Proton transport through water-filled carbon nanotubes

Chemistry for water treatment under nanoconfinement

The global freshwater crisis, exacerbated by escalating pollution, poses a significant threat to human health. Addressing this challenge required innovative strategies to develop highly efficient and process-adaptable materials for water decontamination. In this regard, nanomaterials with confinement structures have emerged as a promising solution, outperforming traditional nanomaterials in terms of efficiency, selectivity, stability, and process adaptability, thereby serving as an ideal platform for designing novel functional materials for sustainable water treatment. This Review focuses on recent advancements and employment of nanoconfinement effects in various water treatment processes, emphasizing the fundamental chemistry underlying nanoconfinement effects. Also, the existing knowledge gaps related to nanoconfinement effects and future prospects for expanding their applications in diverse water treatment scenarios are discussed.

Protons Accumulate at the Graphene-Water Interface

Water's ability to autoionize into hydroxide and hydronium ions profoundly influences surface properties, rendering interfaces either basic or acidic. While it is well-established that protons show an affinity to the air-water interface, a critical knowledge gap exists in technologically relevant surfaces like the graphene-water interface. Here we use machine learning-based simulations with first-principles accuracy to unravel the behavior of hydroxide and hydronium ions at the graphene-water interface. Our findings reveal that protons accumulate at the graphene-water interface, with the hydronium ion predominantly residing in the first contact layer of water. In contrast, the hydroxide ion exhibits a bimodal distribution, found both near the surface and further away from it. Analysis of the underlying electronic structure reveals local polarization effects, resulting in counterintuitive charge rearrangement. Proton propensity to the graphene-water interface challenges the interpretation of surface experiments and is expected to have far-reaching consequences for ion conductivity, interfacial reactivity, and proton-mediated processes.

Nanoconfinement Effects in Electrocatalysis and Photocatalysis

Recently, the enzyme-inspired nanoconfinement effect has garnered significant attention for enhancing the efficiency of electrocatalysts and photocatalysts. Despite substantial progress in these fields, there remains a notable absence of comprehensive and insightful articles providing a clear understanding of nanoconfined catalysts. This review addresses this gap by delving into nanoconfined catalysts for electrocatalytic and photocatalytic energy conversion. Initially, the effect of nanoconfinement on the thermodynamics and kinetics of reactions is explored. Subsequently, the primary and secondary structures of nanoconfined catalysts are categorized, their properties are outlined, and typical methods for their construction are summarized. Furthermore, an overview of the state-of-the-art applications of nanoconfined catalysts is provided, focusing on reactions of hydrogen and oxygen evolution, oxygen reduction, carbon dioxide reduction, hydrogen peroxide production, and nitrogen reduction. Finally, the current challenges and future prospects in nanoconfined catalysts are discussed. This review aims to provide in-depth insights and guidelines to advance the development of electrocatalytic and photocatalytic energy conversion technology by nanoconfined catalysts.

Why Proton Grotthuss Diffusion Slows down at the Air-Water Interface while Water Diffusion Accelerates

Excess proton diffusion at aqueous interfaces is crucial for applications including electrocatalysis, aerosol chemistry, and biological energy conversion. While interfaces have been proposed as pathways for channeling protons, proton diffusion at interfaces remains far less understood than in the bulk. Here we focus on the air-water interface and use density functional theory-based deep potential molecular dynamics simulations to reveal the contrasting interface's impacts: excess proton diffusion slows down compared to the bulk, while water diffusion accelerates. This contrast stems from reduced hydrogen-bond coordination at the interface, which facilitates water diffusion and transient unstable proton rattling but impedes the stable proton hops central to Grotthuss diffusion. As a result, at the interface, excess protons and water molecules diffuse at comparable rates, in stark departure from bulk behavior. This mechanistic insight delineates distinct limiting regimes for bulk-enhanced interfacial proton diffusion, with important implications for interfacial chemistry.

Dimensionally Extending from 1D MX-Chain to Ladder and Nanotube Systems: Structural and Electronic Properties

Molecular one-dimensional (1D) electron systems have attracted much attention due to their unique electronic state, physical and chemical properties derived from high-aspect-ratio structures. Among 1D materials, mixed-valence halogen-bridged transition-metal chain complexes (MX-chains) based on coordination assemblies are currently of particular interest because their electronic properties, such as mixed-valence state and band gap, can be controlled by substituting components and varying configurations. In particular, chemistry has recently noted that dimensionally extending MX-chains through organic rung ligands can introduce and modulate electronic coupling of metal atoms between chains, i. e., interchain interactions. In this review, for the first time, we highlight the recent progress on MX systems from the viewpoint of dimensionally extending from 1D chain to ladder and nanotube, mainly involving structural design and electronic properties. Overall, dimensional extension can not only tune the electronic properties of MX-chain, but also build the unique platform for studying transport dynamics in confined space, such as proton conduction. Based on these features, we envision that the MX-chain systems provide valuable insights into deep understanding of 1D electron systems, as well as the potential applications such as nanoelectronics.

