A Strategy for Low Thermal Conductivity and Enhanced Thermoelectric Performance in SnSe: Porous SnSe_1–<i>x</i>S_<i>x</i> Nanosheets

Ju, Hyun,Kim, Myeongjin,Park, Dabin,Kim, Jooheon American Chemical Society 2017 Chemistry of materials Vol.29 No.7

<P>A higher figure of merit (ZT) can be achieved for tin selenide (SnSe)-based thermoelectric materials by significantly reducing the thermal conductivity (kappa) via three promising strategies: substitution with isoelectric atoms, exfoliation of nanosheets (NSs) from a bulk ingot, and chemical transformation of the material into a porous structure. Specifically, SnSe1-xSx NSs are prepared from bulk ingots by hydrothermal Li intercalation and subsequent exfoliation. The substitution of S atoms into SnSe and the fabrication of SnSe1-xSx NSs contribute to the scattering of phonons at a number of atomic disorders and nanosized boundaries, leading to effective reduction of the lc value and an improved ZT. The introduction of porosity into the material through the chemical transformation process results in further reduction of kappa, which leads to a higher ZT. The fabricated porous SnSe0.8S0.2 NS has a maximal ZT value of 0.12 at 310 K, which is significantly higher than that of pristine SnSe.</P>

A facile fabrication of n-type Bi₂Te₃ nanowire/graphene layer-by-layer hybrid structures and their improved thermoelectric performance

Ju, Hyun,Kim, Myeongjin,Kim, Jooheon Elsevier 2015 CHEMICAL ENGINEERING JOURNAL -LAUSANNE- Vol.275 No.-

<P><B>Abstract</B></P> <P>n-Type Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowire/graphene composite films were prepared with a facile wet chemical synthesis and sintering process, and the effect of Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowire content on the thermoelectric properties of the composite films were investigated. The one-dimensional Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowires were homogeneously intercalated and dispersed between the prepared graphene layers, forming the Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowire/graphene layer-by-layer hybrid structure. These composite films showed enhanced thermoelectric properties compared to the pristine graphene film fabricated with the same methods but without the Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowires. This was attributed to both the large Seebeck coefficient, which is intrinsic to the Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowires, and the reduced thermal conductivity, which is attributed to the exceptional phonon scattering at the interfaces between the nanowires and graphene layers. The maximum thermoelectric efficiency (<I>ZT</I>) of the Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowire/graphene composites was achieved with a Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowire content of 20wt.%. This composite had a dramatically improved <I>ZT</I> value of 0.2, which was ∼27 times larger than that of the pristine graphene film.</P> <P><B>Highlights</B></P> <P> <UL> <LI> n-Type Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowire/graphene layer-by-layer composites are fabricated. </LI> <LI> Synthesized composite films show high thermoelectric performance. </LI> <LI> Bi<SUB>2</SUB>Te<SUB>3</SUB> nanowires contribute to the increase in Seebeck coefficient of the composite. </LI> <LI> Phonons are significantly scattered at the interfaces between nanowires and graphene. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Enhanced thermoelectric performance of highly conductive poly(3,4-ethylenedioxythiophene)/carbon black nanocomposites for energy harvesting

Ju, Hyun,Kim, Myeongjin,Kim, Jooheon Elsevier 2015 MICROELECTRONIC ENGINEERING Vol.136 No.-

