N. Bazzanella

Catalytic effect on hydrogen desorption in Nb-doped microcrystalline MgH2

Mg and Nb-doped Mg films were deposited by rf magnetron sputtering. Morphological and structural analysis were performed by scanning electron microscopy and x-ray diffraction. The desorption kinetics has been investigated by using a Sievert-type apparatus. The overall activation energy and the reaction order controlling desorption are (141±5)kJmol−1H and n≈4 for Mg and (51±5)kJmol−1H and n≈1 for Nb-doped (5at.%) Mg. It is suggested that Nb atoms dispersed in the MgH2 grains catalyzes the dissociation of the hydride phase and that the rate limiting step in the H2 desorption is given by the H atomic migration through interconnected transformed domains of h‐Mg.

High quality factor 1-D Er 3+ -activated dielectric microcavity fabricated by RF-sputtering

Rare earth-activated 1-D photonic crystals were fabricated by RF-sputtering technique. The cavity is constituted by an Er3+-doped SiO2 active layer inserted between two Bragg reflectors consisting of ten pairs of SiO2/TiO2 layers. Scanning electron microscopy is employed to put in evidence the quality of the sample, the homogeneities of the layers thickness and the good adhesion among them. Near infrared transmittance and variable angle reflectance spectra confirm the presence of a stop band from 1500 nm to 2000 nm with a cavity resonance centered at 1749 nm at 0° and a quality factor of 890. The influence of the cavity on the ⁴I₁₃/₂ -->⁴I₁₅/₂ emission band of Er3+ ion is also demonstrated.

Hydrogen kinetics in magnesium hydride: On different catalytic effects of niobium

The hydrogen desorption kinetics from pure and Nb-doped MgH2 samples was studied as function of the Nb concentration (6×10−4<[Nb∕Mg]<5×10−2). Structural and kinetics analyses indicate that Nb acts as catalyst both when Nb atoms aggregate forming NbH clusters dispersed in the MgH2 and also when Nb is contained as atomic impurity. It is suggested that the local atomic environment around the Nb atom acts as seed for h-Mg phase nucleation which constitutes the rate limiting step for the hydrogen kinetics while, when the Nb concentration exceeds about 1at.%, the rate limiting step is hydrogen diffusion.

Nb clusters formation in Nb-doped magnesium hydride

Extended x-ray absorption fine structure spectroscopy, x-ray diffraction and transmission electron microscopy were used to analyze the Nb coordination and clustering in Nb-doped (5 at. %) h-Mg film samples deposited by rf magnetron sputtering. Results show that the catalytic effect of the Nb doping in the H2 absorption and desorption kinetics is connected with the formation of Nb nanoclusters dispersed in the host matrix. The H2 desorption from β-MgH2 is favored by local elastic stresses produced by β-NbH0.89 clusters on the MgH2 matrix that reduces the stability of the hydride phase and by preferential paths in the nanocomposite hydride.

Pulsed-laser deposition of nanostructured iron oxide catalysts for efficient water oxidation.

Amorphous iron oxide nanoparticles were synthesized by pulsed-laser deposition (PLD) for functionalization of indium-tin oxide surfaces, resulting in electrodes capable of efficient catalysis in water oxidation. These electrodes, based on earth-abundant and nonhazardous iron metal, are able to sustain high current densities (up to 20 mA/cm2) at reasonably low applied potential (1.64 V at pH 11.8 vs reversible hydrogen electrode) for more than 1 h when employed as anodes for electrochemical water oxidation. The good catalytic performance proves the validity of PLD as a method to prepare nanostructured solid-state materials for catalysis, enabling control over critical properties such as surface coverage and morphology.

Catalytic effect of mixed Zr–Fe additives on the hydrogen desorption kinetics of MgH2

Mg films containing Fe–Zr mixed additives were prepared by rf sputtering and their hydrogen sorption kinetic was studied by Sievert technique. We observed that mixed Fe–Zr additives improve the H2 desorption kinetics better than singly Fe or Zr. X-ray diffraction analysis evidences that mixed additives optimize the Fe distribution that forms smaller clusters than in Mg sample with single Fe catalyst. The presence of mixed additives, with Fe having larger diffusivity than Zr, contributes to maintain the atomic Zr dispersion in the Mg matrix, thus, favoring the presence of a greater number of nucleation sites for MgH2 to Mg transformation.

