Optical and Electrochemical Properties of a Nanostructured ZnO Thin Layer Deposited on a Nanoporous Alumina Structure via Atomic Layer Deposition.

This study explores the optical and electrochemical properties of a ZnO coating layer deposited on a nanoporous alumina structure (NPAS) for potential multifunctional applications. The NPAS, synthesized through an electrochemical anodization process, displays well-defined nanochannels with a high aspect ratio (~3000). The ZnO coating, achieved via atomic layer deposition, enables the tuning of the pore diameter and porosity of the NPAS, thereby influencing both the optical and electrochemical interfacial properties. A comprehensive characterization using photoluminescence, spectroscopy ellipsometry and impedance spectroscopy (with the sample in contact with NaCl solutions) provides insights into optical and electrochemical parameters, including the refractive index, absorption coefficient, and electrolyte-ZnO/NPAS interface processes. This research demonstrates potential for tailoring the optical and interfacial properties of nanoporous structures by selecting appropriate coating materials, thus opening avenues for their utilization in various technological applications.

Enhancing the Electrochemical Performance in Symmetrical Solid Oxide Cells through Nanoengineered Redox-Stable Electrodes with Exsolved Nanoparticles.

Symmetrical solid oxide cells (SSOCs) have recently gained significant attention for their potential in energy conversion due to their simplified cell configuration, cost-effectiveness, and excellent reversibility. However, previous research efforts have mainly focused on improving the electrode performance of perovskite-type electrodes through different doping strategies, neglecting microstructural optimization. This work presents novel approaches for the nanostructural tailoring of (La0.8Sr0.2)0.95Fe1-xTixO3-δ (LSFTx, x = 0.2 and 0.4) electrodes using a single-step spray-pyrolysis deposition process. By incorporating these electrodes into a Ce0.9Gd0.1O1.95 (CGO) porous backbone or employing a nanocomposite architecture with nanoscale particle size, we achieved significant improvements in the polarization resistance (Rp) compared with traditional screen-printed electrodes. To further boost the fuel oxidation performance, a Ni-doping strategy, coupled with meticulous microstructural optimization, was implemented. The exsolution of Ni nanoparticles under reducing conditions resulted in remarkable Rp values as low as 0.34 and 0.11 Ω cm2 in air and wet H2 at 700 °C, respectively. Moreover, an electrolyte-supported cell with symmetrical electrodes demonstrated a stable maximum power density of 617 mW cm-2 at 800 °C. These findings highlight the importance of combining electrode composition optimization with advanced morphology control in the design of highly efficient and durable SSOCs.

Modification of the Physical Properties of a Nafion Film Due to Inclusion of n-Dodecyltriethylammonium Cation: Time Effect.

This study investigates the effects of modifying commercial Nafion-212 thin films with dodecyltriethylammonium cation (DTA+) on their electrical resistance, elastic modulus, light transmission/reflection and photoluminescence properties. The films were modified through a proton/cation exchange process for immersion periods ranging from 1 to 40 h. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to analyze the crystal structure and surface composition of the modified films. The electrical resistance and the different resistive contributions were determined via impedance spectroscopy. Changes in the elastic modulus were evaluated using stress-strain curves. Additionally, optical characterization tests, including light/reflection (250-2000 nm) and photoluminescence spectra, were also performed on both unmodified and DTA+-modified Nafion films. The results reveal significant changes in the electrical, mechanical and optical properties of the films, depending on the exchange process time. In particular, the inclusion of the DTA+ into the Nafion structure improved the elastic behavior of the films by significantly decreasing the Young modulus. Furthermore, the photoluminescence of the Nafion films was also enhanced. These findings can be used to optimize the exchange process time to achieve specific desired properties.

Unraveling the Influence of the Electrolyte on the Polarization Resistance of Nanostructured La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes.

