High Ion Selectivity Research Articles

The synergistic effect of space and surface charge on nanoconfined ion transport and nanofluidic energy harvesting

Diverse nanoconfined ion transport behaviors have attracted increasing attention in constructing ion-channel-inspired nanoconfined systems for high nanofluidic energy conversion performance. Conventional nanofluidic systems cannot simultaneously obtain high ion-selectivity and high ion-flux in a large region due to the decreasing effect of the electrical double layer (EDL) formed on the charged surface. Here, space charge is introduced in the nanoconfined system to effectively solve this problem by employing the synergistic effect of surface and space charges. The charge effect contribution on controlled-ion-transport is further decoupled into surface charge, space charge and synergistic effects for the first time. The surface charge induces charge separation within the EDL in a smaller diameter channel while the space charge dominates the ion-selectivity by the ion-exchange mechanism in a larger diameter channel. The synergistic effect, contributed by EDL and ion-exchange effects, is extremely important in large (around 100 nm), short nanochannel (around 100 nm), and high concentration fold (around 1000-fold) conditions to achieve high ion-selectivity, ion-flux, and nanofluidic energy conversion performances, which is systematically analyzed by theoretical simulation results. These results reveal the mechanism for enhanced ion transport behaviors and provide a solid foundation for designing high-performance nanofluidic devices in fields of desalination, sensor, energy conversion and storage.

Effective and continuous removal of Cr(VI) from brackish wastewater by flow-electrode capacitive deionization (FCDI)

Effective treatment of Cr(VI)-containing wastewater is a great challenge faced by many industries with this situation further complicated by the coexistence of other ions such as Cl−. This study demonstrated the suitability and effectiveness of utilizing flow-electrode capacitive deionization (FCDI) for effective and continuous removal of Cr(VI) from brackish wastewaters. The effects of various influencing factors were systematically investigated, including applied current density, feed concentration of coexisting Cl−, hydraulic residence time (HRT), activated carbon loading, initial solution pH and other common coexisting anions. Results indicated that Cr(VI) was preferentially removed compared to Cl− with a high ion selectivity achievable, and Cr(VI) and Cl− had significantly different removal behaviors in many cases. The steady-state effluent Cl− concentration dropped almost linearly with the increase of current density, while the steady-state effluent Cr(VI) concentration dropped to a value under a small current density and decreased insignificantly in spite of the increase in current density. A larger HRT or activated carbon loading favorably contributed to a lower steady-state effluent concentrations of Cr(VI) and Cl−. Initial pH of feed water around 5–6 without pH adjustment was the most advantageous to the removal of Cr(VI). Different coexisting anions had inhibitory effects on the Cr(VI) removal to varying degrees and the carbonate species was found to have the greatest inhibitory effect. This study is of value in promoting FCDI as a viable technology for treatment of Cr(VI)-containing brackish wastewater.

Scalable bacterial cellulose biofilms with improved ion transport for high osmotic power generation

The manufacture of large-sized material with tunable nanochannel size and high ion selectivity is always a challenge for osmotic power generation. Herein, we develop negatively charged carboxymethyl bacterial cellulose membranes (BC-CMC) and positively charged chitosan quaternary ammonium bacterial cellulose membranes (BC-HACC) with adjustable charge density and nanochannel size by in situ culture. The scalable membranes are suitable for rapidly ion selective transmission process. When applying the charged BC membranes for an osmotic energy harvesting device, an output power density of 2.25 W m −2 can be reached. Further connecting 15 units of the charged BC device, the output voltage can reach up to 2.53 V, which can directly power the electronic devices. This work highlights the advantage of large-scale preparation by the biosynthesis method, which can simultaneously tune the surface properties and nanochannel size of BC to regulate the ion transport behavior. We offer an easy and scalable method to obtain low-cost membranes for high osmotic energy conversion device, providing the feasibility for their large-scale application. • This paper offers a simple in situ biosynthetic modification to obtain charged BC membranes. • The scalable membranes are suitable for ion selective transmission process. • The BC-RED device can achieve an output power density of 2.25 W m −2 . • Our work develops low-cost, large-scale charged BC membranes potential in practical osmotic energy collection.

