Ion Transport Phenomena Research Articles

Light-driven directional ion transport for enhanced osmotic energy harvesting.

Light-driven ion (proton) transport is a crucial process both for photosynthesis of green plants and solar energy harvesting of some archaea. Here, we describe use of a TiO2/C3N4 semiconductor heterojunction nanotube membrane to realize similar light-driven directional ion transport performance to that of biological systems. This heterojunction system can be fabricated by two simple deposition steps. Under unilateral illumination, the TiO2/C3N4 heterojunction nanotube membrane can generate a photocurrent of about 9 μA/cm2, corresponding to a pumping stream of ∼5500 ions per second per nanotube. By changing the position of TiO2 and C3N4, a reverse equivalent ionic current can also be realized. Directional transport of photogenerated electrons and holes results in a transmembrane potential, which is the basis of the light-driven ion transport phenomenon. As a proof of concept, we also show that this system can be used for enhanced osmotic energy generation. The artificial light-driven ion transport system proposed here offers a further step forward on the roadmap for development of ionic photoelectric conversion and integration into other applications, for example water desalination.

Nonlinear Ion Transport through Ultrathin Metal–Organic Framework Nanosheet

AbstractThe rational design of artificial solid‐state nanopores is of great importance in the discovery of intriguing ion transport phenomena. 2D metal–organicframework (2D MOF) nanosheets with single crystallinity, aligned nanochannels, ultrathin thickness, and diverse functionalities are highly potential solid‐state nanopores. An electrophoretic method is developed to successfully fabricate MOF nanopores supported by SiNx substrate, which is confirmed by high‐resolution transmission electron microscopy. A giant gap around 4 V together with ionic current rectification is discovered in nonlinear voltage‐activated current‐voltage curves, revealing the synergy of the hydrophobic effect and charge effect in MOF nanopores. The charge effect embodies the different contribution current which results from the enrichment and depletion of ions in MOF nanopores by COMSOL simulation. Moreover, 2D MOF nanosheets with different surface charges, hydrophobicity, and pore sizes demonstrate the universality of nanopore fabrication and further confirm the synergistic mechanism. The nonlinear ion transport in the ultrathin MOF nanosheets will provide an opportunity to explore further applications in solid‐state nanopores.

Ionic Conductivity in Copper Selenides

For the first time, the results of investigations of ion transport phenomena in high-temperature modifications of single-crystalline copper selenide are presented depending on the temperature and the composition within the homogeneity region as well as the effect of the degree of perfection of single-crystalline samples on the ion transfer. It is found that the activation energy estimated from the temperature dependences of the ionic conductivity for perfect single crystals is significantly higher than in polycrystalline samples and increases with a deviation from stoichiometry. For single crystals, lower values of ionic conductivity are observed. The ionic conductivity in the temperature range of 573–723 K is 1.5–2 times higher for polycrystalline samples than for single crystals.

Ultrashort nanopores of large radius can generate anomalously high salinity gradient power

Using ultrashort nanopores in salinity gradient power seems promising, however, their poor ion selectivity is disadvantageous. We show that this difficulty can be circumvented by adopting a two-dimensional material having a high surface charge density. Through raising the surface charge density, both the electric power and the transference number can both be enhanced effectively. For example, for a cylindrical nanopore having length 2 nm, radius 2 nm and surface charge density −1000 mC/m2, the transference number can approach ca. 0.97 and the electric power ca. 200 pW if the salt concentration ratio across the nanopore is (1000/1), much higher than previous reported values of 3.13 pW in similar systems having longer pores (∼1000 nm) and lower surface charge density (∼-60 mC/m2). The underlying mechanisms of the present novel salinity gradient power system are investigated in detail for the first time. In particular, the profiles for the concentration of ions and its flux inside the nanopore are examined to explain the ion transport phenomena observed. Anomalously, if the surface charge density is sufficiently high (e.g., −1000 mC/m2), a nanopore of radius as large as 50 nm can still generate appreciable electric power (ca. 45 pW).

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.

Microstructure and Transport Phenomena

Lithium ion mobility is one of the key properties that all battery materials need to exhibit for a Li-ion battery that functions reversibly and effectively. Within the battery, active materials are responsible for reversible intercalation, that requires the transport of Li-ions within the solid lattice. Outside the active material, the electrolyte shuttles Li-ions between the two electrodes. In between those materials, ion transport phenomena at the interfaces has been reported as having potentially significant impact on battery performance.We are here reporting our efforts in tracking Li+ transport in LiFePO4 and solid electrolyte composites. Having found a significant role of microstructure in ionic transport, this presentation is further dedicated to our recent exploration of microstructure and improved electrode interfaces. Using active microscopy techniques, new methods are being developed in the imaging and quantification of microstructural properties. These techniques are applied to the comparison of a new conductive binder system to the traditional PVDF/C electrode matrix. These studies are contributing to the underexplored issue of microstructural effects on battery performance, and highlight alternatives to composite electrode composition and microstructure control.