Large variations in the polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes are reported in the literature, which are usually related to different preparation methods, sintering temperatures, and resulting microstructures. However, the influence of the electrolyte on the electrochemical activity and the rate-limiting steps of LSCF remains unclear. In this work, LSCF nanostructured electrodes with identical microstructure are prepared by spray-pyrolysis deposition onto different electrolytes: Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM), and Bi1.5Y0.5O3-δ (BYO). The ionic conductivity of the electrolyte has a great influence on the electrochemical performance of LSCF due to the improved oxide ion transport at the electrode/electrolyte interface, as well as the extended ionic conduction paths for the electrochemical reactions on the electrode surface. In this way, the polarization resistance of LSCF decreases as the ionic conductivity of the electrolyte increases in the following order: YSZ > LSGM > CGO > BYO, with values ranging from 0.21 Ω cm2 for YSZ to 0.058 Ω cm2 for BYO at 700 °C. In addition, we demonstrate by distribution of relaxation times and equivalent circuit models that the same rate-limiting steps for the ORR occur regardless of the electrolyte. Furthermore, the influence of the current collector material on the electrochemical performance of LSCF electrodes is also analyzed.

LaCrO3-CeO2-Based Nanocomposite Electrodes for Efficient Symmetrical Solid Oxide Fuel Cells.

La0.98Cr0.75Mn0.25O3-δ-Ce0.9Gd0.1O1.95 (LCM-CGO) nanocomposite layers with different LCM contents, between 40 and 60 wt %, are prepared in a single step by a spray-pyrolysis deposition method and evaluated as both air and fuel electrodes for solid oxide fuel cells (SOFCs). The formation of fluorite (CGO) and perovskite (LCM) phases in the nanocomposite electrode is confirmed by different structural and microstructural techniques. The intimate mixture of LCM and CGO phases inhibits the grain growth, retaining the nanoscale microstructure even after annealing at 1000 °C with a grain size lower than 50 nm for LCM-CGO compared to 200 nm for pure LCM. The synergetic effect of nanosized LCM and CGO by combining their high electronic and ionic conductivity, respectively, leads to efficient and durable symmetrical electrodes. The best electrochemical properties are found for 50 wt % LCM-CGO, showing polarization resistance values of 0.29 and 0.09 Ω cm2 at 750 °C in air and H2, respectively, compared to 2.05 and 1.9 Ω cm2 for a screen-printed electrode with the same composition. This outstanding performance is mainly ascribed to the nanoscale electrode microstructure formed directly on the electrolyte at a relatively low temperature. These results reveal that the combination of different immiscible phases with different crystal structures and electrochemical properties could be a promising strategy to design highly efficient and durable air and fuel electrodes for SOFCs.

Improvement of the Proton Conduction of Copper(II)-Mesoxalate Metal-Organic Frameworks by Strategic Selection of the Counterions.

Three copper(II)/mesoxalate-based MOFs with formulas (H3O)[Cu9(Hmesox)6(H2O)6Cl]·8H2O (1), (NH2Me2)0.4(H3O)0.6[Cu9(Hmesox)6(H2O)6Cl]·8H2O (2), and (enH2)0.25(enH)1.5[Cu6(Hmesox)3(mesox)(H2O)6Cl0.5]Cl0.5·5.25H2O (3) were synthesized (H4mesox = mesoxalic acid = 2,2-dihydroxypropanedioic acid, en = ethylenediamine). Essentially, all of the compounds display the same anionic network with a different arrangement of the cations, which have a remarkable effect on the proton conduction of the materials, ranging from 1.16 × 10-4 S cm-1 for 1 to 1.87 × 10-3 S cm-1 for 3 (at 80 °C and 95% RH). These compounds also display antiferromagnetic coupling among the copper(II) ions through both the carboxylate and alkoxido bridges. The values of the principal magnetic coupling constants were calculated by density functional theory (DFT), leading to congruent values that confirm the predominant antiferromagnetic nature of the interactions.

Exploiting the Multifunctionality of M2+/Imidazole-Etidronates for Proton Conductivity (Zn2+) and Electrocatalysis (Co2+, Ni2+) toward the HER, OER, and ORR.

