Junbom Kim

Ordered mesoporous carbon–carbon nanotube nanocomposites as highly conductive and durable cathode catalyst supports for polymer electrolyte fuel cells

Ordered mesoporous carbon–carbon nanotube (OMC–CNT) nanocomposites were prepared and used as catalyst supports for polymer electrolyte fuel cells. The OMC–CNT composites were synthesized via a nanocasting method that used ordered mesoporous silica as a template and Ni–phthalocyanine as a carbon source. For comparison, sucrose and phthalocyanine were used to generate two other OMCs, OMC(Suc) and OMC(Pc), respectively. All three carbons exhibited hexagonally ordered mesostructures and uniform mesopores. Among the three carbons the OMC–CNT nanocomposites showed the highest electrical conductivity, which was due to the nature of their graphitic framework as well as their lower interfacial resistance. The three carbons were then used as fuel cell catalyst supports. It was found that highly dispersed Pt nanoparticles (ca. ∼1.5 nm in size) could be dispersed on the OMCs via a simple impregnation–reduction method. The activity and kinetics of the oxygen reduction reaction (ORR), measured by the rotating ring-disk electrode technique revealed that the ORR over the Pt/OMC catalysts followed a four-electron pathway. Among the three Pt/OMC catalysts, the Pt/OMC–CNT catalyst resulted in the highest ORR activity, and after an accelerated durability test the differences in the ORR activities of the three catalysts became more pronounced. In single cell tests, the Pt/OMC–CNT-based cathode showed a current density markedly greater than those of the other two cathodes after a high-voltage degradation test. These results were supported by the fact that the Pt/OMC–CNT-based cathode had the lowest resistance, which was probed by electrochemical impedance spectroscopy (EIS). The results of the single cell tests as well as those of the EIS-based measurements indicate that the rigidly interconnected structure of the OMC–CNT as well as their highly conductive frameworks are concomitantly responsible for the OMC–CNT nanocomposites exhibiting higher current density and durability than the other two carbons.

Durability test with fuel starvation using a Pt/CNF catalyst in PEMFC

In this study, a catalyst was synthesized on carbon nanofibers [CNFs] with a herringbone-type morphology. The Pt/CNF catalyst exhibited low hydrophilicity, low surface area, high dispersion, and high graphitic behavior on physical analysis. Electrodes (5 cm2) were prepared by a spray method, and the durability of the Pt/CNF was evaluated by fuel starvation. The performance was compared with a commercial catalyst before and after accelerated tests. The fuel starvation caused carbon corrosion with a reverse voltage drop. The polarization curve, EIS, and cyclic voltammetry were analyzed in order to characterize the electrochemical properties of the Pt/CNF. The performance of a membrane electrode assembly fabricated from the Pt/CNF was maintained, and the electrochemical surface area and cell resistance showed the same trend. Therefore, CNFs are expected to be a good support in polymer electrolyte membrane fuel cells.

Optimum Ratio between Nafion and 20, 40 wt% Pt/C Catalysts for MEAs

To enhance the performance of a MEA (membrane electrode assembly) in a polymer electrolyte membrane fuel cell (PEMFC), optimum contents of Nafion ionomer as electrolyte in the 20 and 40 wt% Pt/C used in electrodes were examined. Variety characterization techniques were applied to examine optimum Nafion contents: cell performance test, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). According to Pt wt% supported on carbon support, it has been observed that polarization, ohmic, and mass transfer resistances were changed so that the cell performance was significantly dependent on the content of Nafion ionomer. Optimum Nafion ionomer contents in the 20 wt% Pt/C and 40 wt% Pt/C were showed 35 wt% and 20 wt%, respectively. This is due to different surface area of the Pt/C catalyst, and formation of triple phase boundary seems to be affected by the Nafion contents.

