Scientific publications

Electrochemical double layer capacitors (EDLCs) are inherently high power devices when compared to rechargeable batteries. While capacitance and energy storage ability are mainly increased by optimizing the electrode active material or the electrolyte, the power capability could be improved by including conductive additives in the electrode formulations. This publication deals with the use of four different carbon additives – two carbon blacks and two graphites – in standard activated carbon based EDLC electrodes. The investigations include: (i) physical characterization of carbon powder mixtures such as surface area, press density, and electrical resistivity measurements, and (ii), electrochemical characterization via impedance spectroscopy and cyclic voltammetry of full cells made with electrodes containing 5 wt.% of carbon additive and compared to cells made with pure activated carbon electrodes in organic electrolyte. Improved cell performance was observed in both impedance and cyclic voltammetry responses. The results are discussed considering the main characteristics of the different carbon additives, and important considerations about electrode structure and processability are drawn.

Most efforts to improve the energy density of supercapacitors are currently dedicated to optimized porosity or hybrid devices employing pseudocapacitive elements. Little attention has been given to the effects of the low charge carrier density of carbon on the total material capacitance. To study the effect of graphitization on the differential capacitance, carbon onion (also known as onion-like carbon) supercapacitors are chosen. The increase in density of states (DOS) related to the low density of charge carriers in carbon materials is an important effect that leads to a substantial increase in capacitance as the electrode potential is increased. Using carbon onions as a model, it is shown that this phenomenon cannot be related only to geometric aspects but must be the result of varying graphitization. This provides a new tool to significantly improve carbon supercapacitor performance, in addition to having significant consequences for the modeling community where carbons usually are approximated to be ideal metallic conductors. Data on the structure, composition, and phase content of carbon onions are presented and the correlation between electrochemical performance and electrical resistance and graphitization is shown. Highly graphitic carbons show a stronger degree of electrochemical doping, making them very attractive for enhancing the capacitance.

Hybrid materials represent one of the most growing new material classes at the edge of technological innovations. Unique possibilities to create novel material properties by synergetic combination of inorganic and organic components on the molecular scale makes this materials class interesting for application-oriented research of chemists, physicists, and materials scientists. The modular approach for combination of properties by the selection of the best suited components opens new options for the generation of materials that are able to solve many technological problems. This review will show in selected examples how science and technological driven approaches can help to design better materials for future applications.

Amphiphilic surface-functionalized titania nanoparticles were prepared in a Pickering emulsion applying hydrophobic or hydrophilic agents containing a phosphonate anchor group. The Pickering emulsion approach allows for the formation of anisotropic nanoparticles with a high degree of surface-functionalization that mimic the behavior of surfactants. Therefore, the efficiency of the formed particles in stabilizing emulsions was studied and it could be shown that the stability of emulsions substantially increases by addition of amphiphilic particles. Due to the photocatalytic activity of anatase nanoparticles the emulsions stability against creaming can be decreased by irradiation with UV light. The critical micelle concentration (CMC) of suspensions of amphiphilically modified nanoparticles was determined by conductivity measurements. The prepared surface-functionalized nanoparticles show similar characteristics as amphiphilic block copolymers.

A novel strategy for a directed nanoparticle coupling to isolated Stephanopyxis turris valves is presented. After pyrolysis, the valves exhibit incomplete wetting due to their characteristic T-shaped profiles as a prerequisite for a regioselective coupling reaction. A micromanipulation system allows for precise handling and their immobilization onto an adhesive substrate and manipulation into arrays.

The surface stability of yttria-doped tetragonal polycrystalline zirconia is critical for load-bearing biomedical applications. In this work, the small-scale mechanical behavior of this material is probed by employing the in situ micro-cantilever bending technique to near-surface regions. Micro-cantilevers are milled by the focused ion beam technique in the as-sintered condition as well as after hydrothermal degradation by water vapor and tested in order to investigate the effect of degradation on the local flexural response. Results demonstrate that the technique is reliable for assessing the mechanical properties of thin superficial layers and their dependence on orientation. In the non-degraded material, the flexural strength is surprisingly higher than in standard-size specimens and transformation-induced plasticity takes place during testing, inducing defects that become critical at the failure stress. The strength and stiffness of cantilevers obtained from the degraded surface are indeed much lower, and the magnitude of the effect clearly depends on their orientation with respect to the surface. These results are discussed in terms of the presence and spatial distribution of microcracks nucleated during hydrothermal degradation.

