500 Naturwissenschaften
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Scientific Unit
Improvement of hybrid supercapacitors by optimization of electrode design and material properties
(2018)
Lithium-ion batteries and supercapacitors have become indispensable energy storage devices for the steadily growing electrification. Both technologies possess unique advantages and disadvantages due to the inherently different energy storage principles involved. Hybrid supercapacitors (HSC) combine the advantages of the individual devices and have thus attracted considerable attention in recent years. In this thesis, electrode hybridization was investigated based on an optimized and synergetic combination of commercial lithium-ion battery and supercapacitor materials. This cost-effective approach is highly versatile since the electrode recipe can be precisely adjusted to a certain application via simple variation of the active material ratio or active material combination. A special focus of this thesis was set on the reference electrode design for HSC characterization, the influence of electrode microstructure on the electrochemical performance and the improvement of the rate performance of hybrid supercapacitors via enhancing the electrical conductivity of the active material. The latter was achieved via adjustment of the oxygen defect concentration and the associated titanium valence state of lithium titanate. This enabled the reduction of the carbon concentration of lithium titanate electrodes to 5 mass%, while yielding a high electrode capacity of about 70 mAh/g (82 mAh/g normalized to the active mass) at ultra-high C-rates of 100 C. When combined with an activated carbon / lithium manganese oxide composite cathode, an excellent energy and power performance of 70 Wh/kg and 47 kW/kg, respectively, was obtained (82 Wh/kg and 55 kW/kg normalized to the active mass), while maintaining 83 % of its energy ratings after 5,000 cycles at 10 C (78 % after 15,000 cycles at 100 C).
Transparent electrodes (TEs) are key components of modern optoelectronic devices like touch screens, solar cells, and OLEDs, but an inherent trade-off between high electrical conductivity and optical transparency limits the available material range. Indium tin oxide (ITO) has been dominating the market, but cannot provide the mechanical flexibility that novel devices based on polymer substrates require; high process temperatures required for high-grade ITO exceed the thermal budget of many polymers. Solutionprocessed metal grids from nanoscale building blocks are a promising alternative providing superior mechanical flexibility at cost-effective and scalable fabrication with low thermal budget. For this dissertation, ultrathin gold nanowires (AuNWs) from wet-chemical synthesis were explored as novel base material for TEs. Plasma sintering was shown to ameliorate the wires’ high contact resistances and poor stability. A novel nanoimprinting process was developed to pattern AuNWs into grids. The method relies on the large flexibility of the AuNWs and their ability to self-assemble into continuous hierarchical superstructures in the cavities of a pre-patterned elastomeric stamp. The process yielded ordered grids with submicron linewidth at low thermal budget, thus going beyond state-of-theart printed grids. The grids also showed competitive optoelectronic properties and superior mechanical flexibility to the incumbent materials and were applied as TEs in touch sensors.
Adhesion to substrates with surface roughness is a research field with many unsolved questions. A more thorough understanding of the underlying principles is important to develop new technologies with potential implications for instance in robotics, industrial automatization and wearable interfaces. Nature is a vast source of inspiration as animals have mastered climbing on various surfaces at high speed with several attachment and detachment events in a short time. In this work, new designs for dry adhesives inspired by natural blueprints are presented. Different strategies were explored to understand and tune adhesion on a range of substrates from smooth glass to polymers with skin-like roughness. Both the material properties and the geometry of the dry adhesives were utilized to improve adhesion strength. Three concepts are presented in this work: (i) composite structures with tunable interface, (ii) soft pressure sensitive adhesive layers, and (iii) funnel-shaped microstructures. This thesis aims for better understanding of the adhesion behavior as a function of several important factors including hold time, substrate material and roughness. The new concepts for bioinspired structures investigated in the present thesis will contribute to the development of performant, reversible adhesives for a variety of applications where surface roughness is involved.
