The Impact of Fe Addition on the Electronic Conductivity of Gadolinium Doped Ceria

Abstract

Gd-doped ceria with transition metal oxide co-doping is especially interesting for catalytical applications in the intermediate temperature range between 400–600°C. Ce0.9-xFexGd0.1O2-δ samples with x = 0.0 to 0.05 were produced via a sol-gel synthesis route. XRD measurements proved the materials to be single phase with fluorite structure, but electron microscopy investigations showed enrichment of Gd and Fe at the grain boundaries and the existence of small-scale secondary phases for 5 mol% Fe. The effect of Fe co-doping on the ionic and electronic conductivity was monitored using impedance spectroscopy and DC polarization-relaxation measurements applying ion blocking micro contacts (Hebb-Wagner setup). Already the addition of 1 mol% Fe raised the total conductivity of the material by two decades (about 10−4 S/cm at 750°C compared to 5·10−6 S/cm for Ce0.9Gd0.1O2-δ). In addition, already for 1 mol% Fe, but very clearly for 2 and 5 mol% Fe addition, a temperature and oxygen partial pressure dependent maximum in the p-type conductivity has been found, which is attributed to the Fe2+/3+ redox reaction and a related small polaron hopping process.