Influence of particle size and agglomeration in solid oxide fuel cell cathodes using manganite nanoparticles

In this work we studied the influence of particle size and agglomeration in the performance of solid oxide fuel cell cathodes made with nanoparticles of La0.8Sr0.2MnO3. We followed two synthesis routes based on the Liquid Mix method. In both procedures we introduced additional reagents in order to separated the manganite particles. We evaluated cathodic performance by Electrochemical Impedance Spectroscopy in symmetrical (CATHODE/ELECTROLYTE/CATHODE) cells. Particle size was tuned by the temperature used for cathode sintering. Our results show that deagglomeration of the particles, serves to improve the cathodes performance. However, the dependence of the performance with the size of the particles is not clear, as different trends were obtained for each synthesis route. As a common feature, the cathodes with the lowest area specific resistance are the ones sintered at the largest temperature. This result indicates that an additional factor related with the quality of the cathode/electrolyte sintering, is superimposed with the influence of particle size, however further work is needed to clarify this issue. The enhancement obtained by deagglomeration suggest that the use of this kind of methods deserved to be considered to develop high performance electrodes for solid oxide fuel cells.Fil: Martinelli, Hernán. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; ArgentinaFil: Lamas, Diego Germán. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Leyva, Ana G. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Sacanell, Joaquin Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Polarity studies of single polyelectrolyte layers in polyelectrolyte multilayers probed by steady state and life time doxorubicin fluorescence

Hypothesis: Polarity in polyelectrolyte multilayers (PEMs) may vary from the inner to the top layers of the film as the charge compensation of the layers is more effective inside the PEMs than in outer layers. Doxorubicin hydrochloride (DX) is used here to sense polarity at the single polyelectrolyte level inside PEMS. Experimental: DX is complexed electrostatically to a polyanion, either polystyrene sulfonate (PSS) or polyacrylic acid (PAA) and assembled at selected positions in a multilayer of the polyanion and polyally lamine hydrochloride (PAH) as polycation. Local polarity in the layer domain is evaluated through changes in the intensity ratio of the first to second band of spectra of DX (I1/I2 ratio) by steady state flu orescence, and by Lifetime fluorescence. Findings: PAH/PSS multilayers, show a polarity similar to water with DX/PSS as top layer, decreasing to I1/ I2 ratios similar to organic solvents as the number of polyelectrolyte layers assembled on top increases. For PAH/PAA multilayers, polarity values reflect a more polar environment than water when DX/PAA is the top layer, remaining unaltered by the assembly of polyelectrolyte layers on top. Results show that different polar environments may be present in a PEM when considering polarity at the single layer level.Fil: Martinelli, Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina. Centro de Investigación Cooperativa en Biomateriales; EspañaFil: Tasca, Elisamaria. Centro de Investigación Cooperativa en Biomateriales; España. Università degli Studi di Roma "La Sapienza"; ItaliaFil: Andreozzi, Patrizia. Università degli Studi di Firenze; Dipartimento di Chimica “Ugo Schiff”; Italia. Centro de Investigación Cooperativa en Biomateriales; EspañaFil: Libertone, Sara. Centro de Investigación Cooperativa en Biomateriales; EspañaFil: Ritacco, Hernán Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Giustini, Mauro. Università degli Studi di Roma "La Sapienza"; Italia. Università degli Studi di Firenze; Dipartimento di Chimica “Ugo Schiff” ; ItaliaFil: Moya, Sergio Enrique. Centro de Investigación Cooperativa en Biomateriales; España. Università degli Studi di Roma "La Sapienza"; Itali

Uso de materiales reciclados en compuestos cementicios

Context: Concrete production is characterized by a significant demand for energy and raw materials, and by emitting large amounts of greenhouse gases (GHG). Moreover, the processes of construction, maintenance, and demolition of buildings generate huge quantities of waste that require costly and environmentally sensitive disposal procedures. Therefore, several solutions are being investigated to reduce the environmental impact of the processes associated with the life cycle of concrete. Methodology: Through experimental tests, the physical and mechanical properties of three sustainable concrete mixtures were studied (concrete with recycled aggregates, concrete with partial replacement of Portland cement by fly ash, and cementitious composites reinforced with recycled fibers). Results: The use of coarse recycled aggregates degraded the mechanical characteristics of the concrete due to their greater porosity and water absorption capacity. However, the mixture made out of recycled aggregates and fly ash showed a synergistic effect, mitigating the adverse consequences mentioned. The post-cracking response of concrete reinforced with recycled steel fibers was characterized by a lower tenacity and ductility compared to composites with industrial fibers. Specifically, mixtures with recycled fibers showed a more pronounced softening stage. This revealed a lower efficiency of recycled fibers compared to industrial fibers. Conclusions: The experimental results showed that the incorporation of recycled materials led to a deterioration in the physical and mechanical behavior of the analyzed composites. However, the resulting properties exceeded the recommended minimum values required for their application as structural materials.Contexto: La producción de hormigón se caracteriza por una importante demanda de energía y materias primas, emitiendo grandes cantidades de gases de efecto invernadero (GEI). Asimismo, la construcción, mantenimiento y demolición de edificios genera enormes cantidades de residuos que requieren costosos y ambientalmente sensibles procedimientos de disposición final. Por tanto, en la actualidad se están investigando diversas soluciones para reducir el impacto ambiental de los procesos asociados al ciclo de vida del hormigón. Metodología: Se estudiaron, mediante ensayos experimentales, las propiedades físicas y mecánicas de los siguientes materiales sustentables: hormigón con agregados reciclados, hormigón con reemplazo parcial de cemento Portland por cenizas volantes y compuestos cementicios reforzados con fibras recicladas. Resultados: El uso de agregados gruesos reciclados degradó las características mecánicas del hormigón debido a su mayor porosidad y capacidad de absorción de agua. Sin embargo, su combinación con cenizas volantes mostró un efecto sinérgico, mitigando las consecuencias adversas mencionadas. La respuesta posfisuración del hormigón reforzado con fibras de acero recicladas se caracterizó por una menor tenacidad y ductilidad respecto a los compuestos con fibras industriales. Específicamente, las mezclas con fibras recicladas mostraron una etapa de ablandamiento más pronunciada. Esto reveló una eficiencia menor de las fibras recicladas con respecto a las industriales. Conclusiones: Los resultados experimentales demostraron que la incorporación de materiales reciclados condujo a un deterioro en el comportamiento físico y mecánico-resistente de los compuestos analizados. No obstante, las propiedades resultantes superaron los valores mínimos recomendados para su aplicación como materiales estructurales

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

Combined fit to the spectrum and composition data measured by the Pierre Auger Observatory including magnetic horizon effects

The measurements by the Pierre Auger Observatory of the energy spectrum and mass composition of cosmic rays can be interpreted assuming the presence of two extragalactic source populations, one dominating the flux at energies above a few EeV and the other below. To fit the data ignoring magnetic field effects, the high-energy population needs to accelerate a mixture of nuclei with very hard spectra, at odds with the approximate E$^{-2}$ shape expected from diffusive shock acceleration. The presence of turbulent extragalactic magnetic fields in the region between the closest sources and the Earth can significantly modify the observed CR spectrum with respect to that emitted by the sources, reducing the flux of low-rigidity particles that reach the Earth. We here take into account this magnetic horizon effect in the combined fit of the spectrum and shower depth distributions, exploring the possibility that a spectrum for the high-energy population sources with a shape closer to E$^{-2}$ be able to explain the observations