Self-standing Membranes Research Articles

Electrospun, non-woven fiber membranes of porous polyimides with high carbon dioxide uptakes and selectivities

Microporous organic polyimides are well suited for carbon dioxide separation from gas mixtures based on their polar surface and their tendency towards ultramicroporosity. Nevertheless, their application potential is limited due to an inherent insolubility and infusibility, preventing an easy processing into functional objects like membranes. By establishing a three-step synthesis procedure for the literature known network NPI-I, here, a solution to this challenge is demonstrated. The central step is electrospinning of a solution of the linkers tetrakis(4-aminophenyl)methane and naphthalene 1,4,5,8-tetracarboxylic dianhydride and the auxiliary polymer polyvinylpyrrolidone into a precursor fiber mat. The mat was then heated to polymerize the linkers into the polyimide. Subsequently, PVP was removed by pyrolysis obtaining a robust, flexible and self-standing membrane. The NPI-I fiber mat exhibits a remarkable microporosity with a BET surface area of 222 m 2 /g, a total pore volume of 0.121 cm 3 /g and a high amount of ultramicropores. Its CO 2 uptake of 3.0 mmol/g (0 °C, 1 bar) and its CO 2 /CH 4 selectivity of about 20 (0 °C, 1 bar) even exceed the literature values for bulk NPI-I. This study reveals that PVP acts as a template on molecular level influencing, primarily the micropore formation, reducing the BET surface area by roughly a factor of three compared to the bulk material. In contrast, the ultramicroporosity of the porous polyimides remains mainly unaffected. We envision that this three-step synthesis can be transferred to a broad variety of porous polymers. With electrospinning the large-scale production of self-standing membranes becomes realistic rendering the application of porous polymers for gas separation more likely. • Shaping of ultramicroporous polyimides into non-woven, robust and flexible fiber membranes with a hierarchical porosity • Development of a three-step synthesis procedure based on electrospinning with thermally labile auxiliary polymer • Auxiliary polymer acts as template which influences the micro- and mesoporosity of the fibrous membranes • Synthesis procedure retains the ultramicroporous character of the polyimides

Interfacial-assembly engineering of asymmetric magnetic-mesoporous organosilica nanocomposites with tunable architectures.

The asymmetric morphology of nanomaterials plays a crucial role in regulating their physical and chemical properties, which can be tuned by two key factors: (i) interfacial interaction between seed particles and growth materials (anisotropic island nucleation) and (ii) reaction kinetics of the growth material (growth approach). However, controllable preparation of asymmetric nanoarchitectures is a daunting challenge because it is difficult to tune the interfacial energy profile of a nanoparticle. Here, we report an interfacial-assembly strategy that makes use of different surfactant/organosilica-oligomer micelles to actively regulate interfacial energy profiles, thus enabling controllable preparation of well-defined asymmetric nanoarchitectures (i.e., organosilica nano-tails) on magnetic Fe3O4 nanoparticles. For our magnetic nanocomposite system, the assembly structure of surfactant/organosilica-oligomer micelles and the interfacial electrostatic interaction are found to play critical roles in controlling the nucleation and architectures of asymmetric magnetic-mesoporous organosilica nanocomposite particles (AMMO-NCPs). Surfactant/organosilica-oligomer micelles with a one-dimensional wormlike linear structure could strengthen the interfacial assembly behavior between seed particles and growth materials, and thus achieved the longest tail length (25 μm) exceeding the previously reported highest recorded value (2.5 μm) of one order of magnitude. In addition, clickable AMMO-NCPs can employ a thiol-ene click reaction to modify their surface with a broad range of functional groups, such as amines, carboxyls, and even long alkyl chains, which allows for expanding functionalities. We demonstrate that C18 alkyl-grafted AMMO-NCPs can self-assemble into self-standing membranes with robust superhydrophobicity. In addition, carboxyl-modified AMMO-NCPs exhibit excellent adsorption capacity for cationic compounds. This study paves the way for designing and synthesizing asymmetric nanomaterials, which possess immense potential for future engineering applications in nanomaterial assembly, nanoreactors, biosensing, drug delivery, and beyond.

