Ionic Cross-Linked MOF-Polymer Mixed-Matrix Membranes for Suppressing Interfacial Defects and Plasticization Behavior.

To address the plasticization phenomenon and MOF-polymer interfacial defects, we report the synthesis of ionic cross-linked MOF MMMs from a dual brominated polymer and MOF components by using N,N'-dimethylpiperazine as the cross-linker. We synthesized brominated MIL-101(Cr) nanoparticles by using mixed linkers and prepared brominated polyimide (6FDA-DAM-Br) to form ionic cross-linked MMMs. The gas permeation properties of the polyimide, ionic cross-linked MOF-polymer MMMs, and non-cross-linked MOF-polymer MMMs with various MOF weight loadings were investigated systematically to effectively understand the effects of MOF weight loading and ionic cross-linking. The ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly enhanced gas permeability with an H2 permeability of 1640 Barrer and CO2 permeability of 1981 Barrer and slightly decreased H2/CH4, H2/N2, CO2/CH4 and CO2/N2 selectivities of 16.9, 15.4, 20.5, and 18.6, respectively. The H2 and CO2 permeabilities are approximately 2-3 fold higher than those of the pure polyimide (6FDA-DAM) membrane. Moreover, the ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly increased resistance to plasticization. This is because the brominated MOF incorporation boosted molecular transport and polymer chain rigidity, and ionic cross-linking further reduced the number of interfacial defects and polymer chain mobility.

Covalent and non-covalent approaches to suppress plasticization of polymer membranes for gas separation.

Polymer membranes represent an attractive platform for energy-efficient gas separation, but they are known to suffer from plasticization during continuous gas-separation processes. This phenomenon is caused by the spontaneous relaxation of individual polymer chains arising from the swelling effect induced by high-pressure highly soluble gases such as CO2, and it weakens the stability of the membrane, leading to a significant loss of selectivity during the separation of mixed gases. Thus, minimizing the disadvantages of polymer membranes is essential to ensure reliable gas-separation performance for practical applications. This feature article summarizes the theory underlying the plasticization of polymer membranes and introduces covalent and non-covalent approaches to suppress plasticization behaviour on a molecular level.

High-Loading Poly(ethylene glycol)-Blended Poly(acrylic acid) Membranes for CO2 Separation.

Poly(ethylene glycol) (PEG) is an amorphous material of interest owing to its high CO2 affinity and potential usage in CO2 separation applications. However, amorphous PEG often has a low molecular weight, making it challenging to form into the membrane. The crystalline high average molar mass poly(ethylene oxide) (PEO) cannot exhibit CO2 separation characteristics. Thus, it is crucial to employ low molecular weight PEG in high molecular weight polymers to increase the CO2 affinity for CO2 separation membranes. In this work, poly(acrylic acid) (PAA)/PEG blend membranes with a PEG-rich phase were simply fabricated by physical mixing with an ethanol solvent. The carbonyl group of the PAA and the hydroxyl group of the PEG formed a hydrogen bond. Furthermore, the thermal stability, glass transition temperature, and surface hydrophilicity of PAA/PEG blend membranes with various PEG concentrations were further characterized. The PAA/PEG(1:9) blend membrane exhibited an improved CO2 permeability of 51 Barrer with high selectivities relative to the other gas species (H2, N2, and CH4; CO2/H2 = 6, CO2/N2 = 63, CO2/CH4 = 21) at 35 °C and 150 psi owing to the enhanced CO2 affinity with the amorphous PEG-rich phase. These PAA/PEG blend membrane permeation characteristics indicate a promising prospect for CO2 capture applications.

Poly(benzyl ether)-type additive to engineer glassy polyimide membranes for enhanced gas separations.

This conceptual study demonstrates the reinforcement of glassy polyimide membranes by incorporating a poly(benzyl ether)-type additive. Traces of the sterically bulky additive alter the overall physical properties of the entire matrix and further enhance the separation properties of small gas molecules.

