Low Initial Efficiency Research Articles

Ppm-Level Thermally Switchable Yttrium Phenoxide Catalysts for Moisture-Insensitive and Controllably Immortal Polymerization of rac-Lactide

Exploring the moisture-insensitive catalysis system for the ring-opening polymerization of cyclic esters is very valuable for the synthesis of polyesters in industry. To reduce metal residue in final polymers, using ppm-level catalysts for the moisture-insensitive and controllable ROP system is still a big challenge until now. In this work, as a general strategy, a series of D3 symmetrical binuclear yttrium phenoxides were applied in the moisture-insensitive ring-opening polymerization of rac-lactide. The structures of these yttrium complexes can be reversibly switched between highly active and less active species in the presence of water via changing temperature. These catalysts are so robust in air, the loading of catalyst can be as low as 15 ppm, and one molecular catalyst can mediate the synthesis of more than 1000 polymer chains. The low initiation efficiency of water was proved unambiguously; hence, the polymerizations are controllable with a highly efficient initiator of alcohol in moisture conditi...

(Invited) Developing P Based Anode Materials for Na Ion Batteries

The Na abundance and its low cost make Na ion batteries (NIBs) promising alternatives for Lithium ion batteries (LIBs) in large scale energy storage application. However, the size of Na ions is larger than that of Li ion, making graphite no longer suitable as anode material. In addition, the higher redox potential of Na+/Na (vs. that of Li+/Li) indicates a smaller voltage window achievable (when similar cathode is employed). These factors urge the search for anode material of high capacity for NIBs. Among various material choices, P based material caught much research attention in recent years. Phosphorus is a most promising candidates for anode applications in sodium ion batteries due to its highest theoretical capacity （2596 mA h/g）and reasonably low redox voltage (~0.6 V vs. Na/Na+), implying achievable high energy density. Nevertheless, experimentally pure phosphorous anode suffers from fast capacity decay during cycling and low initial coulomb efficiency. These are caused by intrinsic characteristics of the phosphorous, including (1) low electrical conductivity (~10-14 S/cm); (2) instability of the Na-P phases; and (3) electrode pulverization due to sodiation induced volume expansion (390% to its original volume). In the present work, we will discuss two different approaches (forming metal phosphides or dispersing phosphorus in conducting matrix) that can improve the electrochemical properties of the anode. We show that although phosphide formation can largely stabilize the electrode for longer cycle life, the stable chemical bonds of metal-phosphorous make the sodiation difficult, and thus significantly reduce the capacity of the electrode. Manipulation of material chemistry may lead to an optimized balance between the stabilization of P and their active sodiation, resulting in high capacity, high rate performance and long cycle life. The authors are grateful to the financial support from GRF of RGC under project No. 14316716

Association of fibrous filters for aerosol filtration in predominant Brownian diffusion conditions

Abstract In order to improve the dust retention capacity of air filters and increase their lifetime, the filtration performance of associations of fibrous filters has been investigated. This study highlighted the interest of the addition of a medium or coarse filter, composed of microfibers, upstream of a HEPA filter. Collecting nanostructured particles in the depth of this upstream filter, rather than directly on the surface of a HEPA filter, permits to significantly reduce the pressure drop increase while maintaining a global mass collection efficiency close to the unit. On the one hand, the association of a medium filter and a HEPA filter induces a real energy gain, compared to the implementation of a single HEPA filter. On the other hand, the implementation of a coarse filter, presenting a too low initial collection efficiency, upstream of a HEPA filter leads to mixed results.

