Recursos de colección

Caltech Authors (170.931 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 172

  1. Efficient solar hydrogen generation in microgravity environment

    Brinkert, Katharina; Richter, Matthias H.; Akay, Ömer; Liedtke, Janine; Giersig, Michael; Fountaine, Katherine T.; Lewerenz, Hans-Joachim
    Long-term space missions require extra-terrestrial production of storable, renewable energy. Hydrogen is ascribed a crucial role for transportation, electrical power and oxygen generation. We demonstrate in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel. The photoelectrochemical cell consists of an integrated catalyst-functionalized semiconductor system that generates hydrogen with current densities >15 mA/cm^2 in the absence of buoyancy. Conditions are described adverting the resulting formation of ion transport blocking froth layers on the photoelectrodes. The current limiting factors were overcome...

  2. Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

    Moreno-Hernandez, Ivan A.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO_x) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO_x layer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm^(−2) of simulated sunlight. The SnO_x layer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0 m KOH(aq) or 1.0 m H_2SO_4(aq). Ideal regenerative solar‐to‐O_2(g) efficiencies of 4.1% and 3.7%, respectively, are obtained in...

  3. Temperature Dependent Electron-Phonon Scattering and Electron Mobility in SrTiO_3 Perovskite from First Principles

    Zhou, Jin-Jian; Hellman, Olle; Bernardi, Marco
    Structural phase transitions and soft phonon modes pose a longstanding challenge to computing electron-phonon (e-ph) interactions in strongly anharmonic crystals. Here we develop a first-principles approach to compute e-ph scattering and charge transport in materials with anharmonic lattice dynamics. Our approach employs renormalized phonons to compute the temperature-dependent e-ph coupling for all phonon modes, including the soft modes associated with ferroelectricity and phase transitions. We show that the electron mobility in cubic SrTiO_3 is controlled by scattering with longitudinal optical phonons at room temperature and with ferroelectric soft phonons below 200~K. Our calculations can accurately predict the temperature dependence of...

  4. Broadband sensitization of lanthanide emission with indium phosphide quantum dots for visible to NIR downshifting

    Swabeck, Joseph K.; Fischer, Stefan; Bronstein, Noah D.; Alivisatos, A. Paul
    Semiconductor quantum dot sensitized lanthanide ions hold great promise in producing a broadly absorbing and sharply emitting luminophore, but their synthesis has proven to be difficult. We report the first synthesis of core/shell/shell InP/Ln_xY_(1-x)F_3/ShF_3 (Ln = Yb, Nd, Sh = Lu, Y) nanocrystals that exhibit a broad visible absorption coupled to a sharp near-infrared emission. Additionally, this is the first report of Nd being coupled to a quantum dot absorber. We characterize the system with a variety of electron microscopy and x-ray techniques that prove this unique structure. Optical meas-urements confirm the correlation of the Ln3+ emission to the quantum...

  5. Reduction of aqueous CO_2 to 1-Propanol at MoS_2 electrodes

    Francis, Sonja A.; Velazquez, Jesus M.; Ferrer, Ivonne M.; Torelli, Daniel A.; Guevarra, Dan; McDowell, Matthew T.; Sun, Ke; Zhou, Xinghao; Saadi, Fadl H.; John, Jimmy; Richter, Matthias H.; Hyler, Forrest P.; Papadantonakis, Kimberly M.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS_2 or thin-film MoS_2 electrodes yields 1-propanol as the major CO_2 reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and t-butanol were also produced. At an applied potential of -0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO_2 to 1-propanol were ~3.5% for MoS_2 single crystals and ~1% for thin films with low edge-site densities. Reduction of CO_2 to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e^- and 18...

  6. An Efficient Relaxed Projection Method for Constrained Non-negative Matrix Factorization with Application to the Phase-Mapping Problem in Materials Science

    Bai, Junwen; Ament, Sebastian; Perez, Guillaume; Gregoire, John; Gomes, Carla
    In recent years, a number of methods for solving the constrained non-negative matrix factorization problem have been proposed. In this paper, we propose an efficient method for tackling the ever increasing size of real-world problems. To this end, we propose a general relaxation and several algorithms for enforcing constraints in a challenging application: the phase-mapping problem in materials science. Using experimental data we show that the proposed method significantly outperforms previous methods in terms of ℓ_2-norm error and speed.