<P><B>Abstract</B></P> <P>The thermoelectric performance of <I>para</I>-methylbenzenesulfonate (<I>p</I>-MeBzs) doped highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT) can be improved by the use of carbon black fillers. Thermoelectric nanocomposites were prepared via chemical polymerization. Dodecylbenzenesulfonic acid (DBSA) was introduced before polymerization to act simultaneously as a surfactant for formation of micelles of carbon black and as a doping agent. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) were employed to characterize the morphology of PEDOT coated carbon black and PEDOT. Electrical conductivity of the composites improved with increase in weight percentage of carbon black from 0% to 30%. Extended chain conformations and increase in electron delocalization reduces the carrier hopping barriers. These contribute to the enhancement of charge carrier mobility. Although electrical conductivity is directly proportional to the increase in the filler content, Seebeck coefficient is more or less constant. Relatively small changes of thermal conductivity can be attributed to the phonon scattering effect in both the carbon black and the thermally insulating PEDOT layers. This study reports that the power factor of the composite was estimated to be 0.993μW/mK<SUP>2</SUP> for 10wt% filler content and was more than 1.7 times higher than that for pure PEDOT, and the maximum figure of merit (<I>ZT</I>) value was 0.0012 at room temperature.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Synthesized 0–30% carbon black/PEDOT composites show high thermoelectric efficiency. </LI> <LI> PEDOT layer coating the filler has an affinity to help its dispersion into the matrix. </LI> <LI> Strong conductive network between filler and matrix improves electrical conductivity. </LI> <LI> The Seebeck coefficient remains almost constant with filler load. </LI> <LI> Small changes in thermal conductivity are attributed to the phonon scattering effect. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Introduction of Co₃O₄ into activated honeycomb-like carbon for the fabrication of high performance electrode materials for supercapacitors

Kim, Myeongjin,Oh, Ilgeun,Ju, Hyun,Kim, Jooheon The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.13

<P>In this work, a three-dimensional hierarchical carbon framework/Co3O4 hybrid composite was fabricated. The three-dimensional hierarchical carbon framework was constructed by thermal vapor deposition on the silica nanosphere templates and etching these templates. The resulting carbon framework was activated using phosphoric acid to control its surface area and porosity. The degree of activation of the carbon framework was optimized by measuring the specific capacitance. The carbon framework electrode activated with 3 M phosphoric acid (HCCA(3)) exhibited the highest specific capacitance (134 F g(-1) at 10 mV s(-1)). Subsequently, Co3O4 was formed on the carbon framework via the hydrothermal method. The resulting product HCCA(3)/Co3O4 showed a dramatic enhancement in the specific capacitance (456 F g(-1) at 1 A g(-1)) compared with the pristine Co3O4 and HCCA(3) electrodes. The proposed HCCA(3)/Co3O4 composite can be used for the fabrication of high-performance electrodes.</P>

Highly efficient bifunctional catalytic activity of bismuth rhodium oxide pyrochlore through tuning the covalent character for rechargeable aqueous Na-air batteries

Kim, Myeongjin,Ju, Hyun,Kim, Jooheon The Royal Society of Chemistry 2018 Journal of materials chemistry. A, Materials for e Vol.6 No.18

<P>Na-air batteries have received significant attention as possible candidates for alternative battery systems due to their high specific energy density (1683 W h kg<SUP>−1</SUP>). However, the undesirable sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) limit the practicality of the production of rechargeable Na-air batteries. Recently, pyrochlore oxides (A2B2O7) have received great attention as effective bifunctional electrocatalysts. However, the comprehensive understanding of catalytic activity with the development of new pyrochlore catalysts is unsatisfactory due to the limited use of B-site cations. Here, we report the use of a novel nanocrystalline bismuth rhodium oxide (Bi2Rh2O6.8) crystal with a pyrochlore structure as a bifunctional electrocatalyst. Moreover, the surface of Bi2Rh2O6.8was modified<I>via</I>phosphate-ion functionalization (P-Bi2Rh2O6.8) to enhance its surface chemical reactivity, resulting in fast and efficient charge transfer with high ORR and OER activities. During electrocatalysis, the functionalized H2PO4<SUP>−</SUP>ion can not only significantly enhance the surface reactivity for a fast and efficient electron/charge-transfer reaction but also facilitate the oxidation of Bi and Rh ions and boost the electron donation by improving the electron transport. Finally, the first successful translation of the bifunctional electrocatalytic activities of P-Bi2Rh2O6.8to a practical device, an aqueous Na-air battery, was demonstrated.</P>

Single crystalline thallium rhodium oxide pyrochlore for highly improved round trip efficiency of hybrid Na-air batteries

Kim, Myeongjin,Ju, Hyun,Kim, Jooheon The Royal Society of Chemistry 2018 Dalton Transactions Vol.47 No.42

