Ab Initio Studies of Hydrogen Physisorption on Clear and Li-Doped Carbon Nanotubes

Abstract

The hydrogen adsorption on internal and external surfaces of clear and Lidoped carbon nanotubes of different radii are investigated to assess the effects of concavity and doping on hydrogen uptake and binding energy. We make density functional calculations with the exchange-correlation functionals PBE and CA. Modeling of H 2 adsorption on clear carbon tubes shows that only in case of internal sorption on narrow (5,5) nanotube energy of adsorption fall within the desirable range of 300-400 meV per H 2 molecule. But in this case hydrogen uptake is too low and constitutes about 1,6 wt %. Doping with Li atom increases the adsorption energy of hydrogen molecule by 30-100 meV and in case of external sorption this energy enlarges several times. Nevertheless, the optimal range of binding energy can be achieved only in case of hydrogen adsorption inside quite narrow (5,5) and (7,7) Li-doped nanotubes.Keywords: carbon nanotubes; lithium sorption; hydrogen adsorption; firstprinciples calculations; density functional theory. IntroductionHydrogen is considered as one of the most perspective energy carriers as it has a high energy content per mass, it can be generated from clean and green sources and its combustion produces only water as a byproduct [1]. However, for the utilization of H 2 in the national energy systems the difficulties concerning storage and transporting of hydrogen fuels should be overcome.These problems can be solved by using compact and safe hydrogen storages. To create such storages it is necessary to find appropriate hydrogen sorbent. Carbon-based nanomaterials are thoroughly investigated candidates for H 2 storage due to their low density, porosity, high thermal and chemical stability, the simplicity and low cost of production [2]. Carbon nanotubes (CNTs) have not only properties mentioned above but also high mechanical strength, unique electrical and capillary features and therefore they attract a great attention of researches [3]. However, in spite of high surface area clear CNTs can adsorb only up to 1 wt % at room temperature [4] due to the weak bond between H 2 molecules and tube [5].The possible solution of this problem is more active adsorption sites formation, for instance, by doping of carbon nanotubes with metal atoms [6, 7]. The perspective element for such use is lithium, because it has high nucleation barrier [8, 9]. Recently, Li-doped CNTs have been investigated theoretically and experimentally in search of effective material for Li-ion battery [10, 11]. Much less is known about the interaction of these structures with hydrogen. To fill this knowledge gap ab initio modeling based on density functional theory of H 2 physisorption on clear and Li-doped carbon nanotubes was made.

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

“…Previously, we showed that doping CNTs with Li atom increases the adsorption energy of H 2 molecule for single molecule adsorption, and the E bind of the hydrogen molecule adsorbed inside the nanotube is in the desirable range . However, the hydrogen uptake of a CNT@Li-based material could be accessed once we defined the maximum H 2 binding capability of the Li adatom.…”

“…16−25 Previously, we showed that doping CNTs with Li atom increases the adsorption energy of H 2 molecule for single molecule adsorption, and the E bind of the hydrogen molecule adsorbed inside the nanotube is in the desirable range. 26 However, the hydrogen uptake of a CNT@Li-based material could be accessed once we defined the maximum H 2 binding capability of the Li adatom. To be more precise, we need to know the structural and energetic characteristics of CNT@Li +kH 2 complexes, k = 1, 2, ..., k max , where k max is the maximum quantity of H 2 molecules.…”