Synthesis and characterization of polymer embedded LaNi5 composite material for hydrogen storage

Composite materials consisting of powders of a standard hydride forming an intermetallic compound (LaNi5) dispersed into a hydrogen-permeable elastomer (polysiloxane) have been produced. The H2 storage capacity and the hydrogen desorption kinetics have been measured with composite samples having a metal content of 50 and 83 wt%. At a metal content of 50 wt%, the composite material consists of separated LaNi5 particles embedded into the polymeric matrix: this composite sample shows low H2 storage capacity that is attributed to the chemistry of the metal–polymer interface limiting the H2 absorption by the metallic particles. The amount of absorbed hydrogen significantly increases at the metal content of 83%, when a percolative distribution is assumed by the metallic particles which are connected through a high volumetric density of metal–metal interfaces. The rate limiting process in the hydrogen desorption kinetics is attributed to the H diffusion process in the metallic fraction of the composite.

Porous versus Compact Nanosized Fe(III)-Based Water Oxidation Catalyst for Photoanodes Functionalization

Integrated absorber/electrocatalyst schemes are increasingly adopted in the design of photoelectrodes for photoelectrochemical cells because they can take advantage of separately optimized components. Such schemes also lead to the emergence of novel challenges, among which parasitic light absorption and the nature of the absorber/catalyst junction features prominently. By taking advantage of the versatility of pulsed-laser deposition technique, we fabricated a porous iron(III) oxide nanoparticle-assembled coating that is both transparent to visible light and active as an electrocatalyst for water oxidation. Compared to a compact morphology, the porous catalyst used to functionalize crystalline hematite photoanodes exhibits a superior photoresponse, resulting in a drastic lowering of the photocurrent overpotential (about 200 mV) and a concomitant 5-fold increase in photocurrents at 1.23 V versus reversible hydrogen electrode. Photoelectrochemical impedance spectroscopy indicated a large increase in trapped surface hole capacitance coupled with a decreased charge transfer resistance, consistent with the possible formation of an adaptive junction between the absorber and the porous nanostructured catalyst. The observed effect is among the most prominent reported for the coupling of an electrocatalyst with a thin layer absorber.

Hydrogen sorption in metal-polymer composites: The role of interfaces

We studied the hydrogen storage capacity and sorption kinetics of composite materials made of hydride forming metal particles (LaNi5 or Pd, particle size of ∼1 μm) embedded into hydrogen permeable polymers. Experimental analysis shows that (i) the composite material consisting of LaNi5 particles dispersed into polysiloxane (PS-LaNi5) shows negligible H2 storage capacity while the LaNi5 particles dispersed into polyethylene (PE-LaNi5) are completely hydrogenated and (ii) the Pd particles dispersed both into polysiloxane (PS-Pd) and polyvinylpyrrolidone (PVP-Pd) are completely hydrogenated. The interfacial interactions in the PE-LaNi5 and in the PS-Pd composite materials have weak Van der Waals character while strong interfacial interactions occur in the PS-LaNi5 and in the PVP-Pd composites due to the formation of chemical bonds between polymer side groups and the metal surface atoms. Results indicate that in the metal-polymer composites the hydrogenation of the metallic phase cannot be obtained when the i...

High temperature efficient deuterium permeation and oxidation (Al,Ti)N barriers deposited on stainless steel

We have shown that reduction of deuterium permeation through AISI 316L stainless steel can be obtained by the deposition of an (Al,Ti)N coating: a 1.7-μm-thick coating was able to reduce the deuterium permeation flux by a factor 100–1000 in the 473–873 K temperature range. This result is related to the very low diffusivity of the deuterium migrating species in the nitride: in the examined temperature range the evaluated values of the deuterium diffusion coefficient are on the order of 10−12–10−11 cm2/s with an activation energy as low as 0.37±0.03 eV: this activation energy may be related to the defect microstructure of the sample. The (Al,Ti)N layer, which is produced on an industrial scale as an anticorrosive and wear resistant coating, has shown good properties as an oxidation barrier during thermal treatment in air at temperatures up to 873 K.