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Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 160

  1. 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 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 candidates: Co, Rh, and Ir,...

  2. 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...

  3. 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.; Haber, J. A.; Neaton, J. B.; Gregoire, J. M.
    Improving the efficiency of solar-power 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...

  4. 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...

  5. 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...

  6. 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.

  7. 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...

  8. Accelerated experimental materials discovery through integration with theory and artificial intelligence

    Gregoire, John
    Solar photochem. is a promising energy technol. limited by the performance and compatibility of its component materials. A variety of approaches for discovering solar fuels materials have been and continue to be deployed, with photoactive components proving to be particularly difficult to identify due to their need to simultaneously exhibit a broad range of chem. and phys. properties. The development of high throughput screening techniques in the Joint Center for Artificial Photosynthesis has opened new avenues for materials discovery with unique challenges and opportunities. Most notably, rapid synthesis and screening enables evaluation of candidate materials proposed by high throughput computational screening, and some expts. produce datasets that...

  9. Surface Reconstruction of Polycrystalline Cu Electrodes in Aqueous KHCO_3 Electrolyte at Potentials in the Early Stages of CO_2 Reduction

    Kim, Youn-Geun; Baricuatro, Jack H.; Soriaga, Manuel P.
    The reconstruction of the Cu(pc) polycrystalline surface at potentials that correspond to the early stages of CO_2 reduction in 0.1 M KHCO_3 was investigated by electrochemical scanning tunneling microscopy (ECSTM) at −0.90 V (SHE). A kinetically hindered surface reconstruction of the topmost layers of Cu(pc) into the (100) face was observed, reminiscent of the transformation previously reported at the same electrode potential in 0.1 M KOH. Evidently, the same reconstructed surface, Cu(pc)-[Cu(100)], can be generated in either 0.1 M KHCO_3 (pH 8) or 0.1 M KOH (pH 13). In addition, only minimal structural disruption was observed when the reconstructed surface...

  10. Performance and failure modes of Si anodes patterned with thin-film Ni catalyst islands for water oxidation

    Sun, Ke; Ritzert, Nicole L.; John, Jimmy; Tan, Haiyan; Hale, William G.; Jiang, Jingjing; Moreno-Hernandez, Ivan; Papadantonakis, Kimberly M.; Moffat, Thomas P.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Silicon photoanodes patterned with thin-film Ni catalyst islands exhibited stable oxygen evolution for over 240 h of continuous operation in 1.0 mol L^(−1) KOH under simulated sunlight conditions. Buried-junction np^+-Si(111) photoanodes with an 18.0% filling fraction of a square array of Ni microelectrodes, np^+-Si(111)|NiμE_(18.0%), demonstrated performance equivalent to a Ni anode in series with a photovoltaic device having an open-circuit voltage of 538 ± 20 mV, a short-circuit current density of 20.4 ± 1.3 mA cm^(−2), and a photovoltaic efficiency of 6.7 ± 0.9%. For the np^+-Si(111)|NiμE_(18.0%) samples, the photocurrent density at the equilibrium potential for oxygen evolution was 12.7...

  11. Dual-Gated Active Metasurface at 1550 nm with Wide (>300°) Phase Tunability

    Shirmanesh, Ghazaleh Kafaie; Sokhoyan, Ruzan; Pala, Ragip A.; Atwater, Harry A.
    Active metasurfaces composed of electrically reconfigurable nanoscale subwavelength antenna arrays can enable real-time control of scattered light amplitude and phase. Achievement of widely tunable phase and amplitude in chip-based active metasurfaces operating at or near 1550 nm wavelength has considerable potential for active beam steering, dynamic hologram rendition, and realization of flat optics with reconfigurable focal lengths. Previously, electrically tunable conducting oxide-based reflectarray metasurfaces have demonstrated dynamic phase control of reflected light with a maximum phase shift of 184° ( Nano Lett. 2016, 16, 5319). Here, we introduce a dual-gated reflectarray metasurface architecture that enables much wider (>300°) phase tunability....

  12. Charge transport in organic molecular semiconductors from first principles: The bandlike hole mobility in a naphthalene crystal

    Lee, Nien-En; Zhou, Jin-Jian; Agapito, Luis A.; Bernardi, Marco
    Predicting charge transport in organic molecular crystals is notoriously challenging. Carrier mobility calculations in organic semiconductors are dominated by quantum chemistry methods based on charge hopping, which are laborious and only moderately accurate. We compute from first principles the electron-phonon scattering and the phonon-limited hole mobility of naphthalene crystal in the framework of ab initio band theory. Our calculations combine GW electronic bandstructures, ab initio electron-phonon scattering, and the Boltzmann transport equation. The calculated hole mobility is in very good agreement with experiment between 100 – 300 K, and we can predict its temperature dependence with high accuracy. We show...

  13. A novel preparation method for ZnO/γ-Al_2O_3 nanofibers with enhanced absorbability and improved photocatalytic water-treatment performance by Ag nanoparticles

    Cheng, Zhiqiang; Zhao, Shengzhe; Han, Lihao
    A novel method for synthesizing ZnO/γ-Al_2O_3 nanofibers by electrospinning and subsequent calcination is reported. The prepared nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The ZnO/γ-Al_2O_3 nanofibers exhibited excellent capacity for adsorbing organics with a negative zeta potential such as methyl orange (95.8%) and heavy metal ions such as Cr(VI) in aqueous solution. The mechanism of adsorption was investigated, and the adsorption results were fitted using the Langmuir and Freundlich models. Once silver nanoparticles (Ag NPs) were decorated on the surface of the nanofibers...

