Ki Ro Yoon

One-Dimensional RuO2/Mn2O3 Hollow Architectures as Efficient Bifunctional Catalysts for Lithium–Oxygen Batteries

Rational design and massive production of bifunctional catalysts with fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics are critical to the realization of highly efficient lithium-oxygen (Li-O2) batteries. Here, we first exploit two types of double-walled RuO2 and Mn2O3 composite fibers, i.e., (i) phase separated RuO2/Mn2O3 fiber-in-tube (RM-FIT) and (ii) multicomposite RuO2/Mn2O3 tube-in-tube (RM-TIT), by controlling ramping rate during electrospinning process. Both RM-FIT and RM-TIT exhibited excellent bifunctional electrocatalytic activities in alkaline media. The air electrodes using RM-FIT and RM-TIT showed enhanced overpotential characteristics and stable cyclability over 100 cycles in the Li-O2 cells, demonstrating high potential as efficient OER and ORR catalysts.

Rational Design of Efficient Electrocatalysts for Hydrogen Evolution Reaction: Single Layers of WS2 Nanoplates Anchored to Hollow Nitrogen-Doped Carbon Nanofibers.

To exploit the benefits of nanostructuring for enhanced hydrogen evolution reaction (HER), we employed coaxial electrospinning to synthesize single-layered WS2 nanoplates anchored to hollow nitrogen-doped carbon nanofibers (WS2@HNCNFs) as efficient electrocatalysts. For comparison, bulk WS2 powder and single layers of WS2 embedded in nitrogen-doped carbon nanofibers (WS2@NCNFs) were synthesized and electrochemically tested. The distinctive design of the WS2@HNCNFs enables remarkable electrochemical performances showing a low overpotential with reduced charge transfer resistance, a small Tafel slope, and excellent durability. The experimental results highlight the importance of nanostructure engineering in electrocatalysts for enhanced HER.

A High-Capacity and Long-Cycle-Life Lithium-Ion Battery Anode Architecture: Silver Nanoparticle-Decorated SnO2/NiO Nanotubes

The combination of high-capacity and long-term cyclability has always been regarded as the first priority for next generation anode materials in lithium-ion batteries (LIBs). To meet these requirements, the Ag nanoparticle decorated mesoporous SnO2/NiO nanotube (m-SNT) anodes were synthesized via an electrospinning process, followed by fast ramping rate calcination and subsequent chemical reduction in this work. The one-dimensional porous hollow structure effectively alleviates a large volume expansion during cycling as well as provides a short lithium-ion duffusion length. Furthermore, metallic nickel (Ni) nanoparticles converted from the NiO nanograins during the lithiation process reversibly decompose Li2O during delithiation process, which significantly improves the reversible capacity of the m-SNT anodes. In addition, Ag nanoparticles uniformly decorated on the m-SNT via a simple chemical reduction process significantly improve rate capability and also contribute to long-term cyclability. The m-SNT@Ag anodes exhibited excellent cycling stability without obvious capacity fading after 500 cycles with a high capacity of 826 mAh g-1 at a high current density of 1000 mA g-1. Furthermore, even at a very high current density of 5000 mA g-1, the charge-specific capacity remained as high as 721 mAh g-1, corresponding to 60% of its initial capacity at a current density of 100 mA g-1.

Synthesis of Ni-based co-catalyst functionalized W:BiVO4 nanofibers for solar water oxidation

We report on the synthesis of highly porous, 1-D tungsten-doped BiVO4 nanofibers (W:BiVO4 NFs). To facilitate photocatalysis, we introduced nickel nanoparticles (NiOx NPs) as co-catalysts on the surface of W:BiVO4 NFs. The outstanding water oxidation performance of the NiOx NP-functionalized W:BiVO4 NFs was obtained through (i) the control of polymers/precursors to achieve porous W:BiVO4 NFs (for highly increased surface area), (ii) the control of the tungsten-doping level (for fast charge transfer), and (iii) the optimization of the loading amounts of NiOx NPs (for efficient charge pathway suppression of charge recombination).

