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Abstract: The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48Å and porosity ∼60% was investigated as a function of pressure at T =298K using quasi-elastic neutron scattering (QENS). The diffusivity of methane shows a clear effect of confinement: it is about two orders of magnitude lower than in bulk at the same thermodynamic conditions and is close to the diffusivity of liquid methane at 100K (i.e. ∼90K below the liquid–gas critical temperature T C ≈191K). The diffusion coefficient (D) of methane initially increases with pressure by a factor of ∼2.5 from 3.47±0.41×10−10 m2 s−1 at 0.482MPa to D =8.55±0.33×10−10 m2 s−1 at 2.75MPa and starts to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid is based on the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure is explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume is less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. Subsequent decrease of D, is associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results are compared with the available QENS data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites. [Copyright &y& Elsevier]

We investigate the extent to which the alkyl chain on the cation of an imidazolium-based neat room-temperature ionic liquid influences mobility and electrochemical behavior in nanoporous supercapacitors. Changing the cation chain length from an ethyl (n = 2) to a butyl (n = 4) to a hexyl (n = 6) group affects the electrolyte dynamics and their accumulation densities under dynamic charge-discharge processes. We relied on molecular dynamics (MD) computational simulations and classical density functional theory (cDFT) calculations of our system to reinforce the experimental results obtained from electrochemical measurements and quasi-elastic neutron scattering (QENS). We contrast the different dynamics of ionic liquids in bulk and confined states and demonstrate the effect of the cation dimension on resulting arrangements of positive and negative ions in pores. We correlate these fundamental properties with device performance metrics in an effort to properly tailor high-performance carbon supercapacitor electrodes with non-flammable and electrochemically stable electrolytes. [ABSTRACT FROM AUTHOR]

We investigate the aggregation, diffusion, and resulting electrochemical behavior of ionic liquids inside carbon electrodes with complex pore architectures and surface chemistries. Carbide-derived carbons (CDCs) with bimodal porosities and defunctionalized or oxidized electrode surfaces served as model electrode materials. Our goal was to obtain a fundamental understanding of room-temperature ionic liquid ion orientation, mobility, and electrosorption behavior. Neat 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide confined in CDCs was studied using an integrated experimental and modeling approach, consisting of quasielastic neutron scattering, small-angle neutron scattering, X-ray pair distribution function analysis, and electrochemical measurements, which were combined with molecular dynamics simulations. Our analysis shows that surface oxygen groups increase the diffusion of confined electrolytes. Consequently, the ions become more than twice as mobile in oxygen-rich pores. Although greater self-diffusion of ions translates into higher electrochemical mobilities in oxidized pores, bulk-like behavior of ions dominates in the larger mesopores and increases the overall capacitance in defunctionalized pores. Experimental results highlight strong confinement and surface effects of carbon electrodes on electrolyte behavior, and molecular dynamics simulations yield insight into diffusion and capacitance differences in specific pore regions. We demonstrate the significance of surface defects on electrosorption dynamics of complex electrolytes in hierarchical pore architectures of supercapacitor electrodes. [ABSTRACT FROM AUTHOR]

We report results of vibrational neutron spectroscopy investigation aimed to identify the state of hydrogen adsorbed in ultramicroporous carbon. The mobility of hydrogen confined in carbon pores was probed as a function of temperature and pressure using inelastic neutron scattering, and the molecular translational and rotational motions were studied. At low loading rotation of H2 molecules adsorbed in the smallest carbon pores (-4-5 Å) is severely hindered, suggesting that the interaction between H2 and the host matrix is anisotropic. At higher loading, H2 molecules behave as a nearly free rotor, implying lower anisotropic interactions with adsorption sites. At 77 K where bulk H2 is a gas, deconvolution of elastic/quasielastic signal provide evidence of pressure-dependent fractions of immobile (solid-like) and partially mobile (liquid-like) hydrogen, which correlate with the excess adsorption isotherm at 77 K. Effective H2 density in pores changes from solid-like to liquid-like with increasing pressure at 77 K. Surprisingly, immobile and partially mobile H2 is present even at temperatures as high as -110 K where bulk hydrogen exists only in gas form. [ABSTRACT FROM AUTHOR]

