Impedance of rough capacitive electrodes

Abstract The impedance of solid electrodes in the absence of faradaic reactions usually deviates from purely capacitive behaviour. The widely accepted explanations of this “capacitance dispersion” are based on the assumption that owing to surface roughness, or porosity, or spatially inhomogeneous capacitance density, the current density along the surface is not homogeneous, and thus capacitance dispersion is purely of geometric origin. We show that this view is not correct in the case of rough electrodes because capacitance dispersion due to irregular geometry appears at much higher frequencies than is usual in electrochemical methodologies. We present impedance spectra measured on platinum electrodes of various roughnesses in aqueous solutions to demonstrate that capacitance dispersion on rough electrodes is of interfacial origin and is due to adsorption effects. The old finding that the rougher the surface the larger the capacitance dispersion, can be rationalized alternatively in such a way that increasing roughness may broaden the time constant distribution of adsorption kinetics and may therefore increase the capacitance dispersion.

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