Sulfonated reduced graphene oxide modification layers to improve monovalent anions selectivity and controllable resistance of anion exchange membrane

Sulfonated reduced graphene oxide encapsulated perovskite-type ErCoFe oxide nanoparticles for efficient electrochemical water oxidation

Perovskite oxides play a vital role as electrocatalysts in water oxidation due to their flexible and unique electronic structures. In this work, Er-based perovskites ErCo1-xFexO3-δ (x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) denoted as EC, ECF-0.9, ECF-0.7, ECF-0.5, ECF-0.3, and EF, respectively, are synthesized by the sol-gel method. Then, ECF-0.9 is supported on sulfonated reduced graphene oxide (S-rGO) by a hydrothermal method, with weight ratios of 1 : 1 and 3 : 1 of ECF/0.9 to S-rGO (shown as ECF-0.9/S-rGO(50%) and ECF-0.9/S-rGO(75%), respectively). The structural properties and the morphology of the synthesized materials are studied using a series of different techniques. The prepared perovskites are then used as electrode materials for electrochemical water oxidation. ECF-0.9 reveals better activity than pure EF, EC, and other perovskite oxides in terms of overpotential, Tafel slope, electrochemically active surface area (ECSA), and charge transfer resistance (Rct) values. Interestingly, when the optimized perovskite oxide catalyst ECF-0.9 is decorated on S-rGO sheets, the water oxidation activity is significantly improved. ECF-0.9/S-rGO(75%) exhibits superior activity for water oxidation with an overpotential of 290 mV@10 mA cm-2 and a Tafel slope of 41 mV dec-1. Finally, overall water splitting with ECF-0.9/S-rGO(75%) as the anode electrode shows a low electrolysis voltage of 1.60 V, alongside excellent stability for 20 h.

Enhanced reduction of graphene oxide via laser-dispersion coupling: Towards large-scale, low-defect graphene for crease-free heat-dissipating membranes in advanced flexible electronics

The advancement of flexible electronics demands improved components, necessitating heat dissipation membranes (HDMs) to exhibit high thermal conductivity while maintaining structural integrity and performance stability even after extensive deformation. Herein, we have devised a laser-modulated reduction technique for graphene oxide (GO), enabling the fabrication of high-quality, large-scale, low-defect graphene, which yields high-performance HDMs after orderly deposition. The work underscores the crucial role of the laser wavelength and dispersion liquid's coupling intensity in influencing the morphology and properties of graphene. Optimal coupling effect and energy conversion are realized when a laser of 1064 nm wavelength irradiates a triethylene glycol (TEG)/N,N-Dimethylformamide (DMF) dispersion. This unique synergy generates high transient energy, which facilitates the deprotonation process and ensures a swift, comprehensive GO reduction. In contrast to conventional water-based laser reduction methods, the accelerated reaction magnifies the size of the graphene sheets by mitigating the ablation effect. After membrane construction with an ordered structure, the corresponding membrane exhibits a high thermal conductivity of 1632 W m-1 K-1, requiring only ∼1/10 of the total preparation time required by other reported methods. Remarkably, the resulting HDM demonstrates superior resilience against creasing and folding, maintaining excellent smoothness and negligible reduction in thermal conductivity after violent rubbing. The combination of exceptional flexibility and thermal conductivity in HDMs paves the way for long-term practical use in the flexible electronics industry.

Oxidative MnO2 Template Assisted Electrochemical Fabrication of Graphene/Polypyrrole Supercapacitor Electrodes

Improving the morphological structure of active materials is a reliable strategy for the fabrication of high-performance supercapacitor electrodes. In this study, we introduce a feasible approach to constructing the graphene/polypyrrole (PPy) composite film implanted onto the current collector through a two-step electrochemical deposition method utilizing MnO2 as an intermediary template. The reduced graphene oxide (rGO) hydrogel film is first hydrothermally grown on a carbon cloth (CC) substrate to obtain a porous rGO@CC electrode on which MnO2 is electrodeposited. Then the as-prepared rGO/MnO2@CC electrode is subjected to the electrochemical polymerization of pyrrole, with MnO2 acting as an oxidizing template to facilitate the oxidative polymerization of pyrrole, ultimately yielding an rGO/PPy composite film on CC. The PPy synthesized via this methodology exhibits a distinctive interconnected structure, resulting in superior electrochemical performance compared with the electrode with PPy directly electrodeposited on rGO@CC. The optimized electrode achieves an impressive specific capacitance of 583.6 F g-1 at 1 A g-1 and retains 83% of its capacitance at 20 A g-1, with a capacitance loss of only 9.5% after 5000 charge-discharge cycles. The corresponding all-solid-state supercapacitor could provide a high energy density of 22.5 Wh kg-1 and a power density of 4.6 kW kg-1, with a capacitance retention of 82.7% after 5000 charge-discharge cycles. Furthermore, the device also demonstrates good flexibility performance upon bending at 90 and 180°. This work presents an innovative method for the preparation of carbon material/conducting polymer electrodes with specific structural characteristics and superior performance.

