1,721,036 research outputs found
Controlled Synthesis of Pt Nanowires with Ordered Large Mesopores for Methanol Oxidation Reaction
No full textPublisher's PDFCatalysts for methanol oxidation reaction (MOR) are at the heart of key green-energy fuel cell
technology. Nanostructured Pt materials are the most popular and effective catalysts for MOR.
Controlling the morphology and structure of Pt nanomaterials can provide opportunities to greatly
increase their activity and stability. Ordered nanoporous Pt nanowires with controlled large mesopores
(15, 30 and 45 nm) are facilely fabricated by chemical reduction deposition from dual templates using
porous anodic aluminum oxide (AAO) membranes with silica nanospheres self-assembled in the
channels. The prepared mesoporous Pt nanowires are highly active and stable electrocatalysts for MOR.
The mesoporous Pt nanowires with 15 nm mesopores exhibit a large electrochemically active surface
area (ECSA, 40.5 m2 g−1), a high mass activity (398 mA mg−1) and specific activity (0.98 mA cm−2), and
a good If/Ib ratio (1.15), better than the other mesoporous Pt nanowires and the commercial Pt black
catalyst.University of Delaware. Department of Chemical & Biomolecular Engineering
Correcting the Hydrogen Diffusion Limitation in Rotating Disk Electrode Measurements of Hydrogen Evolution Reaction Kinetics
No full textPulsiher's PDFRotating disk electrode (RDE) method is widely employed in studies on the hydrogen oxidation/evolution reaction (HOR/HER) owing to its well-defined mass transport behaviors. While it is accepted that the measured HOR current is controlled by both the electrode kinetics and the diffusion of H2, HER is typically assumed to be free of diffusion limitation. Here we demonstrate that HER could also be diffusion limited when the electrode kinetics is fast, as evidenced by the rotation speed dependent HER current on Pt in acid (pH = 1) and the overlap of the HER polarization curve with the concentration overpotential curve. The HER diffusion limitation originates from the insufficient mass transport of produced H2 from the electrode surface to the bulk electrolyte and the highly reversible nature of HOR/HER. Kinetic analyses based on HER polarization curves on Pt in acid without correcting for the diffusion limitation could lead to inaccurate Tafel slopes and mechanistic interpretations, and significantly underestimated HER activities. A general data analysis protocol based on the reversible Koutecky-Levich equation is developed to obtain accurate kinetic information of HOR/HER even when electrode kinetics is facile. This new method is compared with other existing methods on Pt disk electrodes at different pHs and thin-film electrodes with different Pt loadings.University of Delaware. Center for Catalytic Science & Technology
Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy
No full textPublisher's PDFUnderstanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction
(HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel
cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed
for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13),
suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current
density and HBE is also observed on the four metals, indicating that they may share the same elementary steps
and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential
of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence
against the promoting effect of adsorbed OH on HOR/HER.University of Delaware. Department of Chemical & Biomolecular Engineering.University of Delaware. Center for Catalytic Science & Technology
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Extended Two Dimensional Nanotube and Nanowire Surfaces as Fuel Cell Catalysts
Full text linkExtended network nanomaterials of platinum (Pt), silver (Ag), palladium (Pd), and gold (Au) are synthesized and characterized as proton exchange membrane (PEMFC), hydroxide exchange membrane (HEMFC), and direct alcohol (DAFC) fuel cell catalysts.Porous Pt nanotubes (PPtNTs), 5 nm thick, are synthesized by the galvanic displacement of Ag nanowires (AgNWs) for PEMFCs and DAFCs. PPtNTs produce oxygen reduction (ORR) and durability characteristics significantly higher than supported Pt nanoparticles (Pt/C); prior to and following durability testing, PPtNTs produce specific ORR activities 3.1 and 19.9 times greater than Pt/C. The methanol oxidation peak specific activity of PPtNTs is 2.0 times and 1.3 times greater than Pt/C and a polycrystalline Pt electrode. AgNWs, 25-60 nm in diameter, are synthesized as HEMFC catalysts. In contrast to Ag nanoparticles, the ORR specific activity of AgNWs increases and peroxide production decreases with diameter reduction. AgNWs (25 nm) exceed the mass and specific ORR activity of 2.4 nm AgNPs by 16 % and 5.3 times. Pd and Au NTs, 5 nm thick, are synthesized as HEFMC and DAFC catalysts. PdNTs and AuNTs exceed the ORR specific activity of Pt/C by 42% and 21%. PdNTs produce an earlier onset potential than all conventional catalysts in methanol and ethylene glycol oxidation; PdNTs further meet or exceed the specific activity of Pt/C in methanol, ethanol, and ethylene glycol oxidation.Pt coated PdNTs (Pt/PdNTs) are synthesized as PEMFC catalysts by the incomplete galvanic displacement of PdNTs. Pt/PdNTs express an ORR activity comparable to PtNTs while containing 9-18 wt% Pt. Pt/PdNTs exceed the cost and specific activity benchmarks of the US Department of Energy (DOE) and produce a Pt normalized mass activity 11.6 times greater than Pt/C. Pt coated copper NWs (Pt/CuNWs) are synthesized by the incomplete galvanic displacement of copper nanowires. Pt/CuNWs exceed the cost and specific activity DOE benchmarks; durability testing reveals improved retention of surface area and ORR activity in comparison to Pt/C.Templated synthesis allows for the control of growth directions and lattice spacing. The use of templated extended network nanomaterials has improved the electrocatalytic activity and durability of conventional fuel cell catalysts
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A Single-Step Monomeric Photo-Polymerization and Crosslinking via Thiol-Ene Reaction for Hydroxide Exchange Membrane Fabrication
No full textPubllisher's PDF.A single step monomeric photo-polymerization and crosslinking via thiol-ene reaction is developed for the preparation of hydroxide exchange membranes (HEMs) in a ternary system with a triallyl triazine, a quaternary ammonium diallyl, and a dithiol. This facile method enables reproducible tuning of the ion exchange capacity and crosslink density. These HEMs demonstrate reasonable hydroxide conductivity, limited alkaline stability, and good thermal stability and have lower water uptakes than other photo-crosslinked HEMs produced with much longer reaction times. Furthermore, this new fabrication method allows the incorporation of catalyst nanoparticles in the hydroxide exchange materials to form thin catalyst layers that are resistant to dissolution in methanol which suggests these polymers can be used in direct alcohol fuel cells (DAFCs).University of Delaware. Department of Chemical & Biomolecular Engineering.University of Delaware. Department of Materials Science and Engineering
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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