Double Reinforced Energy Storage of Graphene By KOH Activation and Nitrogen Doping
Authors: Wang Yiran , Guo Jiang , Lyu Hailong , Wei Suying , Guo Zhanhu 
Published: February 2020

For the goal of synthesizing high performance supercapacitor electrode materials to meet the pressing requirements of energy storage, nitrogen doped activated graphene (N-AG) was successfully synthesized by employing two steps as KOH activating and following nitrogen doping using a hydrothermal method. Continuously increased ID/IG ratios were observed in Raman spectra after KOH activation and N-doping process, indicating increased defects due to the etching effect of KOH and the introduced nitrogen defects on graphene sheet. Stacked graphene with damaged sheets on the surface was clearly seen in transmission electron microscopy (TEM) image after the KOH etching. The successful dopant of N was demonstrated by X-ray photoelectron spectroscopy (XPS) spectra and energy-dispersive X-ray spectroscopy (EDS) mapping, confirming this effective hydrothermal N doping method. The N-AG exhibited largely enhanced capacitance (186.63 F/g) and cycling stability compared with that of nitrogen doped graphene (N-G, 50.88 F/g) and activated graphene (AG, 58.38 F/g) due to the combined positive effects of KOH activation and N-doping. This facile hydrothermal method is of great significance of scaling the production and the synthesized N-AG through combining the KOH activation and nitrogen doping processes represents a promising alternative candidate to commonly used electrode materials in supercapacitor.

Experimental

Graphite oxide (GO) was synthesized following the modified Hummers method.1 For the synthesis of nitrogen doped graphene (N-G) and KOH activated graphene (AG), 26.0 g homogeneous GO solution (0.338 g GO) was first transferred to a 40 mL teflon lined hydrothermal autoclave, 3 mL NH4OH solution or well dissolved KOH solution (4.05 g KOH, mass KOH : GO = 12:1) was added, the solution was diluted to 40 mL by DI water and sealed maintaining at 150 ºC for 3 hrs. After cooling down, the precipitates were collected by filtration and washed with water for 3 times, the products were dried naturally for 72 hours. Reduced graphene oxide (RGO) was also obtained following the same procedure using simple GO solutions as a control experiment. For the synthesis of N-AG, 0.2 g synthesized AG powders were uniformly grinded and dissolved in 30 mL water, which undergo a 30 min sonication to achieve a uniform distribution, the AG solution was then transferred to a 40 mL autoclave added with 1.80 mL NH4OH solution (same NH4OH/AG ratio as that of NH4OH/GO). Similar procedures were following proceeded to obtain the N-AG powder products.

Results and Discussion

The Raman spectroscopy and TEM have successfully demonstrated the increased defects and edge parts on graphene after the activation with KOH. Typical N peaks deconvoluted from XPS and uniform distribution of N observed from EDS mapping clearly indicate the successful doping of N using this facile hydrothermal method. Activation and N-doping processes can both contribute to the enhancement of energy storage through increasing active electronic sites and surface area. Increased capacitance, energy and power densities as well as cycling stability after the activation and N-doping processes are confirmed by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements,2demonstrating the effectiveness of this hydrothermal method and potential of this N-AG for practical energy storage applications.

Conclusion

A facile hydrothermal synthesis method combining KOH activation and N-doping was successfully employed to synthesize nitrogen doped activated graphene, which is economically and effectively at a very high yield, implying feasibility for mass production. The synthesized N-AG exhibited relatively decreased oxygen containing groups with increased defects due to the etching nature of KOH and N containing defects were also introduced with this effective N doping method. These unique properties endow them as promising electrodes for supercapacitors with high capacity, excellent rate capability, and long-term stability. Due to the possibility of scalable synthesis and prominent properties, the N-AG can offer attractive opportunities for both fundamental study and potential industrial applications in catalysis, adsorption, energy storage, and energy conversion systems.

Acknowledgments

The financial supports from University of Tennessee Knoxville are kindly acknowledged.

Figure 1. Charge−discharge of RGO, N-G, AG and N-AG with a current density as 1 A/g. Background is a schematic representation of the synthesis process.

References

1. Y. Wang, Q. He, X. Zhang, J. Guo, J. Zhu and Z. Guo, J. Mater. Chem. C, 2, 9478 (2014).

2. Y. Wang, H. Wei, J. Wang, J. Liu, and Z. Guo, J. Mater. Chem. A, 3, 20778 (2015).

Figure 1