The Mass-Balancing between Positive and Negative Electrodes for Optimizing Energy Density of Supercapacitors.

Supercapacitors (SCs) are some of the most promising energy storage devices, but their low energy density is one main weakness. Over the decades, superior electrode materials and suitable electrolytes have been widely developed to enhance the energy storage ability of SCs. Particularly, constructing asymmetric supercapacitors (ASCs) can extend their electrochemical stable voltage windows (ESVWs) and thus achieve high energy density. However, only full utilization of the electrochemical stable potential windows (ESPWs) of both positive and negative electrodes can endow the ASC devices with a maximum ESVW by using a suitable mass-ratio between two electrodes (the mass-balancing). Nevertheless, insufficient attention is directed to mass-balancing, and even numerous misunderstandings and misuses have appeared. Therefore, in this Perspective, we focus on the mass-balancing: summarize theoretic basis of the mass-balancing, derive relevant relation equations, analyze and discuss the change trends of the specific capacitance and energy density of ASCs with mass-ratios, and finally recommend some guidelines for the normative implementation of the mass-balancing. Especially, the issues related to pseudocapacitive materials, hybrid devices, and different open circuit potentials (OCPs) of the positive and negative electrodes in the mass-balancing are included and emphasized. These analyses and guidelines can be conducive to understanding and performing mass-balancing for developing high-performance SCs.

Metal-Organic Framework Derived Bi-O-Sn/C Nanostructure: Tailoring the Adsorption Site of Dominant Intermediate for Highly Efficient CO2 Electroreduction to Formate.

Electrochemical CO2 reduction into high-value-added formic acid/formate is an attractive strategy to mitigate global warming and achieve energy sustainability. However, the adsorption energy of most catalysts for the key intermediate *OCHO is usually weak, and how to rationally optimize the adsorption of *OCHO is challenging. Here, an effective Bi-Sn bimetallic electrocatalyst (Bi1 -O-Sn1 @C) where a Bi-O-Sn bridge-type nanostructure is constructed with O as an electron bridge is reported. The electronic structure of Sn is precisely tuned by electron transfer from Bi to Sn through O bridge, resulting in the optimal adsorption energy of intermediate *OCHO on the surface of Sn and the enhanced activity for formate production. Thus, the Bi1 -O-Sn1 @C exhibits an excellent Faradaic efficiency (FE) of 97.7% at -1.1 V (vs RHE) for CO2 reduction to formate (HCOO- ) and a high current density of 310 mA cm-2 at -1.5 V, which is one of the best results catalyzed by Bi- and Sn-based catalysts reported previously. Impressively, the FE exceeds 93% at a wide potential range from -0.9 to -1.4 V. In-situ ATR-FTIR, in-situ Raman, and DFT calculations confirm the unique role of the bridge-type structure of Bi-O-Sn in highly efficient electrocatalytic reduction of CO2 into formate.

Zr-Based MOF-Stabilized CO2-Responsive Pickering Emulsions for Efficient Reduction of Nitroarenes.

A Pickering emulsion is a natural microreactor for interfacial catalysis in which an emulsifier is critical. Recently, a metal-organic framework (MOF) has attracted attention to emulsify water-organic mixtures for constructing a Pickering emulsion. However, a few stimuli-responsive Pickering emulsions based on MOFs have been reported, and the MOF emulsifiers cannot be regenerated at room temperature. Herein, the Zr-MOF with a rodlike morphology is synthesized using ionic liquid as a modulator and then modified with n-(trimethoxysilylpropyl)imidazole (C3im) to prepare a series of functionalized Zr-MOFs (MOF-C3im). It is found that MOF-C3im is an excellent emulsifier to construct stable and CO2-responsive Pickering emulsions even at low content (>0.20 wt %). Notably, the emulsification and demulsification of the emulsions can be easily and reversibly switched by bubbling of CO2 and N2 alternatively at room temperature because CO2 and imidazole molecules anchored on the Zr-MOF underwent a reversible acid-base reaction, resulting in an obvious change in the wettability of the emulsifier. As a proof of concept, the reduction reactions of nitrobenzene have been successfully carried out in these Pickering emulsions, demonstrating the efficient integration as a microreactor for chemical reaction, product separation, and emulsifier recycling under ambient conditions. This strategy provides an innovative option to develop stimulus-responsive Pickering emulsions for sustainable chemical processes.

