Effect of tri- and tetravalent metal doping on the electrochemical properties of lanthanum tungstate proton conductors.

Rare-earth tungstates (La(28-y)W(4+y)O(54+δ)□(2-δ)) have attracted attention recently because of their relatively high proton-electron conductivity and high stability in a CO2 environment. Since doping on the tungsten-site may increase the conductivity, a new series of compounds with composition La(5.5)W(1-x)M(x)O(11.25-δ) (M = Al, Ti and Zr; x = 0, 0.05 and 0.10) have been investigated. The crystal structure of these materials has been studied using X-ray and time-of-flight neutron powder diffraction by Rietveld analysis. The concentration of oxygen vacancies for hydration in the structure has been indirectly determined by thermogravimetric analysis, and the total conductivity in several pO2, pH2O and pD2O atmospheres has been studied by impedance spectroscopy. An increase in the conductivity is observed, ranging from 4.1 mS cm(-1) for the undoped sample to 9.2 mS cm(-1) for La(5.5)W(0.9)Ti(0.1)O(11.25-δ), in wet N2 at 800 °C.

Novel microstructural strategies to enhance the electrochemical performance of La0.8Sr0.2MnO3-δ cathodes.

Novel strategies based on spray-pyrolysis deposition are proposed to increase the triple-phase boundary (TPB) of La0.8Sr0.2MnO3-δ (LSM) cathodes in contact with yttria-stabilized zirconia (YSZ) electrolyte: (i) nanocrystalline LSM films deposited on as-prepared YSZ surface; (ii) the addition of poly(methyl methacrylate) microspheres as pore formers to further increase the porosity of the film cathodes; and (iii) the deposition of LSM by spray pyrolysis on backbones of Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), and Bi1.5Y0.5O3-δ (BYO) previously fixed onto the YSZ. This last method is an alternative to the classical infiltration process with several advantages for large-scale manufacturing of planar solid oxide fuel cells (SOFCs), including easier industrial implementation, shorter preparation time, and low cost. The morphology and electrochemical performance of the electrodes are investigated by scanning electron microscopy and impedance spectroscopy. Very low values of area specific resistance are obtained, ranging from 1.4 Ω·cm(2) for LSM films deposited on as-prepared YSZ surface to 0.06 Ω-cm(2) for LSM deposited onto BYO backbone at a measured temperature of 650 °C. These electrodes exhibit high performance even after annealing at 950 °C, making them potentially suitable for applications in SOFCs at intermediate temperatures.

Proton conductors based on alkaline-earth substituted La(28-x)W(4+x)O(54+3x/2).

Lanthanum tungstates, "La6WO12", are mixed ion proton-electronic conductors with very interesting properties for technological applications and better phase stability compared to alkaline earth perovskites. A new series of compounds La(27.04-x)M(x)W(4.96)O(55.44-x/2□8.56+x/2) (M = Ca(2+), Sr(2+) and Ba(2+)) are investigated with the aim of increasing the concentration of oxygen vacancies and studying their effects on the structure and transport properties. The materials have been studied by high-resolution laboratory X-ray powder diffraction and scanning electron microscopy combined with energy dispersive spectroscopy (EDS). High temperature X-ray powder diffraction and thermal analysis in wet and dry N2 gas did not show any evidence of phase transition up to 800 °C. The total conductivity was studied by impedance spectroscopy under dry and wet atmospheres and as a function of the oxygen partial pressure. The electronic contribution to the conductivity was determined by the Hebb-Wagner polarization method. The generation of extrinsic vacancies in the lattice with alkaline earth doping leads to a decrease of the ionic conductivity for high doping level, suggesting a proton trapping mechanism.