Surface-Modified Pore-Filled Anion-Exchange Membranes for Efficient Energy Harvesting via Reverse Electrodialysis

In this study, novel pore-filled anion-exchange membranes (PFAEMs) modified with polypyrrole (PPy) and reduced graphene oxide (rGO) were developed to improve the energy harvesting performance of reverse electrodialysis (RED). The surface-modified PFAEMs were fabricated by varying the contents of PPy and rGO through simple spin coating and chemical/thermal treatments. It was confirmed that the PPy and PPy/rGO layers introduced on the membrane surface did not significantly increase the electrical resistance of the membrane and could effectively control surface characteristics, such as structural tightness, hydrophilicity, and electrostatic repulsion. The PPy/rGO-modified PFAEM showed excellent monovalent ion selectivity, more than four times higher than that of the commercial membrane (AMX, Astom Corp., Tokyo, Japan). This means that the PPy/rGO layer can effectively reduce the permeation of multivalent ions with a high charge intensity and a relatively large hydration radius compared to monovalent ions. The results of evaluating the performance of the surface-modified PFAEMs by applying them to a RED cell revealed that the decrease in potential difference occurring in the membrane was reduced by effectively suppressing the uphill transport of multivalent ions. Consequently, the PPy/rGO-modified membrane exhibited a 5.43% higher power density than the AMX membrane.

Ionic Control of Functional Zeolitic Imidazolate Framework-Based Membrane for Tailoring Selectivity toward Target Ions

Ion exchange membranes with strong ionic separation performance have strategic importance for resource recovery and water purification, but the current state-of-the-art membranes suffer from inadequate ion selective transport for the target ions. This work proposes a new class of zeolitic imidazolate framework (ZIF)-based anion exchange membranes (named as S@ZIF-AMX) with suppressed multivalent anion mobility and enhanced target ion transport via an ionic control strategy under alternating current driven assembly. In electrodialysis with an initial concentration of 50 mM of NaBr, NaCl, Na₂SO₄, and Na₃PO₄ (mixed feed) and a current density of 10 mA cm^-2, the S@ZIF-AMX membrane demonstrated an excellent transport of the target ion (Cl^-) based on the synergy between the Cl^- regulated ZIF cavity and the electrostatic interaction with sulfonic groups. The separation efficiency and permselectivity of PO₄^3-/Cl^- through S@ZIF-AMX largely increased to 83% and 32, respectively, compared to 42% and 4.0 of the pristine AMX membrane (a commercial anion exchange membrane), respectively. Furthermore, the separation between SO₄^2- and Cl^- was also enhanced, the separation efficiency and permselectivity of SO₄^2-/Cl^- increased from 11% and 1.4 to 45% and 4.3, respectively. In addition, the combined strategy developed in the S@ZIF-AMX membrane was proven effective in promoting Cl^- transport by shifting the separation equilibrium of the ion pair Br^-/Cl^-, which is known to be extremely challenging. This work provides a new design strategy toward pushing the limits of current ion exchange membranes for target ion separation in water, resource, and energy applications.

Anion Exchange Membranes with 1D, 2D and 3D Fillers: A Review

Hydroxide exchange membrane fuel cells (AEMFC) are clean energy conversion devices that are an attractive alternative to the more common proton exchange membrane fuel cells (PEMFCs), because they present, among others, the advantage of not using noble metals like platinum as catalysts for the oxygen reduction reaction. The interest in this technology has increased exponentially over the recent years. Unfortunately, the low durability of anion exchange membranes (AEM) in basic conditions limits their use on a large scale. We present in this review composite AEM with one-dimensional, two-dimensional and three-dimensional fillers, an approach commonly used to enhance the fuel cell performance and stability. The most important filler types, which are discussed in this review, are carbon and titanate nanotubes, graphene and graphene oxide, layered double hydroxides, silica and zirconia nanoparticles. The functionalization of the fillers is the most important key to successful property improvement. The recent progress of mechanical properties, ionic conductivity and FC performances of composite AEM is critically reviewed.

