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In operando, ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has been used to evaluate surface states of gadolinia-doped ceria (GDC) thin-film electrodes during H2 oxidation and H2O electrolysis, on yttria-stabilized zirconia (YSZ)-supported solid oxide electrochemical cells (SOCs). Porous nickel (Ni) and gold (Au) overlayers deposited on separate GDC thin films served as current collectors and potential electrocatalysts to facilitate heterogeneous chemistry for H2 oxidation or H2O electrolysis. Electrochemical characterization of the GDC thin-film electrodes complemented in operando XPS measurements of the O 1s spectra to correlate electrochemical overpotentials with surface chemistry near the Ni/GDC and Au/GDC interfaces. Shifts in O 1s binding energies across the metal/GDC/YSZ interfaces signified changes of local surface potential and provided a means of estimating kinetic parameters associated with charge transfer reactions. Effective oxygen partial pressure and surface potential impacted oxide vacancy and ceria polaron concentrations in the GDC, resulting in different reactivities of the GDC under the tested conditions. Both the Ni/GDC and Au/GDC demonstrated much higher currents for H2O electrolysis vs. H2 oxidation for comparable metal/GDC overpotentials due to increased electronic conductivity of the GDC under positive potentials and associated spreading of the electrochemically active region away from the triple-phase boundary. Higher electrochemical activity of the Ni/GDC electrode is attributed to the increased H2 activation on Ni in promoting charge transfer reactions (particularly for H2 oxidation). These results provide a basis for developing more informed reaction mechanisms for both H2 oxidation and H2O electrolysis of GDC-based composite electrodes in SOCs.

Nickel-based catalysts play an important role in the chemical transformation of CO2. A fundamental understanding of the interaction between CO2 and Ni surfaces at atomic level is necessary. In this study, the interfacial reactions of CO2 and CO2 + H2O mixture on Ni(111) as well Ni(100) surfaces were investigated using ambient pressure X-ray photoelectron spectroscopy as well as theoretical calculations. The results indicate that the distributions of dissociation products are very different on the Ni(111) and Ni(100) surfaces. Carbonate, CO, and graphitic carbon formed on both Ni(111) and Ni(100) surfaces in the presence of 0.2 Torr CO2. However, more than 90% adsorption species on the Ni(111) surface is the carbonate, whereas the Ni(100) surface is mainly covered by adsorbed CO∗ and graphitic carbon. The co-adsorption of H2O weakens the influence of structure effect on the interfacial interaction between CO2 and Ni surfaces. Furthermore, the density functional theory calculations suggest that the activation of CO2 on Ni(111) and Ni(100) tends to follow different reaction paths, which are consistent with the experimental results.

Interactions at the gas-solid interface drive physicochemical processes in many energy and environmental applications; however, the challenges associated with characterization and development of these dynamic interactions in complex systems limit progress in developing effective materials. Therefore, structure-property-performance correlations greatly depend on the development of advanced techniques and analysis methods for the investigation of gas-solid interactions. In this work, adsorption behavior of O2 and humidified O2 on nitrogen-functionalized carbon (N-C) materials was investigated to provide a better understanding of the role of nitrogen species in the oxygen reduction reaction (ORR). N-C materials were produced by solvothermal synthesis and N-ion implantation, resulting in a set of materials with varied nitrogen amount and speciation in carbon matrices with different morphologies. Adsorption behavior of the N-C samples was characterized by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments. A new analysis method for the interpretation of AP-XPS data was developed, allowing both the determination of overall adsorption behavior of each N-C material and identification of which nitrogen species were responsible for adsorption. The complementary information provided by in situ DRIFTS and AP-XPS indicates that O2 adsorption primarily takes place on either electron-rich nitrogen species like pyridine, hydrogenated nitrogen species, or graphitic nitrogen. Adsorption of O2 and H2O occurs competitively on solvothermally prepared N-Cs, whereas adsorption of H2O and O2 occurs at different sites on N-ion implanted N-Cs, highlighting the importance of tuning the composition of N-C materials to promote the most efficient ORR pathway.

Carbon deposition from CO and other carbon-containing fuels is a major cause of the performance degradation of catalysts and electrocatalysts in many energy conversion devices, including low-temperature solid oxide cells (LT-SOCs). In this work, we present direct observation of carbon deposition on thin-film CeO2 electrodes at LT-SOC operating temperatures (450 °C) in a CO/CO2 atmosphere by in operando X-ray photoelectron spectroscopy. In contrast to the general view that CeO2 is a carbon tolerant material, significant carbon formation was observed on CeO2 during CO2 electrolysis, with no other catalyst present. Moreover, carbon deposition on CeO2 demonstrated an intriguing threshold onset formation against surface Ce3+ concentration. With the aid of Monte Carlo simulations, we propose the neighboring Ce3+-Ce3+ pairs to be a critical catalytic structure that facilitates carbon deposition from CO. Finally, we propose mitigation of carbon deposition on CeO2 by doping CeO2 with non-redox-active cations, and proved this concept using 50% Gd- and 50% Zr-doped CeO2 as an example system. These findings provide an in-depth understanding of the mechanism of carbon deposition on CeO2 during electrochemical reactions and can guide the design of carbon-resistant CeO2-based electrocatalysts.

