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.

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.