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Item 3D Impedance Modelling of Metal Anodes in Solid-State Batteries − Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models(2022-09-08) Eckhardt, Janis K.; Fuchs, Till; Burkhardt, Simon; Klar, Peter J.; Janek, Jürgen; Heiliger, ChristianA non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal (porous) electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation.Item Amorphous and highly nonstoichiometric titania (TiOx) thin films close to metal-like conductivity(2014) Leichtweiss, Thomas; Henning, Ralph A.; Koettgen, Julius; Schmidt, Rüdiger M.; Holländer, Bernd; Martin, Manfred; Wuttig, Matthias; Janek, JürgenOxygen-deficient titanium oxide films (TiOx) have been prepared by pulsed laser deposition at room temperature. Samples in their as-deposited state have an average composition of TiO1.6, are optically absorbing and show electronic conductivities in the range of 10 S cm-1. The films are metastable and consist of grains of cubic titanium monoxide (gamma-TiO) embedded in an amorphous TiO1.77 matrix. Upon annealing in an argon atmosphere the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S cm-1) at about 450 [degree]C whereas the local structure is changed: nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid state disproportionation. The highly conductive state can be frozen by quenching. During heat treatment in an argon atmosphere a stoichiometric rutile TiO2 surface layer forms due to oxidation by residual oxygen. The combination of a highly conductive TiOx film with such an approximately 20 nm thick rutile cover layer leads to a surprisingly high efficiency for the water-splitting reaction without the application of an external potential.Item Bimodal mesoporous titanium dioxide anatase films templated by a block polymer and an ionic liquid: influence of the porosity on the permeability(2013) Sallard, Sébastien; Schröder, Michael; Boissière, Cédric; Dunkel, Christian; Etienne, Mathieu; Walcarius, Alain; Oekermann, Torsten; Wark, Michael; Smarsly, Bernd M.In the present paper, we report the synthesis of bimodal mesoporous anatase TiO2 films by the EISA (Evaporation-Induced Self-Assembly) method using sol-gel chemistry combining two porogen agents, a low molecular weight ionic template and a neutral block copolymer. The surfactant template (C(16)mimCl) generates non-oriented worm-like pores (8 to 10 nm) which connect the regularly packed ellipsoidal mesopores (15 to 20 nm diameter) formed by an amphiphilic block copolymer of the type poly(isobutylene)-b-poly(ethylene oxide) (PIB-PEO). The surfactant template can also significantly influence the size and packing of the ellipsoidal mesopores. The mesostructural organization and mesoporosity of the films are studied by Environmental Ellipsometry-Porosimetry (EEP), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and electron microscopy techniques. Electrochemical characterization is performed to study the permeability of the films to liquid solutions, using two types of probe moieties (K3FeIII(CN)(6) and Ru(bpy)(3)(2+)) by the wall-jet technique. An optimum ratio of C(16)mimCl/PIB-PEO provides anatase films with a continuous bimodal mesopore structure, possessing a permeability up to two times higher than that of the mesoporous films templated by PIB-PEO only (with partially isolated mesopores). When C(16)mimCl is used in large quantities, up to 20% weight vs. PIB-PEO, large overall porous volume and surface area are obtained, but the mesostructure is increasingly disrupted, leading to a severe loss of permeability of the bimodal films. A dye-sensitized solar cell set-up is used with anatase films as the photoelectrode. The photosensitizer loading and the total energy conversion efficiency of the solar cells using the mesoporous films templated by an optimal ratio of the two porogen agents C(16)mimCl and PIB-PEO can be substantially increased in comparison with the solar cells using mesoporous films templated by PIB-PEO only.Item Chemie und Licht : Eine weihnachtliche Experimentalvorlesung(2001) Albert, Barbara; Janek, JürgenDass viele chemische Reaktionen unter Lichterscheinung verlaufen, ist jedem geläufig. Chemische Reaktionen, bei denen es leuchtet, stehen mit einer groß,en Bandbreite für Demonstrationsversuche zur Verfügung: Wollen wir Explosionen sehen? Flammen? Kaltes Licht? Wir haben uns zu Weihnachten für die ruhigeren Varianten dieser Experimente entschieden und versucht, durch chemische Leuchterscheinungen hervorgerufene Effekte zum einen zu erklären, vor