One-Pot STRATEGY for CONTROLLABLE GROWTH of Cnts on GRAPHENE AS High-PERFORMENCE SUPERCAPACITOR ELECTRDE Material

One-pot pyrolysis strategy was reported to prepare graphene-carbon nanotube (GN-CNT) hybrid. We realized controllable growth of CNT on graphene nanosheets with this novel method. The strategy for the fabrication of GN-CNT hybrids is illustrated in Figure 1a. Figure 1b presents the SEM image of GN-CNT, from which interlinked CNTs on the graphene flakes can be clearly seen, indicating a well-defined GN-CNT hybrid. 

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Figure 1a) Schematic representation of the procedure for the preparation of the GN-CNT hybrids; b) SEM images of GN- CNT.

Urea was utilized as carbon source and the produced cobalt (verified by XRD in Figure 2a) acted as the catalyst for growing CNTs. In addition, urea also prevents the restack of graphene sheets, as well as introduces nitrogen heteroatom into the carbon network (verified by XPS in Figure 2b). N₂ adsorption/desorption analyses in Figure 2c revealed that the as-prepared GN-CNT hybrid has high specific surface area of 903 m² g^-1, nearly 4.5 fold of that of graphene (GN) (205 m² g^-1). We attribute the significant increase to an effective intercalation and distribution of CNTs between graphene nanosheets, together with the generation of a mass of pores caused by the pyrolysis of urea, releasing a large amount gases. Further, Raman spectrum ( Figure 2d) also shows that GN-CNT hybrid has much higher disorder graphic structure, and fewer graphene layers comparing to that of  GN.

The GN-CNT hybrids show promising performance for super-capacitors (SCs) applications in 6 M KOH electrolyte. In order to state the advantage of GN-CNT in SCs application, GN, a physical mixture of graphene and CNTs and GN-Co-Urea_3.0 were used for comparisons. Figure 3a presents the cyclic voltammetry of all the samples. A promising improvement of specific capacitance was observed for GN-CNT based SCs. Figure 3b shows the galvanostatic charge/discharge curves of GN-CNT_6.0. Specific capacitance of 472 F g^-1 was calculated for GN-CNT_6.0 at a current density of 1 A g^-1. Long-term stability was also performed in Figure 3c. After 5000 cycles, a capacitance increase of ca. 15% of the initial capacitance for GN-CNT_6.0is observed, indicating excellent electrochemical durability of the sample. Further, electrochemical impedance spectroscopy demonstrates that internal resistance also could be reduced due to the combination CNTs with graphene.

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Figure 2 a) XRD patterns of GN and GN-CNT_6.0; b) N₂ adsorption-desorption isotherms for GN-CNT_6.0 and GN; c) High-resolution of N1s XPS spectra of GN-CNT_6.0; d) Raman spectra of GN-CNT_6.0 and GN; the inset shows an expanded view in the region of 2500-3000 cm^-1.

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Figure 3 a) CVs of GN-CNT hybrids, GN, GN-CNT_blend and GN-Co-Urea_3.0 in 6 M KOH solution at 5 mV s^-1; b) galvanostatic charge/discharge curves for GN-CNT_6.0 at various discharge current densities; c) Average specific capacitance versus cycle number for GN-CNT hybrids, GN-CNT_blend and GN at a galvanostatic charge and discharge current density of 10 A g^-1; d) Complex plane plot of the impedance; inset: an expanded view in the region of high freq