<P>The development of bifunctional electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is essential for electrical energy storage systems (EES). Sodium-air (Na-air) batteries are emerging as a new type of EES, due to their high specific energy density and high sodium ion conductivity. However, the undesirable sluggish kinetics of the ORR and OER limit the practical production of rechargeable Na-air batteries. Although pyrochlore oxides (A2B2O7) exhibit great potential for highly-active bifunctional electrocatalysts, the lack of studies regarding A and B-site cations has hindered the development of new pyrochlore catalysts with a comprehensive understanding of the catalytic activity. Here, we demonstrate the use of a single crystalline thallium rhodium pyrochlore oxide (Tl2Rh2O7) as a highly efficient bifunctional electrocatalyst. The high catalytic activity of Tl2Rh2O7 can be attributed to the oxidation of Tl and Rh ions in Tl2Rh2O7 during the ORR and OER. The oxidized Tl and Rh ions can donate their electrons, which easily migrate to the surface and inner layers, following the low-resistance electron pathway during the electrocatalytic reaction. The hybrid Na-air batteries using the prepared Tl2Rh2O7 achieve greatly improved round-trip efficiency with a high power density for 50 cycles.</P>

Oxygen-doped porous silicon carbide spheres as electrode materials for supercapacitors

Kim, Myeongjin,Ju, Hyun,Kim, Jooheon The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.4

<P>Oxygen-containing functional groups were introduced onto the surface of the micro-and meso-porous silicon carbide sphere (MMPSiC) in order to investigate the relationship between the electric double layer properties and pseudo-capacitive properties; the degree of oxidation of MMPSiC was also optimized. Although the oxygenated surface functionalities can lead to a decrease in the surface area of MMPSiC, the oxygen functional groups attached to the external surface can participate in the redox reaction, resulting in the enhancement of the total super-capacitive performance. The MMPSiC electrode oxidized for 24 h exhibits a high charge storage capacity with a specific capacitance of 301.1 F g(-1) at a scan rate of 5 mV s(-1), with 86.8% rate performance from 5 to 500 mV s(-1) in 1 M KCl aqueous electrolyte. This outstanding capacitive performance of the MMPSiC electrode oxidized for 24 h can be attributed to the harmonious synergistic effect between the electric double layer capacitive contribution of MMPSiC and the pseudo-capacitive contribution of the oxygen-containing functional groups. These encouraging results demonstrate that the MMPSiC electrode oxidized for 24 h is a promising candidate for high performance electrode materials for supercapacitors.</P>

인도네시아 자카르타 고층 아파트 거주자의 주거행태 분석

박명진(Park, Myeongjin),주서령(Ju, Seo Ryeung) 한국주거학회 2019 한국주거학회 학술대회논문집 Vol.31 No.2

As part of a series studying high-rise apartments in Jakarta, this study aims to understand how people live in these apartments and how their design reflects the housing behavior of residents, thereby revealing how the behavior of Indonesian has been transformed along with their society and environmental contexts. A field survey conducted in January 2019 includes the measurement of 10 apartment and interviews with the residents using a semi-structured questionnaire. Most of the residents explained that they had moved into apartments as a consequence of traffic congestion on the way to work. A large proportion of them felt that, although apartments were smaller than landed houses, they were satisfied with the convenience of the public facilities provided by apartments. Among the diverse housing activities undertaken by residents, washing entailed the greatest discrepancy between behavior and spatial setting. Spaces for washing machines were not provided in four units, the residents of which had to use external laundry services. Based on a precedent study, Indonesian prefer natural drying under sunlight. However, in many cases, yards or utility spaces open to the outside air were either not provided or too small. It is ideal for activities to be elaborately reflected in housing design. However, in high-rise apartment plans in Jakarta, there are some discrepancies between housing design and behavior. To be accepted as popular housing typologies, high-rise apartments should be carefully designed to address tropical climatic conditions and to adhere to housing behavior in Indonesia.

