Ultrathin atomic layer deposited ceria films (<20 nm) are capable of H2 heterolytic activation at room temperature, undergoing a significant reduction regardless of the absolute pressure, as measured under in-situ conditions by near ambient pressure X-ray photoelectron spectroscopy. ALD-ceria can gradually reduce as a function of H2 concentration under H2/O2 environments, especially for diluted mixtures below 10 %. At room temperature, this reduction is limited to the surface region, where the hydroxylation of the ceria surface induces a charge transfer towards the ceria matrix, reducing Ce4+ cations to Ce3+. Thus, ALD-ceria replicates the expected sensing mechanism of metal oxides at low temperatures without using any noble metal decorating the oxide surface to enhance H2 dissociation. The intrinsic defects of the ALD deposit seem to play a crucial role since the post-annealing process capable of healing these defects leads to decreased film reactivity. The sensing behavior was successfully demonstrated in sensor test structures by resistance changes towards low concentrations of H2 at low operating temperatures without using noble metals. These promising results call for combining ALD-ceria with more conductive metal oxides, taking advantage of the charge transfer at the interface and thus modifying the depletion layer formed at the heterojunction.

Understanding the oxygen evolution reaction (OER) and Ir dissolution mechanisms in amorphous, hydrous iridium oxides (am-hydr-IrOx) is hindered by the reliance on crystalline iridium oxide theoretical models to interpret its behaviour. This study presents a comprehensive investigation of hydrous iridium oxide thin films (HIROFs) as a model for am-hydr-IrOx to elucidate electronic and structural transformations under OER conditions of proton exchange membrane water electrolyzers (PEM-WE). Employing in situ and operando Ir L3-edge X-ray absorption spectroscopy supported by density functional theory calculations, we introduce a novel surface H-terminated nanosheet model that better characterizes the short-range structure of am-hydr-IrOx compared to previous crystalline models, which exhibits elongated Ir-O bond lengths compared to rutile-IrO2. This atomic model unveils the electronic and structural transformations of am-hydr-IrOx, progressing from H-terminated nanosheets to structures with multiple Ir vacancies and shorter bond-lengths at OER potentials. Notably, Ir dissolution emerges as a spontaneous, thermodynamically driven process, initiated at potentials lower than OER activation, which requires a parallel mechanistic framework describing Ir dissolution by Ir defect formation. Moreover, our results provide mechanistic insights into the activity-stability relationship of am-hydr-IrOx by systematically screening the DFT-calculated OER activity of diverse Ir and O chemical environments. This work challenges conventional perceptions of iridium dissolution and OER mechanisms in am-hydr-IrOx, providing an alternative perspective within a dual-mechanistic framework.

Currently, only iridium oxide shows favourable activity and stability under the harsh oxygen evolution reaction (OER) conditions for application in proton exchange membrane water electrolysis (PEM-WE) [1-3]. There is a well established inverse relationship between OER activity and stability of iridium oxide-based OER catalysts given its degree of crystallinity and hydration [4-6]. Crystalline, anhydrous iridium oxides are more stable but have diminished OER activity compared to amorphous, hydrous iridium oxides, which are more active. Different OER mechanisms have been theorized to occur on these materials, explaining their difference in performance [6]. Several spectroscopic investigations have been performed to explore the properties of such different iridium oxides, in particular to identify the chemical and electronic state of their active sites. However, a consensus regarding the active site structure and OER mechanism has yet to be reached [7-10]. Using scanning electron microscopy, x-ray photoelectron spectroscopy, and combining in-situ Ir L 3-edge x-ray absorption spectroscopy with density functional theory (DFT) calculations and ab initio thermodynamics, we have investigated in-situ electrochemically grown porous hydrous iridium oxide thin films (HIROF) as a model system to examine the chemical and electronic structure of the highly active, hydrous iridium oxide species. In-situ extended x-ray absorption fine structure(EXAFS) results show that HIROF grows preferentially in a form most often associated with the OER catalytically active site of hydrous iridium oxides. Calculations over different possible structures allowed us to identify a unique structural group with enhanced hydrogenation that best fits the EXAFS data. In-situ x-ray absorption near edge structure (XANES) results reveal a lower onset potential for the redox behaviour of HIROF compared to rutile IrO2. Based on this study, we propose a new structural model explaining the high activity and poorer stability of hydrous iridium oxides compared to crystalline IrO2. References: [1] A. Buttler and H. Spliethoff, Renew. Sustain. Energy Rev., 2018, 82, pp. 2440-2454. [2] Schlögl, ChemSusChem, 2010, 3, pp. 209-222. [3] M. Schalenbach, J. Electrochem. Soc., 2016, 163, pp. F3197-P3208. [4] S. Cherevko, Electrochem. Commun., 2014, 48, pp. 81-85. [5] S. Cherevko, J. Electroanal. Chem., 2016(1), 773, pp 69-78. [6] S. Cherevko, J. Electroanal. Chem., 2016(2), 774, pp 102-110. [7] V. Pfeifer. Surf. Interface Anal., 2016, 48, pp. 261-273. [8] A. Minguzzi, Chem. Sci., 2014, 5, pp- 3591-3597. [9] A.H. Reksten, Phys. Chem. Chem. Phys., 2020, 22, pp. 18868-18881. [10] J.J. Veslasco-Vélez, J. Am. Chem. Soc., 2021, 143, pp. 12524-12534.