Radical polymerization of styrene mediated by 9-(1-phenylethyl)-9-((1-phenylethyl)thio)-9<italic>H</italic>-thioxanthene

Rapid progress in controlled/living radical polymerization (CLRP) has been achieved since the end of the last century. However, the demerits and limitations of existed methods impede their industrial applications on a large scale. Therefore, it is prominent to develop new CLRP methods that are better in controlled/living features, more simple, feasible and environmental-friendly. In this article, a new thioether, 9-(1-phenylethyl)-9-((1-phenylethyl)thio)-9 H -thioxanthene (DPETTX) was synthesized. Then, the polymerization behavior of St initiated by DPETTX and both DPETTX and a small amount of 2,2′-azobisisoheptonitrile (ABVN) was studied. Results show that DPETTX can initiated the polymerization of St at a high temperature ( 100 °C ) with low initiation efficiency, while St polymerization at a low temperature ( 55 °C ) was possible in the presence of a small amount of ABVN and number average molecular weight ( M n) increased continuously with increasing conversion of St. M n increased from 27 k to 46 k when a molar ratio of ABVN/DPETTX=0.05/1 was adopted. This co-initiating system based on DPETTX and a small amount of ABVN paves a new way for developing new CLRP methods.

Enhanced capacity of chemically bonded phosphorus/carbon composite as an anode material for potassium-ion batteries

Potassium-ion batteries are attracting great attention as a promising alternative to lithium-ion batteries due to the abundance and low price of potassium. Herein, the phosphorus/carbon composite, obtained by a simple ball-milling of 20 wt% commercial red phosphorus and 80 wt% graphite, is studied as a novel anode for potassium-ion batteries. Considering the high theoretical specific capacity of phosphorus and formation of stable phosphorus-carbon bond, which can alleviate the volume expansion efficiently, the phosphorus/carbon composite exhibits a high charge capacity of 323.5 mA h g−1 after 50 cycles at a current density of 50 mA g−1 with moderate rate capability and cycling stability. By the X-ray diffraction analysis, the alloying–dealloying mechanism of phosphorus is proposed to form a KP phase. Meanwhile, prepotassiation treatment is conducted to improve the low initial coulomb efficiency.

Simultaneous surface modification method for 0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion batteries: Acid treatment and LiCoPO4 coating

Li-rich layered cathode materials have been considered the most promising candidates for large-scale Li-ion batteries due to their low cost and high reversible capacity. However, these materials have many drawbacks that hinder commercialization, such as low initial efficiency and cyclability at elevated temperatures. To overcome these barriers, we propose an efficient and effective surface modification method, in which chemical activation (acid treatment) and LiCoPO4 coating were carried out simultaneously. During the synthesis, the lithium ions were extracted from the lattice, leading to improved Columbic efficiency, and these ions were used for the formation of LiCoPO4. The Ni and Co doped spinel phase was formed at the surface of the host material, which gives rise to the facile pathway for lithium ions. The LiCoPO4 and highly doped spinel on the surface acted as double protection layers that effectively prevented side reactions on the surface at 60 °C. Moreover, the transition metal migration of the modified cathode was weakened, due to the presence of the spinel structure at the surface. Consequently, the newly developed Li-rich cathode material exhibited a high 1st efficiency of 94%, improved capacity retention of 82% during 100 cycles at 60 °C, and superior rate capability of 62% at 12C (1C = 200 mA/g) rate at 24 °C. In addition, the thermal stability of the modified cathode was significantly improved as compared to that of a bare counterpart at 4.6 V, showing a 60% decrease in the total heat generation.

Improving Grafting Efficiency of Dicarboxylic Anhydride Monomer on Polylactic Acid by Manipulating Monomer Structure and Using Comonomer and Reducing Agent

Maleic anhydride (MA) grafted polylactic acid (PLA) acting as reactive compatibilizer for PLA blends and composites has been reported. However, melt free-radical grafting of MA on PLA is often subject to steric and electron effects of the substituents in the monomer and low initiation efficiency, yielding low grafting efficiency (Eg). In this work, five dicarboxylic anhydride monomers, including MA, itaconic anhydride (IA), cis-1,2,3,6-tetrahydrophthalic anhydride (TA), 4-allyltrimellitate anhydride (ATA), and 4-methacryloxyethyl trimellitate anhydride (META), were grafted onto PLA, and the effect of steric hindrance on Eg was assessed. It is noted that Eg values followed the order of ATA > META ≫ MA ≥ TA > IA. The introduction of styrene as a comonomer selectively increased the Eg values of three electron-deficient monomers, MA, IA, and META, while Sn(Oct)2 as a reducing agent increased Eg for all monomers. Both styrene and Sn(Oct)2 exhibited a synergistic effect when used in grafting MA, IA, and META.

