Publikationen

A solvent-free synthesis of hierarchical porous carbons is conducted by a facile and fast mechanochemical reaction in a ball mill. By means of a mechanochemical ball-milling approach, we obtained titanium(IV) citrate-based polymers, which have been processed via high temperature chlorine treatment to hierarchical porous carbons with a high specific surface area of up to 1814 m2 g−1 and well-defined pore structures. The carbons are applied as electrode materials in electric double-layer capacitors showing high specific capacitances with 98 F g−1 in organic and 138 F g−1 in an ionic liquid electrolyte as well as good rate capabilities, maintaining 87% of the initial capacitance with 1 M TEA-BF4 in acetonitrile (ACN) and 81% at 10 A g−1 in EMIM-BF4.

A new carbon model derived from in situ small-angle X-ray scattering (SAXS) enables a quantitative description of the voltage-dependent arrangement and transport of ions within the nanopores of carbon-based electric double-layer capacitors. In the first step, ex situ SAXS data for nanoporous carbon-based electrodes are used to generate a three-dimensional real-space model of the nanopore structure using the concept of Gaussian random fields. This pore model is used to derive important pore size characteristics, which are cross-validated against the corresponding values from gas sorption analysis. In the second step, simulated in situ SAXS patterns are generated after filling the model pore structure with an aqueous electrolyte and rearranging the ions via a Monte Carlo simulation for different applied electrical potentials. These simulated SAXS patterns are compared with in situ SAXS patterns recorded during voltage cycling. Experiments with different cyclic voltammetry scan rates revealed a systematic time lag between ion transport processes and the applied voltage signal. Global transport into and out of nanopores was found to be faster than the accommodation of the local equilibrium arrangement in favor of sites with a high degree of confinement.

A detailed understanding of confinement and desolvation of ions in electrically charged carbon nanopores is the key to enable advanced electrochemical energy storage and water treatment technologies. Here, we present the synergistic combination of experimental data from in situ small-angle X-ray scattering with Monte Carlo simulations of length-scale-dependent ion arrangement. In our approach, the simulations are based on the actual carbon nanopore structure and the global ion concentrations in the electrodes, both obtained from experiments. A combination of measured and simulated scattering data provides compelling evidence of partial desolvation of Cs+ and Cl− ions in water even in mixed micro–mesoporous carbons with average pore size well above 1 nm. A tight attachment of the aqueous solvation shell effectively prevents complete desolvation in carbons with subnanometre average pore size. The tendency of counter-ions to change their local environment towards high confinement with increasing voltage determines conclusively the performance of supercapacitor electrodes.

This work establishes molybdenum disulfide/carbon nanotube electrodes for the desalination of high molar saline water. Capitalizing on the two-dimensional layered structure of MoS2, both cations and anions can be effectively removed from a feed water stream by faradaic ion intercalation. The approach is based on the setup of capacitive deionization (CDI), where an effluent water stream is desalinated via the formation of an electrical double-layer at two oppositely polarized carbon electrodes. Yet, CDI can only be effectively applied to low concentrated solutions due to the intrinsic limitation of the electrosorption mechanism. By replacing the conventional porous carbon with MoS2/CNT binder-free electrodes, deionization of sodium and chloride ions was achieved by ion intercalation instead of ion electrosorption. This enabled stable desalination performance over 25 cycles in various molar concentrations, with salt adsorption capacities of 10, 13, 18, and 25 mg g-1 in 5, 25, 100, and 500 mM NaCl aqueous solutions, respectively. This novel approach of faradaic deionization (FDI) paves the way towards a more energy-efficient desalination of brackish water and even sea water.

Capacitive deionization (CDI) is a promising technology for the desalination of brackish water due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in oxygen-free electrolyte, but by contrast poor stability when oxygen is present due to a gradual oxidation of the carbon anode. This oxidation leads to increased concentration of oxygen-containing surface functional groups within the micropores of the carbon anode, increasing parasitic co-ion current and decreasing SAC. In this work, activated carbon (AC) was chemically modified with titania to achieve additional catalytic activity for oxygen-reduction reactions on the electrodes, preventing oxygen from participating in carbon oxidation. Using this approach, we show that the SAC can be increased and the cycling stability prolonged in electrochemically highly demanding oxygen-saturated saline media (5 mM NaCl). The electrochemical oxygen reduction reaction (ORR) occurring in our CDI cell was evaluated by the number of electron transfers during charging and discharging. It was found that, depending on the amount of titania, different ORR pathways take place. A loading of 15 mass% titania presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR.