Development of Flexible, Conductive and Biocompatible Chitosan-Based Miniaturized Bioelectrodes for Enzymatic Glucose Biofuel Cells

The need for new clean energy sources for portable devices in biomedical, agro-food industry and environmental related sectors boosts scientists towards the development of new strategies for energy harvesting for their application in biodevices development. In this sense, enzymatic biofuel cells (BFCs) have gained much attention in the last years. This work faces the challenge of develop new generation of BFCs able to be adapted to remote and personal monitoring devices within the framework of wearable technologies. To this aim, one of the main challenges consists of the development of conductive and biocompatible electrodes, which constitute a challenge itself due to the non-conductive capabilities of most of the biocompatible supports. Additionally, bioelectrodes may achieve good mechanical properties and resilience in order to be suitable for the envisioned application, which involves exposure to deformation during long-term use. Furthermore, it is desirable that the systems developed are versatile enough to be adapted to miniaturized supports for new personal wearable devices development. In the present work, self-standing chitosan-carbon black membranes have been synthesized and modified with suitable enzymes for the assembly of an enzymatic glucose BFC. The membranes have been adapted to be integrated in miniaturized interdigitated gold electrodes as the step forward to miniaturized systems, modified with enzymes and metallic particles clusters and tested for energy harvesting from glucose solutions. The miniaturized system produces a power density of 0.64 µW/cm2 that is enhanced to 2.75 µW/cm2 in the presence of the metallic clusters, which constitute a 76% incensement. Such preliminary demonstrations highlight the good response of metals in bioelectrode configuration. However, energy harvesting real application of the developed miniaturized electrodes need still improvements but pave the way for the use of BFC as an energy source in wearable technologies due to their good mechanical, electrical and biocompatible properties.

Interfacing MXene Flakes on a Magnetic Fiber Network as a Stretchable, Flexible, Electromagnetic Shielding Fabric.

A unique self-standing membrane composed of hierarchical thermoplastic polyurethane (TPU)/polyacrylonitrile (PAN) fibers is prepared by the electrospinning technique, followed by a simple dip-coating process. Fe3O4 nanoparticles are uniformly anchored on TPU/PAN fibers during the electrospinning process, enabling the membrane to achieve effective electromagnetic interference shielding (EMI SE) performance. Such a hybrid membrane has a high magnetization of 18.9 emu/g. When MXene (Ti3C2Tx) layers are further loaded on the TPU/PAN/Fe3O4NPs hybrid membrane, its EMI SE performance in the X band can exceed 30 dB due to the hydrogen bonds generated between the macromolecular chain of PAN and the functional group (Tx) on the surface of MXene. Simultaneously, the interfacial attraction between MXene and the TPU/PAN/Fe3O4NPs substrate is enhanced. The EMI SE mechanism of the hybrid membrane indicates that this film has great potential in the fields of wearable devices and flexible materials

Preparation and decontamination performance of a flexible self-standing hydrogel photocatalytic membrane

