Publications by authors named "Yung-Eun Sung"

117 Publications

Amphiphilic Ti porous transport layer for highly effective PEM unitized regenerative fuel cells.

Sci Adv 2021 Mar 24;7(13). Epub 2021 Mar 24.

Center for Hydrogen, Fuel Cell Research, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea.

Polymer electrolyte membrane unitized regenerative fuel cells (PEM-URFCs) require bifunctional porous transport layers (PTLs) to play contradictory roles in a single unitized system: hydrophobicity for water drainage in the fuel cell (FC) mode and hydrophilicity for water supplement in the electrolysis cell (EC) mode. Here, we report a high-performance amphiphilic Ti PTL suitable for both FC and EC modes, thanks to alternating hydrophobic and hydrophilic channels. To fabricate the amphiphilic PTL, we used a shadow mask patterning process using ultrathin polydimethylsiloxane (PDMS) brush as a hydrophobic surface modifier, which can change the Ti PTL's surface polarity without decreasing its electrical conductivity. Consequently, performance improved by 4.3 times in FC (@ 0.6 V) and 1.9 times in EC (@ 1.8 V) from amphiphilic PTL. To elucidate reason for performance enhancement, discrete gas emission through the hydrophobic channels in amphiphilic PTL was verified under scanning electrochemical microscopy.
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http://dx.doi.org/10.1126/sciadv.abf7866DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7990350PMC
March 2021

Self-supported mesoscopic tin oxide nanofilms for electrocatalytic reduction of carbon dioxide to formate.

Chem Commun (Camb) 2021 Apr 1;57(28):3445-3448. Epub 2021 Mar 1.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea.

Here we report a self-supported SnO nanofilm prepared by a robust electrochemical process as an electrocatalyst for the CO reduction reaction. The SnO film had a large surface area originating from its nano-architecture and manifested high selectivity toward formate (over 60%), which resulted in CO-to-formate current density up to 33.66 mA cm that is among the state-of-the-art. We unveiled that the high performance of the SnO nanofilm is attributable to the presence of a metastable oxide under reductive conditions in addition to the abovementioned advantages.
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http://dx.doi.org/10.1039/d1cc00927cDOI Listing
April 2021

Methanol Tolerant Pt-C Core-Shell Cathode Catalyst for Direct Methanol Fuel Cells.

ACS Appl Mater Interfaces 2020 Oct 25;12(40):44588-44596. Epub 2020 Sep 25.

Department of Chemical Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Republic of Korea.

Methanol crossover is one of the largest problems in direct methanol fuel cells (DMFCs). Methanol passing from the anode to the cathode through the membrane is oxidized at the cathode, degrading the DMFC performance, and the intermediates of the methanol oxidation reaction (MOR) cause cathode catalyst poisoning. Therefore, it is essential to develop a cathode catalyst capable of inhibiting MOR while promoting the oxygen reduction reaction (ORR), which is a typical cathode reaction in DMFCs. In this study, a carbon-encapsulated Pt cathode catalyst was synthesized for this purpose. The catalyst was simply synthesized by heat treatment of Pt-aniline complex-coated carbon nanofibers. The carbon shell of the catalyst was effective in inhibiting methanol from accessing the Pt core, and this effect became more prominent as the graphitization degree of the carbon shell increased. Meanwhile, the carbon shell allowed O to permeate regardless of the graphitization degree, enabling the Pt core to participate in ORR. The synthesized catalyst showed higher performance and stability in single-cell tests under various conditions compared to commercial Pt/C.
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http://dx.doi.org/10.1021/acsami.0c07812DOI Listing
October 2020

Structural and Thermodynamic Understandings in Mn-Based Sodium Layered Oxides during Anionic Redox.

Adv Sci (Weinh) 2020 Aug 2;7(16):2001263. Epub 2020 Jul 2.

Center for Nanoparticle Research Institute for Basic Science (IBS) Seoul 08826 Republic of Korea.

A breakthrough utilizing an anionic redox reaction (O/O) for charge compensation has led to the development of high-energy cathode materials in sodium-ion batteries. However, its reaction results in a large voltage hysteresis due to the structural degradation arising from an oxygen loss. Herein, an interesting P2-type Mn-based compound exhibits a distinct two-phase behavior preserving a high-potential anionic redox (≈4.2 V vs Na/Na) even during the subsequent cycling. Through a systematic series of experimental characterizations and theoretical calculations, the anionic redox reaction originating from O 2p-electron and the reversible unmixing of Na-rich and Na-poor phases are confirmed in detail. In light of the combined study, a critical role of the anion-redox-induced two-phase reaction in the positive-negative point of view is demonstrated, suggesting a rational design principle considering the phase separation and lattice mismatch. Furthermore, these results provide an exciting approach for utilizing the high-voltage feature in Mn-based layered cathode materials that are charge-compensated by an anionic redox reaction.
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http://dx.doi.org/10.1002/advs.202001263DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7435253PMC
August 2020

Direct Synthesis of Intermetallic Platinum-Alloy Nanoparticles Highly Loaded on Carbon Supports for Efficient Electrocatalysis.

