Small-molecule organic electrode materials on carbon-coated aluminum foil for high-performance sodium-ion batteries.

Organic molecular electrode materials are promising candidates in batteries. However, direct application of small molecule materials usually suffers from drastic capacity decay and inefficient utilization of active materials because of their high solubility in organic electrolytes and low electrical conductivity. Herein, a simple strategy is found to address the above issues through coating the small-molecule organic materials on a commercialized carbon-coated aluminum foil (CCAF) as the enhanced electrode. Both the experimental and calculation results confirm that the relatively rough carbon coating on the aluminum foil not only exhibits superior adsorption capacity of small-molecule organic electrode materials with a tight contact interface but also provides continuous electronic conduction channels for the facilitated charge transfer and accelerated reaction kinetics. In addition, the carbon coating also inhibits Al corrosion in electrochemical process. As a result, by using the tetrahydroxy quinone-fused aza-phenazine (THQAP) molecule as an example, the THQAP-CCAF electrode exhibits an excellent rate performance with a high capacity of 220 and 180 mAh g-1 at 0.1 and 2 A/g, respectively, and also a remarkable cyclability with a capacity retention of 77.3% even after 1700 cycles in sodium-ion batteries. These performances are much more superior than that of batteries with the THQAP on bare aluminum foil (THQAP-AF). This work provides a substantial step in the practical application of the small-molecule organic electrode materials for future sustainable batteries.

An Ultrathin Composite Polymer Electrolyte Dual-Reinforced by a Polymer of Intrinsic Microporosity (PIM-1) and Poly(tetrafluoroethylene) (PTFE) Porous Membrane.

The performances of solid-state polymer electrolytes are urgently required to be further improved for high energy density lithium metal batteries. Herein, a highly reinforced ultrathin composite polymer electrolyte (PLPP) is successfully fabricated in a large scale by densely filling the well-dispersed mixture of polyethylene oxide (PEO), Li-salt (LiTFSI) and a polymer of intrinsic microporosity (PIM-1) into porous poly(tetrafluoroethylene) (PTFE) matrix. Based on the macro-plus-micro synergistic enhancement of the PTFE with excellent mechanical properties and the soluble PIM-1 with suitable functional groups, the PLPP electrolyte exhibits excellent properties including mechanical stress, thermal stability, lithium-ion transference number, voltage window and ionic conductivity, which are all superior to the typical PEO/LiTFSI electrolytes. As a result, the Li/PLPP/Li symmetric cell can stably cycle for > 2000 h, and the LiFePO4/PLPP/Li full cell exhibits excellent rate performance (>10 C) and high cycling stability with an initial capacity of 158.8 mAh g-1 and a capacity retention of 78.8% after 300 cycles. In addition, the excellent mechanical properties as well as the wide voltage window reasonably result in the stable operation of full cells with either high-loading cathode up to 28.1 mg cm-2 or high voltage cathode with high energy density.

A porous polyacrylonitrile (PAN)/covalent organic framework (COF) fibrous membrane photocatalyst for highly efficient and ultra-stable hydrogen evolution.

Photocatalytic water splitting has been regarded as one of the most promising technologies to generate hydrogen as an ideal energy carrier in the future. However, most of the experience for such process are derived from the researches based on the suspension powder photocatalysts under a stirring condition and a practical scaling application is urgently calling for the high-efficient panel reactors based on the membrane photocatalysts. Herein, we develop a new series of flexible and ultrastable membrane photocatalysts through a controllable growth of covalent organic framework (COF) photocatalysts on the polyacrylonitrile (PAN) electrospun fiber membrane. Multiple characterization techniques verify the successful anchoring of the COF-photocatalysts on the PAN fibers, forming a three-dimensional porous PAN/COF membrane photocatalyst with excellent light absorption ability, high specific surface area, and good hydrophily. As a result, the optimized PAN/COF membrane photocatalyst exhibits excellent hydrogen evolution rate up to 1.25 mmol g-1h-1 under visible-light irradiation without stirring, which is even higher than that of the corresponding suspension COF-powder photocatalyst with stirring. In particular, the PAN/COF membrane photocatalyst demonstrates a much more superior hydrogen evolution stability and also a much better recyclability. This study gives some experience for the practical scaling application of solar-driven water splitting.

Reversible Regulation of Polar Gas Molecules by Azobenzene-Based Photoswitchable Metal-Organic Framework Thin Films.

