Operando X-ray Photoelectron Spectroscopy Research Articles

X-ray and photoelectron spectroscopy of surface chemistry; from bonding via femtosecond to operando

For the 60th anniversary of Surface Science, I present here a personal account of some of the most significant contributions I have made to the field over the past three decades. The utilisation of X-rays serves as the foundation for these studies, encompassing X-ray spectroscopy for the mapping of surface chemical bonds, probing of surface reactions on ultrafast timescales, and X-ray photoelectron spectroscopy under operando conditions. The direct projection of electronic states onto the adsorbed atom allowed the detection of bonding and anti-bonding states within the d-band model. The selective probing of orbitals of different symmetries on the two atoms in adsorbed N2 provided a fundamental understanding of the nature of diatomic bonding to surfaces. Ultrafast optical pumping and X-ray laser techniques allowed the study of CO undergoing desorption leading to the observation of the precursor state. Pump-probed studies of co-adsorbed CO and O on Ru enabled the means to detect transition state species during catalytic CO oxidation. The use of operando X-ray photoelectron spectroscopy at near-atmospheric pressures opened the door to probe the surface chemistry and gain insight into the reaction mechanism during hydrogenation reactions to produce ammonia, hydrocarbons, methanol and ethanol. By inserting an electrochemical cell into the spectroscopic chamber, both fuel cell and water splitting electrocatalysis could be studied giving insight about the reaction mechanism.

Unveiling Direct Electrochemical Oxidation of Methane at the Ceria/Gas Interface.

Solid oxide fuel cells (SOFCs) stand out in sustainable energy systems for their unique ability to efficiently utilize hydrocarbon fuels, particularly those from carbon-neutral sources. CeO2-δ (ceria) based oxides embedded in SOFCs are recognized for their critical role in managing hydrocarbon activation and carbon coking. However, even for the simplest hydrocarbon molecule, CH4, the mechanism of electrochemical oxidation at the ceria/gas interface is not well understood and the capability of ceria to electrochemically oxidize methane remains a topic of debate. This lack of clarity stems from the intricate design of standard metal/oxide composite electrodes and the complex nature of electrode reactions involving multiple chemical and electrochemical steps. This study presents a Sm-doped ceria thin-film model cell that selectively monitors CH4 direct-electro-oxidation on the ceria surface. Using impedance spectroscopy, operando X-ray photoelectron spectroscopy, and density functional theory, it is unveiled that ceria surfaces facilitate C─H bond cleavage and that H2O formation is key in determining the overall reaction rate at the electrode. These insights effectively address the longstanding debate regarding the direct utilization of CH4 in SOFCs. Moreover, these findings pave the way for an optimized electrode design strategy, essential for developing high-performance, environmentally sustainable fuel cells.

An electrochemical flow cell for operando XPS and NEXAFS investigation of solid–liquid interfaces

Suitable reaction cells are critical for operando near ambient pressure (NAP) soft x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) studies. They enable tracking the chemical state and structural properties of catalytically active materials under realistic reaction conditions, and thus allow a better understanding of charge transfer at the liquid–solid interface, activation of reactant molecules, and surface intermediate species. In order to facilitate such studies, we have developed a top-side illuminated operando spectro-electrochemical flow cell for synchrotron-based NAP-XPS/-NEXAFS studies. Our modular design uses a non-metal (PEEK) body, and replaceable membranes which can be either of x-ray transparent silicon nitride (SiN x ) or of water permeable polymer membrane materials (e.g. NafionTM). The design allows rapid sample exchange and simultaneous measurements of total electron yield, Auger electron yield and fluorescence-yield. The developed system is highly modular and can be used in the laboratory or directly at the beamline for operando XPS/ x-ray absorption spectroscopy investigations of surfaces and interfaces. We present examples to demonstrate the capabilities of the flow cell. These include an operando NEXAFS study of the Cu-redox chemistry using a SiN x /Ti-Au/Cu working electrode assembly (WEA) and a NAP-XPS/-NEXAFS study of water adsorption on a NafionTM polymer membrane based WEA (NafionTM/C/IrO x catalyst). More importantly, the spectro-electrochemical flow cell is available for user community of B07 beamlines at Diamond Light Source.

