Soo-Kil Kim

Role of Electronic Perturbation in Stability and Activity of Pt-Based Alloy Nanocatalysts for Oxygen Reduction

The design of electrocatalysts for polymer electrolyte membrane fuel cells must satsify two equally important fundamental principles: optimization of electrocatalytic activity and long-term stability in acid media (pH <1) at high potential (0.8 V). We report here a solution-based approach to the preparation of Pt-based alloy with early transition metals and realistic parameters for the stability and activity of Pt(3)M (M = Y, Zr, Ti, Ni, and Co) nanocatalysts for oxygen reduction reaction (ORR). The enhanced stability and activity of Pt-based alloy nanocatalysts in ORR and the relationship between electronic structure modification and stability were studied by experiment and DFT calculations. Stability correlates with the d-band fillings and the heat of alloy formation of Pt(3)M alloys, which in turn depends on the degree of the electronic perturbation due to alloying. This concept provides realistic parameters for rational catalyst design in Pt-based alloy systems.

Designing a Highly Active Metal‐Free Oxygen Reduction Catalyst in Membrane Electrode Assemblies for Alkaline Fuel Cells: Effects of Pore Size and Doping‐Site Position

To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70% of the maximum power density obtained using Pt/C.

One-Pot Synthesis of Intermetallic Electrocatalysts in Ordered, Large-Pore Mesoporous Carbon/Silica toward Formic Acid Oxidation

This study describes the one-pot synthesis and single-cell characterization of ordered, large-pore (>30 nm) mesoporous carbon/silica (OMCS) composites with well-dispersed intermetallic PtPb nanoparticles on pore wall surfaces as anode catalysts for direct formic acid fuel cells (DFAFCs). Lab-synthesized amphiphilic diblock copolymers coassemble hydrophobic metal precursors as well as hydrophilic carbon and silica precursors. The final materials have a two-dimensional hexagonal-type structure. Uniform and large pores, in which intermetallic PtPb nanocrystals are significantly smaller than the pore size and highly dispersed, enable pore backfilling with ionomers and formation of the desired triple-phase boundary in single cells. The materials show more than 10 times higher mass activity and significantly lower onset potential for formic acid oxidation as compared with commercial Pt/C, as well as high stability due to better resistivity toward CO poisoning. In single cells, the maximum power density was higher than that of commercial Pt/C, and the stability highly improved, compared with commercial Pd/C. The results suggest that PtPb-based catalysts on large-pore OMCSs may be practically applied as real fuel cell catalysts for DFAFC.

Facile preparation of carbon-supported PtNi hollow nanoparticles with high electrochemical performance

To design Pt-based materials with a hollow structure via a galvanic reaction would be one of the effective ways to prepare electro- catalysts with high activity. The galvanic reaction between Pt ions and metal template is usually conducted under limited conditions, which makes the preparation of Pt hollow nanoparticles laborious. Here, we introduce a one-step and one-pot synthetic approach for the preparation of carbon-supported PtNi alloy hollow nanoparticles with a narrow size distribution. Prepared PtNi alloys were characterized by a nonporous shell consisting of a Pt-enriched surface layer and an inner alloy layer of Pt and Ni. Due to its unique structural advantages, this material showed excellent electrocatalytic performance for oxygen reduction (3.3- and 7.8-fold enhanced mass and specific activities compared to those of a commercial carbon-supported Pt nanoparticle). A possible mechanism for the formation of PtNi hollow structure is suggested.

Promoting effects of La for improved oxygen reduction activity and high stability of Pt on Pt–La alloy electrodes

The design of polymer electrolyte fuel cell electrocatalysts depends on two equally important fundamental principles: the optimization of electrocatalytic activities as well as the long-term stability under operating conditions (e.g., pH 0.8 V). Pt-based alloys with transition metals (i.e., Pt–La) address both of these key issues. The oxygen reduction kinetics depends on the alloy composition which, in turn, is related to the d-band center position. The stability of the oxygen reduction reaction is predictable by correlation of the d-band fillings and vacancies of Pt–M (M = Ti, Fe, Zr and La).

