Commercial Cation Exchange Membrane Research Articles

A Stable Polymer-Blended Anion-Exchange Membrane for Enhanced Performance in Redox Flow Batteries

Redox flow batteries (RFBs) operate with a membrane, responsible for the separation of cationic active species and allowing the passage of counter-ions, to preserve electroneutrality on each side of RFBs. Commercial cation-exchange membranes (CEMs), such as Nafion®, possess excellent ionic conductivity and chemical stability. However, they face challenges in blocking cationic active species common in RFBs, leading to significant capacity loss over time. Anion-exchange membranes (AEMs) efficiently block active species with the assistance of the Donnan effect. Nevertheless, commercial AEMs lack acid stability and conductivity compared to their cationic counterparts.Polytetrafluoroethylene (PTFE)-reinforced quaternary ammonium cardo poly(ether)ketone (QPEK-C)-based membranes with Al2O3 additives (PQPAM), a heterogeneous AEM developed in our lab1, possess excellent ionic conductivity with low permeability of active species and good mechanical properties. PQPAM has demonstrated better acid stability than commercial AEMs. However, its acidic and oxidative stability require improvement, as evidenced by degradation after 250 hours of in-situ harsh RFB cycling due to the heterogeneous nature of PQPAM and the exposure of the susceptible QPEK-C-rich side to the electrolytes2. To address this issue, we propose a strategy to enhance the chemical stability of QPEK-C-based membranes by inhibiting the exposure of QPEK-C to oxidation agents through blending with a hydrophobic engineering thermoplastic like polybenzimidazole (PBI)3,4. Furthermore, PBI's known ability to protonate in acidic media will reinforce ionic conductivity and the separation of active species, thus improving the Donnan effect, in addition to enhancing chemical and mechanical stability.Elemental analyses and microscopy images confirm a homogeneous blending of PBI and QPEK-C, promising stability improvement. The results of ex-situ tests on ionic conductivity, cation permeability, and chemical stability align with the mentioned hypotheses, showcasing that blend of PBI/QPEK-C outperforms pure PBI and QPEK-C AEMs. Additionally, the mechanical properties of the PBI/QPEK-C blended AEMs are improved compared to pure QPEK-C AEMs and PBI/QPEK-C blended AEMs outperform pure PBI and QPEK-C AEMs in in-situ cycling tests with our patented titanium-cerium RFBs5. Moreover, this work will explore other strategies to improve the blended AEM properties, such as utilizing inorganic fillers or PTFE reinforcement.

A comparative study on the influences of sulfonated- and phosphorated-graphene oxide on polyvinylidene fluoride cation-exchange membranes for electrodialysis

The –SO3H and –PO3H2 groups were grafted on graphene oxide (GO) by adding sulfuric acid and phosphoric acid at the same weight percentage, respectively. The physicochemical properties of the sulfonated GO (SGO) and phosphorated GO (PGO) were compared using different techniques. SGO and PGO were individually incorporated into the PVDF matrix (at the same wt%) to compare their effects on the properties of the resulting PVDF/SGO and PVDF/PGO cation-exchange membranes (CEMs). Membranes' characteristics including ionic conductivity and water content, area resistance, and the rate at which Na+ ions diffuse throughout the membranes changed considerably by altering the type of ion exchange sites, and more desirable values were obtained using PGO. Nevertheless, PVDF/SGO provided an ion exchange capacity (IEC) of 5.5 meq g−1, while it was about 1.7 times lower than PVDF/PGO (9.4 meq g−1). It also provided a promising ionic conductivity (17.24 mS cm−1), transport number (93.65 %), permselectivity (89.58 %), and low area resistance (2.7 Ω cm2), which exceeded those of the commercial CEMs such as Ralex. Moreover, PVDF/SGO exhibited desirable mechanical characteristics, and a low weight loss (2.4 %) and low resistance change (22.2 %) after 9 h being submerged in the Fenton reagent, however, these parameters were better for PVDF/PGO.

