Sang-Cheol Han

Effect of Multiwalled Carbon Nanotubes on Electrochemical Properties of Lithium/Sulfur Rechargeable Batteries

In order to bestow high electronic conductivity and prevent dissolution of sulfur into the electrolyte, multiwalled carbon nanotubes (MWNTs) were prepared by thermal chemical vapor deposition as an inactive additive material for elemental sulfur positive electrodes for lithium/sulfur rechargeable batteries. The initial discharge capacity of elemental sulfur positive electrode with MWNT is 485 mAh/g sulfur at 2.0 V vs. The cycle life and rate capability of sulfur cathode is increased with addition of MWNT. The MWNT shows a vital role on polysulfide adsorbtion and is a good electric conductor for a sulfur cathode.

Effects of Nanosized Adsorbing Material on Electrochemical Properties of Sulfur Cathodes for Li/S Secondary Batteries

In order to prevent polysulfide dissolution into liquid electrolytes and to promote the Li/S redox reaction (16Li + S 8 ↔ Li 2 Sn ↔ Li 2 S), nanosized Mg 0.6 Ni 0.4 O, which has the catalytic effect of chemical bond dissociating and is expected to have an adsorbing effect due to the effect of retaining liquid electrolyte of MgO in a Li/iron sulfide secondary battery, 16 was prepared by the sol-gel method as an electrochemically inactive additive for an elemental sulfur cathode for Li/S rechargeable batteries. The Li/S battery using an elemental sulfur cathode with a nanosized Mg 0.6 Ni 0.4 O added showed the improvement of not only the discharge capacity but also cycle durability (maximum discharge capacity: 1185 mAh/g sulfur, C 50 /C 1 = 85%).The rate capability of the sulfur cathode was also increased with the addition of the nanosized Mg 0.6 Ni 0.4 O. From the msults. it is confirmad that the nanosized Mg 0.6 Ni 0.4 O had the polysulfide adsorbing effect and the catalytic elfect of promoting Lt/S redox reaction. Furthermore, it is found that the nanosized Mg 0.6 Ni 0.4 O also increased the porosity of the sulfur cathode.

Production of hydrogen from sodium borohydride in alkaline solution: development of catalyst with high performance

Abstract The hydrogen production from hydrolysis of sodium borohydride in alkaline solution has been extensively studied. As a result, we have studied that stylene-butadiene-rubber as a binder of catalyst electrode is very effective because their hydrophilic property promotes the infiltration of liquid fuel into the catalyst. The filamentary Ni mixed Co catalyst with superior performance of short initial waiting time and fast hydrolysis of sodium borohydride has been developed and showed a maximum hydrogen production rate of 96.3 ml / min g . Also because hydrogen gas can be generated at room temperature and has a high purity more than 99.99%, it can be directly used as a fuel to PEMFC.

Effects of Ti on the cycle life of amorphous MgNi-based alloy prepared by ball milling

Amorphous MgNi alloys have a critical problem of poor cyclic behavior in spite of their high capacity for the negative electrode of a Ni/MH rechargeable battery. In order to improve the cycle life of the alloys, the degradation mechanism of amorphous MgNi alloy is suppressed. It is found that the surface property of the MgNi alloy is a critical factor for improvement of cycle life because an increase of the thickness of the magnesium oxide layer on the alloy surface results in a decrease of the discharge capacity of the alloy. To prevent further oxidation of magnesium, both titanium surface coating and substitution methods are adopted. Ti substitution is found to be very effective for improving the cycle life while Ti surface coating proves to be hardly effective. Magnesium in the Mg50Ni50 alloy is replaced by titanium. The amount of titanium (x in Mg1−xTixNi1.0) is varied from 0.1 to 0.3. In the case of x=0.3 (Mg0.7Ti0.3Ni1.0), the alloy shows the best cycle life. To explain this phenomenon, both electrochemical and phenomenological analyses are conducted. Electrochemical impedance spectroscopy (EIS) analysis shows that the charge transfer resistance (Rct) between the Mg0.7Ti0.3Ni1.0 alloy and the electrolyte does not increase during charge–discharge cycles. It is also found by auger electron spectroscopy (AES) that the thickness of the surface oxide layer on the Mg0.7Ti0.3Ni1.0 alloy is thinner than that of the oxide layer on the MgNi alloy. At the same time, a nickel-enriched layer is found on the surface of the Mg0.7Ti0.3Ni1.0 alloy. X-ray photoelectron spectroscopy (XPS) also shows that magnesium in the Mg0.7Ti0.3Ni1.0 alloy exists in metallic state while titanium forms an oxide layer. These results indicate that the titanium oxide layer not only prevents further oxidation of magnesium, but also induces a nickel enriched layer on the alloy surface.

