Abstract
Herein, results of thermodynamic analysis of some theoretical and experimental [thermal desorption (TDS), scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), high-resolution electron energy loss spectroscopy/low-energy electron diffraction (HREELS/LEED), photoelectron spectroscopy (PES), angle-resolved photoemission spectroscopy (ARPES), Raman spectroscopy and others] data on “reversible” hydrogenation and dehydrogenation of some graphene-layer-nanostructures are presented. In the framework of the formal kinetics and the approximation of the first order rate reaction, some thermodynamic quantities for the reaction of hydrogen sorption (the reaction rate constant, the reaction activation energy, the per-exponential factor of the reaction rate constant) have been determined. Some models and characteristics of hydrogen chemisorption on graphite (on the basal and edge planes) have been used for interpretation of the obtained quantities, with the aim of revealing the atomic mechanisms of hydrogenation and dehydrogenation of different graphene-layer-systems. The cases of both non-diffusion rate limiting kinetics and diffusion rate limiting kinetics are considered. Some open questions and perspectives remain in solving the actual problem in effective hydrogen on-board storage; using the graphite nanofibers (GNFs) is also considered. Key words: Epitaxial and membrane graphenes, other graphene-layer-systems, hydrogenation-dehydrogenation, thermodynamic characteristics, atomic mechanisms, the hydrogen on-board efficient storage problem. 
Highlights
As suggested in Waqar (2007), atomic hydrogen intercalates between layers in the graphite net through holes in graphene hexagons, because of the small diameter of atomic hydrogen, compared to the hole’s size, and is converted to a H2 gas form which is captured inside the graphene blisters, due to the relatively large kinetic diameter of hydrogen molecules. Such interpretation is in contradiction with that noted in Introduction results (Xiang et al, 2010; Jiang et al, 2009), that it is almost impossible for a hydrogen atom to pass through the six-member ring of graphene at room temperature
In this study, the results of the thermodynamic analysis (Equations 11 and 12) are presented, which may be used for interpretation of related data (Figures 6 to 8, 11 to 16, 19 to 21)
A number of researchers noted above have not taken into account the calculation results (Xiang et al, 2010) showing that the barrier for the penetration of a hydrogen atom through the six-member ring of a perfect graphene is larger than 2.0 eV
Summary
As noted in a number of articles 2007 through 2014, hydrogenation of graphene-layers-systems, as a prototype of covalent chemical functionality and an effective tool to open the band gap of graphene, is of both fundamental and applied importance (Geim and Novoselov, 2007; Palerno, 2013). Such different (in some sense, extraordinary) interpretation is consisted with the above analytical data (Table 1A) on activation energies of hydrogen adsorption for the epitaxial graphene samples (∆H(ads.)epitax.[5] ≈ 0.3 ± 0.2 eV), which is much less than the similar one for the free standing graphene membranes (Elias et al, 2009) (∆H(ads.)membr.[5] = 1.0 ± 0.2 eV) It may be understood for the case of chemisorotion [of “F”, “G” and/or “H” types (Figure 4)] on the internal graphene surfaces [neighboring to the substrate (SiO2) surfaces], which obviously proceeds without the diamond-like strong distortion of the graphene network, unlike graphene (Sofo et al, 2007). Such plastic deformation of the nanoregins during hydrogenation of GNFs may be accompanied with some mass transfer resulting in such thickness (db) of the walls. (6) The related data (Figure 25) allows us to reasonably assume a break-through in results (Nechaev and Veziroglu, 2013) on the possibility (and physics) of intercalation of a high density molecular hydrogen (up to solid H2) into closed (in the definite sense) nanoregions in hydrogenated GNFs (Gupta et al, 2004; Park et al, 1999), relevant for solving of the current problem (Akiba, 2011; Zuettel, 2011; DOE targets, 2012) of the hydrogen on-board effective storage. (7) Some fundamental aspects - open questions on engineering of "super" hydrogen storage carbonaceous nanomaterials, relevance for clean energy applications, are considered in (Nechaev and Veziroglu, 2013) and in this study, as well
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