Properties And Catalytic Mechanism Research Of N/S Doped 3-D Porous Graphene For Oxygen Reduction Reaction

Oxygen reduction reaction?ORR?is an extremely important process in energy conversion device such as metal–air batteries and fuel cells which are hindered by the sluggish ORR.Until now,commercial Pt/C is the most efficient catalysts to catalyze the inherently sluggish oxygen reduction reaction.However,the limited natural abundance of Pt along with its high cost has hindered the widespread commercial success of the new energy technologies.Graphene,owning great electrical properties,mechanical properties,optical properties,has attracted scientists' attention from 2004.Introduction of transition metal and heteroatom into graphene could improve greatly the electrochemical performance thus it is regarded the one of the most promising non-precious metals catalysts.Nevertheless,incident stack and reunite of graphene sheets will largely diminish the specific surface area,resulting lower electrochemical property.At present,it has been reported the hard template method is believed to be efficient way to convert two-dimensional?2D?graphene sheets into three-dimensional?3D?porous structures.But hard template method,using Ni or silica as template,is tedious and complicated.In this paper,we have resoundingly applied a one-step synthesis procedure to prepare 3-D porous N/S-doped graphene without needing addition of any extra Ni or silica hard template and studied their electrochemical activities towards ORR in alkaline medium.SEM,TEM,low temperature nitrogen absorption/desorption,XRD and XPS are used to characterize the structure and compositions of catalysts.The kinetic of ORR has been studied via CV,RDE,RRDE and zinc-air battery techniques.We are trying to find the key factors that affect ORR and the possible structure of active sites.The main achievements are summarized as follows:?1?We have developed an efficient new one-step synthesis method to prepare 3D porous N/S doped graphene,in which the 3-aminopropyltriethoxysilane/trimethoxysilylpropanethiol was used as ‘template' and N/S resources.In alkaline solutions,the ORR onset potential of 3D-PNG is 0.973 V vs.RHE and limiting current density is 7.154 mA cm^-2.The ORR activity of 3D-PNG is comparable with that of the Pt-based catalyst.Meanwhile,the sulfur doped graphene?PSG?shows little lower ORR activity.3D-PNG also exhibits an outstanding ORR process that occurs mainly through a 4-electron transfer pathway reducing oxygen to water in alkaline medium.In addition,the 3D-PNG catalyst also shows an excellent tolerance to methanol poisoning effects.The applied one-step synthesis method without addition of template can effectively decrease the aggregation of graphene sheets and thus increase the surface area.The large surface area and porous structure have an advantage of active sites exposure and rapid transportation of electro-reactants/products.Through the zinc-air battery test,the peak power density of 3D-PNG is found to be as high as 275 mW cm^-2 that was superior to Pt/C(120mW cm^-2).?2?The type of transition metal precursors plays a key role in the performance of catalysts.We have prepared Me-PNG?Me=Fe,Co,Ni?catalyst using FeSO4·7H2O,CoSO4·7H2O,NiSO4·6H2O as different transition metal.The effects of transition metal precursors on the performance of Me-PNG?Me=Fe,Co,Ni?are also investigated.The Fe-PNG prepared with FeSO4·7H2O as precursors exhibits the best ORR activity.Fe-PNG has a high onset potential of 0.197 V and large diffusion-limiting current density of 5.9 mA cm^-2 in alkaline solution,better than that of Co-PNG and Ni-PNG catalyst.Fe-PNG has high energy conversion efficiency with 4e-transfer pathway in alkaline medium.There is 2e-transfer pathway during the process that Co-PNG and Ni-PNG catalyze the ORR.?3?Furthermore,we have investigated the influence of reduction way and pickling on N-doped graphene.The NG-1 prepared by high-temperature reduction and washing with acid exhibits the best onset potential.The NG-2 prepared by high-temperature reduction but without pickling shows the best kinetics for ORR with 4e-pathway.The NG-3 reduced with sodium borohydride possesses lower ORR performance may be caused by deficient structure of graphene resulted from the remanent oxygen-containing groups.