In the search for rational design strategies for oxygen evolution reaction (OER) catalysts, linking the catalyst structure to activity and stability is key. However, highly active catalysts such as ...IrO x and RuO x undergo structural changes under OER conditions, and hence, structure–activity–stability relationships need to take into account the operando structure of the catalyst. Under the highly anodic conditions of the oxygen evolution reaction (OER), electrocatalysts are often converted into an active form. Here, we studied this activation for amorphous and crystalline ruthenium oxide using X-ray absorption spectroscopy (XAS) and electrochemical scanning electron microscopy (EC-SEM). We tracked the evolution of surface oxygen species in ruthenium oxides while in parallel mapping the oxidation state of the Ru atoms to draw a complete picture of the oxidation events that lead to the OER active structure. Our data show that a large fraction of the OH groups in the oxide are deprotonated under OER conditions, leading to a highly oxidized active material. The oxidation is centered not only on the Ru atoms but also on the oxygen lattice. This oxygen lattice activation is particularly strong for amorphous RuO x . We propose that this property is key for the high activity and low stability observed for amorphous ruthenium oxide.
The combination of operando and computational X-ray spectroscopies has shown promise for building accurate models of active catalyst surfaces. Operando spectroscopy captures metastable active ...surfaces and computational spectroscopy uses this information to aid in building models for first principles reaction simulations. Herein, we review recent efforts and outline future opportunities to study the oxygen evolution reaction (OER) by combining operando spectroscopies and first principles modeling. We begin by showcasing how explicit simulation of operando-collected spectra has helped validate an OER mechanism over Ir-based catalysts involving electron-deficient oxygen, or OI−. We continue by reviewing efforts on 3d transition metal (TM) oxyhydroxides, where operando studies again suggest OI− is critical. While for these materials, changes in OI− coverage have been argued to cause qualitative mechanistic differences, comparative operando and computational spectroscopic studies are still lacking. We close by outlining how such comparative studies would aid in testing mechanistic claims on 3d TM oxyhydroxides.
•Operando spectroscopy offers insight into active electrocatalytic surfaces.•Direct simulation of spectra allows validation of spectral assignments.•Combining operando and computational spectroscopy leads to mechanistic understanding.•Spectroscopy reveals importance of electron-deficient oxygen in OER catalysts.
Understanding the atomic-scale mechanistic details of the oxygen evolution reaction (OER) remains an unresolved challenge in electrochemistry owing to the complexity of the OER. In this short review ...we discuss how, with the advent of new experimental and computational methodologies, the OER can be treated with increasingly sophisticated models to aid in our complete understanding. For the case of steady state catalyst surfaces, we define a six-rung ladder of complexity to frame how far this understanding reaches and in which aspects our understanding could still improve.
Photoelectron spectroscopy offers detailed information about the electronic structure and chemical composition of surfaces, owing to the short distance that the photoelectrons can escape from a dense ...medium. Unfortunately, photoelectron based spectroscopies are not directly compatible with the liquids required to investigate electrochemical processes, especially in the soft X-ray regime. To overcome this issue, different approaches based on photoelectron spectroscopy have been developed in our group over the last few years. The performance and the degree of information provided by these approaches are compared with those of the well established bulk sensitive spectroscopic approach of total fluorescence yield detection, where the surface information gained from this approach is enhanced using samples with large surface to bulk ratios. The operation of these approaches is exemplified and compared using the oxygen evolution reaction on IrO
x
catalysts. We found that all the approaches, if properly applied, provide similar information about surface oxygen speciation. However, using resonant photoemission spectroscopy, we were able to prove that speciation is more involved and complex than previously thought during the oxygen evolution reaction on IrO
x
based electrocatalysts. We found that the electrified solid-liquid interface is composed of different oxygen species, where the terminal oxygen atoms on iridium are the active species, yielding the formation of peroxo species and, finally, dioxygen as the reaction product. Thus, the oxygen-oxygen bond formation is dominated by peroxo species formation along the reaction pathway. Furthermore, the methodologies discussed here open up opportunities to investigate electrified solid-liquid interfaces in a multitude of electrochemical processes with unprecedented speciation capabilities, which are not accessible by one-dimensional X-ray spectroscopies.
Different
operando
approaches based in PES were used and compared under OER conditions on IrO
x
. ResPES indicates that single coordinated oxygen atoms (μ
1
-O) are the active species yielding the formation of peroxo (μ
1
-OO) and finally O
2
as product.
The role of the nitrogen incorporation into titanium, its chemical nature, the location in the titanium lattice and its electrochemical performance were investigated by a combination of several ...spectroscopy and microscopy techniques using samples prepared by CVD of NH3 at different temperatures and successive electrochemically tested in 1M of HClO4. We found that nitrogen is incorporated in either the interstitial or substitutional site of the lattice depending on the preparation temperature modifying strongly its corrosion resistance which was ascribed to the N 2p hybridization with the Ti 3d orbitals. It was found that at low temperature the N 2p orbitals were more likely to hybridize with Ti3d-t2g orbitals while higher temperature favors the hybridization with the Ti3d-eg orbitals. This is responsible for the corrosion resistance shown by the samples prepared at higher temperature.