Multi-component ion equilibria and transport in ion-exchange membranes

At the interface between an ion-exchange membrane and a multi-electrolyte solution, charged species redistribute themselves to minimize the free energy of the system. In this paper, we explore the Donnan equilibrium of membranes with quaternary electrolyte (Na+/Mg2+/K+/Ca2+/Cl−) solutions, experimentally. The data was used to calculate the ion activity coefficients for six commercial cation-exchange membranes (CEMs). After setting one of the activity coefficients to an arbitrary value, we used the remaining (N-1) activity coefficients as fitting parameters to describe the equilibrium concentrations of (N) ionic species with a mean relative error of 3 %. At increasing solution ionic strengths, the equivalent ion fractions of monovalent counter-ions inside the membrane increased at the expense of the multivalent ones in alignment with the Donnan equilibrium theory. The fitted activity coefficients were employed in a transport model that simulated a Donnan dialysis experiment involving all four cations simultaneously. The arbitrary value assigned to one activity coefficient affects the calculated Donnan potential at the membrane interface. Nevertheless, this arbitrary value does not affect the prediction of the ion concentrations inside the membrane and consequently does not affect the modeled ion fluxes.

Energy harvesting from acid mine drainage using a highly proton/ion-selective thin polyamide film

A huge chemical potential difference exists between the acid mine drainage (AMD) and the alkaline neutralization solution, which is wasted in the traditional AMD neutralization process. This study reports, for the first time, the harvest of this chemical potential energy through a controlled neutralization of AMD using H+-conductive films. Polyamide films with controllable thickness achieved much higher H+ conductance than a commercially available cation exchange membrane (CEM). Meanwhile, the optimal polyamide film had an excellent H+/Ca2+ selectivity of 63.7, over two orders of magnitude higher than that of the CEM (0.3). The combined advantages of fast proton transport and high proton/ion selectivity greatly enhanced the power generation of the AMD battery. The power density was 3.1 W m–2, which is over one order of magnitude higher than that of the commercial CEM (0.2 W m–2). Our study provides a new sustainable solution to address the environmental issues of AMD while simultaneously enabling clean energy production.

Modelling of selective ion partitioning between ion-exchange membranes and highly concentrated multi-ionic brines

In recent years, the growing interest in the use of Ion-Exchange Membranes, in the treatment of highly concentrated multi-ionic brines for the selective recovery of critical elements, has prompted the research of fundamental models capable of predicting the IEMs selectivity towards like-charged species. Prior studies have proposed ion partitioning models limited to single-salt solutions that were validated only up to moderate salt concentrations. In this work, we developed a novel multi-ionic extension of the Manning counter-ion condensation model, aiming to predict the sorption selectivity of like-charged counter-ions. Furthermore, the peNRTL model was coupled with the extended Manning model to broaden its applicability range, encompassing membrane equilibrated with very highly concentrated solutions. Novel experimental ion sorption tests with single-salt and binary solutions including NaCl, KCl, MgCl2, and CaCl2 at high concentrations were performed with the commercial cation-exchange membrane Fumasep FKE-50. To the best of Authors’ knowledge, the proposed model for the first time successfully provided quantitative predictions of multi-ionic ion partitioning for all the systems investigated up to extremely high external salt concentrations. The outcomes of this work suggest a strong influence of the local non-electrostatic interactions on the activity coefficients in the membrane phase at high external concentration and highlight the key role of counter-ions hydration state in the condensation phenomenon.

Structural Characterization and Physicochemical Properties of Functionally Porous Proton-Exchange Membrane Based on PVDF-SPA Graft Copolymers.