The development of tunable molecule separation membranes requires materials with remote controllability and ultra-high separation capability. In this paper, a novel photoswitchable metal organic framework (MOF) thin film (Cu2(AzoBPDC)2) was prepared by liquid phase epitaxial layer-by-layer assembly to realize the reversible remote-controlled switching. The azobenzene side groups in the Cu2(AzoBPDC)2 thin film showed excellent reversible photoswitching performance under UV (365 nm) and Vis (450 nm) irradiation, achieving the remote-controlled mode of the diffusion flux of polar gas molecules in the MOF thin film.

Interfacial Self-assembly of Organics/MXene Hybrid Cathodes Toward High-Rate-Performance Sodium Ion Batteries.

Conjugated quinones are promising cathode materials for sodium-ion batteries. However, the contemporary primary conjugated quinones cathodes still hold to limited capacity, poor rate performance and low cyclability, due to the poor electronic and ionic conductivity. Herein, a series of high-performance conjugated-quinones@MXene hybrid cathodes is constructed by an in situ polymerization-assembly strategy based on the hydrogen bond and S-Ti interaction. The PAQS@Ti3C2Tx MXene hybrid, as a typical example, exhibits sandwiched structure with intimate PAQS@MXene contact, resulting in efficient interfacial mass transfer. The assembled MXene is able to build interconnected conductive channels in the hybrid cathodes for continuous and fast electrons/ions transport, which is verified by both the experimental results and density functional theory (DFT) calculations. As a result, the optimal PAQS@MXene hybrid electrode delivers excellent electrochemical performances with high capacity (∼242 mA h g-1 at 100 mA g-1), superior fast-charge/discharge ability (∼148 and 121 mA h g-1 at 5 and 10 A g-1, respectively), and ultralong cycle life (capacity as high as 57 mA h g-1 after 9000 cycles at 5 A g-1), which are more superior to that of the pure PAQS electrodes. Besides, the analogous PPTS@Ti3C2Tx MXene hybrid cathode also shows better performances compared to the pure materials.

Fabrication of zirconium-based metal-organic frameworks@tungsten trioxide (UiO-66-NH2@WO3) heterostructure on carbon cloth for efficient photocatalytic removal of tetracycline antibiotic under visible light.

Designing recyclable photocatalysts with high activity and stability has drawn considerable attention in the fields of sewage treatment. Herein, a series of heterojunctions constructed by zirconium-based metal-organic frameworks (UiO-66-NH2) and tungsten trioxide (WO3) is immobilized on carbon cloth via a facile solvothermal method, resulting in highly recyclable photocatalysts. Multiple characterization techniques, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, verify the successful synthesis of UiO-66-NH2 nanospheres on the surface of needlelike WO3 modified carbon cloth. Results show that the optimal heterojunction photocatalyst exhibits excellent photocatalytic degradation efficiency for the removal of tetracycline (TC) from water, for which nearly 100% of TC is degraded within 60 min under visible light. Trapping experiments and electron spin resonance (ESR) spectra analyses demonstrate that the superoxide radicals O2- and photogenerated hole h+ play a dominant role in the degradation process. Excellent photocatalytic activity is dominantly attributed to the effective separation of photoinduced carriers in this type-Ⅱ heterostructure system. Moreover, the possible photocatalytic oxidation degradation pathway is confirmed by analyzing intermediates using liquid chromatography mass spectrometry (LC-MS). This study offers a highly efficient strategy to design recyclable heterojunction photocatalysts for the degradation of refractory antibiotics in sewage.

In Situ Fabrication of Porous MOF/COF Hybrid Photocatalysts for Visible-Light-Driven Hydrogen Evolution.

Construction of porous metal-organic framework (MOF)/covalent organic framework (COF) hybrid photocatalysts for enriched structures and unprecedented properties is still a great challenge but highly desirable. Herein, a new series of Cu3(HHTP)2-MOF/Tp-Pa-1-COF hybrids with different MOF content are successfully fabricated. The as-prepared MOF/COF hybrids exhibit intimate interaction based on the coordination of Cu ions with the carbonyl oxygen and enamine nitrogen groups in Tp-Pa-1. The integrated conductive Cu3(HHTP)2 is able to act as an excellent electron extractor instead of noble metal cocatalysts to significantly promote the charge transfer and inhibit the recombination of photogenerated electron-hole pairs. As a results, the optimized photocatalyst with Cu3(HHTP)2:Tp-Pa-1 ratio of 1:15 achieves the highest hydrogen evolution rate of 1.76 mmol·h-1·g-1 under visible-light irradiation, which is about 93 times higher than that of the pure Tp-Pa-1 and even slightly higher than that of the Tp-Pa-1 with Pt (3 wt %) as a cocatalyst.