Measurement of surface tension (s.t.) and critical micelle concentration (c.m.c.) of a surfactant in dynamic condition is important for several engineering applications, for which, the interface between two or more different phases does not remain constant but alters and replenishes continuously with flow of the fluids so that equilibrium may not be reached between the bulk and the interface. There are however not many methods for measuring these quantities in dynamic experiments which mimics the real dynamic situations. In this report, we present a novel two-phase flow pattern inside a triple-helical micro-channel using which, we show that it may be possible to measure the dynamic s.t. of a liquid. When two immiscible liquids such as oil and water are pumped into it, at a certain range of flow rates, oil flows as the continuous phase, whereas water remains in it as a wavy filament, the wavelength of which varies with the flow rates of oil and water but also on the interfacial tension between these two liquids. We show that wavelength decreases with increase in concentration of a solute attaining a minima at the c.m.c. A simple scaling analysis captures most experimental observations.

Inspired by recent experiments on hierarchically structured adhesives, we analyze here the effect of spatial variation in surface topography and shear modulus of an elastomeric adhesive on its ability to adhere strongly to a flexible contactor. The undulation of surface and modulus both were assumed to be periodic with periodicity, which is either identical or different for the two parameters; for identical periodicity, the phase lag between the respective undulations is also systematically varied. Calculations show that during continuous lifting of the flexible contactor from complete initial contact, the interfacial crack between the two adherents does not propagate continuously but intermittently, with crack arrest and initiation at the vicinity of minimum thickness and modulus of the layer; the torque required to initiate an arrested crack increases significantly over that required to propagate it on a smooth adhesive surface. The adhesion strength estimated from the corresponding force vs displacement plot is calculated to be higher than that achieved on a smooth and featureless adhesive surface. For in-phase variation in topography and shear modulus of the layer, the adhesive strength is found to be higher than for nonzero phase lag between the two parameters. The adhesion strength is found to diminish also for nonidentical periodicity between modulus and surface undulation. We have derived a scaling law for relating adhesion strength to several of these parameters.

The efficient sunlight-induced crosslinking of 1,2-polybutadienes to generate fluorescent patterns with spatial resolution is reported. The employed photochemical conjugation method is based on a nitrile imine-mediated tetrazole-ene cycloaddition (NITEC), which proceeds under UV-light irradiation ([small lambda]max = 312 nm) at ambient temperature in the absence of any catalyst. The NITEC reaction between 1-pentene and a newly designed di-linker consisting of two photosensitive diaryl-substituted tetrazoles joined by a tetraethylene glycol spacer was investigated in an initial study. Detailed characterization of the small molecule model study was performed by size exclusion chromatography (SEC), UV-Vis and fluorescence spectroscopy, as well as electrospray-ionization mass spectrometry (ESI-MS), which is also employed for monitoring the reaction kinetics (100 % conversion in 20 min). Finally, light-triggered crosslinking is accomplished with two 1,2-polybutadienes of different molar masses. The crosslinking reaction parameters, such as concentration, di-linker fraction and reaction time were optimized via a SEC analysis and gravimetric determination of gel fractions. The applicability of the novel crosslinking technology for generating spatially controlled highly fluorescent gel patterns is demonstrated in a solvent-free reaction in 2 h in sunlight. The current study, in summary, thus introduces an efficient light-triggered technology platform for crosslinking polymers carrying non-activated double bonds.

We measured the shape change of periodic ridge surface profiles in gelatin organogels resulting from deformation driven by their solid-vapor surface stress. A gelatin organogel was molded onto poly-dimethylsiloxane (PDMS) masters having ridge heights of 1.7 and 2.7 [small mu ]m and several periodicities. Gel replicas were found to have a shape deformed significantly compared to their PDMS master. Systematically larger deformations in gels were measured for lower elastic moduli. Measuring the elastic modulus independently, we estimate a surface stress of 107 +/- 7 mN m-1 for the organogels in solvent composed of 70 wt% glycerol and 30 wt% water. Shape changes are in agreement with a small strain linear elastic theory. We also measured the deformation of deeper ridges (with height 13 [small mu ]m), and analysed the resulting large surface strains using finite element analysis.