Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. ...However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with Ir(III). These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts.
Electronic metal–support interactions (EMSI) describe the electron flow between metal sites and a metal oxide support. It is generally used to follow the mechanism of redox reactions. In this study ...of CuO‐CeO2 redox, an additional flow of electrons from metallic Cu to surface carbon species is observed via a combination of operando X‐ray absorption spectroscopy, synchrotron X‐ray powder diffraction, near ambient pressure near edge X‐ray absorption fine structure spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. An electronic metal–support–carbon interaction (EMSCI) is proposed to explain the reaction pathway of CO oxidation. The EMSCI provides a complete picture of the mass and electron flow, which will help predict and improve the catalytic performance in the selective activation of CO2, carbonate, or carbonyl species in C1 chemistry.
During the oxidation of CO over the surface of a CuO‐CeO2 catalyst, electrons are simultaneously transferred from Cu0 to lattice Ce4+ and surface carbon species deposited from CO. These “electronic metal–support–carbon interactions” (EMSCI) are thought to play an important role in the reactions involving the support and surface carbon species in C1 chemistry.
Water splitting performed in acidic media relies on the exceptional performance of iridium-based materials to catalyze the oxygen evolution reaction (OER). In the present work, we use
X-ray ...photoemission and absorption spectroscopy to resolve the long-standing debate about surface species present in iridium-based catalysts during the OER. We find that the surface of an initially metallic iridium model electrode converts into a mixed-valent, conductive iridium oxide matrix during the OER, which contains O
and electrophilic O
species. We observe a positive correlation between the O
concentration and the evolved oxygen, suggesting that these electrophilic oxygen sites may be involved in catalyzing the OER. We can understand this observation by analogy with photosystem II; their electrophilicity renders the O
species active in O-O bond formation,
the likely potential- and rate-determining step of the OER. The ability of amorphous iridium oxyhydroxides to easily host such reactive, electrophilic species can explain their superior performance when compared to plain iridium metal or crystalline rutile-type IrO
.
Tremendous effort has been devoted towards elucidating the fundamental reasons for the higher activity of hydrated amorphous Ir
oxyhydroxides (IrO
) in the oxygen evolution reaction (OER) in ...comparison with their crystalline counterpart, rutile-type IrO
, by focusing on the metal oxidation state. Here we demonstrate that, through an analogy to photosystem II, the nature of this reactive species is not solely a property of the metal but is intimately tied to the electronic structure of oxygen. We use a combination of synchrotron-based X-ray photoemission and absorption spectroscopies,
calculations, and microcalorimetry to show that holes in the O 2p states in amorphous IrO
give rise to a weakly bound oxygen that is extremely susceptible to nucleophilic attack, reacting stoichiometrically with CO already at room temperature. As such, we expect this species to play the critical role of the electrophilic oxygen involved in O-O bond formation in the electrocatalytic OER on IrO
. We propose that the dynamic nature of the Ir framework in amorphous IrO
imparts the flexibility in Ir oxidation state required for the formation of this active electrophilic oxygen.
Creating active and stable electrodes is an essential step toward efficient and durable electrolyzers. To achieve this goal, understanding what aspects of the electrode structure dictate activity and ...catalyst dissolution is key. Here, we investigate these aspects by studying trends in the activity, stability, and operando structure of iridium oxides during the oxygen evolution reaction. Using operando X-ray photoelectron and X-ray absorption spectroscopy, we determined the near-surface structure of oxides ranging from amorphous to crystalline during the reaction. We show that applying oxygen evolution potentials universally yields deprotonated μ2-O moieties and a μ1-O/μ1-OH mixture, with universal deprotonation energetics but in different amounts. This quantitative difference mainly results from variations in deprotonation depth: surface deprotonation for crystalline IrO2 versus near-surface deprotonation for semicrystalline and amorphous IrO x . We argue that both surface deprotonation and subsurface deprotonation modify the barrier for the oxygen evolution and Ir dissolution reactions, thus playing an important role in catalyst performance.
Abstract
Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, ...we achieve selective oxygen (O
2
) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3
d
orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a Cu(I)O
2
3−
site selectively adsorbs molecular O
2
, forming a rarely reported electrophilic η
2
-O
2
species at 298 K. Assisted by neighbouring Ce(III) cations, η
2
-O
2
is finally reduced to two O
2−
, that create two Cu–O–Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s
−1
at 373 K and 0.01 bar
P
CO
. The unique electronic structure of Cu(I)O
2
3−
site suggests its potential in selective oxidation.
Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based ...anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrO x centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ1-O and μ1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.
In this contribution, we report the development of in situ electrochemical cells based on proton exchange membranes suitable for studying interfacial structural dynamics of energy materials under ...operation by near ambient pressure X-ray photoelectron spectroscopy. We will present both the first design of a batch-type two-electrode cell prototype and the improvements attained with a continuous flow three-electrode cell. Examples of both sputtered metal films and carbon-supported metal nanostructures are included demonstrating the high flexibility of the cells to study energy materials. Our immediate focus was on the study of the oxygen evolution reaction, however, the methods described herein can be broadly applied to reactions relevant in energy conversion and storage devices.
Iridium oxide-based catalysts are uniquely active and stable in the oxygen evolution reaction. Theoretical work attributes their activity to oxyl or μ1-O species. Verifying this intermediate ...experimentally has, however, been challenging. In the present study, these challenges were overcome by combining theory with new experimental strategies. Ab initio molecular dynamics of the solid–liquid interface were used to predict spectroscopic features, whereas sample architecture, developed for surface-sensitive X-ray spectroscopy of electrocatalysts in confined liquid, was used to search for these species under realistic conditions. Through this approach, we have identified μ1-O species during oxygen evolution. Potentiodynamic X-ray absorption additionally shows that these μ1-O species are created by electrochemical oxidation currents in a deprotonation reaction.
In situ x-ray spectroscopies offer a powerful way to understand the electronic structure of the electrode-electrolyte interface under operating conditions. However, most x-ray techniques require ...vacuum, making it necessary to design spectro-electrochemical cells with a delicate interface to the wet electrochemical environment. The design of the cell often dictates what measurements can be done and which electrochemical processes can be studied. Hence, it is important to pick the right spectro-electrochemical cell for the process of interest. To facilitate this choice, and to highlight the challenges in cell design, we critically review four recent, successful cell designs. Using several case studies, we investigate the opportunities and limitations that arise in practical experiments.
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