Interactions of hydrogen molecules with complexes of lithium cation and aromatic nitrogen-containing heterocyclic anions

Highly stable salt functional groups consisting of lithium cation and aromatic anions (C _n H _n N_5?n ?Li) are studied for hydrogen storage using ab initio calculations, force field development, and grand canonical Monte Carlo simulations. Second-order Møller–Plesset perturbation theory with the resolution of identity approximation calculations are calibrated at the CCSD(T)/complete basis set (CBS) level of theory. The calibrations on different types of binding sites are different, but can be used to correct the van der Waals interactions systematically. The anion and salt functional groups provide multiple binding sites. With increased number of nitrogen atoms in the aromatic anion, the number of binding sites increases but the average binding energy decreases. Among the functional groups considered, CHN₄-Li exhibits the largest number of binding sites (14) and a weak average binding energy of 5.7 kJ mol^–1 with CCSD(T)/CBS correction. The calculated adsorption isotherms demonstrate that the introduction of the functional group significantly enhances hydrogen uptake despite relatively weak average binding energy. Therefore, it is concluded that searching for functional groups with the larger number of binding sites is another key factor for enhancing the hydrogen storage capacity, given that other conditions such as free volume and surface area are fixed.