1,720,975 research outputs found
Transient studies at microelectrodes
No full textTransient studies of electrochemical systems at microelectrodes allow analysis under rapid mass transport conditions. The small active area allows rapid resolution of charging currents, giving access to meaningful information even at short times. Sampled current voltammetry at microelectrodes (MSCV) is a multistep technique whereby data is collected from a series of potential step experiments along the redox wave of interest. Varying the sampling time allows comparison of how a reaction proceeds at varying timescales, whilst simultaneously showing the potential dependence. Selection of an appropriate sampling time tunes the rate of mass transport to give quasireversible conditions, allowing facile kinetic analysis using quasireversible models. Application to the oxygen reduction reaction (ORR) revealed unreasonably large currents at short times. This work suggests that pre-adsorbed oxygen at the electrode surface is responsible. The presence of the pre-adsorbed oxygen was confirmed by its direct reduction in argon purged solution, and its strong dependence on the metal substrate. The resultant peak potentials were used to calculate the binding energies of varying metals towards oxygen (?G?), which are in excellent agreement with the literature. This is useful, as ?G? is a popular descriptor for oxygen reduction activity. Once the pre-adsorbed oxygen is consumed, MSCVs for the ORR can be used for standard kinetic analysis using Tafel or Koutecky-Levich analyses, with the advantage of the electrode being oxide free before each data point is recorded
Pourbaix diagrams as a root for the simulation of polarization curves for corroding metal surfaces
No full textThe corrosion of metals is a much researched area thanks to its importance in the development of materials for a wide range of applications. A common means of experimental analysis of the corrosion rate of a specific metal is the recording of polarization curves and subsequent Tafel analysis. Comparison of such data with simulated examples provides useful validation of experimental data, as well as a better understanding of the specific reactions occurring at the metal surface. Many existing models require knowledge of parameters such as the exchange current density and Tafel slope, which requires the experiment to be conducted before the model is run. Here, we propose a model based around the Pourbaix diagram, where input parameters are either simply calculated from reaction schemes, or are easily accessible from thermodynamic data tables.In this work we use Pourbaix diagrams as a means for simulating a polarization curve at a corroding iron surface. Pourbaix diagrams show the boundaries between the changing thermodynamically stable species at a metal surface in an aqueous environment as a function of the applied potential and pH. The position of these boundaries can therefore be used to model the onset of the corresponding oxidation and reduction reactions by combining the equation of the appropriate boundary line with Butler-Volmer kinetics. At the same time, the change in pH local to the metal surface is monitored by simulating the flux of protons generated during the oxidation process, and the impact of this on the corrosion potentials and rate is taken into account. This is of great importance as the corrosion rate and the corrosion product varies according to the pH at the metal surface.In this way, we show a simple means for the simple simulation of a polarisation curve at an iron surface, which is in excellent agreement with an experimentally recorded curve under the same conditions. This same method can then be applied to more complex metal alloys such as stainless steels, by combining the Pourbaix diagrams for the appropriate alloy components. This allows the model to be used as a standalone analytical tool for the prediction of corrosion behaviour of novel alloys before they are developed, as well as for the validation of experimental data obtained from existing samples
Template-free electrochemical deposition of tellurium nanowires with eutectic solvents
No full textElectrochemical deposition of tellurium from deep eutectic solvents (DES) at gold film electrodes produced films of tellurium nanowires. The deposition was evenly distributed over the gold surface, with an average diameter of ~70 nm and length of ~1 μm. Deposition was extremely sensitive to the applied potential, tellurium concentration and deposition bath temperature, with deviations away from optimised conditions preventing the formation of the desired nanostructure. Interestingly, replacing the chloride in a popular eutectic solvent with bromide or iodide had a significant impact on the resultant film structure, with bromide giving no clearly defined nanostructure but iodide giving nanoplatelets. This demonstrates a strong control of tellurium nanostructure by halide ions, offering a way to two distinct nanostructures from the same core experimental set up
