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Wu, Feng

Publications and source records attributed to Wu, Feng.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Spin-phonon relaxation from a universal ab initio density-matrix approach

Designing new quantum materials with long-lived electron spin states urgently requires a general theoretical formalism and computational technique to reliably predict intrinsic spin relaxation times. We present a new, accurate and universal first-principles methodology based on Lindbladian dynamics of density matrices to calculate spin-phonon relaxation time of solids with arbitrary spin mixing and crystal symmetry. This method describes contributions of Elliott-Yafet and D’yakonov-Perel’ mechanisms to spin relaxation for systems with and without inversion symmetry on an equal footing. We show that intrinsic spin and momentum relaxation times both decrease with increasing temperature; however, for the D’yakonov-Perel’ mechanism, spin relaxation time varies inversely with extrinsic scattering time. We predict large anisotropy of spin lifetime in transition metal dichalcogenides. The excellent agreement with experiments for a broad range of materials underscores the predictive capability of our method for properties critical to quantum information science.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Optimization of Oxygen Electrode Combined with Soluble Catalyst to Enhance the Performance of Lithium-Oxygen Battery

Lithium-air batteries possess a high theoretical energy density among the current battery technologies, and are expected to become the driving power for electric vehicles. However, the practical application of lithium air batteries is handicapped by their poor cycle life, high over-charge potential and low energy efficiency. Numerous studies have performed to improve the kinetics of the electrochemical reaction. Making a good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalyst system is under intense scrutiny. In this work, we report a rational design of air electrode, which derives from biomass and presents a three-dimensional porous carbon architecture with high specific surface and large volume expansion coefficient. Combining with a soluble catalyst, this oxygen electrode presents an ultrahigh areal specific capacity of 7.95 mAh/cm 2 and a long cycling life up to 1000 cycles with suppressed polarization (about 0.46 V). Finally, the greatly improved overall performance is due to the synergetic effect of the novel designed oxygen electrode capable of mediating the redox mediators to trigger the ORR and OER process.

25 ENERGY STORAGE↗
DOE OSTI2020

Carbon doping switching on the hydrogen adsorption activity of NiO for hydrogen evolution reaction

Hydrogen evolution reaction (HER) is more sluggish in alkaline than in acidic media because of the additional energy required for water dissociation. Numerous catalysts, including NiO, that offer active sites for water dissociation have been extensively investigated. Yet, the overall HER performance of NiO is still limited by lacking favorable H adsorption sites. Here we show a strategy to activate NiO through carbon doping, which creates under-coordinated Ni sites favorable for H adsorption. DFT calculations reveal that carbon dopant decreases the energy barrier of Heyrovsky step from 1.17 eV to 0.81 eV, suggesting the carbon also serves as a hot-spot for the dissociation of water molecules in water-alkali HER. As a result, the carbon doped NiO catalyst achieves an ultralow overpotential of 27 mV at 10 mA cm -2 , and a low Tafel slope of 36 mV dec -1 , representing the best performance among the state-of-the-art NiO catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Carrier recombination mechanism at defects in wide band gap two-dimensional materials from first principles

The identification and design of defects in two-dimensional (2D) materials as promising single photon emitters (SPEs) requires a deep understanding of the underlying carrier recombination mechanisms. Yet, the dominant mechanism of carrier recombination at defects in 2D materials has not been well understood, and some outstanding questions remain: How do recombination processes at defects differ between 2D and 3D systems? What factors determine defects in 2D materials as excellent SPEs at room temperature? In order to address these questions, we developed first-principles methods to accurately calculate the radiative and nonradiative recombination rates at defects in 2D materials, using h-BN as a prototypical example. In this work, we reveal the carrier recombination mechanism at defects in 2D materials being mostly dominated by defect-defect state recombination in contrast to defect-bulk state recombination in most 3D semiconductors. In particular, we disentangle the nonradiative recombination mechanism into key physical quantities: the zero-phonon line and Huang-Rhys factor. At the end, we identified that strain can effectively tune the electron-phonon coupling at defect centers and drastically change the nonradiative recombination rates. Our theoretical development serves as a general platform for understanding carrier recombination at defects in 2D materials, while providing pathways for engineering of quantum efficiency of SPEs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Improving the reversibility of the H2-H3 phase transitions for layered Ni-rich oxide cathode towards retarded structural transition and enhanced cycle stability