A Series of Molecule-Intercalated MoS2 as Anode Materials for Sodium Ion Batteries.

Molybdenum disulfide (MoS2) with a graphite-like layer structure has attracted substantial interest as an anode material for sodium ion batteries (SIBs), but its inherent poor electrical conductivity and slow sodium ion transportation are the two important factors that limit its use in SIBs. Here, we report a general approach to synthesize a series of molecule-intercalated MoS2 with a precisely controlled interlayer distance of 0.62 to 1.24 nm in which the electrical conductivity could be also widely and finely adjusted from 1.3 × 10-4 to 3.5 × 10-2 S cm-1 via the insertion of different molecules. By adjusting the interlayer space and enhancing the electrical conductivity, the highest initial sodium ion storage capacity of 465 mA h g-1 (vs 195 mA h g-1 for the pure MoS2 anode) and the highest capacity of 420 mA h g-1 (vs 31 mA h g-1 for the pure MoS2 anode) after 600 cycles at a rate of 100 mA g-1 were obtained. The excellent performance is credited to the rapid Na+ and electron transport and higher material utilization derived from the synergistic effect of the expanded interlayer space and the higher electronic conductivity. The results provide some inspiration for the design and construction of superior layered anode materials for sodium-ion batteries.

Assembled Exciton Dynamics in Porphyrin Metal-Organic Framework Nanofilms.

Metal-organic frameworks (MOFs) provide a novel strategy to precisely control the alignment of molecules to enhance exciton diffusion for high-performance organic semiconductors. In this paper, we characterize exciton dynamics in highly ordered and crystalline porphyrin MOF nanofilms by time-resolved photoluminescence and femtosecond-resolved transient absorption spectroscopy. Results suggest that porphyrin MOF nanofilms could be a promising candidate for high-performance organic photovoltaic semiconductors in which the diffusion coefficient and diffusion length of excitons are 9.0 × 10-2 cm2 s-1 and 16.6 nm, respectively, comparable with or even beyond that of other excellent organic semiconductors. Moreover, by monitoring real-time exciton dynamics it is revealed that excitons in MOF nanofilms undergo high-efficient intermolecular hopping and multiexciton annihilation due to the short intermolecular distance and aligned molecular orientation in MOF structure, thus providing new insights into the underlying physics of exciton dynamics and many-body interaction in molecular assembled systems.

Giant and Multistage Nonlinear Optical Response in Porphyrin-Based Surface-Supported Metal-Organic Framework Nanofilms.

Benefitting from the strong intrinsic nonlinear optical (NLO) property of the individual porphyrin molecule, the integration of porphyrin molecules into tightly aligned arrays may lead to intuitively promising high-performance materials of tailorable NLO effect. In order to verify this speculation, we prepare crystalline and highly oriented porphyrin-based surface-supported metal-organic framework nanofilms (SURMOFs) and then characterize their NLO performance. Results reveal that porphyrin-based SURMOFs exhibit the highest saturable absorption (SA) yet recorded with a third-order NLO absorption coefficient up to -10-3 cm/W, about 7 orders stronger than porphyrin solvents in which the porphyrin molecules are disordered, under a certain excitation strength. Further increasing the excitation strength shows that the NLO absorption property of the porphyrin-based SURMOFs can be effectively modulated from SA to reverse saturable absorption, followed by a reemerging SA. The multiple-stage NLO switching is assigned to the interplay of simultaneous one-photon SA, two-photon absorption, and two-photon SA effects. The superior and modulatable NLO property as well as the designable and ordered crystalline structure suggest that porphyrin-based SURMOFs might be employed as a new class of high-performance NLO materials with potential applications in novel optical switches or logic gates to realize the all-optical information process.

Nitrogen and Oxygen Co-doped Graphitized Carbon Fibers with Sodiophilic-Rich Sites Guide Uniform Sodium Nucleation for Ultrahigh-Capacity Sodium-Metal Anodes.