A Lamellar MXene (Ti3 C2 Tx )/PSS Composite Membrane for Fast and Selective Lithium-Ion Separation

A two-dimensional (2D) laminar membrane with Li⁺ selective transport channels is obtained by stacking MXene nanosheets with the introduction of poly(sodium 4-styrene sulfonate) (PSS) with active sulfonate sites, which exhibits excellent Li⁺ selectivity from ionic mixture solutions of Na⁺ , K⁺ , and Mg²⁺ . The Li⁺ permeation rate through the MXene@PSS composite membrane is as high as 0.08 mol m^-2  h^-1 , while the Li⁺ /Mg²⁺ , Li⁺ /Na⁺ , and Li⁺ /K⁺ selectivities are 28, 15.5, and 12.7, respectively. Combining the simulation and experimental results, we further confirm that the highly selective rapid transport of partially dehydrated Li⁺ within subnanochannels can be attributed to the precisely controlled interlayer spacing and the relatively weaker ion-terminal (-SO₃ ^- ) interaction. This study deepens the understanding of ion-selective permeation in confined channels and provides a general membrane design concept.

Selectivity of Transport Processes in Ion-Exchange Membranes: Relationship with the Structure and Methods for Its Improvement

Nowadays, ion-exchange membranes have numerous applications in water desalination, electrolysis, chemistry, food, health, energy, environment and other fields. All of these applications require high selectivity of ion transfer, i.e., high membrane permselectivity. The transport properties of ion-exchange membranes are determined by their structure, composition and preparation method. For various applications, the selectivity of transfer processes can be characterized by different parameters, for example, by the transport number of counterions (permselectivity in electrodialysis) or by the ratio of ionic conductivity to the permeability of some gases (crossover in fuel cells). However, in most cases there is a correlation: the higher the flux density of the target component through the membrane, the lower the selectivity of the process. This correlation has two aspects: first, it follows from the membrane material properties, often expressed as the trade-off between membrane permeability and permselectivity; and, second, it is due to the concentration polarization phenomenon, which increases with an increase in the applied driving force. In this review, both aspects are considered. Recent research and progress in the membrane selectivity improvement, mainly including a number of approaches as crosslinking, nanoparticle doping, surface modification, and the use of special synthetic methods (e.g., synthesis of grafted membranes or membranes with a fairly rigid three-dimensional matrix) are summarized. These approaches are promising for the ion-exchange membranes synthesis for electrodialysis, alternative energy, and the valuable component extraction from natural or waste-water. Perspectives on future development in this research field are also discussed.

Surface Modifications of Anion Exchange Membranes for an Improved Reverse Electrodialysis Process Performance: A Review

Reverse electrodialysis (RED) technology represents a promising electro-membrane process for renewable energy harvesting from aqueous streams with different salinity. However, the performance of the key components of the system, that is, the ion exchange membranes, is limited by both the presence of multivalent ions and fouling phenomena, thus leading to a reduced generated net power density. In this context, the behavior of anion exchange membranes (AEMs) in RED systems is more severely affected, due to the undesirable interactions between their positively charged fixed groups and, mostly negatively charged, foulant materials present in natural streams. Therefore, controlling both the monovalent anion permselectivity and the membrane surface hydrophilicity is crucial. In this respect, different surface modification procedures were considered in the literature, to enhance the above-mentioned properties. This review reports and discusses the currently available approaches for surface modifications of AEMs, such as graft polymerization, dip coating, and layer-by-layer, among others, mainly focusing on preparing monovalent permselective AEMs with antifouling characteristics, but also considering hydrophilicity aspects and identifying the most promising modifying agents to be utilized. Thus, the present study aimed at providing new insights for the further design and development of selective, durable, and cost-effective modified AEMs for an enhanced RED process performance, which is indispensable for a practical implementation of this electro-membrane technology at an industrial scale.

