Flexible Porous Materials Research Articles

Porous flexible molecular-based piezoelectric composite achieves milliwatt output power density

Molecular ferroelectrics have made breakthrough progress in intrinsic piezoelectric response that can be on par with advanced inorganic piezoelectric ceramics. However, their successful applications in high-density energy harvesting and self-powered flexible devices have been great challenge, owing to the low elastic moduli, intrinsically brittle, and fracture proneness of such material systems under mechanical loading. Here, we have developed a flexible porous composite piezoelectric material by using soft thermoplastic polyurethane (TPU) and molecular ferroelectric materials. Benefiting from the porous structure of TPU, the flexible piezoelectric composites enable effectively large doping ratio (50%) of [Me3NCH2Cl]CdCl3 (TMCM-CdCl3) and highly efficient stress absorption, coupled with the excellent piezoelectric properties of TMCM-CdCl3, to realize a superior power density (636.9 µW cm−2 or 1273.9 µW cm−3). This output is 2000 times higher than that of flexible piezoelectric materials represented by poly(vinylidene fluoride) (PVDF). We believe that the outstanding performance of the porous composite piezoelectric material would pave a feasible way for real industrial applications of molecular ferroelectrics.

Flexible Mesopores in Nanoscrolls: Extraordinarily Large Alteration of Pore Sizes and Their Reversibility.

Flexible porous materials have gained considerable interest for their potential applications in selective absorption and controlled release/storage of specific molecules or compounds. Here, nanoscrolls are proposed as a type of inorganic solids with reversibly flexible mesopores. Nanoscrolls exhibit a rolled-up structure composed of nanosheets with a 1D rod-like morphology, possessing two distinct nanospaces. The first space comprises 1D tubular mesopores located at the center of the rod, while the second space exists in the interlayer regions on the wall of the mesopore, resulting from the layer stacking caused by the scrolling of nanosheets. By replacing the interlayer cations on the nanoscroll walls with other cations, a drastic alteration in the size of the 1D mesopores is observed. For instance, exchanging bulky dodecylammonium cations with small NH4 + cations leads to a substantial change in pore size, with differences ranging from 10 to 20nm-a notably larger variation compared to previous reports on flexible porous materials. Importantly, the alteration of pore size induced by the exchange reaction is found to be reversible. This reversible alteration in pore size holds promise for applications in host-guest chemistry involving large moieties such as nanoparticles and enzymes.

Sulfonate Functional Ultramicroporous Materials with Suitable Pore Size and Layer-Stacked Structure for C4 Olefins Purification.

Selective recognition of 1,3-butadiene from complex olefin isomers is vital for 1,3-butadiene purification, but the lack of porous materials with suitable pore structures results in poor selectivity and low capacity in C4 olefin separation. Herein, two sulfonate-functionalized organic frameworks, ZU-601 and ZU-602, are designed and show impressive separation performance toward C4 olefins. Benefiting from the suitable aperture size caused by the flexibility of coordinated organic ligand, ZU-601, ZU-602 that are pillared with different sulfonate anions could discriminate C4 olefin isomers with high uptake ratio: 1,3-butadiene/1-butene (207), 1,3-butadiene/trans-2-butene (10.1). Meanwhile, their layer-stacked structure enables the utilization of both intra- and interlayer space, enhancing the accommodation of guest molecules. ZU-601 exhibits record high 1,3-butadiene adsorption capacity of 2.90 mmol g-1 (0.5 bar, 298 K) among the reported flexible porous materials with high 1,3-butadiene/1-butene selectivity. The breakthrough experiments confirm their superior separation ability even for all five C4 olefin isomers, and the molecular-level structural change is well elucidated via powder, crystal analysis, and simulation studies. The work provides ideas toward advanced materials design with simultaneous high separation capacity and high separation selectivity for challenging separations.

Temperature-Dependent Separation of CO2 from Light Hydrocarbons in a Porous Self-Assembly of Vertexes Sharing Octahedra.