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M2(ETID)(Im)3]·nH2O (M = Co2+ and Ni2+; n = 0, 1, 3) and [Zn2(ETID)2(H2O)2](Im)2, as well as the corresponding Co2+/Ni2+ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co2+ and Ni2+ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co2+, Ni2+ derivatives) or charge-compensating protonated species (Zn2+ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10-4 S·cm-1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H2-Ar at 700-850 °C, core-shell electrocatalysts consisting of Co2+-, Ni2+-phosphides or Co2+/Ni2+-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co2+ phosphides consist of CoP and Co2P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni2+ phosphide is composed of Ni2P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) reveals that the anhydrous Co2(ETID)(Im)3 pyrolyzed at 800 °C (CoP/Co2P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn-air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec-1 in 0.5 M H2SO4. Ni2+- and Co2+/Ni2+-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.

Zinc Polyaleuritate Ionomer Coatings as a Sustainable, Alternative Technology for Bisphenol A-Free Metal Packaging.

Sustainable coatings for metal food packaging were prepared from ZnO nanoparticles (obtained by the thermal decomposition of zinc acetate) and a naturally occurring polyhydroxylated fatty acid named aleuritic (or 9,10,16-trihydroxyhexadecanoic) acid. Both components reacted, originating under specific conditions zinc polyaleuritate ionomers. The polymerization of aleuritic acid into polyaleuritate by a solvent-free, melt polycondensation reaction was investigated at different times (15, 30, 45, and 60 min), temperatures (140, 160, 180, and 200 °C), and proportions of zinc oxide and aleuritic acid (0:100, 5:95, 10:90, and 50:50, w/w). Kinetic rate constants calculated by infrared spectroscopy decreased with the amount of Zn due to the consumption of reactive carboxyl groups, while the activation energy of the polymerization decreased as a consequence of the catalyst effect of the metal. The adhesion and hardness of coatings were determined from scratch tests, obtaining values similar to robust polymers with high adherence. Water contact angles were typical of hydrophobic materials with values ≥94°. Both mechanical properties and wettability were better than those of bisphenol A (BPA)-based resins and most likely are related to the low migration values determined using a hydrophilic food simulant. The presence of zinc provided a certain degree of antibacterial properties. The performance of the coatings against corrosion was studied by electrochemical impedance spectroscopy at different immersion times in an aqueous solution of NaCl. Considering the features of these biobased lacquers, they can be potential materials for bisphenol A-free metal packaging.

Nanostructured BaCo0.4Fe0.4Zr0.1Y0.1O3-δ Cathodes with Different Microstructural Architectures.

Lowering the operating temperature of solid oxide fuel cells (SOFCs) is crucial to make this technology commercially viable. In this context, the electrode efficiency at low temperatures could be greatly enhanced by microstructural design at the nanoscale. This work describes alternative microstructural approaches to improve the electrochemical efficiency of the BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) cathode. Different electrodes architectures are prepared in a single step by a cost-effective and scalable spray-pyrolysis deposition method. The microstructure and electrochemical efficiency are compared with those fabricated from ceramic powders and screen-printing technique. A complete structural, morphological and electrochemical characterization of the electrodes is carried out. Reduced values of area specific resistance are achieved for the nanostructured cathodes, i.e., 0.067 Ω·cm2 at 600 °C, compared to 0.520 Ω·cm2 for the same cathode obtained by screen-printing. An anode supported cell with nanostructured BCFZY cathode generates a peak power density of 1 W·cm-2 at 600 °C.

Enhanced Intermediate-Temperature Electrochemical Performance of Air Electrodes for Solid Oxide Cells with Spray-Pyrolyzed Active Layers.

The potential of interactive layers of mixed-conducting oxides for improving the performance of air electrodes of solid oxide cells in the intermediate-temperature range is demonstrated. Active layers of Ce0.9Gd0.1O2-δ (CGO), Ce0.8Pr0.2O2-δ (CPO), and SrFe0.9Mo0.1O3-δ (SFM) with thickness in the range 200-400 nm are deposited on CGO-based electrolyte by spray pyrolysis, followed by deposition of a SFM/CGO composite air electrode by painting. The morphologies and phase composition of the active layers are examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis. The electrochemical performance of the electrolyte-electrode assemblies is determined by impedance spectroscopy in the range 600-800 °C. Significant improvements in the performance of the electrode process and the geometrically normalized ohmic conductance are observed for the assembly with a CPO active layer with mixed-oxide-ionic-electronic conductivity, especially in the low-temperature range, attributable to extension of the surface path of the electrochemical reactions. The CGO intermediate layer also improves performance but to a lesser degree, most likely due to better ionic-current collection in comparison to the assemblies with either SFM as the active layer or no active layer.

