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Amorphous polymers are often used for gas separation but have a trade-off between gas permeability and selectivity. Here, the effect of chemical composition and temperature on gas permeability and solubility in well-ordered LC polymer membranes is investigated. Membranes with various compositions of a monomethacrylate LC (M1) with a crown ether functionality to enhance CO2 solubility and a smectic diacrylate (M2) cross-linker were fabricated, while all having the same order (smectic C) and alignment (planar). Single gas sorption and permeation data show for the membranes with 30 wt % M1 a higher CO2 solubility coefficient compared to membranes without M1, which results in a higher CO2 permeability and selectivity. For membranes that contain more than 30 wt % M1 decreasing layer spacings lead to reduced gas solubilities that result in lower gas permeabilities without additional selectivity gain toward CO2. The effect of temperature is demonstrated by comparing single gas sorption and permeation data below and above the Tg of the membranes. The diffusion coefficient increases above the Tg of the membranes with increasing M1 content leading to higher CO2 permeabilities and selectivities. These results show that not only the chemical composition but also the layer spacing of the smectic structures determines the gas separation performance of smectic LC polymer membranes.

The surface properties of porous carbonaceous electrodes govern the performance, durability, and ultimately the cost of redox flow batteries (RFBs). State-of-the-art carbon fiber-based electrode interfaces suffer from limited kinetic activity and incomplete wettability, fundamentally limiting the performance. Surface treatments for electrodes such as thermal and acid activation are a common practice to make them more suitable for aqueous RFBs; however, these treatments offer limited control over the desired functional properties. Here, we propose, for the first time, electrografting as a facile, rapid, and versatile technique to enable task-specific functionalization of porous carbonaceous electrodes for use in RFBs. Electrografting allows covalent attachment of organic molecules on conductive substrates upon application of an electrochemical driving force, and the vast library of available organic molecules can unlock a broad range of desired functional properties. To showcase the potential of electrografting for RFBs, we elect to investigate taurine, an amine with a highly hydrophilic sulfonic acid tail. Oxidative electrografting with cyclic voltammetry allows covalent attachment of taurine through the amine group to the fiber surface, resulting in taurine-functionalized carbon cloth electrodes. In situ polarization and impedance spectroscopy in single-electrolyte flow cells reveal that taurine-treated cloth electrodes result in 40% lower charge transfer and 25% lower mass transfer resistances than off-the-shelf cloth electrodes. We find that taurine-treated electrode interfaces promote faster Fe3+ reduction reaction kinetics as the electrochemical surface area normalized current densities are 2-fold and 4-fold higher than oxidized and untreated glassy carbon surfaces, respectively. Improved mass transfer of taurine-treated electrodes is attributed to their superior wettability, as revealed by operando neutron radiography within a flow cell setup. Through demonstrating promising results for aqueous systems with the model molecule taurine, this work aims to bring forth electrografting as a facile technique to tailor electrode surfaces for other RFB chemistries and electrochemical technologies.

So-called specific ion effects strongly impact physicochemical phenomena in among others, morphologies of polymeric membranes. This can be correlated to the identity of the ions, rather than just to their charge or concentration. We use this effect to tune the properties of layer-by-layer membranes prepared with the polyelectrolytes polydiallyldimethylammonium chloride and polystyrene sodium sulfonate. To systematically study this, membranes were prepared using different salts (NaBr, NH4Cl, MgCl2, NaCl, LiCl, MgSO4, NaH2PO4, (NH4)2SO4, Na2SO4). The specific ion effects were evaluated thanks to the classification of salts and PEs into kosmotropes (well hydrated) and chaotropes (poorly hydrated) following the law of matching water affinities (LMWA). The impact of the type of anions and cations on the multilayer formation and membrane performances was evaluated. Membranes prepared with kosmotropic salts achieved stable performances after only 4 bilayers, and almost 6 times higher water permeabilities than membranes prepared with chaotropic salts without compromising and often even improving the membrane's size exclusion behaviour. The salts used tuned the membranes charge: Kosmotropic salts delivered negatively charged membranes with Na2SO4 and MgCl2 retentions till 97% and 10% respectively, while chaotropic salts formed more positively charged membranes with reversed behaviour with Na2SO4 and MgCl2 rejection of 20 and 89%.

The development of functionalized polyolefins for use as stimuli-responsive commodity polymers has recently received much attention. In this work, a microporous polypropylene (PP) scaffold is used to align and fortify a smectic liquid crystalline network (LCN) which can switch its gas permeability upon pH changes. The LCN is a photopolymerized liquid crystalline mixture of a dimerized benzoic acid derivative monoacrylate and a diacrylate crosslinker. In the hydrogen-bonded state, the composite membrane shows a high-molecular order and a low permeability for He, N2, and CO2 gases. By pH switching from the hydrogen-bonded state to the salt form, the molecular order is reduced, and the gas permeability is increased by one order of magnitude. This increase is mainly attributed to a loss in order of the system, increasing the free volume, resulting in an increased diffusibility. By exposing the composite film to basic or acidic environments, reversible switching between low and high gas permeability states is observed, respectively. The physical constraints imposed by the PP scaffold strengthens the membrane while the reversible switching inside the liquid crystalline polymer is maintained. This switching of gas permeation properties is not possible with the fragile freestanding LCN films alone.

Layer-by-layer (LbL) assembly of the alternating adsorption of oppositely charged polyions is an extensively studied method to produce nanofiltration membranes. In this work, the concept of chaotropicity of the polycation and its counterion is introduced in the LbL field. In general, the more chaotropic a polyion, the lower its effective charge, charge availability, and hydrophilicity. Here, this is researched for the well-known PDADMAC (polydiallyldimethylammonium chloride) and PAH (poly(allylamine) hydrochloride), and the synthesized PAMA (polyallylmultimethylammonium), with two different counterions (I- and Cl-). Higher chaotropicity (PDADMAC > PAMA-I > PAMA-Cl > PAH) translates into a reduced charge availability and a more pronounced extrinsic charge compensation, resulting in more mass adsorption and a higher pure water permeability. PAMA-containing membranes show the most interesting results in the series. Due to its molecular structure, the chaotropicity of this polycation perfectly lies between PDADMAC and PAH. Overall, the chaotropicity of PAMA membranes allows for the formation of the right balance between extrinsic and intrinsic charge compensation with PSS. Moreover, modifying the nature of the counterions of PAMA (I- or Cl-) allows to tune the density of the multilayer and results in lower size exclusion abilities with PAMA-I compared to PAMA-Cl (higher MWCO and lower MgSO4 retention). In general, the contextualization of the polyion interaction within the specific (poly)ion effects expands the understanding of the influence of the charge density of polycations without ignoring the chemical nature of the functional groups in their monomer units.

