Soft X-ray Absorption Spectroscopy Research Articles

Key Role of Oxidising Species for Water Oxidation on Iridium Oxides Revealed By Time-Resolved Optical and X-Ray Spectroscopies

Iridium oxide is the state-of-the-art electrocatalyst for water oxidation in polymer electrolyte membrane (PEM) electrolysers, crucial for green hydrogen production. Despite its extensive industrial use, the water oxidation mechanism on this metal oxide and the key factors controlling the reaction rate remain unclear. Understanding these controls at a fundamental level on such benchmark metal oxides is vital for designing more active and stable electrocatalysts for water oxidation in PEM electrolysers.In this talk, I will present our research on probing oxygen evolution reaction (OER) relevant species on iridium-based catalysts. This involves a combination of time-resolved operando optical spectroscopy, soft and hard X-ray absorption spectroscopy (XAS), and electrochemical mass spectrometry. Initially, I will discuss the intermediate and catalytically active states of iridium oxides. This is based on correlating optically detected states with oxygen molecular products identified through on-chip electrochemical mass spectrometry. Subsequently, I will demonstrate how we used optical spectroscopy to quantify these states as a function of potential. The nature of these states, including the Ir oxidation state and surface adsorbates on iridium sites, will be elucidated using a combination of time-resolved Ir-L edge XAS, O-K edge XAS, and supported by Density Functional Theory (DFT).With these quantification results of active species for OER, I will compare the intrinsic kinetics of two state-of-the-art iridium oxide structures - amorphous IrOx versus crystalline rutile IrO2.[1] The effect of the electrolyte on the intrinsic activity of iridium oxides will also be discussed.[2] Finally, based on this molecular-level understanding, I will present a modified volcano model for OER catalyst design. This model considers the impact of adsorbate-adsorbate interactions on binding energetics, demonstrating critical design principles for high intrinsic activity OER catalysts.The authors acknowledge the funding support from the Imperial College-Chinese Scholarship Council Studentship and bp-ICAM 92, which made this research possible.[1] Caiwu Liang, Reshma Rao, Karine Svane et al. Unravelling the effects of active site densities and energetics on the water oxidation activity of iridium oxides, 07 March 2023, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-2605628/v1][2] Caiwu Liang, Yu Katayama, Yemin Tao, Asuka Morinaga, Benjamin Moss, Verónica Celorrio, et al. Role of electrolyte pH on water oxidation for iridium oxides. ChemRxiv. Cambridge: Cambridge Open Engage; 2023

(Invited) Probing Electrochemical Interfaces in Catalytic and Energy Materials and Process Using Operando Soft X-Ray Spectroscopy

Advanced energy technology arises from the understanding in fundamental science, thus rest in large on in-situ/operando characterization tools for observing the physical and chemical interfacial processes. Synchrotron based X-ray spectroscopic techniques offers unique characterization in many important energy materials of energy conversion, energy storage and catalysis in regards to the functionality, complexity of material architecture, chemistry and interactions among constituents within. However, it is challenging to reveal the real mechanism of the chemical processes.In the operando soft X-ray spectroscopy characterization of interfacial phenomena in energy materials and devices, it has been found that the microstructure and composition of materials as well as the microstructure evolution process have a great influence on performances in a variety of fields, e.g., energy conversion and energy storage materials, chemical and catalytic processes. This presentation will show how to best use the in-situ/operando soft X-ray spectroscopy characterization techniques, including soft x-ray absorption spectroscopy (XAS) and resonant inelastic soft X-ray scattering (RIXS) to investigate the real electrochemical mechanism during the operation. The experimental results show how in-situ/operando soft X-ray spectra characterization techniques uncover the phase conversion, chemical and environmental change of elements and other very important information of solid/gas and solid/liquid interfaces in real time, thus further enhance the understanding of real reaction mechanism.

