In Situ Dispersive EXAFS in Electrocatalysis: The Investigation of the Local Structure of IrO x in Chronoamperometric Conditions as a Case Study (original) (raw)
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Electronic structure vs. electrocatalytic activity of iridium oxide
Journal of Electroanalytical Chemistry, 2010
The electrochemical oxidation and reduction of hexacyanoferrate redox couples at anodically formed iridium oxide (AIROF) and nanostructured iridium oxide produced by hydrolysis, hIrO 2 , are compared. Whereas AIROF displays the characteristics of a semiconducting electrode in its reduced state, hIrO 2 is similar to a metal electrode. In view of also significant differences between the two oxides' optical and intercalation properties, this strongly indicates that hIrO 2 has a different bulk electronic structure. Yet, the charge-normalised electrocatalytic activity for the oxygen-evolution reaction at AIROF and hIrO 2 is the same. This indicates that the electrocatalytic properties of iridium oxide are a function of the local structure at the oxide surface and not uniquely related to the bulk electronic structure.
Journal of Electroanalytical Chemistry, 2021
Hydrated iridium oxyhydroxide (IrO x) films were electrochemically deposited from an alkaline oxalic solution at constant anodic potentials and by applying a potential cycling protocol, in both cases with variation of the electrodeposi-tion time. From UV-vis spetroscopy of the solution for the deposition and their characterization it was concluded that a mixture of Ir(III)/Ir(IV) monomers participates in the deposition of IrO x film. X-ray photoelectron spectroscopy (XPS) of IrO x films indicated that both types of films contained hydrated Ir(IV) hydroxide as the dominant species, but in the film deposited by potential cycling the presence of the additional Ir(III) species was evident. The scanning electon mi-croscopy (SEM) analysis of the surface morphology revealed that films deposited by potential cycling were more uniform than the films deposited at a constant potential. The amount of electrochemically active Ir-species on the surface of deposited IrO x films was estimated from the voltammetric charge of the Ir(III)/Ir(IV) transition. Depending on the film electrodeposition parameters, the values between 15 and 1080 nmol cm −2 were obtained. The electrochemically active surface area (ECSA) of IrO x films was calculated from cyclic voltammetry and electrochemical impedance spec-troscopy (EIS) measurements and ranged from 3 to 131 cm 2 per 1 cm 2 of geometric surface area for various films. The activity and stability of IrO x films toward oxygen evolution reaction (OER) was investigated in 0.5 M H 2 SO 4 solution under potentiostatic conditions. The intrinsic activity, stated as turnover frequency and specific current density normalized per ECSA, showed that the OER activity of IrO x films deposited by potential cycling are up to two and a half times higher than the activity of films deposited at a constant anodic potential. Potentiostatic stability test showed a decrease in OER current over time for both type of the films. Determination of ECSA, the amount of electroactive Ir species, XPS spectrum and SEM imaging after the test indicated that the decrease in OER activity was caused by partial dissolution and delamination of the film as well as by oxidation of highly active hydroxide Ir(III) species.
Journal of the Electrochemical Society, 2014
The great interest of the electrochemical industry toward materials that exhibit high electrocatalytic activity and long service-life in electrolytic processes has constantly prompted fundamental research, enhancing the understanding of electrode behavior and the improvement of their performance. One of the most important class of industrial devices is represented by the so-called DSAs, which consist of an electrocatalytic oxide film (usually based on RuO 2 and/or IrO 2 ) deposited on a suitable metal support (titanium). To increase the stability of the material, other oxides (TiO 2 , SnO 2 , etc.) are added to the electrocatalytic one, with the double purpose of increasing the corrosion resistance of the coating and of diluting the main oxide (to minimize the production costs). In this work, mixtures of 35 mol% IrO 2 -65 mol% TiO 2 deposited on titanium supports have been studied by ex-situ (RBS, XRD, ERDA, AFM) and on-site (CV) techniques. Electrodes prepared by an updated sol-gel method and reactive sputtering, at two different temperatures (350 and 450 • C), were investigated with the aim of correlating structural information with electrochemical performances, in terms of number of active electrocatalytic iridium sites. Based on roughness indexes, morphological properties, and of data obtained by nuclear methods for surface analysis and cyclic voltammetry, the behavior of iridium sites has been discussed, attempting a qualitative distinction of roles of roughness and intrinsic catalytic activity in a model-electrochemical reaction, like oxygen evolution reaction. Sputtered samples appear, in general, more compact than sol-gel ones, and a low temperature of pyrolysis favours a more extended electroactive surface. The study of OER demonstrates that the kinetics of gas evolution proceeds via the electrochemical oxide formation pathway (Volmer-Heyrovsky mechanism) on all samples.
Observing the oxidation state turnover in heterogeneous iridium-based water oxidation catalysts
Chemical Science, 2014
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.
Physical Chemistry Chemical Physics, 2019
Iridium and ruthenium oxide are active electrocatalysts for oxygen evolution. The relation between preparation method, structure, and behavior of mixed oxides of iridium and ruthenium are of interest in order to obtain active and stable catalysts. In this work the structure of mixed Ru-Ir oxides synthesized by the polymeric precursor method, which involves the formation of a gel containing the metal precursors and subsequent heat-treatment in air, was studied for the Ir x Ru 1−x O 2 system. An in-depth analysis of X-ray diffraction (XRD) and X-ray absorption (XAS) data, including EXAFS and linear combination of XANES, shows that the polymeric precursor synthesis method is capable of providing an intimate mixing of Ir and Ru in the catalyst. In addition to the oxide phase, metal phases, i.e. with Ru or Ir or both in oxidation state zero (Ir(fcc) and Ru(hcp)), were also found in the product materials. Facing complex structures such as some of those synthesized here, we have shown that a representation of shells with more than one atom type are efficiently represented using mixed sites, i.e. including scattering contributions from several elements in a site corresponding to a partial occupancy of the site by these elements, this method forming a very efficient basis for analyzing EXAFS data.
Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1984
X-ray photoelectron spectroscopic measurements on the state of Ir and O species in electrochemically and thermally formed iridium oxide films are reported. The electrochemically formed films were grown to three thicknesses by means of potential cycling. The monolayer oxide film which exhibits electrochemical irreversibility in its formation and reduction already causes a change of electronic state of the Ir substrate surface atoms corresponding to formation of an Ir(OH)4 species. However, if any electrochemically reversible, thick film oxide, 100 × a monolayer is formed, it behaves like bulk IrO 2 powder with regard to 4f electron shell binding energies. The state of O (ls) is also different in this material from that in thinner oxide films, and contains clearly distinguishable ionic 0 2-rather than OH-, bound water, or adsorbed 02, although the latter type of species is also present. Appreciable SO 2-ion incorporation or irreversible adsorption can also be detected. No difference in the oxidation state of Ir can be detected on (i.e., over -