Selective Cation Exchange Research Articles

High-Performance Monovalent Selective Cation Exchange Membranes with Ionically Cross-Linkable Side Chains: Effect of the Acidic Groups.

Inspired by the charge-governed protein channels located in the cell membrane, a series of polyether ether ketone-based polymers with side chains containing ionically cross-linkable quaternary ammonium groups and acidic groups have been designed and synthesized to prepare monovalent cation-selective membranes (MCEMs). Three acidic groups (sulfonic acid, carboxylic acid, and phenolic hydroxyl) with different acid dissociation constant (pKa) were selected to form the ionic cross-linking structure with quaternary ammonium groups in the membranes. The ionic cross-linking induced the nanophase separation and constructed ionic channels, which resulted in excellent mechanical performance and high cation fluxes. Interesting, the cation flux of membranes increased as the ionization of acidic groups increase, but the selectivity of MCEMs did not follow the same trend, which was mainly dependent on the affinity between the functional groups and the cations. Carboxyl group-containing MCEMs exhibited the best selectivity (9.01 for Li+/Mg2+), which was higher than that of the commercial monovalent cation-selective CIMS membrane. Therefore, it is possible to prepare stable MCEMs through a simple process using ionically cross-linkable polymers, and tuning acidic groups in the membranes provided an attractive approach to improving the cation flux and selectivity of MCEMs.

Flexible lithium selective composite membrane for direct lithium extraction from high Na/Li ratio brine

Monovalent selective ion-exchange membranes have been widely used for lithium extraction from brine with a high Mg/Li ratio. However, extracting Li+ directly from raw brine with high Na/Li ratio remains a significant challenge. This study introduces a flexible lithium selective composite membrane by incorporating lithium-ion conductor with a quaternized polymer. The lithium-ion conductor provides pathways for lithium transmission, while the quaternized polymer effectively hinders the leakage of impurity cations. As a result, with the applied voltage of 2.0 V, the membrane achieved a high Li+/Na+ separation coefficient of 35.48 with a Li+ flux of 8.33 × 10−10 mol cm−2 s−1 in the simulated Yiliping brine, which are 56.3 and 2.8 times higher than that of commercial monovalent selective cation exchange membrane (CIMS). We proposed a lithium transport mechanism involving an adsorption-dehydration-diffusion process through the membrane. Theoretical calculations indicate that the combined effects of significant adsorption energy (−0.47 eV), low dehydration energy (515 kJ mol−1), and low diffusion barrier (0.37 eV) leads to the high selectivity for lithium. These findings provide promising prospects for the large-scale preparation of lithium ion-selective membranes, enabling the direct lithium extraction from raw brine.

Harnessing ion resource recovery: Design of selective cation exchange membranes via a synergistic ionic control method

Effective recovery of valuable ion resources such as lithium is gaining increasing importance. Monovalent selective cation exchange membranes (MCEM) used in electrodialysis have shown great potential, but their development is hampered by the classical trade-offs related to membrane permeability, selectivity and energy efficiency. We reported a new route to design high performance MCEM, utilizing the synergy of ionic control (IC) through cation-π bonding of polydopamine (PDA) and alternating current (AC), which has not been reported before. A set of deliberately controlled membrane fabrication conditions were chosen, with different ionic control (i.e., no cation, K+, Li+) and with/without AC for systematic comparison. Combining with morphology & electrochemical measurements, the membrane with electro-assisted K+ control, i.e., PDAM-K+/AC, exhibited respective 72 % and 51 % higher Li+ and K+ transport rate, higher permselectivity of 9.54 (Li+/Mg2+) and 17.86 (K+/Mg2+), and 3.3-fold lower surface electrical resistance (SER) compared to other modified membranes (e.g., PDAM). Also, PDAM-K+/AC exhibited no sign of scaling, supported by its much increased limiting current density. The results supported the hypothesis of the synergistic effect between cation-π bonding and AC electro-deposition, which altered the polymerization dynamics and produced more ordered membrane structure. The control strategy showed a viable route to fabricate ion exchange membranes for imparting monovalent selectivity, reducing scaling and avoiding energy penalty.

