▎ 摘　　要

Grain boundaries (GBs) are common features in materials and can tune their physical and/or chemical properties. GBs in graphene could degrade electrical or thermal properties and, thus, are widely supposed to be avoided in related applications. However, the potential to utilize GBs in graphene remains to be explored. Here we reveal that, contrary to traditional negative effects, GBs can efficiently tune ion transport properties, but this remains poorly understood. By controlling the GB density in graphene, we achieve precise tuning of proton permeance with a range of 2 orders of magnitude. We study the reactivity of GBs and attain 3-6 times higher permeance of ions for GB-rich samples than for GB-poor samples. We obtain a proton conductivity 3-4 orders of magnitude higher than commercial Nafion membranes with higher selectivity. Our work discloses the crucial roles of GBs to tailor ion transport and shows the potential of designing GBs in 2D material for membrane applications.