Abstract
Carbon materials, such as graphene nanoflakes, carbon nanotubes, and fullerene, can be widely used to store hydrogen, and doping these materials with lithium (Li) generally increases their H2-storage densities. Unfortunately, Li is expensive; therefore, alternative metals are required to realize a hydrogen-based society. Sodium (Na) is an inexpensive element with chemical properties that are similar to those of lithium. In this study, we used density functional theory to systematically investigate how hydrogen molecules interact with Na-doped graphene nanoflakes. A graphene nanoflake (GR) was modeled by a large polycyclic aromatic hydrocarbon composed of 37 benzene rings, with GR-Na-(H2)n and GR-Na+-(H2)n (n = 0–12) clusters used as hydrogen storage systems. Data obtained for the Na system were compared with those of the Li system. The single-H2 GR-Li and GR-Na systems (n = 1) exhibited binding energies (per H2 molecule) of 3.83 and 2.72 kcal/mol, respectively, revealing that the Li system has a high hydrogen-storage ability. This relationship is reversed from n = 4 onwards; the Na systems exhibited larger or similar binding energies for n = 4–12 than the Li-systems. The present study strongly suggests that Na can be used as an alternative metal to Li in H2-storage applications. The H2-storage mechanism in the Na system is also discussed based on the calculated results.
Highlights
Measures that counter climate change are of considerable international urgency [1,2,3,4,5]
Lithium is expensive because it is generally shipped from various international locations to China for processing into various products; alternative metals are required to realize a hydrogen-based society
We showed that electron transfer from Li to graphene plays an important role in the adsorption of H2
Summary
Measures that counter climate change are of considerable international urgency [1,2,3,4,5]. Lithium is expensive because it is generally shipped from various international locations to China for processing into various products; alternative metals are required to realize a hydrogen-based society. In this regard, sodium (Na) is an inexpensive metal with chemical properties similar to those of Li. Interactions between sodium with carbon materials have been investigated both theoretically and experimentally. In order to determine the hydrogen storage ability of GR-Na as an alternative metalcontaining system, we used the DFT method in the present study to investigate interactions between molecular hydrogen and Na-doped graphene nanoflakes (GR-Na-(H2 )n ; n = 1–12). Elucidating the storage mechanisms of inexpensive metals is important for future hydrogentransport applications; we examined in detail whether or not sodium can be used as an alternative to lithium for this purpose
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