Redox switching and oxygen evolution electrocatalysis in polymeric iron oxyhydroxide films

Abstract

Outstanding issues regarding the redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been examined. Charge percolation through the hydrous layer has been quantified, using cyclic voltammetry, in terms of a charge transport diffusion coefficient D CT which admits a value of ca. 3 Â 10 À10 cm 2 s À1 . Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec À1 and ca. 120 mV dec À1 are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

“…In such cases, the experimentally observed kinetic parameters represent a combination of the parameters expected for the competing pathways, the exact value of which depends on the fraction of catalytic sites following each individual pathway. Then again, Lyons and Brandon 92 have shown that the fractional reaction order m OH À = 1.5 observed for an 'aged' passive oxide coated iron electrode can be rationalised by admitting Temkin rather than Langmuir adsorption conditions. (b) Open circuit potential decay.…”

“…Lyons and coworkers 17,92 have applied the Aoki model to the voltammetric data obtained for hydrous Fe oxide coated electrodes. In particular, they note that the i p /v 1/2 proportionality can be readily obtained at reasonable values of sweep rate for thick layers whereas the i p /v proportionality, suggesting the operation of Nernstian equilibrium throughout the dispersed layer during redox switching, is observed over an extended range of sweep rates for thin layers.…”

“…25, R far is calculated from the fitting parameters as R far = R p + R s . 92,96,98 At an oxygen evolution overpotential Z where simple Tafel behaviour prevails, the current density i is related to Z via the following expression, 92,96,98,296 …”

“…332 On the other hand, charge percolation proceeds comparatively easily and quickly through the outer, hydrous, polymeric oxide region; the average rate of charge diffusion for a hydrous oxide covered Fe electrode in base is comparable to that of electrodes modified by redox polymers such as poly(pyrrole). 17,92 Interestingly, a duplex model was also specifically proposed for the anodic oxide formed on Au electrodes 334 -the very system for which the twin barrier model was originally applied to the OER. 332 The final mechanism to be discussed pertains to the hydrous oxide coated metal substrates.…”

An electrochemical impedance study of the oxygen evolution reaction at hydrous iron oxide in base

“…It has been noted that dispersed hydrated oxide materials exhibit good electrocatalytic potentiality due to their skeletal nature as the latter structural feature permits a major increase in the number of oxyions participating in the electrode reaction. 11,30 Indeed, with microdispersed species the boundary between the solid and aqueous phase may be nebulous as the two phases virtually intermingle. This will be an important consideration when a model for the OER is developed.…”