Ion selectivity of graphene nanopores

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Unipolar Ionic Diode Nanofluidic Membranes Enabled by Stepped Mesochannels for Enhanced Salinity Gradient Energy Harvesting

Developing ionic diode membranes featuring asymmetric structures is in high demand for salinity gradient energy harvesting. These membranes offer benefits in mitigating ion concentration polarization, thereby promoting ion permeability. However, most reported works focus on the role of heterogeneous charge-based bipolar ionic diode membranes for ion concentration polarization suppression, with comparatively less attention given to maintaining ion selectivity. Herein, unipolar ionic diode nanofluidic mesoporous silica membranes featuring stepped mesochannels were developed via a micellar sequential oriented interfacial self-assembly strategy as a salinity gradient energy harvester. Due to the asymmetric mesochannels and unipolar structure (both sides carry negative charge), the ionic diode membranes exhibit a strong rectification ratio of ∼15.91 to facilitate unidirectional ion transport while maintaining excellent cation selectivity (cation transfer number of ∼0.85). Besides, the vertically aligned mesochannels significantly reduce ion transport resistance, generating a high ionic flux. Consequently, the unipolar ionic diode nanofluidic membranes demonstrate a power output of 5.88 W/m2 between artificial sea and river water. The unipolar feature gives notable enhancements of 296% and 144% in power output compared to the symmetric membrane and bipolar ionic diode membrane, respectively. This work opens up new routes for designing ionic diode membranes for salinity gradient energy harvesting.

Effect of Membrane Thickness on Ion Transport in pH-Regulated Zero-Depth Interfacial Nanopores

Zero-depth interfacial nanopores, which are formed by two crossed nanoscale channels at their intersection interface, have been proposed to increase the spatial resolution of solid-state nanopores. However, research on zero-depth interfacial nanopores is still in its early stages. Although it has been shown that the current passing through an interfacial nanopore is largely independent of the membrane thickness, existing studies have not fully considered the impact of membrane thickness on other ion transport characteristics within these nanopores. In this paper, we investigate the electrokinetic ion transport phenomenon in the zero-depth interfacial nanopores, especially focusing on the influence of membrane thickness on the ion transport phenomenon. Our model incorporates the Poisson-Nernst-Planck equations and the Navier-Stokes equations, featuring a pH-regulated surface charge density. We find that when the thickness of the nanochannels is close to the interface size of the formed interfacial nanopore, the phenomenon of ion transport in the interfacial nanopore is similar to that in a conventional cylindrical nanopore. However, when the thickness of the nanochannels is much greater than the interface size of the formed interfacial nanopore, several distinct phenomena occur. The surface charge density on the inner walls of the interfacial nanopores has a small peak at the interface of the two crossing nanochannels, and the anion concentration changes greatly between the two nanochannels; that is, a much greater anion concentration forms in the nanochannel near the anode side than in the nanochannel near the cathode side. When the surface charge is nonzero, the electric field within the interfacial nanopore creates three extreme points, and the directions of the local electric fields are opposite at the ends of the membrane.

Recent progress in on-surface synthesis of nanoporous graphene materials

Nanoporous graphene (NPG) materials are generated by removing internal degree-3 vertices from graphene and introducing nanopores with specific topological structures, which have been widely explored and exploited for applications in electronic devices, membranes, and energy storage. The inherent properties of NPGs, such as the band structures, field effect mobilities and topological properties, are crucially determined by the geometric structure of nanopores. On-surface synthesis is an emerging strategy to fabricate low-dimensional carbon nanostructures with atomic precision. In this review, we introduce the progress of on-surface synthesis of atomically precise NPGs, and classify NPGs from the aspects of element types, topological structures, pore shapes, and synthesis strategies. We aim to provide a comprehensive overview of the recent advancements, promoting interdisciplinary collaboration to further advance the synthesis and applications of NPGs.

Nanoarchitectonics in Advanced Membranes for Enhanced Osmotic Energy Harvesting

Osmotic energy, often referred to as "blue energy", is the energy generated from the mixing of solutions with different salt concentrations, offering a vast, renewable, and environmentally friendly energy resource. The efficacy of osmotic power production considerably relies on the performance of the transmembrane process, which depends on ionic conductivity and the capability to differentiate between positive and negative ions. Recent advancements have led to the development of membrane materials featuring precisely tailored ion transport nanochannels, enabling high-efficiency osmotic energy harvesting. In this review, ion diffusion in confined nanochannels and the rational design and optimization of membrane architecture are explored. Furthermore, structural optimization of the membrane to mitigate transport resistance and the concentration polarization effect for enhancing osmotic energy harvesting is highlighted. Finally, an outlook on the challenges that lie ahead is provided, and the potential applications of osmotic energy conversion are outlined. This review offers a comprehensive viewpoint on the evolving prospects of osmotic energy conversion.

Gate-All-Around Nanopore Osmotic Power Generators

Nanofluidic channels in a membrane represent a promising avenue for harnessing blue energy from salinity gradients, relying on permselectivity as a pivotal characteristic crucial for inducing electricity through diffusive ion transport. Surface charge emerges as a central player in the osmotic energy conversion process, emphasizing the critical significance of a judicious selection of membrane materials to achieve optimal ion permeability and selectivity within specific channel dimensions. Alternatively, here we report a field-effect approach for in situ manipulation of the ion selectivity in a nanopore. Application of voltage to a surround-gate electrode allows precise adjustment of the surface charge density at the pore wall. Leveraging the gating control, we demonstrate permselectivity turnover to enhanced cation selective transport in multipore membranes, resulting in a 6-fold increase in the energy conversion efficiency with a power density of 15 W/m2 under a salinity gradient. These findings not only advance our fundamental understanding of ion transport in nanochannels but also provide a scalable and efficient strategy for nanoporous membrane osmotic power generation.