Ozark Graphene Nanopore for Efficient Water Desalination.

A nanoporous graphene membrane is crucial to energy-efficient reverse osmosis water desalination given its high permeation rate and ion selectivity. However, the ion selectivity of the common circular graphene nanopore is dependent on the pore size and scales inversely with the water permeation rate. Larger, circular graphene nanopores give rise to the high water permeation rate but compromise the ability to reject ions. Therefore, the pursuit of a higher permeation rate while maintaining high ion selectivity can be challenging. In this work, we discover that the geometry of graphene nanopore can play a significant role in its water desalination performance. We demonstrate that the ozark graphene nanopore, which has an irregular slim shape, can reject over 12% more ions compared with a circular nanopore with the same water permeation rate. To reveal the physical reason behind the outstanding performance of the ozark nanopore, we compared it with circular, triangular, and rhombic pores from perspectives including interfacial water density, energy barrier, water/ion distribution in pores, the ion-water RDF in pores, and the hydraulic diameter. The ozark graphene nanopore further explores the potential of graphene for efficient water desalination.

Nanofiltration through pH-regulated bipolar cylindrical nanopores for solution containing symmetric, asymmetric, and mixed salts

The rejection of symmetric (1:1 and 2:2), asymmetric (1:2 and 2:1), and mixed (1:1 + 1:2 and 1:1 + 2:1) salts by a polyamide membrane is investigated theoretically taking account of its pH-regulated and bipolar nature. We consider the most compact way of distributing pores (honeycomb-like) on the membrane surface so that the influence of pore-pore distance on rejection performance can be taken into account. Assuming large pore-pore distance (or low porosity) so that the concentration polarization effect is negligible, we can investigate the influence of the intrinsic properties of the membrane on its performance. The results of numerical simulation reveal that, due to high resulting surface potential and high ionic selectivity, asymmetric salts exhibit better rejection performance than symmetric salts. The variation of rejection with the applied pressure shows a local maximum, and the associated mechanism discussed. The variations of local pH and filtration potential are examined to help understanding the ionic transport mechanisms in nanofiltration. The separation of mixed monovalent and divalent ions of the same polarity under the conditions of low ionic strength and low applied pressure is discussed for assessing the feasibility of ion recovery and purification.

Transport and photophysical studies on porphyrin-containing sulfonated poly(etheretherketone) composite membranes

The development of efficient and reliable membranes for redox flow batteries (RFBs) with high ionic conductivity and selectivity is challenging. Aiming the development of better membranes, we have synthesized porphyrin-containing sulfonated poly(etheretherketone) (SPEEK). In particular, the effect of water soluble 5,10,15,20-tetrakis(N-methyl-4-pyridinyl)porphyrin iodide (TMePyPI4) and its Cu(II) complex, 5,10,15,20-tetrakis(N-methyl-4-pyridinyl)porphyrinate copper(II) iodide, (CuTMePyPI4), at 0.1–0.3 wt%, have been assessed. Photophysical studies have shown that porphyrins are aggregated in the J-type. The obtained composite membranes show high macroscopic homogeneity and lower ability to sorb water when compared with pristine SPEEK membranes but comparable protonic conductivity. The hydronium diffusion and permeability coefficients suggest that at low porphyrin concentrations an enhancement of proton channels occurs, whilst at high concentration that effect decreases due to the formation of porphyrin aggregates. A similar effect occurs in the permeation of vanadium ions through composite membranes with a remarkable lower permeability and diffusion coefficients when compared with those obtained for Nafion 115 membranes. This along with the improved thermal stability of porphyrin-containing SPEEK composite membranes indicate that these advanced materials can be used for improving the performance of RFBs.

