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Chen, Chi

Publications and source records attributed to Chen, Chi.

45 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Accurate force field for molybdenum by machine learning large materials data

In this work, we present a highly accurate spectral neighbor analysis potential (SNAP) model for molybdenum (Mo) developed through the rigorous application of machine learning techniques on large materials data sets. Despite Mo's importance as a structural metal, existing force fields for Mo based on the embedded atom and modified embedded atom methods still do not provide satisfactory accuracy on many properties. We will show that by fitting to the energies, forces and stress tensors of a large density functional theory (DFT)-computed dataset on a diverse set of Mo structures, a Mo SNAP model can be developed that achieves close to DFT accuracy in the prediction of a broad range of properties, including energies, forces, stresses, elastic constants, melting point, phonon spectra, surface energies, grain boundary energies, etc. We will outline a systematic model development process, which includes a rigorous approach to structural selection based on principal component analysis, as well as a differential evolution algorithm for optimizing the hyperparameters in the model fitting so that both the model error and the property prediction error can be simultaneously lowered. We expect that this newly developed Mo SNAP model will find broad applications in large-scale, long-time scale simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Controlled synthesis of MnOOH multilayer nanowires as anode materials for lithium-ion batteries

MnOOH multilayer nanowires have been successfully synthesized by a hydrothermal method. It is found that the uniform multilayer structure of nanowires ran through the entire nanowire, which is formed via a layer by layer. The electrochemical properties of MnOOH multilayer nanowires as an anode material for Li-ion batteries (LIB) were investigated, and excellent capacity retention, superior cycling performance, and high rate capability were achieved. Specifically, the reversible capacity of MnOOH multilayer nanowires is 521 mAh/g after 500 cycles at 0.1 C, with excellent electrochemical stability. The multilayer nanowire electrodes exhibit short electron path lengths, high internal dislocation densities and large surface to volume ratio, resulting in increased specific capacity, cycling stability and rate performance in the energy storage devices, which serves as an indication of their potential application in LIBs. - Highlights: •MnOOH multilayer nanowires were synthesized by a hydrothermal method. •The uniform multilayer structure of nanowires was formed via layer by layer. •The reversible capacity of product shows 521 mAh/g after 500 cycles at 0.1 C. •MnOOH multilayer nanowires showed higher property as anode material in LIB.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Pseudocapacitance and excellent cyclability of 2,5-dimethoxy-1,4-benzoquinone on graphene

Electrochemically active organic materials are emerging as low cost, naturally abundant and sustainable alternatives to their metal-based counterparts. However, their usage in energy storage systems is mainly hindered by their poor conductivity, which results in capacitance fade upon cycling. In this paper, we present a redox-active xerogel composed of 2,5-dimethoxy-1,4-benzoquinone (DMQ) decorated on reduced graphene oxide (rGO) sheets via a hydrothermal method as a high capacitance and long cycle life pseudocapacitive electrode. DMQ not only provided stable redox-active centers but also served as a spacer to avoid rGO sheets aggregation and led to a three-dimensional (3D) hierarchical electrode architecture. When a binder-free 50 μm thick rolled film was tested as a pseudocapacitive electrode, it exhibited an excellent capacitance of 650 F g -1 at 5 mV s -1 (780 F cm -3 ) in 1 M sulfuric acid, outperforming a large number of reported organic and inorganic electrodes. Most importantly, optimized electrodes showed an excellent capacitance retention of 99% after 25 000 cycles at 50 mV s -1 . Density functional theory (DFT) calculations are further used to understand the charge storage mechanism, the preferred orientation of the adsorbed molecules, charge density distribution and density of states. In conclusion, our combined experimental and theoretical findings demonstrate that the careful selection of the conductive substrate, electrode architecture and organic molecules plays a crucial role in achieving high capacitance and long cycling performance.

25 ENERGY STORAGE↗
DOE OSTI2014

Lithium-decorated oxidized graphyne for hydrogen storage by first principles study

The geometric stability and hydrogen storage capacity of Li decorated oxidized γ-graphyne are studied based on the first-principles calculations. It is found that oxygen atoms trend to bond with acetylenic carbons and form C=O double bonds on both sides of graphyne. The binding energy of single Li atom on oxidized graphyne is 3.29 eV, owning to the strong interaction between Li atom and O atom. Meanwhile, the dispersion of Li is stable even under a relatively high density. One attached Li atom can at least adsorb six hydrogen molecules around. Benefitting from the porous structure of graphyne and the high attached Li density, a maximum hydrogen storage density 12.03 wt. % is achieved with four Li atoms in graphyne cell. The corresponding average binding energy is 0.24 eV/H{sub 2}, which is suitable for reversible storage. These results indicate that Li decorated graphyne can serve as a promising hydrogen storage material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Toward epitaxially grown two-dimensional crystal hetero-structures: Single and double MoS{sub 2}/graphene hetero-structures by chemical vapor depositions

Uniform large-size MoS{sub 2}/graphene hetero-structures fabricated directly on sapphire substrates are demonstrated with layer-number controllability by chemical vapor deposition (CVD). The cross-sectional high-resolution transmission electron microscopy (HRTEM) images provide the direct evidences of layer numbers of MoS{sub 2}/graphene hetero-structures. Photo-excited electron induced Fermi level shift of the graphene channel are observed on the single MoS{sub 2}/graphene hetero-structure transistors. Furthermore, double hetero-structures of graphene/MoS{sub 2}/graphene are achieved by CVD fabrication of graphene layers on top of the MoS{sub 2}, as confirmed by the cross-sectional HRTEM. These results have paved the possibility of epitaxially grown multi-hetero-structures for practical applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Synergistic effect of V/N codoping by ion implantation on the electronic and optical properties of TiO{sub 2}

Performance of the material depends directly on the electronic and energy band structure, to improve the photoactivity of TiO{sub 2} and decrease carrier recombination centers induced by monodoping, the TiO{sub 2} thin film has been modified with V and N codopants by ion implantation for tailing and controlling the electronic structure and energy band structure. Compared to monodopant, codopants of V and N exhibit a synergistic effect in the photoactivity enhancement of TiO{sub 2}. X-ray photoelectron spectroscopy (XPS) studies demonstrate that the implanted V and N ions are introduced into the lattice of TiO{sub 2} through V and N substituting Ti and O, respectively. The electronic structure of V/N codoped TiO{sub 2} was calculated by First-principles calculations based on density-functional theory, the results show the band edges of TiO{sub 2} can be tailored by V and N codopants. UV-vis spectra consistently show the absorption edge of V/N codoped TiO{sub 2} film is widen to visible light region. More importantly, the photoactivity of TiO{sub 2} film has been significantly improved after V/N codoping. The enhanced photocatalytic performance is believed to be due to the V and N codopants induced synergistic effect that not only enhances the absorption of visible light but also promotes the separation of photogenerated electrons and holes in TiO{sub 2}. Besides, there exists an optimum for V/N ions implantation fluence. The capability of improving TiO{sub 2} photoactivity by V/N codoping could open up new opportunities in the development of highly efficient photocatalysts and photoelectrodes for solar energy and environmental applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Frequency spectroscopy of irreversible electrochemical nucleation kinetics

An approach is developed for probing the thermodynamics and kinetics of the irreversible electrochemical reactions on solid surfaces based on frequency-voltage spectroscopy. For a model Li-ion conductor surface, the two regimes for bias-controlled behavior are demonstrated and ascribed to the difference in the critical nucleus size. The electrostatic and electrochemical phenomena at tip-surface junction are analyzed. These studies suggest an experimental pathway for exploring local electrochemical activity in solids.

Kumar, Amit↗