Synthesis and applications of carbon nanomaterials: from quanta dots to microspheres
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In this work, activated carbon fibers and activated carbon spheres were pre- pared through physical activation with CO2 and steam. The precursors used were polyaniline (PANI) and resorcinol-formaldehyde polymer spheres (RF poly- mer spheres). Various activation conditions, such as activation time, activation temperature and partial pressure of CO2 were investigated in the CO2 activation process, as well as the effect of carbonization temperature. It was found that a carbonization temperature of 650°C resulted in the largest BET surface area and pore volume after the activation process. Moreover, the results suggests that the BET surface area, total pore volume and mesopore volume increases with increas- ing activation time, while the micropore volume goes through a maximum at an activation time of three hours. Also, the BET surface area, total pore volume and mesopore volume increases with increasing activation temperature, whereas the micropore volume goes through a maximum at 950°C. The results shows that the fiber structure is mostly retained during the CO2 activation process, however a decrease in macroporosty and destruction of fibers occurs with an activation time of 4 hours at an activation temperature 950°C. Also, an activation temperature of 1000°C results in a decrease in macroporosity.In the combined CO2 and steam activation process, the BET surface area goes through a maximum at a water temperature of 40°C. The results show that at each activation time, the obtained BET surface area is higher for the combined CO2 and steam activation process compared to that obtained using pure CO2 ac- tivation. Also, at each activation temperature, the obtained BET surface area is higher for the combined CO2 and steam activation process compared to that ob- tained using pure CO2 activation. In the optimization part, a high BET surface area of 2552.6 m2g°1 is obtained with a burn-off equal to 83.6 wt.%.Supercapacitors constructed by electrode materials prepared from the activated carbon fibers and activated carbon spheres provided a specific capacitance at low current densites, in which the highest specific capacitance obtained was 416.5 F/g at a current density of 0.1 A/g. However, the supercapacitors showed a poor rate capability, where activated carbon spheres provided the best rate capability of only 53.3 % at a current density of 3.45 A/g. Also, the cycling stability was very low for the supercapacitors prepared in this work, where activated carbon spheres showed to best cycling stability of only 48 % retention of the initial capacitance after 1200 cycles. CV curves indicated that pseudocapacitance was present in all of the supercapacitors. This was reasonable due to the presence of nitrogen and oxygen in the activated carbons.