Biodegradation of polystyrene by bacteria from the soil in common environments.

Polystyrene (PS), a major plastic waste, is difficult to biodegrade due to its unique chemical structure that comprises phenyl moieties attached to long linear alkanes. In this study, we investigated the biodegradation of PS by mesophilic bacterial cultures obtained from various soils in common environments. Two new strains, Pseudomonas lini JNU01 and Acinetobacter johnsonii JNU01, were specifically enriched in non-carbonaceous nutrient medium, with PS as the only source of carbon. Their growth after culturing in basal media increased more than 3-fold in the presence of PS. Fourier transform infrared spectroscopy analysis, used to confirm the formation of hydroxyl groups and potentially additional chemical bond groups, showed an increase in the amount of oxidized PS samples. Moreover, field emission scanning electron microcopy analysis confirmed PS biodegradation by biofilms of the screened microbes. Water contact angle measurement additionally offered insights into the increased hydrophilic characteristics of PS films. Bioinformatics and transcriptional analysis of A. johnsonii JNU01 revealed alkane-1-monooxygenase (AlkB) to be involved in PS biodegradation, which was confirmed by the hydroxylation of PS using recombinant AlkB. These results provide significant insights into the discovery of novel functions of Pseudomonas sp. and Acinetobacter sp., as well as their potential as PS decomposers.

MOF-Based Membranes for Gas Separations.

Metal-organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.

Mixed-Matrix Membranes Formed from Imide-Functionalized UiO-66-NH2 for Improved Interfacial Compatibility.

Mixed-matrix membranes (MMMs) formed by dispersing metal-organic framework (MOF) particles in polymers have attracted significant attention because these composite systems can potentially surpass the separation performance of pure polymers alone. However, performance improvements are often unrealized because of poor interfacial compatibility between the MOF and the polymer, which results in interfacial defects. From a practical perspective, strategies are needed to address these defects so that MMMs can be deployed in real-world separation processes. From a fundamental perspective, strategies are needed to reliably form defect-free MMMs so that transport models can be applied to estimate pure MOF property sets, thereby enabling the development of robust structure-property relationships. To address these interfacial challenges, we have developed a method to surface-functionalize a UiO-66-NH2 MOF with a nanoscopic shell of covalently tethered 4,4'-(hexafluoroisopropylidene)diphthalic anhydride-Durene oligomers. When combined with a high-molecular-weight polymer of identical chemical structure to that of the imide-functional MOF surface, defect-free MMMs with uniform particle dispersions can be formed. With this technique, both permeabilities and selectivities of select gases in the MMMs were improved at loadings ranging from 5 to 40 wt %. At a 40 wt % loading, CO2 permeability and CO2/CH4 selectivity were enhanced by 48 and 15%, respectively. Additionally, pure MOF permeabilities for H2, N2, O2, CH4, and CO2 were predicted by the Maxwell model.

Mixed-Matrix Membranes Formed from Multi-Dimensional Metal-Organic Frameworks for Enhanced Gas Transport and Plasticization Resistance.

Mixed-matrix membranes (MMMs) formed by incorporating metal-organic frameworks (MOFs) into polymers have a general limitation in that the MOFs are typically formed into rather simple dimensionalities (such as 1D, 2D, or 3D). Each design approach has intrinsic-albeit independent-benefits, such as network percolation (1D), access to high-aspect ratios (2D), and ease of processability (3D). However, a design strategy is needed to combine multiple dimensionalities and, thereby, access the full range of transport and compositing benefits of these high-performance materials. Herein, a facile method to form multi-dimensional HKUST-1 nanoparticles is introduced by using a modulator to tune the MOF nucleation and growth mechanism. At 30 wt % multidimensional MOF loading, the MMM shows CO2 permeabilities of approximately 2500 Barrer, which represents a 2.5-fold increase compared to that of a pure polymer without a large loss of selectivity for CO2 /CH4 and CO2 /N2 . Additionally, almost no plasticization pressure response is observed for CO2 up to 750 psi, suggesting an unusual stability to high activity feeds.

