1,721,031 research outputs found
Composite based on nickel-functionalized carbon nitride and carbon nanotubes as an efficient electrocatalyst for the oxygen evolution reaction
Full text linkSingle metal atom catalysts (SACs) are receiving widespread attention in electrochemical energy conversion reactions due to the rational use of metal resources and maximum atom utilization efficiency. The role of the support in stabilizing the single atoms is crucial for their catalytic stability. Carbon nitride (CN) is an excellent support for SACs but its low electrical conductivity is not appropriate for electrochemical applications. Here, we report an engineered composite material based on multiwall carbon nanotubes (MWCNTs) and single nickel atoms stabilized on CN (Ni–CN) as efficient and robust electrocatalyst for the oxygen evolution reaction (OER). Composites with different mass Ni–CN:MWCNT ratios have been prepared to optimize the contribution of both materials, and characterized by X-ray diffraction, transmission electron microscopy, X-ray absorption, and X-ray photoemission spectroscopy. Results confirmed the self-assembly of both materials and the condensation of the triazine-based structure of CN into heptazine-based onto the MWCNTs’ surface during the synthesis, as well as the presence of single Ni atoms in the composites. The co-presence of NiO nanoparticles was detected for the samples with the highest Ni content. The ratio of NiO nanoparticles to single-atom Ni centers was governed by the Ni–CN:MWCNT ratio employed during synthesis. Electrochemical characterization showed a synergistic effect between Ni–CN and MWCNTs that boosted the OER activity of the composites respect to the individual components. The 1:2 ratio turned out to be the optimal one for the composite preparation, maximizing the combined effects of the catalytic activity of the Ni centers and the electrical conductivity of MWCNTs. The mass activity obtained by this composite was 30 times higher than that of the Ni–CN starting material, attributable to its superior electrical conductivity and improved accessibility of Ni active sites. This study underscores the potential of composite materials to advance SACs toward large-scale application
Enhancing the Oxygen Electroreduction Activity through Electron Tunnelling: CoOx Ultrathin Films on Pd(100)
No full textNitrogen- and Fluorine-Doped Carbon Nanohorns as Efficient Metal-Free Oxygen Reduction Catalyst: Role of the Nitrogen Groups
Full text linkThe search of active, stable and low costs catalysts for the oxygen reduction reaction (ORR) is crucial for the extensive use of fuel cells and metal–air batteries. The development of metal-free catalysts, instead of platinum-based materials, can dramatically reduce the cost and increase the efficiency of these devices. In this work, carbon nanohorns (CNHs) have been covalently functionalized with N-containing heterocycles by the Tour reaction protocol and tested as metal-free ORR catalysts. The insertion of N-functionalities favored the complete reduction of oxygen to hydroxyl ions, while their absence favored the production of hydrogen peroxide. With the aim of determining the N-species responsible for the ORR activity of CNHs, photoemission and electrochemical measurements were combined. Results suggest that protonated N is the main species involved in the ORR process, facilitating the adsorption of oxygen, with their consequent reduction to neutral hydrogenated N species
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
Oxygen reduction reaction at La<sub>x</sub>Ca<sub>1-x</sub>MnO<sub>3</sub> nanostructures: interplay between A-site segregation and B-site valency
No full textThe mean activity of surface Mn sites at LaxCa1-xMnO3 nanostructures towards the oxygen reduction reaction (ORR) in alkaline solution is assessed as a function of the oxide composition. Highly active oxide nano-particles were synthesised by an ionic liquid-based route, yielding phase-pure nanoparticles, across the entire range of compositions, with sizes between 20 and 35 nm. The bulk vs. surface composition and structure are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). These techniques allow quantification of not only changes in the mean oxidation state of Mn as a function of x, but also the extent of A-site surface segregation. Both trends manifest themselves in the electrochemical responses associated with surface Mn sites in 0.1 M KOH solution. The characteristic redox signatures of Mn sites are used to estimate their effective surface number density. This parameter allows comparing, for the first time, the mean electrocatalytic activity of surface Mn sites as a function of the LaxCa1-xMnO3 composition. The ensemble of experimental data provides a consistent picture in which increasing electron density at the Mn sites leads to an increase in the ORR activity. We also demonstrate that normalisation of electrochemical activity by mass or specific surface area may result in inaccurate structure–activity correlations
