Byung Won Cho

Continuous determination of biochemical oxygen demand using microbial fuel cell type biosensor

A mediator-less microbial fuel cell (MFC) was used as a biochemical oxygen demand (BOD) sensor in an amperometric mode for real-time wastewater monitoring. At a hydraulic retention time of 1.05 h, BOD values of up to 100 mg/l were measured based on a linear relationship, while higher BOD values were measured using a lower feeding rate. About 60 min was required to reach a new steady-state current after the MFCs had been fed with different strength artificial wastewaters (Aws). The current generated from the MFCs fed with AW with a BOD of 100 mg/l was compared to determine the repeatability, and the difference was less than 10%. When the MFC was starved, the original current value was regained with a varying recovery time depending on the length of the starvation. During starvation, the MFC generated a background level current, probably due to an endogenous metabolism.

Electrochemical properties of PAN-based carbon fibers as anodes for rechargeable lithium ion batteries

Abstract Polyacrylonitrile(PAN)-based carbon fibers were tested as anodes for lithium ion rechargeable batteries. PAN-based fibers were first stabilized under tension in air at about 200°C (stabilization tension) and then carbonized in different gas environments (carbonization atmospheres) at heat treatment temperatures (HTT) between 700 and 1500°C. The carbon fiber electrodes were prepared at various conditions using the stabilized PAN fibers and then their electrochemical characteristics were investigated. The PAN-based carbon fiber prepared at an oxidative stabilization tension of ca. 10 MPa showed the highest discharge capacity in our experimental range. We found that the effective diffusion coefficient of lithium in the carbon fiber electrode was influenced by the carbonization environment employed. The electrochemical intercalation process depended on mass transfer of lithium into carbon layer which is rate-determining during the electrode charge–discharge process. The effect of HTT on discharge capacity varied depending on the combined effect of both the amount of intercalation sites available and the electric conductivity of the carbon fiber used.

Enhanced charge collection and reduced recombination of CdS∕TiO2 quantum-dots sensitized solar cells in the presence of single-walled carbon nanotubes

This paper reports the modification of CdS∕TiO2 quantum-dot sensitized solar cells by using single-walled carbon nanotubes (SWCNTs) on indium-doped tin oxide (ITO) electrodes. The presence of the SWCNT layers on an ITO electrode increased the short-circuit current under the irradiation condition and also reduced the charge recombination process under the dark condition. The power conversion efficiency of CdS∕TiO2 on ITO increased 50.0% in the presence of SWCNTs due to the improved charge-collecting efficiency and reduced recombination.

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide

Zn-ion batteries are emerging energy storage systems eligible for large-scale applications, such as electric vehicles. These batteries consist of totally environmentally-benign electrode materials and potentially manufactured very economically. Although Zn/α-MnO2 systems produce high energy densities of 225 Wh kg−1, larger than those of conventional Mg-ion batteries, they show significant capacity fading during long-term cycling and suffer from poor performance at high current rates. To solve these problems, the concrete reaction mechanism between α-MnO2 and zinc ions that occur on the cathode must be elucidated. Here, we report the intercalation mechanism of zinc ions into α-MnO2 during discharge, which involves a reversible phase transition of MnO2 from tunneled to layered polymorphs by electrochemical reactions. This transition is initiated by the dissolution of manganese from α-MnO2 during discharge process to form layered Zn-birnessite. The original tunneled structure is recovered by the incorporation of manganese ions back into the layers of Zn-birnessite during charge process.

Radio-Frequency Magnetron Sputtering Power Effect on the Ionic Conductivities of Lipon Films

gas atmosphere with different radio-frequency magnetronsputtering power from 80 to 160 W with 20 W step increase. Lipon films deposited at lower sputtering power showed higher ionicconductivities than the films deposited at higher sputtering power. The results of impedance measurements showed that nitrogenincorporation into the glass structure increased the ionic conductivity and this nitrogen content in the Lipon films increased as thesputtering power decreased. In addition, the Auger electron spectroscopy depth profile showed that the increased nitrogen contentin the Lipon films was not the result of the target surface poisoning effect but the result of reactive incorporation of nitrogen.© 2001 The Electrochemical Society. @DOI: 10.1149/1.1420926# All rights reserved.Manuscript submitted May 29, 2001; revised manuscript received September 21, 2001. Available electronically November 8,2001.

