Engineering topics
Li, Jun
Publications and source records attributed to Li, Jun.
Discharge Permit-1845 (Quarterly Status Report July 2020)
Abstract not provided.
Ultrahigh-Loading of Ir Single Atoms on NiO Matrix to Dramatically Enhance Oxygen Evolution Reaction
Abstract not provided
Advanced electrolytes for fast-charging high-voltage lithium-ion batteries in wide-temperature range
LiNixMnyCo1-x-yO2 (NMC) cathode materials with Ni=0.8 have attracted great interest for application in high energy-density lithium (Li)-ion batteries (LIBs) because of their high specific capacities at high voltages. However, the practical application of Ni-rich NMC in LIBs under high charge voltages (e.g. 4.4 V and above) still faces big challenges due to the severe capacity fading, which is directly related to the instability of the cathode/electrolyte interface. In this work, we develop new localized high-concentration electrolytes (LHCEs) based on carbonate solvents, which are compatible with graphite (Gr) anode and have a high oxidation potential over 4.9 V vs. Li/Li+. The optimal LHCE enables the Gr||NMC811 cells with 2.8 mAh cm-2 cathode areal capacity loading to achieve an excellent cycling stability over 600 cycles with a capacity retention of 94.2 % under a charge cut-off voltage of 4.4 V at 25 ?C, good rate capabilities under fast charging or discharging up to 3C rate, and superior low-temperature discharge performance down to -30 ?C with a capacity retention of 85.6% at C/5 rate. The findings in this work shed light a very promising strategy to develop new electrolytes for practical high-energy LIBs with Ni-rich NMC cathodes.
Spherical trihedral metallo-borospherenes
The discovery of borospherenes unveiled the capacity of boron to form fullerene-like cage structures. While fullerenes are known to entrap metal atoms to form endohedral metallofullerenes, few metal atoms have been observed to be part of the fullerene cages. Here we report the observation of a class of remarkable metallo-borospherenes, where metal atoms are integral parts of the cage surface. We have produced La 3 B 18 - and Tb 3 B 18 - and probed their structures and bonding using photoelectron spectroscopy and theoretical calculations. Global minimum searches revealed that the most stable structures of Ln 3 B 18 - are hollow cages with D 3h symmetry. The B 18 -framework in the Ln 3 B 18 - cages can be viewed as consisting of two triangular B6 motifs connected by three B 2 units, forming three shared B 10 rings which are coordinated to the three Ln atoms on the cage surface. These metallo-borospherenes represent a new class of unusual geometry that has not been observed in chemistry heretofore.
Memristive Behavior Enabled by Amorphous–Crystalline 2D Oxide Heterostructure
The emergence of memristive behavior in amorphous–crystalline 2D oxide heterostructures, which are synthesized by atomic layer deposition (ALD) of a few-nanometer amorphous Al 2 O 3 layers onto atomically thin single-crystalline ZnO nanosheets, is demonstrated. The conduction mechanism is identified based on classic oxygen vacancy conductive channels. ZnO nanosheets provide a 2D host for oxygen vacancies, while the amorphous Al 2 O 3 facilitates the generation and stabilization of the oxygen vacancies. The conduction mechanism in the high-resistance state follows Poole–Frenkel emission, and in the the low-resistance state is fitted by the Mott–Gurney law. From the slope of the fitting curve, the mobility in the low-resistance state is estimated to be ≈2400 cm 2 V -1 s -1 , which is the highest value reported in semiconductor oxides. When annealed at high temperature to eliminate oxygen vacancies, Al is doped into the ZnO nanosheet, and the memristive behavior disappears, further confirming the oxygen vacancies as being responsible for the memristive behavior. The 2D heterointerface offers opportunities for new design of high-performance memristor devices.
