Search for: All records

Abstract Direct ethanol fuel cells have been widely investigated as nontoxic and low-corrosive energy conversion devices with high energy and power densities. It is still challenging to develop high-activity and durable catalysts for a complete ethanol oxidation reaction on the anode and accelerated oxygen reduction reaction on the cathode. The materials’ physics and chemistry at the catalytic interface play a vital role in determining the overall performance of the catalysts. Herein, we propose a Pd/Co@N-C catalyst that can be used as a model system to study the synergism and engineering at the solid-solid interface. Particularly, the transformation of amorphous carbon to highly graphitic carbon promoted by cobalt nanoparticles helps achieve the spatial confinement effect, which prevents structural degradation of the catalysts. The strong catalyst-support and electronic effects at the interface between palladium and Co@N-C endow the electron-deficient state of palladium, which enhances the electron transfer and improved activity/durability. The Pd/Co@N-C delivers a maximum power density of 438 mW cm −2 in direct ethanol fuel cells and can be operated stably for more than 1000 hours. This work presents a strategy for the ingenious catalyst structural design that will promote the development of fuel cells and other sustainable energy-related technologies. 

Metalorganic chemical vapor deposition (MOCVD) is a promising technique for wafer-scale synthesis of MoS2 monolayers for 2D field-effect transistors (2D-FETs) and related devices. Epitaxial growth of MoS2 on sapphire provides films that are crystallographically well-oriented but typically contain low-angle grain boundaries (e.g., mirror twins), voids, and other defects depending on growth conditions and substrate characteristics. In this study, we investigate microstructure, optical properties, and field-effect characteristics of wafer-scale MoS2 monolayers grown by MOCVD on c-plane sapphire over a narrow window of growth temperatures (900–1000 °C). The density of low-angle grain boundaries in the MoS2 monolayer was found to decrease dramatically from 50% areal coverage for films grown at 900 °C to 5% at 1000 °C. This decrease in low-angle grain boundary density is correlated with an increase in the room-temperature photoluminescence intensity of A excitons and a decrease in the full-width-half maximum (FWHM) of the Raman A1g peak, which are typically indicative of a general reduction in defects in MoS2. However, the best transport properties (e.g., mean field-effect mobility mFE = 17.3 cm2/V s) were obtained in MoS2 monolayers grown at an intermediate temperature of 950 °C. It was found that as the growth temperature increased, small regions bound by high-angle boundaries begin to appear within the monolayer and increase in areal coverage, from ∼2% at 900 °C to ∼5% at 950 °C to ∼10% at 1000 °C. The growth temperature of 950 °C, therefore, provides an intermediate condition where the combined effects of low-angle and high-angle boundaries are minimized. The results of this study provide guidance on MOCVD growth and characterization that can be used to further optimize the performance of MoS2 2D-FETs.

 

Although El Niño‐Southern Oscillation (ENSO) and its global impacts through teleconnection have been known for decades, if and how the mean currents and mesoscale eddies in the Caribbean Sea are linked to ENSO remains an open question. Here, by analyzing satellite observations and an ocean reanalysis product, we found a close connection between mean currents, eddies in the Caribbean Sea and ENSO on interannual timescales. Strong El Niño events result in enhanced north‐south sea surface height differences and consequently stronger mean currents in the Caribbean Sea, and the opposite happens during La Niña events. The eddy kinetic energy responds to ENSO via eddy‐mean flow interaction, primarily through baroclinic instability, which releases the available potential energy stored in the mean currents to mesoscale eddies. Our results suggest some predictability of the mean currents and eddies in the Caribbean Sea, particularly during strong El Niño and La Niña events. 

High entropy oxides are a class of materials distinguished by the use of configurational entropy to drive material synthesis. These materials are being examined for their exciting physiochemical properties and hold promise in numerous fields, such as chemical sensing, electronics, and catalysis. Patterning and integration of high entropy materials into devices and platforms can be difficult due to their thermal sensitivity and incompatibility with many conventional thermally-based processing techniques. In this work, we present a laser-based technique, laser-induced thermal voxels, that combines the synthesis and patterning of high entropy oxides into a single process step, thereby allowing patterning of high entropy materials directly onto substrates. As a proof-of-concept, we target the synthesis and patterning of a well-characterized rock salt-phase high entropy oxide, (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O, as well as a spinel-phase high entropy oxide, (Mg_0.2Ni_0.2Co_0.2Cu_0.2Zn_0.2)Cr₂O₄. We show through electron microscopy and x-ray analysis that the materials created are atomically homogenous and are primarily of the rock salt or spinel phase. These findings show the efficacy of laser induced thermal voxel processing for the synthesis and patterning of high entropy materials and enable new routes for integration of high entropy materials within microscale platform and devices.

 

In this paper, we investigate the reliability in an unmanned aerial vehicle (UAV) assisted caching-based downlink network where non-orthogonal multiple access (NOMA) transmission and finite blocklength (FBL) codes are adopted. In this network, the ground user equipments (GUEs) request contents from a distant base station (BS) but there are no direct links from the BS to the GUEs. A UAV with limited cache size is employed to assist the BS to complete the communication by either first requesting the uncached contents from the BS and then serving the GUEs or directly sending the cached contents to the GUEs. In this setting, we first introduce the decoding error rate in the FBL regime as well as the caching policy at the UAV, and subsequently we construct an optimization problem aiming to minimize the maximum end-to-end decoding error rate among all GUEs under both coding length and maximum UAV transmission power constraints. A two-step alternating algorithm is proposed to solve the problem and numerical results demonstrate that our algorithm can solve the optimization problem efficiently. More specifically, loosening the FBL constraint, enlarging the cache size and having a higher transmission power budget at the UAV lead to an improved performance. 

Proton transfer is crucial in various chemical and biological processes. Because of significant nuclear quantum effects, accurate and efficient description of proton transfer remains a great challenge. In this Communication, we apply constrained nuclear–electronic orbital density functional theory (CNEO-DFT) and constrained nuclear–electronic orbital molecular dynamics (CNEO-MD) to three prototypical shared proton systems and investigate their proton transfer modes. We find that with a good description of nuclear quantum effects, CNEO-DFT and CNEO-MD can well describe the geometries and vibrational spectra of the shared proton systems. Such a good performance is in significant contrast to DFT and DFT-based ab initio molecular dynamics, which often fail for shared proton systems. As an efficient method based on classical simulations, CNEO-MD is promising for future investigations of larger and more complex proton transfer systems.