Unraveling the evolution of Cathode–Solid electrolyte interface using operando X-ray Photoelectron spectroscopy

Understanding the evolution of the solid electrolyte-electrode interface is currently one of the most challenging obstacles in the development of solid-state batteries (SSBs). Here, we develop an X-ray Photoelectron Spectroscopy (XPS) that allows for operando measurement during cycling. Based on theoretical analysis and the modulated electrode and detector co-grounding mode, the displacement of binding energy can be correlated with the surface electrostatic potential of the material, revealing the charge distribution and composition evolution of the space charge layer between the cathode and the electrolyte. In the investigation of typical LiCoO2 (LCO)/Li6PS5Cl (LPSC)/Li–In batteries, we observed the static potential difference and oxidative decomposition between LPSC and LCO, and the effectiveness of the LiNbO3 coating in reducing potential difference and inhibiting the diffusion of Co and oxidation of S species. Furthermore, our study also revealed that the potential drop between LiNi0·8Co0·1Mn0·1O2 and LPSC is smaller than that of LCO, whilst that between Li3InCl6 and LCO remains near zero. The proposed operando XPS method offers a novel approach for real-time monitoring of interface potential and species formation, providing rational guidance for the interface engineering in SSBs.

Operando X-ray photoelectron spectroscopy cell for water electrolysis: A complete picture of iridium electronic structure during oxygen evolution reaction

Operando investigations are crucial for understanding various catalytical processes. We present a newly designed cell for operando X-ray Photoelectron Spectroscopy water electrolysis. All measurements are done on a laboratory X-ray source, which makes the cell easily accessible to a wide audience. We demonstrate the cell operation on a magnetron-sputtered iridium catalyst for the anode of a Proton Exchange Membrane Water Electrolyzer. The main challenges consist of the anode water intake through the membrane from the cathode and the electrical contact. We can oxidize metallic Ir0 into a stable IrIV and reduce it back to metal, which agrees well with the ex-situ measurements. Furthermore, the operando tracking of the Ir 4f and O 1s spectra during the oxidation uncovers the presence of intermediates of the Oxygen Evolution Reaction on Ir, and thus allows the improvement of the understanding of its mechanism. The cell can be used to further study other catalysts for low-temperature water electrolyzers, both noble and non-noble metal-based.

The Chemistry of Interfacial Ions: In Situ XPS and XAS

The choice of electrolyte ions has significant impact on the performance of electrochemical systems, affecting properties such as the electrocatalytic activity and capacitance. To understand how ions influence electrochemical processes, a key step is to determine the chemical behavior of ions at the electrode-electrolyte interface. Here, I will show how X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) allow one to obtain such information. Traditionally, these techniques are vacuum-based, only providing a detailed chemical analysis before or after electrochemistry. However, advanced spectro-electrochemical cell design now makes it possible to interface the vacuum needed for XPS and soft XAS with wet electrochemical environments, enabling us to study the electrode-electrolyte interface during electrocatalytic reactions. Using recent case studies from my group, I will show how operando XPS and soft XAS are able to detect the interfacial ion concentration, the hydration shell structure of the ions, and the electrostatic potential in the double layer. These insights allowed us, for example, to follow the intercalation of Na+ ions into NiFeOx and IrOx in alkaline electrolytes, and to obtain a measure of the double layer layer thickness at a RuOx electrode in 0.1 M H2SO4. Finally, I will give a perspective on how to bring operando XPS from the synchrotron to the laboratory, which will be an important step towards widespread implementation of the technology. Figure 1