Electroless deposition of tellurium nanowires in eutectic solvents using immobilised silver islands
No full textIn this work we demonstrate a new approach towards the electroless deposition of tellurium nanowires in deep eutectic solvents. Unlike most electroless deposition where the substrate is sacrificed to drive the reduction, our process uses immobilised silver epoxy islands on gold films to give localised galvanic displacement of the silver, resulting in an even growth of wires across the entire gold electrode surface. We demonstrate the strong dependence of the nanostructure on the experimental conditions, with changes in bath temperature, tellurium concentration and the halide component of the solvent leading to sizeable alterations in the nanowire geometry. This demonstrates electroless deposition as a promising synthetic route towards low-dimensional tellurium nanostructures
Experiment-supported model development for data treatment of diffusion and activation limited polarization curves of magnesium and steel alloys
No full textCharacterization of any corroding system begins with determining its corrosion potential and rate. These two values serve as a preliminary measure of its surface passivation and kinetic activity which may then be investigated in more detail using local electrochemical or spectroscopic techniques. The potentiodynamic polarization curve (PDP) is the most common technique for simultaneous extraction of these two values, since it provides additional kinetic information in the form of Tafel slopes and can further be used to measure the pitting potential of a system. In the past, numerical analysis of these curves has proven challenging where mass transport limitations influence the currents measured. In this presentation we discuss a finite element model that has been developed to analyse the kinetics of corroding magnesium and steel alloys during PDP's where both activation and diffusion-controlled currents are present. Furthermore, the origins of the mass transport limitations present in these systems have been investigated in more detail through an analysis of the concentration profiles involved
Sampled-current voltammetry at microdisk electrodes: kinetic information from pseudo steady state voltammograms
Full text linkIn sampled-current voltammetry (SCV), current transients acquired after stepping the potential along the redox wave of interest are sampled at a fixed time to produce a sigmoidal current–potential curve akin to a pseudo steady state voltammogram. Repeating the sampling for different times yields a family of sampled-current voltammograms, one for each time scale. The concept has been used to describe the current–time-potential relationship at planar electrodes but rarely employed as an electroanalytical method except in normal pulse voltammetry where the chronoamperograms are sampled once to produce a single voltammogram. Here we combine the unique properties of microdisk electrodes with SCV and report a simple protocol to analyze and compare the microdisk sampled-current voltammograms irrespective of sampling time. This is particularly useful for microelectrodes where cyclic voltammograms change shape as the mass transport regime evolves from planar diffusion at short times to hemispherical diffusion at long times. We also combine microdisk sampled-current voltammetry (MSCV) with a conditioning waveform to produce voltammograms where each data point is recorded with the same electrode history and demonstrate that the waveform is crucial to obtaining reliable sampled-current voltammograms below 100 ms. To facilitate qualitative analysis of the voltammograms, we convert the current–potential data recorded at different time scales into a unique sigmoidal curve, which clearly highlights kinetic complications. To quantitatively model the MSCVs, we derive an analytical expression which accounts for the diffusion regime and kinetic parameters. The procedure is validated with the reduction of Ru(NH3)63+, a model one electron outer sphere process, and applied to the derivation of the kinetic parameters for the reduction of Fe3+ on Pt microdisks. The methodology reported here is easily implemented on computer controlled electrochemical workstations as a new electroanalytical method to exploit the unique properties of microelectrodes, in particular at short times
Extending the Lifetime of pH Microelectrode with Stabilized Palladium Hydride
No full textWe report a new fabrication method to produce palladium hydride pH microelectrode using a chemical approach to synthesize the palladium hydride. In contrast to electrochemically generated palladium hydride microelectrodes, chemically generated palladium hydride microelectrodes are longer lasting and importantly have a good analytical performance under aerobic conditions. Chemically generated palladium hydride microelectrodes perform best in acid to neutral electrolytes devoid of Cl−. They can readily be produced on 10 μm diameter disk platinum microelectrodes, which makes them attractive candidates for future localized electrochemical studies.</p