While the layered Ni-rich LiNi x Co y Mn 1-x-y O 2 (0.7 + ions delithiated during charging will leave the structure of the layered Ni-rich materials more vulnerable. In this report, we propose that improving the reversibility of H2-H3 phase transition for Ni-rich materials is effective to tackle this challenge. It has been confirmed that the generation of microcracks and structural transformations have been suppressed since the H2-H3 phase transition becomes reversible, while which shows little effect on capacity delivery. Consequently, using Ni-rich LiNi 0.9 Co 0.1 O 2 as the cathode material, the 100 th capacity retention cycling at 38mAg -1 has been improved remarkably from 69.7% to 97.9% by adopting this strategy. Hence, it should be a novel solution to realize both high capacity and stable cyclability for the Ni-rich cathodes.

25 ENERGY STORAGE↗
DOE OSTI2019

Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media

Hydrogen evolution reaction is an important process in electrochemical energy technologies. Herein, ruthenium and nitrogen codoped carbon nanowires are prepared as effective hydrogen evolution catalysts. The catalytic performance is markedly better than that of commercial platinum catalyst, with an overpotential of only –12 mV to reach the current density of 10 mV cm -2 in 1 M KOH and –47 mV in 0.1 M KOH. Comparisons with control experiments suggest that the remarkable activity is mainly ascribed to individual ruthenium atoms embedded within the carbon matrix, with minimal contributions from ruthenium nanoparticles. Consistent results are obtained in first-principles calculations, where RuC x N y moieties are found to show a much lower hydrogen binding energy than ruthenium nanoparticles, and a lower kinetic barrier for water dissociation than platinum. Among these, RuC 2 N 2 stands out as the most active catalytic center, where both ruthenium and adjacent carbon atoms are the possible active sites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Electrochemically activated spinel manganese oxide for rechargeable aqueous aluminum battery

Aluminum is a naturally abundant, trivalent charge carrier with high theoretical specific capacity and volumetric energy density, rendering aluminum-ion batteries a technology of choice for future large-scale energy storage. Nonetheless, the frequent collapse of the host structure of the cathode materials and sluggish kinetics of aluminum ion diffusion have thus far hampered the realization of practical battery devices. Here, we synthesize Al x MnO 2 •nH 2 O by an in-situ electrochemical transformation reaction to be used as a cathode material for an aluminum-ion battery with a configuration of Al/Al(OTF) 3- H 2 O/Al x MnO 2 •nH 2 O. This cell is not only based on aqueous electrolyte chemistry but also delivers a high specific capacity of 467 mAh g -1 and a record high energy density of 481 Wh kg -1 . The high safety of aqueous electrolyte, facile cell assembly and the low cost of materials suggest that this aqueous aluminum-ion battery holds promise for large-scale energy applications.

25 ENERGY STORAGE↗
DOE OSTI2018

Fundamental Principles for Calculating Charged Defect Ionization Energies in Ultrathin Two-Dimensional Materials

