In this work the oxidation states assumed by Ir in oxide systems used as heterogeneous catalysts for water oxidation are determined by means of in situ X-ray Absorption Spectroscopy (XAS). Using a highly hydrated iridium oxide film allows the maximum number of Ir sites to be involved in the electrochemical processes occurring at the catalysts during water oxidation (oxygen evolution reaction, OER). X-ray Absorption Near Edge Structure (XANES) spectra clearly indicate the co-existence of Ir(III) and Ir(V) at the electrode potentials where OER occurs. This represents a fundamental step both in the understanding of the water oxidation mechanism catalysed by heterogeneous Ir oxide systems, and in the possible tailoring of electrocatalysts for OER.

The construction and use of ''dynamic potential-pH diagrams'' (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO 2 , Co 3 O 4 , Co 3 O 4 electrodeposited in a phosphate medium (Co-Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e.g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.

In this paper, we present a comprehensive study on low hydration Ir/IrO 2 electrodes, made of an Ir core and an IrO 2 shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir−O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of transmission electron microscopy (TEM) and synchrotron radiation techniques such as high-resolution (HR) and pair distribution function (PDF) quality Xray powder diffraction (XRPD), X-ray absorption spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady-state I/E curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrO x . The main conclusion of this study is that the Ir core is noninteracting with the IrO x shell, the latter being able to easily accommodate Ir in different oxidation states, as previously suggested for the hydrated form, thus explaining the activity as electrocatalysts. In addition, in operando XAS experiments assessed that the catalytic cycle involves Ir(III) and (V), as previously established for the highly hydrated IrO x material.

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The determination of the number of active sites is a key issue in the evaluation of electrode materials for any electrochemical application. Nonetheless, and particularly in the case of powder materials, a commonly accepted method to determine the actual density of active sites is not yet available, mainly because a method to quantify the amount of material under investigation is missing. In this study, we propose the use of the cavity microelectrode (C-ME, i.e., a cylindrical recessed microdisk) of known volume, which enables the study of a known amount of material, thus allowing the quantitative evaluation of its properties. The validation of the method implied (i) the preparation of C-MEs with different radii and depths; (ii) the calibration of the relevant volumes by means of a "standard" powder, whose number of active sites can be easily determined by cyclic voltammetry; and (iii) their use for the quantification of specific parameters that identify the electrochemical properties of mixed IrO(2)-SnO(2) powders. The results evidence that the quantity of charge relative to the number of pseudocapacitance sites and the currents for the oxygen evolution reaction are proportional to the cavity volumes. This strategy allows the direct comparison of different materials for their rapid and accurate screening. In addition, thanks to the small amount of material required for the sample (typically 10-100 ng), the method can be safely listed among the nondestructive techniques.

We report on the effects of self-assembled monolayer (SAM) dilution and thickness on the electron transfer (ET) event for cytochrome c (CytC) electrostatically immobilized on carboxyl terminated groups. We observed biphasic kinetic behavior for a logarithmic dependence of the rate constant on the SAM carbon number (ET distance) within the series of mixed SAMs of C(5)COOH/C(2)OH, C(10)COOH/C(6)OH, and C(15)COOH/C(11)OH that is in overall similar to that found earlier for the undiluted SAM assemblies. However, in the case of C(15)COOH/C(11)OH and C(10)COOH/C(6)OH mixed SAMs a notable increase of the ET standard rate constant was observed, in comparison with the corresponding unicomponent (omega-COOH) SAMs. In the case of the C(5)COOH/C(2)OH composite SAM a decrease of the rate constant versus the unicomponent analogue was observed. The value of the reorganization free energy deduced through the Marcus-like data analysis did not change throughout the series; this fact along with the other observations indicates uncomplicated rate-determining unimolecular ET in all cases. Our results are consistent with a model that considers a changeover between the alternate, tunneling and adiabatic intrinsic ET mechanisms. The physical mechanism behind the observed fine kinetic effects in terms of the protein-rigidifying omega-COOH/CytC interactions arising in the case of mixed SAMs are also discussed.

In this paper, we introduce the concept and the methodology of quantitative rapid screening (QRS) of catalysts. It is based on the use of the cavity-microelectrode (C-ME), a tool that hosts a known amount of powder and can be filled and emptied quickly, thus allowing the quantitative, rapid, fine characterization of different materials. Here, C-MEs are used for selecting a suitable material to be used as electrocatalyst for the oxygen evolution reaction (water oxidation) in acidic environment, a key process for the majority of the industrial electrolytic applications including the production of high purity hydrogen. A matrix of materials, each having the same low iridium oxide content, is quantitatively screened for finding the most promising one. C-MEs allowed us to measure the effective number of active Ir sites and their surface concentration. The success of this strategy is proven by the good performance of the ''best'' material when tested in a proton exchange membrane water electrolyzer, that allowed high hydrogen fluxes at a low cell potential ($4000 dm 3 h À1 m À2 at less than 1.9 V).