J Am Chem Soc 2020 Aug 5;142(33):14190-14200. Epub 2020 Aug 5.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea.

Compared to nanostructured platinum (Pt) catalysts, ordered Pt-based intermetallic nanoparticles supported on a carbon substrate exhibit much enhanced catalytic performance, especially in fuel cell electrocatalysis. However, direct synthesis of homogeneous intermetallic alloy nanocatalysts on carbonaceous supports with high loading is still challenging. Herein, we report a novel synthetic strategy to directly produce highly dispersed MPt alloy nanoparticles (M = Fe, Co, or Ni) on various carbon supports with high catalyst loading. Importantly, a unique bimetallic compound, composed of [M(bpy)] cation (bpy = 2,2'-bipyridine) and [PtCl] anion, evenly decomposes on carbon surface and forms uniformly sized intermetallic nanoparticles with a nitrogen-doped carbon protection layer. The excellent oxygen reduction reaction (ORR) activity and stability of the representative reduced graphene oxide (rGO)-supported L1-FePt catalyst (37 wt %-FePt/rGO), exhibiting 18.8 times higher specific activity than commercial Pt/C catalyst without degradation over 20 000 cycles, well demonstrate the effectiveness of our synthetic approach toward uniformly alloyed nanoparticles with high homogeneity.
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http://dx.doi.org/10.1021/jacs.0c05140DOI Listing
August 2020

Design considerations for lithium-sulfur batteries: mass transport of lithium polysulfides.

Nanoscale 2020 Jul;12(28):15466-15472

School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 08826, Republic of Korea. and Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea.

Irreversible loss of soluble lithium polysulfides (LiPSs) is a major obstacle deteriorating the performance of lithium-sulfur batteries. Multiple innovative approaches have recently been developed to resolve these LiPS issues. Melt-diffusion of sulfur into porous carbon is a representative solution for preventing the diffusion out of LiPSs, which aims to coordinate the sulfur on the electrochemically active site, accordingly. However, it has been overlooked that the mass transport motion of LiPSs has a crucial role in achieving high-performance. In this paper, we highlight the importance of the mass transport of soluble sulfur in the cathode structure by introducing various starting materials, i.e., solid sulfur using melt-diffusion and a catholyte, using 3-dimensional ordered macroporous carbon. The capacity of the sulfur cathode using melt-diffusion is well conserved in carbon with small pores because LiPSs are slowly diffused away, however, the catholyte derived sulfur cathode shows superior performance in carbon with large pores due to their rapid mass transport. The comparison with the four different combinations that control the pore size and mass transport reveals that proper selection of the initial state of starting materials using porous carbons demonstrates the optimal cell performance.
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http://dx.doi.org/10.1039/d0nr02936jDOI Listing
July 2020

Bi-MOF derived micro/meso-porous [email protected] nanoplates for high performance lithium-ion batteries.

Nanoscale 2020 Jul;12(28):15214-15221

School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 08826, Republic of Korea and Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea.

Micro/meso-porous [email protected] nanoplates are synthesized by pyrolyzing Bi-based metal-organic frameworks (MOFs) prepared by a microwave-assisted hydrothermal method to overcome huge volume expansion and pulverization of anode materials during battery operation. The [email protected] nanoplates are composed of ∼10-50 nm Bi nanoparticles in an amorphous carbon shell. The material shows very high capacity (556 mA h g-1) after 100 cycles at 100 mA g-1 and good cycling performance. Moreover, the [email protected] nanoplates perform well at high current densities and have excellent cyclic stability; their capacity is 308 mA h g-1 after 50 cycles and 200 mA h g-1 after 1000 cycles at 3000 mA g-1. The outstanding performance of this anode is due to the nanosized Bi and amorphous carbon shell. The nanosized Bi reduces the diffusion length of Li ions, while the amorphous carbon shell improves the electrical conductivity of the anode and also restrains the pulverization and aggregation of the metal during cycling. The proposed hierarchical micro/meso-porous materials derived from MOFs are a new type of nanostructures that can aid the development of novel Bi-based anodes for LIBs.
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http://dx.doi.org/10.1039/d0nr03219kDOI Listing
July 2020

Operando Identification of the Chemical and Structural Origin of Li-Ion Battery Aging at Near-Ambient Temperature.

J Am Chem Soc 2020 Aug 15;142(31):13406-13414. Epub 2020 Jul 15.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea.

