Effects of pressure, temperature, and geometric structure of pillared graphene on hydrogen storage capacity

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Effects of pressure, temperature, and geometric structure

of pillared graphene on hydrogen storage capacity

Cheng-Da Wu

1

, Te-Hua Fang

*

, Jian-Yuan Lo

1

Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80807, Taiwan

a r t i c l e i n f o

Article history:

Received 22 May 2012 Received in revised form 9 July 2012

Accepted 10 July 2012 Available online 26 July 2012 Keywords: Pillared graphene Hydrogen Adsorption Pressure Molecular dynamics

a b s t r a c t

The adsorption of molecular hydrogen on a three-dimensional pillared graphene structure under various environments is studied using molecular dynamics simulations. The effects of pressure, temperature, and the geometric structure of pillared graphene are evaluated in terms of molecular trajectories, binding energy, binding force, and gravimetric hydrogen storage capacity (HSC). The simulation results show that in a pillared graphene structure, the HSC of the graphene sheets is better than that of the carbon nanotube (CNT) pillars. An insufficient gap between graphene sheets decreases the HSC because hydrogen adsorbed at the edges of a pillared graphene structure prevents hydrogen from entering the structure. A low temperature, a high pressure, and a large gap between graphene sheets maximize the HSC. The HSC is only slightly improved by increasing the CNT diameter.

1. Introduction

Carbon-based systems that comprise carbon atoms with sp2 hybridization (i.e., carbon nanotubes (CNTs) and fullerenes) have been widely studied and applied in micro/nano-systems due to their excellent mechanical, thermal, and electronic properties[1e4]. A single layer of graphene has been synthe-sized using the mechanical exfoliation of graphite[5]. Carbon-based systems are considered as potential candidates for energy storage[6]due to their high adsorption surface area.

Hydrogen is a potential successor to gasoline due to its many advantages, including clean combustion and the high-est energy content per weight unit of any known fuel. However, hydrogen is a gas with a very low density under ambient conditions. Understanding the mechanisms of hydrogen adsorption on graphene under various environ-ments would benefit various fields, including motor vehicles,

fusion reactor design, and hydrogen storage [7]. Molecular dynamics (MD) simulations of the grapheneehydrogen inter-face can be used to explore the nature of surinter-face adsorption. Atomistic simulation avoids experimental noise and turbu-lence problems. It can be used to analyze molecular trajecto-ries and thermodynamic properties. Many nanosystems have been analyzed using MD, such as studies on nanowetting properties[8], nanoimprinting[9], nanoforming[10], and dip-pen nanolithography [11,12]. Herrero and Ramirez [13]

studied the diffusion of hydrogen in graphite and found that hydrogen atoms jump from a C atom to a neighboring one with an activation energy of about 0.4 eV. Compared to atomic hydrogen, molecular hydrogen has a much smaller activation energy and faster diffusion speed. Lamari and Levesque[14]

studied hydrogen adsorption on graphene and found that at a temperature of 77 K and a pressure of 1 MPa, the excess hydrogen physisorption is estimated to be equal tow7 wt%,

* Corresponding author. Tel./fax:þ886 7 3814526 5336.

E-mail addresses:nano112002@gamil.com(C.-D. Wu),fang.tehua@msa.hinet.net(T.-H. Fang),1099303134@kuas.edu.tw(J.-Y. Lo). 1_Tel.:_{þ886 7 3814526 5336; fax: þ886 7 3831373.}

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and decreases with increasing temperature. Du et al. [15]

studied the separation of hydrogen and nitrogen gases by porous graphene membranes. They found that the separation mechanism is related to the van der Waals (VDW) interactions of the gases with the graphene membrane.

This work investigates the mechanism of molecular hydrogen adsorption on a pillared graphene structure using MD simulations. Pillared graphene is considered to be superior to CNTs as a hydrogen storage material due to its higher surface area and storage capacity. To optimize the operating parameters, the effects of pressure, temperature, and the geometric structure of pillared graphene are studied. The results are discussed in terms of molecular trajectories, binding energy, binding force, and gravimetric hydrogen storage capacity (HSC).

2. Methodology

Fig. 1(a) shows a schematic of three-dimensional pillared graphene array that combines two allotropes of carbon, namely CNTs and graphene sheets. In the nanostructure, CNT pillars occupy the empty space between two graphene sheets. The graphene sheets are perfectly arranged in hexagonal form.Fig. 1(b) shows the initial MD model of a hydrogen/pil-lared graphene interface. The pilhydrogen/pil-lared graphene structure is placed at the middle of the system, and hydrogen with a face-centered cubic (fcc) structure is placed above and below the pillared graphene. The hydrogen molecules are treated as single spherical molecules to simplify the model. The two

graphene sheets have dimensions of 7 nm (length) 7 nm (width). The length of a (10,10) CNT is 1.2 nm. There are 4000 hydrogen molecules in the simulated system, whose dimen-sions are 10 nm (length) 10 nm (width) 12 nm (height). Periodic boundary conditions are applied in all three dimen-sions. To focus on the hydrogen adsorption mechanism, the graphene is considered to be ideally rigid to increase compu-tational efficiency; this is reasonable due to graphene’s extremely strong bonding energy. Pressure, temperature, the separation distance (gap) between the graphene sheets, and the pore size of the CNT are varied in ranges of 4e15 MPa, 77e300 K, 0.6e1.5 nm, and 0.81e2.44 nm, respectively, to determine their effects on HSC. Before adsorption simula-tions, all hydrogen underwent an MD equilibrium run of 100 ps in order to achieve energy relaxation and uniform distribution in the system. The VDW interactions of C/H2and H2/H2are described by the Lennard-Jones potential[16]. A cut-off radius of 1.0 nm is used to evaluate the number of hydrogen molecules adsorbed on the graphene. The time step is fixed at 1 fs. The ensemble in the system is isothermal-isobaric (NPT).

3. Results and discussion

3.1. Effect of pressure

To focus on the effect of system pressure on hydrogen adsorp-tion, the system was maintained at an isothermal state at 77 K and the gap of the pillared graphene structure was set to 1.2 nm. Fig. 1e (a) Schematic of three-dimensional pillared graphene array. (b) Initial MD model of a hydrogen/pillared graphene interface.

Fig. 2e Snapshots of hydrogen adsorption on pillared graphene at a temperature of 77 K at a time of 1500 ps for pressures of (a) 4, (b) 6, (c) 8, and (d) 15 MPa.

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