Study On Nitrogen Doped Carbon Nanotubes Catalyst For Fuel Cells

As highly efficient and environmentally friendly electrochemical power generationdevice, fuel cells are more and more concerned. The oxygen reduction reaction (ORR)is one of the most crucial factors determining the performance of a fuel cell. Highlyactive catalysts for ORR have long been regarded as a key to optimize the performanceof fuel cells. Platinum-based materials have long been investigated as active catalystsfor ORR. Although much progress has been made, this noble metal catalyst and its alloycan hardly meet the demands which allow for widespread commercialization of fuelcells because of high costs and poor durability. Thus, nonprecious-metal and metal-freecatalysts for ORR have attracted enormous interest as an alternative to platinum-basedcatalysts. Nitrogen-doped carbon nanotubes (NCNT) have been demonstrated to showexcellent electrocatalytic activity for ORR. Generally, NCNT can be prepared bythermal chemical vapor deposition (CVD) of nitrogen-containing precursors. It is still achallenge to obtain NCNT with high yield, acceptable cost, few byproducts, andcontrolling content. Meanwhile, toxic amine precursors contained in the off-gas canpollute environment.In order to overcome the problems of manufacturing NCNT, a modified thermalchemical vapor deposition method with two heating zones was designed to grow NCNT.Solid carbon precursor and nitrogen precursor are placed in the low heating zone.Meanwhile, in the high heating zone, a quartz boat is put to collect the NCNT products.The low flow rate of carrier gas can make the precursors fully react in the high heatingzone and prevent precursors from wasting. Furthermore, corrosion to quartz tubefurnace and pollution of off-gas can be avoided with this method.NCNT catalyst at different manufacturing temperature was prepared by thefollowing steps: Melamine as solid nitrogen precursor, and ferrocene as carbonprecursor and catalyst were placed in the low heating zone, where the temperature washold at about500℃greater than the boiling point of both precursors to ensure fullevaporation of the precursors. In the high heating zone, where the temperature was keptat750℃、800℃and850℃, a quartz boat was put to collect the NCNT products. Thedepositing time is about40min. The product was purified by oxygenizing it in wet air at225℃for10hours and boiling it in nitric acid for5hours. SEM analysis，TEManalysis and EDS analysis revealed that the aligned NCNT with bamboo-like structure was obtained at800℃and nitrogen has been successfully doped into nanotubes. TheORR test indicated that NCNT has high activity for ORR. The nitrogen content of thisNCNT was about2.44%from XPS test.NCNT catalyst at different depositing time was by the following steps: Melamineas solid nitrogen precursor, and ferrocene as carbon precursor and catalyst were placedin the low heating zone, where the temperature was hold at about500℃greater thanthe boiling point of both precursors to ensure full evaporation of the precursors. In thehigh heating zone, where the temperature was kept at800℃, a quartz boat was put tocollect the NCNT products. The depositing time was10min,20min and40minrespectively. SEM analysis and TEM analysis revealed that many aligned and uniformNCNT with bamboo-like structure was obtained at20min depositing time. The ORRtest also indicated that NCNT has high activity for ORR. The nitrogen content of thisNCNT was about4.55%from XPS analysis. The ID/IGratio of NCNT is4.6fromRaman spectra.Finally, urea instead of melamine as the nitrogen precursor, ferrocene as carbonprecursor and catalyst were placed in the low heating zone, the temperature of highheating zone was kept at800℃. The depositing time was20min. However, the disorderNCNT was obtained. The nitrogen content of this NCNT was about2.10%from XPSanalysis. The ID/IGratio of NCNT is2.5from Raman spectra. The ORR test indicatedthat this NCNT has poor activity for ORR than the above NCNT. Maybe it is directlybound up with the structure of precursors and nitrogen content.