We explore different electrode microstructures and the associated implications on the electrochemical stability of activated carbon/lithium titanate (Li4Ti5O12, LTO) composite electrodes by incrementally increasing the LTO content. At low LTO concentrations, the electrochemical stability is progressively improved with respect to neat activated carbon based electrodes. This trend is abruptly changed for high LTO concentrations (72 mass%) as the electrolyte starts to decompose unexpectedly far below the electrochemical stability boundaries of the single materials. We attribute this to a loss of electrical percolation and local degradation spots caused by peculiarities of the carbon distribution: Initially the sub-micrometer-sized LTO solely occupies spaces between the large, micrometer-sized activated carbon. With increasing LTO content the activated carbon particles get separated in an insulating LTO matrix. Electrochemical stability can be reestablished with electronic conduction paths of well distributed sub-micrometer-sized carbon black particles. By this way, cell degradation can be reduced and the cycle life of cells with high LTO concentration is prolonged from 10 to >36,000 cycles. Finally, we propose a simple method to distinguish cell fading caused by electrolyte decomposition from cell fading caused by poor electrical percolation.

Advanced medical devices, treatments and therapies demand an understanding of the role of interfacial properties on the cellular response. This is particularly important in the emerging fields of cell therapies and tissue regeneration. In this study, we evaluate the role of surface nanotopography on the fate of human dental pulp derived stem cells (hDPSC). These stem cells have attracted interest because of their capacity to differentiate to a range of useful lineages but are relatively easy to isolate. We generated and utilized density gradients of gold nanoparticles which allowed us to examine, on a single substrate, the influence of nanofeature density and size on stem cell behavior. We found that hDPSC adhered in greater numbers and proliferated faster on the sections of the gradients with higher density of nanotopography features. Furthermore, greater surface nanotopography density directed the differentiation of hDPSC to osteogenic lineages. This study demonstrates that carefully tuned surface nanotopography can be used to manipulate and guide the proliferation and differentiation of these cells. The outcomes of this study can be important in the rational design of culture substrates and vehicles for cell therapies, tissue engineering constructs and the next generation of biomedical devices where control over the growth of different tissues is required.

In this study, we propose a novel estimate of listening effort using electroencephalographic data. This method is a translation of our past findings, gained from the evoked electroencephalographic activity, to the oscillatory EEG activity. To test this technique, electroencephalographic data from experienced hearing aid users with moderate hearing loss were recorded, wearing hearing aids. The investigated hearing aid settings were: a directional microphone combined with a noise reduction algorithm in a medium and a strong setting, the noise reduction setting turned off, and a setting using omnidirectional microphones without any noise reduction. The results suggest that the electroencephalographic estimate of listening effort seems to be a useful tool to map the exerted effort of the participants. In addition, the results indicate that a directional processing mode can reduce the listening effort in multitalker listening situations.

Spherosilicates and polyhedral oligomeric silsesquioxanes represent unique well-defined rigid building blocks for molecular and hybrid materials. Drawbacks in their synthesis are often low yields and the restricted presence of functional groups either based on incomplete transformation of all corners or the reactivity of the functional groups. Particularly amine-functionalization reveals some synthetic challenges. In this study we report the synthesis of a new class of octafunctionalized hydrogen bond forming spherosilicates via a facile route based on octabromo alkyl functionalized cubic spherosilicates. Four different alkyl chain lengths, namely C4, C5, C6 and C11, were realized starting from [small omega]-alkenylbromides via hydrosilylation of Q8M8H. Using sodium azide in a mixture of acetonitrile : DMF = 10 : 1, the octaazide was obtained quantitatively and could be rapidly transformed in an octaamine cube via catalytic hydrogenation over Pd/C in absolute ethanol. The following reaction to hydrogen bond forming spherosilicates was performed in situ by adding propyl isocyanate. All transformations proceed quantitatively at the eight corners of the cube, which was evidenced by NMR spectroscopy and ESI-MS measurements. The Q8-target compound can be separated after each reaction step over simple chemical workup while no cage rearrangement was observed. The structures were confirmed using 1H, 13C, 29Si-NMR, FT-IR, elemental analysis and ESI-MS. The method opens a high yield route (overall isolated yield 83-88%) for structural building blocks in hybrid materials.