Tod A. Pascal

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

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.

Electronic-Mechanical Coupling in Graphene from in situ Nanoindentation Experiments and Multiscale Atomistic Simulations

We present the in situ nanoindentation experiments performed on suspended graphene devices to introduce homogeneous tensile strain, while simultaneously carrying out electrical measurements. We find that the electrical resistance shows only a marginal change even under severe strain, and the electronic transport measurement confirms that there is no band gap opening for graphene under moderate uniform strain, which is consistent with our results from the first-principles informed molecular dynamics simulation.

The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy

The molecular structure of the electrical double layer determines the chemistry in all electrochemical processes. Using x-ray absorption spectroscopy (XAS), we probed the structure of water near gold electrodes and its bias dependence. Electron yield XAS detected at the gold electrode revealed that the interfacial water molecules have a different structure from those in the bulk. First principles calculations revealed that ~50% of the molecules lie flat on the surface with saturated hydrogen bonds and another substantial fraction with broken hydrogen bonds that do not contribute to the XAS spectrum because their core-excited states are delocalized by coupling with the gold substrate. At negative bias, the population of flat-lying molecules with broken hydrogen bonds increases, producing a spectrum similar to that of bulk water. The water double-layer structure at an electrode changed from ordered to disordered when the applied bias was switched. Dissecting the electrical double layer The structure of water within a nanometer of an electrode surface is known as the electrical double layer. This layer creates a strong electrical field that can affect electrochemical reactions. Velasco-Velez et al. explored the structure of the electrical double layer at a bare gold electrode. With no applied potential and at positive potentials, the layer is highly structured (resembling ice) with few dangling hydrogen bonds. However, at negative potentials, the layer was more like bulk water, and half of the water molecules lie flat on the surface. Science, this issue p. 831

New Insights into the Structure of the Vapor/Water Interface from Large-Scale First-Principles Simulations

We present extensive ab initio simulations of the molecular arrangements at the vapor/water interface, which provide valuable insights into the interface structure. In particular, the simulations address the controversy of whether there is a significant amount of nondonor configurations at this prototypical interface, using a novel Car-Parrinello-like ab initio molecular dynamics approach. The interface is modeled by a system of 384 water molecules for 125 ps in a two-dimensional periodic slab, the most extensive ab initio molecular dynamics simulation to date. In contrast to previous theoretical simulations and X-ray absorption spectroscopy, but consistent with sum-frequency generation experiments, we observe no evidence for a significant occurrence of acceptor-only species at the vapor/water interface. Besides a distinct surface relaxation effect, we find that only the topmost layers of the interface obey structural order.

X-ray Absorption Spectra of Dissolved Polysulfides in Lithium-Sulfur Batteries from First-Principles.

The X-ray absorption spectra (XAS) of lithium polysulfides (Li2Sx) of various chain lengths (x) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (x - 2) internal atoms in the chain, except in the case of Li2S2, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS.

Polysulfide-Blocking Microporous Polymer Membrane Tailored for Hybrid Li-Sulfur Flow Batteries

Redox flow batteries (RFBs) present unique opportunities for multi-hour electrochemical energy storage (EES) at low cost. Too often, the barrier for implementing them in large-scale EES is the unfettered migration of redox active species across the membrane, which shortens battery life and reduces Coulombic efficiency. To advance RFBs for reliable EES, a new paradigm for controlling membrane transport selectivity is needed. We show here that size- and ion-selective transport can be achieved using membranes fabricated from polymers of intrinsic microporosity (PIMs). As a proof-of-concept demonstration, a first-generation PIM membrane dramatically reduced polysulfide crossover (and shuttling at the anode) in lithium-sulfur batteries, even when sulfur cathodes were prepared as flowable energy-dense fluids. The design of our membrane platform was informed by molecular dynamics simulations of the solvated structures of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) vs lithiated polysulfides (Li2Sx, where x = 8, 6, and 4) in glyme-based electrolytes of different oligomer length. These simulations suggested polymer films with pore dimensions less than 1.2-1.7 nm might incur the desired ion-selectivity. Indeed, the polysulfide blocking ability of the PIM-1 membrane (∼0.8 nm pores) was improved 500-fold over mesoporous Celgard separators (∼17 nm pores). As a result, significantly improved battery performance was demonstrated, even in the absence of LiNO3 anode-protecting additives.

Design of Covalent Organic Frameworks for Methane Storage

We designed 14 new covalent organic frameworks (COFs), which are expected to adsorb large amounts of methane (CH(4)) at 298 K and up to 300 bar. We have calculated their delivery uptake using grand canonical Monte Carlo (GCMC) simulations. We also report their thermodynamic stability based on 7.5 ns molecular dynamics simulations. Two new frameworks, COF-103-Eth-trans and COF-102-Ant, are found to exceed the DOE target of 180 v(STP)/v at 35 bar for methane storage. Their performance is comparable to the best previously reported materials: PCN-14 and Ni-MOF-74. Our results indicate that using thin vinyl bridging groups aid performance by minimizing the interaction methane-COF at low pressure. This is a new feature that can be used to enhance loading in addition to the common practice of adding extra fused benzene rings. Most importantly, this report shows that pure nonbonding interactions, van der Waals (vdW) and electrostatic forces in light elements (C, O, B, H, and Si), can rival the enhancement in uptake obtained for microporous materials derived from early transition metals.

Chemistries for Patterning Robust DNA MicroBarcodes Enable Multiplex Assays of Cytoplasm Proteins from Single Cancer Cells

The optimization of chemistries to enable the patterning of miniaturized DNA barcodes using microfluidics flow channels is described (see picture). Experiment and theory reveal that solvent mixtures in which counterions are strongly associated with the negatively charged DNA oligomers may be harnessed to produce high quality, high density DNA microarray patterns over a large area.

Thermodynamics of liquids: standard molar entropies and heat capacities of common solvents from 2PT molecular dynamics

We validate here the Two-Phase Thermodynamics (2PT) method for calculating the standard molar entropies and heat capacities of common liquids. In 2PT, the thermodynamics of the system is related to the total density of states (DoS), obtained from the Fourier Transform of the velocity autocorrelation function. For liquids this DoS is partitioned into a diffusional component modeled as diffusion of a hard sphere gas plus a solid component for which the DoS(υ) → 0 as υ→ 0 as for a Debye solid. Thermodynamic observables are obtained by integrating the DoS with the appropriate weighting functions. In the 2PT method, two parameters are extracted from the DoS self-consistently to describe diffusional contributions: the fraction of diffusional modes, f, and DoS(0). This allows 2PT to be applied consistently and without re-parameterization to simulations of arbitrary liquids. We find that the absolute entropy of the liquid can be determined accurately from a single short MD trajectory (20 ps) after the system is equilibrated, making it orders of magnitude more efficient than commonly used perturbation and umbrella sampling methods. Here, we present the predicted standard molar entropies for fifteen common solvents evaluated from molecular dynamics simulations using the AMBER, GAFF, OPLS AA/L and Dreiding II forcefields. Overall, we find that all forcefields lead to good agreement with experimental and previous theoretical values for the entropy and very good agreement in the heat capacities. These results validate 2PT as a robust and efficient method for evaluating the thermodynamics of liquid phase systems. Indeed 2PT might provide a practical scheme to improve the intermolecular terms in forcefields by comparing directly to thermodynamic properties.