Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR

Chemical Functional Groups Regulate Ion Concentrations and pHs in Nanopores

Understanding ion behaviors in functionalized nanopores is essential to deciphering reactions in both natural and engineered systems, such as sediments, biological ion channels, and membranes. While many efforts have shown the modified ion behaviors in the functionalized nanopores, a direct measurement and analysis to show how chemical functional groups affect ion concentrations in nanopores are critically needed. Herein, we present a plasmonic nanosensor that can measure the local concentrations of protons, anions (phosphate, nitrate, sulfate, and arsenate), and cations (mercury, lead, and copper) in functionalized nanopores, and we compare their concentrations in nanopores with the corresponding bulk concentrations. Notably, chemical functional groups induced ion concentrations differently in nanopores. In pristine nanopores and methyl- and phenyl-functionalized nanopores, we discovered an unexpected concurrence of an enhanced anion concentration and a suppressed cation concentration. In addition, the nanopore pH is dependent on bulk solution compositions and can be lower by 2.5 units, even when the bulk solution is well-buffered. In contrast, for hydrophilic (amine, thiol, and carboxyl) nanopores, pH depended on the pKa of the functional groups, and the heavy metal concentrations depended on chemical interactions with the functional groups. Our findings provide a better understanding of water chemistry in nanopores and can help precisely control ions in nanopores to benefit the design of membrane-based desalination techniques, CO2 storage, and porous catalysts.

On the Physical Origins of Reduced Ionic Conductivity in Nanoconfined Electrolytes

Ion transport through nanoscale pores is at the heart of numerous energy storage and separation technologies. Despite significant efforts to uncover the complex interplay of ion-ion, ion-water, and ion-pore interactions that give rise to these transport processes, the atomistic mechanisms of ion motion in confined electrolytes remain poorly understood. In this work, we use machine learning-based molecular dynamics simulations to characterize ion transport with first-principles-level accuracy in aqueous NaCl confined to graphene slit pores. We find that ionic conductivity decreases as the degree of confinement increases, a trend governed by changes in both ion self-diffusion and dynamic ion-ion correlations. We show that the self-diffusion coefficients of our confined ions are strongly influenced by the overall electrolyte density, which changes nonmonotonically with slit height based on the layering of water molecules within the pore. We further observe a shift in the ions' diffusion mechanism toward more vehicular motion as the degree of confinement increases. Despite the ubiquity of ideal solution (Nernst-Einstein) assumptions in the field, we find that nonideal contributions to transport become more pronounced under confinement. This increase in nonideal ion correlations arises not simply from an increase in the fraction of associated ions, as is commonly assumed, but from an increase in ion pair lifetimes. By building a mechanistic understanding of confined electrolyte transport, this work provides insights that could guide the design of nanoporous materials optimized for efficient and selective ion transport.

Advanced characterization of confined electrochemical interfaces in electrochemical capacitors

The advancement of high-performance fast-charging materials has significantly propelled progress in electrochemical capacitors (ECs). Electrochemical capacitors store charges at the nanoscale electrode material-electrolyte interface, where the charge storage and transport mechanisms are mediated by factors such as nanoconfinement, local electrode structure, surface properties and non-electrostatic ion-electrode interactions. This Review offers a comprehensive exploration of probing the confined electrochemical interface using advanced characterization techniques. Unlike classical two-dimensional (2D) planar interfaces, partial desolvation and image charges play crucial roles in effective charge storage under nanoconfinement in porous materials. This Review also highlights the potential of zero charge as a key design principle driving nanoscale ion fluxes and carbon-electrolyte interactions in materials such as 2D and three-dimensional (3D) porous carbons. These considerations are crucial for developing efficient and rapid energy storage solutions for a wide range of applications.

Scaling Behavior and Conductance Mechanisms of Ion Transport in Atomically Thin Graphene Nano/Subnanopores

Ion transport through atomically thin nano/subnanopores, such as those in monolayer graphene, presents challenges to traditional ion conduction models, primarily due to extreme confinement effects and hydration interactions. Under these conditions, existing models fail to account for conductance behaviors at the nano- and subnanometer scales. In this study, we perform a combined experimental and theoretical investigation of ion transport in monolayer graphene nano/subnanopores across varying salt concentrations. We introduce a conductance model that accurately predicts the observed scaling behavior by addressing the interaction between counterions and the edges of atomically thin pores, where counterion movement is constrained by the pore's structure. This model also quantifies the hydration energy barrier, highlighting the impact of the hydration shell structures on ion transport efficiency. Our findings reveal that hydrated potassium ions traverse these pores with higher efficiency than previously estimated, offering new insights into ion transport mechanisms under atomic-scale confinement.

Scaling Behavior of Ionic Conductance Dependent on Surface Charge Inside a Single-Digit Nanopore

The ionic conductance in a charged nanopore exhibits a power-law behavior in low salinity-as has been verified in many experiments (G0∝c0α)-which is governed by surface charges. The surface charge inside a nanopore determines the zeta potential and ion distributions, which have a significant impact on ion transport, especially in a single-digit nanopore with potential leakage. However, precisely measuring surface charge density in a single-digit nanopore remains a challenge. Here, we propose a methodology for exploring the power-law variation of ionic conductance, with potential leakage taken into account. We conducted experiments to measure the ionic current using silicon nitride nanopores and employed a continuous theory to explore the relationship between pore-bound concentration and surface charges. Considering that the influence of potential leakage on concentration follows a power-law relationship, we established a coefficient (α) to examine the controlling factors of potential leakage and modified the conductance model to obtain the ion mobility inside a nanopore.

