Young-Woo Choi

Numerical study on the coal gasification characteristics in an entrained flow coal gasifier

The coal gasification process of a slurry feed type, entrained-flow coal gasifier was numerically predicted in this paper. By dividing the complicated coal gasification process into several simplified stages such as slurry evaporation, coal devolatilization and two-phase reactions coupled with turbulent flow and two-phase heat transfer, a comprehensive numerical model was constructed to simulate the coal gasification process. The k–e turbulence model was used for the gas phase flow while the Random-Trajectory model was applied to describe the behavior of the coal slurry particles. The unreacted-core shrinking model and modified Eddy break-up (EBU) model, were used to simulate the heterogeneous and homogeneous reactions, respectively. The simulation results obtained the detailed information about the flow field, temperature and species concentration distributions inside the gasifier. Meanwhile, the simulation results were compared with the experimental data as a function of O2/coal ratio. It illustrated that the calculated carbon conversions agreed with the measured ones and that the measured quality of the syngas was better than the calculated one when the O2/coal ratio increases. This result was related with the total heat loss through the gasifier and uncertain kinetics for the heterogeneous reactions.

Investigation of hollow nitrogen-doped carbon spheres as non-precious Fe–N4 based oxygen reduction catalysts

The development of inexpensive non-precious oxygen reduction catalysts has become one of the most important efforts in polymer electrolyte membrane fuel cells. In this report, we synthesized a non-precious electrocatalyst from a single precursor, iron(III) diethylene triaminepentaacetate, using a heat-treatment effect to prepare an active catalyst. A series of catalysts were prepared at different temperatures leading to different degrees of graphitization, heteroatom content and activity. In 0.1 M KOH electrolyte solution, the oxygen reduction reaction (ORR) onset potential of the HNCS71 catalyst was as high as 0.97 V, and half-wave potentials were only 20 mV lower than those for Pt/C. X-ray absorption measurements of the Fe K-edge showed the structure of Fe–N4 centers, formed in HNCS71, which were responsible for the ORR activity. An alkaline exchange membrane fuel cell fabricated with HNCS71 as the cathode was tested in a H2–O2 single cell and showed a maximum power density of ∼68 mW cm−2. The 100 hour fuel cell durability test of the HNCS71 cathode showed a decay in the current density of about 14% at 0.4 V. Therefore, the HNCS catalyst appears to be a promising new class of non-precious catalysts for fuel cell applications.

Sulfonated poly(arylene ether sulfone) membrane containing sulfated zirconia for high-temperature operation of PEMFCs

A new composite membrane that consists of sulfonated poly(arylene ether sulfone) (SPES) and a highly functionalized inorganic filler of sulfated zirconia (s-ZrO2) was synthesized and evaluated for enhancement of water retention as well as proton conductivity at high temperatures above 120 °C in the absence of water. Since the SPES has less hydrophobic/hydrophilic difference between the polymer backbone and –SO3H ionic groups, the proton channels are relatively small and not easily distributed, which results in no significant change at highly saturated conditions. Therefore, the effective interconnection between the –SO3H ionic groups and bound water adsorbed on the surface of the s-ZrO2 is the key in achieving a higher proton transfer for PEM fuel cells at high temperatures above 120 °C. Analysis by small angle X-ray scattering (SAXS) of recast membranes showed that SPES random copolymers with various s-ZrO2 contents have a similar ionic cluster size and less pronounced phase separation regardless of the dry/swollen condition of the membrane. However, the phase images obtained by atomic force microscopy (AFM) showed that the connectivity of ionic groups varies, depending on the content of s-ZrO2 in the polymer matrix. Larger and more interconnected hydrophilic regions were found with the SPES membrane when there was no significant particle aggregation in the hydrophilic moieties of the polymer matrix. In this study, the composite membrane with 5 wt% s-ZrO2 had the highest proton conductivity at 120 °C due to better ionic interactions as well as the presence of bound water combined with the –SO3H ionic groups.

A highly durable cross-linked hydroxide ion conducting pore-filling membrane

A highly durable, anion exchange membrane with a uniform thickness of 23.4 μm was fabricated by the pore-filling approach in a porous substrate. The resulting membrane has a hydroxide conductivity in excess of 40 mS cm−1 at 20 °C and does not exhibit significant changes in ion conductivity and IEC in 5 M solution of NaOH at 50 °C for 1500 h.

High power density of reverse electrodialysis with pore-filling ion exchange membranes and a high-open-area spacer

In the present study, a novel reverse electrodialysis (RED) stack with ultrathin lab-made pore-filling membranes and a high-open-area spacer was proposed to enhance the gross power density. The proposed stack had much lower internal resistance than a typical RED stack at optimum flow rates of seawater and river water. Therefore, a gross power density of 2.4 W m−2 was achieved.

Facile surface modification of anion-exchange membranes for improvement of diffusion dialysis performance.

