Recursos de colección

Caltech Authors (143.226 recursos)

Repository of works by Caltech published authors.

Group = JCAP

Mostrando recursos 1 - 20 de 169

  1. Embedded Mean-Field Theory with Block-Orthogonalized Partitioning

    Ding, Feizhi; Manby, Frederick R.; Miller, Thomas F., III
    Embedded mean-field theory (EMFT) provides a simple, flexible framework for describing subsystems at different levels of mean-field theory. Subsystems are defined by partitioning a one-particle basis set, with a natural choice being the atomic orbital (AO) basis. Although generally well behaved, EMFT with AO partitioning can exhibit unphysical collapse of the self-consistent solution. To avoid this issue, we introduce subsystem partitioning of a block-orthogonalized (BO) basis set; this eliminates the unphysical collapse without significantly increasing computational cost. We also investigate a non-self-consistent implementation of EMFT, in which the density matrix is obtained using BO partitioning and the final energy evaluated...

  2. Evaluation of flow schemes for near-neutral pH electrolytes in solar-fuel generators

    Singh, Meenesh R.; Xiang, Chengxiang; Lewis, Nathan S.
    The electrochemical performance of three different types of membrane-containing electrolyte-flow schemes for solar-driven water splitting has been studied quantitatively using 1-dimensional and 2-dimensional multi-physics models. The three schemes include a recirculation scheme with a well-mixed bulk electrolyte, a recirculation scheme with laminar flow fields, and a fresh-feed scheme with laminar flow fields. The Nernstian potential loss associated with pH gradients at the electrode surfaces, the resistive loss between the cathode and anode, the product-gas crossovers, and the required pumping energy in all three schemes have been evaluated as a function of the operational current density, the flow rates for the...

  3. Reliable Performance Characterization of Mediated Photocatalytic Water-Splitting Half Reactions

    Han, Lihao; Lin, Meng; Haussener, Sophia
    Photocatalytic approaches using two sets of semiconductor particles and a pair of redox-shuttle mediators are considered as a safe and economic solution for solar water splitting. Here, accurate experimental characterization techniques for photocatalytic half reactions are reported, investigating the gas as well as the liquid products. The methods are exemplified utilizing photocatalytic titania particles in an iron-based aqueous electrolyte for effective oxygen evolution and mediator reduction reactions under illumination. Several product characterization methods, including an optical oxygen sensor, pressure sensor, gas chromatography, and UV/Vis spectroscopy are used and compared for accurate, high-resolution gas-products and mediator conversion measurements. Advantages of each...

  4. Grand canonical electronic density-functional theory: Algorithms and applications to electrochemistry

    Sundararaman, Ravishankar; Goddard, William A., III; Arias, Tomas A.
    First-principles calculations combining density-functional theory and continuum solvation models enable realistic theoretical modeling and design of electrochemical systems. When a reaction proceeds in such systems, the number of electrons in the portion of the system treated quantum mechanically changes continuously, with a balancing charge appearing in the continuum electrolyte. A grand-canonical ensemble of electrons at a chemical potential set by the electrode potential is therefore the ideal description of such systems that directly mimics the experimental condition. We present two distinct algorithms: a self-consistent field method and a direct variational free energy minimization method using auxiliary Hamiltonians (GC-AuxH), to solve...

  5. Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment

    Yan, Qimin; Yu, Jie; Suram, Santosh K.; Zhou, Lan; Shinde, Aniketa; Newhouse, Paul F.; Chen, Wei; Li, Guo; Persson, Kristin A.; Gregoire, John M.; Neaton, Jeffrey B.
    The limited number of known low-band-gap photoelectrocatalytic materials poses a significant challenge for the generation of chemical fuels from sunlight. Using high-throughput ab initio theory with experiments in an integrated workflow, we find eight ternary vanadate oxide photoanodes in the target band-gap range (1.2–2.8 eV). Detailed analysis of these vanadate compounds reveals the key role of VO_4 structural motifs and electronic band-edge character in efficient photoanodes, initiating a genome for such materials and paving the way for a broadly applicable high-throughput-discovery and materials-by-design feedback loop. Considerably expanding the number of known photoelectrocatalysts for water oxidation, our study establishes ternary metal...

  6. Experimental and Ab Initio Ultrafast Carrier Dynamics in Plasmonic Nanoparticles

    Brown, Ana M.; Sundararaman, Ravishankar; Narang, Prineha; Schwartzberg, Adam M.; Goddard, William A., III; Atwater, Harry A.
    Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our...

  7. Full atomistic reaction mechanism with kinetics for CO reduction on Cu(100) from ab initio molecular dynamics free-energy calculations at 298 K

    Cheng, Tao; Xiao, Hai; Goddard, William A., III
    A critical step toward the rational design of new catalysts that achieve selective and efficient reduction of CO_2 to specific hydrocarbons and oxygenates is to determine the detailed reaction mechanism including kinetics and product selectivity as a function of pH and applied potential for known systems. To accomplish this, we apply ab initio molecular metadynamics simulations (AIMμD) for the water/Cu(100) system with five layers of the explicit solvent under a potential of −0.59 V [reversible hydrogen electrode (RHE)] at pH 7 and compare with experiment. From these free-energy calculations, we determined the kinetics and pathways for major products (ethylene and...

