Search

The electrochemical reduction of oxygen on Pd nanoparticle-decorated nitrogen-doped graphene (Pd/NG) catalyst has been investigated in acid and alkaline solutions using the rotating disk electrode (RDE) method. The Pd/NG nanocomposites were prepared by NaBH 4 reduction. The surface characterisation of Pd/NG composites was performed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TEM images showed the uniform distribution of Pd nanoparticles over the N-doped graphene nanosheets. The RDE results indicated that Pd/NG catalysts possess a high electrocatalytic activity towards the oxygen reduction reaction (ORR). The prepared materials catalyse a 4-electron reduction of oxygen. The Tafel behaviour of O 2 reduction on Pd/NG catalysts was similar to that of bare Pd. The Pd/NG composite showed excellent ORR performance in alkaline media and is a promising material to be used as a cathode catalyst for alkaline membrane fuel cells.

The electrochemistry of copper(I) and copper(II) chloride complexes in the RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, BMPTf2N, has been studied with constant current electrolysis and cyclic staircase voltammetry at temperatures between 21.0 and 96.0 °C and in different concentrations of chloride. The chloride concentration was controlled by addition of 1-butyl-1-methylpyrrolidinium chloride, BMPCl. An important finding is the evidence of a three-coordinated complex, Cu(I)Cl32− which has not been found in organic chloroaluminates without a significant increase in temperature. Two Cu(I) species were found at molar ratios of [Cl−]/[Cu]

Kinetics of electron transfer of the Cu(I)/Cu(II) redox couple at a platinum electrode has been studied with chronoamperometry, cyclic voltammetry and impedance spectroscopy in a deep eutectic solvent consisting of choline chloride and ethylene glycol. At 25 °C, the reaction was found to be quasi-reversible with a relatively high rate constant k0 of 9.5 ± 2 × 10−4 cm s−1, and a charge transfer coefficient α of 0.25 ± 0.05. Diffusion coefficients for the Cu(I) and Cu(II) complexes were determined to be 2.7 ± 0.1 × 10−7 and 1.5 ± 0.1 × 10−7 cm2 s−1, respectively. The viscosity of the electrolyte was 41 ± 3 mPa s. The temperature dependency was also investigated. The activation energy of mass transfer was found to be 27.7 ± 1 kJ mol−1 and that of electron transfer 39 ± 7 kJ mol−1. Speciation of the Cu(I) and Cu(II) complexes was determined using UV–VIS spectroscopy, and the prevailing Cu(I) complex was found to be [CuCl3]2− and that of Cu(II) [CuCl4]2−. Highlights ► Kinetics of Cu2+/Cu+ redox process in a deep eutectic solvent. ► Comparison of cyclic voltammetry and impedance spectroscopy. ► Diffusion coefficient of Cu+ twice that of Cu2+. ► Heterogeneous kinetics fast, transfer coefficient is 0.25. ► Activation energy of mass transfer identical to activation energy of viscous flow.

Glassy carbon electrodes were grafted with carboxyphenyl groups by reduction of 4-carboxyphenyldiazonium tetrafluoroborate and these modified electrodes were characterised by cyclic voltammetry and ac impedance measurements. Cu(II) was reacted with the carboxyphenyl groups in the film to give a surface voltammetric response for the immobilised Cu(II)/Cu(I) couple. The results indicated an ECEC mechanism, in which the chemical steps correspond to the change of coordination environment following the electron transfer steps. The relaxation half-life time for the Cu(I) species formed after electron transfer was estimated at (140 ± 11) s. The large value of the peak width of ∼200 mV was analysed by modelling the voltammograms and the large value of the full width at half maximum (FWHM) could be explained by dispersion in the formal potentials of Cu centres present in a variety of environments in the films studied. An ECEC mechanism (scheme of square) is proposed for the electron transfer reaction considering that the chemical step after reduction of the Cu(II) complex corresponds to conformational changes within the attached layer. Experimental data clearly show that the oxidation of the reduced film can take place from different Cu(I) complexes formed along the reduction to the fully relaxed Cu(I) species.

The novel use of micro ITIES to study electron transfer (ET) has been investigated. Micropipettes and microholes were used to support the interface and the organic solvents considered were 1,2-DCE and o -NPOE. ET involving ferrocene, dimethylferrocene and tetracyanoquinodimethane as the organic redox couples was studied. The ferri/ferrocyanide redox couple was used in all cases as the aqueous redox couple. Micro ITIES were found not to be amenable to kinetic determination. ET is complicated by reactions involving the base electrolytes used, partitioning of redox products, equilibrium reactions and interfacial precipitation, which will need to be considered if accurate kinetic data is to be realised.

