Structural and microstructural stability of ceria – gadolinia electrolyte exposed to reducing environments of high temperature fuel cells

Abstract

Doped CeO₂ is widely used in intermediate temperature solid oxide fuel cells (500–650 °C) due to its high ionic conductivity, low reactivity to other cell components and ability to facilitate charge transfer reactions at the electrode/electrolyte interface. However, on exposure to hydrogen above 650 °C doped cerates can be reduced leading to catastrophic microstructure failure and loss of mechanical integrity. The effect of other fuels such as C and CO on the stability of ceria based electrolytes remains largely unexplored despite the increased significance in developing fuel cells that operate on these fuels. In this paper a systematic investigation has been carried out on the effect of carbon monoxide on the electrical conductivity, ionic transport, crystal structure and microstructure of Ce_0.8Gd_0.2O_2−x, with particular emphasis on the mechanisms of reduction and the long term stability of the material for use in a direct carbon fuel cell (DCFC) where this material will be exposed to a reducing environment containing little or no hydrogen. These investigations have been carried out at temperatures typically found during the operation of a DCFC (800 °C) and the results have been compared with similar investigation carried out in dry hydrogen atmosphere. A wide range of techniques including synchrotron X-ray powder diffraction, high resolution transmission and scanning electron microscopy, four-probe DC conductivity measurements and electrochemical impedance analysis have been used to investigate the structural, microstructural and electrical properties of Ce_0.8Gd_0.2O_2−x exposed to the operating environments of a DCFC.