The gecko is of high interest for scientists due to its ability to attach and to move on different surfaces with various roughnesses. To date, research groups worldwide aim to study adhesion mechanisms of gecko-like structures and to mimic gecko adhesion. However, most investigations have been performed in controlled environments and under near to ideal conditions, which present a significant constraint for transferring the results to applications. Therefore, two important parameters have been the subject of investigations in the present work, the surface roughness and elevated temperatures. For the first time, the impact of roughness on the adhesion of gecko-like, micropatterned structures was systematically studied. Two adhesive regimes, which are dependent on the pillar geometry and the roughness parameters, were discovered: an adhesive and a non –adhesive regime. The influence of the temperature on adhesion was studied on micropatterned samples fabricated out of three materials, which are interesting candidates for industrial applications. Promising correlations were determined between the temperature dependent mechanical properties and the adhesion values: the glass transition temperature was identified as the temperature of maximum adhesion. These results can support the improvement of bioinspired adhesives for industrial applications.
Switchable microtopographies based on the two-way shape memory effect in nickel-titanium alloys
(2016)
Nickel-titanium (NiTi) shape memory alloys are functional materials that are capable of undergoing a reversible temperature-induced shape change. Specifically in martensitic NiTi alloys, a reversible two-way shape memory effect can be induced using indentation techniques enabling a temperature-induced change in topography. Combining switchable topographies with nano- or microstructures could expand the properties of functional surfaces, and in addition make the surfaces responsive to their environment. For example, it would be possible to change the adhesive properties of surfaces with switchable dry adhesive microstructures or to control celladhesion on implant materials with specific nano- and micro-switchable structures. In this study, the indentation induced two-way shape memory effect was investigated in different NiTi alloys. In particular, the effects of alloy microstructure, deformation parameters (training) and thermal treatments on switchability were explored. In an austenitic NiTi alloy a specific thermal treatment led to the formation of coherent precipitates, which were shown to be crucial for the two-way shape memory behavior; exceeding the phase transformation temperature considerably decreased the switchability of the topography. At higher temperatures the stabilized martensite, which is required for an oriented phase transformation and consequently for the two-way shape memory behavior, transforms to austenite. An embossing and an electrochemical forming process were developed to prepare switchable topographies on larger areas. Both methods led to surface arrays on NiTi with two-way shape memory topographies. Finally, two approaches were presented, which use the switchable topographies to enable switching of a formerly passive surface function. In combination with bioinspired dry adhesive structures, the switchable NiTi topography led to a reversible, temperature-induced change of the adhesive properties of the surface. Secondly, the two-way shape memory effect was transferred to an alloy system with a phase transformation temperature near body temperature and a small width of hysteresis. By this, a switchable topography was induced, which is controllable within a physiological temperature range. The only issue impeding the use of this switchable surface for experiments on cell-surface interactions is an increased leakage of harmful copper ions. Therefore, surface passivation through oxidation is presented as a method to reduce the ion leakage.
Die Auswirkungen technisch relevanter Nanopartikel auf die Gesundheit des Menschen stellen eine Thematik von hoher gesellschaftlicher aber auch wissenschaftlicher Relevanz dar. Ziel des Vorhabens war es, durch Kombination chemischer, physikalischer und biologischer Kenntnisse und Techniken, Wechselwirkungen zwischen Zellen und Nanopartikeln zu analysieren und Mechanismen aufzuklären, welche eine möglicherweise toxische Wirkung von Nanopartikeln gegenüber menschlichen Zellen herbeiführen. Dieses Ziel sollte durch Einsatz moderner lichtmikroskopischer Techniken, vor allem der neu entwickelten STimulated Emission Depletion (STED-) Mikroskopie, einer hochauflösenden Variante der Konfokalmikroskopie erreicht werden. Mit Hilfe der STED-Technik können fluoreszenzmarkierte Nanopartikel und nanoskalige zelluläre Komponenten ortsaufgelöst sichtbar gemacht werden. Ziel war es, die Lokalisation und den Status der Partikel innerhalb der Zellen detailgenau zu analysieren. Neben der Partikellokalisation sollten die Auswirkungen der eingesetzten Partikel auf die Zellen anhand morphologischer Untersuchungen und biochemischer Tests erfasst werden. Um die Lokalisation der Partikel mit der biochemischen Antwort der Zellen, aber auch mit den Partikeleigenschaften zu korrelieren (Abbildung 1), war der Einsatz wohl definierter Nanopartikel erforderlich.[...]