Fluorinated proton-exchange membranes (PEMs) based on graft copolymers of dehydrofluorinated polyvinylidene fluoride (D-PVDF), 3-sulfopropyl acrylate (SPA), and 1H, 1H, 2H-perfluoro-1-hexene (PFH) were prepared via free radical copolymerization and characterized for fuel cell application. The membrane morphology and physical properties were studied via small-(SAXS) and wide-angle X-ray scattering (WAXS), SEM, and DSC. It was found that the crystallinity degree is 17% for PEM-RCF (co-polymer with SPA) and 16% for PEM-RCF-2 (copolymer with SPA and PFH). The designed membranes possess crystallite grains of 5-6 nm in diameter. SEM images reveal a structure with open pores on the surface of diameters from 20 to 140 nm. Their transport and electrochemical characterization shows that the lowest membrane area resistance (0.9 Ωcm2) is comparable to perfluorosulfonic acid PEMs (such as Nafion®) and polyvinylidene fluoride (PVDF) based CJMC cation-exchange membranes (ChemJoy Polymer Materials, China). Key transport and physicochemical properties of new and commercial membranes were compared. The PEM-RCF permeability to NaCl diffusion is rather high, which is due to a relatively low concentration of fixed sulfonate groups. Voltammetry confers that the electrochemical behavior of new PEM correlates to that of commercial cation-exchange membranes, while the ionic conductivity reveals an impact of the extended pores, as in track-etched membranes.

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.

On the Ionic Conductivity of Cation Exchange Membranes in Mixed Sulfates Using the Two-Phase Model.

The concentration dependence of the conductivity of ion exchange membranes (IEMs), as well as other transport properties, has been well explained by the contemporary two-phase model (Zabolotsky et al., 1993) considering a gel phase and an inter-gel phase filled with electroneutral solution. Here, this two-phase model has been adopted and first applied in electrolytes containing mixed counter-ions to investigate the correlation between the membrane ionic conductivity and its microstructure. For three representative commercial cation exchange membranes (CEMs), the total membrane conductivity (κT) when in equilibrium with mixed MgSO4 + Na2SO4 and H2SO4 + Na2SO4 electrolytes could be well predicted with the experimental composition of counter-ions in the gel and inter-gel phase, as well as the counter-ion mobility in the gel phase when the membrane is in a single electrolyte. It is found that the volume fraction of the inter-gel phase (f2) has little impact on the predicted results. The accuracy of the model can be largely improved by calculating the inter-gel phase conductivity (κin) with the ionic mobility being the same as that in the external solution (obtained via simulation in the OLI Studio), rather than simply as equivalent to the conductivity of the external solution (κs). Moreover, a nonlinear correlation between the CEMs' conductivities and the counter-ion composition in the gel phase is observed in the mixed MgSO4 + Na2SO4 solution, as well as for the Nafion117 membrane in the presence of sulfuric acid. For CEMs in mixed MgSO4 + Na2SO4 electrolytes, the calculated conductivity values considering the interaction parameter σ, similar to the Kohlrausch's law, are closer to the experimental ones. Overall, this work provides new insights into membrane conductivity with mixed counter-ions and testifies to the applicability of the contemporary two-phase model.

Solar-driven watersteam/brine production and brine-driven electricity generation by photothermal fabric coupled with osmotic membrane

The combination of solar-driven seawater evaporation and electricity generation has received increasing attention, and the traditional routes are based on the combination of the evaporator with thermogalvanic cells, thermoelectric or hydrovoltaic modules. But these combinations are relatively complex, and solid salt will crystallize during long-term seawater evaporation, limiting their practical applications. To solve the above problems, herein we have proposed a new kind of evaporation/electricity generation device by combining the photothermal fabric hanging-model evaporator and osmotic membrane-based generator. As a model of photothermal fabric, cotton fabric has been dyed with Chinese ink, and then the cotton-carbon fabric has high hydrophilicity and excellent photoabsorption efficiency (97.1%). Subsequently, the cotton-carbon fabric is set in an arc-shaped design between two seawater tanks for the construction of the hanging-model evaporator. Under the illumination of one sun, the evaporation rate of the hanging-model evaporator is as high as 1.94 kg m−2 h−1 without solid-salt accumulation, and the high-concentration brine (Na+ concentration: 1.07 ×105 mg L−1) can drip from the fabric bottom, realizing solar-driven watersteam and brine production with standing stability. Simultaneously, the brine is separated from the original seawater by a commercial cation exchange membrane, and two Ag/AgCl electrodes are used to construct an osmotic generator. It shows a high open voltage of 52 mV and the highest output power density of 116.52 mW m−2. Therefore, the present integration of the hanging-model evaporator and osmotic generator establishes a new path to continuously produce watersteam and brine under the sun and then to confer brine-driven electricity generation.

