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Choi, Jihwan,Jeong, Tae-Gyung,Lee, Donggeun,Oh, Si Hyoung,Jung, Yousung,Kim, Yong-Tae Elsevier 2019 Journal of Power Sources Vol.435 No.-
<P><B>Abstract</B></P> <P>The heterogeneous catalysis is widely recognized to promote polysulfide fragmentations can play a key role for the rate capability enhancement of lithium sulfur batteries (LSB). In this study, we report the most active catalysts (Ta<SUB>2</SUB>O<SUB>5</SUB>) for LSB which showed an enhanced rate capability (914 mAh g<SUP>−1</SUP> at 5 C) in comparison with the previously reported catalysts, TiN (865 mAh g<SUP>−1</SUP> at 5 C). The addition of Ta<SUB>2</SUB>O<SUB>5</SUB> into the carbon-coated separator facilitates the chemical disproportionation of reaction intermeditates, Li<SUB>2</SUB>S<SUB>4</SUB> into Li<SUB>2</SUB>S<SUB>8</SUB> and Li<SUB>2</SUB>S effectively, which leads to the greatly lengthened second plateau in discharge profile. The heterogeneous catalysis of Ta<SUB>2</SUB>O<SUB>5</SUB> is clearly verified with the UV–Vis spectra directly showing the change of Li<SUB>2</SUB>S<SUB>4</SUB> concentration and the battery performance tests without any active elemental sulfur.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Heterogeneous catalysts as reaction intermediates can enhance battery performance. </LI> <LI> Ta<SUB>2</SUB>O<SUB>5</SUB> is one of the most promising heterogeneous catalysts for Li-S battery. </LI> <LI> Strong chemical bonding between S and Ta<SUB>2</SUB>O<SUB>5</SUB> result in catalytic effect of Ta<SUB>2</SUB>O<SUB>5</SUB>. </LI> <LI> Ta<SUB>2</SUB>O<SUB>5</SUB> can accelerate slow disproportionation reaction of LiPS. </LI> <LI> Modified separator can help achieving long cycle life and enhanced rate capability. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Resource Allocation for H-FDD OFDMA Systems
Choi, Jihwan P.,Jaesub Shin,Jungwon Lee,Hui-Ling Lou,Ji-Woong Choi IEEE 2014 IEEE communications letters Vol.18 No.7
<P>Half-frequency-division duplexing (H-FDD) has recently attracted attention since it can serve as a cost-effective solution for machine-to-machine (M2M) communication networks. This letter proposes a method to jointly optimize resource allocation, mobile station grouping, and frame partitioning for H-FDD orthogonal frequency-division multiplexing access systems. We develop a near-optimal and low-complexity algorithm that is also applicable to frequency-division duplexing and time-division duplexing as special cases. Simulation results demonstrate that our algorithm with reduced computation can provide performance close to that of the optimal but time-consuming exhaustive search.</P>
Cross-Layer Routing and Scheduling for Onboard Processing Satellites with Phased Array Antenna
Choi, Jihwan P.,Chang, Seok-Ho,Chan, Vincent W. S. IEEE 2017 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.16 No.1
<P>The advanced multibeam satellite equipped with phased array antenna and solid state power amplifiers can generate flexible beams, by managing interbeam interference to serve a very large number of users effectively over its coverage area. Onboard processing (OBP) functionality can enhance the flexibility of a large-scale antenna by speeding up computational processes and saving precious radio link spectrum. In this paper, we derive a cross-layer OBP design of switching/routing, beamforming, and user scheduling as taking advantage of fine spatial resolution capability of phased array antenna satellites. We evaluate tradeoff between OBP computational complexity and throughput performance, showing that the additional complexity of an increased number of switch ports and phased array antenna gain patterning is compensated by high throughput gain achieved by mitigating interference. Our analysis shows that throughput gain for the next generation satellite system can be as high as 40, compared with the conventional multiple beam antenna with travelling wave tube amplifiers, and that beamforming is critical for achieving high spectral efficiency in the crowded service area. We then investigate the impacts of onboard switching and phased array antenna beamforming to practical routing protocols, such as open short path first and routing information protocol.</P>
Tungsten Carbide as a Highly Efficient Catalyst for Polysulfide Fragmentations in Li-S Batteries
Choi, Jihwan,Jeong, Tae-Gyung,Cho, Byung Won,Jung, Yousung,Oh, Si Hyoung,Kim, Yong-Tae American Chemical Society 2018 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.122 No.14