Boron oxynitride two-colour fluorescent dots and their incorporation in a hybrid organic-inorganic film
No full textHighly Active MoS2‐MXenes Hybrid Electrocatalysts Towards the Oxygen Reduction Reaction
Full text linkIn this work, catalytic performance of MoS2 supported on Mo2TiC2 and Ti3C2 MXenes towards the oxygen reduction reaction (ORR) was studied. These materials were synthesised through an etching route of MAX phases as precursors of MXenes, followed by a hydrothermal treatment to coat them with MoS2. This procedure generated MoS2 nanospheres covering the MXenes, which were verified by SEM-EDX mapping, Raman spectroscopy and XPS. Activity of these materials towards ORR was studied by RRDE, comparing its performance with that of individual MoS2 and MXenes. As main features, it was demonstrated that production of HOO- occurring on the composites decreases in comparison with individual materials. Long-term stability tests performed in O-2-saturated electrolyte showed a drop in the activity of the materials due to the increase in the production of HOO-, although no morphological or compositional changes were observed. Despite this result, the improvement of the catalytic properties of the individual materials means that these composites can be considered as candidates to be used as supports of active non-noble metal nanoparticles in the cathodes of anion exchange membrane fuel cells (AEMFCs)
Insights into the active nickel centers embedded in graphitic carbon nitride for the oxygen evolution reaction
No full textExperimental and theoretical studies have demonstrated that the use of single atom catalysts (SACs) for energy conversion processes is very promising. However, their stability under catalytic conditions is the main issue that hinders their commercial use. In this work, we report an oxygen evolution catalyst based on single nickel atoms stabilized in triazine-based carbon nitride (CN) and a detailed study of the evolution of the Ni centers under catalytic conditions. The nanostructured materials have been characterized by combining experimental techniques, such as X-ray diffraction, transmission electron microscopy, X-ray absorption and X-ray photoemission spectroscopy, with DFT theoretical calculations to determine the CN structure, the metal adsorption sites, the coordination of the Ni atoms, and the changes undergone under catalytic conditions. Electrochemical characterization showed a linear increase of the catalytic activity with Ni loading. The stability of the materials was studied by HR-TEM and XAS post-catalysis measurements and DFT simulations. Results indicated a partial chemical restructuring of the single Ni atoms under catalytic conditions with the formation of Ni-O-Ni moieties, stabilized in the CN cavities, which are the real catalytic species
Quaternary ammonium-functionalized carbon nanotubes/alginate nanocomposite hydrogels support myoblast growth and differentiation
Full text linkCarbon nanotube (CNT) composite hydrogels are promising materials for tissue engineering due to the biocompatibility of the matrix and the electrical conductivity of the filler, which is crucial for promoting the growth and functions in electroactive tissues. While pristine CNTs are insoluble, we synthesized and fully characterized a water-soluble CNT derivative (fCNT) bearing quaternary ammonium groups, and we homogeneously dispersed it within alginate-based hydrogels. Through external and internal gelation we obtained two plain and two fCNT-filled hydrogels (HG1 and HG2 and HG1-fCNT and HG2-fCNT, respectively), and we compared the physical properties of the four different materials. A measurement setup and an approach were specifically designed for the electrical characterization of our hydrogel samples, showing that the addition of a low amount (0.1 mg mL−1) of fCNT enhanced the conductivity of the hydrogel from internal gelation (HG2-fCNT) by more than one order of magnitude, from 5.7 × 10−10 to 2.8 × 10−8 S cm−1. Even more interestingly, HG2-fCNT featured a faster transmission of low frequency signals (with time scales from 1 ms to 100 ms, typical of electroactive biological tissues) than the other samples. Finally, the behavior of the four hydrogels as scaffolds for muscle tissue engineering was compared through studies of myoblast viability, proliferation, and differentiation. A relevant improvement in differentiation (more than doubling the number and area of myotubes and the fusion index) was obtained by adding the fCNT in the case of HG2-fCNT, in line of its superior electrical properties. These outcomes hint at the feasibility of using the fCNT combined with the alginate hydrogel in order to support the myoblast growth and proliferation
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