Graphene Oxide/Polyethyleneimine-Modified Cation Exchange Membrane for Efficient Selective Recovery of Ammonia Nitrogen from Wastewater.

Competition for the migration of interfering cations limits the scale-up and implementation of the Donnan dialysis process for the recovery of ammonia nitrogen (NH4+-N) from wastewater in practice. Highly efficient selective permeation of NH4+ through a cation exchange membrane (CEM) is expected to be modulated via tuning the surface charge and structure of CEM. In this work, a novel CEM was designed to form a graphene oxide (GO)-polyethyleneimine (PEI) cross-linked layer by introducing self-assembling layers of GO and PEI on the surface of a commercial CEM, which rationally regulates the surface charge and structure of the membrane. The resulting positively charged membrane surface exhibits stronger repulsion for divalent cations compared to monovalent cations according to Coulomb's law, while, simultaneously, GO forms π-metal cation conjugates between metal cations (e.g., Mg2+ and Ca2+), thus limiting metal cation transport across the membrane. During the DD process, higher NH4+ concentrations resulted in a longer time to reach Donnan equilibrium and higher NH4+ flux, while increased Mg2+ concentrations resulted in lower NH4+ flux (from 0.414 to 0.213 mol·m-2·h-1). Using the synergistic effect of electrostatic interaction and non-covalent cross-linking, the designed membrane, referred to as GO-PEI (20) and prepared by a 20 min impregnation in the GO-PEI mixture, exhibited an NH4+ transport rate of 0.429 mol·m-2·h-1 and a Mg2+ transport rate of 0.003 mol·m-2·h-1 in single-salt solution tests and an NH4+/Mg2+ selectivity of 15.46, outperforming those of the unmodified and PEI membranes (1.30 and 5.74, respectively). In mixed salt solution tests, the GO-PEI (20) membrane showed a selectivity of 15.46 (~1.36, the unmodified membrane) for NH4+/Mg2+ and a good structural stability after 72 h of continuous operation. Therefore, this facile surface charge modulation approach provides a promising avenue for achieving efficient NH4+-selective separation by modified CEMs.

Monovalent Cation Exchange Membranes with Janus Charged Structure for Ion Separation

Monovalent cation exchange membranes (M-CEMs) have been extensively applied in environmental remediation and energy harvesting such as the extraction of Na + or Li + from brine and seawater. However, owing to the limitations of membrane structures and materials, M-CEMs have a low perm-selectivity issue. Herein, we proposed a facile approach to construct a novel M-CEM with a Janus-charged structure, consisting of a positively-charged trimesic acid/polyethylenimine surface thin layer and a negatively charged commercial cation exchange membrane (CEM). Selectrodialysis results indicated that the Janus-charged M-CEMs could effectively suppress the migration of anions, which often occurred in porous CEMs, thereby enabling the novel Janus-charged M-CEMs to possess high perm-selectivity and high total cation fluxes. Compared with state-of-the-art M-CEMs, the Janus-charged M-CEM exhibited the highest perm-selectivity of 145.77 for Na + /Mg 2+ beyond the contemporary “Upper Bound” plot as well as the excellent perm-selectivity of 14.11 for Li + /Mg 2+ , indicating its great potentials in ion separation. This study can provide novel insights into the design of Janus-charged M-CEMs for ion separation in diverse environmental and energy applications.