Efficient Capture and Low Energy Release of NH3 by Azophenol Decorated Photoresponsive Covalent Organic Frameworks.

In NH3 capture technologies, the desorption process is usually driven by high temperature and low pressure (such as 150-200 °C under vacuum), which accounts for intensive energy consumption and CO2 emission. Developing light responsive adsorbent is promising in this regard but remains a great challenge. Here, we for the first time designed and synthesized a light responsive azophenol-containing covalent organic framework (COF), COF-HNU38, to address this challenge. We found that at 25 °C and 1.0 bar, the cis -COF exhibited a NH3 uptake capacity of 7.7 mmol g-1 and a NH3/N2 selectivity of 158. In the adsorbed NH3, about 29.0 % could be removed by vis-light irradiated cis-trans isomerization at 25 °C, and the remaining NH3 might be released at 25 °C under vacuum. Almost no decrease in adsorption capacity was observed after eight adsorption-desorption cycles. As such, an efficient NH3 capture and low energy release strategy was established thanks to the multiple hydrogen bond interactions (which are strong in total but weak in individuals) between NH3 and the smart COF, as well as the increased polarity and number of hydrogen bond sites after the trans-cis isomerization.

Bodipy-Based Metal-Organic Frameworks Transformed in Solid States from 1D Chains to 2D Layer Structures as Efficient Visible Light Heterogeneous Photocatalysts for Forging C-B and C-C Bonds.

Boron dipyrromethene (also known as bodipy), as a class of versatile and robust fluorophores and a structural analogue of porphyrins, has received a great deal of interests in the field of light-harvesting and energy-transfer processes. However, the fabrication of bodipy monomers into metal-organic frameworks (MOFs) and the exploitation of their potential still lags behind the porphyrin MOFs. In this work, two bodipy-based MOFs, BMOF 1D with 1D chain structure and BMOF 2D with 2D layer structure, were assembled by using dicarboxyl-functionalized bodipy ligands. BMOF 1D can also be converted to BMOF 2D by inserting additional ligands into BMOF 1D to cross-link the adjacent chains into the rhombic grid layer. During this process, spontaneous exfoliation occurred simultaneously and resulted in the formation of several hundred nanometer thickness BMOF 2D (nBMOF 2D), which can be further exfoliated into one-layer MOF nanosheets (BMON 2D) by using the ultrasonic liquid exfoliation method in a high yield. Featuring the distinct bodipy scaffolds in the porous frameworks, both BMOF 2D and BMON 2D displayed high reactivity and recyclability in the photocatalytic inverse hydroboration and cross-dehydrogenative coupling reactions to afford α-amino organoborons and α-amino amides in moderate to high yields. This work not only highlights the cascade utilization of ligand installation and ultrasonic liquid exfoliation methods to provide the single-layer MOF sheets in high yields but also advances the bodipy-based MOFs as a new type of heterogeneous photocatalysts in the forging of C-B and C-C bonds driven by visible light.

Ingenious Artificial Leaf Based on Covalent Organic Framework Membranes for Boosting CO2 Photoreduction.

Covalent organic frameworks (COFs) hold the potential in converting CO2 with water into value-added fuels and O2 to save the deteriorating ecological environment. However, reaching high yield and selectivity is a grand challenge under metal-, photosensitizer-, or sacrificial reagent-free conditions. Here, inspired by microstructures of natural leaves, we designed triazine-based COF membranes with the integration of steady light-harvesting sites, efficient catalytic center, and fast charge/mass transfer configuration to fabricate a novel artificial leaf for the first time. Significantly, a record high CO yield of 1240 µmol g-1 in a 4 h reaction, approximately 100% selectivity, and a long lifespan (at least 16 cycles) were achieved under gas-solid conditions without using any metal, photosensitizer, or sacrificial reagent. Unlike the existing knowledge, the chemical structural unit of triazine-imide-triazine and the unique physical form of the COF membrane are predominant for such a remarkable photocatalysis. This work opens a new pathway to simulating photosynthesis in leaves and may motivate relevant research in the future.