Characterization of Diffusioosmotic Ion Transport for Enhanced Concentration-Driven Power Generation via Charge Heterogeneity in Nanoporous Membranes

Nanoscopic mass/ion transport through heterogeneous nanostructures with various physicochemical environments occurs in both natural and artificial systems. Concentration gradient-driven mass/ion transport mechanisms, such as diffusioosmosis (DO), are primarily governed by the structural and electrical features of the nanostructures. However, these phenomena under various electrical and chemical conditions have not been adequately investigated. In this study, we fabricated a pervaporation-based particle-assembled membrane (PAM)-integrated micro-/nanofluidic device that facilitates easy tuning of the surface charge heterogeneity in nanopores/nanochannels. The nanochannels in the device consisted of two heterogeneous and in-series PAMs. The device was used to quantitatively measure electric signals generated by DO within the nanochannels with a single electrolyte or a combination of two electrolytes. Then, we characterized ion transport by changing surface charge heterogeneity and applying various electrolytic conditions, characterizing the concentration-driven power generation under these conditions. We found that not only does the charge heterogeneity provide additional resistance to ion transport but also the manipulation of the heterogeneity enables the effective modulation of ion transport and optimization of concentration-driven power generators regarding ion selectivity. In conjunction with the surface charge heterogeneity, the electrolytic conditions significantly affected the net flux of ion transport by enhancing or even negating the ion selectivity. Hence, we anticipate that both the platform and results will provide a deeper understanding of ion transport in nanostructures within complex environments by optimizing and improving practical concentration-driven applications, such as energy conversion/harvesting, molecular focusing/separation, and ionic diodes and memristors.

Dynamics of the Boundary Layer in Pulsed CO2 Electrolysis

Electrochemical reduction of CO2 poses a vast potential to contribute to a defossilized industry. Despite tremendous developments within the field, mass transport limitations, carbonate salt formation, and electrode degradation mechanisms still hamper the process performance. One promising approach to tweak CO2 electrolysis beyond today's limitations is pulsed electrolysis with potential cycling between an operating and a regeneration mode. Here, we rigorously model the boundary layer at a silver electrode in pulsed operation to get profound insights into the dynamic reorganization of the electrode microenvironment. In our simulation, pulsed electrolysis leads to a significant improvement of up to six times higher CO current density and 20 times higher cathodic energy efficiency when pulsing between -1.85 and -1.05 V vs SHE compared to constant potential operation. We found that elevated reactant availability in pulsed electrolysis originates from alternating replenishment of CO2 by diffusion and not from pH-induced carbonate and bicarbonate conversion. Moreover, pulsed electrolysis substantially promotes carbonate removal from the electrode by up to 83 % compared to constant potential operation, thus reducing the risk of salt formation. Therefore, this model lays the groundwork for an accurate simulation of the dynamic boundary layer modulation, which can provide insights into manifold electrochemical conversions.

Formation of compounds with diverse polyelectrolyte morphologies and nonlinear ion conductance in a two-dimensional nanofluidic channel

Aqueous electrolytes subjected to angstrom-scale confinement have recently attracted increasing interest because of their distinctive structural and transport properties, as well as their promising applicability in bioinspired nanofluidic iontronics and ion batteries. Here, we performed microsecond-scale molecular dynamics simulations, which provided evidence of nonlinear ionic conductance under an external lateral electric field due to the self-assembly of cations and anions with diverse polyelectrolyte morphologies (e.g., extremely large ion clusters) in aqueous solutions within angstrom-scale slits. Specifically, we found that the cations and anions of Li2SO4 and CaSO4 formed chain-like polyelectrolyte structures, whereas those of Na2SO4 and MgSO4 predominantly formed a monolayer of hydrated salt. Additionally, the cations and anions of K2SO4 assembled into a hexagonal anhydrous ionic crystal. These ion-dependent diverse polyelectrolyte morphologies stemmed from the enhanced Coulomb interactions, weakened hydration and steric constraints within the angstrom-scale slits. More importantly, once the monolayer hydrated salt or ionic crystal structure was formed, the field-induced ion current exhibited an intriguing gating effect at a low field strength. This abnormal ion transport was attributed to the concerted movement of cations and anions within the solid polyelectrolytes, leading to the suppression of ion currents. When the electric field exceeded a critical strength, however, the ion current surged rapidly due to the dissolution of many cations and anions within a few nanoseconds in the aqueous solution.