Defects in two-dimensional (2D) materials are becoming prominent candidates for quantum emitters and scalable optoelectronic applications. However, several physical properties that characterize their behavior, such as charged defect ionization energies, are difficult to simulate with conventional first-principles methods, mainly because of the weak and anisotropic dielectric screening caused by the reduced dimensionality. We establish fundamental principles for accurate and efficient calculations of charged defect ionization energies and electronic structure in ultrathin 2D materials. We propose to use the vacuum level as the reference for defect charge transition levels (CTLs) because it gives robust results insensitive to the level of theory, unlike commonly used band-edge positions. Furthermore, we determine the fraction of Fock exchange in hybrid functionals for accurate band gaps and band-edge positions of 2D materials by enforcing the generalized Koopmans’ condition of localized defect states. We found that the obtained fractions of Fock exchange vary significantly from 0.2 for bulk hexagonal boron nitride (h-BN) to 0.4 for monolayer h-BN, whose band gaps are also in good agreement with experimental results and calculated GW results. The combination of these methods allows for the reliable and efficient prediction of defect ionization energies (the difference between CTLs and band-edge positions). We motivate and generalize these findings with several examples including different defects in monolayer to few-layer h-BN, monolayer MoS 2 , and graphane. Lastly, we show that increasing the number of layers of h-BN systematically lowers defectionization energies, mainly through CTLs shifting towards vacuum, with conduction band minima kept almost unchanged.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced lithium storage capability of FeF 3 ·0.33H 2 O single crystal with active insertion site exposed

Iron fluoride cathode for lithium batteries intrigues researchers for decades due to high capacity and low cost, but suffers from poor electronic and ionic conductivity. Hierarchical micro/nano construction with preferred orientation growth can shorten ion diffusion pathway and facilitate electron transportation without inducing side reactions. FeF 3 ·0.33H 2 O hierarchical microspheres self-assembled by octagonal single crystal slices are prepared. Benefiting from the artful structure, SAED and XRD characterizations confirm that the octagonal single crystal slices provides more lithium ions intercalation positions (4c sites). Electrochemical tests demonstrate that the well designed FeF 3 ·0.33H 2 O single crystal (FFH-S) delivers 30 mAh g -1 higher capacity (172 mAh g -1 at 0.1C) than the common FeF 3 ·0.33H 2 O polycrystal (FFH-P). In-situ XRD combined with theoretic calculation elucidate the difference in lithium storage capacity. Instructive argument that single crystal octagon slices with [110] oriented growth provide more active sites for lithium ions intercalation contributes to the preparation of high performance iron fluoride cathode materials for rechargeable battery.

25 ENERGY STORAGE↗
DOE OSTI2018

Mechanistic insights of enhanced spin polaron conduction in CuO through atomic doping

Abstract The formation of a “spin polaron” stems from strong spin-charge-lattice interactions in magnetic oxides, which leads to a localization of carriers accompanied by local magnetic polarization and lattice distortion. For example, cupric oxide (CuO), which is a promising photocathode material and shares important similarities with high T c superconductors, conducts holes through spin polaron hopping with flipped spins at Cu atoms where a spin polaron has formed. The formation of these spin polarons results in an activated hopping conduction process where the carriers must not only overcome strong electron−phonon coupling but also strong magnetic coupling. Collectively, these effects cause low carrier conduction in CuO and hinder its applications. To overcome this fundamental limitation, we demonstrate from first-principles calculations how doping can improve hopping conduction through simultaneous improvement of hole concentration and hopping mobility in magnetic oxides such as CuO. Specifically, using Li doping as an example, we show that Li has a low ionization energy that improves hole concentration, and lowers the hopping barrier through both the electron−phonon and magnetic couplings' reduction that improves hopping mobility. Finally, this improved conduction predicted by theory is validated through the synthesis of Li-doped CuO electrodes which show enhanced photocurrent compared to pristine CuO electrodes. We conclude that doping with nonmagnetic shallow impurities is an effective strategy to improve hopping conductivities in magnetic oxides.

Smart, Tyler J.↗
DOE OSTI2018

Analytic Analysis for Dynamic System Frequency in Power Systems Under Uncertain Variability

In this paper, a new systematic method is proposed to analyze the system frequency in power systems under uncertain variability (UV). UV gradually increases in power systems, which originates from the renewable energy generation, random loads, frequency measurement, and communication channels, exerting noteworthy impacts on dynamic system frequency. The UV is modeled as a stochastic process in this paper. Then, the stochastic differential equations are used to describe the dynamic system frequency response (SFR) of a power system under UV, which is based on the SFR model. To assess system frequency dynamics under UV, an index of intra-range probability is put forward. Based on stochastic analysis theory, an analytic method is proposed to analyze the system frequency under UV, by which the intra-range probability can be analytically solved. Compared with Monte Carlo simulation in a typical SFR case and the Iceland electricity transmission network, the proposed method shows almost the same results using much less computation resource. Finally, insights for improving the SFR under UV are provided. The proposed method can be used to quickly verify whether the system frequency could withstand the UV and stay in the security range.

