Elisabetta Achilli | University of Pavia (original) (raw)

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Papers by Elisabetta Achilli

Electrochimica Acta, 2023

Charge transfer reactions in electrodeposited iridium oxide films (EIROF) are investigated by mea... more Charge transfer reactions in electrodeposited iridium oxide films (EIROF) are investigated by means of operando energy dispersive X-ray absorption spectroscopy (EDXAS), where oxidation and reduction conditions are selected to drive the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) reactions in acidic solutions. The Ir(III)/Ir(IV) couple is related to a well-known electrochromic phenomenon, while the Ir(IV)/Ir(V) couple might play an important role in the catalysis of the oxygen evolution reactions (OER). In the experiments, current intensity and time-resolved X-ray absorption spectroscopy (XAS) are simultaneously recorded upon application of appropriate potential steps, leading to the independent determination of both the relevant reaction rates and the rate-determining steps. This is allowed by the fast acquisition time (∼10−2 s) at the ESRF Energy Dispersive XAS (EDXAS) ID24 beam-line, in combination with the highly hydrated amorphous iridium oxide electrode material, which in turn allows to maximize the fraction of Ir sites participating in the electrochemical processes. If the experimental conditions exclude the possibility of having either oxygen evolution (or reduction), the Degree of Reaction (DoR), determined by both electrochemistry and XAS, exhibits exponential time dependence, clearly pointing to diffusion-controlled processes. Vice versa, under concomitant OER + oxidation of iridium centers or ORR + iridium reduction, the electrochemical and XAS DoRs highlight different phenomena, providing fully complementary information of the ongoing electrode reactions. In all cases, data elaboration allows to determine the diffusion coefficient of H+ ions within the catalyst layer, that is compared and confirmed by data obtained by electrochemical impedance spectroscopy (EIS). The high values of D obtained for EIROF is compared to values obtained on other IrO2 materials can help in explaining the relevant high electrocatalytic activity.

The employment of Luminescent Solar Concentrators (LSCs) is a well known strategy for the integra... more The employment of Luminescent Solar Concentrators (LSCs) is a well known strategy for the integration of photovoltaic technologies in buildings. One of the main challenges in this area lies in the developement of effective solar cells, whose band gap and structures are adjusted to match the emisson peak of the related LSCs. The cells are placed at the edges of the concentrator plate and should be designed to benefit of quasi-monochromatic irradiation. In this work we investigate the growth and characterization of high-efficiency (Al)InGaP-based solar cells designed to be coupled with LSCs: their performance is evaluated upon variation of their structure and composition and compared with Si cell efficiency. In particular, by EQE theoretical simulations, we evidence how InGaP structures optmized for LSCs differ from those to be used under direct solar spectrum. Results obtained not only unveil the characteristics for an improved performance of the cells, but also represent an advance in the elucidation of the behaviour of InGaP-based devices in LSCs for future optimization.

ACS Applied Materials & Interfaces, 2016

Light-driven water splitting is one of the most promising approaches for using solar energy in li... more Light-driven water splitting is one of the most promising approaches for using solar energy in light of more sustainable development. In this paper, a highly efficient p-type copper(II) oxide photocathode is studied. The material, prepared by thermal treatment of CuI nanoparticles, is initially partially reduced upon working conditions and soon reaches a stable form. Upon visible-light illumination, the material yields a photocurrent of 1.3 mA cm(-2) at a potential of 0.2 V vs a reversible hydrogen electrode at mild pH under illumination by AM 1.5 G and retains 30% of its photoactivity after 6 h. This represents an unprecedented result for a nonprotected Cu oxide photocathode at neutral pH. The photocurrent efficiency as a function of the applied potential was determined using scanning electrochemical microscopy. The material was characterized in terms of photoelectrochemical features; X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, fixed-energy X-ray absorption voltammetry, and extended X-ray absorption fine structure analyses were carried out on pristine and used samples, which were used to explain the photoelectrochemical behavior. The optical features of the oxide are evidenced by direct reflectance spectroscopy and fluorescence spectroscopy, and Mott-Schottky analysis at different pH values explains the exceptional activity at neutral pH.

Electrochimica Acta, 2016

Journal of Synchrotron Radiation, 2016

Three-dimensional printed multi-purpose electrochemical devices for X-ray absorption spectroscopy... more 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.

Journal of Electroanalytical Chemistry, 2016

Acta Crystallographica Section A Foundations and Advances, 2014

Journal of Spectroscopy, 2014

An in situ study with dispersive EXAFS (Extended X-Ray Absorption Spectroscopy) at the Ir-L III e... more An in situ study with dispersive EXAFS (Extended X-Ray Absorption Spectroscopy) at the Ir-L III edge is performed to characterize Electrodeposited Iridium Oxide Films (EIROF) under chronoamperometric conditions. The technique monitors the local chemical environment and electronic structure of iridium during the oxidation of Ir(III) to Ir(IV) with a time resolution of milliseconds. The study is performed in both acidic and basic media. The Fourier transforms of the time-resolved EXAFS signals clearly show that the short-range structure of Ir is similar to that of rutile-type IrO 2 and is maintained during the reaction, thus accounting for the flexibility of the structure of the electrode material in accommodating different oxidation states. From a more general point of view, the work demonstrates the capabilities of in situ experiments based on state-of-the-art dispersive EXAFS in clarifying the mechanistic aspects of electrochemical processes.

Chemical Science, 2014

In this work the oxidation states assumed by Ir in oxide systems used as heterogeneous catalysts ... more In this work the oxidation states assumed by Ir in oxide systems used as heterogeneous catalysts for water oxidation are determined by mean s of in-situ X-ray Absorption Spectroscopy (XAS). Using a highly hydrated iridium oxide film allows to having the maximum number of Ir sites involved in the electrochemical processes occurring at the catalysts while water oxidation (oxygen evolution reaction, OER) occurs. X-ray Absorption Near Edge Structure (XANES) spectra clearly indicates the co-existence of Ir(III) and Ir(V) at 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.

Analytical Chemistry, 2013

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV repre... more 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.