Control of proton transport and hydrogenation in double-gated graphene

The basal plane of graphene can function as a selective barrier that is permeable to protons1,2 but impermeable to all ions3,4 and gases5,6, stimulating its use in applications such as membranes1,2,7,8, catalysis9,10 and isotope separation11,12. Protons can chemically adsorb on graphene and hydrogenate it13,14, inducing a conductor-insulator transition that has been explored intensively in graphene electronic devices13-17. However, both processes face energy barriers1,12,18 and various strategies have been proposed to accelerate proton transport, for example by introducing vacancies4,7,8, incorporating catalytic metals1,19 or chemically functionalizing the lattice18,20. But these techniques can compromise other properties, such as ion selectivity21,22 or mechanical stability23. Here we show that independent control of the electric field, E, at around 1 V nm-1, and charge-carrier density, n, at around 1 × 1014 cm-2, in double-gated graphene allows the decoupling of proton transport from lattice hydrogenation and can thereby accelerate proton transport such that it approaches the limiting electrolyte current for our devices. Proton transport and hydrogenation can be driven selectively with precision and robustness, enabling proton-based logic and memory graphene devices that have on-off ratios spanning orders of magnitude. Our results show that field effects can accelerate and decouple electrochemical processes in double-gated 2D crystals and demonstrate the possibility of mapping such processes as a function of E and n, which is a new technique for the study of 2D electrode-electrolyte interfaces.

Porous Nanographenes, Graphene Nanoribbons, and Nanoporous Graphene Selectively Synthesized from the Same Molecular Precursor

We demonstrate a family of molecular precursors based on 7,10-dibromo-triphenylenes that can selectively produce different varieties of atomically precise porous graphene nanomaterials through the use of different synthetic environments. Upon Yamamoto polymerization of these molecules in solution, the free rotations of the triphenylene units around the C-C bonds result in the formation of cyclotrimers in high yields. In contrast, in on-surface polymerization of the same molecules on Au(111) these rotations are impeded, and the coupling proceeds toward the formation of long polymer chains. These chains can then be converted to porous graphene nanoribbons (pGNRs) by annealing. Correspondingly, the solution-synthesized cyclotrimers can also be deposited onto Au(111) and converted into porous nanographenes (pNGs) via thermal treatment. Thus, both processes start with the same molecular precursor and end with a porous graphene nanomaterial on Au(111), but the type of product, pNG or pGNR, depends on the specific coupling approach. We also produced extended nanoporous graphenes (NPGs) through the lateral fusion of highly aligned pGNRs on Au(111) that were grown at high coverage. The pNGs can also be synthesized directly in solution by Scholl oxidative cyclodehydrogenation of cyclotrimers. We demonstrate the generality of this approach by synthesizing two varieties of 7,10-dibromo-triphenylenes that selectively produced six nanoporous products with different dimensionalities. The basic 7,10-dibromo-triphenylene monomer is amenable to structural modifications, potentially providing access to many new porous graphene nanomaterials. We show that by constructing different porous structures from the same building blocks, it is possible to tune the energy band gap in a wide range.

Ab Initio Molecular Dynamics Investigation on the Permeation of Sodium and Chloride Ions Through Nanopores in Graphene and Hexagonal Boron Nitride Membranes

Nanoporous membranes promise energy-efficient water desalination. Hexagonal boron nitride (h-BN), like graphene, exhibits outstanding physical and chemical properties, making it a promising candidate for water treatment. We employed Car-Parrinello molecular dynamics simulations to establish an accurate modeling of Na+ and Cl- permeation through hydrogen passivated nanopores in graphene and h-BN membranes. We demonstrate that ion separation works well for the h-BN system by imposing a barrier of 0.13 eV and 0.24 eV for Na+ and Cl- permeation, respectively. In contrast, for permeation of the graphene nanopore, the Cl- ion faces a minimum of energy of 0.68 eV in the nanopore plane and is prone toward blockade of the nanopore, while the Na+ ion experiences a slight minimum of 0.03 eV. Overall, the desalination performance of h-BN nanopores surpasses that of their graphene counterparts.

Bottom-Up Synthesis of Twisted Porous Graphene through a Heterogeneous Scholl Reaction and Its Supercapacitor Application

Anthracene- and pyrene-based twisted porous graphene (AN-Pyre-PG) with an ordered pore structure has been synthesized through bottom-up solution phase synthesis from a conjugated microporous polymer (AN-Pyre-CMP) via a heterogeneous Scholl cyclization reaction. The regular-ordered pores embedded within the graphene structures were analyzed through a Raman spectrum, different morphological analyses, and theoretical studies. A significant change in surface area from AN-Pyre-CMP to AN-Pyre-PG was observed, from 143 to 640 m2/g, respectively. Surface area-driven capacitive properties were also observed. Twisted-structure and ordered porous graphene shows better specific capacitance compared to CMP. AN-Pyre-PG shows a specific capacitance of 629 F g-1 at 1 A g-1, with 91% retention of capacitance after 3000 charge-discharge cycles, whereas AN-Pyre-CMP shows a maximum specific capacitance of 200 F g-1 was observed at 2 A g-1.

Addressing Challenges in Ion-Selectivity Characterization in Nanopores

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors. Quantitative characterization of ion selectivities in nanopores often employs the Nernst or Goldman-Hodgkin-Katz (GHK) equation to interpret transmembrane potentials. However, the direction of the measured transmembrane potential drop is not specified in these equations, and selectivity values calculated using absolute values of transmembrane potentials do not directly reveal the ion for which the membrane is selective. Moreover, researchers arbitrarily choose whether to use the Nernst or GHK equation and overlook the significant differences between them, leading to ineffective quantitative comparisons between studies. This work addresses these challenges through (a) specifying the transmembrane potential (sign) and salt concentrations in terms of working and reference electrodes and the solutions in which they reside when using the Nernst and GHK equations, (b) reporting of both Nernst-selectivity and GHK-selectivity along with solution compositions and transmembrane potentials when comparing different nanopores/channels, and (c) performing simulations to define an ideal selectivity for nanochannels. Experimental and modeling studies provide significant insight into these fundamental equations and guidelines for the development of nanopore/channel-based devices.

Nonequilibrium Anion Detection in Solid-Contact Ion-Selective Electrodes

Low-cost and portable nitrate and phosphate sensors are needed to improve farming efficiency and reduce environmental and economic impact arising from the release of these nutrients into waterways. Ion selective electrodes (ISEs) could provide a convenient platform for detecting nitrate and phosphate, but existing ionophore-based nitrate and phosphate selective membrane layers used in ISEs are high cost, and ISEs using these membrane layers suffer from long equilibration time, reference potential drift, and poor selectivity. In this work, we demonstrate that constant current operation overcomes these shortcomings for ionophore-based anion-selective ISEs through a qualitatively different response mechanism arising from differences in ion mobility rather than differences in ion binding thermodynamics. We develop a theoretical treatment of phase boundary potential and ion diffusion that allows for quantitative prediction of electrode response under applied current. We also demonstrate that under pulsed current operation, we can create functional solid-contact ISEs using lower-cost molecularly imprinted polymers (MIPs). MIP-based nitrate sensors provide comparable selectivity against chloride to costlier ionophore-based sensors and exhibit >100,000 times higher selectivity against perchlorate. Likewise, MIP-based solid contact ion-selective electrode phosphate sensors operated under pulsed current provide competitive selectivity against chloride, nitrate, perchlorate, and carbonate anions. The theoretical treatment and conceptual demonstration of pulsed-current ISE operation we report will inform the development of new materials for membrane layers in ISEs based on differences in ion mobility and will allow for improved ISE sensor designs.

