Yongchai Kwon

Current trends for the floating liquefied natural gas (FLNG) technologies

Natural gas (NG) and liquefied NG (LNG), which is one trade type of NG, have attracted great attention because their use may alleviate rising concerns about environmental pollution produced by classical fossil fuels and nuclear power plants. However, when gas reserves are located in stranded areas and a portion of the offshore reserves is a significant amount of the total gas reserves, LNG is not suitable because (i) installation of pipelines for the transfer of NG to onshore LNG facilities is expensive and difficult, and (ii) it still has environmental and security problems. As a result, there are many efforts to excavate and monetize these stranded and offshore reserves with floating facilities where offshore liquefaction of NG is possible. Therefore, the development of floating LNG (FLNG) technology is becoming important. Although the FLNG technologies have advantages over conventional LNG technologies, there are still several roadblocks. To overcome the challenges, modular designs related to the main and typical stages of the FLNG process — gas pretreatment, liquefaction and regasification topsides, hulls, mooring, and transfer systems should be enhanced. Regarding FLNG ongoing operations and future plans, there are six nations (Argentina, Brazil, Kuwait, UAE, UK, and USA) operating FLNG, and a variety of FLNG liquefaction projects will be finished soon. Shell and Petrobras are making rapid strides to build FLNG facilities, and Flex LNG, Hoegh LNG, SBM Linde, MODEC, and Saipem are also building their FLNGs. In this review paper, we initially review the LNG concept and compare it with FLNG. In turn, new and typical FLNG technologies are introduced and the main challenges are also explained with insight into how these challenges are overcome. The main market drivers for FLNG industry are also considered.

Fabrication of Mediatorless/Membraneless Glucose/Oxygen Based Biofuel Cell using Biocatalysts Including Glucose Oxidase and Laccase Enzymes

Mediatorless and membraneless enzymatic biofuel cells (EBCs) employing new catalytic structure are fabricated. Regarding anodic catalyst, structure consisting of glucose oxidase (GOx), poly(ethylenimine) (PEI) and carbon nanotube (CNT) is considered, while three cathodic catalysts consist of glutaraldehyde (GA), laccase (Lac), PEI and CNT that are stacked together in different ways. Catalytic activities of the catalysts for glucose oxidation and oxygen reduction reactions (GOR and ORR) are evaluated. As a result, it is confirmed that the catalysts work well for promotion of GOR and ORR. In EBC tests, performances of EBCs including 150 μm-thick membrane are measured as references, while those of membraneless EBCs are measured depending on parameters like glucose flow rate, glucose concentration, distance between two electrodes and electrolyte pH. With the measurements, how the parameters affect EBC performance and their optimal conditions are determined. Based on that, best maximum power density (MPD) of membraneless EBC is 102 ± 5.1 μW · cm−2 with values of 0.5 cc · min−1 (glucose flow rate), 40 mM (glucose concentration), 1 mm (distance between electrodes) and pH 3. When membrane and membraneless EBCs are compared, MPD of the membraneless EBC that is run at the similar operating condition to EBC including membrane is speculated as about 134 μW · cm−2.

Effect of nafion membrane thickness on performance of vanadium redox flow battery

The performance of vanadium redox ow batteries (VRFBs) using different membrane thicknesses was evaluated and compared. The associated experiments were conducted with Nafion® 117 and 212 membranes that have 175 and 50 μm of thickness, respectively. The charge efficiency (CE) and energy efficiency (EE) of VRFB using Nafion® 117 were higher than those of VRFB using Nafion® 212, while power efficiency was vice versa. In terms of amounts of charge and discharge that are measured in different charging current densities, the amounts in VRFB using Nafion® 212 are more than that in VRFB using Nafion® 117. To further characterize the effect of membrane thickness on VRFB performance, electrochemical impedance spectroscopy (EIS) and UV-vis. spectrophotometer (UV-vis) were used. In EIS measurements, VRFB using Nafion® 117 was more stable than that using Nafion® 212, while in UV-vis measurements, vanadium crossover rate of VRFB usingNafion® 212 (0.0125M/hr) was higher than that of VRFB using Nafion® 117 (0.0054 M/hr). These results are attributed to high crossover rate of vanadium ion in VRFB using Nafion® 212. With these results, vanadium crossover plays more dominant role than electrochemical reaction resistance in deciding performance of VRFB in condition of different membranes.

