Interaction Mediator Assisted Synthesis of Mesoporous Molybdenum Carbide: Mo-Valence State Adjustment for Optimizing Hydrogen Evolution.

To overcome inherent limitations of molybdenum carbide (MoxC) for hydrogen evolution reaction (HER), i.e., low density of active site and nonideal hydrogen binding strength, we report the synthesis of valence-controlled mesoporous MoxC as a highly efficient HER electrocatalyst. The synthesis procedure uses an interaction mediator (IM), which significantly increases the density of active site by mediating interaction between PEO-b-PS template and Mo source. The valence state of Mo is tuned by systematic control of the environment around Mo by controlled heat treatment under air before thermal treatment at 1100 °C. Theoretical calculations reveal that the hydrogen binding is strongly influenced by Mo valence. Consequently, MoxC achieves a significant increase in HER activity (exceeding that of Pt/C at high current density ∼35 mA/cm2 in alkaline solution). In addition, a volcano-type correlation between HER activity and Mo valence is identified with various experimental indicators. The present strategies can be applied to various carbide and Mo-based catalysts, and the established Mo valence and HER relations can guide development of highly active HER electrocatalysts.

An extremely low Pt loading cathode for a highly efficient proton exchange membrane water electrolyzer.

Direct self-terminated Pt electrodeposition on carbon paper enables precise control of loading Pt mass, from the sub-microgram to the sub-milligram scale. This can provide insight into the low limits of Pt use for reasonable performance of a proton exchange membrane water electrolyzer.

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.

Effect of morphology of electrodeposited Ni catalysts on the behavior of bubbles generated during the oxygen evolution reaction in alkaline water electrolysis.

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.

Reduction process of dislocation and standby leakage current for embedded flash memory using nano-scale integration.

We determined that the use of densification, sacrificial oxidation, gate oxidation and source/drain implantation has the capability to reduce the dislocation. A dislocation-free process is proposed, and its mechanism presented in embedded flash memory. The dislocation decreased when n-type ions were implanted at a low energy level for source and drain. A dry oxidation process using only oxygen without hydrogen and oxidation for logic gates led to the formation of a sacrificial oxide on the rapid thermal oxidation (RTP) methods without densification after gap-filling as reducing dislocation processes. These methods dramatically reduced the standby leakage current.

Effect of particle size of PtRu nanoparticles embedded in WO3 on electrocatalysis.

Size-controlled PtRu nanoparticles embedded in WO3 were prepared by simultaneous multigun sputtering on pure targets of Pt, Ru, and WO3. The mean diameter of the PtRu nanoparticles, as confirmed by high-resolution transmission electron microscopy, can be varied from -2.3 to -3.6 nm by varying the RF power ratio of PtRu and WO3. On the basis of transmission electron diffraction results for the PtRu nanoparticles embedded in WO3, it was confirmed that PtRu exists as an alloy metal phase, whereas the WO3 matrix is present as an amorphous phase. Size-controlled PtRu/WO3 electrodes were found to exhibit unique electronic properties depending on their size, which affected the potential of zero total charge and the methanol oxidation reaction. The mass activity of PtRu/WO3 for methanol oxidation was determined by the interplay of the surface electronic factors at the metal-solution interface; the oxophilicity of the nanoparticles increased with decreasing particle size.

Supported core@shell electrocatalysts for fuel cells: close encounter with reality.

Core@shell electrocatalysts for fuel cells have the advantages of a high utilization of Pt and the modification of its electronic structures toward enhancement of the activities. In this study, we suggest both a theoretical background for the design of highly active and stable core@shell/C and a novel facile synthetic strategy for their preparation. Using density functional theory calculations guided by the oxygen adsorption energy and vacancy formation energy, Pd3Cu1@Pt/C was selected as the most suitable candidate for the oxygen reduction reaction in terms of its activity and stability. These predictions were experimentally verified by the surfactant-free synthesis of Pd3Cu1/C cores and the selective Pt shell formation using a Hantzsch ester as a reducing agent. In a similar fashion, Pd@Pd4Ir6/C catalyst was also designed and synthesized for the hydrogen oxidation reaction. The developed catalysts exhibited high activity, high selectivity, and 4,000 h of long-term durability at the single-cell level.

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.

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.

Observation of methanol behavior in fuel cells in situ by NMR spectroscopy.

The chemical conversion of methanol in direct methanol fuel cells was followed in situ by NMR spectroscopy. Comparing data of the methanol oxidation on Pt and PtRu anode catalysts allowed the role of Ru in both Faradaic and non-Faradaic reactions to be investigated. The spatial distributions of chemicals could also be determined. (Picture: T1-T4=inlet and outlet tubes.).

Enhanced stability and activity of Pt-Y alloy catalysts for electrocatalytic oxygen reduction.

We report Pt-based alloys with early transition metals. Significant electrocatalysis occurs during oxygen reduction reaction (ORR) at the Pt-Y alloy electrodes, and the extent depends on the alloy composition. The Pt-Y alloy electrode activity is related to the d-band center position, and the lattice strain and stability for oxygen reduction reaction.

Facile synthesis of highly active and stable Pt-Ir/C electrocatalysts for oxygen reduction and liquid fuel oxidation reaction.

A facile room temperature synthesis technique has been developed for Pt-Ir/C electrocatalysts for applications to low-temperature fuel cells. The prepared Pt(x)Ir(y) electrocatalyst was highly stable and active toward the oxygen reduction reaction (ORR), as well as liquid fuel oxidation reaction with high CO tolerance.