Understanding Sorption of Aqueous Electrolytes in Porous Carbon by NMR Spectroscopy

Ion adsorption at solid-water interfaces is crucial for many electrochemical processes involving aqueous electrolytes including energy storage, electrochemical separations, and electrocatalysis. However, the impact of the hydronium (H3O+) and hydroxide (OH-) ions on the ion adsorption and surface charge distributions remains poorly understood. Many fundamental studies of supercapacitors focus on non-aqueous electrolytes to avoid addressing the role of functional groups and electrolyte pH in altering ion uptake. Achieving microscopic level characterization of interfacial mixed ion adsorption is particularly challenging due to the complex ion dynamics, disordered structures, and hierarchical porosity of the carbon electrodes. This work addresses these challenges starting with pH measurements to quantify the adsorbed H3O+ concentrations, which reveal the basic nature of the activated carbon YP-50F commonly used in supercapacitors. Solid-state NMR spectroscopy is used to study the uptake of lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) aqueous electrolyte in the YP-50F carbon across the full pH range. The NMR data analysis highlights the importance of including the fast ion-exchange processes for accurate quantification of the adsorbed ions. Under acidic conditions, more TFSI- ions are adsorbed in the carbon pores than Li+ ions, with charge compensation also occurring via H3O+ adsorption. Under neutral and basic conditions, when the carbon's surface charge is close to zero, the Li+ and TFSI- ions exhibit similar but lower affinities toward the carbon pores. Our experimental approach and evidence of H3O+ uptake in pores provide a methodology to relate the local structure to the function and performance in a wide range of materials for energy applications and beyond.

Local electroneutrality breakdown for electrolytes within varying-section nanopores

We determine the local charge dynamics of a [Formula: see text] electrolyte embedded in a varying-section channel. By means of an expansion based on the length scale separation between the axial and transverse direction of the channel, we derive closed formulas for the local excess charge for both, dielectric and conducting walls, in 2D (planar geometry) as well as in 3D (cylindrical geometry). Our results show that, even at equilibrium, the local charge electroneutrality is broken whenever the section of the channel is not homogeneous for both dielectric and conducting walls as well as for 2D and 3D channels. Interestingly, even within our expansion, the local excess charge in the fluid can be comparable to the net charge on the walls. We critically discuss the onset of such local electroneutrality breakdown in particular with respect to the correction that it induces on the effective free energy profile experienced by tracer ions.

Ion-Specific Nanoconfinement Effect in Multilayered Graphene Membranes: A Combined Nuclear Magnetic Resonance and Computational Study

Ion adsorption within nanopores is involved in numerous applications. However, a comprehensive understanding of the fundamental relationship between in-pore ion concentration and pore size, particularly in the sub-2 nm range, is scarce. This study investigates the ion-species-dependent concentration in multilayered graphene membranes (MGMs) with tunable nanoslit sizes (0.5-1.6 nm) using nuclear magnetic resonance and computational simulations. For Na⁺-based electrolytes in MGMs, the concentration of anions in graphene nanoslits increases in correlation with their chaotropic properties. As the nanoslit size decreases, the concentration of chaotropic ion (BF₄^-) increases, whereas the concentration of kosmotropic ions (Cit^3-, PO₄^3-) and other ions (Ac^-, F^-) decreases or changes slightly. Notably, anions remain more concentrated than counter Na⁺ ions, leading to electroneutrality breakdown and unipolar anion packing in MGMs. A continuum modeling approach, integrating molecular dynamic simulation with the Poisson-Boltzmann model, elucidates these observations by considering water-mediated ion-graphene non-electrostatic interactions and charge screening from graphene walls.

Where Are Sodium Ions in AOT Reverse Micelles? Fluoride Anion Probes Nanoconfined Ions by 19F Nuclear Magnetic Resonance Spectroscopy

Confining water to nanosized spaces creates a unique environment that can change water's structural and dynamic properties. When ions are present in these nanoscopic spaces, the limited number of water molecules and short screening length can dramatically affect how ions are distributed compared to the homogeneous distribution assumed in bulk aqueous solution. Here, we demonstrate that the chemical shift observed in ¹⁹F NMR spectroscopy of fluoride anion, F^-, probes the location of sodium ions, Na⁺, confined in reverse micelles prepared from AOT (sodium dioctyl sulfosuccinate) surfactants. Our measurements show that the nanoconfined environment of reverse micelles can lead to extremely high apparent ion concentrations and ionic strength, beyond the limit in bulk aqueous solutions. Most notably, the ¹⁹F NMR chemical shift trends we observe for F^- in the reverse micelles indicate that the AOT sodium counterions remain at or near the interior interface between surfactant and water, thus providing the first experimental support for this hypothesis.

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.

Biphasic concentration patterns in ionic transport under nanoconfinement revealed in steady-state and time-dependent properties

Ion permeation across nanoscopic structures differs considerably from microfluidics because of strong steric constraints, transformed solvent properties, and charge-regulation effects revealed mostly in diluted solutions. However, little is known about nanofluidics in moderately concentrated solutions, which are critically important for industrial applications and living systems. Here, we show that nanoconfinement triggers general biphasic concentration patterns in a myriad of ion transport properties by using two contrasting systems: a biological ion channel and a much larger synthetic nanopore. Our findings show a low-concentration regime ruled by classical Debye screening and another one where ion-ion correlations and enhanced ion-surface interactions contribute differently to each electrophysiological property. Thus, different quantities (e.g., conductance vs noise) measured under the same conditions may appear contradictory because they belong to different concentration regimes. In addition, non-linear effects that are barely visible in bulk conductivity only in extremely concentrated solutions become apparent in nanochannels around physiological conditions.