Isomerization of DASA Molecules in the Nanopores of Metal-Organic Frameworks: What Determines Its Reversibility?

In recent years, light-responsive molecules have been incorporated in metal-organic frameworks (MOFs) to fabricate light-responsive intelligent devices, where reversible isomerization of the guest molecules in the nanopores is crucial. However, how to design a porous environment of MOFs to achieve a reversible isomerization remains unknown until now. In this work, donor-acceptor Stenhouse adducts (DASAs), a new kind of visible light responsive compound, were confined in the nanopores of different MOFs to study their isomerization upon visible-light irradiation/mild heating. We found that the polarity of the pore environment is the key to control the reversibility of isomerization of such guest molecules. Under the guidance of this principle, MIL-53(Al) was screened to investigate the proton conductivity and switching performance of the DASA-confined MOF. The proton conductance was up to 0.013 S cm-1 at 80 °C and 98 % RH, and at least 30 switching cycles were achieved thanks to the Grotthuss-type mechanism and the low polarity of MIL-53(Al) pore environment.

Activation and Inhibition of Isomerization of a Cationic Azobenzene Surfactant in the Large Void Space of Polyglycerol Dendron Micelles.

Owing to the unique geometric structure of dendritic amphiphiles with voluminous dendrons, their micelles can harbor a large void space, which provides a new research focus and approach for micellar functionalization. In this work, we used the void space to construct a UV responsive micelle system of the mixed dendritic amphiphile (C12-(G3)2) and cationic azobenzene surfactant (C4AzoTAB). The synthesized C12-(G3)2 that possesses double third generation polyglycerol (PG) dendrons and a single alkyl chain is expected to highlight the large void space within the inside of the micelles. Thus, the aims of this work are to achieve the isomerization of C4AzoTAB in situ and to deeply understand the intermolecular interaction in the mixed micelles. The effect of the large void room with a wall decorated with the ether oxygen atoms on the isomerization of C4AzoTAB was studied by isomerization kinetics, conductivity measurements, isothermal titration calorimetry (ITC), and 1H NMR and 2D NOESY spectroscopies. The isomerization behavior of C4AzoTAB in C12-(G3)2 micelles was presented in terms of its kinetic constant, counterionic association, interaction enthalpy, and position and orientation of C4AzoTAB. The results of NMR and conductivity show that the quaternary ammonium group of C4AzoTAB situates on the surface of the mixed micelles with C12-(G3)2 both before and after UV-irradiation, while the position of azobenzene group in C12-(G3)2 micelles depends on its conformation. The C12-(G3)2 micelles can inhibit the UV response of the trans-isomer and activate the thermal relaxation of the cis-isomer, which has a potential application in the field of light-controlled smart nanocarriers.

Spatial-Temporal Self-Attention Enhanced Graph Convolutional Networks for Fitness Yoga Action Recognition.

Fitness yoga is now a popular form of national fitness and sportive physical therapy. At present, Microsoft Kinect, a depth sensor, and other applications are widely used to monitor and guide yoga performance, but they are inconvenient to use and still a little expensive. To solve these problems, we propose spatial-temporal self-attention enhanced graph convolutional networks (STSAE-GCNs) that can analyze RGB yoga video data captured by cameras or smartphones. In the STSAE-GCN, we build a spatial-temporal self-attention module (STSAM), which can effectively enhance the spatial-temporal expression ability of the model and improve the performance of the proposed model. The STSAM has the characteristics of plug-and-play so that it can be applied in other skeleton-based action recognition methods and improve their performance. To prove the effectiveness of the proposed model in recognizing fitness yoga actions, we collected 960 fitness yoga action video clips in 10 action classes and built the dataset Yoga10. The recognition accuracy of the model on Yoga10 achieves 93.83%, outperforming the state-of-the-art methods, which proves that this model can better recognize fitness yoga actions and help students learn fitness yoga independently.

