M. Yu. Chaika

The stabilization of nanodisperse silver in a sulfo cation exchanger

Silver-ion exchanger (electron ion exchanger, EI) composites with equivalent silver and hydrogen counterion contents were prepared by chemical deposition. Microscopic and X-ray data showed that silver nanoparticles and their ensembles isolated from each other and stabilized by a polymeric matrix were formed. Contact of Ag0-EI in the H+ form with solutions of silver salts caused the occurrence of two processes, ion exchange and metal recrystallization. These processes were interrelated because they involved one common particle, the silver counterion. Recrystallization proceeded by the electron-ion mechanism, but, because of matrix isolation of silver particles, electron transfer occurred inside separate structural elements (ensembles of particles) rather than over the whole composite volume. The transfer of silver ions largely occurred over ionogenic matrix centers, which substantially decreased their mobility. The low electronic conductivity of the composite and limited mobility of counterions were charge stabilization factors, which hindered recrystallization and, along with matrix stabilization, contributed to the retention of nanosized silver particles.

Electroreduction of molecular oxygen on carbon electrode modified by dispersed silver

Silver particles are formed by electrochemical deposition on the carbon electrode surface. It is found that the deposition process occurs according to the progressive nucleation mechanism, which results in formation of silver particles with the size of 95 to 190 nm as dependent on the electrodeposition time. The values of silver particle size and support surface coverage by metal obtained on the basis of microphotographs indicate that cathodic polarization in the presence of dissolved oxygen results in particle size redistribution due to the reaction of silver particle dissolution with further deposition simultaneously with oxygen electroreduction. The reaction of molecular oxygen electroreduction on a carbon electrode with deposited dispersed silver occurs via a mixed two- and four-electron mechanism. The observed limiting reaction current is of diffusion nature.

Electroreduction of molecular oxygen on dispersed copper in an ion-exchange matrix

Electrochemical reduction of molecular oxygen was studied on a [dispersed copper]-[macroporous KU-23 15/100S sulfocation exchanger with various metal concentrations] composite electrode. It was found that a high proton concentration in the ion-exchange matrix causes a decrease in the oxygen reaction overvoltage. The nanostructured state of copper particles causes stabilization of the intermediate product, i.e., hydrogen peroxide. Using the rotating disk electrode method, it was detected that the process is limited by external diffusion of oxygen to composite grains. The oxygen reaction is mostly concentrated on the grain surface and surface layers; oxygen is reduced in the bulk due to dispersed copper oxidation.

Copper electrodeposition into ion-exchange materials

Electrodeposition of copper into spherical granules of ion-exchange materials KU-23 and KU-2 out of acid sulfate solutions is studied by a method of cyclic voltammetry. It is discovered that the discharge of copper ions in an ion-exchange matrix is characterized by a cathodic overvoltage that is higher than the overvoltage of the same process on a graphite substrate by 0.08 V, which is most probably connected with a limited mobility of ions localized at fixed groups [RSO3−]. The cyclic voltammogram exhibits an additional cathodic peak in the potential region corresponding to the reduction of single-charged copper ions that form as a result of their accumulation inside pores of the ion-exchange matrix during anodic dissolution of metal deposited previously. It is fixed microscopically that the process of deposition begins at the graphite substrate/ion-exchanger interface and passes into bulk upon the formation of an electron-conducting layer saturated with copper. Preliminary saturation of the ion-exchanger by copper deposited chemically facilitates uniform electrodeposition of copper over the entire volume of pores of the ion-exchange matrix.

Space localization of the electrode reaction in copper-ion-exchanger nanocomposites

The formation of copper nanoparticles in a KU-23 15/100 sulfocation-exchanger was studied. It was demonstrated that the formation of copper as assemblies from nanoparticles with sizes of 3 to 10 nm during chemical synthesis is determined by the nature of the polymer and does not depend on the amount of metal precipitated. The percolation threshold of electron conductivity, which determines the formation of electrochemical activity of nanocomposites, was discovered. It was determined that the electroreduction of molecular oxygen takes place on the surface and in the subsurface zone of a nanocomposite grain, the size of which is determined by the local concentration of metal particles in the ion-exchanger phase.