The surface chemical states of Pd(100) during CO oxidation were investigated using ambient pressure x-ray photoelectron spectroscopy and mass spectroscopy. Under the reactant ratio of CO/O 2 = 0.1, i.e. an oxygen-rich reaction condition, the formation of surface oxides was observed with the onset of CO oxidation reaction at T = 525 K. As the reactant ratio (CO/O 2 ) increased from 0.1 to 1.0, ∼ 90 % surface oxides remains on surface during the reaction. Upon the formation of surface oxides, the core level shift of oxygen gas phase peak was observed, indicating that change of surface work function. As CO oxidation takes places, i.e. making a transition from CO covered surface to the oxidic surface, the work functions of surface oxide on Pd(100) and Pt(110) display opposite behavior.

Ceramic materials based on the garnet structure Li7La3Zr2O12 (LLZO) show great promise as lithium-ion conducting electrolytes for solid-state lithium batteries. However, these materials exhibit surface degradation when exposed to air and moisture, which adversely impacts their functioning in operating devices. In this work, we use several depth-profiling and in situ techniques to probe the nature of the surface reactions that occur when aluminum (Al)-substituted LLZO is exposed to air. These experiments show that a proton exchange reaction occurs near the surface of the LLZO and leads to change in its chemistry and structure, concomitant with the formation of Li2CO3. But these reactions can be readily reversed by heating samples at 250 °C under an inert atmosphere to recover LLZO surface chemistry and structure. Symmetrical cells containing samples treated this way exhibited much lower area specific impedances than those containing air-exposed LLZO without the treatment, confirming the reversal of the degradation process. Our results show a process to rejuvenate LLZO surface, and this opens the possibility of integrating this material in solid-state devices.

Electrocatalysis of the oxygen evolution reaction is central to several energy technologies including electrolyzers, solar fuel generators, and air-breathing batteries. Strong acid electrolytes are desirable for many implementations of these technologies, although the deployment of such device designs is often hampered by the lack of non-precious-metal oxygen evolution electrocatalysts, with Ir-based oxides comprising the only known catalysts that exhibit stable activity at low overpotential. During our exploration of the Mn-Sb-O system for precious-metal-free electrocatalysts, we discovered that Mn can be incorporated into the rutile oxide structure at much higher concentrations than previously known, and that these Mn-rich rutile alloys exhibit great catalytic activity with current densities exceeding 50 mA cm-2 at 0.58 V overpotential and catalysis onset at 0.3 V overpotential. While this activity does not surpass that of IrO2, Pourbaix analysis reveals that the Mn-Sb rutile oxide alloys have the same or better thermodynamic stability under operational conditions. By combining combinatorial composition, structure, and activity mapping with synchrotron X-ray absorption measurements and first-principles materials chemistry calculations, we provide a comprehensive understanding of these oxide alloys and identify the critical role of Sb in stabilizing the trivalent Mn octahedra that have been shown to be effective oxygen evolution reaction (OER) catalysts.

Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.

Achieving a molecular-level understanding of interfacial (photo)electrochemical processes is essential in order to tailor novel and highly-performing catalytic systems. The corresponding recent development of in situ and operando tools has posed new challenges on experimental architectures. In this study, we use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to probe the solid/liquid electrified interface of a polycrystalline Pt sample in contact with an alkaline electrolyte during hydrogen and oxygen evolution reactions. Using the “dip-and-pull” technique to probe the interface through a thin liquid layer generated on the sample surface, we observe that the electrolyte meniscus becomes unstable under sustained driving of an electrolyte-consuming reaction (such as water oxidation). The addition of an electrochemically inert supporting electrolyte mitigates this issue, maintaining a stable meniscus layer for prolonged reaction times. In contrast, for processes in which the electrolyte is replenished in the reaction pathway (i.e. water reduction in alkaline conditions), we find that the solid/liquid interface remains stable without addition of a secondary supporting electrolyte. The approach described in this work allows the extension of operando AP-XPS capabilities using the “dip-and-pull” method to a broader class of reactions consuming ionic species during complex interfacial faradaic processes.

