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Author ORCID ID is 000000033132190X
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  1. Controlling the structure of catalysts at the atomic level provides an opportunity to establish detailed understanding of the catalytic form-to-function and realize new, non-equilibrium catalytic structures. Here, advanced thin-film deposition is used to control the atomic structure of La 2/3Sr 1/3MnO 3, a well-known catalyst for the oxygen reduction reaction. The surface and sub-surface is customized, whereas the overall composition and d-electron configuration of the oxide is kept constant. Although the addition of SrMnO 3 benefits the oxygen reduction reaction via electronic structure and conductivity improvements, SrMnO 3 can react with ambient air to reduce the surface site availability. Placingmore » SrMnO 3 in the sub-surface underneath a LaMnO 3 overlayer allows the catalyst to maintain the surface site availability while benefiting from improved electronic effects. The results show the promise of advanced thin-film deposition for realizing atomically precise catalysts, in which the surface and sub-surface structure and stoichiometry are tailored for functionality, over controlling only bulk compositions.« less
  2. The bulk-to-surface Sr segregation can seriously compromise the stability of oxygen electrocatalysis in La 1-xSr xCoO 3-δ and limit its practical applications such as in solid oxide fuel cells. We show via in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) that the surface Sr-segregation is a kinetically fast process and the equilibrium surface Sr-concentration follows Arrhenius law from 250 to 520 °C at a fixed p O2 = 1 × 10 -3 atm. We also show that application of a nanoscaled, atomic layer deposition (ALD) derived ZrO 2 overcoat can effectively suppress the Sr-segregation by reducing the surface concentration ofmore » oxygen vacancies. Electrochemical impedance spectroscopy (EIS) study further confirms that the ALD-ZrO 2-coated LSCo epitaxial film exhibits a much lower and more stable polarization resistance than the uncoated one at 550 °C for >300 hours, suggesting that Sr-segregation is the source of the higher resistance.« less
  3. Perovskite oxides are promising materials for photoabsorbers and electrocatalysts for solar-driven water oxidation.
  4. Here, the water-gas shift (WGS) reaction (where carbon monoxide plus water yields dihydrogen and carbon dioxide) is an essential process for hydrogen generation and carbon monoxide removal in various energy-related chemical operations. This equilibrium-limited reaction is favored at a low working temperature. Potential application in fuel cells also requires a WGS catalyst to be highly active, stable, and energy-efficient and to match the working temperature of on-site hydrogen generation and consumption units. We synthesized layered gold (Au) clusters on a molybdenum carbide (α-MoC) substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction. Water was activated over α-MoCmore » at 303 kelvin, whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity at low temperatures.« less
    Cited by 27Full Text Available
  5. 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 chemistrymore » 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.« less
  6. Understanding the interaction of CO 2 with perovskite metal oxide surfaces is crucial for the design of various perovskite (electro)chemical functionalities, such as solid oxide fuel cells, catalytic oxidation reactions, and gas sensing. In this study, we experimentally investigated the reactivity of CO 2 with a series of cobalt-based perovskites (i.e., LaCoO 3, La 0.4Sr 0.6CoO 3, SrCoO 2.5, and Pr 0.5Ba 0.5CoO 3–δ) by a combined ambient-pressure XPS (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) approach. Isobaric measurements by AP-XPS on epitaxial pulsed laser deposition-grown (100)-oriented thin films under 1 mTorr CO 2 showed the formation andmore » uptake of adsorbed adventitious-like C–C/C–H, –CO species, monodentate carbonate, and bidentate (bi)carbonates. DRIFTS measurements on powder samples under CO 2 atmosphere revealed the presence of multiple configurations of carbonate in the asymmetric O–C–O stretching region with peak splittings of ~100 and ~300 cm –1 correlated to the monodentate- and bidentate-bound carbonate adsorbates, respectively. The synergy between chemical state identification by AP-XPS and vibrational state detection by DRIFTS allows both the carbonaceous species type and the configuration to be identified. We further demonstrate that the surface chemistry of the A-site cation strongly influences CO 2 reactivity; the La, Sr, and Ba cations in the LaCoO 3, La 0.4Sr 0.6CoO 3, SrCoO 2.5, and Pr 0.5Ba 0.5CoO 3 thin films showed significant carbon adsorbate speciation. Additionally, we link the La 0.4Sr 0.6CoO 3 surface chemistry to its surface reactivity toward formation of bidentate (bi)carbonate species via exchange of lattice oxygen with carbonate oxygen. In conclusion, we show that the perovskite electronic structure ultimately dictates the driving force for formation of oxidized oxo-carbonaceous species (CO 3) versus reduced species (C–C/C–H). Furthermore, a higher O 2p-band center relative to the Fermi level was correlated with a higher degree of (bi)carbonate formation relative to the other carbonaceous species observed (C–C/C–H and –CO) due to a more facile charge transfer from oxygen states at the Fermi level to free CO 2 gas.« less