Ion current rectification in combination with ion current saturation

Over the decades, nanochannels have been widely used for single molecule detection, smart sensors, and energy transfer and storage based on its unique ion transport properties. Although various ion transport phenomena of nanochannels have been reported, the discovery of new ion transport phenomena is still of great significance for understanding material transport of nanochannels and development of nanodevices with unique working capabilities. This article reports a novel nanochannel ion transport phenomenon - ion current rectification in combination with ion current saturation (ICR-S), which arised from a mesoporous titania conical microplug generated in situ in the glass micropipette tip cavity by space confinement evaporation. The ion current of forward voltage is greater than that of reverse voltage, and the saturation currents appear in both the forward and reverse voltages, the ratio of forward and reverse saturation current can reaches to 10. In addition, the influence of pH, ionic strength, and micropipette angle on ICR-S is also investigated.

A single-ion conducting covalent organic framework for aqueous rechargeable Zn-ion batteries.

Despite their potential as promising alternatives to current state-of-the-art lithium-ion batteries, aqueous rechargeable Zn-ion batteries are still far away from practical applications. Here, we present a new class of single-ion conducting electrolytes based on a zinc sulfonated covalent organic framework (TpPa-SO3Zn0.5) to address this challenging issue. TpPa-SO3Zn0.5 is synthesised to exhibit single Zn2+ conduction behaviour via its delocalised sulfonates that are covalently tethered to directional pores and achieve structural robustness by its β-ketoenamine linkages. Driven by these structural and physicochemical features, TpPa-SO3Zn0.5 improves the redox reliability of the Zn metal anode and acts as an ionomeric buffer layer for stabilising the MnO2 cathode. Such improvements in the TpPa-SO3Zn0.5–electrode interfaces, along with the ion transport phenomena, enable aqueous Zn–MnO2 batteries to exhibit long-term cyclability, demonstrating the viability of COF-mediated electrolytes for Zn-ion batteries.

Light-Powered Directional Nanofluidic Ion Transport in Kirigami-Made Asymmetric Photonic-Ionic Devices.

Nacre-mimetic 2D nanofluidic materials with densely packed sub-nanometer-height lamellar channels find widespread applications in water-, energy-, and environment-related aspects by virtue of their scalable fabrication methods and exceptional transport properties. Recently, light-powered nanofluidic ion transport in synthetic materials gained considerable attention for its remote, noninvasive, and active control of the membrane transport property using the energy of light. Toward practical application, a critical challenge is to overcome the dependence on inhomogeneous or site-specific light illumination. Here, asymmetric photonic-ionic devices based on kirigami-tailored graphene oxide paper are fabricated, and directional nanofluidic ion transport properties therein powered by full-area light illumination are demonstrated. The in-plane asymmetry of the graphene oxide paper is essential to the generation of photoelectric driving force under homogeneous illumination. This light-powered ion transport phenomenon is explained based on a modified carrier diffusion model. In asymmetric nanofluidic structures, enhanced recombination of photoexcited charge carriers at the membrane boundary breaks the electric potential balance in the horizontal direction, and thus drives the ion transport in that direction under symmetric illumination. The kirigami-based strategy provides a facile and scalable way to fabricate paper-like photonic-ionic devices with arbitrary shapes, working as fundamental elements for large-scale light-harvesting nanofluidic circuits.

Asymmetry Switching Behavior of the Binary Memristor

The physical model of binary oxide memristor is developed using the Finite Element Method (FEM). Ion transport, through the device, is modeled according to the non-linear ionic transport phenomenon in nano-scale devices for high electric field. The main physical mechanisms incorporated in memristor operation are considered using the simulations obtained from the FEM model. Using this model, the asymmetry between set and reset switching times of memristor is discussed. Finally, the effects of temperature and device geometry on the switching behavior of memristor are investigated.

Ion transport in backbone-embedded polymerized ionic liquids.

