Yousung Jung

Scaled opposite-spin second order Møller–Plesset correlation energy: An economical electronic structure method

A simplified approach to treating the electron correlation energy is suggested in which only the alpha-beta component of the second order Møller-Plesset energy is evaluated, and then scaled by an empirical factor which is suggested to be 1.3. This scaled opposite-spin second order energy (SOS-MP2), where MP2 is Møller-Plesset theory, yields results for relative energies and derivative properties that are statistically improved over the conventional MP2 method. Furthermore, the SOS-MP2 energy can be evaluated without the fifth order computational steps associated with MP2 theory, even without exploiting any spatial locality. A fourth order algorithm is given for evaluating the opposite spin MP2 energy using auxiliary basis expansions, and a Laplace approach, and timing comparisons are given.

Unprecedented high-temperature CO2 selectivity in N2-phobic nanoporous covalent organic polymers

Post-combustion CO(2) capture and air separation are integral parts of the energy industry, although the available technologies remain inefficient, resulting in costly energy penalties. Here we report azo-bridged, nitrogen-rich, aromatic, water stable, nanoporous covalent organic polymers, which can be synthesized by catalyst-free direct coupling of aromatic nitro and amine moieties under basic conditions. Unlike other porous materials, azo-covalent organic polymers exhibit an unprecedented increase in CO(2)/N(2) selectivity with increasing temperature, reaching the highest value (288 at 323 K) reported to date. Here we observe that azo groups reject N(2), thus making the framework N(2)-phobic. Monte Carlo simulations suggest that the origin of the N(2) phobicity of the azo-group is the entropic loss of N(2) gas molecules upon binding, although the adsorption is enthalpically favourable. Any gas separations that require the efficient exclusion of N(2) gas would do well to employ azo units in the sorbent chemistry.

Strengthening effect of single-atomic-layer graphene in metal–graphene nanolayered composites

Graphene is a single-atomic-layer material with excellent mechanical properties and has the potential to enhance the strength of composites. Its two-dimensional geometry, high intrinsic strength and modulus can effectively constrain dislocation motion, resulting in the significant strengthening of metals. Here we demonstrate a new material design in the form of a nanolayered composite consisting of alternating layers of metal (copper or nickel) and monolayer graphene that has ultra-high strengths of 1.5 and 4.0 GPa for copper-graphene with 70-nm repeat layer spacing and nickel-graphene with 100-nm repeat layer spacing, respectively. The ultra-high strengths of these metal-graphene nanolayered structures indicate the effectiveness of graphene in blocking dislocation propagation across the metal-graphene interface. Ex situ and in situ transmission electron microscopy compression tests and molecular dynamics simulations confirm a build-up of dislocations at the graphene interface.

Entropy and the driving force for the filling of carbon nanotubes with water

The spontaneous filling of hydrophobic carbon nanotubes (CNTs) by water observed both experimentally and from simulations is counterintuitive because confinement is generally expected to decrease both entropy and bonding, and remains largely unexplained. Here we report the entropy, enthalpy, and free energy extracted from molecular dynamics simulations of water confined in CNTs from 0.8 to 2.7-nm diameters. We find for all sizes that water inside the CNTs is more stable than in the bulk, but the nature of the favorable confinement of water changes dramatically with CNT diameter. Thus we find (i) an entropy (both rotational and translational) stabilized, vapor-like phase of water for small CNTs (0.8–1.0 nm), (ii) an enthalpy stabilized, ice-like phase for medium-sized CNTs (1.1–1.2 nm), and (iii) a bulk-like liquid phase for tubes larger than 1.4 nm, stabilized by the increased translational entropy as the waters sample a larger configurational space. Simulations with structureless coarse-grained water models further reveal that the observed free energies and sequence of transitions arise from the tetrahedral structure of liquid water. These results offer a broad theoretical basis for understanding water transport through CNTs and other nanostructures important in nanofluidics, nanofiltrations, and desalination.

The High Performance of Crystal Water Containing Manganese Birnessite Cathodes for Magnesium Batteries.

Rechargeable magnesium batteries have lately received great attention for large-scale energy storage systems due to their high volumetric capacities, low materials cost, and safe characteristic. However, the bivalency of Mg(2+) ions has made it challenging to find cathode materials operating at high voltages with decent (de)intercalation kinetics. In an effort to overcome this challenge, we adopt an unconventional approach of engaging crystal water in the layered structure of Birnessite MnO2 because the crystal water can effectively screen electrostatic interactions between Mg(2+) ions and the host anions. The crucial role of the crystal water was revealed by directly visualizing its presence and dynamic rearrangement using scanning transmission electron microscopy (STEM). Moreover, the importance of lowering desolvation energy penalty at the cathode-electrolyte interface was elucidated by working with water containing nonaqueous electrolytes. In aqueous electrolytes, the decreased interfacial energy penalty by hydration of Mg(2+) allows Birnessite MnO2 to achieve a large reversible capacity (231.1 mAh g(-1)) at high operating voltage (2.8 V vs Mg/Mg(2+)) with excellent cycle life (62.5% retention after 10000 cycles), unveiling the importance of effective charge shielding in the host and facile Mg(2+) ions transfer through the cathodes interface.