Integrated with heat-generating devices, a Li-ion battery (LIB) often operates at 20-40 °C higher than the ordinary working temperature. Although macroscopic investigation of the thermal contribution has shown a significant reduction in the LIB performance, the molecular level structural and chemical origin of battery aging in a mild thermal environment has not been elucidated. On the basis of the combined experiments of the electrochemical measurements, Cs-corrected electron microscopy, and in situ analyses, we herein provide operando structural and chemical insights on how a mild thermal environment affects the overall battery performance using anatase TiO as a model intercalation compound. Interestingly, a mild thermal condition induces excess lithium intercalation even at near-ambient temperature (45 °C), which does not occur at the ordinary working temperature. The anomalous intercalation enables excess lithium storage in the first few cycles but exerts severe intracrystal stress, consequently cracking the crystal that leads to battery aging. Importantly, this mild thermal effect is accumulated upon cycling, resulting in irreversible capacity loss even after the thermal condition is removed. Battery aging at a high working temperature is universal in nearly all intercalation compounds, and therefore, it is significant to understand how the thermal condition contributes to battery aging for designing intercalation compounds for advanced battery electrode materials.
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http://dx.doi.org/10.1021/jacs.0c02203DOI Listing
August 2020

Sn(salen)-derived SnS nanoparticles embedded in N-doped carbon for high performance lithium-ion battery anodes.

Chem Commun (Camb) 2020 Jul;56(58):8095-8098

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea and School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea.

By simple pyrolysis of a tin salen complex [Sn(salen)] and sulfur powder at 700 °C, SnS nanoparticles with ∼20 nm thickness homogeneously embedded in nitrogen-doped carbon are prepared. When applied as lithium-ion battery anodes, the SnS/N-C nanocomposites exhibited long cycling stability and excellent rate capability.
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http://dx.doi.org/10.1039/d0cc02871aDOI Listing
July 2020

Understanding the Behaviors of λ-MnO in Electrochemical Lithium Recovery: Key Limiting Factors and a Route to the Enhanced Performance.

Environ Sci Technol 2020 07 1;54(14):9044-9051. Epub 2020 Jul 1.

School of Chemical and Biological Engineering and Institute of Chemical Processes (ICP), Seoul National University, Seoul 08826, Republic of Korea.

Recently developed electrochemical lithium recovery systems, whose operation principle mimics that of lithium-ion battery, enable selective recovery of lithium from source waters with a wide range of lithium ions (Li) concentrations; however, physicochemical behaviors of the key component-Li-selective electrode-in realistic operation conditions have been poorly understood. Herein, we report an investigation on a λ-MnO electrode during the electrochemical lithium recovery process with regards to the Li concentration in source water and operation rate of the system. Three distinctive stages of λ-MnO originating from different limiting factors for lithium recovery are defined with regard to the rate of Li supply from the electrolyte: depleted, transition, and saturated regions. By characterization of λ-MnO at different stages using diverse X-ray techniques, the importance of Li concentration in the vicinity of the electrode surface is revealed. On the basis of this understanding, increasing the density of the electrode/electrolyte interface is suggested as a realistic and general route to enhance the overall lithium recovery performance and is experimentally corroborated at a wide range of operation environments.
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http://dx.doi.org/10.1021/acs.est.9b07646DOI Listing
July 2020

Epitaxially Strained CeO /Mn O Nanocrystals as an Enhanced Antioxidant for Radioprotection.

Adv Mater 2020 Aug 10;32(31):e2001566. Epub 2020 Jun 10.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, 08826, Republic of Korea.

Nanomaterials with antioxidant properties are promising for treating reactive oxygen species (ROS)-related diseases. However, maintaining efficacy at low doses to minimize toxicity is a critical for clinical applications. Tuning the surface strain of metallic nanoparticles can enhance catalytic reactivity, which has rarely been demonstrated in metal oxide nanomaterials. Here, it is shown that inducing surface strains of CeO /Mn O nanocrystals produces highly catalytic antioxidants that can protect tissue-resident stem cells from irradiation-induced ROS damage. Manganese ions deposited on the surface of cerium oxide (CeO ) nanocrystals form strained layers of manganese oxide (Mn O ) islands, increasing the number of oxygen vacancies. CeO /Mn O nanocrystals show better catalytic activity than CeO or Mn O alone and can protect the regenerative capabilities of intestinal stem cells in an organoid model after a lethal dose of irradiation. A small amount of the nanocrystals prevents acute radiation syndrome and increases the survival rate of mice treated with a lethal dose of total body irradiation.
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http://dx.doi.org/10.1002/adma.202001566DOI Listing
August 2020

Enhancement of service life of polymer electrolyte fuel cells through application of nanodispersed ionomer.

Sci Adv 2020 Jan 31;6(5):eaaw0870. Epub 2020 Jan 31.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea.

