Soma Vesztergom

Electrochemical control of single-molecule conductance by Fermi-level tuning and conjugation switching.

Controlling charge transport through a single molecule connected to metallic electrodes remains one of the most fundamental challenges of nanoelectronics. Here we use electrochemical gating to reversibly tune the conductance of two different organic molecules, both containing anthraquinone (AQ) centers, over >1 order of magnitude. For electrode potentials outside the redox-active region, the effect of the gate is simply to shift the molecular energy levels relative to the metal Fermi level. At the redox potential, the conductance changes abruptly as the AQ unit is oxidized/reduced with an accompanying change in the conjugation pattern between linear and cross conjugation. The most significant change in conductance is observed when the electron pathway connecting the two electrodes is via the AQ unit. This is consistent with the expected occurrence of destructive quantum interference in that case. The experimental results are supported by an excellent agreement with ab initio transport calculations.

The interface between HOPG and 1-butyl-3-methyl-imidazolium hexafluorophosphate.

The interface between highly oriented pyrolytic graphite (HOPG) and 1-butyl-3-metyl-imidazolium hexafluorophosphate (BMIPF6) has been studied using cyclic voltammetry, electrochemical impedance spectroscopy, immersion charge measurements and in situ scanning tunneling microscopy (in situ STM). The results are compared with those obtained with Au(100) in BMIPF6 (Phys. Chem. Chem. Phys., 2011, 13, 11627). The main result is that the high frequency capacitance spectra on the two systems are similar to each other, however at low frequencies some slow interfacial processes cause the appearance of a second capacitance arc on Au(100), which is absent for HOPG. The slow processes are attributed to the rearrangement of the Au surface structure and to the formation of ionic liquid adlayers--these are visualized by in situ STM.

The electrochemical degradation of poly(3,4-ethylenedioxythiophene) films electrodeposited from aqueous solutions

Abstract In this review, results of recent studies on the electrochemical stability and degradation properties of poly(3,4-ethylenedioxythiophene) films are summarized, with particular emphasis on the structural changes induced by overoxidation and electrochemical degradation. The most important electrodeposition methods for the preparation of PEDOT films in surfactant free aqueous media have also been summarized, and several experimental techniques suitable for monitoring the degradation process have been discussed. Morphological changes in PEDOT films during overoxidation have been analyzed. Overoxidation mechanisms proposed in the literature have been surveyed.

Electrical cross-talk in four-electrode experiments

AbstractThe subject of this paper is electrical cross-talk, an interference between the current/voltage characteristics of the two working electrodes in four-electrode (generator/collector) systems. Cross-talk arises in electrochemical cells of finite resistance due to the superposition of the electrical fields of the working electrodes, and often causes difficulties in the interpretation of measurement results. In this paper, we present an algorithm for modelling simple generation/collection experiments with a rotating ring–disk electrode (RRDE) immersed into a finite resistance solution of a redox couple. We show that based on the analysis of the Kirchhoff (Laplace) matrix of the simulation mesh, the effect of electrical cross-talk may be accounted for in such experiments. The intensity of cross-talk is found to be heavily influenced by the selection of the reference point for potential measurements; in practice, this is the position of the reference electrode or the tip of the Luggin probe. The devised model is validated by means of a simple and demonstrative experiment. Graphical Abstractᅟ

Characterization of the Capacitance of a Rotating Ring–Disk Electrode

The use of rotating ring–disk electrodes as generator-collector systems has so far been limited to the detection of Faradaic currents at the ring. As opposed to other generator-collector configurations, non-Faradaic detection has not yet been carried out with rotating ring–disk electrodes. In this study, a.c. perturbation based detection for measurement of the ring impedance is introduced. By using a conducting polymer-modified disk electrode in combination with a bare gold ring as a model, it is shown that the measured ring capacitance correlates with the polarization of the polymer film, most probably due to counter-ion exchange. A method of calculating the ring capacitance based on a small-signal sinusoid perturbation is described and the most important instrumental limitations are identified.

