Universität Stuttgart
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Item Open Access Die protonenleitende Membran - Schlüsselkomponente einer Brennstoffzelle(2001) Roduner, Emil; Kerres, JochenDie Brennstoffzellentechnologie wird wohl eine der Schlüsseltechnologien dieses Jahrhunderts werden, da die Energieausnutzung von Brennstoffzellenaggregaten deutlich höher ist als der Wirkungsgrad von Verbrennungsmotoren. Mögliche Anwendungsfelder von Brennstoffzellen sind sowohl stationäre Einsätze in Blockkraftwerken als auch mobile Anwendungen als Energiequelle in Fahrzeugen wie Personen- und Lastkraftwagen, Bussen oder Schienenfahrzeugen und nicht zuletzt in mobilen Elektronikgeräten wie Funktelefonen oder Laptops.Item Open Access What is heat? : can heat capacities be negative?(2023) Roduner, EmilIn the absence of work, the exchange of heat of a sample of matter corresponds to the change of its internal energy, given by the kinetic energy of random translational motion of all its constituent atoms or molecules relative to the center of mass of the sample, plus the excitation of quantum states, such as vibration and rotation, and the energy of electrons in excess to their ground state. If the sample of matter is equilibrated it is described by Boltzmann’s statistical thermodynamics and characterized by a temperature T. Monotonic motion such as that of the stars of an expanding universe is work against gravity and represents the exchange of kinetic and potential energy, as described by the virial theorem, but not an exchange of heat. Heat and work are two distinct properties of thermodynamic systems. Temperature is defined for the radiative cosmic background and for individual stars, but for the ensemble of moving stars neither temperature, nor pressure, nor heat capacities are properly defined, and the application of thermodynamics is, therefore, not advised. For equilibrated atomic nanoclusters, in contrast, one may talk about negative heat capacities when kinetic energy is transformed into potential energy of expanding bonds.Item Open Access Towards a molecular understanding of cation‐anion interactions and self‐aggregation of adeninate salts in DMSO by NMR and UV spectroscopy and crystallography(2021) Buyens, Dominique M. S.; Pilcher, Lynne A.; Roduner, EmilRare anionic forms of nucleic acids play a significant biological role and lead to spontaneous mutations and replication and translational errors. There is a lack of information surrounding the stability and reactivity of these forms. Ion pairs of mono‐sodium and ‐potassium salts of adenine exist in DMSO solution with possible cation coordination sites at the N1, N7 and N9 atoms of the purine ring. At increasing concentrations π‐π stacked dimers are the predominant species of aggregates followed by higher order aggregation governed by coordination to metal cations in which the type of counter ion present has a central role in the aggregate formation.Item Open Access Symmetry and electronic properties of metallic nanoclusters(2023) Roduner, EmilSpherical nanoclusters with countable member atoms and delocalized valence orbitals are superatoms with properties analogous to those of simple atoms. This is reflected, in particular, in their optical spectra and magnetic properties, in a similar sense to transition metal ions and complexes. Clusters can be of low-spin or high-spin with considerable contributions to magnetism by the large cluster orbital magnetic moment. Due to the large radius of the clusters, they can be diamagnetic with an unusually high diamagnetic susceptibility. Gold and platinum, which in the bulk are non-magnetic, show pronounced superparamagnetism associated with their high-spin nature, and the magnetic moment can be trapped in symmetry-breaking environments so that hysteresis pertains far beyond room temperature. A significant deviation from hydrogen-like orbitals results from the shape of the confining potential, which has the effect that the orbital quantum number ℓ is not limited to values less than the principal quantum number n.Item Open Access Preserving cultural diversity in rural Africa using renewable energy(2023) Roduner, Emil; Rohwer, Egmont R.Ninety percent of the large interior, rural part of Africa is not an abundant user of fossil fuels and is not connected to an electricity grid. This limits habitability and leads to significant migration to larger cities in attempts to improve economic and social welfare, which happens at the cost of its rich cultural diversity by inevitable adaption and mixing of societies. A direct transition from a firewood to an off‐grid renewable electricity age can mitigate this detrimental development. This perspective discusses the interdisciplinary requirements linking cultural, sociological, economic, and technical aspects for a transition to modern life without loss of valuable traditions. Photovoltaic power and wind energy can provide local affordable electricity in off‐grid locations. Intermediate storage for day–night cycles is catered for by novel types of batteries. Purifying and recycling water, refrigerating food and medicine, and benefitting from contact with the world via electronic media permit a tremendous increase in living conditions and significantly lower the pressure of migration into cities. Access to energy is a fundamental requirement for the preservation of the rich cultural diversity with family and tribal bindings, local languages, traditions, and religions, and allows for a more moderate transition to a modern society.Item Open Access Uncovering thermally activated purple-to-blue luminescence in Co-modified MgAl-layered double hydroxide(2024) Gevers, Bianca R.; Roduner, Emil; Leuteritz, Andreas; Labuschagné, Frederick J. W. J.Thermally activated blue-to-purple luminescence of Co-modified nano-sandrose MgAl-layered double hydroxides (LDHs) is concentration dependent, occurring only for MgCoAl-LDH with a molar metal cation concentration of 15% Co. Temperature sweep luminescence spectroscopy between 83 K and 298 K shows that the luminescence is strongest at room temperature, increasing with an activation energy of 1 kJ mol-1 between these temperatures. The luminescence occurs in a broad, but fine-structured band below the conduction band (CB) edge at 3.0 eV after excitation at 5.0 eV.Item Open Access Technical principles of atmospheric carbon dioxide reduction and conversion : economic considerations for some developing countries(2020) Roduner, Emil; Rohwer, Egmont R.Since natural photosynthesis in our biosphere does not have the capacity to cope with the additional atmospheric CO2 due to combustion of fossil fuels, CO2 has to be actively removed. Efficient methods are currently being developed, but the captured gas has to be dumped in safe and permanent storage environments. Alternatively, it has to be purified before it can be recycled catalytically, using renewable energy, to high-value chemicals as feedstock for the synthesis of polymers, fine chemicals, or in large quantities liquid solar fuels. The combustion of solar fuels is carbon-neutral. If produced at locations where renewable energy is cheap, they become an important economic opportunity. The requirement to achieve a carbon-zero energy supply also for air traffic allows planning for an as yet unknown higher price compared to that of fossil fuels. Use of solar fuels in closed cycle applications may also relieve the energy situation in the large number of off-grid households in rural Africa. The availability of energy, in particular of electricity, is essential for advanced living conditions, prevents migration to urban areas, and therefore protects a rich variation of tribal cultural, religious and social traditions.