Abstract
From a systematic study of the concentration driven diffusion of positive and negative ions across porous 2D membranes of graphene and hexagonal boron nitride (h-BN), we prove their cation selectivity. Using the current–voltage characteristics of graphene and h-BN monolayers separating reservoirs of different salt concentrations, we calculate the reversal potential as a measure of selectivity. We tune the Debye screening length by exchanging the salt concentrations and demonstrate that negative surface charge gives rise to cation selectivity. Surprisingly, h-BN and graphene membranes show similar characteristics, strongly suggesting a common origin of selectivity in aqueous solvents. For the first time, we demonstrate that the cation flux can be increased by using ozone to create additional pores in graphene while maintaining excellent selectivity. We discuss opportunities to exploit our scalable method to use 2D membranes for applications including osmotic power conversion.
Highlights
Ion selective membranes are key targets for the advancement of separation-based technologies with the aim to reduce their flow resistance while maintaining high selectivity
While single-crystal monolayer graphene has been shown to be intrinsically impermeable to gases,[7] technologically relevant large-area (>few cm2) 2D chemical vapor deposited (CVD) films typically exhibit a range of defects through which ions can pass in solution, indicating a pathway toward their use as ion selective membranes.[8]
We have previously shown that the resistance is a direct measure of the defect density in graphene,[8] and selectivity can be directly extracted from the investigating the current−voltage (I−V) curves.[21]
Summary
Ion selective membranes are key targets for the advancement of separation-based technologies with the aim to reduce their flow resistance while maintaining high selectivity. A number of methods have been demonstrated to control perforation of graphene membranes including ion bombardment, ozone treatment, and oxidative etching.[9−11] Atomic and molecular transport through the pores has been characterized by ionic current as well as optical and conductivity measurements.[8,12,13] Despite these positive results, it remains unclear how selectivity arises, exemplified by the lack of established methods for controlling selective permeance.[14] A fundamental understanding of selectivity is required for engineering new 2D materials into functional membranes.[15]. We set out to understand transport and selectivity through CVD-grown 2D membranes in aqueous solutions, focusing first on the contribution of intrinsic defects to selectivity These are unavoidable in industrially relevant large-area membranes, so their effect must be well characterized. We propose a mechanism for how charge selectivity arises in these pores and demonstrate a pathway to maximize ion flux while maintaining excellent selectivity
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