The membrane immobilization of photocatalyst is a kind of win-win mode which can not only avoid the loss of catalyst but also give the membrane antibacterial and self-cleaning properties. Nevertheless, the inefficiency and low durability remain two major obstacles to hinder its practical applications. Herein, a facile micro-solvent combustion synthesis (MSCS) and sodium alginate (SA) crosslinking method were proposed to fabricate an integrated and self-standing membrane of nano-ZnCeOx combined with porous graphitic carbon nitride (ZnCe@PCNM). In the photocatalytic-membrane separation system, the water flux of ZnCe@PCNM was 800 l m−2•h−1•bar−1. Remarkably, the separation efficiency of ZnCe@PCNM could reach more than 90% during the 4-h continuous membrane separation process. The water flux of ZnCe@PCNM under visible light decreased only 21% which was less than that in the absence of light (53%), and the original flux could be completely restored after photocatalysis, signifying that the prepared ZnCe@PCNM had good self-cleaning ability in membrane separation. Meanwhile, ZnCe@PCNM exhibited 99% removal efficiency and 0.06131 min−1 reaction rate in photocatalysis experiments under 1-h visible light. The removal efficiency of ZnCe@PCNM was 1.2-fold and 2.8-fold that of carbon nitride particles (CN) and hydrogel membrane morphology (CNM), respectively. The optical properties test of ZnCe@PCNM showed that its excellent photocatalytic activity was derived from the rapid electron transfer caused by cross-linked structure, heterojunction and the oxygen vacancies induced by nano-ZnCeOx. The radical quenching experiment and EPR experiment showed that •O2− and •OH both were the main factors in reaction.

Towards the application of carboxymethyl chitin/ionic liquid blend as polymer electrolyte membrane for aqueous batteries

Carboxymethyl chitin (CMChit) has the potential to be used as a solid polymer electrolyte (SPE) based on its ionic conductivity value of the order of 10−6 S·cm−1 in self-standing membranes. In controlled humidity of 65RH%, carboxymethyl chitin based membrane blended with 1-Butyl-3-methylimidazolium acetate (BMIM[Ac]) ionic liquid (IL) (40 wt%) showed a threshold value of ionic conductivity in the order of 10−4 S·cm−1 and electrochemical stability was up to 2.93 V. The effects of the relative humidity and ionic liquid weight fraction on the ionic conductivity and structural changes were investigated in detail. Furthermore, the X-ray diffraction (XRD) diffractogram indicated a clear reduction of crystallinity of the CMChit. The Field-emission scanning electron microscopy (FESEM) observation of the cross-sections confirmed the homogeneity of the prepared blend. This electrolyte was tested in symmetric cells based on Zn//SPE//Zn and showed good reversibility and potential for application in proton-conducting batteries.

High-Temperature Proton Conduction in Covalent Organic Frameworks Interconnected with Nanochannels for Reverse Electrodialysis.

The crystalline porous organic framework offers a highly ordered and stable structure under hydrated conditions at high temperatures. Here, we demonstrated a method for preparing high-performance membrane buildup using "heterogeneous networks" and "polymer phase-separated nanochannels". A well-interconnected "nanochannel" with a "crystalline organic framework" forms a highly stable hybrid membrane above 80 °C under 100% hydration under acidic and basic conditions. The prepared structure provides a self-standing membrane that easily overcomes the problem faced by conventional high ion-exchange capacity (IEC)-based membranes such as swelling, gelling, fragility, and dissolving at elevated temperatures. Apart from structural stability, it also shows better chemical stability with enhanced proton conduction at elevated temperatures. This proton conduction with better structural stability in the high IEC sample confirms from thermal analysis, whereas it also offers relatively low in-plane membrane swelling as compared to the conventional membranes. These hybrid membranes were further combined with the FAA-3 membrane to manufacture a reverse electrodialysis system for generating a power output. We also evaluated the maximum power density (Pmax) of the stack theoretically and experimentally. The determined net power density (Pnet) is reported to be 0.45 W m-2 at a flow rate of 40 mL min-1. These results confirm that the developed membrane can withstand robustly under realistic ambient conditions maintaining stable cell performance.

Carbon-black-based self-standing porous electrode for 500 Wh/kg rechargeable lithium-oxygen batteries

While lithium-oxygen batteries exhibit high theoretical energy densities that by far exceed those of conventional lithium-ion batteries, there are a limited number of examples that actually demonstrate cells with high energy density. The limitation of the positive electrode with a capacity high enough to sustain repeated discharge/charge cycles under high areal capacity conditions hinders the implementation of the lithium-oxygen batteries with practically high energy density. In this study, we develop a carbon black powder-based self-standing membrane, which exhibits a discharge capacity of ~7,000 mAh/gelectrode even under high current density conditions (>0.4 mA/cm2). Using the developed self-standing carbon membrane as the positive electrode, a 500-Wh/kg class rechargeable lithium-oxygen battery was fabricated and a repeated discharge/charge cycle was demonstrated at 0.1 C-rate. The results obtained in this study provide new material research directions to realize high energy density rechargeable lithium-oxygen batteries and facilitate their future development.

