Low dimensions for hydrogen storage media: exceeding U.S. DOE uptake target on Li-functionalized carbyne with hydrogen binding energies needed for good cyclability.

The problem of decaying rotation of a disk floating on the surface of a viscoelastic fluid in a cylindrical container is solved by numerical methods. The motion is found to have the form of decaying oscillations observed previously for water. In addition to the viscosity coefficient, the constructed mathematical model of the viscoelastic fluid has two more independent parameters: shear modulus and time of relaxation of elastic stresses. Elastic parameters of water are determined through comparisons with experimental data.Introduction. Rotational motion of a floater made in the form of a disk floating on the fluid surface in a cylindrical container is studied in the present work by numerical methods (Fig. 1). The disk is instantaneously set into motion (rotation around its own centerline coinciding with the container centerline). When the disk acquires steady-state rotation, the action of the external moment is terminated, and the disk motion starts to decay owing to viscous friction on the fluid. It was found in the experiments [1] that the decay has an oscillating character (Fig. 2). Based on this fact, water may be assumed to have viscoelastic properties, though it is normally considered as a Newtonian fluid. The problem considered here was numerically solved in [2] in the approximation of the Newtonian fluid; changes in the initial conditions and physical parameters of the disk and the container were found to initiate no backward motion of the disk. The data of [1] are interpreted in the present work on the basis of several linear models of a viscoelastic fluid. The most suitable model turned out to be the model of viscoelasticity [3] with an exponential function of decaying of elastic stresses. As compared with the Newtonian fluid model, the model developed in [3] has two more independent parameters: shear modulus G and time of relaxation of elastic stresses T . The fluid considered is interpreted as a substance with the stress dependent on the strain and strain rate. In turn, the strain is determined by the differences in the material configuration at consecutive times.Mathematical Model. The stress tensor of the examined fluid is presented as σ = σ vis + σ el . The elastic part of the stress tensor of the isotropic fluid with the convective elasticity

We investigated Li-doped carbon nanotubes (CNTs) as a promising hydrogen storage media. In this computational model, we considered isolated lithium atom adsorbed on a CNT wall as an adsorption site for hydrogen. We focused on the influence of size effects on the structural and energetic characteristics of CNT­(n,n)@Li+kH2 complexes where n = 5, 7, 9; k = 1, ..., 6; N c = 4, 5, 6 (N c is translation length of CNT, expressed in terms of a number of CNT unit cells). We proved that modeled CNT length substantially influences internal sorption of Li and hydrogen on the narrow tube (5,5), which subsequently alters the adsorption energies of H2 molecules and causes the deformation of the carbon framework. Moreover, the size effects are not pronounced in the case of external sorption for all considered CNT translation lengths and diameters. We have not observed any noticeable qualitative difference between internal and external hydrogen sorption in the nanotube wider than CNT­(5,5). In the case of external adsorption on all considered nanotubes, doping with Li increases hydrogen adsorption energies of up to four H2 molecules by 100 meV in comparison with pure CNTs. And the local density approximation estimations (∼250 meV/H2) of adsorption energy on Li-decorated CNTs exceed the lowest requirement proposed by the U.S. Department of Energy (200 meV/H2). In the case of internal sorption on Li-functionalized tubes, the generalized gradient approximation also gives hydrogen adsorption energies in the desired range of 200–600 meV/H2. However, steric hindrances could prevent sufficient hydrogen uptakes (less than 2 wt % inside CNT­(5,5)). We believe that our findings on the size effects are important for estimation of CNT’s hydrogen storage properties.

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In this study, we have employed density functional theory with a range of van der Waals corrections to study geometries, electronic structures, and hydrogen (H 2 ) storage properties of carbon ene−yne (CEY) decorated with selected alkali (Na, K) and alkaline-earth metals (Mg, Ca). We found that all metals, except Mg, bind strongly by donating a major portion of their valence electrons to the CEY monolayers. Thermal stabilities of representative systems, Ca-decorated CEY monolayers, have been confirmed through ab initio molecular dynamics simulations (AIMD). We showed that each metal cation adsorbs multiple H 2 with binding energies (E bind ) considerably stronger than on pristine CEY. Among various metal dopants, Ca stands out with the adsorption of five H 2 per each Ca having E bind values within the desirable range for effective adsorption/desorption process. The resulting gravimetric density for CEY@Ca has been found around 6.0 wt % (DFT-D3) and 8.0 wt % (LDA), surpassing the U.S. Department of Energy's 2025 goal of 5.5 wt %. The estimated H 2 desorption temperature in CEY@Ca exceeds substantially the boiling point of liquid nitrogen, which confirms its potential as a practical H 2 storage medium. We have also employed thermodynamic analysis to explore the H 2 adsorption/desorption mechanism at varied conditions of temperature and pressure for real-world applications.