Rich Micropores Research Articles

Three-Dimensional Flower-Shaped Activated Porous Carbon /Sulfur Composites As Cathode Materials for Lithium-Sulfur Batteries

Lithium sulfur battery, which use sulfur as a cathode and Li as an anode, have taken more and more attention for its high theoretical specific capacity of 1675 mA h g−1 and high theoretical specific energy of 2600 Wh kg−1. Despite these particular advantages, several troublesome issues associated with the sulfur cathode severely limits the practical use of sulfur in an electrode, such as, the highly electrical insulating nature of sulfur, the redox shuttle of dissolved polysulfide ions and the volume expansion of sulfur cathode materials. In our work, three-dimensional flower-shaped activated porous carbon/sulfur composites (FA-PC/S) are fabricated for the first time via a simple method utilizing flower-shaped ZnO as a template and pitch as the carbon precursor, followed by carbonization-activation and thermal treatment. A typical flower-shaped FA-PC spherical structure is composed of many nanopetals intersecting with one another. Because of the unique three-dimensional flower-shaped porous spherical structure, FA-PC has high reactivity, high electrical conductivity and a short transport length for the Li ion. Furthermore, the rich micropores of the FA-PC offer enough space to accommodate the volume expansion that occurs during the discharge process of the encapsulated sulfur as well as confine the electrochemical reaction products of the sulfur cathode within the micropores. The composites are also characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis, Raman spectroscopy, X-ray powder diffraction (XRD), and thermogravimetric (TG) measurements. The results show that sulfur is well dispersed and encapsulated homogeneously in the micropores of the flower-shaped activated porous carbon (FA-PC) with excellent electrical conductivity, high surface area and large pore volume. The electrochemical tests show that the FA-PC/S composites with 60 wt. % S have a high initial discharge capacity of 1388 mA h g-1 at 100 mA g-1, a good cycling stability (reversible discharging capacity of approximately 600 mA h g-1 at 1600 mA g-1) and excellent rate capability.

Electropolymerized Conjugated Microporous Poly(zinc‐porphyrin) Films as Potential Electrode Materials in Supercapacitors

A film-state conjugated microporous polymer constructed with zinc porphyrin cores and dimeric carbazole linkages is fabricated by electropolymerization. With its rich micropores and excellent reversible redox processes, the novel conjugated microporous polymer film is fabricated into a supercapacitor that demonstrates high capacitance up to 142 F g−1 and good capacitance retention at high current densities.

High-yield bamboo-like porous carbon nanotubes with high-rate capability as anodes for lithium-ion batteries

Bamboo-like porous carbon nanotubes (BCNTs) are prepared on a large scale through the pyrolysis of bamboo-like highly crosslinked polydivinylbenzene nanotubes. As anode materials for lithium-ion batteries, the BCNTs exhibit excellent electrochemical performance, attaining a high discharge capacity of 335 mA h g−1 at a current of 100 mA g−1. Even at a higher charge–discharge rate of 1000 mA g−1, a high discharge capacity of 230 mA h g−1 can be obtained for up to 300 cycles, which indicates the material has an excellent high-rate performance and cycle stability due to the separated hollow compartment structure and rich micropores on the surface of the BCNTs with the high BET surface area of 490.8 m2 g−1.

Application of biochar from sewage sludge to plant cultivation: Influence of pyrolysis temperature and biochar-to-soil ratio on yield and heavy metal accumulation

Applying biochar products from sewage sludge (SS) pyrolysis as soil amendment for plant cultivation was investigated in this study with special attention paid to heavy metal accumulation in the plants when pyrolysis temperature and biochar-to-soil mass ratio (C:S) were changed. Biochar obtained at four different temperatures were adopted as soil amendment for Allium sativum L. garlic plant cultivation. Experimental results revealed that biochars were rich in nutrient contents and they improved garlic yields. Although contents of heavy metals including As, Zn, Pb, Ni, Cd, Cr and Cu, etc. were elevated in the biochars compared to local soil, they fell within the acceptable limits for land application and SS is a suitable biochar resource, especially biochar produced at 450°C had rich micropores, relatively stable functional groups in structure and rugged surface to contact well with soil, conducive to its usage as a biochar. The garlic grew faster when planted in the biochar-amended soil and had higher final dry matter yields than those planted in the reference soil, especially biochar produced at 450°C corresponding to the highest final yields. The C:S ratio related to the highest garlic yields changed when the pyrolysis temperature was changed and this ratio was 1:4 for the biochar produced at 450°C. General heavy metal accumulation in the garlic occurred only for the most enriched Zn and Cu, and mainly in the roots & bulbs; in addition this bioaccumulation was increasing as leaching from biochar increased but not increasing with C:S ratio. The garlic planted in soil amended with biochar of 450°C contained the lowest level of heavy metals compared to other biochars. Those results indicated that heavy metal accumulation in plants can be inhibited through proper pyrolysis temperature choice and prevention of heavy metal leaching from the SS biochar.