Synthesis of ITO nanoparticles at room temperature using plasma treatment process and use it as back reflector in a-Si flexible solar cell

At present main challenges of flexible thin film solar cell is its low initial efficiency. Use of suitable back reflector is one of the major ways of improving its performance. We synthesized ITO (Indium tin-oxide) nanoparticles by plasma treatment in a vacuum chamber using low pressure, high power technique at room temperature. We investigated the variation of size and shape of the nanoparticles with the process pressure, power density and process time and optimized it for solar cell application. Structural and optical characterization of the nanoparticles were carried out by FESEM and UV-VIS-NIR spectrophotometer. In this paper we introduce ITO nanoparticles as back reflector in silicon based thin film flexible solar cell. It has been found that by using suitable ITO nanoparticles it is possible to increase the solar cell short circuit current (Isc) by 8.9%. This may be due to the better light reflection from the back after application of nanoparticles. The maximum efficiency obtained with ITO nanoparticles back reflector is 8.27% which is 4.68% higher than that obtained without ITO nanoparticles at back.

The effect of three electron donors on the initiator system efficiency of photopolymer film photosensitized by methylene blue

Photopolymer film has been widely used in optical devices. However, the low initiator efficiency limits its wider development. In this study, Methylene blue (MB)/N-methyldiethanolamine (MDEA) and MB/1,4-diazabicyclo [2.2.2]octane (DABCO) were first adopted as initiator system in photopolymer film and compared their initiator system efficiencies with the common initiator system MB/triethanolamine (TEA) by a self-established single-beam exposure setup. The MB/MDEA showed the highest performance, while the MB/DABCO was the poorest. The quantum yield, bleaching rate and saturated exposure of the MB/MDEA were 3.31 times, 2.62 times and 2.49 times, respectively, higher than those of the MB/TEA. This order of initiator system efficiencies (MB/MDEA>MB/TEA>MB/DABCO) is exactly consistent with the order of proton transfer efficiencies of the electron donors (MDEA>TEA>DABCO). Furthermore, high proton transfer efficiency and excellent migration of electron donors are helpful to improve initiator system efficiency via surface micromorphology analysis.

Performance study of desiccant coated heat exchanger air conditioning system in winter

Conventional air source heat pump system faces several challenges when adopted in winter season. Solid desiccant air conditioning system can provide humidification and heating power simultaneously and can be driven by low grade thermal energy; it provides a good alternative for air source heat pump systems. However, conventional solid desiccant air conditioning system adopts desiccant wheel with high cost as core component, which hinders the development of such system. Recently, desiccant coated heat exchanger (DCHE) with low initial cost and high efficiency was developed and this paper aims to investigate performance of DCHE air conditioning system adopted in Shanghai winter season. Performance of the system is predicted by a developed mathematical model where supply air states, mass of humidification and coefficient of performance (COP) are adopted as performance indices to evaluate the feasibility and energy utilization ratio of the system. Effects of regeneration water temperature on system performance are analyzed. It is found that under the simulation condition, relatively low regeneration temperature (such as 20°C) cannot meet the designed standard and relatively high regeneration temperature (such as 40°C) provides too much extra heating power, thus moderate regeneration temperature around 30°C is recommended. Meanwhile, switch time is a crucial operation parameter for the system to obtain satisfied supply air, switch time from 40s to 80s and from 70s to 240s are recommended for transient and average supply air states, respectively. Both mass of humidification and COP increase with increasing regeneration temperature under simulation condition. Also, influences of ambient air temperature and humidity ratio on system performance are discussed to study the feasibility of such system regarding different climatic conditions. Results show that higher humidity ratio of ambient air results in increased humidity ratio of supply air, temperature of ambient air has neglectable effect on supply air. In conclusion, DCHE air conditioning system can be adopted for winter operation with moderate selection of regeneration temperature as well as switch time.

