Seung-Keun Park

Rational Design and Synthesis of Extremely Efficient Macroporous CoSe2–CNT Composite Microspheres for Hydrogen Evolution Reaction

Uniquely structured CoSe2 -carbon nanotube (CNT) composite microspheres with optimized morphology for the hydrogen-evolution reaction (HER) are prepared by spray pyrolysis and subsequent selenization. The ultrafine CoSe2 nanocrystals uniformly decorate the entire macroporous CNT backbone in CoSe2 -CNT composite microspheres. The macroporous CNT backbone strongly improves the electrocatalytic activity of CoSe2 by improving the electrical conductivity and minimizing the growth of CoSe2 nanocrystals during the synthesis process. In addition, the macroporous structure resulting from the CNT backbone improves the electrocatalytic activity of the CoSe2 -CNT microspheres by increasing the removal rate of generated H2 and minimizing the polarization of the electrode during HER. The CoSe2 -CNT composite microspheres demonstrate excellent catalytic activity for HER in an acidic medium (10 mA cm-2 at an overpotential of ≈174 mV). The bare CoSe2 powders exhibit moderate HER activity, with an overpotential of 226 mV at 10 mA cm-2 . The Tafel slopes for the CoSe2 -CNT composite and bare CoSe2 powders are 37.8 and 58.9 mV dec-1 , respectively. The CoSe2 -CNT composite microspheres have a slightly larger Tafel slope than that of commercial carbon-supported platinum nanoparticles, which is 30.2 mV dec-1 .

Metal–organic framework-derived CoSe2/(NiCo)Se2 box-in-box hollow nanocubes with enhanced electrochemical properties for sodium-ion storage and hydrogen evolution

Multishell structured metal selenide nanocubes, namely, Co/(NiCo)Se2 box-in-box structures with different shell compositions, were successfully synthesized by applying zeolitic imidazolate framework-67 (ZIF-67) as a template. This strategy involved the fabrication of cube-shaped ZIF-67/Ni–Co layered double hydroxides with a yolk–shell structure and then transformation into Co/(NiCo)Se2 with a box-in-box structure by a selenization process under Ar/H2 conditions. During the selenization step, hollow structured CoSe2 cores were generated by Ostwald ripening, resulting in the formation of Co/(NiCo)Se2 with a box-in-box structure composed of an inner CoSe2 shell and an outer (NiCo)Se2 shell. Due to the synergetic effect of the unique structure and multicomponent selenide composition, the Co/(NiCo)Se2 with the box-in-box structure offered excellent dual functionality as both an anode for sodium ion batteries (SIBs) and an electrocatalyst for the hydrogen evolution reaction (HER). Electrochemical tests on the Co/(NiCo)Se2 with the box-in-box structure demonstrated a low Tafel slope (39.8 mV dec−1) and excellent stability. In addition, it delivered a high specific capacity of 497 mA h g−1 after 80 cycles, with a current density of 0.2 A g−1 and excellent cycling stability as an anode material for SIBs.

Selenium-infiltrated metal–organic framework-derived porous carbon nanofibers comprising interconnected bimodal pores for Li–Se batteries with high capacity and rate performance

The rational design of cathode materials for lithium–selenium (Li–Se) batteries is essential to achieve high-performance electrochemical properties with long cycle life and excellent rate capability. In this paper, novel porous carbon nanofibers with bimodal pores (micro/meso), as efficient cathode hosts for Li–Se batteries, were successfully synthesized by carbonization of electrospun zeolitic imidazole framework-8/polyacrylonitrile (ZIF-8/PAN) nanofibers and further chemical activation. Mesopores originated from carbonization of ZIF-8 embedded in the carbon nanofiber, and micropores were further introduced via KOH activation. During the activation step, micropores were introduced to the ZIF-8-derived meso porous carbon cages and within the carbon nanofibers, resulting in the formation of bimodal porous carbon nanofibers with enlarged pore volumes. Owing to their mesopores for easy access of electrolyte and high utilization of chain-like selenium with low-range ordering within the micropore, the selenium-loaded bimodal porous carbon nanofibers exhibited high discharge capacity and superb rate performance. The discharge capacities of the nanofibers at the 2nd and 300th cycle at a current density of 0.5C were 742 and 588 mA h g−1, respectively. The capacity retention calculated from the 2nd cycle was 79.2%. In addition, a discharge capacity of 568 mA h g−1 was obtained at an extremely high current density of 10.0C.

