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Item type:Publication, Access status: with files , Relationship between structure and mechanical properties in hierarchical nanoporous gold(2026); Hierarchical nanoporosity represents a novel type of material which enables the combination of mechanical stability with a reduced solid fraction. The impact of a 2nd hierarchy level on the mechanical behavior is not yet fully understood. This thesis aims to unveil the various effects of structural parameters in hierarchical nanoporous gold on the mechanical properties. Monolithic samples of hierarchical nanoporous gold (HNPG) are produced by a three-step procedure which consists of a 1st electrochemical dealloying, a coarsening step by annealing and a 2nd electrochemical dealloying. The variation of synthesis parameters enables the independent adjustment of the ligament size and the solid fraction on the upper and the lower hierarchy level. Scanning electron microscopy (SEM) as well as small- and ultra-small-angle X-ray scattering (SAXS/USAXS) are employed for the structural analysis. In preparation for SAXS/USAXS, thin walls are cut out of the bulk material by plasma focused ion beam (PFIB). Two distinct length scales are identified for hierarchical nanoporous gold. The comparison between the ligament size or diameter determined by SEM and the ligament spacing determined by SAXS/USAXS results in a conversion factor which is dependent on the solid fraction. Furthermore, the specific surface area, which is detected on the basis of the scattering intensity, shows a good agreement with the specific surface area expected from the ligament spacing. A mechanical characterization is performed by macroscopic compression tests. Coarsening on the lower hierarchy level is accompanied by a distinct decrease of stiffness and strength if the lower level solid fraction is about 0.30, which is explained by the influence of the lower level connectivity. A reduction of stiffness and strength is detected if the ratio between the upper and the lower level ligament size is reduced to about 2. The Young's modulus is lower than expected on the basis of the modified Roberts-Garboczi-law, which is associated with the early yielding of ligaments. This assumption is supported by the general impairment of the Young's modulus by the strength. During compression, the flow stress of several HNPG variants reaches the maximum value which is estimated according to the theoretical shear strength of gold and the modified Roberts-Garboczi-law. Altogether, this thesis provides a comprehensive study on the multiple interactions between structure and mechanical behavior in hierarchical nanoporosity.Publicationtype: Doctoral Thesis50 25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electrochemo-mechanical coupling in electrocatalytic oxidation of methanol on platinum(2025); This study investigates the impact of elastic strain on platinum electrode surfaces and its effects on both the adsorption of reactants and the electrocatalytic oxidation of methanol in alkaline solutions. By exploring the coupling between external mechanical modulation and electrocatalytic activity, the research aims to uncover how strain influences both the reaction currents (Faraday currents) and the adsorption enthalpy of methanol and its intermediates. To explore this coupling effect, two experimental approaches are employed. First, the impact of surface strain on adsorption enthalpy is assessed through surface stress variations, measured via cantilever bending experiments. Second, the strain effect on the overall reaction current is examined using Dynamic Electro-Chemo-Mechanical Analysis (DECMA). Here, a lock-in technique is utilized to capture strain-modulated reaction currents, quantifying them as functions of the applied potential. The study also discusses the role of uncompensated resistance in parameter adjustments, which is crucial for accurately interpreting the intrinsic coupling between externally applied strain and reaction currents on platinum thin film electrode surfaces. Methanol oxidation reaction (MOR) is selected as the model system due to its importance in various industrial applications, including fuel cells and chemical production. To gain a deeper understanding of the complex electrocatalytic processes involved in MOR, classical electrochemical characterization techniques are employed, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). These techniques are applied under different parameters and methanol concentrations to capture a comprehensive view of the reaction mechanisms. The findings reveal that external strain significantly influences the methanol oxidation current. Tensile strain enhances reactivity at low overpotentials, while compressive strain becomes more effective at higher overpotentials, a condition more typical in practical applications. However, the adsorption enthalpy of methanol exhibits negligible dependence on strain, suggesting that the observed changes