Effective fluoride removal from brackish groundwaters by flow-electrode capacitive deionization (FCDI) under a continuous-flow mode

Herein, we demonstrated the suitability and effectiveness of utilizing flow-electrode capacitive deionization (FCDI) for treatment of fluoride-contaminated brackish groundwater. By comparing operational modes of short-circuited closed-cycle (SCC), isolated closed-cycle (ICC) and single cycle (SC), it was found that SCC mode was the most advantageous. In SCC configuration, the effects of different parameters on the removal of F− and Cl− were investigated including current density, hydraulic residence time (HRT), activated carbon (AC) loading and feed concentration of coexisting NaCl. Results indicated that the steady-state effluent Cl− concentration dropped with elevated applied current, and the decreasing rate got faster with the increase of HRT or AC loading. However, for the steady-state effluent F− concentration, it dropped to a value under a small applied current and maintained stable in spite of the increase in applied current, and both HRT and AC loading had insignificant effects on the steady-state effluent F− concentration. F− was preferentially removed from the treated water compared with Cl−, and a higher ion selectivity could be obtained at lower applied current and smaller HRT with the trade-off being that operation under these circumstances would generate outlet water with little change in conductivity compared to the influent. The removal efficiencies of F− and Cl− both decreased with increasing feed concentration of coexisting NaCl. This study should be of value in establishing FCDI as a viable technology for treatment of fluoride-contaminated brackish groundwater.

Excellent ion selectivity of Nafion membrane modified by PBI via acid-base pair effect for vanadium flow battery

As the key material of Vanadium flow battery (VFB), the properties of ion exchange membrane limit the battery performance and cost of VFB to some extent. Nafion membrane of Dupont, one of the most employed membranes in VFB, is also exposed to some issues such as high cost and low ion selectivity. To improve ion selectivity of Nafion membrane, polybenzimidazole (PBI), functional groups of which can be ionized to a positive charge and prevent vanadium ions from crossing the membrane, is introduced to prepare reinforced Nafion membrane (Nafion/PBI membrane). By this way, excellent conductivity and vanadium ion selectivity of Nafion membrane are optimized via the “acid-base pair” effect. Furthermore, a series of PBI contents have been optimized and the battery with Nafion/PBI membrane of 1 wt% PBI exhibits excellent performance (coulombic efficiency: 97.72%, energy efficiency: 81.31% at 200 mA cm−2). In the long-term cycles, higher capacity retention and superior stability of the battery with Nafion/PBI membrane (1 wt% PBI) are delivered than Nafion 212 membrane. Thus, Nafion/PBI membrane is indeed a promising candidate to promote the whole properties of VFB by surmounting the trade-off between proton conductivity and vanadium ion permeability.

Heterogeneous MXene/PS‐b‐P2VP Nanofluidic Membranes with Controllable Ion Transport for Osmotic Energy Conversion

AbstractMembrane‐based osmotic power harvesting is a strategy for sustainable power generation. 2D nanofluids with high ion conductivity and selectivity are emerging candidates for osmotic energy conversion. However, the ion diffusion under nanoconfinement is hindered by homogeneous 2D membranes with monotonic charge regulation and severe concentration polarization, which results in an undesirable power conversion performance. Here, an asymmetric nanochannel membrane with a two‐layered structure is reported, in which the angstrom‐scale channels of 2D transition metal carbides/nitrides (MXenes) act as a screening layer for controlling ion transport, and the nanoscale pores of the block copolymer (BCP) are the pH‐responsive arrays with an ordered nanovoid structure. The heterogeneous nanofluidic device exhibits an asymmetric charge distribution and enlarged 1D BCP porosity under acidic and alkaline conditions, respectively; this improves the gradient‐driven ion diffusion, allowing a high‐performance osmotic energy conversion with a power density of up to 6.74 W m−2 by mixing artificial river water and seawater. Experiments and theoretical simulations indicate that the tunable asymmetric heterostructure contributes to impairing the concentration polarization and enhancing the ion flux. This efficient osmotic energy generator can advance the fundamental understanding of the MXene‐based heterogeneous nanofluidic devices as a paradigm for membrane‐based energy conversion technologies.

