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ABSTRACT Gadolinia-doped ceria electrolyte is being investigated as an alternative electrolyte for solid oxide fuel cells operating at temperatures below 700'C. Measurements were made to determine the effects that small additions of Ca, Pr and Fe to gadolinia-doped ceria have on the bulk and grain boundary conductivities. These small additions (1-2%) of dopant did not alter the bulk conductivity significantly but resulted in a large increase in the grain boundary conductivity. The grain boundary conductivity was similar for all three electrolyte compositions. These results are explained by the possible formation of second phases at the grain boundary, which can incorporate impurity elements. The electronic conductivity in these electrolyte materials was also evaluated, but it was found that the Ca, Pr and Fe additions do not reduce the electronic conductivity compared to a standard Gd-doped ceria sample. INTRODUCTION At present, zirconia-based materials are the only electrolytes being evaluated in large-scale tests for the solid oxide fuel cell (SOFC)[1]. Fuel cell systems incorporating zirconia electrolytes operate at high temperatures (1000°C) to achieve acceptable current densities and useful power output. With the rapid development of the polymer membrane fuel cell in the last few years, the cost of the SOFC must be reduced for it to remain competitive as a future renewable energy source. This paper reports on a new electrolyte composition that might help

achieve that goal. The majority of the total cost for a complete SOFC system comprises the balance-of-plant materials and their fabrication. In particular, the high operating temperatures necessitate the use of expensive ceramic components for parts such as ducting, air and fuel injection tubes, and interconnects. As the temperature is lowered to around 800'C, ferritic steels can be utilised; however, although fabrication of components is somewhat easier than with ceramics, their material cost is still high. Thus, it is desirable to lower the operating temperatures below 700'C, where readily available stainless steels can be used. At these relatively low temperatures, zirconia-based electrolytes are not sufficiently conducting to provide a useful power output. An alternative electrolyte based on ceria has been studied for many years as a replacement for the zirconia electrolytes at low temperatures. The ceria-based electrolyte is attractive because of its higher ionic conductivity in air. A key problem with ceria-based electrolytes, is the reducibility of the Ce ion when exposed to the reducing conditions experienced on the anode side of the SOFC. More recent studies on ceria-based electrolytes have concentrated on maximising the ionic conductivity to enable the electrolyte to operate at lower temperatures, with a current density similar to that of zirconia [2]. Steele [3] reported that the problem of the cerium ion reduction, decreases as the temperature is decreased and, below about 650'C, becomes small enough to permit the use of ceria electrolytes in SOFC