Molten Metal Anodes for Direct Carbon-Solid Oxide Fuel Cells

The aim of this thesis was to enable the direct utilization of solid carbonaceous fuels like coal and biomass, in solid oxide fuel cells (SOFC). Since SOFCs are based on ceramic oxygen-ion conducting electrolytes, it is possible, in principle, to generate electricity by the direct oxidation of solid fuels in these fuel cells. The electrochemical conversion of these fuels can realize high efficiencies in power generation and decrease environmental impact by allowing easy CO2 capture and minimizing NOx and SOx emissions. The main challenge in this technology involves finding an anode system to enable the facile transfer of oxygen from the SOFC electrolyte to the solid fuel. In this study, several molten-metal/metal-oxide systems were examined as anodes for such direct carbon fuel cells (DCFC). The molten metal candidates tested were, Tin(Sn), Bismuth(Bi), Indium(In), Lead(Pb) and Antimony(Sb). The oxygen transport properties of these molten metal systems were studied by placing them in direct contact with the SOFC electrolyte. The open circuit voltage (OCV) related to the anode oxidation reaction and its contribution to the cell resistance were the two major considerations for anode viability. Characterization of the electrochemical oxidation of the metal at the electrolyte interface showed that, metals like Sn and In had high OCVs but exhibited a time-dependent increase in non-ohmic resistance due to formation of solid oxide layers of their high-melting oxides, which blocked further ion transfer. Bi had low resistances because of its ionically conductive oxide, but had a low OCV. The key anode characteristic for low resistance was observed to be a low melting oxide from the study of Pb and Sb. Sb was the most intriguing because it had a reasonable OCV and oxidized most types of solid fuels below 973 K. High power DCFCs based on molten Sb anodes were demonstrated using various types of solid fuels at stable power levels for over 250 h at 973 K. These long-term stability tests revealed thinning of scandia-stabilized zirconia electrolytes, but no thinning was observed using yttria-stabilized zirconia. Hence, efficient and long-term stable DCFCs using molten Sb are possible with the correct choice of electrolyte material