MinJoong Kim

High-Performance Sb/Sb2O3 Anode Materials Using a Polypyrrole Nanowire Network for Na-Ion Batteries

Three-dimensional porous Sb/Sb2 O3 anode materials are successfully fabricated using a simple electrodeposition method with a polypyrrole nanowire network. The Sb/Sb2 O3 -PPy electrode exhibits excellent cycle performance and outstanding rate capabilities; the charge capacity is sustained at 512.01 mAh g(-1) over 100 cycles, and 56.7% of the charge capacity at a current density of 66 mA g(-1) is retained at 3300 mA g(-1) . The improved electrochemical performance of the Sb/Sb2 O3 -PPy electrode is attributed not only to the use of a highly porous polypyrrole nanowire network as a substrate but also to the buffer effects of the Sb2 O3 matrix on the volume expansion of Sb. Ex situ scanning electron microscopy observation confirms that the Sb/Sb2 O3 -PPy electrode sustains a strong bond between the nanodeposits and polypyrrole nanowires even after 100 cycles, which maintains good electrical contact of Sb/Sb2 O3 with the current collector without loss of the active materials.

Cobalt-carbon nanofibers as an efficient support-free catalyst for oxygen reduction reaction with a systematic study of active site formation

Recently, major efforts have been devoted to exploring cheap and active non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells for large-scale applications. Herein, we report electrospun cobalt-carbon nanofibers (Co-CNFs) as an efficient catalyst for the ORR, together with a systematic study of the active site formation. The ORR activity of the Co-CNFs increases with increasing Co content up to approximately 30 wt%, at which high ORR activity is exhibited, comparable with a commercial Pt/C catalyst in alkaline media. XPS and structural analysis reveals a Co–pyridinic Nx bond at the edge plane, and more Co nanoparticles were found in the Co-CNFs as the Co content was increased. These sites can behave as the primary and the secondary active sites for the ORR, according to a dual-site mechanism. The ORR activity of the Co-CNFs may deteriorate even if only one of these sites is limited. The high ORR activity of the Co-CNF catalysts results from the synergetic effect of dual site formation for the ORR.

Porous Co–P foam as an efficient bifunctional electrocatalyst for hydrogen and oxygen evolution reactions

A high-performance bifunctional Co–P foam catalyst was successfully synthesized by facile one-step electrodeposition at a high cathodic current density. The synthetic approach includes fast generation of hydrogen bubbles as well as fast deposition of Co–P, which played a key role in forming a porous Co–P foam structure. The Co–P foam exhibits remarkable electrocatalytic activity and stability in both acidic and alkaline solution. Its HER activity was recorded with an overpotential of 50 mV in 0.5 M H2SO4 and 131 mV in 1 M KOH at 10 mA cm−2, which is comparable to that of commercial Pt/C (η@10 mA cm−20.5 M H2SO4: 33 mV, η10 mA, 1 M KOH: 80 mV). The Co–P foam (η@10 mA cm−2: 300 mV) exhibits better OER activities than Ir/C (η@10 mA cm−2: 345 mV) and RuO2 (η@10 mA cm−2: 359 mV) in 1 M KOH solution. The excellent performance of the Co–P foam as an HER and OER catalyst can be attributed to the charge separation between Co and P in Co–P foam as well as the porous foam structure providing a large electrochemically active surface area (ECSA). The ECSA of the Co–P foam was calculated to be 118 cm2, which was 2.4 times higher than that of a Co–P film (49 cm2).

Single-step synthesis of polypyrrole nanowires by cathodic electropolymerization

Polypyrrole nanowires are successfully fabricated with a one-step process by cathodic electropolymerization from an aqueous solution without templates and chemical additives. The method utilizes electrochemically generated NO+ to oxidize the neutral pyrrole monomers, making it possible to use oxidizable metal substrates, such as Cu and Ni. The synthesized nanowires are directly deposited on the substrate in the form of a thin film consisting of fine polypyrrole nanowires with a nanoporous and interconnected network structure. The growth kinetics of the polypyrrole nanowires was investigated by analyzing the effects of the chemical composition of the electrolyte and the synthesis time on the formation of polypyrrole nanowires. It was found that the polymerization process of pyrrole is very sensitive to the reactivity of radical cations. For a radical cation with high reactivity, the polypyrrole nanospheres are synthesized near the electrode in the solution. In contrast, for a radical cation with sufficiently low reactivity, the polypyrrole nanowires are grown on the priorly deposited polypyrrole nanospheres.

Facile synthesis of SnO2-polypyrrole hybrid nanowires by cathodic electrodeposition and their application to Li-ion battery anodes

SnO2-polypyrrole hybrid nanowires are synthesized in a one-step process by a simple electrochemical method from an aqueous solution at room temperature. This novel and facile technique involves the rapid electropolymerization of pyrrole and the relatively slow chemical deposition of SnO2, which leads to the incorporation of uniformly dispersed SnO2 nanoparticles inside polypyrrole nanowires. Notably, the synthesized nanowires are directly deposited as a thin film on the substrate in a three-dimensional porous and interconnected network structure composed of numerous fine nanowires. This open architecture is highly desirable in energy storage devices because of its excellent mass transfer and high specific surface area, and therefore, the SnO2-polypyrrole hybrid nanowires are evaluated for their use as high-performance anode materials in Li-ion batteries. Over 200 cycles, the hybrid nanowires show superior cyclic performance and a charge capacity higher than 0.307 mA h cm−2, most likely because the polypyrrole matrix effectively prevents the agglomeration of the SnO2 nanoparticles and elastically buffers the volumetric change in the nanoparticles that occurs during cycling.