Yeast and carbon nanotube based biocatalyst developed by synergetic effects of covalent bonding and hydrophobic interaction for performance enhancement of membraneless microbial fuel cell

Membraneless microbial fuel cell (MFC) employing new microbial catalyst formed as yeast cultivated from Saccharomyces cerevisiae and carbon nanotube (yeast/CNT) is suggested. To analyze its catalytic activity and performance and stability of MFC, several characterizations are performed. According to the characterizations, the catalyst shows excellent catalytic activities by facile transfer of electrons via reactions of NAD, FAD, cytochrome c and cytochrome a3, while it induces high maximum power density (MPD) (344mW·m-2). It implies that adoption of yeast induces increases in catalytic activity and MFC performance. Furthermore, MPD is maintained to 86% of initial value even after eight days, showing excellent MFC stability.

Vanadium Redox Flow Batteries Using meta-Polybenzimidazole-Based Membranes of Different Thicknesses

15, 25, and 35 μm thick meta-polybenzimidazole (PBI) membranes are doped with H2SO4 and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H2SO4, PBI absorbs about 2 mol of H2SO4 per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, meta-PBI shows a negligible crossover of V3+ and V4+ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBIs nanosized pores. It turns out that charge efficiency (CE) of VRFBs using meta-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick meta-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin meta-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H2SO4-doped meta-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 μm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm-2.

Highly sensitive glucose biosensor using new glucose oxidase based biocatalyst

Glucose, which is a primary energy source of living organisms, can induce diabetes or hypoglycemia if its concentration in blood is irregular. It is therefore important to develop glucose biosensor that reads the concentration of glucose in blood precisely. In the present work, we suggest new glucose oxidase (GOx) based catalysts that can improve the sensitivity of the glucose biosensor and make glucose measurements over a wide concentration ranges possible. For synthesizing such catalysts, a composite including pyrenecarboxaldehyde (PCA) and GOx is attached to substrate including carbon nanotube (CNT) and polyethyleneimine (PEI) (CNT/PEI/[PCA/GOx]). Catalytic activity and stability of the catalyst are then evaluated. According to the investigation, the catalyst shows excellent glucose sensitivity of 47.83 μAcm−2mM−1, low Michaelis-Menten constant of 2.2 mM, and wide glucose concentration detection, while it has good glucose selectivity against inhibitors, such as uric acid and ascorbic acid. Also, its activity is maintained to 95.7% of its initial value even after four weeks, confirming the catalyst is stable enough. The excellence of the catalyst is attributed to hydrophobic interaction, C=N bonds, and π-hydrogen interaction among GOx, PCA and PEI/CNT. The bindings play a role in facilitating electron transport between GOx and electrode.

A correlation of results measured by cyclic voltammogram and impedance spectroscopy in glucose oxidase based biocatalysts

A new biocatalyst consisting of glucose oxidase (GOx) and polyethylenimine (PEI) immobilized on carbon nanotube (CNT) (CNT/PEI/GOx) was developed, while cyclic voltammogram (CV) behaviors of several related catalysts including the CNT/PEI/GOx were analyzed in terms of charge transfer resistances (Rcts) obtained by measuring Nyquist plots using electrochemical impedance spectroscopy (EIS). A qualitative correlation between the flavin adenine dinucleotide (FAD) redox reactivity measured by the CV and Rct was established. As factors affecting both the FAD reactivity and Rct, concentrations of GOx, glucose, and phosphate buffer solution, electrolyte pH and ambient condition were considered and evaluations of the catalysts using the CV curves and Nyquist plots confirmed that a pattern in the FAD reactivity was closely linked to that in the Rct, implying that FAD reactivities of the catalysts are predicted by the measurements of their Rcts. Even regarding performance of the enzymatic biofeul cells (EBCs) using the reacted catalysts, a pattern of the Rcts is compatible with that in the maximum power densities (MPDs) of the EBCs.

