Preparation And Electrochemical Properties Of Carbon Nanomaterials As Oxygen Reduction Reaction Catalysts For Metal Air Fuel Cells

Fuel cell has the advantages of high efficiency,pollution-free,and it is an ideal energy device.As a kind of fuel cells,magnesium-Air fuel cell has attracted much attention due to its rich source,low cost,non-toxic and environmental protection.In the cathode of fuel cells,oxygen reduction reaction?ORR?occurs.However,the performance of fuel cells is limited because of the extremely high cathodic overpotential.Therefore,the research and development of highly active cathode catalyst for oxygen reduction is desiderated.In this article,two-dimensional nitrogen doped graphene?2D-NG@H-MMT?was prepared by the in situ oxidative polymerization of aniline and more amino benzene compounds?ph?NH₂?_x?inside the layer gaps of hydrogen montmorillonite?H-MMT?,and then pyrolyzed at high temperature as catalysts for oxygen reduction reaction.The composition,structure and microstructure of the catalysts were characterized by SEM?TEM?XPS?Raman and nitrogen adsorption-desorption isotherms.The results show that the nitrogen doped graphene presents wrinkled,silk-veil-like,and porous graphene structures.Many edge planes can be observed on the surface of the graphene nanosheets.The pore size is about 20 nm.The electrochemical tests show that the original reduction potential of 2D-NG@H-MMT?AN?is0.88 V?vs.RHE?,which is 110 mV higher than that of nitrogen doped graphite.The number of transferred electrons is 3.93.And peak power of MAFC is 72 mWcm^-2.It proves that the catalytic performance of 2D-NG@H-MMT?AN?is higher than nitrogen doped graphite.The layers of two-dimensional graphene are easy to stack together,so its advantages can not be reflected.In order to solve this problem,three-dimensional nitrogen doped graphene was prepared and researched.O-phenylendiamine?OPD?or?ph?NH₂?_x?was used as the carbon sources and nitrogen source,and silica colloid?the partical size is is 5 to 10 nm?was used as the templates.Three-dimensional nitrogen doped graphene is prepared by the in situ oxidative polymerization of OPD or?ph?NH₂?_x?,and then pyrolyzed at high temperature.By discussing the influence of mass ratios of raw materials and the pyrolysis temperature on the performance of catalysts,the optimum parameter is obtained,and that is OPD:SiO₂=1:2,pyrolysis@900°C.The composition,structure and microstructure of the catalysts were characterized by SEM?TEM?XPS?Raman and nitrogen adsorption-desorption isotherms.The results show that it shows wrinkled,silk-veil-like,and porous graphene structures.The number of graphene layers is 4–6.Nitrogen mainly exists in the form of pyridinic nitrogen and pyrrolic nitrogen.The electrochemical tests show that the original reduction potential of3D-NG@SiO₂ is 0.947 V?vs.RHE?.The number of transferred electrons is 3.92?@0.567 V?.3D-NG@SiO₂ possesses good catalytic activity and durability.The peak power of MAFC is 78mWcm^-2.The discharge performance of MAFC is good.The synergy between transition metal and nitrogen atoms can further improve the electrochemical performance of catalysts.In this paper,metal and nitrogen co-doped graphene was prepared by pyrolyzing the mixture of PPDA(or other?ph?NH₂?_x?and iron salt or cobalt salt.It presents silk-veil-like and porous graphene structures.Such structures can provide large specific area,convenient electron transfer channels,and numerous exposed active sites.The electrochemical tests show that among all the metal and nitrogen doped graphene we prepared,3D-Fe-Co-PDA presents the best catalytic performance.In alkaline conditions,its original reduction potential is 0.967 V?vs.RHE?.The number of transferred electrons is 3.95.In acidic conditions,its original reduction potential is 0.869 V?vs.RHE?.The number of transferred electrons is 3.86.Its stability and methanol resistance is excellent.The peak power of MAFC is81.59 mWcm^-2.Polymer materials can also be used as the source of nitrogen and carbon source to prepare nitrogen doped graphene.They are soluble and their solution is easily to paint on the surface of templates.In the carbide process,it is not easy to volatilize.So the layers of graphene can be regulated by controlling the mass ratios of polybenzimidazole?ABPBI?and templates.ABPBI@MMT is prepared by the pyrolysis of ABPBI in layered templates under high temperature.Fe-ABPBI@MMT is prepared by the pyrolysis of ABPBI and FeCl₃·6H₂O in layered templates.Two catalysts present the typical multilayer graphene structure.Fe-N_x is formed in Fe-ABPBI@MMT,and its presence can increase the number of active sites and further improve the catalyst performance.The electrochemical tests show that the catalyst performance of Fe-ABPBI@MMT is better than ABPBI@MMT.This is because the synergies between iron and planar N provides further improvement in catalytic properties.In alkaline conditions,its original reduction potential is 1.01 V?vs.RHE?.The number of transferred electrons is 3.95.In acidic conditions,its original reduction potential is 0.826 V?vs.RHE?.The number of transferred electrons is 3.86.The peak power of MAFC is 90.2 mWcm^-2.