Praperation of magnetic Fe,N co-doped nano-sized graphitized carbon aerogel with hierarchical porous structure and its electrochemical and adsorption performance

Abstract

<p indent=0mm>Nano-sized carbon materials have received intensive preoccupation owing to their extraordinary properties such as high specific surface area, abundant porous structure, and excellent electronic properties. Doping non-metallic element N can effectively modulate the electronic properties of graphene, and doping metallic element Fe can further improve their catalytic activity. To date, applications of Fe-N/C materials are mostly concentrated in supercapacitors, electrochemical double layer electrode materials or adsorption. In this communication, we present a novel route for fabricating Fe,N co-doped nano-sized graphitized carbon materials (FeNC aerogels) by a chelate-carbonization-activation process using chitosan-Fe³⁺ chelate and KOH as precursor and activator respectively. Chitosan-Fe³⁺ chelate aerogel was first formed via sol-gel process, and Fe³⁺ was chelated by chitosan and evenly distributed in 3D networks of aerogel. Then in a nitrogen atmosphere, Chitosan-Fe³⁺ chelate aerogel was pyrolyzed at elevated temperature, and Fe³⁺ was reduced to Fe nanoparticles under the control of chitosan molecular in this carbonization process. At the last stage of KOH activation process, most of the amorphous carbon was etched, and abundant nanoscale pores appeared. Upon mild heating at 70°C in air, core@shell Fe@Fe₃O₄ nanoparticles were formed due to the reaction of Fe nanoparticles inside the Fe-N/C aerogels with oxygen in air. The morphology, structure, and properties of the obtained FeNC aerogels were characterized using field emission scanning electron microscope (FESEM), high-resolution transmission electron microscope (HRTEM), element mapping, Raman spectra, X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), and vibrating-sample-magnetometer (VSM). FeNC aerogels were confirmed to be Fe,N-contained carbon aerogels with 3D hierarchical porous scaffold including macropores, micropores and mesopores, displayed low density and good magnetic response. The as-prepared FeNC(1-100)-K800 showed extremely large specific surface area <sc>(3103 m²/g),</sc> low density <sc>(27 mg/cm³),</sc> and high nitrogen content (4%). The magnetic response ability of FeNC aerogels increased with the increase of Fe content, and Fe was doped as Fe@Fe₃O₄ and Fe-N. FeNC aerogels exhibited good electrocatalytic oxidation-reduction activity, which increased with the increase of Fe doping amount, graphitization degree, and specific surface area. The obtained FeNC(1-20)-K800 with the highest Fe doping amount showed the highest initial potential of <sc>0.98 V,</sc> half-wave potential of <sc>0.82 V</sc> in<sc>0.1 mol/L</sc> KOH electrolyte saturated with O₂, and the highest initial potential of <sc>0.92 V,</sc> half-wave potential of <sc>0.62 V</sc> in<sc>0.05 mol/L</sc> H₂SO₄ electrolyte, respectively. Meanwhile, FeNC aerogels showed good performance of the electric double layer capacity and good cycling stability, and the specific capacitance enhanced with the increase of specific surface area. The specific capacitance, energy density, and power density of FeNC(1-40)-K800 was <sc>253 F/g,</sc> 35.1 W h/kg, and <sc>249.7 W/kg</sc> respectively. And its specific capacitance retention rates were 92.9% and 88.6% respectively after 6000 times and 10000 times continuous charge-discharge cycles at a current density of <sc>10 A/g.</sc> Furthermore, The saturated adsorption capacities of four kinds of dyes: Methyl orange (anionic dye), Rhodamine B (neutral dye), crystal violet and malachite green (cationic dyes), on FeNC(1-20)-K800 all exceeded <sc>400 mg/g,</sc> and the saturated adsorption capacity of malachite green reached up to <sc>816 mg/g.</sc> FeNC aerogel prepared here is a new type of carbon materials with application potential in the field of new energy and environmental remediation. The novel aerogel preparation process we preposed is suitable for large-scale industrial preparation without environmental risks because complex nitrogen and iron treatment steps and expensive equipments are not needed.