ELECTROCHEMICAL MODIFICATION OF GRAPHITE SURFACE

Yarema TEVTUL, T. FILIPCHUK, Sonia GUTT, Silviu STROE

Abstract

Thermodynamic and electrochemical parameters of intercalation and deintercalation of hydrosulfate ions into the graphite substrate have been investigated using electrodes after preliminary intercalation and without this stage and involving methods of the cyclic voltammetry, electronic microscopy and general principles of the irreversible thermodynamics. Potentials of the intercalation and deintercalation starting points, potentials of the anodic and cathodic maximums have been determined and changes in the free Gibbs energy for the intercalation and deintercalation processes, changes in the enthalpy and enthropy of these processes, diffusion and migration fluxes of the hydrosulfate ions were calculated using the experimental data. Criteria of Schtakelberg, Tomesh and Shevchik and measurements of an amount of electricity required for the processes were used to evaluate the reversibility ratio of intercalation and deintercalation. It was found that raise in the intercalation-deintercalation cycles number results in increase in the limit anodic current value. This increase may be caused by partial decomposition of the graphite substrate structure and faster intercalation of the hydrosulfate ions during next cycles. Evaluation of the diffusion and migration fluxes of hydrosulfate ions has been made using an electronic microscopy. Thermodynamic moving forces of the fluxes have been analyzed. The higher is the concentration of sulfuric acid, the higher is the surface concentration of sulfur. A value of the flux of the electrochemical intercalation of hydrosulfate ions from a 1 M solution of sulfuric acid is about 42 times higher comparing to the physical intercalation flux value. A contribution of the latter flux into the total intercalation flux is only 3.8 % from the electrochemical intercalation flux contribution.

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