CONCENTRATION-DEPENDENCE OF THE RATE OF HYDROGEN EVOLUTION ON SOLID GALLIUM ELECTRODES IN CONCENTRATED PERCHLORIC-ACID

Hydrogen overpotential was investigated on solid gallium electrode in solutions of perchloric acid at six different concentrations between 4 .5-9.0 mol/kg in the range 263-208 K, in the system of liquid phase \G a and solid phase \Ga. It has been found that the Igj-eta curves at co nstant concentration of HClO4 are independent from the states of the p hases. In both cases linear Tafel lines were observed, with transfer c oefficients, beta, of about 0.5. At the j vs. m(HClO4) curves a maximu m was observed; in the range 4.5-6.5 mol/kg the j increased with the H ClO4 content, then decreased. The j-m(HCLO4) and 1/R-m(HClO4) plots ha ve similar shapes. The similarity suggests that the charge carriers an d the species taking part in the rate determining steps of the electro chemical reaction may be identical. This similarity of the curves does not depend on the phase states of the electrolyte. The decrease of th e electrical conductivity (1/R) at greater concentrations than 6.2-6.6 mol/kg HCLO4 can be explained in term of the decrease of water activi ty and/or the increase of the numbers of undissociated HClO4 molecules . Both effects result in the decrease of the prototropic conduction. W e have also found experimentally that the activation energy of conduct ivity in the case of solid phase of the electrolyte is higher than in liquid phase. This can be explained by assuming that the charge carrie rs in both cases are the same, i.e., the H3O+ ions, and the proton tra nsfer from H3O+ ions to H2O molecules demands higher energy in solid s tate.