This thesis investigates how specific ion effects impact the properties of several materials both fundamentally and technologically. Ions are used to probe fine modification in chemical interactions and associated to that, self-assembly, structural characteristics and ultimately performances of the materials investigated. Formed morphologies are characterized and their performance is linked to the molecular reality of ion specificity for various applications. More in detail, ions are used to tune the strength of H-bond interactions inducing the formation of organic ionic plastic crystalline (OIPC) phases and to control the ionic conductivity of solid polymer electrolytes (SPEs). Also, specific ion effects are used to modulate structure and performances of polymeric layer-by-layer (LbL) membranes. The findings obtained in this work provide a fundamental understanding on how specific ion effects can be used to tailor properties and performance of several materials. A summary of the contents of this thesis is provided below. Chapter 1 introduces the historical background on specific ion effects and provides a theoretical platform to evaluate their impact. The key points of related theories and the fundamental reasoning behind ion specificity are presented and connected. Practical implications of the use of specific ion effects in several fields are reported. With an eye to the expansion of the classical ion classification and to the evaluation of specific ion effects in non-aqueous systems, the motivation of the thesis is outlined. In Chapter 2 a new non-globular molecule containing 15-crown-5 ether moieties and amidic functionalities is synthesized and characterized. The formation of H-bonds between the amide groups is detected and the strength of these interactions is tuned by complexing the system with different sodium salts containing chaotropic anions. The type of chaotropic anion has an effect on the self-assembly of the non-globular molecules by weakening or disrupting of the H-bonds between the amidic functionalities. This leads to the formation of organic ionic plastic crystalline (OIPC) phases with rotational freedom and to the tuning of the thermal and morphological properties of the OIPC. In Chapter 3 a study of the effect of chaotropic anions on the properties of a solid polymer electrolyte is performed. Even though the mechanism of ion conduction in solid polymer electrolytes has been extensively studied in the past, the role of the counterion (i.e. anion) interacting with both the polymer and the cation has not been explored in detail. This study shows that the anion can severely impact the conduction as well. The effect of the anion (I-, SCN-, BF4- and PF6-) on the thermal, structural and electrical properties of a solid polymer electrolyte (15C5BA polymer) is investigated. The results clearly show that the nature of the anions can be used as a parameter to actually tune the properties of the polymer electrolyte. Chapter 4 describes the impact of the use of salts with a different chaotropic or kosmotropic nature on the self-assembly behaviour of oppositely charged strong polyelectrolytes to ultimately fabricate layer-by-layer membranes. The formation of a multilayer arrangement is detected and investigated in terms of mass adsorption and surface charge. The use of salts with a different chaotropic/kosmotropic nature in the coating solution leads to the formation of layers with various densities and charge based on the salt used during preparation. The impact of chaotropic or kosmotropic salts is also evident in the performances of the layer-by-layer membranes prepared. It is observed that water permeability and salt retentions are significantly influenced by the nature of the salts used in the coating solution. While the use of chaotropic salts results in membranes with a more positively charged surface, high retention of MgCl2 and relatively distinct odd-even effects, membranes prepared with kosmotropic salts exhibit a more negative surface with a higher retention for Na2SO4, plateauing of the odd-even effects and water permeabilities that reach up to 6 times those of the membranes prepared with chaotropic salts. These findings confirm that specific ion effects are a valuable tool to tailor membrane performances in technological applications. In Chapter 5 an expansion of the classification of ions into kosmotropic and chaotropic is addressed to include not just simple ions but also more complex polyions (polyelectrolytes). To do so, three polycations (poly(allylamine) hydrochloride (PAH), poly allylmultimethylammonium (PAMA) and poly diallyldimethylammonium chloride (PDADMAC) with different chaotropicity are selected and are used for the preparation of layer-by-layer membranes with the same polyanion (poly(sodium-4-styrenesulfonate or PSS). The chaotropicity of the monomer unit of the polycations used is studied in relation to their charge density. Results show that the polycation with the strongest chaotropic character is subject to higher extrinsic charge compensation and for this reason, delivers membranes with the highest water permeability. Moreover, the chaotropicity of the polycation has an effect on the surface charge of the membranes and on their retention behaviour. Finally, in Chapter 6 the main conclusions of the findings obtained in this thesis are reported. Furthermore, an outlook on the use of specific ion effects as tool to tailor materials properties for applications is provided with a careful look on the expansion of the concept to polyions and mixed salt systems.

Electrode-assisted techniques are well suited for the separation of ions from solutions with reduced energy and chemical consumption. This emerging platform can benefit greatly from the convection enhanced and zero-gap reactor designs unlocked by macroporous electrodes, but immobilizing ion-selective layers on such complex 3D architectures is challenging. Electropolymerization of conductive polymers is proposed as a coating methodology to fabricate highly conformal coatings with electrochemically switchable ion-exchange functionality. To demonstrate this, the synthetic parameters, resulting morphology, and ionic separation performance of poly(3,4-ethylenedioxythiophene) (PEDOT) on commercially available carbon paper electrodes are studied. Electropolymerization of PEDOT in organic solvents results in rougher morphologies with high sensitivity to electrochemical protocols employed. When polymerized in aqueous solutions of poly(4-styrenesulfonate) (PSS−), the resulting PEDOT/PSS blend polymer forms smooth coatings with a controllable thickness down to 0.1 µm. With appropriate voltage bias, PEDOT/PSS coated electrodes can take up and release Ni2+ in the presence of excess Na+. Increasing the coating thickness decreases the adsorption capacity due to mass transfer limitations, and the maximum adsorption capacity for Ni2+ is reached for the thinnest coatings at 228 mg g−1. Through a systematic study of PEDOT/PSS coated carbon paper, electropolymerization is identified to be a promising avenue for porous electrode functionalization.

The microstructure of porous electrodes determines multiple performance-defining properties, such as the available reactive surface area, mass transfer rates, and hydraulic resistance. Thus, optimizing the electrode architecture is a powerful approach to enhance the performance and cost-competitiveness of electrochemical technologies. To expand our current arsenal of electrode materials, we need to build predictive frameworks that can screen a large geometrical design space while being physically representative. Here, we present a novel approach for the optimization of porous electrode microstructures from the bottom-up that couples a genetic algorithm with a previously validated electrochemical pore network model. In this first demonstration, we focus on optimizing redox flow battery electrodes. The genetic algorithm manipulates the pore and throat size distributions of an artificially generated microstructure with fixed pore positions by selecting the best-performing networks, based on the hydraulic and electrochemical performance computed by the model. For the studied VO2+/VO2+ electrolyte, we find an increase in the fitness of 75 % compared to the initial configuration by minimizing the pumping power and maximizing the electrochemical power of the system. The algorithm generates structures with improved fluid distribution through the formation of a bimodal pore size distribution containing preferential longitudinal flow pathways, resulting in a decrease of 73 % for the required pumping power. Furthermore, the optimization yielded an 47 % increase in surface area resulting in an electrochemical performance improvement of 42 %. Our results show the potential of using genetic algorithms combined with pore network models to optimize porous electrode microstructures for a wide range of electrolyte composition and operation conditions.

