Structure and Stability of a Copper(II) Lactate Complex in Alkaline Solution: a Case Study by Energy-Dispersive X-ray Absorption Spectroscopy.

Energy-dispersive X-ray absorption spectroscopy was applied, aimed at solving the problem of the structure and stability of a copper(II) lactate complex in alkaline solution, used as a precursor for the electrodeposition of Cu2O. The application of multiple scattering calculations to the simulation of the X-ray absorption near-edge structure part of the spectra allowed an accurate resolution of the structure: the copper(II) cation is surrounded by four lactate ions in a distorted tetrahedral environment, with the lactate anions acting as monodentate ligands. This results in an atomic arrangement where copper is surrounded by four oxygen atoms located at quite a short distance (ca. 1.87 Å) and four oxygen atoms located quite far apart (ca. 3.1-3.2 Å). The complex was finally found to be stable in a wide range of applied potentials.

Observation of charge transfer cascades in α-Fe2O3/IrOx photoanodes by operando X-ray absorption spectroscopy.

Electrochemical devices for energy conversion and storage are central for a sustainable economy. The performance of electrodes is driven by charge transfer across different layer materials and an understanding of the mechanistics is pivotal to gain improved efficiency. Here, we directly observe the transfer of photogenerated charge carriers in a photoanode made of hematite (α-Fe2O3) and a hydrous iridium oxide (IrOx) overlayer, which plays a key role in photoelectrochemical water oxidation. Through the use of operando X-ray absorption spectroscopy (XAS), we probe the change in occupancy of the Ir 5d levels during optical band gap excitation of α-Fe2O3. At potentials where no photocurrent is observed, electrons flow from the α-Fe2O3 photoanode to the IrOx overlayer. In contrast, when the composite electrode produces a sustained photocurrent (i.e., 1.4 V vs. RHE), a significant transfer of holes from the illuminated α-Fe2O3 to the IrOx layer is clearly demonstrated. The analysis of the operando XAS spectra further suggests that oxygen evolution actually occurs both at the α-Fe2O3/electrolyte and α-Fe2O3/IrOx interfaces. These findings represent an important outcome for a better understanding of composite photoelectrodes and their use in photoelectrochemical systems, such as hydrogen generation or CO2 reduction from sunlight.

3D-printed photo-spectroelectrochemical devices for in situ and in operando X-ray absorption spectroscopy investigation.

Three-dimensional printed multi-purpose electrochemical devices for X-ray absorption spectroscopy are presented in this paper. The aim of this work is to show how three-dimensional printing can be a strategy for the creation of electrochemical cells for in situ and in operando experiments by means of synchrotron radiation. As a case study, the description of two cells which have been employed in experiments on photoanodes for photoelectrochemical water splitting are presented. The main advantages of these electrochemical devices are associated with their compactness and with the precision of the three-dimensional printing systems which allows details to be obtained that would otherwise be difficult. Thanks to these systems it was possible to combine synchrotron-based methods with complementary techniques in order to study the mechanism of the photoelectrocatalytic process.

The Influence of Carbonaceous Matrices and Electrocatalytic MnO2 Nanopowders on Lithium-Air Battery Performances.

Here, we report new gas diffusion electrodes (GDEs) prepared by mixing two different pore size carbonaceous matrices and pure and silver-doped manganese dioxide nanopowders, used as electrode supports and electrocatalytic materials, respectively. MnO2 nanoparticles are finely characterized in terms of structural (X-ray powder diffraction (XRPD), energy dispersive X-ray (EDX)), morphological (SEM, high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM)/TEM), surface (Brunauer Emmet Teller (BET)-Barrett Joyner Halenda (BJH) method) and electrochemical properties. Two mesoporous carbons, showing diverse surface areas and pore volume distributions, have been employed. The GDE performances are evaluated by chronopotentiometric measurements to highlight the effects induced by the adopted materials. The best combination, hollow core mesoporous shell carbon (HCMSC) with 1.0% Ag-doped hydrothermal MnO2 (M_hydro_1.0%Ag) allows reaching very high specific capacity close to 1400 mAh·g-1. Considerably high charge retention through cycles is also observed, due to the presence of silver as a dopant for the electrocatalytic MnO2 nanoparticles.

Fixed energy X-ray absorption voltammetry.

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.

IrO2-based disperse-phase electrocatalysts: a complementary study by means of the cavity-microelectrode and ex-situ X-ray absorption spectroscopy.

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.

Quantitative studies on electrode material properties by means of the cavity microelectrode.

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.

Ultrasound biomicroscopy of residual vitreous base after vitreoretinal surgery.

Residual vitreous base after vitreoretinal surgery was evaluated by ultrasound biomicroscopy (UBM). Twenty aphakic and pseudoaphakic patients (20 eyes) undergoing surgery for different vitreoretinal diseases were evaluated by high-frequency (50 MHz), high-resolution (50 microm) UBM, performed the day before surgery, weekly up to 1 month after surgery and then monthly. One week after surgery, the vitreous remnants were 'hardly visible' in 3 cases, 'visible' in 6 cases and 'highly visible' in 11. At the end of the follow-up (2.2 months), the 'hardly visible' cases increased to 6 and the 'visible' cases to 12, while the 'highly visible' cases decreased to 2. UBM demonstrated that vitreous base remnants were present in spite of accurate surgery; a spontaneous volume reduction of vitreous was observed during the follow-up.