Kyu Tae Lee

Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium–Sulfur Batteries

Rechargeable lithium–sulfur (Li–S) batteries are attracting increasing attention due to their high theoretical specific energy density, which is 3 to 5 times higher than that of Li-ion batteries based on intercalation chemistry. Since the electronic conductivity of sulfur is extremely low, conductive carbon materials with high accessible porosity to “wire” and contain the sulfur are an essential component of the positive electrode. During the past decades, attempts have been made to fabricate C/S composites using carbon black, activated carbons (ACs), and carbon nanotubes (CNTs). Although improvements resulted, the cathodes suffered from inhomogeneous contact between the active material and the electronic conductors. A major step forward in fabricating a uniform C/S composite was reported in 2009. Some of us employed CMK-3, an ordered mesoporous carbon (OMC) featuring high specific surface area and large pore volume as a scaffold. As much as 70 wt% sulfur was incorporated into the uniform 3–4 nm mesopores, and the cells exhibited reversible capacities up to 1350 mAhg , albeit at moderate rates. Inspired by this, another OMC, a bulk bimodal mesoporous carbon (BMC-1) was investigated as a Li-S cathode. The favorable pore dimensions and large pore volume greatly improved the rate performance. An electrode with 40 wt% S showed a high initial discharge capacity of 1135 mAhg 1 at a current rate of 1 C (defined as discharge/ charge in one hour). However, similar to other reports, the capacity is sensitive to the sulfur ratio, dropping to 718 mAhg 1 at a sulfur content of 60 wt%. These results suggest that the texture of the mesoporous carbon could be further enhanced. Recently, Archer et al. reported nanoscale hollow porous C/S spheres prepared through vapor infusion. These materials displayed good cyclability and capacity at a C/5 rate, illustrating the advantages of nanosized porous carbon in the sulfur cathodes. Here we report the synthesis of unique nanoscale spherical OMCs with extremely high bimodal porosities. The particles were investigated as a cathode material and sulfur host in Li–S batteries where they showed high initial discharge capacity and good cyclability without sacrificing rate capability. Unlike bulk porous carbons, these carbon– sulfur sphere electrodes did not display significant capacity fading with the increase of sulfur content in the cathodes. We show that the nanoscale morphology of these materials is of key importance for ensuring very efficient use of the sulfur content even at high cycling rates. Morphology control is a central issue in OMC synthesis. There are numerous examples of mesoporous bulk materials obtained either by hard-templating or soft-templating, including thin films, membranes or free fibers. Most syntheses use evaporation-induced self-assembly (EISA) followed by thermal treatment for template-removal and carbonization. It is a challenge to either create solution-based OMC nanoparticle syntheses or to adapt the established EISA methods to nanoparticles. Only few examples of OMC nanoparticles have been reported so far which are mostly unsuitable for applications in Li–S cells due to low pore volume and/or surface area. Approaches include templating with PMMA colloidal crystals or mesoporous silica nanoparticles, aerosol-assisted synthesis, ultrasonic emulsification or hydrothermal synthesis. Ordered arrays of fused mesoporous carbon spheres were reported by Liu et al. using a macroporous silica as template. Recently Lei et al. reported the synthesis of 65 nm mesoporous carbon nanospheres, with both 2.7 nm mesopores and high textural porosity (surface area of 2400 mg ). These showed promising supercapacitor properties. Our spherical OMC nanoparticles of 300 nm in diameter, prepared by a novel method, can be dispersed in water by sonification to form stable colloidal suspensions. The spherical mesoporous carbon nanoparticles were obtained in a twostep casting process. An opal structure of PMMA spheres was cast with a silica precursor solution to form a silica inverse opal. The inverse opal was then used as template for a triconstituent precursor solution containing resol as the carbon precursor, tetraethylorthosilicate (TEOS) as the silica precursor and the block copolymer Pluronic F127 as a structure-directing agent. Carbonization was followed by etching of the silica template and the silica in the carbon/silica nanocomposite, resulting in the formation of OMC with hierarchical porosity. Through the presence of silica in the [*] J. Schuster, B. Mandlmeier, Prof. Dr. T. Bein Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstrasse 5–13 (Gerhard Ertl Building), 81377 Munich (Germany) E-mail: tbein@cup.uni-muenchen.de Homepage: http://bein.cup.uni-muenchen.de G. He, T. Yim, K. T. Lee, Prof. Dr. L. F. Nazar Department of Chemistry, University of Waterloo 200 University Avenue West, Waterloo, Ontario N2L 3G1 (Canada) E-mail: lfnazar@uwaterloo.ca [] These authors contributed equally to this work.

Nanocrystalline intermetallics on mesoporous carbon for direct formic acid fuel cell anodes.

