In this paper a small signal capacitance model for an electrochemical
electrode is developed. This model takes into consideration the conventional
Helmholtz layer and diffusion layer capacitances in addition to the capacitance
of a homogeneous middle layer between the two previous layers. The small
signal capacitance calculated by this model increases with the electrode
potential reaching a maximum at certain specific voltage and then decreases in
qualitative agreement with the measured C-V curve. By quantitative
comparison of the theoretical and experimental C-V curves satisfactory
agreement was found at the lower potentials. At the higher potentials where
the reaction rate is appreciable the measured capacitance is smaller than the
theoretical one. This is attributed to the formation of gas bubbles leading to a
continuous decrease of electrode area with increased reaction rate.