In polymer electrolyte fuel cells (PEFCs), protons from the anode are transferred to the cathode through the ionomer membrane. By impregnating the ionomer into the electrodes, proton pathways are extended and high proton transfer efficiency can be achieved. Because the impregnated ionomer mechanically binds the catalysts within the electrode, the ionomer is also called a binder. To yield good electrochemical performance, the binder should be homogeneously dispersed in the electrode and maintain stable interfaces with other catalyst components and the membrane. However, conventional binder materials do not have good dispersion properties. In this study, a facile approach based on using a supercritical fluid is introduced to prepare a homogeneous nanoscale dispersion of the binder material in aqueous alcohol. The prepared binder exhibited high dispersion characteristics, crystallinity, and proton conductivity. High performance and durability were confirmed when the binder material was applied to a PEFC cathode electrode.
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http://dx.doi.org/10.1126/sciadv.aaw0870DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6994205PMC
January 2020

Design and synthesis of multigrain nanocrystals via geometric misfit strain.

Nature 2020 01 15;577(7790):359-363. Epub 2020 Jan 15.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, South Korea.

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a CoO nanocube core that carries a MnO shell on each facet. The individual shells are symmetry-related interconnected grains, and the large geometric misfit between adjacent tetragonal MnO grains results in tilt boundaries at the sharp edges of the CoO nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.
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http://dx.doi.org/10.1038/s41586-019-1899-3DOI Listing
January 2020

Atomic-level tuning of Co-N-C catalyst for high-performance electrochemical HO production.

Nat Mater 2020 04 13;19(4):436-442. Epub 2020 Jan 13.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, Republic of Korea.

Despite the growing demand for hydrogen peroxide it is almost exclusively manufactured by the energy-intensive anthraquinone process. Alternatively, HO can be produced electrochemically via the two-electron oxygen reduction reaction, although the performance of the state-of-the-art electrocatalysts is insufficient to meet the demands for industrialization. Interestingly, guided by first-principles calculations, we found that the catalytic properties of the Co-N moiety can be tailored by fine-tuning its surrounding atomic configuration to resemble the structure-dependent catalytic properties of metalloenzymes. Using this principle, we designed and synthesized a single-atom electrocatalyst that comprises an optimized Co-N moiety incorporated in nitrogen-doped graphene for HO production and exhibits a kinetic current density of 2.8 mA cm (at 0.65 V versus the reversible hydrogen electrode) and a mass activity of 155 A g (at 0.65 V versus the reversible hydrogen electrode) with negligible activity loss over 110 hours.
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http://dx.doi.org/10.1038/s41563-019-0571-5DOI Listing
April 2020

Short-Chain Polyselenosulfide Copolymers as Cathode Materials for Lithium-Sulfur Batteries.

ACS Appl Mater Interfaces 2019 Dec 27;11(49):45785-45795. Epub 2019 Nov 27.

Photo-Electronic Hybrids Research Center , Korea Institute of Science and Technology , Seoul 02792 , Republic of Korea.

Copolymerization of sulfur, which forms sulfur-rich polymers, has recently opened a new era in the lithium-sulfur (Li-S) battery research as improved battery performances could be achieved compared to pure sulfur (S). By means of organic chemistry, sulfur copolymers with desired features and chemical structures could be rationally designed and synthesized. In this study, sulfur-rich polymers consisting of short-chain tetrasulfide (R-S-R) (PTS) and selenotrisulfide (R-SeS-R) (PTSeS) bonds are suggested as cathode materials for Li-S batteries. Intrinsically short poly(seleno)sulfide bonds along with covalent anchoring effect effectively suppress the parasitic shuttle effect originating from soluble long-chain lithium polysulfides formed from pure S. Furthermore, a comparative study demonstrates the indisputable advantage of the selenium doping, which enhances the electrical conductivity of the polymer and following battery performances. In terms of cycling performance, both PTSeS and PTS with ∼2 mg cm polymer loading exhibit small capacity decays of 0.078 and 0.052% per cycle until 500 cycles at 0.5C, respectively. However, active material utilization and high rate performance are substantially superior in PTSeS due to the enhanced electron transfer kinetics. This work would provide useful design principles for fabrication of sulfur-based polymers with even greater applicability in future Li-S batteries.
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http://dx.doi.org/10.1021/acsami.9b17209DOI Listing
December 2019

Membrane/Electrode Interface Design for Effective Water Management in Alkaline Membrane Fuel Cells.

ACS Appl Mater Interfaces 2019 Sep 11;11(38):34805-34811. Epub 2019 Sep 11.

Fuel Cell Research Center , Korea Institute of Science and Technology (KIST) , Seoul 02792 , Republic of Korea.