42 ENGINEERING↗
DOE OSTI2018

Combining Landau–Zener theory and kinetic Monte Carlo sampling for small polaron mobility of doped BiVO 4 from first-principles

Transition metal oxides such as BiVO 4 are promising photoelectrode materials for solar-to-fuel conversion applications. However, their performance is limited by the low carrier mobility (especially electron mobility) due to the formation of small polarons. Recent experimental studies have shown improved carrier mobility and conductivity by atomic doping; however the underlying mechanism is not understood. A fundamental atomistic-level understanding of the effects on small polaron transport is critical to future material design with high conductivity. We studied the small polaron hopping mobility in pristine and doped BiVO 4 by combining Landau–Zener theory and kinetic Monte Carlo (kMC) simulation fully from first-principles, and investigated the effect of dopant–polaron interactions on the mobility. We found that polarons are spontaneously formed at V in both pristine and Mo/W doped BiVO 4 , which can only be described correctly by density functional theory (DFT) with the Hubbard correction (DFT+U) or hybrid exchange-correlation functional but not local or semi-local functionals. We found that DFT+U and dielectric dependant hybrid (DDH) functionals give similar electron hopping barriers, which are also similar between the room temperature monoclinic phase and the tetragonal phase. The calculated electron mobility agrees well with experimental values, which is around 10 -4 cm 2 V -1 s -1 . We found that the electron polaron transport in BiVO4 is neither fully adiabatic nor nonadiabatic, and the first and second nearest neighbor hoppings have significantly different electronic couplings between two hopping centers that lead to different adiabaticity and prefactors in the charge transfer rate, although they have similar hopping barriers. Without considering the detailed adiabaticity through Landau–Zener theory, one may get qualitatively wrong carrier mobility. We further computed polaron mobility in the presence of different dopants and showed that Cr substitution of V is an electron trap while Mo and W are “repulsive” centers, mainly due to the minimization of local lattice expansion by dopants and electron polarons. The dopants with “repulsive” interactions to polarons are promising for mobility improvement due to larger wavefunction overlap and delocalization of locally concentrated polarons.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Unconventional Relation between Charge Transport and Photocurrent via Boosting Small Polaron Hopping for Photoelectrochemical Water Splitting

Doping in semiconductor photoelectrodes controls defect formation and carrier transport that critically determine the device performance. Here we report an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO 4 photoanodes. Using the single-crystalline thin film approach, we identify that Mo doping significantly condenses the optimization regime between carrier transport and photon collection. For Mo-doped BiVO 4 films, an unprecedentedly thin layer (50 nm), less than one-third of the pristine BiVO 4 thickness, delivers larger photocurrents by overcoming the charge transport limitation, representing a regime not covered in conventional models. We provide direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of a small polaron, with the latter applying substantial impact on the photoelectrochemical performance. Density functional theory calculations reveal that fully ionized Mo dopants establish a strong electrostatic interaction with a small polaron, which helps reduce its hopping barrier by minimizing the local lattice expansion. Here, our results deliver mechanistic insights on the interplay between extrinsic doping and carrier transport, and provide guidance in developing advanced semiconductor photoelectrodes.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Expanding Interlayer Spacing of Hard Carbon by Natural K + Doping to Boost Na-Ion Storage

Heteroatom-doped carbon is an attractive material for anodes in lithium/sodium ion batteries as a replacement for the traditional graphite anodes. However, the complex fabrication process and high-cost force these carbon materials far from the practical applications. Here we report a low-cost, natural potassium-doped carbon material, which is directly carbonized from the coconut endocarp – a kind of the high-potassium contained biomass materials. The obtained carbon structure features an expanded d(002)-spacing (0.4 nm) originating from the super-high potassium content (6654 mg kg-1). Because of the improvement on charge transfer kinetics and electrical properties, the potassium-doped carbon anode exhibits promising electrochemical performance in sodium ion batteries, including high initial reversible capacity (314 mAh g-1) and good cycle stability (289 mAh g-1 after 200 cycles). Additionally, this work opens up a new approach for the design of heteroatom-doped carbon materials from the view of the natural environmental friendly.

