SUBSTANTIATION OF TECHNOLOGICAL INDICATORS OF APPLICATION OF A GAS-DIFFUSION CATHODE IN ELECTROCHEMICAL SYNTHESIS OF HYPOCHLORITE SOLUTIONS

Abstract

A gas diffusion electrode was used to implement depolarization of the cathodic process with atmospheric oxygen to improve the production of sodium hypochlorite by electrolysis of an aqueous solution of sodium chloride. As materials for the implementation of depolarization of the cathode process on a porous cathode from the grid, we selected: manganese oxides, cobalt oxides, ruthenium oxides. These oxides are characterized by low overvoltage of the oxygen reaction. Oxides of selected metals were applied to a mesh current lead by thermal decomposition of coating solutionsю. The gas diffusion electrode consisted of a lined titanium current lead, a dispersant of gas made of porous graphite, and an external mesh working element, on which cathodic reactions occurred. The preparation of a catalytically active layer of oxide-metal coatings was carried out by thermal decomposition of coating solutions. This method fully complies with the requirements for oxide-metal electrodes for the electrolysis of aqueous solutions of sodium chloride: the ability to control the composition of the composite coating in a wide range of component concentrations. On the current-voltage cyclic dependences of the cathodic process, for all the materials studied, certain areas of oxygen reduction and combined oxygen reduction and hydrogen evolution are observed. The first section of oxygen reduction is observed to the equilibrium potentials of the hydrogen reaction (approximately –0.42 V). The oxygen reduction rate is small and amounts to 3...5 mA/cm2. There is no difference in the current-voltage dependence due to the high potential sweep speed, which does not lead to oxygen depletion in the case of cathode operation without air supply. In the second section (at potentials, more negative equilibrium potentials of the hydrogen reaction), a significant increase in the rate of the cathodic reaction due to hydrogen evolution is observed. Oxygen, in this case, is reduced at the limiting current density. In the third section (more than –1.5 V), the speed of the cathodic process is almost completely determined by the rate of hydrogen evolution. The effect of air supply to the gas diffusion cathode is observed when comparing the reverse stroke of cyclic current–voltage dependences. On the surface of the steel mesh, an increase in the reverse current is observed in the potential range –1.0 to 0 V. Which indicates an increase in adsorbed particles involved in the cathodic process. As shown earlier, this range of potentials corresponds to the 1st and 2nd sections of the obtained dependences in which the predominant oxygen reduction occurs. Therefore, an increase in the reverse current, with potentials more positive than 1.0 V, can be explained by the effect of oxygen adsorption on the surface of gas-permeable mesh steel cathodes when air is supplied. The addition of hypochlorite ion has practically no effect on the current density in the first and second sections of the current – voltage dependences. A decrease in the cathodic current density is observed at potentials more negative from the equilibrium potential of the hydrogen reaction. This indicates a certain inhibition of the process of hydrogen evolution. In the third section, the current density also decreases. This indicates that 0.08 mol∙dm3 hypochlorite ions do not participate in cathodic reduction. Recommended current density for the studied design of the gas diffusion cathode is 15 mA/cm2 at a temperature of 291...293 K. The cathodic recovery of hypochlorite ions, under these conditions, is reduced by 55...60 %.