Sang Hyun Ahn

Korea Institute of Science and Technology, Sŏul, Seoul, South Korea

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Publications (50)114.67 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: To reduce the use of noble metals, iridium oxide (IrO2) catalysts are prepared on carbon paper (CP) by electrodeposition for use as the anodes in polymer electrolyte membrane water electrolysis (PEMWE). The activities of the fabricated electrodes toward the oxygen evolution reaction are evaluated. The loading amount and morphology of the IrO2 deposits are varied by controlling the electrodeposition potential (Edep) and time (tdep). When electrodeposited at 0.7 V, the IrO2 loading ranges from 0.007 to 0.464 mg cm−2 (tdep: 1 ∼ 30 min). In the PEMWE test at 90 °C, the IrO2/CP electrode with an IrO2 loading of 0.1 mg cm−2 shows the highest performance (1.92 A cm−2 at 1.8 V) in which high IrO2 utilization is achieved without significant exposure of the substrate carbon surface. The developed IrO2/CP electrodes provide cell performances comparable to those in previous reports with a higher IrO2 loading, indicating markedly enhanced mass activity.
    Applied Catalysis B Environmental 12/2015; 179. DOI:10.1016/j.apcatb.2015.05.027 · 6.01 Impact Factor
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    ABSTRACT: NiW alloy catalysts were prepared on a Cu foil substrate by using a co-electrodeposition method for use in the hydrogen evolution reaction (HER) in alkaline water electrolysis. The compositions of the alloy catalysts were controlled over a wide range by varying the molar ratio [W6+]/[Ni2+] in the electrolyte. At high W contents (∼41 at %) in the deposited catalyst, the morphologies of the NiW alloys showed drastic changes compared with those of electrodeposited Ni, in particular a significant decrease in surface coverage and aggregates’ sizes. All of NiW alloys deposited in a citrate-containing bath demonstrated an amorphous structure, regardless of W content. Electrochemical measurements employing repeated cyclic voltammetry (CV) showed enhanced HER activities for the NiW alloy catalysts compared with the pure Ni catalyst. The maximum HER activity was obtained with a W content of 41 at %; the alloying effects, including morphological changes, and the high affinity for hydrogen absorption shown by this alloy's amorphous structure are dominant factors in the enhanced HER activity. The results presented herein provide advanced information of significant relationship between material properties and HER activity of NiW alloys.
    Applied Surface Science 05/2015; 349. DOI:10.1016/j.apsusc.2015.05.040 · 2.54 Impact Factor
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    ABSTRACT: We have investigated phosphate adsorption on the Au surface by in situ electrochemical surface-enhanced Raman spectroscopy to assess the possible use of Au-based alloys as catalysts for the oxygen reduction reaction (ORR) in high-temperature proton exchange membrane fuel cells. Compared with its strong adsorption in neutral or basic electrolytes, the adsorption of phosphate species on the Au surface were significantly altered in acidic electrolyte. Thus, PtxAu100-x catalysts of various compositions were prepared by electrodeposition and exhibited better ORR activities than pure Pt in phosphoric acid electrolyte, which can be explained in terms of alloying effects and the different phosphate adsorption behavior.
    Applied Catalysis B Environmental 04/2015; 165:495-502. DOI:10.1016/j.apcatb.2014.10.042 · 6.01 Impact Factor
  • Applied Catalysis B Environmental 03/2015; 174-175. DOI:10.1016/j.apcatb.2015.03.010 · 6.01 Impact Factor
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    ABSTRACT: Electrochemical reduction of CO2 to HCOOH was performed on a Sn electrode using a proton exchange membrane-embedded electrolysis cell. The effects of reaction conditions such as catholyte and anolyte types, reduction potential, catholyte pH, and reaction temperature on the amount of HCOOH and its faradaic efficiency were investigated. Four different electrolytes (KOH, KHCO3, KCl, KHSO4) were chosen as the candidate catholyte and anolyte; the most suitable electrolyte was chosen by monitoring the amount of HCOOH and faradaic efficiency. The effect of the pH of the selected catholyte on the conversion of CO2 to HCOOH was also investigated. In addition, the reaction temperature was varied and its effect was studied. From the observations made, we determined the optimal reaction conditions for the production of HCOOH via the electrochemical reduction of CO2 by a systematic approach.