Direct Organization of Morphology-Controllable Mesoporous SnO2 Using Amphiphilic Graft Copolymer for Gas-Sensing Applications.

A simple and flexible strategy for controlled synthesis of mesoporous metal oxide films using an amphiphilic graft copolymer as sacrificial template is presented and the effectiveness of this approach for gas-sensing applications is reported. The amphiphilic graft copolymer poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) is used as a sacrificial template for the direct synthesis of mesoporous SnO2. The graft copolymer self-assembly is shown to enable good control over the morphology of the resulting SnO2 layer. Using this approach, mesoporous SnO2 based sensors with varied porosity are fabricated in situ on a microheater platform. This method reduces the interfacial contact resistance between the chemically sensitive materials and the microheater, while a simple fabrication process is provided. The sensors show significantly different gas-sensing performances depending on the SnO2 porosity, with the highly mesoporous SnO2 sensor exhibiting high sensitivity, low detection limit, and fast response and recovery toward hydrogen gas. This printable solution-based method can be used reproducibly to fabricate a variety of mesoporous metal oxide layers with tunable morphologies on various substrates for high-performance applications.

Nanowire-Assembled Hierarchical ZnCo2O4 Microstructure Integrated with a Low-Power Microheater for Highly Sensitive Formaldehyde Detection.

Nanowire-assembled 3D hierarchical ZnCo2O4 microstructure is synthesized by a facile hydrothermal route and a subsequent annealing process. In comparison to simple nanowires, the resulting dandelion-like structure yields more open spaces between nanowires, which allow for better gas diffusion and provide more active sites for gas adsorption while maintaining good electrical conductivity. The hierarchical ZnCo2O4 microstructure is integrated on a low-power microheater platform without using binders or conductive additives. The hierarchical structure of the ZnCo2O4 sensing material provides reliable electrical connection across the sensing electrodes. The resulting sensor exhibits an ultralow detection limit of 3 ppb toward formaldehyde with fast response and recovery as well as good selectivity to CO, H2, and hydrocarbons such as n-pentane, propane, and CH4. The sensor only consumes ∼5.7 mW for continuous operation at 300 °C with good long-term stability. The excellent sensing performance of this hierarchical structure based sensor suggests the advantages of combining such structures with microfabricated heaters for practical low-power sensing applications.

Selective Ion Transporting Polymerized Ionic Liquid Membrane Separator for Enhancing Cycle Stability and Durability in Secondary Zinc-Air Battery Systems.

Rechargeable secondary zinc-air batteries with superior cyclic stability were developed using commercial polypropylene (PP) membrane coated with polymerized ionic liquid as separators. The anionic exchange polymer was synthesized copolymerizing 1-[(4-ethenylphenyl)methyl]-3-butylimidazolium hydroxide (EBIH) and butyl methacrylate (BMA) monomers by free radical polymerization for both functionality and structural integrity. The ionic liquid induced copolymer was coated on a commercially available PP membrane (Celguard 5550). The coat allows anionic transfer through the separator and minimizes the migration of zincate ions to the cathode compartment, which reduces electrolyte conductivity and may deteriorate catalytic activity by the formation of zinc oxide on the surface of the catalyst layer. Energy dispersive X-ray spectroscopy (EDS) data revealed the copolymer-coated separator showed less zinc element in the cathode, indicating lower zinc crossover through the membrane. Ion coupled plasma optical emission spectroscopy (ICP-OES) analysis confirmed over 96% of zincate ion crossover was reduced. In our charge/discharge setup, the constructed cell with the ionic liquid induced copolymer casted separator exhibited drastically improved durability as the battery life increased more than 281% compared to the pure commercial PP membrane. Electrochemical impedance spectroscopy (EIS) during the cycle process elucidated the premature failure of cells due to the zinc crossover for the untreated cell and revealed a substantial importance must be placed in zincate control.

