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

Abstract

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec À1 and 40 mV dec À1 are found at low overpotentials depending on the oxide growth conditions, with an apparent Tafel slope of ca. 120 mV dec À1 at high overpotentials. Reaction orders of ca. 0.5 and 1.0 are observed at low and high overpotentials, again depending on the oxide growth conditions. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. This work brings together current research in heterogeneous electrocatalysis and homogeneous molecular catalysis for water oxidation.

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

“…Indeed, the surfaquo groups are specifically implicated as the catalytic centres in the hydrous oxide layers. 96,98 The latter assertion being supported by the pH dependence of the OER at hydrous iron oxide covered electrodes. In Fig.…”

“…However, it has been noted previously that the A3/C2 peaks exhibit the usual characteristics of a hydrated or hyperextended oxide. Lyons and coworkers 84,96,98 have reported super-Nernstian potential-pH shifts for peaks A3/C2 of the order of dE/dpH = À2.303(3RT/2F) = À0.088 V per pH unit at T = 298 K, indicating that the oxide acquires a net negative charge in the oxidised state relative to the reduced state. Since traces of this peak appear even during the first sweep of a fresh, previously unused electrode, it is possible that at the outer regions of the oxide film the hydroxylation process outlined in eqn (27) is succeeded by a further stage which involves coordination of additional H 2 O and OH À species.…”

“…Lyons and coworkers 96,98,99 found that hydrous Fe oxide layers exhibited distinct Tafel slopes for the OER depending on the concentration of NaOH used to prepare the film. In particular, it was observed that hydrous layers grown in high base concentrations had an associated Tafel slope of ca.…”

“…Indeed, the latter authors noted that by purposely dehydrating the hydrous Fe oxide layer, a film with a characteristic Tafel slope of 40 mV dec À1 could be transformed into one displaying a 60 mV dec À1 slope. 96,98 …”

“…These authors noted that the former Tafel slope is characteristic of a fresh electrode, whereas the latter value can be observed for an electrode that has undergone a significant number of polarisation experiments. More recently, Doyle and Lyons 96,98,99 have shown that the concentration of NaOH used in the preparation of hydrous Fe oxide layers has a significant influence on the observed Tafel slope for these electrode systems. It was observed that hydrous layers grown in high base concentrations had an associated Tafel slope of ca.…”