Understanding and controlling adhesive interactions on the molecular scale is one of the main challenges in the field of nanotechnology. A new surface functionalization was developed in this thesis for investigating the molecular origin of adhesive interactions from single molecular level to assemblies of multiple bonds. The surface functionalization is based on supramolecular bonds established by the inclusion of ditopic connector molecules into two cyclodextrin (CD) molecules, one attached to a tip of an atomic force microscope and the other attached to a flat silicon surface. By using different connector molecules, the dynamics in friction and adhesion can be tuned. The dynamics of the molecular system were studied with respect to single bond kinetics and the flexibility of the attachment. The control of adhesion and friction was achieved by using photosensitive connector molecules which are sensitive to an external light stimuli. In order to enhance the applicability of the surface functionalization, the CD molecules were attached onto stiff polymers which can bridge the surface roughness of real contacts. The results of this thesis provide a deeper understanding of the molecular mechanisms underlying adhesive friction and open a new pathway for actively controlling friction and adhesion.
Numerical analysis of interfacial stress distributions and adhesion behaviour of fibrillar surfaces
(2016)
The climbing abilities of geckos have inspired many researchers to develop reusable, reversible adhesives. The fabrication of such synthetic adhesives has been well investigated. However, a full theoretical description is still lacking. The objective of the thesis is to improve the theoretical understanding of the mechanics of fibrillar adhesion and also to uncover the various factors influencing the adhesion of the compliant fibrils adhered to a rigid surface using finite element analysis. The effect of fibril geometry on the adhesion was examined. Straight punch and mushroom fibrils were examined numerically and it was found that mushroom fibrils show better adhesion compared to straight punch. Mushroom fibrils with higher stalk to cap ratio and smaller flap height show better adhesion when the corner singularity is considered as driving force for delamination. For these fibrils the detachment will begin from centre instead of corner. Some other shapes were also studied by introducing a fillet radius at the corner joining stalk and the cap. We propose a novel composite fibril with a stiff stalk and a softer tip to replicate the benefits shown by mushroom fibrils but with reduced manufacturing complications. The influence of Young’s modulus and tip height were studied along with different interfacial shapes joining the stiff stalk and soft tip. It is found that higher Young’s modulus ratio and smaller soft tip height result in higher adhesion strength. The results support the rational optimization of synthetic micropatterned adhesives.
Hydrogel biomaterials for wound care dressing and tissue gluing need to adhere to tissue and on-demand disappear. Advanced tissue adhesives also envision the encapsulation of therapeutic drugs or cells to promote the healing process. The design of hydrogels with all these functionalities is challenging. In this PhD, hydrogels that fulfil several of the previous properties for wound dressing at reasonable chemical complexity is presented. These hydrogels can be formed in situ and encapsulate cells, they can adhere to tissue and detach after use by light exposure at cytocompatible doses. The developed photodegradable hydrogels are based on 4-star PEG end-catechol precursors for crosslinking, and intercalate photocleavable o-nitrobenzyl groups in their structure. These gels can form at mild oxidative conditions and encapsulate cells or microparticles. UV-vis light exposure (λ= 365 or 405 nm) photocleavables the nitrobenzyl moiety and promotes degradation. This can occur at cytocompatible doses, and enables on-demand detachment from tissue and release of the encapsulated materials or cells. These biomaterials are interesting for the development of advanced tissue adhesives and cell therapies, by expanding the range of functionality of existing choices.