in electrocatalytic activity are not primarily driven by alterations in methanol adsorption strength. These insights underscore the importance of considering strain effects in the design of high-performance electrocatalysts. Understanding and quantifying how mechanical strain impacts overall reaction rates provides valuable guidance for developing more efficient catalytic materials.Publicationtype: Doctoral Thesis68 36 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Mechanical behavior of nanoporous gold functionalized with surface oxide and organic films(2025); This thesis investigates functional mechanical properties of smart stimuli-sensitive hybrids based on nanoporous gold and organic layers. As a metal skeleton, unimodal and hierarchical nanoporous nested networks fabricated via an electrochemical dealloying were employed. Three electrochemistry-inspired approaches are integrated to surface modification of nanoporous gold – reversible capacitive assembly of ionic charges in the electric double layer, electrosorption of oxygen-species, and chemisorption of electroactive ferrocene-terminated alkanethiol self-assembled monolayers. To probe functionalities such as actuation and tunable stiffness, in situ experiments are carried out in electrochemical environment. In situ dilatometry is employed to investigate the impact of the electrochemicallycontrolled interface on the actuation behavior of the hybrids. The findings demonstrate remarkable differences in the strain responses of the materials with respect to the pore morphology and surface state. The contribution of the surface stress to the reproducible macroscopic actuation is experimentally confirmed for all hybrids under study. The negative values of the electrocapillary coupling parameters are determined pointing to the compressive surface stress in the metal surface upon the capacitive charging and electrosorption of oxygen-species. In nanoporous gold modified with the ferrocene-bearing self-assembled molecules, the actuation strain non-linearly scales with the ferrocene surface fraction testifying to the impact of the redox-events in the monolayer on the macroscopic actuation of the nanoporous network. The estimation of the mean actuation coefficient in the hybrids with the electroactive self-assembled monolayers pointed out to an enhanced strain response compared to those induced by conductive polymers. Along with actuation, the thesis explores the size-dependence of the effective elastic modulus of the hybrids via in situ dynamic mechanical analysis. Experimental observations in this study at a structural size below 100 nm support a hypothesis that the effective elastic behavior is dominated by the surface excess elasticity at this scale when specific adsorption is involved. Subsequently, the electro-elastic coupling parameters have been estimated for bulk nanoporous networks. Overall, this thesis discusses the electro-chemo-mechanical coupling in the nanoporous hybrids. It demonstrates that hierarchical nanoporous gold serves as a robust platform for enhancing the functional properties of nanoporous materials. Incorporating organic films allows nanoporous gold to convert chemical processes into mechanical motion, paving the way for further research and practical applications of nanoscale materials with tunable properties.Publicationtype: Doctoral Thesis62 40 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Krümmungs-Kapazitäts-Beziehungen gewellter Elektroden in elektrochemischen Doppelschichtkondensatoren(2023); Like capacitors, the storage mechanism of electrochemical double-layer capacitors is based on the formation of electrostatic fields between the electrodes and a double layer formed by the ions in the electrode-electrolyte interface. Beside cycle stability and the charging and discharging speed the performance is also characterized by the capacity of the electrodes. However the capaitance is strongly influenced by the size of the electrode surface on which it is possible for the electrolyte ions to accumulate in the form of a double layer. Due to their high surface area, amorphous carbons are considered typical electrode materials for common supercapacitors. Among others, these electrodes are characterized by convex and concave curvatures, among other things. However, the interaction of convex and concave regions and their superposition have rarely been investigated. In addition this work demonstrates a large impact of nanoscale surface curvatures, either convex or concave, on the capacitive performance of supercapacitors. Molecular dynamics simulations of trilayer graphene electrodes with convex and concave curved regions of different curvature radii are performed to describe this interaction in more detail. A 1.5 M solution of the ionic liquid 1-butyl-3-methylimidazoliumhexafluorophosphate [BMI][PF6] and acetonitil as solvent serves as the electrolyte. The definition of constant potentials at the electrodes allows the determination of the charge of each