Electronic Structure Investigation of BSCF Catalysts during Electrochemical Water Electrolysis by Using Time Zero Analysis and Advanced Operando X-Ray Absorption Spectroscopy

The rapid increase in global energy demands and environmental awareness regarding the excessive use of fossil fuel has spurred significant academic interest in the development of alternative energy conversion and storage technologies that prioritize both efficiency and environmental sustainability. Among all kinds of renewable energy, hydrogen energy holds the potential to create a carbon free clean society, and a possible solution to the environmental issues.1 Electrochemical water splitting is considered as a greener approach for pure H2, which involved cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). The anodic OER is kinetically sluggish and considered as a rate determining step of the water splitting reaction. Therefore, it is necessary to develop OER catalysts with high activities.Perovskite oxides knows as a promising oxygen electrode catalyst due to their low cost, flexibility, and tailorable properties. However, most perovskite oxides possess poor electrical conductivity at room temperatures, and this limits their oxygen catalytic activity. In our work Ba0.5Sr0.5CoxFe1-xO3-δ (BSCF) catalysts with different cobalt and iron amounts where X=0, 0.2, 0.8, 1 were synthesized and investigated by time zero analysis and flow rate infinitization analysis which are electrochemical testing method that uses pressure to remove generated oxygen gas bubbles and provides accurate activity of the catalyst.2We find the volcano plot of the Fe amount and the OER activities, showing samples with X=0.8 have the best OER activities as Figure 1. Electronic structure of B-Site transition metal elements, oxygen vacancies, crystal structures and electrical conductivity are the key factors for determining the activities of Perovskite catalysts in OER.3Also it is quite important to determine the real active sites during the OER process by using advance operando techniques.Therefore, in order to investigate the in-depth mechanism of OER activities of Perovskite catalysts, the newly designed operando soft X-ray Absorption Spectroscopy (XAS) cells was utilized to analyze the subtle change as shown in Figure 2 on the oxygen K-edge (531 eV - 532 eV). Ba0.5Sr0.5Co0.8Fe0.2O3-δ has the most obvious changes in integral surface area at the O K-edge pre-edge region, which leads to the best activity among other BSCF samples. According to the Energy Dispersive X-ray Spectroscopy and Lattice Oxygen Evolution Mechanism, ions on the A sites (Ba2+,Sr2+) tend to dissolve and ions on the B sites (Co, Fe) are forming hydroxides (BOOH).Keywords: oxygen evolution reaction, BSCF catalysts, Time zero analysis, operando X-ray absorption spectroscopy Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Reference: (1) Rose, P. K.; Neumann, F. Hydrogen refueling station networks for heavy-duty vehicles in future power systems. Transportation Research Part D: Transport and Environment 2020, 83, 102358.(2) Nagasawa, K.; Matsuura, I.; Kuroda, Y.; Mitsushima, S. A Novel Evaluation Method of Powder Electrocatalyst for Gas Evolution Reaction. Electrochemistry 2022, 90 (1), 017012-017012.(3) Zhu, Y.; Zhou, W.; Shao, Z. Perovskite/carbon composites: applications in oxygen electrocatalysis. Small 2017, 13 (12), 1603793. Figure 1

Operando Soft X-ray Absorption of LaMn1-x Co x O3 Perovskites for CO Oxidation.

We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn1-x Co x O3 catalysts subjected to a mixture of CO and O2 at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn2+ to Mn3+ and Mn4+, while Co largely maintains a valence state of Co2+. In the case of LaCoO3, we identify high-spin and low-spin Co3+ species combined with Co2+. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.

Capacity Fade of Graphite/NMC811: Influence of Particle Morphology, Electrolyte, and Charge Voltage

LiNi0.8Mn0.1Co0.1O2 (NMC811) is an important Li-ion battery cathode material; however, there is a tradeoff between delivered capacity and capacity retention. As the charge potential increases the capacity rises but at the expense of capacity retention. The decrease in capacity retention has been ascribed to several factors including particle cracking, surface reconstruction, transition metal dissolution, and electrolyte reactivity. The present study compares 4.1 and 4.3 V charging limits in commercially relevant graphite/NMC811 pouch cells for single crystal (SC) and polycrystalline (PC) NMC811 with ethylene carbonate (EC)-containing or EC-free electrolytes. The electrochemistry is rationalized through analysis of electrochemical impedance spectroscopy, positive electrode X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, X-ray diffraction, and negative electrode mapping by X-ray fluorescence. Graphite/SC-NMC811 cells show high-capacity retention at 4.1 V but exhibit degradation at 4.3 V charging potentials. The EC-free electrolyte cells led to higher capacity fade, especially when charged to 4.3 V. Cathode dissolution and deposition on the negative electrode from PC-NMC811 cells was higher than for samples from SC-NMC811 cells. This study reveals the impact of material type, charge voltage, and electrolyte composition on the reactions at the positive electrode, their influence on the negative electrode, and evolution with cycle number.