The oxidation of the aqueous H3PO3 in contact with Pt was investigated for a fundamental understanding of the Pt/aqueous H3PO3 interaction with the goal of providing a comprehensive basis for the further optimization of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Ion-exchange chromatography (IEC) experiments suggested that in ambient conditions, Pt catalyzes H3PO3 oxidation to H3PO4 with H2O. X-ray photoelectron spectroscopy (XPS) on different substrates, including Au and Pt, previously treated in H3PO3 solutions was conducted to determine the catalytic abilities of selected metals toward H3PO3 oxidation. In situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) combined with the "dip-and-pull" method was performed to investigate the state of H3PO3 at the Pt|H3PO3 interface and in the bulk solution. It was shown that whereas H3PO3 remains stable in the bulk solution, the catalyzed oxidation of H3PO3 by H2O to H3PO4 accompanied by H-2 generation occurs in contact with the Pt surface. This catalytic process likely involves H3PO3 adsorption at the Pt surface in a highly reactive pyramidal tautomeric configuration.

The spatial distribution and concentration of lanthanide activator and sensitizer dopant ions are of key importance for the luminescence color and efficiency of upconverting nanoparticles (UCNPs). Quantifying dopant ion distributions and intermixing, and correlating them with synthesis methods require suitable analytical techniques. Here, X-ray photoelectron spectroscopy depth-profiling with tender X-rays (2000-6000 eV), providing probe depths ideally matched to UCNP sizes, is used to measure the depth-dependent concentration ratios of Er3+ to Yb3+ , [Er3+ ]/[Yb3+ ], in three types of UCNPs prepared using different reagents and synthesis methods. This is combined with data simulations and inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurements of the lanthanide ion concentrations to construct models of the UCNPs' dopant ion distributions. The UCNP sizes and architectures are chosen to demonstrate the potential of this approach. Core-only UCNPs synthesized with XCl3 ·6H2 O precursors (β-phase) exhibit a homogeneous distribution of lanthanide ions, but a slightly surface-enhanced [Er3+ ]/[Yb3+ ] is observed for UCNPs prepared with trifluroacetate precursors (α-phase). Examination of Yb-core@Er-shell UCNPs reveals a co-doped, intermixed region between the single-doped core and shell. The impact of these different dopant ion distributions on the UCNP's optical properties is discussed to highlight their importance for UCNP functionality and the design of efficient UCNPs.

