Characteristics of Molten Alloys as Anodes in Solid Oxide Fuel Cells

Abstract

Molten alloys with 50-mol% In-Sb, Sn-Sb, Sb-Bi, and Sb-Pb were examined as anodes for solid oxide fuel cells at 973 K. The cells were operated in the battery mode, without added fuel, in order to understand the oxidation characteristics of these alloys at electrolyte interfaces. Cells using 50-mol% In-Sb and Sn-Sb mixtures exhibited open-circuit voltages (OCV) of 1.0 and 0.93 V, values that are identical to that of cell with pure In and Sn respectively. Also similar to the pure In and Sn anodes, the impedances of these cells were initially low but increased dramatically after drawing a small amount of charge, implying formation of In2O3 and SnO2 layers at the electrolyte interface. The 50-mol% Sb-Bi cell had an OCV of 0.73 V initially, close to the OCV observed with pure Sb. The OCV remained constant until a charge identical to that required for oxidation of all the Sb had been passed, after which the OCV dropped to 0.43 V, similar to the value for pure Bi. SEM analysis of the cell after conversion of the Sb showed two distinct phases, with metallic Bi at the bottom and Sb2O3 at the top. The electrochemical oxidation of 50-mol% Sb-Pb alloys exhibited an OCV that changed continuously with conversion, from 0.73 V initially to 0.67 V following the addition of charge corresponding to oxidation of 120% the Sb. The total cell impedance remained low for this entire period. EDS measurements on the sectioned Sb-Pb cell suggested that both Sb and Pb were oxidized simultaneously to form a mixed oxide of Pb and Sb.