Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution

The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograms of HIROFs display two reversible redox couples commonly assigned to the Ir­(III)/Ir­(IV) and Ir­(IV)/Ir­(V) transitions, respectively. However, compared to the first, the second redox couple has significantly less charge associated with it. This effect is interpreted as partial oxidation of Ir­(IV) as limited by nearest neighbor repulsion of resulting Ir­(V) sites. Thus, the redox process is divided into two steps: one preceding and one overlapping the oxygen evolution reaction (OER). Here, the “super-nernstian” pH dependence of the redox processes in the HIROF is used to expose how pH controls the overpotential for oxygen evolution, as evidenced by the complementary increased formation of Ir­(V) oxide. A recently formulated binuclear mechanism for the OER is employed to illustrate how hydrogen bonding may suppress the OER, thus implicitly favoring Ir­(V) oxide formation above the thermodynamic onset potential for the OER at low pH

Potential-Dependent Structural Memory Effects in Au–Pd Nanoalloys

Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au–Pd surface composition and distribution of Pd within a Au_0.7Pd_0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H₂O₂ as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au–Pd surface composition and in remarkable structural memory effects