Cross-Linked Composite Gel Polymer Electrolyte Based on an H-Shaped Poly(ethylene oxide)-Poly(propylene oxide) Tetrablock Copolymer with SiO2 Nanoparticles for Solid-State Supercapacitor Applications.

Achieving high ionic conductivity, wide voltage window, and good mechanical strength in a single material remains a key challenge for polymer-based electrolytes for use in solid-state supercapacitors (SCs). Herein, we report cross-linked composite gel polymer electrolytes (CGPEs) based on multi-cross-linkable H-shaped poly(ethylene oxide)–poly(propylene oxide) (PEO-PPO) tetrablock copolymer precursors, SiO2 nanoparticles, and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, an ionic liquid (IL). Self-standing CGPE membranes with a high IL content were prepared using in situ cross-linking reactions between the silane groups present in the precursor and the SiO2 surface. The incorporation of an optimal amount of SiO2 increased the cross-linking density of the resulting CGPE while reducing polymer-chain ordering and, consequently, increasing both ionic conductivity and mechanical strength. As a result, the CGPE with 0.1 wt % SiO2 exhibited a high ionic conductivity (2.22 × 10–3 S cm–1 at 25 °C), good tensile strength (453 kPa), and high thermal stability up to 330 °C. Finally, an all-solid-state SC assembled with the prepared CGPE showed a high operating voltage (3 V), a large specific capacitance (103.9 F g–1 at 1 A g–1), and excellent durability (94% capacitance retention over 10,000 charge/discharge cycles), which highlights its strong potential as a solid-state electrolyte for SCs.

Influence of Polymer Topology on Gas Separation Membrane Performance of the Polymer of Intrinsic Microporosity PIM-Py

The polymer synthesis of 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile-3,3,3′,3′-tetramethyl-1,1′-spirobisindane-5,5′,6,6′-tetrol copolymer, termed PIM-Py, was investigated under different solvent (dimethylformamide (DMF) and dimethylacetamide/dichlorobenzene) and temperature (65–160 °C) conditions to produce a range of topologically different polymer samples. Characterization of the polymers, particularly with proton NMR spectroscopy and multiple detector SEC analysis, indicated that, like PIM-1, the polymerizations proceeded with a degree of polymer chain branching. This is attributed to the occurrence of monosubstitution reactions, instead of disubstitution, which eventually leads to a significant proportion of colloidal network formation. However, all polymer samples remained soluble/dispersible in chloroform at the concentration required to cast self-standing films. This work reports the first examination of PIM-Py as a membrane for gas separation applications. The most structurally diverse PIM-Py samples produced films that exhibited selectivity/permeability balances in single gas permeation studies above the 2008 Robeson upper bound for the CO2/N2 gas pair. Indeed, a film cast from the highest colloidal network content sample surpassed the recently introduced 2019 CO2/N2 upper bound. After 143 days of aging, a 40 μm self-standing membrane still exhibited a single gas CO2 permeability of 4480 barrer and an ideal CO2/N2 selectivity of 45. The polymers produced in lower temperature reactions in DMF exhibited gas separation performances very similar to a structurally regular “normal” PIM-1 polymer, sitting on or around the 2008 Robeson upper bound line. Single gas permeation measurements to determine CO2/CH4 selectivity showed similar trends across the range of polymer samples, without generally reaching high selectivities as for the CO2/N2 pair. Mixed gas CO2/CH4 permeation measurements with aging were also completed for PIM-Py membranes, which indicated similar gas separation performance to a structurally regular PIM-1 polymer. This study would suggest that, like PIM-1, gas separation performance of PIM-Py is greatly influenced by the topological balance toward branched and network material within the polymer sample.

