Temperature-programmed reduction and dispersive X-ray absorption spectroscopy studies of CeO2-based nanopowders for intermediate-temperature Solid-Oxide Fuel Cell anodes

In this work, we study the influence of the average crystallite size and dopant oxide on the reducibility of CeO2-based nanomaterials. Samples were prepared from commercial Gd2O3-, Sm2O3- and Y2O3-doped CeO2 powders by calcination at different temperatures ranging between 400 and 900C and characterized by X-ray powder diffraction, transmission electron microscopy and BET specific surface area. The reducibility of the samples was analyzed by temperature-programmed reduction and in situ dispersive X-ray absorption spectroscopy techniques. Our results clearly demonstrate that samples treated at lower temperatures, of smallest average crystallite size and highest specific surface areas, exhibit the best performance, while Gd2O3-doped ceria materials display higher reducibility than Sm2O3- and Y2O3-doped CeO2

Electrochemical behavior of nanostructured La0.8Sr0.2MnO3 as cathodes for solid oxide fuel cells

LaSrMnO (LSM) is one of the most commonly used cathodes in Solid Oxide Fuel Cells (SOFC). In spite of the fact that nanostructured cathodes are expected to display improved performance, the high operating temperature (∼ 1000°C) of LSM-based SOFCs hinders their stability. In the present work, we have developed nanostructured cathodes prepared from LSM nanotubes of enhanced performance, allowing its use at lower temperatures (∼ 800°C). We observed that our cathodes have qualitative improvements compared with microstructured materials: firstly, the diffusion in the gas phase is optimized to a negligible level and secondly, evidence of ionic conduction is found, which is extremely rare in LSM cathodes. We propose that this important change in the electrochemical properties is due to the nanostructuration of the cathode and deserves further attention, including the exploration of other materials

Oxygen Reduction Mechanisms in Nanostructured La0.8Sr0.2MnO3 Cathodes for Solid Oxide Fuel Cells

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of LaSrMnO (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: (i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and (ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first-principles calculations point toward an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface

Propiedades eléctricas y magnéticas, separación de fases y comportamiento dinámico en manganitas

