Andreas Bund

Ion Current Rectification at Nanopores in Glass Membranes

The origin of ion current rectification observed at conical-shaped nanopores in glass membranes immersed in KCl solutions has been investigated using finite-element simulations. The ion concentrations and fluxes (due to diffusion, migration, and electroosmotic convection) were determined by the simultaneous solution of the Nernst-Planck, Poisson, and Navier-Stokes equations for the two-ion (K+ and Cl-) system. Fixed surface charge on both the internal and external glass surfaces that define the pore structure was included to account for electric fields and nonuniform ion conductivity within the nanopores and electric fields in the external solution near the pore mouth. We demonstrate that previous observations of ion current rectification in conical-shaped glass nanopores are a consequence of the voltage-dependent solution conductivity in the vicinity of the pore mouth, both inside and outside of the pore. The simulations also demonstrate that current rectification is maximized at intermediate bulk ion concentrations, a combination of (i) the electrical screening of surface charge at high concentrations and (ii) a fixed number of charge-carrying ions in the pore at lower concentration, which are physical conditions where the voltage dependence of the conductivity disappears. In addition, we have quantitatively shown that electroosmotic flow gives rise to a significant but small contribution to current rectification.

Electrochemical supercapacitors based on a novel graphene/conjugated polymer composite system

An efficient method for the preparation of a highly conducting hybrid material from graphene oxide nanosheets (GNS) and a novel conjugated polymer, poly(3,4-propylenedioxythiophene), is demonstrated. A functionalized monomer based on 3,4-propylenedioxythiophene, namely ProDOT–OH, was covalently functionalized with GNS, followed by oxidative polymerization to prepare GNS-f-PProDOT composites. The covalent functionalization process of GNS with the monomer ProDOT–OH was activated through the simple esterification reaction between the acyl chloride derivative on the nanosheets and the pendant hydroxyl group present in the monomer. Furthermore, the monomer functionalized GNS were co-polymerized with thiophene resulting in hybrid graphene nanostructures coated with highly conducting co-polymers with a room temperature electrical conductivity as high as 22.5 S cm−1. The resulting hybrid materials were characterized using a range of analytical techniques. The specific capacitance value of the composite and the co-polymer hybrids at a scan rate of 10 mV s−1 has been determined to be 158 and 201 F g−1 respectively and hence particularly promising for supercapacitors.

Investigations on metal depositions and dissolutions with an improved EQCMB based on quartz crystal impedance measurements

With the experimental set-up frequency and damping changes of resonating quartz crystals in electrochemical experiments can be measured at a time resolution of about 500 ms. The damping effects accompanying metal depositions and dissolutions (Ag and Cu) on polycrystalline Au electrodes of 6 and 10 MHz AT quartzes were studied and discussed in terms of roughness and sliding effects.

Novel amino-acid-based polymer/multi-walled carbon nanotube bio-nanocomposites: highly water dispersible carbon nanotubes decorated with gold nanoparticles

A well-reproducible and completely green route towards highly water dispersible multi-walled carbon nanotubes (MWNT) is achieved by a non-invasive, polymer wrapping technique, where the polymer is adsorbed on the MWNTs surface. Simply mixing an amino-acid-based polymer derivative, namely poly methacryloyl beta-alanine (PMBA) with purified MWNTs in distilled water resulted in the formation of PMBA-MWNT nanocomposite hybrids. Gold nanoparticles (AuNPs) were further anchored on the polymer-wrapped MWNTs, which were previously sonicated in distilled water, via the hydrogen bonding interaction between the carboxylic acid functional groups present in the polymer-modified MWNTs and the citrate-capped AuNPs. The surface morphologies and chemistries of the hybrids decorated with nanoparticles were characterized by transmission electron microscopy (TEM) and UV-visible absorption spectroscopy. Additionally, the composites were also prepared by the in situ free radical polymerization of the monomer, methacryloyl beta-alanine (MBA), with MWNTs. Thus functionalized MWNTs were studied by thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM) and TEM. Both methods were effective in the nanotube functionalization and ensured good dispersion and high stability in water over three months. Due to the presence of the high densities of carboxylic acid functionalities on the surface of CNTs, various colloidal nanocrystals can be attached to MWNTs.

Mechanism of electrostatic gating at conical glass nanopore electrodes.

