Chain-Like Semiconductive Fillers for Dielectric Enhancement and Loss Reduction of Polymer Composites.

Dielectric loss is a crucial factor in determining the long-term endurance for security and energy loss of dielectric composites. Here, chain-like semiconductive fibers of titanium oxide, indium, and niobium-doped titanium oxide are used for enhancing the complex dielectric properties of a polymer through composite construction, which involves significant interface enhancements. The chain-like fibers significantly enhance the dielectric constant owing to the special morphology of the fillers and their interfacial polarization, especially at higher temperatures. The dielectric loss and electrical conductivity of the composites are substantially reduced across the entire investigated temperature range, achieved by passivating the fiber surface with an alumina shell using atomic layer deposition. The as-deposited alumina shell transformed from an amorphous to a crystalline phase through thermal annealing results in a porous shell and more effective suppression of the loss tangent and electrical conductivity. A plausible mechanism for loss suppression is associated with carrier movement along the surface of the fibers and bulk, leading to a higher loss tangent. The alumina shell blocks the carrier transport path, particularly at the interfaces, resulting in a reduced interfacial polarization contribution and energy storage loss. This study provides a method for inhibiting dielectric loss by fabricating fillers with special surfaces.

Carbon Contamination Prevention during Spark Plasma Sintering.

Carbon contamination from graphite molds during spark plasma sintering (SPS) considerably affects the properties of the sintered materials, especially transparent ceramics. Herein, transparent Y3Al5O12 (YAG) ceramics were prepared via SPS using Mo and Ta foils, separately and in tandem, as protective barriers against carbon contamination. The effects of Ta and Mo foils on the transparency and microstructure of the ceramics, and their protection mechanisms were studied. Experimental results show that a reaction layer formed at the Ta-YAG interface with a YTaO4-Al2O3 eutectic composition suppresses carbon penetration into the ceramic, increasing its transparency. By contrast, Mo foils, when used as protective barriers, allow carbon diffusion into the ceramic, resulting in the formation of nonuniform microstructural features. However, it does not form a reactive layer and, hence, is removed easily from the YAG surface. Multilayered Ta-Mo barrier exhibits improved outcomes if the Ta thickness is more than ∼100 µm. This behavior is attributed to the interior diffusion-blocking mechanism of Ta. Similar optical performance was demonstrated by both approaches. The results prove that carbon contamination in SPS-derived samples can be effectively prevented. Additionally, this study reports on a novel strategy of bonding oxide ceramics to metals by adding a Ta layer at the joint interface.

Performance of Cu/ZnO Nanosheets on Electrospun Al2O3 Nanofibers in CO2 Catalytic Hydrogenation to Methanol and Dimethyl Ether.

The synthesis of methanol and dimethyl ether (DME) from carbon dioxide (CO2) and green hydrogen (H2) offers a sustainable pathway to convert CO2 emissions into value-added products. This heterogeneous catalytic reaction often uses copper (Cu) catalysts due to their low cost compared with their noble metal analogs. Nevertheless, improving the activity and selectivity of these Cu catalysts for these products is highly desirable. In the present study, a new architecture of Cu- and Cu/Zn-based catalysts supported on electrospun alumina nanofibers were synthesized. The catalysts were tested under various reaction conditions using high-throughput equipment to highlight the role of the hierarchical fibrous structure on the reaction activity and selectivity. The Cu or Cu/ZnO formed a unique structure of nanosheets, covering the alumina fiber surface. This exceptional morphology provides a large surface area, up to ~300 m2/g, accessible for reaction. Maximal production of methanol (~1106 gmethanolKgCu-1∙h-1) and DME (760 gDMEKgCu-1∙h-1) were obtained for catalysts containing 7% wt. Cu/Zn with a weight ratio of 2.3 Zn to Cu (at 300 °C, 50 bar). The promising results in CO2 hydrogenation to methanol and DME obtained here point out the significant advantage of nanofiber-based catalysts in heterogeneous catalysis.

Highly Transparent Polycrystalline MgO via Spark Plasma Sintering.

