Minjoon Park

Integrating NiCo Alloys with Their Oxides as Efficient Bifunctional Cathode Catalysts for Rechargeable Zinc-Air Batteries

The lack of high-efficient, low-cost, and durable bifunctional electrocatalysts that act simultaneously for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is currently one of the major obstacles to commercializing the electrical rechargeability of zinc-air batteries. A nanocomposite CoO-NiO-NiCo bifunctional electrocatalyst supported by nitrogen-doped multiwall carbon nanotubes (NCNT/CoO-NiO-NiCo) exhibits excellent activity and stability for the ORR/OER in alkaline media. More importantly, real air cathodes made from the bifunctional NCNT/CoO-NiO-NiCo catalysts further demonstrated superior performance to state-of-the-art Pt/C or Pt/C+IrO2 catalysts in primary and rechargeable zinc-air batteries.

All‐Solid‐State Cable‐Type Flexible Zinc–Air Battery

A cable-type flexible Zn-air battery with a spiral zinc anode, gel polymer electrolyte (GPE), and air cathode coated on a nonprecious metal catalyst is designed in order to extend its application area toward wearable electronic devices.

Corn protein-derived nitrogen-doped carbon materials with oxygen-rich functional groups: a highly efficient electrocatalyst for all-vanadium redox flow batteries

Recent studies on all-vanadium redox flow batteries (VRFBs) have focused on carbon-based materials for cost-effective electrocatalysts to commercialize them in grid-scale energy storage markets. We report an environmentally friendly and safe method to produce carbon-based catalysts by corn protein self-assembly. This new method allows carbon black (CB) nanoparticles to be coated with nitrogen-doped graphitic layers with oxygen-rich functionalities (N-CB). We observed increased catalytic activity of this catalyst toward both V2+/V3+ and VO2+/VO2+ ions, showing a 24% increased mass transfer process and ca. 50 mV higher reduction onset potential compared to CB catalyst. It is believed that the abundant oxygen active sites and nitrogen defects in the N-CB catalyst are beneficial to the vanadium redox reaction by improving the electron transfer rate and giving faster vanadium ion transfer kinetics.

Nanostructured Electrocatalysts for All-Vanadium Redox Flow Batteries.

Vanadium redox reactions have been considered as a key factor affecting the energy efficiency of the all-vanadium redox flow batteries (VRFBs). This redox reaction determines the reaction kinetics of whole cells. However, poor kinetic reversibility and catalytic activity towards the V(2+)/V(3+) and VO(2+)/VO2(+) redox couples on the commonly used carbon substrate limit broader applications of VRFBs. Consequently, modified carbon substrates have been extensively investigated to improve vanadium redox reactions. In this Focus Review, recent progress on metal- and carbon-based nanomaterials as an electrocatalyst for VRFBs is discussed in detail, without the intention to provide a comprehensive review on the whole components of the system. Instead, the focus is mainly placed on the redox chemistry of vanadium ions at a surface of various metals, different dimensional carbons, nitrogen-doped carbon nanostructures, and metal-carbon composites.

13.2% efficiency Si nanowire/PEDOT:PSS hybrid solar cell using a transfer-imprinted Au mesh electrode

In recent years, inorganic/organic hybrid solar cell concept has received growing attention for alternative energy solution because of the potential for facile and low-cost fabrication and high efficiency. Here, we report highly efficient hybrid solar cells based on silicon nanowires (SiNWs) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using transfer-imprinted metal mesh front electrodes. Such a structure increases the optical absorption and shortens the carrier transport distance, thus, it greatly increases the charge carrier collection efficiency. Compared with hybrid cells formed using indium tin oxide (ITO) electrodes, we find an increase in power conversion efficiency from 5.95% to 13.2%, which is attributed to improvements in both the electrical and optical properties of the Au mesh electrode. Our fabrication strategy for metal mesh electrode is suitable for the large-scale fabrication of flexible transparent electrodes, paving the way towards low-cost, high-efficiency, flexible solar cells.

Organic-Catholyte-Containing Flexible Rechargeable Lithium Batteries

Organic-catholyte-containing flexible rechargeable lithium batteries are developed using fused cyclic quinone derivatives. The structural dependence of the quinone isomers in the liquid catholyte is studied using a combined experimental and theoretical approach. Stable electrochemical performance even under severe bending/stretching deformations is successfully demonstrated by prototype batteries containing liquid catholytes.

Long-term durable silicon photocathode protected by a thin Al2O3/SiOx layer for photoelectrochemical hydrogen evolution

The long-term steady production of H2 is vital for p-Si photocathodes. However, the oxidation of a Si photoelectrode substantially deteriorates its performance, over time. This study demonstrates that a thin Al2O3 layer deposited over the Si can prevent oxidation and also reduce overpotential, via a surface passivation effect.

