Synthesis, Characterization And Hydrogen Storage Of Monodisperse Carbon Nanoparticles

Because fossil fuel reserves are diminishing rapidly and the use of fossil fuels brings serious environmental problems, there is an urgent need for people to develop new renewable clean energy. Hydrogen has many advantages such as it has high calorific value, its product is water, has no pollution, can be recycled, and it does not exist exhaustion problem. Therefore, the hydrogen energy system as a way to reduce greenhouse gas emissions and other harmful substances becomes the 21st century green energy. However, the storage of hydrogen becomes the bottleneck of large-scale application of hydrogen technology. The United States department of energy (DOE) established primary aim for hydrogen storage media to reach hydrogen storage-release of 6 wt% in room temperature or close to room temperature condition. Carbon-based materials have certain potential at adsorption of hydrogen and become a new hot spot of physical hydrogen storage material because it has many micropore, large specific surface area, high surface tension and high adsorption potential. To our knowledge, the research of hydrogen storage using carbon spheres has been little reported. This paper reported a new method to prepare monodisperse micropore carbon spheres which had adjustable micropore size and multistage pore structure. Because of its optimized pore size, high specific surface area and large pore volume, these carbon spheres showed excellent hydrogen storage properties. In this paper, our main research was as follows:(1) We adopted a new approach to adjust the micropore structure of carbon microspheres by changing the mass density of the carbon source-hypercrosslinked polystyrene spheres (HCPSs). The carbon microspheres with optimized pore structure showed excellent hydrogen storage properties. We synthesized the crosslinked core@shell poly(styrene-co-divinylbenzene) spheres (PS-DVB spheres) by soap-free emulsion polymerization. The uncrosslinked PS core was removed via dissolution by CCl4 and hollow PS-DVB capsules were formed. Then HCPSs with different mass densities were prepared by hypercrosslingking of the solid PS-DVB spheres or PS-DVB capsules. At last, the carbon microspheres with different micropore structure were prepared by carbonization of the HCPSs with different mass densities at 700℃. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that we successfully prepared monodisperse carbon microspheres and a large number of micropores and worm-like channels were homogeneously dispersed in the carbon matrix of carbon spheres. XRD patterns and Raman spectra indicated that the carbon spheres were amorphous graphite phase. High-resolution TEM results and nitrogen adsorption/desorption results indicated that when the content of DVB was not less than 40 wt%, we could adjusted the microporous structure parameters of the carbon microspheres through the mass density of HCPSs. The carbon microspheres showed high specific surface area (679m2/g), large pore volume (0.74 cm3/g) and optimized micropore size (0.51-0.85) nm. These porous texture parameters were advantageous for hydrogen storage. It was noticeable that high H2 uptake capacity of 4.55 wt% was obtained at 77 K and 800 mmHg (1.06 bar), which had nearly twice maximum hydrogen storage capacity (2.4 wt%) of current porous carbon reported in similar conditions (77 K,0.93 bar).(2) Monodisperse micropore carbon hemispheres with great pore volume were synthesized by a novel route. First, hollow PS-DVB capsules were prepared through removing uncross-linked PS cores of core@shell PS-DVB spheres. Second, hollow PS-DVB capsules were sulfonated and then were coated by SiO2. Finally, hollow carbon hemispheres were obtained after carbonizing the sulfonated hollow PS-DVB capsule/SiO2 composite and removing SiO2 layers. The samples were characterized by SEM, TEM, XRD, Raman spectra and nitrogen adsorption/desorption measurements. These monodisperse carbon hemispheres had hollow single-layer shells. Large numbers of micropores and worm-like channels were homogeneously dispersed in the single-layer shells. These carbon hemispheres had high BET surface area (676 m2/g), extremely large pore volume (2.63 cm3/g) and micropore size (0.55-0.75 nm) which was larger than that of single carbon atoms cross-linking agent (CCl4) of carbon microspheres. The H2 uptake capacity of 1.62 wt% was obtained at 77 K and 800 mmHg (1.06 bar). The formation mechanism of carbon hemispheres and the effect of DVB content in core@shell PS-DVB spheres on the microstructure and morphologies of carbon hemispheres were studied.