Optically transparent ceramics and MgO in particular are promising materials for a wide range of optical applications. This study introduces exceptionally highly transparent MgO ceramics produced via spark plasma sintering (SPS) at relatively low temperature and pressure by optimal incorporation of LiF as a sintering additive. The effect of LiF content on the microstructural and optical properties is presented with emphasis on its function as a densification aid and an agent for minimizing residual carbon contamination. Fully dense MgO discs, 20 mm in diameter and 2 mm thick, with ∼80% in-line transmission at 800 nm and >85% transmission in the infrared range (2-6 µm), are attained. These results demonstrate outstanding transparency in SPS polycrystalline MgO in the 800 nm range, only 7% below the theoretical value. In addition, this work strengthens our understanding of the LiF action mechanism during MgO sintering and its influence on texture development in the SPS-pressing direction. These findings pave the way for fabrication of large, fully dense samples with nearly theoretical transparency.

Composite Indium Tin Oxide Nanofibers with Embedded Hematite Nanoparticles for Photoelectrochemical Water Splitting.

Hematite is a classical photoanode material for photoelectrochemical water splitting due to its stability, performance, and low cost. However, the effect of particle size is still a question due to the charge transfer to the electrodes. In this work, we addressed this subject by the fabrication of a photoelectrode with hematite nanoparticles embedded in close contact with the electrode substrate. The nanoparticles were synthesized by a solvothermal method and colloidal stabilization with charged hydroxide molecules, and we were able to further use them to prepare electrodes for water photo-oxidation. Hematite nanoparticles were embedded within electrospun tin-doped indium oxide nanofibers. The fibrous layer acted as a current collector scaffold for the nanoparticles, supporting the effective transport of charge carriers. This method allows better contact of the nanoparticles with the substrate, and also, the fibrous scaffold increases the optical density of the photoelectrode. Electrodes based on nanofibers with embedded nanoparticles display significantly enhanced photoelectrochemical performance compared to their flat nanoparticle-based layer counterparts. This nanofiber architecture increases the photocurrent density and photon-to-current internal conversion efficiency by factors of 2 and 10, respectively.

Metal nanoparticles entrapped in metal matrices.

We developed synthetic methods for the doping of metals (M) with metallic nanoparticles (NPs). To the best of our knowledge - unlike oxides, polymers and carbon-based supports - metals were not used so far as supporting matrices for metallic NPs. The composites (denoted M1-NPs@M2) comprise two separate phases: the metallic NPs (the dopant) and the entrapping 3D porous metallic matrix, within which the NPs are intimately held and well dispersed. Two different general synthetic strategies were developed, each resulting in a group of materials with characteristic structure and properties. The first strategy uses pre-prepared NPs and these are entrapped during reductive formation of the metallic matrix from its cation. The second strategy is in situ growth of the doped metallic NPs within the pre-prepared entrapping metallic matrix. These two methods were developed for two types of entrapping metallic matrices with different morphologies: porous aggregated metallic matrices and metallic foams. The leading case in this study was the use of Pt as the NP dopant and Ag as the entrapping matrix, using all of the four combinations - entrapment or growth within aggregated Ag or Ag foam matrices. Full physical and chemical properties analysis of these novel types of materials was carried out, using a wide variety of analytical methods. The generality of the methods developed for these bi-metallic composites was investigated and demonstrated on additional metallic pairs: Au NPs within Ag matrices, Pd NPs within Ni matrices and Ir-NPs within a Rh matrix. As the main application of metallic NPs is in catalysis, the catalytic activity of M1-NPs@M2 is demonstrated successfully for entrapped Pt within Ag for reductive catalytic reactions, and for Pd within Ni for the electrocatalytic hydrogen oxidation reaction.

Progress and Prospective of Nitrogen-Based Alternative Fuels.

Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this review, we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the production, distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based molecules in the energy sector, outlining their use as energy carriers in relevant fields.

Electrospun Anion-Conducting Ionomer Fibers-Effect of Humidity on Final Properties.

Anion-conducting ionomer-based nanofibers mats are prepared by electrospinning (ES) technique. Depending on the relative humidity (RH) during the ES process (RHES), ionomer nanofibers with different morphologies are obtained. The effect of relative humidity on the ionomer nanofibers morphology, ionic conductivity, and water uptake (WU) is studied. A branching effect in the ES fibers found to occur mostly at RHES < 30% is discussed. The anion conductivity and WU of the ionomer electrospun mats prepared at the lowest RHES are found to be higher than in those prepared at higher RHES. This effect can be ascribed to the large diameter of the ionomer fibers, which have a higher WU. Understanding the effect of RH during the ES process on ionomer-based fibers' properties is critical for the preparation of electrospun fiber mats for specific applications, such as electrochemical devices.

Electrospun Fe-Al-O Nanobelts for Selective CO2 Hydrogenation to Light Olefins.