Membrane Design Principles for Ion-Selective Electrodialysis: An Analysis for Li/Mg Separation.

Selective electrodialysis (ED) is a promising membrane-based process to separate Li+ from Mg2+, which is the most critical step for Li extraction from brine lakes. This study theoretically compares the ED-based Li/Mg separation performance of different monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes at the coupon scale using a unified mass transport model, i.e., a solution-friction model. We demonstrated that monovalent selective CEMs with a dense surface thin film like a polyamide film are more effective in enhancing the Li/Mg separation performance than those with a loose but highly charged thin film. Polyamide film-coated CEMs when used in ED have a performance similar to that of polyamide-based NF membranes when used in NF. NF membranes, when expected to replace monovalent selective CEMs in ED for Li/Mg separation, will require a thin support layer with low tortuosity and high porosity to reduce the internal concentration polarization. The coupon-scale performance analysis and comparison provide new insights into the design of composite membranes used for ED-based selective ion-ion separation.

Ion selective cation exchange membrane with tailored electrochemical properties by incorporating carboxymethyl cellulose nanogels

Ion selective cation exchange membrane with tailored electrochemical properties by incorporating carboxymethyl cellulose nanogels

Surface implantation of ionically conductive polyhydroxyethylaniline on cation exchange membrane for efficient heavy metal ions separation

Extraction of heavy metal ions from aqueous could enable recycling of wastewater and reusing of high-value ion resources. Recently, electrodialysis (ED) technology has been applied in the removal of heavy metal ions from wastewater, however, developing high-efficient and stable heavy metal ions selective cation exchange membrane (CEM) remains an elusive challenge. Herein, we demonstrate the fabrication of high-efficient cations selective CEM with the surface implantation of ionically conductive polyhydroxyethylaniline (PANI-OH) on a commercial CEM membrane (CTG) containing sulfonic acid groups via the electrostatic attraction. Implanted hydrophilic and positively charged PANI-OH facilitates fast and selective transfer of Cu2+ over Zn2+ cations through the membrane. Meanwhile, the synergistic effect of rigid conjugate structure and hydrogen-bond interaction of PANI-OH renders the resultant membrane CTG-PANI-OH with stable structure and separation performance. As such, the Cu2+/Zn2+ permselectivity of CTG-PANI-OH membrane is ~1.70, which is about 190 % of that of the pristine CTG membrane. This work provides a facile design and strategy for the construction of high-efficient cations selective CEMs with great application prospects in the purification and extraction of heavy metal ions from wastewater.

Selective recovery of silver from copper impurities by electrodialysis: Tailoring monovalent selective cation exchange membranes by monomolecular layer deposition

Selective recovery of silver from secondary resources enriched with copper impurities is a well-known challenge but is urgently needed by the industry. This study addresses the challenge by developing a highly efficient monovalent selective cation exchange membrane (CEM) with a specifically tailored polyelectrolyte deposition, enabling efficient Ag+/Cu2+ separation in electrodialysis. Based on the Ag+/Cu2+ separation mechanism in electrodialysis, a selected polyelectrolyte, polyethyleneimine (PEI), was deposited on a standard CEM as a monomolecular layer with precisely controlled polymer chain stretch patterns and optimized morphologies. The effects of deposition conditions, such as solution pH and ionic strength, were studied to ensure the desired surface properties. The selectivity performance of the developed membranes was tested using an equimolar binary mixture; a high Ag+/Cu2+ selectivity of > 20 was documented, exhibiting superior selectivity performance compared to commercial monovalent selective CEMs.