(Invited) Electrochemistry Happens at the Interface

Earth-abundant, active, selective and stable electrocatalysts are the cornerstone in our transition to defossilisation of the chemical industry, sector coupling, and sustainable energy. The catalyst surface is known to transform at the interface to the electrolyte, especially under operation conditions. Only the properties of the transformed interface coupled to the underlying layer gives rise to observed metrics like activity and stability. This complicates predictive electrocatalyst materials design – a challenge which can only be tackled by collaborative and interdisciplinary efforts, at the Interface between Chemistry, Physics, Materials Science, and Engineering. I believe this presents one of the main tasks for the young electrochemistry researchers in Europe and around the world, especially since the same challenges also arise in related technologies like Li-ion batteries.In my talk I will discuss how the old wisdom from semiconductor research “the Interface is the device” can be translated into the current challenge “Electrochemistry happens at the Interface”. We need new approaches in designing and studying these electrochemical interfaces for fundamental insights that may in a next step yield better performance in future applications. The talk will be split in two parts.Firstly, I will introduce how single crystalline surfaces can be obtained using epitaxial thin films, and discuss how these can be used to derive atomic-level structure-property relations by synergetic experimental and theoretical investigation. Such films can be fabricated with unit-cell or even atomic-layer precision and enable direct comparison to single facets typically investigated in density functional theory.Secondly, I will discuss opportunities and challenges in surface-sensitive operando characterization in a liquid medium.1 Information from the outermost surface of a catalyst can be obtained through a standing-wave approach2,3 or extraction of a surface-only signal from careful thickness-dependent studies.4 In my ERC project “Interfaces at work”, we are also developing interface-sensitive, laboratory-based operando X-ray photoelectron spectroscopy approaches based on the new XPS user facility at the University of Twente.Throughout the talk, I will refer to examples from LaNiO3 thin films, which are atomically flat both before and after application as electrocatalysts for the OER during water electrolysis. We selectively tuned the surface cationic composition in epitaxial growth. The Ni-termination is approximately twice as active for the OER as the La-termination.2 Our ex situ and operando characterization confirmed that the surface transformation pathways – and therefore the electrochemical functionality – depend on a single atomic layer at the surface. A second example will be the introduction of multi-cation compositions in so-called high entropy perovskite oxides (HEO), which can maximize the catalytic activity. The HEO LaCr0.2Mn0.2Fe0.2Co0.2Ni0.2O3-δ outperforms all of its parent compounds (single TM-site element in the LaTMO3 perovskite) by orders of magnitude.5 X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during adsorption of reaction intermediates. Rao, R. R., van den Bosch, I. C. G. & Baeumer, C. Operando X-ray characterization of interfacial charge transfer and structural rearrangements. in Reference Module in Chemistry, Molecular Sciences and Chemical Engineering 1–24 (Elsevier, 2023). doi:10.1016/B978-0-323-85669-0.00068-4.Baeumer, C. et al. Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis. Nat Mater 20, 674–682 (2021).Martins, H. P. et al. Near total reflection x-ray photoelectron spectroscopy: quantifying chemistry at solid/liquid and solid/solid interfaces. J Phys D Appl Phys 54, 464002 (2021).Baeumer, C. Operando characterization of interfacial charge transfer processes. J Appl Phys 129, 170901 (2021).Kante, M. V et al. A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation. ACS Nano (2023) doi:10.1021/acsnano.2c08096. Figure 1

A laboratory-based multifunctional near ambient pressure X-ray photoelectron spectroscopy system for electrochemical, catalytic, and cryogenic studies.

A versatile multifunctional laboratory-based near ambient pressure x-ray photoelectron spectroscopy (XPS) instrument is presented. The entire device is highly customized regarding geometry, exchangeable manipulators and sample stages for liquid- and solid-state electrochemistry, cryochemistry, and heterogeneous catalysis. It therefore delivers novel and unique access to a variety of experimental approaches toward a broad choice of functional materials and their specific surface processes. The high-temperature (electro)catalysis manipulator is designed for probing solid state/gas phase interactions for heterogeneous catalysts including solid electrolyzer/fuel cell electrocatalysts at pressures up to 15mbar and temperatures from room temperature to 1000 °C. The liquid electrochemistry manipulator is specifically designed for in situ spectroscopic investigations of polarized solid/liquid interfaces using aqueous electrolytes and the third one for experiments for ice and ice-like materials at cryogenic temperatures to approximately -190 °C. The flexible and modular combination of these setups provides the opportunity to address a broad spectrum of in situ and operando XPS experiments on a laboratory-based system, circumventing the limited accessibility of experiments at synchrotron facilities.

Dynamic tracking of exsolved PdPt alloy/perovskite catalyst for efficient lean methane oxidation

Supported Pd based catalysts are considered as the efficient candidates for low-carbon alkane oxidation for their outstanding capability to break C-H bond. Whereas, the irreversible deactivation of Pd based catalysts was still frequently observed. Herein, we reinforced the extruded Pd nanoparticles with quantitive Pt to assemble the evenly distributed PdPt nanoalloy onto ferrite perovskite (PdPt-LCF) matrix with strengthened robustness of metal/oxide support interface. We further co-achieved the enhanced performance, anti-overoxidation as well as resistance of vapor-poisoning in durability measurement. The operando X-ray photoelectron spectroscopy (O-XPS) combined with various morphology characterizations confirms that the accumulation of surface deep-oxidation species of Pd4+ is the culprit for fast activity loss in exsolved Pd system, especially at high temperature of 400 °C. Conversely, it could be completely suppressed by in-situ alloying Pd with equal amount of Pt, which helps maintain the metastable Pd2+/Pd shell and metallic solid-solution core structure. The density function theory (DFT) calculations further buttress that the dissociation of CH was facilitated on alloy/perovskite interface which is, on the contrary, resistant toward O–H bond cleavage, as compared to Pd/perovskite. Our work suggests that the modification of exsolved metal/oxide catalytic interface could further enrich the toolkit of heterogeneous catalyst design.

