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Ab initio first-principles calculations were carried out to investigate lithium-dispersed two-dimensional carbon allotropes, viz. graphyne and graphdiyne, for their applications as lithium storage and hydrogen storage materials. The lithiation potentials (vs Li/Li + ) and specific capacities in these materials are found to be enhanced considerably as compared to the conventional graphite-based electrode materials. Lithium metal binding to these carbon materials is found to be enhanced considerably and is more than the cohesive energy of lithium. Each lithium atom in these metal-dispersed materials is found to carry nearly one unit positive charge and bind molecular hydrogen with considerably improved adsorption energies. Our calculated hydrogen adsorption enthalpies (−3.5 to −2.8 kcal/mol) are very close to the optimum adsorption enthalpy proposed for ambient temperature hydrogen storage (−3.6 kcal/mol). We have also shown that the band gaps in these planar carbon allotropes can be tuned by varying the number of acetylenic bridging units which will have versatile applications in nanoelectronics.

It is demonstrated that the doping of alkali metal atoms on fullerene, C60, remarkably enhances the molecular hydrogen adsorption capacity of fullerenes, which is higher than that of conventionally known other fullerene complexes. This effect is observed to be more pronounced for sodium than lithium atom. The formation of stable complex forms of a sodium-doped fullerene molecule, Na8C60, and the corresponding hydrogenated species, [Na(H2)6]8C60, with 48 hydrogen molecules has been demonstrated to lead to a hydrogen adsorption density of approximately 9.5 wt %. One of the main factors favoring the interactions involved is attributed to the pronounced charge transfer from the sodium atom to the C60 molecule and electrostatic interaction between the ion and the dihydrogen. The suitability of these complexes for developing fullerene-based hydrogen storage materials is discussed.

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The concepts of local temperature, local entropy, and local free energy density'are introduced within the framework of the ground-state density-functional theory of many-electron systems, and a complete local thermodynamic picture is then developed. A view emerges of the electron cloud, as analogous to a classical inhomogeneoUs fluid moving under gradients of temperature, pressure, and an effective potential, described by a locally Maxwellian distribution.Density functional theory (1, 2) provides a way to characterize a nonhomogeneous many-electron system in terms of the electron density and through its connection with quantum hydrodynamics (3) permits one to view an electron cloud as a fluid in three-dimensional space. From this picture, one can obtain much physical insight through the study of local behavior rather than global quantities such as the total internal energy. However, although the total energy is determined uniquely by the electron density, the nonlocal nature of the energy functional prevents the density at a particular point from providing a complete characterization of the electron fluid at that point. To obtain a local description of a quantum system, it therefore is necessary to introduce certain other local (ir-dependent) quantities.

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Hydrogen generation through photocatalytic water splitting with the aid of renewable solar energy is an important step toward the development of sustainable and alternative energy. In the present study, using the first-principles calculations, we have explored the s-triazine based two-dimensional porous graphitic carbon nitride (g-CN) materials as a potential photocatalyst for water splitting. For calculating the band structures more accurately, we have employed hybrid density functionals. The calculated band gap of the single layer g-CN is found to be 2.89 eV, which decreases to ∼2.75 eV in multilayered structure. To improve the visible light activity, the effect of doping with different nonmetals on the electronic structure has been investigated. Among the different dopants studied, phosphorus is found to be more effective to reduce the band gap to 2.31 eV. The band edge potentials obtained from density functional calculations are corrected for vacuum potentials. The band alignments with respect to the water redox levels show that the thermodynamic criterion for the overall water splitting is satisfied. We have also carried out analogous studies on the heptazine based carbon nitride, g-C 3 N 4 , and the calculated band gaps, as well as the position of the valence band maximum, are consistent with the reported experimental results validating the computational method we have used. Based on our theoretical investigations, we can predict that the s-triazine based carbon nitride materials should be a potential photocatalyst for water splitting under visible light.