Tailored Combination of Low Dimensional Catalysts for Efficient Oxygen Reduction and Evolution in Li–O2 Batteries

The development of efficient bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key issue pertaining high performance Li-O2 batteries. Here, we propose a heterogeneous electrocatalyst consisting of LaMnO3 nanofibers (NFs) functionalized with RuO2 nanoparticles (NPs) and non-oxidized graphene nanoflakes (GNFs). The Li-O2 cell employing the tailored catalysts delivers an excellent electrochemical performance, affording significantly reduced discharge/charge voltage gaps (1.0 V at 400 mA g(-1) ), and superior cyclability for over 320 cycles. The outstanding performance arises from (1) the networked LaMnO3 NFs providing ORR/OER sites without severe aggregation, (2) the synergistic coupling of RuO2 NPs for further improving the OER activity and the electrical conductivity on the surface of the LaMnO3 NFs, and (3) the use of GNFs providing a fast electronic pathway as well as improved ORR kinetics.

Formation of a Surficial Bifunctional Nanolayer on Nb2O5 for Ultrastable Electrodes for Lithium-Ion Battery

Safe and long cycle life electrode materials for lithium-ion batteries are significantly important to meet the increasing demands of rechargeable batteries. Niobium pentoxide (Nb2 O5 ) is one of the highly promising candidates for stable electrodes due to its safety and minimal volume expansion. Nevertheless, pulverization and low conductivity of Nb2 O5 have remained as inherent challenges for its practical use as viable electrodes. A highly facile method is proposed to improve the overall cycle retention of Nb2 O5 microparticles by ammonia (NH3 ) gas-driven nitridation. After nitridation, an ultrathin surficial layer (2 nm) is formed on the Nb2 O5 , acting as a bifunctional nanolayer that allows facile lithium (Li)-ion transport (10-100 times higher Li diffusivity compared with pristine Nb2 O5 microparticles) and further prevents the pulverization of Nb2 O5 . With the subsequent decoration of silver (Ag) nanoparticles (NPs), the low electric conductivity of nitridated Nb2 O5 is also significantly improved. Cycle retention is greatly improved for nitridated Nb2 O5 (96.7%) compared with Nb2 O5 (64.7%) for 500 cycles. Ag-decorated, nitridated Nb2 O5 microparticles and nitridated Nb2 O5 microparticles exhibit ultrastable cycling for 3000 cycles at high current density (3000 mA g-1 ), which highlights the importance of the surficial nanolayer in improving overall electrochemical performances, in addition to conductive NPs.

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li–O2 Batteries

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O2) batteries, the irreversible formation of Li2O2, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co4N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co4N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co4N-decorated CNF (Co4N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O2 cells. During cycling, metallic Co4N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co4N nanorods promote reversible formation/decomposition of film-type Li2O2, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g-1). Moreover, pouch-type Li-air cells using Co4N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Three-Dimensional Nanofibrous Air Electrode Assembled With Carbon Nanotubes-Bridged Hollow Fe2O3 Nanoparticles for High-Performance Lithium–Oxygen Batteries

Lithium-oxygen batteries have been considered as one of the most viable energy source options for electric vehicles due to their high energy density. However, they are still faced with technical challenges, such as low round-trip efficiency and short cycle life, which mainly originate from the cathode part of the battery. In this work, we designed a three-dimensional nanofibrous air electrode consisted of hierarchically structured carbon nanotube-bridged hollow Fe2O3 nanoparticles (H-Fe2O3/CNT NFs). Composite nanofibers consisted of hollow Fe2O3 NPs anchored by multiple CNTs offered enhanced catalytic sites (interconnected hollow Fe2O3 NPs) and fast charge-transport highway (bridged CNTs) for facile formation and decomposition of Li2O2, leading to outstanding cell performance: (1) Swagelok cell exhibited highly reversible cycling characteristics for 250 cycles with a fixed capacity of 1000 mAh g-1 at a current density of 500 mA g-1. (2) A module composed of two pouch-type cells stably powered an light-emitting diode lamp operated at 5.0 V.