Photoinduced Cu(0)‐Mediated Atom Transfer Radical Polymerization

Photoinduced Cu(0)‐mediated atom transfer radical polymerization (ATRP) of methyl methacrylate is investigated by using Cu(0)/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as the catalyst, ethyl 2‐bromopropionate as the initiator, and dimethyl sulfoxide as the solvent. The effect of light on the conventional Cu(0)‐mediated ATRP and molecular weight characteristics of the obtained polymers are described. Although the polymerization can be achieved in the absence of light, low initiation efficiency of the process is observed. By introducing UV light and additional Cu(II) deactivator, not only the rate of polymerization is increased, but also well‐controlled polymers with narrow‐molecular‐weight distributions at ambient conditions are obtained. The chain extension study clearly confirms high end‐group fidelity of the polymer that is active in the further polymerization process. image

Research on the Air Filters’ Maintenance for Diesel Engines

The wear of internal combustion engines is strongly influenced by dust particles from the atmosphere by that pass through air filter and reach inside the kinematic couplings of the engine. Air filters have relatively low initial efficiency. As they are used, air filters are impregnated with a layer of fine dust and their filtering efficiency is increasing. This paper aims to investigate the technical state of air filters that equip Renault K9K engine that are being replaced during the preventive maintenance schedule according to manufacturer recommendation.The study refers to the measurement in laboratory conditions on a specialized rig of the restriction generated by the air filter housing, as well as the restriction generated by new and used air filters. The used filters were collected from vehicles that came to service stations for air filters replacement as part of the preventive maintenance schedule.The results are used to establish to what extent the air filters replaced during planned maintenance could still be used, if they were replaced too late or at the appropriate time.

Systematic study on the impact of water on the performance and stability of perovskite solar cells

Sequentially spin-coated inverted perovskite solar cells were constructed with systematically varied concentrations of water in the PbI2precursor solution. Surprisingly, small concentrations of water improved long-term stability, although with lower initial efficiency.

Platform openness and the productivity of content providers: A meta-frontier analysis

This paper analyzes the platform environments in which content providers (CPs) may succeed by using a meta-frontier analysis that compares the efficiency of different groups in identical industries. The results illustrate that a group focusing on an iOS platform achieves a high average efficiency with low variance within the group because the iOS ecosystem manages the content novelty and uncertainty risk in the selection process. This quality control enables a CP to maximize value once the CP enters the ecosystem. From the meta-frontier viewpoint, however, Android-group firms have a higher efficiency level than iOS-group firms. Android transfers risk management to CPs who can conduct additional trial and error, causing CPs to endure the tough selection process. This explains the low initial technical efficiency, but in the long term, this group has the potential to achieve high efficiency. In addition, the group providing content to both platforms was the most efficient group because of the economies of scale.

Elastic a-silicon nanoparticle backboned graphene hybrid as a self-compacting anode for high-rate lithium ion batteries.

Although various Si-based graphene nanocomposites provide enhanced electrochemical performance, these candidates still yield low initial coloumbic efficiency, electrical disconnection, and fracture due to huge volume changes after extended cycles lead to severe capacity fading and increase in internal impedance. Therefore, an innovative structure to solve these problems is needed. In this study, an amorphous (a) silicon nanoparticle backboned graphene nanocomposite (a-SBG) for high-power lithium ion battery anodes was prepared. The a-SBG provides ideal electrode structures-a uniform distribution of amorphous silicon nanoparticle islands (particle size <10 nm) on both sides of graphene sheets-which address the improved kinetics and cycling stability issues of the silicon anodes. a-Si in the composite shows elastic behavior during lithium alloying and dealloying: the pristine particle size is restored after cycling, and the electrode thickness decreases during the cycles as a result of self-compacting. This noble architecture facilitates superior electrochemical performance in Li ion cells, with a specific energy of 468 W h kg(-1) and 288 W h kg(-1) under a specific power of 7 kW kg(-1) and 11 kW kg(-1), respectively.