The recent development of ultrathin anion exchange membranes and optimization of their operating conditions have significantly enhanced the performance of alkaline-membrane fuel cells (AMFCs); however, the effects of the membrane/electrode interface structure on the AMFC performance have not been seriously investigated thus far. Herein, we report on a high-performance AMFC system with a membrane/electrode interface of novel design. Commercially available membranes are modified in the form of well-aligned line arrays of both the anode and cathode sides by means of a solvent-assisted molding technique and sandwich-like assembly of the membrane and polydimethylsiloxane molds. Upon incorporating the patterned membranes into a single-cell system, we observe a significantly enhanced performance of up to ∼35% compared with that of the reference membrane. The enlarged interface area and reduced membrane thickness from the line-patterned membrane/electrode interface result in improved water management, reduced ohmic resistance, and effective utilization of the catalyst. We believe that our findings can significantly contribute further advancements in AMFCs.
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http://dx.doi.org/10.1021/acsami.9b08075DOI Listing
September 2019

Alternative Assembly of α-Synuclein Leading to Protein Film Formation and Its Application for Developing Polydiacetylene-Based Sensing Materials.

Langmuir 2019 09 28;35(36):11923-11931. Epub 2019 Aug 28.

School of Chemical and Biological Engineering, Institute of Chemical Processes, College of Engineering , Seoul National University , Seoul 08826 , Korea.

Understanding the self-assembly process of amyloidogenic protein is valuable not only to find its pathological implication but also to prepare protein-based biomaterials. α-Synuclein (αS), a pathological component of Parkinson's disease, producing one-dimensional (1D) amyloid fibrils, has been employed to generate two-dimensional (2D) protein films by encouraging an alternative self-assembly process. At a high temperature of 50 °C, αS molecules self-assembled into 2D films instead of 1D amyloid fibrils, whereas the fibrils were the major product at 37 °C. Based on circular dichroism and Fourier transform infrared spectroscopy analyses, the film was produced via a structural transition from the initial random to still undefined but mostly the turn or loop structure, which was distinctive from the β-sheet formation observed with the amyloid fibrils. The αS 2D film was also routinely prepared at the oil-water interface and used as a matrix to produce polydiacetylene-based sensing materials. 10,12-Pentacosadiynoic acids (PCDA) were aligned on the film and photopolymerized to form a π-conjugated molecular assembly yielding a blue color. Its colorimetric transition to red was induced by increasing the temperature. This functionalized protein film increased its height from 40 to 55 nm upon PCDA immobilization and exhibited enhanced physical and chemical stability. In addition, the modified film showed remarkably high electrical conductivity only in the red state. This film, therefore, can be considered as a robust protein-based hybrid biomaterial capable of simultaneously recognizing various external stimuli (heat, pH, and solvents) with changes in color and conductivity, and it is expected to be utilized as a basic material for the development of biocompatible sensors.
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http://dx.doi.org/10.1021/acs.langmuir.9b01593DOI Listing
September 2019

Changes in the oxidation state of Pt single-atom catalysts upon removal of chloride ligands and their effect for electrochemical reactions.

Chem Commun (Camb) 2019 May;55(45):6389-6392

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea.

Single atomic Pt supported on TiC was prepared from chloride Pt precursors, then the chloride ligands were intentionally removed by increasing the reduction temperature. The 0.2 wt% Pt/TiC catalyst reduced at 300 °C had more reduced Pt single-atoms with fewer chloride ligands and exhibited the highest currents for H2O2 formation in the electrochemical oxygen reduction reaction. Controlling the oxidation state of the single-atoms is very important to maximize the activity of the single-atom catalysts.
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http://dx.doi.org/10.1039/c9cc01593kDOI Listing
May 2019

Acoustic emission analysis of the compressive deformation of iron foams and their biocompatibility study.

Mater Sci Eng C Mater Biol Appl 2019 Apr 12;97:367-376. Epub 2018 Dec 12.

School of Materials Science and Engineering, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 136-702, Republic of Korea.

We synthesized Fe foams using water suspensions of micrometric FeO powder by reducing and sintering the sublimated Fe oxide green body to Fe under 5% H/Ar gas. The resultant Fe foam showed aligned lamellar macropores replicating the ice dendrites. The compressive behavior and deformation mechanism of the synthesized Fe foam were studied using an acoustic emission (AE) method, with which we detected sudden localized structural changes in the Fe foam material. The evolution of the deformation mechanism was elucidated using the adaptive sequential k-means (ASK) algorithm; specifically, the plastic deformation of the cell struts was followed by localized cell collapse, which eventually led to fracturing of the cell walls. For potential biomedical applications, the corrosion and biocompatibility characteristics of the two synthesized Fe foams with different porosities (50% vs. 44%) were examined and compared. Despite its larger porosity, the superior corrosion behavior of the Fe foam with 50% porosity can be attributed to its larger pore size and smaller microscopic surface area. Based on the cytotoxicity tests for the extracts of the foams, the Fe foam with 44% porosity showed better cytocompatibility than that with 50% porosity.
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http://dx.doi.org/10.1016/j.msec.2018.12.035DOI Listing
April 2019

Design Principle of Fe-N-C Electrocatalysts: How to Optimize Multimodal Porous Structures?

