242 research outputs found
Low-Temperature and Gram-Scale Synthesis of Two-Dimensional Fe–N–C Carbon Sheets for Robust Electrochemical Oxygen Reduction Reaction
No full textThe
Fe–N–C-based carbon materials, which are generally
formed by high-temperature annealing, have been highlighted as a promising
alternative to expensive Pt electrocatalysts for oxygen reduction
reaction. However, the delicate formation of active sites remains
an issue because of decomposition and transformation of the macrocycle
during heat treatment. Accordingly, we developed a low-temperature
and gram-scale approach to synthesizing iron phthalocyanine (Pc)-embedded
two-dimensional carbon sheets by annealing at 450 °C. The low-temperature
annealing process, which is motivated by the synthesis of carbon nanoribbons,
is suitable for maintaining the Fe–N–C structure while
enhancing coupling with carbon. Our two-dimensional carbon sheets
show higher ORR activity than commercial Pt catalyst in alkaline media.
Furthermore, the feasibility of real application to alkaline membrane
electrolyte fuel cell is verified by superior volumetric current density.
In durability point of view, the initial activity is retained up to
3000 potential cycles without appreciable activity loss; this excellent
performance is attributed to the structural stabilization and electron
donation from the carbon sheet, which occurs via strong electronic
coupling. We believe that this low-temperature and large-scale synthesis
of a carbon structure will provide new possibilities for the development
of electrochemical energy applications
In situ fabrication of highly porous foam-like Zn nanostructures on gas diffusion layer for selective electrocatalytic reduction of carbon dioxide to carbon monoxide
No full text© 2022 The Korean Society of Industrial and Engineering ChemistryElectrochemical reduction of CO2 is regarded as a promising technique for converting unwanted CO2 into high-value chemicals. Among the various electrocatalysts that are crucial for overcoming sluggish reduction processes, Zn has been continuously studied because of its suitable catalytic activity and abundance in the earth's crust. In this study, we fabricated highly porous foam-like Zn nanostructures on a gas diffusion layer (GDL) using hydrothermal growth and in situ reduction process. The prepared electrode showed a CO partial current density of 20.9 mA∙cm−2 at −1.10 V, which is approximately 10 times higher than that of the bare Zn foil. Moreover, the fabricated electrode can be directly applied to the large-scale flow cell system without further modification. The flow cell system with the fabricated electrode showed a current density of approximately 200 mA∙cm−2 and CO faradaic efficiency of 75% on a 2 h long experiment at 2.7 V cell voltage, which clearly confirms the possibility of highly porous Zn nanostructures on GDL.11Nsciescopuskc
CO electro-oxidation reaction on Pt nanoparticles: Understanding peak multiplicity through thiol derivative molecule adsorption
No full textThe electro-oxidation of CO adsorbed on Pt nanoparticles is an important reaction in fuel cells. Despite extensive research, the underlying concept for peak multiplicity is not yet clearly understood. We investigated CO electro-oxidation features by scan-rate-dependent Tafel analysis and a model system experiment based on adsorption of 3-mercaptopropionic acid (3-MPA). The results suggest that the first main oxidation peak corresponds to the competition between OH adsorption and the interaction between adsorbed CO and OH. Modifying the Pt surface with 3-MPA can reduce the OH coverage, which in turn reduces the width of the first peak. However, the peak potential is not significantly dependent on OH coverage. Considering that the free surface is large enough for OH adsorption on Pt at high potential, the second peak shows few features that depend on OH coverage; the second peak is mainly influenced by the Pt-CO interaction. (C) 2016 Elsevier B.V. All rights reserved1111sciescopu
Influence of cooling rate on iron loss behavior in 6.5 wt% grain-oriented silicon steel
No full textThe influence of cooling rate on iron loss behaviors has been investigated in 6.5 wt% grain oriented silicon steels. To fabricate 6.5 wt% grain-oriented silicon steels and control the microstructure, 3.0 wt% grain-oriented silicon steels covered with SiO2 materials were annealed at 1200 degrees C and then cooled to room temperature using oil quenching, air cooling, or furnace cooling. The magnetic loss of furnace-cooled samples was reduced by 25% compared with oil-quenched samples due to lower anomalous and hysteresis losses. Microstructural analysis showed that these loss behaviors were strongly related to the formation and growth of ordered phases, i.e., B2 and D0(3). These correlations could be ascribed to the formation of antiphase boundaries, which acted as pinning sites of the magnetic domain walls. (C) 2013 Published by Elsevier B.V.This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Republic of Korea (No. 20124030200130)
