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.

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.

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV represents a novel in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials and consists of recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the absorption coefficient. Although the information given by FEXRAV obviously does not supply the detailed information of X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS), it allows to quickly map the oxidation states of the element under consideration within the selected potential windows. This leads to the rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window) and is preliminary to more deep X-ray absorption spectroscopy (XAS) characterizations, like XANES or EXAFS. In addition, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis.

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.

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).

A preliminary investigation on a new class on electrocatalytic materials for the electroreduction of organic halides is presented and discussed. The electrocatalysts are based on silver nanoparticles (Ag_NP), ad hoc synthesised by chemical reduction of an aqueous silver salt in the presence of six different stabilising agents. The colloids are then supported on carbon powder (10% loading) for further characterisation and use. The electrocatalytic properties of the Ag_NP/carbon composites towards the dehalogenation of halocompounds are tested by cyclic voltammetry and by preparative electrolysis. The hydrodehalogenation of trichloromethane, extensively studied by this group, is selected as a model reaction, because of its relevance for the detoxification of wastes. The voltammetric characterisation is performed in an aqueous solution, supporting the composites on cavity microelectrodes. Gas‐diffusion electrodes (GDE) based on the most promising Ag_NP composite – and, for reference, on a commercial Ag/C oxygen reduction electrocatalyst – are then tested in an electrolytic process for the progressive conversion of gaseous trichloromethane to less chlorinated compounds, and ultimately to methane.

In this work, IrO(2)-based powders are screened by cyclic voltammetry for the determination of the electrochemical active sites and for the qualitative evaluation of the iridium atoms speciation. All results are obtained using a cavity-microelectrode as powder holder, thus exploiting the features of this innovative tool, whose best potentialities have been recently introduced by our group. All the studied materials have been prepared by the sol-gel technique and differ in calcination temperature and method of mixing the metal oxide precursors. The electrochemical results are complemented with the information obtained by X-ray absorption spectroscopy (XAS), that give insights on the local structure of each selected sample, confirming the trends found by cyclic voltammetry and give new and unexpected insights on the powder structural features.

Mixed tin-iridium oxide (Sn 0.85 Ir 0.15 O 2 ) nanoparticles at low Ir content (15 mol%) were prepared by the sol-gel preparative route, varying calcination temperatures in the range 450-550°C. The crystal structures, the phase composition and crystallite sizes were analyzed by X-ray powder diffraction (XRD). The local order of the materials was investigated by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis revealed the variation of the Ir surface state with the temperature of firing. The morphology of crystallites and the aggregates were analyzed by high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM), respectively. Nitrogen physisorption by BET method was adopted to evaluate the particle surface area and the mesopore volume distribution. Electrochemical properties of the Ti-supported powders were evaluated by cyclic voltammetry (CV) and quasi steady-state voltammetry.