Unlocking the potential of polymeric desalination membranes by understanding molecular-level interactions and transport mechanisms

Polyamide reverse osmosis (PA-RO) membranes achieve remarkably high water permeability and salt rejection, making them a key technology for addressing water shortages through processes including seawater desalination and wastewater reuse. However, current state-of-the-art membranes suffer from challenges related to inadequate selectivity, fouling, and a poor ability of existing models to predict performance. In this Perspective, we assert that a molecular understanding of the mechanisms that govern selectivity and transport of PA-RO and other polymer membranes is crucial to both guide future membrane development efforts and improve the predictive capability of transport models. We summarize the current understanding of ion, water, and polymer interactions in PA-RO membranes, drawing insights from nanofiltration and ion exchange membranes. Building on this knowledge, we explore how these interactions impact the transport properties of membranes, highlighting assumptions of transport models that warrant further investigation to improve predictive capabilities and elucidate underlying transport mechanisms. We then underscore recent advances in in situ characterization techniques that allow for direct measurements of previously difficult-to-obtain information on hydrated polymer membrane properties, hydrated ion properties, and ion-water-membrane interactions as well as powerful computational and electrochemical methods that facilitate systematic studies of transport phenomena.

Effective Modulation of Ion Mobility through Solid-State Single-Digit Nanopores

Many experimental studies have proved that ion dynamics in a single-digit nanopore with dimensions comparable to the Debye length deviate from the bulk values, but we still have critical knowledge gaps in our understanding of ion transport in nanoconfinement. For many energy devices and sensor designs of nanoporous materials, ion mobility is a key parameter for the performance of nanofluidic equipment. However, investigating ion mobility remains an experimental challenge. This study experimentally investigated the monovalent ion dynamics of single-digit nanopores from the perspective of ionic conductance. In this article, we present a theory that is sufficient for a basic understanding of ion transport through a single-digit nanopore, and we subdivided and separately analyzed the contribution of each conductance component. These conclusions will be useful not only in understanding the behavior of ion migration but also in the design of high-performance nanofluidic devices.

Probing Nanoconfined Ion Transport in Electrified 2D Laminate Membranes with Electrochemical Impedance Spectroscopy

The recent emergence of electrically conductive nanoporous membranes based on graphene and other 2D materials opens up new opportunities to revisit some longstanding nanoconfined ion transport problems under electrification. This work probes the ionic resistance in electrified multilayered graphene membranes with electrochemical impedance spectroscopy. This study demonstrates that the combination of additive-free feature and tunable slit pore sizes in the sub-10 nm range in graphene-based membranes has made it possible to deconvolute the different ionic processes from the impedance obtained and examine the exclusive influence of pore size on the ionic resistance in a quantitative manner. The trends revealed for the ionic resistance at the pore entrance and inside the pores under severe nanoconfinement (<2 nm) are found to be generally consistent with the microscale theoretical simulations previously reported. It also allows a quantitative analysis of the relative effects of the external polarization potential and ion identity under nanoconfinement. The results suggest that the classic electrochemical impedance spectroscopy technique, when applied to appropriate nanoporous electrode materials, can provide rich information about nanoconfined ion transport phenomena under electrification for fundamental understanding and application development.

Electric Potential of Ions in Electrode Micropores Deduced from Calorimetry

The internal energy of capacitive porous carbon electrodes was determined experimentally as a function of applied potential in aqueous salt solutions. Both the electrical work and produced heat were measured. The potential dependence of the internal energy is explained in terms of two contributions, namely the field energy of a dielectric layer of water molecules at the surface and the potential energy of ions in the pores. The average electric potential of the ions is deduced, and its dependence on the type of salt suggests that the hydration strength limits how closely ions can approach the surface.

Understanding the Chemical Shifts of Aqueous Electrolyte Species Adsorbed in Carbon Nanopores

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. Nuclear magnetic spectroscopy is a very useful tool to characterize ion adsorption in such systems thanks to its nuclei specificity and the ability to distinguish between ions in the bulk and in pores. We use complementary methods (density functional theory, molecular dynamics simulations, and a mesoscopic model) to investigate the relative importance of various effects on the chemical shifts of adsorbed species: ring currents, ion organization in pores of various sizes, specific ion-carbon interactions, and hydration. We show that ring currents and ion organization are predominant for the determination of chemical shifts in the case of Li⁺ ions and hydrogen atoms of water. For the large Rb⁺ and Cs⁺ ions, the additional effect of the hydration shell should be considered to predict chemical shifts in agreement with experiments.

Nanoscale Pore-Pore Coupling Effect on Ion Transport through Ordered Porous Monolayers

Distinct from the conventional view that nanopores are considered independent channels for mass transport, recent study on the covalent organic framework (COF)-based monolayers characteristic of an ordered nanopore array exhibits a series of interesting properties originating from the strong interactions between adjacent pores. These interactions are determined to be highly dependent on interpore distance and pose a significant influence on the ion transport, accounting for the exceptional membrane performance including both selectivity and conductance. In this Perspective, we discuss the recently discovered nanoscale pore-pore coupling as well as the exciting features of porous nanostructures. We also look at the challenges and future opportunities of ion transport in ordered porous monolayers in the aspects of both fundamental research and practical use.

