Polymer electrolyte fuel cells (PEFCs) have emerged as a promising renewable emission-free technology to solve the worldwide increasing demand for clean and efficient energy conversion. Despite large efforts in academia and automotive industry, the commercialization of PEFC vehicles still remains a great challenge. Critical issues are high material costs, insufficient catalytic activity as well as longterm durability. Especially due to the sluggish kinetics of the oxygen reduction reaction (ORR), high Pt loadings on the cathode are still necessary which leads to elevated costs. Alloys of Pt with other less precious metals (Co, Ni, Fe, Cu, etc.) show improved ORR activities compared to pure Pt catalysts. However, state-of-the-art carbon-supported catalysts suffer from severe Pt and carbon corrosion during the standard operation of PEFCs, affecting their reliability and long-term efficiency. Multimetallic aerogels constitute excellent candidates to overcome these issues. Due to their large open pores and high inner surface areas combined with electrical conductivity, they are ideal for applications in electrocatalysis. In addition, they can be employed without any catalyst support. Therefore, the fabrication of bimetallic Pt-M (M=Ni, Cu, Co, Fe) aerogels for applications in fuel cell catalysis was the focus of this thesis. Based on a previously published synthesis for Pt–Pd aerogels, a facile one-step procedure at ambient conditions in aqueous solution was developed. Bimetallic aerogels with nanochain diameters of as small as 4 nm and Brunauer-Emmett-Teller (BET) surface areas of up to 60 m2/g could be obtained. Extensive structure analysis of Pt–Ni and Pt–Cu aerogels by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX) and electrochemical techniques showed that both metals were predominantly present in their metallic state and formed homogeneous alloys. However, metal (hydr)oxide byproducts were observed in aerogels with higher contents of non-precious metal (>25 %). Moreover, electronic and geometric structures were similar to those of carbon-supported Pt alloy catalysts. As a result, ORR activites were comparable, too. A threefold improvement in surface-specific activity over Pt/C catalysts was achieved. The mass-specific activites met or exceeded the U.S. Department of Energy (DOE) target for automotive PEFC applications. Furthermore, a direct correlation between non-precious metal content in the alloy and ORR activity was discovered. Aerogels with nonprecious metal contents >25% turned out to be susceptible to dealloying in acid leaching experiments, but there was no indication for the formation of extended surface structures like Pt-skeletons. A Pt3Ni aerogel was successfully employed as the cathode catalyst layer in a differential fuel cell (1 cm2), which is a crucial step towards technical application. This was the first time an unsupported metallic aerogel was implemented in a PEFC. Accelerated stress tests that are usually applied to investigate the support stability of fuel cell catalysts revealed the excellent stability of Pt3Ni alloyed aerogels. In summary, the Pt alloy aerogels prepared in the context of this work have proven to be highly active oxygen reduction catalysts with remarkable stability.

We report controllable synthesis of Pd aerogels with high surface area and porosity by destabilizing colloidal solutions of Pd nanoparticles with variable concentrations of calcium ions. Enzyme electrodes based on Pd aerogels co-immobilized with glucose oxidase show high activity toward glucose oxidation and are promising materials for applications in bioelectronics.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES

We report controllable synthesis of Pd aerogels with high surface area and porosity by destabilizing colloidal solutions of Pd nanoparticles with variable concentrations of calcium ions. Enzyme electrodes based on Pd aerogels co-immobilized with glucose oxidase show high activity toward glucose oxidation and are promising materials for applications in bioelectronics.

No description available.