Biomimetic Nanocomposite Membranes with Ultrahigh Ion Selectivity for Osmotic Power Conversion.

Ion transport in nanoconfinement exhibits significant features such as ionic rectification, ionic selectivity, and ionic gating properties, leading to the potential applications in desalination, water treatment, and energy conversion. Two-dimensional nanofluidics provide platforms to utilize this phenomenon for capturing osmotic energy. However, it is challenging to further improve the power output with inadequate charge density. Here we demonstrate a feasible strategy by employing Kevlar nanofiber as space charge donor and cross-linker to fabricate graphene oxide composite membranes. The coupling of space charge and surface charge, enabled by the stabilization of interlayer spacing, plays a key role in realizing high ion selectivity and the derived high-performance osmotic power conversion up to 5.06 W/m2. Furthermore, the output voltage of an ensemble of the membranes in series could reach 1.61 V, which can power electronic devices. The system contributes a further step toward the application of energy conversion.

Significantly Enhanced Performance of Nanofluidic Osmotic Power Generation by Slipping Surfaces of Nanopores

High-performance osmotic energy conversion (OEC) with a perm-selective porous membrane requires both high ionic selectivity and permeability simultaneously. Here hydrodynamic slip is considered on the surfaces of nanopores to break the trade-off between ionic selectivity and permeability, because it decreases the viscous friction at solid–liquid interfaces which can promote ionic diffusion during OEC. Taking advantage of simulations, influences from individual slipping surfaces on the OEC performance have been investigated, i.e., the slipping inner surface (surfaceinner) and exterior surfaces on the low- and high-concentration sides (surfaceL and surfaceH). Results show that the slipping surfaceL is crucial for high-performance OEC. For nanopores with various lengths, the slipping surfaceL simultaneously increases both ionic permeability and selectivity of nanopores, which results in both significantly enhanced electric power and energy conversion efficiency. For nanopores longer than 30 nm, the slipping surfaceinner plays a dominant role in the increase of electric power, which induces a considerable decrease in energy conversion efficiency due to enhanced transport of both cations and anions. Considering the difficulty in hydrodynamic slip modification to the surfaceinner of nanopores, the surface modification to the surfaceL may be a better choice to achieve high-performance OEC. Our results provide feasible guidance to the design of porous membranes for high-performance osmotic energy harvesting.

Layered double hydroxide membrane with high hydroxide conductivity and ion selectivity for energy storage device

Membranes with fast and selective ions transport are highly demanded for energy storage devices. Layered double hydroxides (LDHs), bearing uniform interlayer galleries and abundant hydroxyl groups covalently bonded within two-dimensional (2D) host layers, make them superb candidates for high-performance membranes. However, related research on LDHs for ions separation is quite rare, especially the deep-going study on ions transport behavior in LDHs. Here, we report a LDHs-based composite membrane with fast and selective ions transport for flow battery application. The hydroxide ions transport through LDHs via vehicular (standard diffusion) & Grotthuss (proton hopping) mechanisms is uncovered. The LDHs-based membrane enables an alkaline zinc-based flow battery to operate at 200 mA cm−2, along with an energy efficiency of 82.36% for 400 cycles. This study offers an in-depth understanding of ions transport in LDHs and further inspires their applications in other energy-related devices.

