The electronic structure of iridium oxide electrodes active in water splitting

Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with IrIII. These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts

Surface analysis by means of reflection, fluorescence and diffuse scattering of hard X-ray

Layered structures play a fundamental role in modern technology. The characterization of these layers includes their composition, composition profile and their geometry. The structure of external and internal interfaces is of special interest. At grazing incidence, all X-ray techniques become surface and interface sensitive. This is the basis for a number of novel analytical tools which will be presented in this paper: X-ray reflection, fluorescence and diffuse scattering. Analytical expressions for these three quantities are given. The influence of interface roughness is included. The information obtained from these techniques is the thickness of the layers, their density, the interface roughness both perpendicular and parallel to the interface, and the depth profile of the individual atomic species. A number of examples will illustrate the capability of the techniques. Comparisons with results from other techniques will show their advantages and drawbacks

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 to 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