Classical Models of Hydroxide for Proton Hopping Simulations

Hydronium (H3O+) and hydroxide (OH-) ions perform structural diffusion in water via sequential proton transfers ("Grotthuss hopping"). This phenomenon can be accounted for by interspersing stochastic proton transfer events in classical molecular dynamics simulations. The implementation of OH--mediated proton hopping is particularly challenging because classical force fields are known to produce overcoordinated solvation structures around the OH- ion. Here, we first explore the ability of two-particle point-charge models to reproduce both the solvation free energy and coordination number in TIP3P water. We find that this is possible only with unphysical changes in the nonbonded parameters which create problems in proton hopping simulations. We then construct a classical OH- model with the charge of oxygen distributed among three auxiliary particles. This model favors a lower coordination number by accepting three hydrogen bonds and weakly donating one. The model was implemented in the MOBHY module of the CHARMM program and was fit to reproduce the experimental aqueous diffusion coefficient of OH-. This parameterization gave reasonable electrophoretic mobilities and the expected accelerated transport under nanoconfinement.

Topological Frustration Triggers Ultrafast Dynamics of Monolayer Water Confined in Graphene Slit Pores

Nanoconfined water exhibits astonishing properties that offer new opportunities in physics, biology and technology like energy-storage applications. Here we study such nanoconfined water using ab initio molecular dynamics simulations to elucidate the structure and dynamics of water monolayers in graphene-based slit pores. The significant population of dangling (or free) O-H bonds pointing toward the two confining walls, leads to topological frustration in the hydrogen bond network. This provides a novel channel for ultrafast diffusion distinct from what has been observed in bulk or interfacial water.

Advanced 1D heterostructures based on nanotube templates and molecules

Recent advancements in materials science have shed light on the potential of exploring hierarchical assemblies of molecules on surfaces, driven by both fundamental and applicative challenges. This field encompasses diverse areas including molecular storage, drug delivery, catalysis, and nanoscale chemical reactions. In this context, the utilization of nanotube templates (NTs) has emerged as promising platforms for achieving advanced one-dimensional (1D) molecular assemblies. NTs offer cylindrical, crystalline structures with high aspect ratios, capable of hosting molecules both externally and internally (Mol@NT). Furthermore, NTs possess a wide array of available diameters, providing tunability for tailored assembly. This review underscores recent breakthroughs in the field of Mol@NT. The first part focuses on the diverse panorama of structural properties in Mol@NT synthesized in the last decade. The advances in understanding encapsulation, adsorption, and ordering mechanisms are detailed. In a second part, the review highlights the physical interactions and photophysics properties of Mol@NT obtained by the confinement of molecules and nanotubes in the van der Waals distance regime. The last part of the review describes potential applicative fields of these 1D heterostructures, providing specific examples in photovoltaics, luminescent materials, and bio-imaging. A conclusion gathers current challenges and perspectives of the field to foster discussion in related communities.

Molecular transport enhancement in pure metallic carbon nanotube porins

Nanofluidic channels impose extreme confinement on water and ions, giving rise to unusual transport phenomena strongly dependent on the interactions at the channel-wall interface. Yet how the electronic properties of the nanofluidic channels influence transport efficiency remains largely unexplored. Here we measure transport through the inner pores of sub-1 nm metallic and semiconducting carbon nanotube porins. We find that water and proton transport are enhanced in metallic nanotubes over semiconducting nanotubes, whereas ion transport is largely insensitive to the nanotube bandgap value. Molecular simulations using polarizable force fields highlight the contributions of the anisotropic polarizability tensor of the carbon nanotubes to the ion-nanotube interactions and the water friction coefficient. We also describe the origin of the proton transport enhancement in metallic nanotubes using deep neural network molecular dynamics simulations. These results emphasize the complex role of the electronic properties of nanofluidic channels in modulating transport under extreme nanoscale confinement.

Confinement Effects on Proton Transfer in TiO2 Nanopores from Machine Learning Potential Molecular Dynamics Simulations

Improved understanding of proton transfer in nanopores is critical for a wide range of emerging applications, yet experimentally probing mechanisms and energetics of this process remains a significant challenge. To help reveal details of this process, we developed and applied a machine learning potential derived from first-principles calculations to examine water reactivity and proton transfer in TiO2 slit-pores. We find that confinement of water within pores smaller than 0.5 nm imposes strong and complex effects on water reactivity and proton transfer. Although the proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. In a broader context, our study demonstrates that proton transfer in hydrophilic nanopores can be enhanced by controlling pore size, surface chemistry, and topology.

Simple Mechanism for the Observed Breakdown of the Nernst-Einstein Relation for Ions in Carbon Nanotubes

In this Letter, we present a simple mechanism that explains the recent experimental observation of the breakdown of the Nernst-Einstein (NE) relation for an ion moving in a carbon nanotube of subnanometer diameter. We argue that the friction acting on the ion is largely independent of the ion velocity, i.e., dry friction, and demonstrate, based on the Langevin equation for a particle subject to both dry and viscous friction, that the NE relation is violated when dry friction dominates. We predict that the ratio of the diffusion constant to the mobility of the ion is a few orders of magnitude smaller than the value predicted by the NE relation, in quantitative agreement with experiment.