Embedding Sulfur in MOF‐Derived Microporous Carbon Polyhedrons for Lithium–Sulfur Batteries

As a promising rechargeable battery system, lithium– sulfur (Li–S) batteries can deliver an exceptionally high theoretical specific capacity of 1672 mAhg 1 and an energy density of 2500 Whkg 1 with the low-cost and environmentfriendly sulfur as the cathode material. Although the potential use of sulfur as a cathode material has long been discovered, several severe drawbacks have hindered the realization of Li–S batteries. One limitation is the insulating nature of sulfur with a very low conductivity of 5 10 30 Scm , which results in low utilization of sulfur. Another well-known problem is associated with the easy dissolution of polysulfides, the intermediate products formed during the electrochemical reaction, in organic electrolytes. The dissolved polysulfides “shuttle” between the electrodes, leading to the low Coulombic efficiency and deposition of a highly resistive layer on the surface of electrodes. These detrimental issues result in unsatisfactory electrochemical performance with rapid fading of capacity. Several approaches have been proposed to overcome the above-mentioned challenges in Li–S batteries, such as developing novel electrolytes and electrode materials. Among these efforts, using sulfur-containing composites instead of pure sulfur as the cathode materials has been demonstrated as an effective way towards high-performance Li–S batteries. Polymers and porous carbons are the common candidates to form composites with sulfur, which immobilize the loaded sulfur, and probably also the derived polysulfides via physical and/or chemical interactions. In addition, the electrical conductivity of composite materials is also better than that obtained with pristine sulfur. In particular, porous carbon materials have attracted intensive attention due to their good compatibility with sulfur, easy accessibility, and the abundance of candidates with diverse porosity and structures. Mesoporous carbon materials have been widely studied as the host materials to confine sulfur. For example, nanocomposites consisting of sulfur and ordered mesoporous carbon or mesoporous hollow carbon spheres have shown improved sulfur utilization and cycling stability. Nonetheless, continuous capacity fading upon prolonged cycling is still commonly observed, and the use of optimized ether-based electrolytes seems to be indispensable. Recent reports on carbon materials with rich micropores have revealed distinct characteristics. 25] Sulfur embedded in microporous carbon shows a pronounced discharge plateau at a lower potential of about 1.8 V versus Li/Li, which is different from the two plateaus of a typical sulfur cathode. More importantly, these microporous carbon/sulfur nanocomposites generally show outstanding capacity retention upon cycling and good compatibility with conventional carbonate-based electrolytes. However, the origins of the unusual characteristics of microporous carbon are not fully understood yet. In recently years, syntheses of porous carbon materials from metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have attracted growing attention due to the facile preparation procedures, high carbon yield, and unique porous structures. For example, carbonization of MOF-5 with furfuryl alcohol results in nanoporous carbon, which shows excellent supercapacitive performance. The carbon materials with fiber-like morphology prepared from Al-based PCPs exhibit remarkably high porosity. In particular, MOFs and PCPs are very attractive as both the template and the precursor for the fabrication of microporous carbon. Compared with many other highly porous carbon materials, such as those prepared by post-activation processes, the porous carbon derived from MOFs and PCPs exhibits highly uniform porosity, largely originating from the ordered crystalline structures of the MOFs and PCPs. However, the interesting application of these carbon materials derived from MOFs and PCPs for Li–S batteries needs to be further explored. Herein, we report the facile synthesis of microporous carbon polyhedrons (MPCPs) using unique MOF polyhedrons as both the template and precursor, and their use as carbon host to incorporate sulfur for Li–S batteries. The asprepared MPCPs with abundant and uniform micropores serve as an ideal model system for investigating the electrochemical behaviors of sulfur embedded in microporous [a] H. B. Wu, S. Wei, Dr. L. Zhang, Prof. R. Xu, Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459 (Singapore) E-mail : rxu@ntu.edu.sg xwlou@ntu.edu.sg Homepage: http://www.ntu.edu.sg/home/xwlou [b] H. B. Wu, Prof. H. H. Hng School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301689.

