Phase Separation and Ion Diffusion in Ionic Liquid, Organic Solvent, and Lithium Salt Electrolyte Mixtures

The highly desirable characteristics of ternary mixtures of ionic liquids, organic solvents, and metal salts make them a promising candidate for use in various electrothermal energy storage and conversion systems. In this study, using large-scale classical molecular dynamics simulations, we looked into 10 different ternary electrolyte mixtures using combinations of [EMIM]+, [BMIM]+, and [OMIM]+ cations with [NO3]−, [BF4]−, [PF6]−, [ClO4]−, [TFO]−, and [NTf2]− anions, tetraglyme, and Li salt to study the effect of ionic liquid composition on the phase behavior of ternary electrolyte mixtures. We uncovered that in these electrolytes, phase separation is mainly a function of pairwise binding energy of the constituents of the mixture. To corroborate this theory, several simulations are performed at various temperatures ranging from 260 to 500 K for each mixture, followed by calculating the binding energy of ionic liquid pairs using density functional theory. Our results verify that the transition temperature for the phase separation of each system is indeed a function of the pairwise binding energy of its ionic liquid pairs. It is also found that in some cases, the diffusion coefficient of the Li+ ions decreased even with the increase in the temperature, an effect that is attributed to the presence of condensed ionic domains in the electrolyte. This study provides a new insight for the design of multicomponent electrolyte mixtures for a wide range of energy applications

Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor Deposition

Wrinkle structures are commonly seen on graphene grown by the chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nanotribological properties of wrinkles and the influence of wrinkle structures on the wrinkle-free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ∼194% compare to that of the parallel direction. Our systematic investigation shows that the ripples and “puckering” mechanism, which dominates the friction of exfoliated graphene, plays even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune the graphene friction property by nano/microstructure engineering such as introducing wrinkles

Stable and Selective Humidity Sensing Using Stacked Black Phosphorus Flakes

Black phosphorus (BP) atomic layers are known to undergo chemical degradation in humid air. Yet in more robust configurations such as films, composites, and embedded structures, BP can potentially be utilized in a large number of practical applications. In this study, we explored the sensing characteristics of BP films and observed an ultrasensitive and selective response toward humid air with a trace-level detection capability and a very minor drift over time. Our experiments show that the drain current of the BP sensor increases by ∼4 orders of magnitude as the relative humidity (RH) varies from 10% to 85%, which ranks it among the highest ever reported values for humidity detection. The mechanistic studies indicate that the operation principle of the BP film sensors is based on the modulation in the leakage ionic current caused by autoionization of water molecules and ionic solvation of the phosphorus oxoacids produced on moist BP surfaces. Our stability tests reveal that the response of the BP film sensors remains nearly unchanged after prolonged exposures (up to 3 months) to ambient conditions. This study opens up the route for utilizing BP stacked films in many potential applications such as energy generation/storage systems, electrocatalysis, and chemical/biosensing

Characteristic Work Function Variations of Graphene Line Defects

Line defects, including grain boundaries and wrinkles, are commonly seen in graphene grown by chemical vapor deposition. These one-dimensional defects are believed to alter the electrical and mechanical properties of graphene. Unfortunately, it is very tedious to directly distinguish grain boundaries from wrinkles due to their similar morphologies. In this report, high-resolution Kelvin potential force microscopy (KPFM) is employed to measure the work function distribution of graphene line defects. The characteristic work function variations of grain boundaries, standing-collapsed wrinkles, and folded wrinkles could be clearly identified. Classical and quantum molecular dynamics simulations reveal that the unique work function distribution of each type of line defects is originated from the doping effect induced by the SiO₂ substrate. Our results suggest that KPFM can be an easy-to-use and accurate method to detect graphene line defects, and also propose the possibility to tune the graphene work function by defect engineering

Power Dissipation of WSe₂ Field-Effect Transistors Probed by Low-Frequency Raman Thermometry

The ongoing shrinkage in the size of two-dimensional (2D) electronic circuitry results in high power densities during device operation, which could cause a significant temperature rise within 2D channels. One challenge in Raman thermometry of 2D materials is that the commonly used high-frequency modes do not precisely represent the temperature rise in some 2D materials because of peak broadening and intensity weakening at elevated temperatures. In this work, we show that a low-frequency E_2g² shear mode can be used to accurately extract temperature and measure thermal boundary conductance (TBC) in back-gated tungsten diselenide (WSe₂) field-effect transistors, whereas the high-frequency peaks (E_2g¹ and A_1g) fail to provide reliable thermal information. Our calculations indicate that the broadening of high-frequency Raman-active modes is primarily driven by anharmonic decay into pairs of longitudinal acoustic phonons, resulting in a weak coupling with out-of-plane flexural acoustic phonons that are responsible for the heat transfer to the substrate. We found that the TBC at the interface of WSe₂ and Si/SiO₂ substrate is ∼16 MW/m² K, depends on the number of WSe₂ layers, and peaks for 3–4 layer stacks. Furthermore, the TBC to the substrate is the highest from the layers closest to it, with each additional layer adding thermal resistance. We conclude that the location where heat dissipated in a multilayer stack is as important to device reliability as the total TBC

