2024 AIChE Annual Meeting

(735ag) Zero-Dimensional Pores in Graphene for Ion-Ion Separation

Strong confinement of solute inside zero-dimensional pores of graphene can differentiate ions based on their ability to partition from the bulk to the pore. Graphene film is an ideal selective layer for this if high-precision pores can be incorporated. However, precise pore size control in Å-regime has proven challenging for ion-ion differentiation due to lack of methods to finely tune pore size in this regime. Further, it is extremely challenging to avoid larger non-selective pores at the tail-end of the pore size distribution which reduces ion-ion selectivity.

Herein, we report a novel Å-scale pore size tuning tool, which incorporates a high density of ion-ion selective pores between 3.5 and 8.5 Å while minimizes the nonselective pores above 10 Å. These pores impose a strong confinement for ions, which results in extremely high selectivity from centimeter-scale porous graphene between monovalent and bivalent ions and near complete blockage of ions with hydration diameter, DH, greater than 9.0 Å. Ion diffusion study reveals the presence of an energy barrier corresponding to partial dehydration of ions with barrier increasing with DH. We observe a reversal of K+/Li+ selectivity at elevated temperature and attribute this to the relative size of dehydrated ions.

Further, we discuss a novel strategy to overcome this challenge using an electrochemical repair strategy that successfully masks larger pores in large-area graphene. 10-nm-thick electropolymerized conjugated microporous polymer (CMP) layer is successfully deposited on graphene, thanks to a strong π-π interaction in these two materials. While the CMP layer itself is not selective, it effectively masks graphene pores, leading to an unprecedented Li+/Mg2+ selectivity from zero-dimensional pores with a high Li+ ion permeation rate surpassing the performance of reported materials for ion-ion separation.

These results underscore the promise of porous two-dimensional materials for solute-solute separation when Å-scale pores can be incorporated in a precise manner. The scalable repair strategy enables the fabrication of monolayer graphene membranes with customizable pore sizes, limiting the contribution of nonselective pores, and offering graphene membranes a versatile platform for a broad spectrum of challenging separations.