allem aber assoziativ in Zusammenhänge zu stellen die auch Nicht-Chemikern eingängig sind.Item Chemische Triebkräfte : Von der Verbrennung zum Herzschlag(2004) Janek, Jürgen; Seibert, Albert; Hofmann, Jan P.; Schwertfeger, HartmutWas hat eine einfache chemische Verbrennung mit dem Herzschlag zu tun? Diese Frage soll im Rahmen einer physikalisch-chemischen Experimentalvorlesung beantwortet werden. Dabei fügen sich anscheinend weit voneinander entfernte Phänomene in ein klar strukturiertes Bild. Es wird deutlich, dass der menschliche Körper aus thermodynamischer Sicht eine biologische 'Wärmekraftmaschine' ist, deren Wirkungsgrad höher als der Wirkungsgrad mechanische Wärmekraftmaschinen ('Dampfmaschinen'). Spröde anmutende thermodynamische Sätze füllen sich mit Leben - die Begriffe 'Energie, Wärme, Entropie' werden anschaulich. Anhand einfacher Demonstrationsexperimente wird ein Bogen über 250 Jahre Forschung im Bereich der chemischen Energiewandlung gespannt.Item Coherent analysis of disordered mesoporous adsorbents using small angle X-ray scattering and physisorption experiments(2014) Stoeckel, Daniela; Wallacher, Dirk; Zickler, Gerald A.; Perlich, Jan; Tallarek, Ulrich; Smarsly, Bernd M.Characterization of mesoporous adsorbents is traditionally performed in terms of the pore size distribution with bulk methods like physisorption and mercury intrusion. But their application relies on assumptions regarding the basic pore geometry. Although novel tools have enabled the quantitative interpretation of physisorption data for adsorbents having a well-defined pore structure the analysis of disordered mesoporosity still remains challenging. Here we show that small angle X-ray scattering (SAXS) combined with chord length distribution (CLD) analysis presents a precise and convenient approach to determine the structural properties of two-phase (solid-void) systems of mesopores. Characteristic wall (solid) and pore (void) sizes as well as surface areas are extracted without the need to assume a certain pore shape. The mesoporous structure of modern, commercially available fully porous and core-shell adsorbent particles is examined by SAXS/CLD analysis. Mean pore size and surface area are compared with results obtained from nitrogen physisorption data and show excellent agreement.Item Complete experimental rovibrational eigenenergies of HCN up to 6880 cm(-1) above the ground state(2011) Mellau, Georg Ch.The [H, C, N] molecular system is a very important model system to many fields of chemical physics and the experimental characterization of highly excited vibrational states of this molecular system is of special interest. This paper reports the experimental characterization of all 3822 eigenenergies up to 6880 cm(-1) relative to the ground state in the HCN part of the potential surface using high temperature hot gas emission spectroscopy. The spectroscopic constants for the first 71 vibrational states including highly excited bending vibrations up to nu(2) = 10 are reported. The perturbed eigenenergies for all 20 rotational perturbations in the reported eigenenergy range have been determined. The 11 070 eigenenergies up to J = 90 for the first 123 vibrational substates are included as supplement to this paper. We show that a complete ab initio rovibrational analysis for a polyatomic molecule is possible. Using such an analysis we can understand the molecular physics behind the Schrodinger equation for problems for which perturbation theoretical calculations are no more valid. We show that the vibrational structure of the linear HCN molecule persists approximately up to the isomerization barrier and only above the barrier the accommodation of the vibrational states to the double well structure of the potential takes place.Item Controlled electrochemical growth of silver microwires(2005) Rohnke, Marcus; Best, Timo; Janek, JürgenThe cathodic deposition of silver whiskers as microwires in the electrochemical cell Ag/AgBr/Ag has been investigated. From growth experiments observed with an optical microscope, we conclude that the growth process occurs exclusively at the interface of the silver wires with the solid electrolyte. The basal contacts of the microwires show different morphologies without clear correlation to the substrate. The silver crystals often show a hexagonal morphology, but XRD gives no definite information on the structure of the wires. The measured exchange current density is in the order of 100 mA/cm2. The growth rate and the diameter of the microwires can be controlled by variation