    International Journal of Hydrogen Energy 10/2014; 39(29):16506–16512. DOI:10.1016/j.ijhydene.2014.03.145 · 2.93 Impact Factor
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    ABSTRACT: Carbon-supported Pt nanoparticles have been widely used as electrocatalysts for electrochemical hydrogen pumps. However, Pt surfaces are susceptible to poisoning under CO2 atmosphere, and as a result, need greater applied cell voltages. Instead of Pt as an anode catalyst in electrochemical hydrogen pumps, we synthesized Ir-based catalysts and characterized them by XRD, XPS, TEM, and TGA. The electrochemical characteristics of the Ir catalysts were evaluated by a halfcell test, and their catalytic activities toward the hydrogen oxidation and hydrogen evolution reactions were evaluated by micro polarization analysis. The exchange current density of the Ir catalyst that was heat treated at 300 degrees C was larger than that of commercial Pt. CO2 stripping analysis confirmed that the Ir catalyst was not affected by CO2, unlike the Pt catalyst. Focusing on H-2 separation from H-2/CO2 gas, an evaluation using a single-cell test indicated that the Ir catalyst performed better than the Pt catalyst.
    Applied Catalysis B: Environmental 10/2014; s 158–159:348–354. DOI:10.1016/j.apcatb.2014.04.016 · 5.63 Impact Factor
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    ABSTRACT: NiCu alloys have been suggested as potential candidates for catalysts in glucose oxidation. In this study, NiCu alloys with different compositions were prepared on a glassy carbon substrate by changing the electro-deposition potential to examine the effect of Ni/Cu ratio S in alloys on catalytic activity toward glucose oxidation. Cyclic voltammetry and chronoamperometry showed that NiCu alloys had higher catalytic activity than pure Ni and Cu catalysts. Especially, Ni59Cu41 had superior catalytic activity, which was about twice that of Ni at a given oxidation potential. X-ray analyses showed that the oxidation state of Ni in NiCu alloys was increased with the content of Cu by lattice expansion. Ni components in alloys with higher oxidation state were more effective in the oxidation of glucose.
    Bulletin- Korean Chemical Society 07/2014; 35(7):2019-2024. DOI:10.5012/bkcs.2014.35.7.2019 · 0.84 Impact Factor
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    ABSTRACT: Silicon-based micro-reactors for the fuel processing of liquefied natural gas (LNG) were fabricated using silicon technologies. The micro-LNG steam reformer achieves a LNG conversion of 77.4%, and the hydrogen composition of the product was 73.3% at 600 degrees C. The product gas was supplied to consecutive micro-reactors to carry out carbon monoxide removal through a high-temperature water gas shift (HTS) reaction and a low-temperature water gas shift (LTS) reaction. Under operating conditions we investigated, the micro-HTS and LTS reactors demonstrated the highest carbon monoxide conversion of 61.5% at 450 degrees C and 77.5% at 300 degrees C, respectively. The final product gas of the micro-fuel processor was composed of 75.7% hydrogen and 0.7% carbon monoxide.
    Chemical Engineering Journal 07/2014; 247:9–15. DOI:10.1016/j.cej.2014.02.108 · 4.32 Impact Factor
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    ABSTRACT: A membrane electrode assembly (MEA) for use in an alkaline water electrolyzer is fabricated by sandwiching an anion exchange membrane (AEM) between two electrodes prepared by the electrodeposition of Ni (used as a catalyst) on carbon papers (CPs) acting as gas diffusion layers. The conditions for Ni electrodepositon are determined on the basis of the results of half-cell tests for hydrogen and oxygen evolution reactions. An MEA fabricated by the direct electrodeposition of an extremely low Ni amount of 8.5 mu g(Ni) cm(-2) exhibits a high cell performance of 150 mA cm(-2) at 1.9 V when a 1.0 M KOH solution is supplied to the cathode. Electrodeposited Ni is uniformly distributed on the surface of CP fibers, thereby providing a large electrochemical surface area for gas evolution reactions and thus maintaining high catalyst utilization. It is suggested that control of bubble generation and AEMs with higher ionic conductivity would further increase MEA performance. (c) 2014 Elsevier B.V. All rights reserved.