Scalability of nanopore osmotic energy conversion

Artificial nanofluidic networks are emerging systems for blue energy conversion that leverages surface charge-derived permselectivity to induce voltage from diffusive ion transport under salinity difference. Here the pivotal significance of electrostatic inter-channel couplings in multi-nanopore membranes, which impose constraints on porosity and subsequently influence the generation of large osmotic power outputs, is illustrated. Constructive interference is observed between two 20 nm nanopores of 30 nm spacing that renders enhanced permselectivity to osmotic power output via the recovered electroneutrality. On contrary, the interference is revealed as destructive in two-dimensional arrays causing significant deteriorations of the ion selectivity even for the nanopores sparsely distributed at an order of magnitude larger spacing than the Dukhin length. Most importantly, a scaling law is provided for deducing the maximal membrane area and porosity to avoid the selectivity loss via the inter-pore electrostatic coupling. As the electric crosstalk is inevitable in any fluidic network, the present findings can be a useful guide to design nanoporous membranes for scalable osmotic power generations.

Controllable Fabrication of Sub-10 nm Graphene Nanopores via Helium Ion Microscopy and DNA Detection

Solid-state nanopores have become a prominent tool in the field of single-molecule detection. Conventional solid-state nanopores are thick, which affects the spatial resolution of the detection results. Graphene is the thinnest 2D material and has the highest spatial detection resolution. In this study, a graphene membrane chip was fabricated by combining a MEMS process with a 2D material wet transfer process. Raman spectroscopy was used to assess the quality of graphene after the transfer. The mechanism behind the influence of the processing dose and residence time of the helium ion beam on the processed pore size was investigated. Subsequently, graphene nanopores with diameters less than 10 nm were fabricated via helium ion microscopy. DNA was detected using a 5.8 nm graphene nanopore chip, and the appearance of double-peak signals on the surface of 20 mer DNA was successfully detected. These results serve as a valuable reference for nanopore fabrication using 2D material for DNA analysis.

Interfacial Assembly of 2D Graphene-Derived Ion Channels for Water-Based Green Energy Conversion

The utilization of sustained and green energy is believed to alleviate increasing menace of global environmental concerns and energy dilemma. Interfacial assembly of 2D graphene-derived ion channels (2D-GDICs) with tunable ion/fluid transport behavior enables efficient harvesting of renewable green energy from ubiquitous water, especially for osmotic energy harvesting. In this review, various interfacial assembly strategies for fabricating diverse 2D-GDICs are summarized and their ion transport properties are discussed. This review analyzes how particular structure and charge density/distribution of 2D-GDIC can be modulated to minimize internal resistance of ion/fluid transport and enhance energy conversion efficiency, and highlights stimuli-responsive functions and stability of 2D-GDIC and further examines the possibility of integrating 2D-GDIC with other energy conversion systems. Notably, the presented preparation and applications of 2D-GDIC also inspire and guide other 2D materials to fabricate sophisticated ion channels for targeted applications. Finally, potential challenges in this field is analyzed and a prospect to future developments toward high-performance or large-scale real-word applications is offered.

Nanoporous Amorphous Carbon Monolayer Derived from Fullerene Film

Carbon materials derived from fullerene are reported recently with unique structures and properties. However, only micrometer size samples can be obtained that limits the studying and exploration for membrane applications. Here, the preparation of a centimeter-size nanoporous amorphous carbon monolayer is reported by rapid pyrolyzing a Langmuir-Blodgett film of fullerene. The sample is fully characterized and the results indicate that the amorphous carbon monolayer derived from fullerene is metastable and insulating. The ionic transmembrane transport study demonstrates that the membrane is porous and cation selective with a selectivity of 26%. This work provides new insights into the controlled synthesis of large-size metastable amorphous carbon monolayer.

Superassembled MXene-carboxymethyl chitosan nanochannels for the highly sensitive recognition and detection of copper ions

Copper ions (Cu2+), as a crucial trace element, play a vital role in living organisms. Thus, the detection of Cu2+ is of great significance for disease prevention and diagnosis. Nanochannel devices with an excellent nanoconfinement effect show great potential in recognizing and detecting Cu2+ ions. However, these devices often require complicated modification and treatment, which not only damages the membrane structure, but also induces nonspecific, low-sensitivity and non-repeatable detection. Herein, a 2D MXene-carboxymethyl chitosan (MXene/CMC) freestanding membrane with ordered lamellar channels was developed by a super-assembly strategy. The introduction of CMC provides abundant space charges, improving the nanoconfinement effect of the nanochannel. Importantly, the CMC can chelate with Cu2+ ions, endowing the MXene/CMC with the ability to detect Cu2+. The formation of CMC-Cu2+ complexes decreases the space charges, leading to a discernible variation in the current signal. Therefore, MXene/CMC can achieve highly sensitive and stable Cu2+ detection based on the characteristics of nanochannel composition. The linear response range for Cu2+ detection is 10-9 to 10-5 M with a low detection limit of 0.095 nM. Notably, MXene/CMC was successfully applied for Cu2+ detection in real water and fetal bovine serum samples. This work provides a simple, highly sensitive and stable detection platform based on the properties of the nanochannel composition.

Tuning Pore Size in Graphene in the Angstrom Regime for Highly Selective Ion-Ion Separation

Zero-dimensional pores spanning only a few angstroms in size in two-dimensional materials such as graphene are some of the most promising systems for designing ion-ion selective membranes. However, the key challenge in the field is that so far a crack-free macroscopic graphene membrane for ion-ion separation has not been realized. Further, methods to tune the pores in the Å-regime to achieve a large ion-ion selectivity from the graphene pore have not been realized. Herein, we report an Å-scale pore size tuning tool for single layer graphene, which incorporates a high density of ion-ion selective pores between 3.5 and 8.5 Å while minimizing 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 the hydration diameter, DH, greater than 9.0 Å. The ion diffusion study reveals the presence of an energy barrier corresponding to partial dehydration of ions with the barrier increasing with DH. We observe a reversal of K+/Li+ selectivity at elevated temperature and attribute this to the relative size of the dehydrated ions. 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.