All-solid-state batteries promise significant safety and energy density advantages over liquid-electrolyte batteries. The interface between the cathode and the solid electrolyte is an important contributor to charge transfer resistance. Strong bonding of solid oxide electrolytes and cathodes requires sintering at elevated temperatures. Knowledge of the temperature dependence of the composition and charge transfer properties of this interface is important for determining the ideal sintering conditions. To understand the interfacial decomposition processes and their onset temperatures, model systems of LiCoO2 (LCO) thin films deposited on cubic Al-doped Li7La3Zr2O12 (LLZO) pellets were studied as a function of temperature using interface-sensitive techniques. X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and energy-dispersive X-ray spectroscopy data indicated significant cation interdiffusion and structural changes starting at temperatures as low as 300 °C. La2Zr2O7 and Li2CO3 were identified as decomposition products after annealing at 500 °C by synchrotron X-ray diffraction. X-ray absorption spectroscopy results indicate the presence of also LaCoO3 in addition to La2Zr2O7 and Li2CO3. On the basis of electrochemical impedance spectroscopy and depth profiling of the Li distribution upon potentiostatic hold experiments on symmetric LCO|LLZO|LCO cells, the interfaces exhibited significantly increased impedance, up to 8 times that of the as-deposited samples after annealing at 500 °C. Our results indicate that lower-temperature processing conditions, shorter annealing time scales, and CO2-free environments are desirable for obtaining ceramic cathode|electrolyte interfaces that enable fast Li transfer and high capacity.

The development of solar fuel generating materials would greatly benefit from a molecular level understanding of the semiconductor/electrolyte interface and changes in the interface induced by an applied potential and illumination by solar light. Ambient pressure photoelectron spectroscopy techniques with both soft and hard X-rays, AP-XPS and AP-HAXPES respectively, have the potential to markedly contribute to this understanding. In this paper we initially provide two examples of current challenges in solar fuels material development that AP-XPS and AP-HAXPES can directly address. This will be followed by a brief description of the distinguishing and complementary characteristics of soft and hard X-ray AP-XPS and AP-HAXPES and best approaches to achieving monolayer sensitivity in solid/aqueous electrolyte studies. In particular we focus on the detection of surface adsorbed hydroxyl groups in the presence of aqueous hydroxide anions in the electrolyte, a common situation when investigating photoanodes for solar fuel generating applications. The paper concludes by providing an example of a combined AP-XPS and AP-HAXPES study of a semiconductor/aqueous electrolyte interface currently used in water splitting devices specifically the BiVO4/aqueous potassium phosphate electrolyte interface.

Single crystals of (Nd1−xCex)2Fe14B, some reaching ∼6×8×8mm3 in volume, are grown out of Fe-(Nd, Ce) flux. This crystal growth method allows for large (Nd1−xCex)2Fe14B single crystals to be synthesized using a simple flux growth procedure. Chemical and structural analyses of the crystals indicate that (Nd1−xCex)2Fe14B forms a solid solution until at least x=0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements at 300 K show only small decreases with increasing Ce content in saturation magnetization (Ms) and anisotropy field (HA), and Curie temperature (TC). First principles calculations are carried out to understand the effect of Ce substitution on the electronic and magnetic properties. For a multitude of applications, it is expected that the advantage of incorporating lower-cost and more abundant Ce will outweigh the small adverse effects on magnetic properties. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content.

Ambient-pressure X-ray photoelectron spectroscopy (APXPS) has contributed greatly to a wide range of research fields, including environmental science [1], catalysis [2], and electrochemistry [3], to name a few. The use of this technique at synchrotron facilities primarily focused on probing the solid/gas interface; however, it quickly advanced to the probing of liquid/vapor interfaces [4, 5] and solid/liquid interfaces through an X-ray-transparent window [6–8]. Most recently, combining APXPS with “Tender” X-rays (~2.5 keV to 8 keV) on beamline 9.3.1 at the Advanced Light Source in Lawrence Berkeley National Laboratory (which can generate photoelectrons with much longer inelastic mean free paths) has enabled us to probe the solid/liquid interface without needing a window [9]. This innovation allows us to probe interfacial chemistries of electrochemically controlled solid/liquid interfaces undergoing charge transfer reactions [9]. These advancements have transitioned APXPS from a traditional surface science tool to an essential interface science technique.

The oxygen evolution reaction (OER) is a critical component of industrial processes such as electrowinning of metals and the chlor-alkali process. It also plays a central role in the development of a renewable energy field for generation a solar fuels by providing both the protons and electrons needed to generate fuels such as H2 or reduced hydrocarbons from CO2. To improve these processes, it is necessary to expand the fundamental understanding of catalytically active species at low overpotential, which will further the development of electrocatalysts with high activity and durability. In this context, performing experimental investigations of the electrocatalysts under realistic working regimes (i.e., under operando conditions) is of crucial importance. Here, we study a highly active quinary transition-metal-oxide-based OER electrocatalyst by means of operando ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy performed at the solid/liquid interface. We observe that the catalyst undergoes a clear chemical-structural evolution as a function of the applied potential with Ni, Fe, and Co oxyhydroxides comprising the active catalytic species. While CeO2 is redox inactive under catalytic conditions, its influence on the redox processes of the transition metals boosts the catalytic activity at low overpotentials, introducing an important design principle for the optimization of electrocatalysts and tailoring of high-performance materials. (Chemical Equation Presented).