Wu, Feng↗
DOE OSTI2018

Stable Carbon-Selenium Bonds for Enhanced Performance in Tremella -Like 2D Chalcogenide Battery Anode [Stable Carbon-Selenium Bonds for Enhanced Performance in Tremella -Like Two-Dimensional Chalcogenide Battery Anode]

Two-dimensional cobalt selenide based on conversion reaction has attracted much attention due to its open layered structure and high specific capacity. However, effectively suppressing the fast capacity fade caused by the irreversible Se dissolution and serious volume change during the cycling process is still a challenges. Herein, we tune the concentration of dispersion liquid under supercritical condition to induce the CoSe crystal to grow along the graphene oxide (GO), and finally obtain the Tremella -like CoSe-rGO hybrid. The nature of epitaxial growth leads to the formation of stable C–Se bonds, which maintains favorable conductive connection between CoSe and reduced GO (rGO) as well as enhances the mechanical strength of active material to suppress Se dissolution and volume expansion during Na/Li intercalation and deintercalation. The unique microstructural merits of the hybrid result in superior sodium/lithium storage performance (400.8 mAh g -1 at 1 A g -1 after 100 cycles for NIBs and 769.6 mAh g -1 at 2 A g -1 after 500 cycles for LIBs). Moreover, the transmission X-ray microscopy technique is firstly used to directly observe the Se segregation in cobalt selenide and it being suppressed by the C–Se bonds.

25 ENERGY STORAGE↗
DOE OSTI2018

A facile recovery process for cathodes from spent lithium iron phosphate batteries by using oxalic acid

A simplistic and novel leaching process is developed to dispose spent lithium iron phosphate (LiFePO 4 ) batteries. In this paper, oxalic acid is selected as a leaching reagent to recover lithium as a resource and remove phosphorus from LiFePO 4 batteries, benefiting from its low natural effects. The physical properties of spent cathode materials (before leaching) and residues (after leaching) are identified by X-ray diffraction (XRD) and scanning electronic microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The total amounts of Li and Fe are analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The process occurs under conditions of a 0.3 mol·L -1 oxalic acid concentration, a temperature of 80°C, a reaction time of 60 min and a solid/liquid ratio of 60 g· L -1 . It can effectively precipitate ~92% ferrum in terms of FeC 2 O 4 ·2H 2 O from LiFePO 4 , with the leaching efficiency of Li achieving up to 98%. This method demonstrates a new strategy for dealing with spent lithium iron phosphate batteries with a low cost and low environmental impact.

25 ENERGY STORAGE↗
DOE OSTI2018

Insights into the Na + Storage Mechanism of Phosphorus-Functionalized Hard Carbon as Ultrahigh Capacity Anodes

Hard carbon as a typical anode material for sodium ion batteries has received much attention in terms of its low cost and renewability. Herein, phosphorus-functionalized hard carbon with a specific honeycomb briquette shaped morphology is synthesized via electrospinning technology. When applied as an anode material for Na + storage, it exhibits an impressively high reversible capacity of 393.4 mA h g -1 with the capacity retention up to 98.2% after 100 cycles. According to first-principle calculation, the ultrahigh capacity of the as-prepared anode is ascribed to the enhancement of Na-absorption through formation of P=O and P-C bonds in graphitic layers when doped with phosphorus. Furthermore, the increase of electron density around the Fermi level is found to be mainly caused by O atoms instead of P atoms.

25 ENERGY STORAGE↗