The mechanism of electroreduction of nitrate ions on a hybrid electrode nanodispersed copper-MK-40 membrane

Based on a MK-40 sulfocation-exchange membrane, a hybride electrode material containing nanodispersed copper is prepared. The methods of scanning electron microscopy and X-ray diffraction (XRD) analysis reveal the formation of copper agglomerates measuring 250–470 nm and consisting of individual particles of 20–30 nm. The procedure of multistage chemical deposition of copper into the ion-exchange carrier makes it possible to obtain a continuous cluster of metal particles which determines the electron conducting properties of the resulting hybrid material. The electrochemical activity of the nanocomposite electrode is studied in the reaction of nitrate ion electroreduction. Nanodispersed copper deposited into the membrane is shown to intensify the electroreduction of nitrate ions by a factor of 1.5–2 as compared with a compact copper electrode. The electroreduction of nitrate ions on compact copper is shown to involve 6 electrons, whereas the electroreduction on the nanocomposite involves 8 electrons. The electroreduction products of nitrate ions are identified by the IR spectroscopy method.

Percolation effects with copper electrodeposition in ion-exchange material

The effect of preliminary doping of sulfo cation-exchange material KU-23 15/100S with a chemically deposited metal (Ag, Cu) on the rate of copper ion electroreduction is investigated. It is shown that the threshold dependence of the reaction current of copper electroreduction on the amount of doped metal is due to the formation of a single percolation cluster in an ion-exchange grain and, as a result, to the appearance of electron conductivity. It is established that preliminary doping changes the nucleation mechanism from a gradual to an instantaneous one. Results from a local X-ray spectral microanalysis provide the basis for concluding that copper electrodeposition is uniform throughout the volume of an ion-exchange grain. The average size of the copper particles formed is 100 nm.

Hybrid electrode materials based on ion-exchange matrix containing copper nanoparticles and carbon fibers for the electroreduction of nitrate ions

Copper-containing composite electrodes have been obtained by the chemical deposition of copper into the film on an electrode composed of an ion-exchange matrix and carbon fibers. The size of copper particles determined by X-ray diffraction analysis amounts to 23–29 nm and is not practically changed with a growth in the copper concentration in the electrode material. The active area of the surface of copper particles is determined electrochemically using the underpotential electrodeposition of lead ions. It is shown that the rate of electrochemical reduction of nitrate ions increases with the concentration of copper particles and reaches its maximum at 90 μg/cm2. The electroreduction of nitrate ions proceeds in the mode of mixed kinetics and is an irreversible process.

The Diffusion Coefficient of Molecular Oxygen in Macroporous Sulfocationite

An electrochemical method for experimentally estimating the diffusion flux of molecular oxygen through a spherical porous medium by taking chronoamperograms with an electrode of a special design was suggested. The corresponding mathematical apparatus based on numerically solving the molecular diffusion equation in spherical coordinates was developed, which enabled the diffusion coefficient and medium porosity to be determined from the experimental data. The method was tested by applying it to the system KU-23 15/100 macroporous sulfocationite—aqueous solution of molecular oxygen.

Electroreduction of molecular oxygen on the silver/perfluorinated ion-exchange membrane MF-4SK/dispersed carbon nanocomposite

By chemical deposition of silver in an ion-exchange matrix, the Ag0/MF-4SK/C nanocomposite has been fabricated, which is capable of changing the amount of deposited silver by varying the concentration of ionic groups in the MF-4SK membrane. Transmission electron microscopy has revealed the presence of silver particles of a 3 nm size. On an electrode coated with the MF-4SK membrane, electroreduction of oxygen occurs on the carbon substrate and the membrane is a diffusion barrier. Since the polymer matrix contains counterions H+ localized near fixed −SO3 groups, the hydrogen ion takes part in the stage of charge transfer to the oxygen molecule. An increase in the dioxygen electroreduction current on the Ag0/MF-4SK/C nanocomposite relative to the MF-4SK/C composite is due to the presence of silver particles, which result in acceleration of the reaction by the catalytic action, and to an increased contribution of the four-electron process as compared with the two-electron process characteristic of carbon materials.