Polyaniline encapsulated sulphonated graphene oxide based cation exchange membrane for electrodialytic separation of mono- and bi-valent ions

Polyaniline encapsulated sulphonated graphene oxide (SGO-PANI) was prepared by ion exchange process. The SGO-PANI was incorporated in sulphonated poly(ether ether ketone) (SPEEK) matrix to fabricate the mono-valent selective cation exchange membrane. The structural morphology of SGO-PANI was studied by FTIR, XPS, SEM and XRD spectra. Resultant SPEEK@SGO-PANI membrane exhibited excellent stabilities and physicochemical and electrochemical properties suitable for electrodialysis (ED) applications. Encapsulation of strong acid with a weak base seems to improve electrostatic repulsion between membrane surfaces and of bi-valent ions, which are responsible for significant flux reduction. Chronopotentiometric and current–voltage responses of SPEEK@SGO-PANI membrane confirmed its ion-conducting nature along with 3.32 mA cm−2 limiting current density (Ilim). Reported SPEEK@SGO-PANI membrane showed a high electrodialytic flux of Na+ compared to bi-valent ions (Mg2+, Ca2+, and Cd2+). Extremely low energy consumption and high current efficiency (81.18%) at 6.0 V applied voltage for Na+ (0.10 M) revealed the promising nature of the reported membrane. Observed mono-valent ion selectivity aroused due to improved electrostatic repulsion of bi-valent ions with the membrane surface.

High-performance nanofiltration concentrate treatment by a five-chamber bioelectrochemical system

A combination of bioelectrochemical systems and electrodialysis has been considered an effective strategy for removing salts from the nanofiltration (NF) concentrate of electroplating wastewater; however, the recovery efficiency of multivalent metals is low. Herein, a new process based on microbial electrolysis desalination and chemical-production cell with five chambers (MEDCC-FC) has been proposed for the simultaneous desalination and recovery of the multivalent metals from NF concentrate. The MEDCC-FC was found to be significantly superior to the MEDCC with the monovalent selective cation exchange membrane (MEDCC-MSCEM) and MEDCC with the cation exchange membrane (MEDCC-CEM), in terms of the elevated desalination efficiency, multivalent metal recovery efficiency, current density, and coulombic efficiency, and decreased energy consumption and membrane fouling. Within 12 h, the MEDCC-FC provided the desirable outcome, indicated by a maximum current density of 6.88 ± 0.06 A/m2, desalination efficiency of 88 ± 10%, metals recovery efficiency of >58%, and total energy consumption of 1.17 ± 0.11 kWh for the per kg total dissolved solids removal. Mechanistic studies revealed that the integration of CEM and MSCEM in the MEDCC-FC promoted the separation and recovery of multivalent metal. These findings revealed that the proposed MEDCC-FC was promising in treating NF concentrate of electroplating wastewater towards advantages of effectiveness, economic viability, and flexibility.

Ultra-selective chelating membranes for recycling of cobalt from lithium-ion spent battery effluents by electrodialysis

The recovery of valuable resources from spent effluents is critical to ensure sustainable growth and reduce commodity shortages at global levels. Spent batteries are largely untapped resources containing multiple high-value metal ions that may be harnest and resued indefinitely to develop a circular battery economy. Challenges however lay with the cost of purification and recovery strategies and the limited selectivity of existing processes. This work presents the development of selective cation exchange membranes suitable for the selective recovery of cobalt from mixtures of lithium, cobalt, and nickel from spent battery effluents. Efficient cation exchange membranes were generated by modifying poly(vinyl chloride) films with ethylene diamine followed by grafting of 5-chloro-8-hydroxyquinoline (5C8Q) chelating agents and two synthesis routes were compared including surface and bulk grafting and the membrane separation properties were benchmarked against CIMS Neosepta. Besides being stable across a pH range between 2 and 8, relevant to spent battery lixiviat, the surface grafted membranes exhibited the lowest energy of sorption in comparison to bulk membranes, which was ten times lower than that of the commercial CIMS Neosepta reference membranes. The transfer of ions from model single salt solutions across the surface grafted membranes allowed for over 60 % of cobalt recovery within 1 h, while nickel and lithium were transferred only up to 18 % and 0.2 %, respectively. In addition, the electrodialysis-based separation of multi-component solutions remained very cost-effective with cobalt ion removal amounts up to 58 %, leading to Co/Li separation factors as high as 247. This novel chelating agent strategy opens avenues for the development of highly efficient and selective membrane materials, which are able to operate in extreme pH environments and supporting ultra-selective ionic transfer.

