Semi-analytical modelling of linear scan voltammetric responses for soluble-insoluble system: The case of metal deposition

The absence of general theoretical models describing linear sweep voltammetry (LSV) or cyclic voltammetry (CV) responses for soluble-insoluble systems such as one-step electrodeposition reactions under quasi-reversible condition makes it difficult to extract quantitative kinetic information from experimental voltammograms. In this work, a semi-analytical method for modelling LSV responses for one–step electrodeposition process is described, for a case where instantaneous nucleation takes place, such as metal deposition on same metal. Voltammetric peaks were analyzed following variation of both dimensionless rate constants and charge transfer coefficients in a broad range. Therefore, kinetic curves for electron transfer processes were established and fitted perfectly by sigmoidal Boltzmann function and linear models. With these models, LSV or CV experimental data can be used to measure electrodeposition reactions kinetics whatever the degree of reversibility. The Cu(I)/Cu(0) couple in acetonitrile was selected as an experimental example. The model developed in this work predicts accurately the current response for Cu electrodeposition reaction and an excellent experiment–theory agreement was found.

Polymer nanospray interfaces for on-line electrochemically induced modifications

Tatiana Rohner

The aim of this work has been to design, and fabricate a miniaturized electrospray-type interface for mass spectrometry analysis, which allows the nebulization of solutions by applying a high potential difference between the source and the mass spectrometer. The fabrication of the nanospray emitter is based on UV-laser photoablation of polymer substrates. Thick-film channel flow electrodes can be integrated in the microfluidic system, allowing the application of a high voltage to generate electrospray from the open outlet of the microchannel. The polymer-based interface first presented is characterized by a flat edge interface. However, a V-shaped nozzle has been found to present higher stability and ease of use. This design also allows the generation of spray by applying high voltage through an electrode located in a solution reservoir, thereby inducing an electroosmotic pumping. Taking advantage of the intrinsic electrolytic behavior of electrospray, a specific electrochemically induced tagging of free cysteine residues located in proteins can be performed on-line. A p-hydroquinone buffer is used to perform a continuous infusion of protein solution. When in contact with the high-voltage microelectrode, p-hydroquinone undergoes oxidation, thereby producing protonated p-benzoquinone. The latter reacts with free cysteine residues, following a Michael-type addition. The modification of the protein can be followed thanks to MS analyses. A complete study of the mechanisms involved in the electrochemically induced tagging with hydroquinone has been carried out by cyclic voltammetry and digital simulation. The process of the tagging consists of an ECE mechanism, since the formed adduct is stabilized in the reduced state, and is thus free to be oxidized. The rate constant of the chemical reaction has been extracted, by performing cyclic voltammetry studies and modeling the whole system by digital simulations. Finite element simulations have also permitted to highlight several mechanistic aspects of the electrotagging. The nanospray interface has been modeled as a channel-flow electrode cell. After validation of the model with respect to the experimental results, kinetics and mass transport conditions have been investigated. The flow profile, as well as the kinetic rate constant, has been shown to be of great importance in the electrochemically induced tagging. The use of a sacrificial electrode as metallic ion source to generate electrospray has also been investigated. Transition metal electrodes, i.e. copper, zinc, nickel and iron, as well as silver, were used to get on-line complexation with model peptides. It has been demonstrated that the use of in-reservoir sacrificial electrode was an efficient method to generate metal ions in order to form complexes with peptides, without the addition of metallic salts. These studies have demonstrated that polymer nanospray interfaces can be advantageously used as analytical devices, together with their primary function, i.e. ionization sources for protein analyses by mass spectrometry.

EPFL2003

Electrochemical incineration of chloromethylphyenoxy herbicides in acid medium by anodic oxidation with boron-doped diamond electrodes

Christos Comninellis, Béatrice Marselli Fuerer, Pierre-Alain Michaud

The electrochemical degradation of saturated solutions of herbicides 4-chloro-2-methylphenoxyacetic acid, 2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(4-chloro-2-methylphenoxy)propionic acid in 1 M HClO4 on a boron-doped diamond (BDD) thin film anode has been studied by chronoamperometry, cyclic voltammetry and bulk electrolysis. At low anodic potentials polymeric products are formed causing the fouling and deactivation of BDD. This is reactivated at high potentials when water decomposes producing hydroxyl radical as strong oxidant of organics. Electrolyses in a batch recirculation system at constant current density ≥8 mA cm−2 yielded overall decontamination of all saturated solution. The effect of current density and herbicide concentration on the degradation rate of each compound, the specific charge required for its total mineralization and instantaneous current efficiency have been investigated. Experimental results have been compared with those predicted by a theoretical model based on a fast anodic oxidation of initial herbicides, showing that at 30 mA cm−2 their degradation processes are completely controlled by mass transfer. Kinetic analysis of the change of herbicide concentration with time during electrolysis, determined by high-performance liquid chromatography, revealed that all compounds follow a pseudo first-order reaction. Aromatic intermediates and generated carboxylic acids have been identified using this technique and a general pathway for the electrochemical incineration of all herbicides on BDD is proposed.

2006

Anodic oxidation of phenol on Ti/IrO2 electrode: Experimental studies

Stéphane Fierro

The electrochemical oxidation of acidic solutions of phenol on a Ti/IrO2 anode has been investigated by cyclic voltammetry and bulk electrolysis in a single-compartment cell. In the potential region of oxygen evolution, anodic oxidation resulted in electrode passivation (as evidenced by voltammetric measurements) allegedly due to the formation of a polymeric film on its surface. Phenol degradation increased with increasing temperature in the range investigated 30-80 degrees C and it was affected by the addition of Cl- and Br- anions in the supporting electrolyte. Complete conversion of 10 mM phenol was achieved after 37 Ah L-1 of charge passed at 80 degrees C under galvanostatic conditions (50 mA cm(-2)) in absence of Cl-. In contrast only 10 Ah L-1 were needed in the presence of 35 mM Cl-. The presence of chloride can induce reactions involving chlorohydroxyl radicals and electrogenerated oxidants such as free chlorine. On the other hand, addition of Br- slightly inhibited degradation possibly due to bromide scavenging of electrogenerated active species. Phenol degradation proceeded through the formation of three dominant, aromatic intermediates, namely 1,4-benzoquinone, hydroquinone and pyrocatechol, while total oxidation to CO2 was not significant unless harsh conditions (i.e. high temperatures and charges) were employed. (C) 2010 Elsevier B.V. All rights reserved.

Elsevier2010

Polymer nanospray interfaces for on-line electrochemically induced modifications

Tatiana Rohner

EPFL2003