Ion-exchange Membranes Research Articles

A triphasic membrane-less battery based on salting-out effect employing metal-free redox materials

The utilization of membrane-less redox flow batteries (RFBs) offers a promising avenue to mitigate the dependence on ion exchange membranes. However, there is a dearth of membrane-less RFBs developed that simultaneously exhibit high cyclability and efficiency, leveraging naturally abundant elements. Here we firstly demonstrated a triphasic membrane-less battery based on salting-out effect utilizing metal-free redox materials. With 2,2,6,6-tetramethylpiperidoxyl and methyl viologen in the salting-out propylene carbonate solution as redox materials in catholyte and anolyte respectively, after being separated by the immiscible salting-out electrolytes, this stable triphasic membrane-less battery can deliver an open circuit voltage of 1.11 V and achieve 98.1 % theoretical capacity. Furthermore, the prolonged galvanostatic cycling maintained high Coulombic efficiencies (>99.5 %) and negligible capacity decay over about 660 cycles (552 h) under static condition. We also employed a hydrogel to immobilize the above immiscible electrolyte which functions as a separator, successfully demonstrating a flow cell with enhanced mechanical stability and good compatibility using the conventional battery stack. This particular triphasic battery not only mitigate the issues of poor mechanical stability and compatibility in membrane-less batteries, but also provides the conceptual validation of membrane-less RFB for next-generation energy storage devices.

Enhancing Surface Charge Density of Graphene Oxide Membranes through Al(OH)4− Anion Incorporation for Osmotic Energy Conversion

Graphene oxide (GO) has been extensively studied for fabricating ion exchange membranes. This material is of interest due to its surface‐governed charge which, combined with the interlayer distance between the GO flakes stack, offers ion selectivity. However, obtaining high‐performing membranes with high ion selectivity and low ionic resistance remains challenging. To address this issue, Al(OH)4− anions are incorporated into graphene oxide membranes to increase their spontaneous negative surface charge. The anions are successfully formed and encapsulated through a reaction with the alumina support under alkaline conditions during the membrane fabrication. A modeling of the system proves the anchoring of the Al(OH)4− anions within the GO matrix. The incorporation of these anions significantly improves the permselectivity and reduces the ionic resistance, reaching approximately 95% and 2 Ω cm2, respectively. The GO‐modified membranes also present mono‐valent selectivity, which can boost reverse electrodialysis power densities.

Modeling Sorption of Water and Vanadium Cations by Ion-Exchange Membranes

Modeling Sorption of Water and Vanadium Cations by Ion-Exchange Membranes

A two-step electrochemical method for separating Mg(OH)2 and CaCO3: Application to RO reject and polluted groundwater

The process of removing Ca2+ and Mg2+ ions typically results in the co-precipitation of Ca2+ and Mg2+ along with other salt waste. To improve water treatment efficiency towards a zero-waste goal, it is crucial to separate Ca2+ and Mg2+, and recover them in their purified form. This study proposes a two-step electrochemical approach that separately recovers Ca2+ as CaCO3 and Mg2+ as Mg(OH)2. The first step uses an undivided cell with 3D electrodes and controlled flow directions to selectively precipitate CaCO3 on the electrode, keeping the cell removal efficiency. The second step employs a two-compartment cell with a cationic exchange membrane to recover Mg(OH)2. This approach was evaluated on RO reject water with high Ca2+ to Mg2+ ratio and industrial effluent-polluted groundwater with a low ratio. Treatment of domestic RO reject water using undivided cell specifically recovered 64% of CaCO3, although the low conductivity of the RO reject water limited further Mg2+ recovery. Conversely, treating industrial effluent-polluted groundwater with this two-step process successfully recovered 80% of CaCO3 and 94% of Mg(OH)2. SEM, EDAX, and XRD analysis confirmed the quality of the recovered products.

Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures.

