Eunjoo Yoo

Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries

The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS.

Enhanced Cyclic Performance and Lithium Storage Capacity of SnO2/Graphene Nanoporous Electrodes with Three-Dimensionally Delaminated Flexible Structure

To fabricate nanoporous electrode materials with delaminated structure, the graphene nanosheets (GNS) in the ethylene glycol solution were reassembled in the presence of rutile SnO(2) nanoparticles. According to the TEM analysis, the graphene nanosheets are homogeneously distributed between the loosely packed SnO(2) nanoparticles in such a way that the nanoporous structure with a large amount of void spaces could be prepared. The obtained SnO(2)/GNS exhibits a reversible capacity of 810 mAh/g; furthermore, its cycling performance is drastically enhanced in comparison with that of the bare SnO(2) nanoparticle. After 30 cycles, the charge capacity of SnO(2)/GNS still remained 570 mAh/g, that is, about 70% retention of the reversible capacity, while the specific capacity of the bare SnO(2) nanoparticle on the first charge was 550 mAh/g, dropping rapidly to 60 mAh/g only after 15 cycles. The dimensional confinement of tin oxide nanoparticles by the surrounding GNS limits the volume expansion upon lithium insertion, and the developed pores between SnO(2) and GNS could be used as buffered spaces during charge/discharge, resulting in the superior cyclic performances.

Enhanced Electrocatalytic Activity of Pt Subnanoclusters on Graphene Nanosheet Surface

Graphene nanosheet (GNS) gives rise to an extraordinary modification to the properties of Pt cluster electrocatalysts supported on it. The Pt/GNS electrocatalyst revealed an unusually high activity for methanol oxidation reaction compared to Pt/carbon black catalyst. The Pt/GNS electrocatalyst also revealed quite a different characteristic for CO oxidation among the measured catalyst samples. It is found that Pt particles below 0.5 nm in size are formed on GNS, which would acquire the specific electronic structures of Pt, modifying its catalytic activities.

Li−Air Rechargeable Battery Based on Metal-free Graphene Nanosheet Catalysts

Metal-free graphene nanosheets (GNSs) were examined for use as air electrodes in a Li-air battery with a hybrid electrolyte. At 0.5 mA cm(-1), the GNSs showed a high discharge voltage that was near that of the 20 wt % Pt/carbon black. This was ascribed to the presence of sp(3) bonding associated with edge and defect sites in GNSs. Moreover, heat-treated GNSs not only provided a similar catalytic activity in reducing oxygen in the air, but also showed a much more-stable cycling performance than GNSs when used in a rechargeable Li-air battery. This improvement resulted from removal of adsorbed functional groups and from crystallization of the GNS surface into a graphitic structure on heat treatment.

N-Doped graphene nanosheets for Li–air fuel cells under acidic conditions

Recently, Li-air battery has attracted much more attention due to its extremely high specific energy density. However, there are still some critical problems such as air electrode’s clogging and organic liquid electrolyte’s decomposition during the discharge process. To overcome these problems, our group has developed a new type rechargeable Li–air battery based on a hybrid electrolyte with an aqueous electrolyte in the air electrode side [1-3]. Such new Li-air battery has an alkaline electrolyte on the air electrode side. However, in theory, the highest energy densities in Li-air battery are achieved by using strongly acidic solution. In this study, the N-doped graphene nanosheet (N-doped GNS) was examined for use as an air electrode in a Li-air fuel cell with a hybrid electrolyte under acidic conditions. The GNS was doped with nitrogen by heating in flow NH3 at various temperatures from 600 to 850 °C for 2 h. The electrochemical test of N-doped GNS was measured in the 1M Li2SO4 + 0.5M H2SO4 as the aqueous electrolyte. A solid-state electrolyte Li(1+x+y)Alx(Ti, Ge)2_xSiyP(3_y)O12 (LISICON) film was used as a separating membrane between the organic and aqueous electrolytes to prevent intermixing of the two solutions. The cells were discharged at a current density of 0.5 mA/cm for 24 h. Figure 1 shows schematic representation of Li–air fuel cell based on N-doped GNS with a hybrid electrolyte. Several researchers have reported that the nitrogen plays an important role in the active site of carbon materials for oxygen reduction reaction (ORR) catalysts [4,5]. It is therefore important to understand the exact roles of nitrogen in the carbon ORR catalysts.

