Èlektrohimiâ

Pt(WC_1–x)/Cu electrodes were obtained by deposition of platinum onto the surface of tungsten carbides under open circuit conditions. A tungsten carbide layer with a thickness of ca. 20 μm was pre-formed on the surface of copper plates by thermolysis of a gas mixture WF₆ + H₂ + C₃H₈. During the deposition process, platinum nanoparticles are formed on the surface of tungsten carbides. The source of electrons for the reduction of Pt(II) species is the oxidation of tungsten carbides surface layers. The morphology of the prepared electrodes was studied by scanning electron microscopy (SEM), the chemical composition of the surface layers by X-ray photoelectron spectroscopy (XPS), and the phase composition by X-ray phase analysis (XRD). The deposition of small amounts of platinum (0.002–0.24 mg Pt/cm² of the geometric electrode surface) resulted in a significant increase in the hydrogen evolution reaction (HER) rate. The catalytic activity for the sample with 0.24 mg/cm² platinum loading approached that of the Pt/Pt electrode. The voltammetric characteristics of the HER on the obtained Pt(WC_1–x)/Cu electrodes were determined, and it was assumed that hydrogen evolution proceeds on catalytically active platinum nanoparticles.

In this work, using the Distribution of Relaxation Times (DRT) function, we analyzed the changes in the electrochemical impedance spectra of lithium-carbon cells during cathodic polarization of a carbon electrode. Soft carbon and graphite were studied as carbon materials. It is shown that the analysis of electrochemical impedance spectra of lithium-carbon cells using the distribution function of relaxation times allows us to establish the number of electrochemical elements and calculate their parameters. Application of DRT functions for modeling of electrochemical impedance showed that there are 8 electrochemical elements in lithium-carbon cells and allowed to quantify their parameters. The obtained results are in good agreement with theoretical ideas about the structure of carbon materials and electrochemical processes occurring during their polarization. The analysis of electrochemical impedance spectra of lithium-carbon cells using the relaxation time distribution function is a more objective method compared to the method of equivalent electrical circuits.

Сyclic charge/discharge process of a hydrogen-bromine battery has been studied. Porous titanium felt coated with mixed IrO₂ – TiO₂ oxide coverage in contact with aqueous HBr/Br₂ solution has been used as positive (“cathode”) electrode. Hydrogen gas diffusion electrode with Pt/C catalytic layer served as negative electrode while the hydrogen ion is transferred between them via perfluorinated sulfocation-exchange membrane GP-IEM 103. Morphology, phase, and chemical composition of the cathode material have been characterized using scanning electron microscopy with X-ray spectral microanalysis, Raman spectroscopy and X-ray photoelectron spectroscopy. Condition for switching between the charging and discharging stages within each cycle (based on upper limit for voltage) has been chosen to minimize the amount of bromide and polybromide anions relative to molecular bromine formed at the end of the charging stage (oxidation of Br^–), instead of the traditionally used approach which includes only partial conversion of bromide to bromine in order to increase the stability of the latter in the form of polybromide complexes. Charge-discharge tests of the hydrogen-bromine battery are carried out in the galvanostatic mode at three current densities: 25, 50 and 75 mA/cm². Comparison of the charge and average voltage values in the course of the electrical energy generation (discharge stage) and storage (charge stage) shows that the highest efficiency of the cycle is achieved at the current density of 50 mA/cm². This value of the charge/discharge current density also corresponds to the maximal use of the redox capacity of the electrolyte. It has been found that the stability of the mixed-oxide cathode material used in contact with bromine compounds in acidic environment exceeds significantly that of the carbon paper. The main reason of the decrease of the battery capacity from cycle to cycle is the molecular bromine absorption by elements of the system in contact with the catholyte: components of the membrane-electrode assembly (MEA), pipelines and elements of the pump that ensures circulation.

A recently proposed express method for experimental determination of diffusion coefficients of electroactive ions inside membrane and their distribution coefficients at the membrane/solution interface (Russ. J. Electrochem., 2022, 58, 1103) is based on interpretation of the measured non-stationary current across the system: electrode/membrane/electrolyte solution after a potential step with the use of theoretical expressions for the time dependence of the current, including the steady-state regime. In previous publications, the application of this method to study the bromide anion transport across membrane is carried out under conditions of the selective permeability (permselectivity) of the membrane for non-electroactive counter-ions where the electric field intensity inside it is suppressed by their high concentration so that the movement of electroactive co-ions (bromide anions), having a much lower concentration inside the membrane, takes place via the pure diffusion mechanism, for which solutions are available in an analytical form. If the concentrations of electroactive co-ions and background counter-ions inside the membrane are comparable between one another their transport occurs under the influence of both diffusion and migration contributions to their fluxes. In particular, such a situation takes place in a ternary system of monovalent ions where both ions of the background electrolyte M⁺ and A^–, as well as the electroactive anion X, penetrate into the membrane from the external solution, their concentrations inside the membrane being comparable to each other. The paper has derived analytical expressions for the steady-state distributions of the concentrations of all ionic components and of the electric field inside the membrane as a function of the amplitude of the passing direct current and of the ion concentrations in the bulk solution, as well as for the intensity of the limiting diffusion-migration current. In particular, it has been shown that at a low concentration of co-ions at the membrane/electrolyte solution interface (compared to the concentration of fixed charged groups of the membrane (X_m << C_f), the migration contribution to the flux of electroactive ions can be neglected so that the formulas derived in this work for the ternary electrolyte are reduced approximately to the corresponding expressions for the pure diffusional transport. If the opposite condition is fulfilled (X_m / C_f>>1), migration contributions to ion fluxes lead to a modification of the expression for the limiting diffusion-migration current.