The rotating diffusion cell was used to study ion transfer across the interface between two immiscible electrolyte solutions. Tetrabutylammonium was chosen as the transferring cation and lithium chloride as the supporting electrolyte in aqueous phase. Tetrabutylammonium tetrakis-(4-fluorophenyl)-borate in 2-nitrophenyl-octylether was used as the organic electrolyte solution supported in the porous membrane. The quasi-steady state current–voltage curves were measured both by applying potential steps and by imposing a slow potential sweep. The analysis of experimental results was based on the comparison with the theoretical current–voltage curves and on the Koutecky–Levich plots. The apparent electrical resistance at low applied potentials was also considered. Some experimental limitations which make the rotating diffusion cell suitable only for a limited range of values of the standard rate constant to be determined were identified. In particular, it was shown that the method requires an accurate evaluation of the different contributions to the ion permeability. Nevertheless, an estimation of the rate constant for ion transfer in the system studied was obtained.

The effect of an electric current on the concentration polarization of the external bathing solutions and the permselectivity of poly(acrylic acid)-grafted porous ion-exchange membranes has been studied. The experimental approach is based on the transient behavior of the total electric potential drop through the membrane cell when a current step is imposed from external nonpolarizable electrodes. When this voltage drop is recorded as a function of time, a transition time characteristic of each membrane system is obtained. From this time, the counterion transport number for the salt solution (KClH2O) in the membrane can be obtained. The theoretical modeling is based on the time-dependent Nernst-Planck equations. It is shown that the transport number, and then the membrane permselectivity, decreases with the electric current. The higher the membrane grafting ratio and the lower the external salt concentration the larger the permselectivity changes.

The influence of thiourea on the nucleation and growth of copper onto glassy carbon from acid sulphate solutions was studied using potentiostatic steps and cyclic voltammetry. The electrolyte was a 0.07 M CuSO4–1.8 M H2SO4 solution containing 0, 1, 10, 100 or 1000 mg l−1 thiourea. In solutions without thiourea an instantaneous nucleation takes place at the surface. Addition of thiourea changes the initial nucleation to a progressive one. After the initial period of progressive nucleation, the nucleation process is terminated and a transition from progressive to instantaneous nucleation takes place. Thiourea increases the nucleation rate and the number density of nuclei at the surface. Since the deposit is smooth and planar at high nuclear number densities, it can be concluded that thiourea effectively promotes smoothness of the surface. The highest nuclear number densities are obtained at 10 mg l−1 thiourea. At higher concentrations of thiourea the nucleation process is disturbed by the high amount of thiourea molecules adsorbed. A convenient diagnostic criterion for thiourea concentrations too high to produce a smooth and homogeneous surface was found in the cyclic voltammograms. An extra anodic stripping peak, possibly due to dissolution of copper—thiourea complexes adsorbed on the surface, appears at too high thiourea concentrations.

An improved method to determine limiting currents IL in cation exchange membranes based on the use of a microhole array attached onto the membrane surface is applied to the Nafion® 117 membrane in dilute aqueous NaCl solutions. The values for IL obtained from the current—voltage curves are used to estimate the cation transport numbers ts1 corresponding to the membrane operating under limiting current conditions. The changes of ts1 with the NaCl concentration give some insights into the membrane selectivity, and cannot be described by the conventional Donnan theory for charged membranes. Instead, a two phase model accounting for different conductive phases in the membrane is able to describe the experimental trends.

The effects of a copper electrode coated with a cation-exchanger, Nafion, on copper deposition in CuSO4H2SO4 solutions have been studied using the impedance technique. It was found that only the impedance due to mass transport of ions to the electrode was clearly afected; it was increased by the film. A theoretical model for an electrode coated with ion-exchanger was derived. The behaviour of the Nafion film was well predicted by this model.

The phenomenological equations for diffusion and electric conduction have been compared in ionic and component forms. It is concluded that electric conduction and diffusion can be treated as two independent phenomena which are superimposed. The transformation between the ionic and component flows depends on the conventions used in defining transport numbers. The ionic forces become functions of the gradient of the electrolyte chemical potentials, transport numbers, the electric current and the conductivity. No ionic thermodynamic quantities are needed for the description of the transport phenomena. The phenomenological equations determine the concentration distributions only when the electric current is given. Instead of the exact phenomenological equations the Nernst-Planck equations can be used for the approximate solution. This approximate solution is given for the system H2O + CaCl2 + HNO3 in the case of free diffusion with and without the electric current. The formation of a sharp moving boundary caused by the electric current is demonstrated.