Electrochemical Detection of Selective Anion Transport through Subnanopores in Liquid-Crystalline Water Treatment Membranes

The anion-selective transport through subnanoporous liquid-crystalline (LC) water treatment membranes was quantitatively detected by the deposition and electrochemical analysis of the LC membrane on the GaN electrode. The time course of the capacitance and Warburg resistance of the LC membrane suggest that the interaction of the LC membrane with monovalent Cl- ions is distinctly different from that with SO42- ions. A continuous decay in capacitance suggests the condensation of Cl- ions in subnanopores, whereas the interaction between SO42- ions and the inner wall of subnanopores is much weaker. The chronoamperometry data further suggest that SO42- ions are transported through subnanoporous channels 10 times faster than Cl- ions. These results, together with the previous X-ray emission spectroscopy, suggest that SO42- ions, which possess similar hydrogen-bonded structures to the hydrogen-bonded networks inside the subnanopores, can exchange the associated water molecules and hop along the network of water molecules, but Cl- ions bind and accumulate inside subnanopores. The well-controlled supramolecular self-assembly of LC building blocks opens a large potential toward the fine adjustment of hydrogen-bonding networks in nanospace providing materials new functions, which cannot be realized by bulk water.

Proton Coulomb Blockade Effect Involving Covalent Oxygen-Hydrogen Bond Switching

Instead of the canonical Grotthuss mechanism, we show that a knock-on proton transport process is preferred between organic functional groups (e.g., -COOH and -OH) and adjacent water molecules in biological proton channel and synthetic nanopores through comprehensive quantum and classical molecular dynamics simulations. The knock-on process is accomplished by the switching of covalent O─H bonds of the functional group under externally applied electric fields. The proton transport through the synthetic nanopore exhibits nonlinear current-voltage characteristics, suggesting an unprecedented proton Coulomb blockade effect. These findings not only enhance the understanding of proton transport in nanoconfined systems but also pave the way for the design of a variety of proton-based nanofluidic devices.

Stabilization of hydrogen-bonded molecular chains by carbon nanotubes

We study numerically nonlinear dynamics of several types of molecular systems composed of hydrogen-bonded chains placed inside carbon nanotubes with open edges. We demonstrate that carbon nanotubes provide a stabilization mechanism for quasi-one-dimensional molecular chains via the formation of their secondary structures. In particular, a polypeptide chain (Gly)N placed inside a carbon nanotube can form a stable helical chain (310-, α-, π-, and β-helix) with parallel chains of hydrogen-bonded peptide groups. A chain of hydrogen fluoride molecules ⋯FH⋯FH⋯FH can form a hydrogen-bonded zigzag chain. Remarkably, we demonstrate that for molecular complexes (Gly)N∈CNT and (FH)N∈CNT, the hydrogen-bonded chains will remain stable even at T=500 K. Thus, our results suggest that the use of carbon nanotubes with encapsulated hydrogen fluoride molecules may be important for the realization of high proton conductivity at high temperatures.

Extreme Ion-Transport Inorganic 2D Membranes for Nanofluidic Applications

Inorganic 2D materials offer a new approach to controlling mass diffusion at the nanoscale. Controlling ion transport in nanofluidics is key to energy conversion, energy storage, water purification, and numerous other applications wherein persistent challenges for efficient separation must be addressed. The recent development of 2D membranes in the emerging field of energy harvesting, water desalination, and proton/Li-ion production in the context of green energy and environmental technology is herein discussed. The fundamental mechanisms, 2D membrane fabrication, and challenges toward practical applications are highlighted. Finally, the fundamental issues of thermodynamics and kinetics are outlined along with potential membrane designs that must be resolved to bridge the gap between lab-scale experiments and production levels.

Theory of the force of friction acting on water chains flowing through carbon nanotubes

A simple model for the friction experienced by the one-dimensional water chains that flow through subnanometer diameter carbon nanotubes is studied. The model is based on a lowest order perturbation theory treatment of the friction experienced by the water chains due to the excitation of phonon and electron excitations in both the nanotube and the water chain, as a result of the motion of the chain. On the basis of this model, we are able to demonstrate how the observed flow velocities of water chains through carbon nanotubes of the order of several centimeters per second can be accounted for. If the hydrogen bonds between the water molecules are broken (as would occur if there were an electric field oscillating with a frequency equal to the resonant frequency of the hydrogen bonds present), it is shown that the friction experienced by the water flowing in the tube can be much smaller.

Search for a Grotthuss mechanism through the observation of proton transfer

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between 'chains' of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5-0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Dynamics and Proton Conduction of Heterogeneously Confined Imidazole in Porous Coordination Polymers

The nanoconfinement of proton carrier molecules may contribute to the lowing of their proton dissociation energy. However, the free proton transportation does not occur as easily as in liquid due to the restricted molecular motion from surface attraction. To resolve the puzzle, herein, imidazole is confined in the channels of porous coordination polymers with tunable geometries, and their electric/structural relaxations are quantified. Imidazole confined in a square-shape channels exhibits dynamics heterogeneity of core-shell-cylinder model. The core and shell layer possess faster and slower structural dynamics, respectively, when compared to the bulk imidazole. The dimensions and geometry of the nanochannels play an important role in both the shielding of the blocking effect from attractive surfaces and the frustration filling of the internal proton carrier molecules, ultimately contributing to the fast dynamics and enhanced proton conductivity.