    Applied Catalysis B Environmental 07/2014; s 154–155:197–205. DOI:10.1016/j.apcatb.2014.02.021 · 6.01 Impact Factor
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    ABSTRACT: As a catalyst for the hydrogen evolution reaction (HER) in alkaline water splitting, NiCo alloys of various compositions were prepared through electrodeposition onto Cu substrates. The composition of each alloy catalyst was varied by controlling the molar ratio of Co2+ ions in the electrolyte. With an increase in the Co content, the morphologies of the NiCo alloys were progressively changed from a round to polygonal shape. The NiCo alloys all exhibited a Ni rich surface, as confirmed by the bulk-to-surface compositional ratio and degree of alloying. The catalytic activities of the NiCo alloys toward the HER of water splitting were electrochemically tested in a KOH electrolyte, and the specific activities were characterized by considering the electrochemical surface areas of Ni and Co. The effect of alloying was demonstrated to be a significant enhancement of HER activity, resulting from a change in the electronic structures of Ni and Co.
    Applied Surface Science 07/2014; 307:146–152. DOI:10.1016/j.apsusc.2014.03.197 · 2.54 Impact Factor
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    ABSTRACT: Objectives Although diffuse splenic 18F-fluorodeoxyglucose (F-18 FDG) uptake exceeding hepatic activity, is considered abnormal, its clinical significance is rarely discussed in the literature. The aim of this study was to determine the contributing factors causing diffusely increased splenic FDG uptake in patients with cholangiocarcinoma. Methods From January 2010 to March 2013, 140 patients (84 men, 56 women) were enrolled in this study. All patients had been diagnosed with cholangiocarcinoma and underwent F-18 FDG positron emission tomography/computed tomography (PET/CT) for the pretreatment staging work up. Clinical records were reviewed retrospectively. Various hematological parameters, C-reactive protein (CRP) level, CEA, CA19-9, pancreatic enzymes and liver function tests were conducted within 2 days after the F-18 FDG PET/CT study. Results Diffuse splenic uptake was observed in 23 patients (16.4%). Of those, 19 patients (82.6%) underwent endoscopic retrograde cholangiopancreastography (ERCP) 7 days before F-18 FDG PET/CT. The CRP level (p < 0.001) and white blood cell count (p = 0.023) were significantly higher in the group of patients with diffuse splenic FDG uptake. The hemoglobin (p < 0.001) and the hematocrit (p < 0.001) were significantly lower in patients with diffuse splenic FDG uptake. Pancreatic enzymes, liver function test results, and tumor markers were not significantly different between the patients who did or did not have diffusely increased splenic FDG uptake. The significant factors for diffuse splenic F-18 FDG uptake exceeding hepatic F-18 FDG uptake on multivariate analysis included: performing ERCP before F-18 FDG PET-CT (odds ratio [OR], 77.510; 95% CI, 7.624-132.105), and the presence of leukocytosis (OR, 12.436; 95% CI, 2.438-63.445) or anemia (OR, 1.211; 95% CI, 1.051-1.871). Conclusion In conclusion, our study demonstrated that concurrent inflammation could be associated with diffusely increased splenic FDG uptake. We suggest that performing ERCP before F-18 FDG PET/CT could cause acute inflammation which may induce splenic FDG activity.