Multifunctional Applications of Blow-Spinning Setaria viridis Structured Fibrous Membranes in Water Purification

With increasing water pollution and human health problems caused by oily wastewater, the fabrication of oil-water separation materials has become an urgent task. However, most of the reported materials have a single function and poor performance. In this paper, a multifunctional zinc oxide/polyaniline/polyacrylonitrile (ZnO/PANI/PAN) nanofibrous membrane with needle-like ZnO nanorods was prepared by in situ synthesis of PANI and a hydrothermal reaction on a highly stable self-standing PAN blow-spinning fibrous membrane. Due to the electronic synergistic effect of ZnO and PANI, the fibrous membrane exhibits excellent antibacterial activity and visible-light degradation ability of organic dyes. Moreover, the micro-/nanosized pores of the ZnO/PANI/PAN fibrous membranes also guarantee its excellent emulsion separation performance, including an ultrahigh surfactant-free emulsion permeate flux (∼8597.40 L/(m2 h)), ultrahigh surfactant-stabilized emulsion permeate flux (∼2253.50 L/(m2 h)), and excellent separation efficiency (above 99%). Furthermore, the composite membrane maintains stable underwater superoleophobicity and hydrophilicity under adverse conditions, shows good biological safety, and is harmless to the water environment. These excellent properties endow the ZnO/PANI/PAN nanofibrous membranes with great potential in treating oily wastewater.

Novel single-ion conducting electrolytes based on vinylidene fluoride copolymer for lithium metal batteries

A single-ion polymer electrolyte (SIPE) based on PVDF-based random copolymer (Li-PVDF) was synthesized in water from the radical copolymerization of vinylidene fluoride (VDF) with perfluoro-2-methyl-3-oxa-5-sulfonimido[-3-oxa-5-sulfonyl fluoride] vinyl ether (VEPFSIS) initiated by potassium persulfate. The purified polymer, characterized by 1H and 19F NMR spectroscopy, ICP-MS, and size-exclusion chromatography, contains 96 mol% VDF with an average molar mass of 128 kg mol−1. The Li-PVDF was cast into self-standing membranes and then swollen with a 50/50 v/v mixture of ethylene carbonate and propylene carbonate. The membrane was analyzed using DSC, TGA, pulse field gradient (PFG)-NMR and electrochemical methods. The solvent-doped membrane exhibited a lithium ion transport number close to 1 with lithium ion conductivities higher than 10−4 S cm−1 at 30 °C, an electrochemical stability up to 4.3 V vs. Li+/Li and an excellent reversibility upon continuous lithium plating/striping cycles. In addition, prototype Li/NMC battery cells using Li-PVDF as both electrolyte and cathode binder in the NMC based composite cathode showed reversible cycling and good performance at 60 °C. The results demonstrated the promising potential of this new single-ion polymer family in intrinsically safe all solid-state lithium metal based batteries.

Ultra-low temperature flexible supercapacitor based on hierarchically structured pristine polypyrrole membranes

