Peter J. Mahon

Measurement and modelling of the high-power performance of carbon-based supercapacitors

Abstract Supercapacitors are now being looked at for use in higher power applications such as mobile telecommunications and hybrid electric vehicles. We have examined two different supercapacitors, one is a commercial sample and the other is a supercapacitor of our own design. Four different testing methods including Impedance Spectroscopy, Constant Current Charging, Cyclic Voltammetry and Power Cycling were applied to each supercapacitor and the results are reviewed. Parameter values obtained from Impedance Spectroscopy are excellent for comparing supercapacitors under equilibrium conditions but correlate poorly with data obtained from the more useful Power Cycling Charts (PCC). The choice of the current step size in Constant Current Charging and the scan rate in Cyclic Voltammetry has a large bearing on the results obtained from these techniques. The strong voltage dependence of the parameters for the commercial sample prevented analysis using Cyclic Voltammetry. It was also clearly demonstrated by Power Cycling that the commercial sample had the poorer power performance of the two supercapacitors tested. It is concluded that for high-powered applications such as telecommunications and wireless protocols the most useful comparison of supercapacitor capability is through the PCC.

Suppressed Polysulfide Crossover in Li–S Batteries through a High-Flux Graphene Oxide Membrane Supported on a Sulfur Cathode

Utilization of permselective membranes holds tremendous promise for retention of the electrode-active material in electrochemical devices that suffer from electrode instability issues. In a rechargeable Li–S battery—a strong contender to outperform the Li-ion technology—migration of lithium polysulfides from the sulfur cathode has been linked to rapid capacity fading and lower Coulombic efficiency. However, the current approaches for configuring Li–S cells with permselective membranes suffer from large ohmic polarization, resulting in low capacity and poor rate capability. To overcome these issues, we report the facile fabrication of a high-flux graphene oxide membrane directly onto the sulfur cathode by shear alignment of discotic nematic liquid crystals of graphene oxide (GO). In conjunction with a carbon-coated separator, the highly ordered structure of the thin (∼0.75 μm) membrane and its inherent surface charge retain a majority of the polysulfides, enabling the cells to deliver very high initial dis...

Molecular Organization of the Nanoscale Surface Structures of the Dragonfly Hemianax papuensis Wing Epicuticle

The molecular organization of the epicuticle (the outermost layer) of insect wings is vital in the formation of the nanoscale surface patterns that are responsible for bestowing remarkable functional properties. Using a combination of spectroscopic and chromatographic techniques, including Synchrotron-sourced Fourier-transform infrared microspectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS) depth profiling and gas chromatography-mass spectrometry (GCMS), we have identified the chemical components that constitute the nanoscale structures on the surface of the wings of the dragonfly, Hemianax papuensis. The major components were identified to be fatty acids, predominantly hexadecanoic acid and octadecanoic acid, and n-alkanes with even numbered carbon chains ranging from C14 to C30. The data obtained from XPS depth profiling, in conjunction with that obtained from GCMS analyses, enabled the location of particular classes of compounds to different regions within the epicuticle. Hexadecanoic acid was found to be a major component of the outer region of the epicuticle, which forms the surface nanostructures, and was also detected in deeper layers along with octadecanoic acid. Aliphatic compounds were detected throughout the epicuticle, and these appeared to form a third discrete layer that was separate from both the inner and outer epicuticles, which has never previously been reported.

Voltammetric modelling via extended semiintegrals

Abstract The modelling of many voltammetric experiments can be carried out expeditiously by making use of semiintegration or its converse, semidifferentiation. The virtue of this approach is that the modelling, be it algebraic, simulative or numerical, takes place in one dimension only—that of time—rather than in the dual dimensions of space and time. However, the applicability of pure semiintegration is limited to experiments in which transport is by planar semiinfinite diffusion, preceded by a state in which no current flows, and without concurrent homogeneous reactions. In this article it is demonstrated that all these limitations may be overcome by broadening the concept of semiintegration to include other convolutions that reduce to semiintegration in the short-time limit. Appropriate convolutions are derived for spherical and cylindrical geometries, for thin-layer and Nernst-layer electrodes, for faradaic processes complicated by homogeneous reactions of the EC, CE and ECE varieties, and for voltammetry preceded by a steady state, but this list does not exhaust the possibilities. Although controlled-current experiments are most readily modelled by the extended semiintegral approach, a powerful procedure is described by which numerical one-dimensional modelling is applicable to controlled-potential voltammetry. Three worked examples are presented in detail: constant-current chronopotentiometry at a wire electrode; a Nernst diffusion layer problem in which the current is shared by a faradaic path and by double-layer charging; and cyclic voltammetry complicated by a following chemical reaction.

Global kinetic analysis of cyclic voltammograms at a spherical electrode

Abstract The global method for analyzing electrochemical processes controlled by diffusion and by the kinetics of heterogeneous electron transfer is redeveloped or application to electrodes haveing a spherical geometry. Heretofore, the method has been restricted to experiments in which plannar diffusion is the sole mode of transport to the electrode surface. The global method is based on a three-dimensional concept and has the advantage of using all of the data available from a voltammogram to measure the heterogeneous charge-transfer rate constant, the reversible halfwave potential, the diffusion coefficient, and the charge-transfer coefficient. Cyclic voltammetry of 2-methyl-2-nitropropane reduction at a mercury sphere is reported. The parameters measured by spherical global analysis are concordant and in excellent agreement with literature values.

Applications of Convolution Voltammetry in Electroanalytical Chemistry

The robustness of convolution voltammetry for determining accurate values of the diffusivity (D), bulk concentration (C(b)), and stoichiometric number of electrons (n) has been demonstrated by applying the technique to a series of electrode reactions in molecular solvents and room temperature ionic liquids (RTILs). In acetonitrile, the relatively minor contribution of nonfaradaic current facilitates analysis with macrodisk electrodes, thus moderate scan rates can be used without the need to perform background subtraction to quantify the diffusivity of iodide [D = 1.75 (±0.02) × 10(-5) cm(2) s(-1)] in this solvent. In the RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, background subtraction is necessary at a macrodisk electrode but can be avoided at a microdisk electrode, thereby simplifying the analytical procedure and allowing the diffusivity of iodide [D = 2.70 (±0.03) × 10(-7) cm(2) s(-1)] to be quantified. Use of a convolutive procedure which simultaneously allows D and nC(b) values to be determined is also demonstrated. Three conditions under which a technique of this kind may be applied are explored and are related to electroactive species which display slow dissolution kinetics, undergo a single multielectron transfer step, or contain multiple noninteracting redox centers using ferrocene in an RTIL, 1,4-dinitro-2,3,5,6-tetramethylbenzene, and an alkynylruthenium trimer, respectively, as examples. The results highlight the advantages of convolution voltammetry over steady-state techniques such as rotating disk electrode voltammetry and microdisk electrode voltammetry, as it is not restricted by the mode of diffusion (planar or radial), hence removing limitations on solvent viscosity, electrode geometry, and voltammetric scan rate.

Toward rational design of organic dye sensitized solar cells (DSSCs): An application to the TA-St-CA dye

A computer aided rational design has been performed on TA-St-CA dye sensitizer in order to improve the desirable properties for new organic dye sensitized solar cell (DSSC). A number of electron-donating (ED) and electron-withdrawing (EW) units based on Dewars rules are substituted into the π-conjugated oligo-phenylenevinylene bridge of the reference TA-St-CA dye. The effects of these alternations on the molecular structures and the electron absorption spectra are calculated using time-dependant density functional theory (TDDFT). It is found that chemical modifications using electron donating (ED) substitutions exhibit advantages over the electron withdrawing (EW) substitutes to reduce the HOMO-LUMO energy gap as well as the electron distribution of the frontier orbitals of the new dyes. Dewars rule is a useful guideline for rational design of new dye sensitizers with desired HOMO-LUMO gap. The impact on the optical spectra of new dyes are, however, less significant.

Incorporating electrode kinetics into the convolutive modeling of reactions at planar, cylindrical and spherical electrodes

Abstract Convolutive modeling provides a valuable alternative to digital simulation as a means of predicting the outcome of a voltammetric experiment, for comparison with the laboratory version. Methods based on either of two conjugate functions are described, differing in the direction that the convolution takes. Other than the requirement of uniform accessibility of the electrode surface, convolutive modeling places few restrictions on the range of experiments that may be modeled. The electrode reaction may have any degree of reversibility and may, or may not, be coupled to a first-order chemical process. The diffusivities of the reactant and product may be equal or unequal. The current may be the controlled electrical variable, or the current may be monitored in an experiment in which the potential is controlled. A range of experimental techniques is addressed in this article, including current-reversal chronopotentiometry and cyclic voltammetry without and with a following chemical reaction. Algorithms are reported for each of the two convolution routes. Examples treated in detail include both planar and spherical diffusion fields. The heterogeneous rate constant was varied in all instances, reversible, quasi-reversible, and near-irreversible cases being considered. Differences between the predictions of the two routes was found to be insignificant, both of the theoretical voltammograms agreeing excellently with analytical formulas, where these are available for comparison. In the case of quasi-reversible cyclic voltammetry, the prediction of the convolutive model was evaluated by global analysis: the input parameters were recovered with only minor discrepancies.

Natural Insect and Plant Micro-/Nanostructsured Surfaces: An Excellent Selection of Valuable Templates with Superhydrophobic and Self-Cleaning Properties

Insects and plants are two types of organisms that are widely separated on the evolutionary tree; for example, plants are mostly phototrophic organisms whilst insects are heterotrophic organisms. In order to cope with environmental stresses, their surfaces have developed cuticular layers that consist of highly sophisticated structures. These structures serve a number of purposes, and impart useful properties to these surfaces. These two groups of organisms are the only ones identified thus far that possess truly superhydrophobic and self-cleaning properties. These properties result from their micro- and nano-scale structures, comprised of three-dimensional wax formations. This review analyzes the surface topologies and surface chemistry of insects and plants in order to identify the features common to both organisms, with particular reference to their superhydrophobic and self-cleaning properties. This information will be valuable when determining the potential application of these surfaces in the design and manufacture of superhydrophobic and self-cleaning devices, including those that can be used in the manufacture of biomedical implants.

A fresh approach to voltammetric modelling.

A modular semianalytic procedure, alternative to simulation, is described for predicting the transient current response to any applied potential signal under a wide variety of voltammetric conditions. Diverse geometries may be treated; the electrode reaction may have any degree of reversibility; homogeneous chemical reactions may be involved. Conceptually, the method consists of splitting the overall problem into three components, each of which is solved separately. Examples presented include cyclic voltammetry and (Osteryoung) square-wave voltammetry.