Understanding the interplay between surface chemistry, electronic structure, and reaction mechanism of the catalyst at the electrified solid/liquid interface will enable the design of more efficient materials systems for sustainable energy production. The substantial progress in operando characterization, particularly using synchrotron based X-ray spectroscopies, provides the unprecedented opportunity to uncover surface chemical and structural transformations under various (electro)chemical reaction environments. In this work, we study a polycrystalline platinum surface under oxygen evolution conditions in an alkaline electrolyte by means of ambient pressure X-ray photoelectron spectroscopy performed at the electrified solid/liquid interface. We elucidate previously inaccessible aspects of the surface chemistry and structure as a function of the applied potential, allowing us to propose a reaction mechanism for oxygen evolution on a platinum electrode in alkaline solutions.

The energy difference between the oxide and bulk peaks in X-ray photoelectron spectroscopy (XPS) spectra was investigated for both GeO2/Ge and SiO2/Si structures with thickness-controlled water films. This was achieved by obtaining XPS spectra at various values of relative humidity (RH) of up to ∼15%. The increase in the energy shift is more significant for thermal GeO2 on Ge than for thermal SiO2 on Si above ∼10-4% RH, which is due to the larger amount of water molecules that infiltrate into the GeO2 film to form hydroxyls. Analyzing the origins of this energy shift, we propose that the positive charging of a partially hydroxylated GeO2 film, which is unrelated to X-ray irradiation, causes the larger energy shift for GeO2/Ge than for SiO2/Si. A possible microscopic mechanism of this intrinsic positive charging is the emission of electrons from adsorbed water species in the suboxide layer of the GeO2 film to the Ge bulk, leaving immobile cations or positively charged states in the oxide. This may be related to the reported negative shift of flat band voltages in metal-oxide-semiconductor diodes with an air-exposed GeO2 layer.

Operando synchrotron radiation photoelectron spectroscopy in the tender X-ray energy range has been used to obtain information on the energy-band relations of semiconductor and metal-covered semiconductor surfaces while in direct contact with aqueous electrolytes under potentiostatic control. The system that was investigated consists of highly doped Si substrates that were conformally coated with ∼70 nm titania films produced by atomic-layer deposition. TiO2/electrolyte and Si/TiO2/Ni/electrolyte interfaces were then analyzed by synchrotron radiation photoelectron spectroscopy. The PES data allows for determination of the flat-band position and identification of potential regions in which Fermi level pinning, depletion, or accumulation occurred. Operando X-ray absorption spectroscopy (XAS) techniques were additionally used to investigate the properties of heterogeneous electrocatalysts for the oxygen-evolution reaction. Operando XAS including the pre-edge, edge and EXAFS regions allowed the development of a detailed picture of the catalysts under operating conditions, and elucidated the changes in the physical and electronic structure of the catalyst that accompanied increases in the applied potential. Specifically, XAS data, combined with DFT studies, indicated that the activity of the electrocatalyst correlated with the formation of Fe dopant sites in γ-NiOOH.

The Second International Workshop on Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) was held at Lawrence Berkeley National Laboratory (LBNL) in Berkeley, CA, from December 7–9, 2015. It brought together more than 100 participants from 17 countries. The workshop followed the inaugural meeting at the French synchrotron SOLEIL in December 2014, which was organized by François Rochet. The strong interest in these workshops reflects the growth of the APXPS community over the last decade, with instruments now operational at more than 12 synchrotrons around the world (see SRN, Vol. 27, No. 2, pp. 14–23 (2014)), and a steady increase in the number of laboratory instruments. APXPS has established itself as an important method for the investigation of surfaces and interfaces under in situ and operando conditions, including liquid/vapor and liquid/solid interfaces.