• A self-standing PPy electrode (PPy-N) with a hierarchical structure was constructed. • PPy-N possessed superior electrochemical properties and low-temperature flexibility. • PPy-N assembled supercapacitors stably powered electronics in a dry ice bath (<-50 ℃) even cyclic bending. Low-temperature flexible supercapacitors (LFSCs) are urgently needed because most supercapacitors become rigid and prone to damage at extremely low temperatures, such as in the winter of the northern atmosphere, at high altitude, and in space. Although extensive investigations on anti-freezing electrolytes have significantly promoted the development of low-temperature resistant supercapacitors, a usable LFSC remains a major challenge because of the inferior capacitance and flexibility of the existing electrodes at low-temperature. Herein, a supercapacitor prototype that remained flexible and energy storage functional at −50 ℃ was demonstrated, thanks to a hierarchically-structured self-standing pristine polypyrrole membrane (PPy-N) that showed high flexibility and electrochemical performances at ultra-low temperature. With PPy-N as the electrodes, the assembled supercapacitor possessed a high cyclic stability (88.9% capacitance retention after 1000 cycles) as well as an outstanding flexibility (only 9.6% capacitance decay when bent to 180°) at −50 ℃. Furthermore, the PPy-N supercapacitors in series stably powered electronic devices even when repeatedly bent in a dry ice bath. Such remarkable performances make this flexible PPy-N electrode a promising candidate for the LFSCs.

Composite electrolytes based on electrospun PVDF and ionic plastic crystal matrices for Na-metal battery applications

Sodium ion batteries are widely considered to be a feasible, cost-effective, and sustainable energy storage alternative to Lithium, especially for large-scale energy storage applications. Next generation, safer electrolytes based on ionic liquid (IL) and organic ionic plastic crystals (OIPCs) have been demonstrated as electrochemically stable systems which show superior performance in both Li and Na applications. In particular, phosphonium‐based systems outperform most studied nitrogen‐based ILs and OIPCs. In this study triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide ([P1i444][FSI]) OIPC mixed with 20 mol% of NaFSI or NaTFSI were combined with an electrospun polyvinylidene fluoride (PVDF) support to create self-standing electrolyte membranes, and their thermal phase behaviour and ionic conductivity were investigated and compared with the bulk electrolytes. The ability of the solid-state composite electrolytes to support the cycling of sodium metal with good efficiency and without breakdown were examined in sodium metal symmetrical coin cells. The sodium transference number was determined to be 0.21. The electrochemical performance of Na/Na3V2(PO4)3 cells incorporating the composite electrolytes, including good cycling stability and rate capability, is also reported. Interestingly, the mixed anion systems appear to outperform the composite electrolyte containing only FSI anions, which may relate to electrolyte interactions with the PVDF fibres.

Post-Process-Functionalized Catalytic Electrospun and 2D-Printed Structures for Wolf–Lamb-Type Catalysis

A process for preparing active, strong base and acid self-standing individual membranes and printed structures is established by electrospinning and two-dimensional (2D) printing of the correspondi...

Preparation, Characterization, and Photocatalytic Properties of Self-Standing Pure and Cu-Doped TiO2 Nanobelt Membranes.

A traditional hydrothermal method was modified to synthesize ultra-long sodium titanate nanobelts by simultaneously stirring the solution. The ultra-long sodium titanate nanobelts were converted to hydrogen titanate nanobelts through an ion exchanging way. A method was then used to prepare self-standing flexible large-area membranes; they were then subject to post-annealing at different temperatures to obtain a self-standing TiO2 nanobelt membrane with a slight decrease in flexibility. Cu-doped TiO2 membranes were prepared by ion exchanging and post-annealing in the same manner. X-ray diffractions, scanning electron microscopy, field-emission scanning electron microscopy, field-emission transmission electron microscopy, Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, and UV–vis spectroscopy were used to characterize the samples. Photodegradation of methylene blue (MB) water solutions was used to evaluate the photocatalytic activity. It was seen that the pure sample presented obvious visible-light responding photocatalytic activity, possibly due to the self-sensitization of the MB molecule. The UV-induced photocatalytic activity is higher because of the photoinduced holes and electrons. It was suggested that the Cu dopant induced intra-gap states from electron traps and recombination centers, resulting in the decrease in both of the visible and UV induced photocatalysis.