In Situ Defect‐Free Vertically Aligned Layered Double Hydroxide Composite Membrane for High Areal Capacity and Long‐Cycle Zinc‐Based Flow Battery

AbstractRechargeable aqueous zinc‐based flow batteries (ZFBs) are promising candidates for large scale energy storage devices. However, the challenges from zinc dendrites and limited areal capacity considerably impede their wide application. Here, an in situ vertical growth of layered double hydroxide membrane (LDH‐G) is constructed to enable long‐life ZFBs. Owing to the high hydroxide ion conductivity and ion selectivity nature of LDH nanosheets, specifically, the precise control of directional ion transport in vertical arrangement LDHs, a superior battery performance can be realized. Moreover, the defect‐free LDHs layer serves as a buffer layer to enable a uniform Zn deposition, which effectively enhances the areal capacity of the battery. As a result, the designed membrane endows an alkaline zinc‐iron flow battery with excellent rate performance and cycling stability, maintaining an energy efficiency of 80% at 260 mA cm−2 for 800 cycles, which is the highest performance ever reported. Most importantly, the LDHs layer enables the battery for 1200 h long‐cycle stability with a uniform Zn deposition and high areal capacity of 240 mAh cm−2. This work realizes an in situ growth of 3D LDHs arrays on the polymer substrate, which provides a strategy toward high areal capacity and dendrite‐free Zn deposition for ZFBs.

High ion selectivity Aquivion-based hybrid membranes for all vanadium redox flow battery

High ion selectivity Aquivion-based hybrid membranes for all vanadium redox flow battery

Free‐Standing Covalent Organic Framework Membrane for High‐Efficiency Salinity Gradient Energy Conversion

AbstractBoth high ionic conductivity and selectivity of a membrane are required for efficient salinity gradient energy conversion. An efficient method to improve energy conversion is to align ionic transport along the membrane thickness to address low ionic conductivity in traditional membranes used for energy harvesting. We fabricated a free‐standing covalent organic framework membrane (TpPa‐SO3H) with excellent stability and mechanical properties. This membrane with one‐dimensional nanochannels and high charge density demonstrated high ionic conductivity and selectivity. Its power density reached up to 5.9 W m−2 by mixing artificial seawater and river water. Based on our results, we attribute the high energy conversion to the high ion conductivity through aligned one‐dimensional nanochannels and high ion selectivity via the size of the nanochannel at ≈1 nm in the membrane. This study paves the way for designing covalent organic framework membranes for high salinity gradient energy conversion.

Free-Standing Covalent Organic Framework Membrane for High-Efficiency Salinity Gradient Energy Conversion.

Both high ionic conductivity and selectivity of a membrane are required for efficient salinity gradient energy conversion. An efficient method to improve energy conversion is to align ionic transport along the membrane thickness to address low ionic conductivity in traditional membranes used for energy harvesting. We fabricated a free-standing covalent organic framework membrane (TpPa-SO3 H) with excellent stability and mechanical properties. This membrane with one-dimensional nanochannels and high charge density demonstrated high ionic conductivity and selectivity. Its power density reached up to 5.9 W m-2 by mixing artificial seawater and river water. Based on our results, we attribute the high energy conversion to the high ion conductivity through aligned one-dimensional nanochannels and high ion selectivity via the size of the nanochannel at ≈1 nm in the membrane. This study paves the way for designing covalent organic framework membranes for high salinity gradient energy conversion.

Photoluminescence quenching of thermally treated waste-derived carbon dots for selective metal ion sensing