Improvement of Li/Mg monovalent ion selectivity of cation-exchange membranes by incorporation of cerium or zirconium phosphate particles

To improve the selectivity of cation-exchange membranes to the transfer of lithium with respect to magnesium during the electrodialysis desalination of lithium and magnesium sulfates solutions, the surface of a commercial cation-exchange membrane based on sulfated polystyrene was modified with cerium(III, IV) and zirconium phosphates. Upon incorporation of phosphate particles, the Li/Mg selectivity coefficients of the membranes increased up to 113%.

Preparation of High-Performance Semihomogeneous Cation Exchange Membranes for Electrodialysis via Solvent-Free Polyethylene Particle-Confined Monomer Polymerization

The inherent two-phase structure of heterogeneous ion exchange membranes leads to defects in high area resistance and powder, which has greatly limited the application in electrodialysis. In this research, we have prepared a series of semihomogeneous cation exchange membranes from polyethylene-polystyrene (PE–PS) resin particles, which were synthesized via monomer absorption and PE particle-confined polymerization. The processing of semihomogeneous cation exchange membranes was conducted via thermoforming the PE–PS resin particles into films and subsequent sulfonation. The influence of the monomer content on water uptake, IEC, membrane area resistance, and electrodialysis performance was studied. The results proved that due to the excellent mechanical and film-forming characteristics of PE, the sulfonated membranes were still endowed with great mechanical properties, and the heating conditions enhanced the monomer absorption in polyethylene particles, which ensured high IEC for excellent electrodialysis performance after sulfonation. It was observed that the semihomogeneous cation exchange membrane (CEM-60) showed the highest NaCl desalination ratio (92.3%) and exhibited higher current efficiency (97.68%) and lowest surface area resistance (1.82 Ω·cm2), which exceeded the commercial cation exchange membrane CAM (6.96 Ω·cm2). These results suggested a promising future of semihomogeneous membranes.

Donnan Dialysis for Recovering Ammonium from Fermentation Solutions Rich in Volatile Fatty Acids.

For the production of polyhydroxyalkanoates (PHA) using nitrogen-rich feedstocks (e.g., protein-rich resources), the typical strategy of restricting cell growth as a means to enhance overall PHA productivity by nitrogen limitation is not applicable. In this case, a possible alternative to remove the nitrogen excess (NH4+/NH3) is by applying membrane separation processes. In the present study, the use of Donnan dialysis to separate ammonium ions from volatile fatty acids present in the media for the production of PHA was evaluated. Synthetic and real feed solutions were used, applying NaCl and HCl receiver solutions separated by commercial cation-exchange membranes. For this specific purpose, Fumasep and Ralex membranes showed better performance than Ionsep. Sorption of ammonium ions occurred in the Ralex membrane, thus intensifying the ammonium extraction. The separation performances with NaCl and HCl as receiver solutions were similar, despite sorption occurring in the Ralex membrane more intensely in the presence of NaCl. Higher volumetric flow rates, NaCl receiver concentrations, and volume ratios of feed:receiver solutions enhanced the degree of ammonium recovery. The application of an external electric potential difference to the two-compartment system did not significantly enhance the rate of ammonium appearance in the receiver solution. The results obtained using a real ammonium-containing solution after fermentation of cheese whey showed that Donnan dialysis can be successfully applied for ammonium recovery from such solutions.

Accounting for Ion Pairing Effects on Sulfate Salt Sorption in Cation Exchange Membranes.

