Preparation And Electrocatalytic Properties Of Nitrogen-doped Graphene And Its Iron-based Hybrid Nanostructure

Fossil fuels depletion and the ever-increasing environmental pollution have raised urgent demand for the exploitation and utilization of renewable clean energy resources.Fuel cell,an environmental friendly and highly efficient energy conversion device,has been under the spotlight in the past several years.Platinum(Pt)based catalysts are the best for the oxygen reduction reaction(ORR)on the cathode of fuel cells.However,the scarcity of Pt natural resource and the prohibitive cost of Pt based catalysts have severely hindered the wide-spread application of fuel cells.Therefore,numerous efforts are underway for the development of low-cost,highly-active and durable Pt-free catalysts.Among them,nitrogen doped graphene(NG)is a promising candidate owing to its unique physical and chemical properties.In this dissertation,graphene oxide(GO)has been selected as the starting material because of its relatively low cost and gram-scale to kilogram-scale production.A facile and effective approach has been utilized forthe synthesis of pyridinic nitrogen dominated graphene(pyri-NG)and pyri-NG supported iron-based nanostructures(i.e.Fe-NG).The effects of synthetic parameters on the morphology,structure,and chemical compostion of the materials have beeninvestigaged.Meanwhile,the ORR catalytic activity of the materials in alkaline electrolyte has beenstudied.Considering the debates on the ORR active sites for NG and Fe/N/C catalysts,the primary ORR active centers of pyri-NG and Fe-NG are clarified.The results are as follows:First,low temperature solution-based chemical reduction of GO is used for the preparation of reduced graphene oxide(r GO).Then,KOH activation is employed for the introduction of rich pores on r GO so as to improve the specific surface area of r GO.This activation method is facile and highly compatible with industrial prodction when compared with other pore-introducing methods.The subsequentlow-vaccum high-temperature annealing of r GO in ammonia gas(NH3)is utilized for the etching of carbon layers and the doping of nitrogen.This NH3 annealing method has several advantages such as simplicity,controllable synthetic conditions,as well as high repeatability.The nitrogen doping level can be tuned via annealing at different temperatures for a certain period of time.Without the sacrifice of conductivity,higher ORR catalytic activity can be achieved by the preparation of NG with higher nitrogen doping level and higher specific surface area.Second,r GO aerogels with highly porous three-dimesional(3D)structure are obtained via solution-based chemical reduction and freeze drying.The r GO aerogels are then subject to low-vaccum high-temperature annealing in NH3.Pyri-NG(up to 80% of a total nitrogen of 9.5 at.%)can be obtained simply bytuning the annealing temperature and annealing time.The starting material and the synthetic parameters are optimized to achieve even higher doping level of pyridinic nitrogen(up to 90.4 % of a total nitrogen of 12 at.%).The pyri-NG displayed high limiting current density and high durability close to those of the commercial Pt/C 20 wt.% catalyst.Our results confirm that pyridinic nitrogen plays dominant role toward the ORR catalytic activity.Furthermore,r GO aerogels anchored with highly dispersed iron precursor are prepared via solution-based method.After annealing in NH3 at elevated temperature,Fe-NG are obtained.The Fe-NG with an optimal loading amount of iron exhibits synergetic enhancement of ORR catalytic activity.Our results support that Fe-N-C structure in Fe-NG may constitute the primary ORR active sites.In addition,holes are introduced on the carbon layer of Fe-NG,which may also facilitate ORR.The best Fe-NG catalyst displays almost the same onset potential and half-wave potential as the commercial Pt/C 20 wt.% catalyst.The durability are comparable to that of Pt/C and the limiting current density and methanol resistance ability are much better than those of Pt/C.Our work provides a facile and effective avenue for the development of high performance Pt-free catalyst that are promising to replace the commercial Pt based catalysts.