Bioinspired light-driven chloride pump with helical porphyrin channels

Halorhodopsin, a light-driven chloride pump, utilizes photonic energy to drive chloride ions across biological membranes, regulating the ion balance and conveying biological information. In the light-driven chloride pump process, the chloride-binding chromophore (protonated Schiff base) is crucial, able to form the active center by absorbing light and triggering the transport cycle. Inspired by halorhodopsin, we demonstrate an artificial light-driven chloride pump using a helical porphyrin channel array with excellent photoactivity and specific chloride selectivity. The helical porphyrin channels are formed by a porphyrin-core star block copolymer, and the defects along the channels can be effectively repaired by doping a small number of porphyrins. The well-repaired porphyrin channel exhibits the light-driven Cl- migration against a 3-fold concentration gradient, showing the ion pumping behavior. The bio-inspired artificial light-driven chloride pump provides a prospect for designing bioinspired responsive ion channel systems and high-performance optogenetics.

Ion current rectification properties of non-Newtonian fluids in conical nanochannels

Ionic current rectification generated by the geometric asymmetry of conical nanochannels has gradually attracted attention, but most studies have been limited to Newtonian fluids. In this study, the ionic current rectification characteristics in conical nanochannels filled with non-Newtonian fluids are investigated by numerical simulations. Electroosmotic flow and ion transport in Sisko fluids are solved using the Poisson-Nernst-Planck equations and the Navier-Stokes equations. The effects of the Debye parameter, power-law indexes and applied voltage on the ionic current, axial potential, ion concentration, radial velocity and rectification ratio in the nanopores are investigated. When κRt = 1, the current rectification ratio increases with the increase of the power-law index. However, when κRt = 6, the current rectification ratio first increases and then decreases with the increase of the power law index, reaching the maximum value at n = 1.0. These findings have positive implications for the construction of some nanodevices such as nanofluidic diodes.

Preanchoring Enabled Directional Modification of Atomically Thin Membrane for High-Performance Osmotic Energy Generation

Salinity gradient energy is an environmentally friendly energy source that possesses potential to meet the growing global energy demand. Although covalently modified nanoporous graphene membranes are prospective candidates to break the trade-off between ion selectivity and permeability, the random reaction sites and inevitable defects during modification reduce the reaction efficiency and energy conversion performance. Here, we developed a preanchoring method to achieve directional modification near the graphene nanopores periphery. Numerical simulation revealed that the improved surface charge density around nanopores results in exceptional K+/Cl- selectivity and osmotic energy conversion performance, which agreed well with experimental results. Ionic transport measurements showed that the directionally modified graphene membranes achieved an outstanding power density of 81.6 W m-2 with an energy conversion efficiency of 35.4% under a 100-fold salinity gradient, outperforming state-of-the-art graphene-based nanoporous membranes. This work provided a facile approach for precise modification of nanoporous graphene membranes and opened up new ways for osmotic power harvesting.

Covalently Functionalized Nanopores for Highly Selective Separation of Monovalent Ions

Biological ion channels possess prominent ion transport performances attributed to their critical chemical groups across the continuous nanoscale filters. However, it is still a challenge to imitate these sophisticated performances in artificial nanoscale systems. Herein, this work develops the strategy to fabricate functionalized graphene nanopores in pioneer based on the synergistic regulation of the pore size and chemical properties of atomically thin confined structure through decoupling etching combined with in situ covalent modification. The modified graphene nanopores possess asymmetric ion transport behaviors and efficient monovalent metal ions sieving (K+ /Li+ selectivity ≈48.6). Meanwhile, it also allows preferential transport for cations, the resulting membranes exhibit a K+ /Cl- selectivity of 76 and a H+ /Cl- selectivity of 59.3. The synergistic effects of steric hindrance and electrostatic interactions imposing a higher energy barrier for Cl- or Li+ across nanopores lead to ultra-selective H+ or K+ transport. Further, the functionalized graphene nanopores generate a power density of 25.3 W m-2 and a conversion efficiency of 33.9%, showing potential application prospects in energy conversion. The theoretical studies quantitatively match well with the experimental results. The feasible preparation of functionalized graphene nanopores paves the way toward direct investigation on ion transport mechanism and advanced design in devices.

Ion Trapping and Thermionic Emission across Sub-nm Pores

Ionic transport through a graphene biomimetic subnanometer (sub-nm) pore of arbitrary shape and realistically decorated by intrinsic negatively charged sites is investigated by all-atom molecular dynamics (MD) simulations. In the presence of external electric fields, cation trapping-assisted translocation occurs in the vicinity of the 2D subnanometer pore, while the anion current is blocked by the negative charges. The adsorbed cations in such asymmetrically charged nanopores are located on the top of the nanopore instead of blocking the pore, as suggested previously in highly symmetric pores such as crown ethers. Our analysis of the different types of energy involved in ion translocations indicates that electrostatics is the dominant factor controlling ion transfer across these sub-nm pores. A physical model based on the thermionic emission formalism to account for the free energy barriers to ion flow reproduces the I-V characteristics.

Voltage and concentration gradients across membraneless interface generated next to hydrogels: relation to glycocalyx

Next to many hydrophilic surfaces, including those of biological cells and tissues, a layer of water that effectively excludes solutes and particles can be generated. This interfacial water is the subject of research aiming for practical applications such as removal of salts, pathogens or manipulation of biomolecules. However, the exact mechanism of its creation is still elusive because its persistence and extension contradict hydrogen-bond dynamics and electric double layer predictions. The experimentally recorded negative voltage of this interfacial water remains to be properly explained. Even less is known about the nature of such water layers in biological systems. We present experimental evidence for ion and particle exclusion as a result of separation of ionic charges with distinct diffusion rates across a liquid junction at the gel/water interface and the subsequent repulsion of ions of a given sign by a like-charged gel surface. Gels represent features of biological interfaces (in terms of functional groups and porosity) and are subject to biologically relevant chemical triggers. Our results show that gels with -OSO3- and -COO- groups can effectively generate ion- and particle-depleted regions of water reaching over 100 μm and having negative voltage up to -30 mV. Exclusion distance and electric potential depend on the liquid junction potential at the gel/water interface and on the concentration gradient at the depleted region/bulk interface, respectively. The voltage and extension of these ion- and particle-depleted water layers can be effectively modified by CO2 (respiratory gas) or KH2PO4 (cell metabolite). Possible implications pertain to biologically unstirred water layers and a cell's bioenergetics.