Enhanced performance of mixed-matrix membranes through a graft copolymer-directed interface and interaction tuning approach.

Herein, a high performance mixed-matrix membrane (MMM) is reported with simultaneously large improvements in the CO2 permeability by 880 % from 70.2 to 687.7 Barrer (1 Barrer=1×10(-10)  cm(3) cm cm(-2) s(-1) cmHg(-1) ) and CO2 /N2 selectivity by 14.4 % from 30.5 to 34.9. These findings represent one of the most dramatic improvements ever reported for MMMs. These improvements are obtained through an interface and interaction tuning approach based on an amphiphilic grafted copolymer. Poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer plays a key role as a soft organic matrix to provide good permeation properties, uniform distribution of zeolite imidazole frameworks-8 (ZIF-8), and better interfacial contact with inorganic compounds. In particular, the CO2 /C3 H8 and CO2 /C3 H6 selectivities reached 10.5 and 42.7, respectively, for PVC-g-POEM/ZIF (40 %) MMMs; this indicates that it could be a promising membrane material for the purification of C3 hydrocarbons.

Mesoporous TiO2 Bragg stack templated by graft copolymer for dye-sensitized solar cells.

Organized mesoporous TiO2 Bragg stacks (om-TiO2 BS) consisting of alternating high and low refractive index organized mesoporous TiO2 (om-TiO2) films were prepared to enhance dye loading, light harvesting, electron transport, and electrolyte pore-infiltration in dye-sensitized solar cells (DSSCs). The om-TiO2 films were synthesized via a sol-gel reaction using amphiphilic graft copolymers consisting of poly(vinyl chloride) backbones and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM as templates. To generate high and low index films, the refractive index of om-TiO2 film was tuned by controlling the grafting ratio of PVC-g-POEM via atomic transfer radical polymerization (ATRP). A polymerized ionic liquid (PIL)-based DSSC fabricated with a 1.2-µm-thick om-TiO2 BS-based photoanode exhibited an efficiency of 4.3%, which is much higher than that of conventional DSSCs with a nanocrystalline TiO2 layer (nc-TiO2 layer) (1.7%). A PIL-based DSSC with a heterostructured photoanode consisting of 400-nm-thick organized mesoporous TiO2 interfacial (om-TiO2 IF) layer, 7-µm-thick nc-TiO2, and 1.2-µm-thick om-TiO2 BS as the bottom, middle and top layers, respectively, exhibited an excellent efficiency of 7.5%, which is much higher than that of nanocrystaline TiO2 photoanode (3.5%).

Structural control of hierarchically-ordered TiO2 films by water for dye-sensitized solar cells.

A facile way of controlling the structure of TiO(2) by changing the amount of water to improve the efficiency of dye-sensitized solar cells (DSSCs) is reported. Hierarchically ordered TiO(2) films with high porosity and good interconnectivity are synthesized in a well-defined morphological confinement arising from a one-step self-assembly of preformed TiO(2) (pre-TiO(2)) nanocrystals and a graft copolymer, namely poly(vinyl chloride)-g-poly(oxyethylene methacrylate). The polymer-solvent interactions in solution, which are tuned by the amount of water, are shown to be a decisive factor in determining TiO(2) morphology and device performance. Systematic control of wall and pore size is achieved and enables the bifunctionality of excellent light scattering properties and easy electron transport through the film. These properties are characterized by reflectance spectroscopy, incident photon-to-electron conversion efficiency, and electrochemical impedance spectroscopy analyses. The TiO(2) photoanode that is prepared with a higher water ratio, [pre-TiO(2)]:[H(2)O]=1:0.3, shows a larger surface area, greater light scattering, and better electron transport, which result in a high efficiency (7.7 %) DSSC with a solid polymerized ionic liquid. This efficiency is much greater than that of commercially available TiO(2) paste (4.0 %).

