T. A. Kravchenko

Composite electrodes of electrochemical capacitors based on carbon materials with different structure

For composite electrodes based on active carbon DCL Supra 30, ordered mesoporous carbon, and synthetic carbon material Sibunit, the electrical double layer capacitance is studied. The original carbon samples are characterized by the methods of gas adsorption, X-ray diffraction, and transmission electron microscopy. The mesoporous structure of the material synthesized by the template method provides the maximum rate of ion transport in pores and demonstrates an insignificant decrease in the specific capacitance (9.5% in an aqueous electrolyte and 1.1% in an nonaqueous electrolyte) with an increase in the polarizing current.

Kinetics and dynamics of redox sorption

ABSTRACT The paper reviews the current state-of-the-art of the kinetics and dynamics of redox soiption. The comparison of existing and proposed theoretical models in description of experimental results obtained by studying different redox processes on redoxites, i.e. sorbents with functional redox centers, is presented and discussed. The practical application of the results obtained is demonstrated.

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.

Activation of mesostructured electrode materials for electrochemical capacitors

Mesostructured carbon materials for electrodes of electrochemical capacitors are synthesized by the method of template-assisted synthesis. To enhance their capacitance characteristics, their surface is chemically activated by etching in fused KOH and NaOH. The structure of materials is studied by the methods of X-ray diffraction and gas adsorption. Their electrochemical characteristics are studied within the composition of electrodes for electrochemical capacitors with aqueous and nonaqueous electrolytes by impedance spectroscopy. It is found that the chemical activation approximately doubles the carbon specific surface area (up to 1700 m2/g), decreases the amount of the mesostructured phase, and leads to spatial disorientation of carbon rods. Moreover, the specific capacitance of mesoporous carbon materials increases from 90 to 160 F/g. An increase in the alkali concentration increases both the specific surface area of samples and their specific capacitance in aqueous and nonaqueous electrolytes, which correlates with the shift of the micropore-to-mesopore ratio of the carbon matrix to micropores. It is shown that the nature of metal hydroxide affects the structural parameters of activated carbon materials which determine the relationship between the specific capacitance and the polarization current.

Cooperative interactions of metal nanoparticles in the ion-exchange matrix with oxygen dissolved in water

The kinetics of the reduction of molecular oxygen dissolved in water with nanocomposites consisting of an ion-exchange matrix and copper nanoparticles deposited in it in various amounts was studied. As the metal content in the polymer increased, the amount of reduced oxygen initially increased and then reached the limiting value. At a certain metal content, ionization of individual particles with formation of metal counterions changes to the oxidation of particles assembly giving layers of oxide products. The mechanism changes at the percolation threshold of the electron conductivity of the nanocomposite and determines the maximum amount of absorbed oxygen.

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.

Redox sorption of oxygen on a layered cathode-polarized nanocomposite metal-ion exchanger

The redox sorption of molecular oxygen from a flow of deionized water onto a cathode-polarized granular layer of nanocomposite copper-ion exchanger is considered. A mathematical description of it in terms of external diffusion is given. In contrast to better-known approaches, conditions are created that are as close as possible to the limiting diffusion current; this effect can be achieved by dividing the granular layer into shallow layers, each of which is then polarized with a near-limiting current. This allows water to be obtained with a particular value of deoxygenation close to the theoretically calculated value in stationary sorption membrane electrolyzers equipped with a unit containing a nanocomposite copper-ion exchanger. It is established that the lower deoxygenation value relative to the one calculated from the limiting current is associated with the additional reduction of oxygen with copper nanoparticles.

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.

A macrokinetic model of redox sorption on metal–ion exchanger nanocomposites at electrochemical polarization

A conceptual macrokinetic model of redox sorption on metal–ion exchanger nanocomposites upon electrochemical polarization is formulated and a corresponding mathematical model is constructed. The solution to a multi-point boundary value problem for the concentration of a sorbed substance (oxygen) is given. The concentration front of the sorbed substance is characterized by a concentration gradient in the near-surface layer of the solution, by layers of the products of metal oxidation in the composite forming due to both external and internal diffusion transfer, and by chemical and electrochemical reactions at the interphase boundaries. A considerable reduction in the concentration gradient of the sorbate in layers of the products of oxidation of metal and the growth of the diffusion layer of the solution with polarizing currents weaker than the limiting diffusion current are noted.