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) employing a phosphoric acid H3PO4-doped membrane are considered to be promising sustainable electrochemical energy storage. The high-temperature operation has several advantages, such as a higher tolerance to CO poisoning, allowing coupling of HT-PEMFCs with reformers [1-3], as well the possibility for heat and electric energy co-generation [1,2]. However, during operation, phosphorus oxo-acids (e.g.: H3PO3) are generated on the anode. These impurities adsorb on the Pt catalyst [4-6], thus possibly negatively affecting the HT-PEMFCs performance. A detailed understanding of the H3PO3-catalyst (Pt) interaction is hence necessary for further HT-PEMFC optimization. However, besides an investigation of the H3PO3 adsorption behavior on Pt [6,7], literature on the behavior of the H3PO3 in contact with Pt (with/without polarization) is scarce. In this work, the oxidation mechanism of H3PO3 was investigated using a combination of in situ x-ray spectroscopy techniques that directly probe the H3PO3/Pt interface interaction, complemented by ex situ x-ray photoelectron spectroscopy (XPS) and ion-exchange chromatography (IEC). IEC gave insights into the effect of Pt on the stability of deaerated aqueous H3PO3 solutions. XPS was conducted on H3PO3/support structures (including Au and Pt supports) to determine to what extent the support affects the H3PO3 oxidation. Furthermore, in-situ dip and pull near-ambient pressure (NAP-)XPS was conducted to investigate the state of H3PO3 at the H3PO3/Pt interface and in solution bulk. It was observed that at the H3PO3/Pt interface, H3PO3 was chemically oxidized to H3PO4, while in the bulk solution it remains stable, as shown in Figure 1. Moreover, in situ x-ray absorption spectroscopy at the P K-edge was conducted at different concentrations of H3PO3 in aqueous solutions (i.e., different amounts of H2O) in contact with Pt, to determine the role of H2O in the oxidation of H3PO3. A higher degree of oxidation was observed for the less concentrated H3PO3, implying that H2O participates in the oxidation mechanism of H3PO3 to H3PO4. References: [1] Chandan et al., A.. J. Power Sources 2013, 231, 264–278. [2] Asensio, et al. Chem. Soc. Rev. 2010, 39 (8), 3210. [3] Q. Li et al, J. Electrochem. Soc. 2003, 150 (12), A1599. [4] Sugishima et al, J. Electrochem. Soc. 1994, 141 (12), 3332. [5] Doh et al. ChemElectroChem. 2014, 1 (1), 180–186. [6] Prokop et al, Electrochimica Acta 2015, 160, 214–218. [7] Prokop et al, Electrochimica Acta 2016, 212, 465–472. Figure 1

We present a new technique for investigating complex model electrocatalysts by means of electrochemical in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). Using a specially designed miniature capillary device, we prepared a three-electrode electrochemical cell in a thin-layer configuration and analyzed the active electrode/electrolyte interface by using "tender" X-ray synchrotron radiation. We demonstrate the potential of this versatile method by investigating a complex model electrocatalyst. Specifically, we monitored the oxidation state of Pd nanoparticles supported on an ordered Co3O4(111) film on Ir(100) in an alkaline electrolyte under potential control. We found that the Pd oxide formed in the in situ experiment differs drastically from the one observed in an ex situ emersion experiment at similar potential. We attribute these differences to the decomposition of a labile palladium oxide/hydroxide species after emersion. Our experiment demonstrates the potential of our approach and the importance of electrochemical in situ AP-XPS for studying complex electrocatalytic interfaces.

