Differences in water and vapor transport through angstrom-scale pores in atomically thin membranes

Scalable Bottom-Up Synthesis of Nanoporous Hexagonal Boron Nitride (h-BN) for Large-Area Atomically Thin Ceramic Membranes

Nanopores embedded within monolayer hexagonal boron nitride (h-BN) offer possibilities of creating atomically thin ceramic membranes with unique combinations of high permeance (atomic thinness), high selectivity (via molecular sieving), increased thermal stability, and superior chemical resistance. However, fabricating size-selective nanopores in monolayer h-BN via scalable top-down processes remains nontrivial due to its chemical inertness, and characterizing nanopore size distribution over a large area remains extremely challenging. Here, we demonstrate a facile and scalable approach of exploiting the chemical vapor deposition (CVD) process temperature to enable direct incorporation of subnanometer/nanoscale pores into the monolayer h-BN lattice, in combination with manufacturing compatible polymer casting to fabricate centimeter-scale nanoporous atomically thin ceramic membranes. We leverage diffusive transport of analytes including size-selective Ficoll sieving to characterize subnanometer-scale and nanoscale defects that manifest as pores in centimeter-scale h-BN membranes, overcoming previous limitations in large-area characterization of nanoscale defects in h-BN. Our approach opens a new frontier to advance atomically thin membranes to 2D ceramic materials, such as h-BN via facile and direct formation of nanopores, for size-selective separations.

Overcoming the Conductance versus Crossover Trade-off in State-of-the-Art Proton Exchange Fuel-Cell Membranes by Incorporating Atomically Thin Chemical Vapor Deposition Graphene

Permeance-selectivity trade-offs are inherent to polymeric membranes. In fuel cells, thinner proton exchange membranes (PEMs) could enable higher proton conductance and increased power density with lower area-specific resistance (ASR), smaller ohmic losses, and lower ionomer cost. However, reducing thickness is accompanied by an increase in undesired species crossover harming performance and long-term efficiency. Here, we show that incorporating atomically thin monolayer graphene synthesized via scalable chemical vapor deposition (CVD) and tunable defect density into PEMs (Nafion, ∼5-25 μm thick) can allow for reduced H2 crossover (∼34-78% of Nafion of a similar thickness) while maintaining adequate areal proton conductance for applications (>4 S cm-2). In contrast to most prior work using >50 μm symmetric Nafion sandwich structures, we elucidate the interplay of graphene defect density and Nafion proton transport resistance on the performance of Nafion|graphene composite membranes and find high-quality low-defect density CVD graphene (G) supported on Nafion 211 (∼25 μm); i.e., N211|G has a high areal proton conductance (∼6.1 S cm-2) and the lowest H2 crossover (∼0.7 mA cm-2). Fully functional centimeter-scale N211|G fuel-cell membranes demonstrate performance comparable to that of state-of-the-art Nafion N211 at room temperature as well as standard operating conditions (∼80 °C, ∼150-250 kPa-abs) with H2/air (power density ∼0.57-0.63 W cm-2) and H2/O2 feed (power density ∼1.4-1.62 W cm-2) and markedly reduced H2 crossover (∼53-57%).

Protein-Enabled Size-Selective Defect-Sealing of Atomically Thin 2D Membranes for Dialysis and Nanoscale Separations

Atomically thin 2D materials present the potential for advancing membrane separations via a combination of high selectivity (from molecular sieving) and high permeance (due to atomic thinness). However, the creation of a high density of precise nanopores (narrow-size-distribution) over large areas in 2D materials remains challenging, and nonselective leakage from nanopore heterogeneity adversely impacts performance. Here, we demonstrate protein-enabled size-selective defect sealing (PDS) for atomically thin graphene membranes over centimeter scale areas by leveraging the size and reactivity of permeating proteins to preferentially seal larger nanopores (≥4 nm) while preserving a significant amount of smaller nanopores (via steric hindrance). Our defect-sealed nanoporous atomically thin membranes (NATMs) show stability up to ∼35 days during size-selective diffusive separations with a model dialysis biomolecule fluorescein isothiocyanate (FITC)-Ficoll 70 in phosphate buffer saline (PBS) solution as well as outperform state-of-the-art commercially available dialysis membranes (molecular-weight-cutoff ∼3.5-5 kDa and ∼8-10 kDa) with significantly higher permeance for smaller solutes KCl (∼0.66 nm) ∼5.1-6 × 10-5 ms-1 and vitamin B12 (B12, ∼1.5 nm) ∼2.8-4 × 10-6 ms-1 compared to small protein lysozyme (Lz, ∼4 nm) ∼4-6.4 × 10-8 m s-1, thereby allowing unprecedented selectivity for B12/Lz ∼70 and KCl/Lz ∼1280. Our work introduces proteins as nanoscale tools for size-selective defect sealing in atomically thin membranes to overcome persistent issues and advance separations for dialysis, protein desalting, small molecule separations/purification, and other bioprocesses.