A semi-interpenetrating network polymer electrolyte membrane prepared from non-self-polymerized precursors for ambient temperature all-solid-state lithium-ion batteries

Solid polymer electrolytes (SPEs) are currently attracting widespread interest for lithium battery applications due to high safety, low volatility, and the ability to suppress Li dendrites. Herein, a novel all-solid-state polymer electrolyte with semi-interpenetrating network (semi-IPN) structure has been prepared. The semi-IPN SPE is fabricated by thiol-tosylate polycondensation of pentaerythritol terakis (3-mercaptopropionate) (PETMP) with non-self-polymerized precursors, ditosylate poly (ethylene glycol) (DTsPEG) and p-toluenesulfonyl poly (ethylene glycol) methyl ether (TsPEGME), and then blended with a high molecular weight polyethylene oxide (PEO, Mw ​= ​300 ​kg ​mol−1). The resulting self-standing membrane achieves an excellent ionic conductivity (1.3 ​× ​10−4 ​S ​cm−1) at ambient temperature, and exhibits an electrochemical stability window up to 4.3 ​V vs. Li/Li+. The LiFePO4/semi-IPN-SPE/Li battery exhibits a first discharging capacity of 143.1 ​mA ​h ​g−1 ​at 0.2C and the discharge capacity keeps 134 ​mA ​h ​g−1 after 100 charge and discharge cycles.

Self-Standing Spray-Deposited Nafion Membranes for Fuel Cell Operations

The characteristics of a polymer electrolyte membrane (PEM) are crucial for the success of PEM based fuel cells and electrolyzers. Membranes play a significant role regarding ohmic losses, fuel permeability and mechanical stability. For the resulting catalyst coated membrane (CCM), it is important to create an ideal contact between membrane and electrode to avoid gaps and enhance interfacial surface area. Production of CCMs for fuel cells or electrolyzers is mainly based on two techniques: Decal Transfer: Fabrication of an electrode onto a carrier substrate and subsequent transfer by hot-pressing it onto a membrane [1].Direct deposition: Spray-deposition onto heated and vacuum fixed membranes [2] or doctor-blade coating onto pre-swollen membranes [3]. Both techniques bear several problems: During hot-pressing membranes are exposed to high temperatures and pressure. This can lead to deformations or cracks within the membrane. At the same time, transfer of electrode material can be insufficient and membrane electrode interface can suffer from delamination which causes gaps in terms of proton conductivity.Direct deposition of electrode material onto membranes entails difficult processing. Swelling and shrinking during coating and drying can result in inhomogeneous electrodes. Instead of coating membranes with electrodes, ultrasonic spray-deposition was employed to produce self-standing membranes. Being able to produce membranes allows the consecutive fabrication of all layers in a CCM and makes commercial membranes obsolete. It also reduces problems when an electrode is spray-deposited because the membrane is usually produced just before the electrode that is supposed to be spray-deposited. Thus, the membrane is attached to either a carrier substrate (PTFE) or another electrode that was spray-deposited at the very beginning.Fabricating membranes by spray-deposition gives freedom in terms of several processing aspects: Modification of the ionomer solution composition. The following variations were used: Solutions based on water/alcohol and water/alcohol + high boiling point solvents such as ethylene glycol, dimethyl sulfoxide, dimethylformamide and dimethylacetamide.Application of thermal treatmentsVariation of the membrane thicknesses This approach reveals possibilities that would not be available with commercial Nafion membranes with predefined properties. Self-standing Nafion membranes, based on different ionomer solution compositions and thermal treatments were produced (120-130 µm). All samples were employed for CCM fabrication (Anode: 3.0 mg/cm² PtRu; Cathode: 0.7 mg/cm² PtNi). Further, they were characterized, regarding hydrogen permeability, ohmic resistances and single-cell polarization curves in direct methanol fuel cell (DMFC) operation. All samples were compared to commercial Nafion 115 membranes.With some samples the performances of CCMs using spray-deposited membranes could be matched with Nafion 115. At the same time they showed lower ohmic resistances and partly lower hydrogen crossover values. Additionally, thinner membranes equivalent to Nafion 212 (50 µm) and Nafion 211 (25 µm) were fabricated and tested. They showed even better performances. While 50 µm thick membranes had moderate permeability levels comparable to 127 µm thick membranes, 25 µm samples showed high permeabilities. To reach high performances with relatively low permeation values, thin composite membranes (25 µm) were produced. These consisted of Nafion and graphene oxide, where graphene oxide was supposed to work as a blocking layer against permeation [4] while supporting proton conductivity.It is also possible to deposit Nafion solutions or Nafion (composite) dispersions directly onto electrodes instead of producing self-standing membranes [5]. This would allow the fabrication of a whole CCM consecutively [6]. Even though this technique was used for fabrication of CCMs for DMFCs, it can also be transferred to hydrogen-based fuel cells (PEMFC) [6] or water-electrolysis (PEMWE) [7]. X. Ren, M. Wilson, S. Gottesfeld, J. Electrochem. Soc., 143 (1996) L12–L15. L. Sun, R. Ran, Z. Shao, Int. J. Hydrogen Energy, 35 (2010) 2921–2925. I.-S. Park, W. Li, A. Manthiram, J. Power Sources, 195 (2010) 7078–7082. L. Sha Wang, A. Nan Lai, C. Xiao Lin, Q. Gen Zhang, A. Mei Zhu, Q. Lin Liu, J. Memb. Sci., 492 (2015) 58–66. M. Klingele, M. Breitwieser, R. Zengerle, S. Thiele, J. Mater. Chem. A, 3 (2015) 11239–11245. M. Klingele, B. Britton, M. Breitwieser, S. Vierrath, R. Zengerle, S. Holdcroft, S. Thiele, Electrochem. Commun., 70 (2016) 65–68. P. Holzapfel, M. Bühler, C. Van Pham, F. Hegge, T. Böhm, D. McLaughlin, M. Breitwieser, S. Thiele, Electrochem. Commun., 110 (2020) 106640. Figure 1