    06/2014; 48(2):137-143. DOI:10.1007/s13139-013-0246-0
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    ABSTRACT: A Pt-shell-Pd-core/C catalyst is prepared via electroless deposition and galvanic displacement. The catalyst is active toward the electro-oxidation of methanol and is more stable against COad-poisoning than a commercial Pt/C catalyst. The stable activity of Pt-shell-Pd-core/C is ascribed to the tuned electronic property of the Pt over-layer in the Pt-shell-Pd-core/C, which leads to weak binding with COad and increases the kinetics of OHad formation. The weakened binding property of the surface Pt with COad and the facile oxidation of COad by OHad were confirmed by a spectroscopic analysis and in a COad-stripping experiment, respectively. The electro-oxidation of COad by Had is the rate-determining step of methanol oxidation. Therefore, the accelerated formation of OHad contributes to the overall oxidation reaction, preventing COad-poisoning. In addition, Pt-shell-Pd-core/C maintains its activity longer than Pt/C does during a prolonged cycle experiment. Copyright
    International Journal of Hydrogen Energy 03/2014; 39(8):3681–3689. DOI:10.1016/j.ijhydene.2013.12.118 · 2.93 Impact Factor
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    ABSTRACT: Although many patients with hepatocellular carcinoma experience recurrence within 2 years after hepatectomy, some patients with T1 and T2 hepatocellular carcinoma show recurrence-free survival for more than 5 years after surgery. This study was designed to analyze the optimal follow-up period on patients with T1 and T2 hepatocellular carcinoma (HCC) showing recurrence-free survival 5 years after surgery. One hundred seventy patients underwent hepatectomy from January 1995 to December 1999. Numbers of patients with T1 and T2 HCC were 76 and 73, respectively. The recurrence patterns of patients experiencing recurrence more than 5 years after liver resection were analyzed in aspect of clinicopathological features and follow-up period. Thirteen patients experienced recurrence more than 5 years after surgery. Only age was found as a significant factor for recurrence. Eight patients were checked regularly with 6-month intervals and the others were checked with 12-month or more intervals. The size of the recurrent tumors in the 6-month interval group had a median of 1.1 cm (range, 1 to 4.2 cm) and the size of the recurrent tumors in the 12-month or more interval group had a median of 3 cm (range, 1.6 to 4 cm). The tumor size was significantly smaller in the 6-month interval group. Though the patients with early stage HCC showed high overall survival, some patients experienced a late recurrence of more than 5 years after surgery. Patients less than 60 years old with early stage HCC should be checked regularly with 6-month intervals even over 5 years after liver resection.
    Journal of the Korean Surgical Society 12/2013; 85(6):269-74. DOI:10.4174/jkss.2013.85.6.269 · 0.62 Impact Factor
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    ABSTRACT: Carbon capture and storage (CCS) technologies have been intensively researched and developed to cope with climate change, by reducing atmospheric CO2 concentration. The electrochemical hydrogen pumps with phosphoric acid doped polybenzimidazole (PBI) membrane are evaluated as a process to concentrate CO2 and produce pure H2 from anode outlet gases (H2/CO2 mixture) of molten carbonate fuel cells (MCFC). The PBI-based hydrogen pump without humidification (160 °C) can provide higher hydrogen separation performances than the cells with perfluorosulfonic-acid membranes at a relative humidity of 43% (80 °C), suggesting that the pre-treatment steps can be decreased for PBI-based systems. With the H2/CO2 mixture feed, the current efficiency for the hydrogen separation is very high, but the cell voltage increase, compared to the pure hydrogen operation, mainly due to the larger polarization resistance at electrodes, as confirmed by electrochemical impedance spectroscopy (EIS). The performance evaluation with various Pt loadings indicates that the hydrogen oxidation reaction at anodes is rate determining, and therefore the Pt loading at cathodes can be decreased from 1.1 mg/cm2 to 0.2 mg/cm2 without significant performance decay. The EIS analysis also confirms that the polarization resistances are largely dependent on the Pt loading in anodes.
    International Journal of Hydrogen Energy 11/2013; 38(34):14816-14823. DOI:10.1016/j.ijhydene.2013.08.142 · 2.93 Impact Factor
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    ABSTRACT: NiCu alloy catalysts for alkaline water electrolysis were prepared by an electrodeposition method varying the alloy composition. When the deposition potential became more positive, the bulk and surface Cu content in NiCu alloys as well as the catalyst particle size gradually increased, which were confirmed by various spectroscopic and electrochemical techniques. The surface coverage of the catalysts was found to be a function of the deposition potential, as well. The catalytic activities of the prepared NiCu alloys to hydrogen evolution reaction (HER) were investigated with cyclic voltammetry in a 6.0 M KOH electrolyte at 298 K, and the mass activities of NiCu alloys were correlated with bulk and surface Cu contents to investigate the Cu alloying effect.
