Internal and External Transport of Redox Species across the Porous Thin-Film Electrode/Electrolyte Interface

Transport of redox species (VO2+/VO2+, V2+/V3+, Ti3+/Ti4+, and Fe2+/Fe3+) across the electrode/electrolyte interface is investigated in a thin-film rotating disk electrode configuration using electrochemical impedance spectroscopy (EIS). The transport features depend on the constituents of the thin-film catalyst layer and on the rate constant of the redox reaction. On Nafion-free porous electrodes, semi-infinite linear and finite transport features are observed under static and hydrodynamic conditions of the electrode, respectively. Depending on the rate constant of the electrochemical reaction, an equivalent circuit consisting of either resistance (R) and constant phase element (Q) or the Warburg short (W-s) element is proposed to explain the finite transport features. Addition of Nafion (binder) in the electrode offers extra resistance to the transport of redox species, which helps resolve EIS features of the transport of redox species through the porous thin-film electrode and that through the bulk of the electrolyte. The features of the transport of redox species through the porous electrode media are independent of the hydrodynamic conditions.