| dc.description.abstract | Hydrogen energy has been internationally recognized as the mainstream of clean energy for the future. Since hydrogen reacts with oxygen in the fuel cells to produce energy, only water is produced. Thus the entire reaction process does not emit greenhouse gases such as carbon dioxide, which is the main cause of the global warming. However, one of the bottle necks to the realisation of hydrogen economy is the lack of safe and effective way to store hydrogen. As compared to the pressurized hydrogen gas storage, solid-state hydrogen storage is considered to be a safe and effective way to store hydrogen.
In this research, the aim has been to develop a cost-effective material with good hydrogen storage properties. It has been suggested that g-C3N4 nanotubes have the potential to store up to 5.45 wt.% of hydrogen under room temperature and moderate pressure. Hence, this study aims to further enhance the hydrogen storage capacity of g-C3N4 nanotubes using a high-energy planetary ball mill. In this work, g-C3N4 nanotubes were produced by grinding the starting materials (melamine and cyanuric acid) at various speeds, specifically 50, 70, 80, 90, 100, 110, and 240 rpm, for a duration of 2 or 5 hours, followed by calcinating at 550 °C for 4 hours. It was observed that the resulting g-C3N4 nanotubes exhibited a variation in specific surface area proportional to the grinding speed. Additionally, it has been observed that the tubular g-C3N4 only formed when the ball milling speed was above 80 rpm. Furthermore, the diameter of these tubes was inversely proportional to the speed of the ball mill. When the speed of the planetary ball milling went up to 240 rpm, the size of the tube could only be resolved under the transmission electron microscope. The above observations suggest that the use of a high-energy planetary ball mill can effectively produce g-C3N4 with tubular structures. The hydrogen storage capacities of the samples were determined using a PCI at room temperature and under a pressure up to 13 MPa. It was found that all these samples produced under the planetary ball milling speed of 80 to 110 rpm yielded a hydrogen capacity around 1.0 wt.%. However, for the g-C3N4 produced using the highest speed of 240 rpm in the planetary ball milling, it has the highest specific surface area of 122.2 cm2/g and the highest storage capacity of 1.724 wt.% at a pressure of 11.7 MPa. This suggests that grinding through a high-energy planetary ball mill can alter the surface charge of the material, thereby effectively enhancing its hydrogen storage performance.
A second approach to fabricate g-C3N4 nanotubes has been adopted in this work. In this self-assembling method, melamine was mixed with ethylene glycol and nitric acid to form the supramolecular intermediate, and then the intermediate was calcinated at 550 °C for 2 hours. The so-produced g-C3N4 nanotubes have more complete tubular structures (sample labeled as CNNT-65). When compared with g-C3N4 produced in the previous method, it was observed that CNNT-65 which has a lower specific surface area of 11.3 cm2/g. However, it exhibited a similar hydrogen storage capacity to C-110 (with a specific surface area of 33.6 cm2/g), under similar hydrogen charging pressure. This might be attributed to the more complete nanotube structure.
We concluded that with both of the two methods above, g-C3N4 nanotubes could be obtained. Also, the hydrogen storage capacity of 1.724 wt.% and the cost-effective starting materials made the so-produced g-C3N4 very useful for hydrogen storage applications.
Keywords: Hydrogen storage; graphitic carbon nitride; nanotube; microstructure; high-energy ball milling | en_US |