Nanoparticulate ceria–zirconia anode materials for intermediate temperature solid oxidefuelcells using hydrocarbon fuels

Abstract

Solid Oxide Fuel Cells (SOFCs) represent an attractive technology for the conversion of chemical to electrical energy because of their high efficiencies and low environmental impact, and because of the useful, high grade heat also generated. Direct utilisation of hydrocarbons in SOFCs would contribute to the more sparing utilisation of remaining fossil fuel reserves. In the longer term, this technology could be extended to work with more sustainable biofuel and waste-derived feedstocks. In this work, nanoparticulate Ceria–Zirconia mixed oxides, Ce_1−xZr_xO₂ (x = 0.1, 0.25, 0.5, 0.75 and 0.9), were studied with a view to their application as anode materials in intermediate temperature (IT) SOFCs using hydrocarbon fuels. Impedance spectra were recorded in symmetrical cells under reducing conditions using gadolinium-doped ceria (GDC) as the electrolyte material. The spectra were analysed in terms of a double fractal finite length Gerischer impedance model. The model parameters were found to have monotonic dependences on temperature and more complex relationships with respect to Zr content. Diffusion-related processes and the electrochemical reaction were fastest for intermediate Zr contents while the chemical exchange reaction rate increased with decreasing Zr content. As a result, anode catalysts with 10 and 25 mol% Zr showed the lowest polarisation resistances of only 0.17 and 4.52 Ω cm² at 700 °C in humidified 5% H₂ and humidified 5% CH₄, respectively. These values represented an approximately two-fold improvement on the pure ceria electrode. This performance compares very favourably with the currently most promising candidate anode materials applied in SOFCs for use with hydrocarbon fuels.