  7. Data Acquisition Protocols and Reporting Standards for Studies of the Electrochemical Reduction of Carbon Dioxide

    Clark, Ezra L.; Resasco, Joaquin; Landers, Alan; Lin, John; Chung, Linh-Thao; Walton, Amber; Hahn, Christopher; Jaramillo, Thomas F.; Bell, Alexis T.
    Objective evaluation of the performance of electrocatalysts for CO_2 reduction has been complicated by a lack of standardized methods for measuring and reporting activity data. In this perspective, we advocate that standardizing these practices can aid in advancing research efforts toward the development of efficient and selective CO_2 reduction electrocatalysts. Using information taken from experimental studies, we identify variables that influence the measured performance of CO_2 reduction electrocatalysts and propose procedures to improve the accuracy and reproducibility of reported data. We recommend that catalysts be measured under conditions which do not introduce artifacts from impurities, either from the electrolyte or...

  8. Excitonic effects in photovoltaic materials with large exciton binding energies

    Omelchenko, Stefan T.; Tolstova, Yulia; Atwater, Harry A.; Lewis, Nathan S.
    We investigate the effect of excitons on charge transport in photovoltaic materials with large exciton binding energies using Cu_2O as a model system. We develop a thermodynamic model to estimate the fraction of excitons in Cu_2O at quasi-equilibrium and find that over 20% of the generated population of carriers during photovoltaic operation could be excitons. Experiments show the presence of excitons at room temperature under visible light excitation and current collection due to excitons during device operation. This work demonstrates that excitons can play a fundamental role in photovoltaic materials with large exciton binding energies and lays the foundation for...

  9. The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO_2 Reduction Conditions: An Experimental and Theoretical Study

    Landers, Alan T.; Fields, Meredith; Torelli, Daniel A.; Xiao, Jianping; Hellstern, Thomas R.; Francis, Sonja A.; Tsai, Charlie; Kibsgaard, Jakob; Lewis, Nathan S.; Chan, Karen; Hahn, Christopher; Jaramillo, Thomas F.
    A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO_2 reduction (CO_2R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO_2R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO_2R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO_2R. On Ni-MoS_2, a limitation is the kinetic barrier for the...

  10. Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH

    Cheng, Tao; Wang, Lu; Merinov, Boris V.; Goddard, William A., III
    Hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) are both 2 orders slower in alkaline electrolyte than in acidic electrolyte, but no explanation has been provided. The first step toward understanding this dramatic pH-dependent HOR/HER performance is to explain the pH-dependent hydrogen binding to the electrode, a perplexing behavior observed experimentally. In this work, we carried out Quantum Mechanics Molecular Dynamics (QMMD) with explicit considerations of solvent and applied voltage (U) to in situ simulate water/Pt(100) interface in the condition of under-potential adsorption of hydrogen (H_(UPD)). We found that as U is made more negative, the electrode tends to...

  11. Advancing semiconductor–electrocatalyst systems: application of surface transformation films and nanosphere lithography

    Brinkert, Katharina; Richter, Matthias H.; Akay, Ömer; Giersig, Michael; Fountaine, Katherine T.; Lewerenz, Hans-Joachim
    Photoelectrochemical (PEC) cells offer the possibility of carbon-neutral solar fuel production through artificial photosynthesis. The pursued design involves technologically advanced III–V semiconductor absorbers coupled via an interfacial film to an electrocatalyst layer. These systems have been prepared by in situ surface transformations in electrochemical environments. High activity nanostructured electrocatalysts are required for an efficiently operating cell, optimized in their optical and electrical properties. We demonstrate that shadow nanosphere lithography (SNL) is an auspicious tool to systematically create three-dimensional electrocatalyst nanostructures on the semiconductor photoelectrode through controlling their morphology and optical properties. First results are demonstrated by means of the photoelectrochemical...

