1,720,990 research outputs found
Second-order kinetics for EC' reactions at a spherical microelectrode
No full textA second-order (nonlinear) model is derived for steady-state kinetics of an EC' (catalytic electrochemical) reaction at a spherical microelectrode in the case where the electron transfer process is followed by a homogeneous chemical reaction regenerating the electroactive species. An asymptotic analysis of the model is performed, and the asymptotic results are compared with those from a numerical solution of the full nonlinear model. It is shown that in the fast reaction limit, where the current at the electrode takes its maximum possible value, the concentrations of the reactants are controlled by diffusion both close to and far from the electrode, with significant chemical activity occurring only in a narrow zone standing off the electrode. Also, it is shown that an equation obtained from a different asymptotic limit may be used to predict the limiting current at the microelectrode in all circumstances. The reasons for the surprising measure of agreement at the surface of the electrode are discussed, the predictions from the model of the limiting current are compared (favourably) with experimental results, and the model is compared with the standard pseudo-first-order model, which, although also based on a linearization of the governing equations, has a restricted range of validity
Scanning electrochemical microscopy: Amperometric probing of diffusional ion fluxes through porous membranes and human dentine
No full textThe scanning electrochemical microscope (SECM) was used to probe highly localised fluxes of electroactive species diffusing through the channels of porous materials. The validity and feasibility of the approach were demonstrated with the help of model experiments carried out on a track-etched polycarbonate membrane. This sample was chosen for its low porosity (5%) and its quasi-perfect 10 mu m diameter pores. While diffusion of redox species was induced by a large concentration gradient across the sample, other mass transport mechanisms were eliminated. The arrival of redox species, [Fe(CN)(6)](4-), was detected amperometrically by the SECM tip positioned a few micrometres above the mouth of the pores. Two basic SECM techniques were considered. Approach curves (tip current versus tip-substrate distance Z) were recorded to assess the extent of diffusion in the solution close to the substrate and to establish the optimum tip-substrate distance for the detection of the pores. Maps (tip current versus planar coordinates X, Y) were recorded to assess the distribution of diffusional fluxes. Images of localised diffusional fields were obtained and analyzed in terms of single ion fluxes associated with individual pores. More complex experiments were carried out with human dentine samples which had been characterised by atomic force microscopy (AFM). Dentine has a very high porosity (75%) with tubules varying from 2 to 5 mu m in diameter. It was chosen to challenge the ability of the SECM to image the fluxes of electroactive species diffusing through closely spaced pores and because ion transport through dentinal tubules is thought to be one of the major mechanisms for nerve stimulation in dentine hypersensitivity. As expected the diffusion fields from individual channels overlapped and showed the limitation of the SECM. However, the tip response was highly sensitive to the presence of obstruction inside the pores. Spatially resolved low current regions were analyzed in terms of blocked pores. Numerical simulations of diffusion through model porous substrates were used to predict the time dependence and spatial distribution of diffusional patterns in the presence and absence of the SECM tip. The experimental approach reported forms the basis of novel investigations for the assessment of the treatment of dentine hypersensitivity
The unexpected activity of Pd nanoparticles prepared using a non-ionic surfactant template
No full textPd deposits on vitreous carbon substrates were prepared by electrodeposition from liquid crystal phases (both micellar and hexagonal phases) consisting of self-assembled non-ionic surfactant molecules. The morphology of the deposits varied significantly with the concentration of the surfactant but all are made up of aggregated nanoparticles circa 9 nm in diameter. The deposits from the micellar phase of the surfactant offer the largest electroactive area and specific activity for the hydrogen evolution, oxygen evolution and reduction reactions and formic acid and ethanol oxidations. Unexpectedly the deposits lead to an increase in catalytic activity far in excess of that expected from an enhancement in electroactive are
Electron tunnelling at the Pt(100)|water interface
No full textElectron tunnelling through the Pt(100)\water interface was studied by numerical methods. Molecular dynamics simulation was used to generate equilibrated configurations of the system. Then the potential barrier profiles, experienced by a tunnelling electron, were obtained by calculating the potential energy of the electron due to the surrounding water molecules. Finally the tunnelling probability profile of the electron was computed in one dimension perpendicular to the platinum surface above different adsorption sites. The potential barrier profiles show that tunnelling is highly improbable from a top site and that it is also hindered from a bridge site. At a hollow site, however, the probability of tunnelling through the adsorbed water layer is highly increased; it is only restrained by the second water layer. The potential barrier in the bulk is nearly constant as it is expected for homogeneous dielectrics. The tunnelling probability profiles show that on average there is a steep decrease of the probability next to the Pt(100) surface and that it increases if the tip is 0.2 to 0.3 nm farther away in accordance with experimental observations. This indicates that our calculations give at least a qualitatively correct approximation of the tunnelling barrier. The agreement between theory and experiment also strongly supports the validity of our present picture of the structure of the Pt(100)\water interface