    International Journal of Hydrogen Energy 10/2013; 38(31):13493-13501. DOI:10.1016/j.ijhydene.2013.07.103 · 2.93 Impact Factor
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    ABSTRACT: We analyzed the wear characteristics according to dispersion level of MWCNT in YD-128/MWCNT composite. Specimens for this study were fabricated using mechanical stirrer after blending of YD-128 and MWCNT. To change the dispersion level, the mixture of YD-128/MWCNT was stirred using mechanical stirrer during different times, such as, 15, 30, 60, and 120 minutes. Because the direct measuring of dispersion level is difficult in the case of solid composite, we suggest an indirect method for checking qualitatively the dispersion level as follows. Firstly, using the AEH (asymptotic expansion homogenization) in-house code, we analyzed numerically the mechanical stiffness of composite using RVEs (representative volume elements) which are modeled with different dispersion level. According to the numerical results of RVEs, we verified that the mechanical stiffness is higher as the dispersion degree is better. Then, through the experimental tensile test of the fabricated specimens using UTM, we obtained that the mechanical stiffness is higher as the stirring time is longer. Consequently, we could ensure that the dispersion degree of the fabricated specimens is better as the mechanical stirring time is longer. Finally, we assessed the wear test using abrading machine with fabricated specimens. We confirmed that the abrasion loss is decreased according to the increasing of dispersion degree in the case of YD-128/MWCNT composites.
    Journal of Mechanical Science and Technology 10/2013; 27(10):3131-3138. DOI:10.1007/s12206-013-0833-0 · 0.70 Impact Factor
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    ABSTRACT: There is a growing concern about the therapeutic equivalence of the generic tacrolimus formulation (GEN Tacrolimus) to the reference tacrolimus (REF Tacrolimus) in solid organ transplantation. A prospective, randomized study of 126 de novo renal transplant patients was conducted to compare the efficacy, safety and pharmacokinetic (PK) profiles between GEN tacrolimus (n = 63) and REF tacrolimus (n = 63). The PK of tacrolimus was evaluated on Day 10 and 6 months under steady-state condition. Crossover study was carried out in 66 patients at 6 months. On Day 10, 117 patients completed PK profiles (54 GEN tacrolimus and 63 REF tacrolimus) and GEN tacrolimus showed comparable C0 (9.8 ± 2.5 versus 9.7 ± 3.0 ng/mL, P = 0.80) but significantly higher dose-normalized Cmax (309.1 ± 191.9 versus 192.5 ± 95.2 ng/mL/mg/kg, P < 0.001). The dose-normalized AUC0-12 tended to be higher in the GEN tacrolimus than in the REF tacrolimus group (1513.4 ± 935.4 versus 1262.5 ± 593.5 ng.h/mL/mg/kg, P = 0.084). Because of this early and high Cmax with a rapid decline in GEN tacrolimus concentration, the trough concentration was maintained lower than that of REF tacrolimus. At 6 months, GEN tacrolimus showed equivalent dose-normalized AUC0-12 (1882.2 ± 935.6 versus 1718.1 ± 946.3 ng.h/mL/mg/kg, P = 0.429) but still higher dose-normalized Cmax (346.3 ± 184.4 versus 273.2 ± 148.9 ng/mL/mg/kg, P = 0.056), despite a reduced trough concentration (5.7 ± 1.6 versus 6.9 ± 2.2 ng/mL, P = 0.004). PK profiles evaluated at 9 months showed that generic substitution also resulted in an 'early and high Cmax'. Efficacy and safety data were comparable over the 9-month study period. Therapeutic equivalence and the PK of GEN tacrolimus should be evaluated in patients undergoing de novo renal transplantation.