2D materials towards energy conversion processes in nanofluidics

Hierarchically assembled 2D material membranes are extremely promising platforms for energy conversion processes in nanofluidics. In this perspective, we discuss recent advances in the production of smart 2D material membranes that come close to mimicking biological energy conversion processes and how these efforts translate into the design of water purification systems, artificial photosynthesis, and solar energy conversion devices. As we depict here, 2D material membranes synergistically modulate the intrinsic active sites (nanopores), electron transport, mass transfer, and mechanical and chemical stability aiming at cost-effective and highly efficient smart membranes.

Meta-Aerogel Ion Motor for Nanofluid Osmotic Energy Harvesting

Osmotic power, also known as "blue energy", is a vast, sustainable, and clean energy source that can be directly converted into electricity by nanofluidic membranes. However, the key technological bottleneck for large-scale osmotic electricity is that macroscopic-scale bulky membrane cannot synergistically satisfy the demands of high power density and low resistance without sacrificing scalability and mechanical robustness. Here, inspired by the anatomy and working principle of electric eels, which harness osmotic energy through embedded neuron-mediated fibril nanochannels with nanoconfined transport dynamics. Fibrous nanofluidic meta-aerogel ion motors, 3D-assembled from nanofluidic cable fibers with actuatable stimulation/transport "ion highways" are engineered. The meta-aerogel exhibits the integrated coupling effect of boosted ion propulsion and surface-charge-dominated selective ion transport. Driven by osmosis, the meta-aerogel ion motor can produce an unprecedented output power density of up to 30.7 W m-2 under a 50-fold salinity gradient. Advancing ultra-selective ion transport in nanofluidic meta-aerogels may provide a promising roadmap for blue energy harvesting.

Ultra-mechanosensitive Chloride Ion Transport through Bioinspired High-Density Elastomeric Nanochannels

Mechanosensitive ion channels play crucial roles in physiological activities, where small mechanical stimuli induce the membrane tension, trigger the ion channels' deformation, and are further transformed into significant electrochemical signals. Artificial ion channels with stiff moduli have been developed to mimic mechanosensory behaviors, exhibiting an electrochemical response by the high-pressure-induced flow. However, fabricating flexible mechanosensitive channels capable of regulating specific ion transporting upon dramatic deformation has remained a challenge. Here, we demonstrate bioinspired high-density elastomeric channels self-assembled by polyisoprene-b-poly4-vinylpyridine, which exhibit ultra-mechanosensitive chloride ion transport resulting from nanochannel deformation. The PI-formed continuous elastic matrix can transmit external forces into internal tensions, while P4VP forms transmembrane chloride channels that undergo dramatic deformation and respond to mechanical stimuli. The integrated and flexible chloride channels present a dramatic and stable electrochemical signal toward a low pressure of 0.2 mbar. This research first demonstrates the artificial mechanosensory chloride channels, which could provide a promising avenue for designing flexible and responsive channel systems.

Robust Cellulose Nanocrystal-Based Self-Assembled Composite Membranes Doped with Polyvinyl Alcohol and Graphene Oxide for Osmotic Energy Harvesting

Osmotic energy from the salinity gradients represents a promising energy resource with stable and sustainable characteristics. Nanofluidic membranes can be considered as powerful alternatives to the traditional low-performance ion exchange membrane to achieve high-efficiency osmotic energy harvesting. However, the development of a highly efficient and easily scalable core membrane component from low-cost raw materials remains challenging. Here, a composite membrane based on the self-assembly of cellulose nanocrystals (CNCs) with polyvinyl alcohol (PVA) and graphene oxide (GO) nanoflakes as additives is developed to provide a solution. The introduction of soft PVA polymer significantly improves the mechanical strength and water stability of the composite membrane by forming a nacre-like structure. Benefiting from the abundant negative charges of CNC nanorods and GO nanoflakes and the generated network nanochannels, the composite membrane demonstrates a good cation-selective transport capacity, thus contributing to an optimal osmotic energy conversion of 6.5 W m-2 under a 100-fold salinity gradient and an exemplary stability throughout 25 consecutive days of operation. This work provides an option for the development of nanofluidic membranes that can be easily produced on a large scale from well-resourced and sustainable biomass materials for high-efficiency osmotic energy conversion.

Superhigh and Robust Ion Selectivity in Membranes Assembled with Monolayer Clay Nanosheets

It is crucial to control the ion transport in membranes for various technological applications such as energy storage and conversion. The emerging functional two-dimensional (2D) nanosheets such as graphene oxide and MXenes show great potential for constructing ordered nanochannels, but the assembled membranes suffer from low ion selectivity and stability. Here a class of robust charge-selective membranes with superhigh cation/anion selectivity, which are assembled with monolayer nanosheets of cationic/anionic clays that inherently have permanent and uniform charges on each layer is reported. The transport number of cations/anions of cationic vermiculite nanosheet membranes (VNMs)/anionic Co-Al layered double hydroxide (CoAl-LDH) nanosheet membranes is over 0.90 in different NaCl concentration gradients, outperforming all the reported ion-selective membranes. Importantly, this excellent ion selectivity can persist at high-concentration salt solutions, under acidic and alkaline conditions, and for a wide range of ions of different sizes and charges. By coupling a pair of cation-selective vermiculite membrane and anion-selective CoAl-LDH membrane, a reverse electrodialysis device which shows an output power density of 0.7 W m-2 and energy conversion efficiency of 45.5% is constructed. This work provides a new strategy to rationally design high-performance ion-selective membranes by using 2D nanosheets with inherent surface charges for controllable ion-transport applications.

Superassembly of 4-Aminothiophenol-Modified Mesoporous Titania-Alumina Oxide Heterochannels for Smart Ion Transport Based on the Photo-Induced Electron-Transfer Process

Light-responsive nanochannels have attracted extensive attention due to their noninvasive external field control and intelligent ion regulation. However, the limited photoresponsive current and the low photoelectric conversion efficiency still restrict their development. Here, a light-controlled nanochannel composed of 4-aminothiophenol and gold nanoparticles-modified mesoporous titania nanopillar arrays and alumina oxide (4-ATP-Au-MTI/AAO) is fabricated by the interfacial super-assembly strategy. Inspired by the process of electron transfer between photosystem I and photosystem II, the efficient electron transfer between TiO₂, AuNPs, and 4-ATP under light is achieved by coupling the photoresponsive materials and functional molecules. Under illumination, 4-ATP is oxidized to p-nitrothiophenol (PNTP), which brings about changes in the wettability of the nanochannel, resulting in significant improvement (252.8%) of photoresponsive current. In addition, under the action of the reductant, the nanochannels can be restored to the initial dark state, enabling multiple reversible cycles. This work opens a new route for the fabrication of high-performance light-controlled nanochannels by coupling light-responsive materials and light-responsive molecules, which may guide the development of photoelectric conversion nanochannel systems.

