Do-Hwan Nam

Template-Free Electrochemical Synthesis of Sn Nanofibers as High-Performance Anode Materials for Na-Ion Batteries

Sn nanofibers with a high aspect ratio are successfully synthesized using a simple electrodeposition process from an aqueous solution without the use of templates. The synthetic approach involves the rapid electrochemical deposition of Sn accompanied by the strong adsorption of Triton X-100, which can function as a growth modifier for the Sn crystallites. Triton X-100 is adsorbed on the {200} crystallographic planes of Sn in an elongated configuration and suppressed the preferential growth of Sn along the [100] direction. Consequently, the Sn electrodeposits are forced to grow anisotropically in a direction normal to the (112) or (1̅12) plane, forming one-dimensional nanofibers. As electrode materials for the Na-ion batteries, the Sn nanofibers exhibit a high reversible capacity and an excellent cycle performance; the charge capacity is maintained at 776.26 mAh g(-1) after 100 cycles, which corresponds to a retention of 95.09% of the initial charge capacity. The superior electrochemical performance of the Sn nanofibers is mainly attributed to the high mechanical stability of the nanofibers, which originate from highly anisotropic expansion during sodiation and the pore volumes existing between the nanofibers.

Structural enhancement of Na3V2(PO4)3/C composite cathode materials by pillar ion doping for high power and long cycle life sodium-ion batteries

Structurally stabilized Na3V2(PO4)3/C composite cathode materials with excellent electrochemical performance can be obtained by incorporating functional pillar ions into the structure. As pillar ions, K-ions have a larger ionic radius compared to Na-ions, and play an important role in enlarging the Na-ion diffusion pathway and in increasing the lattice volume by elongating the c-axis, thereby improving the rate performance. Furthermore, since the incorporated K-ions rarely participate in the electrochemical extraction/insertion reactions, they can stabilize the Na3V2(PO4)3 structure by suppressing significant lattice volume changes or structural distortion, even in a wide range of voltage windows accompanying multiple transitions of V ions and phase distortions. We investigated how the K-ion doping level affected the crystal structure and electrochemical properties of Na3V2(PO4)3 cathode materials for Na-ion batteries.

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.

Electrochemically Synthesized Sb/Sb2O3 Composites as High-Capacity Anode Materials Utilizing a Reversible Conversion Reaction for Na-Ion Batteries

Sb/Sb2O3 composites are synthesized by a one-step electrodeposition process from an aqueous electrolytic bath containing a potassium antimony tartrate complex. The synthesis process involves the electrodeposition of Sb simultaneously with the chemical deposition of Sb2O3, which allows for the direct deposition of morula-like Sb/Sb2O3 particles on the current collector without using a binder. Structural characterization confirms that the Sb/Sb2O3 composites are composed of approximately 90 mol % metallic Sb and 10 mol % crystalline Sb2O3. The composite exhibits a high reversible capacity (670 mAh g(-1)) that is higher than the theoretical capacity of Sb (660 mAh g(-1)). The high reversible capacity results from the conversion reaction between Na2O and Sb2O3 that occurs additionally to the alloying/dealloying reaction of Sb with Na. Moreover, the Sb/Sb2O3 composite shows excellent cycle performance with 91.8% capacity retention over 100 cycles, and a superior rate capability of 212 mAh g(-1) at a high current density of 3300 mA g(-1). The outstanding cycle performance is attributed to an amorphous Na2O phase generated by the conversion reaction, which inhibits agglomeration of Sb particles and acts as an effective buffer against volume change of Sb during cycling.

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.

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.

Effects of Substrate Morphology and Postelectrodeposition on Structure of Cu Foam and Their Application for Li-Ion Batteries

bubbles are split into small bubbles by the nodules. In addition, the adhesive properties of the Cu foamdeposited on the nodular Cu foil are improved by the mechanical interlocking effects of the nodules. As a result of the postelec-trodeposition treatment, the mechanical strength of the Cu foam is significantly enhanced primarily due to the covering of thepost-Cu electrodeposit on the dendrite crystallites of the Cu foam. The Li capacity and cycle performances of a Sn anode areclearly improved when Sn is electrodeposited on the covered Cu foam on the nodular Cu foil compared with that on the uncoveredCu foam: 451 mAh g

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.

One-step synthesis of a Si/CNT–polypyrrole composite film by electrochemical deposition

A Si/CNT–polypyrrole thin film was prepared by electrochemical deposition from an aqueous solution containing Si/CNT composite colloids. The proposed co-deposition mechanism involves the rapid electropolymerization of polypyrrole nanowires and the electrophoretic deposition of Si/CNT particles. The CNT coating on the Si particles enables electrophoresis of the Si particles and promotes the incorporation of the particles into the polypyrrole film.