Modification of nanostructures via laser processing is of great interest for a wide range of applications such as aerospace and the storage of nuclear waste. The primary goal of this dissertation is to improve the understanding of nanostructures through two primary routes: the modification of aerogels and pulsed laser ablation in ethanol. A new class of materials, patterned aerogels, was fabricated by photopolymerizing selected regions of homogeneous aerogel monoliths using visible light. The characterization and fabrication of functionally graded, cellular and compositionally anisotropic aerogels and ceramics is discussed. Visible light was utilized due to it’s minimal absorption and scattering by organic molecules and oxide nanoparticles within wet gels. This allowed for the fabrication of deeply penetrating, well resolved patterns. Similarly, nanoporous monoliths with a typical aerogel core and a mechanically robust exterior ceramic layer were synthesized from silica aerogels cross-linked with polyacrylonitrile. Simple variations of the exposure geometry allowed fabrication of a wide variety of anisotropic materials without requiring layering or bonding. Nanoparticle solutions were prepared by laser ablation of metal foils (Fe and Mo) in ethanol. Ablation of Fe generated Fe3O4 and Fe3C nanoparticles which were superparamagnetic with a saturation magnetization Ms = 124 emu/g. Zero field cooled (ZFC) measurements collected at an applied field of 50 Oe displayed a maximum magnetic susceptibility at 120 K with a broad distribution. Field cooled (FC) measurements showed a thermal hysteresis indicative of temperature dependent magnetic viscosity. Pulsed laser ablation of a Mo foil in ethanol generated inhomogeneous nanoparticles where Mo and MoC coexisted within the same aggregate. Formation of these unique nanoparticles is likely due to phase separation that occurs when a high temperature carbide phase cools after the laser pulse terminates. Similarly, magnetic nanoparticle suspensions were generated by pulsed laser ablation of Fe and Mo in ethanol. The formation of several carbide phases with no discernable alloy formation was seen. A decrease in magnetization with a decrease in Fe concentration was seen which was reconciled with the decreased Fe content in the system. However, at Fe concentrations below ~ 40%, an increase in Ms and Hc was observed which was reconciled with the disappearance of the ε–Fe3C. TEM analysis showed the formation of core-shell nanoparticles and Energy Filtered TEM showed the distribution of Fe-based nanoparticles in the suspensions.

This is a report on the study of the drying of nanoporous polymer foam material fabricated by photolithogtaphic methods. Three drying methods were employed, which were air drying, supercritical drying and freeze drying. After fabrication and drying, physical properties of the polymer foams were measured. These measurements included density of the material, Young’s modulus, surface area, and the shape of the skeletal particles. The measurements determined the effect of the polymer concentration and the effect of drying methods. It was determined that polymer concentration had a much larger effect on the properties of the materials than the drying method.