Dual-functionalized mesoporous TiO(2) hollow nanospheres for improved CO(2) separation membranes.

Simultaneous improvement in CO2 permeability and CO2/N2 selectivity was obtained from mixed matrix membranes (MMMs) containing dual-functionalized mesoporous TiO2 hollow nanospheres (f-MTHS). Dual functionality resulted in the increased CO2 affinity and improved interfacial properties between inorganic nanofillers and the polymer matrix.

Improved electron transfer and plasmonic effect in dye-sensitized solar cells with bi-functional Nb-doped TiO2/Ag ternary nanostructures.

TiO2 nanoparticles are surface-modified via atom transfer radical polymerization (ATRP) with a hydrophilic poly(oxyethylene)methacrylate (POEM), which can coordinate to the Ag precursor, i.e. silver trifluoromethanesulfonate (AgCF3SO3). Following the reduction of Ag ions, a Nb2O5 doping process and calcination at 450 °C, bi-functional Nb-doped TiO2/Ag ternary nanostructures are generated. The resulting nanostructures are characterized by energy-filtering transmission electron microscopy (EF-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy. The dye-sensitized solar cell (DSSC) based on the Nb-doped TiO2/Ag nanostructure photoanode with a polymerized ionic liquid (PIL) as the solid polymer electrolyte shows an overall energy conversion efficiency (η) of 6.9%, which is much higher than those of neat TiO2 (4.7%) and Nb-doped TiO2 (5.4%). The enhancement of η is mostly due to the increase of current density, attributed to the improved electron transfer properties including electron injection, collection, and plasmonic effects without the negative effects of charge recombination or problems with corrosion. These properties are supported by intensity modulated photocurrent/voltage spectroscopy (IMPS/IMVS) and incident photon-to-electron conversion efficiency (IPCE) measurements.

One-step synthesis of vertically aligned anatase thornbush-like TiO2 nanowire arrays on transparent conducting oxides for solid-state dye-sensitized solar cells.

Herein, we report a facile synthesis of high-density anatase-phase vertically aligned thornbush-like TiO2 nanowires (TBWs) on transparent conducting oxide glasses. Morphologically controllable TBW arrays of 9 µm in length are generated through a one-step hydrothermal reaction at 200 °C over 11 h using potassium titanium oxide oxalate dehydrate, diethylene glycol (DEG), and water. The TBWs consist of a large number of nanoplates or nanorods, as confirmed by SEM and TEM imaging. The morphologies of TBWs are controllable by adjusting DEG/water ratios. TBW diameters gradually decrease from 600 (TBW600) to 400 (TBW400) to 200 nm (TBW200) and morphologies change from nanoplates to nanorods with an increase in DEG content. TBWs are utilized as photoanodes for quasi-solid-state dye-sensitized solar cells (qssDSSCs) and solid-state DSSCs (ssDSSCs). The energy-conversion efficiency of qssDSSCs is in the order: TBW200 (5.2%)>TBW400 (4.5%)>TBW600 (3.4%). These results can be attributed to the different surface areas, light-scattering effects, and charge transport rates, as confirmed by dye-loading measurements, reflectance spectroscopy, and incident photon-to-electron conversion efficiency and intensity-modulated photovoltage spectroscopy/intensity-modulated photocurrent spectroscopy analyses. TBW200 is further treated with a graft-copolymer-directed organized mesoporous TiO2 to increase the surface area and interconnectivity of TBWs. As a result, the energy-conversion efficiency of the ssDSSC increases to 6.7% at 100 mW cm(-2) , which is among the highest values for N719-dye-based ssDSSCs.

One-dimensional hierarchical nanostructures of TiO(2) nanosheets on SnO(2) nanotubes for high efficiency solid-state dye-sensitized solar cells.