Solid-state batteries are a valuable option for sustainable energy storage. Their performance is strongly dependent on the ionic conductivity of the solid-state electrolyte (SSE). The mechanism of ion conduction in solid polymer electrolytes has been extensively studied in terms of flexibility of the polymer chains and their interactions with mobile cations that hop between conduction sites carrying the charge. The role of the counterion (i.e. anion) interacting with both the polymer and the cation has not been explored in detail. In this study we show however, that the anion can severely impact the conduction as well. The effect of the anion (I−, SCN−, BF4− and PF6−) on the thermal, structural and electrical properties of a solid polymer electrolyte (15C5BA polymer) was investigated. The results clearly show that the nature of the anions can be used as a parameter to actually tune the properties of the polymer electrolyte.

For preserving food packed in environmentally friendly and recyclable paperboard packages, it is important to have sufficient gas barrier performance of the paperboard container. Paperboard has poor intrinsic barrier properties and to overcome this deficiency, so a barrier coating is needed that does not hinder the recycling of the paperboard substrate. However, the gas barrier properties and the recyclability of such coatings have been rarely studied. Here, both the gas barrier performance and the removal of an alkali-soluble resin (ASR)-stabilized waterborne barrier coatings from paperboard are investigated. For barriers for gases, such as nitrogen, carbon dioxide, and oxygen, defect-free coatings are needed which is achieved by applying three coating layers. The oxygen transmission rate (OTR) of the three-layered coating on paperboard was 920 cm 3/(m 2∙day). For water vapor barriers, two coating layers already show a strong improvement, as water follows a different penetration mechanism than the other tested gases. The water vapor transmission rate WVTR of double coated paperboard was 240 g/(m 2∙day). Preliminary results show that the coating is removed by immersion of the coated paperboard in an aqueous alkaline solution at room temperature. This causes de-protonation of the carboxylic acids of the ASR and subsequent re-dispersion of the coating in water. Removing double-layer coatings from the paperboard is more challenging, possibly due to the coating/coating interface between the two coating layers and enhanced adhesion between coating and paperboard.

Organic ionic plastic crystals (OIPCs) are a class of soft materials showing positional order while still allowing orientational freedom. Due to their motional freedom in the solid state, they possess plasticity, non-flammability and high ionic conductivity. OIPC behavior is typically exhibited by ‘simple’ globular molecules allowing molecular rotation, whereas the interactions that govern the formation of OIPC phases in complex non-globular molecules are less understood. To better understand these interactions, a new family of non-globular OIPCs containing a 15-crown-5 ether moiety was synthetized and characterized. The 15C5BA molecule prepared does not exhibit the sought-after behavior because of its non-globular nature and strong intermolecular H-bonds that restrict orientational motion. However, the OIPC behavior was successfully obtained through complexation of the crown-ether moiety with sodium salts containing chaotropic anions. Those anions weaken the interactions between the molecules, allowing rotational freedom and tuning of the thermal and morphological properties of the OIPC.

Compressing porous carbon electrodes is a common approach to improve flow battery performance, but the resulting impact on electrode structure, fluid dynamics, and cell performance is not well understood. Herein, microtomographic imaging, load cell testing, and flow cell diagnostics are employed to characterize how compression-induced changes impact pressure drop, polarization, and mass-transfer scaling. Five different compressions are tested, spanning ranges typically used in literature, for AvCarb 1071 cloth (0%, 9%, 20%, 25%, 32%) and Freudenberg H23 paper (0%, 8%, 12%, 17%, 22%). It is found that the two electrode structures have distinct responses to compression, resulting in differing optimal conditions identified for each material; specifically, the Freudenberg H23 exhibits lower combined ohmic, charge-transfer, and mass-transport values at 8% compression, resulting in improved electrochemical performance across all compressive values, as compared to the optimal AvCarb 1071 compression (20%). Overall, Freudenberg H23 exhibits a greater sensitivity to compression with peak electrochemical activity correlating with increased permeability, whereas AvCarb 1071 is insensitive to compressive loads but produces lower electrochemical performance. Herein, the trade-offs of mechanical robustness on fluid-dynamic and electrochemical performance between the two electrodes are demonstrated by the aforementioned findings, suggesting each could be used for specific operating environments.

The development of high-performance membrane materials for non-aqueous redox flow batteries (NAqRFBs) could unlock a milestone towards widespread commercialization of the technology. Understanding of transport phenomena through membrane materials requires diagnostic tools able to monitor the concentrations of redox active species. While membrane characterization in aqueous media focused the attention of the scientific community, dedicated efforts for non-aqueous electrolytes remain poorly developed. Here, we develop new methodologies to assess critical membrane properties, namely ion exchange capacity and species transport, applied to NAqRFBs. In the first part, we introduce a method based on 19F-NMR to quantify ion exchange capacity of membranes with hydrophobic anions commonly used in non-aqueous systems (e.g., PF6 − and BF4 −). We find a partial utilization of the ion exchange capacity compared to the values reported using traditional aqueous chemistry ions, possibly limiting the performance of NAqRFB systems. In the second part, we study mass transport with a microelectrode placed on the electrolyte tank. We determine TEMPO crossover rates through membranes by using simple calibration curves that relate steady-state currents at the microelectrode with redox active species concentration. Finally, we show the limitations of this approach in concentrated electrolyte systems, which are more representative of industrial flow battery operation.

The effect of layer spacing and halogenation on the gas separation performances of free-standing smectic LC polymer membranes is being investigated by molecular engineering. LC membranes with various layer spacings and halogenated LCs were fabricated while having a planar aligned smectic morphology. Single permeation and sorption data show a correlation between gas diffusion and layer spacing, which results in increasing gas permeabilities with increasing layer spacing while the ideal gas selectivity of He over CO2 or He over N2 decreases. The calculated diffusion coefficients show a 6-fold increase when going from membranes with a layer spacing of 31.9 Å to membranes with a layer spacing of 45.2 Å, demonstrating that the layer spacing in smectic LC membranes mainly affects the diffusion of gasses rather than their solubility. A comparison of gas sorption and permeation performances of smectic LC membranes with and without halogenated LCs shows only a limited effect of LC halogenation by a slight increase in both solubility and diffusion coefficients for the membranes with halogenated LCs, resulting in a slightly higher gas permeation and increased ideal gas selectivities towards CO2. These results show that layer spacing plays an important role in the gas separation performances of smectic LC polymer membranes.