Design of flexible porous materials where the diffusion of guest molecules is regulated by the dynamics of contracted pore aperture is challenging. Here, a flexible porous self-assembly consisting of 1D channels with dynamic bottleneck gates is reported. The dynamic pendant naphthimidazolylmethyl moieties at the channel necks provide kinetic gate function, that enables unusual adsorption for light hydrocarbons. The adsorption for CO2 is mainly dominated by thermodynamics with the uptakes decreasing with increasing temperature, whereas the adsorptions for larger hydrocarbons are controlled by both thermodynamics and kinetics resulting in an uptake maximum at a temperature threshold. Such an unusual adsorption enables temperature-dependent separation of CO2 from the corresponding hydrocarbons.

Flexible hydrogen-bonded organic frameworks (HOFs): opportunities and challenges.

Flexible behavior is one of the most fascinating features of hydrogen-bonded organic frameworks (HOFs), which represent an emerging class of porous materials that are self-assembled via H-bonding between organic building units. Due to their unique flexibility, HOFs can undergo structural changes or transformations in response to various stimuli (physical or chemical). Taking advantage of this unique structural feature, flexible HOFs show potential in multifunctional applications such as gas storage/separation, molecular recognition, sensing, proton conductivity, biomedicine, etc. While some other flexible porous materials have been extensively studied, the dynamic behavior of HOFs remains relatively less explored. This perspective highlights the inherent flexible properties of HOFs, discusses their different flexible behaviors, including pore size/shape changes, interpenetration/stacking manner, H-bond breaking/reconstruction, and local dynamic behavior, and highlights their potential applications. We believe that this perspective will not only contribute to HOF chemistry and materials science, but will also facilitate the ongoing extensive research on dynamic porous materials.

Degradation of indoor formaldehyde by the flexible porous material loaded photocatalyst

Indoor formaldehyde purification is a crucial subject that has received much attention to enhance indoor air quality. Photocatalytic technology can degrade formaldehyde into harmless small molecules and avoid secondary pollution. To improve the photocatalytic purification efficiency, the photocatalysts were loaded on the flexible porous material to degrade indoor formaldehyde. The TiO2-4% carbon nanotubes/nonwoven fabric showed 20% higher photocatalytic performance than pure TiO2. The influence of airflow organization on the photocatalytic process was analyzed by experiment and CFD method. The formaldehyde degradation efficiency could be increased by more than 10% with the air flow of same-side downward and upward return. The interactions between ventilation and photocatalysis were investigated. Under continuous ventilation, the photocatalytic process can also effectively improve the purification efficiency of formaldehyde, and the purification rate of indoor formaldehyde can reach 97% under the combined action of ventilation and photocatalysis in 30 min.

Unveiling the amorphization of sodalite topology zeolitic imidazolate frameworks and zeolites by pressure and stress

AbstractAs a flexible porous material, the amorphous behavior of zeolitic imidazolate framework‐8 (ZIF‐8) has garnered considerable interest. However, the association between its strain behavior and topology has not yet been exhaustively studied computationally. We perform molecular dynamic simulations on both ZIF‐8 and its topological silicate isomer sodalite (SOD) to investigate the pressure‐induced development of the mid‐ and short‐range structures. We find that both ZIF‐8 and SOD undergo two successive transformations. The first is the amorphization process, which is characterized by the breakdown of the mid‐range structure without a significant change in the elastic modulus. The other type of densification involves a change in the short‐range structure, during which the mechanical properties are improved as the short‐range disorder increases.

Exploring Multifunctional Hydrogen-Bonded Organic Framework Materials.