Hydroxide Diffusion in Functionalized Cylindrical Nanopores as Idealized Models of Anion Exchange Membrane Environments: An Ab Initio Molecular Dynamics Study

Anion exchange membranes (AEMs) have attracted significant interest for their applications in fuel cells and other electrochemical devices in recent years. Understanding water distributions and hydroxide transport mechanisms within AEMs is critical to improving their performance as concerns hydroxide conductivity. Recently, nanoconfined environments have been used to mimic AEM environments. Following this approach, we construct nanoconfined cylindrical pore structures using graphane nanotubes (GNs) functionalized with trimethylammonium cations as models of local AEM morphology. These structures were then used to investigate hydroxide transport using ab initio molecular dynamics (AIMD). The simulations showed that hydroxide transport is suppressed in these confined environments relative to the bulk solution although the mechanism is dominated by structural diffusion. One factor causing the suppressed hydroxide transport is the reduced proton transfer (PT) rates due to changes in hydroxide and water solvation patterns under confinement compared to bulk solution as well as strong interactions between hydroxide ions and the tethered cation groups.

Influence of sulfonated SBA - 15 on fuel cell performance of sulfonated polysulfone electrolyte membranes

The prepared mesoporous SBA-15 (Santa Barbara Amorphous-15) was sulfonated and used as filler for the preparation of sulfonated polysulfone based composite electrolyte membranes. The SBA-15 and polysulfone were sulfonated using 3-mercaptopropyl trimethoxysilane and trimethylsilyl chlorosulfonate, respectively. The different weight percentages (1, 3, and 5 wt%) of sulfonated SBA-15 (SSBA-15) were used to prepare composite electrolyte membranes. Water uptake, ion exchange capacity, swelling ratio and proton conductivity of the composite membranes were studied for assessing the suitability of the electrolyte membranes for use in fuel cells. Characterization techniques such as FT-IR, XRD, SEM, TEM and Brunauer–Emmett– Teller were used to study the physico-chemical properties of the electrolyte membranes. TEM and BET analysis showed that SBA -15 retained its mesoporous structure even after sulfonation process. The prepared membranes were then tested in an in-house built single-cell fuel cell using hydrogen as fuel and oxygen as the oxidant. The fuel cell study showed that the presence of Sulfonated SBA-15 in the polymer matrix provided additional ion exchange sites and retained water for proton transfer which resulted in higher power density of 815 mW/cm2 with SPSU + 3% SSBA-15 membrane as compared with Nafion 117®.

Biomimetic Artificial Proton Channels

One of the most common biochemical processes is the proton transfer through the cell membranes, having significant physiological functions in living organisms. The proton translocation mechanism has been extensively studied; however, mechanistic details of this transport are still needed. During the last decades, the field of artificial proton channels has been in continuous growth, and understanding the phenomena of how confined water and channel components mediate proton dynamics is very important. Thus, proton transfer continues to be an active area of experimental and theoretical investigations, and acquiring insights into the proton transfer mechanism is important as this enlightenment will provide direct applications in several fields. In this review, we present an overview of the development of various artificial proton channels, focusing mostly on their design, self-assembly behavior, proton transport activity performed on bilayer membranes, and comparison with protein proton channels. In the end, we discuss their potential applications as well as future development and perspectives.

Directed walk in probability space that locates mean field solutions to spin models

Despite their formal simplicity, most lattice spin models cannot be easily solved, even under the simplifying assumptions of mean field theory. In this paper, we present a method for generating mean field solutions to classical continuous spins. We focus our attention on systems with nonlocal interactions and nonperiodic boundaries, which require careful handling with existing approaches, such as Monte Carlo sampling. Our approach utilizes functional optimization to derive a closed-form optimality condition and arrive at self-consistent mean field equations. We show that this approach significantly outperforms conventional Monte Carlo sampling in convergence speed and accuracy. To convey the general concept behind the approach, we first demonstrate its application to a simple system: a finite one-dimensional dipolar chain in an external electric field. We then describe how the approach naturally extends to more complicated spin systems and to continuum field theories. Furthermore, we numerically illustrate the efficacy of our approach by highlighting its utility on nonperiodic spin models of various dimensionality.