EFFECT OF TEMPERATURE AND TIME ON MICROSTRUCTURE AND SURFACE FUNCTIONAL GROUPS OF ACTIVATED CARBON FIBERS PREPARED FROM LIQUEFIED WOOD

Activated carbon fibers were prepared from liquefied wood through stream activation. The effects of activation temperature and time on the microstructure and surface functional groups of the liquefied wood activated carbon fibers (LWACFs) were studied using analysis of burning behavior, X-ray diffraction, nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy, and SEM. The results showed that the burn-off value of the LWACFs increased gradually with the increase in temperature or time. All the LWACFs were far from being structurally graphitized, and in general, as temperature or time increased, the degree of graphitization and thickness of crystal structure increased. In addition, the LWACFs possessed rich micropores, and their specific surface area, pore volume, micropore size, and mesopore quantity were directly related to the activation temperature or time. The maximum specific surface area was found to be 2641 m2/g. The fractal dimension values of all samples were close to 3, indicating that their surfaces were very rough. Furthermore, with an increase in temperature or time, the elemental content of carbon increased, while that of oxygen decreased. Meanwhile, as the temperature or time increased, the relative content of graphitic carbon decreased, whereas that of carbon bonded to oxygen-containing functions increased. The surface of samples prepared at higher temperature or with longer time formed a considerable amount of holes.

Phenolic-based carbon nanofiber webs prepared by electrospinning for supercapacitors

Phenolic-based carbon nanofiber webs (PCNFWs) were prepared by electrospinning resole-type phenolic resin/PVA blend solution, followed by curing and carbonization. The PCNFWs presented rich micropores and a BET surface area of 416m2/g without any activation process. The electrochemical properties of the PCNFWs were investigated in two- or three-electrode cell. In spite of moderate surface area, the PCNFWs showed excellent capacitance performance. The specific capacitance was up to 171F/g at 5mV/s and remained 84% at 100mV/s. The excellent electrochemical performance was further confirmed through comparison with commercial activated carbon cloth (1201m2/g). The encouraging results showed the potential of the PCNFWs as supercapacitor electrode material.

Fischer–Tropsch synthesis over Co/montmorillonite—Insights into the role of interlayer exchangeable cations

To develop an efficient catalyst for controlling the product distribution of the Fischer–Tropsch (FT) synthesis, the natural Na-type montmorillonite (Na-MMT) was modified by ion-exchanging the interlayer Na + with selected cations, NH 4 +, Co 2+, and Al 3+. The degree of the interlayer Na + replaced by the selected cations was regulated by changing the ion-exchange conditions. A 20 wt.% Co/MMT catalyst was prepared by the incipient impregnation method. The materials were characterized by XRD, SEM/EDS, N 2 adsorption–desorption at low temperature, H 2-TPR, and NH 3-TPD techniques, and the results indicate that the MMT interlayer structure was well reserved for all the ion-exchanged MMTs. Moreover, acidic sites, rich micropores and open mesopores were created after exchanging the Na-MMT with NH 4 +, Co 2+, or Al 3+. The Co-supported catalysts were investigated for the FT synthesis in a fixed-bed reactor under the conditions of 1.0 MPa, 235 °C, H 2/CO = 2, and W/F = 5.02 g h mol −1. The results show that the Co/Na-MMT is an inefficient catalyst for the FT synthesis. In contrast, the Co/ion-exchanged MMT showed sharply increased CO conversion. The CO conversion was dependent on the ion-exchange conditions used, however. Moreover, the product selectivity over Co/MMT, i.e., 40–47% C 4–C 12 hydrocarbons and 13–20% C 21 + hydrocarbons, significantly deviates from the Anderson–Schulz–Flory distribution in the FT synthesis over the typical Co/SiO 2 catalyst. Based on the catalysts characterization, the difference of the FT activity as a result of interlayer cation can be explained as the different reduction behavior of the catalysts. Due to the cracking of the long-chain FT hydrocarbons, a narrow product distribution over the Co/MMT catalyst was observed, which correlates well with the NH 3-TPD results.

Designed synthesis and characterization of ordered mesoporous materials with high micropore content

Highly ordered mesoporous material SBA-15 with rich micropores has been successfully prepared by changing the hydrothermal treatment acidity and temperature to adjust the interaction between silanols and block copolymers at the inorganic/organic interface in the self-assembled mesostructured solids, so that the amount of hydrophilic PEO blocks inserted into the silica walls can be adjusted. The characterization results show that when the hydrothermal treatment pH value and temperature are 2.2 and 100 °C, respectively, the sample calcined at 300 °C shows a micropore volume as high as 0.176 cm3/g, which is 21.6% of the total pore volume.