J Am Chem Soc 2019 Feb 18;141(5):2035-2045. Epub 2019 Jan 18.

Center for Nanoparticle Research , Institute for Basic Science (IBS) , Seoul 08826 , Republic of Korea.

The effect of porous structures on the electrocatalytic activity of N-doped carbon is studied by using electrochemical analysis techniques and the result is applied to synthesize highly active and stable Fe-N-C catalyst for oxygen reduction reaction (ORR). We developed synthetic procedures to prepare three types of N-doped carbon model catalysts that are designed for systematic comparison of the porous structures. The difference in their catalytic activity is investigated in relation to the surface area and the electrochemical parameters. We found that macro- and mesoporous structures contribute to different stages of the reaction kinetics. The catalytic activity is further enhanced by loading the optimized amount of Fe to prepare Fe-N-C catalyst. In both N-doped carbon and Fe-N-C catalysts, the hierarchical porous structure improved electrocatalytic performance in acidic and alkaline media. The optimized catalyst exhibits one of the best ORR performance in alkaline medium with excellent long-term stability in anion exchange membrane fuel cell and accelerated durability test. Our study establishes a basis for rationale design of the porous carbon structure for electrocatalytic applications.
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http://dx.doi.org/10.1021/jacs.8b11129DOI Listing
February 2019

Recent Progress in the Design and Synthesis of Nitrides for Mesoscopic and Perovskite Solar Cells.

ChemSusChem 2019 Feb 25;12(4):772-786. Epub 2019 Jan 25.

School of Chemical and Biological Engineering, Seoul National University, Seoul, 08826, Republic of Korea.

With growing concerns about global warming and the energy crisis, a variety of photovoltaic devices have attracted worldwide attention as alternative energy sources. Among them, organic-inorganic hybrid photovoltaics, typically mesoscopic and perovskite solar cells, are promising, owing to their potential for low-cost energy production, which mainly comes from unlimited combinations of materials optimized for each step of solar energy conversion. However, the commercialization of organic-inorganic hybrid solar cells is hampered by costly electrocatalysts or hole-transport materials. Currently, state-of-the-art dye- or quantum-dot-sensitized solar cells and perovskite solar cells necessitate noble metals and high-price polymeric materials. In an attempt to resolve this issue, various kinds of metal compounds have been investigated, and nitrides have been actively reported to possess a number of favorable properties for the aforementioned purpose, such as excellent electrical conductivity and superb electrocatalytic performance. Herein, the use of nitrides as cost-effective electrocatalysts or hole-transport materials in organic-inorganic hybrid solar cells is reviewed. Nitrides with a variety of morphologies and scales are discussed, together with the synergistic effect in the case of diverse composites. In addition, prospects and challenges for applying nitride materials are briefly suggested.
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http://dx.doi.org/10.1002/cssc.201802251DOI Listing
February 2019

Engineering Titanium Dioxide Nanostructures for Enhanced Lithium-Ion Storage.

J Am Chem Soc 2018 Dec 26;140(48):16676-16684. Epub 2018 Nov 26.

Center for Nanoparticle Research , Institute for Basic Science (IBS) , Seoul 08826 , Korea.

Various kinds of nanostructured materials have been extensively investigated as lithium ion battery electrode materials derived from their numerous advantageous features including enhanced energy and power density and cyclability. However, little is known about the microscopic origin of how nanostructures can enhance lithium storage performance. Herein, we identify the microscopic origin of enhanced lithium storage in anatase TiO nanostructure and report a reversible and stable route to achieve enhanced lithium storage capacity in anatase TiO. We designed hollow anatase TiO nanostructures composed of interconnected ∼5 nm sized nanocrystals, which can individually reach the theoretical lithium storage limit and maintain a stable capacity during prolonged cycling (i.e., 330 mAh g for the initial cycle and 228 mAh g for the 100th cycle, at 0.1 A g). In situ characterization by X-ray diffraction and X-ray absorption spectroscopy shows that enhanced lithium storage into the anatase TiO nanocrystal results from the insertion reaction, which expands the crystal lattice during the sequential phase transition (anatase TiO → LiTiO → LiTiO). In addition to the pseudocapacitive charge storage of nanostructures, our approach extends the utilization of nanostructured TiO for significantly stabilizing excess lithium storage in crystal structures for long-term cycling, which can be readily applied to other lithium storage materials.
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http://dx.doi.org/10.1021/jacs.8b09487DOI Listing
December 2018

Edge-Terminated MoS Nanoassembled Electrocatalyst via In Situ Hybridization with 3D Carbon Network.

Small 2018 Sep 10;14(36):e1802191. Epub 2018 Aug 10.