Our research investigates the impact of auxiliary structures on ion transport in electrochemical systems such as batteries and microscale desalination units, whose importance for sustainable development has increased dramatically in recent decades. The electrochemical systems typically feature ion-selective surfaces, such as electrodes and ion exchange membranes, where ion depletion can cause performance issues including metal dendrite formation and flow instability. Recent research has shown that auxiliary structures in these electrochemical systems can enhance ion transfer near ion-selective surfaces, thereby resolving the instability problem and improving the energy conversion efficiency of the system. Our study leverages recent advancements in nanoscale electrokinetics to model these auxiliary structures as pillar arrays near an ion exchange membrane in a microchannel. We examine how these structures enhance ion transports relative to the characteristic length scale of microchannel depth and pillars' proximity to the ion-selective surface. Results show that the effect of the pillars varies significantly with their placement. Specifically, in deeper microchannels, where electrokinetic convection is stronger, the closer the auxiliary structure is to the ion-selective membrane, the better the ion transfer. However, in the thinner microchannel, the proximity of the auxiliary structure to the ion selective membrane has a less significant correlation with the ion transfer. Therefore, this finding highlights the importance of spatial arrangement of the auxiliary structures in improving the performance of electrochemical devices. Conclusively, this study can help to better understand energy conversion systems such as fuel cells, salinity gradient power generation systems, and electrochemical desalination systems, where auxiliary structures can be used in the vicinity of ion-selective surfaces. Especially, our fundamental electrokinetic study provides an effective means for designing the efficient electrochemical platforms utilizing micro/nanofluidics.

Improving ammonia nitrogen removal and recovery by BMED stack optimization: The effect of ion exchange membrane thickness

Bipolar membrane electrodialysis (BMED) has emerged as an efficient and promising technology for the removal and recovery of ammonia nitrogen. Despite its potential, the optimization of BMED systems for enhanced performance and energy efficiency remains a critical challenge, particularly in the context of varying ion exchange membrane (IEM) thicknesses. Membranes with a thickness of < 80 µm have low resistance and are mainly used for processes requiring low energy consumption. Membranes with a thickness of 80–250 µm have moderate resistance and high ion selectivity and are widely used for electrodialysis applications. Membranes with a thickness of > 250 µm have higher ion selectivity than commercial membranes with a thickness of 80–250 µm and are suitable for processes that demand high purity. This study investigates the effect of IEM thickness ranging from 16–400 µm on BMED performance for ammonia nitrogen removal and recovery, and energy consumption. The BMED configuration comprising five cell triplets consisting of a cation exchange membrane, bipolar membrane, and anion exchange membrane was employed. The thickness of IEMs was divided into three categories: 16–17 µm (PCEM/ PAEM), 100–150 µm (CD100/AD100, InnoSep-C/InnoSep-A), and 380–400 µm (MC-3470/MA-3475). In membranes with a thickness of 100–150 µm, the ammonia nitrogen removal was exceptionally high at 99.2 %, with a recovery of 98.7 % (CD100/AD100: 100 µm) at a current density of 80 mA/cm2, and the system exhibited low specific energy consumption (SEC) of 8.87 kWh/kg-N (InnoSep-C/InnoSep-A: 150 µm) at a current density of 20 mA/cm2. However, when using the thinnest IEMs (PCEM: 16 µm, PAEM: 17 µm), an increase in total ammonia nitrogen loss (10.21–22.01 %) led to a reduction in ammonia nitrogen recovery. Conversely, when using the thickest IEMs (MC-3470: 380 µm, MA-3475: 400 µm), the SEC was extremely high (30.98–126.67 kWh/kg-N), making them unsuitable for the BMED process for ammonia nitrogen removal and recovery. Based on these experimental results, it is suggested to use an optimal thickness of IEM within the range of 80–250 µm in the BMED systems to achieve a balanced performance, maximizing ammonia nitrogen removal and recovery while minimizing the SEC. Consequently, the present study provides a comprehensive understanding of the effects of the IEM thicknesses on the BMED systems and could offer a practical perspective for optimizing the BMED stack with respect to improving the removal and recovery rate of ammonia nitrogen and reducing the energy consumption.