electrode atom depending on the time-dependent electrolyte configuration. The results show a variable behavior of the differential capacitance with respect to different curvature radii both for the overall electrode and when the concave and convex regions were plotted separately. Furthermore, this work describes the influence of the electrode curvature on the orientation of the cations adsorbed on the electrode surface and the composition of the ionic layer accumulated in the electrode-electrolyte-interface and correlates this influence with the differential capacitance peaks. The evaluation of more exhaustive simulations of selected systems by using the Unbinned Weighted Histogram Analysis Method (UWHAM) provides a higher statistical accuracy and allows the results to be presented as a continuous function of the applied potential. These calculations also reveals potential-dependent fluctuations of the differential capacitance, the compostion of the ionic layer and the cation orientation. If the curvature radii are of the same order of magnitude as the ions, the restricted space in the electrode-electrolyte interface causes a reduced number of possibilities for the orientation of the ions within this interface. This leads to changes in free energy compared to flat electrodes or electrodes with significantly larger curvature radii. Furthermore, the capacitance peaks of the negative electrode can be related not only to variations in the composition of the Helmholtz layer of the negative electrode, but also to variations at the positive electrode. It is found that both the applied potential and the electrode curvature determine the occurrence of capacitive peaks in the convex and concave regions and thus determine the capacitive performance of the electrode. Furthermore, by calculating a charge-charge covariance matrix from the charge fluctuations of the electrode atoms, the influence of individual atomic electrode charges or defined regions of the electrode on the distinct nontrivial features of the differential capacitance can be quantified in a sophisticated manner. This method can also be applied to more complex electrode models with different pore geometries, thus showing the influence of different morphological characteristics on the capacitance. For this reason, the method presented here can serve as a tool in defining design criteria of future electrodes.Publicationtype: Doctoral Thesis66 60 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Elektrisch angesteuerte Gold-Polymer-Gold-Ultrafiltrationsmembranen zur Aufbereitung NOM-haltiger Wässer(2022-04-08); Humanity is facing a water crisis due growing water demand of an increasing population, economic development, water pollution, and climate change. Ultrafiltration (UF) is a promising technology for countering the global water crisis due to its high removal potential for pathogens and turbidity at high recovery and low demand of chemicals. However, it suffers from membrane fouling and low removal performance for organic water constitutes such as natural organic matter (NOM). The selectivity for NOM increases when the pore size of the UF membrane is decreased but this also leads to reduction of permeability and energy efficiency. This problem is called the selectivity-permeability trade-off. Electrically conductive UF membranes are a new approach which might offer a solution to these problems. Most NOM are negatively charged. By application of a negative or positive electrical potential to the membrane surface, a repulsive or attractive force is induced on the charged substances in the feed water, respectively, which influences the rejection and fouling behavior of these membranes. In this work, an ultra-thin gold coating was applied on the active and support layer of flat-sheet polymer membranes to achieve electrical conductivity. Due to the coating of both sides of the membrane, no additional counter electrode was necessary to apply an external potential. An intrinsically negatively charged polyethersulfone (PES, UP150) and an intrinsically positively charged polyamide (M5) membrane were used for electro-repulsive and electro-sorptive filtration experiments, respectively. In the first part of this thesis, the membranes were characterized before and after the gold-coating regarding its filtration and electrochemical properties. Sputter coating only slightly changed the filtration properties of the membranes. The molecular weight cut-off was almost not affected by the gold coating. However, the pure-water permeability was reduced by 15 % and 40 % for the M5 and UP150 membrane, respectively. In the second part of this thesis, electro-repulsive filtration was conducted with model NOM solutions and natural lake water with the UP150 membrane. At filtration of Hohloh lake water the permeability decreased by 49 % (± 2 %) when no external potential was applied (0 V). However, when negative potential was applied the permeability only decreased by 17 % (± 3 %) (at -2.5 V). The application of