Elsevier

Journal of Power Sources

Volume 271, 20 December 2014, Pages 32-41
Journal of Power Sources

Hydrogen storage reactions on titanium decorated carbon nanocones theoretical study

https://doi.org/10.1016/j.jpowsour.2014.07.158Get rights and content

Highlights

  • DFT study of hydrogen storage on Ti functionalized CNC.

  • Ti can bind up to 6 hydrogen molecules with surface coverage 0.94.

  • Reversible and irreversible reactions are characterized.

  • Expected hydrogen storage capacity 14.34 wt% and rate constants ratio 1.35.

  • Physisorption and thermodynamics meet the ultimate targets of DOE for vehicle operation.

Abstract

Hydrogen storage reactions on Ti decorated carbon nanocones (CNC) are investigated by using the state of the art density functional theory calculations. The single Ti atom prefers to bind at the bridge site between two hexagonal rings, and can bind up to 6 hydrogen molecules with average adsorption energies of −1.73, −0.74, −0.57, −0.45, −0.42, and −0.35 eV per hydrogen molecule. No evidence for metal clustering in the ideal circumstances, and the hydrogen storage capacity is expected to be as large as 14.34 wt%. Two types of interactions are recognized. While the interaction of 2H2 with Ti–CNC is irreversible at 532 K, the interaction of 3H2 with Ti–CNC is reversible at 392 K. Further characterizations of the former two reactions are considered in terms of projected densities of states, simulated infrared and proton magnetic resonance spectra, electrophilicity, and statistical thermodynamic stability. The free energy of the highest hydrogen storage capacity reaction between 6H2 and Ti–CNC meets the ultimate targets of department of energy at (233.15 K) and (11.843 atm) with surface coverage (0.941) and (direct/inverse) rate constants ratio (1.35).

Introduction

Hydrogen powered fuel cell is a technology being pursued as an alternative to gasoline. As an energy carrier for use in vehicle applications, hydrogen has several advantages. It has the highest density by weight and can be produced renewably. In this direction a wide range of materials are currently being considered as potential reversible hydrogen storage materials. Of these, carbon based materials can be structured into a variety of forms including carbon nanotubes, fibers, fullerenes, and activated carbons. They are attractive materials for hydrogen storage, as they have the potential to be low cost, light weight, reversible, and possess facile hydrogen charge–discharge kinetics. However, they share a common limitation of weak van der Waals interaction between molecular hydrogen and the host material which is translated to an operating temperatures at or near that of liquid nitrogen. Therefore, to enhance hydrogen binding, small amounts of hydrogen dissociating catalysts such as transition metals are added to generate atomic hydrogen.

The hydrogen sorption properties of carbon nanotubes have been investigated extensively in recent years. However, there has been considerable controversy over the storage properties of some of these materials since the publication of the first report of the potential for room temperature storage of hydrogen by carbon nanotubes [1]. The results for hydrogen storage in carbon nanotubes have led to the emergence of carbon nanohorns. Single walled carbon nanohorns (SWCNHs) consist of single walled graphitic structures formed out of a single graphene sheet rolled up to form conical (horn like shapes), that aggregate to form globular rosette structures with sizes of about 80–100 nm. CNHs exhibit very large surface areas approaching 1500 m2 g1. Because of low cost, high purity and high surface area CNHs became attractive candidates for hydrogen storage. Heats of adsorption corresponding to 100–120 meV have been reported and attributed to enhanced interaction of H2 molecules at the conical tip of nanohorn [2].

CNHs are subclass of the carbon nanocones CNCs family. They are the fifth allotropic form of carbon, and have been selected for investigating hydrogen storage capacity because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures [3]. However, hydrogen storage on CNCs has been relatively little explored theoretically. Ming-Liang Liao [4] investigated hydrogen adsorption behaviors of single walled carbon nanocones SWCNCs by molecular dynamics simulations. A. Gotzias et al. [5] examined hydrogen adsorption on CNHs and CNCs by using the grand canonical Monte Carlo method. Q. Wang et al. [6] tested the capability to store hydrogen by using the gradient corrected density functional theory.