Small-band-gap (Eg < 2 eV) semiconductors must be stabilized for use in integrated devices that convert solar energy into the bonding energy of a reduced fuel, specifically H2(g) or a reduced-carbon species such as CH3OH or CH4. To sustainably and scalably complete the fuel cycle, electrons must be liberated through the oxidation of water to O2(g). Strongly acidic or strongly alkaline electrolytes are needed to enable efficient and intrinsically safe operation of a full solar-driven water-splitting system. However, under water-oxidation conditions, the small-band-gap semiconductors required for efficient cell operation are unstable, either dissolving or forming insulating surface oxides. We describe herein recent progress in the protection of semiconductor photoanodes under such operational conditions. We specifically describe the properties of two protective overlayers, TiO2/Ni and NiOx, both of which have demonstrated the ability to protect otherwise unstable semiconductors for >100 h of continuous solar-driven water oxidation when in contact with a highly alkaline aqueous electrolyte (1.0 M KOH(aq)). The stabilization of various semiconductor photoanodes is reviewed in the context of the electronic characteristics and a mechanistic analysis of the TiO2 films, along with a discussion of the optical, catalytic, and electronic nature of NiOx films for stabilization of semiconductor photoanodes for water oxidation.

Transition metal perovskite oxides are promising electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells, but a lack of fundamental understanding of oxide surfaces impedes the rational design of novel catalysts with improved device efficiencies. In particular, understanding the surface chemistry of oxides is essential for controlling both catalytic activity and long-term stability. Thus, elucidating the physical nature of species on perovskite surfaces and their catalytic enhancement would generate new insights in developing oxide electrocatalysts. In this article, we perform near-ambient pressure XPS of model brownmillerite Sr2Co2O5 (SCO) epitaxial thin films with different crystallographic orientations. Detailed analysis of the Co 2p spectra suggests that the films lose oxygen as a function of temperature. Moreover, deconvolution of the O 1s spectra shows distinct behavior for (114)-oriented SCO films compared to (001)-oriented SCO films, where an additional bulk oxygen species is observed. These findings indicate a change to a perovskite-like oxygen chemistry that occurs more easily in (114) SCO than (001) SCO, likely due to the orientation of oxygen vacancy channels out-of-plane with respect to the film surface. This difference in surface chemistry is responsible for the anisotropy of the oxygen surface exchange coefficient of SCO and may contribute to the enhanced ORR kinetics of La0.8Sr0.2CoO3-δ thin films by SCO surface particles observed previously.

Chemical analysis of solid-liquid interfaces under electrochemical conditions has recently become feasible due to the development of new synchrotron radiation techniques. Here we report the use of "tender" X-ray ambient-pressure X-ray photoelectron spectroscopy (APXPS) to characterize a thin film of Ni-Fe oxyhydroxide electrodeposited on Au as the working electrode at different applied potentials in 0.1 M KOH as the electrolyte. Our results show that the as-prepared 7 nm thick Ni-Fe (50% Fe) film contains Fe and Ni in both their metallic as well as oxidized states, and undergoes further oxidation when the sample is subjected to electrochemical oxidation-reduction cycles. Metallic Fe is oxidized to Fe3+ and metallic Ni to Ni2+/3+. This work shows that it is possible to monitor the chemical nature of the Ni-Fe catalyst as a function of potential when the corresponding current densities are small. This allows for operando measurements just above the onset of OER; however, current densities as they are desired in photoelectrochemical devices (∼1-10 mA cm-2) could not be achieved in this work, due to ohmic losses in the thin electrolyte film. We use a two-dimensional model to describe the spatial distribution of the electrochemical potential, current density, and pH as a function of the position above the electrolyte meniscus, to provide guidance toward enabling the acquisition of operando APXPS at high current density. The shifts in binding energy of water with applied potential predicted by the model are in good agreement with the experimental values.

The electrical and spectroscopic properties of the TiO2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic controlto obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2 surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO2/Ni electrodes and to contextualize the AP-XPS results.

Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux.Chemical and structural analysis of the crystals indicates that(Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with aVegard-like variation of the lattice constants with x. Refinements of singlecrystal neutron diffraction data indicate that Ce has a slight site preference(7:3) for the 4g rare earth site over the 4f site. Magnetization measurementsshow that for x = 0.38 the saturation magnetization at 400 K, a temperatureimportant to applications, falls from 29.8 for the parent Nd2Fe14B to 27.6(mu)B/f.u., the anisotropy field decreases from 5.5 T to 4.7 T, and the Curietemperature decreases from 586 to 543 K. First principles calculations carriedout within density functional theory are used to explain the decrease inmagnetic properties due to Ce substitution. Though the presence of thelower-cost and more abundant Ce slightly affects these important magneticcharacteristics, this decrease is not large enough to affect a multitude ofapplications. Ce-substituted Nd2Fe14B is therefore a potential high-performancepermanent magnet material with substantially reduced Nd content.

