Jieun Hwang

A simple, one-pot synthesis of molybdenum oxide-reduced graphene oxide composites in supercritical methanol and their electrochemical performance

A simple and green supercritical methanol (scMeOH) route is developed to tightly anchor molybdenum oxide (MoO2) nanoparticles on reduced graphene oxide (RGO). In scMeOH, graphene oxide is reduced, and MoO2 nanoparticles with sizes of 10–20 nm are simultaneously deposited on the basal plane of RGO in a short time without using any reducing agents or additives. When tested as an anode in lithium ion batteries, the MoO2–RGO composites show enhanced electrochemical performance compared to bare MoO2. The composite with a MoO2 loading of 37.0 wt% delivers a high reversible discharge capacity of 793 mA h g−1 at 50 mA g−1 and an excellent rate performance of 205 mA h g−1 at 2.5 A g−1. After 100 cycles of high rate testing of up to 50 A g−1, the MoO2–RGO composite recovers most of its initial capacity. The improved electrochemical performance of MoO2–RGO can be attributed to the tight anchoring of nanosized MoO2 on RGO and the mesoporous structure of the composite. Consequently, the transport length of Li diffusion into the MoO2 phase is shortened, charge transfer kinetics at the electrode–electrolyte interface is facilitated, and the volume expansion associated with the conversion reaction can be accommodated.

Carbon with expanded and well-developed graphene planes derived directly from condensed lignin as a high-performance anode for sodium-ion batteries

In this study, we demonstrate that lignin, which constitutes 30-40 wt % of the terrestrial lignocellulosic biomass and is produced from second generation biofuel plants as a cheap byproduct, is an excellent precursor material for sodium-ion battery (NIB) anodes. Because it is rich in aromatic monomers that are highly cross-linked by ether and condensed bonds, the lignin material carbonized at 1300 °C (C-1300) in this study has small graphitic domains with well-developed graphene layers, a large interlayer spacing (0.403 nm), and a high micropore surface area (207.5 m2 g-1). When tested as an anode in an NIB, C-1300 exhibited an initial Coulombic efficiency of 68% and a high reversible capacity of 297 mA h g-1 at 50 mA g-1 after 50 cycles. The high capacity of 199 mA h g-1 at less than 0.1 V with a flat voltage profile and an extremely low charge-discharge voltage hysteresis (<0.03 V) make C-1300 a promising energy-dense electrode material. In addition, C-1300 exhibited an excellent high-rate performance of 116 mA h g-1 at 2.5 A g-1 and showed stable cycling retention (0.2% capacity decay per cycle after 500 cycles). By comparing the properties of the lignin-derived carbon with oak sawdust-derived and sugar-derived carbons and a low-temperature carbonized sample (900 °C), the reasons for the excellent performance of C-1300 were determined to result from facilitated Na+-ion transport to the graphitic layer and the microporous regions that penetrate through the less defective and enlarged interlayer spacings.

Effect of Local Frost Characteristics on Thermal Performance of Fin Tube Heat Exchanger Under Frosting Condition

The present study aimed to measure and predict frost characteristics and heat transfer rate of fin-tube specimen, and predicted them of fin-tube heat exchanger. Test conditions were air temperature from 0.5 to 7 °C, absolute humidity from 3.18 to 4.01 g/kgDA, air velocity from 0.5 to 2.5m/s, and coolant temperature from −15 to −5 °C. The coolant was ethylene-glycol aqueous solution with concentration of 50%. Correlation equations for the average frost thickness and frost surface temperature were proposed. The proposed correlation equation for the average frost thickness and average frost surface temperature agreed with the measured data within the maximum deviation of 20% and 15%. The average frost thickness in front and front mass were predicted and compared with the measured data. They agreed within ±14 and ±23%, respectively. The heat transfer rate was predicted and compared with the measured data. They agreed within ±13% when the total heat transfer rate was over 70W.Copyright

Prediction of Non-Uniform Frost Distribution on a Fin Tube Heat Exchanger

Heat exchanger experiences frost on its surface when it operates below 0°C under heating condition of the heat pump. Since frost blocks air flow through the fin tube heat exchanger, it increases air-side pressure drop and deteriorates heat transfer rate of the heat exchanger. Prediction of the frost profiles on the heat exchanger is needed to minimize the unfavorable effect on the heat exchanger by frost. The present study predicts non-uniform frost distribution on the surface of fin-tube heat exchanger and shows its accuracy by comparing with measured profiles.Fin and tube heat exchanger for heat pump was considered for the frost prediction under practical refrigerant and air conditions. Non-uniform frost pattern was predicted by using segment by segment method of the heat exchanger. Heat transfer rate and exit temperature of air and refrigerant for each segment were calculated by applying e-NTU method. Air volume flow rate in the front of the heat exchanger was decreased as frost goes on. It was utilized for the prediction of the frost formation. Numerically predicted results were compared with measured local data. They agreed within ±10.4% under the ISO 5151 condition.Copyright