Direct Identification of O─O Bond Formation Through Three-Step Oxidation During Water Splitting by Operando Soft X-ray Absorption Spectroscopy.

Anionic redox allows the direct formation of O─O bonds from lattice oxygens and provides higher catalytic in the oxygen evolution reaction (OER) than does the conventional metal ion mechanism. While previous theories have predicted and experiments have suggested the possible O─O bond, it has not yet been directly observed in the OER process. In this study, operando soft X-ray absorption spectroscopy (sXAS) at the O K-edge and the operando Raman spectra is performed on layered double CoFe hydroxides (LDHs) after intercalation with [Cr(C2O4)3]3-, and revealed a three-step oxidation process, staring from Co2+ to Co3+, further to Co4+ (3d6L), and ultimately leading to the formation of O─O bonds and O2 evolution above a threshold voltage (1.4V). In contrast, a gradual oxidation of Fe is observed in CoFe LDHs. The OER activity exhibits a significant enhancement, with the overpotential decreasing from 300 to 248mV at 10 mA cm-2, following the intercalation of [Cr(C2O4)3]3- into CoFe LDHs, underscoring a crucial role of anionic redox in facilitating water splitting.

Regulating electronic structure of anionic oxygen by Ti4+ doping to stabilize layered Li-rich oxide cathodes for Li-ion batteries

Abstract Layered Li-rich oxide cathodes enable to activate lattice oxygen anions redox in the charge compensation process and provide superior high specific capacity over 250 mAh g−1 due to their unique configuration, and thus attracting great attentions as promising cathode candidates for Li-ion batteries. However, how to better stabilize the bulk lattice oxygen framework and surface structure, and slow down the release of oxygen, is still major bottleneck to develop high performance Li-rich materials. Transition metal ions with outer d0 electronic configuration have distortable configuration, which can accommodate the local structure and chemical environment of the material, and then improve structural stability. Herein this work, the d0 transition metal Ti4+ is used as doping element to improve the chemical and structural stability, capacity retention and lithium ion diffusion kinetics of Li-rich material. The role of Ti in the material modification is revealed through synchrotron-based soft x-ray absorption spectroscopy, XRD, XPS and electrochemical tests. The improvement in structural stability can be attributed to that Ti doping can adjust the hybridization of O2p and TM3d to regulate the local electronic structure of both bulk lattice oxygen and surface oxygen vacancies. It is hoped that this work should shed light on the development of high-performance cathode materials for Li-ion Batteries.

Metal-ligand dual-site single-atom nanozyme mimicking urate oxidase with high substrates specificity

In nature, coenzyme-independent oxidases have evolved in selective catalysis using isolated substrate-binding pockets. Single-atom nanozymes (SAzymes), an emerging type of non-protein artificial enzymes, are promising to simulate enzyme active centers, but owing to the lack of recognition sites, realizing substrate specificity is a formidable task. Here we report a metal-ligand dual-site SAzyme (Ni-DAB) that exhibited selectivity in uric acid (UA) oxidation. Ni-DAB mimics the dual-site catalytic mechanism of urate oxidase, in which the Ni metal center and the C atom in the ligand serve as the specific UA and O2 binding sites, respectively, characterized by synchrotron soft X-ray absorption spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, and isotope labeling. The theoretical calculations reveal the high catalytic specificity is derived from not only the delicate interaction between UA and the Ni center but also the complementary oxygen reduction at the beta C site in the ligand. As a potential application, a Ni-DAB-based biofuel cell using human urine is constructed. This work unlocks an approach of enzyme-like isolated dual sites in boosting the selectivity of non-protein artificial enzymes.

