltage (OCV) remaining at 0.92V, 400mV higher than the OCV developed by a cell infiltrated with ceria alone. Tests were performed up to 700°C without registering significant degradation for the ceria/tin anodes.
X-ray diffraction experiments in a hot chamber were made to confirm that even in the presence of ceria tin is present in operating conditions as a molten metal while no evidence of the presence of solid solutions was found; SEM/EDX analysis shows that tin localizes preferentially on the ceria particles. This is in stark contrast with the behavior of tin when alone in the anodic scaffold, in which case EDX spectra reveal that the metal is more uniformly spread inside the anode, and rapidly segragates to the surface. This highlights the stabilization of molten metallic tin in the presence of ceria and suggests the existance of a synergystic relationship between ceria and tin which includes an increased oxygen exchange rate for ceria, which leads to increased cell performance, and a stabilization of tin as a molten metal on ceria nanoparticles. Ex situ characterizations of the tin/Ceria-SDC composites are in progress to understand the nature of this interaction and to evaluate the use of Tin/Ceria based materials with alternative fuels.
 C. Chatzichristodoulou, P.T. Blennow, M. Sogaard, P.V. Hendriksen, M.B. Mogensen in: A. Trovarelli, P. Fornasiero (Eds.), Catalysis by ceria and related materials, Imperial College Press, London, (UK), 2013, pp. 623-758.
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