Amino Acid Assisted Preparation Of Iron Based/Nitrogen Doped Graphene Composites And Its Electrochemical Performances

Carbon materials are widely used in electrochemical electrode materials with various variety,low cost,large surface area,rich pore structure and good chemical stability.Owing to unique two-dimensional structure and excellent physicochemical properties,graphene becomes a potential supercapacitor and fuel cell electrode material.However,as the electrode material of supercapacitor,its theoretical specific capacity is small,and catalytic activity is poor as the catalyst of fuel cell,thus limiting its application.The previous work in our lab was focus on the graphene doping modification,and doped three dimensional ordered assembly graphene was realized using amino acid as hetero atomic source,which exhibited excellent electrochemical properties.Considering that amino acids are a kind of green morphological adjuvant of nanomaterials,this paper is focus on the functionalized modification of iron-based graphene composite.Iron oxide/nitrogen doped graphene composite electrode materials were prepared with amino acid as morphological assisted agent and hetero atom source.The supercapacitor and electrocatalytic oxidation reduction properties of the materials were studied.The research contents and main conclusions are as follows:?1?Using three different acidity amino acids as the morphology assisted agent and the nitrogen source,Fe₂O₃/NG composites with the different morphologies were prepared by one-step hydrothermal method.The effects of different acid amino acids for the morphology and ultimate supercapacitance of the composite were investigated.The results showed that the composite materials prepared by acidic amino acid aspartate showed the three-dimensional crosslinked porous network structure and the largest specific surface area(121.5 m² g^-1),and Fe₂O₃ nanoparticles are spindle.The specific surface area of the composites prepared by neutral and especially alkaline amino acid is relatively small,and the Fe₂O₃ nanoparticles are nearly cubic.The Fe₂O₃/NG-Aspartic shows a high specific capacitance,long cycle stability,better rate capability,high energy and power densities.Under the current density of 2 A g^-1,the specific capacity can reach to 1053 F g^-1,when the current density is 20 A g^-1,the specific capacity of 593 F g^-1 can still be maintained.This was due to its three-dimensional porous network structure and large specific surface area.The three-dimensional porous network favored the fast diffusion and adsorption of the potassium ion on the large electrode surface.?2?Although nitrogen-doped graphene prepared by acidic aspartic acid has the larger specific surface area and three-dimensional porous network structure,its final electrocatalytic performance is poor.The main reason is the conductivity of the material is poor due to bad crystallization.In order to obtain the materials with excellent ORR performance by improving the electrical conductivity and enhancing the interaction between non-precious metal and nitrogen doped graphene,we have carried out this part of the work.With the aspartic acid as morphology adjuvant and nitrogen source,we first prepared a three-dimensionally porous network?-Fe₂O₃/NG hydrogel by hydrothermal method.After freezing and drying,and then?-Fe₂O₃/NG composite was prepared by high temperature carbonization.The oxygen reduction catalytic performance of composite were investigated.The results showed that the?-Fe₂O₃/NG composite material has the positive initial potential?0.91 V?,better limit current density(5.01 m A cm^-2)and nearly four electron transfer process?electron transfer number is 3.88?,and better anti-methanol performance and durability than Pt/C electrode.This is attributed to three-dimensional porous network structure of materials,the strong interaction between non-precious metals and nitrogen doping graphene base,higher content of nitrogen and the degree of graphitization of nitrogen doped graphene.?3?In order to avoid the high temperature calcining process to destroy the pore structure of graphene material and cause the accumulation of iron oxide on the graphene sheet,in the second part of this thesis,we first prepare hydrogel and further prepare aerogel precursor by freeze drying.However,the process is complicated,time-consuming and high drying technology required.Furthermore the pore structure of the material is limited to the pore structure of the aerogel after drying.For this reason,we choose iron nitrate as iron source,aiming to prevent the agglomeration of graphene with the gas produced by the decomposition of nitrate ions in the heat treatment process.It is hoped that the iron oxide/graphene composite with excellent electrochemical properties can be prepared the conventional drying technology.We used aspartic acid as the auxiliary agent,nitrogen and carbon source,graphene as carbon source and the supporting substrate of iron based compounds,and ferric nitrate as the iron source and pore forming agent to prepare Fe-N-C/NG composite by conventional drying and heat treatment.The oxygen reduction properties of the prepared samples were tested.The results show that Fe-N-C/NG composites have loose porous structure,larger specific surface area(245.0 m² g^-1).The Fe-N-C/NG composite has the positive initial potential?0.91 V?,better limit current density(5.97mA cm^-2),and nearly four electron transfer process?electron transfer number is 3.88?,and better anti-methanol performance and durability than Pt/C electrode.