With the use of bipolar membranes (BPMs) in an expanding range of applications, there is an urgent need to understand and improve the catalytic performance of BPMs for water dissociation, as well as to increase their physical and chemical stability. In this regard, electrospinning BPMs with 2D and 3D junction structures have been suggested as a promising route to produce high-performance BPMs. In this work, we investigate the effect of entangling anion and cation exchange nanofibers at the junction of bipolar membranes on the water dissociation rate. In particular, we compare the performance of different tailor-made BPMs with a laminated 2D junction and a 3D electrospun entangled junction, while using the same type of anion and cation exchange polymers in a single/dual continuous electrospinning manufacturing method. The bipolar membrane with a 3D entangled junction shows an enhanced water dissociation rate as compared to the bipolar membrane with laminated 2D junction, as measured by the decreased bipolar membrane potential. Moreover, we investigate the use of a third polymer, that is, poly(4-vinylpyrrolidine) (P4VP), as a catalyst for water dissociation. This polymer confirmed that a 3D entangled junction BPM (with incorporated P4VP) gives a higher water dissociation rate than does a 2D laminated junction BPM with P4VP as the water dissociation catalyst. This work demonstrates that the entanglement of the anion exchange polymer with P4VP as the water dissociation catalyst in a 3D junction is promising to develop bipolar membranes with enhanced performance as compared to the conventionally laminated membranes.

Bottom-up methods for the fabrication of nanoporous polymer membranes have numerous advantages. However, it remains challenging to fabricate nanoporous membranes that are mechanically robust and have aligned pores, that is, with a low tortuosity. Here, a mechanically robust thin-film composite membrane was fabricated consisting of a two-dimensional (2D) porous smectic liquid crystalline polymer network inside an anisotropic, microporous polymer scaffold. The polymer scaffold allows for relatively straightforward planar alignment of the smectic liquid crystalline mixture, which consisted of a diacrylate cross-linker and a dimer forming benzoic acid-based monoacrylate. Polymerized samples displayed a smectic A (SmA) phase, which formed the eventual 2D porous channels after base treatment. The aligned 2D nanoporous membranes showed a high rejection of anionic solutes bigger than 322 g/mol. Cleaning and reusability of the system were demonstrated by intentionally fouling the porous channels with a cationic dye and subsequently cleaning the membrane with an acidic solution. After cleaning, the membrane properties were unaffected; this, combined with numerous pressurizing cycles, demonstrated reusability of the system.

Forward osmosis (FO), using the osmotic pressure difference over a membrane to remove water, can treat highly foul streams and can reach high concentration factors. In this work, electrospun TFC membranes with a very porous open support (porosity: 82.3%; mean flow pore size: 2.9 µm), a dense PA-separating layer (thickness: 0.63 µm) covalently attached to the support and, at 0.29 g/L, having a very low specific reverse salt flux (4 to 12 times lower than commercial membranes) are developed, and their FO performance for the concentration of apple juice, manure and whey is evaluated. Apple juice is a low-fouling feed. Manure concentration fouls the membrane, but this results in only a small decrease in overall water flux. Whey concentration results in instantaneous, very severe fouling and flux decline (especially at high DS concentrations) due to protein salting-out effects in the boundary layer of the membrane, causing a high drag force resulting in lower water flux. For all streams, concentration factors of approximately two can be obtained, which is realistic for industrial applications.

Porous electrodes are core components that determine the performance of redox flow batteries. Thus, optimizing their microstructure is a powerful approach to reduce system costs. Here we present a pore network modeling framework that is microstructure and chemistry agnostic, iteratively solves transport equations in both half-cells, and utilizes a network-in-series approach to simulate the local transport phenomena within porous electrodes at a low computational cost. In this study, we critically assess the versatility and robustness of pore network models to enable the modeling of different electrode geometries and redox chemistries. To do so, the proposed model was validated with two commonly used carbon fiber-based electrodes (a paper and a cloth), by extracting topologically equivalent networks from X-ray tomograms, and evaluated for two model redox chemistries (an aqueous iron-based and a non-aqueous TEMPO-based electrolyte). We find that the modeling framework successfully captures the experimental performance of the non-aqueous electrolyte but is less accurate for the aqueous electrolyte which was attributed to incomplete wetting of the electrode surface in the conducted experiments. Furthermore, the validation reveals that care must be taken when extracting networks from the tomogram of the woven cloth electrode, which features a multiscale microstructure with threaded fiber bundles. Employing this pore network model, we elucidate structure-performance relationships by leveraging the performance profiles and the simulated local distributions of physical properties and finally, we deploy simulations to identify efficient operation envelopes.

Electrochemical CO2reduction is a promising process to store intermittent renewable energy in the form of chemical bonds and to meet the demand for hydrocarbon chemicals without relying on fossil fuels. Researchers in the field have used gas diffusion electrodes (GDEs) to supply CO2to the catalyst layer from the gas phase. This approach allows us to bypass mass transfer limitations imposed by the limited solubility and diffusion of CO2in the liquid phase at a laboratory scale. However, at a larger scale, pressure differences across the porous gas diffusion layer can occur. This can lead to flooding and electrolyte breakthrough, which can decrease performance. The aim of this study is to understand the effects of the GDE structure on flooding behavior and CO2reduction performance. We approach the problem by preparing GDEs from commercial substrates with a range of structural parameters (carbon fiber structure, thickness, and cracks). We then determined the liquid breakthrough pressure and measured the Faradaic efficiency for CO at an industrially relevant current density. We found that there is a trade-off between flooding resistance and mass transfer capabilities that limits the maximum GDE height of a flow-by electrolyzer. This trade-off depends strongly on the thickness and the structure of the carbon fiber substrate. We propose a design strategy for a hierarchically structured GDE, which might offer a pathway to an industrial scale by avoiding the trade-off between flooding resistance and CO2reduction performance.

The suppression of CO2-induced plasticization in polyimide membranes at supercritical conditions up to 120 bar is investigated. Three approaches (polymer blending, thermal treatments and chemical crosslinking) known from relatively low-pressure applications are applied and their effectiveness to suppress membrane plasticization at high CO2 pressures and under supercritical conditions is systematically identified. CO2 sorption measurements reveal that especially Henry sorption promotes plasticization and that the corresponding Henry sorption parameter (kD) correlates with the d-spacing and Tg of the membranes. A lower d-spacing and higher Tg results in a reduced kD parameter and thus a higher resistance to plasticization. A high interchain rigidity is required to suppress plasticization at the highly plasticizing liquid-like CO2 densities. Chemical and thermo-oxidative crosslinking results in the largest decrease in interchain mobility and therefore shows the highest resistance to plasticization, but also a significantly lower permeability. Thermally treating the membranes in N2 retains a high permeability, while still displaying significant plasticization resistance. Polymer blending does increase the plasticization resistance, but strongly reduces the permeability. All three methods manage to suppress plasticization at supercritical conditions, but crosslinking offers superior plasticization resistance. However, proper tailoring strategies are required to combine a high plasticization resistance with a high permeability.