En esta Tesis se presenta un estudio experimental de las propiedades eléctricas y magnéticas en óxidos de manganeso con valencia mixta también conocidos como manganitas. El trabajo se focaliza en el estudio de la coexistencia intrínseca de distintas fases que presentan algunas manganitas, estado conocido como separación de fases (SF). Este estado está típicamente formado por regiones ferromagnéticas (FM) metálicas y antiferromagnéticas (AFM) con orden de carga (CO) el cual es un tipo de aislante. Realizamos mediciones sobre muestras policristalinas de La0.5Ca0.5MnO3, y de los sistemas La_5/8-yPryCa_3/8MnO_3 y La_5/8-yNdyCa_3/8MnO_3 las cuales constituyen manganitas prototípicas en la problemática de separación de fases. Estudiamos la influencia del campo magnético en La0.5Ca0.5MnO3 a través de mediciones de resistividad eléctrica (r) y magnetización (M), en función de la temperatura y el campo magnético. Realizamos un modelo para explicar el crecimiento de la fase FM inducido por la aplicación de campo magnético, principal responsable de la Magnetorresistencia de Bajo Campo (< 1 Tesla) que presentan las manganitas con SF. Realizamos experimentos en los cuales es posible inducir un crecimiento de las regiones FM y no FM en el rango en que hay SF y observamos que existe un efecto de memoria del campo magnético relacionado con ese crecimiento. El efecto observado está directamente relacionado con el comportamiento dinámico que presenta el estado de SF y con las características del compuesto que se utilice. El carácter dinámico del estado de SF también fue estudiado en La_0.5Ca_0.5MnO_3 a través de la realización de repetidos ciclados térmicos de baja temperatura (30 – 300 K) y mediciones de las propiedades físicas en función del tiempo a temperatura fija. Ambos experimentos evidenciaron cambios asociados con una reducción de la fase FM. Presentamos un modelo para explicar el efecto de ciclados térmicos, según el cual el mismo se origina por un mecanismo a nivel de la interfase entre las regiones FM y CO. Las relajaciones lentas observadas sugieren que el sistema evoluciona a través de una distribución de barreras de energía que separan a los estados FM y CO de forma jerárquica. Estudiamos el rol de la sustitución química en el sistema La_5/8-yNdyCa_3/8MnO_3. Vimos que la sustitución de La por Nd produce una desestabilización del estado FM homogéneo correspondiente al compuesto con y = 0, induciendo SF para dopajes y = 0.3 – 0.4. Para dopajes mayores (y = 0.5 – 0.625) el estado de SF desaparece para dar lugar a un estado AFM – CO homogéneo. Esto está relacionado con que la introducción de iones de Nd, más pequeños que los de La, provoca distorsiones en los ángulos de unión Mn – O – Mn, las cuales favorecen al CO. En el rango de dopajes en que se observa la SF, obtuvimos evidencias de un comportamiento dinámico característico de este estado, previamente reportado en el compuesto La_5/8-yPryCa_3/8MnO_3 (y = 0.4). El comportamiento dinámico de las manganitas con SF es también el responsable de la particular histéresis térmica que presentan las propiedades físicas de algunos de estos compuestos. Realizamos mediciones de r vs. T para distintos valores de corriente aplicada en La_5/8-yPryCa_3/8MnO_3 (y = 0.34) que muestran una reducción de r al aumentar la corriente en el rango en que hay SF. Mostramos fuertes evidencias en favor de que esa reducción, que comúnmente se atribuye a la ruptura del estado de CO, se debe a un artificio que resulta de la combinación de un calentamiento de la muestra con la irreversibilidad de la dependencia de ··vs. T. En la última parte del trabajo, presentamos un modelo termodinámico del estado de SF teniendo en cuenta sus propiedades estáticas y dinámicas. A partir de mediciones de calor específico en La_5/8-yPryCa_3/8MnO_3 con y = 0.4, obtuvimos las energías libres de cada una de las fases que coexisten. El estado de SF se modeló bajo la hipótesis de que el desorden químico y estructural da origen a densidades de energía libre inhomogéneas uniformemente distribuidas en el volumen de la muestra para cada una de las fases. Los cálculos contemplan las características de un estado fuera del equilibrio típicas del estado de SF, para permitir la comparación de mediciones magnéticas con las predicciones del modelo. Finalmente, presentamos un diagrama de fases que incluye las propiedades estáticas y dinámicas del sistema, mostrando la existencia de regímenes bloqueados