Beyond steric selectivity of ions using ångström-scale capillaries

Ion-selective channels play a key role in physiological processes and are used in many technologies. Although biological channels can efficiently separate same-charge ions with similar hydration shells, it remains a challenge to mimic such exquisite selectivity using artificial solid-state channels. Although there are several nanoporous membranes that show high selectivity with respect to certain ions, the underlying mechanisms are based on the hydrated ion size and/or charge. There is a need to rationalize the design of artificial channels to make them capable of selecting between similar-sized same-charge ions, which, in turn, requires an understanding of why and how such selectivity can occur. Here we study ångström-scale artificial channels made by van der Waals assembly, which are comparable in size with typical ions and carry little residual charge on the channel walls. This allows us to exclude the first-order effects of steric- and Coulomb-based exclusion. We show that the studied two-dimensional ångström-scale capillaries can distinguish between same-charge ions with similar hydrated diameters. The selectivity is attributed to different positions occupied by ions within the layered structure of nanoconfined water, which depend on the ion-core size and differ for anions and cations. The revealed mechanism points at the possibilities of ion separation beyond simple steric sieving.

An anomalous anion transfer order in graphene oxide membranes induced by anion-π interactions

Selective transport of anions across membranes has become an important goal in chemistry and biology. Here, we found an anomalous anion transfer order within the graphene oxide membrane: Cl^- > Br^- > F^- > I^-. This is at odds with the conventional ranking of the transfer order, which usually decreases as the radii of the anions increase, i.e., F^- > Cl^- > Br^- > I^-. The abnormal transportation of F^- can be ascribed to the strong anion-π interactions between F^- and graphene oxide sheets. Such unexpectedly strong anion-π interaction resulted in the lower movement of F^- in the graphene oxide membrane and caused the anomalous anion transfer order. Our findings not only provide experimental evidence of anion-π interactions, but also improve our understanding of anion-π interactions in the selective transport of anions across a two-dimensional membrane.

Enhanced osmotic transport in individual double-walled carbon nanotube

The transport of fluid and ions across nanotubes or nanochannels has attracted great attention due to the ultrahigh energy power density and slip length, with applications in water purification, desalination, energy conversion and even ion-based neuromorphic computing. Investigation on individual nanotube or nanochannel is essential in revealing the fundamental mechanism as well as demonstrating the property unambiguously. Surprisingly, while carbon nanotube is the pioneering and one of the most attractive systems for nanofluidics, study on its response and performance under osmotic forcing is lacking. Here, we measure the osmotic energy conversion for individual double-walled carbon nanotube with an inner radius of 2.3 nm. By fabricating a nanofluidic device using photolithography, we find a giant power density (up to 22.5 kW/m²) for the transport of KCl, NaCl, and LiCl solutions across the tube. Further experiments show that such an extraordinary performance originates from the ultrahigh slip lengths (up to a few micrometers). Our results suggest that carbon nanotube is a good candidate for not only ultrafast transport, but also osmotic power harvesting under salinity gradients.

Light-Driven Ion Transport through Single-Heterojunction Nanopores

Inspired by natural photosynthesis, light has become an emerging ionic behavior regulator and ion-pumping source. Nanoprocessing technology has allowed the bridge between the light-regulated nanofluids and the optoelectronic properties of two-dimensional (2D) materials, which inspires applications like energy harvesting and enhances fundamental understandings in nanofluidics. However, unlike light-induced ion pumping based on densely layered membranes with multiple nanochannels, experimental implementation on atomically thin materials featuring only a single nanochannel remains challenging. Here, we report light-induced ion pumping based on a single artificial heterojunction nanopore. Under light illumination, the induced current through a single nanopore reaches tens of picoamperes. The hole-electron separation originating from the optoelectrical property of a van der Waals PN junction is proposed to capture the light-driven ion transport. Further, different methods are adopted to modify the ion behavior and response time, presenting potential applications in fluidic photoenergy harvesting, photoelectric ion transport control, and bionic artificial neurons.

Two-Dimensional Materials Applied in Membranes of Redox Flow Battery

Redox flow batteries (RFBs) are one of the most promising techniques to store and convert green and renewable energy, benefiting from their advantages of high safety, flexible design and long lifespan. Membranes with fast and selective ions transport are required for the advances of RFBs. Remarkably, two-dimensional (2D) materials with high mechanical and chemical stability, strict size exclusion and abundantly modifiable functional groups, have attracted extensive attentions in the applications of energy fields. Herein, the improvements and perspectives of 2D materials working for ionic transportation and sieving in RFBs membranes are presented. The characteristics of various materials and their advantages and disadvantages in the applications of RFBs membranes particularly are focused. This review is expected to provide a guidance for the design of membranes based on 2D materials for RFBs.

Cell-inspired selective potassium removal towards hyperkalemia therapy by microphase-isolated core-shell microspheres

Hyperkalemia is a common metabolic problem in patients with chronic kidney disease. Although oral medications and hemodialysis are clinically applied for lowering serum potassium, the intrinsic limitations encourage alternative therapy in the trend of adsorbent-based miniaturized blood purification devices. Cells serve as the biological K⁺ storage units that accumulate K⁺ through multiple mechanisms. Inspired by cells, our strategy aims at favorable permeation and enrichment of K⁺ in the microsphere. We incorporate cation-affinitive groups into core-shell structures with submicron-sized phase separation. These nano-spaced side-groups cooperate to form interlinked clusters, where crown ethers with Angstrom-scale ring for size-matched complexation, while ionic sulfonic acid groups for hydrophilicity and charge-buffering. The unique structure with such non-covalent interactions facilitates K⁺ for permeation across the shell and binding to the core while also ensuring mechanical strength and anti-swelling durability in biofluids. The microspheres exhibit high selectivity ratios of K⁺ (S_K/Na, S_K/Ca, S_K/Mg up to 9.8, 21.6, and 17.7). As column adsorbents for hemoperfusion simulation, they effectively lower elevated K⁺ levels to the normal range (clearance rates up to 44.4%/45.3% for hyperkalemic human serum/blood). Blood compatibility tests show low protein adsorption, preferable hemocyte compatibility, and anticoagulation property in vitro. This promising strategy has clinical potential for hyperkalemia in high-risk patients. STATEMENT OF SIGNIFICANCE: Hyperkalemia (serum potassium >5 mmol/L) is a common complication in chronic renal failure patients. The limitations of existing treatments prompt a shift to wearable artificial kidney technology for clinical convenience and efficacy. Existing treatments have limitations, and we turn to adsorbent-based miniaturized blood purification devices in the prospect of wearable artificial kidney technology. There exists a lack of ion-specific adsorbents applied in extracorporeal circuits to redress electrolyte imbalances like hyperkalemia. Inspired by cells, we aim at the favorable permeation and enrichment of K⁺ by microspheres. The microspheres have a microphase-isolated core-shell structure, whose nano-spaced groups form cation-affinitive clusters. Selective K⁺ removal and blood compatibility are achieved. We expect this strategy to enlighten alternative hyperkalemia therapy for these high-risk patients.

