A Cationic Diode Based on Asymmetric Nafion Film Deposits

Nanoarchitectonics in Advanced Membranes for Enhanced Osmotic Energy Harvesting

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

Tuning and Coupling Irreversible Electroosmotic Water Flow in Ionic Diodes: Methylation of an Intrinsically Microporous Polyamine (PIM-EA-TB)

Molecularly rigid polymers with internal charges (positive charges induced by amine methylation) allow electroosmotic water flow to be tuned by adjusting the charge density (the degree of methylation). Here, a microporous polyamine (PIM-EA-TB) is methylated to give a molecularly rigid anion conductor. The electroosmotic drag coefficient (the number of water molecules transported per anion) is shown to increase with a lower degree of methylation. Net water transport (without charge flow) in a coupled anionic diode circuit is demonstrated based on combining low and high electroosmotic drag coefficient materials. The AC-electricity-driven net process offers water transport (or transport of other neutral species, e.g., drugs) with net zero ion transport and without driver electrode side reactions.

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

Understanding Transient Ionic Diode Currents and Impedance Responses for Aquivion-Coated Microholes

Ionic diode based devices or circuits can be applied, for example, in electroosmotic pumps or in desalination processes. Aquivion ionomer coated asymmetrically over a Teflon film (5 μm thickness) with a laser-drilled microhole (approximately 10 μm diameter) gives a cationic diode with a rectification ratio of typically 10-20 (measured in 0.01 M NaCl with ±0.3 V applied bias). Steady state voltammetry, chronoamperometry, and electrochemical impedance spectroscopy data are employed to characterize the ionic diode performance parameters. Next, a COMSOL 6.0 finite element model is employed to quantitatively assess/compare transient phenomena and to extract mechanistic information by comparison with experimental data. The experimental diode time constant and diode switching process associated with a distorted semicircle (with a typical diode switching frequency of 10 Hz) in the Nyquist plot are reproduced by computer simulation and rationalized in terms of microhole diffusion-migration times. Fundamental understanding and modeling of the ionic diode switching process can be exploited in the rational/optimized design of new improved devices.

Scanning Ion Conductance Microscopy of Nafion-Modified Nanopores

Single nanopores in silicon nitride membranes are asymmetrically modified with Nafion and investigated with scanning ion conductance microscopy, where Nafion alters local ion concentrations at the nanopore. Effects of applied transmembrane potentials on local ion concentrations are examined, with the Nafion film providing a reservoir of cations in close proximity to the nanopore. Fluidic diodes based on ion concentration polarization are observed in the current-voltage response of the nanopore and in approach curves of SICM nanopipette in the vicinity of the nanopore. Experimental results are supported with finite element method simulations that detail ion depletion and enrichment of the nanopore/Nafion/nanopipette environment.

Current oscillations from bipolar nanopores for statistical monitoring of hydrogen evolution on a confined electrochemical catalyst

Taking advantage of bipolar electrochemistry and a glass nanopipette, continuous single bubbles can be controlled which are generated and detached from a nanometer-sized area of confined electrochemical catalysts. The observed current oscillations offer opportunities to rapidly collect data for the statistical analysis of single-bubble generation on and departure from the catalysts.

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

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

Polymers of Intrinsic Microporosity in the Design of Electrochemical Multicomponent and Multiphase Interfaces

Polymers of intrinsic microporosity (or PIMs) provide porous materials due to their highly contorted and rigid macromolecular structures, which prevent space-efficient packing. PIMs are readily dissolved in solvents and can be cast into robust microporous coatings and membranes. With a typical micropore size range of around 1 nm and a typical surface area of 700-1000 m² g^-1, PIMs offer channels for ion/molecular transport and pores for gaseous species, solids, and liquids to coexist. Electrode surfaces are readily modified with coatings or composite films to provide interfaces for solid|solid|liquid or solid|liquid|liquid or solid|liquid|gas multiphase electrode processes.

