Hydrogen adsorption on lithium clusters coordinated to a gC3N4 cavity

A new type of renewable and environmentally-friendly source of energy is the combustion of molecular hydrogen (H2). [1] However, transportation and storage of hydrogen remains the biggest challenge for its commercial applications. [2] The usual H2 storage techniques, such as liquefaction, storage in metal hydrides, and solid-state absorption are still not suitable, because safety, weight and cost issues remain to be solved. [3] Thus, for developing the hydrogen economy it is imperative to consider hydrogen storage in materials with high gravimetric/volumetric density and good adsorption/desorption. [4] In this sense, van der Waals interaction under ambient conditions is an attractive alternative, since hydrogen may be easily desorbed afterwards. Carbon-based materials have characteristics that can meet these requirements, i.e., large surface area, low production cost, low weight and good chemical stability. [5,6]

A practical way to predict the reactivity of carbon-based materials towards hydrogen adsorption is by using the density functional theory (DFT), which was already reported for H2 storage with graphene [[7], [8], [9]] and graphene doped with Pt [10] and Pd. [11] Fullerenes and super activated carbon, [12] as well as graphitic carbon nitride (gC3N4) [13], were recently identified as promising candidates for hydrogen storage. The gC3N4 is a 2D structure have well-defined cavities lined with nitrogen atoms, making this material reactive towards positive or partially positive chemical species. gC3N4 consists of two types of structural units: the tri-s-triazine (heptazine) and the s-triazine. As the former is more stable under environmental conditions, [14,15] it was considered more attractive for this study. The gC3N4 can be easily synthesized by thermal polymerization of abundant nitrogen-rich precursors such as melamine, dicyandiamide, cyanamide, urea, or thiourea. [[16], [17], [18]] The material reactivity towards neutral or negatively charged molecules can be modified by insertion of electropositive atoms, e.g. lithium. [3,15,[19], [20], [21]] Ibarra-Rodríguez and Sánchez recently studied the adsorption of different gases on a graphitic carbon nitride fragment having a single Li atom in its cavity. [22] The influence of different metal clusters (Lin, Ben, and Bn, n = 2–6) on the reactivity of the gC3N4-fragment cavity has also been studied for the adsorption of a styrene molecule. [20] The clusters were coordinated in the center of the cavity forming B – N, Be – N and Li – N bonds. [20] Liu et al. studied the 2Li/BN-yne system, finding that it was able to adsorb six hydrogen molecules. [23] On the other hand, Seenithurai and Chai studied several Li-coordinated n-acene systems, finding that each Li atom can adsorb up to two H2 molecules with energies in the range of 0.17–0.41 eV. [24]

More reports on gC3N4 for H2 storage can be found in the literature. [[25], [26], [27], [28], [29]] However, none of them focuses on the incorporation of Li clusters (Lin, n > 2) into gC3N4 structure. Therefore, it is timely and important to address the role of coordinated Li atoms for effective hydrogen storage. Although we cannot simulate the final macroscopic device using DFT or similar techniques, it becomes clear that the design of the adsorption cell has to consider the questions of hydrogen diffusion and saturation of the adsorbing element. If the latter is manufactured in the shape of a solid block, one can envision the difficulty of establishing proper hydrogen flux to the areas of the device that are still capable of adsorption. One of the possible solutions to this problem concerns the use of powders or emulsions, with adsorber material present at a very reduced size. This will allow to form a loose and unconsolidated system of adsorptive particles, facilitating hydrogen diffusion and moreover, benefiting from their increased total surface area. Hence, we decided to focus our attention on such type of system geometry.

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