High capacity carbon dioxide adsorption by inexpensive covalent organic polymers

Efficient CO2 scrubbing without a significant energy penalty remains an outstanding challenge for the fossil fuel-burning industry where aqueous amine solutions are still widely used. Porous materials have long been evaluated for next generation CO2 adsorbents. Porous polymers, robust and inexpensive, show promise as feasible materials for the capture of CO2 from warm exhaust fumes. We report the syntheses of porous covalent organic polymers (COPs) with CO2 adsorption capacities of up to 5616 mg g−1 (measured at high pressures, i.e. 200 bar) and industrially relevant temperatures (as warm as 65 °C). COPs are stable in boiling water for at least one week and near infinite CO2/H2 selectivity is observed.

Improved reversibility in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of metal alloy catalyst

Most Li-O2 batteries suffer from sluggish kinetics during oxygen evolution reactions (OERs). To overcome this drawback, we take the lesson from other catalysis researches that showed improved catalytic activities by employing metal alloy catalysts. Such research effort has led us to find Pt3Co nanoparticles as an effective OER catalyst in Li-O2 batteries. The superior catalytic activity was reflected in the substantially decreased overpotentials and improved cycling/rate performance compared to those of other catalysts. Density functional theory calculations suggested that the low OER overpotentials are associated with the reduced adsorption strength of LiO2 on the outermost Pt catalytic sites. Also, the alloy catalyst generates amorphous Li2O2 conformally coated around the catalyst and thus facilitates easier decomposition and higher reversibility. This investigation conveys an important message that understanding elementary reactions and surface charge engineering of air-catalysts are one of the most effective approaches in resolving the chronic sluggish charging kinetics in Li-O2 batteries.

Hydrogen-Bond-Assisted Controlled C–H Functionalization via Adaptive Recognition of a Purine Directing Group

We have developed the Rh-catalyzed selective C-H functionalization of 6-arylpurines, in which the purine moiety directs the C-H bond activation of the aryl pendant. While the first C-H amination proceeds via the N1-chelation assistance, the subsequent second C-H bond activation takes advantage of an intramolecular hydrogen-bonding interaction between the initially formed amino group and one nitrogen atom, either N1 or N7, of the purinyl part. Isolation of a rhodacycle intermediate and the substrate variation studies suggest that N1 is the main active site for the C-H functionalization of both the first and second amination in 6-arylpurines, while N7 plays an essential role in controlling the degree of functionalization serving as an intramolecular hydrogen-bonding site in the second amination process. This pseudo-Curtin-Hammett situation was supported by density functional calculations, which suggest that the intramolecular hydrogen-bonding capability helps second amination by reducing the steric repulsion between the first installed ArNH and the directing group.

Anomalous Manganese Activation of a Pyrophosphate Cathode in Sodium Ion Batteries: A Combined Experimental and Theoretical Study

Sodium ion batteries (SIBs) have many advantages such as the low price and abundance of sodium raw materials that are suitable for large-scale energy storage applications. Herein, we report an Mn-based pyrophosphate, Na(2)MnP(2)O(7), as a new SIB cathode material. Unlike most Mn-based cathode materials, which suffer severely from sluggish kinetics, Na(2)MnP(2)O(7) exhibits good electrochemical activity at ~3.8 V vs Na/Na(+) with a reversible capacity of 90 mAh g(-1) at room temperature. It also shows an excellent cycling and rate performance: 96% capacity retention after 30 cycles and 70% capacity retention at a c-rate increase from 0.05C to 1C. These electrochemical activities of the Mn-containing cathode material even at room temperature with relatively large particle sizes are remarkable considering an almost complete inactivity of the Li counterpart, Li(2)MnP(2)O(7). Using first-principles calculations, we find that the significantly enhanced kinetics of Na(2)MnP(2)O(7) is mainly due to the locally flexible accommodation of Jahn-Teller distortions aided by the corner-sharing crystal structure in triclinic Na(2)MnP(2)O(7). By contrast, in monoclinic Li(2)MnP(2)O(7), the edge-sharing geometry causes multiple bonds to be broken and formed during charging reaction with a large degree of atomic rearrangements. We expect that the similar computational strategy to analyze the atomic rearrangements can be used to predict the kinetics behavior when exploring new cathode candidates.

A fast doubly hybrid density functional method close to chemical accuracy using a local opposite spin ansatz

We develop and validate the XYGJ-OS functional, based on the adiabatic connection formalism and Görling-Levy perturbation theory to second order and using the opposite-spin (OS) ansatz combined with locality of electron correlation. XYGJ-OS with local implementation scales as N3 with an overall accuracy of 1.28 kcal/mol for thermochemistry, bond dissociation energies, reaction barrier heights, and nonbonded interactions, comparable to that of 1.06 kcal/mol for the accurate coupled-cluster based G3 method (scales as N7) and much better than many popular density functional theory methods: B3LYP (4.98), PBE0 (4.36), and PBE (12.10).