Experimental investigation of hydrogen adsorption in doped silicon-carbide nanotubes

Highlights

• Pure SiCNTs were synthesized using a gas/solid reaction.

• The SiCNTs were doped with titanium (Ti).

• The SiCNTs were doped with Potassium (K).

• Hydrogen uptake capacity of the pure and doped SiCNTs was gravimetrically measured.

Abstract

Silicon-carbide nanotubes were synthesized by the reaction between SiO vapor as the Si source and carbon nanotubes as the carbon precursor. The resulting nanotubes were then purified with a hot, concentrated NaOH solution in order to remove the amorphous silica from their surface. The purified silicon-carbide nanotubes were doped with either potassium (K) or titanium (Ti). The hydrogen adsorption isotherms of the nanotubes were measured gravimetrically with the aid of a magnetic suspension microbalance. The results indicate that the K-doped silicon-carbide nanotubes exhibit promise as a hydrogen adsorption material, storing three times more hydrogen than the carbon nanotubes.

Introduction

Over the past decade or so extensive efforts have been devoted to the development of nanoscale carbonaceous materials for hydrogen storage applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Despite the promising progress made in using nanostructured materials for this purpose, the availability of efficient hydrogen storage materials still remains a key barrier to the commercial application of hydrogen as a fuel. Use of silicon-carbide nanotubes (SiCNTs), instead of carbon nanotubes (CNT), is a potential solution to this challenge [17], [18], [19], [20], [21]. In our previous work [22], [23], we showed that converting CNTs into SiCNTs does increase their hydrogen storage capacity by more than 50%; however, the hydrogen uptake of the resulting SiCNTs was still far below the target set forth by the United States Department of Energy (DOE) for vehicular applications, which is 6–7 wt.%.

In order to investigate methods for increasing the hydrogen storage capacity of nanoscale materials, in addition to experimental efforts, complimentary theoretical and computational studies have also been undertaken over the past few years. Computer simulation studies of doped nanomaterials, for example, have indicated considerable improvement in their hydrogen storage capacity brought forth by doping [24], [25], [26], [27], [28], [29]. Dag et al. [25], for example, used first-principles calculations based on the plane-waves approach to investigate the interaction between hydrogen and pure and doped CNTs. According to their study, doping of the CNTs with transition metals, such as titanium (Ti), and alkali metals, such as lithium (Li) and potassium (K), can increase significantly the capacity of the nanotubes for hydrogen sorption and storage. In another first-principles study Yildirim and Ciraci [26] showed that a single Ti atom coated on the outer surface of the CNTs can bind with up to four hydrogen molecules. The adsorption of the first hydrogen molecule was shown to be dissociative, while the next three would adsorb in their molecular state. In addition to serving as dopants for the CNTs, transition and alkali metals have also been recommended as efficient dopants for the SiCNTs in order to improve their hydrogen storage capacity [18], [27], [28], [29]. Wang and Liew [28], for example, studied alkali-doped SiCNTs with the aid of ab initio density-functional theory. They showed that doping the SiCNTs would increase their hydrogen binding energy from 0.086 eV to 0.211 eV due to the charge transfer from the dopant to the SiCNTs. Four hydrogen molecules with an average binding energy of 0.165 eV can consequently be adsorbed on the doping sites and thus increase the hydrogen uptake capacity of the SiCNTs. In addition, simulations based on pseudopotential methods [29] have indicated that Ti-decorated SiCNTs exhibit superior hydrogen storage capacity. According to Banerjee et al. [29], a Ti atom doped on the SiCNTs' surface is capable of adsorbing two hydrogen atoms and three hydrogen molecules, which is similar to the observed behavior of the Ti-doped CNTs [26].

In the present paper, we report on an experimental study of the effect on hydrogen storage capacity of the SiCNTs doped with K (as an alkali metal) and Ti (as a transition metal). The goal of the work is to determine whether doped SiCNTs have, indeed, enhanced adsorption capacities as the aforementioned molecular simulations seem to indicate. If they do, we seek to understand whether the increased hydrogen storage capacities approach the target set forth by the United States Department of Energy for such hydrogen storage media.

This paper is organized as follows: In the next section, we describe the experimental procedure for doping the SiCNTs, and for measuring the sorption of hydrogen in them. The results are then presented and discussed in Results and Discussion.