Manifold improvement of water oxidation activity of NaCoO2 by selective cation exchange

Water oxidation produces electrons, protons, and oxygen, essential components in fuel production processes like CO2 reduction and hydrogen generation. To date, less cost and more-abundant catalysts showed low activity for the water oxidation reaction. Herein, we developed a simple and effective way to selectively exchange Na+ with H+ in NaCoO2 and created high surface area layered materials for efficient water oxidation. The photochemical water oxidation reaction activity improved from 0.9 × 10−3 s−1 (pristine NCO) to 2.90 × 10−3 s−1 (AT-NCO-2M). The electrochemical overpotential decreased from 600 to 370 mV, and the Tafel slope dropped from 93 to 47 mV/dec. Enhanced water oxidation activity is owing to delamination of stacked layered structure into few layer structure, increased surface area, super hydrophilic nature, and coexistence of Co4+ and Co3+ species. Our findings show the importance of the exchange of Na+ with H+ in Na-based metal oxides for efficient water oxidation.

Fe3+ ions induced rapid co-deposition of polydopamine-polyethyleneimine for monovalent selective cation exchange membrane fabrication

Selective monovalent cation transport is crucial for the concentration of target ion(s) using electro-membrane processes prior to precipitation. Co-deposition of biomimetic adhesive polydopamine (PDA) and the positively charged polyethyleneimine (PEI) has emerged as an effective approach for building monovalent perm-selectivity functional coatings for cation exchange membranes (CEMs). Here we report a facile strategy to fabricate monovalent selective cation exchange membranes through quick co-deposition of PDA-PEI induced by ferric (Fe3+) ions. With CEM substrate ion exchanged with Fe3+ ions, the polymerization of DA on the membrane surface was accelerated. The mechanism of the promoting effect of Fe3+ ions on the PDA-PEI co-deposition was explored by UV spectroscopy, SEM, AFM, zeta potential and area resistance measurements. In this approach, the characteristics of the membrane surface were easily manipulated by varying the Fe3+ concentration. The selective CEM fabricated through 0.0001 M FeCl3 pretreatment and only 30-min PDA-PEI deposition showed remarkably enhanced Na+/Mg2+ selectivity of 8.3, low electrical resistance and excellent stability. This study provides a simple and scalable technique for time-saving and environmentally friendly construction of PDA based coating for membrane modifications.

Influencing the coupling between network building blocks in CdSe/CdS dot/rod aerogels by partial cation exchange.

The assembly of CdSe/CdS dot/rod nanocrystals (NCs) with variable length of ZnS tips into aerogel networks is presented. To this end, a partial region selective cation exchange procedure is performed replacing Cd by Zn starting at the NC tip. The produced aerogel networks are investigated structurally and optically. The networks of tip-to-tip connected NCs have an intricate band structure with holes confined to the CdSe cores while electrons are delocalized within the CdS also within connected building blocks. However, the ZnS tips act as a barrier of variable length and strength between the NC building blocks partly confining the electrons. This results in NC based aerogel networks with tunable strength of coupling between building blocks.

Sustainable K+/Na+ monovalent-selective membranes with hot-pressed PSS-PVA saloplastics

Monovalent selective cation exchange membranes could play an important role in balancing the K+/Na+ ratio in agricultural feed streams to prevent the toxic effects of excess Na+ in the plant and soil systems, especially in greenhouses and dry areas. A polyelectrolyte complex of polystyrenesulfonate and polyvinylamine in the monomer ratio 1:2.5 is hot-pressed to form a dense saloplastic. The plastic takes up 42% w/w water when equilibrated, while ion-exchange capacity measurements show that it is negatively charged with a net ion-exchange capacity of 1.1 ± 0.4. Resistance measurements show a very promising preferred conductivity for K+ over Na+. This was confirmed by measuring K+ and Na+ transport through the membrane under diffusive conditions from an aqueous mixture of KCl and NaCl. Commercial membranes show resistance-based selectivities of 1.32 ± 0.1 to 1.19 ± 0.1, and diffusion based selectivities of 0.99 ± 0.1 to 0.78 ± 0.1. In contrast, the selectivities for the newly developed saloplastic membrane were 1.80 ± 0.33 for the resistance-based selectivity while the diffusion-based selectivity was 1.91 ± 0.1. The procedure is green as toxic solvents and/or halogenating agents, typically used to make cation exchange membranes, are not needed. This work thus highlights how monovalent selective membranes with a relevant K+/Na+ selectivity can be prepared by a simple and sustainable hot-pressing approach.