Characterization of the electrical conductivity of bone and its correlation to osseous structure

The interaction of osseous tissue with electric fields is an important subject. The electrical stimulation of bone promotes osteogenesis, while bone impedance has been proposed as a measure of osteoporosis, to follow fracture healing, or as a method to improve safety of surgical procedures. However, a deeper understanding of the electrical properties of bone and their relation to the architecture of osseous tissue is required to extend the range of use of electrical measurements to clinical studies. In this paper we apply electrical impedance spectroscopy to study the conductivity of fresh bovine tibia and we correlate the measured conductivities with its structural properties. Impedance was measured using a custom-made cell and a potentiostat. Bone conductivity was determined at 100 kHz, where the phase shift was negligible. A good agreement (R2 = 0.83) was found between the measured conductivity and the bone volume fraction, determined on microCT images. Based on this relationship, an equivalent circuit model was created for bone samples. The results of this ex-vivo study are comparable to previous in-vivo observations reporting bone resistivity as a function of bone density. This information can be used to construct a map of the tissue resistivity directly derived from clinical images.

Apparatus and methods for using a rotating ring–disk electrode with potentiodynamic control of both working electrodes

Abstract When studying electrochemical processes, one of the most widely used methods of determining the reaction pathway is an electrochemical assay of products using a rotating ring–disk electrode (RRDE). An RRDE tip consists of two electron conducting parts: a centrally located disk and a ring around it. When brought into contact with an electrolyte solution, the disk and the ring both form electrodes, the potentials of which can be independently controlled by a bi-potentiostat. When the tip is rotated, reactants from the solution arrive at the disk where they undergo an electrode reaction (oxidation or reduction) at a given rate, depending on the rotation speed. The formed products leave the disk surface and due to forced convection make their way towards the ring electrode where they can undergo another electrode reaction and can thus be detected. Normally, one applies potentiostatic control to at least one of the electrodes when carrying out an RRDE experiment; albeit the simultaneous potentiodynamic perturbation of the electrodes offers an increased applicability range. This paper presents the construction of a measuring system capable for the “bi-potentiodynamic” perturbation of two working electrodes, and demonstrates the use of such methods in case of a few chosen example systems.

Experimental methods for the determination of stress changes at electrified solid-liquid interfaces

Abstract In the present study some of the methods developed for the experimental determination of (interfacial or film) stress changes in electrochemical systems containing solid-liquid interfaces are briefly reviewed, as well as the kind and quality of information that can be obtained using these methods are discussed. The “bending beam” (“bending cantilever”, “laser beam deflection”, “wafer curvature”) method and related techniques, the piezoelectric method, the extensometer method, and the method based on the measurement of contact angle are discussed in detail. Special attention has been paid to problems related to the use of optical methods for position sensing.

Transport Matters: Boosting CO2 Electroreduction in Mixtures of [BMIm][BF4]/Water by Enhanced Diffusion

Room-temperature ionic liquids (RTILs) are promising new electrolytes for efficient carbon dioxide reduction. However, due to their high viscosity, the mass transport of CO2 in RTILs is typically slow, at least one order of magnitude slower than in aqueous systems. One possibility to improve mass transport in RTILs is to decrease their viscosity through dilution with water. Herein, defined amounts of water are added to 1-butyl-3methylimidazolium tetrafluoroborate ([BMIm][BF4 ]), which is a hydrophilic RTIL. Electrochemical measurements on quiescent and hydrodynamic systems both indicate enhanced CO2 electroreduction. This enhancement has its origin in thermodynamic/kinetic effects (the addition of water increases the availability of H+ , which is a reaction partner of CO2 electroreduction) and in an increased rate of transport due to lower viscosity. Electrochemically determined diffusion coefficients for CO2 in [BMIm][BF4 ]/water systems agree well with values determined by NMR spectroscopy.