Ceramic nanobelt catalysts consisting of Fe-Al-O spinel modified with potassium were synthesized for CO2 hydrogenation into hydrocarbons. Nanobelts and hollow nanofibers were produced utilizing the internal heat released by oxidation of the organic component within the fibers. This extremely fast and short heating facilitated crystallization of the desired phase, while maintaining small grains and a large surface area. We investigated the effects of mat thickness, composition, and heating rate on the final morphology. A general transformation mechanism for electrospun nanofibers that correlates for the first time the mat's thickness and the rate of oxidation during thermal treatment was proposed. The catalytic performance of carburized ceramic K/Fe-Al-O nanobelts was compared to the K/Fe-Al-O spinel powder. The electrospun catalyst showed a superior carbon dioxide conversion of 48% and a selectivity of 52% to light C2-C5 olefins, while the powder catalyst produced mainly C6+ hydrocarbons. Characterization of steady state catalytic materials by energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and N2-adsorption methods revealed that high olefin selectivity of the electrospun materials is related to a high extent of reduction of surface iron atoms because of more efficient interaction with the potassium promoter.

Lamellar-like Electrospun Mesoporous Ti-Al-O Nanofibers.

Ceramic oxides nanofibers are promising materials as catalysts, electrodes and functional materials. In this report, a unique lamellar-like mesoporous structure was realized for the first time in a new system based on titania and alumina. The final structure was found to be highly dependent on the process conditions which are outlined herein. In view of the similar architecture we recently obtained with Fe-Al-O fibers, the pore formation mechanism we outline herein is general and is applicable to additional systems.

Photoelectrochemical water splitting in separate oxygen and hydrogen cells.

Solar water splitting provides a promising path for sustainable hydrogen production and solar energy storage. One of the greatest challenges towards large-scale utilization of this technology is reducing the hydrogen production cost. The conventional electrolyser architecture, where hydrogen and oxygen are co-produced in the same cell, gives rise to critical challenges in photoelectrochemical water splitting cells that directly convert solar energy and water to hydrogen. Here we overcome these challenges by separating the hydrogen and oxygen cells. The ion exchange in our cells is mediated by auxiliary electrodes, and the cells are connected to each other only by metal wires, enabling centralized hydrogen production. We demonstrate hydrogen generation in separate cells with solar-to-hydrogen conversion efficiency of 7.5%, which can readily surpass 10% using standard commercial components. A basic cost comparison shows that our approach is competitive with conventional photoelectrochemical systems, enabling safe and potentially affordable solar hydrogen production.

Pollutant Abatement of Nitrogen-Based Fuel Effluents over Mono- and Bimetallic Pt/Ru Catalysts.

Mono- and bimetallic alloy Pt and Ru catalysts supported on γ-Al2O3 have been investigated for the reduction of pollutants (NO x , NH3, and CO) generated during the continuous combustion of an aqueous urea ammonium nitrate fuel. A Pt/Ru alloy with a Pt25/Ru75 atomic ratio has been found to have higher activity and selectivity than those of a 50/50 alloy and monometallic catalysts. Among monometallic catalysts, Ru was more selective toward N2 formation, whereas Pt showed a higher selectivity toward NH3 formation. For Ru, it was observed that the oxidizing atmosphere of NO x pollutants caused the formation of RuO2, whereas Ru in the Pt/Ru alloy was stable under these conditions. Temperature (250-500 °C) and pressure (1-8 MPa) studies over Ru and 25/75 Pt/Ru have concluded that the alloy catalyst at 400 °C and 5 MPa reduced the pollutants to a minimum level with high yields of N2 (99.7%) and CO2 (99.9%). It was also observed that the 25/75 Pt/Ru catalyst remained stable up to 100 h of thermal treatment at 400 °C. Minimal pollutants were obtained at a weight hourly space velocity = 11 822 h-1. Characterization studies of the spent catalyst showed that metal particles were sintered over a period of time (8 h) and the γ-Al2O3 support was transformed into θ- and α-phases under the hydrothermal reaction conditions.

Nitrogen-Based Fuels: A Power-to-Fuel-to-Power Analysis.

What are the fuels of the future? Seven representative carbon- and nitrogen-based fuels are evaluated on an energy basis in a power-to-fuel-to-power analysis as possible future chemical hydrogen-storage media. It is intriguing to consider that a nitrogen economy, where hydrogen obtained from water splitting is chemically stored on abundant nitrogen in the form of a nontoxic and safe nitrogen-based alternative fuel, is energetically feasible.