Concurrent probing of electron-lattice dephasing induced by photoexcitation in 1 T -TaSeTe using ultrafast electron diffraction
It has been technically challenging to concurrently probe the electrons and the lattices in materials during nonequilibrium processes, allowing their correlations to be determined. Here, in a single set of ultrafast electron diffraction patterns taken on the charge-density-wave (CDW) material 1T-TaSeTe, we discover a temporal shift in the diffraction intensity measurements as a function of scattering angle. With the help of dynamic models and theoretical calculations, we show that the ultrafast electrons probe both the valence-electron and lattice dynamic processes, resulting in the temporal shift measurements. Our results demonstrate unambiguously that the CDW is not merely a result of the periodic lattice deformation ever present in 1T-TaSeTe but has significant electronic origin. This method demonstrates an approach for studying many quantum effects that arise from electron-lattice dephasing in molecules and crystals for next-generation devices.
Bioinspired Synthesis of Quasi-Two-Dimensional Monocrystalline Oxides
Abstract not provided
Publisher Correction: Copper adparticle enabled selective electrosynthesis of n-propanol
An amendment to this paper has been published and can be accessed via a link at the top of the paper. The original version of this Article incorrectly omitted the received/accepted dates of Received: 05 June 2018; Accepted: 10 October 2018. This has now been corrected in the PDF and HTML versions of the Article.
Tailoring conductive networks within hollow carbon nanospheres to host phosphorus for advanced sodium ion batteries
The formidable sustainability challenges in advancing energy storage technologies call for game-changing research in battery designs. The previous pursuing of novel cathode materials with high redox potentials impedes the vast applications due to the simultaneous electrolyte decomposition at high potentials, though they are expected to deliver high specific capacities. Eventually, people start thinking in an opposite way, desirable anode materials with low redox potentials can also own high specific capacities. Among all the promising candidates, phosphorus-based anodes in sodium ion batteries (SIBs) have received considerable attention owing to the low cost and relatively high natural abundance of phosphorus. More importantly, phosphorus can store three sodium atoms and enable a high theoretical capacity of 2596 mAh g -1 , which overwhelms any other SIB anode currently available. However, the poor electronic conductivity and large volume change of phosphorus during cycling severely deteriorate battery performance. The most widely used strategy is to confine phosphorus within well-designed carbon hosts. Here, we thereby introduce a new type of porous hollow carbon with conductive-network interior as phosphorus host, which not only improves the electrical conductivity, but also creates enough interior surface for maximizing phosphorus utilization and shortening the ion's diffusion distance, compared to those conventional hollow carbon hosts. Therefore, the as-prepared red phosphorus-carbon spheres composites (RP/CS) exhibit superior rate performance (similar to 1083 mAh g -1 at 4 A g -1 , similar to 837 mAh g -1 even at 8 A g -1 ) and excellent cycle life (1027 mAh g -1 at 4 A g -1 more than 2000 cycles).
Mode Specificity in the OH + HO 2 → H 2 O + O 2 Reaction: Enhancement of Reactivity by Exciting a Spectator Mode
A reaction usually involves a few active modes while the other modes are largely preserved throughout the reaction as spectators. Excitation of an active mode is expected to promote the reaction, but depositing energy in a spectator mode typically has no effect, because of the differing ability for energy flow to the reaction coordinate. Here, we report a surprising case of mode specificity in a key radical–radical reaction OH + HO 2 → H 2 O + O 2 , where such canonical expectations fail to hold. Despite its spectator nature, the vibrational excitation of the OH reactant is shown at low collision energies to enhance the reactivity significantly. This unique effect can be attributed to the increased attraction with HO 2 due to the larger dipole of the stretched OH. At low collision energies, the stronger attraction increases the chance of capturing the reactants to form a hydrogen-bonded complex, thus of passing through the submerged barrier. The novel mechanism differs from the conventional vibrational enhancement via coupling to the reaction coordinate at the transition state, enriching our understanding of mode specificity in chemistry.
