Uso de conductores iónicos de tipo óxido para aplicaciones energéticas. Introducción a las pilas de combustible y las membranas de separación de gases.

Los conductores iónicos, y los conductores mixtos de iones y electrones tienen el potencial de servir como membranas en procesos relevantes en tecnologías avanzadas de generación de energía limpia y respetuosa con el medio ambiente. Las pilas de combustible conductoras de ión óxido (solid oxide fuel cells, del inglés SOFCs) o conductoras de protones (proton conducting SOFCS, PC-SOFCs) son algunos ejemplos de generación de energía más limpia en comparación con la tecnología actual, basada en la quema de combustibles fósiles. Los conductores mixtos de protones y electrones se pueden utilizar como membranas de separación de gases a elevada temperatura para la purificación de ese gas, y éstas se pueden integrar en reactores de membrana catalítica o bien implementarse en tecnologías de captura y almacenamiento de CO2Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Optimisation of growth parameters to obtain epitaxial Y-doped BaZrO3 proton conducting thin films

We hereby report developments on the fabrication and characterization of epitaxial thin films of proton conducting Y-doped BaZrO (BZY) by pulsed laser deposition (PLD) on different single crystal substrates (MgO, GdScO, SrTiO, NdGaO, LaAlO and sapphire) using Ni-free and 1% Ni-containing targets. Pure, high crystal quality epitaxial films of BZY are obtained on MgO and on perovskite-type substrates, despite the large lattice mismatch. The deposition conditions influence the morphology, cell parameters and chemical composition of the film, the oxygen partial pressure during film growth being the most determining. Film characterization was carried out using X-ray diffraction, transmission electron and atomic force microscopies, wavelength dispersive X-ray spectroscopy and angle-resolved X-ray photoelectron spectroscopy. All films show a slight tetragonal distortion that is not directly related to the substrate-induced strain. The proton conductivity of the films depends on deposition conditions and film thickness, and for the optimised conditions its total conductivity is slightly higher than the bulk conductivity of the target material (3 mS/cm at 600 °C, in wet 5% H/Ar). The conductivities are, however, more than one order of magnitude lower than the highest reported in literature and possible reasoning is elucidated in terms of local and extended defects in the films

Low-temperature solid-oxide fuel cells based on proton-conducting electrolytes

The need for reducing the operating temperature of solid-oxide fuel cells (SOFCs) imposed by cost reduction has pushed significant progress in fundamental understanding of the individual components, as well as materials innovation and device engineering. Proton-conducting oxides have emerged as potential alternative electrolyte materials to oxygen-ion conducting oxides for operation at low and intermediate temperatures. This article describes major recent developments in electrolytes, electrodes, and complete fuel cell performance for SOFCs based on proton-conducting electrolytes. Although the performance of such fuel cells is still relatively modest, significant improvements in the power density output have been made during the last couple of years, and this trend is expected to continu

Comparison of the local and the average crystal structure of proton conducting lanthanum tungstate and the influence of molybdenum substitution

We report on the comparison of the local and average structure reported recently for proton conducting lanthanum tungstate, of general formula La28−xW4+xO54+δv2−δ, and the impact of molybdenum-substitution on the crystal structure of the material. Partial replacement of W with 10 and 30 mol% Mo is investigated here, i.e. La27(W1−xMox)5O55.5 for x = 0.1 and 0.3. This study addresses the interpretation and the description of a disordered cation and anion sublattice in this material, which enables the understanding of the fundamental properties related to hydration, transport properties and degradation in lanthanum tungstate. The report shows that Mo-substituted lanthanum tungstate is a promising material as a dense oxide membrane for hydrogen separation at intermediate temperatures.Peer reviewe

Comparison of the local and the average crystal structure of proton conducting lanthanum tungstate and the influence of molybdenum substitution

We report on the comparison of the local and average structure reported recently for proton conducting lanthanum tungstate, of general formula LaWOv, and the impact of molybdenum-substitution on the crystal structure of the material. Partial replacement of W with 10 and 30 mol% Mo is investigated here, i.e. La(WMo)O for x = 0.1 and 0.3. This study addresses the interpretation and the description of a disordered cation and anion sublattice in this material, which enables the understanding of the fundamental properties related to hydration, transport properties and degradation in lanthanum tungstate. The report shows that Mo-substituted lanthanum tungstate is a promising material as a dense oxide membrane for hydrogen separation at intermediate temperatures

On the development of proton ceramic fuel cells based on Ca-doped LaNbO4 as electrolyte

