Hydrogen Storage Performance of Graphitic Carbon Nitride(g-C3N4) Nanotubes

Abstract

21st century is the hydrogen century. However, to realize hydrogen economy the safe and effective hydrogen storage remains one of the main challenges. In this work, graphitic carbon nitride (g-C3N4) nanotubes were synthesised through a convenient one-step calcination method. The physical, chemical and hydrogen storage properties were examined. For comparison, multiwall carbon nanotubes (MWCNTs) and g-C3N4 nanosheets and bulk g-C3N4 were also studied. While the MWCNTs were purchased from Shenzhen Nanotech Port Co., Ltd, China, bulk g-C3N4 was synthesised by calcined melamine at 550 degrees following a route in the literature. A novel and facile high temperature calcination method was proposed and developed here to synthesize g-C3N4 nanosheets. All the g-C3N4 materials (nanotubes, nanosheets and bulk) possessed better performance in hydrogen storage than the MWCNTs.The hydrogen storage capacities of the materials studied were obtained at 25 degrees (room temperature) and under a hydrogen pressure of 3.7 MPa. For g-C3N4 nanotubes, the hydrogen storage capability has improved by up to 70% (from 0.46 wt.% for MWCNTs to 0.78 wt.%. The 25 degrees hydrogen storage capacity of bulk g-C3N4 (0.51 wt.%) was also higher than that of MWCNTs. The capacity of g-C3N4 nanosheets (0.73 wt.%) was higher than that of bulk g-C3N4, but lower than the capacity of g-C3N4 nanotubes. The higher calculated isosteric heat of adsorption (Qst) for the three g-C3N4 materials (8.5 kJ/mol for bulk, 10.5 kJ/mol for nanosheets and 12.5 kJ/mol for nanotubes) as compared to that of MWCNTs (6 kJ/mol) implies that the high storage capacities could be attributed to the strong interaction between hydrogen and the g-C3N4 materials. Besides, Qst of g-C3N4 nanotubes was higher than Qst of g-C3N4 nanosheets and bulk g-C3N4, suggesting that g-C3N4 nanotubes could form a much stabler bond with hydrogen. This finding can be used to explain the highest storage capacity of g-C3N4 nanotubes as compared with that of g-C3N4 nanosheets and bulk g-C3N4. Temperature Programming Desorption (TPD) was also applied to study the four samples. Strong signals were detected for the chemisorbed hydrogen in g-C3N4 materials, suggesting except physisorption, chemisorption should also play an important role in the great hydrogen storage performance of them. In the second part of this work, nickel was used in the fabrication of g-C3N4 nanotubes, in an attempt to further improve the hydrogen storage performance. The samples were synthesized by calcining melamine, cyanuric acid, and nickel together. The obtained g-C3N4 nanotubes possessed a highly defective and porous nanotube structure. Consequently, the specific surface area and pore volume have largely improved. Compared with g-C3N4 nanotubes synthesized without using nickel, the 25 degrees storage capacity of g-C3N4 nanotubes synthesized by nickel-assisted approach has further increased by 50 % (from 0.78 wt.% to 1.17 wt.%). As confirmed by hydrogen desorption study and Temperature Programming Desorption (TPD) experiments, the improved capacity could be attributed to the increased physisorption and chemisorption of hydrogen in the nanotubes. Based on this finding, it could be concluded that the created defects and pores provided extra strong adsorption sites for hydrogen, which enabled more hydrogen to be adsorbed, either physically or chemically. In summary, this work is one of the first to study g-C3N4 nanotubes for hydrogen storage through the experimental methods. Evidence showed that g-C3N4 nanotubes could be a potential material for hydrogen storage application at room temperature. A facile, one step fabrication method was disclosed in this work which could be used to synthesize porous g-C3N4 nanosheets with high specific surface area and pore volume. The as synthesized g-C3N4 nanosheets not only had the advantage for hydrogen storage, but also could be expected to have good performance in other fields like photocatalyst. In addition, this research reported a nickel based modification method to create defects and pores on g-C3N4 nanotubes. This methodology should pave a new avenue to modify g-C3N4 based materials for advanced applications like photocatalyst, solar cell and lithium battery.