Author Correction: Anisotropic magnetocaloric effect in Fe 3- x GeTe 2
Correction to: Scientifc Reports https://doi.org/10.1038/s41598-019-49654-4, published online 13 September 2019
Author Correction: Thickness-dependent magnetic order in CrI3 single crystals
Two-dimensional (2D) materials with intrinsic ferromagnetism provide unique opportunity to engineer new functionalities in nano-spintronics. One such material is CrI3, showing long-range magnetic order in monolayer with the Curie temperature (Tc) of 45 K. Here we study detailed evolution of magnetic transition and magnetic critical properties in response to systematic reduction in crystal thickness down to 50 nm. Bulk Tc of 61 K is gradually suppressed to 57 K, however, the satellite transition at T * = 45 K is observed layer-independent at fixed magnetic field of 1 kOe. The origin of T * is proposed to be a crossover from pinning to depinning of magnetic domain walls. The reduction of thickness facilitates a field-driven metamagnetic transition around 20 kOe with out-of-plane field, in contrast to the continuous changes with in-plane field. The critical analysis around Tc elucidates the mean-field type interactions in microscale-thick CrI3.
Hydroxide promotes carbon dioxide electroreduction to ethanol on copper via tuning of adsorbed hydrogen
Producing liquid fuels such as ethanol from CO 2 , H 2 O, and renewable electricity offers a route to store sustainable energy. The search for efficient electrocatalysts for the CO 2 reduction reaction relies on tuning the adsorption strength of carbonaceous intermediates. Here, we report a complementary approach in which we utilize hydroxide and oxide doping of a catalyst surface to tune the adsorbed hydrogen on Cu. Density functional theory studies indicate that this doping accelerates water dissociation and changes the hydrogen adsorption energy on Cu. We synthesize and investigate a suite of metal-hydroxide-interface-doped-Cu catalysts, and find that the most efficient, Ce(OH) x -doped-Cu, exhibits an ethanol Faradaic efficiency of 43% and a partial current density of 128 mA cm -2 . Mechanistic studies, wherein we combine investigation of hydrogen evolution performance with the results of operando Raman spectroscopy, show that adsorbed hydrogen hydrogenates surface *HCCOH, a key intermediate whose fate determines branching to ethanol versus ethylene.
Catalyst synthesis under CO 2 electroreduction favours faceting and promotes renewable fuels electrosynthesis
The electrosynthesis of C 2+ hydrocarbons from CO 2 has garnered recent attention in light of the relatively high market price per unit energy input. Today’s low selectivities and stabilities towards C 2+ products at high current densities curtail system energy efficiency, which limits their prospects for economic competitiveness. Here we present a materials processing strategy based on in situ electrodeposition of copper under CO 2 reduction conditions that preferentially expose and maintain Cu(100) facets, which favour the formation of C 2+ products. We observe capping of facets during catalyst synthesis and achieve control over faceting to obtain a 70% increase in the ratio of Cu(100) facets to total facet area. We report a 90% Faradaic efficiency for C 2+ products at a partial current density of 520 mA cm –2 and a full-cell C 2+ power conversion efficiency of 37%. We achieve nearly constant C2H4 selectivity over 65 h operation at 350 mA cm –2 in a membrane electrode assembly electrolyser.
Norm-conserving pseudopotentials and basis sets to explore Lanthanide chemistry in complex environments
A complete set of pseudopotentials and accompanying basis sets for all lanthanide elements are presented based on the relativistic, norm-conserving, separable, dual-space Gaussian-type pseudopotential protocol of Goedecker, Teter and Hutter (GTH) within the generalized gradient approximation (GGA) and the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE). The corresponding basis sets have been molecularly optimized (MOLOPT) using a contracted form with a single set of Gaussian exponents for s, p and d states. The f states are uncontracted explicitly with Gaussian exponents. Moreover, the Hubbard U values for each lanthanide element, to be used in DFT+U calculations, are also tabulated, allowing for the proper treatment of the strong on-site Coulomb interactions of localized 4f electrons. The accuracy and reliability of our GTH pseudopotentials and companion basis sets optimized for lanthanides is illustrated by a series of test calculations on lanthanide-centered molecules, and solid-state systems, with the most common oxidation states. We anticipate that these pseudopotentials and basis sets will enable larger-scale density functional theory calculations and ab initio molecular dynamics simulations of lanthanide molecules in either gas or condensed phases, as well as of solid state lanthanide-containing materials, allowing to further explore the chemical and physical properties of lanthanide systems.