Improvement of Catalyst Supporting Characteristic on MWCNTs with Different Thermal Treatment for PEMFC

C에서 열처리를 하였을 경우, 온도변화에 따른 작용기의 영향을 FT-IR, XPS를 통해 분석하였다. XPS를 통해 전처리의 온도가 증가할수록 작용기의 농도가 증가하는 것을 확인 하였으며,TGA와 TEM을 통해 Pt의 담지량과 분산도 또한 증가한 것을 확인하였다. 산화과정에서 열적거동은 고담지 Pt 촉매의 제조와 밀접한 관련이 있으며, 본 연구에서 제조된 촉매의 고분산, 고담지에 적절한 합성 o온도는 90C로 관찰되었으며 단위전지에서 가장 좋은 성능을 나타내었다. Abstract : In this study, carbon nanotubes were used as supporter to get high dispersionand high loading of Pt for PEMFC. Thermal oxidation method was applied to carbonnanotubes surface treatment. FT-IR and XPS were used to measure the effect of temperatureon functional group. The increased concentration of functional groups was confirmed by XPSanalysis, and increased Pt loading and dispersion was also observed by TGA and TEM analysiswith increased temperature. Thermal behavior of oxidation is closely related to the manufactureof highly dispersed Pt/MWCNTs. Pt/MWCNTs treatment temperature at 90

Effect of Carbon Dioxide in Fuel on the Performance of PEMFC

Even though fuel cell have high efficiency when pure hydrogen from gas tank is used as a fuel source, it is more beneficial to generate hydrogen from city gas (mainly methane) in residential application such as domestic or office environments. Thus hydrogen is generated by reforming process using hydrocarbon. Unfortunately, the reforming process for hydrogen production is accompanied with unavoidable impurities. Impurities such as CO, , , , , and in hydrogen could cause negative effects on fuel cell performance. Those effects are kinetic losses due to poisoning of the electrode catalysts, ohmic losses due to proton conductivity reduction including membrane and catalyst ionomer layers, and mass transport losses due to degrading catalyst layer structure and hydrophobic property. Hydrogen produced from reformer eventually contains around 73% of , 20% or less of , 5.8% of less of , or 2% less of , and 10ppm or less of CO. This study is aimed at investigating the effect of carbon dioxide on fuel cell performance. The performance of PEM fuel cell was investigated using current vs. potential experiment, long run(10 hr) test, and electrochemical impedance measurement when the concentrations of carbon dioxide were 10%, 20% and 30%. Also, the concentration of impurity supplied to the fuel cell was verified by gas chromatography(GC).

Effect of Coolant on PEMFC Performance in Low Humidification Condition

Proton exchange membrane fuel cell(PEMFC) performance could be affected by various factors such as cell temperature, total pressure, partial pressure of reactants and relative humidity. Hydrogen ion is combined with water to form hydronium ion [] and pass through membrane resulting electricity generation. Cooling system is needed to remove heat and other uses on large scale fuel cell. In case that collant conductivity is increased, fuel cell performance could be decreased because produced electricity could be leaked through coolant. In this study, triple distilled water(TDW) and antifreeze solution containing ethylene glycol was used to observe resistance change. Resistance of TDW was taken 28 days to reach preset value, and effect on fuel cell operation was not observed. Resistance of antifreeze solution was not reached to preset value up to 48 days, but performance failure occurred presumably caused by bipolar plate junction resulting stoppage resistance experiment. Generally PEMFC humidification is performed near-saturated operating conditions at various temperatures and pressures, but non-humidifying condition could be applied in small scale fuel cell to improve efficiency and reduce system cost. However, it was difficult to operate large scale fuel cell without humidifying, especially higher than . In case of small flux such as 0.78 L/min, temperature difference between inlet and outlet was occurred larger than other cases resulting performance decrease. Non-humidifying performance experiments were done at various cell temperature. When both of anode and cathode humidification were removed, cell performance was strongly depended on cell operating temperature.

