On the ideal polarisability of electrodes displaying cpe-type capacitance dispersion

The relevance of the concept of ideal polarisability to solid electrodes displaying constant phase angle behaviour (constant phase element (cpe)) is reconsidered in the context of the recent dispute in this Journal [Zoltowski, 443 (1998) 149; Lang and Heusler, 457 (1998) 257]. It is shown that the cpe as a model of the electrochemical interface is ‘unphysical’ due, first of all to unattainability of the steady state after potential or current perturbation. It is also concluded that the thermodynamic equations governing the equilibria of an ideally polarisable electrode cannot be applied to an electrode modelled by cpe because the notions of the charge and double layer capacitance (Cd) are irrelevant in the framework of the cpe model. As a consequence, procedures of calculation of Cd from impedance data modelled by cpe are unjustified.