We review the key properties of Ca-doped LaNbO4 (LCNO) and related materials for use as proton conducting electrolyte, from the discovery of its proton conductivity, through developments of synthesis and production, anodes and cathodes, to demonstrations of it in laboratory fuel cells, exemplified through published and new results from a range of collaborative projects in Norway and Europe the last years. Important new knowledge has been obtained concerning the reliable synthesis of line compounds like LaNbO4, B-site doping and co-doping, grain boundary resistance, phase transformations, thin film fabrication, and electrodes for proton conducting ceramics. However, as LCNO still requires films too thin for reliable fabrication of large areas and operation over prolonged times, and since cathodes with acceptable electrocatalytic performance have not been identified, one must conclude that high-power fuel cells (and steam electrolysers) based on LCNO are not viable.Funding from the European Union (FP7 ENERGY Project EFFIPRO – Grant Agreement 227560) and The Research Council of Norway (notably projects NANOMAT/NANIONET 182065/S10, NANOMAT/NANOPCFC 182090/S10, and RENERGI/STACKPRO 185322/S30) is kindly acknowledged, as are the collaboration and inputs from all contributing partners.Peer Reviewe

Reduced long term electrical resistance in Ce/Co-coated ferritic stainless steel for solid oxide fuel cell metallic interconnects

The present study is focused on the influence of selected coatings on a ferritic stainless steel (Sanergy HT™, Sandvik) on the evolution of the area specific resistance (ASR) as a function of time at high temperature. The samples are exposed in humidified air at 850 °C for up to 4200 h. It combines long-term ASR measurements with the thermogravimetric behavior and microstructural analysis of the cross sections by scanning electron microscopy. The results show that uncoated and Co-coated Sanergy HT™ exhibit similar oxidation kinetics and comparable ASRs, while a combined Ce/Co coating improves oxidation resistance and, consequently, reduces the ASR significantly. Other reports have earlier shown that Co- (and Ce/Co)-coated Sanergy HT™ reduces the evaporation of volatile chromium species. Overall, the study indicates that Ce/Co-coatings will render substantially improved performance for ferritic steel interconnects for solid oxide fuel cells.This work was financed under the NaCo–SOFC project- “Nano Coatings for Solid Oxide Fuels Cells” funded by the Top-Level Research initiative.Peer Reviewe

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

et al.Rare-earth tungstates (LaWO□) have attracted attention recently because of their relatively high proton-electron conductivity and high stability in a CO environment. Since doping on the tungsten-site may increase the conductivity, a new series of compounds with composition LaWMO (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 pO, pHO and pDO atmospheres has been studied by impedance spectroscopy. An increase in the conductivity is observed, ranging from 4.1 mS cm for the undoped sample to 9.2 mS cm for LaWTiO, in wet N at 800°C.This work was supported by MINECO through the MAT2013-41836-R research grant (Spain) which is co-funded by FEDER. J. M. P. V. thanks Andalucía Tech, and A. M. and N. M. thank the Research Council of Norway (Grant 219731) for the funding. L. dS. G. thanks to the Spanish Ministry of Education, Culture and Sports for her FPU studentship.Peer Reviewe

Optimisation of growth parameters to obtain epitaxial Y-doped BaZrO3 proton conducting thin films

We hereby report developments on the fabrication and characterization of epitaxial thin films of proton conducting Y-doped BaZrO (BZY) by pulsed laser deposition (PLD) on different single crystal substrates (MgO, GdScO, SrTiO, NdGaO, LaAlO and sapphire) using Ni-free and 1% Ni-containing targets. Pure, high crystal quality epitaxial films of BZY are obtained on MgO and on perovskite-type substrates, despite the large lattice mismatch. The deposition conditions influence the morphology, cell parameters and chemical composition of the film, the oxygen partial pressure during film growth being the most determining. Film characterization was carried out using X-ray diffraction, transmission electron and atomic force microscopies, wavelength dispersive X-ray spectroscopy and angle-resolved X-ray photoelectron spectroscopy. All films show a slight tetragonal distortion that is not directly related to the substrate-induced strain. The proton conductivity of the films depends on deposition conditions and film thickness, and for the optimised conditions its total conductivity is slightly higher than the bulk conductivity of the target material (3 mS/cm at 600 °C, in wet 5% H/Ar). The conductivities are, however, more than one order of magnitude lower than the highest reported in literature and possible reasoning is elucidated in terms of local and extended defects in the films