Sodium (Na) metal is an ideal anode for high-energy Na batteries due to the low cost and natural abundance of Na metal. Nevertheless, issues regarding dendritic and mossy Na metal deposits have prevented their practical application. Herein, nitrogen and oxygen co-doped graphitized carbon fibers (DGCF) have been developed as the Na plating hosts to direct Na metal homogeneous nucleation and suppress the growth of Na dendrites. We show experimental results as well as first-principles calculations demonstrating that the uniformly doped nitrogen and oxygen function as sodiophilic sites that direct the sodium-metal nucleation to a smooth dendrite-free anode. The resultant DGCF-Na anode can be cycled stably at 1 mA cm-2 for a high areal capacity of 12.7 mA h cm-2 with an average Coulombic efficiency of 99.8%, and a Na|Na symmetrical cell can be cycled with long-term durability for more than 1200 h at 2 mA cm-2. When coupled with P2-Na2/3Ni1/3Mn1/3Ti1/3O2 and Na3V2(PO4)3 cathodes, the DGCF-Na composite demonstrates good feasibility in full cells.

Water-Stable Nanoporous Polymer Films with Excellent Proton Conductivity.

Achieving high values for proton conductivity in a material critically depends on providing hopping sites arranged in a regular fashion. Record values reported for regular, molecular crystals cannot yet be reached by technologically relevant systems, and the best values measured for polymer membranes suited for integration into devices are almost two orders of magnitude lower. Here, an alternative polymer membrane synthesis strategy based on the chemical modification of surface-mounted, monolithic, crystalline metal-organic framework thin films is demonstrated. Due to chemical crosslinking and subsequent removal of metal ions, these surface-mounted gels (SURGELs) are found to exhibit high proton conductivity (0.1 S cm-1 at 30 °C and 100% RH (relative humidity). These record values are attributed to the highly ordered polymer network structure containing regularly spaced carboxylic acid side groups. These covalently bound organic frameworks outperform conventional, ion-conductive polymers with regard to ion conductivity and water stability. Pronounced water-induced swelling, which causes severe mechanical instabilities in commercial membranes, is not observed.

High Antimicrobial Activity of Metal-Organic Framework-Templated Porphyrin Polymer Thin Films.

Development of surface coatings with high antimicrobial activity is urgently required to fight bacteria and other microorganisms on technical and hygiene relevant surfaces. Control over structure and topology of the surface coatings, combined with the ability to include functional molecules within the structure, is crucial for optimizing their performance. Herein, we describe a novel strategy to synthesize structurally well-defined porphyrin polymer thin films via a template approach. In this approach, bisazido-functionalized porphyrin molecules are preorganized within a metal-organic framework (MOF) structure. Afterward, porphyrin units within the MOF are covalently connected via a secondary linker. Removal of the metal ions of the MOF results in water-stable porphyrin polymer thin films that demonstrate high antibacterial activity against pathogens via visible-light-promoted generation of reactive oxygen species. In addition, this approach offers the inherent possibility to incorporate guest molecules within the structures, to functionalize the surface with biomolecules, and to create hierarchically structured materials.

Molecular weaving via surface-templated epitaxy of crystalline coordination networks.

One of the dream reactions in polymer chemistry is the bottom-up, self-assembled synthesis of polymer fabrics, with interwoven, one-dimensional fibres of monomolecular thickness forming planar pieces of textiles. We have made a major step towards realizing this goal by assembling sophisticated, quadritopic linkers into surface-mounted metal-organic frameworks. By sandwiching these quadritopic linkers between sacrificial metal-organic framework thin films, we obtained multi-heteroepitaxial, crystalline systems. In a next step, Glaser-Hay coupling of triple bonds in the quadritopic linkers yields linear, interwoven polymer chains. X-ray diffraction studies revealed that this topochemical reaction leaves the MOF backbone completely intact. After removing the metal ions, the textile sheets can be transferred onto different supports and imaged using scanning electron microscopy and atomic-force microscopy. The individual polymer strands forming the two-dimensional textiles have lengths on the order of 200 nm, as evidenced by atomic-force microscopy images recorded from the disassembled textiles.

Tunable molecular separation by nanoporous membranes.

