Active States During the Reduction of CO2 by a MoS2 Electrocatalyst

Transition-metal dichalcogenides (TMDCs) such as MoS₂ are Earth-abundant catalysts that are attractive for many chemical processes, including the carbon dioxide reduction reaction (CO2RR). While many studies have correlated synthetic preparation and architectures with macroscopic electrocatalytic performance, not much is known about the state of MoS₂ under functional conditions, particularly its interactions with target molecules like CO₂. Here, we combine operando Mo K- and S K-edge X-ray absorption spectroscopy (XAS) with first-principles simulations to track changes in the electronic structure of MoS₂ nanosheets during CO2RR. Comparison of the simulated and measured XAS discerned the existence of Mo-CO₂ binding in the active state. This state perturbs hybridized Mo 4d-S 3p states and is critically mediated by sulfur vacancies induced electrochemically. The study sheds new light on the underpinnings of the excellent performance of MoS₂ in CO2RR. The electronic signatures we reveal could be a screening criterion toward further gains in activity and selectivity of TMDCs in general.

A KMnO4-Generated Colloidal Electrolyte for Redox Mediation and Anode Protection in a Li-Air Battery

The rechargeable lithium-oxygen (Li-O₂) battery has the highest theoretical specific energy density of any rechargeable batteries and could transform energy storage systems if a practical device could be attained. However, among numerous challenges, which are all interconnected, are polarization due to sluggish kinetics, low cycle life, small capacity, and slow rates. In this study, we report on use of KMnO₄ to generate a colloidal electrolyte made up of MnO₂ nanoparticles. The resulting electrolyte provides a redox mediator for reducing the charge potential and lithium anode protection to increase cycle life. This electrolyte in combination with a stable binary transition metal dichalcogenide alloy, Nb_0.5Ta_0.5S₂, as the cathode enables the operation of a Li-O₂ battery at a current density of 1 mA·cm^-2 and specific capacity ranging from 1000 to 10 000 mA·h·g^-1 (corresponding to 0.1-1 mA·h·cm^-2) in a dry air environment with a cycle life of up to 150. This colloidal electrolyte provides a robust approach for advancing Li-air batteries.

Nanostructured Conductive Metal Organic Frameworks for Sustainable Low Charge Overpotentials in Li-Air Batteries

Lithium-oxygen batteries are among the most attractive alternatives for future electrified transportation. However, their practical application is hindered by many obstacles. Due to the insulating nature of Li₂ O₂ product and the slow kinetics of reactions, attaining sustainable low charge overpotentials at high rates becomes a challenge resulting in the battery's early failure and low round trip efficiency. Herein, outstanding characteristics are discovered of a conductive metal organic framework (c-MOF) that promotes the growth of nanocrystalline Li₂ O₂ with amorphous regions. This provides a platform for the continuous growth of Li₂ O₂ units away from framework, enabling a fast discharge at high current rates. Moreover, the Li₂ O₂ structure works in synergy with the redox mediator (RM). The conductivity of the amorphous regions of the Li₂ O₂ allows the RM to act directly on the Li₂ O₂ surface instead of catalyst edges and then transport through the electrolyte to the Li₂ O₂ surface. This direct charge transfer enables a small charge potential of <3.7 V under high current densities (1-2 A g^-1 ) sustained for a long cycle life (100-300 cycles) for large capacities (1000-2000 mAh g^-1 ). These results open a new direction for utilizing c-MOFs towards advanced energy storage systems.

High Performance Air Breathing Flexible Lithium-Air Battery

Lithium-oxygen (Li-O₂ ) batteries possess the highest theoretical energy density (3500 Wh kg^-1 ), which makes them attractive candidates for modern electronics and transportation applications. In this work, an inexpensive, flexible, and wearable Li-O₂ battery based on the bifunctional redox mediator of InBr₃ , MoS₂ cathode catalyst, and Fomblin-based oxygen permeable membrane that enable long-cycle-life operation of the battery in pure oxygen, dry air, and ambient air is designed, fabricated, and tested. The battery operates in ambient air with an open system air-breathing architecture and exhibits excellent cycling up to 240 at the high current density of 1 A g^-1 with a relative humidity of 75%. The electrochemical performance of the battery including deep-discharge capacity, and rate capability remains almost identical after 1000 cycle in a bending fatigue test. This finding opens a new direction for utilizing high performance Li-O₂ batteries for applications in the field of flexible and wearable electronics.

2D High-Entropy Transition Metal Dichalcogenides for Carbon Dioxide Electrocatalysis

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.

2D Copper Tetrahydroxyquinone Conductive Metal-Organic Framework for Selective CO2 Electrocatalysis at Low Overpotentials

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 (CO₂ RR) 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 CO₂ RR 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.