Synergic Effect of Metal and Fluorine Doping on the Structural and Electrical Properties of La5.4MoO11.1-Based Materials.

Cationic and anionic frameworks of La5.4MoO11.1 proton conductors have been modified by means of metal (Ti4+, Zr4+, and Nb5+) and fluorine (F-) doping. This synergic effect leads to the stabilization of high-symmetry and single-phase polymorphs. The materials have been fully characterized by structural techniques, such as X-ray and neutron powder diffraction and transmission electron microscopy. The fluorine content was determined by ion chromatography. Impedance spectroscopy analysis under different atmospheres (dry and wet N2 and O2 and wet 5% H2-Ar) showed an improvement in the electronic conductivity under reducing conditions, making these materials potential candidates for hydrogen separation membranes.

Relationship between the Structure and Transport Properties in the Ce1-xLaxO2-x/2 System.

La-doped CeO2 materials have been widely investigated for potential applications in different high-temperature electrochemical devices, such as fuel cells and ceramic membranes for hydrogen production. However, the crystal structure is still controversial, and different models based on fluorite, pyrochlore, and/or type-C structures have been considered, depending on the lanthanum content and synthesis method used. In this work, an exhaustive structural analysis of the Ce1-xLaxO2-x/2 system (0.2 < x ≤ 0.7) is performed with different techniques. The average crystal structure, studied by conventional X-ray diffraction, could be considered to be a disordered fluorite; however, the local structure, examined by electron diffraction and Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases. The thermal and electrical properties demonstrate that the materials with x ≥ 0.4 are oxide ion proton conductors in an oxidizing atmosphere and mixed ionic electronic conductors in a reducing atmosphere. The water uptake and proton conductivity increase gradually with the increase in La content, suggesting that the formation of the C-type phase is responsible for the proton conduction in these materials.

Electrochemical stability of (La,Sr)CoO3-δ in (La,Sr)CoO3-δ/(Ce, Gd)O2-δ heterostructures.

A modulated coherent (La,Sr)CoO3-δ/(Ce,Gd)O2-δ heterostructure is characterized for the first time for its electronic and chemical properties. 2D-multilayer architectures are deposited on NdGaO3 (110) single crystal substrate by pulsed laser deposition, resulting in epitaxial structures with in-plane lattice rotation that, via the metal oxides' interfaces, induces mutual structural rearrangements. Our results show that (La,Sr)CoO3-d thin films of 10-100 nm are chemically unstable when exposed to air at 600 °C during electrical cyclic stress-tests. Conversely, improved stability is achieved confining LSC in the nanometric heterostructure. Remarkably, the chemical stabilization occurs without compromising substantially the electrical properties of the LSC component: the heterostructures show unexpected electrical behaviour with dominant electronic contributions, fast conductivity and mixed ionic-electronic properties, depending on the number of interfaces and the nano-scaled layers.

Metal-Doping of La5.4MoO11.1 Proton Conductors: Impact on the Structure and Electrical Properties.

La5.4MoO11.1 proton conductors with different metal doping (Ca2+, Sr2+, Ba2+, Ti4+, Zr4+, and Nb5+) have been prepared and structurally and electrically characterized. Different polymorphs are stabilized depending on the doping and cooling rate used during the synthesis process. The most interesting results are obtained for Nb-doping, La5.4Mo1- xNb xO11.1- x/2, where single compounds are obtained in the compositional range 0 ≤ x ≤ 0.2. These materials are fully characterized by structural techniques such as X-ray and neutron powder diffraction and transmission electron microscopy, which independently confirm the changes of polymorphism. Scanning electron microscopy and impedance spectroscopy measurements in dry/wet gases (N2, O2, and 5% H2-Ar) showed an enhancement of the sinterability and electrical properties of the materials after Nb-doping. Conductivity measurements under very reducing conditions revealed that these materials are mixed ionic-electronic conductors, making them potential candidates for hydrogen separation membranes.