Yildirim and Ciraci [7] investigated Ti decorated CNT as potential high capacity hydrogen storage medium. They found that the first H2 adsorption is dissociative, while other adsorptions are molecular with elongated H–H bonds in agreement with our results for Ti decorated CNC. However, they reported that Ti binds up to four hydrogen molecules with hydrogen storage capacity up to 8 wt% in contrast with our results that Ti binds up to six hydrogen molecules with expected hydrogen storage capacity up to 14.34 wt%. These discrepancies might be attributed to the different curvatures of CNTs and CNCs. Samolia and Kumar [8] carried out theoretical calculations to examine the hydrogen sorption efficiency of Ti functionalized MOF with organic linker replaced with BN linker. They reported that low adsorption and desorption energies suggest the high hydrogen reversibility of the system. This may be correlated with the present reversible adsorption energies of 3–6 H2 molecules relative to the irreversible adsorption energies of 1 and 2 H2 molecules. They also reported that Ti adsorbs 4 H2 molecules by Kubas interaction with average desorption temperature 323 K and storage capacity 7.8 wt%. This desorption temperature may be compared with the present 233.15 K assigned for the highest hydrogen storage capacity reaction between 6H2 and Ti–CNC. The preferred orientation of single H2 molecule adsorbed on Ti decorated CNT was investigated by Shalabi et al. [9]. The preferential adsorption of H2 was found to be on Ti atom located on the most stable top adsorption site of the (5,5) SWCNT, with H2 oriented parallel to tube (x)-[100] axis. The corresponding adsorption energy of −0.44 eV meets the DOE target for physisorption (−0.2 to −0.6 eV) and is close to the present average adsorption energy of 4H2 molecules (−0.45 eV). Carbon based materials decorated by transition metals other than Ti have been also investigated. Nachimuthu et al. [10] found that among the investigated transition metals, the Ni, Pd and Co atoms were suitable for decorating boron doped graphene, which could be adsorbed stably on the surface. They reported −0.68 eV for single hydrogen molecule adsorption on Pd. This value may be compared with the results of Xiao et al. [11]. They reported 0.80 eV for single hydrogen molecule adsorption on Pd with bond direction perpendicular to the axis of the (8,0) SWCNT. They also reported that electrostatic Coulomb attraction and orbital repulsion mediate the interaction between H2 and Pd.

Hydrogen storage in Yttrium decorated single walled carbon nanotube has been investigated by Chakraborty et al. [12]. They predicted that a single Y atom attached on SWCNT can adsorb up to six hydrogen molecules and showed that 100% desorption at comparatively lower temperature can be achieved in a transition metal decorated SWCNT system. Finally, J. Yang et al [13] reviewed the most valuable experimental and computational techniques employed in the field of hydrogen storage materials research.

With reference to the ultimate targets of US department of energy for physisorption, temperature, pressure, and gravimetric density, we investigate the hydrogen storage capacity of Ti metal functionalized single walled carbon nanocones (SWCNCs), the preferable location of the metal, and metal clustering. We have considered the nH2-Ti-CNC and nH2-2Ti–CNC (n = 1–6) activated complexes with special attention to the characterization of reversible and irreversible hydrogen storage reactions and the thermodynamic capabilities of the highest hydrogen storage complex.

Section snippets

Computational details

The DFT calculations were performed by using Becke's three-parameter exchange functional (B3) with Lee Young Parr (LYP) correlation functional [14], [15], [16], [17]. B3LYP correctly reproduces the thermo chemistry of many compounds including transition metal atoms [18], [19], [20]. The advantages of employing DFT calculations for hydrogen storage materials research may be summarized as the accuracy of computed thermodynamic quantities, the efficiency relative to experiment, and thermodynamic

Results and discussion

We constructed a CNC with disclination angle 180° and height 9 Å, consisting of 69 carbon atoms and 27 hexagonal rings. There are 24 bridge sites between hexagonal rings, in addition to 10 more associated with the dangling bonds at the CNC base, available for accommodating Ti atoms. The optimal geometries of CNC, Ti–CNC, nH2–Ti–CNC, and nH2–Ti2–CNC (n = 1–6) are shown in Fig. 1. The most stable site of Ti atom adsorption is the bridge site. The distances between the Ti atom and the nearest

Interactions of 2H2 and 3H2 with Ti–CNC

Two types of interactions between nH2 and Ti–CNC could be identified from Table 1 : (i) irreversible interactions between nH2 (n = 1–2) and Ti–CNC (ii) reversible interactions between nH2 (n = 3–6) and Ti–CNC. The irreversible interactions are outside the desirable energy window (−0.2 to −0.6 eV) recommended by DOE for practical applications, while the reversible interactions are inside. To characterize the nature of the two types of interactions, we considered the following theoretical

Interaction of 6H2 with Ti–CNC

It is possible to identify a range of reaction enthalpies that satisfy the ultimate DOE targets of temperatures and pressures through applications of van't Hoff relation. The former results, Table 1, Table 2, indicate that the highest hydrogen storage capacity reaction can be represented by the formation of the activated complex 6H2–Ti–CNC, where Ti can bind up to 6 hydrogen molecules with average adsorption energy of −0.35 eV per hydrogen molecule, and the gravimetric hydrogen storage capacity

Conclusions

An attempt has been made to guide materials development by translating vehicle operating constraints into thermodynamic constraints. Two types of reactions, namely reversible and irresversible, were characterized in terms of projected densities of states, infrared and proton magnetic resonance spectra, electrophilicity, and statistical thermodynamic stability descriptors. With reference to the ultimate targets of the department of energy for physisorption, gravimetric hydrogen storage capacity,

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