Investigation Of Non-precious Metal Cathode Catalyst Based On Nitrogen Doped Carbon Materials

In the past few years,new energy vehicles were developing rapidly.Among them,fuel cell vehicles are very promising due to their comparable mileage and refueling time towards traditional vehicles.Proton exchange membrane fuel cells(PEMFCs)are very suitable for vehicles since their high power density and fast startup capability.The main factor hindering its large-scale application is the cost,so it is of great significance to develop low cost non-precious metal cathode catalyst.Among all kinds of non-precious metal catalysts,nitrogen doped carbon and metal-nitrogen-carbon catalysts are the most promising ones,since their high performance in both acidic and alkaline electrolyte,and much higher durability than metal-oxides in acid.The synthesis routes of the two kinds of catalysts are varied,but most of the synthetic routes require high temperature heating.The commonly used high temperature tube furnace has low heating efficiency and low heating rate,making the preparation of catalysts waste time and energy.In this paper,nitrogen doped graphene was prepared by microwave heating.Taking the advantage of high microwave absorption rate of graphene,selective heating of the precursors was realized.The total heating time was reduced from hours to less than 1 minute,thus greatly reducing the energy consumption.Throughout the electrochemical test of the samples with different microwave heating time,it was shown that the best oxygen reduction activity was achieved after the catalyst was heated for 10s in the atmosphere of ammonia.Further extended reaction time doesn't improve the ORR performance and the selectivity of the catalyst significantly.It is found that the ORR activity of the catalysts has a high correlation with the mass fraction of the graphite structure nitrogen atom by the results of elemental analysis and the Nls peak of XPS spectrum.It indicates the most active type of nitrogen atoms is the graphitic nitrogen atom.The selectivity of this non-precious metal catalyst is poor,with a hydrogen peroxide yield 35.58%-47.49%,which is more than 7 times that of commercial Pt/C catalysts.Increasing the ORR activity requires increasing the density of active sites.The nitrogen precursor urea is easy to decompose during the heating procedure.Therefore,Maillard reaction is used to make urea and reducing sugar react to form caramel which contains nitrogen.Thus,nitrogen atoms can be retained as much as possible during the pyrolysis process,and the oxygen reduction activity of the catalyst can be improved.TEM images show that the microstructure of the catalyst is mainly the carbon nanotubes grown around the porous carbon black,and some iron nanoparticles are encapsulated by carbon nanotubes.XPS results showed that the iron and nitrogen contents in the catalyst were low while the oxygen reduction activity of the catalysts was high,indicating that there were more active iron and nitrogen atoms in the catalyst.The secondary heat treatment of ammonia partially destroyed the active sites and reduced the activity of the catalyst.5%hydrogen argon mixture heat treatment does not destroy the active sites,and improves the onset potential of the catalyst prepared with BP2000 pretreated with nitric acid.An important factor affecting the ORR activity is caramel and carbon black mass ratio.The optimized mass ratio is 1:1.The mixture degree of BP2000 and caramel can be increased by using the ultrasonic wave,the pretreatment of BP2000 with nitric acid and the addition of Twain 80 as the surfactants,thus improve the ORR activity.The freeze-drying method can avoid agglomeration and significantly enhance the ORR activity.Doping with gold nanoparticles does not help reduce hydrogen peroxide yield.The microwave reflux process of sodium citrate can remove harmful impurities on the catalyst surface and improve the initial ORR activity of the catalyst.The hydrogen peroxide yield of the catalyst is less than 3%.Sulfur doping with thiourea increased the initial oxygen reduction potential of the catalyst,but had a negative effect on the mass transfer capacity.The catalyst showed high oxygen reduction activity in the electrochemical test,but the power density of 254mW/cm2 in a fuel cell test was much lower than commercial Pt/C catalysts.In order to further improve the active site density of the catalyst,a self-supporting non-precious metal catalyst was synthesized.The single component 2-amino thiazole iron complex is used as the precursor,which greatly simplifies the process of the preparation.The agglomeration of precursors during the synthesis was solved by molten salt method,thus the synthesis of self-support sulfur doped Fe-N-C non-precious metal catalysts was realized.TEM and SEM images indicate that the catalyst is of graphene sheet structure,which contains a lot of crimp and pore structure.The surface of the catalyst is distributed with iron nanoparticles less than 10 nm in diameter.The addition of 1g ammonium fluoride in the precursor significantly enhanced the oxygen reduction activity of the catalyst.The increase of activity was mainly due to the increase of the number of micropores in the catalyst.The catalytic activity decreases when the amount of ammonium fluoride added exceeds 1g.The reduction of activity is due to the substitution of some iron ions from the 2-thiazole iron complex by fluorine anions,which changes the structure of the catalyst.The optimized catalyst doped with 1g ammonium fluoride showed good performance in a single cell test at high back pressure.The maximum power density can reach 638mW/cm2 under 1Bar back pressure.