Evolution of copper ions in nanostructured Cu[formula omitted]O according to X-ray absorption spectroscopy data

Soft X-ray absorption spectroscopy (Cu L and O K spectra ) was applied to identify changes in the charge states of copper ions in quasi-statically loaded (torsion under pressure) Cu2O. As was found from our X-ray diffraction and X-ray absorption experiments, an increase in the degree of deformation lead to a decrease in the areas of coherent scattering (sizes of nanograins), to partial amorphization of oxides, and to a slight increase in the concentration of Cu2+ ions on the surface of the Cu2O samples. No new phases were detected using X-ray diffraction measurements. The repeated measurements of the Cu L and O K spectra of the nanostructured Cu2O samples after two years showed significantly more intense signals at high photon energies, indicating the appearance of Cu2+ ions on the surface of nanostructured Cu2O, corresponding to the CuO phase. It was shown that two types of Cu2+ ions on the surface of Cu2O samples with different binding energies of Cu 2p electrons could be formed.

Real-time and operando observation of intermediates on TiO2 photoelectrocatalysis by soft X-ray absorption spectroscopy

The oxygen evolution reaction (OER) on the TiO2 surface at the solid–liquid interface was observed in real-time and operando using fluorescence-yield X-ray absorption spectroscopy (XAS) in the soft X-ray region. The OER was observed during a potential sweep with UV light on or off, focusing on the oxygen K-edge XAS. Although conventional XAS measurements require long measurement times, the spectra during the reaction were obtained every 3 s in the present experiment via wavelength-dispersive XAS technique. An increase in the X-ray absorption intensity at approximately 533.8 eV was observed during the potential sweep only with UV light on. The observed spectral change is discussed in relation to the reaction mechanism of the OER. The present technique can be applied to a wide range of analyses of (photo-) electrocatalysis and electrochemical reactions at solid–liquid interfaces to observe their products and intermediates during the reaction.

Ni-rich LiNi0.905Co0.043Al0.052O2 cathode material for high-energy density Li-ion cells: Tuning lithium content, structural evolution, and full-cell performance

Ni-rich LiNi0.905Co0.043Al0.052O2 oxides with the Li excess (x) up to 7 mol.% used in the synthesis are prepared by a facile dry-mixing method. It is shown that the Li/Ni mixing level and crystallite size are dependent on the Li amount, while Al from the precursor acts as a scavenger of the excessive Li. Operando X-ray diffraction (XRD) studies enable explaining differences in the structural evolution of materials exhibiting high and low Li/Ni mixing, especially in the H2↔H3 transition region. Soft X-ray absorption spectroscopy (XAS) studies show differences in the charge state evolution of the surface and bulk Ni. The optimized oxide with the Li excess x = 2 mol.% delivers excellent electrochemical performance, with an initial discharge capacity of 205.5 mAh g−1 at 0.1C, and 91.4 % capacity retention after 100 cycles at 1C. The unmodified in any other way cathode material allows achieving the discharge capacity of 178.5 mAh g−1 at the high rate of 5C, and works well in the full-cells.

Irradiation multi-scale damage and interface effects of 3D braided carbon fiber/epoxy composites subjected to high dose γ-rays

The serious damage caused to 3D braided carbon fiber (CF)/epoxy (EP) composites in high-energy irradiation environments is a pressing research topic that has not yet been reported. This study analyzed the γ-irradiation multi-scale damage of the matrix, interfaces, and near-interface regions in 3D braided CF/EP composites with three different braiding angles (18°, 28°, 38°) at atomic, microscopic, and macroscopic scales. The molecular structure and atomic charge information alterations of epoxy matrix were characterized using soft X-ray absorption spectroscopy (sXAS). When the irradiation dose reached 1000 KGy, the compressive strength of composites decreased by 20.88 %, 18.07 %, and 16.60 %, respectively, with an increase in the braiding angle. Micro-CT observation, along with statistical computing, confirmed that irradiation causes interface damage and increases the defect volume of 3D braided composites. Nanoindentation was used to compare the modulus of carbon fiber, interface, near-interface regions and matrix in irradiated composites and the function of CF/resin interface as an irradiation defect capture site in the irradiation resistance of resin-matrix composites has been newly established. In addition, the mechanism behind the effect of braiding angle on macroscopic properties was also analyzed.

