Advanced materials are those that have physical or chemical properties that can be controlled intelligently through external stimuli such as mechanical tension, temperature, electric and magnetic fields, among others. Several PPG-Nano research groups have been synthesizing and studying these materials focusing on the energetic, electrochemical, and optical properties of systems with photocatalytic activity, thermal stability, and biocompatibility.
Within this context, we present some of our most recent works.
In recent years, there has been growing interest in developing technologies for direct ethanol fuel cells (DEFCs) in an alkaline medium [, , , ]. The use of ethanol has received great attention because it can easily be produced in large quantities from biological processing of agricultural products and is considered a renewable energy source due to low pollutant emission characteristics . Moreover, ethanol is a strategic fuel because of its low toxicity, since its complete oxidation to CO2 involves 12 electrons per molecule oxidized, resulting in high energy density (8.01 kW h Kg−1 or 6.34 kW h L−1, b.p. 78.4 °C) when compared to hydrogen [, , ,], as well as other alcohols, such as methanol (6.09 kW h Kg−1, b.p. 64.7 °C) [9,10], glycerol (5.0 kW h Kg−1, b.p. 290.0 °C)  and ethylene glycol (5.2 kW h Kg−1, b.p. 197.3 °C) . However, the greatest obstacle to the commercialization of DEFCs is the lack of catalysts that can trigger the organic oxidation at a favorable rate . The search for an active catalyst that yields high current densities during the ethanol oxidation reaction (EOR) is a key goal in research into DEFCs.
In this work, Carbon-supported Pd, Au@Pd core–shell and Au1–xPdx-alloyed nanoparticles were prepared by a chemical reduction method and characterized by different experimental techniques, including X-ray powder diffraction, transmission electron microscopy, scanning-transmission electron microscopy using bright-field and high-angle annular dark field detectors and X-ray energy dispersive spectroscopy. The catalytic mass activity toward ethanol oxidation was assessed by cyclic voltammetry and chronoamperometry at room temperature. The measurements showed that the addition of Au enhances remarkably the electrocatalytic activity of the material, due to the bifunctional effect of Au1–xPdx/C alloys, and the synergetic effect on Au@Pd/C, resulting in a dissolution resistance of core–shell catalysts at potentials of 1.5 V versus reversible hydrogen electrode. In situ Fourier transform infrared spectroscopy measurements showed that the mechanism for ethanol oxidation depends on the electrocatalyst structure and morphology. Acetate was identified as the main product of ethanol electro-oxidation on the studied electrocatalysts. However, the presence of a core–shell structure on Au@Pd/C resulted in enhanced ethanol oxidation selectivity toward CO2. The improvement of activity is attributed to the interaction between Pd shell and Au core.
Read the full article at https://www.sciencedirect.com/science/article/abs/pii/S0926337319302899#fig0045 and access the numerical references.
In this paper is explored the diffusion of lithium ions decoupled from a solid polymer electrolyte matrix is the key for high-energy electrochemical devices with the safety needed for commercial use. This Letter reports how the ion mobility in a single-phase hybrid polyelectrolyte (SPHP) matrix can be tuned by changing an inorganic coordinating atom from silicon (Si) to germanium (Ge).
Nuclear Magnetic Resonance (NMR) results show that the lithium ion activation barrier in the polyelectrolyte with Si can be modulated from 0.26 eV to the unprecedented value of 0.12 eV in the polyelectrolyte with Ge. Density functional theory is used to show that the electronic structures of both polymers are very different, although their chemical structures are very similar, except for the coordinating atom.
As a starting point, a model was built for the synthesized SPHP matrix, containing the Si/Ge tetrahedral core and the subsequent C chains with different oxygen groups. The optimized geometry and symmetry of both polymers are very similar, but their electronic structures are rather different (Figure a–d).
While the LUMO is mostly localized at specific C and O sites in SPHP/Si (a), for SPHP/Ge it is mostly localized around the Ge atom and the neighboring oxygen sites (b).
The origin of this difference can be understood by analysis of their projected density of states. From this, schematic representation of the molecular orbital diagrams of sp hybridization for SPHP/Si and SPHP/Ge are shown in panels (e and f), respectively. SPHP/Si and SPHP/Ge have the same s2p2 electronic configuration in valence, with the Si 3p orbitals having very similar energy as the Ge 4p orbitals.
However, the source of the LUMO difference is mainly related to the energy of the Si and Ge s orbitals; the Ge 4s atomic orbitals are ∼0.8 eV deeper than the Si 3s orbitals. As a consequence, the energy difference between the O p and Ge s orbitals is larger compared to O p and Si s. Taking also into account that the lengths of the covalent radius are distinct, with 1.11 and 1.20 Å for Si and Ge, respectively, the sp hybridization–repulsion in the Ge polymer is decreased.
FT-Raman spectroscopy was performed to observe the “fingerprints” of SPHP/Ge and SPHP/Si, to properly index their molecular vibrations, and to present the main differences between these two systems.
FT-Raman spectra of SPHP/Si and SPHP/Ge and the structural formula of SPHP/Ge (figure). The blue and green shaded regions indicate the differences found ascribed by unequal electronic structure. The red shaded region represents the vibration of the ester group, which is the preferred ion site.