Design of a Metal/Oxide/Carbon Interface for Highly Active and Selective Electrocatalysis
No full textSustainable energy-conversion and chemical-production require catalysts with high activity, durability, and product-selectivity. Metal/oxide hybrid structure has been intensively investigated to achieve promising catalytic performance, especially in neutral or alkaline electrocatalysis where water dissociation is promoted near the oxide surface for (de)protonation of intermediates. Although catalytic promise of the hybrid structure is demonstrated, it is still challenging to precisely modulate metal/oxide interfacial interactions on the nanoscale. Herein, we report an effective strategy to construct rich metal/oxide nano-interfaces on conductive carbon supports in a surfactant-free and self-terminated way. When compared to the physically mixed Pd/CeO2 system, a much higher degree of interface formation was identified with largely improved hydrogen oxidation reaction (HOR) kinetics. The benefits of the rich metal-CeO2 interface were further generalized to Pd alloys for optimized adsorption energy, where the Pd3Ni/CeO2/C catalyst shows superior performance with HOR selectivity against CO poisoning and shows long-term stability. We believe this work highlights the importance of controlling the interfacial junctions of the electrocatalyst in simultaneously achieving enhanced activity, selectivity, and stability
Scaffold-like titanium nitride nanotubes with a highly conductive porous architecture as a nanoparticle catalyst support for oxygen reduction
No full textWe designed a scaffold-like porous titanium nitride (TiN) nanotube (NT) as a catalyst support for Pt to facilitate the oxygen reduction reaction. Bulk titanium nitride, which is known as an electrically conductive material, is compatible with other metals. As the size of TiN particles decreases, however, they lose their intrinsic high electrical conductivity, due to a series of nanoparticle grain boundaries acting as electron reservoirs and traps. A designed grain-boundary-free scaffold-like porous TiN NT which is analogous to the shape of the one-dimensional porous human spine exhibits high electrical conductivity in spite of having a surface area similar to that of TiN nanoparticle (NPs). The electrical conductivity of TiN NTs is ca. 30-fold higher than that of spherical TiN NPs. The electrochemical oxygen reduction measurements between porous TiN NT and TiN NPs after Pt loading clearly exhibit the superiority of TiN NT as a catalyst support. The results from various electrochemical measurements suggest that the electrocatalytic activity per site did not change from a kinetic viewpoint, but the utilization (the amount of triggered catalytic active sites) in the catalyst layer on the electrode decreased. The Pt/TiN NT composite catalyst exhibited higher activity in comparison to TiN NPs as well as conventional Pt/C catalysts. The accelerated durability test (ADT) revealed that this nanotubular supporting material dramatically enhanced the durability of the catalyst and maintained the electrochemically active surface area (ECSA) of Pt nanoparticles, thus exhibiting performance higher than that of the commercial Pt/C catalyst. X-ray spectroscopy results verified the strong metal-support interaction between Pt nanoparticles and the TiN NT support. This approach opens a reliable path for designing innovative transition-metal oxides, nitrides, or carbides as catalyst supports for use in a wide range of energy conversion applications. © 2016 American Chemical Society114151sciescopu
Effect of post heat-treatment of composition-controlled PdFe nanoparticles for oxygen reduction reaction
No full textComposition-controlled and carbon-supported PdFe nanoparticles (NPs) were prepared via a modified chemical synthesis after heat-treatment at high temperature under a reductive atmosphere. This novel synthesis, which combines the polyol reduction method and hydride method, was used to obtain monodispersed PdFe NPs. In addition, to induce structural modifications, the as-prepared PdFe NPs received heat-treatment under a reductive atmosphere. Structural characterization, including high-resolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analysis, indicated that heat-treated PdFe NPs exhibited a higher degree of alloying and surface Pd atomic composition compared with as-prepared ones. Furthermore, new crystalline phases were detected after heat-treatment. Thanks to the structural alterations, heat-treated PdFe NPs showed ∼3 and ∼18 times higher mass- and area-normalized oxygen reduction reaction (ORR) activities, respectively than commercial Pt/C. Single cell testing with heat-treated PdFe catalysts exhibited a ∼2.5 times higher mass-normalized maximum power density than the reference cell. Surface structure analyses, including cyclic voltammetry (CV), COad oxidation, and XPS, revealed that, after heat-treatment, a downshift of the Pd d-band center occurred, which led to a decrease in the affinity of Pd for oxygen species, resulting in more favorable ORR kinetics. © 2015 Elsevier B.V114141sciescopu