Effect of Reverse Voltage on Proton Exchange Membrane Fuel Cell Performance

Reverse voltage could be occurred on proton exchange membrane fuel cell (PEMFC) by fuel starvation and/or uneven gas distribution in flow channel. The performance of PEMFC was decreased by reverse voltage. Performance decrease in local area could be affected by duration and/or extent of reverse voltage. Hydrogen stoichiometric ratio was used to find the experimental condition of abrupt voltage decrease. In case that specific cell was reached to reverse voltage, the cell performance was checked every 5 cycle. LabVIEW was used to make control logic of automatic load off system in preset voltage. Performance change experiment was carried out at constant current condition. When the cell voltage was reached to 0.1 V, electronic load was disconnected to make open circuit voltage for 1 minute. Fuel cell performance was checked every 5 cycle and the degree of performance decrease and/or recovery was estimated. Cell performance was maintained up to 40 cycle in case that the load off condition was set at 0 V.

Clinical outcomes of 23 patients who had repeat pelvic arterial embolisation for uncontrolled post-partum haemorrhage at a single centre

AIM To evaluate the safety and efficacy of repeated pelvic arterial embolisation (PAE) for uncontrolled postpartum haemorrhage (PPH) after a single session of PAE and to compare angiographic findings between the two sessions of PAE. MATERIALS AND METHODS A total of 23 consecutive patients (age range, 23-44 years) who underwent repeated PAE for uncontrolled PPH between March 2001 and January 2016 in Severance Hospital were reviewed. The interval times between the two sessions of PAE, the angiographic findings, embolic materials, arteries embolised during PAE, and the clinical outcomes were reviewed retrospectively. RESULTS Overall clinical success was achieved after repeated PAE in 21 of 23 patients (91.3%). There were no procedure-related, major complications. On angiography, active bleeding from the uterine collateral arteries was more frequently observed in the second session of PAE (p>0.05), and embolisation of the anterior division of the internal iliac artery was significantly higher during the second session of PAE. Use of permanent embolic materials was significantly higher during the second session of PAE. Recanalisation of a previously embolised artery was identified in 14 patients (60.9%) during the second session. CONCLUSION Repeated PAE is safe and effective for managing recurrent bleeding after a single session of PAE. Repeated PAE is related to a higher chance of embolisation of the anterior division of the internal iliac artery, with the use of permanent embolic materials. Recanalisation of a previously embolised artery seems to be a principal source of rebleeding during a repeated session of PAE.

Polyol-free synthesis of uniformly dispersed Pt/graphene oxide electrocatalyst by sulfuric acid treatment

Polyol-free synthesis of highly loaded Pt catalysts on sulfuric-acid-treated graphene oxide (SGO) was reported. Sulfuric acid treatment increased the surface hydroxyl groups on graphene oxide (GO), which contributed to the reduction of Pt precursors in the absence of external reducing agent. By adjusting pH during the Pt reduction, we can get uniformly dispersed 2.5 nm size Pt nanoparticles on GO surface even at 50wt% Pt loading amount. Cyclic voltammetry showed that increased pH resulted in increased electrochemical surface area.

Study for Effective Cooling of Ni-MH Battery Module Using Forced Air Flow

In this study, computational simulation was performed for thermal management of modules consisting of 10 batteries. Simplified structure and equivalent thermal resistance network was applied to maintain the thermal properties. Verification test of the mesh were in progress to ensure the reliability of 2.6 mm in the narrow gap between the battery, resulting in at least three divided mesh between the shape of the grid was required. Type of air from rear of the module, type of air from top of the module and type of air from bottom of the module were applied and effective cooling methods are discussed based on the location of fan and air intake of the modules. Maximum temperature and temperature differences of modules that directly affect the performance of the module were compared, and also behavior of the fluid was confirmed by comparing the air flow. The best maximum temperature is shown type of air from bottom of the module to 40.27 o C and type of air from top of the module shows smallest temperature difference 0.73 o C.