  12. Surface Ligand Promotion of Carbon Dioxide Reduction through Stabilizing Chemisorbed Reactive Intermediates

    Wang, Zhijiang; Wu, Lina; Sun, Kun; Chen, Ting; Jiang, Zhaohua; Cheng, Tao; Goddard, William A., III
    We have explored functionalizing metal catalysts with surface ligands as an approach to facilitate electrochemical carbon dioxide reduction reaction (CO_2RR). To provide a molecular level understanding of the mechanism by which this enhancement occurs, we combine in situ spectroscopy analysis with an interpretation based on quantum mechanics (QM) calculations. We find that a surface ligand can play a critical role in stabilizing the chemisorbed CO_2, which facilitates CO_2 activation and leads to a 0.3 V decrease in the overpotential for carbon monoxide (CO) formation. Moreover, the presence of the surface ligand leads to nearly exclusive CO production. At −0.6 V...

  13. Synergy between Fe and Ni in the optimal performance of (Ni,Fe)OOH catalysts for the oxygen evolution reaction

    Xiao, Hai; Shin, Hyeyoung; Goddard, William A., III
    The oxygen evolution reaction (OER) is critical to solar production of fuels, but the reaction mechanism underlying the performance for a best OER catalyst, Fe-doped NiOOH [(Ni,Fe)OOH], remains highly controversial. We used grand canonical quantum mechanics to predict the OER mechanisms including kinetics and thus overpotentials as a function of Fe content in (Ni,Fe)OOH catalysts. We find that density functional theory (DFT) without exact exchange predicts that addition of Fe does not reduce the overpotential much. However, DFT with exact exchange predicts dramatic improvement in performance for (Ni,Fe)OOH, leading to an overpotential of 0.42 V and a Tafel slope of...

  14. In silico discovery of new dopants for Fe-doped Ni oxyhydroxide (Ni_(1-x)Fe_xOOH) catalysts for oxygen evolution reaction

    Shin, Hyeyoung; Xiao, Hai; Goddard, William A., III
    The oxygen evolution reaction (OER) is critical to efficient water splitting to produce the H_2 fuel for sustainable energy production. Currently, the best non-noble metal OER electrocatalyst in base conditions is the Fe-doped NiOOH (Ni_(1–x)Fe_xOOH), with an overpotential of η = 0.4, but much lower values are desired. We use density functional theory to determine the overall mechanism for the OER of Ni_(1–x)Fe_xOOH, concluding that promoting radical character on the metal–oxo bond is critical to efficient OER. Then we consider replacing Fe with 17 other transition metals of the Fe, Ru, and Os rows, where we find 3 new promising...

  15. Seriatim ECSTM-ECPMIRS of the adsorption of carbon monoxide on Cu(100) in alkaline solution at CO_2-reduction potentials

    Baricuatro, Jack H.; Kim, Youn-Geun; Korzeniewski, Carol L.; Soriaga, Manuel P.
    It was recently demonstrated that the sequential or seriatim application of electrochemical scanning tunneling microscopy (ECSTM) and differential electrochemical mass spectrometry (DEMS) enables the correlation, under actual reaction conditions, of a specific structure on a Cu electrode surface with the generation of a particular CO-reduction product. As an extension of the operando hyphenated-technique approach, we paired ECSTM with electrochemical polarization-modulation IR reflection-absorption spectroscopy (ECPMIRS) to identify a delineating potential that affected the coverage, the molecular orientation, and the adlattice structure of CO adsorbed on Cu(100) in 0.1 M KOH under CO_2-reduction conditions. The results may have significant ramifications on the theory-based...