  8. Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

    Nellist, Michael R.; Chen, Yikai; Mark, Andreas; Gödrich, Sebastian; Stelling, Christian; Jiang, Jingjing; Poddar, Rakesh; Li, Chunzeng; Kumar, Ravi; Papastavrou, Georg; Retsch, Markus; Brunschwig, Bruce S.; Huang, Zhuangqun; Xiang, Chengxiang; Boettcher, Shannon W.
    Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ~200 nm in height and of ~25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended...

  9. Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu_2O

    Omelchenko, Stefan T.; Tolstova, Yulia; Atwater, Harry A.; Lewis, Nathan S.
    Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu_2O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu_2O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu_2O-based photovoltaics. In the case of Cu_2O, the free carrier model underestimates the...

  10. Ray trace optimization of a light trapping filtered concentrator for spectrum splitting photovoltaics

    Lloyd, John V.; Kosten, Emily D.; Warmann, Emily C.; Flowers, Cristofer A.; Atwater, Harry A.
    A ray trace model of the light trapping filtered concentrator spectrum splitting architecture is presented. The scripted ray trace allows for examination of non-idealities in materials and design that were not addressed in previous analytical investigations of this optical design. The design of the angle restricting elements is examined with regards to optical efficiency and system efficiency. In addition, the scripted ray trace enables rapid evaluation of multiple candidate filter sets and optimization of the optical design for each set via a gradient ascent algorithm. A discussion of filter design considerations and insight provided by the ray trace model evaluations...

  11. Synthesis and Characterization of Two-Dimensional Conjugated Polymers Incorporating Electron-Deficient Moieties for Application in Organic Photovoltaics

    Hsiow, Chuen-Yo; Wang, Han-Ying; Lin, Yu-Hsiang; Raja, Rathinam; Rwei, Syang-Peng; Chiu, Wen-Yen; Dai, Chi-An; Wang, Leeyih
    A series of novel p-type conjugated copolymers, PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP, cooperating benzo[1,2-b:4,5-b′]dithiophene (BDT) and terthiophene-vinylene (TTV) units with/without thieno[3,4-c]pyrrole-4,6-dione (TPD) or pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) via Stille polymerization were synthesized and characterized. Copolymer PTTVBDT shows a low-lying HOMO energy level and ordered molecular-packing behavior. Furthermore, two terpolymers, PTTVBDT-TPD and PTTVBDT-DPP, display stronger absorption ability, alower-lying HOMO energy level, and preferred molecular orientation, due to the replacement TTV-monomer units with electron-deficient groups. Furthermore, bulk-heterojunction organic solar cells were fabricated using blends of the PTTVBDT-TPD, and PC_(61)BM gave the best power conversion efficiency of 5.01% under the illumination of AM 1.5G, 100 mW·cm^(−2);...

  12. Mechanism and kinetics of the electrocatalytic reaction responsible for the high cost of hydrogen fuel cells

    Cheng, Tao; Goddard, William A., III; An, Qi; Xiao, Hai; Merinov, Boris; Morozov, Sergey
    The sluggish oxygen reduction reaction (ORR) is a major impediment to the economic use of hydrogen fuel cells in transportation. In this work, we report the full ORR reaction mechanism for Pt(111) based on Quantum Mechanics (QM) based Reactive metadynamics (RμD) simulations including explicit water to obtain free energy reaction barriers at 298 K. The lowest energy pathway for 4 e^− water formation is: first, *OOH formation; second, *OOH reduction to H_2O and O*; third, O* hydrolysis using surface water to produce two *OH and finally *OH hydration to water. Water formation is the rate-determining step (RDS) for potentials above...

  13. An Operando Investigation of (Ni-Fe-Co-Ce)O_x System as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

    Favaro, Marco; Drisdell, Walter S.; Marcus, Matthew A.; Gregoire, John M.; Crumlin, Ethan J.; Haber, Joel A.; Yano, Junko
    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 developing renewable energy field of solar-fuels generation by providing both the protons and electrons needed to generate fuels such as H_2 or reduced hydrocarbons from CO_2. 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 novel electrocatalysts with high activity and durability. In this context, performing experimental investigations of the electrocatalysts under realistic working regimes,...