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•The role of the N location in the Ti lattice and its electrochemical performance was revealed by soft X-ray spectroscopy.•N 2p bonded with Ti 3d-t2g or eg depends strongly on the nitrification temperature.•The incorporation of N to the pristine TiO2 makes it more surface and chemical structure stable.•The stability and demonstrated ability of TiNx to absorb visible light makes it a good candidate for solar water splitting.
Photoelectron spectroscopy offers detailed information about the electronic structure and chemical composition of surfaces, owing to the short distance that the photoelectrons can escape from a dense ...medium. Unfortunately, photoelectron based spectroscopies are not directly compatible with the liquids required to investigate electrochemical processes, especially in the soft X-ray regime. To overcome this issue, different approaches based on photoelectron spectroscopy have been developed in our group over the last few years. The performance and the degree of information provided by these approaches are compared with those of the well established bulk sensitive spectroscopic approach of total fluorescence yield detection, where the surface information gained from this approach is enhanced using samples with large surface to bulk ratios. The operation of these approaches is exemplified and compared using the oxygen evolution reaction on IrO
catalysts. We found that all the approaches, if properly applied, provide similar information about surface oxygen speciation. However, using resonant photoemission spectroscopy, we were able to prove that speciation is more involved and complex than previously thought during the oxygen evolution reaction on IrO
based electrocatalysts. We found that the electrified solid-liquid interface is composed of different oxygen species, where the terminal oxygen atoms on iridium are the active species, yielding the formation of peroxo species and, finally, dioxygen as the reaction product. Thus, the oxygen-oxygen bond formation is dominated by peroxo species formation along the reaction pathway. Furthermore, the methodologies discussed here open up opportunities to investigate electrified solid-liquid interfaces in a multitude of electrochemical processes with unprecedented speciation capabilities, which are not accessible by one-dimensional X-ray spectroscopies.
The
variation in the morphology and electronic structure of copper
during the electroreduction of CO
2
into valuable hydrocarbons
and alcohols was revealed by combining
in situ
surface-
and ...bulk-sensitive X-ray spectroscopies with electrochemical scanning
electron microscopy. These experiments proved that the electrified
interface surface and near-surface are dominated by reduced copper.
The selectivity to the formation of the key C–C bond is enhanced
at higher cathodic potentials as a consequence of increased copper
metallicity. In addition, the reduction of the copper oxide electrode
and oxygen loss in the lattice reconstructs the electrode to yield
a rougher surface with more uncoordinated sites, which controls the
dissociation barrier of water and CO
2
. Thus, according
to these results, copper oxide species can only be stabilized kinetically
under CO
2
reduction reaction conditions.
We report that SS 433 is a binary system containing a supergiant star that is overflowing its Roche lobe with matter accreting onto a compact object (either a black hole or neutron star). Two jets of ...ionized matter with a bulk velocity of approximately 0.26c (where c is the speed of light in vacuum) extend from the binary, perpendicular to the line of sight, and terminate inside W50, a supernova remnant that is being distorted by the jets. SS 433 differs from other microquasars (small-scale versions of quasars that are present within our own Galaxy) in that the accretion is believed to be super-Eddington, and the luminosity of the system is about 1040 ergs per second. The lobes of W50 in which the jets terminate, about 40 parsecs from the central source, are expected to accelerate charged particles, and indeed radio and X-ray emission consistent with electron synchrotron emission in a magnetic field have been observed14,15,16. At higher energies (greater than 100 gigaelectronvolts), the particle fluxes of γ-rays from X-ray hotspots around SS 433 have been reported as flux upper limits. In this energy regime, it has been unclear whether the emission is dominated by electrons that are interacting with photons from the cosmic microwave background through inverse-Compton scattering or by protons that are interacting with the ambient gas. Here we report teraelectronvolt γ-ray observations of the SS 433/W50 system that spatially resolve the lobes. The teraelectronvolt emission is localized to structures in the lobes, far from the centre of the system where the jets are formed. We have measured photon energies of at least 25 teraelectronvolts, and these are certainly not Doppler-boosted, because of the viewing geometry. Lastly, we conclude that the emission—from radio to teraelectronvolt energies—is consistent with a single population of electrons with energies extending to at least hundreds of teraelectronvolts in a magnetic field of about 16 microgauss.
Steady gamma-ray emission up to at least 200 GeV has been detected from the solar disk in the Fermi-LAT data, with the brightest, hardest emission occurring during solar minimum. The likely cause is ...hadronic cosmic rays undergoing collisions in the Sun’s atmosphere after being redirected from ingoing to outgoing in magnetic fields, though the exact mechanism is not understood. An important new test of the gamma-ray production mechanism will follow from observations at higher energies. Only the High Altitude Water Cherenkov (HAWC) Observatory has the required sensitivity to effectively probe the Sun in the TeV range. Here, using 3 years of HAWC data from November 2014 to December 2017, just prior to the solar minimum, we search for 1–100 TeV gamma rays from the solar disk. No evidence of a signal is observed, and we set strong upper limits on the flux at a few 10-12 TeV-1 cm-2 s-1 at 1 TeV. Our limit, which is the most constraining result on TeV gamma rays from the Sun, is ~ 10 % of the theoretical maximum flux (based on a model where all incoming cosmic rays produce outgoing photons), which in turn is comparable to the Fermi-LAT data near 100 GeV. The prospects for a first TeV detection of the Sun by HAWC are especially high during the solar minimum, which began in early 2018.
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