A Comparison of Molten Sn and Bi for Solid Oxide Fuel Cell Anodes

Molten Sn and Bi were examined at 973 and 1073 K for use as anodes in solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes. Cells were operated under "battery" conditions, with dry He flow in the anode compartment, to characterize the electrochemical oxidation of the metals at the YSZ interface. For both metals, the open-circuit voltages (OCVs) were close to that expected based on their oxidation thermodynamics, ~0.93 V for Sn and ~0.48 V for Bi. With Sn, the cell performance degraded rapidly after the transfer of approximately 0.5-1.5 C/cm2 of charge due to the formation of a SnO2 layer at the YSZ interface. At 973 K, the anode impedance at OCV for freshly reduced Sn was approximately 3 Ω cm2 but this increased to well over 250 Ω cm2 after the transfer of 1.6 C/cm2 of charge. Following the transfer of 8.2 C/cm2 at 1073 K, the formation of a 10 µm thick SnO2 layer was confirmed by scanning electron microscopy. With Bi, the OCV anode impedance at 973 K was less than 0.25 Ω cm2 and remained constant until essentially all of the Bi had been oxidized to Bi2O3. Some implications of these results for direct carbon fuel cells are discussed.