A novel chemical composite strategy for synergistically enhancing the activity and stability of strontium-containing air electrode for protonic ceramic cells

Cation segregation on perovskite electrode surfaces usually leads to a decrease in the catalytic activity of electrodes and a shortened cell life. Herein, we have developed a simple and novel chemical composite strategy to achieve selectively surface reconstruction of the Sr-rich regions in perovskite Pr0.4Sr0.6Fe0.9Nb0.1O3-δ (PSFN), constructing strongly interacting interfaces between PSFN and Fe2O3 phases. The enhanced charge transfer between the two phases in PSFN/Fe2O3 is revealed by soft X-ray absorption spectroscopy. Meanwhile, fast surface oxygen exchange rate accelerates the migration and adsorption/desorption of oxygen species. The chemical composite electrode PSFN/Fe2O3 exhibits a smaller polarization resistance (0.35 Ω cm2) than that (0.61 Ω cm2) of the physical composite electrode PSFN + Fe2O3 at 600 °C in air. The distribution of relaxation time analyses demonstrate that enhanced reaction kinetics related to charge transfer and mass transfer are key factors in achieving stable operation of symmetric cell within 1000 h. Importantly, this study provides a simple and universal strategy for surface modification of perovskite oxides with cation separation issue, and highlights the crucial role of constructing strongly interacting interfaces in achieving high activity and stability of air electrodes.

Surface matters: A case study of the scale and impact of oxide surfaces via orbital polarization

Transition metal oxides (TMOs) exhibit a broad spectrum of functional electronic, magnetic, and optical properties, making them attractive for various technological applications. The scale and impact of surface defects and inhomogeneity can extend many unit cells below the surface. Overlooking this aspect of TMO surfaces can result in an incorrect interpretation of their physics and inhibit their maturation into device technology. Soft x-ray absorption spectroscopy (XAS) is a common technique for TMO studies, and different XAS acquisition modes can be used to measure different depth regimes in the sample. Here, we demonstrate a substantial disparity between the near-surface region and the “bulk” of the prototypical TMO SrVO3. By driving the system across two scenarios of orbital polarization, we illustrate how a common XAS surface-sensitive acquisition technique fails to detect the intrinsic orbital polarization. By stark contrast, a “bulk”-sensitive technique successfully captures this effect, elucidating the expected orbital occupation inversion. These results not only underscore the impact of the near-surface region on the correct interpretation of TMO fundamental physics, but further highlight the scale of surface inhomogeneity, a critical aspect of nanoscale functional devices.

Suppressed Electrolyte Decomposition Behavior to Improve Cycling Performance of LiCoO2 under 4.6V through the Regulation of Interfacial Adsorption Forces.

Alleviating the decomposition of the electrolyte is of great significance to improving the cycle stability of cathodes, especially for LiCoO2 (LCO), its volumetric energy density can be effectively promoted by increasing the charge cutoff voltage to 4.6V, thereby supporting the large-scale application of clean energy. However, the rapid decomposition of the electrolyte under 4.6V conditions not only loses the transport carrier for lithium ion, but also produces HF and insulators that destroy the interface of LCO and increase impedance. In this work, the decomposition of electrolyte is effectively suppressed by changing the adsorption force between LCO interface and EC. Density functional theory illustrates the LCO coated with lower electronegativity elements has a weaker adsorption force with the electrolyte, the adsorption energy between LCO@Mg and EC (0.49eV) is weaker than that of LCO@Ti (0.73eV). Meanwhile, based on the results of time of flight secondary ion mass spectrometry, conductivity-atomic force microscopy, in situ differential electrochemical mass spectrometry, soft X-ray absorption spectroscopy, and nuclear magnetic resonance, as the adsorption force increases, the electrolyte decomposes more seriously. This work provides a new perspective on the interaction between electrolyte and the interface of cathode and further improves the understanding of electrolyte decomposition.