Metal-organic frameworks offer tremendous potential for efficient separation of molecular mixtures. Different pore sizes and suitable functionalizations of the framework allow for an adjustment of the static selectivity. Here we report membranes which offer dynamic control of the selectivity by remote signals, thus enabling a continuous adjustment of the permeate flux. This is realized by assembling linkers containing photoresponsive azobenzene-side-groups into monolithic, crystalline membranes of metal-organic frameworks. The azobenzene moieties can be switched from the trans to the cis configuration and vice versa by irradiation with ultraviolet or visible light, resulting in a substantial modification of the membrane permeability and separation factor. The precise control of the cis:trans azobenzene ratio, for example, by controlled irradiation times or by simultaneous irradiation with ultraviolet and visible light, enables the continuous tuning of the separation. For hydrogen:carbon-dioxide, the separation factor of this smart membrane can be steplessly adjusted between 3 and 8.

Metal-Organic-Framework-Derived Dual Metal- and Nitrogen-Doped Carbon as Efficient and Robust Oxygen Reduction Reaction Catalysts for Microbial Fuel Cells.

A new class of dual metal and N doped carbon catalysts with well-defined porous structure derived from metal-organic frameworks (MOFs) has been developed as a high-performance electrocatalyst for oxygen reduction reaction (ORR). Furthermore, the microbial fuel cell (MFC) device based on the as-prepared Ni/Co and N codoped carbon as air cathode catalyst achieves a maximum power density of 4335.6 mW m-2 and excellent durability.

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20†K.

We have studied the magnetic properties of the SURMOF-2 series of metal-organic frameworks (MOFs). Contrary to bulk MOF-2 crystals, where Cu(2+) ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu(2+) ions are connected via carboxylate groups in a zipper-like fashion. This unusual coupling of the spin 1/2 ions within the resulting one-dimensional chains is found to stabilize a low-temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy-to-fabricate, material to be described in a detailed fashion.

Improving the Loading Capacity of Metal-Organic Framework Thin Films Using Optimized Linkers.

The large surface area of metal-organic frameworks (MOFs) sparks great interest for their use in storage applications. While the bulk of MOF applications focuses on incorporation of gases, we demonstrate that these highly porous frameworks are also well-suited for metal ion storage. For well-defined, highly oriented surface-anchored MOF thin films grown on modified gold surfaces using liquid-phase epitaxy (LPE), also referred to as SURMOFs, we determined the loading of two different types of MOF materials with a total of seven types of metal ions (Zn(2+), Ag(+), Pd(2+), Fe(3+), Cd(2+), Ni(2+), and Co(2+)). Measurements using a quartz crystal microbalance (QCM) allowed determination of loading capacities as well as diffusion constants in a quantitative fashion. The adsorption capacities were observed to be highly ion specific; the largest uptake was for Fe(3+) and Pd(2+) ions with six and four metal ions per MOF pore, respectively. By comparing results for SURMOFs fabricated from different types of linkers, we demonstrate that S-containing functionalities in particular drastically improve the storage capacity of MOFs for metal ions.

Photoswitchable Adsorption in Metal-Organic Frameworks Based on Polar Guest-Host Interactions.

Reversible remote-controlled switching of the properties of nanoporous metal-organic frameworks (MOFs) is enabled by incorporating photoswitchable azobenzene. The interaction of the host material with different guest molecules, which is crucial for all applications, is precisely studied using thin MOF films of the type Cu2 (BDC)2 (AzoBipyB). A molecule-specific effect of the photoswitching, based on dipole-dipole interactions, is found.

cis-to-trans isomerization of azobenzene investigated by using thin films of metal-organic frameworks.

The activation barrier for cis-to-trans isomerization is a key parameter for governing the properties of photoswitchable molecules. This quantity can be computed by using theoretical methods, but experimental determination is not straightforward. Photoswitchable molecules typically do not change their conformation in the pure crystalline state. When the molecules are in solution, the switching is affected by the viscosity and polarity of the solvent and when embedded in polymers, the conformational change is affected by the polymer matrix. Here, we describe a novel approach where the photoswitchable group is integrated in a highly crystalline, porous molecular framework. Sufficiently large pore sizes in such metal-organic frameworks, MOFs, allow unhindered switching and the strictly periodic structure of the lattice eliminates virtually all contributions from inhomogeneities. Using IR spectroscopy to probe the conformational state of azobenzene, the energy barrier separating the cis and the trans state could be determined by an Arrhenius analysis of the data accumulated in a temperature regime between 314 K and 385 K. The result, 1.09 ± 0.09 eV, is in very good agreement with the activation energy reported for the thermal cis-to-trans isomerization of free azobenzene as computed by DFT calculations.