Construction of angstrom-scale ion channels with versatile pore configurations and sizes by metal-organic frameworks

Controllable fabrication of angstrom-size channels has been long desired to mimic biological ion channels for the fundamental study of ion transport. Here we report a strategy for fabricating angstrom-scale ion channels with one-dimensional (1D) to three-dimensional (3D) pore structures by the growth of metal-organic frameworks (MOFs) into nanochannels. The 1D MIL-53 channels of flexible pore sizes around 5.2 × 8.9 Å can transport cations rapidly, with one to two orders of magnitude higher conductivities and mobilities than MOF channels of hybrid pore configurations and sizes, including Al-TCPP with 1D ~8 Å channels connected by 2D ~6 Å interlayers, and 3D UiO-66 channels of ~6 Å windows and 9 - 12 Å cavities. Furthermore, the 3D MOF channels exhibit better ion sieving properties than those of 1D and 2D MOF channels. Theoretical simulations reveal that ion transport through 2D and 3D MOF channels should undergo multiple dehydration-rehydration processes, resulting in higher energy barriers than pure 1D channels. These findings offer a platform for studying ion transport properties at angstrom-scale confinement and provide guidelines for improving the efficiency of ionic separations and nanofluidics.

Laser-Assisted Scalable Pore Fabrication in Graphene Membranes for Blue-Energy Generation

The osmotic energy from a salinity gradient (i. e. blue energy) is identified as a promising non-intermittent renewable energy source for a sustainable technology. However, this membrane-based technology is facing major limitations for large-scale viability, primarily due to the poor membrane performance. An atomically thin 2D nanoporous material with high surface charge density resolves the bottleneck and leads to a new class of membrane material the salinity gradient energy. Although 2D nanoporous membranes show extremely high performance in terms of energy generation through the single pore, the fabrication and technical challenges such as ion concentration polarization make the nanoporous membrane a non-viable solution. On the other hand, the mesoporous and micro porous structures in the 2D membrane result in improved energy generation with very low fabrication complexity. In the present work, we report femtosecond (fs) laser-assisted scalable fabrication of μm to mm size pores on Graphene membrane for blue energy generation for the first time. A remarkable osmotic power in the order of μW has been achieved using mm size pores, which is about six orders of magnitudes higher compared to nanoporous membranes, which is mainly due to the diffusion-osmosis driven large ionic flux. Our work paves the way towards fs laser-assisted scalable pore creation in the 2D membrane for large-scale osmotic power generation.

Next-generation graphene oxide additives composite membranes for emerging organic micropollutants removal: Separation, adsorption and degradation

In the past two decades, membrane technology has attracted considerable interest as a viable and promising method for water purification. Emerging organic micropollutants (EOMPs) in wastewater have trace, persistent, highly variable quantities and types, develop hazardous intermediates and are diffusible. These primary issues affect EOMPs polluted wastewater on an industrial scale differently than in a lab, challenging membranes-based EOMP removal. Graphene oxide (GO) promises state-of-the-art membrane synthesis technologies and use in EOMPs removal systems due to its superior physicochemical, mechanical, and electrical qualities and high oxygen content. This critical review highlights the recent advancements in the synthesis of next-generation GO membranes with diverse membrane substrates such as ceramic, polyethersulfone (PES), and polyvinylidene fluoride (PVDF). The EOMPs removal efficiencies of GO membranes in filtration, adsorption (incorporated with metal, nanomaterial in biodegradable polymer and biomimetic membranes), and degradation (in catalytic, photo-Fenton, photocatalytic and electrocatalytic membranes) and corresponding removal mechanisms of different EOMPs are also depicted. GO-assisted water treatment strategies were further assessed by various influencing factors, including applied water flow mode and membrane properties (e.g., permeability, hydrophily, mechanical stability, and fouling). GO additive membranes showed better permeability, hydrophilicity, high water flux, and fouling resistance than pristine membranes. Likewise, degradation combined with filtration is two times more effective than alone, while crossflow mode improves the photocatalytic degradation performance of the system. GO integration in polymer membranes enhances their stability, facilitates photocatalytic processes, and gravity-driven GO membranes enable filtration of pollutants at low pressure, making membrane filtration more inexpensive. However, simultaneous removal of multiple contaminants with contrasting characteristics and variable efficiencies in different systems demands further optimization in GO-mediated membranes. This review concludes with identifying future critical research directions to promote research for determining the GO-assisted OMPs removal membrane technology nexus and maximizing this technique for industrial application.

Engineering Polymeric Nanofluidic Membranes for Efficient Ionic Transport: Biomimetic Design, Material Construction, and Advanced Functionalities

Design elements extracted from biological ion channels guide the engineering of artificial nanofluidic membranes for efficient ionic transport and spawn biomimetic devices with great potential in many cutting-edge areas. In this context, polymeric nanofluidic membranes can be especially attractive because of their inherent flexibility and benign processability, which facilitate massive fabrication and facile device integration for large-scale applications. Herein, the state-of-the-art achievements of polymeric nanofluidic membranes are systematically summarized. Theoretical fundamentals underlying both biological and synthetic ion channels are introduced. The advances of engineering polymeric nanofluidic membranes are then detailed from aspects of structural design, material construction, and chemical functionalization, emphasizing their broad chemical and reticular/topological variety as well as considerable property tunability. After that, this Review expands on examples of evolving these polymeric membranes into macroscopic devices and their potentials in addressing compelling issues in energy conversion and storage systems where efficient ion transport is highly desirable. Finally, a brief outlook on possible future developments in this field is provided.