Electric Migration of Hydrogen Ion in Pore-Voltammetry Suppressed by Nafion Film

Micro-hole voltammetry exhibiting rectified current-voltage curves was performed in hydrochloric acid by varying the lengths and the diameters of the micro-holes on one end of which a Nafion film was mounted. Some voltammetric properties were compared with those in NaCl solution. The voltammograms were composed of two line-segments, the slope of one segment being larger than the other. They were controlled by electric migration partly because of the linearity of the voltammograms and partly the independence of the scan rates. Since the low conductance which appeared in the current from the hole to the Nafion film was proportional to the cross section area of the hole and the inverse of the length of the hole, it should be controlled by the geometry of the hole. The conductance of the hydrogen ion in the Nafion film was observed to be smaller than that in the bulk, because the transport rate of hydrogen ion by the Grotthuss mechanism was hindered by the destruction of hydrogen bonds in the film. In contrast, the conductance for the current from the Nafion to the hole, enhancing by up to 30 times in magnitude from the opposite current, was controlled by the cell geometry rather than the hole geometry except for very small holes. A reason for the enhancement is a supply of hydrogen ions from the Nafion to increase the concentration in the hole. The concentration of the hydrogen ion was five times smaller than that of sodium ion because of the blocking of transport of the hydrogen ion in the Nafion film. However, the rectification ratio of H+ was twice as large as that of Na+.

Switching Anionic and Cationic Semipermeability in Partially Hydrolyzed Polyacrylonitrile: A pH-Tunable Ionic Rectifier

Membrane materials with semipermeability for anions or for cations are of interest in electrochemical and nanofluidic separation and purification technologies. In this study, partially hydrolyzed polyacrylonitrile (phPAN) is investigated as a pH-switchable anion/cation conductor. When switching from anionic to cationic semipermeability, also the ionic current rectification effect switches for phPAN materials deposited asymmetrically onto a 5, 10, 20, or 40 μm diameter microhole in a 6 μm thick polyethylene-terephthalate (PET) film substrate. Therefore, ionic rectifier behavior can be tuned and used to monitor and characterize semipermeability. Effects of electrolyte type and concentration and pH (relative to the zeta potential at approximately 3.1) are investigated by voltammetry, chronoamperometry, and impedance spectroscopy. A computational model provides good qualitative agreement with the observed electrolyte concentration data. High rectification effects are observed for both cations (pH > 3.1) and anions (pH < 3.1) but only at relatively low ionic strengths.

Cationic Rectifier Based on a Graphene Oxide-Covered Microhole: Theory and Experiment

Cation transport through nanochannels in graphene oxide can be rectified to give ionic diode devices for future applications, for example, in desalination. A film of graphene oxide is applied to a 6 μm thick poly(ethylene terephthalate) substrate with a 20 μm diameter microhole and immersed in aqueous HCl solution. Strong diode effects are observed even at high ionic strength (0.5 M). Switching between open and closed states, microhole size effects, and time-dependent phenomena are explained on the basis of a simplified theoretical model focusing on the field-driven transport within the microhole region. In aqueous NaCl, competition between Na⁺ transport and field-driven heterolytic water splitting is observed but shown to be significant only at low ionic strength. Therefore, nanostructured graphene oxide is demonstrated to exhibit close to ideal behavior for future application in ionic diode desalination of seawater.

Asymmetric Nafion-Coated Nanopore Electrode Arrays as Redox-Cycling-Based Electrochemical Diodes

Inspired by the functioning of cellular ion channels, pore-based structures with nanoscale openings have been fabricated and integrated into ionic circuits, for example, ionic diodes and transistors, for signal processing and detection. In these systems, the nonlinear current responses arise either because asymmetric nanopore geometries break the symmetry of the ion distribution, creating unequal surface charge across the nanopore, or by coupling unidirectional electron transfer within a nanopore electrode. Here we develop a high-performance redox-cycling-based electrochemical diode by coating an asymmetric ion-exchange membrane, that is, Nafion, on the top surface of a nanopore electrode array (Nafion@NEA), in which each pore in the array exhibits one or more annular electrodes. Nafion@NEAs exhibit highly sensitive and charge-selective electroanalytical measurements due to efficient redox-cycling reaction, the permselectivity of Nafion, and the strong confinement of redox species in the nanopore array. In addition, the top electrode of dual-electrode Nafion@NEAs can serve as a voltage-controlled switch to gate ion transport within the nanopore. Thus Nafion@NEAs can be operated as a diode by switching voltages applied to the top and bottom electrodes of the NEA, leading to a large rectification ratio, fast response times, and simplified circuitry without the need for external electrodes. By taking advantage of closely spaced and individually addressable electrodes, the redox-cycling electrochemical diode has the potential for application to large-scale production and electrochemically controlled circuit operations, which go well beyond conventional electronic diodes or transistors.