Redox flow batteries are a promising technological option for large-scale energy storage, but their deployment is hampered by suboptimal performance and elevated costs. The reactor performance is determined by the mass, charge, momentum, and heat transport rates coupled with electrochemical reactions. Understanding, measuring, and simulating these metrics is thus necessary for device optimization. In this chapter, we first describe the main transport mechanisms and cell overpotentials relevant to flow batteries with a focus on single cells representative of individual units in a stack. Then, we critically review some experimental methods, including bulk and locally resolved diagnostics, which enable determination of cell overpotentials. Finally, we briefly describe the most promising modeling approaches. Our goal is to provide students and scientists with the fundamental guiding principles of transport phenomena and cell overpotentials that can be leveraged to design advanced materials and reactor architectures with enhanced performance.

High dimensional models typically require a large computational overhead for multiphysics applications, which hamper their use for broad-sweeping domain interrogation. Herein, we develop a modeling framework to capture the through-plane fluid dynamic response of electrodes and flow fields in a redox flow cell, generating a computationally inexpensive two-dimensional (2D) model. We leverage a depth averaging approach that also accounts for variations in out-of-plane fluid motion and departures from Darcy’s law that arise from averaging across three-dimensions (3D). Our Resulting depth-averaged 2D model successfully predict the fluid dynamic response of arbitrary in-plane flow field geometries, with discrepancies of < 5% for both maximum velocity and pressure drop. This corresponds to reduced computational expense, as compared to 3D representations (< 1% of duration and 10% of RAM usage), providing a platform to screen and optimize a diverse set of cell geometries.

High Na+ levels are detrimental for most crops. Selective membranes provide the possibility for the selective removal of Na+ while preserving beneficial ion species. The challenge is to separate two ion species of the same charge. This study evaluates the implementation of an electrodialysis (ED) system equipped with a supported liquid membrane (SLM) and a commercially available monovalent cation-selective membrane (CIMS) in the treatment of greenhouse drainage water. The SLM shows a (minimum) K+ over Na+, Ca2+ and Mg2+ permselectivity of 9, 15 and 30, respectively. Whereas the CIMS holds a high K+ over Ca2+ and Mg2+ permselectivity of 10 and 16, respectively, the K+ over Na+ permselectivity is just 1.3. With the experimentally obtained membrane characteristics at hand, the treatment of drainage water was simulated by a two-steps process with the two membrane types operating in series. Using real-life operational parameters, analysis revealed the optimal configuration and the ability to recover 96% of the K+ and approximately 80% of the water, Ca2+ and Mg2+. Summarized, this study not only shows the efficient separation of two ion species of the same valance but also the implementation of this technology in a real-life application.

Electrochemical CO2reduction has the potential to use excess renewable electricity to produce hydrocarbon chemicals and fuels. Gas diffusion electrodes (GDEs) allow overcoming the limitations of CO2mass transfer but are sensitive to flooding from (hydrostatic) pressure differences, which inhibits upscaling. We investigate the effect of the flooding behavior on the CO2reduction performance. Our study includes six commercial gas diffusion layer materials with different microstructures (carbon cloth and carbon paper) and thicknesses coated with a Ag catalyst and exposed to differential pressures corresponding to different flow regimes (gas breakthrough, flow-by, and liquid breakthrough). We show that physical electrowetting further limits the flow-by regime at commercially relevant current densities (≥200 mA cm-2), which reduces the Faradaic efficiency for CO (FECO) for most carbon papers. However, the carbon cloth GDE maintains its high CO2reduction performance despite being flooded with the electrolyte due to its bimodal pore structure. Exposed to pressure differences equivalent to 100 cm height, the carbon cloth is able to sustain an average FECOof 69% at 200 mA cm-2even when the liquid continuously breaks through. CO2electrolyzers with carbon cloth GDEs are therefore promising for scale-up because they enable high CO2reduction efficiency while tolerating a broad range of flow regimes.

Redox flow batteries (RFBs) are emerging as viable options for grid-scale energy storage, but their elevated costs hamper commercialization. Enhancing the porous carbon electrode performance to improve power density and reduce system costs is an effective strategy toward widespread deployment; however, the porous carbon electrode must satisfy multiple contradictory roles, including providing high surface area, low pressure drop, and facile mass transport, thus motivating electrode engineering efforts. In this work, we systematically explore the non-solvent induced phase separation (NIPS) technique as a platform to synthesize a family of distinct microstructures for use in RFBs. Flow cell studies in commercially relevant redox pairs (i.e., Fe2+/3+, V2+/3+, and V4+/5+) are performed, revealing diverse performance profiles, synthesis-structure-performance relationships, and opportunities for high-power electrode materials. We anticipate that, with further refinement and customization, NIPS electrodes can broadly benefit electrode engineering efforts for electrochemical energy storage and conversion applications.

Contrary to traditional electrolysers which operate continuously at their nominal load, future alkaline electrolysers need to be able to operate over a wide load range due to the variability of renewable electricity supply. We have investigated how the residual ripples from thyristor-based power supplies are influenced by the operating load of the system, and how these ripples affect the efficiency of alkaline electrolysers. For this, a simulation tool was developed which combines a six-pulse bridge thyristor rectifier model with closed-loop current control and semi-empirical electrolysis models. The electrolysis models can simulate the potential response to both direct and high amplitude alternating currents for lab-scale and industrial electrolysers. The electrolysis model of the lab-scale electrolyser was validated with experiments with a square wave current input. The models show that without filters the ripples result in a total system efficiency loss of 1.2–2.5% at full load and of 5.6–10.6% at a part load of 20% depending on the type of electrolyser. The implementation of an optimized L-filter suppresses residual ripples and reduces the efficiency losses to 0.5%–0.8% at full load and to 0.8–1.2% at the minimum load.

