Defects and transport in Ce-doped La27W5O55.5

Abstract

Non-perovskite structured oxides are the promising candidates for fuel cell and/or gas separation membrane applications. Some of them exhibit mixed proton-electron conductivity and are stable towards CO2 and SO2/SO3 containing atmosphere. Rare – earth tungstates such as La28-xW4+xO54+1,5xv2-1,5x attracted attention recently for their significant protonic conductivity at elevated temperatures. It was therefore of interest to study the conductivity of these materials upon substitution of La3+ with Ce4+ in LWO. Lanthanum tungstate with a nominal composition La27W5O55.5v0.5 was doped with 2 mol% and 5 mol% CeO2. These compounds were synthesized by a wet-chemical method, calcined at 1000 °C for 11 hours and sintered at 1500 °C for 5-7 hours. The crystal structure was characterized by XRD. The microstructure and the ratio of the cations were studied by SEM and EPMA, respectively. The defect structure and the transport properties of 2%Ce – and 5%Ce- doped La27W5O55.5 have been studied by means of thermogravimetric measurements and electrical measurements as a function of temperature, oxygen vapor pressure and water vapor pressure in the temperature range 1000 – 300 °C. In order to study the water uptake of the material after the donor doping and measure the standard molar hydration thermodynamic properties, thermogravimetric measurements were performed on 2Ce-LWO. From the total conductivity measurements it was observed that the compounds exhibit mixed ionic-electronic conductivity. The total conductivity of both samples is predominated by protons under wet atmospheres below ∼ 800 °C and the protonic conductivity reached a maximum of ∼1⨯〖10〗^(-3) S/cm and ∼3⨯〖10〗^(-4) S/cm above 800 °C for 2Ce-LWO and 5Ce-LWO, respectively. At high temperatures both samples exhibit n-and p-type electronic conduction under reducing and oxidizing conditions respectively. Donor doping decreases the ionic contribution of La27W5O55.5, whereas it enhances the n-type electronic conductivity for both samples, and it is more significant with the higher level of donor dopant. Impedance spectroscopy was conducted at temperature range of 300 - 1000 °C. The measurements demonstrated relatively resistive grain boundaries under oxidizing conditions, which increases with increasing level of the dopant. Under reducing conditions no grain boundary contribution was detected. This difference was suggested to be due to a positive space charge layer, which was depleted under reducing conditions. EPMA analysis revealed formation of La6W2O15 as a secondary phase. Understanding of this behavior was supported by existence of La1 and La2 sites with different coordination number, where some La2 sites are donor substituted by W, forming intrinsic positive defect, the concentration of which is determined by the crystal structure of La28-xW4+xO54+1,5xv2-1,5x. (see subsection 3.1.1). The formation of the secondary phase may be formed due to donor substitution of both La and WLa2 sites in La27W5O55.5v0.5. This phase was not refined by Rietveld method, but the peaks were close to those reported earlier. The derived defect structure was used as a basis to model the conductivity and the thermogravimetry data to extract thermodynamic and transport parameters. This approach gave a hydration enthalpy and entropy in the range of –105±5 kJ/mol and –123±5 J/molK, respectively, varying slightly between two experimental approaches. The thermogravimetric parameters extracted from the thermogravimetric analysis were in agreement with the values extracted from the conductivity measurements. The enthalpy of mobility of protons and oxide ions for both compounds was in good agreement with reported data, of 60±5 kJ/mol and ∼ 90±10 kJ/mol respectively.