Preparation Of Co(Fe)-N-C Composite And Its Catalytic Performance For Oxygen Reduction Reaction

In modern society,energy crisis and environmental problems have seriously affected human health and restricted social progress.On the road of sustainable development,green new energy technology has become an important lever to change the traditional energy structure and has attracted the attention of the whole world.Low temperature fuel cell can directly convert chemical energy into electric energy,and there is no other product except water.In recent years,this clean and efficient energy conversion technology has developed rapidly in the unanimous efforts of the researchers,and it has become an important branch of the road to the construction of new energy systems.At present,the most primary factor hindering the large-scale commercialization of fuel cells is the high cost and scarcity of cathode noble metal catalysts.Therefore,developing cheap,easily available,highly efficient and stable non-noble metal oxygen reduction cathode catalyst has become a significant research direction in the fuel cell industry.Up to now,the transition metal nitrogen and carbon composite?M-N-C?is considered to be a promising catalyst for oxygen reduction because of its high activity,low price,rich reserves and various kinds.Although the great achievement has made in these M-N/C composite for oxygen reduction reaction?ORR?,there are still many problems,such as controversial active sites,unstable in acid medium and unclear ORR catalytic mechanism,needed to be solved before it can be really used in the practical application.In this work,the carbon-based transition metal nitrogen and carbon compounds with special morphologies were prepared by doping,self-assembly,template and pyrolysis.The structure of the catalyst and the catalytic performance for the oxygen reduction reaction were studied.The results obtained are as follows:?1?Using urea as carbon source and nitrogen source,Co?OAc?₂ as metal source and P123as a soft template,Co,N co-doped carbon nanotube?Co-N-CNTs?with typical tubular structure was successfully prepared after high temperature pyrolysis.The test results show that the catalyst with mass ratio of urea to Co?OAc?₂ to P123 of 3:0.5:1.5 at the temperature of 700?has the most outstanding catalytic performance for ORR.Under acidic condition,the initial potential and half wave potential of the ORR on this catalyst reach 0.95 V and 0.75 V,respectively,which is comparable to the commercial Pt/C catalyst.Co-N-CNTs exhibit better methanol tolerance and stability than Pt/C both in alkaline and acidic solutions.It is proved that the material is a direct 4e-pathway in the oxygen reduction reaction.The element mapping,XPS results and toxicity test show that the Co-N structure is the key active site for the oxygen reduction reaction.?2?By adding certain amount of Fe?acac?₃ during the synthesis of ZIF-8,a novel Fe,N doped porous carbon/carbon nanotube composite?Fe-NC@CNTs?was obtained after high temperature annealing.A large number of channels exist in Fe-NC@CNTs.The formation of carbon tubes not only further stabilize the material structure,but also enhance the graphitization and electrical conductivity of materials.Fe-NC@CNTs-0.4?the amount of Fe added in the raw material is 0.4 mmol?catalyst shows the most outstanding oxygen reduction catalytic activity,with the starting potential and the half wave potential in the alkaline medium reaching 0.995 V and 0.897 V,respectively,and 1.030 V and 0.880 V in the acid medium,which are all superior to the commercial Pt/C catalyst and to most of the other materials reported in the known literatures.The acid leaching experiment and the toxicity test with NaSCN confirm that the catalytic active sites of the composite for the oxygen reduction reaction are mainly Fe-N_x.