Layer-by-layer assembly of polyelectrolyte layers is a versatile method to produce nanofiltration membranes. The membrane formation and subsequently performance is easily tailored with pH as it varies the polyelectrolyte charge density. Here, membranes are fabricated at different pH (4,8, and 9) with branched polyethyleneimine (PEI)/poly(sodium-4-styrenesulfonate) (PSS) layers (symmetric membranes) or a combination of poly(diallyldimethylammonium chloride) (PDADMAC)/PSS base layers terminated with PEI/PSS layers (asymmetric membranes). Overall, increasing the pH lowers the PEI charge density, which increases PEI adsorption, as measured by optical reflectometry and positive zeta potential. For symmetric systems, decreasing the charge density decreases the salt retention, because fewer intrinsic linkages are formed. Contrarily, asymmetric membranes, independent of charge density, show retentions >90% for MgSO4 and Na2SO4. Additionally, the benefit of asymmetric membrane formation is proven by comparing the best membrane performances. Asymmetric membranes prepared at pH = 4 form an open base layer and defect-free dense selective layer, resulting in much higher permeabilities compared to symmetric membranes (∼13 and ∼9 L/(m2hbar)), and significantly improving MgSO4 and Na2SO4 retentions (>95% compared to >90%). By combining two well-known polycations and tailoring the pH, versatile membranes are produced, without the need for synthesis or modification steps while obtaining improved water fluxes and salt retentions.

To prevent greenhouse emissions into the atmosphere, separations like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes gained a key role in gas separations over the past decades, but these polymers are often not organized at a molecular level, which results in a trade-off between permeability and selectivity. In this work, the effect of molecular order and orientation in liquid crystals (LCs) polymer membranes for gas permeation is demonstrated. Using the self-assembly of polymerizable LCs to prepare membranes ensures control over the supramolecular organization and alignment of the building blocks at a molecular level. Robust freestanding LC membranes were fabricated that have various, distinct morphologies (isotropic, nematic cybotactic, and smectic C) and alignment (planar and homeotropic), while using the same chemical composition. Single gas permeation data show that the permeability decreases with increasing molecular order while the ideal gas selectivity of He and CO2 over N2 increases tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly ordered smectic C morphology. The calculated diffusion coefficients showed a 10-fold decrease when going from randomly ordered membranes to ordered smectic C membranes. It is proposed that with increasing molecular order, the free volume elements in the membrane become smaller, which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation performances of planar and homeotropic aligned smectic C membranes shows the effect of molecular orientation by a 3-fold decrease of the diffusion coefficient of homeotropic aligned smectic C membranes resulting in a diminished gas permeation and increased ideal gas selectivities. These results strongly highlight the importance of molecular order and orientation in LC polymer membranes for gas separation.

Controlling the metal-organic framework (MOF)-matrix interface is a useful strategy to improve the gas separation performance of mixed matrix membranes (MMMs). Although polymer blending has been investigated to enhance MMM performances, its true strength, i.e., aligning polymer and additive chemistries to improve interfacial compatibility and ultimately the separation performance, is only poorly investigated. In this work we demonstrate how controlling interfacial chemistries by polymer blending is an effective tool to tune the membrane performance. Three isoreticular zeolitic imidazolate frameworks (ZIFs) and two matrix polymers (Matrimid and polybenzimidazole oPBI (PBI)) were used to prepare the MMMs. The ZIF linker functionality strongly determined the extent of the effect of PBI addition in the MMMs. For both the hydrophobic ZIF-7 and ZIF-8-based MMMs, PBI compatibilized the MOF-matrix interface and increased the CO2/N2 separation factor while slightly decreasing the permeability. Contrarily, the separation performance of the hydrophilic ZIF-90 MMM was not affected by PBI incorporation. Additionally, the MMM permeability followed the trend of the ZIF pore geometries and linker flexibilities. These results proved that MOF-matrix interfacial compatibility can effectively be controlled by polymer blending and that the extent of control is determined by a subtle balance between the MOF linker functionality and matrix chemistry.

In this work, the effectiveness of metal-organic frameworks (MOFs) with sheet-like morphologies was assessed as function of the MOF microporosity and MOF-matrix compatibility. Zeolitic imidazolate frameworks (ZIFs, a MOF subclass) with sheet-like/platelet morphologies were incorporated in Matrimid/PBI matrices resulting in mixed matrix membranes (MMMs). The ZIFs were either permeable (ZIF-301) or impermeable (ZIF-95X) for gases with a kinetic diameter bigger than or equal to the kinetic diameter of CO2. Additionally, MMMs containing impermeable graphene nanosheets were fabricated as reference to confirm the ZIF-95X impermeability. The MMMs containing the ZIF-301 nanoplatelets showed enhanced CO2 permeabilities. Analysis of the N2 and CO2 solubility and diffusivity showed that this permeable additive enhances the solubility of both gases, but only increases the N2 diffusivity through the ZIF-301 MMM. This signified that for MMMs with sheet-like MOFs the MOF micropore volume should only be accessible for one gas such that the sheet-like morphology effectively increases the tortuosity for the other species. Contrarily, the MMMs containing impermeable ZIF-95X and graphene showed declined MMM separation performances. Analysis of the N2 and CO2 diffusivities showed the presence of defective interfaces in these MMMs and proved that an increased tortuosity is only effective if the additive-matrix interface is defect free.

The time-dependent CO2-induced plasticization behavior of glassy Matrimid® 5218 polymer membranes at supercritical conditions up to 120 bar was investigated. Glassy polyimide membranes were conditioned with both gaseous CO2 and liquid-like sc-CO2. The plasticization behavior during permeation and sorption was correlated with the intrinsic membrane properties and the CO2 fluid properties. In the gaseous region the CO2 concentration increased slightly over time, while in the liquid-like sc-CO2 region the CO2 concentration remained constant over time and showed no hysteresis, indicating an induced glass transition. Contrary to the CO2 sorption the CO2 permeability showed more pronounced time-dependent behavior which increases with feed pressure because of polymer membrane plasticization. Despite the strong time-dependency, the CO2 permeability was independent of the feed pressure in the liquid-like sc-CO2 region. This difference in time-dependent behavior between sorption and permeation is due to the presence of a concentration gradient during permeation experiments. In addition, the permeability showed significant hysteresis. Exposure to liquid-like sc-CO2 resulted in a highly plasticized membrane and changed the permeation behavior at all subsequent feed pressures, due to slow polymer chain relaxation rates. Clearly, these relationships proof that the permeation history is a critical aspect for time-dependent plasticization phenomena at high CO2 pressures.

Reverse electrodialysis (RED) is an electro-membrane process to harvest renewable energy from salinity gradients. RED process models have been developed in the past, but they mostly assume that only NaCl is present in the feedwaters, which results in unrealistically high predictions. In the present work, an existing simple model is extended to accommodate the presence of magnesium ions and sulfate in the feedwaters, and potentially even more complex mixtures. All power loss mechanisms deriving from the presence of multivalent ions are included in the new model: increased membrane electrical resistance, uphill transport of multivalent ions from the river to the seawater compartment, and membrane permselectivity loss. This new model is validated with experimental and literature data of membrane electrical resistance (at 10 mol. % MgCl2 for the CEMs and 25 mol. % Na2SO4 for the AEMs), RED stack performance (up to 50 mol. % MgCl2 or Na2SO4 in the feedwaters), and ion transport (at 10 mol. % MgCl2 or Na2SO4 in the feedwaters) showing very good agreement between model predictions and experimental data. Finally, we showed that the developed model not only describes experimental data but can also predict RED performances under a variety of conditions and cross-flow configurations (single-stage with and without electrode segmentation, multi-stage in co-current and counter-current mode) and feedwater compositions (only NaCl, with Na2SO4, with MgCl2, and with MgSO4). It thus provides a very valuable tool to design and evaluate RED process systems.