Electrospun Nb-doped TiO 2 nanofiber support for Pt nanoparticles with high electrocatalytic activity and durability

This study explores a facile method to prepare an efficient and durable support for Pt catalyst of polymer electrolyte membrane fuel cell (PEMFC). As a candidate, Nb-doped TiO2 (Nb-TiO2) nanofibers are simply fabricated using an electrospinning technique, followed by a heat treatment. Doping Nb into the TiO2 nanofibers leads to a drastic increase in electrical conductivity with doping level of up to 25 at. % (Nb0.25Ti0.75O2). Pt nanoparticles are synthesized on the prepared 25 at. % Nb-doped TiO2-nanofibers (Pt/Nb-TiO2) as well as on a commercial powdered carbon black (Pt/C). The Pt/Nb-TiO2 nanofiber catalyst exhibits similar oxygen reaction reduction (ORR) activity to that of the Pt/C catalyst. However, during an accelerated stress test (AST), the Pt/Nb-TiO2 nanofiber catalyst retained more than 60% of the initial ORR activity while the Pt/C catalyst lost 65% of the initial activity. The excellent durability of the Pt/Nb-TiO2 nanofiber catalyst can be attributed to high corrosion resistance of TiO2 and strong interaction between Pt and TiO2.

Synergetic effects of edge formation and sulfur doping on the catalytic activity of a graphene-based catalyst for the oxygen reduction reaction

An edge activated S doped Fe-N-graphene (EA-SFeNG) was successfully synthesized via facile ball milling followed by a pyrolysis process. The oxygen reduction reaction (ORR) performance of EA-SFeNG was dramatically improved by doping S and forming edge sites in Fe-N-graphene; the onset potential was shifted from 0.91 VRHE to 1.0 VRHE with the half-wave potential increased from 0.77 VRHE to 0.848 VRHE. The EA-SFeNG exhibited catalytic performances that are comparable to those of commercial 20 wt% Pt/C (Vonset: 1.05 V, V1/2: 0.865 V); however, its durability was better than that of the Pt catalyst in alkaline media. The excellent ORR activity can be attributed to the increase in defect density and SOx bonding in the EA-SFeNG. Furthermore, we experimentally demonstrate that the work function of the Fe-N-graphene is significantly reduced from 4.06 eV to 4.01 eV by the increase in edge density and doping S, thereby improving the ORR kinetics of EA-SFeNG.

Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell.

The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a conventional decal transfer method and an outer catalyst layer directly coated on a gas diffusion layer. The double-layered structure was used to improve the interfacial contact between the catalyst layer and membrane, to increase catalyst utilization and to modify the removal of product water from the cathode. Based on a series of MEAs with double-layered cathodes with an overall Pt loading fixed at 0.4 mg cm(-2) and different ratios of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg Pt cm(-2) and an outer layer of 0.1 mg Pt cm(-2) exhibited the best performance. This performance was better than that of the conventional single-layered electrode by 13.5% at a current density of 1.4 A cm(-2).

Ga–Doped Pt–Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

Bimetallic PtNi nanoparticles have been considered as a promising electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) owing to their high catalytic activity. However, under typical fuel cell operating conditions, Ni atoms easily dissolve into the electrolyte, resulting in degradation of the catalyst and the membrane-electrode assembly (MEA). Here, we report gallium-doped PtNi octahedral nanoparticles on a carbon support (Ga-PtNi/C). The Ga-PtNi/C shows high ORR activity, marking an 11.7-fold improvement in the mass activity (1.24 A mgPt-1) and a 17.3-fold improvement in the specific activity (2.53 mA cm-2) compared to the commercial Pt/C (0.106 A mgPt-1 and 0.146 mA cm-2). Density functional theory calculations demonstrate that addition of Ga to octahedral PtNi can cause an increase in the oxygen intermediate binding energy, leading to the enhanced catalytic activity toward ORR. In a voltage-cycling test, the Ga-PtNi/C exhibits superior stability to PtNi/C and the commercial Pt/C, maintaining the initial Ni concentration and octahedral shape of the nanoparticles. Single cell using the Ga-PtNi/C exhibits higher initial performance and durability than those using the PtNi/C and the commercial Pt/C. The majority of the Ga-PtNi nanoparticles well maintain the octahedral shape without agglomeration after the single cell durability test (30,000 cycles). This work demonstrates that the octahedral Ga-PtNi/C can be utilized as a highly active and durable ORR catalyst in practical fuel cell applications.

Correction to Ga–Doped Pt–Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction JeongHoon Lim, Hyeyoung Shin, MinJoong Kim, Hoin Lee, Kug-Seung Lee, YongKeun Kwon, DongHoon Song, SeKwon Oh, Hyungjun Kim, and EunAe Cho* Nano Lett., 2018, 18 (4) 2450−2458. DOI: 10.1021/acs.nanolett.8b00028. I Acknowledgments, the statement “Korea Institute of Science and Technology (KIST) under Contract No. 2017081254” should be “National Research Foundation of Korea under Contract No. NRF-2015M1A2A2056558”. Addition/Correction