IMPEDANCE OF ROUGH CAPACITIVE ELECTRODES

The impedance of solid electrodes in the absence of faradaic reactions usually deviates from purely capacitive behaviour. The widely accepte d explanations of this ''capacitance dispersion'' are based on the ass umption that owing to surface roughness, or porosity, or spatially inh omogeneous capacitance density, the current density along the surface is not homogeneous, and thus capacitance dispersion is purely of geome tric origin. We show that this view is not correct in the case of roug h electrodes because capacitance dispersion due to irregular geometry appears at much higher frequencies than is usual in electrochemical me thodologies. We present impedance spectra measured on platinum electro des of various roughnesses in aqueous solutions to demonstrate that ca pacitance dispersion on rough electrodes is of interfacial origin and is due to adsorption effects- The old finding that the rougher the sur face the larger the capacitance dispersion, can be rationalized altern atively in such a way that increasing roughness may broaden the time c onstant distribution of adsorption kinetics and may therefore increase the capacitance dispersion.