The effect of impurity ions on the pH adjustment for vanadium-bearing acid leaching solution by selective electrodialysis

Leaching of vanadium shale with sulfuric acid results in a vanadium acid leaching solution with low pH and many impurity ions. Monovalent selective electrodialysis was employed to adjust the pH of the acid leaching solution by recovering the sulfuric acid. The results demonstrated that CIMS monovalent selective cation exchange membrane had good performance in separating H+ and VO2+ for various vanadium shale acid leaching solutions without Fe3+. The effect of the concentration of impurity ions was investigated. Under the optimum concentration of impurity ions, The treatment time was 125 min, the V rejection ratio reached 98.06%, the water loss rate was controlled at 5.25%, and the pH was adjusted from 0.08 to 1.8. Mg2+ had the best optimization effect on vanadium rejection. Al3+ had the best optimization effect in reducing water loss. F− had the best optimization effect on the treatment time. Meanwhile, the effect of impurity ions on the pH adjustment of acid leaching solution was discussed qualitatively. SEM-EDS analysis confirmed that Fe3+ fouled the membrane and caused the loss of vanadium. Since Fe3+ and PO43− in the acid leaching solution formed FePO4 precipitates. Selective electrodialysis was used to treat vanadium-bearing acid leaching solution, the pH could meet the requirements of separation and purification without consuming any reagent. This process is clean and efficient, and the recycled acid is reused without producing waste water or residue.

Boosting solar water oxidation activity of BiVO4 photoanode through an efficient in-situ selective surface cation exchange strategy

The sluggish kinetics for water oxidation is recognized as one of the major problems for the unsatisfied photoelectrochemical (PEC) performance. Herein, we developed a feasible strategy based on in-situ selective surface cation exchange, for activating surface water oxidation reactivity toward boosted PEC water oxidation of BiVO4 photoanodes with fundamentally improved surface charge transfer. The as-constructed Co/BiVO4 photoanodes exhibit 2.6 times increase in photocurrent density with superior stability, in comparison to those of pristine counterpart. Moreover, the faradaic efficiency of as-fabricated photoanode can be up to ∼ 95% at 1.23 V (vs. RHE). The unique selective replacement of Bi by Co on the surface could modify the electronic structure of BiVO4 with reduced energy barrier of the deprotonation of OH* to O, thus favoring the overall excellent PEC performance of Co/BiVO4 photoanode.

Seawater desalination using the microbial electrolysis desalination and chemical-production cell with monovalent selective cation exchange membrane

The objective of this study was to investigate the feasibility of seawater desalination using the microbial electrolysis desalination and chemical-production cell (MEDCC) with monovalent selective cation exchange membrane (MSCEM) (MEDCC-MSCEM). With dissolved aquarium sea salts as artificial seawater, the maximum current density in the MEDCC-MSCEM reached 19.6 ± 0.3 A/m2, which was 43.1% higher than that in the MEDCC with cation exchange membrane (i.e., the traditional MEDCC as the control). The desalination efficiency within 24 h was 76 ± 7% in the MEDCC-MSCEM. The harvested acids (mainly HCl and H2SO4) and alkali (NaOH with 96% purity) concentrations reached 0.28 ± 0.02 and 0.26 ± 0.02 M, respectively. The total energy consumption within 24 h was much lower in the MEDCC-MSCEM than in the control (3.46 ± 0.41 vs. 4.99 ± 0.46 kWh/kg·TDS). The separation efficiency of Na+:Ca2+ and Na+:Mg2+ in the MSCEM was in the range of 42% - 72% and 53% - 87%, respectively. Effective limitation of Ca2+ and Mg2+ by MSCEM resulted in low precipitation of Ca (OH)2 and Mg(OH)2 in the membrane and cathode, which significantly improved the performance of MEDCC. The MEDCC-MSCEM has great potential in seawater desalination with resource recovery.

