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The present study elucidates the role of surface oxygen functional groups on the electrochemical behavior of porous carbons when used as anodes for Li-ion batteries. To achieve this objective, a carbon xerogel (CX) obtained by pyrolysis of a resorcinol-formaldehyde gel, was modified by different postsynthesis treatments in order to modulate its surface chemistry while maintaining its external surface constant. Various surface modifications were obtained by oxidation in air, in situ polymerization of dopamine, and finally by grafting of a polyethylene oxide layer on the polydopamine coating. While oxidation in air did not affect the pore texture of the CX, modifications by coating techniques substantially decreased the micropore fraction. Detailed electrochemical characterizations of the materials processed as electrodes were performed by capacitance measurements and galvanostatic cycling. Surface chemistry results, from X-ray photoelectron spectroscopy, show that the accessibility and the capacity increase when carbonyl (R-CâO) groups are formed on the CX, but not with oxides and hydroxyls. The amount of surface carbonyls, and in particular, aldehyde (OâCH) groups, is found to be the key parameter because it is directly correlated with the modified CX electrochemical behavior. Overall, the explored surface coatings tend to reduce the micropore volume and add mainly hydroxyl functional groups but hardly change the Li+ insertion/deinsertion capacities, while oxidation in air adds carbonyl groups, increasing the Li+ ion storage capacity, thanks to an improved accessibility to the carbon network, which is not caused by any textural change.
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ZnO is known to be photocatalytic, but with limited performances due to the strong electron-hole recombination after irradiation. The integration of ZnO nanomaterials on a conductive and high surface area carbon substrate is thus a potential alternative to obtain a significant improvement of the photocatalytic performance. Moreover, the carbon functionalization is expected to have a significant role in the adsorption/degradation mechanisms of dye, due to the difference in wettability or surface charge. In this view, ZnO photocatalytic nanoparticles have been deposited on high surface area carbon xerogel substrate (CXG), using a new and original plasma process, consisting in the degradation of a solid organometallic directly on the carbon substrate (no gaseous precursor). In addition to the ZnO nanoparticle formation, the plasma treatment allows the carbon functionalization. The ZnO/CXG composite has been tested for the degradation of Rhodamine B (RhB) in aqueous media and compared with and O2 or NH3 plasma-treated xerogels (without nanoparticles) to identify the significant role of the substrate and its modification in the RhB adsorption and degradation mechanism. The high photocatalytic activity of ZnO/CXG composite is attributed to (i) the formation of small (8-10 nm) and well-crystallized ZnO nanoparticles anchored to the carbon substrate and (ii) to the modification of the xerogel surface chemistry. Indeed, O2 plasma treatment of the CXG promotes the generation of hydroxyl, carbonyl and carboxyl surface functional groups, which are polar and acidic, while the NH3 plasma treatment mainly leads to the formation of polar and basic amino groups. While both plasma treatments promote the formation of polar functional groups, which enhance the CXG wettability, the formation of acidic groups is identified as beneficial for the adsorption of the RhB dye, while basic groups are detrimental.
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Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt-alloy nanoparticles (NPs) supported on high-surface-area carbon attract growing interest. However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt-alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt-alloy/C hollow nanostructures. Our results indicate that metals with standard potential -0.4
RESUMO
The oxygen reduction reaction (ORR) activity of Pt-alloy electrocatalysts depends on (i) the strain/ligand effects induced by the non-noble metal (3d-transition metal or a rare-earth element) alloyed to Pt, (ii) the orientation of the catalytic surfaces, and (iii) the density of structural defects (SDs) (e.g., vacancies, voids, interconnections). These SDs influence the "generalized" coordination number of Pt atoms, the Pt-alloy lattice parameter, and thus the adsorption strength of the ORR intermediates (O*, OH*, OOH*). Here, we discuss a set of parameters derived from COads stripping measurements and the Rietveld refinement of X-ray diffraction (XRD) patterns, aiming to show how the leaching of the non-noble metal and the density of SDs influence the ORR activity of porous hollow PtNi/C nanoparticles (PH-PtNi/C NPs). PH-PtNi/C NPs were aged at T = 353 K in an Ar-saturated 0.1 M HClO4 electrolyte during 20 000 potential cycles between E = 0.6 and 1.0 V versus the reversible hydrogen electrode, with an intermediate characterization after 5000 cycles. The losses in the ORR specific activity were attributed to the dissolution of Ni atoms (modifying strain/ligand effects) and to the increase of the crystallite size (dXRD), resulting in a diminution of the density of grain boundaries. In agreement with the Gibbs-Thompson equation, the electrocatalysts that presented larger crystallites (dXRD > 3 nm) were far more stable than the ones with the smallest crystallites (dXRD < 2 nm). We also observed that performing intermediate characterizations (in an O2-saturated electrolyte) results in activity losses for the ORR.
RESUMO
A novel hybrid material displaying a fast and reversible charge storage capability is prepared by electrografting of an alkoxyamine-bearing acrylate onto a carbon nanotube buckypaper, followed by the quantitative generation of an electroactive polynitroxide.
RESUMO
The electrocatalytic properties of home-made Pt nanoparticles supported onto WO(x) were determined for the electrooxidation of a CO(ads) monolayer and compared with that of a commercial Pt/C having the same Pt particle size. By combining electrochemical and spectroscopic techniques, we found that Pt/WO(x) nanoparticles exhibit a very high tolerance to CO at low electrode potentials (E = 0.1 V vs. RHE), which was never reported in the literature before. CO adsorption at E = 0.1 V vs. RHE on Pt/WO(x) yields CO(2) production as observed by Fourier-transform infrared spectroscopy (FTIR). When the gas bubbling in solution changes from CO to Ar, the current attenuates and the CO(2) production vanishes. This points towards a limited number of "active sites" and a slow step in the electrocatalytic process. When H(2) is used to purge the electrolyte from CO, a steep and continuous increase of the H(2) electrooxidation current is observed pointing towards continuous liberation of the Pt catalytic sites. The high tolerance to CO of Pt/WO(x) is discussed in terms of strong metal-support interaction (SMSI), which involves formation of a metal-oxide film partially covering the Pt nanoparticles (encapsulation) and creation of W-OH groups upon H(+) insertion at low electrode potentials.