Putting together the puzzle of ion transfer in single-digit carbon nanotubes: mean-field meets ab initio

Nature employs channel proteins to selectively pass water across cell membranes, which inspires the search for bio-mimetic analogues. Carbon nanotube porins (CNTPs) are intriguing mimics of water channels, yet ion transport in CNTPs still poses questions. As an alternative to continuum models, here we present a molecular mean-field model that transparently describes ion coupling, yet unlike continuum models, computes ab initio all required thermodynamic quantities for the KCl salt and H⁺ and OH^- ions present in water. Starting from water transfer, the model considers the transfer of free ions, along with ion-pair formation as a proxy of non-mean-field ion-ion interactions. High affinity to hydroxide, suggested by experiments, making it a dominant charge carrier in CNTPs, is revealed as an exceptionally favorable transfer of KOH pairs. Nevertheless, free ions, coexisting with less mobile ion-pairs, apparently control ion transport. The model well explains the observed effects of salt concentration and pH on conductivity, transport numbers, anion permeation and its activation energies, and current rectification. The proposed approach is extendable to other sub-nanochannels and helps design novel osmotic materials and devices.

Proton Generation Using Chitin–Chitinase and Collagen–Collagenase Composites

Hydrogen energy is focused on as next-generation energy without environmental load. Therefore, hydrogen production without using fossil fuels is a key factor in the progress of hydrogen energy. In the present work, it was found that chitin–chitinase and collagen–collagenase composites can generate protons by the hydrolysis of the enzyme. The concentration of the generated proton in the chitin–chitinase and collagen–collagenase composites are 1.68 × 1017 cm−3 and 1.02 × 1017 cm−3, respectively. Accompanying these results, proton diffusion constants in the chitin and collagen membranes are also estimated to be 8.59 × 10−8 cm2/s and 8.69 × 10−8 cm2/s, respectively. Furthermore, we have fabricated the bio-fuel cell using these composites as hydrogen fuel and demonstrated that these composites become a fuel of the fuel cell.

De Novo Synthesis of Free-Standing Flexible 2D Intercalated Nanofilm Uniform over Tens of cm2

Of a variety of intercalated materials, 2D intercalated systems have attracted much attention both as materials per se, and as a platform to study atoms and molecules confined among nanometric layers. High-precision fabrication of such structures has, however, been a difficult task using the conventional top-down and bottom-up approaches. The de novo synthesis of a 3-nm-thick nanofilm intercalating a hydrogen-bonded network between two layers of fullerene molecules is reported here. The two-layered film can be further laminated into a multiply film either in situ or by sequential lamination. The 3 nm film forms uniformly over an area of several tens of cm² at an air/water interface and can be transferred to either flat or perforated substrates. A free-standing film in air prepared by transfer to a gold comb electrode shows proton conductivity up to 1.4 × 10^-4 S cm^-1 . Electron-dose-dependent reversible bending of a free-standing 6-nm-thick nanofilm hung in a vacuum is observed under electron beam irradiation.

Two-dimensional quantum-sheet films with sub-1.2 nm channels for ultrahigh-rate electrochemical capacitance

Dense, thick, but fast-ion-conductive electrodes are critical yet challenging components of ultrafast electrochemical capacitors with high volumetric power/energy densities^1-4. Here we report an exfoliation-fragmentation-restacking strategy towards thickness-adjustable (1.5‒24.0 μm) dense electrode films of restacked two-dimensional 1T-MoS₂ quantum sheets. These films bear the unique architecture of an exceptionally high density of narrow (sub-1.2 nm) and ultrashort (~6.1 nm) hydrophobic nanochannels for confinement ion transport. Among them, 14-μm-thick films tested at 2,000 mV s^-1 can deliver not only a high areal capacitance of 0.63 F cm^-2 but also a volumetric capacitance of 437 F cm^-3 that is one order of magnitude higher than that of other electrodes. Density functional theory and ab initio molecular dynamics simulations suggest that both hydration and nanoscale channels play crucial roles in enabling ultrafast ion transport and enhanced charge storage. This work provides a versatile strategy for generating rapid ion transport channels in thick but dense films for energy storage and filtration applications.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Void Space versus Surface Functionalization for Proton Conduction in Metal-Organic Frameworks

Void space and functionality of the pore surface are important structural factors for proton-conductive metal-organic frameworks (MOFs) impregnated with conducting media. However, no clear study has compared their priority factors, which need to be considered when designing proton-conductive MOFs. Herein, we demonstrate the effects of void space and pore-surface modification on proton conduction in MOFs through the surface-modified isoreticular MOF-74(Ni) series [Ni₂ (dobdc or dobpdc), dobdc=2,5-dihydroxy-1,4-benzenedicarboxylate and dobpdc=4,4'-dihydroxy-(1,1'-biphenyl)-3,3'-dicarboxylate]. The MOF with lower porosity with the same surface functionality showed higher proton conductivity than that with higher porosity despite including a smaller amount of conducting medium. Density functional theory calculations suggest that strong hydrogen bonding between molecules of the conducting medium at high porosity is inefficient in inducing high proton conductivity.

