Graphene with nanoperforation for high-capacity potassium-ion storage: Decoupling structural defect and doping effects of N-doped graphene

Heteroatom-doping has emerged as an effective strategy for enhancing the charge storage of graphene. N-doping treatment of graphene involves doping of N heteroatoms as well as structural defect formation, and it remains unclear which one is more responsible for the enhanced K-ion storage. Therefore, to design high-capacity graphene, their effects must be decoupled to understand the individual contributions to the K-ion storage properties. To decouple these effects, four graphene samples are prepared: nanoperforated graphene (NPG) with pre-formed structural defects, and N-doped NPG (N-NPG) with a high fraction of pyridinic N preferentially doped at the nanoperforation edges, along with two control samples of reduced graphene oxide (RGO) and N-doped RGO (N-RGO). Specific capacity increased in the order of RGO < N-RGO < N-NPG < NPG. Differential capacity analyses prove to be a very effective tool in the decoupling. Notably, the four materials show distinct differential capacity plots, which can be divided into two potential regions. Differential capacity at each region varied systematically in magnitude upon N-doping and structural defects, which enabled decoupling the effects of structural defects and N heteroatoms. According to the decoupling, the capacity enhancement of N-RGO is mainly due to the N heteroatoms rather than structural defects. As for the N-NPG, however, the capacity decrease is due to the elimination of carbon radicals and oxygen functional groups at the edges by N-doping treatment, thereby, reducing the reactivity of edges of the nanoperforations. Density functional theory calculations support the electrochemical performance of NPG from the nanoperforations.