Cuso4 -amine redox-initiated radical polymerization of methyl methacrylate mediated by a CuCl2 complex: Homogeneous reverse ATRP

Kinetic results of CuSO4/2,2'-bipyridine(bPy)-amine redox initiated radical polymerization of methyl methacrylate (MMA) at 70 to 90 °C in dimethylsulfoxide suggest that such initiation is characteristic of a slow rate and a low initiator efficiency, but tertiary amines exhibit a relatively higher rate. UV-Vis spectroscopy confirms the alpha-amino functionality of PMMA chains. CuCl2/bPy successfully mediates the redox-initiated radical polymerization of MMA with aliphatic tertiary amines in a fashion of slow-initiated reverse atom transfer radical polymerization (ATRP), i.e. both the initiator efficiency of aliphatic tertiary amines and the average molecular weight of PMMA increase gradually, while the molecular weight distribution remains narrow but become broader with the conversions. As the PMMA chains contain alpha amino and omega C-Cl moieties, UV-induced benzophenone-initiated radical polymerization and CuICl/bPy-catalyzed ATRP initiated from PMMA lead to block copolymers from terminal functionalities. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2562-2578

Preparation and characterization of anode materials using expanded graphite/pitch composite for high-power Li-ion secondary batteries

Expanded graphite was prepared by the intercalation of natural graphite using ammonium peroxodisulfate as an oxidizing agent and a high purity of sulfuric acid as an intercalate. The amounts of oxidizing agent and intercalate were changed to determine the preparation conditions of expanded graphite as the anode material for high-power Li-ion batteries. The expanded graphite was tested as the anode material and further composited with the different amounts of petroleum pitch to improve the electrochemical properties. Although the expanded graphite anode showed the improved electrochemical properties such as initial reversible capacities of around 400 mAh/g and a charge capacity at 5 C-rate of 83 mAh/g as compared with those for the natural graphite anode of 378 and 19.4 mAh/g, respectively, it still had some weak points for a high-power anode material such as low initial efficiency and potential plateaus with the stage characteristic. The anode composites with high performance could be obtained by compositing the expanded graphite and the petroleum pitch at the ratio of 1:2, showing an improved initial efficiency of 78 % and decreased potential plateaus with 389 mAh/g of the initial reversible capacity.

Mechanistic Aspects of Initiation and Deactivation in N-Heterocyclic Olefin Mediated Polymerization of Acrylates with Alane as Activator

Highly active catalyst systems based on N-heterocyclic olefin (NHO) as initiator were developed for polymerizing various polar monomers such as methyl methacrylate (MMA), n-butyl methacrylate (BMA), N,N-dimethylacrylamide (DMAA) and N,N-diphenylacrylamide (DPAA) at ambient temperature. The preactivation of these polar monomers by Lewis acid such as Al(C6F5)3 or AlCl3 is a prerequisite for their rapid transformation, since a stable adduct easily forms by the interaction of nucleophilic NHO and Lewis acidic activator. The formation of NHO/Al(C6F5)3 adduct was confirmed by 1H NMR spectroscopy and X-ray single crystal analysis. The length of AlAl(C6F5)3–CNHO bond is in the range of 2.002–2.018 Å, dependent on the substitute groups of N-heterocyclic ring. Highly molecular weight (Mn = 7.50 × 105 g/mol) and narrow molecular weight distribution (Mw/Mn = 1.04–1.27) were achieved with dissymmetry NHO as initiator and Al(C6F5)3 as activated agent. Although binary NHO/Al(C6F5)3 catalyst system could polymerize both MMA and BMA with high activity, the attempt to synthesize their block copolymers proved to be unsuccessful. Electrospray ionization time-of-flight mass spectrometry (ESI–TOF MS) study provided important information on the polymer-chain ends. It was found that NHO as the initiation group bounded to one end of a polymer chain and an unexpected six-membered lactone ring appeared at another chain end. The formation of lactone end is ascribed to the nucleophilic backbiting of the polymeric anion to the carboxyl carbon of the adjacent unit, in companion with the release of the methoxyl group. The low initiation efficiency of NHO is attributed to the formation of the stable NHO/alane adduct during the polymerization, while the production of lactone end results in complete deactivation in polymer chain propagation.

