Study On Preparation And Performance Of Biomass-derived Carbon-based Oxygen Reduction Reaction Catalyst

Pt alloy and other noble metal catalysts are widely utilized in the cathode oxygen reduction reaction（ORR）of anion exchange membrane fuel cells（AEMFC）.Due to the limitation of precious metal reserves and market price factors,ORR catalysts account for more and more costs in AEMFC reactors.Therefore,it is great engineering significance to develop ORR catalysts with high electrocatalytic activity,strong cycle stability,and low price to promote the application of AEMFC.Biomass-derived carbon-based materials have the advantages of easy access,low cost,rich in heteroatoms,controllable morphology,and so on.It is a highly competitive ORR catalytic material.In this thesis,N-doped and N,P,S ternary-doped porous carbon-based ORR catalysts are synthesized by high-temperature pyrolysis using yuba and spinach leaves as raw materials.The effects of structure and morphology on ORR catalytic performance are systematically studied.The main conclusions are as follows:（1）High N content（sample Y-850）and multi-type heteroatom（N,P,S）doped porous carbon materials（sample S-850）are prepared by pyrolysis at 850℃using yuba with protein content up to 53%and spinach leaves rich in vitamins,folic acid and other nutrients,respectively.The total N content in Y-850 is 4.29 at%（graphite-N content is 3.23 at%）.The N,P and S contents of S-850 are 5.43 at%（graphite-N content is 1.21 at%）,0.71 at%and 0.30at%,respectively.In the O₂ saturated 0.1 M KOH solution,the half-wave potential and limit diffusion current density of Y-850 are-0.163 V and-4.51 m A cm^-2,respectively,while the corresponding values of S-850 are-0.182 V and-4.33 m A cm^-2,respectively.Although the ORR catalytic performance of the above two carbon materials is slightly inferior to that of commercial 20%Pt/C,their stability and resistance to methanol/CO toxicity are better than those of commercial 20%Pt/C.（2）In order to improve the ORR catalytic performance of Y-850 and S-850,Na PO₃ and urea as P and N sources are mixed with yuba and spinach leave and carry out pyrolysis,respectively.The results show that P atom is doped,and N atom content is improved,and the ORR performance of the two catalysts is improved.Further doping of P,N atoms into biomass derived carbon materials can effectively improve the catalytic performance of ORR.（3）After ball milling the raw materials of yuba and spinach leaves,it is found that the ball milling process did not affect the porous structure of the surface of the prepared carbon particles,which can effectively reduce the particle size of the prepared carbon materials,increase the specific surface area,and thus improve their ORR performance.（4）In order to further improve the ORR performance and stability of biomass-derived carbon materials,Y-850 is treated by ultrasonic hydrothermal+centrifugal separation treatment（abbreviated as ultrasonic+centrifugal treatment）,and the sample is obtained（sample YUC-850）.Under the continuous bombardment of micro-jet formed by ultrasonic cavitation,the isolated small particles with low N content and sediment on the surface of Y-850 are gradually peeled off,and the surface with high N content is exposed.The total N content increased from4.29 at%to 6.32 at%,and the graphite-N ratio increased from 75%to 79%（XPS results）.In O₂ saturated 0.1 M KOH solution,the half-wave potential and limiting diffusion current density of YUC-850 are-0.139 V and-5.21 m A cm^-2,respectively,which are comparable to those of20%Pt/C.After 5000 cycles of CV test,the half-wave potential and limiting diffusion current density of YUC-850 only attenuate-31 m V and-0.28 m A cm^-2,and the current density of YUC-850 only attenuate 7%and 13%after potentiostatic polarization and CO toxicity test.The results show that ultrasonic+centrifugal treatment can significantly improve the ORR performance,stability,and toxicity resistance of Y-850.By analyzing the surface element distribution and content changes of carbon particles before and after ultrasonic+centrifugal treatment,the mechanism of improving ORR catalytic performance is clarified.The effectiveness of ultrasonic+centrifugal treatment in improving the performance of S-850 has been verified.（5）Based on the mechanism of improving catalyst performance by ultrasonic+centrifugal treatment,a method is proposed to recover the catalytic performance of degraded biomass-derived carbon material ORR by ultrasonic+centrifugal treatment.After 5000 CV cycles of YUC-850 in O₂ saturated 0.1 M KOH solution（accelerated durability test,sample YUC-850-ADT）,compared with YUC-850（based on its test data）,The surface total N content,half-wave potential and limiting current density of YUC-850-ADT are reduced to 80.3%,76.3%and 94.6%of the original values,respectively.After ultrasonic+centrifugal treatment（sample YUC-850-ADT-U）,the total N content,half-wave potential and limit diffusion current density of the sample surface are restored to 92.3%,88.5%and 97.9%of the original values,higher than the performance after accelerated durability test.After repeated accelerated durability test and ultrasonic+centrifugal treatment steps,the recovery degree of catalytic performance of ORR gradually decreased with the increase of repetition times.The performance recovery process of ultrasonic+centrifugal treatment has achieved the same effect on yuba derived carbon materials（Y-850）,spinach leave derived carbon materials（S-850 and SUC-850）.（6）The above research results show that the ORR catalytic performance of the derived carbon materials can be improved by ball milling,heteroatomic doping,and ultrasonic+centrifugal treatment.Among them,ultrasonic+centrifugal treatment has a significant effect on improving ORR catalytic performance and ORR catalytic performance recovery after aging of biomass derived carbon materials,which has good engineering application value.