MOF-Templated N-Doped Carbon-Coated CoSe2 Nanorods Supported on Porous CNT Microspheres with Excellent Sodium-Ion Storage and Electrocatalytic Properties

Three-dimensional (3D) porous microspheres composed of CoSe2@N-doped carbon nanorod-deposited carbon nanotube (CNT) building blocks (CoSe2@NC-NR/CNT) can be successfully synthesized using CNT/Co-based metal-organic framework (ZIF-67) porous microspheres as a precursor. This strategy involves the homogeneous coating of ZIF-67 polyhedrons onto porous CNT microspheres prepared by spray pyrolysis and further selenization of the composites under an Ar/H2 atmosphere. During the selenization process, the ZIF-67 polyhedrons on the CNT backbone are transformed into N-doped carbon-coated CoSe2 nanorods by a directional recrystallization process, resulting in a homogeneous deposition of CoSe2@NC nanorods on the porous CNT microspheres. Such a unique structure of CoSe2@NC-NR/CNT microspheres facilitates the transport of ions, electrons, and mass and provides a conductive pathway for electrons during electrochemical reactions. Correspondingly, the composite exhibits a superior dual functionality as both an electrocatalyst for the hydrogen evolution reaction (HER) and an electrode for sodium-ion batteries (SIBs). The CoSe2@NC-NR/CNT microspheres exhibit a small Tafel slope (49.8 mV dec-1) and a superior stability for HER. Furthermore, the composite delivers a high discharge capacity of 555 mA h g-1 after 100 cycles at a current density of 0.2 A g-1 and a good rate capability for SIBs.

Metal-Organic-Framework-Derived N-Doped Hierarchically Porous Carbon Polyhedrons Anchored on Crumpled Graphene Balls as Efficient Selenium Hosts for High-Performance Lithium–Selenium Batteries

Developing carbon scaffolds showing rational pore structures as cathode hosts is essential for achieving superior electrochemical performances of lithium-selenium (Li-Se) batteries. Hierarchically porous N-doped carbon polyhedrons anchored on crumpled graphene balls (NPC/CGBs) are synthesized by carbonizing a zeolitic imidazolate framework-8 (ZIF-8)/CGB composite precursor, producing an unprecedented effective host matrix for high-performance Li-Se batteries. Mesoporous CGBs obtained by one-pot spray pyrolysis are used as a highly conductive matrix for uniform polyhedral ZIF-8 growth. During carbonization, ZIF-8 polyhedrons on mesoporous CGBs are converted into N-doped carbon polyhedrons showing abundant micropores, forming a high-surface-area, high-pore-volume hierarchically porous NPC/CGB composite whose small unique pores effectively confine Se during melt diffusion, thereby providing conductive electron pathways. Thus, the integrated NPC/CGB-Se composite ensures high Se utilization originating from complete electrochemical reactions between Se and Li ions. The NPC/CGB-Se composite cathode exhibits high discharge capacities (998 and 462 mA h g-1 at the 1st and 1000th cycles, respectively, at a 0.5 C current density), good capacity retention (68%, calculated from the 3rd cycle), and excellent rate capability. A discharge capacity of 409 mA h g-1 is achieved even at an extremely high (15.0 C) current density.

Synthesis of hierarchical structured Fe2O3 rod clusters with numerous empty nanovoids via the Kirkendall effect and their electrochemical properties for lithium-ion storage

The Kirkendall effect has been widely applied to prepare hollow/porous metal-oxide-based composites. We propose a new mechanism for the transformation of hierarchical structured metal selenides into their corresponding metal oxides with unique structures via the Kirkendall effect. Based on this mechanism, hierarchical structured iron oxide clusters comprising one-dimensional nanorods with numerous empty nanovoids were successfully prepared. FeSe2 rod clusters synthesized by a one-pot hydrothermal process were post-treated under an air atmosphere. During the oxidation process, FeSe2@FeOx–Se@Fe2O3 and FeOx–Se@Fe2O3 intermediates are formed owing to the different diffusion rates of iron cations, the selenium component, and oxygen gas. As the oxidation proceeded, elimination by evaporation of the SeO2 layer formed by the reaction of the diffused-out metalloid Se and oxygen gas and oxidation of FeOx resulted in porous Fe2O3 nanorods with numerous interconnected empty nanovoids. When employed as a lithium-ion battery anode, hierarchical Fe2O3 rod clusters exhibited high reversible discharge capacity, good cycling stability, and excellent rate performance. Following 200 cycles, their discharge capacity is 1318 mA h g−1 at a current density of 1 A g−1. Additionally, the rod clusters delivered a discharge capacity of 745 mA h g−1 at a high current density of 10 A g−1.