Revealing the effect of electrolyte coordination structures on the intercalation chemistry of batteries

In-depth understanding of the electrolyte-dependent intercalation chemistry in batteries through direct operando/in situ characterizations is crucial for the development of the high-performance batteries. Herein, taking the Al/graphite battery as a model system, the effect of electrolyte coordination structure on the intercalation processes has been investigated over the batteries with either 1-hexyl-3-methylimidazolium chloride (HMICl)-AlCl3 or 1-ethyl-3-methylimidazolium chloride (EMICl)-AlCl3 ionic liquid electrolyte using operando X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. With a weaker anion-cation interaction in HMI-based electrolyte, the XPS-derived atomic ratio between co-intercalated N and intercalated Al is 0.9, which is lower than 1.6 for EMI-based electrolyte. Attributed to the additional de-solvation process, the batteries with the HMI-based electrolyte show a lower ionic diffusion rate, capacity, and cycling performance, which agree with the operando characterization results. Our findings highlight the critical role of the electrolyte coordination structure on the (co–)intercalation chemistry.

State of the Surface During CO Hydrogenation over Ni(111) and Ni(211) Probed by Operando X-ray Photoelectron Spectroscopy

The state of the surface near-region during CO hydrogenation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C.

Understanding fundamentals of electrochemical reactions with tender X-rays: A new lab-based operando X-ray photoelectron spectroscopy method for probing liquid/solid and gas/solid interfaces across a variety of electrochemical systems

Electrocatalysis is key to improving energy efficiency, reducing carbon emissions, and providing a sustainable way of meeting global energy needs. Therefore, elucidating electrochemical reaction mechanisms at the electrolyte/electrode interfaces is essential for developing advanced renewable energy technologies. However, the direct probing of real-time interfacial changes, i.e., the surface intermediates, chemical environment, and electronic structure, under operating conditions is challenging and necessitates the use of in situ methods. Herein, we present a new lab-based instrument commissioned to perform in situ chemical analysis at liquid/solid interfaces using ambient pressure X-ray photoelectron spectroscopy (APXPS). This setup takes advantage of a chromium source of tender X-rays and is designed to study liquid/solid interfaces by the “dip and pull” method. Each of the main components was carefully described, and the results of performance tests are presented. Using a three-electrode setup, the system can probe the intermediate species and potential shifts across the liquid electrolyte/solid electrode interface. In addition, we demonstrate how this system allows the study of interfacial changes at gas/solid interfaces using a case study: a sodium–oxygen model battery. However, the use of APXPS in electrochemical studies is still in the early stages, so we summarize the current challenges and some developmental frontiers. Despite the challenges, we expect that joint efforts to improve instruments and the electrochemical setup will enable us to obtain a better understanding of the composition–reactivity relationship at electrochemical interfaces under realistic reaction conditions.

Surface Exchange Reaction of Mixed Conductive La0.65Ca0.35FeO3−δ during Oxygen Evolution and Incorporation as Traced by Operando X-ray Photoelectron Spectroscopy

High oxygen permeability of mixed conductive La0.65Ca0.35FeO3-δ (LCF) is applicable to pure oxygen gas generators and cathodes for solid oxide fuel cells, etc.; however, lower surface exchange reactions at temperatures below 800 °C reduce permeability. To understand the microscopic surface reaction mechanism, operando soft X-ray photoelectron spectroscopy of an LCF film surface was conducted during the evolution and incorporation of oxygen. LCF film was prepared on yttria-stabilized zirconia and a current was applied throughout the film at ∼600 °C. From operando X-ray photoelectron spectra, surface oxide species involved in the surface exchange reaction obviously appeared on the film during the evolution of oxygen from the surface. The number of surface oxide species abruptly decreased during incorporation of oxygen. By applying the current from a negative to positive value, the numbers of surface oxide species and ligand holes near Fe3+ ions on the surface both significantly increased. The results infer that ligand holes in the Fe 3d-O 2p hybrid orbitals correspond to active reaction sites at which surface oxide species change to oxygen molecules. Increasing the number of active reaction sites is key to improving oxygen evolution of mixed conductive oxides.