2D Carbonaceous Materials for Molecular Transport and Functional Interfaces: Simulations and Insights

ConspectusCarbon-based two-dimensional (2D) functional materials exhibit potential across a wide spectrum of applications from chemical separations to catalysis and energy storage and conversion. In this Account, we focus on recent advances in the manipulation of 2D carbonaceous materials and their composites through computational design and simulations to address how the precise control over material structure at the atomic level correlates with enhanced functional properties such as gas permeation, selectivity, membrane transport, and charge storage. We highlight several key concepts in the computational design and tuning of 2D structures, such as controlled stacking, ion gating, interlayer pillaring, and heterostructure charge transfer.The process of creating and adjusting pores within graphene sheets is vital for effective molecular separation. Simulations show the power of controlling the offset distance between layers of porous graphene in precisely regulating the pore size to enhance gas separation and entropic selectivity. This strategy of controlled stacking extends beyond graphene to include covalent organic frameworks (COFs) such as covalent triazine frameworks (CTFs). Experimental assembly of the layers has been achieved through electrostatic interactions, thermal transformation, and control of side chain interactions.Graphene can interface with ionic liquids in various forms to enhance its functionality. A computational proof-of-concept showcases an ion-gating concept in which the interaction of anions with the pores in graphene allows the anions to dynamically gate the pores for selective gas transport. Realization of the concept has been achieved in both porous graphene and carbon molecular sieve membranes. Ionic liquids can also intercalate between graphene layers to form interlayer pillaring structures, opening the slit space. Grand canonical Monte Carlo simulations show that these structures can be used for efficient gas capture and separation. Experiments have demonstrated that the interlayer space can be tuned by the density of the pillars and that, when fully filled with ionic liquids and forming a confined interface structure, the graphene oxide membrane achieves much higher selectivity for gas separations. Moreover, graphene can interface with other 2D materials to form heterostructures where interfacial charge transfers take place and impact the function. Both ion transport and charge storage are influenced by both the local electric field and chemical interactions.Fullerene can be used as a building block and covalently linked together to construct a new type of 2D carbon material beyond a one-atom-thin layer that also has long-range-ordered subnanometer pores. The interstitial sites among fullerenes form funnel-shaped pores of 2.0-3.3 Å depending on the crystalline phase. The quasi-tetragonal phases are shown by molecular dynamics simulations to be efficient for H2 separation. In addition, defects such as fullerene vacancies can be introduced to create larger pores for the separation of organic solvents.In conclusion, the key to imputing functions to 2D carbonaceous materials is to create new interactions and interfaces and to go beyond a single-atom layer. First-principles and molecular simulations can further guide the discovery of new 2D carbonaceous materials and interfaces and provide atomistic insights into their functions.

Polyoxometalate Clusters Confined in Reduced Graphene Oxide Membranes for Effective Ion Sieving and Desalination

Efficient 2D membranes play a critical role in water purification and desalination. However, most 2D membranes, such as graphene oxide (GO) membranes, tend to swell or disintegrate in liquid, making precise ionic sieving a tough challenge. Herein, the fabrication of the polyoxometalate clusters (PW12) intercalated reduced graphene oxide (rGO) membrane (rGO-PW12) is reported through a polyoxometalate-assisted in situ photoreduction strategy. The intercalated PW12 result in the interlayer spacing in the sub-nanometer scale and induce a nanoconfinement effect to repel the ions in various salt solutions. The permeation rate of rGO-PW12 membranes are about two orders of magnitude lower than those through the GO membrane. The confinement of nanochannels also generate the excellent non-swelling stability of rGO-PW12 membranes in aqueous solutions up to 400 h. Moreover, when applied in forward osmosis, the rGO-PW12 membranes with a thickness of 90 nm not only exhibit a high-water permeance of up to 0.11790 L m-2 h-1 bar-1 and high NaCl rejection (98.3%), but also reveal an ultrahigh water/salt selectivity of 4740. Such significantly improved ion-exclusion ability and high-water flux benefit from the multi-interactions and nanoconfinement effect between PW12 and rGO nanosheets, which afford a well-interlinked lamellar structure via hydrogen bonding and van der Waals interactions.