Hydraulic Permeation-Induced Water Concentration Gradients in Ion Exchange Membranes

Although substantial literature has detailed transport through swollen membranes, there are still conflicting reports regarding the fundamental physics of hydraulic water transport in such materials, with a long-standing debate centered on the applicability of the pore-flow and solution-diffusion mechanisms. The critical thermodynamic distinction between these models is the presence of a membrane-phase concentration gradient. For pure solvent transport, the pore-flow model presumes that a membrane-phase pressure gradient drives transport in the absence of a concentration gradient. In contrast, the solution-diffusion model proposes that a membrane-phase concentration gradient drives transport and that there is a negligible pressure gradient through the membrane. To address this topic for ion exchange membranes, this study reports measurements of water concentration profiles in pressurized films of Nafion and a commercial membrane based on sulfonated poly(styrene-co-divinylbenzene). Several films were stacked together in a custom-built permeation cell, with the feed side pressurized to between 68.9 and 137.9 bar, and the permeate side held at atmospheric conditions. At steady state, the experiment was stopped, the films were separated, and the water content of each film was measured, revealing a water concentration gradient through the stack thickness that increased with increasing feed pressure. The experimental data were in good agreement with theoretical predictions based on Fick’s law and polymer-solvent swelling theories (i.e., Flory-Huggins and Flory-Rehner). These results are consistent with the solution-diffusion model and cannot be explained by a pore-flow mechanism.

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

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

Waste Acid Recovery Utilizing Monovalent Cation Permselective Membranes through Selective Electrodialysis

Selective electrodialysis (SED) has surfaced as a highly promising membrane separation technique in the realm of acid recovery owing to its ability to effectively separate monovalent ions through the utilization of a potential difference. However, the current SED process is limited by conventional commercial monovalent cation permselective membranes (MCPMs). This study systematically investigates the use of an independently developed MCPM in the SED process for acid recovery. Various factors such as current density, volume ratio, initial ion concentration, and waste acid systems are considered. The independently developed MCPM offers several advantages over the commercial monovalent selective cation-exchange membrane (CIMS), including higher recovered acid concentration, better ion flux ratio, improved acid recovery efficiency, increased recovered acid purity, and higher current efficiency. The SED process with the MCPM achieves a recovered acid of 95.9% and a concentration of 2.3 mol·L–1 in the HCl/FeCl2 system, when a current density of 20 mA·cm–2 and a volume ratio of 1:2 are applied. Similarly, in the H2SO4/FeSO4 system, a purity of over 99% and a concentration of 2.1 mol·L–1 can be achieved in the recovered acid. This study thoroughly examines the impact of operation conditions on acid recovery performance in the SED process. The independently developed MCPM demonstrates outstanding acid recovery performance, highlighting its potential for future commercial utilization.

Achievement of Efficient and Stable Nonflow Zinc-Bromine Batteries Assisted by Rational Decoration upon the Two Electrodes.

Aqueous zinc-bromine batteries (ZBBs) are highly promising because of the advantages of safety and cost. Compared with flow ZBBs, static ones without the assistance of pumping and tank components possess decreased cost and increased energy density and efficiency. Yet, the issues of Zn dendrites and shuttle effect of polybromide ions (Brn-) are more serious in nonflow ZBBs. Meanwhile, the hydrogen evolution reaction (HER) and the sluggish kinetics of the Br2/Br- couple are also in-negligible. Herein, a compressive approach, the cation-exchange membrane (CEM) coating on Zn anodes and N-defect decoration toward carbon felt cathodes, is developed. The CEM with cation-only function can inhibit the formation of Zn dendrites via tuning the Zn2+ flow at the interface, block the noncationic substances, and hence prevent the shuttle of Br2/Brn- and the water decomposition-concerned HER. The optimized nonflow ZBBs can deliver high Coulombic, voltage, and energy efficiencies of 94.1, 92.8, and 87.4%, respectively, which can be well remained in 1000 cycles. Meanwhile, the output voltage is as high as 1.7 V at 10 mA cm-2 with a high areal capacity of 2 mA h cm-2, and a LED with a rated voltage of 1.6 V can be powered successfully, exhibiting high application value.

Advancements in Bipolar Membrane Electrodialysis Techniques for Carbon Capture.

