Perspective Outlook on Operando Photoelectron and Absorption Spectroscopy to Probe Catalysts at the Solid Liquid Electrochemical Interface

Operando X-ray Photoelectron and Absorption Spectroscopy (XPS and XAS) using soft (up to 2 KeV) and tender (2–10 KeV) X-rays applied to study functional materials for energy conversion has gone through great development in the last years and several approaches to different cell designs combined with instrumentation development now allow successful characterization of electrode-electrolyte interfaces under working conditions. An overview of the current state and challenges are presented along with an outlook into the direction that future development should take, which we expect would allow us to expand and complete our understanding of the liquid-solid electrochemical interfaces

On the operando structure of ruthenium oxides during the oxygen evolution reaction in acidic media

In the search for rational design strategies for oxygenevolutionreaction (OER) catalysts, linking the catalyst structure to activityand stability is key. However, highly active catalysts such as IrO x and RuO x undergostructural changes under OER conditions, and hence, structure-activity-stabilityrelationships need to take into account the operando structure ofthe catalyst. Under the highly anodic conditions of the oxygen evolutionreaction (OER), electrocatalysts are often converted into an activeform. Here, we studied this activation for amorphous and crystallineruthenium oxide using X-ray absorption spectroscopy (XAS) and electrochemicalscanning electron microscopy (EC-SEM). We tracked the evolution ofsurface oxygen species in ruthenium oxides while in parallel mappingthe oxidation state of the Ru atoms to draw a complete picture ofthe oxidation events that lead to the OER active structure. Our datashow that a large fraction of the OH groups in the oxide are deprotonatedunder OER conditions, leading to a highly oxidized active material.The oxidation is centered not only on the Ru atoms but also on theoxygen lattice. This oxygen lattice activation is particularly strongfor amorphous RuO x . We propose that thisproperty is key for the high activity and low stability observed foramorphous ruthenium oxide.Catalysis and Surface Chemistr

CMOS-compatible nanoscale gas-sensor based on field effect

The integration of a solid state gas sensor of the metal oxide sensor type into CMOS technology still is a challenge because of the high temperatures during metal oxide annealing and sensor operation that do not comply with silicon device stability. In the presence of an external electric field sensor sensitivity can be controlled through a change of the Fermi energy level and consequently it is possible to reduce the operation temperature. Based in this effect, a novel field effect gas sensor was developed resembling a reversed insulated gate field effect transistor (IGFET) with the thickness of gas sensing layer in the range of the Debye length (L-D). Under these conditions the controlling electrical field reaches the sensitive surface and a modulation of the Fermi energy level occurs, producing an effective control of gas sensitivity and sensor response. In this paper several aspects are treated, like technological fabrication process, complete sensor characterization by means of an electrical model and sensor response measurements. Other effects as base-line drift effects and layer thickness implications also are studied

Probing Operating Electrochemical Interfaces by Photons and Neutrons

The operation of all electrochemical energy-related systems, such as supercapacitors, batteries, fuel cells, etc. depends largely on the processes occurring at electrochemical interfaces at which charge separation and chemical reactions occur. Evolution of structure and composition at the interface between electrodes and electrolytes affects all the device′s functional parameters including power and long-term performance stability. The analytical techniques capable of exploring the interfaces are still very limited, and more often only ex situ studies are performed. This sometimes leads to a loss of important pieces of the puzzle, hindering the development of novel technologies, as in many cases intermediates and electrochemical reaction products cannot be “quenched” for post-process analyses. Techniques capable of operando probing of electrochemical interfaces by photons and neutrons have become an extensively growing field of research. This review aims at highlighting approaches and developing ideas on the adaptation of photoelectron, X-ray absorption, vibrational spectroscopy, nuclear magnetic resonance, and X-ray and neutron reflectometry in electrochemical studies

CMOS-compatible field effect nanoscale gas-sensor: Operation and annealing models

Complete modelling of electrically controlled nanoscale gas sensors with Poisson, Wolkenstein, Fokker-Planck and continuity is presented. Based on a plausible Drift explanation we developed suitable models for sensitivity control and operational modes. An onset for CMOS-complying annealing procedures is given

Exploring the incorporation of nitrogen in titanium and its influence on the electrochemical corrosion resistance in acidic media

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 1 M 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 Ti_3d-t_2g orbitals while higher temperature favors the hybridization with the Ti_3d-e_g orbitals. This is responsible for the corrosion resistance shown by the samples prepared at higher temperature

Notable Reactivity of Acetonitrile Towards Li2O2 LiO2 Probed by NAP XPS During Li O 2 Battery Discharge

One of the key factors responsible for the poor cycleability of Li–O2 batteries is a formation of byproducts from irreversible reactions between electrolyte and discharge product Li2O2 and/or intermediate LiO2. Among many solvents that are used as electrolyte component for Li–O2 batteries, acetonitrile (MeCN) is believed to be relatively stable towards oxidation. Using near ambient pressure X-ray photoemission spectroscopy (NAP XPS), we characterized the reactivity of MeCN in the Li–O2 battery. For this purpose, we designed the model electrochemical cell assembled with solid electrolyte. We discharged it first in O2 and then exposed to MeCN vapor. Further, the discharge was carried out in O2 + MeCN mixture. We have demonstrated that being in contact with Li–O2 discharge products, MeCN oxidizes. This yields species that are weakly bonded to the surface and can be easily desorbed. That’s why they cannot be detected by the conventional XPS. Our results suggest that the respective chemical process most probably does not give rise to electrode passivation but can decrease considerably the Coulombic efficiency of the battery