    Nephrology Dialysis Transplantation 09/2013; DOI:10.1093/ndt/gft300 · 3.49 Impact Factor
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    ABSTRACT: The activities of Au-modified Cu electrodes toward glucose oxidation are evaluated according to their fabrication conditions and physico-chemical properties. The Au-modified Cu electrodes are fabricated by the galvanic displacement of Au on a Cu substrate and the characteristics of the Au particles are controlled by adjusting the displacement time. From the glucose oxidation tests, it is found that the Au modified Cu has superior activity to the pure Au or Cu film, which is evidenced by the negative shift in the oxidation potential and enhanced current density during the electrochemical oxidation. Though the activity of the Au nanoparticles is a contributing factor, the enhanced activity of the Au-modified Cu electrode is due to the increased oxidation number of Cu through the electron transfer from Cu to more electronegative Au. The depletion of electron in Cu facilitates the oxidation of glucose. The stability of the Au-modified Cu electrode was also studied by chronoamperometry.
    Bulletin- Korean Chemical Society 09/2013; 34(9). DOI:10.5012/bkcs.2013.34.9.2685 · 0.84 Impact Factor
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    ABSTRACT: We have investigated the release of active sites blocked by bubbles attached on the surface of catalysts during the oxygen evolution reaction (OER) in alkaline water electrolysis, via the modulation of the wetting properties of the four different morphologies of a nickel catalyst.
    Chemical Communications 09/2013; 49(81). DOI:10.1039/c3cc44891f · 6.72 Impact Factor
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    ABSTRACT: The effect of Pt loading on the performance of proton exchange membrane fuel cells with atmospheric air feed was evaluated at various relative humidities. The membrane electrode assemblies (MEAs) were fabricated by decal methods with high Nafion ionomer content (30 and 40 wt.%). When the Pt loading was decreased, the performance of the MEAs with an ionomer content of 30 wt.% gradually decreased, mainly due to the insufficient active Pt surfaces with low proton conductivity. With a higher ionomer content of 40 wt.%, the activation overpotential was not significantly increased by the decrease in Pt loading, and the concentration overpotential could be largely reduced by decreasing the Pt loading to 0.25 mg/cm(2). When the Pt loading was further decreased to 0.15 mg/cm(2), even though the flooding became more severe, the cell performance at 0.6 V and intermediate relative humidity of 55% was about 71.6%, compared to the MEA with a high Pt loading of 0.35 mg/cm(2) (ionomer content: 30 wt.%). The cell performance could be further enhanced by decreasing the ionomer content in the anode to enhance the water back diffusion. Copyright
    International Journal of Hydrogen Energy 08/2013; 38(23):9826-9834. DOI:10.1016/j.ijhydene.2013.05.123 · 2.93 Impact Factor

Publication Stats

182 Citations
114.67 Total Impact Points

Institutions

  • 2013–2015
    • Korea Institute of Science and Technology
      • Fuel Cell Research Center
      Sŏul, Seoul, South Korea
    • Wonkwang University
      • Division of Mechanical and Automotive Engineering
      Riri, North Jeolla, South Korea
    • Wonju Severance Christian Hospital
      Genshū, Gangwon-do, South Korea
    • Seoul National University Hospital
      • Department of Surgery
      Seoul, Seoul, South Korea
  • 2014
    • Pusan National University
      • Department of Nuclear Medicine
      Tsau-liang-hai, Busan, South Korea
  • 2009–2013
    • Hanyang University
      • Division of Chemical Engineering and Bioengineering
      Sŏul, Seoul, South Korea
    • University of Seoul
      • School of Computer Science
      Sŏul, Seoul, South Korea
  • 2008–2013
    • Seoul National University
      • • Institute of Chemical Processes
      • • Department of Chemical and Biological Engineering
      • • Department of Surgery
      Sŏul, Seoul, South Korea
  • 2012
    • Yonsei University Hospital
      • Department of Internal Medicine
      Sŏul, Seoul, South Korea
  • 2007
    • Dongguk University
      Sŏul, Seoul, South Korea