  7. 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 LiCoO 2 (LCO) thin films deposited on cubic Al-doped Li 7La 3Zr 2O 12 (LLZO) pellets were studied as a function ofmore » 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. La 2Zr 2O 7 and Li 2CO 3 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 LaCoO 3 in addition to La 2Zr 2O 7 and Li 2CO 3. 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. Here, our results indicate that lower-temperature processing conditions, shorter annealing time scales, and CO 2-free environments are desirable for obtaining ceramic cathode|electrolyte interfaces that enable fast Li transfer and high capacity.« less
  8. Photoelectrochemical water splitting is a promising pathway for the direct conversion of renewable solar energy to easy to store and use chemical energy. The performance of a photoelectrochemical device is determined in large part by the heterogeneous interface between the photoanode and the electrolyte, which we here characterize directly under operating conditions using interface-specific probes. Utilizing X-ray photoelectron spectroscopy as a noncontact probe of local electrical potentials, we demonstrate direct measurements of the band alignment at the semiconductor/electrolyte interface of an operating hematite/KOH photoelectrochemical cell as a function of solar illumination, applied potential, and doping. Here, we provide evidence formore » the absence of in-gap states in this system, which is contrary to previous measurements using indirect methods, and give a comprehensive description of shifts in the band positions and limiting processes during the photoelectrochemical reaction.« less
  9. Au(111) does not bind CO and O 2 well. The deposition of small nanoparticles of MgO, CeO 2, and TiO 2 on Au(111) produces excellent catalysts for CO oxidation at room temperature. In an inverse oxide/metal configuration there is a strong enhancement of the oxide–metal interactions, and the inverse catalysts are more active than conventional Au/MgO(001), Au/CeO 2(111), and Au/TiO 2(110) catalysts. An identical trend was seen after comparing the CO oxidation activity of TiO2/Au and Au/TiO 2 powder catalysts. In the model systems, the activity increased following the sequence: MgO/Au(111) < CeO 2/Au(111) < TiO 2/Au(111). Ambient pressure X-raymore » photoelectron spectroscopy (AP-XPS) was used to elucidate the role of the titania–gold interface in inverse TiO 2/Au(111) model catalysts during CO oxidation. Stable surface intermediates such as CO(ads), CO 3 2–(ads), and OH(ads) were identified under reaction conditions. CO 3 2–(ads) and OH(ads) behaved as spectators. The concentration of CO(ad) initially increased and then decreased with increasing TiO 2 coverage, demonstrating a clear role of the Ti–Au interface and the size of the TiO 2 nanostructures in the catalytic process. Overall, our results show an enhancement in the strength of the oxide–metal interactions when working with inverse oxide/metal configurations, a phenomenon that can be utilized for the design of efficient catalysts useful for green and sustainable chemistry.« less
  10. Electrochemistry is necessarily a science of interfacial processes, and understanding electrode/electrolyte interfaces is essential to controlling electrochemical performance and stability. Undesirable interfacial interactions hinder discovery and development of rational materials combinations. By example, we examine an electrolyte, magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI) 2) dissolved in diglyme, next to the Mg metal anode, which is purported to have a wide window of electrochemical stability. However, even in the absence of any bias, using in situ tender X-ray photoelectron spectroscopy, we discovered an intrinsic interfacial chemical instability of both the solvent and salt, further explained using first-principles calculations as driven by Mg 2+ dicationmore » chelation and nucleophilic attack by hydroxide ions. The proposed mechanism appears general to the chemistry near or on metal surfaces in hygroscopic environments with chelation of hard cations and indicates possible synthetic strategies to overcome chemical instability within this class of electrolytes.« less

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