negative potential to the membrane active layer led to less fouling and an increased NOM rejection at cross-flow mode. The molecular weight cut-off was shifted from 150 kDa at no applied potential to 5 kDa at -2.5 V (cell potential). Therefore, it could be seen that the duplex-coated membrane configuration was almost as effective as conventional counter electrode configuration in fouling mitigating and rejection enhancement. In the third part, electro-sorptive dead-end filtration experiments showed that the application of a positive potential led to adsorption of NOM and negatively charged organic dye molecules. When the potential was reversed to negative potential, the previously adsorbed substances could be desorbed. The process of electrosorptive UF worked with the intrinsically positively charged M5 membrane but not with the intrinsically negatively charged UP150. The molecular weight cut-off of the M5 membrane was shifted from approx. 1000 kDa at no applied potential to approx. 0.7 kDa at +2.5 V (cell potential). Therefore, the electrosorptive UF achieved a NOM rejection performance in the range of commercially available nanofiltration membranes. At the same time, the positively charged M5 membrane showed permeability in the range of loose UF membranes and fouling was not observed to be problematic. The additional energy consumption for the application of the external potential was low with 0.03 kWh/m³ of permeate. Overall, electro-repulsive and electro-sorptive UF membranes, both, broke the selectivity-permeability trade-off of the UF process. Whereas, the electrosorptive enhancement of NOM removal was more pronounced than the electro-repulsive.Publicationtype: Doctoral Thesis259 985 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Investigation of capillary effects on the grain scale by means of in situ experiments, imaging and numerical simulations(2022)In the framework of this habilitation thesis, the application of in situ (imaging) experiments, i. e., experiments run in an imaging environment, for studying geomechanical problems with focus on unsaturated granular soils is motivated and discussed. By means of computed tomography (CT), a non destructive imaging technique frequently applied in materials science, but also by means of much simpler imaging techniques, and custom-built experimental set-ups with geomechanical background, different processes in unsaturated granular soils are studied. In the thesis, different miniaturised set-ups for such "in situ (CT) experiments" are presented and applied to study (1) the shear strength and hydro-mechanical coupling and (2) the water retention behaviour of unsaturated granular media, based on the example of a coarse grained model sand and packings of artificial soda-lime glass beads. The data obtained from a temporal sequence of 2D or 3D images in in situ imaging experiments is visualised and analysed to investigate the microscale soil behaviour. By studying "capillary state variables", e. g., fluid clusters and interfacial properties, such as interfacial areas, the acquired image data allow for quantitative insights into the shear behaviour and water retention behaviour on the grain or pore scale. This helps to improve our understanding of links between the microscopic and macroscopic soil behaviour which is represented by shear strength, capillary cohesion, volume change or the water retention curve. Besides a discussion of different other experimental applications of in situ CT experiments with a geomechanical background, the thesis gives an outlook on numerical modelling based on CT data as geometry model input in particle or mesoscopic models which could lead to a so-called "virtual soil lab", allowing to study the hydro-mechanical behaviour of soils on a digital simulation-based basis. In this context, the application of the Finite-Discrete Element Method (FDEM) with granular CT data as input as well as the multiphase Lattice Boltzmann Method (LBM) for the simulation of multiphase flow in digitised pore spaces from CT data is presented.Publicationtype: Doctoral Thesis286 328 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Korrosionssimulation von pressgehärteten Stählen und Filiformfäden auf Aluminium(2021); A simulation model for the prediction of corrosion damage on coated press-hardened steel and aluminium, which may be parts of the automobile body, is developed. The properties of the metallic alloys and coatings are measured with metallographic and electrochemical methods (polarization curves, galvanostatic dissolution, impedance spectroscopy). A new approach to model the influence of corrosion products on the corrosion rate is presented.Publicationtype: Doctoral Thesis382 1626 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electrochemically tunable mechanical behavior of bulk nanoporous gold/polypyrrole(2021); This thesis explores the viscoelastic modulus and actuation behavior of an electroactuator based on nanoporous gold coated with polypyrrole film (NPG/PPy). The