Dehydrated Biomimetic Membranes with Skinlike Structure and Function

Novel vapor-permeable materials are sought after for applications in protective wear, energy generation, and water treatment. Current impermeable protective materials effectively block harmful agents but trap heat due to poor water vapor transfer. Here we present a new class of materials, vapor permeable dehydrated nanoporous biomimetic membranes (DBMs), based on channel proteins. This application for biomimetic membranes is unexpected as channel proteins and biomimetic membranes were assumed to be unstable under dry conditions. DBMs mimic human skin's structure to offer both high vapor transport and small molecule exclusion under dry conditions. DBMs feature highly organized pores resembling sweat pores in human skin, but at super high densities (>1012 pores/cm2). These DBMs achieved exceptional water vapor transport rates, surpassing commercial breathable fabrics by up to 6.2 times, despite containing >2 orders of magnitude smaller pores (1 nm vs >700 nm). These DBMs effectively excluded model biological agents and harmful chemicals both in liquid and vapor phases, again in contrast with the commercial breathable fabrics. Remarkably, while hydrated biomimetic membranes were highly permeable to liquid water, they exhibited higher water resistances after dehydration at values >38 times that of commercial breathable fabrics. Molecular dynamics simulations support our hypothesis that dehydration induced protein hydrophobicity increases which enhanced DBM performance. DBMs hold promise for various applications, including membrane distillation, dehumidification, and protective barriers for atmospheric water harvesting materials.

Selective Photonic Gasification of Strained Oxygen Clusters on Graphene for Tuning Pore Size in the Å Regime

Controlling the size of single-digit pores, such as those in graphene, with an Å resolution has been challenging due to the limited understanding of pore evolution at the atomic scale. The controlled oxidation of graphene has led to Å-scale pores; however, obtaining a fine control over pore evolution from the pore precursor (i.e., the oxygen cluster) is very attractive. Herein, we introduce a novel "control knob" for gasifying clusters to form pores. We show that the cluster evolves into a core/shell structure composed of an epoxy group surrounding an ether core in a bid to reduce the lattice strain at the cluster core. We then selectively gasified the strained core by exposing it to 3.2 eV of light at room temperature. This allowed for pore formation with improved control compared to thermal gasification. This is because, for the latter, cluster-cluster coalescence via thermally promoted epoxy diffusion cannot be ruled out. Using the oxidation temperature as a control knob, we were able to systematically increase the pore density while maintaining a narrow size distribution. This allowed us to increase H2 permeance as well as H2 selectivity. We further show that these pores could differentiate CH4 from N2, which is considered to be a challenging separation. Dedicated molecular dynamics simulations and potential of mean force calculations revealed that the free energy barrier for CH4 translocation through the pores was lower than that for N2. Overall, this study will inspire research on the controlled manipulation of clusters for improved precision in incorporating Å-scale pores in graphene.

Anomalous water molecular gating from atomic-scale graphene capillaries for precise and ultrafast molecular sieving

The pressing crisis of clean water shortage requires membranes to possess effective ion sieving as well as fast water flux. However, effective ion sieving demands reduction of pore size, which inevitably hinders water flux in hydrophilic membranes, posing a major challenge for efficient water/ion separation. Herein, we introduce anomalous water molecular gating based on nanofiltration membranes full of graphene capillaries at 6 Å, which were fabricated from spontaneous π-π restacking of island-on-nanosheet graphitic microstructures. We found that the membrane can provide effective ion sieving by suppressing osmosis-driven ion diffusion to negligible levels (~10-4 mol m-2 h-1); unexpectedly, ultrafast bulk flow of water (45.4 L m-2 h-1) was still functional with ease, as gated on/off by adjusting hydrostatic pressures within only 10-2 bar. We attribute this seemingly incompatible observation to graphene nanoconfinement effect, where crystal-like water confined within the capillaries hinders diffusion under osmosis but facilitates high-speed, diffusion-free water transport in the way analogous to Newton's cradle-like Grotthus conduction. This strategy establishes a type of liquid-solid-liquid, phase-changing molecular transport for precise and ultrafast molecular sieving.