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

Abstract

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.

Resolving charge-transfer and mass-transfer processes of VO2+/VO2+redox species across the electrode/electrolyte interface using electrochemical impedance spectroscopy for vanadium redox flow battery

“…19 Recently, we reported the transport mechanism of redox species across the porous thin-lm electrode/electrolyte interface and explained the conditions in which either W s element or (R 2 , Q 2 ) elements should be used to t the nite transport feature. 30 Similar EIS features with equimolar concentration of V 4+ /V 5+ redox species at EP are reported in the literature. 29,41,42 However, several others have reported such features (one semi-circle and $45 line) by recording EIS even at measured OCP or at some nite overpotential in the electrolyte containing one of the species of the redox couple (either V 4+ or V 5+ ) and not a solution of both.…”

“…The semi-innite linear mass-transport feature is usually tted with the Warburg element (W) and thus, the feature is also known as Warburg diffusion. [29][30][31] From the inset to Fig. 2, it is observed that the linear t to the LF data is at $45 .…”

“…2). 30 However, a series combination of R s , HF (R 1 , Q 1 ), and LF (R 2 , Q 2 ) elements is used to t the EIS pattern recorded under hydrodynamic conditions of the electrode. The HF (R 1 , Q 1 ) element is associated with the electron-transfer process through the electrode-electrolyte interface.…”

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

“…5 The general preparation procedure of such thin-film electrodes includes the addition of polymer binders to porous carbon to improve the mechanical integrity of the thin-film electrodes. 6–14 The transport and the subsequent adsorption of redox species into a porous carbon-modified electrode surface may include three consecutive steps. The first step is the advection or bulk transport of the ions to the hydrodynamic boundary layer surrounding the electrode/electrolyte interface.…”

PVDF-Modified Nafion Membrane for Improved Performance of MFC

“…Molasses wastewater is a kind of wastewater with high pollutant concentration, which is rich in sugars, proteins, amino acids, vitamins, and other organic matter. Molasses wastewater, which has high concentrations of COD and complex components, could cause serious environmental pollution in the ecosystem [40]. At present, the COD removal rate of the existing molasses wastewater treatment methods is generally not very high.…”