Improvement to the equation for the steady-state limiting currents at a microelectrode - Ec' processes (1st and 2nd-order reactions)
No full textThe aim of this communication is to present a more general expression to cover the situation when each species is allowed an individual diffusion coefficient and also to show the enhanced agreement with experimental data
Applications of the boundary element method in electrochemistry: scanning electrochemical microscopy, part 2
No full textBoundary element method (BEM) simulations are presented for a range of scanning electrochemical microscopy applications. Calculations are performed to quantify the effects of the surrounding shield for a range of tip geometries and produce three-dimensional images of electrodes embedded in substrate surfaces. Approach curves are presented for a range of experimentally exploited probes, including the sphere-cap electrode. In addition, the BEM is used to generate a line scan across the interface between a conducting and nonconducting substrate for different tip geometries. The comparative resolution at a fixed tip-substrate separation for a microdisk and microhemisphere probe is noted. Finally, three-dimensional images of raised and recessed hemispherical electrodes embedded in nonconducting flat substrates are generated and the results compared to the image of a microdisk electrode. <br/
An instrument for simultaneous EQCM impedance and SECM measurements
No full textA novel combination of an electrochemical quartz crystal microbalance (EQCM) and a scanning electrochemical microscope (SECM) has been built. Unlike conventional EQCMs, the instrument described here allows rapid in situ measurement of the modulus of the quartz crystal's transfer function. Data analysis in the complex plane for the Butterworth-Van Dyke (BVD) equivalent circuit yields the real and the imaginary components R (damping resistance) and XL (reactive inductance) of the crystal's electroacoustic impedance around its resonant frequency of 10 MHz, The influence of different tip shapes of an approaching microelectrode on the electroacoustic impedance of the quartz crystal was studied and found to be minimal for certain geometries, The capability of the EQCM/SECM instrument was tested in cyclic voltammetric plating/stripping experiments using a copper(I) chloride solution of high concentration in 1 M HCl, Four parameters, XL, R, the substrate, and the tip current, can be recorded simultaneously. Depletion layer effects were observed and could be corrected for to yield accurate current efficiencies for potentiodynamic and potentiostatic copper plating. The amperometric response of the SECM tip positioned closely to the substrate reflects the concentration changes of electroactive ions in the diffusion layer of the substrate electrode
Scanning electrochemical microscopy: potentiometric probing of ion fluxes
No full textThe scanning electrochemical microscope has been used to probe the diffusion layer by measuring the potential difference between a large reference electrode located in the bulk of the solution and a microindicator electrode positioned a few mum away from a substrate electrode. Ag micro-discs (10 and 50 mum diameter) have been used as potentiometric sensors to probe the concentration profile due to the diffusion of Ag+ to and from a planar Ag electrode. A similar experiment was carried out with an Ag microcylinder (50 mum diameter) as substrate electrode. In a separate set of experiments, Ag/AgCl microdiscs (10 and 50 mum diameter) were used to monitor the flux of Cl- consumed and generated by a film of polyaniline electrodeposited on Pt. The tip potential was recorded while cycling the potential of the PANI film past the first oxidation peak. Direct measurement of the ingress and egress of Cl- ions supports previously reported mechanisms for the oxidation of PANI films
A study of the preconcentration and stripping voltammetry of Pb(II) at carbon electrodes
Full text linkBoth glassy carbon electrodes and carbon fibre microelectrodes were shown to accumulate Pb2+ ions at open circuit, the amount of Pb2+ accumulated being determined by anodic stripping voltammetry in 0.1 M HCl following the accumulation step. The mechanism of Pb2+ accumulation was investigated by studying the effects of the pH of the solution, the concentration of Pb2+ and the preconcentration time on the amount of Pb2+ accumulated. For a 5 min preconcentration time at open circuit, using a glassy carbon electrode (4 mm diameter) and a carbon fibre microelectrode (8 mm diameter), detection of around 10 nM Pb2+ was obtained for both electrodes. The pH dependence of the amount of Pb2+ accumulated suggests that the Pb2+ ions are complexed by carboxylate groups at the surface of the carbon electrodes. This conclusion is supported by results obtained using basal plane and edge plane pyrolytic graphite electrodes and by studies of the effects of electrochemical oxidation of the glassy carbon surface on the magnitude of the lead stripping peak
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