Low methanol permeable crosslinked sulfonated poly(phenylene oxide) membranes with hollow glass microspheres for direct methanol fuel cells

Ahn, Kisang,Kim, Myeongjin,Kim, Kiho,Oh, Ilgeun,Ju, Hyun,Kim, Jooheon Elsevier 2015 Polymer Vol.56 No.-

<P><B>Abstract</B></P> <P>Organic/inorganic composite membranes based on sulfonated poly(phenylene oxide) (SPPO) and hollow glass microspheres (HGMs) were prepared for use as proton exchange membranes in direct methanol fuel cells (DMFCs). First, SPPO was successfully introduced onto the surfaces of the HGMs (SPPO-HGMs) to increase dispersion of SPPO-HGMs in the SPPO matrix. The SPPO composite membranes were fabricated with varying amounts of SPPO-HGMs. Then the composite SPPO membranes were crosslinked to decrease the methanol permeability and increase the mechanical properties, and the crosslinking time was controlled to determine the optimal crosslinking time. As a result, a 12 h crosslinking time at 80 °C was selected as the optimum. The SPPO-HGM composite membranes exhibited proton conductivities ranging from 0.0294 to 0.0201 S cm<SUP>−1</SUP> and methanol permeability ranging from 5.20 × 10<SUP>−7</SUP> to 1.99 × 10<SUP>−7</SUP> cm<SUP>2</SUP> s<SUP>−1</SUP> at 20 °C.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Crosslinked Sulfonated poly(phenylene oxide) is used as matrix of proton exchange membrane. </LI> <LI> SPPO is introduced onto the surface of hollow glass microsphere via intermolecular interaction. </LI> <LI> Introduced SPPO onto the surface of HGM increase dispersion of HGM in the crosslinked SPPO matrix. </LI> <LI> HGM is used as filler to reduce the methanol permeability. </LI> <LI> Reduced methanol permeability leads to improved selectivity of SPPO membrane. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Fabrication of low-methanol-permeability sulfonated poly(phenylene oxide) membranes with hollow glass microspheres for direct methanol fuel cells

Ahn, Kisang,Kim, Myeongjin,Kim, Kiho,Ju, Hyun,Oh, Ilgeun,Kim, Jooheon Elsevier 2015 Journal of Power Sources Vol.276 No.-

<P><B>Abstract</B></P> <P>Organic/inorganic composite membranes, based on sulfonated poly(phenylene oxide) (SPPO) and hollow glass microspheres (HGMs), with various compositions are prepared for use as proton exchange membranes in direct methanol fuel cells (DMFCs). Reaction time between chlorosulfonic acid solution and PPO is controlled to improve proton conductivity of the SPPO membrane. As a result, SPPO at 38.2% sulfonation is selected as the optimum degree of sulfonation. Afterwards, SPPO is successfully introduced onto the surfaces of HGMs to increase their dispersion in the SPPO matrix. The ion exchange capacities (IEC) and proton conductivities of the membranes decrease with increasing amounts of the SPPO-HGMs, because of the decrease of ionic sites with increasing HGM content. The SPPO-HGM composite membranes exhibit proton conductivities ranging from 0.0350 to 0.0212 S cm<SUP>−1</SUP> and low methanol permeability ranging from 1.02 × 10<SUP>−6</SUP> to 3.41 × 10<SUP>−7</SUP> cm<SUP>2</SUP> s<SUP>−1</SUP> at 20 °C. Furthermore, the SPPO-HGM 9 wt%/SPPO membrane presents a maximum power density of 81.5 mW cm<SUP>−2</SUP> and open circuit voltage of 0.70 V.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Sulfonated poly(phenylene oxide) is used as matrix of proton exchange membrane. </LI> <LI> SPPO is introduced onto the surface of hollow glass microsphere via intermolecular interaction. </LI> <LI> Introduced SPPO onto the surface of HGM increase dispersion of HGM in the SPPO matrix. </LI> <LI> HGM is used as filler to reduce the methanol permeability. </LI> <LI> Reduced methanol permeability leads to improved selectivity of SPPO membrane. </LI> </UL> </P>