Revealing ion transport in supercapacitors with Sub-2 nm two-dimensional graphene channels

Unique ion transport behaviors in two-dimensional (2D) nanochannels have sparked strong interests in exploring 2D nanomaterials for supercapacitor applications, which rely much on the formation of electric double layers (EDLs) at the solid-liquid interface. However, there is still a crucial missing part on understanding how ions with different kinetic properties could affect the formation of EDLs. Here, we examine the real-time ion transport in the formation of EDLs by electrochemical quartz crystal microbalance (EQCM) and observe distinct charge transport behaviors between activated carbon with a tortuous pore structure and graphene films with 2D channel spacing (<2 nm). Using molecular dynamics (MD) simulations, we find that ions with a higher diffusion coefficient predominantly affect the EDLs formation, leading to the anion- or cation-dominated ion exchange process in the sub-2 nm 2D graphene channels. By expanding to different electrolytes, the kinetics-controlled ion transport mechanism is further confirmed by experimental observations and MD simulations. Such findings will bring new insights for improved electrochemical performance of 2D nanomaterials.