AC-IMPEDANCE STUDIES OF HIGHLY ORIENTED PYROLYTIC-GRAPHITE IN 1M NAOHSOLUTION

AC impedance studies of the highly oriented pyrolytic graphite (HOPG) basal plane were carried out in 1M NaOH at 25-degrees-C. The data were interpreted in terms of equivalent circuits generally associated with a pure intrinsic semiconductor-electrolyte interface. A five-element equivalent-circuit model was proposed and described satisfactorily the electrochemical properties of the HOPG basal plane at different poten tials and frequencies. The space-charge capacitance, the surface state capacitance parameter, the resistance related to the surface states, the charge-transfer resistance, and the solution resistance were deter mined. The influence of hydrogen evolution on the ac impedance respons e of the HOPG basal plane also was studied. This reaction produces sur face defects by increasing the edge-plane content. The ac technique ma y be used in the diagnostics of the surface changes.