Carbon-Supported and Unsupported Pt Anodes for Direct Borohydride Liquid Fuel Cells

Investigations have been conducted on direct borohydride liquid fuel cells (DBFCs) based on the electio-oxidation of sodium borohydride, NaBH 4 . A comparative study on the use of carbon-supported and unsupported Pt anods catalysts for DBFCs has been made. The effects on anode and fuel cell performance of catalyst loading, binder content, fuel concentration, and pH of supporting solution dissolving NaBH 4 are studied. A maximum power density of 44.2 mW cm -2 to unsupported catalyst of 7 mg cm -2 (under room temperature and air breathing) has been obtained. Cell performance by using the anode with 1.50 mg cm -2 carbon-supported Pt anode catalyst is comparable to that by using the anode with 6 mg cm -2 unsupported Pt anode catalyst. It is found that the carbon-supported catalysts are more cost effective and have higher catalytic activity than the unsupported catalysts. The coulombic efficiencies calculated from the energy density (theoretical capacity 5880 vs. measured capacity) for unsupported and carbon-supported Pt anode catalysts are 62.3 and 68.1%, respectively. The DBFC developed in this work has better performance than the conventional fuel cells using hydrocarbon liquid fuels like methanol.

The thermodynamic properties of Ti–Zr–Cr–Mn Laves phase alloys

Abstract Several TiMn 2 -based C14 Laves phase alloys were prepared and their hydrogen storage properties were studied in order to develop a suitable material for metal hydride heat pump applications. The metal hydride should have a suitable plateau pressure of under 10 atm and low sloping. The plateau pressure was effectively decreased by the partial substitutions of Zr for Ti, while the slope was increased. It was found that the decrease of stoichiometry and the substitution of V, Cu were very effective in improving the sloping properties, and the improvement of sloping properties was mainly attributed to the strain energy effect. After a careful balancing of the substitution effects of the alloying elements, it was found that (Ti 0.8 Zr 0.2 ) 1.05 Mn 0.8 Cr 1.05 V 0.05 Cu 0.1 alloy showed good plateau characteristics with very small hysteresis and sloping.

The development of compressor-driven metal hydride heat pump (CDMHHP) system as an air conditioner

Abstract The compressor-driven metal hydride heat pump (CDMHHP) using commercialized oil-type compressor is built to demonstrate the practical applicability of metal hydride air-conditioning system. Extensive research has been carried out on the design, construction and operating performance of the system. The experimental results indicate that the present CDMHHP can produce a maximum cooling power of 353 kcal/kg - alloy h and an energy efficiency (COP) of 1.8 with a half-cycle time of 3 min . The minimum cooling temperature is 6°C at 7 Nm 3 / min airflow rate.

The electrochemical evaluation of ball milled MgNi-based hydrogen storage alloys

The electrochemical properties of amorphous MgNi-based hydrogen storage alloys synthesized by ball milling were evaluated. In order to improve the cycle life, the surface of the amorphous Mg50Ni50 alloy was coated with Ti, Al and Zr by ball milling. Among them Ti was very effective to improve cycle life. And two kinds of MgNi-based amorphous alloys were designed by the substitution of Ti and other elements for Mg of MgNi-based alloys, which were composed of four components. Thus, the cycle life of electrodes with these quaternary amorphous alloys was strongly improved.

Simulation of the metal hydride heat pump system with the single and double reactors

Abstract The main obstacles for the practical use of a metal hydride heat pump (MHHP) system are a low-heat transport rate of the metal hydride and a relatively large irreversible heat loss during the hydriding and dehydriding processes. Very extensive investigations are required to overcome these obstacles, which are both time-consuming and costly. In order to save both time and cost, computer simulation can be introduced in the development of such MHHP systems. In particular, simulation can provide useful technical knowledge for improving the system by optimum reactor construction and identification of optimum operational parameters. In this work, a simulation method for the MHHP system, based on the three-vector equilibrium space of temperature, concentration and enthalpy, is proposed and tested practically. Using a computer simulation program for a single reactor MHHP system driven by a given pressure or flow rate of hydrogen, it was possible to describe equilibrium properties of hydride as they are changed by variations in temperature and concentration. Simulation of the MHHP system with two reactors, i.e. one metal hydride pair, allowed the thermodynamics of a conventional system to be described theoretically.

Hydrogen storage properties of TiMn2-based alloys for metal hydride heat pump

Abstract Several TiMn 2 -based C14 Laves phase alloys were prepared and their hydrogen storage properties were studied in order to develop a suitable material for metal hydride heat pump applications. The metal hydride should have a suitable plateau pressure under 10 atm and low sloping. The plateau pressure was effectively decreased by the partial substitutions of Zr for Ti, while the slope was increased. It was found that the decrease of stoichiometry and the substitution of V, Cu were very effective in improving the sloping properties and the improvement of sloping properties was mainly attributed to the strain energy effect. After careful balancing of the substitution effects of the alloying elements, it was found that (Ti 0.8 Zr 0.2 ) 1.05 Mn 0.8 Cr 1.05 V 0.05 Cu 0.1 alloy showed good plateau characteristics with considerably small hysteresis and sloping.