Multi-weight susceptible-infected model for predicting COVID-19 in China.

The mutant strains of COVID-19 caused a global explosion of infections, including many cities of China. In 2020, a hybrid AI model was proposed by Zheng et al., which accurately predicted the epidemic in Wuhan. As the main part of the hybrid AI model, ISI method makes two important assumptions to avoid over-fitting. However, the assumptions cannot be effectively applied to new mutant strains. In this paper, a more general method, named the multi-weight susceptible-infected model (MSI) is proposed to predict COVID-19 in Chinese Mainland. First, a Gaussian pre-processing method is proposed to solve the problem of data fluctuation based on the quantity consistency of cumulative infection number and the trend consistency of daily infection number. Then, we improve the model from two aspects: changing the grouped multi-parameter strategy to the multi-weight strategy, and removing the restriction of weight distribution of viral infectivity. Experiments on the outbreaks in many places in China from the end of 2021 to May 2022 show that, in China, an individual infected by Delta or Omicron strains of SARS-CoV-2 can infect others within 3-4 days after he/she got infected. Especially, the proposed method effectively predicts the trend of the epidemics in Xi'an, Tianjin, Henan, and Shanghai from December 2021 to May 2022.

Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold.

Utilizing weak interactions to effectively recover and separate precious metals in solution is of great importance but the practice remains a challenge. Herein, we report a novel strategy to achieve precise recognition and separation of gold by regulating the hydrogen-bond (H-bond) nanotrap within the pore of covalent organic frameworks (COFs). It is found that both COF-HNU25 and COF-HNU26 can efficiently capture AuIII with fast kinetics, high selectivity, and uptake capacity. In particular, the COF-HNU25 with the high density of H-bond nanotraps exhibits an excellent gold uptake capacity of 1725 mg g-1 , which is significantly higher than that (219 mg g-1 ) of its isostructural COF (COF-42) without H-bond nanostrap in the pores. Importantly, the uptake capacity is strongly correlated to the number of H-bonds between phenolic OH in the COF and [AuCl4 ]- in water, and multiple H-bond interactions are the key driving force for the excellent gold recovery and reusability of the adsorbent.

Supramolecular Vector/Drug Coassemblies of Polyglycerol Dendrons and Rutin Enhance the pH Response.

A coassembly strategy for a supramolecular vector/drug was proposed with a biocompatible ternary dodecyl-bi(third-generation polyglycerol (PG) dendrons) (C12-(G3)2) amphiphile, dodecyl sulfobetaine (SB3-12) surfactant, and poorly water-soluble drug rutin. C12-(G3)2 and rutin will mutually enhance their pH response by protonation and deprotonation of dendritic PG and rutin's ionization as the pH changes from the acidic gastric lumen to the weakly alkaline intestine. SB3-12 may increase the payload and bring about sustained release for rutin by intermolecular interactions. Self-assembling behaviors of C12-(G3)2, SB3-12, sodium dodecyl sulfate (SDS), and dodecyl trimethylammonium bromide (DTAB) and their hybrids with rutin were characterized by UV-vis spectroscopy, a fluorescence probe, and 1H NMR. UV-vis and 1H NMR were used to identify the position and orientation of rutin in the vectors. The functions of the vector/drug were confirmed by measuring the solubility and in vitro release of rutin. The ternary coassembling vector/drug easily imparted functions of pH-responsive and sustained release without complex synthetic processes. The nanocaves framed by PG dendrons in the micelles provide pH-responsive compartments for rutin and SB3-12 in the supramolecular vector/drug anchors that accommodate rutin by weak interactions. The finely matched supramolecular vector/drug coassemblies exhibit the pH-responsive function for a potential application in the treatment of inflammatory bowel disease.