Nano- and micro-sized TiN as the electrocatalysts for ORR in Li–air fuel cell with alkaline aqueous electrolyte

Due to its higher electrical conductivity compared with carbon and its outstanding corrosion resistance, titanium nitride (TiN) has recently been studied intensively for applications as an active electrode as well as catalyst support materials in supercapacitors, fuel cells and Li–air batteries. In this work, we studied the electrocatalytic activities of nano- and micro-sized TiN toward the oxygen reduction reaction (ORR) in an alkaline media using a thin film–rotating-disk electrode (RDE) technique, and investigated their performances as active air electrodes on a newly developed Li–air fuel cell with a hybrid electrolyte. The results show that both nano- and micro-sized TiN exhibit electrocatalytic activities toward ORR in alkaline media but with different mechanisms. The ORR catalyzed by micro-sized TiN proceeds via the serial “2e− + 2e−” pathway in a consecutive manner with the reduction of HO2− starting at a higher electrode potential as a discrete step. On the other hand, the ORR catalyzed by nano-sized TiN proceeds via a dual-path, where the two serial “2e−” steps proceed with smaller intervals and manifest an overall mixed appearance by coexistence of the parallel and serial “2e−” steps. The extent to which the two steps are in parallel or consecutive reveals a potential-dependent feature. Furthermore, both nano- and micro-sized TiN particles demonstrate evident electrocatalytic activities toward ORR in the Li–air fuel cell, with the nano-sized TiN showing a much better catalytic activity, which is comparable to that of the nano-sized Mn3O4.

Influence of CO2 on the stability of discharge performance for Li–air batteries with a hybrid electrolyte based on graphene nanosheets

The long-term discharge performance of graphene nanosheets for Li–air batteries with hybrid electrolytes shows different behaviour depending on the surface state of graphene. The functional groups on graphene play a more important role than the defect sites for the formation of Li2CO3 and electrochemical performance in Li–air batteries with hybrid electrolytes.

Hybrid electrolyte Li-air rechargeable batteries based on nitrogen- and phosphorus-doped graphene nanosheets

Nitrogen doped GNSs (N-doped GNSs) and phosphorus doped GNSs (P-doped GNSs) are examined as cathode electrodes for hybrid electrolyte Li-air batteries under basic conditions. The N-doped GNSs not only show a high discharge voltage that is near that of 20 wt% Pt/carbon black, but also provide better rate performance in the discharge process than that of the P-doped GNSs.

Extraction of Radioactive Cs and Sr from Nitric Acid Solutions with 25,27-Bis(1-octyloxy)calix(4)-26,28-Crown-6 and Dicyclohexyl-18-Crown-6: Effect of Nature of the Organic Solvent

Extraction of microamounts of radioactive cesium and strontium by 25,27-bis(1-octyloxy)calix[4]-26,28-crown-6 (DOC[4]C6) and dicyclohexyl-18-crown-6 (DCH18C6) from nitric acidic solutions into alcohols, ketones and some other solvents was studied. We concentrated on collecting detailed data possibly useful for subsequent evaluations of appropriate schemes and mechanisms. Although not testing in fullness the system with DOC[4]C6 and DCH18C6 in 1-octanol, we present some evidence of its suitability such as fast kinetics, chemical and radiation stability, and invariance of parameters (process stability) after several times repeated batch extraction-strip cycles. Some peculiarities of a possible mechanism of extraction in these systems are reported.

Carbon Cathodes in Rechargeable Lithium-Oxygen Batteries Based on Double-Lithium-Salt Electrolytes.

The use of carbon materials as air electrodes in lithium-oxygen (Li-O2 ) batteries is known to be advantageous owing to their good conductivity and because they offer sites suitable for the reversible electrode reactions. However, the exact influence of carbon materials on the electrochemical performance of Li-O2 batteries is not clear. In this study the electrochemical performance of four different types of carbon materials (multiwalled carbon nanotubes (MWCNTs), CMK-3, graphene nanosheets (GNSs), and Ketjen Black (KB)) as air electrodes is examined. We find that a Li-O2 cell based on an electrode of multiwalled carbon nanotubes (MWCNTs) demonstrates good rate performance and cycle stability, when using LiNO3 -LiTFSI/DMSO as electrolyte. Li-O2 cells based on such MWCNT electrodes, with a cut-off capacity of 1000 mAh g(-1) at 500 mA g(-1) , can undergo around 90 cycles without obvious losses of capacity. Even when the discharge depth is increased to 2000 mA h g(-1) , stable cycling is maintained for 45 cycles at a charge potential below 4.0 V.