We use atomistic computer simulations to examine ion-transport phenomena for backbone polymerized cationic liquids with bistrifluoromethylesulfonylimide (TFSI-) counterions. We consider a system in which the polymerized cation moiety is the imidazolium ring and study the structural characteristics and ion mobilities for cases in which the cations are separated by four, six, and eight methylene units on the backbone. A pendant polymerized ionic liquid, 1-butyl-3-vinylimidazolium, is compared to the backbone series across ion coordination and hopping features. The anion diffusivity in backbone polymerized cationic liquids is found to decrease with increasing spacer length, which is shown to result from a decrease in intramolecular and intermolecular hopping frequencies due to an increasing distance separating imidazolium moieties. In comparison with pendant polymerized ionic liquids, we observe that the participation rates of intermolecular hopping events in the backbone polymers far exceed that of the pendant, and the intrapolymeric ionic coordination profile shows the TFSI- of the pendant polymer with a high propensity for coordination by multiple imidazolium, compared with one monomer from a given polymer for the backbone series. Despite these differences, backbone polymerized ionic liquids are seen to possess correlated diffusivity and ion-association relaxation times, in a manner similar to the results observed in past studies for pendant variants.

Single Lithium-Ion Conducting Covalent Organic Frameworks

The demands for high-energy density power sources are growing from the beginning on the advent of portable devices. Since the beginning, liquid electrolyte was adopted owing to their high ionic conductivity, excellent wetting to electrode components. However, liquid electrolyte contains risks since it exhibits highly flammable and sometimes explosive characteristics. To resolve safety issue for demanded high-energy density batteries, all-solid-state battery, which utilizes solid electrolyte instead of liquid electrolyte, hold particular promise as solid-state electrolytes feature low flammability and highly compaction nature for high-energy density battery performance.So far, all-solid-state batteries have generally been powered using inorganic sulfides/oxides or polymer-based electrolyte. In addition to these conventional approaches, solid-state Li-ion conductors based on porous crystalline materials (i. e. covalent organic frameworks (COFs) and metal–organic frameworks (MOFs)) have garnered attention due to their directional ion conduction pathways through the ordered pores and versatile structural design. Porous crystalline materials, however, additionally incorporate lithium salts and/or solvents inside the pores of frameworks so that these COFs or MOFs act as conducting medium, not solid-state ion conductors in specific. Herein, we demonstrate a lithium sulfonated covalent organic framework (denoted as TpPa-SO3Li) as a solvent-free, single lithium-ion conductors. Benefiting from the well-designed directional ion channels, high number density of lithium-ions, and covalently tethered anion groups, TpPa-SO3Li exhibits an ionic conductivity of 2.7 × 10–5 S cm–1 with a lithium-ion transference number of 0.9 at room temperature, and an activation energy of 0.18 eV. Moreover, unusual ion transport phenomena of TpPa-SO3Li allow reversible and stable lithium plating/stripping on lithium metal electrodes. These features show promising results to be adopted in next-generation all-solid-state batteries and also demonstrate potential usage for lithium metal batteries.

Novel L AGP − PEI Composite Electrolyte

Solid electrolytes (SE) have major advantages as an alternative to liquid electrolytes, in terms of reliability, safety, and easy design. SEs are generally defined as electronically insulating solid materials with high mobility and selective transport of charged ionic species within their structure. In general, they can be divided into two main categories: polymeric and inorganic. In this study we focus on the development of a novel composite Li1.5Al0.5Ge1.5(PO4)3 – Polyethyleneimine (LAGP−PEI) polymer-in-ceramic electrolyte, which contains high concentrations of ionic charge carriers with a minimal polymer concentration. Commercial nanoparticles of lithium aluminum germanium phosphate (Li1.5Al0.5Ge1.5(PO4)3, LAGP) were used as a ceramic matrix. Polyethyleneimine (PEI) was tested as a binder and lithium-ion-conducting medium. The combination of nanosize LAGP glass ceramics and PEI polymer was chosen to provide mechanical flexibility with the benefit of the high ionic conductivity of LAGP. The composite electrolytes were prepared by electrophoretic deposition (EPD). TGA, DSC, XRD, ESEM, NMR and EIS tests were used for the characterization of the samples. The dielectric properties of pristine LAGP and LAGP-based quasi-solid electrolytes, containing different concentrations of LiTFSI-based Pyr14 ionic liquid ions, were studied with the use of the BDS 80 (NOVOCONTROL) with automatic temperature control by a QUATRO Cryosystem. The complex, non-Debye dielectric response of the composite electrolyte has been described in terms of several distributed relaxation processes separated by different frequency and temperature ranges. While at low temperatures, the main contribution is from LAGP, at the middle- and high-temperature regions, the superposition of a few non-Arrhenius processes is observed. 7Li diffusion of the composite electrolyte measured by NMR at 90°C was about 6±2 E-11 m2/s. The correlation of ion-transport phenomena with XRD, DSC and ESEM data will be presented in detail. Acknowledgements The research was funded by the Israel Ministry of Science and Technology, grant 53886 and by the Israel Science Foundation (INREP project).

