Li-Functionalized Carbon Nanotubes for Hydrogen Storage: Importance of Size Effects

Abstract

We investigated Li-doped carbon nanotubes (CNTs) as a promising hydrogen storage media. In this computational model, we considered isolated lithium atom adsorbed on a CNT wall as an adsorption site for hydrogen. We focused on the influence of size effects on the structural and energetic characteristics of CNT­(n,n)@Li+kH2 complexes where n = 5, 7, 9; k = 1, ..., 6; N c = 4, 5, 6 (N c is translation length of CNT, expressed in terms of a number of CNT unit cells). We proved that modeled CNT length substantially influences internal sorption of Li and hydrogen on the narrow tube (5,5), which subsequently alters the adsorption energies of H2 molecules and causes the deformation of the carbon framework. Moreover, the size effects are not pronounced in the case of external sorption for all considered CNT translation lengths and diameters. We have not observed any noticeable qualitative difference between internal and external hydrogen sorption in the nanotube wider than CNT­(5,5). In the case of external adsorption on all considered nanotubes, doping with Li increases hydrogen adsorption energies of up to four H2 molecules by 100 meV in comparison with pure CNTs. And the local density approximation estimations (∼250 meV/H2) of adsorption energy on Li-decorated CNTs exceed the lowest requirement proposed by the U.S. Department of Energy (200 meV/H2). In the case of internal sorption on Li-functionalized tubes, the generalized gradient approximation also gives hydrogen adsorption energies in the desired range of 200–600 meV/H2. However, steric hindrances could prevent sufficient hydrogen uptakes (less than 2 wt % inside CNT­(5,5)). We believe that our findings on the size effects are important for estimation of CNT’s hydrogen storage properties.

Current Research Trends and Perspectives on Solid-State Nanomaterials in Hydrogen Storage

“… Overview and examples of the solid hydrogen storage systems. (a) Nanoporous carbon materials (carbon nanotube (CNT), (b) MOF, (c) COF, (d) PAFs, (e) the structure of a nanoporous organic polymer (PIM-1) and the composite with a nanoporous filler, and (f) nanohydrides (sodium alanate (NaAlH 4 )) confined in the nanopores of a MOF (MOF-74) [ 27 , 51 – 56 ]. Reproduced with permission [ 56 ].…”

An Overview of the Recent Progress in Modifications of Carbon Nanotubes for Hydrogen Adsorption

“…Although using the modified synthesis method without loading metals has been reported to facilitate the hydrogen adsorption capacity of CNTs [65,127], in fact, some residual metal catalyst has played an important role in the reported high capacities of hydrogen adsorption. Two possibilities explaining the catalysis effect of metal dopants on the hydrogen storage properties of CNTs have been reported: spillover mechanism ( Figure 6) [122,[128][129][130][131] and the Kubas type of interaction of hydrogenation (Figure 7) [47,[132][133][134]. Through the spillover mechanism, the hydrogen adsorbed by the CNT-metal dopant system is more than the sum of hydrogen adsorbed by CNT and metal dopant separately under the same conditions.…”

“…Compared with hydrogen adsorbed by CNTs, E ad of H 2 molecules adsorbed by metal-doped CNTs increases to 0.2-0.6 eV/H 2 caused by electron transfer from metal atoms to carbon atoms, which is between strong chemisorption and weak physisorption states and is ideal for the reversible adsorption/desorption of H 2 at ambient conditions [47,102,148,149]. The increase in E ad is determined by metal dopants and the size effect of CNTs, as well as the interaction between hydrogen molecules, adsorbed on metal dopants [132][133][134]. It is worth emphasizing that 0.2 eV/H 2 is the lowest requirement proposed by the U.S. DOE [150].…”

“…At present, the added metal catalysts are mainly Pd [117,125,129,130,135,140,141,143,[151][152][153][154][155][156][157][158][159], Mg [160][161][162], Ni [98,136,139,163,164], Ti [34,137,138,165,166], Li [132,144,150,167], Al [102,168,169], etc.…”

Enhanced Room‐Temperature Synthesis of Li@f‐MWCNTs for Hydrogen Storage Application

“…But the hydrogen technology faces a major setback in terms of Hydrogen storage, especially for on‐board transportation. The storage of hydrogen needs to be efficient, safe and compact . This is possible only by solid‐state storage of hydrogen through adsorption on materials.…”