Lithium-CO 2 batteries are attractive energy storage systems for fulfilling the demand of future large-scale applications such as electric vehicles due to their high specific energy density compared to lithium-ion batteries. However, a major challenge with Li-CO 2 batteries is attaining reversible formation and decomposition of the Li 2 CO 3 and carbon discharge products, along with a lack of mechanistic understanding of the associated charge and discharge reaction mechanisms. In this study, we developed a fully reversible Li-CO 2 battery with overall carbon neutrality using molybdenum disulfide nanoflakes as a cathode catalyst combined with an ionic liquid and dimethyl sulfoxide hybrid electrolyte. This combination of materials produces a multicomponent composite (Li 2 CO 3 /C) product rather than formation of separated carbon and Li 2 CO 3 nanoparticles. The battery shows a superior long cycle life of 500 for a fixed 500 mAh/g capacity per cycle, which is by far the best cycling stability reported in Li-CO 2 batteries, respectively. The long cycle life demonstrates for the first time that covalent CO bond making and breaking chemical transformations can be used in energy storage systems, in addition to the widely studied alkali metal (Li, Na, K)-oxygen ionic-bond making and breaking transformations. Theoretical calculations are used to deduce a mechanism for the reversible discharge/charge processes and explain how the carbon interface with Li 2 CO 3 provides the electronic conduction needed for the oxidation of Li 2 CO 3 , as well as the carbon to generate the CO 2 on charge. The achievement of a reversible, long cycle life Li-CO 2 battery opens the way for use of CO 2 in advanced energy storage systems. Lithium-ion batteries are widely used as electrochemical energy storage systems for consumer electronics [1] ; however, technologies with higher specific energy are needed for electrified transportation applications [2]. Therefore, beyond Li-ion battery chemistries such as rechargeable Li-O 2 batteries have recently garnered much attention This article is protected by copyright. All rights reserved. 3 due to their higher theoretical energy density [3,4]. Li-O 2 batteries generally have limited cyclability, though several studies have reported new concepts that have achieved long cycle life [5,6]. Although far less studied, the Li-CO 2 battery is another beyond Li-ion technology with a theoretical energy density of 1876 Wh/kg [7,8] , far exceeding that of Li-ion batteries (~265 Wh/kg). This type of battery involves CO 2 reduction and evolution reactions during discharge and charge, respectively, on the surface of a porous cathode with an electrolyte based on lithium salts.

High‐entropy alloys combine multiple principal elements at a near equal fraction to form vast compositional spaces to achieve outstanding functionalities that are absent in alloys with one or two principal elements. Here, the prediction, synthesis, and multiscale characterization of 2D high‐entropy transition metal dichalcogenide (TMDC) alloys with four/five transition metals is reported. Of these, the electrochemical performance of a five‐component alloy with the highest configurational entropy, (MoWVNbTa)S2, is investigated for CO2 conversion to CO, revealing an excellent current density of 0.51 A cm−2 and a turnover frequency of 58.3 s−1 at ≈ −0.8 V versus reversible hydrogen electrode. First‐principles calculations show that the superior CO2 electroreduction is due to a multi‐site catalysis wherein the atomic‐scale disorder optimizes the rate‐limiting step of CO desorption by facilitating isolated transition metal edge sites with weak CO binding. 2D high‐entropy TMDC alloys provide a materials platform to design superior catalysts for many electrochemical systems.

Metal–organic frameworks (MOFs) are promising materials for electrocatalysis; however, lack of electrical conductivity in the majority of existing MOFs limits their effective utilization in the field. Herein, an excellent catalytic activity of a 2D copper (Cu)‐based conductive MOF, copper tetrahydroxyquinone (CuTHQ), is reported for aqueous CO2 reduction reaction (CO2RR) at low overpotentials. It is revealed that CuTHQ nanoflakes (NFs) with an average lateral size of 140 nm exhibit a negligible overpotential of 16 mV for the activation of this reaction, a high current density of ≈173 mA cm−2 at −0.45 V versus RHE, an average Faradaic efficiency (F.E.) of ≈91% toward CO production, and a remarkable turnover frequency as high as ≈20.82 s−1. In the low overpotential range, the obtained CO formation current density is more than 35 and 25 times higher compared to state‐of‐the‐art MOF and MOF‐derived catalysts, respectively. The operando Cu K‐edge X‐ray absorption near edge spectroscopy and density functional theory calculations reveal the existence of reduced Cu (Cu+) during CO2RR which reversibly returns to Cu2+ after the reaction. The outstanding CO2 catalytic functionality of conductive MOFs (c‐MOFs) can open a way toward high‐energy‐density electrochemical systems.