Bipolar membrane electrodialysis (BMED) is a promising technology for the capture of carbon dioxide (CO2) from seawater, offering a sustainable solution to combat climate change. BMED efficiently extracts CO2 while generating valuable byproducts like hydrogen and minerals, contributing to the carbon cycle. The technology relies on ion-exchange membranes and electric fields for efficient ion separation and concentration. Recent advancements focus on enhancing water dissociation in bipolar membranes (BPMs) to improve efficiency and durability. BMED has applications in desalination, electrodialysis, water splitting, acid/base production, and CO2 capture and utilization. Despite the high efficiency, scalability, and environmental friendliness, challenges such as energy consumption and membrane costs exist. Recent innovations include novel BPM designs, catalyst integration, and exploring direct air/ocean capture. Research and development efforts are crucial to unlocking BMED's full potential in reducing carbon emissions and addressing environmental issues. This review provides a comprehensive overview of recent advancements in BMED, emphasizing its role in carbon capture and sustainable environmental solutions.

Efficient removal of short-chain perfluoroalkyl substances (PFAS) using asymmetric membrane capacitive deionization

Concerns have been raised over the persistence and potential health risks of perfluoroalkyl substances (PFAS). PFAS pose significant challenges in water treatment owing to their chemical stability and the difficulty of treating them with conventional purification methods. Although traditional water treatment methods for contaminant removal have been explored, their effectiveness against PFAS remains limited. This gap in knowledge highlights the need for innovative approaches. This study introduces asymmetric membrane capacitive deionization (ACDI) as a novel method for short-chain PFAS removal to address the shortcomings of existing techniques. ACDI refers to a configuration in which an anion exchange membrane is removed from a conventional membrane capacitive deionization with both a cation exchange membrane and an anion exchange membrane. Among the PFAS, perfluorobutanesulfonic acid (PFBS), which is known to be difficult to remove because of its short chain, has been the focus. The ACDI system demonstrated superior deionization capacity (28.75 mg/g) and energy efficiency (energy consumption of 0.46 Wh/g) compared to conventional CDI and membrane CDI systems. In addition, the effects of key operating parameters, such as voltage (0.5 ∼ 1.2 V), initial concentration (50 ∼ 500 mg/L), and flow rate (1 ∼ 5 mL/min), on the PFBS removal were systematically investigated and superior at 1.2 V, 500 mg/L, and 2 mL/min. This study suggests the need for further exploration of real-world applications considering the presence of competing ions in wastewater. This study provides valuable insights into advancing CDI technology for the sustainable treatment of industrial wastewater containing PFAS.

Visualized in-sensor computing

In artificial nervous systems, conductivity changes indicate synaptic weight updates, but they provide limited information compared to living organisms. We present the pioneering design and production of an electrochromic neuromorphic transistor employing color updates to represent synaptic weight for in-sensor computing. Here, we engineer a specialized mechanism for adaptively regulating ion doping through an ion-exchange membrane, enabling precise control over color-coded synaptic weight, an unprecedented achievement. The electrochromic neuromorphic transistor not only enhances electrochromatic capabilities for hardware coding but also establishes a visualized pattern-recognition network. Integrating the electrochromic neuromorphic transistor with an artificial whisker, we simulate a bionic reflex system inspired by the longicorn beetle, achieving real-time visualization of signal flow within the reflex arc in response to environmental stimuli. This research holds promise in extending the biomimetic coding paradigm and advancing the development of bio-hybrid interfaces, particularly in incorporating color-based expressions.

Efficient chlorine evolution of CuO modified RuO2 electrode and its application in preparation of electrochemical disinfectant

Electrochemical chlorine evolution reaction (CER) is one of the most important practical applications of electrochemical reaction, which can be used for electrochemical disinfection. The electrochemical disinfectant of acidic electrolyzed water (AEW) is obtained by electrolyzing extremely dilute chlorine-containing solution in ion-exchange membrane electrolyzer. In the anode area, HClO is produced by CER, which has a strong bactericidal effect. Due to the low concentration of chloride ion, it is necessary to improve the CER selectivity of the electrode in order to improve the efficiency of chlorine electrolysis. In this paper, CuO/RuO2 electrode was prepared by the two-step thermal decomposition method, and the surface morphology and electronic structure of RuO2 electrode were modified. Compared with the ordinary RuO2 electrode, the CER activity of CuO/RuO2 electrode is improved. For the oxygen evolution reaction (OER), the activity of CuO/RuO2 electrode is significantly lower than that of RuO2 electrode. The CER mechanism of CuO/RuO2 electrode is the second electron transfer, and the OER mechanism is the formation and transformation of OHads. The potential difference between CER and OER of CuO/RuO2-3 electrode is 105 mV, which is 52 mV higher than that of RuO2 electrode, so the selectivity of CER of CuO/RuO2 electrode is remarkably improved. During the preparation of AEW, the HClO of AEW2 (prepared by CuO/RuO2-3 electrode) is 0.72 mmol/L, which is 2.5 times that of AEW1 (prepared by RuO2 electrode). In the sterilization experiments on E. coli and B. subtilis, the germicidal efficacy of AEW2 (3.71 and 2.99 log10 CFU/mL) is obviously superior to that of AEW1 (0.76 and 0.61 log10 CFU/mL).

