CHARACTERIZING ELECTROCHEMICAL SYSTEMS IN THE FREQUENCY-DOMAIN

The use of various impedance spectroscopies, including electrochemical impedance spectroscopy (EIS), photoelectrochemical impedance spectros copy (PEIS), and those defined using various electrochemical, hydrodyn amical, and mechanical conjugate variables, for studying physico-elect rochemical systems is briefly reviewed with emphasis on identifying an d defining a common theoretical basis. In particular, we explore the c oncepts of ''conjugate properties'' and ''reciprocity'', the applicabi lity of the Kramers-Kronig transforms, and the distinction between an impedance and a transfer function. We argue that the interpretation of impedance data in terms of electrical analogs and in terms of classic al linear systems theory (I,ST) is possibly too restrictive, and that a more general basis is required to accommodate the broad range of ''i mpedance spectroscopies'' that are now being developed. (C) 1997 Elsev ier Science Ltd.