Selective subatomic separations Membranes are thin materials used to selectively separate gases or liquids and are used on a range of scales from benchtop experiments to industrial processes. Challenges arise in separating materials with very similar sizes or chemical properties, particularly at the smallest scales. Kidambi et al . review advances in using atomically thin two-dimensional materials such as graphene or hexagonal boron nitride for the separation of subatomic species, including electrons, hydrogen isotopes, and gases. The authors explore the scope to scale up the sizes of these membranes and their potential use in applications relating to energy, microscopy, and electronics. —MSL

Inspired by recent reports on possible proton conductance through graphene, we have investigated the behavior of pristine graphene and defect engineered graphene membranes for ionic conductance and selectivity with the goal of evaluating a possibility of its application as a proton selective membrane. The averaged conductance for pristine chemical vapor deposited (CVD) graphene at pH1 is ∼4 mS/cm 2 but varies strongly due to contributions from the unavoidable defects in our CVD graphene. From the variations in the conductance with electrolyte strength and pH, we can conclude that pristine graphene is fairly selective and the conductance is mainly due to protons. Engineering of the defects with ion beam (He + , Ga + ) irradiation and plasma (N 2 and H 2 ) treatment showed improved areal conductance with high proton selectivity mostly for He-ion beam and H 2 plasma treatments, which agrees with primarily vacancy-free type of defects produced in these cases confirmed by Raman analysis.

Angstrom-scale pores introduced into atomically thin 2D materials offer transformative advances for proton exchange membranes in several energy applications. Here, we show that facile kinetic control of scalable chemical vapor deposition (CVD) can allow for direct formation of angstrom-scale proton-selective pores in monolayer graphene with significant hindrance to even small, hydrated ions (K+ diameter ∼6.6 Å) and gas molecules (H2 kinetic diameter ∼2.9 Å). We demonstrate centimeter-scale Nafion|Graphene|Nafion membranes with proton conductance ∼3.3–3.8 S cm–2 (graphene ∼12.7–24.6 S cm–2) and H+/K+ selectivity ∼6.2–44.2 with liquid electrolytes. The same membranes show proton conductance ∼4.6–4.8 S cm–2 (graphene ∼39.9–57.5 S cm–2) and extremely low H2 crossover ∼1.7 × 10–1 – 2.2 × 10–1 mA cm–2 (∼0.4 V, ∼25 °C) with H2 gas feed. We rationalize our findings via a resistance-based transport model and introduce a stacking approach that leverages combinatorial effects of interdefect distance and interlayer transport to allow for Nafion|Graphene|Graphene|Nafion membranes with H+/K+ selectivity ∼86.1 (at 1 M) and record low H2 crossover current density ∼2.5 × 10–2 mA cm–2, up to ∼90% lower than state-of-the-art ionomer Nafion membranes ∼2.7 × 10–1 mA cm–2 under identical conditions, while still maintaining proton conductance ∼4.2 S cm–2 (graphene stack ∼20.8 S cm–2) comparable to that for Nafion of ∼5.2 S cm–2. Our experimental insights enable functional atomically thin high flux proton exchange membranes with minimal crossover.

Selective proton (H+) permeation through the atomically thin lattice of graphene and other 2D materials offers new opportunities for energy conversion/storage and novel separations. Practical applications necessitate scalable synthesis via...