ConspectusHydrogen-bonded organic framework (HOF) materials have provided a new dimension and bright promise as a new platform for developing multifunctional materials. They can be readily self-assembled from their corresponding organic molecules with diverse functional sites such as carboxylic acid and amine groups for their hydrogen bonding and aromatic ones for their weak π···π interactions to stabilize the frameworks. Compared with those established porous materials such as zeolites, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs), it is much more difficult to stabilize HOFs and thus establish their permanent porosities given the fact that hydrogen bonds are typically weaker than ionic, coordination, and covalent bonds. But it provides the uniqueness of HOF materials in which they can be easily recovered and regenerated through simple recrystallization. HOF materials can also be easily and straightforwardly processed and very compatible with the biomolecules, making them potentially very useful materials for industrial and biomedical applications. The reversible and weak bonding nature of the hydrogen bonds can be readily utilized to construct flexible porous HOF materials in which we can tune the temperature and pressure to control their porosities and, thus, their diverse applications, for example, on gas separations, gas storage, drug delivery, and sensing. Some specific organic functional groups are quite directional for the hydrogen bond formations; for example, carboxylic acid prefers to form a directional dimer, which has enabled us to readily construct reticular porous HOF materials whose pores can be systematically tuned. In this Account, we outline our journey of exploring this new type of porous material by establishing one of the first porous HOFs in 2011 and thus developing its diverse applications. We have been able to use organic molecules with different functional sites, including 2,4-diaminotriazine (DAT), carboxylic acid (COOH), aldehyde (CHO), and cyano (CN), to construct porous HOFs. Through tuning the pore sizes, introducing specific binding sites, and making use of the framework flexibility, we have realized a series of HOF materials for the gas separations of C2H2/C2H4, C2H4/C2H6, C3H6/C3H8, C2H2/CO2, CO2/N2, and Xe/Kr and enantioselective separation of alcohols. To make use of optically active organic molecules, we have developed HOF materials for their luminescent sensing and optical lasing. Our research endeavors on multifunctional HOF materials have initiated extensive research in this emerging research topic among chemistry and materials sciences communities. We foresee that not only many more HOF materials will be developed but novel functions will be fulfilled beyond our imaginations soon.

Highly conductive and flexible porous carbon nanofibers cloth for high-performance supercapacitor

Porous carbon nanofibers combine the superiorities such as good electrical conductivity, intrinsic and structural flexibility, as well as potential mass production, rendering it an important material for flexible and wearable electronics. Nonetheless, the influence of porous carbon nanofiber's nanostructure on electrochemical performances and mechanical flexibility remains an important open question in the design of high-performance and flexible supercapacitors. To address this matter, herein, we report a systematic study to fabricate the porous carbon nanofibers cloth with both high electrical conductivity and flexibility. Our results indicate that the stress concentration during the bending of porous carbon nanofibers can be efficiently relieved through reducing the size of graphite microcrystals and introducing pores between graphite microcrystals, resulting in good flexibility. Meanwhile, the fabricated porous carbon nanofiber achieves high electrical conductivity (Max. 1600 S/m), which can be directly used as flexible supercapacitor electrode material without introducing a conductive agent and binder. Finally, the flexible supercapacitor electrode based on this porous carbon nanofiber exhibits a high specific capacitance of 184 F/g at a current density of 0.5 A/g. Our win-win strategy provides new insight into the fabrication of flexible porous carbon nanofiber materials, showing great application prospects in the field of flexible supercapacitors.

Identifying the Gate-Opening Mechanism in the Flexible Metal-Organic Framework UTSA-300.

Atomic-level understanding of the gate-opening phenomenon in flexible porous materials is an important step toward learning how to control, design, and engineer them for applications such as the separation of gases from complex mixtures. Here, we report such mechanistic insight through an in-depth study of the pressure-induced gate-opening phenomenon in our earlier reported metal-organic framework (MOF) Zn(dps)2(SiF6) (dps = 4,4'-dipyridylsulfide), also called UTSA-300, using isotherm and calorimetry measurements, in situ infrared spectroscopy, and ab initio simulations. UTSA-300 is shown to selectively adsorb acetylene (C2H2) over ethylene (C2H4) and ethane (C2H6) and undergoes an abrupt gate-opening phenomenon, making this framework a highly selective gas separator of this complex mixture. The selective adsorption is confirmed by pressure-dependent in situ infrared spectroscopy, which, for the first time, shows the presence of multiple C2H2 species with varying strengths of bonding. A rare energetic feature at the gate-opening condition of the flexible MOF is observed in our differential heat energies, directly measured by calorimetry, showcasing the importance of this tool in adsorption property exploration of flexible frameworks and offering an energetic benchmark for further energy-based fundamental studies. Based on the agreement of this feature with ab initio-based adsorption energies of C2H2 in the closed-pore structure UTSA-300a ("a" refers to the activated form), this feature is assigned to the weakening of the H-bond C-H···F formed between C2H2 and fluorine of the MOF. Our analysis identifies the weakening of this H-bond, the expansion of the closed-pore MOF upon successive C2H2 coadsorption until its volume is close to that of the open-pore MOF, and the spontaneous gate opening to energetically favor C2H2 adsorption in the open-pore structure as crucial steps in the gate-opening mechanism in this system.