Gegenstand der vorliegenden Arbeit ist die Synthese, Charakterisierung und Optimierung von Kohlenstoff-Aerogelen (C-Aerogele) für den Einsatz als Hochtemperaturwärmedämmung (> 1000°C). C-Aerogele sind offenporöse monolithische Festkörper, die durch Pyrolyse von organischen Aerogelen entstehen. Die Synthese dieser organischen Vorstufen erfolgt über das Sol-Gel-Verfahren. Zur Charakterisierung der Morphologie wurde die innere Struktur der Aerogele mittels Raster- und Transmissionselektronenmikroskopie, Röntgendiffraktometrie (XRD), Raman-Spektroskopie, Stickstoffsorption und Röntgenkleinwinkelstreuung (SAXS) untersucht. Die thermischen Eigenschaften der Aerogele wurden mit Hilfe von Laser-Flash Messungen, dynamischer Differenzkalorimetrie (DSC), thermographischen und infrarot-optischen (IR) Messungen quantifiziert. Die innere Struktur von Aerogelen besteht aus einem dreidimensionalen Gerüst von Primärpartikeln, die während der Sol-Gel Synthese ohne jede Ordnung aneinander wachsen. Die zwischen den Partikeln befindlichen Hohlräume bilden die Poren. Die mittlere Partikel- und Porengröße eines Aerogels kann durch die Konzentration der Ausgangslösung und der Katalysatorkonzentration einerseits und durch die Synthesetemperatur und –dauer andererseits eingestellt werden. Der Bereich der mittleren Partikel- und Porengröße, der in dieser Arbeit synthetisierten Aerogele, erstreckt sich von einigen 10 Nanometern bis zu einigen Mikrometern. Die Dichten der Proben wurden im Bereich von 225 kg/m3 bis 635 kg/m3 variiert. Die Auswirkungen der Pyrolysetemperatur auf die Struktur und die thermischen Eigenschaften der C-Aerogele wurden anhand einer Probenserie erstmalig systematisch untersucht. Die Proben wurden dazu bei Temperaturen von 800°C bis 2500°C pyrolysiert bzw. temperaturbehandelt (geglüht). Um die einzelnen Beiträge zur Wärmeleitfähigkeit trennen und minimieren zu können, wurden die synthetisierten Aerogele thermisch mit mehreren Meßmethoden unter unterschiedlichen Bedingungen charakterisiert. Temperaturabhängige Messungen der spezifischen Wärmekapazität cp im Bereich von 32°C bis 1500°C ergaben für C-Aerogele verglichen mit den Literaturdaten von Graphit einen ähnlichen Verlauf. Allerdings steigt cp etwas schneller mit der Temperatur an, was auf eine „weichere“ Struktur hindeutet. Die maximale Abweichung beträgt etwa 11%. Messungen an einer Serie morphologisch identischer Aerogelproben, die im Temperaturbereich zwischen 800°C und 2500°C pyrolysiert bzw. geglüht wurden, ergeben eine Zunahme der Festkörperwärmeleitfähigkeit mit der Behandlungstemperatur um etwa einen Faktor 8. Stickstoffsorptions-, XRD-, Raman- und SAXS-Messungen an diesen Proben zeigen, dass dieser Effekt wesentlich durch das Wachstum der graphitischen Bereiche (Mikrokristallite) innerhalb der Primärpartikel des Aerogels bestimmt wird. Berechnungen auf Basis von Messungen der Temperaturleitfähigkeit weisen außerdem auch auf Veränderungen der Mikrokristallite hin. Gasdruckabhängige Messungen der Wärmeleitfähigkeit und der Vergleich zwischen Messungen unter Vakuum und unter Normaldruck an verschiedenen Aerogelmorphologien liefern Aussagen über den Gasanteil der Wärmeleitfähigkeit. Dabei zeigt sich, dass sich der Gasanteil der Wärmeleitfähigkeit in den Poren des Aerogels verglichen mit dem freien Gas durch die geeignete mittlere Porengröße erwartungsgemäß erheblich verringern lässt. Diese Ergebnisse stimmen in Rahmen der Messunsicherheit mit der Theorie überein. Durch infrarot-optische Messungen an C-Aerogelen konnte der Extinktionskoeffizient bestimmt und daraus der entsprechende Beitrag der Wärmestrahlung zur Wärmeleitfähigkeit berechnet werden. Temperaturabhängige Messungen der thermischen Diffusivität erlaubten mit der zur Verfügung stehenden Laser-Flash Apparatur die Bestimmung der Wärmeleitfähigkeit bis zu Temperaturen von 1500°C. Die Temperaturabhängigkeit der Wärmeleitfähigkeit der C-Aerogele zeigt eine Charakteristik, die mit den separat gemessenen bzw. berechneten Beiträgen zur Wärmeleitfähigkeit und der Theorie im Rahmen der Messunsicherheit gut übereinstimmen. Auf der Basis der gewonnenen Messdaten ist es möglich, die Wärmeleitfähigkeit von Aerogelen für Anwendungen über die maximale Messtemperatur von 1500°C durch Extrapolation vorherzusagen. Die niedrigste Wärmeleitfähigkeit der im Rahmen dieser Arbeit synthetisierten C-Aerogele beträgt danach etwa 0,17 W/(m•K) bei 2500°C unter Argonatmosphäre. Kommerziell erhältliche Hochtemperatur-Wärmedämmstoffe, wie z. B. Kohlefaserfilze oder Kohlenstoffschäume weisen Wärmeleitfähigkeiten im Bereich von etwa 0,7 bis 0,9 W/(m•K) bei einer Temperatur von 2000°C auf. Die Messungen zeigen, dass die vergleichsweise niedrigen Wärmeleitfähigkeiten von C-Aerogelen bei hohen Temperaturen durch die Unterdrückung des Gas- und Strahlungsbeitrags der Wärmeleitfähigkeit bedingt sind. / The scope of the present work is the synthesis, the characterisation and the optimisation of carbon (c-) aerogels as high temperature insulation (> 1000°C). Carbon aerogels are open porous monolithic solids which are produced by pyrolysis of organic aerogels. These organic precursors are synthesized via the sol-gel route. For the structural characterisation of the aerogels the samples were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, nitrogen sorption measurements and small angle X-ray scattering (SAXS). The thermal properties of the aerogels were quantified by laser flash measurements, differential scanning calorimetry (DSC), thermographic and infrared optical measurements. The inner structure of the aerogels consists of a three dimensional skeleton of primary particles which grow during the sol-gel synthesis and are connected to each other without any orientation. The voids between the particles are the pores. The mean particle and pore size of the aerogel can be tailored specifically via the concentration of the catalyst and the degree of dilution of the educt solution on the one hand and the synthesis time and -temperature on the other hand. The range of the mean particle and pore sizes of the aerogels synthesized within this work extends from some tens of nanometers to some microns. The density of the samples was varied in the range from 225 kg/m3 to 635 kg/m3. The impact of pyrolysis and annealing temperature on the morphology and the thermal properties of carbon aerogels was investigated for the first time systematically on one series of samples. For that purpose the samples were pyrolysed and annealed in the range of 800 to 2500°C. To separate and minimize the individual contributions, the thermal conductivity of the synthesized c-aerogels were thermally characterized by different measuring methods under various conditions. The measurements of the specific heat in the range of 32 to 1500°C yield values similar to the literature data of graphite; however slightly systematic higher values of up to 11% were observed as expected for “softer” solids with high interfacial surface areas. Measurements of a series of carbon aerogels with identical morphology, however different annealing temperatures, show an increase of the solid thermal conductivity with increasing annealing temperature of up to a factor of about 8 for temperatures between 800°C and 2500°C. Nitrogen sorption-, XRD-, Raman-, and SAXS-measurements reveal that this effect is dominated by the growth of graphitic domains (microcrystallites) within the primary particles of the aerogel. In addition calculations based on measurements of the thermal diffusivity indicate changes of the microcrystallites. Measurements of the thermal conductivity of aerogels with different morphologies as a function of gas pressure and the comparison of the data taken under vacuum and normal pressure yield informations about the gaseous contribution to the thermal conductivity. As expected, the gaseous thermal conductivity within the pores of the aerogel can be reduced compared to the free gas when the pore size is in the range of the mean free path of the gas molecules or smaller. The results agree with the theory within the measuremental uncertainties. Infrared optical measurements provide the extinction coefficient of carbon aerogels, from which the radiative contribution to the thermal conductivity could be determined. The laser flash equipment available at the ZAE Bayern allows measurements of the thermal diffusivity up to 1500°C from which the thermal conductivity can be determined. The thermal conductivity of carbon aerogels as a function of temperature is well described by a superposition of the single contributions determined separately and the theoretical predictions within the uncertainties. Based on the experimental data it is possible to extrapolate the thermal conductivity of carbon aerogels for applications beyond the maximum temperature investigated (1500°C). Thus the lowest thermal conductivity of the carbon aerogels synthesized in the scope of this work is about 0,17 W/(m•K) at 2500°C in argon atmosphere. This value is about a factor 4 lower than for the best commercially available insulation material.