Nanoconfined Space: Revisiting the Charge Storage Mechanism of Electric Double Layer Capacitors

The electric double layer capacitor (EDLC) has been recognized as one of the most appealing electrochemical energy storage devices. Nanoporous materials with relatively high specific surface areas are generally used as the electrode materials for electric double layer capacitors (EDLCs). The past decades have witnessed anomalous phenomena of EDLCs under nanoconfined space, which to a large degree doubt the conventional recognition. However, there are currently still no deep insights and consensus on the mechanism of these striking discoveries. In this Perspective, we start with a brief introduction to contextualize the significance of EDLCs, especially with electrode materials of nanoconfined space. Next, we briefly review the landmark studies in light of the charge storage mechanism of EDLCs, mainly focusing on the study of nanoporous materials for EDLCs. Subsequently, we reexamine the basic concepts under nanoconfined space and some representative in situ characterization techniques applied to understand the charge storage mechanism of EDLCs. Finally, we provide general conclusions and insights into the future research directions in the field of EDLCs.

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.

Reduced Ionic Conductivity but Enhanced Local Ionic Conductivity in Nanochannels

The ionic transport in nanoscale channels with the critical size comparable to ions and solvents shows excellent performance on electrochemical desalination, ion separation, and supercapacitors. However, the key quantity ionic conductivity (σ) in the nanochannel that evaluates how easily the electric current is driven by an external voltage is still unknown because of the challenges in experimental measurement. In this work, we present an atomistic simulation-based study, which shows that how the ion concentration, nanoconfinement, and heterogeneous solvation modify the ionic conductivity in a two-dimensional graphene nanochannel. We find that σ in the confined channel is lower than that in the bulk (σ_b) at the same concentration along with enhanced ion-ion correlation. However, surprisingly, the local σ near the channel wall is more conductive than σ_b and is about 2-3 folds of the inner layer due to the highly concentrated charge carriers. Based on the layered feature of σ along the width of the channel, we propose a model that contains two dead (or depletion) layers, two highly conductive layers, and one inner layer to describe the ionic dynamics in the nanochannels. Our findings may open the way to unique nanofluidic functionalities, such as energy harvesting/storage and controlling transport at single-molecule and ion levels using the liquid layer near the wall.

Image-charge effects on ion adsorption near aqueous interfaces

Electrostatic interactions near surfaces and interfaces are ubiquitous in many fields of science. Continuum electrostatics predicts that ions will be attracted to conducting electrodes but repelled by surfaces with lower dielectric constant than the solvent. However, several recent studies found that certain "chaotropic" ions have similar adsorption behavior at air/water and graphene/water interfaces. Here we systematically study the effect of polarization of the surface, the solvent, and solutes on the adsorption of ions onto the electrode surfaces using molecular dynamics simulation. An efficient method is developed to treat an electrolyte system between two parallel conducting surfaces by exploiting the mirror-expanded symmetry of the exact image-charge solution. With neutral surfaces, the image interactions induced by the solvent dipoles and ions largely cancel each other, resulting in no significant net differences in the ion adsorption profile regardless of the surface polarity. Under an external electric field, the adsorption of ions is strongly affected by the surface polarization, such that the charge separation across the electrolyte and the capacitance of the cell is greatly enhanced with a conducting surface over a low-dielectric-constant surface. While the extent of ion adsorption is highly dependent on the electrolyte model (the polarizability of solvent and solutes, as well as the van der Waals radii), we find the effect of surface polarization on ion adsorption is consistent throughout different electrolyte models.

Atomic-scale ion transistor with ultrahigh diffusivity

Biological ion channels rapidly and selectively gate ion transport through atomic-scale filters to maintain vital life functions. We report an atomic-scale ion transistor exhibiting ultrafast and highly selective ion transport controlled by electrical gating in graphene channels around 3 angstroms in height, made from a single flake of reduced graphene oxide. The ion diffusion coefficient reaches two orders of magnitude higher than the coefficient in bulk water. Atomic-scale ion transport shows a threshold behavior due to the critical energy barrier for hydrated ion insertion. Our in situ optical measurements suggest that ultrafast ion transport likely originates from highly dense packing of ions and their concerted movement inside the graphene channels.

Uncovering a Universal Molecular Mechanism of Salt Ion Adsorption at Solid/Water Interfaces

Solid/water interfaces, in which salt ions come in close proximity to solids, are ubiquitous in nature. Because water is a polar solvent and salt ions are charged, a long-standing puzzle involving solid/water interfaces is how do the electric fields exerted by the salt ions and the interfacial water molecules polarize the charge distribution in the solid and how does this polarization, in turn, influence ion adsorption at any solid/water interface. Here, using state-of-the-art polarizable force fields derived from quantum chemical simulations, we perform all-atomistic molecular dynamics simulations to investigate the adsorption of various ions comprising the well-known Hofmeister series at the graphene/water interface, including comparing with available experimental data. Our findings reveal that, in vacuum, the ionic electric field-induced polarization of graphene results in a significantly large graphene-ion polarization energy, which drives all salt ions to adsorb to graphene. On the contrary, in the presence of water molecules, we show that the ions and the water molecules exert waves of molecular electric fields on graphene which destructively interfere with each other. This remarkable phenomenon is shown to cause a water-mediated screening of more than 85% of the graphene-ion polarization energy. Finally, by investigating superhydrophilic and superhydrophobic model surfaces, we demonstrate that this phenomenon occurs universally at all solid/water interfaces and results in a significant weakening of the ion-solid interactions, such that ion specific effects are governed primarily by a competition between the ion-water and water-water interactions, irrespective of the nature of the solid/water interface.