of the current.Item Conversion reactions for sodium-ion batteries(2013) Klein, Franziska; Jache, Birte; Bhide, Amrtha; Adelhelm, PhilippResearch on sodium-ion batteries has recently been rediscovered and is currently mainly focused on finding suitable electrode materials that enable cell reactions of high energy densities combined with low cost. Naturally, an assessment of potential electrode materials requires a rational comparison with the analogue reaction in lithium-ion batteries. In this paper, we systematically discuss the broad range of different conversion reactions for sodium-ion batteries based on their basic thermodynamic properties and compare them with their lithium analogues. Capacities, voltages, energy densities and volume expansions are summarized to sketch out the scope for future studies in this research field. We show that for a given conversion electrode material, replacing lithium by sodium leads to a constant shift in cell potential Delta E-(Li-Na)(o) depending on the material class. For chlorides Delta E-(Li-Na)(o) equals nearly zero. The theoretical energy densities of conversion reactions of sodium with fluorides or chlorides as positive electrode materials typically reach values between 700 W h kg(-1) and 1000 W h kg(-1). Next to the thermodynamic assessment, results on several conversion reactions between copper compounds (CuS, CuO, CuCl, CuCl2) and sodium are being discussed. Reactions with CuS and CuO were chosen because these compounds are frequently studied for conversion reactions with lithium. Chlorides are interesting because of Delta E-(Li-Na)(o) approximate to 0 V. As a result of chloride solubility in the electrolyte, the conversion process proceeds at defined potentials under rather small kinetic limitations.Item Coulometric titration at low temperatures-nonstoichiometric silver selenide(2003) Beck, Gesa K.; Janek, JürgenA modified coulometric titration technique is described for the investigation of nonstoichiometric phases at low temperatures. It allows to obtain titration curves at temperatures where the conventional coulometric titration technique fails because of too small chemical diffusion coefficients of the mobile component. This method for indirect coulometric titration is applied to silver selenide between -100 and 100 °C. The titration curves are analyzed on the basis of a defect chemical model and provide thermodynamic data of the a-silver selenide phase. The boundaries of the homogeneous phase field are determined.Item Determination of local potentials in mixed conductors : two examples(1995) Rosenkranz, C.; Janek, JürgenTwo basic experiments for the measurement of local (electro-)chemical potentials in crystalline mixed conductors, which are exposed to external forces, are reported, (a) The profile of the chemical potential of silver metal in a Wagner-Hebb polarization cell Ag ¦AgBr ¦Pt is measured by means of local potential probes. Deviations from calculated profiles are discussed in terms of either a possible electronic overvoltage at the anodic interface AgBr ¦ Pt or a change of the defect structure of AgBr in the region near the anode, (b) In a second experiment, the ionic overvoltage at an interface AgBr ¦ AgCl is determined, again by means of local potential probes.Item Diamondoid-modified DNA(2011) Wang, Yan; Tkachenko, Boryslav A.; Schreiner, Peter R.; Marx, AndreasWe prepared novel C5-modified triphosphates and phosphoramidites with a diamondoid functionally linked to the nucleobase. Using primer extension experiments with different length templates we investigated whether the modified triphosphates were enzymatically incorporated into DNA and whether they were further extended. We found that all three modified nucleotides can be incorporated into DNA using a single-nucleotide incorporation experiment, but only partially using two templates that demand for multiple incorporation of the modified nucleotides. The modified phosphoramidites were introduced into oligonucleotides utilizing DNA synthesizer technology. The occurring oligonucleotide structures were examined by circular dichroism (CD) and melting temperature (T(m)) measurements and were found to adapt similar helix conformations as their unmodified counterparts.Item Electrocatalysis on Pt/YSZ electrodes(2001) Luerssen, Bjoern; Janek, Jürgen; Imbihl, R.Electrochemical polarization of porous metal electrodes on solid electrolytes often leads to an increase of their catalytic activity in heterogeneous reactions. Thin microstructured Pt films on single crystalline YSZ (yttria stabilized zirconia) are investigated as structurally and geometrically well-defined model systems in order to understand the origin of this effect. Photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM) have been applied as spatially resolving methods in situ to study the processes in the vicinity of the three-phase boundary (tpb). Measurements with SPEM show that atomic oxygen is created under anodic polarization and covers the Pt film homogeneously. A specific spillover species is not found, rather the formation of atomic oxygen is detected, which has the same O1s binding energy as chemisorbed oxygen from the gas phase.Item Electrochemical activation of molecular nitrogen at the Ir/YSZ interface(2011) Valov, Ilia; Luerssen, Björn; Mutoro, Eva; Gregoratti, Luca; De Souza, Rogar A.; Bredow, Thomas; Günther, Sebastian; Barinov, Alexei; Dudin, Pacel; Martin, Mandred; Janek, JürgenNitrogen is often used as an inert background atmosphere in solid state studies of electrode and reaction kinetics, of solid state studies of transport phenomena, and in applications e.g. solid oxide fuel cells (SOFC), sensors and membranes. Thus, chemical and electrochemical reactions of oxides related to or with dinitrogen are not supposed and in general not considered. We demonstrate by a steady state electrochemical polarisation experiments complemented with in situ photoelectron spectroscopy (XPS) that at a temperature of 450 degrees C dinitrogen can be electrochemically activated at the three phase boundary between N(2), a metal microelectrode and one of the most widely used solid oxide electrolytes-yttria stabilized zirconia (YSZ)-at potentials more negative than E = -1.25 V. The process is neither related to a reduction of the electrolyte nor to an adsorption process or a purely chemical reaction but is electrochemical in nature. Only at potentials more negative than E = -2 V did new components of Zr 3d and Y 3d signals with a lower formal charge appear, thus indicating electrochemical reduction of the electrolyte matrix. Theoretical model calculations suggest the presence of anionic intermediates with delocalized electrons at the electrode/electrolyte reaction interface. The ex situ SIMS analysis confirmed that nitrogen is incorporated and migrates into the electrolyte beneath the electrode.Item Electrochemical blackening of yttria-stabilized zirconia : morphological instability of the moving reaction front(1999) Janek, Jürgen; Korte, CarstenElectrochemical reduction was performed on both polycrystals of calcia-stabilized zirconia (CSY) and single crystals of yttria-stabilized zirconia (YSZ) at 500°C. A glass-encapsulation was used to prevent access of molecular oxygen from the gas phase and to realize a virtually two-dimensional geometry for the reacting crystal. The resulting blackening process which occurs by the advancement of a morphologically unstable reaction front was observed in-situ in a heating-stage by the use of an optical microscope.Item Electrochemically induced oxygen spillover and diffusion on Pt(111): PEEM imaging and kinetic modelling(2011) Mutoro, Eva; Hellwig, Christian; Luerssen, Björn; Guenther, Sebastian; Bessler, Wolfgang G.; Janek, JürgenElectrochemically induced oxygen spillover and diffusion in the Pt(O(2))|YSZ system is investigated in a combined experimental and theoretical study. The spreading of spillover oxygen is imaged by photoelectron emission microscopy (PEEM) on dense and epitaxial Pt(111) thin film electrodes prepared by pulsed laser deposition (PLD). Two different models are used to obtain surface diffusion coefficients from the experimental data, (i) an analytical solution of Fick´s 2nd law of diffusion, and (ii) a numerical reaction-diffusion model that includes recombinative desorption of O(2) into the gas phase. The resulting diffusion coefficient has an activation energy of 50 kJ mol(-1) and a preexponential factor of 0.129 cm(2) s(-1) with an estimated uncertainty of +/- 20% for the activation energy and +/- 50% for the absolute value. The Fickian model slightly overpredicts diffusion coefficients due to the neglect of oxygen desorption. Experimental and theoretical results and limitations are discussed and compared to previous work.Item An EMF cell with a nitrogen solid electrolyte-on the transference of nitrogen ions in yttria-stabilized zirconia(2011) Lee, Doh-Kwon; Fischer, Claus C.; Valov, Ilia; Reinacher, Jochen; Stork, Alexandra; Lerch, Martin; Janek, JürgenThe mobility and electrochemical activity of nitrogen inside and/or at the surface