Wettability of vertically-oriented graphenes with different intersheet distances

Abstract

Vertically-oriented graphenes (VGs), i.e., graphene nanosheets arranged perpendicularly to the substrate surface, possess great promise as electrode materials for supercapacitors, mainly due to the vertical orientation on the substrate, open intersheet space, and exposed sharp edges. In this work, the dependence of VGs' wettability on their surface morphologies is investigated with experiments and numerical simulations. VGs with different intersheet distances are fabricated with plasma-enabled methods employing different plasma sources. The results show that the contact angle of VGs changes from 111° to 34.5° with a decreasing intersheet distance from ∼306.2 to ∼14.5 nm, indicating that the graphene intersheet distance plays an critical role on the wettability of VGs. Lattice Boltzmann (LB) simulation is further conducted to explore the mechanism on the flow and transport of electrolytes within VG channels. The behavior of electrolyte flow and its permeation into VG interiors is found to be strongly dominated by the capillary force across the air–liquid interface, while the gravity of the bulk electrolyte and the viscous force produced by the graphene surface could be ignored. A 3-fold improvement in the accessible surface area of VGs is achieved with reducing intersheet distance. Electrochemical results indicate that the hydrophilic VG electrode with a small intersheet distance of 14.5 nm exhibits a high specific capacitance (up to 147 F g⁻¹) at a high cyclic voltammetry scan rate of 500 mV s⁻¹, due to the effective wetting and utilization of VG surfaces. The results of the current work could provide instructive information in the morphology optimization of VGs for high performance energy storage applications.