Shape- and size-controlled supported metal and intermetallic nanocrystallites are of increasing interest because of their catalytic and electrocatalytic properties. In particular, intermetallics PtX (Xxa0=xa0Bi, Pb, Pd, Ru) are very attractive because of their high activity as fuel-cell anode catalysts for formic acid or methanol oxidation. These are normally synthesized using high-temperature techniques, but rigorous size control is very challenging. Even low-temperature techniques typically produce nanoparticles with dimensions much greater than the optimum <6xa0nm required for fuel cell catalysis. Here, we present a simple and robust, chemically controlled process for synthesizing size-controlled noble metal or bimetallic nanocrystallites embedded within the porous structure of ordered mesoporous carbon (OMC). By using surface-modified ordered mesoporous carbon to trap the metal precursors, nanocrystallites are formed with monodisperse sizes as low as 1.5xa0nm, which can be tuned up to ∼3.5xa0nm. To the best of our knowledge, 3-nm ordered mesoporous carbon-supported PtBi nanoparticles exhibit the highest mass activity for formic acid oxidation reported to date, and over double that of Pt-Au.

Sodium Terephthalate as an Organic Anode Material for Sodium Ion Batteries

Disodium terephthalate and its various derivatives are synthesized via simple acid-base chemistry for anode materials in Na ion batteries. They show excellent electrochemical performance, including little capacity fading over 90 cycles, ideal redox potential, and excellent rate performance, making them promising candidates for Na ion batteries.

Tavorite Lithium Iron Fluorophosphate Cathode Materials: Phase Transition and Electrochemistry of LiFePO4F – Li2FePO4F

We have synthesized LiFePO 4 F by a simple solid-state route as a pure single phase, which we show is isostructural with that of the minerals tavorite and amblygonite, and we report the first isolation of its fully lithium-inserted crystalline analog, Li 2 FePO 4 F. We show that the latter adopts a triclinic P1 tavorite-type framework that is very closely related to the parent phase. The redox activity between these two compositions is very facile and occurs with an 8% change in volume to result in a reversible and stable capacity of about 145 mAh/g. The electrochemical cycling at both room temperature and 55 °C is very stable.

Proof of Intercrystallite Ionic Transport in LiMPO4 Electrodes (M ) Fe, Mn)

Homogeneous-sized LiMPO(4) (M = Fe, Mn) nanorods and bulk particles were synthesized, and the thermodynamic characteristics of their mixtures as electrodes were analyzed to study the lithiation/delithiation mechanism for the general case of nanoparticles with a heterogeneous particle size distribution. We show that ionic transport occurs between nano and bulk particles in a cell at equilibrium, due to their electrochemical potential difference that originates from their different thermodynamic properties. This means that one phase in a single particle is preferred to two phases in a single particle during lithiation/delithiation of LiMPO(4) from the viewpoint of thermodynamics if the electrode is composed of differing particle sizes.

Chemical‐Assisted Thermal Disproportionation of Porous Silicon Monoxide into Silicon‐Based Multicomponent Systems

Under the surface: Ag nanoparticles are deposited onto the surface of commercially available SiO particles, and subsequent chemical etching results in the formation of nanoporous SiO without changing the chemical and physical properties of the original SiO. Moreover, chemical-assisted thermal annealing produces a shape-preserving Si-based multicomponent system, which exhibits high-performance electrochemical properties.

Versatile double hydrophilic block copolymer: dual role as synthetic nanoreactor and ionic and electronic conduction layer for ruthenium oxide nanoparticle supercapacitors

The facile synthetic approach to ruthenium oxide nanoparticles using double hydrophilic block copolymers (DHBCs) and their application toward the supercapacitor are presented. Nanostructured hydrous ruthenium oxide (RuO2) nanoparticles are synthesized using a double hydrophilic block copolymer of poly(ethylene oxide)-block-poly(acrylic acid) (PEO-b-PAA) as a template, forming a micelle upon addition of the ruthenium precursor, which then transformed into RuO2 nanoparticles of controlled dimension with reducing agents. The synthesized hydrous RuO2·xH2O nanoparticles are very stable for several months without any noticeable aggregates. Furthermore, we have demonstrated their utility in application as supercapacitors. Through annealing at 400 °C, we found that the crystallinity of RuO2 nanoparticles increases considerably with a simultaneous transformation of the surrounding double hydrophilic block copolymer into ionic and electronic conducting buffer layers atop RuO2 nanoparticles, which contribute to the significant enhancement of the overall specific capacitance from 106 to 962 F g−1 at 10 mV s−1. The RuO2 nanoparticles annealed at 400 °C also exhibit a superior retention of capacitance over 1000 cycles at very high charge–discharge rates at 20 A g−1. We envision that the double hydrophilic block copolymer will provide a facile and general tool in creating functional nanostructures with controlled dimensions that are useful for various applications.