A Highly Selective Novel Green Cation Exchange Membrane Doped with Ceramic Nanotubes Material for Direct Methanol Fuel Cells

Herein, a pair of inexpensive and eco-friendly polymers were blended and formulated based on poly (ethylene oxide) (PEO) and poly (vinyl alcohol) (PVA). FTIR, XRD, EDX and TEM techniques were used to describe a Phosphated titanium oxide (PO4TiO2) nanotube synthesised using a straightforward impregnation-calcination procedure. For the first time, the produced nanoparticles were inserted as a doping agent into this polymeric matrix at a concentration of (1–3) wt.%. FTIR, TGA, DSC and XRD were used to identify the formed composite membranes. Furthermore, because there are more hydrogen bonds generated between the polymer’s functional groups and oxygen functional groups PO4TiO2, oxidative stability and tensile strength are improved with increasing doping addition and obtain better results than Nafion117. The permeability of methanol reduced as the weight % of PO4TiO2 increased. In addition, the ionic conductivity of the membrane with 3 wt.% PO4-TiO2 is raised to (28 mS cm−1). The optimised membrane (PVA/PEO/PO4TiO2-3) had a higher selectivity (6.66 × 105 S cm−3 s) than Nafion117 (0.24 × 105 S cm−3 s) and can be used as a proton exchange membrane in the development of green and low-cost DMFCs.

(Invited) Synthesis and Optoelectronic Applications of Colloidal Nanorod Heterostructures

Engineering band structure and heterointerfaces has been and continues to be critical for advances in electronics, optoelectronics, photovoltaics, and displays among many high-tech applications. Achieving atomic precision through widely-accessible and cost-effective means would allow rapid advances in these critical areas. The evolution of colloidal semiconductor nanocrystals from single-composition, “spherical” particles to complex heterostructures of diverse shapes provides many opportunities for precision band structure engineering through scalable solution synthesis. With anisotropic shapes that can be exploited for assembly, charge carrier manipulation and optical anisotropy, etc., incorporating heterojunctions and other functional interfaces into colloidal nanorod heterostructures represents an especially promising direction. In this talk, general challenges to the synthesis of complex-yet-well-defined colloidal nanorod heterostructures will first be discussed. Approaches such as spatially selective solution epitaxy, catalytic growth, cation exchange and combinations thereof can be exploited to achieve unique heterostructures with useful properties. Examples of colloidal nanorod heterostructures of II-VI as well as I-III-VI semiconductors will be discussed. In addition, light-emitting diodes (LEDs) incorporating these materials are shown to not only radically improve existing function but also impart new capabilities without adding complexity in manufacturing.

Brine management from desalination plants for salt production utilizing high current density electrodialysis-evaporator hybrid system: A case study in Kuwait

The potential of producing salt and low salinity water by treating synthetic brine was investigated in a pilot-scale electrodialysis-evaporator hybrid system as part of brine management. An electrodialyzer with 25 cell pairs of monovalent selective anion exchange membranes (AEMs) and monovalent selective cation exchange membranes (CEMs) were used. Effect of three different current densities (300 to 500 A/m2) and two different flowrates (400 L/h and 450 L/h) on brine treatment were evaluated in the current study. The results indicated that the aforementioned membranes were efficient in separating monovalent ions over divalent ions. Moreover, higher current density operation was favoured during the electrodialysis (ED) operation as the operation time for producing salt was shorter. Flowrate is a crucial parameter for the operation of the ED system, where higher flowrate (by 12.5%) reduced the operation time (by 30 min), reduced energy consumption (by 32%) and reduced system resistance (by 32.42%) leading to an increase in current efficiency (by 31.4%). The concentrated brine effluent from ED was fed to an evaporator to produce coarse salt with 84.75% salt recovery and low salinity water of 1526 ppm. Thus, ED-evaporator hybrid system was found to be a good option to treat reject brine from desalination plants.