Synthesis Of Nitrogen Doped Carbon-Based Catalyst Derived From Biomass Pyrolysis And Study On Their Catalytic Performance For Oxygen Reduction Reaction

In order to alleviate the energy crisis and environmental problems which were caused by the rapid development of human society and globalization,more and more fuel cells have been developed.ORR catalyst is an important part of fuel cell.The commercial application of fuel cell is limited,because of the scarcity of platinum,high cost,poor stability and methanol resistance of commercial platinum carbon electrocatalyst.It is urgent to find a cheaper and efficient ORR catalyst as an alternative to replace the commercial platinum based catalyst.The ORR activity of the catalyst can be significantly improved by introducing nitrogen into the carbon skeleton of the carbon material.The carbon materials treated with HNO3,H2SO4 and H2O2 can increase the amount of oxygen-containing functional groups,and then increase the amount of nitrogen doping after pyrolysis.However,this treatment process not only improves the requirements of corrosion resistance of the equipment,and increases the preparation process and cost,but also requires a large amount of fresh water.In addition,the discharged wastewater has a great threat to environment.Therefore,this dissertation aims to provide two strategies for the preparation of green,low-cost and efficient carbon based ORR catalysts derived from biomass pyrolysis.Firstly,biochar(BC)was obtained from biomass sawdust by low temperature oxygen limiting heat treatment as carbon support,and then Co@NCBC was prepared by introducing Co and N.In addition,Co@NCSD was obtained by using sawdust(SD)as carbon support.The ORR activity of the catalyst was determined by a series of electrochemical characterization.The results show that Co@NCBC is better than commercial platinum carbon electrocatalyst in stability and methanol tolerance.The corresponding current-time(i-t)response of Co@NCBC exhibited a high relative current of 98.1%after 10,000 s,and after the addition of 10 mL methanol,the ORR current for Co@NCBC is only slightly changed about 3.5%.Moreover,Co@NCBC displays an obvious ORR activity,and the half wavepotential(E1/2)of 0.860 V vs RHE.The improvement of ORR performance is attributed to the higher nitrogen content(6.00 at%)and active nitrogen content(4.73 at%),the improvement of carbon material microstructure(SBET=409.75 cm2/g)and the high dispersion of Co NPs.Low temperature oxygen limiting heat treatment can increase the number of oxygen-containing functional groups on the surface of carbon materials,and then promote the nitrogen content of carbon materials in NH3 carbonization atmosphere,which is a relatively green and environmentally friendly preparation strategy.Secondly,nitrogen doped porous biochar N/C800-800 and N/C700-900 derived from chitosan by pyrolysis under CO2-NH3 carbonization atmosphere.Then,a series of electrochemical tests show that N/C800-800 and N/C700-900 exhibited better ORR performance than other nitrogen doped porous biochar,and N/C700-900(E1/2=0.88 V vs RHE)>N/C800-800(E1/2=0.86 V vs RHE)?20 wt%Pt/C(E1/2=0.86 V vs RHE).Moreover,the stability and methanol tolerance of these two catalysts are better than commercial platinum carbon electrocatalyst.The corresponding current-time(i-t)response of N/C800-800 and N/C700-900 exhibited current of 85.4%and 94.8%after 10,000 s,and the ORR current for N/C800-800 and N/C700-900 are only slightly changed.After a series of characterization,the oxygen content of carbon materials can be increased under CO2 carbonization atmosphere.So,in the next step,the nitrogen content of biochar will be increased under NH3 carbonization atmosphere.Furthermore,the microstructure of biochar(the maximum SBET=981.84 cm2/g)is improved after CO2-NH3 carbonization atmosphere,which will be conducive to improving the electrochemical performance of nitrigen doped porous biochar.In addition,we found that the preparation of high performance ORR catalyst is more important to improve the surface microstructure than to increase the nitrogen content of biochar.In the whole preparation,we choose a relatively green preparation process and no aqueous solution is used.