Synthesis and Electrochemical Properties of Nitrogen-Doped Partially Graphitized Carbon/Cobalt Iron Oxides Composite

With the renewable biopolymer chitosan (CTS) as a structure directing agent and organic precursor, facile coprecipitation method was applied for the cobalt and iron nitrates in solution to prepare CTS/cobalt iron layered double hydroxides composite. The LDHs sample was calcinated in a tubular furnace under Ar atmosphere via heating ramps of 5 degrees C min(-1) from room temperature to 200 degrees C and kept for 1 h, then heated to 600 degrees C and remained for 2 h. After the sample was cooled naturally to room temperature, it was heated again to 250 degrees C under air atmosphere and kept for 12 h to oxidize the transition metal elements. As a result, nitrogen-doped partially graphitized carbon/cobalt iron transition metal oxides nanocomposite (N-PGC/CoFe-TMOs) was obtained. X-ray diffraction, Raman spectroscopy, N-2 adsorption-desorption analysis, scanning electron microscopy, high resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy were carried out to characterize the structure, morphology and elemental composition of the product. Cyclic voltammetry and galvanostatic charge-discharge measurements were conducted to evaluate the electrochemical properties of N-PGC/CoFeTMOs. Experimental results showed that the CTS precursor was converted into partially graphitized carbon by pyrolysis with the help of catalysis graphitization action of transition metal elements. At the same time, the derived carbon material was successfully doped with nitrogen in situ and the N/C atomic ratio was about 1/18. N-PGC/CoFe-TMOs possessed bimodal porous texture including macropores and mesopores, exhibited combined characters of electrical double-layer supercapacitor and pseudocapacitor when used as supercapacitor electrode material. At the current density of 2 A.g(-1), N-PGC/CoFe-TMOs composite delivered a large discharge capacity of 671.1 F.g(-1), far higher than 283.3 F.g(-1) of pure cobalt iron oxides, indicating the typical synergistic effect between nitrogen-doped partially graphitized carbon and transition metal oxides. Even at the high current density of 10 A.g(-1), N-PGC/CoFe-TMOs composite still remained a specific capacity of 573.3 F.g(-1). After 5000 charge-discharge cycles at 10 A.g(-1), the capacitance retention was 66.4%. The reported synthesis method in this work is simple and universal, and calcination process combines the nitrogen-doping, partially graphitized carbon formation with redox-active transition metal oxides synthesis in one step, endowing the product with excellent electrochemical properties.