Preparation And Purification Of Carbon Nanotubes And Its Application In Oxygen Reduction Reaction At The Cathode Of Fuel Cells

As a one-dimensional tubular carbon material with excellent conductivity and stability,carbon nanotubes(CNTs)have received extensive attention in many fields.The huge market demand puts forward higher requirements on the yield and quality of CNTs.The chemical vapor deposition(CVD)method is currently the most promising method for preparing CNTs.Its demand for metal oxide supports and subsequent removal of metal oxide supports have caused higher costs for CNTs.More importantly,metal oxide supports have trace residues that are difficult to completely remove,which limits the application of CNTs in some fields.Based on this,we designed and studied the preparation of activated carbon(AC)using coconut shell as a raw material,and then using AC as support to prepare CNTs by CVD.Then we proposed a novel purification method to CNTs,and the obtained CNTs have controllable defects and opened tips.Finally,we tested the application of CNTs with controllable defects and opening tips in the cathode oxygen reduction reaction(ORR)of fuel cells.The main work are as follows:(1)Using coconut shell as raw material,study the effect of the activation of KOH/H3PO4 dual activator on the specific surface area and pore size distribution of the prepared AC.Then we selected three ACs with different pore size distributions as supports,and supported transition metals Fe,Co,and Ni as catalysts to catalytic CH4 cracking to prepare CNTs,the results show that Ni/50 nm AC(M3)has the best CNTs growth effect.This is because the larger pore size of the pore structure is conducive to gas diffusion,and when AC is used as support,the carbon diffusion rate of active metal particles is a key factor in the growth of CNTs.(2)Use KOH-Ca(OH)2-CO2 to purify the prepared CNTs crude product.The mixing of KOH,Ca(OH)2 and CNTs crude product will form a bimetallic salt K2Ca(CO3)2,which is conducive to the dispersion of K and the intercalation of the carbon layer,thereby catalyze the oxidative ablation of non-graphitic carbon by CO2,and alleviate the damage of K aggregation to CNTs.The results show that when KOH:Ca(OH)2=2:1,the purification effect is the best.Almost all the amorphous carbon is oxidized,and CNTs with fewer defects and opening tips are obtained.Finally,the test is carried out for the possibility of its application in ORR.(3)Explore different calcium-containing compounds and KOH to catalyze CO2 to oxidize and etch CNTs,create controllable defect sites on the surface of CNTs,and then co-doped with Fe,N,and S to obtain effective high N,S doping ORR catalyst.The results show that Fe,N,S-PCNTs-4 catalyst exhibits excellent ORR activity in alkaline medium,with a half-wave potential(E1/2)0.92 V and diffusion limiting current density(jL)6.2 mA cm-2,It also has a low Tafel slope of 68 mV dec-1 and a high dynamic current density of 34.8 mA cm-2.In addition,the catalyst also exhibits excellent long-term stability and methanol tolerance.