1,720,967 research outputs found
Electrochemical water oxidation using a copper complex
No full textThis study highlights the importance of proton coupled electron transfer (PCET) during electrochemical-driven water oxidation catalysis employing a copper complex.</p
Integrated Devices for Photoelectrochemical Water Splitting Using Adapted Silicon Based Multi-Junction Solar Cells Protected by ALD TiO2 Coatings
No full textIn this study, we present different silicon based integrated devices for photoelectrochemical water splitting, which provide enough photovoltage to drive the reaction without an external bias. Thin films of titanium dioxide, prepared by atomic layer deposition (ALD), are applied as a surface passivation and corrosion protection. The interfaces between the multi-junction cells and the protective coating were optimized individually by etching techniques and finding optimal parameters for the ALD process. The energy band alignment of the systems was studied by X-ray photoelectron spectroscopy (XPS). Electrochemically deposited platinum particles were used to reduce the HER overpotential. The prepared systems were tested in a three-electrode arrangement under AM 1.5 illumination in 0.1 M KOH. In final tests the efficiency and stability of the prepared devices were tested in a two-electrode arrangement in dependence of the pH value with a ruthenium-iridium oxide counter electrode. For the tandem-junction device solar to hydrogen efficiencies (STH) up to 1.8% were reached, and the triple-junction device showed a maximum efficiency of 4.4%
A metal-free heterogeneous photocatalyst for the selective oxidative cleavage of CC bonds in aryl olefins via harvesting direct solar energy
No full textAtom Surface Dynamics of Manganese Oxide under Oxygen Evolution Reaction-Like Conditions Studied by In Situ Environmental Transmission Electron Microscopy
No full textHydrogen production by electrochemical water splitting is limited by the sluggish oxygen evolution reaction (OER). In order to improve our understanding of the underlying mechanisms, information about the atomic surface structure of the active state of the electrode is required. Here, we present environmental transmission electron microscopy studies of Ca-birnessite (K0.20Ca0.21MnO2.21·1.4H2O) electrodes under conditions close to those of the OER. Remarkably, in H2O vapor, a highly dynamic state of the surface and subsurface develops with a thickness of the formed dynamic layer of up to 0.6 nm, which is absent in O2 and inert gases. Electron beam-induced effects are carefully studied, showing high stability of the material against radiation damage in high vacuum until a dose rate of 42,000 e–/(Å2 s). In contrast, in H2O, the dynamic surface layer develops and forms a stationary state even at low dose rates, down to 5000 e–/(Å2 s). Electron energy-loss spectroscopy reveals an increase in the Mn oxidation state in H2O and in O2 ambient. Our results are interpreted as the formation of a few-angstrom-thick, dynamic, and hydrated surface layer of birnessite in H2O, with an increased Mn valence state. Such a dynamic surface layer with a flexible Mn coordination and valence state might be optimal for oxygen evolution due to the higher effective interaction volume beyond the surface area and a flexible bond coordination of partially hydrated Mn species
Structure of Nanocrystalline, Partially Disordered MoS2+δ Derived from HRTEM—An Abundant Material for Efficient HER Catalysis
No full textMolybdenum sulfides (MoSx, x > 2) are promising catalysts for the hydrogen evolution reaction (HER) that show high hydrogen evolution rates and potentially represent an abundant alternative to platinum. However, a complete understanding of the structure of the most active variants is still lacking. Nanocrystalline MoS2+δ was prepared by a solvothermal method and immobilized on graphene. The obtained electrodes exhibit stable HER current densities of 3 mA cm−2 at an overpotential of ~200 mV for at least 7 h. A structural analysis of the material by high-resolution transmission electron microscopy (HRTEM) show partially disordered nanocrystals of a size between 5–10 nm. Both X-ray and electron diffraction reveal large fluctuations in lattice spacing, where the average c-axis stacking is increased and the in-plane lattice parameter is locally reduced in comparison to the layered structure of crystalline MoS2. A three-dimensional structural model of MoS2+δ could be derived from the experiments, in which [Mo2S12]2− and [Mo3S13]2− clusters as well as disclinations represent the typical defects in the ideal MoS2 structure. It is suggested that the partially disordered nanostructure leads to a high density of coordinatively modified Mo sites with lower Mo–Mo distances representing the active sites for HER catalysis, and, that these structural features are more important than the S:Mo ratio for the activity
Stability and Degradation Mechanism of Si-based Photocathodes for Water Splitting with Ultrathin TiO2 Protection Layer
No full textUsing transmission and scanning electron microscopy, we study mechanisms which determine the stability of Silicon photocathodes for solar driven water splitting. Such tandem or triple devices can show a promising stability as photocathodes if the semiconductor surface is protected by an ultrathin TiO2 protection layer. Using atomic layer deposition (ALD) with Cl-precursors, 4–7 nm thick TiO2 layers can be grown with high structural perfection. The layer can be electrochemically covered by Pt nanoparticels serving as electro-catalysts. However, Cl-remnants which are typically present in such layers due to incomplete oxidation, are the origin of an electrochemical degradation process. After 1 h AM1.5G illumination in alkaline media, circular shaped corrosion craters appear in the topmost Si layer, although the TiO2 layer is intact in most parts of the crater. The crater development is stopped at local inhomogenities with a higher Pt coverage. The observations suggests that reduced Titanium species due to Cl−/O2− substitution are nucleation sites of the initial corrosion steps due to enhanced solubility of reduced Ti in the electrolyte. This process is followed by electrochemical dissolution of Si, after direct contact between the electrolyte and the top Si layer surface. To increase the stability of TiO2 protected photocathodes, formation of reduced Ti species must be avoided
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
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