Coupling solute interactions with functionalized graphene membranes: towards facile membrane-level engineering

Optimizing ion transport through nanoporous graphene membranes with intricate engineering at nanoscale levels finds applications ranging from ion segregation to desalination. Such membrane-level engineering often requires futuristic and state-of-the-art micro- and nanofabrication infrastructure making it less accessible to widespread applications. In this study, the effective membrane pore size is modulated using macroscopic membrane functionalization, which, when combined with the solute concentration, can prove to be facile nanoscale engineering towards achieving selectivity. By performing robust molecular dynamics (MD) simulations of aqueous NaCl solution through a nanoporous graphene membrane, we demonstrate that varying membrane wettability influences the structural organization of ions and water molecules both in the vicinity and inside the nanopore, which is manifested in the form of altered permeation characteristics. Moreover, the disparate solvation characteristics of the ionic species in conjunction with the variable van der Waals interactive forces affect the ion-selective nature (Cl^- over Na⁺) of the membrane. The relative hydrophilization, resulting from the effective functionalization of the nanoporous graphene membrane, not only allows greater control over the permeation characteristics of ions and water molecules mediated by an altered depletion ratio but also gives rise to the ion-selective nature of the membrane, thus providing a sound understanding of the transport properties of ion-water solutions through nanoporous materials.

High-Throughput Nanopore Fabrication and Classification Using Xe-Ion Irradiation and Automated Pore-Edge Analysis

Large-area nanopore drilling is a major bottleneck in state-of-the-art nanoporous 2D membrane fabrication protocols. In addition, high-quality structural and statistical descriptions of as-fabricated porous membranes are key to predicting the corresponding membrane-wide permeation properties. In this work, we investigate Xe-ion focused ion beam as a tool for scalable, large-area nanopore fabrication on atomically thin, free-standing molybdenum disulfide. The presented irradiation protocol enables designing ultrathin membranes with tunable porosity and pore dimensions, along with spatial uniformity across large-area substrates. Fabricated nanoporous membranes are then characterized using scanning transmission electron microscopy imaging, and the observed nanopore geometries are analyzed through a pore-edge detection and analysis script. We further demonstrate that the obtained structural and statistical data can be readily passed on to computational and analytical tools to predict the permeation properties at both individual pore and membrane-wide scales. As an example, membranes featuring angstrom-scale pores are investigated in terms of their emerging water and ion flow properties through extensive all-atom molecular dynamics simulations. We believe that the combination of experimental and analytical approaches presented here will yield accurate physics-based property estimates and thus potentially enable a true function-by-design approach to fabrication for applications such as osmotic power generation and desalination/filtration.

Essence of the Enhanced Osmotic Energy Conversion in a Covalent Organic Framework Monolayer

Low membrane conductivity originated from a high membrane thickness has long been the "Achilles heel" of the conventional polymeric membrane, greatly hampering the improvement of the output power density in osmotic power generation. Herein, we demonstrate a molecularly-thin two-dimensional (2D) covalent organic framework (COF) monolayer membrane, featured with ultimate thickness, high pore density, and tight pore size distribution, which performs as a highly efficient osmotic power generator. Despite the large pore size up to 3.8 nm and relatively low surface charge density of 2.2 mC m^-2, the monolayer COF membrane exhibits a high osmotic current density of 16.7 kA m^-2 and an output power density of 102 W m^-2 under 50 times the NaCl salinity gradient (0.5 M/0.01 M). This superior power density could be further improved to 170 W m^-2 in the real seawater/river water gradient system. When the large pore size and low surface charge density are considered, this superior performance is not expected. Computational studies further reveal that the ultimate membrane permeability originated from the high membrane porosity, rather than ion selectivity, plays a dominant role in the production of high current density, especially under high salinity. This work provides an alternative strategy to realize improved output power density in ultrapermeable membranes.

Kinetic Control of Angstrom-Scale Porosity in 2D Lattices for Direct Scalable Synthesis of Atomically Thin Proton Exchange Membranes

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.

Ultrathin and Ultrastrong Kevlar Aramid Nanofiber Membranes for Highly Stable Osmotic Energy Conversion

An ion-selective membrane can directly convert the osmotic energy to electricity through reverse electrodialysis. However, developing an advanced membrane that simultaneously possesses high power density, excellent mechanical strength, and convenient large-scale production for practical osmotic energy conversion, remains challenging. Here, the fabrication of ultrathin and ultrastrong Kevlar aramid nanofiber (KANF) membranes with interconnected three-dimensional (3D) nanofluidic channels via a simple blade coating method is reported. The negatively charged 3D nanochannels show typical surface-charge-governed nanofluidic ion transport and exhibit excellent cation selectivity. When applied to osmotic energy conversion, the power density of the KANF membrane-based generator reaches 4.8 W m^-2 (seawater/river water) and can be further increased to 13.8 W m^-2 at 328 K, which are higher than most of the state-of-the-art membranes. Importantly, a 4-µm-thickness KANF membrane shows ultrahigh tensile strength (565 MPa) and Young's modulus (25 GPa). This generator also exhibits ultralong stability over 120 days, showing great potential in practical energy conversions.

Flexible iontronics based on 2D nanofluidic material

Iontronics focuses on the interactions between electrons and ions, playing essential roles in most processes across physics, chemistry and life science. Osmotic power source as an example of iontronics, could transform ion gradient into electrical energy, however, it generates low power, sensitive to humidity and can’t operate under freezing point. Herein, based on 2D nanofluidic graphene oxide material, we demonstrate an ultrathin (∼10 µm) osmotic power source with voltage of 1.5 V, volumetric specific energy density of 6 mWh cm⁻³ and power density of 28 mW cm⁻³, achieving the highest values so far. Coupled with triboelectric nanogenerator, it could form a self-charged conformable triboiontronic device. Furthermore, the 3D aerogel scales up areal power density up to 1.3 mW cm⁻² purely from ion gradient based on nanoconfined enhancement from graphene oxide that can operate under −40 °C and overcome humidity limitations, enabling to power the future implantable electronics in human-machine interface.

Osmotic power source based on 2D nanofluidic graphene oxide could overcome humidity and temperature limitations due to high areal power density purely from ion gradient. Here, authors couple it with triboelectric nanogenerator, and demonstrate a self-chargeable conformable tribo-iontronic device.