A lignin-derived flexible porous carbon material for highly efficient polyselenide and sodium regulation.

Low-cost and sustainable sodium-selenium (Na-Se) batteries are promising energy storage media for the advancement of electromobility and large-scale energy storage. However, the sluggish kinetics of Se cathodes and the unpredictable metal electrodeposition of Na at the anode remain critical challenges. In this work, we reveal the catalytic effect of atomic Fe on the conversion of polyselenides (SPSs) to Na2Se by density functional theory (DFT) calculations. Then, we prepare a lignin-derived flexible porous carbon matrix loaded with atomic Fe (Fe-BC/rGO, BC: lignin-derived porous carbon material; rGO: reduced graphene oxide) as a Se host to further verify the DFT calculation results. Due to the encapsulation of Se into the porous carbon matrix, the catalytic effect of atomic Fe on the conversion of SPSs to Na2Se and the continuous electron/ion transportation path, the prepared Se@Fe-BC/rGO cathode can deliver a high reversible capacity of 213 mA h g-1 at 2 A g-1, which is much better than the electrochemical performance of a Se cathode without atomic Fe loading (Se@BC/rGO). In addition, we further reveal the advantageous effect of the presence of the Fe-BC/rGO film in regulating the interfacial Na electrodeposition at the anode. Due to the porous structure and the catalytic effect of atomic Fe, a very low nucleation overpotential of 15.3 mV is achieved at a current density of 1 mA cm-2, which is much lower than that of the BC/rGO film. Therefore, this work provides a low-cost and sustainable strategy for simultaneously solving the challenges of the Se cathode and the Na metal anode for future Na-Se batteries.

High-Yielding Flow Synthesis of a Macrocyclic Molecular Hinge.

Many molecular machines are built from modular components with well-defined motile capabilities, such as axles and wheels. Hinges are particularly useful, as they provide the minimum flexibility needed for a simple and pronounced conformational change. Compounds with multiple stable conformers are common, but molecular hinges almost exclusively operate via dihedral rotations rather than truly hinge-like clamping mechanisms. An ideal molecular hinge would better reproduce the behavior of hinged devices, such as gates and tweezers, while remaining soluble, scalable, and synthetically versatile. Herein, we describe two isomeric macrocycles with clamp-like open and closed geometries, which crystallize as separate polymorphs but interconvert freely in solution. An unusual one-pot addition cyclization reaction was used to produce the macrocycles on a multigram scale from inexpensive reagents, without supramolecular templating or high-dilution conditions. Using mechanistic information from NMR kinetic studies and at-line mass spectrometry, we developed a semicontinuous flow synthesis with maximum conversions of 85–93% and over 80% selectivity for a single isomer. The macrocycles feature voids that are sterically protected from guests, including reactive species such as fluoride ions, and could therefore serve as chemically inert hinges for adaptive supramolecular receptors and flexible porous materials.

Elucidation of Diffusivity of Hydrogen Isotopes in Flexible MOFs by Quasi-Elastic Neutron Scattering.

Kinetic-quantum-sieving-assisted H2 :D2 separation in flexible porous materials is more effective than the currently used energy-intensive cryogenic distillation and girdle-sulfide processes for isotope separation. It is believed that material flexibility results in a pore-breathing phenomenon under the influence of external stimuli, which helps in adjusting the pore size and gives rise to the optimum quantum-sieving phenomenon at each stage of gas separation. However, only a few studies have investigated kinetic-quantum-sieving-assisted isotope separation using flexible porous materials. In addition, no reports are available on the microscopic observation of isotopic molecular transportation during the separation process under dynamic transition. Here, the experimental observation of a significantly faster diffusion of deuterium than hydrogen in a flexible pore structure, even at high temperatures, through quasi-elastic neutron scattering, is reported. Unlike rigid structures, the extracted diffusion dynamics of hydrogen isotopes within flexible frameworks show that the diffusion difference between the isotopes increases with an increase in temperature. Owing to this unique inverse trend, a new strategy is suggested for achieving higher operating temperatures for efficient isotope separation utilizing a flexible metal-organic framework system.