The mechanism of molecule-based electrostatic gating of redox fluxes at conical glass nanopore (GNP) electrodes has been investigated using finite-element simulations. The results demonstrate that the fluxes of cationic redox molecules through the nanopore orifice can be reduced to negligibly small values when the surface charge of the nanopore is switched from a negative to a positive value. Electrostatic charge reversal can be affected by ionization of surface-bound moieties in response to environmental stimuli (e.g., photoionization or acid protonation), but only if the negative charge of the glass is included in the analysis. Numerical simulations of the responses of GNP electrodes are based on a simultaneous solution of the Poisson and Nernst-Planck equations and are in excellent agreement with our previously reported experimental results for electrostatic gating of the fluxes of Ru(NH 3) 6 (3+) and Fe(bpy) 3 (2+) at GNP electrodes with orifice radii between 15 and 100 nm. The gating mechanism is discussed in terms of three components: (1) migration of ionic redox species in the depletion layer adjacent to the electrode surface; (2) migrational transport along the charged pore walls; (3) electrostatic rejection of charged molecules at the pore orifice. The numerical results indicate that all three components are operative, but that ion migration along the pore walls is dominant.

Determination of the complex shear modulus of polymer solutions with piezoelectric resonators

The new procedure, which was worked out for the determination of complex shear moduli of polymer solutions, comprises an experimental setup working with impedance measurements at liquid covered quartz crystals (network analysis in the MHz range) and a calibration of these sensors concerning their surface roughness and an additional damping from the coupling of shear and compressional waves. As an example the frequency dispersion of the viscosity and shear modulus of aqueous poly(ethylene oxide) and dextran solutions is determined. The steady state value of the viscosity, which depending on concentration (0.1–40 wt.%) lies between 1 and 20 mPa s decreases by about 50% at 90 MHz.

Combining AFM and EQCM for the in situ investigation of surface roughness effects during electrochemical metal depositions

In this communication we clearly show that the surface roughness of Cu films strongly influences the signal of an EQCM. These effects were studied with a novel combination of AFM and EQCM, which proved to be a powerful tool for the in situ investigation of surface roughness effects during electrochemical metal depositions and dissolutions.

Electrocodeposition of Nickel Nanocomposites Using an Impinging Jet Electrode

The electrocodeposition of nickel alumina nanocomposites was investigated using an impinging jet electrode. The effects of jet flow rate, particle loading, and current density on the particle incorporation were studied. The amount of codeposited particles was determined using both electrogravimetric measurements and energy dispersive X-ray analysis. A maximum particle incorporation of about 5 wt % was found for a flow rate of 2.5 L min -1 and a current density of 10 A dm -2 . The microstructure of the coatings was investigated via X-ray diffraction. As a result of increasing current density and particle incorporation, a loss of (100) texture and a relative enhancement of the (111), (220), and (311) reflections appeared. The microhardness of the nickel films increased significantly with the inclusion of alumina nanoparticles.

Electrochemical investigations on the influence of electrolyte composition of Watts baths with special regard to throwing power

The influence of a variety of substances on technically relevant parameters of a nickel electroplating electrolyte (Watts bath) has been investigated. Special attention has been paid to the throwing power (TP), as well as visual appearance, current efficiency and codeposition of foreign atoms. Systems reported in the literature (e.g., inorganic salts to increase conductivity or complexing agents to increase polarization) were compared with typical reducing agents normally used in electroless Ni deposition. The mechanism of TP improvement in the case of sodium hypophosphite and dimethylamine-borane has been examined with the electrochemical quartz crystal microbalance (EQCM). Using the EQCM it was shown that there is a synergistic effect between the electrochemical and electroless deposition process. In addition, the activation energy of the latter was determined from temperature dependent measurements.

Liquid metal batteries - materials selection and fluid dynamics

Liquid metal batteries are possible candidates for massive and economically feasible large-scale stationary storage and as such could be key components of future energy systems based mainly or exclusively on intermittent renewable electricity sources. The completely liquid interior of liquid metal batteries and the high current densities give rise to a multitude of fluid flow phenomena that will primarily influence the operation of future large cells, but might be important for todays smaller cells as well. The paper at hand starts with a discussion of the relative merits of using molten salts or ionic liquids as electrolytes for liquid metal cells and touches the choice of electrode materials. This excursus into electrochemistry is followed by an overview of investigations on magnetohydrodynamic instabilities in liquid metal batteries, namely the Tayler instability and electromagnetically excited gravity waves. A section on electro-vortex flows complements the discussion of flow phenomena. Focus of the flow related investigations lies on the integrity of the electrolyte layer and related critical parameters.