In-Situ Combination of Bipolar Membrane Electrodialysis with Monovalent Selective Anion-Exchange Membrane for the Valorization of Mixed Salts into Relatively High-Purity Monoprotic and Diprotic Acids

The crystalized mixed salts from the zero liquid discharge process are a hazardous threat to the environment. In this study, we developed a novel electrodialysis (SBMED) method by assembling the monovalent selective anion-exchange membrane (MSAEM) into the bipolar membrane electrodialysis (BMED) stack. By taking the advantages of water splitting in the bipolar membrane and high perm-selectivity of MSAEM for the Cl- ions against the SO42- ions, this combination allows the concurrent separation of Cl-/SO42- and conversion of mixed salts into relatively high-purity monoprotic and diprotic acids. The current density has a significant impact on the acid purity. Both the monoprotic and diprotic acid purities were higher than 80% at a low current density of 10 mA/cm2. The purities of the monoprotic acids decreased with an increase in the current density, indicating that the perm-selectivity of MSAEM decreases with increasing current density. An increase in the ratio of monovalent to divalent anions in the feed was beneficial to increase the purity of monoprotic acids. High-purity monoprotic acids in the range of 93.9-96.1% were obtained using this novel SBMED stack for treating simulated seawater. Therefore, it is feasible for SBMED to valorize the mixed salts into relatively high-purity monoprotic and diprotic acids in one step.

Characterization of Poly(Acrylic) Acid-Modified Heterogenous Anion Exchange Membranes with Improved Monovalent Permselectivity for RED

The performance of anion-exchange membranes (AEMs) in Reverse Electrodialysis is hampered by both presence of multivalent ions and fouling phenomena, thus leading to reduced net power density. Therefore, we propose a monolayer surface modification procedure to functionalize Ralex-AEMs with poly(acrylic) acid (PAA) in order to (i) render a monovalent permselectivity, and (ii) minimize organic fouling. Membrane surface modification was carried out by putting heterogeneous AEMs in contact with a PAA-based aqueous solution for 24 h. The resulting modified membranes were firstly characterized by contact angle, water uptake, ion exchange capacity, fixed charge density, and swelling degree measurements, whereas their electrochemical responses were evaluated through cyclic voltammetry. Besides, their membrane electro-resistance was also studied via electrochemical impedance spectroscopy analyses. Finally, membrane permselectivity and fouling behavior in the presence of humic acid were evaluated through mass transport experiments using model NaCl containing solutions. The use of modified PAA-AEMs resulted in a significantly enhanced monovalent permselectivity (sulfate rejection improved by >35%) and membrane hydrophilicity (contact angle decreased by >15%) in comparison with the behavior of unmodified Ralex-AEMs, without compromising the membrane electro-resistance after modification, thus demonstrating the technical feasibility of the proposed membrane modification procedure. This study may therefore provide a feasible way for achieving an improved Reverse Electrodialysis process efficiency.

Mussel-Inspired Surface Functionalization of AEM for Simultaneously Improved Monovalent Anion Selectivity and Antibacterial Property

A facile membrane surface modification process for improving permselectivity and antimicrobial property was proposed. A polydopamine (PDA) coating was firstly fabricated on pristine anion exchange membrane (AEM), followed by in situ reduction of Ag without adding any extra reductant. Finally, 2,5-diaminobenzene sulfonic acid (DSA) was grafted onto PDA layer via Michael addition reaction. The as-prepared AEM exhibited improved permselectivity (from 0.60 to 1.43) and effective inhibition of bacterial growth. In addition, the result of the long-term (90-h continuous electrodialysis) test expressed the excellent durability of the modified layer on membrane surface, because the concentration of Cl- and SO₄²- in diluted chamber fluctuated ~0.024 and 0.030 mol·L-1 with no distinct decline. The method described in this work makes the full use of multifunctional PDA layer (polymer-like coating, in situ reduction and post-organic reaction), and a rational design of functional AEM was established for better practical application.

Comparative studies on fouling of homogeneous anion exchange membranes by different structured organics in electrodialysis

Five negatively charged organic compounds with different structures, sodium methane sulfonate (MS), sodium benzene sulfonate (BS), sodium 6-hydroxynaphthalene-2-sulfonate (NSS), sodium dodecyl sulfate (SDS), and sodium dodecyl benzene sulfonate (SDBS), were used to examine the fouling of an anion exchange membrane (AEM) in electrodialysis (ED), to explore the effect of molecular characteristics on the fouling behavior on the AEM and changes in the surface and electrochemical properties of the AEM. Results indicated that the fouling degree of the AEM by the different organics followed the order: SDBS > SDS > NSS > BS > MS. SDBS and SDS formed a dense fouling layer on the surface of the AEM, which was the main factor in the much more severe membrane fouling, and completely restricted the transmembrane ion migration. The other three organics caused fouling of the AEM by adsorption on the surface and /or accumulation in the interlayer of the AEM, and exhibited almost no influence on the transmembrane ion migration. It was also concluded that the organics with benzene rings caused more severe fouling of the AEM due to the stronger affinity interaction and steric effect between the organics and the AEM compared with organics with aliphatic chains.