Lithium Storage Characteristics and Possible Applications of Graphene Materials

Graphene materials are materials with a flat mono/few layer of carbon atoms tightly packed to a two-dimensional honeycomb lattice. Graphene materials are expected to be applied in lithium ion batteries due to their unique structural, mechanical and electrical properties. As an anode material, the charge/discharge characteristics of graphene materials is similar to those of low-temperature soft carbon materials, such as high capacity, low initial efficiency and large voltage hys- teresis. Although attractive results have been achieved for graphene as anode materials for LIBs, detailed lithium storage mechanisms are still not clear. The effects of the following several structural parameters including disorder degree, surface area, micropores, interlayer spacing, C/O ratio and layer number on the lithium storage properties are discussed. Thermally reduced graphene materials with a highly disordered structure and high surface area has exceptionally high reversible capac- ity. Micropores in graphene materials have a great impact on their electrochemical performance. Although these micropores can provide additional sites for increased reversible lithium storage, it can also results in severe capacity fading and voltage hysteresis. Oxygen functional groups and larger interlayer spacing may provide higher reversible capacity of graphene, but the micropores and defect-based reversible storage may be the main contribution. Effect of layer number on lithium storage mechanisms of graphene and the conclusion are still in debate. Graphene with rich oxygen functional groups is a promising cathode material with high capacity and rate performance for lithium storage. High specific capacity of graphene cathode is mainly ascribed to lithiation reaction of oxygen functional groups, such as epoxide and carbonyl groups. Lithiation of oxygen functional groups still requires further study for a full understanding. Based on the lithium storage characteristics of graphene anode and cathode, lithium ion capacitors with high energy density and graphene composite cathode materials for lithium ion batteries may be designed and developed in the future. Graphene based lithium ion capacitors facilitate the reversible lithium storage, which significantly improves the energy density of lithium ion capacitors compared to those of conventional systems based on activated carbon. LiFePO4 modified with graphene layers has reached 208 mAh/g in specific capacity. The excess capacity is attributed to the reversible reduction-oxidation reaction between the lithium ions of the electrolyte and the exfoli- ated graphene flakes.

Effective Addition of Organic Chloride Salts on Atom Transfer Radical Polymerization in Fluoroalcohols

Atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in the presence of organic chloride salts, imidazolium chlorides, using alkyl bromide initiator was carried out in 2,2,2-trifluoroethanol (TFE) at 333 K. The ATRP using a sufficient large amount of chloride salts proceeded in a controlled manner to produce polymers with narrow molecular weight distributions (Mw/Mn < 1.2). The model reaction using an alkyl bromide and the imidazolium chloride indicated that a halogen exchange reaction occurred during the ATRP in TFE to form a chloride-terminated propagating species from the alkyl bromide. ATRP of MMA in TFE using an alkyl chloride initiator and CuCl catalyst, however, gave low initiation efficiency and loss of control over the polymerization. The combination of the alkyl bromide initiator, CuBr, and a sufficient amount of organic chloride salts was very effective to improve the control of ATRP in protic solvents, especially in fluoroalcohols. A methacrylate monomer bearing imidazolium chloride moiety was synthesized and polymerized by ATRP without added chloride salts to produce the polymers with well-controlled molecular weights and narrow polydispersities.