  16. Combinatorial Alloying Improves Bismuth Vanadate Photoanodes via Reduced Monoclinic Distortion

    Newhouse, P. F.; Guevarra, D.; Umehara, M.; Reyes-Lillo, S. E.; Zhou, L.; Boyd, D. A.; Suram, S. K.; Cooper, J. K.; Haber, J. A.; Neaton, J. B.; Gregoire, J. M.
    Improving the efficiency of solar-powered oxygen evolution is both critical for development of solar fuels technologies and challenging due to the broad set of properties required of a solar fuels photoanode. Bismuth vanadate, in particular the monoclinic clinobisvanite phase, has received substantial attention and has exhibited the highest radiative efficiency among metal oxides with a band gap in the visible range. Efforts to further improve its photoelectrochemical performance have included alloying one or more metals onto the Bi and/or V sites, with progress on this frontier stymied by the difficulty in computational modelling of substitutional alloys and the high dimensionality...

  17. Alkaline-stable nickel manganese oxides with ideal band gap for solar fuel photoanodes

    Suram, Santosh K.; Zhou, Lan; Shinde, Aniketa; Yan, Qimin; Yu, Jie; Umehara, Mitsutaro; Stein, Helge S.; Neaton, Jeffrey B.; Gregoire, John M.
    Combinatorial (photo)electrochemical studies of the (Ni–Mn)O_x system reveal a range of promising materials for oxygen evolution photoanodes. X-ray diffraction, quantum efficiency, and optical spectroscopy mapping reveal stable photoactivity of NiMnO_3 in alkaline conditions with photocurrent onset commensurate with its 1.9 eV direct band gap. The photoactivity increases upon mixture with 10–60% Ni_6MnO_8 providing an example of enhanced charge separation via heterojunction formation in mixed-phase thin film photoelectrodes. Density functional theory-based hybrid functional calculations of the band edge energies in this oxide reveal that a somewhat smaller than typical fraction of exact exchange is required to explain the favorable valence band...

  18. Sunlight-driven hydrogen formation by membrane-supported photoelectrochemical water splitting

    Lewis, Nathan S.
    We are developing an artificial photosynthetic system that will utilize sunlight and water as inputs and will produce hydrogen and oxygen as outputs using a modular, parallel development approach in which the three distinct primary components-the photoanode, the photocathode, and the product-sepg. but ion-conducting membrane-are fabricated and optimized sep. before assembly into a water-splitting system. The design principles incorporate two sep., photosensitive semiconductor/liq. junctions that will collectively generate the 1.7-1.9 Vat open circuit to support both the oxidn. of H2_O (or OH-) and the redn. of H+ (or H_2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multielectron transfer catalysts, needed...

  19. High throughput, multi-pH evaluation of earth-abundant pseudo-quaternary metal oxide catalysts for the oxygen evolution reaction

    Haber, Joel; Guevarra, Dan; Jones, R. J. R.; Kan, Kevin; Gregoire, John
    The development of efficient, stable photoanodes for solar fuels generators requires integration of a light absorbing semiconductor with an efficient oxygen evolution reaction catalyst. However, the abs. and relative performance of different OER catalyst compns. vary with electrolyte pH. The operational conditions for the solar fuel generators is often dictated by the pH stability range of the light absorber, not the conditions optimal for OER catalysis. We are evaluating the activity and stability against corrosion of dozens of comprehensive multi-metal oxide compn. spaces from pH 1-13 using high throughput electrochem. methods. This talk will describe these methods and characteristic results.

  20. Development of solar fuels photoanodes through combinatorial integration of multifunctional Fe-Ce oxide coatings on BiVO4 as a function of coating composition, loading, and electrolyte

    Haber, Joel; Guevarra, Dan; Shinde, Aniketa; Zhou, Lan; Toma, Francesca; Gregoire, John
    The development of an efficient, stable photoanode to provide protons and electrons to the (photo)cathode remains a primary materials challenge in the establishment of a scalable technol. for artificial photosynthesis. The typical photoanode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, hole transport, and elimination of deleterious surface recombination sites. To provide a more efficient exploration of metal oxide coatings for a given light absorber, we introduce a high throughput methodol. wherein a uniform BiVO4 library is coated with multi-metal oxide coatings. This presentation will focus on the variation in...

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