Electric control of ionic transport in sub-nm nanopores

The ion transport behavior through sub-nm nanopores (length (L) ≈ radius (R)) on a film is different from that in nanochannels (L ≫ R), and even more different from the bulk behavior. The many intriguing phenomena in ionic transport are the key to the design and fabrication of solid-state nanofluidic devices. However, ion transport through sub-nm nanopores is not yet clearly understood. We investigate the ionic transport behavior of sub-nm nanopores from the perspective of conductance via molecular dynamics (MD) and experimental methods. Under the action of surface charge, the average ion concentration inside the nanopore is much higher than the bulk value. It is found that 100 mM is the transition point between the surface-charge-governed and the bulk behavior regimes, which is different from the transition point for nanochannels (10 mM). Moreover, by investigating the access, pores, surface charge, electroosmosis and potential leakage conductance, it is found that the conductive properties of the nanopore at low bulk concentration are determined by the surface charge potential leaks into the reservoir. Specifically, there is a huge increase in cation mobility through a cylindrical nanopore, which implies potential applications for the fast charging of supercapacitors and batteries. Sub-nm nanopores also show a strong selectivity toward Na⁺, and a strong repellence toward Cl⁻. These conclusions presented here will be useful not only in understanding the behavior of ion transport, but also in the design of nanofluidic devices.

The ion transport behavior through sub-nm nanopores (length (L) ≈ radius (R)) on a film is different from that in nanochannels (L ≫ R), and even more different from the bulk behavior.

Electrolytes in regimes of strong confinement: surface charge modulations, osmotic equilibrium and electroneutrality

In the present work, we study an electrolyte solution confined between planar surfaces with nanopatterned charged domains, which has been connected to a bulk ionic reservoir. The system is investigated through an improved Monte Carlo (MC) simulation method, suitable for simulation of electrolytes in the presence of modulated surface charge distributions. We also employ a linear approach in the spirit of the classical Debye-Hückel approximation, which allows one to obtain explicit expressions for the averaged potentials, ionic profiles, effective surface interactions and the net ionic charge confined between the walls. Emphasis is placed on the limit of strongly confined electrolytes, in which case local electroneutrality in the inter-surface space might not be fulfilled. In order to access the effects of such a lack of local charge neutrality on the ion-induced interactions between surfaces with modulated charge domains, we consider two distinct model systems for the confined electrolyte: one in which a salt reservoir is explicitly taken into account via the osmotic equilibrium with an electrolyte of fixed bulk concentration, and a second one in which the equilibrium with a charge neutral ionic reservoir is implicitly considered. While in the former case the osmotic ion exchange might lead to non-vanishing net charges, in the latter model charge neutrality is enforced through the appearance of an implicit Donnan potential across the charged interfaces. A strong dependence of the ion-induced surface interactions on the employed model system is observed at all surface separations. These findings strongly suggest that due care is to be taken while choosing among different scenarios to describe the ion exchange in electrolytes confined between charged surfaces, even in cases when the monopole (non zero net charge) surface contributions are absent.

Nanoconfinement-Enforced Ion Correlation and Nanofluidic Ion Machinery

Machines operating at the atomic level are of fundamental interests for information manipulation and communication. However, preparation of thermodynamically stable states and regulation of transitions between them at a low energy cost are challenging. We report that, by enforcing nanoconfinement and surface gating, one can control the configurations and dynamics of ions for computational tasks. The layered structures of water confined in nanochannels render the spatial and temporal correlation between ions, offering a number of distinct states with paired configurations. Free energy barriers for transitions between them are on the order of k_BT, allowing modulation through external fields or surface charges at a low energy cost. Ionic switches, rectifiers, and logical gates are constructed following the physical rules elucidated at the molecular level, opening an avenue toward artificial nanofluidic functionalities such as efficient ionic machinery by configuring the ionic pairs and controlled mass/charge transport by tuning the strength of correlation.

Breakdown of electroneutrality in nanopores

Ion transport in extremely narrow nanochannels has gained increasing interest in recent years due to unique physical properties at the nanoscale and the technological advances that allow us to study them. It is tempting to approach this confined regime with the theoretical tools and knowledge developed for membranes and microfluidic devices, and naively apply continuum models, such as the Poisson-Nernst-Planck and Navier-Stokes equations. However, it turns out that some of the most basic principles we take for granted in larger systems, such as the complete screening of surface charge by counter-ions, can break down under extreme confinement. We show that in a truly one-dimensional system of ions interacting with three-dimensional electrostatic interactions, the screening length is exponentially large, and can easily exceed the macroscopic length of a nanotube. Without screening, electroneutrality breaks down within the nanotube, with fundamental consequences for ion transport and electrokinetic phenomena. In this work, we build a general theoretical framework for electroneutrality breakdown in nanopores, focusing on the most interesting case of a one-dimensional nanotube, and show how it provides an elegant interpretation for the peculiar scaling observed in experimental measurements of ionic conductance in carbon nanotubes.

