Elsevier

Materials Letters

Volume 81, 15 August 2012, Pages 215-218
Materials Letters

High hydrogen storage capacity of heteroatom-containing porous carbon nanospheres produced from cross-linked polyphosphazene nanospheres

https://doi.org/10.1016/j.matlet.2012.04.152Get rights and content

Abstract

Heteroatom-containing porous carbon nanospheres were fabricated by forming polyphosphazene nanospheres and carbonizing them with NaOH as activating agent. They were then examined as a material for hydrogen storage. N2 sorption and H2 sorption measurements showed that the carbon nanospheres possess a BET surface area of 1140 m2 g 1, a total pore volume of 0.90 m3 g 1, an ultramicropore volume of 0.30 m3 g 1, a bimodal pore size distribution (3–5 nm and 0.6–0.8 nm diameter pores), and a gravimetric hydrogen uptake of 2.7 wt.% at 77 k and 1 atm.

Graphical abstract

Highlights

► Polyphosphazene nanospheres were used as carbon precursors. ► The carbon nanospheres possess a large surface area and a bimodal pore size distribution. ► Small amounts of heteroatoms such as N, O, P, and S were doped in carbon nanospheres. ► The carbon nanospheres exhibit a high hydrogen uptake of 2.7 wt.% at 77 k and 1 atm.

Introduction

Hydrogen storage is an essential prerequisite in the development of hydrogen-fueled vehicles. As an attractive candidate for hydrogen storage, porous carbon materials have attracted considerable attention due to their good adsorptive capacity, low density, low cost, and high surface area. Generally, high surface area and pore volume are essential for enhancing the hydrogen storage capacity [1], [2]. Also, narrow pore size distributions for carbon materials in which the pore sizes are mainly smaller than 1 nm are expected [3], [4]. In addition, the heteroatoms (such as N, P, S, and B) in carbon materials can play the role of activator, resulting in the doped carbon materials exhibiting remarkable potential in hydrogen storage [5]. Hence, finding novel nanostructured carbon materials and tailoring their pore structures and textures for hydrogen storage are of great significance.

Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) is a typical cross-linked polymer that forms carbon materials during carbonization [6]. Nanostructured PZS can easily be synthesized in bulk quantities under ambient conditions through polycondensation of comonomers hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulfonyldiphenol (BPS) [6], [7]. NaOH or KOH is a good activating agent, which helps to the development of porosity in carbon frameworks, leading to a higher specific surface area and pore volume [8], [9]. In this study, we selected PZS nanospheres as carbon precursor and NaOH as activating agent for preparing porous carbon nanospheres with high hydrogen storage capacity.

Section snippets

Materials

Hexachlorocyclotriphosphazene (HCCP) (Aldrich) was recrystallized from dry hexane followed by sublimation twice before use. 4,4′-Sulfonyldiphenol (BPS), triethylamine (TEA), sodium hydroxide (NaOH), acetonitrile, and ethanol were commercially obtained and used as received.

Preparation of PZS nanospheres

PZS nanospheres were synthesized according to our previous report [7]. Briefly, 4 mL TEA was added to a solution of 0.8 g HCCP and 1.78 g BPS in 210 mL acetonitrile under ultrasonic irradiation (150 W, 40 Hz) at room temperature.

Results and discussion

Fig. 2(A) and (B) shows the SEM and TEM images of the PZS nanospheres with average diameter of 420 nm prepared through the precipitation polymerization of HCCP with BPS. After the PZS nanospheres were activated with NaOH and carbonized at high temperature, the spherical morphology of the carbonized samples was retained as shown in Fig. 2(C). The diameter of the carbonized samples was about 350 nm, showing an overall reduction in size dimension due to mass-transfer flow during the carbonization.

Conclusions

We have demonstrated an approach to fabricate heteroatom-containing porous carbon nanospheres for hydrogen storage using polyphosphazene nanospheres as carbon precursor and NaOH as activating agent. As highlighted here, our preparation route is simple, rapid, and easy to scale-up. Also, as-fabricated carbon nanospheres show a gravimetric hydrogen uptake up to 2.7 wt.% at 77 k and 1 atm. This study might open a new synthesis route to the heteroatom-containing porous carbon materials with high

Acknowledgments

We are grateful to the National Natural Science Foundation of China (nos. 51003098, 51173170), the National Science Foundation for Post-doctoral Scientists of China (nos. 20100480055, 201104398), the Henan Foundation for Key Program of Science and Technology (no. 12A430014), and the financial support from the Program for New Century Excellent Talents in Universities (NCET).

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