  14. Hot Hole Collection and Photoelectrochemical CO_2 Reduction with Plasmonic Au/p-GaN Photocathodes

    DuChene, Joseph S.; Tagliabue, Giulia; Welch, Alex J.; Cheng, Wen-Hui; Atwater, Harry A.
    Harvesting nonequilibrium hot carriers from plasmonic-metal nanostructures offers unique opportunities for driving photochemical reactions at the nanoscale. Despite numerous examples of hot electron-driven processes, the realization of plasmonic systems capable of harvesting hot holes from metal nanostructures has eluded the nascent field of plasmonic photocatalysis. Here, we fabricate gold/p-type gallium nitride (Au/p-GaN) Schottky junctions tailored for photoelectrochemical studies of plasmon-induced hot-hole capture and conversion. Despite the presence of an interfacial Schottky barrier to hot-hole injection of more than 1 eV across the Au/p-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochemical properties consistent with the injection of hot holes from Au nanoparticles...

  15. Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO_2 Reduction

    Chapovetsky, Alon; Welborn, Matthew; Luna, M.; Haiges, Ralf; Miller, Thomas F., III; Marinescu, Smaranda C.
    The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO_2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO_2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO_2, and the number of pendant secondary amines, respectively. Density functional...

  16. High Rate Electrochemical Reduction of Carbon Monoxide to Ethylene using Cu-Nanoparticle-Based Gas Diffusion Electrodes

    Han, Lihao; Zhou, Wu; Xiang, Chengxiang
    Gas diffusion electrodes (GDEs) with high electrochemically active surface areas (ECSAs) and triple-phase boundaries for efficient gas, electron, and ion transport offer a unique opportunity for high-rate electrochemical CO reduction (COR) in relative to traditional aqueous configurations. Cu-nanoparticle-based GDEs were fabricated by applying a mixture of carbon powders, copper acetate aqueous solution, and Teflon onto a Cu gauze substrate. The catalyst-coated substrate was air-dried, mechanically pressed, and subsequently annealed under forming gas to produce GDEs. Two distinctive types of GDE configurations, a flow-through configuration and a flow-by configuration, were constructed, characterized, and tested to quantitatively evaluate the effects of reactant...

  17. Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense Co–P Catalyst Films on Structured p-Si Photocathodes

    Kempler, Paul A.; Gonzalez, Miguel A.; Papadantonakis, Kimberly M.; Lewis, Nathan S.
    Planar and three-dimensionally structured p-Si devices, consisting of an electrodeposited Co–P catalyst on arrays of Si microwires or Si micropyramids, were used as photocathodes for solar-driven hydrogen evolution in 0.50 M H_2SO_4(aq) to assess the effects of electrode structuring on parasitic absorption by the catalyst. Without the use of an emitter layer, p-Si/Co–P microwire arrays produced a photocurrent density of −10 mA cm^(–2) at potentials that were 130 mV more positive than those of optimized planar p-Si/Co–P devices. Champion p-Si/Co–P microwire array devices exhibited ideal regenerative cell solar-to-hydrogen efficiencies of >2.5% and were primarily limited by the photovoltage of the...

  18. Relative costs of transporting electrical and chemical energy

    Saadi, Fadl H.; Lewis, Nathan S.; McFarland, Eric W.
    Transportation costs of energy resources are important when determining the overall economics of future energy infrastructure. The majority of long distance energy transmission occurs via merchant ships and pipelines carrying oil or natural gas. In contrast, future energy scenarios often envision vastly altered energy transportation scenarios including very high degrees of grid electrification and widespread installation of hydrogen pipelines. The unit cost of energy transportation varies by over two orders of magnitude. In particular, the costs of electricity and hydrogen transmission are substantially higher than the cost of oil and natural gas transportation. If carbon pricing is to be used...

  19. Effects of Electrolyte Buffer Capacity on Surface Reactant Species and Reaction Rate of CO_2 in Electrochemical CO_2 Reduction

    Hashiba, Hiroshi; Weng, Lien-Chun; Chen, Yikai; Sato, Hiroki K.; Yotsuhashi, Satoshi; Xiang, Chengxiang; Weber, Adam Z.
    In the aqueous electrochemical reduction of CO_2, the choice of electrolyte is responsible for the catalytic activity and selectivity, although there remains a need for more in-depth understanding of electrolyte effects and mechanisms. In this study, using both experimental and simulation approaches, we report how the buffer capacity of the electrolytes affects the kinetics and equilibrium of surface reactant species and resulting reaction rate of CO_2 with varying partial CO_2 pressure. Electrolytes investigated include KCl (non-buffered), KHCO3 (buffered by bicarbonate), and phosphate buffered electrolytes. Assuming 100% methane production, the simulation successfully explains the experimental trends of maximum CO_2 flux in...

  20. Pb-activated Amine-assisted Photocatalytic Hydrogen Evolution Reaction on Organic-Inorganic Perovskites

    Wang, Lu; Xiao, Hai; Cheng, Tao; Li, Youyong; Goddard, William A., III
    We report here the reaction mechanism for explicit aqueous solvent quantum mechanics (QM) studies determining the energetics and reaction barriers for the photocatalytic hydrogen evolution reaction (HER) on CH_3NH_3PbI_3 surface. We find that both the lead (Pb) atoms and the surface organic molecules play essential roles, leading to a two-step Pb-activated amine-assisted (PbAAA) reaction mechanism involving an intermediate lead hydride state. Both H of H_2 product are extracted from surface organic molecules, while two protons from the solution migrate along water chains via the Grotthuss mechanism to replace the H in organic molecule. We obtain a reaction barrier of 1.08...

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