(Invited) Interfacial Dynamics of Anode-Free Solid-State Batteries

There is significant interest in solid-state batteries (SSBs) as an alternative to traditional Li-ion batteries, as they can potentially eliminate the flammable liquid electrolyte and enable Li-metal anodes. However, manufacturing of SSBs with Li metal anodes poses a significant challenge, owing to the highly reactive nature of Li metal. Therefore, there has been a recent increase in interest in “anode-free” configurations, where the Li metal anode is formed in situ using the pre-existing inventory in the cathode. In addition to manufacturing considerations, anode-free configurations also enhance energy density compared to batteries with excess Li metal in the anode. However, compared to pre-formed Li metal/solid electrolyte interfaces, the nucleation, growth, and stripping of Li metal in anode-free SSBs has not been widely studied, and poses unique electro-chemo-mechanical phenomena.To probe the dynamic interfacial evolution of anode-free SSBs, in this study, I will present results using a complimentary set of in situ/operando methodologies. First, I will describe the morphological evolution of Li nucleation and growth at garnet LLZO solid electrolytes, using a newly developed platform for operando 3-D video microscopy [1]. In particular, the influence of mechanical stresses on both the thermodynamic and kinetic properties of Li will be discussed, and unique electrochemical signatures of anode-free plating will described. A mechanistic framework for these electro-chemo-mechanics will be presented, which results in design rules for improving the homogeneity of Li plating during anode formation.To compliment this morphological information, I will also present results using operando x-ray photoelectron spectroscopy (XPS) on sulfide solid electrolytes. This allows for direct observation of SEI formation during in situ anode formation. In particular, we observe a transition in reaction pathways from SEI formation to nucleation of Li metal, which is a function of both the charging protocol and the presence of interlayer coatings [2-3]. Finally, these results will be compared and contrasted to anode-free Li plating in liquid electrolytes, and a discussion will be presented on how to optimize plating morphology and reversibility.[1] E. Kazyak, M. Wang, K. Lee, S. Yadavalli, A. J. Sanchez, M. D. Thouless, J. Sakamoto, N. P. Dasgupta, Submitted (2022).[2] A. L. Davis, R. Garcia-Mendez, K. N. Wood, E. Kazyak, K.-H. Chen, G. Teeter, J. Sakamoto, N. P. Dasgupta, J. Mater. Chem. A 8, 6291 (2020).[3] A. L. Davis, E. Kazyak, D. W. Liao, K. N. Wood, N. P. Dasgupta, J. Electrochem. Soc. 168, 070557 (2021).

Electrochemical Lithiation and Delithiation in Amorphous Si Thin Film Electrodes Studied by Operando X-ray Photoelectron Spectroscopy.

The electrochemical lithiation/delithiation in amorphous Si thin film electrodes deposited on a L6.6La3Zr1.6Ta0.4O12 are dynamically analyzed by operando X-ray photoelectron spectroscopy. In the initial lithiation, the Si 2p peak corresponding to bulk Si0 significantly shifts to a lower binding energy due to the formation of LixSi and then monotonically with increasing capacity, i.e., the Li content x in LixSi. When the lithiation stops at capacity of 2200 mAh g-1 (x = ∼2.3), the peak recovers monotonically to a higher binding energy throughout the successive delithiation. When Li is inserted into LixSi up to 3400 mAh g-1 (x = 3.5), however, the peak drastically shifts in the capacity range of 1520-1920 mAh g-1 (x = 1.6-2.0) in the successive delithiation. This shift is attributed to the phase transition of crystalline Li15Si4 formed in the preceding lithiation to the amorphous phase. The mechanism of initial lithiation/delithiation at each step is summarized on the basis of the state of charge, Li content x in LixSi, and positions of XPS peaks.

In situ x-ray photoelectron spectroscopy analysis of electrochemical interfaces in battery: Recent advances and remaining challenges

An in-depth understanding of charge transfer processes at the electrochemical interfaces is a critical knowledge gap impeding the design of energy storage materials. X-ray photoelectron spectroscopy plays an important role in analyzing electronic structures of heterogeneous interfaces, such as electrode-electrolyte interphases. Correspondingly, ex situ studies based on postmortem analysis of electrode materials using x-ray techniques are widely reported in the literature but often fail to capture intermediate and transient species, which are critical for a predictive understanding of the charge transfer process. The lack of extensive in situ/operando x-ray analysis of buried interfaces in energy storage systems can be mainly attributed to technical limitations, such as the requirement of high vacuum conditions. However, in the past decade, considerable efforts have been devoted to overcoming these technical barriers and enable investigation of the solid/solid and solid/liquid interfaces. This review catalogs some of the recent progresses and new experimental designs in the application of in situ and operando x-ray photoelectron spectroscopy toward characterizing interfacial processes and emergent properties, which can help build the design strategy for advanced batteries. The remaining challenges and future research directions are also discussed, as potential paths forward in this field.

