Mono-, bi- and tri-metallic Fe-based platinum group metal-free electrocatalysts derived from phthalocyanine for oxygen reduction reaction in alkaline media.

In this manuscript, a comprehensive study is presented on Fe-based electrocatalysts with mono, bi, and tri-metallic compositions, emphasizing the influence of processing-structure correlations on the electrocatalytic activity for the oxygen reduction reaction (ORR) in the alkaline medium. These electrocatalysts were synthesized through the mixing of transition metal phthalocyanines (TM-Pc) with conductive carbon support, followed by controlled thermal treatment at specific temperatures (600 °C and 900 °C). An extensive analysis was conducted, employing various techniques, including X-ray Absorption Spectroscopy (XAS), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD), providing valuable insights into the structural characteristics of the synthesized nanoparticles. Importantly, an increase in the Fe-Pc weight percentage from 10% to 30% enhanced the ORR activity, although not proportionally. Furthermore, a comparative analysis between mono, bi, and tri-metallic samples subjected to different functionalization temperatures highlighted the superior electrocatalytic activity of electrocatalysts functionalized at 600 °C, particularly Fe 600 and Fe-Ni-Cu 600. These electrocatalysts featured Eon values of 0.96 V vs. RHE and E1/2 values of 0.9 V vs. RHE, with the added benefit of reduced anionic peroxide production. The potential of these Fe-based electrocatalysts to enhance ORR efficiency is underscored by this research, contributing to the development of more effective and sustainable electrocatalysts for energy conversion technologies.

Ni,Fe,Co-LDH Coated Porous Transport Layers for Zero-Gap Alkaline Water Electrolyzers.

Next-generation alkaline water electrolyzers will be based on zero-gap configuration to further reduce costs related to technology and to improve performance. Here, anodic porous transport layers (PTLs) for zero-gap alkaline electrolysis are prepared through a facile one-step electrodeposition of Ni,Fe,Co-based layered double hydroxides (LDH) on 304 stainless steel (SS) meshes. Electrodeposited LDH structures are characterized using Scanning Electron Microscopy (SEM) confirming the formation of high surface area catalytic layers. Finally, bi and trimetallic LDH-based PTLs are tested as electrodes for oxygen evolution reaction (OER) in 1 M KOH solution. The best electrodes are based on FeCo LDH, reaching 10 mA cm-2 with an overpotential value of 300 mV. These PTLs are also tested with a chronopotentiometric measurement carried out for 100 h at 50 mA cm-2, showing outstanding durability without signs of electrocatalytic activity degradation.

Modeling of Optical Band-Gap Values of Mixed Oxides Having Spinel Structure AB2O4 (A = Mg, Zn and B = Al, Ga) by a Semiempirical Approach.

Spinel oxides with the general formula AB2O4 comprise a large family of compounds covering a very wide range of band-gap values (1 eV < Eg < 8 eV) as a function of the nature of the metallic cations A and B. Owing to this, the physical properties of these materials have been largely exploited both from a fundamental point of view, for their variable electronic properties, and for their possible use in numerous engineering applications. Herein, the modeling of ZnAl2O4, ZnGa2O4, MgAl2O4, and MgGa2O4 cubic spinel oxides has been carried out by using the semiempirical approach based on the difference of electronegativity between oxygen and the average electronegativity of cations present in the oxides. The results of recent theoretical extensions of our semiempirical approach to ternary and quaternary oxides have been tested for spinel oxides with metallic ions occupying both octahedrally and tetrahedrally coordinated sites in different ratios. A detailed analysis of the experimental band-gap values and comparison with the theoretically estimated values has been carried out for ternary ZnAl2O4, ZnGa2O4, MgAl2O4, and MgGa2O4 spinels as well as for double spinels Mg(Al2xGa2-x)O4 and Zn(Al2xGa2-x)O4, and quaternary mixed oxides (ZnxMg(1-x))Al2O4 and (ZnxMg(1-x))Ga2O4. The wide range of band-gap values reported in the literature for simple or double spinels has been related to the different preparation methods affecting the grain dimension of crystalline spinel samples as well as to the presence of crystallographic defects and/or impurities in the spinel matrix. The good agreement between experimental band-gap values and the theoretical ones strongly supports the use of our semiempirical approach in the area of band-gap engineering of new materials.

Effect of TiO2 and Al2O3 Addition on the Performance of Chitosan/Phosphotungstic Composite Membranes for Direct Methanol Fuel Cells.