Understanding improved cycling and thermal stability of compositionally graded Ni-rich layered LiNi0.6Mn0.2Co0.2O2 cathode materials

The concentration gradient is a strategic design, adjusting the distribution of Ni, typically with a higher Ni content in the core and a higher Mn content toward the surface. This design leverages the pivotal role of the Ni/Mn ratio, seeking to optimize cathode performance by balancing Ni's high capacity with Mn's stabilizing effects, particularly at the surface where degradation commonly occurs during cycling. Our study delves into the intricate structural and chemical transformations within concentration gradient cathode materials during electrochemical cycling. Utilizing advanced synchrotron X-ray techniques, including hard and soft X-ray absorption spectroscopy (hXAS, sXAS), and nanoscale X-ray imaging, we investigate buried changes in concentration gradient LiNi0.6Mn0.2Co0.2O2 (CG NMC622). Contrary to conventional assumptions, our findings challenge the notion that cycling stability relies solely on Mn stability. Unraveling the roles of Ni and Mn, we uncover how their individual and collective contributions impact the cathode's overall performance. This investigation transcends established paradigms, shedding light on the crucial mechanisms governing the enhanced cycling stability of Ni-rich layered cathode materials.

Construction of an Electron Capture and Transfer Center for Highly Efficient and Selective Solar-Light-Driven CO2 Conversion.

Exploring high-efficiency photocatalysts for selective CO2 reduction is still challenging because of the limited charge separation and surface reactions. In this study, a noble-metal-free metallic VSe2 nanosheet was incorporated on g-C3N4 to serve as an electron capture and transfer center, activating surface active sites for highly efficient and selective CO2 photoreduction. Quasi in situ X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and femtosecond transient absorption spectroscopy (fs-TAS) unveiled that VSe2 could capture electrons, which are further transferred to the surface for activating active sites. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations revealed a kinetically feasible process for the formation of a key intermediate and confirmed the favorable production of CO on the VSe2/PCN (protonated C3N4) photocatalyst. As an outcome, the optimized VSe2/PCN composite achieved 97% selectivity for solar-light-driven CO2 conversion to CO with a high rate of 16.3 μmol·g-1·h-1, without any sacrificial reagent or photosensitizer. This work offers new insights into the photocatalyst design toward highly efficient and selective CO2 conversion.

Enhancing Ni/Co Activity by Neighboring Pt Atoms in NiCoP/MXene Electrocatalyst for Alkaline Hydrogen Evolution.

Density functional theory (DFT) calculations demonstrate neighboring Pt atoms can enhance the metal activity of NiCoP for hydrogen evolution reaction (HER). However, it remains a great challenge to link Pt and NiCoP. Herein, we introduced curvature of bowl-like structure to construct Pt/NiCoP interface by adding a minimal 1 ‰-molar-ratio Pt. The as-prepared sample only requires an overpotential of 26.5 and 181.6 mV to accordingly achieve the current density of 10 and 500 mA cm-2 in 1 M KOH. The water dissociation energy barrier (Ea) has a ~43 % decrease compared with NiCoP counterpart. It also shows an ultrahigh stability with a small degradation rate of 10.6 μV h-1 at harsh conditions (500 mA cm-2 and 50 °C) after 3000 hrs. X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and X-ray absorption fine structure (XAFS) verify the interface electron transfer lowers the valence state of Co/Ni and activates them. DFT calculations also confirm the catalytic transition step of NiCoP can change from Heyrovsky (2.71 eV) to Tafel step (0.51 eV) in the neighborhood of Pt, in accord with the result of the improved Hads at the interface disclosed by in situ electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) tests.

Surface oxidation/spin state determines oxygen evolution reaction activity of cobalt-based catalysts in acidic environment

Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin CoII are found to be not active due to their unfavorable water dissociation to form CoIII-OH species. By contrast, the presence of low-spin CoIII is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism.

A sub-100 nm thickness flat jet for extreme ultraviolet to soft X-ray absorption spectroscopy.

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.