Treatment uniformity of atmospheric pressure plasma on flexible and porous material surface: A critical review

<sec>Flexible porous materials play an important role in frontier science and technology fields. Surface modification will further endow the materials with diverse and excellent surface properties, and expand the scope of their applications in functional and intelligent wearable devices. Atmospheric pressure plasma technology has many advantages in treating the flexible materials, such as low temperature, low energy consumption, high efficiency, friendly environment, low cost, no change in material itself characteristics, suitability for roll-to-roll preparation, etc. Also, it presents good adaptability in applied environment and target materials. All these advantages meet the requirements of large area and low-cost surface modification of flexible porous materials.</sec><sec>In this paper, we review several researches of atmospheric pressure plasma surface modification of flexible porous materials used in advanced materials, new energy, environmental protection and biomedicine. The problems and challenges of stability and permeability encountered in uniformly treating the flexible and porous materials by atmospheric pressure plasma are presented. Then, we introduce our research work on atmospheric pressure plasma stable discharge, roll-to-roll coating treatment of permeability and uniformity. Finally, we introduce the breakthrough in and ideas on the deposition kinetics of nanoparticle thin films and their microstructure control by atmospheric pressure plasma. </sec><sec>However, there are still many challenges to be overcome in the applications of the methods in current situation. Basic characteristics, discharge modes of atmospheric pressure plasma and the relationships of plasma discharge to structure and property of the various treated materials need to be further explored. It is confirmed that the permeability and uniformity of the atmospheric pressure plasma treatment in flexible porous materials are very important and their in-depth investigations will promote the application of this method—a high efficient, environmentally-friendly and continuous way of realizing functional and intelligent wearable devices in the future.</sec>

Pressure-Gradient Sorption Calorimetry of Flexible Porous Materials: Implications for Intrinsic Thermal Management.

Thermal management is an important consideration for applications that involve gas sorption by flexible porous materials. A pressure-gradient differential scanning calorimetric method was developed to measure the energetics of adsorption and desorption both directly and continuously. The method was applied to the uptake and release of CO2 by the well-known flexible metal-organic frameworks MIL-53(Al) and MOF-508b. High-resolution differential enthalpy plots and total integral enthalpy values for sorption allow comprehensive assessment of the thermal behavior of the materials throughout the entire sorption process. During adsorption, the investigated materials display the ability to offset exothermic adsorption enthalpy against endothermic structural transition enthalpy, and vice versa during desorption. The results show that flexible materials offer reduced total integral heat over a working range when compared to rigid materials.

Fiber-based flexible composite with dual-gradient structure for sound insulation

Sound insulation materials in the traditional sense are rigid, which are usually dense and bulky, while common porous materials have almost no sound insulation ability. Herein, a fiber-based flexible porous composite is presented as sound insulator fabricated via single-sided coating finishing on the one side of a nonwoven framework and has been demonstrated for high performance noise reduction. The enhanced acoustic insulation is identified to be structure dependent of the area density and microstructure of the sound insulator and the type of filler. The flexible sound insulator exhibits ≈100% enhancement in broadband acoustic insulation over the whole frequency range compared to the bare nonwovens. And the most sound transmission loss (STL) of at the peak is up to 37 dB, which is about 30 times that of bare nonwovens. Furthermore, the novel sound insulator has an admirable dual-gradient structure in which the concentration gradient of coating finishing agent is opposite to the pore gradient. Therefore, sound insulation capacity of the sound insulator exhibits obviously anisotropy according to the incident direction of sound waves. This flexible porous material for broad-band sound insulation has an easy and inexpensive scalable production, demonstrating its great potential for lightweight requirements for specific applications.