Two-Dimensional Infrared Spectroscopy and Molecular Dynamics Simulation Studies of Nonaqueous Lithium Ion Battery Electrolytes.

Lithium ion battery (LIB) technology is undoubtedly indispensable to modern life. However, despite enormous and extended effort to improve LIB performance, our understanding of the underlying principles and mechanisms of lithium ion transport in nonaqueous LIB electrolytes remained limited until recently. There is a particular lack of knowledge of the microscopic solvation structures and fluctuation dynamics around charge carriers in real electrolytes. Typical electrolytes found in commercially available LIBs consist of lithium salts and mixed carbonate solvents, with the latter playing an essential role in promoting lithium ion transport and forming an electrically stable solid electrolyte interphase. Although a number of linear spectroscopic studies of LIB electrolytes aiming at understanding the complex nature of lithium ion solvation processes have been reported, the notion that each lithium ion is strongly solvated by carbonate molecules to form a long-lasting solvation sheath structure has remained the subject of intense debate. Here, we present the results of FTIR, fs IR pump-probe, two-dimensional IR spectroscopy, and molecular dynamics simulations reported by us and others and discuss the possible interplay of picosecond solvation dynamics and macroscopic ion transport processes within the framework of the fluctuation-dissipation relationship. Further, by measuring the time-dependent fluctuations and spectral diffusions of carbonate carbonyl stretch modes that act as excellent infrared probes for the local electrostatic environment, we show that lithium cations are not only solvated by carbonate molecules but also interact with counteranions at equilibrium depending on solvent composition. Molecular dynamics simulations support the notion that rapid chemical exchanges between carbonate solvent molecules in the first and outer solvation shells are critical for describing mobile lithium ion transport phenomena. We thus anticipate that time-resolved coherent multidimensional vibrational spectroscopy is capable of providing decisive evidence on the ultrafast solvent dynamics of various electrolytes, which is potentially helpful for designing improved and more efficient LIB electrolytes in the future.

Solvent-Free, Single Lithium-Ion Conducting Covalent Organic Frameworks

Porous crystalline materials such as covalent organic frameworks and metal-organic frameworks have garnered considerable attention as promising ion conducting media. However, most of them additionally incorporate lithium salts and/or solvents inside the pores of frameworks, thus failing to realize solid-state single lithium-ion conduction behavior. Herein, we demonstrate a lithium sulfonated covalent organic framework (denoted as TpPa-SO3Li) as a new class of solvent-free, single lithium-ion conductors. Benefiting from well-designed directional ion channels, a high number density of lithium-ions, and covalently tethered anion groups, TpPa-SO3Li exhibits an ionic conductivity of 2.7 × 10-5 S cm-1 with a lithium-ion transference number of 0.9 at room temperature and an activation energy of 0.18 eV without additionally incorporating lithium salts and organic solvents. Such unusual ion transport phenomena of TpPa-SO3Li allow reversible and stable lithium plating/stripping on lithium metal electrodes, demonstrating its potential use for lithium metal electrodes.

Stretchable and transparent ionic diode and logic gates

Inspired by the asymmetric ion transport phenomenon commonly seen in biology, in this article, we develop transparent and stretchable hydrogel-based ionic diode and logic gates. The ionic diode is made up of polyacrylamide hydrogel doped by two different polyelectrolytes with opposite fixed charges. Due to the asymmetric distribution of the fixed charges, the hydrogel diode shows ion rectification capability. Our experiments further show that large rectification ratio of the diode can be maintained when the diode is under moderate stretch. Using the ionic diode, we further successfully construct two types of ionic logic gates: OR gate and AND gate, which can perform logic operations on binary inputs and produce a single output. We hope the transparent and stretchable ionic diode, as well as the logic gates, can be used as the basic functional elements for advanced and integrated stretchable ionic circuit system.

Tuning Ion Transport through a Nanopore by Self-Oscillating Chemical Reactions.