Enhancing selective ammonium transport in membrane electrochemical systems

Recovering ammonia nitrogen from wastewater is a sustainable strategy that simultaneously addresses both nitrogen removal and fertilizer production. Membrane electrochemical system (MES), which utilizes electrochemical redox reactions to transport ammonium ions through cation exchange membranes, has been considered as an effective technology for ammonia recovery from wastewater. In this study, we develop a mathematical model to systematically investigate the impact of co-existing ions on the transport of ammonium (NH4+) ions in MES. Our analysis elucidates the importance of pH values on both the NH4+ transport and inert ion (Na+) transport. We further comprehensively assess the system performance by varying the concentration of Na+ in the system. We find that while the inert cation in the initial anode compartment competes with NH4+ transport, NH4+ dominates the cation transport in most cases. The transport number of Na+ surpasses NH4+ only if the fraction of Na+ to total cation is extremely high (>88.5%). Importantly, introducing Na+ ions into the cathode compartment significantly enhances the ammonia transport due to the Donnan dialysis. The analysis of selective ion transport provides valuable insights into optimizing both selectivity and efficiency in ammonia recovery from wastewater.

Towards optimized cation-exchange membranes for overlimiting current electrodialysis: Correlation between size of resin particles in membranes and mechanism of ion transport through them

Experimental heterogeneous membranes with different particle sizes of cation-exchange resin KU-2–8 (the diameter of resin particles varied from 20 to 71 μm) were manufactured under the production conditions of LLC “IE “Shchekinoazot”. It is established that at currents higher than the limiting current the main mechanisms of electrolyte ion transport are classical electrodiffusion and electroconvection. It is shown for the first time that at the same degree of membrane polarization i/ilim the fraction of electroconvection contribution increases, and the fraction of contribution of water splitting reaction products decreases with decreasing size of resin particles in membranes. The theoretical evaluation of the influence of the size of resin particles in the membrane on the internal parameters of electromembrane systems MK-40/0.01M NaCl solution has shown that the crucial role in the development of electroconvection is played not by the field effect, but by the peculiarities of electrical heterogeneity of the membrane surface.

Deep eutectic solvents recovery with bipolar membrane electrodialysis in biorefinery: An economic study

The complex composition and shortage of recycle technology restricted application of deep eutectic solvents (DES). Bipolar membrane electrodialysis has been proven efficient for DES recovery in biorefinery. A comprehensive economic study was essential prior to industrial recovery of DES with bipolar membrane electrodialysis (BMED). An economic study of the BMED-based recovery technique was performed by ascertaining influence of major factors on DES recovery charge with sensitivity analysis. Recovery charge of three representative types of DES was found to be changed within 0.83–1.66 $/Kg (choline chloride-ethylene glycol), 1.60–3.88 $/Kg (lactic acid-ethylene glycol), and 1.21–2.44 $/Kg (choline chloride-urea) as each factor altered in the control. Proportion of annual operating duration and loan duration displayed a negative correlation with the total recovery charge of DES. Multiple of membrane price, factor of membrane facility charge, factor of ancillary facility charge, factor of annual maintaining charge, annual reduction of recovery ratio, annual increase of recovery energy consumption, and annual interest rate of loan showed a positive relationship with the recovery charge. Total DES recovery charge increased within 6.0 %–6.4 % when heterogeneous ion-exchange membranes were employed in BMED facility instead of homogeneous ion-exchange membranes. This study verified the economic virtue of BMED-based DES recovery strategy in biorefinery.