In the present study, carbon-dots (CDs) were derived from the thermal oxidation of an agricultural waste, bitter tea residue, to obtain different sp2/sp3 ratios and electronic structures for metal sensing. The CDs obtained from calcination at 700 °C exhibited the highest photoluminescence (PL) quantum yield (QY) of 11.8% among all the samples treated at different temperatures. These CDs had a high degree of graphitization, which resulted in a strong π-π* electron transition, and hence in a high QY. The strong photoluminescence of the CDs could be used to sense the metal ions Ag+, Sr2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, and Sn2+ by monitoring their PL intensity at an excitation wavelength of 320 nm. The metals inhibited the PL intensity in the order Ag+ > Fe2+, Fe3+, Ni2+ > Sr2+, Co2+, Cu2+, Sn2+, which demonstrated that the CDs exhibited high metal ion detection capability and selectivity. The detection of Fe3+ using CDs was performed in the range of 10–100 ppm with a LOD (limit of detection) value of 0.380 ppm. Theoretical calculations demonstrated that Ag+, Sr2+, and Sn2+ induced charge transfer excitation and that Fe2+ and Ni2+ induced d-d transitions via complexation with the sp2 clusters. The charge transfer excitation and d-d transitions hindered the π-π* transition of the sp2 clusters, leading to a quenching effect. On the other hand, Li+, Na+, and K+ ions did not alter the π-π* transition of the sp2 clusters, resulting in a negligible quenching effect. In summary, the oxidation level and electronic structure of CDs derived from bitter tea residue could be tailored, and the CDs were shown to be a facile, sustainable, and eco-friendly material for metal sensing.

One-pot synthesis of silicon-based zirconium phosphate for the enhanced adsorption of Sr (II) from the contaminated wastewater

The composite of zirconium phosphate (ZrP) loaded on mesoporous silica pellet (MSP) was typically prepared by one-pot liquid phase grafting method to develop an effective inorganic adsorbent for strontium (Sr) ion removal from the contaminated water. The as-synthesized product was well characterized by energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), N2 adsorption–desorption isotherms and FT–IR. The adsorption of Sr2+ on ZrP/MSP composite was systematically investigated as the functions of pH, contact time, ionic strength, and various ions interference. It showed that Sr adsorption on ZrP/MSP was highly dependent on pH and the concentration of metal ion, achieving larger adsorption capacity, faster adsorption equilibrium, and higher ion selectivity than those of bare ZrP under batch reaction conditions. The Langmuir model and the pseudo-second-order model separately fitted the adsorption isotherm and kinetic data better than the Freundlich and pseudo-first-order ones. These results suggested that our present methodology may provide a general synthesis of silica supported zirconium phosphate with mesoporous structure in large surface areas which could be used as inorganic composite adsorbent for effective removal of strontium ions from the contaminated solutions.

Foldamer-Based Potassium Channels with High Ion Selectivity and Transport Activity.

Small molecules that independently perform natural channel-like functions show greatly potential in the treatment of human diseases. Taking advantage of aromatic helical scaffolds, we develop a kind of foldamer-based ion channels with lumen size varying from 3.8 to 2.3 Å through a sequence substitution strategy. Our results clearly elucidate the importance of channel size in ion transport selectivity in molecular detail, eventually leading to the discoveries of the best artificial K+ channel by far and a rare sodium-preferential channel as well. High K+ selectivity and transport activity together make foldamers promising in therapeutic applications.

High-performance nanofluidic osmotic power generation enabled by exterior surface charges under the natural salt gradient

High-performance osmotic energy conversion (OEC) requires both high ionic selectivity and permeability in nanopores. Here, through systematical explorations of influences from individual charged nanopore surfaces on the performance of OEC, we find that the charged exterior surface on the low-concentration side (surfaceL) is essential to achieve high-performance osmotic power generation, which can significantly improve the ionic selectivity and permeability simultaneously. Detailed investigation of ionic transport indicates that electric double layers near charged surfaces provide high-speed passages for counterions. The charged surfaceL enhances cation diffusion through enlarging the effective diffusive area, and inhibits anion transport by electrostatic repulsion. Different areas of charged exterior surfaces have been considered to mimic membranes with different porosities in practical applications. Through adjusting the width of the charged ring region on the surfaceL, electric power in single nanopores increases from 0.3 to 3.4 pW with a plateau at the width of ~200 nm. The power density increases from 4200 to 4900 W/m2 and then decreases monotonously that reaches the commercial benchmark at the charged width of ~480 nm. While, energy conversion efficiency can be promoted from 4% to 26%. Our results provide useful guide in the design of nanoporous membranes for high-performance osmotic energy harvesting.