Design And Synthesis Of Metal-free Oxygen Reduction Electrocatalyst With Surface Microstructure Tuning

As one of the most promising automotive clean power solutions,fuel cell has gradually come into our lives.The oxygen reduction reaction(ORR)is the cathode reaction of the fuel cell,which is now still heavily relied on platinum-based catalysts and subsequently leads to the high cost of fuel cell stacks.Meanwhile,due to the scarceness of platinum,it is difficult to meet the increasing demand for fuel cells in various applications in the near future.Therefore,the development of low-cost,highly active and environmental-friendly ORR catalysts is crucial for the future commercial application of fuel cells.Based on the idea of "material plainification",this paper developed a series of N-doped carbon materials via tuning surface microstructure with low-cost and high-quality biomass as raw materials.The research details are as follows:1.To explore the influence of the morphology and catalytic activity of carbon-based electrocatalysts hydrothermally treated in different solution environments,a series of N-doped carbon sphere materials were prepared by the hydrothermal heating and subsequent calcinating in an ammonia atmosphere.The concentrations of glucose solutions were varied and the hydrothermal environment was chosen to be in nucleus-containing,transition-metal-ion-containing or alkaline solutions,respectively.Under different hydrothermal environments,the morphology and size distribution of nano-carbon materials were quite different.The thinner the carbon layer is,the smaller the carbon sphere size is,then the smaller the electron transfer resistance would be,which results in a more heterogeneous reaction interface and a smaller ion transfer resistance,eventually boosting the 4e ORR process.Besides,the ORR catalytic performances of electrocatalysts with either glucose or dopamine as the carbon source and either urea or ammonia as the nitrogen source were compared.It was found that N-doped carbon prepared with glucose and urea as raw materials would achieve their superior ORR performance.This work provided a wealth of practical experience for the preparation of ORR electrocatalysts with biomass as the resource.2.To optimize the surface microstructure of metal-free N-doped carbon-based electrocatalysts,a two-step synthesis process combined hydrothermal polymerization and high-temperature pyrolysis was adopted with glucose as the carbon source,urea as the nitrogen source,and ZnCl2 as the porogenic agent.ZnCl2 could be volatilized at high temperatures,and a large number of micro/mesopores were generated.As the calcination temperature increased from 800℃ to 1000℃,the specific surface area of the N-doped porous carbon catalyst increased from 965.18 m2 g-1 to 1786.41 m2 g-1.A space confinement effect was found in this specific structure,which increased the local density of HOO" or other intermediates of ORR,and subsequently increased their collision frequency with active sites of the catalyst.Thus,it was beneficial for N-doped porous carbon catalysts to obtain the desired high selectivity towards 4e ORR process.The N-doped porous carbon sample of NPC-1000 was pyrolyzed at 1000 ℃,which possessed better onset and half-wave potentials of 0.9249 V(vs.Pt/C 0.9224 V)and 0.8593 V(vs.Pt/C 0.8509 V),respectively,than the commercial Pt/C catalyst with the same loading.Meanwhile,the NPC-1000 sample also exhibited superior resistance to methanol poisoning compared with the commercial Pt/C catalyst,which were beneficial for its potential applications in alkaline fuel cells,direct methanol fuel cells,and metal-air batteries.In this work,not only efficient metal-free carbon-based catalysts were successfully synthesized for ORR,but also the factors related to the catalytic activity besides traditional active sites were systematically explored.Thus,it could provide guidance to understand the role of surface microstructures in ORR electrocatalysts.3.To explore the mechanism of micro/mesoporous structure formation and the contribution of zinc salts to ORR in metal-free N-doped porous carbon catalysts,control experiments of precarbonization and with different zinc sources were conducted.The precursors precarbonized at different temperatures were characterized by FT-IR and XPS.The carbon precursor after precarbonization at 400℃ could not be activated by ZnCl2,and the onset potential and half-wave potential were reduced to 0.8661 V and 0.6692 V,respectively.However,the carbon precursor precarbonized at 600℃ could be well activated with onset potential of 0.9067 V and half-wave potential of 0.8254 V,close to that of the NPC-1000 sample.Compared to zinc-source-free N-doped carbon material,the half-wave potential of the N-doped carbon with zinc oxide addition in the precursor negatively shifted～23 mV,whereas the half-wave potential of the N-doped carbon with zinc nitrate addition in the precursor positively shifted～82 mV,which was similar to that with ZnCl2 addition(positively shifted～87 mV).This observation suggested that different types of zinc porogenic agents had an important effect on the properties of N-doped carbon-based electrocatalysts.In FT-IR spectra of polyglucose and urea,C=O and C-O peaks were weakened or even disappeared after the addition of zinc salts,while the XPS spectra indicated that the peak spacing between Zn 2p1/2 and O is peaks increased from～247 eV without Zn-O interaction to～247.9 eV with Zn-O interaction.Thus,Zn-O interaction occurred when zinc salts were used as the porogenic agent,which affected the electrocatalytic performance of N-doped carbon-based electrocatalysts.In this work,the pore formation mechanism by zinc salt addition was explored,which could be helpful to fully understand the role of ZnCl2 in improving the catalytic performance of carbon-based materials for ORR.