Titanium Dioxide (TiO2) emerged as one of the most popular photocatalysts since 1970's. However, its photocatalytic activity requires UV irradiation due to its large band gap unless further functionalization or modifications are performed. Furthermore, recovery issue has always been a major drawback, if the more effective form nano particles are utilized. The key objectives of this research were synthesizing new TiO2 based photocatalyst systems that are effective with both the UV and the visible light while utilizing novel superior environmentally friendly techniques enabling development of nano-structured photocatalysts that can be easily recovered. In this dissertation research, highly porous nano-structured TiO2/WO3/Fe3+ aerogel composite photocatalyst are prepared, characterized, and tested for model photocatalytic reactions. The photocatalyst structure is tailored to capture environmental pollutants and enable their decomposition in-situ under both UV and visible light through photodecomposition to smaller benign substances. A novel and green method is applied to prepare unique surfactant templated aerogel photocatalysts with highly porous nano-structure, high surface area, and tailored pore size distribution. Sol-gel process followed by supercritical fluids extraction and drying allowed synthesis of highly porous composite TiO2/WO3 aerogel. The surfactant template was completely removed with the aid of a supercritical solvent mixture followed with heat treatment. Fe3+ ion was incorporated within the composite aerogel photocatalyst as dopant either at the sol-gel co-precipitation step or at a subsequent supercritical impregnation process. Supercritical drying followed with heat treatment results in titanium dioxide with the most profound anatase crystal structure. Neutral templates were used to further enhance retention and tuning of the nano-pore structure and the surface properties. The Nitrogen adsorption-desorption isotherms methods were used to follow the removal of solvents and templates as well as tracking the textural properties of the synthesized aerogel. Surfactant-templated aerogels, which show remarkable thermal stability and uniform pore size distribution, exhibit specific surface areas three times more than the highly optimized commercial nano-particles, industry standard TiO2 photocatalyst Degussa P-25, even after heat treatment. The synthesized catalysts were characterized by using SEM, FIB, EDS, XRD, XPS and porosimetry prior to post photocatalytic activity evaluation through a model photocatalytic reaction. The band gaps of the catalysts were also determined by using diffuse reflectance spectroscopy. The model reaction employed Methylene Blue (MB) photo-oxidation under UV and visible light. Resulting aerogel TiO2/WO3/Fe3+ photocatalyst exhibited comparable photocatalytic capability to Degussa P25 under UV light exposure and offered much superior photocatalytic capability under visible light exposure.