Technology-driven layer-by-layer assembly of a membrane for selective separation of monovalent anions and antifouling

Selective separation of monovalent anions with reduced fouling is one of the major challenges for anion exchange membranes (AEM) in electrodialysis (ED). In this research, an alternating current layer-by-layer (AC∼LbL) assembly technology was first proposed and then applied to the construction of a durable multilayer with the selective separation of monovalent anions with reduced fouling. Under an alternating current (AC) electric field, the hydrophilic poly(4-styrenesulfonic acid-co-maleic acid) sodium salt and 2-hydroxypropyltrimethyl ammonium chloride chitosan were homogenized and rapidly assembled on a commercial original AEM and then crosslinked using 1,4-bis(2',3'-epoxypropyl) perfluoro-1-butane. In ED, the permselectivity and the selective separation efficiency [separation parameter between sulfate (SO42-) and chloride (Cl-) ions] of the resulting membrane (AC∼LbL#7.5 AEM) were 4.87 and 62%, respectively, whereas the original AEM had corresponding parameters of 0.81 and -8%, respectively. Furthermore, the AC∼LbL#7.5 AEM still retained a permselectivity of 4.52 and a selective separation efficiency for Cl- of 57% after 96 h of ED operation. In addition, the AC∼LbL#7.5 AEM showed an excellent antifouling property when three types of organic fouling materials: sodium dodecylbenzenesulfonate, bovine serum albumin and humic acid were used as model foulants.

Potential-Dependent, Switchable Ion Selectivity in Aqueous Media Using Titanium Disulfide

The selective removal of ions by an electrochemical process is a promising approach to enable various water-treatment applications such as water softening or heavy-metal removal. Ion intercalation materials have been investigated for their intrinsic ability to prefer one specific ion over others, showing a preference for (small) monovalent ions over multivalent species. In this work, we present a fundamentally different approach: tunable ion selectivity not by modifying the electrode material, but by changing the operational voltage. We used titanium disulfide, which shows distinctly different potentials for the intercalation of different cations and formed binder-free composite electrodes with carbon nanotubes. Capitalizing on this potential difference, we demonstrated controllable cation selectivity by online monitoring the effluent stream during electrochemical operation by inductively coupled plasma optical emission spectrometry of aqueous 50 mm CsCl and MgCl₂ . We obtained a molar selectivity of Mg²⁺ over Cs⁺ of 31 (strong Mg preference) in the potential range between -396 mV and -220 mV versus Ag/AgCl. By adjusting the operational potential window from -219 mV to +26 mV versus Ag/AgCl, Cs⁺ was preferred over Mg²⁺ by 1.7 times (Cs preference).

Robust Multilayer Graphene-Organic Frameworks for Selective Separation of Monovalent Anions

The chemical and mechanical stability of graphene nanosheets was used in this work to design a multilayer architecture of graphene, grafted with sulfonated 4,4'-diaminodiphenyl sulfone (SDDS). Quaternized poly(phenylene oxide) (QPPO) was synthesized and mixed with SDDS (rGO-SDDS-rGO@QPPO), yielding a multilayer graphene-organic framework (MGOF) with positive as well as negative functional groups that can be applied as a versatile electrodriven membrane in electrodialysis (ED). Multilayer graphene-organic frameworks are a new class of multilayer structures, with an architecture having a tunable interlayer spacing connected by cationic polymer material. MGOF membranes were demonstrated to allow for an excellent selective separation of monovalent anions in aqueous solution. Furthermore, different types of rGO-SDDS-rGO@QPPO membranes were found to have a good mechanical strength, with a tensile strength up to 66.43 MPa. The membrane (rGO-SDDS-rGO@QPPO-2) also has a low surface electric resistance (2.79 Ω·cm²) and a low water content (14.5%) and swelling rate (4.7%). In addition, the selective separation between Cl^- and SO₄^2- of the MGOF membranes could be as high as 36.6%.

A novel nanofiltration membrane inspired by an asymmetric porous membrane for selective fractionation of monovalent anions in electrodialysis

The present study describes the synthesis of new nanofiltration membranes inspired by asymmetric porous membranes used as monovalent anion selective membranes for electro-membrane separation.