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Managing the gas–liquid interface within gas-diffusion electrodes (GDEs) is key to maintaining high product selectivities in carbon dioxide electroreduction. By screening silver-catalyzed GDEs over a range of applied current densities, an inverse correlation was observed between carbon monoxide selectivity and the electrochemical double-layer capacitance, a proxy for wetted electrode area. Plotting current-dependent performance as a function of cumulative charge led to data collapse onto a single sigmoidal curve indicating that the passage of faradaic current accelerates flooding. It was hypothesized that high cathode alkalinity, driven by both initial electrolyte conditions and cathode half-reactions, promotes carbonate formation and precipitation which, in turn, facilitates electrolyte permeation. This mechanism was reinforced by the observations that post-test GDEs retain less hydrophobicity than pristine materials and that water-rinsing and drying electrodes temporarily recovers peak selectivity. This knowledge offers an opportunity to design electrodes with greater carbonation tolerance to improve device longevity.

Supercritical CO 2 (scCO 2 ), used in the food industry as a water extraction agent, requires dehydration units for regeneration. The present study assesses the economics of membrane-based dehydration of scCO 2 . In contrast to earlier studies, the contribution, next to the membrane, also the contributions of the mass transfer resistances of the feed and permeate boundary are included, which have a dominant effect on the final process design and economics. In addition, our work also extrapolates the process to industrial scale evaluating different configurations and process conditions. Specifically, the contribution of the membrane and membrane unit costs is discussed in more detail. Including the mass transfer resistances of feed and permeate boundary layer reduces the water flux across the membrane up to a factor 150, implying a larger required membrane surface area for a given water removal rate, and thus higher costs. Using a SPEEK-based membrane, the total drying costs, normalized for the amount of water removed, minimize around a skin layer thickness of 1 μm, i.e., not too thin to permeate and thus spill too much CO 2 and not too thick to hamper the H 2 O flux. Because the feed boundary layer dominates water transport, conditions that minimize its thickness reduce total costs. A reduction of the feed boundary layer dominance can be achieved by adjusting channel height, cross-flow velocity and the density and viscosity of scCO 2 , the latter two by increasing the operational temperature from 45 to 65 °C (at 130 bar). Compared to the benchmark zeolite process currently available, the membrane-based process for drying scCO 2 outlined and optimized in the present study results in a 50% saving of total drying costs. These savings can be achieved by using a dense polymeric membrane with a H 2 O permeability of at least 10,000 Barrer and a CO 2 permeability of at most 10 Barrer.

This paper presents a thorough and affordable numerical framework to design the flow field of Proton Exchange Membrane Fuel Cells using topology optimization. No assumption is made about the layout of the channels, which freely evolves along the optimization process, resulting in non-trivial optimized geometries. The optimization problem is formulated to maximize both the power generation and homogeneity of current density distribution, in the spirit of reduced costs and increased durability. The evolution of the flow field geometry is computed with a gradient-based optimizer with gradients of the objective and constraints computed through the discrete adjoint method. At each optimization iteration, the incompressible Navier-Stokes, advection-diffusion, and Butler-Volmer equations are solved with a 2D finite element model that predicts the flow in the channels, transport of chemical species and electrochemical reactions. The 3D transport effects are accounted for in the 2D model through an original depth-averaging procedure. The results of the 2D prediction are verified against a full 3D model calibrated using numerical and experimental results. The developed design framework can be used to identify flow field layouts that outperform current industrial solutions, catch design trends and provide guidelines to technology practitioners. The topology-optimized designs yield significant power generation enhancements, an improved reactant distribution and a reduced pressure drop as compared to conventional flow fields. Increasing the inlet pressure leads to more and more intricate configurations with complex topologies and highly tortuous channels. However, the cell performance is found to be more sensitive to the topology of the flow distributor at low rather than at high inlet pressures. Considering a measure of the current density homogeneity in the optimization objective allows the identification of layouts in which the gas channels concentrate close to the outlets rather than close to the inlet. These design features slightly affect the amount of power generated, suggesting a viable route for future technological development.

The fabrication of temperature responsive photonic polymers remains a challenge. Here, we report the fabrication of humidity-gated temperature-responsive infrared reflective photonic coatings using an easy-to-process bar-coating technique. At high humidity the hydroscopic cholesteric liquid crystalline polymer is able to absorb water vapour from the air causing swelling of the photonic coating. By increasing the temperature, water is desorbed from the coating, resulting in a reversible 420 nm shift of the photonic reflection band. In particular, it is shown that temperature-responsive single-layered broadband IR reflective coatings, prepared by creation of a pitch gradient of the cholesteric liquid crystals, might be suitable for smart window applications in high relative humidity environments such as greenhouses.

Supercritical carbon dioxide (scCO2) is used in the food industry as a water-extracting drying agent. Once saturated with water, the scCO2 needs to be regenerated. A promising way of drying scCO2 is by using H2O permeable membranes. Ideally, these membranes demonstrate low CO2 permeability. Here, we investigated the CO2 permeability of three types of dense membranes, Nafion, Natural Rubber and PDMS, of which the latter in more detail because of its ease of handling. The experimental conditions, temperature and pressure, resulting in minimum CO2 permeability (=losses) were explored. Even though the absolute CO2 permeability depends on the intrinsic membrane material properties, its trend with increasing feed pressure is defined by the (supercritical) behavior of CO2, notably its density as a function of temperature and pressure. The data points to transitions within the supercritical regime, from the gaseous-like supercritical state to the liquid-like supercritical state, graphically visualized by the Widom line for CO2 density. Sorption measurements with PDMS membranes confirm this behavior that follows the diffusion-solution theory. In the gaseous state, the (normalized) permeability follows the (normalized) solubility, indicating a constant CO2 diffusivity. With increasing pressure and when entering the liquid-like (supercritical) regime, the diffusivity drops, resulting in a (normalized) permeability that starts to lag behind the (normalized) solubility.