Counteranions in the Stimulation Solution Alter the Dynamics of Exocytosis Consistent with the Hofmeister Series

We show that the Hofmeister series of ions can be used to explain the cellular changes in exocytosis observed by single-cell amperometry for different counteranions. The formation, expansion, and closing of the membrane fusion pore during exocytosis was found to be strongly dependent on the counteranion species in solution. With stimulation of chaotropic anions (e.g., ClO₄^–), the expansion and closing time of the fusion pore are longer, suggesting chaotropes can extend the duration of exocytosis compared with kosmotropic anions (e.g., Cl^–). At a concentration of 30 mM, the two parameters (e.g., t_1/2 and t_fall) that define the duration of exocytosis vary with the Hofmeister series (Cl^– < Br^– < NO₃^– ≤ ClO₄^– < SCN^–). More interestingly, fewer (e.g., N_foot/N_events) and smaller (e.g., I_foot) prespike events are observed when chaotropes are counterions in the stimulation solution, and the values can be sorted by the reverse Hofmeister series (Cl^– ≥ Br^– > NO₃^– > ClO₄^– > SCN^–). Based on ion specificity, an adsorption-repulsion mechanism, we suggest that the exocytotic Hofmeister series effect originates from a looser swelling lipid bilayer structure due to the adsorption and electrostatic repulsion of chaotropes on the hydrophobic portion of the membrane. Our results provide a chemical link between the Hofmeister series and the cellular process of neurotransmitter release via exocytosis and provide a better physical framework to understand this important phenomenon.

Solvation-Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

Nanoporous laminar membranes composed of multilayered 2D nanomaterials (2D-NLMs) are increasingly being exploited as a unique material platform for understanding solvated ion transport under nanoconfinement and exploring novel nanoionics-related applications, such as ion sieving, energy storage and harvesting, and in other new ionic devices. Here, the fundamentals of solvation-involved nanoionics in terms of ionic interactions and their effect on ionic transport behaviors are discussed. This is followed by a summary of key requirements for materials that are being used for solvation-involved nanoionics research, culminating in a demonstration of unique features of 2D-NLMs. Selected examples of using 2D-NLMs to address the key scientific problems related to nanoconfined ion transport and storage are then presented to demonstrate their enormous potential and capabilities for nanoionics research and applications. To conclude, a personal perspective on the challenges and opportunities in this emerging field is presented.

Electrochemical impedance of electrodiffusion in charged medium under dc bias

An immobile charged species provides a charged medium for transport of charge carriers that is exploited in many applications, such as permselective membranes, doped semiconductors, biological ion channels, as well as porous media and microchannels with surface charges. In this paper, we theoretically study the electrochemical impedance of electrodiffusion in a charged medium by employing the Nernst-Planck equation and the electroneutrality condition with a background charge density. The impedance response is obtained under different dc bias conditions extending above the diffusion-limiting bias. We find a transition in the impedance behavior around the diffusion-limiting bias and present an analytical approximation for a weakly charged medium under an overlimiting bias.

In Situ Magnetic Resonance Imaging of a Complete Supercapacitor Giving Additional Insight on the Role of Nanopores

Nuclear magnetic resonance is one of the rare techniques able to probe selectively the ions inside the nanoporous network in supercapacitor devices. With a magnetic resonance imaging method able to detect all ions (adsorbed and nonadsorbed), we record one-dimensional concentration profiles of the active ions in supercapacitors with an electrode configuration close to that used in industry. Larger anionic concentration changes are probed upon charge and discharge in a carbide-derived carbon (CDC) with micropores smaller than 1 nm compared to a conventional nanoporous carbon (CC) with a larger distribution of pore sizes, up to 2 nm. They highlight the increased interaction of the anions with CDC and provide a better understanding of the enhanced capacitance in CDC-based supercapacitors.

Dynamic Adsorption of Ions into Like-Charged Nanospace: A Dynamic Density Functional Theory Study

The adsorption processes of ions into charged nanospace are associated with many practical applications. Whereas a large number of microporous materials have been prepared toward efficient adsorption of ions from solutions, theoretical models that allow for capturing the characteristics of ion dynamic adsorption into like-charged nanopores are still few. The difficulty originates from the overlapping of electric potentials inside the pores. Herein, a theoretical model is proposed by incorporating dynamic density functional theory with modified Poisson equation for investigating the dynamic adsorption of ions into like-charged nanoslits. This model is rationalized by comparing the theoretical predictions with corresponding simulation results. Afterward, by analyzing the adsorption dynamics, we show that the overlapping effect is associated with the pore size, ion bulk concentration, and surface charge density, and it plays a dominant role in the coupling between the total adsorption amount of ions and total adsorption time. Specifically, with weak overlapping effect, the total adsorption amount is intuitively proportional to the total adsorption time; however, when the overlapping effect is strong, the total adsorption amount may be inversely proportional to the total adsorption time, indicating that both high adsorption amount and short adsorption time can be achieved simultaneously. This work provides a meaningful insight toward the rational design and optimization of microporous materials for efficient ion adsorption.