y no bloqueados característicos del estado de SF.In this thesis we present an experimental study of the electrical and magnetic properties of mixed valent manganese oxides, also known as manganites. Our work is focused on the analysis of the intrinsic coexistence of different phases presented by several manganites, a state called Phase Separation (PS). This state is usually formed by ferromagnetic (FM) metallic and antiferromagnetic (AFM) charge ordered (CO) insulating regions. We have performed our measurements on ceramic samples of La0.5Ca0.5MnO3, and on samples of the La_5/8-yPryCa_3/8MnO_3 and La_5/8-yNdyCa_3/8MnO_3 systems, all of which are prototypical compounds in the problem of PS. We have studied the influence of the magnetic field on La_0.5Ca_0.5MnO_3 through electrical resistivity (r) and magnetization (M) measurements, as a function of temperature and magnetic field. We have developed a model to explain the growth of the FM fraction that can be induced by magnetic field application, main ingredient of the low field (< 1 Tesla) magnetoresistance presented by phase separated manganites. We have performed different experiments in which we could induce the growth of the FM and non FM regions in the temperature range characterized by PS, and we have observed a memory effect of the applied magnetic field, associated with that growth. This memory effect is directly related with the dynamical behaviour of the phase separated state and with the characteristics of the compound under study. The dynamical character of the phase separated state was also studied on La_0.5Ca_0.5MnO_3 by repeatedly cycling the samples at low temperature (30 – 300 K) and through measurements of the physical properties as a function of time at fixed temperature. Both experiments have evidenced changes that can be associated with a reduction of the FM phase. We have developed a model to explain the thermal cycling effect according to which it has its origin in a mechanism that occurs at the FM/CO interface. The slow relaxations observed suggests that the system is evolving hierarchically through a distribution of energy barriers which separates the FM and CO states. We have also studied the role played by chemical subtitution on the La5/8- yNdyCa3/8MnO3 system. We have observed that the homogeneous FM state, characteristic of the y = 0 compound, is destabilized when replacing La ions by Nd ones, while PS is induced for intermediate doping (y = 0.3 – 0.4). For higher doping (y = 0.5 – 0.625) the phase separated state dissapears and an homogeneous AFM – CO develops. This is related with the fact that the introduction of smaller Nd ions, produces distortions on the Mn – O – Mn bond angles, which favour the CO state. In the doping range characterized by PS, we have obtained evidences of a dynamical behaviour typical of this state, previously reported on the La5/8-yPryCa3/8MnO3 (y = 0.4) compound. The dynamical behaviour of manganites with PS is also responsible for the particular thermal hysteresis displayed on its physical properties. We have performed r vs. T measurements for different electrical current values on La_5/8-yPryCa_3/8MnO_3 (y = 0.34) which show a reduction of r while increasing current in the PS range. We show strong evidence that points that this reduction, commonly attributed to melting of the CO state, is a consequence of an artifact which results from the combination of Joule heating and the irreversible ··vs. T dependence. In the last part of this work, we present a thermodynamic model of the PS state accounting for its static and dynamic properties. Through calorimetric measurements on La_5/8-yPryCa_3/8MnO_3 con y = 0.4, the low temperature free energies of the coexisting phases are evaluated. The phase separated state is modeled by free energy densities uniformly spread over the sample volume. The calculations contemplate the out of equilibrium features of the coexisting phase regime, to allow a comparison between magnetic measurements and the predictions of the model. A phase diagram including the static and dynamic properties of the systems is constructed, showing the existence of blocked and unblocked regimes which are characteristics of the phase separated state in manganites.Fil: Sacanell, Joaquín G.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Nanostructured LnBaCo2O6− (Ln = Sm, Gd) with layered structure for intermediate temperature solid oxide fuel cell cathodes