Hierarchical nanostructures of TiO2 nanosheets on SnO2 nanotubes (SNT@TNS) are uniformly dispersed in an organized mesoporous (OM) TiO2 film with large pores, high porosity, and good interconnectivity. The solid-state dye sensitized solar cells (ssDSSCs) fabricated with 10 wt% SNT@TNS dispersed in a OM-TiO2 film show an energy conversion efficiency of 7.7% at 100 mW cm(-2) , which is one of the highest values for N719-based ssDSSCs and much larger than that of a randomly oriented TiO2 nanoparticles-based cell (4.0%).

Hybrid electrolytes prepared from ionic liquid-grafted alumina for high-efficiency quasi-solid-state dye-sensitized solar cells.

Alumina (Al2O3) nanoparticles were covalently surface-modified with an ionic liquid (IL) to improve their miscibility with ILs such as 1-methyl-3-propylimidazolium iodide (MPII). Hybrids consisting of MPII and the surface-modified IL-Al2O3 nanoparticles were utilized as an I2-free electrolyte for quasi-solid-state dye-sensitized solar cells (DSSCs). The synthesis and properties of the IL-Al2O3 nanoparticles and hybrid electrolytes were characterized by Fourier transform infrared (FT-IR) spectroscopy, UV-visible spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The viscosity of the electrolyte continuously increased with the content of IL-Al2O3, and the fluidity almost disappeared completely when the MPII:IL-Al2O3 weight ratio was 95 : 5 or 90 : 10. The energy conversion efficiencies of DSSCs fabricated with IL-Al2O3 were always greater than those with pristine Al2O3. Such a finding is due to the favorable interactions and good miscibility between MPII and IL-Al2O3, which in turn results in the formation of an interconnected channel pathway for ion transport. Incident photon-to-electron conversion efficiency (IPCE), intensity modulated photocurrent spectroscopy (IMPS)/intensity-modulated photovoltage spectroscopy (IMVS), and electrochemical impedance spectroscopy (EIS) measurements were used to investigate the interfacial properties and electron transport characteristics. Upon utilizing double-layer structures with mesoporous TiO2 beads, the efficiency increased to 7.6% at 100 mW cm(-2), one of the highest values reported for quasi-solid-state DSSCs.

Room temperature solid-state synthesis of a conductive polymer for applications in stable I2-free dye-sensitized solar cells.

A solid-state polymerizable monomer, 2,5-dibromo-3,4-propylenedioxythiophene (DBProDOT), was synthesized at 25 °C to produce a conducting polymer, poly(3,4-propylenedioxythiophene) (PProDOT). Crystallographic studies revealed a short interplane distance between DBProDOT molecules, which was responsible for polymerization at low temperature with a lower activation energy and higher exothermic reaction than 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) or its derivatives. Upon solid-state polymerization (SSP) of DBProDOT at 25 °C, PProDOT was obtained in a self-doped state with tribromide ions and an electrical conductivity of 0.05 S cm⁻¹, which is considerably higher than that of chemically-polymerized PProDOT (2×10⁻6 S cm⁻¹). Solid-state ¹³C NMR spectroscopy and DFT calculations revealed polarons in PProDOT and a strong perturbation of carbon nuclei in thiophenes as a result of paramagnetic broadening. DBProDOT molecules deeply penetrated and polymerized to fill nanocrystalline TiO2 pores with PProDOT, which functioned as a hole-transporting material (HTM) for I2-free solid-state dye-sensitized solar cells (ssDSSCs). With the introduction of an organized mesoporous TiO2 (OM-TiO2) layer, the energy conversion efficiency reached 3.5 % at 100 mW cm⁻², which was quite stable up to at least 1500 h. The cell performance and stability was attributed to the high stability of PProDOT, with the high conductivity and improved interfacial contact of the electrode/HTM resulting in reduced interfacial resistance and enhanced electron lifetime.