Specific ion effects on electrocapillarity in aqueous electrolytes confined within nanochannels

A nanoslit is a long, extremely narrow (nanometers apart) opening between two parallel plates. An overlapped electric double layer is formed when an electrolyte is present inside the slit and there exist distributions of the osmotic pressure and the Maxwell stress across the nanoslit, which lead to the electrocapillarity effect. This feature can be incorporated with the specific ion effects by considering the nonelectrostatic interactions between ions and confining walls, as they significantly influence the potential, electric field, and ion distributions across the nanoslit. In the present work, the electromechanical approach is integrated with the concept of specific ion effects to analyze the behavior of an electrolyte confined in a one-dimensional nanochannel. For a nanochannel, the average outward normal stress exerted on the cross section of a channel (P_{zz}[over ¯]) can be regarded as a measure of electrocapillarity and it is the driving force of the flow. This electrocapillarity measure is analyzed by using the solution of the modified Poisson-Boltzmann equation as a function of the bulk concentration of the electrolyte, the boundary potential, and most importantly, the ion-specific interfacial interactions. The significance of the present work can be manifested by the increasing usage of extremely narrow channels in nanoscaled systems, which will require proper consideration of specific ion effects in determining the behavior of the confined electrolyte.

Surface engineering in PbS via partial oxidation: towards an advanced electrocatalyst for reduction of levulinic acid to γ-valerolactone

Development of mild and efficient strategies for biomass conversion is of great significance, and design of advanced catalysts is crucial for biomass valorization. Herein, we designed PbS-based electrocatalysts through a surface engineering strategy via partial oxidation, and the degree of surface oxidation of PbS to PbSO₄ could be easily tuned by calcination temperature. It was discovered that the prepared electrocatalysts could efficiently catalyze reduction of biomass-derived levulinic acid (LA) to γ-valerolactone (GVL) using water as the hydrogen source. Especially, the electrocatalyst calcined at 400 °C (PbS-400) showed outstanding performance with a current density of 13.5 mA cm^-2 and a GVL faradaic efficiency of 78.6%, which was far higher than the best results reported up to date. Moreover, GVL was the only product from LA reduction, indicating the excellent selectivity. Mechanism investigation showed that LA was converted through electrocatalytic hydrogenation of carbonyl groups of LA and subsequent intramolecular esterification.

Scaling Behavior of Ionic Transport in Membrane Nanochannels

Ionic conductance in membrane channels exhibits a power-law dependence on electrolyte concentration ( G ∼ c^α). The many scaling exponents, α, reported in the literature usually require detailed interpretations concerning each particular system under study. Here, we critically evaluate the predictive power of scaling exponents by analyzing conductance measurements in four biological channels with contrasting architectures. We show that scaling behavior depends on several interconnected effects whose contributions change with concentration so that the use of oversimplified models missing critical factors could be misleading. In fact, the presence of interfacial effects could give rise to an apparent universal scaling that hides the channel distinctive features. We complement our study with 3D structure-based Poisson-Nernst-Planck (PNP) calculations, giving results in line with experiments and validating scaling arguments. Our findings not only provide a unified framework for the study of ion transport in confined geometries but also highlight that scaling arguments are powerful and simple tools with which to offer a comprehensive perspective of complex systems, especially those in which the actual structure is unknown.

Simulations of Coulomb systems confined by polarizable surfaces using periodic Green functions

We present an efficient approach for simulating Coulomb systems confined by planar polarizable surfaces. The method is based on the solution of the Poisson equation using periodic Green functions. It is shown that the electrostatic energy arising from the surface polarization can be decoupled from the energy due to the direct Coulomb interaction between the ions. This allows us to combine an efficient Ewald summation method, or any other fast method for summing over the replicas, with the polarization contribution calculated using Green function techniques. We apply the method to calculate density profiles of ions confined between the charged dielectric and metal surfaces.

Selective observation of charge storing ions in supercapacitor electrode materials

Nuclear magnetic resonance (NMR) spectroscopy has emerged as a useful technique for probing the structure and dynamics of the electrode-electrolyte interface in supercapacitors, as ions inside the pores of the carbon electrodes can be studied separately from bulk electrolyte. However, in some cases spectral resolution can limit the information that can be obtained. In this study we address this issue by showing how cross polarisation (CP) NMR experiments can be used to selectively observe the in-pore ions in supercapacitor electrode materials. We do this by transferring magnetisation from ¹³C nuclei in porous carbons to nearby nuclei in the cations (¹H) or anions (¹⁹F) of an ionic liquid. Two-dimensional NMR experiments and CP kinetics measurements confirm that in-pore ions are located within Ångströms of sp²-hybridised carbon surfaces. Multinuclear NMR experiments hold promise for future NMR studies of supercapacitor systems where spectral resolution is limited.

Electrodiffusion phenomena in neuroscience: a neglected companion

The emerging technological revolution in genetically encoded molecular sensors and super-resolution imaging provides neuroscientists with a pass to the real-time nano-world. On this small scale, however, classical principles of electrophysiology do not always apply. This is in large part because the nanoscopic heterogeneities in ionic concentrations and the local electric fields associated with individual ions and their movement can no longer be ignored. Here, we review basic principles of molecular electrodiffusion in the cellular environment of organized brain tissue. We argue that accurate interpretation of physiological observations on the nanoscale requires a better understanding of the underlying electrodiffusion phenomena.