<P>The sluggish disproportionation of short-chain lithium polysulfides, Li<SUB>2</SUB>S<SUB><I>x</I></SUB>, is known to be one of the major causes to limit the rate capability of lithium-sulfur batteries. Herein, we report that tungsten carbide not only affords strong sulfiphilic surface moieties but also provides an efficient catalysis to enhance the polysulfide fragmentation, leading to a drastic improvement in the electrode kinetics. We show that tungsten carbide acts as a superb anchoring material for the long-chain polysulfide and also promotes the dissociation of short-chain polysulfide during the electroreduction process. This leads to a high-rate performance of the composite cathode loaded with tungsten carbide, delivering a markedly enhanced discharge capacity of 780 mA h g<SUP>-1</SUP> at a high current rate of 5 C, when it is applied with a combination of a carbon-coated separator for the polysulfide confinement. Hence, this work presents a new strategic approach for a high-power lithium-sulfur battery.</P> [FIG OMISSION]</BR>
Challenges for efficient and seamless space-terrestrial heterogeneous networks
Choi, Jihwan P.,Changhee Joo Institute of Electrical and Electronics Engineers 2015 IEEE communications magazine Vol.53 No.5
<P>By interconnecting satellites in the sky with ground stations on earth, space-terrestrial networks can extend the coverage area and increase the throughput for both commercial and nonprofit applications. To realize the seamless and economic sky-to-earth networks, we still have to advance the technologies that bridge different types of channels, and find killer-applications that are essential for our life. In this article we review the current state-of-the-art technologies of multibeam satellites and relaying as fundamental cornerstones, and then address the issues of low-latency random access, mobility support, and multipath protocols for realizing a seamless heterogeneous network. We then illustrate two impactful applications: for emergency communication as a near-future and non-profit application, and for simultaneous transmission of information and power as a long-term application.</P>
Business Strategy and Audit Efforts - Focusing on Audit Report Lags: An Empirical Study in Korea
Jihwan CHOI,Hyung Ju PARK 한국유통과학회 2021 The Journal of Asian Finance, Economics and Busine Vol.8 No.7
This study examines the association between a firm’s business strategy and audit report lags. This study employs 5,072 firm-year observations from 2015 to 2019. Our sample comprises all of the firms listed on the Korea Composite Stock Price Index (KOSPI) market and Korea Securities Dealers Automated Quotation (KOSDAQ). We perform OLS regression analysis to test our hypothesis. The OLS regression analysis was conducted through the SAS and STATA programs. We find that business strategy is positively associated with audit report lags. Especially, we find that defender firms are negatively associated with audit report lags. The findings of this study suggest that prospectorlike firms would increase their performance uncertainty as well as audit risk. Therefore, prospector-like firms interfere with the efficient audit procedures of auditors. On the other hand, our findings indicate that defender-like firms would decrease their performance uncertainty as well as an audit risk because they focus on simple product lines and cost-efficiency. For this reason, auditors will be able to carry out the audit procedures much more easily. Our results present that a prospector-like business strategy degrades audit effectiveness as it exacerbates a company’s financial risk, willingness to accept uncertainty, and the complexity of organizational structure.
Material and Structure analysis and Recycling Processing for Additional Obligation WEEE in 2020
( Jonghyun Choi ),( Insang Jung ),( Jihwan Park ),( Wonhee Choi ) 한국폐기물자원순환학회(구 한국폐기물학회) 2019 ISSE 초록집 Vol.2019 No.-
In 2020, EPR items and categories will be changed. Items are to be 50 from 27 and new categories will be determined. New categories could be similar to the WEEE 2 categories. There are 22 new items in EPR items such as navigation, scanner, electric massage apparatus, treadmill and sewing machine, etc. However, we don’t have any research (weight, composition and hazardous material etc.) about these items. It is necessary to analyse the recycling process (methods, standards, etc.) of the obligation WEEE (EPR items) to be recycled after 2020 (22 items to be added) for proper recycling. Therefore, we dismantled22 kinds of items manually (2~3 products each items) to analyze weight, composition, hazardous materials etc. As a result, average composition of items was Metal (32.6%), Plastic (3.69%), Component (24.9%) and Waste (5.6%). With those results, we found out that the most valuable item was treadmill (9,170 won/ea) and the least item was sewing machine (- 6,289 won/ea). It took the longest time to dismantle sewing machine. Moreover, we suggest efficient recycling method which is better: mechanical process or manual process each item. Most item could be recycled by mechanical process, but several items could not be used mechanical process, because of size, composition, structure and hazardous material (i.e., refrigerant). As this research is initial stage for new items, further research is necessary.