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO4(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO4(010) surface stabilizes the formation of a small electron polaron on the VO4 tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V4+ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Vol. 35, No. 3, 2022, Synchrotron radiation newS Technical RepoRT The Berlin Joint Lab for Electrochemical Interfaces, BElChem: A Facility for In-situ and Operando NAP-XPS and NAP-HAXPES Studies of Electrochemical Interfaces at BESSY II DaviD E. Starr,1 MichaEl hävEckEr,2,3 axEl knop-GErickE,2,3 Marco Favaro,1 SiMonE vaDilonGa,1 MarcEl MErtin,1 GErD rEicharDt,1 Jan-SiMon SchMiDt,1 Frank SiEwErt,1 robErt Schulz,1 JEnS viEFhauS,1 chriStian JunG,1 anD roEl van DE krol1 1Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany 2Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany 3Max-Planck-Institut für Chemische Energiekonversion, Mülheim, Germany Introduction The Berlin Joint Lab for Electrochemical Interfaces (BElChem) is located at the BESSY II synchrotron in Berlin, Germany, and co-run by the Fritz-Haber-Institut, the Max-Planck-Institut of Chemical Energy Conversion and the Helmholtz-Zentrum Berlin. BElChem focuses on providing a molecular-level description of (photo)electrochemical interfaces that are of high relevance for solar fuel production and renewable energy storage. The CO 2 reduction reaction (CO2RR) and the oxygen evolution reaction (OER) are of particular current interest. In BElChem, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and near-ambient pressure hard X-ray photoelectron spectroscopy (NAP-HAXPES) will be used for the in-situ and operando interrogation of the electronic structure and chemical composition of catalytically active solid/gas and solid/liquid interfaces. BElChem will also enable heterogeneous catalytic reactions, such as oxidation and hydrogenation reactions, to be investigated. The BElChem facility consists of two beamlines with two endstations in two separate hutches and an additional sample preparation/ chemical lab. One beamline, the undulator beamline U49/2 PGM (plane grating monochromator), covers the soft X-ray energy range, whereas the other dipole magnet sourced beamline, BElChem-DCM, with a double crystal monochromator (DCM), covers the tender X-ray energy range. Combined, the BElChem beamlines cover a photon energy range nominally from 90 eV to 10 keV. Each endstation has its own electron spectrometer. The endstation frame is composed of two separate parts. On one part, the electron spectrometer is mounted and, on the other, the analysis chamber is mounted. This allows the easy exchange of experimental modules and the ability for users of BElChem to provide tailor-made modules targeting the sample environment relevant for their in-situ or operando measurement. The BElChem facility provides the opportunity to study electrochemical interfaces with two general approaches. Due to the high surface sensitivity and short mean free paths of low kinetic energy photoelectrons generated with soft X-rays, a suitable method to explore the electrode/electrolyte interface with XPS during a (photo)electrochemical reaction is needed. At BElChem, these types of measurements are carried out using dedicated electrochemical cells or setups, and generally make use of thin membranes with arrays of holes that are either open or covered with graphene, to separate the electrochemical cell from the vacuum environment. Different types of cells are available, and which cell is most appropriate will be determined by the properties of the sample and the desired experimental conditions [1]. Using tender X-rays can produce photoelectrons with higher kinetic energies than soft X-rays, facilitating the investigation of buried interfaces. At the BElChem-DCM beamline, two approaches are used to study electrified solid/liquid interfaces. With the dip-and-pull method, thin electrolyte films, on the order of a few tens of nanometers, cover the electrode surface, and tender X-ray photoemission is used to study the buried solid/electrolyte interface [1, 2]. (Photo)electrochemical reactions can also be investigated in situ using a 3-electrode H-cell [3]. The NAPHAXPES measurements are carried out such that both the X-ray excitation and electron detection occur through a thin electrolyte film. In both cases, the simultaneous detection of the activity of the electrode, product analysis, and measurement of the electrode’s chemical composition and electronic structure enables structure-function relationships to be established.

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.

The development of near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) allows scientists to perform spectroscopic investigations of solid–gas interfaces at elevated pressures in a defined gas atmosphere. Due to the high surface sensitivity and element specificity, this technique promises to be a valuable and powerful tool in gas sensor research. A Pt doped SnO2 sample was studied to explore a variety of phenomena in sensor research that can be addressed using NAP-XPS at a synchrotron. The change of several parameters, including chemical shifts, band bending, and valence band structure, could be observed in-situ and highlights the enormous potential of the method in this field. Furthermore, a series of DC resistance measurements has been performed to study the impact of pressure. The decrease of resistance at low pressure strongly indicates that both the electronic characteristics as well as the surface composition under the conditions of NAP-XPS are dissimilar to operando conditions at atmospheric pressure.

The organic component (methylammonium) of CH3NH3PbI3-xClx-based perovskites shows electronic hybridization with the inorganic framework via H-bonding between N and I sites. Femtosecond dynamics induced by core excitation are shown to strongly influence the measured X-ray emission spectra and the resonant inelastic soft X-ray scattering of the organic components. The N K core excitation leads to a greatly increased N-H bond length that modifies and strengthens the interaction with the inorganic framework compared to that in the ground state. The study indicates that excited-state dynamics must be accounted for in spectroscopic studies of this perovskite solar cell material, and the organic-inorganic hybridization interaction suggests new avenues for probing the electronic structure of this class of materials. It is incidentally shown that beam damage to the methylamine component can be avoided by moving the sample under the soft X-ray beam to minimize exposure and that this procedure is necessary to prevent the creation of experimental artifacts.

