1149-1155The processes
of adsorption and evolution of hydrogen as well as its absorption onto the surface
of platinum in both acidic and alkaline media have been studied by the method of
electrochemical impedance spectroscopy. In acidic solutions
the Nyquist plots
consist of two overlapping semi-circles of variable diameters depending on the extent
of the cathodic dc-potential offset. The semicircle at the high frequency side of
the spectrum enlarges upon increasing the cathodic shift and is associated to the
surface Hupd, while the size of the semicircle at the low frequency
end decreases in line with the behavior of the charge transfer resistance due to
the evolution of hydrogen through surface Hopd. Equivalent circuits in
perfect agreement with the experimental fin ding have been proposed and the values
of the circuit elements and their correspondence with the
electrochemistry
of the processes discussed. In alkaline solutions only one semicircle with its diameter
decreasing upon the cathodic shift has been observed which is exclusively due to
the process of evolution of hydrogen through Hopd