Transition metal dichalcogenide (TMDCs) alloys could provide a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been predicted, the vast compositional space of TMDC alloys has remained largely unexplored due to our lack in understanding of their stability when accommodating different cations or chalcogens in a single-phase. Here, we report a theory-guided synthesis approach to achieve unexplored quasi-binary TMDC alloys through computationally predicted stability maps. We have generated equilibrium temperature-composition phase diagrams using first-principles calculations to identify the stability for 25 quasi-binary TMDC alloys, including those involving non-isovalent cations and verify them experimentally by synthesizing a subset of 12 predicted alloys using a scalable chemical vapor transport method. We demonstrate that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: (i) outstanding thermal stability tested up to 1230 K, (ii) exceptionally high electrochemical activity for CO 2 reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, (iii) excellent energy efficiency in a high rate Li-air battery, and (iv) high break-down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.As a class of 2D materials, transition metal dichalcogenides (TMDCs) display diverse physical properties, including topological insulator properties, [1,2] superconductivity, [3][4][5][6] valley polarization, [7][8][9][10] and enhanced electrocatalytic activity for various chemical reactions. [11][12][13][14][15][16][17][18] This diversity arises due to the ability of TMDCs to accommodate different transition-metal elements, such as Mo, W, V, Nb, Ta, Re and others, with the three chalcogens (S, Se, and Te) in stable layered structures -that can be exfoliated to a desired number of 2D layers to control quantum confinement. Their properties can be further tuned

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 shear mode can be used to accurately extract temperature and measure thermal boundary conductance (TBC) in back-gated tungsten diselenide (WSe) field-effect transistors, whereas the high-frequency peaks (E and A) 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 WSe and Si/SiO substrate is ∼16 MW/m K, depends on the number of WSe 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.

electrochemical reactions. [1][2][3][4][5][6][7][8][9][10][11] In particular, molybdenum disulfide (MoS 2 ) and a few members of transition metal dichalcogenides (TMDCs) in contact with ionic-liquid (IL) electrolyte have recently shown a great promise to overcome fundamental electronic and thermokinetic limitations for CO 2 reduction reaction, as well as the oxygen reduction and evolution reactions (ORR/OER). [7][8][9][10] These studies have been conducted on a limited number of TMDCs, and the majority of other TMDCs with a wide range of electronic and potentially catalytic properties have not been investigated. In this study, we report synthesis and characterization of a wide range of TMDCs including sulfides, selenides, and tellurides of group V and VI transition metals and study their electrochemical performance in aprotic medium with Li salts. We employ a wide suite of characterization techniques, such as scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), dynamic light scattering (DLS), and atomic forceThe optimization of traditional electrocatalysts has reached a point where progress is impeded by fundamental physical factors including inherent scaling relations among thermokinetic characteristics of different elementary reaction steps, non-Nernstian behavior, and electronic structure of the catalyst. This indicates that the currently utilized classes of electrocatalysts may not be adequate for future needs. This study reports on synthesis and characterization of a new class of materials based on 2D transition metal dichalcogenides including sulfides, selenides, and tellurides of group V and VI transition metals that exhibit excellent catalytic performance for both oxygen reduction and evolution reactions in an aprotic medium with Li salts. The reaction rates are much higher for these materials than previously reported catalysts for these reactions. The reasons for the high activity are found to be the metal edges with adiabatic electron transfer capability and a cocatalyst effect involving an ionic-liquid electrolyte. These new materials are expected to have high activity for other core electrocatalytic reactions and open the way for advances in energy storage and catalysis. ElectrocatalystsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Recently two-dimensional (2D) transition metal carbides and nitrides (MXenes) have gained significant attention in electronics and electrochemical energy conversion and storage devices where the heat production significantly affects the safety and performance of these devices. In this paper, we have studied the thermal transport in monolayer , the first and most studied MXene, using density functional theory (DFT) and phonon Boltzmann transport equation and quantified the effect of surface termination (bare, fluorine and oxygen) on its lattice thermal conductivity. We found that thermal conductivity of fluorine-terminated (108 W/m.K) is approximately one order of magnitude higher than its oxygen-terminated counterpart (10 W/m.K). Our calculations reveal that the increased thermal conductivity for the fluorine-terminated structure is due to its enhanced specific heat and group velocity and diminished scattering rate of phonons.

Redox meditators (RMs) are soluble catalysts located in an electrolyte that can improve the energy efficiency (reduced overpotential) and cyclability of Li–oxygen (Li–O2) batteries. In this work, 20 RMs within a Li–O2 system with dimethyl sulfoxide and tetraethylene glycol dimethyl ether electrolytes are studied and their electrochemical features such as redox potential, the separation of cathodic and anodic peaks, and their current intensities are measured using cyclic voltammetry (CV) experiments. Six RMs are selected as “primary” choices based on their electrochemical performance, and stability tests are then performed to examine their electrochemical responses after consecutive cycles. Moreover, galvanostatic cycling tests are performed within a Li–O2 battery system assembled with selected six RMs for real case consistency investigations. It is found that results from CV to galvanostatic cycling tests are consistent for halides and organometallic RMs, where the former exhibit much higher stability. However, the organic RMs show high reversibility in CV but low in battery cycling results. Density functional theory calculations are carried out to gain more understanding of the stability and redox potentials of the RMs. This study provides comparative information to select the most reliable RMs for Li–O2 batteries along with new fundamental understanding of their electrochemical activity and stability.