Conductivity measurements of molten metal oxides and their evaluation in a Direct Carbon Fuel Cell (DCFC)

Abstract

ABSTRACT Since Direct Carbon Fuel Cell (DCFC) technology is in a beginning stage, emphasis should be laid on addressing the fundamental aspects. A molten electrolyte is required to facilitate ionic contact between solid carbon fuel and electrolyte in a DCFC Three different metal oxide electrolytes (Bi2O3, V2O5, and TeO2) have been chosen based on their ability to form stable liquids in air at higher temperatures. Conductivity data beyond their melting points was not readily available for most of the metal oxides. Conductivity studies concerning the above mentioned molten metal oxides have been thoroughly investigated in this study. A four probe measurement method using an AC milliohm-meter at 1 KHz validated by Electrochemical Impedance Spectroscopy (EIS) was used to acquire the conductivity data because of its accuracy when compared to two probe measurement widely used in literature. Also, a DC ohmmeter was used to check whether these metal oxides exhibit electronic conductivity. Experimental results corresponding to the accuracy of DC ohmmeter showed that, it accurately detected the electronic component of the electrolyte. These conductivity studies revealed that the molten oxide electrolytes exhibit high ionic conductivity, in particular, beyond their melting points. Of all the three metal oxides, Bi2O3 demonstrated high ionic conductivity but with minor stability issues under CO2 environment. Under CO2 environment Bi2O3 showed a slight decrease in the conductivity. EDX analysis revealed an increase in carbon content by 50 percent per one mole of bismuth which can be attributed to possible carbonate formation. V2O5 exhibited lower ionic conductivity when compared to Bi2O3 but had the advantage of lower cost and higher abundance. Also, the higher volumetric expansion of V2O5 upon cooling from its melting point i.e. 690˚C caused the alumina crucible containing the metal oxide to break leading to leakage problems. Investigating further, quartz was found to be the best material for handling V2O5. Conductivity of TeO2was measured using a quartz crucible since it formed a Al2TeO6 complex by reacting with alumina. Ionic conductivity of TeO2 was almost as high as Bi2O3. Performance of these molten metal oxides in a DCFC has been evaluated. Dense YSZ solid electrolyte and porous LSM cathode used in the DCFC have been prepared by high temperature sintering. Efficiency of Bi2O3 based DCFC was mainly limited by unwanted bismuth ion reduction to bismuth metal at the carbon anode oxidation potential. Shorting of the LSM cathode by molten V2O5 leaking out of the crucible at high temperatures was found to be major concern corresponding to the performance of V2O5 and TeO2 based DCFC. Based on these results, several fundamental material issues that need to be resolved have been identified.