Anchoring LiCl in the Nanopores of Metal-Organic Frameworks for Ultra-High Uptake and Selective Separation of Ammonia.

Ammonia (NH3 ) is an important chemical raw material and a unique carbon-free fuel with high hydrogen energy density. Thus, NH3 capture, storage, and desorption are of significant importance. However, high capacity capture, low energy desorption, and selective separation of NH3 are still challengs so far. Here, we report high-performance hybrid sorbents by anchoring LiCl in the nanopores of MIL-53-(OH)2 metal-organic frameworks (MOFs). It is found that the optimal composite shows a capture capacity of 33.9 mmol g-1 NH3 at 1.0 bar and 25 °C, which far exceeds the current record among the reported porous materials. Notably, the excellent capture capacity at low pressure and high temperature makes it possible to selectively capture NH3 from NH3 /N2 , NH3 /CO2 , and NH3 /H2 O. It is revealed that synergistic action of NH3 coordination to the highly dispersed Li+ in the MOF nanopores and hydrogen bonding of NH3 with Cl- account for such an excellent capture and selectivity performance.

Multi-Stimuli-Induced Mechanical Bending and Reversible Fluorescence Switching in a Single Organic Crystal.

Fluorescent single crystals that respond to multiple external stimuli are of great interest in molecular machines, sensors, and displays. The integration of photo- or acid-induced fluorescence enhancement and bending in one organic crystal, however, has not been reported yet. Herein, we report the interesting plastic photomechanical bending and switching on of the fluorescence of an azine crystal in a single-crystal transformation, due to extended π-conjugation and molecular slippage. Moreover, the fluorescent plastic bending driven by multiple volatile acid vapors was firstly observed, and attributed to the synergistic effect of push-pull electronic structure and hydrogen bonding. The single crystal also shows high elasticity under external force. In addition, reversible fluorescence switching can be triggered by grinding and solvent fuming, as well as by the adsorption and desorption of HCl vapor. The integration of plastic, elastic bending and switch-on fluorescence into one single crystal provides a new strategy for next-generation smart materials.

An Ultrastable Crystalline Acylhydrazone-Linked Covalent Organic Framework for Efficient Removal of Organic Micropollutants from Water.

As an important member of crystalline porous polymers, acylhydrazone-linked covalent organic frameworks (COFs) have gained much attention in recent years. However, the low structural stability imparts a limit on their practical applications. To tackle this problem, we report a simple strategy to increase the chemical stability of acylhydrazone-linked COFs by incorporating azobenzene groups in the conjugated framework. Through reinforcing the π-π stacking interactions between the adjacent layers with increased π-surface, it is surprising to find that the resulting materials exhibit extreme stability in harsh environments, such as in strong acid, strong base, aqueous educing agent and boiling water, even exposed to air for one year. As a proof-of-concept, such frameworks have been used to remove various organic micropollutants such as antibiotics, plastic components, endocrine disruptors, and carcinogens from water with high capacity, fast speed and excellent reusability over a wide pH range at environmentally relevant concentrations. The results provide a new avenue to significantly enhance the stability of COFs for practical applications.

Cobalt Carbonate-Coated Nitrogen-Doped Carbon Nanotubes with a Sea-Cucumber Morphology for Electrocatalytic Water Splitting.

Herein, we report CoCO3-coated nitrogen-doped carbon nanotubes (NCNTs) with a sea cucumber-like morphology for water splitting. The sample with a CoCO3 content of 26.8 wt % (CoCO3/NCNT-1) exhibits excellent performance for the hydrogen evolution reaction in 1.0 M KOH electrolyte with an overpotential of 58 mV to reach 10 mA cm-2, better than the most non-noble metal catalysts reported; meanwhile, it exhibits superior electrocatalytic activity for the oxygen evolution reaction. The excellent performance of the catalyst is attributed to the nanotip effect caused by the sea-cucumber-like morphology. Notably, CoCO3/NCNT-1 can attain turnover frequencies of 2.7 s-1 at an overpotential of 50 mV, higher than that of Pt/C (1.5 s-1). A cell constructed using CoCO3/NCNT-1 as the catalyst of the electrode pair needs a low cell voltage of 1.54 V at 10 mA cm-2, superior to most reported cells. In addition, CoCO3/NCNT-1 can maintain 10 mA cm-2 for overall water splitting for 100 h without activity loss.

