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What contributes to the internal mass-transport resistance of redox species through porous thin-film electrodes?

Transport of redox species through porous thin-film electrodes is investigated using electrochemical impedance spectroscopy (EIS). Redox species of small size and fast electron-transfer kinetics show two arcs in the EIS pattern: a high frequency arc corresponding to the charge-transfer process (electron-transfer) and a low frequency arc corresponding to the mass-transport process (transport of the redox species from the bulk of the solution to the electrode interface). Often, the features of the Nyquist plot corresponding to the transport of the redox species through the porous electrode and that through the bulk of the electrolyte are not resolved. It is shown that the resolution of such features depends on the (1) composition of the porous thin-film, (2) electron-transfer kinetics, (3) interaction of the redox species with the electrode components, and (4) bulkiness of the redox species and (5) its concentration.