Study on the Assembly Mechanisms and Transport Properties of Transmembrane End-Charged Cyclic Peptide Nanotubes

In this work, two end-charged cyclic peptide nanotubes (CPNTs) embedded in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) were designed to simulate transmembrane ion channels. Density functional theory (DFT) computations at the level of M06-2X/6-31G give different assembling modes of the negatively charged ELWL-CPNT and positively charged RLWL-CPNT as (L-L)(D-L)(D-D)(L-L)(D-D)(L-L)(D-D) and (D-D)(L-L)(D-D)(L-L)(D-D)(L-L)(D-D), respectively. Molecular dynamics (MD) simulations indicate that a charge at a CPNT end obviously affects the structure of the channel water chain and the diffusion behavior of K⁺. The regions with the highest probability of H-bond defects in the channel water chains are gap5 and gap2 in ELWL/POPE-CPNT and RLWL/POPE-CPNT, respectively. K⁺ can easily enter either CPNT by desolvation, and behaves more actively in RLWL/POPE-CPNT, shuttling rapidly and frequently between an α-plane zone and an adjacent midplane region. Results of this work reveal that a charge at the end of an ionic channel may significantly alter the transport characteristics of the channel.

Control of Proton-Conductive Behavior with Nanoenvironment within Metal-Organic Materials

Solid-state proton-conductive materials have been of great interest for several decades due to their promising application as electrolytes in fuel cells and electrochemical devices. Metal-organic materials (MOMs) have recently been intensively investigated as a new type of proton-conductive materials. The highly crystalline nature and structural designability of MOMs make them advantageous over conventional noncrystalline proton-conductive materials-the detailed investigation of the structure-property relationship is feasible on MOM-based proton conductors. This review aims to summarize and examine the fundamental principles and various design strategies on proton-conductive MOMs, and shed light on the nanoconfinement effects as well as the importance of hydrophobicity on specific occasions, which have been often disregarded. Besides, challenges and future prospects on this field are presented.

Gas hydrates in confined space of nanoporous materials: new frontier in gas storage technology

Gas hydrates (clathrate hydrates, clathrates, or hydrates) are crystalline inclusion compounds composed of water and gas molecules. Methane hydrates, the most well-known gas hydrates, are considered a menace in flow assurance. However, they have also been hailed as an alternative energy resource because of their high methane storage capacity. Since the formation of gas hydrates generally requires extreme conditions, developing porous material hosts to synthesize gas hydrates with less-demanding constraints is a topic of great interest to the materials and energy science communities. Though reports of modeling and experimental analysis of bulk gas hydrates are plentiful in the literature, reliable phase data for gas hydrates within confined spaces of nanoporous media have been sporadic. This review examines recent studies of both experiments and theoretical modeling of gas hydrates within four categories of nanoporous material hosts that include porous carbons, metal-organic frameworks, graphene nanoslits, and carbon nanotubes. We identify challenges associated with these porous systems and discuss the prospects of gas hydrates in confined space for potential applications.

Enhanced Organic Photocatalysis in Confined Flow through a Carbon Nitride Nanotube Membrane with Conversions in the Millisecond Regime

Bioinspired nanoconfined catalysis has developed to become an important tool for improving the performance of a wide range of chemical reactions. However, photocatalysis in a nanoconfined environment remains largely unexplored. Here, we report the application of a free-standing and flow-through carbon nitride nanotube (CNN) membrane with pore diameters of 40 nm for confined photocatalytic reactions where reactants are in contact with the catalyst for <65 ms, as calculated from the flow. Due to the well-defined tubular structure of the membrane, we are able to assess quantitatively the photocatalytic performance in each of the parallelized single carbon nitride nanotubes, which act as spatially isolated nanoreactors. In oxidation of benzylamine, the confined reaction shows an improved performance when compared to the corresponding bulk reaction, reaching a turnover frequency of (9.63 ± 1.87) × 10⁵ s^-1. Such high rates are otherwise only known for special enzymes and are clearly attributed to the confinement of the studied reactions within the one-dimensional nanochannels of the CNN membrane. Namely, a concave surface maintains the internal electric field induced by the polar surface of the carbon nitride inside the nanotube, which is essential for polarization of reagent molecules and extension of the lifetime of the photogenerated charge carriers. The enhanced flow rate upon confinement provides crucial insight on catalysis in such an environment from a physical chemistry perspective. This confinement strategy is envisioned not only to realize highly efficient reactions but also to gain a fundamental understanding of complex chemical processes.

Water orientation and dynamics in the closed and open influenza B virus M2 proton channels

The influenza B M2 protein forms a water-filled tetrameric channel to conduct protons across the lipid membrane. To understand how channel water mediates proton transport, we have investigated the water orientation and dynamics using solid-state NMR spectroscopy and molecular dynamics (MD) simulations. 13C-detected water 1H NMR relaxation times indicate that water has faster rotational motion in the low-pH open channel than in the high-pH closed channel. Despite this faster dynamics, the open-channel water shows higher orientational order, as manifested by larger motionally-averaged 1H chemical shift anisotropies. MD simulations indicate that this order is induced by the cationic proton-selective histidine at low pH. Furthermore, the water network has fewer hydrogen-bonding bottlenecks in the open state than in the closed state. Thus, faster dynamics and higher orientational order of water molecules in the open channel establish the water network structure that is necessary for proton hopping.