Composite chitosan/phosphotungstic acid (CS/PTA) with the addition of TiO2 and Al2O3 particles were synthesized to be used as proton exchange membranes in direct methanol fuel cells (DMFCs). The influence of fillers was assessed through X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, liquid uptake, ion exchange capacity and methanol permeability measurements. The addition of TiO2 particles into proton exchange membranes led to an increase in crystallinity and a decrease in liquid uptake and methanol permeability with respect to pristine CS/PTA membranes, whilst the effect of the introduction of Al2O3 particles on the characteristics of membranes is almost the opposite. Membranes were successfully tested as proton conductors in a single module DMFC of 1 cm2 as active area, operating at 50 °C fed with 2 M methanol aqueous solution at the anode and oxygen at the cathode. Highest performance was reached by using a membrane with TiO2 (5 wt.%) particles, i.e., a power density of 40 mW cm-2, almost doubling the performance reached by using pristine CS/PTA membrane (i.e., 24 mW cm-2).

The Effect of Anodizing Bath Composition on the Electronic Properties of Anodic Ta-Nb Mixed Oxides.

Anodic oxides were grown to 50 V on Ta-Nb sputtering deposited alloys, with high Nb content, in acetate ions containing an aqueous solution to study the effect of the anodizing bath composition on anodic layers' dielectric properties. Photoelectrochemical measurements proved the presence of a photocurrent in the band gap of photon energy lower than oxides, due to optical transitions involving localized electronic states as a consequence of acetate ions incorporation. Flat band potential value estimates assessed the insulating nature of the anodic oxides grown in the acetate buffer solution. Differential capacitance measurements showed that the highest capacitance value was measured for the sample grown on Ta-66%Nb. This capacitance value was higher with respect to those estimated for pure Ta and pure Nb anodic layers and with respect to the same alloy anodized in NaOH solution, i.e., acetate-free anodizing bath.

Tuning of the Mg Alloy AZ31 Anodizing Process for Biodegradable Implants.

Coatings were grown on the AZ31 Mg alloy by a hard anodizing process in the hot glycerol phosphate-containing electrolyte. Anodizing conditions were optimized, maximizing corrosion resistance estimated by impedance measurements carried out in Hank's solution at 37 °C. A post anodizing annealing treatment (350 °C for 24 h) allowed us to further enhance the corrosion resistance of the coatings mainly containing magnesium phosphate according to energy-dispersive X-ray spectroscopy and Raman analyses. Gravimetric measurements revealed a hydrogen evolution rate within the limits acceptable for application of AZ31 in biomedical devices. In vitro tests demonstrated that the coatings are biocompatible with a preosteoblast cell line.

A Generalized Semiempirical Approach to the Modeling of the Optical Band Gap of Ternary Al-(Ga, Nb, Ta, W) Oxides Containing Different Alumina Polymorphs.

A generalization of the modeling equation of optical band gap values for ternary oxides, as a function of cationic ratio composition, is carried out based on the semiempirical correlation between the differences in the electronegativity of oxygen and the average cationic electronegativity proposed some years ago. In this work, a novel approach is suggested to account for the differences in the band gap values of the different polymorphs of binary oxides as well as for ternary oxides existing in different crystalline structures. A preliminary test on the validity of the proposed modeling equations has been carried out by using the numerous experimental data pertaining to alumina and gallia polymorphs as well as the crystalline ternary Ga(1-x)AlxO3 polymorphs (α-Ga(1-x)AlxO3 and ß-Ga(1-x)AlxO3) covering a large range of optical band gap values (4.50-8.50 eV). To make a more rigorous test of the modeling equation, we extended our investigation to amorphous ternary oxides anodically formed on Al-d-metal alloys (Al-Nb, Al-Ta, and Al-W) covering a large range of d-metal composition (xd-metal ≥ 0.2). In the last case, the novel approach allows one to overcome some difficulties experienced in fitting the optical band gap dependence from the Al-d-metal mixed anodic oxide composition as well as to provide a rationale for the departure, at the lowest d-metal content (xd-metal < 0.2), from the behavior observed for anodic films containing higher d-metal content.

Electrochemical Tantalum Oxide for Resistive Switching Memories.

Redox-based resistive switching memories (ReRAMs) are strongest candidates for the next-generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta2 O5 -based devices by anodizing. This method allows to grow high-quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical-oxide-based devices demonstrate superior properties, i.e., endurance of at least 106 pulse cycles and/or 103 I-V sweeps maintaining a good memory window with a low dispersion in ROFF and RON values, nanosecond fast switching, and data retention of at least 104 s. Multilevel programing capability is presented with both I-V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.