Production of templated carbon nano materials, carbon nanofibers and super capasitors

摘要

i. Porous carbons are usually obtained via carbonization of precursors of natural orudsynthetic origin, followed by activation. To meet the requirements, a novel approach, theudtemplate carbonization method, has been proposed. Replication, the process of filling theudexternal and / or internal pores of a solid with a different material, physically or chemicallyudseparating the resulting material from the template, is a technique that is widely used inudmicroporosity and printing. This method has been used to prepare replica polymers [1,2]udmetals [3] and semiconductors [4] and other materials [5,6]. Zeolites represent an interestingudcase for replication processes, because the dimensions of their cages and channels are quiteudsimilar to those organic molecules that constitute the replica. If such as nanospace in a zeoliteudis packed with carbon and then the carbon are extracted from the zeolite framework, one canudexpect the formation of a porous carbon whose structure reflects the porosity of the originaludzeolite template. Owing to the disordered and inhomogeneous nature of the starting materials,udthe resulting carbon has a wide and poorly controlled distribution of pore sizes. Zeolites withudthree-dimensional pore structures were found to be suitable as templates [7,8], whereasudzeolites with one-dimensional structures were not effective [9]. These carbons obtained usingudzeolite templates with three-dimensional pore structures retained the shapes of zeoliteudparticles, but did not retain their internal periodic structure.udii. Many methods have been proposed for carbon nanofiber (CNF) production, amongudthem, we have chosen chemical vapor deposition (CVD) method for CNF synthesis becauseudof its potential for scaling up the production and low cost[10]. Recent developments showedudthat alignment, positional control on nanometer scale, control over the diameter, as well as theudgrowth rate of the carbon nanotubes (CNT) and CNFs can be achieved by using CVD[11-13].udMany catalysts supports and metal catalysts were proposed for CNF production through CVDudtechnique. Silica (SiO2) [14], alumina (Al2O3) [15], quartz [16], titania (TiO2) or calciumudoxide (CaO) [17] were used as the catalyst support because of their chemical inertness andudhigh-temperature resistance. However, all of these support materials require harsh chemicaludtreatment i.e. concentrated bases (NaOH) or strong acids (HF) to remove them, and theseudreagents may also damage the carbon nanostructure. Additionally, strong acids and bases areudless desirable for large-scale production due to environmental concerns. Our goal inudsynthesizing CNFs is to achieve a control in tailoring the diameter, and morphology at theudsame time. We believe that understanding the chemistry involved in the catalyst and nanofiberudgrowth process is the critical point to be able to produce defectless, property controlled CNFs.udThus, knowing the effect of the catalyst on CVD production of carbon nanofibers is veryudimportant for producing the desired CNFs. A very unique material, NaCl in the field ofudcatalytic CVD process for carbon materials production, was selected as the support materialudwhich provides easy production and easy removal properties to the catalyst system. Togetherudwith the support material, the metal catalyst preparation step was differentiated from theudconventional wet catalyst methods in which a liquid solution containing the catalyst in saltudform is applied to the substrate via spray coating [16,18,19], spin coating [20-22], orudmicrocontact printing [23] as well. The most active metals that were used previously in theudcatalytic CVD process for carbon materials production were Fe, Co [24], and Ni. The reasonudfor choosing these metals as catalyst for CVD growth of nanotubes was the thermodynamicudbehavior of the metals at high temperatures, in which carbon is soluble in these metals andudthis solubility leads to the formation of metal-carbon solutions and therefore the desiredudcarbon nanomaterial formation nucleates. In this study, transition metal based organometallicudcomplex catalysts of Fe, Co, Ni and Cu were synthesized by a new approach of simultaneousudsynthesis of the support material and the catalyst. Therefore an easy production method forudcatalyst to use in CVD was developed by using only wet chemistry.udiii. Electrochemically conducting polymers (ECPs) are of interest in late years and theyudare promising materials for realization of high performance supercapacitors, as they areudcharacterized by high specific capacitances, by high conductivities in the charged states andudby fast charge-discharge processes. The charge processes pertain to the whole polymer massudand not only to the surface. These features suggest the possibility to develop devices with lowudESR and high specific energy and power. However, the long-term stability during cycling is audmajor demand for an industrial application of ECPs. Swelling and shrinkage of ECPs, causedudby the insertion/deinsertion of counter ions required for doping the polymer, is well knownudand may lead to degradation of the electrode during cycling. This obstacle has been overudovercome to some level by using composite materials made of carbon materials such as CNTsudor activated carbons with CPs. Carbon material in the bulk both ensures a good electricaludconductivity even the CP is in its insulating state and improves the mechanical properties ofudthe electrodes. As mentioned in the earlier chapters, using carbon nanotubes, CPs, or both asudcomposites for the active material of the supercapacitor applications comes with someuddisadvantages as well as the advantages. CPs although being a promising energy source forudthe job, lack the flexibility for insertion/deinsertion of the dopant ions resulting in shorterudrecycling life times than desired. CNTs are the employed to gain more flexibility howeverudwhether they are used as active materials solo, or engaged in a composite with a CP, theyudcould not supply enough energy for the job. Therefore, the objective of this study is, to obtainuda new material for supercapacitor active material; by depositing a conducting polymer,udpolypyrrole, on to carbon nanotubes via electropolymerization. By this method, the problemudof bulk charging in conducting polymers is aimed to be overcomed. Since the coating is inudmagnitudes of nanometers, only surface charging will exist, which is desirable forudsupercapacitor applications.