Exsolution is a promising approach for fabricating oxide-supported metal nanocatalysts through redox-driven metal precipitation. A defining feature of exsolved nanocatalysts is their anchored metal-oxide interface, which exhibits exceptional structural stability in (electro)catalysis. However, the electronic interactions at this unique interface remain unclear, despite their known impact on catalytic performance. In this study, we confirm charge transfer between the host oxide and the exsolved metal by demonstrating a two-stage Fermi level ( E F ) evolution on SrTi 0.65 Fe 0.35 O 3-δ (STF) during metallic iron (Fe 0 ) exsolution. Combining ambient pressure X-ray photoelectron spectroscopy with theoretical analysis, we show that E F initially rises due to electron doping from oxygen vacancy formation in STF. Subsequently, upon Fe 0 precipitation, E F stabilizes and becomes insensitive to further oxygen release in STF, driven by E F equilibration and charge transfer between STF and the exsolved Fe 0 . These findings highlight the importance of considering electronic metal-support interactions when optimizing exsolved nanocatalysts.

Hydrogen spillover is an important process in catalytic hydrogenation reactions, facilitating H 2 activation and modulating surface chemistry of reducible oxide catalysts. This study focuses on the operando unveiling of platinum-induced hydrogen spillover on monoclinic tungsten trioxide (γ-WO 3 ), employing ambient pressure X-ray photoelectron spectroscopy, density functional theory calculations and microkinetic modeling to investigate the dynamic evolution of surface states at varied temperatures. At room temperature, hydrogen spillover results in the formation of W 5+ and hydrogen intermediates (hydroxyl species and adsorbed water), facilitated by Pt metal clusters. With increasing temperature, water desorption, reverse hydrogen spillover and surface-to-bulk diffusion of hydrogen atoms compete with each other, leading initially to reoxidation and then further reduction of W atoms in the near-surface. The combined experimental results and simulations provide a comprehensive understanding of the mechanisms underlying hydrogen interaction with reducible metal oxides, lending insights of relevance to the design of enhanced hydrogenation catalysts.

Inelastic photoelectron scattering (IPES) by gas molecules, a critical phenomenon observed in ambient pressure X-ray photoelectron spectroscopy (APXPS), complicates spectral interpretation due to kinetic energy loss in the primary spectrum and the appearance of additional features at higher binding energies. In this study, we systematically investigate IPES in various gas environments using APXPS, providing detailed insights into interactions between photoelectrons emitted from solid surfaces and surrounding gas molecules. Core-level XPS spectra of Au, Ag, Zn, and Cu metals were recorded over a wide kinetic energy range in the presence of CO 2 , N 2 , Ar, and H 2 gases, demonstrating the universal nature of IPES across different systems. Additionally, we analyzed spectra of scattering effects induced by gas-phase interactions without metal solids. In two reported CO 2 -reduction systems (p-GaN/Au/Cu and p-Si/TaO x /Cu), we elucidated that IPES is independent of the composition, structure, or size of the solid materials. Using metal foil platforms, we further developed an analytical model to extract electron excitation cross sections of gas molecules. These findings enhance our understanding of IPES mechanisms and enable the predictions of IPES structures in other solid-gas systems, providing a valuable reference for future APXPS studies and improving the accuracy of spectral analysis in gas-rich catalytic interfaces., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

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. CeO 2-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, CH 4 , 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 CH 4 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 H 2 O formation is key in determining the overall reaction rate at the electrode. These insights effectively address the longstanding debate regarding the direct utilization of CH 4 in SOFCs. Moreover, these findings pave the way for an optimized electrode design strategy, essential for developing high-performance, environmentally sustainable fuel cells., (© 2024 Wiley‐VCH GmbH.)

Colloidal chemistry holds promise to prepare uniform and size-controllable pre-catalysts; however, it remains a challenge to unveil the atomic-level transition from pre-catalysts to active catalytic surfaces under the reaction conditions to enable the mechanistic design of catalysts. Here, we report an ambient-pressure X-ray photoelectron spectroscopy study, coupled with in situ environmental transmission electron microscopy, infrared spectroscopy, and theoretical calculations, to elucidate the surface catalytic sites of colloidal Ni nanoparticles for CO 2 hydrogenation. We show that Ni nanoparticles with phosphine ligands exhibit a distinct surface evolution compared with amine-capped ones, owing to the diffusion of P under oxidative (air) or reductive (CO 2 + H 2 ) gaseous environments at elevated temperatures. The resulting NiP x surface leads to a substantially improved selectivity for CO production, in contrast to the metallic Ni, which favors CH 4 . The further elimination of surface metallic Ni sites by designing multi-step P incorporation achieves unit selectivity of CO in high-rate CO 2 hydrogenation.

