Nitrogen-enriched Polymer Derived Electrocatalysts For Oxygen Reduction Reaction

With the consumption of global traditional fossil energy and the deterioration of the environment,the development of new energy source is becoming more and more important.Among numerous new energy technologies,low-temperature fuel cells,with the advantages of fast start-up,high conversion efficiency,low emissions,and independent of fossil materials,is regarded as a very promising power energy device.However,the kinetic process of oxygen reduction reaction?ORR?in the cathode of low-temperature fuel cell is slow,resulting in low performance and short service life.Therefore,highly efficient electrocatalyst is needed to accelerate the reaction kinetic process.At present,platinum-based catalyst is a very effective ORR catalyst.However,with high price,scarce storage and poor stability of platinum-based catalyst seriously restrict the development of low-temperature fuel cell.Therefore,the development and design of high performance non-platinum-based ORR catalysts is a key factor in the development of low-temperature fuel cells.Recently,Carbon-based non-precious metal catalysts have become a hotspot in the research of non-platinum-based ORR catalysts because of their low price,wide range of raw materials,excellent performance and good stability.Polymer and biomass are used as carbon precursor in the carbon-based non-precious metal catalysts.Polyethyleneimine is a kind of polymer with high nitrogen content,which has great potential as a nitrogen precursor.However,the thermal stability of PEI at high temperature is poor,so it is difficult to use it as a carbon source for carbon-based catalysts.Therefore,this thesis designed a simple and convenient method through the modification of small molecules to PEI,while leaving high nitrogen content to improve thermal stability.This new material functions as a precursor for the preparation of carbon-based non-precious-metal ORR catalyst.The electrochemical tests showed excellent electrocatalytic properties.Combined with the structure characterization and electrochemical performance,this study explores the relationship between the effects of small molecules on PEI and catalytic performance.The results provide some methodological guidance for further design of carbon-based non-precious-metal ORR catalyst.Based on this,the research content of this thesis mainly includes the following aspects:1.The development status and problems of fuel cells are reviewed;the current research contents of carbon-based non-precious-metal oxygen reduction catalysts are introduced.By elaborating the research background,we propose the design ideas for solving the problems and confirm the plan of experiments.2.PEI was modified by methyl 1H-pyrrole-2-carboxylate containing pyrrole heterocyclic or ethyl nicotinate containing pyridine heterocyclic.The molecular weight of the modified polymer was characterized by time-of-flight mass spectrometry,the results showed that the addition of primary amines on the side chain of PEI was basically achieved,and the thermogravimetric analysis showed that the amidation reaction improved the thermal stability of PEI,which laid a foundation for the preparation of subsequent catalysts.3.Using PEI-M1H as carbon source and nitrogen source,commercial conductive carbon Ketjenblack EC-300J as carrier,PEI-MIH and transition metal Fe were compounded,then supported on EC-300J at high temperature,the catalyst Fe/PEI-M1H/C was carried out.The thermogravimetric analysis shows that the thermal stability of the catalyst Fe/PEI-M1H/C is good.The electrochemical performance tests show that the ORR activity of Fe/PEI-M1H/C is close to that of commercial Pt/C in alkaline condition.(The half-wave potential of Fe/PEI-EN/C E_1/2=0.80 V,E_1/2?Pt/C?=0.84 V,and the limit current density of Fe/PEI-EN/C J_L=6.3mA cm^-2,J_L?Pt/C?=6.0mA cm^-2),and the durability of Fe/PEI-M1H/C is better than that of Pt/C.4.The PEI-EN was used as carbon source and nitrogen source to compound transition metal,and Ketjenblack EC-300J was used as carrier.The results of thermogravimetric analysis showed that heat treatment improved the thermal stability of the catalyst.The electrochemical performance test showed that the catalyst exhibited the same ORR performance as Pt/C.The half wave potential of Fe/PEI-EN/C E_1/2=0.83V,E_1/2?Pt/C?=0.84 V,and the limit current density of Fe/PEI-EN/C J_L=6.3mA cm^-2,equal to Pt/C's),the durability test also shows that the stability of the catalyst is better than that of Pt/C.The structure and characterization of the catalyst are compared with that of PEI modified with M1H,the relationship between the structure of modified PEI and the properties of ORR was also discussed.