Study of glycine nitrate precursor method for the synthesis of gadolinium doped ceria (Ce0.8Gd0.2O1.90) as an electrolyte for intermediate temperature solid oxide fuel cells

Abstract

The effects of the Glycine Nitrate Precursor (GNP) synthesis parameters, viz. fuel to oxidant molar ratio, oven temperature during combustion, calcination temperature, and calcination dwell time, on the structural, morphological and electrical properties of nanocrystalline gadolinium doped ceria (GDC) are reported. X-ray diffraction studies confirmed the FCC crystal structure with a lattice parameter of 5.4230 (±0.1) Å under all conditions. Raman spectroscopy confirmed the FCC crystal structure with F_2g symmetry. Depending upon preparation conditions, crystallite sizes between 15–40 nm were obtained, which was confirmed by TEM images. Good crystallinity, lower lattice strain and optimal crystallite size distribution, which result in high ionic conductivity at a stoichiometric and slightly above stoichiometric F/O ratio (1.7 to 2.5), indicated that a high adiabatic reaction temperature is of paramount importance. An oven temperature below 250 °C during the combustion reaction provides an insufficient heat of formation and causes a slow combustion reaction resulting in lattice strain and low crystallinity. Higher oven temperatures also caused low crystallinity and smaller crystallite size, indicating that 250 °C is the optimal oven temperature. A calcination temperature of 900 °C provided high crystallinity, low stress and high ionic conductivity, probably because of the optimal number of oxygen vacancies. The appropriate calcination dwell time was 8 h, even though its influence on the ionic conductivity was low. A good density of 94% of the theoretical density was obtained for pellets sintered at the relatively low sintering temperature of 1350 °C for 8 h at a F/O ratio of 1.7, oven temperature of 250 °C, calcination temperature of 800 °C and 8 h dwell time. These dense GDC pellets had an average grain size of 0.4–3 μm and showed a promising ionic conductivity of 9.7 × 10⁻³ S cm⁻¹ at 700 °C. The activation energy calculated from Arrhenius plots revealed values as low as 0.19 ± (0.01) eV, which is also the lowest reported value for GDC.