Discussion on the feasibility of SOFC anodes based on CeO2-ZrO2 mixed oxides.

Abstract Several papers have showed that ceria is a good anode electrocatalyst for the direct oxidation of methane and light hydrocarbons when the electrode is obtained by infiltration (1,2). The thermal stability and the reducibility of ceria can be improved by doping its lattice with zirconia (3) and some recent studies reported that ceria-zirconia compositions show similar or superior activity to ceria when used as anode components (4). In this work dense pellets with a high (Ce0.80Zr0.20O2, CZ80), medium (Ce0.50Zr0.50O2, CZ50) and low (Ce0.20Zr0.80O2, CZ20) amount of ceria was prepared and their structural, chemical and electrical properties characterized by temperature programmed reduction (TPR) studies, X-ray diffraction technique (XRD), Raman spectroscopy and electrochemical impedance spectroscopy (EIS) before and after redox treatments. Reduction and oxidation cycles, respectively at 1273 K and 873K, promoted the reduction of all compositions investigated at a lower temperature with the following rank CZ80>CZ50>CZ20. The same rank was observed for the electrical conductivity of these materials, however, an enhancement of the electrical conductivity after the redox treatments was observed only for the intermediate composition CZ50. In this case the increase of conductivity by an order of magnitude could be correlated with the significant structural changes occurring in the reduction-oxidation process (5).Therefore, this study allows us to draw a series of preliminary information in the perspective of the applications of these materials as anode components into SOFCs. Among the compositions investigated the CZ80 would be the most suitable candidate because of its better electrical and redox properties with respect to the other compositions. However, the effect of redox cycles on this material may limit its use since as a result of these treatments the material undergoes large mechanical stress that could cause the destruction of the anode compartment. An alternative to this composition could be CZ50, in fact its catalytic properties and electrical properties are promoted as a result of redox treatment even at low temperature and this could have a positive impact on the durability of the cells. The CZ20 is rather less promising material for such application, since despite its redox properties it shows a lower conductivity. Further investigations on button cells prepared by impregnation are in progress to assess the behavior of these materials under SOFC operating conditions.