Light-Responsive, Reversible Emulsification and Demulsification of Oil-in-Water Pickering Emulsions for Catalysis.

Pickering emulsions are an excellent platform for interfacial catalysis. However, developing simple and efficient strategies to achieve product separation and catalyst and emulsifier recovery is still a challenge. Herein, we report the reversible transition between emulsification and demulsification of a light-responsive Pickering emulsion, triggered by alternating between UV and visible light irradiation. The Pickering emulsion is fabricated from Pd-supported silica nanoparticles, azobenzene ionic liquid surfactant, n-octane, and water. This phase behavior is attributed to the adsorption of azobenzene ionic liquid surfactant on the surface of the nanoparticles and the light-responsive activity of ionic liquid surfactant. The Pickering emulsion can be used as a microreactor that enables catalytic reaction, product separation as well as emulsifier and catalyst recycling. Catalytic hydrogenation of unsaturated hydrocarbons at room temperature and atmospheric pressure has been performed in this system to demonstrate product separation and emulsifier and catalyst re-use.

Manipulation of femtosecond laser filamentation by a gaseous lattice.

Manipulation of femtosecond laser filamentation is essential for many potential applications. We report the simulations of the manipulation of femtosecond laser filamentation by introducing a novel gaseous lattice medium with the alternating positive and negative refractive index distribution at different stages of filamentation. The results show that the filament length has greatly been extended and a multi-filament array can be formed by the gas lattice medium. It has been found that additional focusing and discrete diffraction provided by the gas lattice medium contribute to a new dynamic equilibrium in the filamentation. As a result, a varied cross-section pattern, higher field intensity, and electron density along the filamentation are obtained. Our approach provides a new way to manipulate filamentation for many practical photonic applications.

Insight into the formation and permeability of ionic liquid unilamellar vesicles by molecular dynamics simulation.

Unilamellar vesicles in solution could open up new horizons for reaction and material delivery, but the formation mechanism especially for the permeability of the small molecule through the vesicle membrane is still unknown. In this study, the formation and permeability of the unilamellar vesicles formed by the ionic liquid 1-dodecyl-3-methylimidazolium salicylate ([C12mim][Sal]) have been investigated by molecular dynamics simulation. Starting from a random distribution of ionic liquids, the entire process of vesicle formation could be observed on a nanosecond time scale, during which planar and cup-like structures are formed at the intermediate stage. Energy analysis reveals that the electrostatic interactions between cations and anions play a dominant role in forming and stabilizing the vesicle. Radial density distribution functions indicate that the final stable vesicle is a spherical bilayer structure. Besides, it was found that the structure of vesicles is maintained with the increase of temperature, while the water molecules in the vesicles could be completely exchanged quickly. These results suggest that vesicles may be beneficial for the enrichment or release of molecules.

Tunable Aggregation-Induced Emission of Imidazole Hydrazones by pH and Anions.

Aggregation-induced emission (AIE) materials have drawn great attention for applications as organic light-emitting diodes (OLED) and probes. The applications are, however, restricted by the complex syntheses and hydrophobic properties. Herein, a one-step synthesis of an AIE material based on imidazole hydrazone is assessed. Protonation of the imidazole-H leads to emission color change from yellow to green in the solid state. The emission color is recovered upon imidazole-H+ deprotonation. Moreover, the emission wavelength shifts from 532 to 572 nm by anion exchange. In addition, an enhanced emission (ΦF up to 22.6 %) was obtained with the Br- anion compared with NTf2 - , SbCl5 - , PF6 - , and OTf- anions. X-ray crystallography studies together with theoretical calculations show that the enhanced emission of hydrazone salts arises from strong hydrogen bonding between the hydrazone proton and the halide ion (Cl- or Br- ).