Ionic Current Behaviors of Dual Nano- and Micropipettes

Ionic current rectification (ICR) phenomena within dual glass pipettes are investigated for the first time. We demonstrate that the ionic flow presents different behaviors in dual nano- and micropipettes when the two channels are filled with the same electrolyte KCl and hung in air. Bare dual nanopipettes cannot rectify the ionic current because of their geometric symmetry, but the ICR can be directly observed based on bare dual micropipettes. The phenomena based on dual micropipettes could be explained by the simulation of the Poisson-Nernst-Plank equation. After modification with different approaches, the dual nanopipettes have asymmetric charge patterns and show various ICR behaviors. They have been successfully employed to fabricate various nanodevices, such as ionic diodes and bipolar junction transistors. Due to the simple and fast fabrication with high reproducibility, these dual pipettes can provide a novel platform for controlling ionic flow in nano- and microfluidics, fabrication of novel nanodevices, and detection of biomolecules.

Bioinspired smart asymmetric nanochannel membranes

Bioinspired smart asymmetric nanochannel membranes (BSANM) have been explored extensively to achieve the delicate ionic transport functions comparable to those of living organisms. The abiotic system exhibits superior stability and robustness, allowing for promising applications in many fields. In view of the abundance of research concerning BSANM in the past decade, herein, we present a systematic overview of the development of the state-of-the-art BSANM system. The discussion is focused on the construction methodologies based on raw materials with diverse dimensions (i.e. 0D, 1D, 2D, and bulk). A generic strategy for the design and construction of the BSANM system is proposed first and put into context with recent developments from homogeneous to heterogeneous nanochannel membranes. Then, the basic properties of the BSANM are introduced including selectivity, gating, and rectification, which are associated with the particular chemical and physical structures. Moreover, we summarized the practical applications of BSANM in energy conversion, biochemical sensing and other areas. In the end, some personal opinions on the future development of the BSANM are briefly illustrated. This review covers most of the related literature reported since 2010 and is intended to build up a broad and deep knowledge base that can provide a solid information source for the scientific community.

Selective ion-permeation through strained and charged graphene membranes

By means of molecular dynamics simulations and density functional theory calculations, we demonstrate that stretched and charged graphene can act as ion sieve membranes. It is observed that loading 30% strain on graphene can induce pores in the dense electron cloud to allow ions to pass through the aromatic rings. Meanwhile, a charged surface is helpful to peel the hydration layers from the ions and decrease the energy barrier for ion translocation through nanopores. Our results suggest that with a membrane charge density of 6.80 e nm^-2, Li⁺ can be highly purified from the mixed solution including Li⁺, K⁺, Na⁺ and Cl^- ions. Further increasing the charge density to 15.78 e nm^-2 can obtain excellent Na⁺/K⁺ selectivity. The potential of mean force profiles of ion permeation reveal that the potential for each ion is quite different. By fine tuning membrane charge density, pristine monolayer graphene can act as ion sieves with both high permeability and high selectivity.

Cellulose ionics: switching ionic diode responses by surface charge in reconstituted cellulose films

Cellulose films as well as chitosan-modified cellulose films of approximately 5 μm thickness, reconstituted from ionic liquid media onto a poly(ethylene-terephthalate) (PET, 6 μm thickness) film with a 5, 10, 20, or 40 μm diameter laser-drilled microhole, show significant current rectification in aqueous NaCl.