Single crystalline pyrochlore nanoparticles with metallic conduction as efficient bi-functional oxygen electrocatalysts for Zn-air batteries

Oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) electrocatalysts including carbon-, non-precious metal-, metal alloy-, metal oxide-, and carbide/nitride-based materials are of great importance for energy conversion and storage technologies. Among them, metal oxides (e.g., perovskite and pyrochlore) are known to be promising candidates as electrocatalysts. Nevertheless, the intrinsic catalytic activities of pyrochlore oxides are still poorly understood because of the formation of undesirable phases derived from the synthesis processes. Herein, we present highly pure single crystalline pyrochlore nanoparticles with metallic conduction (Pb2Ru2O6.5) as an efficient bi-functional oxygen electrocatalyst. Notably, it has been experimentally shown that the covalency of Ru–O bonds affects the ORR and OER activities by comparing the X-ray absorption near edge structure (XANES) spectra of the metallic Pb2Ru2O6.5 and insulating Sm2Ru2O7 for the first time. Moreover, we followed the interatomic distance changes of Ru–O bonds using in situ X-ray absorption spectroscopy (XAS) to investigate the structural stabilities of the pyrochlore catalysts during electrocatalysis. The highly efficient metallic Pb2Ru2O6.5 exhibited outstanding bi-functional catalytic activities and stabilities for both ORR and OER in aqueous Zn–air batteries.

Prophylaxis for deep vein thrombosis with aspirin or low molecular weight dextran in Korean patients undergoing total hip replacement;a randomized controlled trial

Summary. 150 Korean patients undergoing primary uncemented total hip replacement were randomized into 3 treatment groups for deep vein thrombosis (DVT) prophylaxis. Group A(50) were controls; Group B(50) received aspirin 1.2 g daily in 3 divided doses from 2 days before, to 14 days after surgery; Group C(50), received low molecular weight dextran 500 ml, infused intravenously at 50 ml/hour during surgery, and on each of the following 2 days. Contrast venograms were performed prior to surgery and 7 – 10 days after. The incidence of DVT was 20% in the control group, 12% in the aspirin group (p <0.1 vs control), and 6% in the dextran group (p <0.05 vs control). In patients developing DVT, the ratio of proximal to distal thrombi was increased in the control group as compared to treated groups (4 : 1 in the control group vs 1.5 : 1 in the treated groups). Both aspirin and dextran were well tolerated. Obesity (p <0.05) and long-term administration of steroids (p <0.05) were risk factors for deep vein thrombosis which reached statistical significance in the control group. Intraoperative venograms performed on 10 patients, showed that hip flexion (mean 40.4°) plus adduction (mean 11.5°) plus internal rotation (mean 81.5°), resulted in severe twisting or kinking of the femoral vein with stagnation of blood flow. Low molecular weight dextran significantly reduce the incidence of deep venous thrombosis and aspirin, though less effective, had a similar effect. Internal rotation of the hip joint should be minimized during operation in order to limit stagnation of blood flow in the femoral vein.Résumé. 150 patients coréens operes pour un remplacement total de hanche non-cimentée ont été triés au hasard en 3 groupes de traitement de prophylaxie de la thrombose. Le groupe A(50): groupe de contrôle. Le groupe B a reçu 1.2 g d’aspirine par jour en 3 doses à compter de 2 jours avant l’opération jusqu’à 14 jours après celle-ci. Le groupe C(50) a reçu des petites molécules de dextrane 500 ml par injection intraveineuse de 50 ml/heure pendant l’intervention et durant les 2 jours suivants. Une phlébographie de contrôle a été rèalisé avant l’intervention et 7 – 10 jours après celle-ci. Le taux de thrombose veineuse profonde (TVP) était de 20% dans le groupe du contrôle, 12% dans le groupe «aspirine» (p <0.1 contre contrôle) et 6% dans le groupe de «dextrane» (p <0.05 contre contrôle). Parmi les patients développant une TVP, le ratio proximal/distal était cliniquement augmenté dans le groupe du contrôle comparé avec les groupes traités (4 : 1 dans le groupe du contrôle contre 1.5 : 1 dans les groupes traités). L’aspirine et la dextrane ont bien été tolerées. L’obésité (p <0.05) et l’administration à long-terme de stéroïde étaient des facteurs de risque pour la TVP avec une signification statistique dans le groupe du contrôle. Les phlébographies per-operatoires faites chez 10 patients ont montré que la flexion (moyenne 40.4°) plus l’adduction (moyenne 11.5°) plus la rotation interne (moyenne 81.5°) se sont traduites par une grave torsion de la veine fémorale accompagné d’une stagnation circulatoire. Les petites molécules de dextrane réduisent significativement le taux de trombose veineuse chez les patients coréens qui ont subi une intervention chirurgicale de remplacement de la hanche et l’aspirine, quoique moins efficace, réduit aussi le taux de TVP. Pour limiter la stagnation de la circulation du sang dans la veine fémorale pendant l’intervention chirurgicale, la rotation interne de la hanche doit être minimisée.

Photoelectrochemical water splitting employing a tapered silicon nanohole array

An effective photocathode adopting a tapered Si nanohole (SiNH) array has been developed for photoelectrochemical water splitting. The tapered feature of SiNH photocathodes resulted in a gradation of the refractive indices between Si and air, such that the mismatching of optical impedance was alleviated and light absorption was enhanced. Adjusting the depth of the SiNHs successfully simulated the number of dielectric layers, optimizing the destructive interference for an antireflective coating (ARC). Only a 200 nm-thin NH array was required to absorb ∼96% of solar spectral irradiance for photoelectrochemical applications. This thickness also minimized the undesirable surface recombination loss. When compared to a similar system using a planar technology, the formation of NHs was observed to cause an increase in the optical bandgap. This could generate a surface-passivation effect, resulting in a lowering of dark current and an increase in photovoltage, which could be utilized for an anodic shift of the onset voltage. Due to the addition of tapered SiNHs, the photogenerated current was improved by ∼30% (∼33 mA cm−2) compared to a planar counterpart (∼25 mA cm−2), while the overpotential required for H2 evolution was reduced.