Chelating functional group attached to carbon nanotubes prepared for performance enhancement of vanadium redox flow battery

A hydroxamic acid functionalized carbon nanotube (HAA-CNT) catalyst is prepared to determine its effect on VO2+/VO2+ and V2+/V3+ redox reactions and on the performance of a vanadium redox flow battery (VRFB), while its catalytic activity and VRFB performance are compared with those of pure CNTs and carboxylic acid functionalized CNT (CA-CNT) catalysts. According to the cyclic voltammetry measurements, HAA-CNT shows a better catalytic activity and reaction reversibility for vanadium redox reactions than CNT and CA-CNT because of the chelation ability of the HAA included in the HAA-CNT. With the role of chelating agent, the HAA group interacts well with vanadium ions, promoting their redox reactions. In particular, the HAA-CNT is more effective for the VO2+/VO2+ reaction than the V2+/V3+ reaction because of the larger number of oxygen atoms considered as active sites for the VO2+/VO2+ reaction. The role promoting VO2+/VO2+ reaction is confirmed by measuring charge transfer resistance efficiencies of a single cell VRFB. The chemical structure of the HAA-CNT is determined using X-ray photoelectron spectroscopy (XPS). Eventually, when the HAA-CNT is used as a positive electrode for the VO2+/VO2+ reaction, the performance of single cell VRFB is best.

A Study on The Effects of Three Different Carbon Catalysts on Performance of Vanadium Redox Flow Battery

Abstract >> In this study, we carry out a study on how to improve performance of vanadium redox flow battery(VRFB) through promoting reaction rate of rate determining vanadium reaction ([VO] 2+ /[VO 2 ] + ). In order to dothat, three different carbons like Vulcan (XC-72), CMK3 and MSU-F-C are adopted as the catalysts, while their catalytic activity and reaction reversibility are evaluated using half-cell tests. Their topological images are alsomeasured by TEM. For estimation of the VRFB performance, multiple charge-discharge curves of VRFBs includingthe catalysts are measured by single cell tests. As a result of that, MSU-F-C shows relatively excellent catalytic activity and reaction reversibility as well as large surface area compared to those of Vulcan (XC-72) and CMK3.Also, in terms of the performance of VRFBs including the catalysts, VRFB including MSU-F-C indicates (i) low charging/discharging overpotentials and low internal resistance, (ii) high charge/discharge capacities and (iii) highenergy efficiency. These VRFB performance data are well agreed with results on catalytic activity and reactionreversibility. The reason that MSU-F-C induces superior VRFB performances is attributed to (i) its large surfacearea and (ii) its hydrophilic surface functional groups that mainly consist of hydroxyl bonds that are supposedto play active surface site role for facilitaing [VO]

A Study on Oxygen Reduction Reaction of PtM Electrocatalysts Synthesized on Graphene for Proton Exchange Membrane Fuel Cell

3 KOLON tower annex, 13, Kolon-ro, Gwachun-si, 427-709, Gyeonggi-do, Korea Abstract >> In this research, we investigate electrical performance and electrochemical properties of graphene supported Pt (Pt/G) and PtM (M = Ni and Y) alloy catalysts (PtM/Gs) that are synthesized by modified polyol method. With the PtM/Gs that are adopted for oxygen reduction reaction (ORR) as cathode of proton exchange membrane fuel cells (PEMFCs), their catalytic activity and ORR performance and electrical performance are estimated and compared with one another. Their particle size, particle distribution and electrochemically active surface (EAS) area are measured by TEM and cyclic voltammetry (CV), respectively. On the other hand, regarding ORR activity and electrical performance of the catalysts, (i) linear sweeping voltammetry by rotating disk electrode and rotating ring-disk electrode and (ii) PEMFC single cell tests are used. The TEM and CV measurements demonstrate particle size and EAS of PtM/Gs are compatible with those of Pt/G. In case of PtNi/G, its half-wave potential, kinetic current density, transferred electron number per oxygen molecule and H2O2 production % are excellent. Based on data obtained by half-cell test, when PEMFC singlecell tests are carried out, current density measured at 0.6V and maximum power density of the PEMFC single cell employing PtNi/G are better than those employing Pt/G. Conclusively, PtNi/Gs synthesized by modified polyol shows better ORR catalytic activity and PEMFC performance than other catalysts.