Improved wettability and enhanced ionic transport in highly porous CNT sponge

We investigated the effect of an electric treatment on the wettability of aqueous solution on carbon nanotubes (CNT) and ion transport behaviors in superhydrophobic porous carbon nanotube sponges (CNTS). This electric activation treatment where an electric voltage was applied across highly porous CNT sponge induced an electrowetting effect. This effect significantly reduced interfacial tensions between CNT sidewalls and aqueous liquids. Meanwhile, polar functional groups were also introduced on CNTs. Both electrowetting effect and polar functional groups greatly improved the wettability of aqueous solutions on CNT sidewalls. After the electric treatment, we observed a dramatic increase in the overall rate of ion flow across porous CNT sponges. The formation of solution channels during the electric treatment is responsible for the enhanced ionic transport in porous CNT sponges. The overall rate of ion flow increased with the increases in electric treatment time and voltage. The crucial role of electric treatment parameters in the ion transport provides a new strategy for precisely controlling the ion transport across CNT sponges by tuning electric treatment time or voltage. Importantly, the good wettability of aqueous solution on CNT sidewalls greatly increased the effective surface area of CNT sponges and thus significantly improved the performance of CNTS-based supercapacitors after the electric treatment.

A Step-by-Step Process-Induced Unidirectional Oriented Water Wire in the Nanotube

The orientation of water molecules is a key requirement for the fast transport of water in nanotubes. It has been accepted that the flip of the water chain follows a concerted mechanism, which has led to the view that bidirectional water flux in nanotubes can be transformed into unidirectional transport when the orientation of water molecules is maintained in long nanotubes under the external field. In this Letter, on the basis of molecular dynamics simulations and first-principles calculations, we confirmed that the flip of the water chain is a step-by-step process, which is different from the perceived concerted mechanism. Further analysis indicated that without an external field, it needed more than 20 water molecules to maintain the unidirectional single-file water flow in a carbon nanotube at a duration time of seconds. Considering that the thickness of the cell membrane (normally 5-10 nm) is larger than the length threshold of the unidirectional water wire, this study suggested that it may not require the external field to maintain the unidirectional flow in the water channel at the macroscopic time scale.

Water in Nanopores and Biological Channels: A Molecular Simulation Perspective

This Review explores the dynamic behavior of water within nanopores and biological channels in lipid bilayer membranes. We focus on molecular simulation studies, alongside selected structural and other experimental investigations. Structures of biological nanopores and channels are reviewed, emphasizing those high-resolution crystal structures, which reveal water molecules within the transmembrane pores, which can be used to aid the interpretation of simulation studies. Different levels of molecular simulations of water within nanopores are described, with a focus on molecular dynamics (MD). In particular, models of water for MD simulations are discussed in detail to provide an evaluation of their use in simulations of water in nanopores. Simulation studies of the behavior of water in idealized models of nanopores have revealed aspects of the organization and dynamics of nanoconfined water, including wetting/dewetting in narrow hydrophobic nanopores. A survey of simulation studies in a range of nonbiological nanopores is presented, including carbon nanotubes, synthetic nanopores, model peptide nanopores, track-etched nanopores in polymer membranes, and hydroxylated and functionalized nanoporous silica. These reveal a complex relationship between pore size/geometry, the nature of the pore lining, and rates of water transport. Wider nanopores with hydrophobic linings favor water flow whereas narrower hydrophobic pores may show dewetting. Simulation studies over the past decade of the behavior of water in a range of biological nanopores are described, including porins and β-barrel protein nanopores, aquaporins and related polar solute pores, and a number of different classes of ion channels. Water is shown to play a key role in proton transport in biological channels and in hydrophobic gating of ion channels. An overall picture emerges, whereby the behavior of water in a nanopore may be predicted as a function of its hydrophobicity and radius. This informs our understanding of the functions of diverse channel structures and will aid the design of novel nanopores. Thus, our current level of understanding allows for the design of a nanopore which promotes wetting over dewetting or vice versa. However, to design a novel nanopore, which enables fast, selective, and gated flow of water de novo would remain challenging, suggesting a need for further detailed simulations alongside experimental evaluation of more complex nanopore systems.

Rheology of water in small nanotubes

The properties of water in confinement are very different from those under bulk conditions. In some cases the melting point of ice may be shifted and one may find either ice, icelike water, or a state in which freezing is completely inhibited. Understanding the dynamics and rheology of water in confined media, such as small nanotubes, is of fundamental importance to the biological properties of micro-organisms at low temperatures, to the development of new devices for preserving DNA samples, and for other biological materials and fluids, lubrication, and development of nanostructured materials. We study rheology and dynamics of water in small nanotubes using extensive equilibrium and nonequilibrium molecular dynamics simulations. The results demonstrate that in strong confinement in nanotubes at temperatures significantly below and above bulk freezing temperature water behaves as a shear-thinning fluid at shear rates smaller than the inverse of the relaxation time in the confined medium. In addition, our results indicate the presence of regions in which the local density of water varies significantly over the same range of temperature in the nanotube. These findings may also have important implications for the design of nanofluidic systems.

How Can Protons Migrate in Extremely Compressed Liquid Water?