Lithiation/delithiation of a Silicon Thin Film Electrode for All-Solid-State Batteries Using Operando X-ray Photoelectron Spectroscopy Apparatus

Lithiation/delithiation of a Silicon Thin Film Electrode for All-Solid-State Batteries Using Operando X-ray Photoelectron Spectroscopy Apparatus

Comparative Operando XPS and SEM Spatiotemporal Potential Mapping of Ionic Liquid Polarization in a Coplanar Electrochemical Device.

The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of two powerful analytic techniques, X-ray photoelectron spectroscopy (XPS) and scanning electron microcopy (SEM). Spatiotemporal distribution of the ionic liquid surface potential, upon DC or AC (square wave) biasing, has been monitored via chemical element binding energy shifts using XPS and secondary electron intensity variations using SEM. SEM's high spatial resolution and speedy imaging together with application of a data mining algorithm made mapping of the surface potential distribution across the capacitor possible. Interestingly, despite the differences in the detection principles, both techniques yield similar polarization relaxation time constants. The results demonstrate the power of a synergistic combination of the two techniques with complementary capabilities and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices.

Reactivity and Potential Profile across the Electrochemical LiCoO2-Li3PS4 Interface Probed by Operando X-ray Photoelectron Spectroscopy.

All-solid-state lithium batteries are a promising alternative for next-generation safe energy storage devices, provided that parasitic side reactions and the resulting hindrances in ionic transport at the electrolyte-electrode interface can be overcome. Motivated by the need for a fundamental understanding of such an interface, we present here real-time measurements of the (electro-)chemical reactivity and local surface potential at the electrified interface (Li2S)3-P2S5 (LPS) and LiCoO2 (LCO) using operando X-ray photoelectron spectroscopy (XPS) supplemented by X-ray photoemission electron microscopy (XPEEM). We identify three main degradation mechanisms: (i) reactivity at open circuit potential leading to the formation of reduced Co in the +2 oxidation state at the LCO surface, detected in the Co L-edge, which is further increased upon cycling, (ii) onset of electrochemical oxidation of the LPS at 2.3 V vs InLix detected in the S 2p and P 2p core levels, and (iii) Co-ion diffusion into the LPS forming CoSx species at 3.3 V observed in both S 2p and Co 2p core levels. Concurrently, a local surface overpotential of 0.9 V caused by a negative localized charge layer is detected at the LPS-LCO interface. Furthermore, in agreement with previous theoretical results, the presence of a sharp potential drop at the interface between active materials and solid electrolyte is demonstrated in all-solid-state batteries.

Bridging the Pressure Gap in CO Oxidation.

Performing fundamental operando catalysis studies under realistic conditions is a key to further develop and increase the efficiency of industrial catalysts. Operando X-ray photoelectron spectroscopy (XPS) experiments have been limited to pressures, and the relevance for industrial applications has been questioned. Herein, we report on the CO oxidation experiment on Pd(100) performed at a total pressure of 1 bar using XPS. We investigate the light-off regime and the surface chemical composition at the atomistic level in the highly active phase. Furthermore, the observed gas-phase photoemission peaks of CO2, CO, and O2 indicate that the kinetics of the reaction during the light-off regime can be followed operando, and by studying the reaction rate of the reaction, the activation energy is calculated. The reaction was preceded by an in situ oxidation study in 7% O2 in He and a total pressure of 70 mbar to confirm the surface sensitivity and assignment of the oxygen-induced photoemission peaks. However, oxygen-induced photoemission peaks were not observed during the reaction studies, but instead, a metallic Pd phase is present in the highly active regime under the conditions applied. The novel XPS setup utilizes hard X-rays to enable high-pressure studies, combined with a grazing incident angle to increase the surface sensitivity of the measurement. Our findings demonstrate the possibilities of achieving chemical information of the catalyst, operando, on an atomistic level, under industrially relevant conditions.