We perform an extensive combinatorial search for optimal nanostructured hydrogen-storage materials among various metal-decorated polymers using first-principles density-functional calculations. We take into account the zero-point vibration as well as the pressure- and temperature-dependent adsorption-desorption probability of hydrogen molecules. An optimal material we identify is Ti-decorated cis-polyacetylene with reversibly usable gravimetric and volumetric density of 7.6 wt % and $63\mathrm{kg}/{\mathrm{m}}^3$, respectively, near ambient conditions. We also propose “thermodynamically usable hydrogen capacity” as a criterion for comparing different storage materials.

Using first-principles calculations based on the density-functional theory, we perform a detailed study of the dihydrogen $\left({\mathrm{H}}_2\right)$ binding in cis- and trans-polyacetylene decorated with transition metal atoms. First, we investigate the origin of metal-dihydrogen bonding and observe the hybridization of $e_g$ $\left(t_{2g}\right)$ orbitals of the Ti atom with the $\sigma$ $\left({\sigma }^{*}\right)$ orbitals of the ${\mathrm{H}}_2$ molecules in octahedral geometries, which is consistent with the Kubas model. Second, using a statistical model parametrized by the results of ab initio calculations and experimental data, the adsorption and desorption of molecular hydrogens are calculated at ambient temperature and pressure. We find that the usable capacity at ambient conditions is dramatically reduced from the maximum capacity, the zero-point energy affects the storage capacity significantly, and the optimal binding energy of ${\mathrm{H}}_2$ molecules under practical conditions is $\sim 0.3\mathrm{eV}∕{\mathrm{H}}_2$. Third, we examine the effects of the aggregation and intercalation of the Ti atoms on ${\mathrm{H}}_2$ adsorption.

We propose a route to dispersing hydrogen-adsorbing transition metals (TMs) on a large scale onto vacancy-engineered defective graphenes by employing natural carbon-nitrogen-TM complexes, i.e., TM-containing porphyrins. Based on first-principles density-functional calculations, the TM-porphyrin core\char22{}made of one central TM and four surrounding nitrogen atoms\char22{}can be effectively generated by three defect-engineering processes of graphenes: (1) creation of carbon divacancies, (2) nitrogen substitution of unsaturated carbons, and (3) TM incorporation. The atomistically dispersed Sc, Ti, and V are able to adsorb hydrogen molecules as strongly as 0.2\char21{}0.4 eV with the Kubas coordination. The Fe-porphyrin-like unit in graphenes can also have the Kubas adsorption of hydrogen, if the exchange splitting is reduced by a compressive in-plane strain.