Quantum chemical study on the alkali atom doped calix[4]arene as hydrogen storage material

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Abstract

We have demonstrated that doping alkali cations in charged state can improve the hydrogen adsorption significantly in the molecular form. In addition, the number of hydrogen molecules adsorbed by Li cation doped benzene system was 3 while Na and K doped benzene were able to adsorb up to 6 hydrogen molecules. In general the adsorption energies of alkali atoms and the binding energy/H2 for hydrogen are underestimated by the hybrid B3LYP functional, while MP2 functional provides higher binding energy. The nature of interaction between hydrogen and the alkali center was mainly due to be dipole – quardupole and dipole – induced dipole electrostatic interaction. Further, we extended the present single benzene system to the curved calixarene system. The calixarene ring was able to adsorb up to five alkali atoms, one inside the cavity and 4 on the walls of the cavity and this system was capable to adsorb up to 30 H2 molecules in molecular form at low temperature.

Introduction

Nowadays, worldwide interest is focused on using a clean burning substitute such as hydrogen in place of fossil fuels, due to both economic and environmental reasons [1], [2]. However, their storage is one of the most important challenges impeding its practical application. In the past attempts have made to find new storage materials by theoretical and experimental approach [3], [4], [5]. However, no materials were found to successfully store hydrogen reversible to satisfy the US Department of Energy (DOE) standards. In the past, studies on carbon materials show a weak physisorption for hydrogen and are found to be outrageous for storage materials [6]. Doping of transition metal elements increases the binding energy of hydrogen, but due to the tendency of clustering of transition metals the storage capacity of the materials are reduced enormously [7], [8]. Some metal–organic frameworks (MOFs) show promising results for hydrogen adsorption, but the storage pressure and temperature conditions make their use economically unfavorable [9], [10], [11]. Moreover, the presence of large number of metal sites decreases their gravimetric density, thereby making them ineffective in the onboard hydrogen storage.

In order to improve the gravimetric storage capacity, it is important to design new materials not only having tailored pore dimension and large void volume but also the materials should consist of elements with molecular mass less than that of aluminum [12], [13], [14]. In this context, organic porous solid, calix[4]arene was found to be promising materials and are capable of storing CO2 up to 10 wt.% reversible, while the storage capacity of other small diatomic molecules are less on these materials [15], [16]. The large pore size and weak physisorption of diatomic molecules are responsible for the low gravimetric adsorption density on these organic hosts [17]. In the past, theoretical studies provide better understanding on the adsorption of hydrogen on materials and energy value of 0.1–0.5 eV was predicted to be the ideal value for a system to satisfy the DOE requirement [18]. Recently, Li doping was realized to enhance the storage capacity of materials both by experimental and theoretical means and to provide the desired binding enthalpies in the range of 0.2–0.3 eV for hydrogen [19], [20], [21], [22], [23].

In this paper, we have performed quantum chemical calculations with the density functional theory (DFT) context, to understand the adsorption of hydrogen on the alkali atom doped calix[4]arene. First, we examine the adsorption energies of alkali atoms (Li, Na and K) on the model system benzene and calix[4]arene molecule. Next, we compare the adsorption of hydrogen molecules on the Li functionalized benzene molecules by using MP2 and DFT methods. Finally, we have shown the adsorption energy of sodium doped calix[4]arene molecule to be in the desired rage for a ideal storage material. The paper is organized as bellow, first in the following section we have provided sufficient details on the calculation methods used in the work. Then, we show and discuss the results of our studies and finally we arrive at our conclusion from the present study.

Section snippets

Computational details

All calculations reported in this work were performed with the Gaussian 03 program package [24]. All geometry optimizations for the alkali atom doped benzene system were done without any geometrical constrains at the Møller–Plesset truncated at the second order level (MP2) and with the hybrid B3LYP method with 6-311 + G(d, p) basis set. For the calix[4]arene system optimization were carried out with B3LYP method and 6-31G basis set and for energy calculations B3LYP/6-311 + G(d, P) basis set was used.

Results and discussion

Our previous study on the adsorption of hydrogen on the simple organic molecular systems calix[4]arene shows that hydrogen adsorption occurs due to the very weak interaction between the hydrogen and organic molecules [17]. However, it appears that a simple van der Waals surface is not suitable for hydrogen adsorption; On this ground doping Li cation was realized to create a strong ionic center which is very useful for the adsorption of more hydrogen molecules closer to the surface with a

Conclusion

In conclusion, we have demonstrated that doping alkali cations in charged state can improve the hydrogen adsorption significantly in the molecular form. In addition, the number of hydrogen molecules adsorbed by Li cation doped benzene system was 3 while Na and K doped benzene were able to adsorb up to 6 hydrogen molecules. In general the adsorption energies of alkali atoms and the binding energy/H2 for hydrogen are underestimated by the hybrid B3LYP functional, while MP2 functional provides

Acknowledgements

This work has been supported by New Energy and Industrial Technology Development Organization (NEDO) under “Advanced Fundamental Research Project on Hydrogen Storage Materials”. The authors thank the crew of the Center for Computational Materials Science at Institute for Materials Research, Tohoku University, for their continuous support of the HITACHI SR11000 supercomputing facility.

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