Relation between the surface states of oxide films at Rh electrodes and kinetics of the oxygen evolution reaction

Abstract

Oxide formation on Rh(poly) electrodes in 0.5 mol dm^–3 aqueous H₂SO₄ at potentials, E_p= 0.80–2.40 V, for times, t_p⩽ 10 000 s, proceeds by inverse-logarithmic growth kinetics and leads to the formation of oxide films which reveal two states, OC1 (α state) and OC2 (β state). The former corresponds to Rh(OH)₃ and the latter to RhO(OH); OC2 occurs in submonolayer thicknesses. The OC1 state reaches a limit of 1980 µC cm^–2 which is equivalent to three monolayers (ML) of Rh(OH)₃. The kinetics of the oxygen evolution reaction (OER) at Rh electrodes, covered with thin films of Rh(OH)₃, are evaluated from current density vs. time (i vs. t_p) transients and by determining Tafel relations. At E_p 2.00 V, the i vs. t_p transients show changes in the OER kinetics, as the predominant electrode process, and reveal an initial drop of the current density which subsequently levels off. The Tafel plots reveal two linear regions with slopes of 90 mV at low overpotentials, η, and 120 mV at high η, with a transition potential, E_tr, at 1.73 V. The exchange current densities expressed as log i_o and determined from the linear components of Tafel plots are –9.4 A cm^–2 for low η and –7.3 A cm^–2 for high η. At low η, electron resonance tunnelling is the rate-determining step in the OER whereas at high η the process is limited by the discharge of H₂O to OH.