Composite polymer electrolytes reinforced by non-woven fabrics

Composite electrolytes composed of a blend of polyethylene glycol diacrylate (PEGDA), poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) together with a non-woven fabric have been prepared by means of ultra-violet cross-linking. As the non-woven fabric serves as a mechanical support medium, the composite electrolyte has good integrity up to an initial liquid electrolyte uptake of 1000% (ethylene carbonate (EC)–dimethyl carbonate (DMC)–ethylmethyl carbonate (EMC)–LiPF6). The ionic conductivity of the composite electrolytes reaches 4.5 mS cm −1 at an ambient temperature of around 18 ◦ C and are electrochemically stable up to about 4.8 V versus Li/Li + . The conductivity and interfacial resistance remain almost constant even at 80 ◦ C. Scanning electron micrographs show that the high-temperature behavior is associated with structural stability that is induced by chain entanglement between PVdF, PMMA and PEGDA network. A MCMB|LiCoO 2 cell using the composite electrolytes retains >97% of its initial discharge capacity after 100 cycles at the C/2 rate (150 mA), and delivers more than 80% of full capacity with an average load voltage of 3.6 V at the 2C rate. The cell also shows much better cycle-life than one with a PVdF-coated composite electrolyte at high temperatures because of the better liquid electrolyte retention capability.

Controlling the intercalation chemistry to design high-performance dual-salt hybrid rechargeable batteries

We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg(2+)/Li(+) rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg(2+) and Li(+) co-insertion into the Mo6S8 cathode host using density functional theory calculations, we show that there is a threshold Li(+) activity for the pristine Mo6S8 cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature.

Stabilization of oxygen-deficient structure for conducting Li4Ti5O12-δ by molybdenum doping in a reducing atmosphere.

Li4Ti5O12 (LTO) is recognized as being one of the most promising anode materials for high power Li ion batteries; however, its insulating nature is a major drawback. In recent years, a simple thermal treatment carried out in a reducing atmosphere has been shown to generate oxygen vacancies (VO) for increasing the electronic conductivity of this material. Such structural defects, however, lead to re-oxidization over time, causing serious deterioration in anode performance. Herein, we report a unique approach to increasing the electronic conductivity with simultaneous improvement in structural stability. Doping of LTO with Mo in a reducing atmosphere resulted in extra charges at Ti sites caused by charge compensation by the homogeneously distributed Mo6+ ions, being delocalized over the entire lattice, with fewer oxygen vacancies (VO) generated. Using this simple method, a marked increase in electronic conductivity was achieved, in addition to an extremely high rate capability, with no performance deterioration over time.

An open-framework iron fluoride and reduced graphene oxide nanocomposite as a high-capacity cathode material for Na-ion batteries

Cathode materials with high capacity and good stability for rechargeable Na-ion batteries (NIBs) are few in number. Here, we report a composite of electrochemically active iron fluoride hydrate and reduced graphene oxide (rGO) as a promising cathode material for NIBs. Phase-pure FeF3·0.5H2O is synthesized by a non-aqueous precipitation method and a composite with rGO is prepared to enhance the electrical conductivity. The encapsulation of FeF3·0.5H2O nanoparticles between the rGO layers results in a lightweight and stable electrode with a three-dimensional network. The composite material delivers a substantially enhanced discharge capacity of 266 mA h g−1 compared to 158 mA h g−1 of the bare FeF3·0.5H2O at a current density of 0.05 C. This composite also shows a stable cycle performance with a high capacity retention of >86% after 100 cycles, demonstrating its potential as a cathode material for NIBs.

Manipulating interfaces in a hybrid solar cell by in situ photosensitizer polymerization and sequential hydrophilicity/hydrophobicity control for enhanced conversion efficiency

The polyacetylene photosensitizer with quaternary pyridinium salts was layered on CdS nanoparticles films by in situ polymerization of 2-ethynylpyridine and 4-bromobutyric acid. The hydrophilic nature of the polyacetylene is shown to enhance the interfacial contact and electrical coupling between hydrophilic CdS and the polymer. The hydrophilicity of the polymer was modified toward hydrophobicity by anion exchange in order to adequately layer the hydrophobic poly(3-hexylthiophene) by spin coating, power-conversion efficiency 1.18% (AM1.5, I=100mW∕cm2).