Ion transport in nanofluidic devices and biological ion channels are highly dependent on the local environmental conditions in the electrolyte solution. Many life processes in living systems are in dynamic electrolyte solutions, and many of them are self-oscillated. Tuning ion transport through a nanofluidic diode by the self-oscillating chemical reactions is demonstrated by modeling the electrokinetic ion transport process with a validated continuum model, which includes the time-dependent Poisson-Nernst-Planck equations for the ionic mass transport of multiple ionic species with both volumetric and surface chemical reactions, and Stokes equations for the flow field. A pH oscillator caused by oscillating chemical reactions (i.e., bromate-sulfite-ferrocyanide system) is added at the tip side of the nanopore to periodically change its surface charge properties, consequently tuning the ion selectivity and ion transport through the nanopore. Results show that both the surface charge density of the nanopore and the electrokinetic ion transport phenomena oscillate simultaneously with the pH oscillation generated by the self-oscillating chemical reactions. The numerical results obtained by our model qualitatively agree with the published experimental observations.

Electrokinetically Controlled Asymmetric Ion Transport through 1D/2D Nanofluidic Heterojunctions

AbstractSupported 2D layered materials are widely used in gas separation, water treatment, energy conversion, etc. In conventional viewpoint, the atop 2D membrane functions as an active layer for mass or charge separation in vapor or liquid phase, while the substrate membrane, containing aligned or tortuous 1D micro or nanoscaled fluidic channels, functions as a mechanical support. However, asymmetric transport property induced by the mixed‐dimensional composite structure is an equally very important, yet long‐overlooked element that endows new features to the membrane‐scale nanofluidic systems and contributes to the promotion of overall performance. Here, reported are asymmetric ion transport properties through 1D/2D nanofluidic heterojunction membrane (NFHM) under three different types of electrokinetic driving force. The 1D/2D NFHM comprises of self‐assembled graphene oxide multi‐layers (2DM, negatively charged) supported on a polydopamine‐coated 1D nanopore array (1DM, ampholytic). Intriguingly, it is found that the preferential direction for electric‐field‐driven ionic transport is from the 2DM to the 1DM, while under the concentration difference or the hydraulic flow, the preferred direction for diffusion or streaming ionic current goes in the reversed direction, from the 1DM to the 2DM. A theoretical model based on coupled Poisson–Nernst–Planck and Navier–Stokes equations is employed to explain the asymmetric ion transport phenomena.

Fabrication of high performance low temperature solid oxide fuel cell based on La0.10Sr0.90Co0.20Zn0.80O5-δ cathode

Nanostructure cathode material with composition of La0.10Sr0.90Co0.20Zn0.80O5-δ (LSCZO) was synthesized by wet chemical route. The crystallite size was found 47.55 nm and porosity was observed in the structure. The oxygen ion and electron transport phenomena were studied from anode to cathode and vice versa by electrical conductivity and electrochemical performance studies. The conductivity measurements were carried and found 20.29 and 4.87 S/cm2 in air and hydrogen atmosphere, respectively. The area specific resistance (ASR) was observed 0.07 Ω/cm2 at 550 °C. Arrhenius plots were drawn to calculate activation energy and found to be 0.09 eV. Excellent performance of 850 mW/cm2 has been recorded at 550 °C.

Spatial averaging of electrostatic potential differences in layered nanostructures with ionic hopping conductivity

Laws of spatial averaging on nanoscale for local electrostatic potential differences in ionic conductors are of key importance for the understanding of ion-transport phenomena and effects in space-charge layers. Currently, these laws are not investigated. As a consequence, the leading software packages for nanoscience do not have any competence for modeling of space-charge formation/relaxation in nanostructures with ionic hopping conductivity. So the article addresses the theme. For a layered nanostructure {Xk}, formed by parallel crystallographic Xk planes, an average electrostatic potential difference (<Vk,k+1>) in a non-uniform field is defined in terms of normalized sums Σ of the local works Wk,k+1 for Xk→Xk+1 transitions of mobile ions in a field of a single point charge. At a weak external electrical influence on {Xk}, the <Vk,k+1> can be easily generalized for a non-uniform field created by a set of point charges. The sums ΣWk,k+1 are calculated for small average distances (~0.6 nm) between mobile ions (as in advanced superionic conductors). Computer experiments show: (i) ΣWk,k+1 sums are independent of k-indices with the accuracy ~10%, (ii) probability density functions for local potential differences Vk,k+1 are close to normal distributions and (iii) <Vk,k+1> ≈ VUk,k+1, where VUk,k+1 is a potential difference in a uniform electrostatic field. These nanoscale laws allow manipulating by an average potential difference (<Vk,k+N>) between Xk and Xk+N planes (N > 1), so the modeling of non-stationary ion-transport processes and energy flows in parallel combinations of nanostructures become possible.