Nathan Keilbart

Bio: Nathan Keilbart is an academic researcher from Lawrence Livermore National Laboratory. The author has contributed to research in topics: Materials science & Hydrogen. The author has an hindex of 4, co-authored 13 publications receiving 82 citations. Previous affiliations of Nathan Keilbart include Brigham Young University–Idaho & Louisiana State University.

EPFR Formation from Phenol adsorption on Al2O3 and TiO2: EPR and EELS studies.

Abstract: We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50–100 h) and one slower-decaying species (lifetimes on the order of 1000 h or more). Electron energy loss spectroscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO2(1 1 0) to phenoxyl radicals on the same substrate indicate distinct changes in the π–π∗ transitions from phenol after radical formation. The identical shifts are observed from EELS studies of phenoxyl radicals on ultrathin alumina grown on NiAl(1 1 0), indicating that this shift in the π–π∗ transition may be taken as a general hallmark of phenoxyl radical formation.

Quantum-continuum simulation of the electrochemical response of pseudocapacitor electrodes under realistic conditions

Abstract: Pseudocapacitors are energy-storage devices characterized by fast and reversible redox reactions that enable them to store large amounts of electrical energy at high rates. We simulate the response of pseudocapacitive electrodes under realistic conditions to identify the microscopic factors that determine their performance, focusing on ruthenia $({\mathrm{RuO}}_{2})$ as a prototypical electrode material. Electronic-structure methods are used together with a self-consistent continuum solvation (SCCS) model to build a complete data set of free energies as the surface of the charged electrode is gradually covered with protons under applied voltage. The resulting data set is exploited to compute hydrogen-adsorption isotherms and charge-voltage responses by means of grand-canonical sampling, finding close agreement with experimental voltammetry. These simulations reveal that small changes on the order of $5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{F}/{\mathrm{cm}}^{2}$ in the intrinsic double-layer capacitance of the electrode-electrolyte interface can induce variations of up to $40\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{F}/{\mathrm{cm}}^{2}$ in the overall pseudocapacitance.

Magnesium- and intermetallic alloys-based hydrides for energy storage: modelling, synthesis and properties

Abstract: Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 ‘Energy Storage and Conversion based on Hydrogen’ of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group ‘Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage’. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage.

Koopmans-Compliant Self-Interaction Corrections

Abstract: Self-interaction is a central problem for the accuracy of density-functional approximations in describing the electronic structure of atoms and molecules In this work, we discuss the different types of self-interaction errors commonly encountered in density-functional calculations, providing precise definitions for each of them Based upon these definitions, we derive an orbital-dependent density-functional method, called the Koopmans-compliant approach, which simultaneously corrects the different self-interaction errors, by enforcing piecewise linearity with respect to fractional particle counts and by imposing the correct asymptotic behavior of the one-electron potential in approximate energy functionals We illustrate the very good performance of this new method in predicting the electronic properties of atoms and molecules, while preserving or improving the prediction of total energies and equilibrium geometries These results highlight the accuracy and efficiency of Koopmans-compliant functionals as an attractive solution to the self-interaction problem

Predicting the Pseudocapacitive Windows for MXene Electrodes with Voltage-Dependent Cluster Expansion Models

Abstract: MXene transition-metal carbides and nitrides are of growing interest for energy storage applications. These compounds are especially promising for use as pseudocapacitive electrodes due to their ab...

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials

Abstract: There is an urgent global need for electrochemical energy storage that includes materials that can provide simultaneous high power and high energy density One strategy to achieve this goal is with pseudocapacitive materials that take advantage of reversible surface or near-surface Faradaic reactions to store charge This allows them to surpass the capacity limitations of electrical double-layer capacitors and the mass transfer limitations of batteries The past decade has seen tremendous growth in the understanding of pseudocapacitance as well as materials that exhibit this phenomenon The purpose of this Review is to examine the fundamental development of the concept of pseudocapacitance and how it came to prominence in electrochemical energy storage as well as to describe new classes of materials whose electrochemical energy storage behavior can be described as pseudocapacitive

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

Abstract: The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

Self-interaction errors in density functional calculations of electronic transport

Abstract: All density-functional calculations of single-molecule transport to date have used continuous exchange-correlation approximations. The lack of derivative discontinuity in such calculations leads to the erroneous prediction of metallic transport for insulating molecules. A simple and computationally undemanding atomic self-interaction correction (SIC) opens conduction gaps in I-V characteristics that otherwise are predicted metallic, as in the case of the prototype Au/ditholated-benzene/Au junction.

Computational Insights into Materials and Interfaces for Capacitive Energy Storage.

Abstract: Supercapacitors such as electric double-layer capacitors (EDLCs) and pseudocapacitors are becoming increasingly important in the field of electrical energy storage. Theoretical study of energy storage in EDLCs focuses on solving for the electric double-layer structure in different electrode geometries and electrolyte components, which can be achieved by molecular simulations such as classical molecular dynamics (MD), classical density functional theory (classical DFT), and Monte-Carlo (MC) methods. In recent years, combining first-principles and classical simulations to investigate the carbon-based EDLCs has shed light on the importance of quantum capacitance in graphene-like 2D systems. More recently, the development of joint density functional theory (JDFT) enables self-consistent electronic-structure calculation for an electrode being solvated by an electrolyte. In contrast with the large amount of theoretical and computational effort on EDLCs, theoretical understanding of pseudocapacitance is very limited. In this review, we first introduce popular modeling methods and then focus on several important aspects of EDLCs including nanoconfinement, quantum capacitance, dielectric screening, and novel 2D electrode design; we also briefly touch upon pseudocapactive mechanism in RuO2. We summarize and conclude with an outlook for the future of materials simulation and design for capacitive energy storage.