Scalable fabrication of highly crosslinked conductive nanofibrous films and their applications in energy storage and electromagnetic interference shielding

With their unique properties and potentials in several critical applications, highly crosslinked and conductive nanofibrous films (HCC-NFs) have attracted broad attentions recently. However, it is still a great challenge to produce high-performance and uniform HCC-NFs with controllable thickness on a large-scale. Electrospinning could be used to fabricate nanofiber films, but it falls short in creating intimate mechanical and electrical joints between adjacent nanofibers for crosslink. Herein, a new strategy inspired by the fabric shrinking when drying was applied to produce HCC-NFs. Hydrophilically-engineered electrospun polyacrylonitrile (PAN) nanofiber film was dip-coated in a water solution of PEDOT:PSS, and was then dried naturally. By preventing contraction of the film in its lateral plane while allowing its thickness shrinking, the capillary effect during water evaporation resulted in collapse of the large pore volume in the film and the subsequent joint formations between PEDOT:PSS-coated PAN nanofibers, which are further cemented together by the conductive polymer coating. The electrochemical, electrical and mechanical properties of such fabricated HCC-NFs were studied, and they were successfully applied as electrodes to demonstrate high-rate supercapacitors and as a flexible porous material for electromagnetic interference (EMI) shielding. The studies suggest HCC-NFs produced by this facile method have promising prospects in several applications.

High-throughput screening of metal–organic frameworks for kinetic separation of propane and propene

We apply molecular simulations to screen a database of reported metal-organic framework structures from the computation-ready, experimental (CoRE) MOF database to identify materials potentially capable of separating propane and propene by diffusion. We report a screening workflow that uses descriptor analysis, conventional molecular dynamics (MD), and Nudged Elastic Band (NEB) energy barrier calculations at both classical force field and Density Functional Theory (DFT) levels. For the first time, the effects of framework flexibility on guest transport properties were fully considered in a screening process and led to the identification of candidate MOFs. The hits identified by this proof-of-concept workflow include ZIF-8 and ZIF-67 previously shown to have large differences in propane and propene diffusivities as well as two other materials that have not been tested experimentally yet. This work emphasises the importance of taking into account framework flexibility when studying guest transport in porous materials, demonstrates the potential of the data-driven identification of high-performance materials and highlights the ways of improving the predictive power of the screening workflow.

Exploring the thermodynamic criteria for responsive adsorption processes.

We describe a general model to explore responsive adsorption processes in flexible porous materials. This model combines mean field formalism of the osmotic potential, classical density functional theory of adsorption in slit pore models and generic potential functions which represent the Helmholtz free energy landscape of a porous system. Using this model, we focus on recreating flexible adsorption phenomena observed in prototypical metal-organic frameworks, especially the recently discovered effect of negative gas adsorption (NGA). We identify the key characteristics required for the model to generate unusual adsorption processes and subsequently employ an extensive parametric study to outline conditions under which gate-opening and NGA are observed. This powerful approach will guide the design of responsive porous materials and the discovery of entirely new adsorption processes.

Fabrication of Thermoresponsive Polymer-Functionalized Cellulose Sponges: Flexible Porous Materials for Stimuli-Responsive Catalytic Systems.

In this present work, a thermoresponsive and recyclable catalytic system was prepared by grafting poly( N-isopropylacrylamide)- co-poly(glycidyl methacrylate) (PNIPAM- co-PGM) to a cellulose sponge, which was reinforced by polydopamine (PDA) and (3-aminopropyl)triethoxysilane(APTMS). Au nanoparticles (Au NPs) were loaded via in situ reduction of HAuCl4 with PDA. Fourier transform infrared, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis results revealed that the Au NPs (<10 nm) were homogenously dispersed on the surface of the sponge. Catalytic experiments with sponges prepared without PNIPAM- co-PGM demonstrated an increased reaction rate when the temperature of the reaction medium was elevated. However, in the presence of PNIPAM-i co-PGM in the sponges, the reaction rate was decreased when the reaction temperature was higher than the lower critical solution temperature of the polymer. The sponge could be conveniently separated from the reactions and reused up to 22 cycles.