  14. Nucleation of Graphene Layers On Magnetic Oxides: Co_3O_4(111) and Cr_2O_3(0001) from Theory and Experiment

    Beatty, John; Cheng, Tao; Cao, Yuan; Driver, M. Sky; Goddard, William A., III; Kelber, Jeffry A.
    We report direct grown strongly adherent graphene on Co_3O_4(111) by Carbon molecular beam epitaxy (MBE) at 850 K and DFT findings that the first graphene layer is reconstructed to fit the Co_3O_4 surface, while subsequent layers retain normal graphene structure. This adherence to the Co_3O_4 structure results from partial bonding of half the carbons to top oxygen of the substrate. This structure is validated by X-ray photoelectron spectroscopy and low energy electron diffraction studies, showing layer-by-layer graphene growth with ~ 0.08 electrons/carbon atom transferred to the oxide from the first graphene layer, in agreement with DFT. In contrast, C MBE...

  15. The Reaction Mechanism with Free Energy Barriers at Constant Potentials for the Oxygen Evolution Reaction at the IrO_2 (110) Surface

    Ping, Yuan; Nielsen, Robert J.; Goddard, William A.
    How to efficiently oxidize H_2O to O_2 (Oxygen Evolution Reaction –OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO_2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work we performed first principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the...

  16. Photon and carrier management design for nonplanar thin-film copper indium gallium selenide photovoltaics

    Bukowsky, Colton R.; Grandidier, Jonathan; Fountaine, Katherine T.; Callahan, Dennis M.; Stanbery, Billy J.; Atwater, Harry A.
    Nonplanar structured photovoltaic absorber design has potential to achieve high solar cell efficiency with significantly reduced material use. We report optoelectronic simulations that highlight photon and generated carrier management opportunities for improvement of thin film Cu(In_xGa_(1−x))Se_2 (CIGS) device performance. Structures realized via either self-assembly or patterning via nanoimprint lithography, and also a combination of both are predicted to exhibit significant increases in short circuit current density and open circuit voltage simultaneously. The structures investigated include: 1) self-assembled nonplanar structures that strongly scatter incident light and enhance carrier generation near regions of high electric potential, 2) lithographically-patterned embedded periodic dielectric structures,...

  17. Efficiency limits for photoelectrochemical water-splitting

    Fountaine, Katherine T.; Lewerenz, Hans Joachim; Atwater, Harry A.
    Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community’s focus on detailed balance. However, due to their multicomponent nature, photoelectrochemical devices do not have an equivalent analogue to detailed balance, and reported theoretical efficiency limits vary depending on the assumptions made. Here we introduce a unified framework for photoelectrochemical device performance through which all previous limiting efficiencies can be understood and contextualized. Ideal and experimentally realistic limiting efficiencies are presented, and then generalized using five representative parameters—semiconductor absorption fraction, external radiative efficiency, series resistance, shunt resistance and...

  18. Atomistic Mechanisms Underlying Selectivities in C_1 and C_2 Products from Electrochemical Reduction of CO on Cu(111)

    Xiao, Hai; Cheng, Tao; Goddard, William A., III
    Practical environmental and energy applications of the electrochemical reduction of CO_2 to chemicals and fuels require far more efficient and selective electrocatalysts beyond the only working material Cu, but the wealth of experimental data on Cu can serve to validate any proposed mechanisms. To provide design guidelines, we use quantum mechanics to predict the detailed atomistic mechanisms responsible for C_1 and C_2 products on Cu. Thus, we report the pH dependent routes to the major products, methane and ethylene, and identify the key intermediates where branches to methanol, ketene, ethanol, acetylene and ethane are kinetically blocked. We discovered that surface...

  19. Gastight Hydrodynamic Electrochemistry: Design for a Hermetically Sealed Rotating Disk Electrode Cell

    Jung, Suho; Kortlever, Ruud; Jones, Ryan J. R.; Lichterman, Michael F.; Agapie, Theodor; McCrory, Charles C. L.; Peters, Jonas C.
    Rotating disk electrodes (RDEs) are widely used in electrochemical characterization to analyze the mechanisms of various electrocatalytic reactions. RDE experiments often make use of or require collection and quantification of gaseous products. The combination of rotating parts and gaseous analytes makes the design of RDE cells that allow for headspace analysis challenging due to gas leaks at the interface of the cell body and the rotator. In this manuscript we describe a new, hermetically-sealed electrochemical cell that allows for electrode rotation while simultaneously providing a gastight environment. Electrode rotation in this new cell design is controlled by magnetically coupling the...

  20. Developing a scalable artificial photosynthesis technology through nanomaterials by design

    Lewis, Nathan S.
    An artificial photosynthetic system that directly produces fuels from sunlight could provide an approach to scalable energy storage and a technology for the carbon-neutral production of high-energy-density transportation fuels. A variety of designs are currently being explored to create a viable artificial photosynthetic system, and the most technologically advanced systems are based on semiconducting photoelectrodes. Here, I discuss the development of an approach that is based on an architecture, first conceived around a decade ago, that combines arrays of semiconducting microwires with flexible polymeric membranes. I highlight the key steps that have been taken towards delivering a fully functional solar...

Aviso de cookies: Usamos cookies propias y de terceros para mejorar nuestros servicios, para análisis estadístico y para mostrarle publicidad. Si continua navegando consideramos que acepta su uso en los términos establecidos en la Política de cookies.