In this study, a facile membrane modification method by spin-coating of pyrrole (Py) monomers dissolved in a volatile solvent followed by an interfacial polymerization is proposed. The surface of a commercial anion-exchange membrane (i.e., Neosepta-AFX, Astom Corp., Japan) was successfully modified with polypyrrole (Ppy) to improve the acid recovery performance in diffusion dialysis (DD). The result of DD experiments revealed that both the acid and metal ion transports are significantly influenced by the surface modification. The metal crossover through the membranes was largely reduced while mostly maintaining the acid permeability by introducing a thin Ppy layer with excellent repelling property to cations on the membrane surface. As a result, the anion-exchange membrane modified with the optimum content of Py monomer (5 vol.%) exhibited excellent acid dialysis coefficient (KAcid) and selectivity (KAcid/KMetal) which is approximately twice as high as that of the pristine membrane.

A Study on Hexachromic Ion Selective Electrode Based on Supported Liquid Membranes

An ion selective electrode (ISE) for determining Cr(VI) using supported liquid membranes (SLMs) containing trioctylphosphine oxide (TOPO) was investigated in this study. TOPO, as a carrier, had a high selectivity for Cr(VI) against interfering ions such as sulfate, nitrate, nitrite, and chloride. The composition of the SLM was optimized as 0.193 g TOPO/1 mL NPOE (o-nitrophenyl octyl ether)/0.5 g poly (vinyl chloride) for detection of Cr(VI). The Cr(VI) concentration was measured in the range of1 × 10-3 to 1 × 10-6 M with the SLM prepared in the study. It seemed that Cr(VI) was transported in the SLM as a triply charged ion indicated by the slope of the emf response. Selectivity coefficients and detection limits of Cr(VI) in the presence of interfering ions were determined experimentally using the fixed interference method.

A SAXS study on nanostructure evolution in water free membranes containing ionic liquid: from dry membrane to saturation

Small-angle X-ray scattering (SAXS) technique has been used to study the evolution of ionomer peak in the recast Nafion membranes containing the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF(4)). To the best of our knowledge, this is the first report dealing with the study of membranes containing different concentrations of the ionic liquid (EMIBF(4)), from the dry (no ionic liquid) to the saturation (containing 70 wt% ionic liquid) state to understand the evolution of the ionomer peak and the formation of ionic aggregates in these anhydrous membranes. The small-angle scattering maximum (ionomer peak) has been observed to shift continuously toward lower scattering vector (q) values as the ionic liquid content increases. The ionic conductivity behavior for the membranes containing ionic liquid has been found to be closely related with the change of slope of the double logarithmic plot between the reciprocal of the position of the ionomer peak and the polymer weight fraction. The q region over which Porods law has been obeyed in different membranes was initially narrow and has been observed to widen with an increase in the content of the ionic liquid.

Effect of Annealing of Nafion Recast Membranes Containing Ionic Liquids

The composite membranes comprising of sulfonated polymers as matrix and ionic liquids as ion-conducting medium in replacement of water are studied to investigate the effect of annealing of the sulfonated polymers. The polymeric membranes are prepared on recast Nafion containing the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (). The composite membranes are characterized by thermogravitational analyses, ion conductivity and small-angle X-ray scattering. The composite membranes annealed at for 2 h after the fixed drying step showed better ionic conductivity, but no significant increase in thermal stability. The mean Bragg distance between the ionic clusters, which is reflected in the position of the ionomer peak (small-angle scattering maximum), is larger in the annealed composite membranes containing than the non-annealed ones. It might have been explained to be due to the different level of ion-clustering ability of the hydrophilic parts (i.e., sulfonic acid groups) in the non- and annealed polymer matrix. In addition, the ionic conductivity of the membranes shows higher for the annealed composite membranes containing . It can be concluded that the annealing of the composite membranes containing ionic liquids due to an increase in ion-clustering ability is able to bring about the enhancement of ionic conductivity suitable for potential use in proton exchange membrane fuel cells (PEMFCs) at medium temperatures () in the absence of external humidification.

Hydrocarbon Composite Membranes with Improved Oxidative Stability for PEMFC

양성자교환막연료전지에서 사용할 탄화수소계 전해질 막 (술폰산화 폴리아릴렌에테르설폰)의 산화안정성을 향상시키기 위하여 세륨이온이 도입된 전해질 복합막을 제조하였다. 세륨이온의 함유 농도에 따른 산화안정성의 변화를 관찰하였고, 함수율 및 수소이온전도도에 미치는 영향을 조사하였다. ICP 분석을 통해 세륨이온의 함침여부를 검증하였고, NMR 피크의 화학적 이동으로 금속이온과 술폰산 그룹과의 배위여부를 확인하였다. 세륨이온의 함량이 증가함에 따라 이온전도도 및 함수율은 감소하였으나, 산화안정성은 향상되었다. 특히, 과산화수소폭로 가속화장치를 이용하여, 기존의 펜톤산화실험에 비해 실제 연료전지운전과 매우 유사한 조건에서 전해질 막의 산화안정성을 평가하였다. 【Sulfonated poly(arylene ether sulfone)-cerium composite membranes with improved oxidative stability were prepared for proton exchange membrane fuel cell application. Oxidative stability of the composite membranes changed depending on the amount of incorporated metal. Their water uptake, IEC and proton conductivity were also affected. ICP analysis confirmed trace of cerium ion in the composite membranes and