We present a newly developed end-station at BESSY II dedicated to in situ Spectroscopic Analysis with Tender Xrays (SpAnTeX). The core of the end-station is a new SPECS PHOIBOS 150 HV NAP electron spectrometer. First, we show that the system has successfully achieved high electron transmission and detection efficiency under gas pressures up to 30 mbar and photon energies ranging between 200 eV and 10 keV. Second, using two features of this spectrometer (a new lateral resolution lens and a 3D delay line detector), we show that the endstation enables collection of the photoelectron spatial distribution under realistic working conditions (p >= 20 mbar) with a resolution better than 30 mu m and the possibility to perform time resolved studies using a continuous tender Xray source. We conclude by reporting an example of the possible experiments that can be performed using this new endstation using the Dip-and-Pull technique. Although mainly focused on the characterization of solid/liquid interfaces using AP-HAXPES, the end-station can be used at soft X-ray beamlines for more traditional AP-XPS experiments. The Dip-and-Pull module also demonstrates good electrochemical performance. The wide pressure and photon energy range covered by this end-station also enables investigations of solid/solid, solid/gas, liquid/vapor and liquid/liquid interfaces at pressures up to 30 mbar with tender X-rays.

Bismuth vanadate (BiVO 4 ) is an established n-type oxide semiconductor for photoelectrochemical oxygen evolution. Direct charge carrier recombination at the solid/liquid interface is a major cause of efficiency loss in BiVO 4 -based devices. Intrinsic and extrinsic surface states (SSs) can act as electron and hole traps that enhance the recombination rate and lower the faradaic efficiency. In this study, we investigate the BiVO 4 /aqueous KPi interface using two types of samples. The samples were prepared at two different deposition and annealing temperatures (450 °C and 500 °C) leading to different morphologies and stoichiometries for the two samples. Both samples exhibit SSs in the dark that are passivated under illumination. In situ ambient pressure hard x-ray photoelectron spectroscopy experiments performed under front illumination conditions reveal the formation of a bismuth phosphate (BiPO 4 ) surface layer for the sample annealed at 450 °C, whereas the sample annealed at 500 °C exhibits band flattening without the formation of BiPO 4 . These results imply that the light-induced formation of BiPO 4 may not be responsible for SS passivation. Our study also suggests that slight differences in the synthesis parameters lead to significant changes in the surface stoichiometry and morphology, with drastic effects on the physical-chemical properties of the BiVO 4 /electrolyte interface. These differences may have important consequences for device characteristics such as long-term stability.

α‐SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar‐to‐hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α‐SnWO4/NiOx interface is performed by synchrotron‐based hard X‐ray photoelectron spectroscopy (HAXPES). NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2+ to Sn4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo‐based photoemission spectra simulation, the presence of a thin oxide layer at the α‐SnWO4/NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new‐found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid‐state interfaces in multi‐layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.

Carbon capture and concentration of low partial pressure CO2 in air and flue gas is a key step in carbon abatement strategies. Traditional CO2 capture methods employ temperature or pressure swings; however, electrochemical swings, in which an applied potential modulates nucleophilicity, are also possible to mediate the capture and release of CO2. In contrast to the breadth of electrochemical CO2 reduction research, electrochemically mediated CO2 capture and concentration is an emerging field. Although some aspects are reminiscent of those in CO2 reduction, like local pH gradients and (bi)carbonate equilibria, ultimately electrochemical CO2 capture and concentration poses its own unique challenges that will benefit from insights from intercalative batteries, redox flow batteries, and biomimetic/-inspired design, among other fields. After an introduction to carbon capture and current chemical strategies, this Review highlights promising emerging electrochemical methods to enable CO2 capture and concentration; specifically discussed are organic redox, transition metal redox, and pH swings. It closes with an outlook and discussion of future research challenges for electrochemically mediated capture.