Electrochemical polarization, which often plays a critical role in driving chemical reactions at solid-liquid interfaces, can arise spontaneously through the exchange of ions and/or electrons across the interface. However, the extent to which such spontaneous polarization prevails at nonconductive interfaces remains unclear because such materials preclude measuring and controlling the degree of interfacial polarization via standard ( i.e. , wired) potentiometric methods. Herein, we circumvent the limitations of wired potentiometry by applying infrared and ambient pressure X-ray photoelectron spectroscopies (AP-XPS) to probe the electrochemical potential of nonconductive interfaces as a function of solution composition. As a model class of macroscopically nonconductive interfaces, we specifically probe the degree of spontaneous polarization of ZrO 2 -supported Pt and Au nanoparticles immersed in aqueous solutions of varying pH. Shifts in the Pt-adsorbed CO vibrational band position evince electrochemical polarization of the Pt/ZrO 2 -water interface with changing pH, and AP-XPS reveals quasi-Nernstian shifts of the electrochemical potential of Pt and Au with pH in the presence of H 2 . These results indicate that spontaneous proton transfer via equilibrated H + /H 2 interconversion spontaneously polarizes metal nanoparticles even when supported on a nonconductive host. Consequently, these findings indicate that solution composition ( i.e. , pH) can be an effective handle for tuning interfacial electrical polarization and potential at nonconductive interfaces., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The need to reduce atmospheric CO 2 concentrations necessitates CO 2 capture technologies for conversion into stable products or long-term storage. A single pot solution that simultaneously captures and converts CO 2 could minimize additional costs and energy demands associated with CO 2 transport, compression, and transient storage. While a variety of reduction products exist, currently, only conversion to C 2+ products including ethanol and ethylene are economically advantageous. Cu-based catalysts have the best-known performance for CO 2 electroreduction to C 2+ products. Metal Organic Frameworks (MOFs) are touted for their carbon capture capacity. Thus, integrated Cu-based MOFs could be an ideal candidate for the one-pot capture and conversion. In this paper, we review Cu-based MOFs and MOF derivatives that have been used to synthesize C 2+ products with the objective of understanding the mechanisms that enable synergistic capture and conversion. Furthermore, we discuss strategies based on the mechanistic insights that can be used to further enhance production. Finally, we discuss some of the challenges hindering widespread use of Cu-based MOFs and MOF derivatives along with possible solutions to overcome the challenges., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Jana, Snyder, Crumlin and Qian.)

A new operando spectro-electrochemical setup was developed to study oxygen depletion from the surface of layered transition metal oxide particles at high degrees of delithiation. An NCM111 working electrode was paired with a chemically delithiated LiFePO 4 counter electrode in a fuel cell-inspired membrane electrode assembly (MEA). A propylene carbonate-soaked Li-ion conducting ionomer served as an electrolyte, providing both good electrochemical performance and direct probing of the NCM111 particles during cycling by ambient pressure X-ray photoelectron spectroscopy. The irreversible emergence of an oxygen-depleted phase in the O 1s spectra of the layered oxide particles was observed upon the first delithiation to high state-of-charge, which is in excellent agreement with oxygen release analysis via mass spectrometry analysis of such MEAs. By comparing the metal oxide-based O 1s spectral features to the Ni 2p 3/2 intensity, we can calculate the transition metal-to-oxygen ratio of the metal oxide close to the particle surface, which shows good agreement with the formation of a spinel-like stoichiometry as an oxygen-depleted phase. This new setup enables a deeper understanding of interfacial changes of layered oxide-based cathode active materials for Li-ion batteries upon cycling.

Selective transport of solutes across a membrane is critical for many biological, water treatment and energy conversion and storage systems. When a charged membrane is equilibrated with an electrolyte, an unequal distribution of ions arises between phases, generating the so-called Donnan electrical potential at the solution/membrane interface. The Donnan potential results in the partial exclusion of co-ion, providing the basis of permselectivity. Although there are well-established ways to indirectly estimate the Donnan potential, it has been widely reported that it cannot be measured directly. Here we report the first direct measurement of the Donnan potential of an ion exchange membrane equilibrated with salt solutions. Our results highlight the dependence of the Donnan potential on external salt concentration and counter-ion valence, and show a reasonable agreement with current theoretical models of IEMs, which incorporate ion activity coefficients. By directly measuring the Donnan potential, we eliminate ambiguities that arise from limitations inherent in current models., (© 2022. The Author(s).)

The dissociation of H 2 is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H 2 -D 2 exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H 2 -D 2 exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300-573 K. Pretreatment with flowing H 2 is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H 2 -D 2 exchange rate is 5-7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H 2 pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The apparent activation energies and pre-exponential factors measured for H 2 -D 2 exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Exsolution synthesizes self-assembled metal nanoparticle catalysts via phase precipitation. An overlooked aspect in this method thus far is how exsolution affects the host oxide surface chemistry and structure. Such information is critical as the oxide itself can also contribute to the overall catalytic activity. Combining X-ray and electron probes, we investigated the surface transformation of thin-film SrTi 0.65 Fe 0.35 O 3 during Fe 0 exsolution. We found that exsolution generates a highly Fe-deficient near-surface layer of about 2 nm thick. Moreover, the originally single-crystalline oxide near-surface region became partially polycrystalline after exsolution. Such drastic transformations at the surface of the oxide are important because the exsolution-induced nonstoichiometry and grain boundaries can alter the oxide ion transport and oxygen exchange kinetics and, hence, the catalytic activity toward water splitting or hydrogen oxidation reactions. These findings highlight the need to consider the exsolved oxide surface, in addition to the metal nanoparticles, in designing the exsolved nanocatalysts.

