Structure Control of Nitrogen-Rich Graphene Nanosheets Using Hydrothermal Treatment and Formaldehyde Polymerization for Supercapacitors

Nitrogen-rich graphene nanosheets (NGN) with intentionally crumpled, stacked, and cross-linked sheet structures were developed using hydrothermal and/or formaldehyde polymerization processes. It is revealed that the hydrothermal treatment produced crumpled NGN (6.0 at% N) with a high surface area of 383 m(2)·g(-1). In contrast, the formaldehyde polymerization process yielded stacked NGN (11.3 at% N) with very low surface area. The combination of formaldehyde polymerization synthesis with hydrothermal treatment led to NGN (14.7 at% N) with a cross-linked structure and a moderate surface area of 88 m(2)·g(-1). Interestingly, this cross-linked NGN exhibited the best electrochemical performance compared with other NGN, with a remarkable specific capacitance of 201 F·g(-1) at 0.05 A·g(-1) in 1 M H2SO4 electrolyte, and an excellent retention rate of 96.2% of the initial capacitance after 10 000 charge-discharge cycles at a current density of 5 A·g(-1) was achieved.