Ion exchange membranes (IEMs) are frequently used in water treatment and electrochemical applications, with their ion separation properties largely governed by equilibrium ion partitioning between a membrane and contiguous solution. Despite an expansive literature on IEMs, the influence of electrolyte association (i.e., ion pairing) on ion sorption remains relatively unexplored. In this study, salt sorption in two commercial cation exchange membranes equilibrated with 0.01-1.0 M MgSO4 and Na2SO4 is investigated experimentally and theoretically. Association measurements of salt solutions using conductometric experiments and the Stokes-Einstein approximation show significant concentrations of ion pairs in MgSO4 and Na2SO4 relative to those in simple electrolytes (i.e., NaCl), which is consistent with prior studies of sulfate salts. The Manning/Donnan model, developed and validated for halide salts in previous studies, substantially underpredicts sulfate sorption measurements, presumably due to ion pairing effects not accounted for in this established theory. These findings suggest that ion pairing can enhance salt sorption in IEMs due to partitioning of reduced valence species. By reformulating the Donnan and Manning models, a theoretical framework for predicting salt sorption in IEMs that explicitly considers electrolyte association is developed. Remarkably, theoretical predictions of sulfate sorption are improved by over an order of magnitude by accounting for ion speciation. In some cases, good quantitative agreement is observed between theoretical and experimental values for external salt concentrations between 0.1 and 1.0 M using no adjustable parameters.

Flexible LATP composite membrane for lithium extraction from seawater via an electrochemical route

The lithium resources in seawater are more abundant than that on land, which is expected to meet the growing demand for the energy storage device like the lithium batteries. The electrochemical method is regarded as the most potential application. However, this method relies on membranes with advanced lithium-ion selectivity, and the emergence of super ionic conductor is expected to solve this problem. Herein, we employed polymer poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the flexible framework and Li1.3Al0.3Ti1.7(PO4)3 (LATP) as the lithium ion conductor to prepare a flexible LATP/PVDF-HFP composite membrane (FLCM). Optimized FLCM has high lithium-ion selectivity and can be enriched up to 5.525 ppm Li+ in diluted seawater (lithium extraction rate: 4.777 × 10−3 mol/(m2 h)), which is 48.465 times that of commercial cation-exchange membrane (CEM). In addition, the FLCM exhibits excellent equilibrium selectivity for Li + over Ca2+and Mg2+. The selectivity of Li+/Ca2+ and Li+/Mg2+ is up to 8424.706, 853.395, respectively. In an electrocatalytic-driven Li-ion extraction device with four recycle chambers at 2 V voltage, the concentration of Mg2+and Ca2+ in the catholyte can be reduced to 0.744% and 1.249% of the seawater concentration, respectively. The work will be significant to solve the challenges of lithium extraction from seawater.

Fabrication of S-PBI cation exchange membrane with excellent anti-fouling property for enhanced performance in electrodialysis

A novel anti-fouling cation exchange membrane (CEM) of sulfonated polybenzimidazole (S-PBI) was fabricated by the polymer bulk grafting modification of poly (4,4′-diphenyl ether-5,5′-bibenzimidazole) with sodium styrene sulfonate. FTIR, TGA, XPS, SEM and AFM were used to characterize membrane properties and the results showed that the ion exchange capacity and hydrophilicity of S-PBI increased with increased degrees of grafting and the electric resistance decreased accordingly. When the mass ratio of O-PBI/SSS was 1:4, the electric resistance of S-PBI-4.0 was 4.5 Ω·cm2, while the ion exchange capacity was 2.3 meq/g. The ED experiment with the S-PBI-4.0 membrane exhibited good desalination rate (>96 %), high current efficiency (80.73 %) and low energy consumption (1.13 kW·h/kg), which were very close to the results of a commercial CEM (Neosepta CMX). What's more, because of its good hydrophilicity, the S-PBI-4.0 membrane had better anti-fouling ability than Neosepta CMX, which was manifested as less resistance increase after membrane fouled by bovine serum albumin and smaller resistance after membrane cleaned by deionized water. These results demonstrate that the S-PBI-4.0 membrane is an appropriate candidate CEM and helpful to expand the application of ED in high-salinity organic industrial wastewater treatment.