The formation and thermal stability of Pt surface oxides on a Pt thin film were studied in situ using ambient-pressure X-ray photoelectron spectroscopy. At an oxygen pressure of 73 Pa (550 mTorr), the surface Pt oxide was gradually formed, evidenced by the O 1s peak at 529.5 eV as the Pt film was heated. The Pt oxide peak reached a maximum between 217 and 317 °C and then decreased as the sample temperature was further increased. A similar response was seen on cooling from 480 to 23 °C; the intensity of the Pt oxide peak first increased and then decreased. The remaining Pt surface oxides partially decomposed during ultra-high-vacuum (UHV) pumping and completely decomposed during heating in UHV, which highlights the challenge of characterizing these surfaces with UHV instruments. These results have important implications for the understanding of the surface states of platinum films in different environments and the roles of different catalytic mechanisms.

Polymers are commonly used in applications that require long-term exposure to water and aqueous mixtures, serving as water purification membranes, marine antifouling coatings, and medical implants, among many other applications. Because polymer surfaces restructure in response to the surrounding environment, in situ characterization is crucial for providing an accurate understanding of the surface chemistry under conditions of use. To investigate the effects of surface-active side chains on polymer surface chemistry and resultant interactions with interfacial water (i.e., water sorption), we present synchrotron ambient pressure X-ray photoelectron spectroscopy (APXPS) studies performed on poly(ethylene oxide) (PEO)- and poly(dimethylsiloxane) (PDMS)-based polymer surfaces modified with amphiphilic polypeptoid side chains, previously demonstrated to be efficacious in marine fouling prevention and removal. The polymer backbone and environmental conditions were found to affect polypeptoid surface presentation: due to the surface segregation of its fluorinated polypeptoid monomers under vacuum, the PEO-peptoid copolymer showed significant polypeptoid content in both vacuum and hydrated conditions, while the modified PDMS-based copolymer showed increased polypeptoid content only in hydrated conditions due to the hydrophilicity of the ether monomers and polypeptoid backbone. Polypeptoids were found to bind approximately 2.8 water molecules per monomer unit in both copolymers, and the PEO-peptoid surface showed substantial water sorption that suggests a surface with a more diffuse water/polymer interface. This work implies that side chains are ideal for tuning water affinity without altering the base polymer composition, provided that surface-driving groups are present to ensure activity at the interface. These types of systematic modifications will generate novel polymers that maximize bound interfacial water and can deliver surface-active groups to the surface to improve the effectiveness of polymer materials.

X-Ray photoemission spectroscopy is a commonly applied characterization technique that probes the local chemistry of atoms in molecules and materials via the photoexcitation of electrons from atomic core orbitals. These measurements can be interpreted by comparison with previous literature or through the calculation of core-electron binding energies (CEBEs) for model systems. However, physically and numerically accurate description of the core-excited electronic structures demands specializations beyond routine ground state setups. Inspired by Slater's rules, we focus on developing computationally efficient and physically motivated contractions to reproduce the core-excited atomic orbitals which led to improved numerical accuracy of calculated CEBEs. When applied to carbon 1s excitations in a wide range of molecules, these core-excited basis sets produce total energy differences (ΔSCF) using a hybrid exact-exchange density functional (B3LYP) that can reproduce core-excitation energies within experimental accuracy (∼0.1 eV). Due to missing relativistic effects, heavier elements (N, O) exhibit slightly larger systematic absolute errors, but still maintain a satisfactory 0.2 eV mean average error for relative CEBEs. We also connect the known variability in the core level binding energy with local atomic charge to demonstrate how the transferability of a given model should be measured against a diverse test set. We conclude by exploring one outlier, CO, and the outlook for extending this approach to other elements.

The properties of bulk water and aqueous solutions are known to change in the vicinity of an interface and/or in a confined environment, including the thermodynamics of ion selectivity at interfaces, transition states and pathways of chemical reactions, and nucleation events and phase growth. Here we describe joint progress in identifying unifying concepts about how air, liquid, and solid interfaces can alter molecular properties and chemical reactivity compared to bulk water and multicomponent solutions. We also discuss progress made in interfacial chemistry through advancements in new theory, molecular simulation, and experiments.