The properties of the ultramicrocylindrical ionization chamber for small field used in stereotactic radiosurgery.

Accurate dosimetry of small-field photon beams tends to be difficult to perform due to the presence of lateral electronic disequilibrium and steep dose gradients. In stereotactic radiosurgery (SRS), small fields of 6-30 mm in diameter are used. Generally thermoluminescence dosimetry chips, Farmer, Thimble ion chamber, and film dosimetry are not adequate to measure dose in SRS beams. These techniques generally do not provide the required precision due to their energy dependence and/or poor resolution. It is necessary to construct a small, accurate detector with high spatial resolution for the small fields used in SRS. The ultramicrocylindrical ionization chamber (UCIC) with a gold wall of 2.2 mm in diameter and 4.0 mm in length has dual sensitive volumes of air (8.0 mm3) and borosilicate (2.6 mm3) cavity. Reproducibility, linearity, and radiation damage with respect to absorbed dose, beam profile of small beam, and independence of dose rate of the UCIC are tested by the dose measurements in high energy photon (5, 15 MV) and electron (9 MeV) beams. The UCIC with a unique supporting system in the polystyrene phantom is demonstrated to be a suitable detector for the dose measurements in a small beam size.

Superconformal Cu Electrodeposition Using DPS

3-N,N-Dimethylaminodithiocarbamoyl-l-propanesulfonic acid (DPS) was applied as an accelerator in damascene Cu electrodeposition. When DPS was used with a one-step electrodeposition method, it was incapable of superfillingin damascene structure. Superfilling, however, could be accomplished with two-step electrodeposition method or derivitization method. The differences in the Cu profiles depending on the deposition methods, were associated with the concentration-dependent behaviors of DPS. The acceleration effect of DPS was proportional to its concentration at low concentrations below 50 μM on nonpatterned wafer, but not at higher concentrations. Through the electrochemical analyses, the critical DPS concentration was determined at 50 μM where the acceleration effect of DPS changed. The DPS reaction mechanism was proposed based on the concentration dependency of the acceleration effect. This is strongly related to the change in molecular structure according to concentration. Furthermore, it was found that even with one-step electrodeposition, the Cu filling profile was considerably improved by controlling DPS concentration.

Silver Direct Electrodeposition on Ru Thin Films

Electrodeposition of Ag was performed on Ru thin films following electrochemical reduction of native Ru oxide. Oxide reduction in a tetramethylammonium hydroxide solution was critical for the formation of continuous Ag film, and a large overpotential was important for high-density nucleation. From a kinetics viewpoint, the thermal stability of the Ag film was improved by the application of a more negative potential, which suggested that better nucleation density at the initial stage of growth induced better substrate adhesion. Suppression of growth by addition of an organic additive generated a larger and more uniformly distributed initial population of Ag particles, and as a result a smooth film was obtained.

Two-Step Filling in Cu Electroless Deposition Using a Concentration-Dependent Effect of 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic Acid

This paper describes electroless Cu filling of trenches with different widths ranging from 130 to 300 nm, using a concentrationdependent effect of 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid DPS. With a fixed DPS concentration, it is shown that these trenches with different dimensions cannot be superfilled simultaneously. This is presumably caused by different surface concentrations of the adsorbed additive, which depends on the feature size and surface area. A two-step filling method is employed to superfill those trenches, which is also effective in control of the deposited Cu amounts to obtain uniform growth front regardless of the trench dimensions.

Electrodeposited amorphous Co–P–B ternary catalyst for hydrogen evolution reaction

The catalytic activity of amorphous Co–P–B catalysts electrodeposited on carbon paper (CP) for the hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H2SO4 electrolyte. Based on the measured double layer capacitance (Cdl) and HER activity, it was found that the HER intrinsic activity exhibited volcano behavior when plotted as a function of the B/P ratio in the Co–P–B catalyst. It was revealed that based on the electronic structures of the elements, electron transfer among the three elements is a significant factor for enhancing the intrinsic activity for the HER. The highest activity was obtained with a B/P ratio of ∼1, where electron transfer among the three elements was maximized.