This work addresses the detailed molecular dynamic behavior of miscible blends of Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and their pure counterparts by quasi-elastic neutron scattering measurements (QENS). The study provides the measure of relaxation processes on pico-to-nanosecond time scales. A single relaxation process was observed in pure P3HT and PCBM while two relaxation processes, one fast and one slow, were observed in the blends. The fast process was attributed to the dynamics of P3HT while the slow process was correlated to the dynamics of PCBM. The results show that the relaxation process is a balance between two opposing effects: increased mobility due to thermal activation of P3HT molecules and decrease mobility due to the presence of PCBM which is correlated to the percent crystallinity of P3HT and local packing density of PCBM in the amorphous phase. The threshold for the domination of the thermally activated relaxation is between 5 and 9 vol.% of PCBM loading. Two distinct spatial dependences of the relaxation processes, in which the crossover length scale depends neither on temperature nor composition, were observed for all the samples. They were attributed to the collective motions of the hexyl side chains and the rotational motions of the C–C single bonds of the side chains. These results provide an understanding of the effects of PCBM loading and temperature on the dynamics of the polymer-fullerene blends which provides a tool to optimize the efficiency of charge carrier and exciton transport within the organic photovoltaic (OPV) active layer to improve the high performance of organic solar cells. [ABSTRACT FROM AUTHOR]

Cyanobacteria are prokaryotes that can use photosynthesis to convert sunlight into cellular fuel. Knowledge of the organization of the membrane systems in cyanobacteria is critical to understanding the metabolic processes in these organisms. We examined the wild-type strain of Synechocystis sp.PCC6803 and a series of mutants with altered light-harvesting phycobilisome antenna systems for changes in thylakoid membrane architecture under different conditions. Using small-angle neutron scattering, it was possible to resolve correlation distances of subcellular structures in live cells on the nanometer scale and capture dynamic light-induced changes to these distances. Measurements made from samples with varied scattering contrasts confirmed that these distances could be attributed to the thylakoid lamellar system. We found that the changes to the thylakoid system were reversible between light- and dark-adapted states, demonstrating a robust structural flexibility in the architecture of cyanobacterial cells. Chemical disruption of photosynthetic electron transfer diminished these changes, confirming the involvement of the photosynthetic apparatus. We have correlated these findings with electron microscopy data to understand the origin of the changes in the membranes and found that light induces an expansion in the center-to-center distances between the thylakoid membrane layers. These combined data lend a dynamic dimension to the intracellular organization in cyanobacterial cells. [ABSTRACT FROM AUTHOR]

Abstract: Quasi-elastic neutron scattering was used for characterization of dynamics of molecular hydrogen confined in narrow nanopores of two activated carbon materials: a carbon derived from polyfurfuryl alcohol and an ultramicroporous carbon. Fast, but incomplete ortho–para conversion was observed at 10K, suggesting that scattering originates from the fraction of unconverted ortho isomer which is rotation-hindered because of confinement in nanopores. Hydrogen molecules entrapped in narrow nanopores (<7Å) were immobile below 22–25K. Mobility increased rapidly with temperature above this threshold, which is higher than the melting point of bulk hydrogen (13.9K). Diffusion obeyed fixed-jump length mechanism, indistinguishable between 2D and 3D processes. Thermal activation of diffusion was characterized between ∼22 and 37K, and structure-dependent differences were found between the two carbons. Activation energy of diffusion was higher than that of bulk solid hydrogen. Classical notions of liquid and solid do not longer apply for H2 confined in narrow nanopores. [Copyright &y& Elsevier]

Abstract: Energy barriers for rotation of –NH3 and –BH3 in the orthorhombic phase of ammonia borane, NH3BH3, were determined using quasielastic neutron scattering (QENS). QENS confirms the 3-site jump model of rotational diffusion and yields barrier heights of 23.6±1.0kJ/mol and 14.8±0.4kJ/mol for the borane and amine groups, respectively, which are comparable to barrier heights determined by recent 2H and 15N NMR studies suggesting no significant isotope effect on rotational motion in the orthorhombic phase of ammonia borane. [Copyright &y& Elsevier]