experiments were performed using in situ dynamic mechanical analysis under electrochemical control. Novel mechanical responses of NPG/PPy hybrids have been revealed. First, during the potential sweep, a reversible change in effective elastic modulus of the NPG/PPy hybrid is observed, characterized by alternating stiffening-softening features. This non-monotonous modulus response of NPG/PPy hybrids is different from the linear modulus behavior as a function of electrode potential of non-coated NPG. Moreover, the amplitude of the modulus variations increases with PPy thickness, showing a 7-fold increase for the thickest PPy film (∼ 40 nm) coated NPG as compared to non-coated NPG. It has been suggested that the modulus behavior strongly correlates with the processes in PPy matrix during doping and dedoping of the polymer. The competition between the enhanced intermolecular bonding due to the charged polymer chains-anion interactions and softening of PPy caused by the incorporated solvent has been suggested to lead to the non-monotonous modulus behavior. Second, the actuation strains of NPG/PPy electroactuator under both potential jumps and potential sweeps have been found to be invariant under increasing applied load. The actuation strain of NPG/PPy hybrids comprises of two parts: The free stroke induced by the swelling of PPy which is independent of load and the strain caused by the stiffness variations which is load-dependent. The findings of the modulus behavior mentioned above help to distinguish the strain contributions of these two factors to the total strain. These experiments allow evaluating a contribution of the electrochemically modulated elastic modulus of NPG/PPy to the actuation strain. Depending on the predeformation of the material as well as on the oxidation or reduction state of PPy, this contribution adds an expansion or contraction to the actuation strain. Yet, in the range of the applied loads tested, the actual performance is not affected by the load, demonstrating an enhanced load-bearing efficiency of NPG/PPy electroactuators in contrast to non-coated NPG.Publicationtype: Doctoral Thesis268 957 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fundamentals of the growth mechanism, tailored properties and applications of 3D hollow carbon foams(2019); The discovery of 1D carbon nanostructures in the 20th century and subsequently of 2D carbon nanostructures in the past century, a competition for possible applications of this material with outstanding physical properties began. Due to the geometric restriction of 1D and 2D carbon structures, a variety of manufacturing techniques have been developed around of the millennium to synthesise 3D carbon structures, which are also referred to as carbon aerogels or carbon foams. Based on their morphology, these structures offer decisive advantages over the existing structures and thus potentially new fields of application arise in the field of energy storage systems and catalysts. The growth mechanism of carbon structures has a property-defining influence on their properties. During their growth, carbon atoms arrange in ideal hexagonal lattices, which can also lead to defects that adversely affect the properties. These defects can be reduced by annealing processes such as a high-temperature treatment. Besides to defects, it was also shown that the number of carbon layers significantly affect the properties. This work concerned with the analysis of the growth mechanism of the 3D carbon structure Aerographite. This carbon foam has a tetrapodal morphology and is distinguished by its high mechanical and electrical properties at a density down to ~2 mg/cm3. Here, the influence of the growth mechanism via ex-situ studies and the tailoring of the properties such as electrical conductivity is investigated. The wall thickness of the tetrapods can be specifically varied by means of a targeted process control during the synthesis of Aerographite in the CVD process. A stepwise increase in the electrical conductivity occurs because of a subsequent temperature treatment of Aerographite, as a function of the temperature and its VI holding time. Additionally, the healing of Aerographite by a thermal treatment and the influence of the graphitisation on the properties of Aerographite is in focus in this work. Furthermore, this study aims a variation of the morphology and the manufacturing process for 3D carbon foams and the identification of possible applications. The focus here is on an application-related morphology and a comparatively simplified manufacturing process of the carbon structure. The developed carbon foam has a hierarchical globular morphology and is produced based on a porous ceramic template. Based on thermodynamic calculations, a wide range of catalytically active materials can be added to the green body, which are remains after the CVD process. Finally, a carbon foam with incorporated catalysts can be used for several applications.Publicationtype: Doctoral Thesis439 1160