Nanoparticle Silver Catalysts That Show Enhanced Activity for Carbon Dioxide Electrolysis

Electrochemical conversion of CO₂ has been proposed both as a way to reduce CO₂ emissions and as a source of renewable fuels and chemicals, but conversion rates need improvement before the process will be practical. In this article, we show that the rate of CO₂ conversion per unit surface area is about 10 times higher on 5 nm silver nanoparticles than on bulk silver even though measurements on single crystal catalysts show much smaller variations in rate. The enhancement disappears on 1 nm particles. We attribute this effect to a volcano effect associated with changes of the binding energy of key intermediates as the particle size decreases. These results demonstrate that nanoparticle catalysts have unique properties for CO₂ conversion

In Situ Spectroscopic Examination of a Low Overpotential Pathway for Carbon Dioxide Conversion to Carbon Monoxide

Lowering the overpotential for the electrochemical conversion of CO₂ to useful products is one of the grand challenges in the Department Of Energy report, “Catalysis for Energy”. In a previous paper, we showed that CO₂ conversion occurs at low overpotential on a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄)-coated silver catalyst in an aqueous solution of EMIM-BF4. One of the surprises in the previous paper was that the selectivity to CO was better than 96% on silver, compared with ∼80% in the absence of ionic liquid. In this article, we use sum frequency generation (SFG) to explore the mechanism of the enhancement of selectivity. The study used platinum rather than silver because previous workers had found that platinum is almost inactive for CO production from CO₂. The results show that EMIM-BF₄ has two effects: it suppresses hydrogen formation and enhances CO₂ conversion. SFG shows that there is a layer of EMIM on the platinum surface that inhibits hydrogen formation. CO₂, however, can react with the EMIM layer to form a complex such as CO₂-EMIM at potentials more negative than −0.1 V with respect to a standard hydrogen electrode (SHE). That complex is converted to adsorbed CO at cathodic potentials of −0.25 V with respect to SHE. These results demonstrate that adsorbed monolayers can substantially lower the barrier for CO₂ conversion on platinum and inhibit hydrogen formation, opening the possibility of a new series of metal/organic catalysts for this reaction

Bimodal Phonon Scattering in Graphene Grain Boundaries

Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs

Cathode Based on Molybdenum Disulfide Nanoflakes for Lithium–Oxygen Batteries

Lithium–oxygen (Li–O₂) batteries have been recognized as an emerging technology for energy storage systems owing to their high theoretical specific energy. One challenge is to find an electrolyte/cathode system that is efficient, stable, and cost-effective. We present such a system based on molybdenum disulfide (MoS₂) nanoflakes combined with an ionic liquid (IL) that work together as an effective cocatalyst for discharge and charge in a Li–O₂ battery. Cyclic voltammetry results show superior catalytic performance for this cocatalyst for both oxygen reduction and evolution reactions compared to Au and Pt catalysts. It also performs remarkably well in the Li–O₂ battery system with 85% round-trip efficiency and reversibility up to 50 cycles. Density functional calculations provide a mechanistic understanding of the MoS₂ nanoflakes/IL system. The cocatalyst reported in this work could open the way for exploiting the unique properties of ionic liquids in Li–air batteries in combination with nanostructured MoS₂ as a cathode material

Tailoring the Edge Structure of Molybdenum Disulfide toward Electrocatalytic Reduction of Carbon Dioxide

Electrocatalytic conversion of carbon dioxide (CO₂) into energy-rich fuels is considered to be the most efficient approach to achieve a carbon neutral cycle. Transition-metal dichalcogenides (TMDCs) have recently shown a very promising catalytic performance for CO₂ reduction reaction in an ionic liquid electrolyte. Here, we report that the catalytic performance of molybdenum disulfide (MoS₂), a member of TMDCs, can be significantly improved by using an appropriate dopant. Our electrochemical results indicate that 5% niobium (Nb)-doped vertically aligned MoS₂ in ionic liquid exhibits 1 order of magnitude higher CO formation turnover frequency (TOF) than pristine MoS₂ at an overpotential range of 50–150 mV. The TOF of this catalyst is also 2 orders of magnitude higher than that of Ag nanoparticles over the entire range of studied overpotentials (100–650 mV). Moreover, the <i>in situ</i> differential electrochemical mass spectrometry experiment shows the onset overpotential of 31 mV for this catalyst, which is the lowest onset potential for CO₂ reduction reaction reported so far. Our density functional theory calculations reveal that low concentrations of Nb near the Mo edge atoms can enhance the TOF of CO formation by modifying the binding energies of intermediates to MoS₂ edge atoms