Center for Nanoparticle Research, Institute for Basic Science (IBS), School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul, 08826, Republic of Korea.

Transition metal dichalcogenides, especially MoS , are considered as promising electrocatalysts for hydrogen evolution reaction (HER). Since the physicochemical properties of MoS and electrode morphology are highly sensitive factor for HER performance, designed synthesis is highly pursued. Here, an in situ method to prepare a 3D carbon/MoS hybrid catalyst, motivated by the graphene ribbon synthesis process, is reported. By rational design strategies, the hybrid electrocatalysts with cross-connected porous structure are obtained, and they show a high HER activity even comparable to the state-of-the-art MoS catalyst without appreciable activity loss in long-term operations. Based on various physicochemical techniques, it is demonstrated that the synthetic procedure can effectively guide the formation of active site and 3D structure with a distinctive feature; increased exposure of active sites by decreased domain size and intrinsically high activity through controlling the number of stacking layers. Moreover, the importance of structural properties of the MoS -based catalysts is verified by controlled experiments, validating the effectiveness of the designed synthesis approach.
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http://dx.doi.org/10.1002/smll.201802191DOI Listing
September 2018

Cross-Linked Sulfonated Poly(arylene ether sulfone) Containing a Flexible and Hydrophobic Bishydroxy Perfluoropolyether Cross-Linker for High-Performance Proton Exchange Membrane.

ACS Appl Mater Interfaces 2018 Jul 19;10(26):21788-21793. Epub 2018 Jun 19.

Department of Chemical and Biological Engineering , Seoul National University , 599 Gwanak-ro , Gwanak-gu , Seoul 151-744 , Republic of Korea.

Here we show a simple and effective cross-linking method to prepare a high performance cross-linked sulfonated poly(arylene ether sulfone) (C-SPAES) membrane using bishydroxy perfluoropolyether (PFPE) as a cross-linker for fuel cell applications. The C-SPAES membrane shows much improved physicochemical stability due to the cross-linked structure and reasonably high proton conductivity compared to the non-cross-linked SPAES membrane due to the incorporation of flexible PFPE and the effective phase-separated morphology between the hydrocarbon and perfluorinated moieties forming well-connected networks. Under intermediate-temperature and low humidity conditions (90 °C, 50% RH, and 150 kPa), the membrane electrode assembly employing the C-SPAES membrane reveals an outstanding cell performance (1.17 W cm at 0.65 V) ascribed to its reasonably high proton conductivity and enhanced interfacial compatibility between the perfluorinated moieties in the electrode and C-SPAES membrane. Furthermore, a hydration-dehydration cycling test result at 90 °C reveals that the C-SPAES membrane has notable durability against rigorous operating conditions.
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http://dx.doi.org/10.1021/acsami.8b05139DOI Listing
July 2018

Solvothermal-Derived S-Doped Graphene as an Anode Material for Sodium-Ion Batteries.

Adv Sci (Weinh) 2018 May 14;5(5):1700880. Epub 2018 Feb 14.

Center for Nanoparticle Research Institute for Basic Science (IBS) School of Chemical and Biological Engineering Seoul National University Seoul 08826 Republic of Korea.

Sodium-ion batteries (SIBs) have attracted enormous attention in recent years due to the high abundance and low cost of sodium. However, in contrast to lithium-ion batteries, conventional graphite is unsuitable for SIB anodes because it is much more difficult to intercolate the larger Na ions into graphite layers. Therefore, it is critical to develop new anode materials for SIBs for practical use. Here, heteroatom-doped graphene with high doping levels and disordered structures is prepared using a simple and economical thermal process. The solvothermal-derived graphene shows excellent performance as an anode material for SIBs. It exhibits a high reversible capacity of 380 mAh g after 300 cycles at 100 mA g, excellent rate performance 217 mAh g at 3200 mA g, and superior cycling performance at 2.0 A g during 1000 cycles with negligible capacity fade.
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http://dx.doi.org/10.1002/advs.201700880DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5979751PMC
May 2018

Highly Efficient Bifacial Dye-Sensitized Solar Cells Employing Polymeric Counter Electrodes.

ACS Appl Mater Interfaces 2018 Mar 27;10(10):8611-8620. Epub 2018 Feb 27.

Center for Nanoparticle Research , Institute for Basic Science (IBS) , Seoul 08826 , Republic of Korea.