High-Rate Long Cycle-Life Li-Air Battery Aided by Bifunctional InX3 (X = I and Br) Redox Mediators

Redox mediators (RMs) are solution-based additives that have been extensively used to reduce the charge potential and increase the energy efficiency of Li-oxygen (Li-O₂) batteries. However, in the presence of RMs, achieving a long cycle-life operation of Li-O₂ batteries at a high current rate is still a major challenge. In this study, we discover a novel synergy among InX₃ (X = I and Br) bifunctional RMs, molybdenum disulfide (MoS₂) nanoflakes as the air electrode, dimethyl sulfoxide/ionic liquid hybrid electrolyte, and LiTFSI as a salt to achieve long cycle-life operations of Li-O₂ batteries in a dry air environment at high charge-discharge rates. Our results indicate that batteries with InI₃ operate up to 450 cycles with a current density of 0.5 A g^-1 and 217 cycles with a current density of 1 A g^-1 at a fixed capacity of 1 A h g^-1. Batteries with InBr₃ operate up to 600 cycles with a current density of 1 A g^-1. These batteries can also operate at a higher charge rate of 2 A g^-1 up to 200 cycles (for InBr₃) and 160 cycles (for InI₃). Our experimental and computational results reveal that while X₃^- is the source of the redox mediator, LiX at the MoS₂ cathode, In³⁺ reacts on the lithium anode side to form a protective layer on the surface, thus acting as an effective bifunctional RM in a dry air environment. This evidence for a simultaneous improvement in the current rates and cycle life of a battery in a dry air atmosphere opens a new direction for research for advanced energy storage systems.

Quasi-Binary Transition Metal Dichalcogenide Alloys: Thermodynamic Stability Prediction, Scalable Synthesis, and Application

Transition metal dichalcogenide (TMDCs) alloys could have 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 demonstrated, the vast compositional space of TMDC alloys has remained largely unexplored due to the lack of understanding regarding their stability when accommodating different cations or chalcogens in a single-phase. Here, a theory-guided synthesis approach is reported to achieve unexplored quasi-binary TMDC alloys through computationally predicted stability maps. Equilibrium temperature-composition phase diagrams using first-principles calculations are generated to identify the stability of 25 quasi-binary TMDC alloys, including some involving non-isovalent cations and are verified experimentally through the synthesis of a subset of 12 predicted alloys using a scalable chemical vapor transport method. It is demonstrated 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 the CO₂ 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.

Controlling Nanoscale Thermal Expansion of Monolayer Transition Metal Dichalcogenides by Alloy Engineering

2D materials, such as transition metal dichalcogenides (TMDs), graphene, and boron nitride, are seen as promising materials for future high power/high frequency electronics. However, the large difference in the thermal expansion coefficient (TEC) between many of these 2D materials could impose a serious challenge for the design of monolayer-material-based nanodevices. To address this challenge, alloy engineering of TMDs is used to tailor their TECs. Here, in situ heating experiments in a scanning transmission electron microscope are combined with electron energy-loss spectroscopy and first-principles modeling of monolayer Mo_{1- x} W_x S₂ with different alloying concentrations to determine the TEC. Significant changes in the TEC are seen as a function of chemical composition in Mo_{1- x} W_x S₂ , with the smallest TEC being reported for a configuration with the highest entropy. This study provides key insights into understanding the nanoscale phenomena that control TEC values of 2D materials.

A Long-Cycle-Life Lithium-CO2 Battery with Carbon Neutrality

Lithium-CO₂ 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. However, a major challenge with Li-CO₂ batteries is to attain reversible formation and decomposition of the Li₂ CO₃ and carbon discharge products. A fully reversible Li-CO₂ battery is developed with overall carbon neutrality using MoS₂ nanoflakes as a cathode catalyst combined with an ionic liquid/dimethyl sulfoxide electrolyte. This combination of materials produces a multicomponent composite (Li₂ CO₃ /C) product. The battery shows a superior long cycle life of 500 for a fixed 500 mAh g^-1 capacity per cycle, far exceeding the best cycling stability reported in Li-CO₂ batteries. The long cycle life demonstrates that chemical transformations, making and breaking covalent CO bonds can be used in energy-storage systems. Theoretical calculations are used to deduce a mechanism for the reversible discharge/charge processes and explain how the carbon interface with Li₂ CO₃ provides the electronic conduction needed for the oxidation of Li₂ CO₃ and carbon to generate the CO₂ on charge. This achievement paves the way for the use of CO₂ in advanced energy-storage systems.

New Class of Electrocatalysts Based on 2D Transition Metal Dichalcogenides in Ionic Liquid

The 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.

Enhanced Thermal Boundary Conductance in Few-Layer Ti3 C2 MXene with Encapsulation

Van der Waals interactions in 2D materials have enabled the realization of nanoelectronics with high-density vertical integration. Yet, poor energy transport through such 2D-2D and 2D-3D interfaces can limit a device's performance due to overheating. One long-standing question in the field is how different encapsulating layers (e.g., contact metals or gate oxides) contribute to the thermal transport at the interface of 2D materials with their 3D substrates. Here, a novel self-heating/self-sensing electrical thermometry platform is developed based on atomically thin, metallic Ti₃ C₂ MXene sheets, which enables experimental investigation of the thermal transport at a Ti₃ C₂ /SiO₂ interface, with and without an aluminum oxide (AlO_x ) encapsulating layer. It is found that at room temperature, the thermal boundary conductance (TBC) increases from 10.8 to 19.5 MW m^-2 K^-1 upon AlO_x encapsulation. Boltzmann transport modeling reveals that the TBC can be understood as a series combination of an external resistance between the MXene and the substrate, due to the coupling of low-frequency flexural acoustic (ZA) phonons to substrate modes, and an internal resistance between ZA and in-plane phonon modes. It is revealed that internal resistance is a bottle-neck to heat removal and that encapsulation speeds up the heat transfer into low-frequency ZA modes and reduces their depopulation, thus increasing the effective TBC.