In this work, we present the combination of two characteristics that are beneficial for solid oxide fuel cell (SOFC) cathodic performance in one material. We developed and evaluated for the first time nanostructured layered perovskites of formulae LnBaCo2O6-d with Ln = Sm and Gd (SBCO and GBCO, respectively) as SOFC cathodes, finding promising electrochemical properties in the intermediate temperature range. We obtained those nanostructures by using porous templates to confine the chemical reagents in regions of 200-800 nm. The performance of nanostructured SBCO and GBCO cathodes was analyzed by electrochemical impedance spectroscopy technique under different operating conditions using Gd2O3-doped CeO2 as electrolyte. We found that SBCO cathodes displayed lower area-specific resistance than GBCO ones, because bulk diffusion of oxide ions is enhanced in the former. We also found that cathodes synthesized using smaller template pores exhibited better performance

Temperature-Programmed Reduction and Dispersive X-Ray Absorption Spectroscopy Studies of CeO2-Based Nanopowders for Intermediate-Temperature Solid-Oxide Fuel Cell Anodes

In this work, we study the influence of the average crystallite size and dopant oxide on the reducibility of CeO2-based nanomaterials. Samples were prepared from commercial Gd2O3-, Sm2O3-and Y2O3-doped CeO2 powders by calcination at different temperatures ranging between 400 and 900C and characterized by X-ray powder diffraction, transmission electron microscopy and BET specific surface area. The reducibility of the samples was analyzed by temperature-programmed reduction and in situ dispersive X-ray absorption spectroscopy techniques. Our results clearly demonstrate that samples treated at lower temperatures, of smallest average crystallite size and highest specific surface areas, exhibit the best performance, while Gd2O3-doped ceria materials display higher reducibility than Sm2O3-and Y2O3-doped CeO2.Fil: Bellora, Marina Soledad. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sacanell, Joaquín. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Huck Iriart, Cristián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Soldati, Analía Leticia. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Larrondo, Susana Adelina. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; ArgentinaFil: Lamas, Diego Germán. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Electrochemical behavior of nanostructured La0.8Sr0.2MnO3 as cathodes for solid oxide fuel cells

LaSrMnO (LSM) is one of the most commonly used cathodes in Solid Oxide Fuel Cells (SOFC). In spite of the fact that nanostructured cathodes are expected to display improved performance, the high operating temperature (∼ 1000°C) of LSM-based SOFCs hinders their stability. In the present work, we have developed nanostructured cathodes prepared from LSM nanotubes of enhanced performance, allowing its use at lower temperatures (∼ 800°C). We observed that our cathodes have qualitative improvements compared with microstructured materials: firstly, the diffusion in the gas phase is optimized to a negligible level and secondly, evidence of ionic conduction is found, which is extremely rare in LSM cathodes. We propose that this important change in the electrochemical properties is due to the nanostructuration of the cathode and deserves further attention, including the exploration of other materials

Oxygen Reduction Mechanisms in Nanostructured La0.8Sr0.2MnO3 Cathodes for Solid Oxide Fuel Cells

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of LaSrMnO (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: (i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and (ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first-principles calculations point toward an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface

Iron-based nanoparticles prepared from yerba mate extract. Synthesis, characterization and use on chromium removal

Iron-based nanoparticles were synthesized by a rapid method at room temperature using yerba mate (YM) extracts with FeCl3 in different proportions. Materials prepared from green tea (GT) extracts were also synthesized for comparison. These materials were thoroughly characterized by chemical analyses, XRD, magnetization, SEMEDS, TEM-SAED, FTIR, UV?Vis, Raman, Mössbauer and XANES spectroscopies, and BET area analysis. It was concluded that the products are nonmagnetic iron complexes of the components of the extracts. The applicabilityof the materials for Cr(VI) (300 μM) removal from aqueous solutions at pH 3 using two Cr(VI):Fe molar ratios (MR), 1:3 and 1:0.5, has been tested. At Cr(VI):Fe MR = 1:3, the best YM materials gave complete Cr(VI) removal after two minutes of contact, similar to that obtained with commercial nanoscale zerovalent iron (N25), with dissolved Fe(II), and with a likewise prepared GT material. At a lower Cr(VI):Fe MR (1:0.5), although Cr(VI) removal was not complete after 20 min of reaction, the YM nanoparticles were more efficient than N25, GT nanoparticles and Fe(II) in solution. The results suggest that an optimal Cr(VI):Fe MR ratio could be reached when using the new YM nanoparticles, able to achieve a complete Cr(VI) reduction, and leaving very low Cr and Fe concentrations in the treated solutions. The rapid preparation of the nanoparticles would allow their use in removal of pollutants in soils and groundwater by direct injection of the mixture of precursors.Fil: García, Fabiana Elena. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); ArgentinaFil: Senn, Alejandro Marcelo. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Meichtry, Jorge Martin. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Scott, Thomas B.. No especifíca;Fil: Pullin, Huw. Cardiff University; Reino UnidoFil: Leyva, Ana G.. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. - Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia.; ArgentinaFil: Halac, Emilia Beatriz. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín; ArgentinaFil: Ramos, Cinthia Paula. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. - Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia.; ArgentinaFil: Sacanell, Joaquín. Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. - Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia.; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; ArgentinaFil: Mizrahi, Martin Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Requejo, Felix Gregorio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Litter, Marta Irene. Universidad Nacional de San Martín. Instituto de Investigación en Ingeniería Ambiental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentin