Single walled carbon nanotube channel flow electrode : hydrodynamic voltammetry at the nanomolar level

The use of single walled carbon nanotube (SWNT) band electrodes in a channel flow cell, for low concentration detection, with hydrodynamic voltammetry is reported. A two dimensional SWNT network electrode is combined with a one piece channel flow cell unit, fabricated by microstereolithography. This configuration provides well defined hydrodynamics over a wide range of volume flow rates (0.05–25 mL min− 1). Limiting current measurements, from linear sweep voltammograms, are in good agreement with the channel electrode Levich equation, for the one electron oxidation of ferrocenylmethyl trimethylammonium (FcTMA+), over a wide concentration range, 1 × 10− 8 M to 2.1 × 10− 5 M, with a detection limit of 5 nM. At the highest flow rates, some influence of the slightly recessed electrode geometry arising from the SWNT electrode fabrication is noted. However, this can be accounted for by a full simulation of the hydrodynamics and solution of the resulting convection–diffusion equation. Application of this hydrodynamic configuration to the voltammetric detection of dopamine is also demonstrated

In situ scanning electrochemical probe microscopy for energy applications

High resolution electrochemical imaging methods provide opportunities to study localized phenomena on electrode surfaces. Here, we review recent advances in scanning electrochemical microscopy (SECM) to study materials involved in (electrocatalytic) energy-related applications. In particular, we discuss SECM as a powerful screening technique and also advances in novel techniques based on micro- and nanopipets, such as the scanning micropipet contact method and scanning electrochemical cell microscopy and their use in energy-related research

New approaches for the study of dissolution kinetics at the microscopic level

This thesis is concerned with the development, application and theoretical treatment of the scanning electrochemical microscope (SEeM), with the aim of obtaining new insights into the kinetics and mechanisms of ionic crystal dissolution processes. The ultramicroelectrode (UME) probe of the SEeM, placed at close distances to the surface of an ionic single crystal face in contact with a saturated solution, was used to induce and monitor the dissolution processes of interest. This was achieved by stepping the potential at the UME from a value at which no electrode reaction occurred to one where a component of the saturated solution was electrolysed at a diffusion-controlled rate. The resulting undersaturation induced the dissolution process and dissolving material, after traversing the tip/substrate gap, was subsequently collected at the UME probe. The current-time behaviour provided quantitative information on the local dissolution rate. The SEeM was successfully used to determine the dissolution characteristics of the (010) face of monoclinic potassium ferrocyanide trlhydrate. A second-order dependence on the interfacial undersaturation was found, consistent with the classical Burton, Cabrera and Frank dissolution model. This investigation proved that the SEeM was capable of delivering high mass transport rates under well-defined conditions and demonstrated that the dissolution of an un symmetric salt could be described by classical theories. In addition, through the development of SEeM dissolution rate imaging, it was shown that it was possible to map the dissolution activity across single pits in the crystal surface with micrometre resolution. The kinetics and mechanism controlling the dissolution of silver chloride is a classical system which, despite a number of studies, remains unresolved. SECM studies of the dissolution of pellets and electrochemically grown films of Agel in aqueous solutions, both in the absence and presence of supporting electrolyte (where the supporting electrolyte does not contain a ion common to Agel), were carried out and the corresponding mass transfer theories developed. In the latter case dissolution was found to be diffusion-controlled, due to the build up of electroinactive ions in the tip/substrate gap, suppressing the attainment of high interfacial undersaturations. In contrast, in the absence of supporting electrolyte, where the principle of electroneutrality prevented this process, the dissolution kinetics were determined unequivocally. In order to significantly increase the spatial resolution of electrochemically induced SECM imaging, a new integrated electrochemical-atomic force microscopy (IEAFM) probe was developed, which simultaneously measured the topography of the surface while electrochemically inducing dissolution under conditions which closely mimicked those of SEeM experiments. Using this technique, it was demonstrated, for the first time, that dissolution of an ionic crystal surface (the (100) face of potassium bromide), under conditions of very low interfacial undersaturation, occurred by the dynamic unwinding of steps at the sites of screw dislocations. Through use of the high spatial resolution and well-defined mass transport characteristics of the SEeM, it was possible to determine the dissolution characteristics, in an area of a crystal surface devoid of dislocations and defects, i.e. a 'perfect' surface. Studies on the (100) face of copper sulfate pentahydrate demonstrated that dissolution, in low dislocation density areas, occurred via an oscillatory mechanism. A new hydrodynamic technique, the microjet electrode, was developed and found to be capable of achieving mass transfer coefficients up to 0.82 cm s-l. The ability of the technique to characterise fast surface processes was demonstrated through kinetic studies of the oxidation of ferrocyanide ions at a Pt electrode. Possible modifications to the technique in order to facilitate the characterisation of dissolution processes were considered

Impact of adsorption on scanning electrochemical microscopy voltammetry and implications for nanogap measurements

Scanning electrochemical microscopy (SECM) is a powerful tool that enables quantitative measurements of fast electron transfer (ET) kinetics when coupled with modeling predictions from finite-element simulations. However, the advent of nanoscale and nanogap electrode geometries that have an intrinsically high surface area-to-solution volume ratio realizes the need for more rigorous data analysis procedures, as surface effects such as adsorption may play an important role. The oxidation of ferrocenylmethyl trimethylammonium (FcTMA+) at highly oriented pyrolytic graphite (HOPG) is used as a model system to demonstrate the effects of reversible reactant adsorption on the SECM response. Furthermore, the adsorption of FcTMA2+ species onto glass, which is often used to encapsulate ultramicroelectrodes employed in SECM, is also found to be important and affects the voltammetric tip response in a nanogap geometry. If a researcher is unaware of such effects (which may not be readily apparent in slow to moderate scan voltammetry) and analyzes SECM data assuming simple ET kinetics at the substrate and an inert insulator support around the tip, the result is the incorrect assignment of tip–substrate heights, kinetics, and thermodynamic parameters. Thus, SECM kinetic measurements, particularly in a nanogap configuration where the ET kinetics are often very fast (only just distinguishable from reversible), require that such effects are fully characterized. This is possible by expanding the number of experimental variables, including the voltammetric scan rate and concentration of redox species, among others

Controlling palladium morphology in electrodeposition from nanoparticles to dendrites via the use of mixed solvents

By changing the mole fraction of water (χwater) in the solvent acetonitrile (MeCN), we report a simple procedure to control nanostructure morphology during electrodeposition. We focus on the electrodeposition of palladium (Pd) on electron beam transparent boron-doped diamond (BDD) electrodes. Three solutions are employed, MeCN rich (90% v/v MeCN, χwater = 0.246), equal volumes (50% v/v MeCN, χwater = 0.743) and water rich (10% v/v MeCN, χwater = 0.963), with electrodeposition carried out under a constant, and high overpotential (−1.0 V), for fixed time periods (50, 150 and 300 s). Scanning transmission electron microscopy (STEM) reveals that in MeCN rich solution, Pd atoms, amorphous atom clusters and (majority) nanoparticles (NPs) result. As water content is increased, NPs are again evident but also elongated and defected nanostructures which grow in prominence with time. In the water rich environment, NPs and branched, concave and star-like Pd nanostructures are now seen, which with time translate to aggregated porous structures and ultimately dendrites. We attribute these observations to the role MeCN adsorption on Pd surfaces plays in retarding metal nucleation and growth

Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage

The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close (not, vert, similar1 μm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface. The topography of the samples was determined using Ru(CN)64−, a solute to which the cartilage matrix was impermeable. This revealed a number of pit-like depressions corresponding to the distribution of chondrocytes, which were also observed by atomic force and light microscopy. Subsequent imaging of the same area of the cartilage sample for the diffusion-limited reduction of oxygen indicated enhanced, but heterogeneous, permeability of oxygen across the cartilage surface. In particular, areas of high permeability were observed in the cellular and pericellular regions. This is the first time that inhomogeneities in the permeability of cartilage toward simple solutes, such as oxygen, have been observed on a micrometer scale

Facet-resolved electrochemistry of polycrystalline boron-doped diamond electrodes : microscopic factors determining the aqueous solvent window in aqueous potassium chloride solutions

A systematic examination of the microscopic factors affecting the aqueous solvent (electrolyte) window of polycrystalline (p) boron-doped diamond (BDD) electrodes in chloride-containing salt solutions is undertaken using scanning electrochemical cell microscopy (SECCM), in conjunction with electron backscatter diffraction (EBSD) and Raman microscopy. A major focus is to determine the effect of local boron doping level, within the same orientation grains, on the solvent window response. EBSD is used to select the predominant (110) orientated areas of the surface with different boron-doped facets, thereby eliminating crystallographic effects from the electrochemical response. Voltammetric SECCM is employed, whereby a cyclic voltammogram (CV) is recorded at each pixel mapped by the meniscus-contact SECCM cell. The data obtained can be played as an electrochemical movie of potential-resolved current maps of the surface to reveal spatial variations of electroactivity, over a wide potential range, including the solvent (electrolyte) window. Local heterogeneities are observed, indicating that the solvent window is mainly linked to local dopant levels, with lower dopant levels leading to a wider window, i.e. slower electrode kinetics for solvent/electrolyte electrolysis. Furthermore, the effects of O- and H-surface termination of the BDD surface are investigated, for the same electrode (in the same area). The surface termination is a particularly important factor: the solvent window of an H-terminated surface is wider than for O-termination for similar boron dopant levels. Further, the anodic potential window of the O-terminated surface is greatly diminished due to chloride electro-oxidation. These studies provide new perspectives on the local electrochemical properties of BDD and highlight the importance of probing the electrochemistry of BDD at the level of a single crystalline grain (facet) in order to unravel the factors that control the solvent (aqueous) window of these complex heterogeneous electrodes

Finite element modeling of the combined faradaic and electrostatic contributions to the voltammetric response of monolayer redox films

The voltammetric response of electrodes coated with a redox-active monolayer is computed by finite element simulations based on a generalized model that couples the Butler–Volmer, Nernst–Planck, and Poisson equations. This model represents the most complete treatment of the voltammetric response of a redox film to date and is made accessible to the experimentalist via the use of finite element modeling and a COMSOL-generated report. The model yields a full description of the electric potential and charge distributions across the monolayer and bulk solution, including the potential distribution associated with ohmic resistance. In this way, it is possible to properly account for electrostatic effects at the molecular film/electrolyte interface, which are present due to the changing charge states of the redox head groups as they undergo electron transfer, under both equilibrium and nonequilibrium conditions. Specifically, our numerical simulations significantly extend previous theoretical predictions by including the effects of finite electron-transfer rates (k0) and electrolyte conductivity. Distortion of the voltammetric wave due to ohmic potential drop is shown to be a function of electrolyte concentration and scan rate, in agreement with experimental observations. The commonly used Laviron analysis for the determination of k0 fails to account for ohmic drop effects, which may be non-negligible at high scan rates. This model provides a more accurate alternative for k0 determination at all scan rates. The electric potential and charge distributions across an electrochemically inactive monolayer and electrolyte solution are also simulated as a function of applied potential and are found to agree with the Gouy-Chapman-Stern theory

Simulation of the cyclic voltammetric response of an outer-sphere redox species with inclusion of electrical double layer structure and ohmic potential drop

A finite-element model has been developed to simulate the cyclic voltammetric (CV) response of a planar electrode for a 1e outer-sphere redox process, which fully accounts for cell electrostatics, including ohmic potential drop, ion migration, and the structure of the potential-dependent electric double layer. Both reversible and quasi-reversible redox reactions are treated. The simulations compute the time-dependent electric potential and ion distributions across the entire cell during a voltammetric scan. In this way, it is possible to obtain the interdependent faradaic and non-faradaic contributions to a CV and rigorously include all effects of the electric potential distribution on the rate of electron transfer and the local concentrations of the redox species Oz and Rz−1. Importantly, we demonstrate that the driving force for electron transfer can be different to the applied potential when electrostatic interactions are included. We also show that the concentrations of Oz and Rz−1 at the plane of electron transfer (PET) significantly depart from those predicted by the Nernst equation, even when the system is characterised by fast electron transfer/diffusion control. A mechanistic rationalisation is also presented as to why the electric double layer has a negligible effect on the CV response of such reversible systems. In contrast, for quasi-reversible electron transfer the concentrations of redox species at the PET are shown to play an important role in determining CV wave shape, an effect also dependant on the charge of the redox species and the formal electrode potential of the redox couple. Failure to consider electrostatic effects could lead to incorrect interpretation of electron-transfer kinetics from the CV response. Simulated CVs at scan rates between 0.1 and 1000 V s−1 are found to be in good agreement with experimental data for the reduction of 1.0 mM Ru(NH3)63+ at a 2 mm diameter gold disk electrode in 1.0 M potassium nitrate