Enhancement of the enzymatic biosensor response through targeted electrode surface roughness
No full textThe field of enzymatic biosensors applied to brain electrochemistry has rapidly expanded over the last few years, thanks in part to their excellent selectivity to specific target species. Much current research is therefore focused on enhancing the electrochemical signal, which often involves the detection of stoichiometric amounts of H2O2 formed as part of the enzyme mechanism. This opens the possibility of enhancing a biosensor's performance by facilitating the H2O2 oxidation signal through surface modification. Here, we investigate the impact of the roughness of the platinum surface on the biosensor response, where rougher platinum surfaces show greater activity for H2O2 oxidation, and therefore enhanced biosensor sensitivity. Through careful manipulation of the electrode surface roughness, we are able to show a significant improvement to the LOD when using a rougher electrode surface. Additionally, we have shown that this enhanced surface roughness has no detrimental effects toward the electrode response time. This suggests that surface roughness could be a simple and easy to implement means of enhancing the sensitivity of electrode-based enzymatic biosensors, and is an important factor to consider when studying other aspects of biosensor fabrication
<i>(Invited)</i> Detection of D-serine using an enzymatic amperometric biosensor and its localized detection using scanning electrochemical microscopy
No full textD-Serine acts as an endogenous co-agonist for N-methyl-D-aspartate receptors at synapses, making it essential for proper brain development and function. This amino acid has also been linked to several neurodegenerative diseases such as Alzheimer's disease and dementia. Nevertheless, the primary site and mechanism of D-serine release remains unclear. We recently demonstrated the use of an enzymatic amperometric biosensor for the in vivo quantification of endogenous D-serine release in Xenopus laevis tadpoles. We also investigate the effect of the permselective polymer layer thickness on the biosensor's response time and selectivity. Finally, scanning electrochemical microscopy (SECM) is then used with the optimized biosensor to measure localized release of D-serine from a model system. This SECM methodology, which provides high spatial and temporal resolution, could be useful to investigate the primary site and mechanism of D-serine release in other biological samples
Finite element simulation of the coreactant electrogenerated chemiluminescence mechanism
No full textElectrogenerated chemiluminescence (ECL) is an electron transfer between redox products formed at an electrode that results in the formation of an excited state species, which is capable of photon emission. This excited state can be achieved by a reaction between an oxidized and a reduced form of the same luminophore, or via the reaction of the oxidised or reduced luminophore with an electrochemically generated co-reactant. This is of great interest to the biosensing community, as the attachment of multiple ECL-active luminophores to a target molecule is a very attractive signal amplification strategy. This is a complicated process involving multiple reaction steps, and so a thorough understanding of the complete reaction process and the evolution of the excited state luminophore is essential.In order to gain a greater understanding of the ECL mechanism, we use finite element digital simulations to explicitly model each reaction step. This was done for an organometallic ECL standard, tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+), and tripropylamine (TPA), where the simultaneous oxidation of Ru(bpy)32+ and TPA result in the generation of the excited state Ru(bpy)32+*. The geometry and reaction conditions were chosen to match experimental data from our previously developed cuvette based system. Comparison of simulated voltammetry and ECL emission both agreed well with the experimental data, validating the experimental results and also giving insight into the impact of the confined cuvette geometry on the observed voltammetry. Investigations into the simulated concentration profiles also revealed the ECL emission to be confined close to the electrode surface, and so the impact of side reactions at the counter electrode could be omitted. Importantly, each step in the ECL process could be individually analysed and quantified in order to provide a greater understanding of the mechanism as a whole.Figure 1: A) Simulated concentration of the excited state luminophore Ru(bpy)32+* at the position of peak ECL emission. B) Comparison of the experimental ECL emission peak with the simulated concentration of Ru(bpy)32+*, showing good agreement.</p
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