The possibility of large scale utilization of hydrogen as energy carrier depends on the convenient solution of several technological problems. One of the most challenging is to find a safe and feasible way of storing this gas for subsequent utilization in fuel cells [1,2]. Metal hydrides have been considered as interesting candidates for hydrogen storage, but the search for hydrides which fulfil all the technical requirements for hydrogen storage applications is far to be a simple task, remaining an open question so far. Among others, magnesium hydride (MgH2) has attracted great attention from the scientific community as potential hydrogen storage material mainly because of its high gravimetric hydrogen storage capacity (7.6 wt%) and relative low cost of magnesium. In opposition, the slow reaction kinetics and high temperatures needed for hydrogen absorption and release have been hindering its practical use [1e3]. This slow kinetics is especially important during the first absorption (activation) of hydrogen by magnesium. The activation of magnesium normally takes several hours, even days and requires high hydrogen pressures and temperatures above 400 C.
In this paper we report an investigation on the processing of magnesium samples by filing and the hydrogen storage properties of the obtained chips. Filing of Mg was performed manually and mechanized. The effect of the filing tool design on the morphology and hydrogen storage properties of Mg chips was evaluated. The hydrogen absorption and desorption kinetics of manual filed Mg is strongly influenced by the geometry of the filing tool. Chips obtained by using a fine tool have larger surface areas and faster hydrogenation and dehydrogenation kinetics. Moreover, these chips show high resistance to air contamination and can absorb and release hydrogen even after long periods of exposition to air. Chips obtained by mechanized filing also showed suitable hydrogen storage properties and a slight increase in incubation period for hydrogen absorption. After this initial delay, the reaction kinetics of hydrogen absorption was similar for both mechanized and manually filed chips. The hydrogen storage properties of the chips are not affected by the machining speed adopted during mechanized filing. These results clearly indicate the feasibility of adopting filing processing to produce Mg for hydrogen storage and open possibilities to scale-up the production of such material using a simple and economical approach.
Kinetic curves for the (a) first hydrogen absorption (activation) and (b) desorption, and second absorption of manual filed magnesium samples.
SEM images of as-prepared coarse (a and b), medium (c and d), and fine (e and f) magnesium chips.
Kinetic curves of hydrogen absorption (a) and desorption (b) for: freshly processed (signed as “air”) sample; processed and stored in air for 30 days; and after one cycle of hydrogen absorption/desorption followed by air exposure (reactivation).
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Niobium oxide films have attracted much attention in materials science in the past few years. NbO, NbO2 and Nb2O5 are the most stable phases, being niobium pentoxide the one with the lowest Gibbs free energy of formation between them and, hence, the most thermodynamically favorable to form. The growing interest in Nb2O5 arises from its applicability in several advanced devices such as sensors, solar cells, capacitors and smart windows and. In these devices electronic and optical properties of Nb2O5 films such as photoelectric and photocatalytic activity, high permittivity, high refractive index and transparency in the UV–vis–NIR region have been advantageously exploited to produce a variety of optical devices. Furthermore, high wear resistance, good thermal stability and biocompatibility have been reported as attributes that widen the engineering applications of Nb2O5 layers to additional areas such as biomedical devices and barrier coatings.
SEM micrographs of the top surfaces of the uncoated and Nb2O5-coated 316 stainless steel samples: (A) uncoated substrate, before immersion; (B) 15′-coated sample, before immersion; (C) 30′-coated sample, before immersion; (D) uncoated substrate, after polarization; (E) 15′-coated sample, after polarization; (F) 30′-coated sample, after polarization.
CSLM images of corrosion pit formed on the surface of uncoated 316 stainless steel sample: (A) 2D-image; (B) 3D-image; (C) transverse profile.
CSLM images of corrosion pit formed on the surface of 15 min-coated sample: (A) 2D-image; (B) 3D-image; (C) transverse profile
CSLM images of corrosion pit formed on the surface of 30 min-coated sample: (A) 2D-image; (B) 3D-image; (C) transverse profile.
Ceria based compounds are used in a broad range of technol- ogies such as catalysis, environmental, electromechanical, electrochemical, and various emerging energy technologies. Such an extensive use of these materials is due to the unique combination of different properties such as high ionic conductivity, mixed electronic and ionic conductivity, and large oxygen storage and exchange capabilities, which are all linked to the ability of the materials to undergo rapid Ce3+/Ce4+ redox cycles and form oxygen vacancies. Moreover, ceria solid solutions usually show exceptionally high chemical stability, even in extremely harsh and corrosive environments, in the presence of sulphur or at very high operating temperatures.
The valence and size of cations influence mass diffusion and oxygen defects in ceria. Here we show that reduction of Ce4+ to Ce3+, at high temperatures and low oxygen activity, activates fast diffusion mechanisms which depend on the aliovalent cation concentration. As a result, polycrystalline solid solutions with enhanced electrochemical properties are formed.
The mass diffusion in aliovalent (i.e. Gd in 0, 0.1, 10 and 30 mol%) doped ceria is investigated in this work. Since mass diffusion in ceria is drastically inuenced by the solute drag effect, the combined effects of chemical reduction from Ce4+ to Ce3+ and Gd3+ dopant concentration result in signicant vari- ations of mass diffusion mechanisms. In particular, consistent with the solute drag theory, in highly defective ceria obtained at high temperatures (1400 C) and low oxygen activity (10 31 < pO2 < 10 12 atm), these lead to opposite effects of inhibition of densication and grain growth in pure ceria and low dopant concentration (0.1 mol% Gd2O3) and fast densication and grain growth at a high dopant concentration (10 and 30 mol% of Gd2O3). The nal result of the thermal and chemical treatments is a set of solid solutions whose electrochemical properties do not depend on the microstructure or on the nominal dopant concentration.
SEM images of CeO2, CGO0.1, CGO10 and CGO30 compacts after sintering in air and 9% H2–N2 at 1400 C for 4 h. Arrhenius plots for the total conductivity in air calculated by EIS data of CeO2, CGO0.1, CGO10 and CGO30 compacts sintered in air and 9% H2–N2 at 1400 C for 4 h.