Influence of electrolyte on concentration-induced conductivity-permselectivity tradeoff of ion-exchange membranes

In ion-exchange membranes (IEMs), the concentration-induced tradeoff between conductivity and permselectivity constrains process performance. This study investigates the impacts of different electrolytes on the conductivity-permselectivity tradeoff of commercial cation and anion exchange membranes. Nine different electrolyte solutions containing mono-, di-, and trivalent ions, and spanning 1.5 orders of magnitude in concentration were examined. Effective conductivity is found to be determined by valency and mobility of the counterion and is insensitive to the co-ion identity. Apparent permselectivity declines with higher valency of the counterion and with lower valency of the co-ion. Overall, the IEMs exhibited different conductivity-permselectivity tradeoff behaviors across the electrolyte solutions investigated. The disparate tradeoff trends are shown to be governed by counter- and co-ion valencies, and counterion diffusivity. The study sheds light on the principal factors underpinning the tradeoff and advances the understanding of attainable conductivity-permselectivity performance in more complex water chemistries that are pertinent for practical IEM applications.

Novel ecofriendly cation exchange membranes for low-cost electrodialysis of brackish water: Desalination and antiscaling performance

Electrodialysis (ED) has been widely used in desalination, and reducing related costs for brackish water (BW) desalination is the key for yielding it a more competitive technology. Ion-exchange membranes (IEMs) are the core of ED process. The complex preparation process of matrix materials for IEMs is not only an important reason for the high cost of IEMs, but also an important factor regarding environmental protection. This study presented an ecofriendly and simple method for preparing homogeneous cation exchange membranes (CEMs) for BW desalination by ED. Novel homogeneous CEMs were successfully prepared with polysulfone as the skeleton material, polyvinylpyrrolidone (PVP) as hydrophilic agent, and sodium dodecylbenzene sulfonate (SDBS) as functional material. The combined effects of PVP and SDBS were successfully used to strengthen membrane structure and properties. The results of membrane strength and structural examinations showed that the combined effects were the key to the stable existence of functional groups in these CEMs for a novel CEM design idea. These membranes were successfully used in ED and compared with commercial CEMs. The current efficiency of prepared CEMs, commercial Astom CEM, and commercial Tingrun CEM were 93.9, 94.8, and 90.4%, respectively, with the desalination rate >90% at an initial salt concentration of 5 g/L. The comprehensive efficiency of the prepared CEMs for removing salt was better than that of Tingrun CEM, while a little lower than that of Astom CEM. In ED operations for 30 cycles, all properties of the prepared membranes for removing salt were stable. This study conclusively revealed bright prospects for preparing ecofriendly CEMs and improving CEM performance in low-cost ED for BW desalination.

Investigation on various types of ion-exchange membranes in vanadium redox flow batteries: Experiment and modeling

Choosing the appropriate ion exchange membrane in vanadium redox flow batteries plays an important role in achieving optimal system performance. In this article, the implementation of commercial cation exchange membranes (Nafion 115, Nafion 117, and Nafion 212), anion exchange membranes (FAP 450 and QA-PFE), and the combination of them are investigated. In this regard, the effects of membrane type, thickness, electrical conductivity, and the transfer of vanadium species at different current densities are examined carefully. In addition, the influences of the flow rate and the concentration of electrolytes are investigated using a comprehensive developed model. The model predictions are validated against the experimental data of a charge and discharge cycle of cation and anion exchange membranes with a mean relative error of 2.06 %. It is found that FAP 450, despite the relatively low thickness, provides the highest capacity retention (91.19 %) after twenty working cycles. This can be attributed to the anion exchange nature and having the lowest diffusion coefficients of vanadium species. In addition, Nafion 212 provides the highest power density (256 mW cm−2) among the examined membranes. It can be attributed to the inverse relation between system power density and membrane ohmic resistance. Moreover, charging and discharging time, efficiency, maximum power density, and capacity fade of the system are functions of net transfer rate of vanadium species, thickness, and electrical conductivity of the membrane combination. Furthermore, the maximum and minimum values of the peak power densities of the system are 244 and 210 mW cm−2, which are obtained for the combination of Nafion 212-Nafion 212 and Nafion 117-Nafion 117, respectively. Finally, the maximum and minimum values of the capacity retention of the system after twenty working cycles are 93.39 % and 75.25 % for combinations of FAP 450-FAP 450 and Nafion 212-Nafion 212, respectively.