of ionic compounds is of fundamental, as well as of possibly practical, relevance. In order to better understand the role of nitrogen anions in solid electrolytes, we measured the transference number of nitrogen in yttria-stabilized zirconia (YSZ) by a concentration cell technique as a function of oxygen activity at different temperatures in the range of 1023 <= T/K <= 1123. YSZ doped with 1.9 wt% of N (YSZ: N) turned out to have an appreciable nitrogen transference number, which increased from 0 to 0.1 with decreasing oxygen activity in the range of -20 < loga(O2) < -14. The stability of N in YSZ: N, however, has yet to be elucidated under oxidizing conditions.Item Evolution of asymmetric organocatalysis : multi- and retrocatalysis(2012) Wende, Raffael C.; Schreiner, Peter R.The evolution of organocatalysis led to various valuable approaches, such as multicomponent as well as domino and tandem reactions. Recently, organomulticatalysis, i.e., the modular combination of distinct organocatalysts enabling consecutive reactions to be performed in one pot, has become a powerful tool in organic synthesis. It allows the construction of complex molecules from simple and readily available starting materials, thereby maximizing reaction efficiency and sustainability. A logical extension of conventional multicatalysis is a multicatalyst, i.e., a catalyst backbone equipped with independent, orthogonally reactive catalytic moieties. Herein we highlight the impressive advantages of asymmetric organomulticatalysis, examine its development, and present detailed reactions based on the catalyst classes employed, ranging from the very beginnings to the latest multicatalyst systems.Item Formation of gold nanoparticles in polymeric nanowires by low-temperature thermolysis of gold mesitylene(2012) Erk, Christoph; Yau, Man Yan Eric; Lange, Holger; Thomsen, Christian; Miclea, Paul; Wehrspohn, Ralf B.; Schlecht, Sabine; Steinhart, MartinThe formation of polymer nanowires containing metal nanoparticle chains by low-temperature thermolyses of metal precursors has remained challenging. We report the block copolymer-assisted generation of locally regular chains of quasi-spherical gold nanoparticles with narrow particle diameter distribution by mild thermolysis of the non-polar gold precursor gold mesitylene inside the cylindrical nanopores of self-ordered anodic aluminium oxide (AAO). The block copolymer separates the gold mesitylene as well as the developing gold nanoparticles from the AAO pore walls so that surface nucleation and pinning of gold clusters are prevented. Growing quasi-spherical gold nanoparticles locally deform the polymer chains irreversibly adsorbed on the AAO pore walls, and the polymer chains are pushed into the space between the gold nanoparticles. The gold nanoparticles have, therefore, larger diameters and smaller specific surface than hypothetical pluglike gold entities with the same volume, the formation of which is suppressed.Item The functionalization of nanodiamonds (diamondoids) as a key parameter of their easily controlled self-assembly in micro- and nanocrystals from the vapor phase(2014) Gunawan, Maria A.; Poinsot, Didier; Domenichini, Bruno; Dirand, Celine; Chevalier, Sebastien; Fokin, Andrey A.; Schreiner, Peter R.; Hierso, Jean-CyrilleWe detail herein readily accessible processes to control previously unobserved robust self-assemblies of nanodiamonds (diamondoids) in micro- and nanocrystals from their mild vapor deposition. The chemical functionalization of uniform and discernible nanodiamonds was found to be a key parameter, and depending on the type of functional group (hydroxy, fluorine, etc.) and its position on the diamondoid, the structure of the discrete deposits can vary dramatically. Thus, well-defined anisotropic structures such as rod, needle, triangle or truncated octahedron shapes can be obtained, and self-assembled edifices of sizes ranging from 20 nm to several hundred micrometers formed with conservation of a similar structure for a given diamondoid. Key thermodynamic data including sublimation enthalpy of diamondoid derivatives are reported, and the SEM of the self-assemblies coupled with EDX analyses and XRD attest the nature and purity of nanodiamond crystal deposits. This attractive method is simple and outperforms in terms of deposit quality dip-coating methods we used. This vapor phase deposition approach is expected to allow for an easy formation of diamondoid nanoobjects on different types of substrates.
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