Continuous drying processes using supercritical CO2 (scCO2) as a water extraction agent require 24/7 operational dehydration units for scCO2 regeneration. Dehydration units using dense polymeric membranes are considered a cost effective, sustainable alternative to the current zeolite-based units. The focus of previous studies on the membrane-based dehydration of scCO2 was always on the membrane itself whereas boundary layer effects, e.g., concentration polarization, were not taken into account. To quantify the boundary layer effects, simulations were performed using three different membrane materials: SPEEK, Nafion® 117, and PEBAX® 1074. Process conditions during the simulations ranged from 8.0 to 18.0 MPa and 40 to 100 °C. Even though the three types of membranes examined differ in their H2O permeability and H2O over CO2 selectivity, in all cases 80% of the total mass-transfer resistance can be assigned to concentration polarization effects, making it the dominant parameter for water transport. Despite high but differing intrinsic water permeabilities of all three membranes materials, the H2O transport, thus H2O flux through the membrane is significantly reduced by concentration polarization down to similar levels. This makes it necessary to use larger membrane areas, that result in higher CO2 fluxes. As a consequence, material selection is predominantly based on the ability to reject CO2. Optimization of process conditions other than membrane material is briefly discussed.

Reverse electrodialysis (RED) is a process to harvest renewable energy from salinity gradients. Under lab conditions with artificial salt solutions, promising results have been achieved in recent years. However, in large scale industrial applications, natural waters are used and that poses challenges such as fouling. Fouling of anion exchange membranes (AEMs) by organic matter (e.g. humic acids) has been identified as a possible cause that lowers RED performance with natural waters. In this work, natural river and seawater at the Afsluitdijk (The Netherlands) are used to study the RED performance of six different AEMs. These AEMs are characterized before and after RED experiments with natural waters. The effect of natural fouling is found to be specific for each AEM and highly dependent on their respective chemistries and associated membrane properties. Firstly, aromatic AEMs with a low swelling degree showed a permselectivity decrease as well as membrane resistance increase. Secondly, aliphatic AEMs with a medium swelling degree experienced only a membrane resistance increase. Finally, only a decrease in permselectivity was observed for aliphatic AEMs with large swelling degrees. Subsequently, the effect of AEM fouling is compared to the observed decrease in RED performance and this shows that AEM fouling can only explain a minor part of the losses in open circuit voltage (OCV). The RED power densities dropped by 15–20% over 12 days, independent of the AEMs selected, while the reduced AEM performance could only explain 2–4% of this reduction in power density. This demonstrates that next to AEM fouling, also other factors, such as spacer fouling, are expected to be the dominant fouling mechanism, reducing the performance to a much larger extent.

Nanofiltration (NF) membrane processes are attractive to remove multivalent ions. As ion retention in NF membranes is determined by both size and charge exclusion, negatively charged membranes are required to reject negatively charged ions. Layer-by-layer assembly of alternating polycation (PC) and polyanion layers on top of a support is a versatile method to produce membranes. Especially the polyelectrolyte (PE) couple polydiallyldimethylammoniumchloride and poly(sodium-4-styrenesulfonate) (PDADMAC/PSS) is extensively investigated. This PE couple cannot form highly negatively charged membrane surfaces, due to interdiffusion and charge overcompensation of PDADMAC into the PSS layers, which limits the operational window to tailor membrane properties. We propose the use of asymmetric layer formation and show how combining two charge densities of one PC can produce negatively charged NF membranes. Starting from hollow fiber ultrafiltration supports coated with base layers of PDADMAC/PSS, they are coated with PDADMAC/PSS or poly(acrylamide-co-diallyldimethylammoniumchloride), P(AM-co-DADMAC)/PSS layers. P(AM-co-DADMAC) has a charge density of only 32% compared to 100% for PDADMAC. The particular novel membranes coated with P(AM-co-DADMAC) have a highly negatively charged surface and high permeabilities (7–19 L/[m2hbar]), with high retentions for Na2SO4 of up to 95%. These values position the developed membranes in the top range compared to commercial and other layer-by-layer membranes.

Cost-effective dense membranes were developed by blending proton-conductive sulfonated poly(ether ether ketone) (SPEEK) with inert, mechanically stable poly(vinylidene fluoride) (PVDF) for hydrogen-bromine flow batteries (HBFBs). Wire-electrospinning followed by hot-pressing was employed to prepare dense membranes. Their properties and performance were compared to solution-cast membranes of similar composition and thickness. Electrospinning improved the ionic conductivity and bromine diffusion properties by providing interconnected ion-conductive SPEEK nanofiber pathways through a PVDF matrix. Relatively thin (~50–60 μm) electrospun membranes with a SPEEK/PVDF ratio (wt%/wt%) of 90/10 and 80/20 showed comparable Br3− diffusion rates as the relatively thick and commercially available perfluorosulfonic acid (PFSA) membrane (~100 μm) at a 35%–42% lower proton conductivity. The latter can be attributed to the poorer ion conductivity of SPEEK compared to PFSA and the presence of PVDF. The HBFB single cell featured the best polarization behavior and ohmic area resistance with the electrospun membrane containing 80/20 (wt%/wt%) SPEEK/PVDF. However, the low thickness and insufficient chemical/mechanical stability of the ES 80/20 causes a rapid decay in the HBFB cycling performance. This study promotes a life-time comparison study between the low-cost wire-electrospun SPEEK/PVDF blend membranes (~€100 m−2) and the typically used PFSA membranes (~€400 m−2) for a long-term HBFB performance.

Electrodialysis (ED) is receiving increasing attention as promising technology for seawater desalination. However, most of the ED investigations are typically performed using artificial NaCl solutions, while the effect of multivalent ions (such as Mg2+ and Ca2+) on membrane scaling and resistance has been so far overlooked. In this work, we investigate the influence of multivalent ions in seawater on the desalination performance of multistage ED. In particular, natural seawater was used as feed solution, and two different strategies were compared, i.e. by using conventional cation exchange membranes (CEMs), as well as CEMs with preferential removal of multivalent ions. For both CEMs, we found that the removal of calcium and magnesium was higher compared to that of sodium and no effect due to operation at low current density was observed. More magnesium was removed with the multivalent ion permeable CEM. Starting from ~27 g/l (i.e. inlet concentration of the natural seawater source), the upscaled multistage ED system produced a continuous diluate concentration of 1.9 g/l. The system performance was stable over 18 days, with an average energy consumption of 3 kWh/m3, demonstrating the potential of multistage ED seawater desalination.

The authors regret that the printed version of the above article contained an error with respect to the experimental membrane area reported. This however does not affect the content, the discussions nor the conclusion of the paper as only trends are discussed, and these remain unchanged. Absolute values did change though. The correct data are given below. The authors apologize for any inconvenience caused. 1) Experimental section 3.4, second sentence: The correct membrane area is 78.8 cm2 instead of 47.9 cm2. 2) Following this change in membrane area, Figs. 4, 6 and 9 are updated and the correct figures are shown below: [Fiure presented] [Fiure presented] [Fiure presented]