New crystal structure and characterization of lanthanum tungstate "La6WO12" prepared by freeze-drying synthesis.

Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 degrees C). Outside this compositional range, segregation of either La(2)O(3) (La/W > or = 5.8) or La(6)W(2)O(15) (La/W < or = 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln(6)WO(12) (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La(6.63)W(1.17)O(13.43) (Z = 4; calculated density = 6.395 g/cm(3)), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F43m. Lattice parameters vary between 11.173 and 11.188 A, depending on composition. Dense ceramic materials are obtained at 1400 degrees C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La(2)O(3) segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.

A new anode for solid oxide fuel cells with enhanced OCV under methane operation.

A new SOFC anode material based upon oxygen excess perovskite related phases has been synthesised. The material shows better electrochemical performance than other alternative new anodes and comparable performance to the state-of-art of the electrodes, Ni-YSZ cermets, under pure hydrogen. Furthermore, this material shows an enhanced performance under methane operation with high open circuit voltages, i.e. 1.2-1.4 V at 950 degrees C, without using steam excess. The effect of the anode configuration was tested in one and four layer configurations. The optimised electrode polarisation resistances were just 0.12 ohm cm(2) and 0.36 ohm cm(2), at 950 degrees C, in humidified H(2) and humidified CH(4), respectively. Power densities of 0.5 W cm(-2) and 0.35 W cm(-2) were obtained in the same conditions. A very low anodic overpotential of 100 mV at 1 A cm(-2) was obtained under humidified H(2) at 950 degrees C. Samples were tested for two days in reducing and oxidising conditions, alternating heating and cooling processes from 850 degrees C to 950 degrees C, showing stable electrode performance and open circuit voltages. The results show that the substituted strontium titanates are very promising anode materials for SOFC.

A new family of oxide ion conductors based on tricalcium oxy-silicate.

Tricalcium oxy-silicates, Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01, have been prepared as crystalline single phases. Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01 have triclinic and monoclinic structures, respectively. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. At 1023 K, the oxide transport numbers range between 0.97 and 0.85 from reducing (dry 5%-H2-Ar/air gradient) to oxidizing (O2/air gradient) conditions in the 1023-1173 K interval. The thermal analyses showed a large weight loss on heating due to the presence of water in the materials. The monoclinic compound has ionic conductivities higher than those of the triclinic stoichiometric oxy-silicate, as expected due to the introduction of oxide vacancies. Typical total conductivities for these un-optimised solids are 10(-5)-10(-4) S cm(-1) at 1100 K. These compounds may contain a small amount of water, approximately 0.05 H2O moles per chemical formula, and they display an important proton contribution under a humidified atmosphere.

Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation.

Point defects largely govern the electrochemical properties of oxides: at low defect concentrations, conductivity increases with concentration; however, at higher concentrations, defect-defect interactions start to dominate. Thus, in searching for electrochemically active materials for fuel cell anodes, high defect concentration is generally avoided. Here we describe an oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO3) in which we control the oxygen stoichiometry in order to break down the extended defect intergrowth regions and create phases with considerable disordered oxygen defects. We substitute Ti in these phases with Ga and Mn to induce redox activity and allow more flexible coordination. The material demonstrates impressive fuel cell performance using wet hydrogen at 950 degrees C. It is also important for fuel cell technology to achieve efficient electrode operation with different hydrocarbon fuels, although such fuels are more demanding than pure hydrogen. The best anode materials to date--Ni-YSZ (yttria-stabilized zirconia) cermets--suffer some disadvantages related to low tolerance to sulphur, carbon build-up when using hydrocarbon fuels (though device modifications and lower temperature operation can avoid this) and volume instability on redox cycling. Our anode material is very active for methane oxidation at high temperatures, with open circuit voltages in excess of 1.2 V. The materials design concept that we use here could lead to devices that enable more-efficient energy extraction from fossil fuels and carbon-neutral fuels.