On optimizing the experimental setup for estimation of the thermal conductivity of thin films

AbstractThe aim of this study is to show the optimal arrangement of a measurement system for estimating the thermal conductivity of thin films from temperature profiles. For this purpose, two different experimental setup systems, with square and circular cross sections, were designed to estimate the thermal conductivity of thin films and, in particular, of two ion exchange membranes. Both systems were placed horizontally and vertically in order to evaluate the best orientation to more accurately determine thermal conductivity. A three-dimensional numerical simulation was performed using Comsol Multiphysics to predict the heat flow and temperature gradient and to evaluate the effect of the geometry and the orientation on the contact resistances. Each system was first calibrated without the membrane inside in order to estimate all the necessary thermal properties of the different materials of the model. Next, the membrane was placed inside the model, so that the model now includes the thermal conductivity of the membrane as the only unknown parameter. The numerical results were compared with the various measured temperature profiles to estimate the thermal conductivity. The thermal conductivity values of the well-known Nafion 117 membrane and other thicker membrane were determined. A very good agreement with reliable literature values was obtained. The approach presented here, combining experimental and simulated temperature profiles, may provide the basis for a practical alternative to better estimate the thermal conductivity of thin films.

Heavy Metal Removal from Aqueous Solutions Using a Customized Bipolar Membrane Electrodialysis Process.

Lack of safe water availability and access to clean water cause a higher risk of infectious diseases and other diseases as well. Heavy metals (HMs) are inorganic pollutants that cause severe threats to humans, animals, and the environment. Therefore, an effective HM removal technology is urgently needed. In the present study, a customized bipolar membrane electrodialysis process was used to remove HMs from aqueous solutions. The impacts of the feed ionic strength, applied electrical potential, and the type and concentration of HMs (Cd2+, Co2+, Cr3+, Cu2+, and Ni2+) on the process performance were investigated. The results showed that feed solution pH changes occurred in four stages: it first decreased linearly before stabilizing in the acidic pH range, followed by an increase and stabilization in the basic range of the pH scale. HM speciation in the basic pH range revealed the presence of anionic HM species. The presence of HMs on anion exchange membranes confirmed the contribution of these membranes for HM removal in the base channels of the process. While no clear trend was seen in the ionic strength solution, the maximum HM removal was observed when 1.5 g/L NaCl was used. The initial HM concentration showed a linear increase in HMs removal of up to 30 mg/L. A similar trend was seen with an increase in the applied electrical potential of up to 15 V. In general, the amount of HMs removed increased in the following order: Cd2+ ˃ Ni2+ ˃ Co2+ ˃ Cu2+ ˃ Cr3+. Under some operational conditions, however, the removed amount of Cu2+, Co2+, and Ni2+ was similar. The mass balance and SEM-EDX results revealed that the removed HMs were sorbed onto the membranes. In conclusion, this process efficiently separates HMs from aqueous solutions. It showed the features of diluate pH adjustment, reduction in the overall stack electrical resistance, and contribution of anion exchange membranes in multivalent cation removal. The mechanisms involved in HMs removal were diffusion and migration from the bulk solution, followed by their sorption on both cation and anion exchange membranes.

Evaluation Study of Ammonium Removal from Groundwater by Electrodialysis: Case Study of Real Groundwater from the City of Kenitra in Morocco.

In this work, we investigated the feasibility of ammonium removal by electrodialysis (ED), a well-known electro-membrane process, based on the selective migration of anions and cations through anion exchange membranes (AEMs) and cation exchange membranes (CEMs). ED experiments are performed using a laboratory pilot. The ion exchange membranes (IEMs) pair used is AXE/CMX, AXE as an AEMs and CMX as a CEMs. The first tests are performed with real groundwater solutions from Kenitra city (Morocco), spiked with an initial concentration of 3 mg/L NH4Cl. The results gave a specific demineralization (SD) to NH4 + ions of 84.60 %; for a demineralization rate (DR) of 80 % and a time of 60 min. The multivariate principal component analysis (PCA) presents a total inertia of 99.23 %, the majority of the variables of which are positively correlated on the C1 axis with a variance of 95.5 % than that of C2 of 3.78 %. The quality of the diluted water determined by the Legrand-Poirier method showed that the water was aggressive and that the addition of 4.54 mg/L Ca2+ was necessary to balance the water and make it fit for human consumption.