Compression of liquid water up to multi-kbar pressures is known to perturb dramatically its local structure required for charge defects to migrate as topological defects in the hydrogen-bonded network. Our ab initio simulations show that the migration of excess protons is not much affected at 10 kbar, whereas that of proton holes is significantly reduced. Non-Markovian analyses show that this is not due to modifying the free energy barriers of both charge transfer and migration. It is rather pressure-induced modifications of the population of activated states, depending on interstitial water, which rules charge migration at extreme compression.

Slow Proton Transfer in Nanoconfined Water

The transport of protons in nanoconfined environments, such as in nanochannels of biological or artificial proton conductive membranes, is essential to chemistry, biology, and nanotechnology. In water, proton diffusion occurs by hopping of protons between water molecules. This process involves the rearrangement of many hydrogen bonds and as such can be strongly affected by nanoconfinement. We study the vibrational and structural dynamics of hydrated protons in water nanodroplets stabilized by a cationic surfactant using polarization-resolved femtosecond infrared transient absorption spectroscopy. We determine the time scale of proton hopping in the center of the water nanodroplets from the dynamics of the anisotropy of the transient absorption signals. We find that in small nanodroplets with a diameter <4 nm, proton hopping is more than 10 times slower than in bulk water. Even in relatively large nanodroplets with a diameter of ∼7 nm, we find that the rate of proton hopping is slowed by ∼4 times compared with bulk water.

Twisted Eigen Can Induce Proton Transfer at a Hydrophobic-Hydrophilic Interface

The investigation of proton localization at a hydrophobic-hydrophilic interface is an important problem in chemical and materials sciences. In this study, protonated benzene (i.e., benzenium ion) and water clusters [BZH⁺W_n (where n = 1-6)] are selected as prototype models to understand the interfacial interactions and proton transfer mechanism between a carbonaceous surface and water molecules. The excess protons can localize in the vicinity of the hydrophobic-hydrophilic interface, and these clusters are stabilized by various kinds of noncovalent interactions. Calculations are carried out using ab initio (MP2) and density functional theory B3LYP methods to shed more light on geometries, energetics, and spectral signatures of the protonated species [H⁺(H₂O)_n] at the interfaces. These calculations revealed few low-lying isomers, which have not been reported earlier. Scrutiny of the results reveals that proton localization in the hydrophilic environment is more stable than the hydrophobic benzene π-cloud. Furthermore, the occurrence of an O-H⁺···π hydrogen bond significantly influences the O-H⁺···O interactions in the water clusters and also intensively affects the vibrational modes of the Eigen cation. Thus, the aromatic π-clouds can stabilize the Eigen cation and at the same time, a twisted form of Eigen (one O-H⁺···π → two O-H⁺···π) can enhance the proton transfer through the water chain via a Grotthuss-type mechanism. The vibrational spectra of these clusters reveal that there is a large red-shifted frequency for the O-H⁺···O, O-H⁺···π, and O-H···π modes of interaction. The energetic values and vibrational frequencies obtained from the B3LYP method are in close agreement with the MP2 level and experimental values, respectively.

Effect of Metal and Carbon Nanotube Additives on the Thermal Diffusivity of a Silica Gel-Based Adsorption Bed

This article presents a study of the effect of metal particle and carbon nanotube additives on the thermal diffusivity of a silica-gel-based adsorption bed of an adsorption chiller. The structural properties of silica gel and carbon nanotubes were investigated using the volumetric method of low-pressure nitrogen adsorption. Thermal characteristic tests of the prepared mixtures based on a silica gel with 5 wt% and 15 wt% of aluminum, copper, or carbon nanotubes were carried out. The obtained results show that all the materials used as additives in blends in this study achieved higher thermal diffusivities in comparison with the thermal diffusivity of the parent silica gel. However, the best effect was observed for the mixture with 15 wt% aluminum.

Confined water-mediated high proton conduction in hydrophobic channel of a synthetic nanotube

Water confined within one-dimensional (1D) hydrophobic nanochannels has attracted significant interest due to its unusual structure and dynamic properties. As a representative system, water-filled carbon nanotubes (CNTs) are generally studied, but direct observation of the crystal structure and proton transport is difficult for CNTs due to their poor crystallinity and high electron conduction. Here, we report the direct observation of a unique water-cluster structure and high proton conduction realized in a metal-organic nanotube, [Pt(dach)(bpy)Br]₄(SO₄)₄·32H₂O (dach: (1R, 2R)-(–)-1,2-diaminocyclohexane; bpy: 4,4’-bipyridine). In the crystalline state, a hydrogen-bonded ice nanotube composed of water tetramers and octamers is found within the hydrophobic nanochannel. Single-crystal impedance measurements along the channel direction reveal a high proton conduction of 10⁻² Scm⁻¹. Moreover, fast proton diffusion and continuous liquid-to-solid transition are confirmed using solid-state ¹H-NMR measurements. Our study provides valuable insight into the structural and dynamical properties of confined water within 1D hydrophobic nanochannels.

Water confined in natural or synthetic hydrophobic nano-spaces behaves differently than in the bulk. Here the authors investigate water in hydrophobic synthetic 1D nanochannels revealing water clustering in tetramers and octamers and high proton conductivity, along with a continuous liquid to solid transition.