Understanding the Bifunctional Effect for Removal of CO Poisoning: Blend of a Platinum Nanocatalyst and Hydrous Ruthenium Oxide as a Model System
No full textCO poisoning of Pt catalysts is one of the most critical problems that deteriorate the electrocatalytic oxidation and reduction reactions taking place in fuel cells. In general, enhancing CO oxidation properties of catalysts by tailoring the electronic structure of Pt (electronic effect) or increasing the amount of supplied oxygen species (bifunctional effect), which is the typical reactant for CO oxidation, has been performed to remove CO from the Pt surface. However, though there have been a few reports about the understanding of the electronic effect for rapid CO oxidation, a separate understanding of bifunctional modification is yet to be achieved. Herein, we report experimental investigations of CO oxidation in the absence of electronic effect and an extended concept of the bifunctional effect. A model system was prepared by blending conventional Pt/C catalysts with hydrous ruthenium oxide particles, and the CO oxidation behaviors were investigated by various electrochemical measurements, including CO stripping and bulk oxidation. In addition, this system allowed the observation of CO removal by the Eley-Rideal mechanism at high CO coverages, which facilitates further CO oxidation by triggering the CO removal by the Langmuir-Hinshelwood mechanism. Furthermore, effective CO management by this approach in practical applications was also verified by single-cell analysis. © 2016 American Chemical Society121231sciescopu
Effects of Anti-phase Boundary on the Iron Loss of Grain Oriented Silicon Steel
No full textWe present a systematic analysis of the iron loss behavior of grain oriented silicon steels containing different Si contents using transmission electron microscopy. When the silicon content changed in the range of 3-6.5 wt%, the iron loss showed a convex profile and the maximum iron loss was observed in 5.2 wt% silicon steel. This maximum iron loss should be ascribed to the formation of antiphase boundaries that acted as pinning centers in the magnetic domain wall.This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy, 20104010100620 and was supported by grant No.K00041173 from the R&D program for core technology of materials of the Ministry of Knowledge Economy, Republic of Korea
Atomic Structure Modification of Fe‒N‒C Catalysts via Morphology Engineering of Graphene for Enhanced Conversion Kinetics of Lithium–Sulfur Batteries
No full text© 2022 Wiley-VCH GmbH.Single-atom M‒N‒C catalysts have attracted tremendous attention for their application to electrocatalysis. Nitrogen-coordinated mononuclear metal moieties (MNx moities) are bio-inspired active sites that are analogous to various metal-porphyrin cofactors. Given that the functions of metal-porphyrin cofactors are highly dependent on the local coordination environments around the mononuclear active site, engineering MNx active sites in heterogeneous M‒N‒C catalysts would provide an additional degree of freedom for boosting their electrocatalytic activity. This work presents a local coordination structure modification of FeN4 moieties via morphological engineering of graphene support. Introducing highly wrinkled structure in graphene matrix induces nonplanar distortion of FeN4 moieties, resulting in the modification of electronic structure of mononuclear Fe. Electrochemical analysis combined with first-principles calculations reveal that enhanced electrocatalytic lithium polysulfide conversion, especially the Li2S redox step, is attributed to the local structure modified FeN4 active sites, while increased specific surface area also contributes to improved performance at low C-rates. Owing to the synergistic combination of atomic-level modified FeN4 active sites and morphological advantage of graphene support, Fe‒N‒C catalysts with wrinkled graphene morphology show superior lithium–sulfur battery performance at both low and high C-rates (particularly 915.9 mAh g−1 at 5 C) with promising cycling stability.N
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