Self-standing and flexible covalent organic framework (COF) membranes for molecular separation.

Almost all covalent organic framework (COF) materials conventionally fabricated by solvothermal method at high temperatures and pressures are insoluble and unprocessable powders, which severely hinder their widespread applications. This work develops an effective and facile strategy to construct flexible and free-standing pure COF membranes via the liquid-liquid interface-confined reaction at room temperature and atmospheric pressure. The aperture size and channel chemistry of COF membranes can be rationally designed by bridging various molecular building blocks via strong covalent bonds. Benefiting from the highly-ordered honeycomb lattice, high solvent permeances are successfully obtained and follow the trend of acetonitrile > acetone > methanol > ethanol > isopropanol. Interestingly, the imine-linked COF membrane shows higher nonpolar solvent permeances than b-ketoenamine-linked COF due to their difference in pore polarity. Both kinds of COF membranes exhibit high solvent permeances, precise molecular sieving, excellent shape selectivity, and sufficient flexibility for membrane-based separation science and technology.

Highly crystalline ionic covalent organic framework membrane for nanofiltration and charge-controlled organic pollutants removal

Organic pollutants are biological toxicity and should be removed from the environment. Membrane separation offers several advantages in organic pollutants removal such as high pollutants selectivity and high solvent permeability. Covalent organic framework membrane is regarded as an emerging porous membrane for nanofiltration owing to their permanent porosity and well-defined pore structure in various organic media. Herein, a self-standing 2D sulfonate anionic COF membrane with nanometer pore size and -SO3Na groups has been successfully prepared. The surface charge combines with nanopore sieving property endow the COF membrane high nanofiltration separation performances and charge-controlled organic molecular separation. The anionic membrane can effectively intercept > 99% of cationic organic pollutants, while maintaining excellent solvent permeability due to its rigid porous structure and high porosity. The density functional theory (DFT) calculation provides further understanding on the charge-controlled separation in ionic nanoporous membranes. Our work may open up a new avenue for developing charged COF membrane for nanofiltration and organic pollutants separation applications.