Dye-sensitized solar cells (DSCs) are promising solar energy conversion devices with aesthetically favorable properties such as being colorful and having transparent features. They are also well-known for high and reliable performance even under ambient lighting, and these advantages distinguish DSCs for applications in window-type building-integrated photovoltaics (BIPVs) that utilize photons from both lamplight and sunlight. Therefore, investigations on bifacial DSCs have been done intensively, but further enhancement in performance under back-illumination is essential for practical window-BIPV applications. In this research, highly efficient bifacial DSCs were prepared by a combination of electropolymerized poly(3,4-ethylenedioxythiphene) (PEDOT) counter electrodes (CEs) and cobalt bipyridine redox ([Co(bpy)]) electrolyte, both of which manifested superior transparency when compared with conventional Pt and iodide counterparts, respectively. Keen electrochemical analyses of PEDOT films verified that superior electrical properties were achievable when the thickness of the film was reduced, while their high electrocatalytic activities were unchanged. The combination of the PEDOT thin film and [Co(bpy)] electrolyte led to an unprecedented power conversion efficiency among bifacial DSCs under back-illumination, which was also over 85% of that obtained under front-illumination. Furthermore, the advantage of the electropolymerization process, which does not require an elevation of temperature, was demonstrated by flexible bifacial DSC applications.
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http://dx.doi.org/10.1021/acsami.7b17815DOI Listing
March 2018

Electrochemically Synthesized Nanoporous Molybdenum Carbide as a Durable Electrocatalyst for Hydrogen Evolution Reaction.

Adv Sci (Weinh) 2018 01 19;5(1):1700601. Epub 2017 Dec 19.

Center for Nanoparticle ResearchInstitute for Basic Science Seoul 08826 Republic of Korea.

Demands for sustainable production of hydrogen are rapidly increasing because of environmental considerations for fossil fuel consumption and development of fuel cell technologies. Thus, the development of high-performance and economical catalysts has been extensively investigated. In this study, a nanoporous Mo carbide electrode is prepared using a top-down electrochemical process and it is applied as an electrocatalyst for the hydrogen evolution reaction (HER). Anodic oxidation of Mo foil followed by heat treatment in a carbon monoxide (CO) atmosphere forms a nanostructured Mo carbide with excellent interconnections, and these structural characteristics lead to high activity and durability when applied to the HER. Additionally, characteristic behavior of Mo is observed; metallic Mo nanosheets form during electrochemical anodization by exfoliation along the (110) planes. These nanosheets are viable for chemical modification, indicating their feasibility in various applications. Moreover, the role of carbon shells is investigated on the surface of the electrocatalysts, whereby it is suggested that carbon shells serve as a mechanical barrier against the oxidative degradation of catalysts that accompanies unavoidable volume expansion.
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http://dx.doi.org/10.1002/advs.201700601DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5770677PMC
January 2018

Highly Durable and Active Pt-Based Nanoscale Design for Fuel-Cell Oxygen-Reduction Electrocatalysts.

Adv Mater 2018 Oct 23;30(42):e1704123. Epub 2018 Jan 23.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, 08826, South Korea.

Fuel cells are one of the promising energy-conversion devices due to their high efficiency and zero emission. Although recent advances in electrocatalysts have been achieved using various material designs such as alloys, [email protected] structures, and shape control, many issues still remain to be resolved. Especially, material design issues for high durability and high activity are recently accentuated owing to severe instability of nanoparticles under fuel-cell operating conditions. To address these issues, fundamental understanding of functional links between activity and durability is timely urgent. Here, the activity and durability of nanoscale materials are summarized, focusing on the nanoparticle size effect. In addition to phenomenological observation, two major degradation origins, including atomic dissolution and particle size increase, are discussed related to the activity decrease. Based on the fundamental understanding of nanoparticle degradation, recent promising strategies for durable Pt-based nanoscale electrocatalysts are introduced and the role of each design for durability enhancement is discussed. Finally, short comments related to the future direction of nanoparticle issues are provided in terms of nanoparticle synthesis and analysis.
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http://dx.doi.org/10.1002/adma.201704123DOI Listing
October 2018

Guided cracking of electrodes by stretching prism-patterned membrane electrode assemblies for high-performance fuel cells.

Sci Rep 2018 01 19;8(1):1257. Epub 2018 Jan 19.

Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul, 08826, Korea.

Guided cracks were successfully generated in an electrode using the concentrated surface stress of a prism-patterned Nafion membrane. An electrode with guided cracks was formed by stretching the catalyst-coated Nafion membrane. The morphological features of the stretched membrane electrode assembly (MEA) were investigated with respect to variation in the prism pattern dimension (prism pitches of 20 μm and 50 μm) and applied strain (S ≈ 0.5 and 1.0). The behaviour of water on the surface of the cracked electrode was examined using environmental scanning electron microscopy. Guided cracks in the electrode layer were shown to be efficient water reservoirs and liquid water passages. The MEAs with and without guided cracks were incorporated into fuel cells, and electrochemical measurements were conducted. As expected, all MEAs with guided cracks exhibited better performance than conventional MEAs, mainly because of the improved water transport.
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http://dx.doi.org/10.1038/s41598-018-19861-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5775251PMC
January 2018
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