Hyea Kim

Sulfur‐Infiltrated Micro‐ and Mesoporous Silicon Carbide‐Derived Carbon Cathode for High‐Performance Lithium Sulfur Batteries

Novel nanostructured sulfur (S)-carbide derived carbon (CDC) composites with ordered mesopores and high S content are successfully prepared for lithium sulfur batteries. The tunable pore-size distribution and high pore volume of CDC allow for an excellent electrochemical performance of the composites at high current densities. A higher electrolyte molarity is found to enhance the capacity utilization dramatically and reduce S dissolution in S-CDC composite cathodes during cycling.

Lithium Iodide as a Promising Electrolyte Additive for Lithium–Sulfur Batteries: Mechanisms of Performance Enhancement

Lithium Iodide (LiI) is reported as a promising electrolyte additive for lithium-sulfur batteries. It induces formation of Li-ion-permeable protective coatings on both positive and negative electrodes, which prevent the dissolution of polysulfides on the cathode and reduction of polysulfides on the anode. In addition to enhancing the cell cycle stability, LiI addition also decreases the cell overpotential and voltage hysteresis.

A Hierarchical Particle–Shell Architecture for Long‐Term Cycle Stability of Li2S Cathodes

A hierarchical particle-shell architecture for long-term cycle stability of Li2S cathodes is described. Multiscale and multilevel protection prevents mechanical degradation and polysulfide dissolution in lithium-sulfur battery chemistries.

Stabilization of selenium cathodes via in situ formation of protective solid electrolyte layer

The lithium/selenium (Li/Se) rechargeable battery chemistry offers a higher energy density than traditional Li ion battery cells. However, high solubility of polyselenides in suitable electrolytes causes Se loss during electrochemical cycling, and leads to poor cycle stability. This study presents a simple technique to form a protective, solid electrolyte layer on the cathode surface. This protective layer remains permeable to Li ions, but prevents transport of polyselenides, thus dramatically enhancing cell cycle stability. The greatly reduced reactivity of polyselenides with fluorinated carbonates (such as fluoroethylene carbonates [FEC]) permits using their in situ reduction for low-cost formation of protective coatings on Se cathodes.

Electroless Deposition and Characterization of PtxRu1−x Catalysts on Pt/C Nanoparticles for Methanol Oxidation

The electroless deposition of Pt x Ru 1―x catalysts using hydrazine dihydrochloride or formic acid as the reducing agent in a modified Leaman bath was investigated. The effect of potential on the Pt x Ru 1 ―x composition was investigated by potentiostatically depositing Pt x Ru 1―x thin films on gold from acidic chloride electrolytes at potentials between ―0.46 V and 0.34 V (versus normal hydrogen electrode). The physical characteristics and elemental composition of the deposits were determined. An empirical model for the deposition process was developed, taking into account reactant concentration, temperature, and surface potential. The model accurately characterized the deposit composition over a wide Pt/Ru range. The surface potential was estimated to be 0.15 V during electroless deposition using formic acid as the reducing agent based on the empirical model. Deviations from the model were found when hydrazine was used as the reducing agent due to the formation of solution phase ruthenium complexes with hydrazine.

Platinum-Glass Composite Electrode for Fuel Cell Applications

Thin-film electrodes for a low-power direct methanol fuel cell (DMFC) were prepared by incorporating carbon-supported Pt nanoparticles (Pt/C) into a silicon dioxide glass matrix. The SiO 2 matrix was prepared via a sol-gel technique where tetraethyl orthosilicate (TEOS) was hydrolyzed by H 2 O in the presence of methanol. The Pt/C was stirred into the sol and the resulting mixture was applied to a glass membrane substrate and cured. The resulting films were ∼ 2 μm thick. Scanning electron microscopy (SEM) images indicate that the Pt/C was well dispersed, forming glass-separated conductive islands with sheet resistances in excess of 5000 Ω/□. The catalyst islands were interconnected into a conductive sheet by electrolessly depositing platinum from an aqueous plating bath. The Pt/C-SiO 2 glass composite thin-film electrodes showed high methanol oxidation peak currents of ∼ 180 mA/cm 2 when immersed in 0.5 M H 2 SO 4 , 0.5 M methanol electrolyte. The composite electrode was also applied to the anode of a 1 cm 2 passive DMFC and compared to an equivalent passive DMFC with a traditional Nafion-based Pt anode electrode with 10 M MeOH at room temperature. The composite electrode DMFC showed a 50 mV higher open-circuit voltage than the Nafion electrode cell, and the current density was also modestly improved.

Deposition of PtxRu1-x Catalysts for Methanol Oxidation in Micro Direct Methanol Fuel Cells

Platinum-ruthenium electrodes (PtxRu1-x) have been prepared by elec- trochemical and electroless deposition and investigated as catalysts for the oxidation of methanol in acidic solutions. PtxRu1-x deposits were electrochemically deposited from acidic chloride electrolytes at potentials between -0.46 and 0.34 V (vs. NHE). The composition of the electrodeposit was estimated by energy dispersive X-ray spectroscopy and is a strong function of the electrode potential. An empirical model for the deposition process is presented and kinetic parameters are estimated and dis- cussed. Also, the methanol oxidation activity of the PtxRu1-x catalysts was character- ized by cyclic voltammetry in 1.0 M CH3OH, 1.0 M H2SO4 solutions. Electroless PtxRu1-x samples were prepared in a modified Leaman bath with hydrazine dihydrochloride as the reducing agent. The kinetic results for the electro- chemical deposition of PtxRu1-x were directly applied and the deposition potential was estimated as approximately 0.40 V.

Nanostructured composites for high energy batteries and supercapacitors

High power energy storage devices, such as Li-ion batteries and supercapacitors, are critical for the development of zero-emission electric vehicles, large scale smart grid, energy efficient ships and locomotives, wearable devices and portable electronics. This review will focus on our progress with the developments of nanocomposite electrodes capable to improve both the energy and power storage characteristics of the state of the art devices. We review recent advancements in ultra-high capacity conversion-type anodes and cathodes for Li ion batteries as well as carbon-metal oxide and carbon-conductive polymer (nano)composite electrodes for supercapacitors. Various routes to overcome existing challenges will be discussed, including various solution deposition techniques, atomic layer deposition (ALD), chemical vapor deposition (CVD) and electro-deposition. Several designs and implementations of multi-functional electrodes will also be presented.

Lithium Sulfide Cathodes: A Hierarchical Particle–Shell Architecture for Long-Term Cycle Stability of Li2S Cathodes (Adv. Mater. 37/2015)

Hierarchical Li2 S-carbon-nanocomposite particles are synthesized by G. Yushin and co-workers, as described on page 5579, using a simple solution-based method followed by vapor deposition. The multiscale and multilevel protection enabled by the proposed architecture prevents mechanical degradation and polysulfide dissolution in lithium-sulfur batteries. The proposed hierarchical particle-shell design can be effectively utilized for a variety of other conversion-type cathode materials.