Nanocrystalline, mesoporous NiO/Ce0.9Gd0.1O2−δ thin films with tuned microstructures and electrical properties: in situ characterization of electrical responses during the reduction of NiO

Abstract

Crystalline, mesoporous NiO/GDC thin films with thicknesses ranging from 50 nm to 250 nm were synthesized through templated sol–gel chemistry coupled with the dip-coating process and heat-treatment in air. The thin films' microstructure is composed of two interpenetrated networks made of mesopores and inorganic components. Efficient coupling between the temperature and the NiO volume fraction (vol%) allows tuning of both the pore size with dimensions ranging from macro- to meso-size and the NiO or GDC crystallite size with a diameter below 10 nm. X-ray diffraction and impedance spectroscopy performed in 10% H2 in Ar allowed the in situ study of the reduction process of NiO to metallic Ni. Coalescence of Ni particles generated by the reduction step and percolation phenomena controls the resulting conductivity of the final materials. Thin films with an initial content of 50 vol% of NiO exhibit lower electrical properties compared to those with an initial content of 70 vol% NiO. Electrical properties in a reducing atmosphere were also studied as a function of microstructure such as the pore dimension and the thickness of the pore wall. Excellent electrical properties are obtained for Ni70/GDC30-porous thin films synthesized with the block-co-polymer PS40-PEO36 that have a final conductivity of 9 x 104 S m−1 at 500 °C. These mesostructured nanocomposite thin films exhibit a connected pore network that ensures good gas diffusion and good particle–particle contact for GDC and Ni, which gives satisfactory electrical properties. These films have all the attributes to be used as anodes in micro-SOFCs.