Green synthesis and characterisation of nanocrystalline NiO-GDC powders with low activation energy for solid oxide fuel cells

Abstract

This work reports the preparation of nanocrystalline Ni-Gd0.1Ce0.9O1.95 (NiO-GDC) anode powders using a novel single-step co-precipitation synthesis method (carboxylate route) based on ammonium tartrate as a low-cost green precipitant. The thermogravimetric analysis (TGA) of the synthesised powder showed the complete calcination/crystallisation of the resultant precipitates to take place at 500 °C. The prepared NiO-GDC powder was coated on a GDC electrolyte disc and co-sintered at 1300 °C. A mixture of La0.6Sr0.4Co0.2Fe0.8O3−δ and GDC was used as the cathode material and subsequently coated onto the anode-electrolyte bilayer, resulting in the fabrication of a NiO-GDC|GDC|La0.6Sr0.4Co0.2Fe0.8O3−δ-GDC cell. The crystallite size of both NiO and CeO2 phases were estimated using the X-ray powder diffraction (XRD) profiles and were calculated to be ~14 nm. Applied H2 temperature-programmed reduction (H2-TPR) analysis indicated a synergetic effect among different anode composites' constituents, where an intense interaction between the dispersed NiO nanocrystalline particles and the GDC crystallite phase had weakened the metal-oxygen bonds in the synthesised anode composites, resulting in a strikingly high catalytic activity at temperatures as low as 300 °C. The electrochemical impedance spectroscopy (EIS) and the electrochemical performance of the fabricated cells were measured over a broad range of operating temperatures (500–750 °C) and H2/Ar-ratios of the anode fuel (e.g. 100%–15%). Quantitative analysis from the EIS data and the application of the distribution of relaxation times (DRT) method allowed for the estimation of the activation energies of the anodic high and intermediate frequency processes that were 0.45 eV and 0.76 eV, respectively. This is the first report of a NiO-GDC synthesis, where a considerable improvement in activation energy is observed at the low-temperature region. Such low activation energies were later associated with the adsorption/desorption process of water molecules at the surface of NiO-GDC composite, indicating a high activity towards hydrogen oxidation.