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Li-air batteries are considered strong candidates for the next-generation energy storage systems designed for electrical transportation. However, low cyclability and current rates are two major drawbacks that hinder them from further realization. These issues necessitate the discovery of novel materials to significantly enhance the redox process of discharge products. In this study, a novel catalytic system comprised of tin sulfide (SnS) nanoflakes as a solid catalyst and tin iodide (SnI2) as a dual-functional electrolyte additive is discovered. This system enables operating the battery at high current rates up to 10 000 mA g−1 (corresponding to 1 mA cm−2). The SnS catalyst shows outstanding catalytic activity for both oxygen reduction and evolution reactions compared to carbon, noble metals, and other transition metal dichalcogenides. It also exhibits good structural integrity at high rates. The computations indicate numerous possible oxygen reduction sites without oxygen dissociations on the SnS surface through solution mechanism that is likely responsible for the formation of Li2O2. The calculations also indicate that the role of the SnI2 is not only reacting with the lithium anode to provide protection but reducing the charge potential by promoting catalytic decomposition of the Li2O2. This work provides new novel additives for designing high-rate Li-air batteries.

The rechargeable lithium-oxygen (Li-O

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 into 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.

Transition metal dichalcogenides (TMDCs) have garnered much attention recently due to their remarkable performance for different electrochemical systems. In this study, we report on the synthesis a...

Few-layer (FL) transition metal dichalcogenides have drawn attention for nanoelectronics applications due to their improved mobility, owing to the partial screening of charged impurities at the oxi...

Lithium–oxygen (Li–O2) batteries are a promising class of rechargeable Li batteries with a potentially very high achievable energy density. One of the major challenges for Li–O2 batteries is the hi...

We investigate the phase behavior of ternary mixtures of ionic liquid, organic solvent, and lithium salt by molecular dynamics simulations. We find that at room temperature, the electrolyte separates into distinct phases with specific compositions; an ion-rich domain that contains a fraction of solvent molecules and a second domain of pure solvent. The phase separation is shown to be entropy-driven and is independent of lithium salt concentration. Phase separation is only observed at microsecond time scales and greatly affects the transport properties of the electrolyte.

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-O

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 CO

Few-layer (FL) transition-metal dichalcogenides have drawn attention for nanoelectronics applications due to their improved mobility, owing to the partial screening of charged impurities at the oxide interface. However, under realistic operating conditions, dissipation leads to self-heating, which is detrimental to electronic and thermal properties. We fabricated a series of FL-WSe

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 E2g2 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 (E2g1 and A1g) 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 responsi...

Lithium-air batteries are considered to be a potential alternative to lithium-ion batteries for transportation applications, owing to their high theoretical specific energy. So far, however, such systems have been largely restricted to pure oxygen environments (lithium-oxygen batteries) and have a limited cycle life owing to side reactions involving the cathode, anode and electrolyte. In the presence of nitrogen, carbon dioxide and water vapour, these side reactions can become even more complex. Moreover, because of the need to store oxygen, the volumetric energy densities of lithium-oxygen systems may be too small for practical applications. Here we report a system comprising a lithium carbonate-based protected anode, a molybdenum disulfide cathode and an ionic liquid/dimethyl sulfoxide electrolyte that operates as a lithium-air battery in a simulated air atmosphere with a long cycle life of up to 700 cycles. We perform computational studies to provide insight into the operation of the system in this environment. This demonstration of a lithium-oxygen battery with a long cycle life in an air-like atmosphere is an important step towards the development of this field beyond lithium-ion technology, with a possibility to obtain much higher specific energy densities than for conventional lithium-ion 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 Li

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.

International audience; Heat transport across vertical interfaces of heterogeneous 2D materials is usually governed by the weak Van der Waals interactions of the surface-terminating atoms. Such interactions play a significant role in thermal transport across transition metal carbide and nitride (MXene) atomic layers due to their hydrophilic nature and variations in surface terminations. Here, the metallicity of atomically thin Ti3C2Tz MXene, which is also verified by scanning tunneling spectroscopy for the first time, is exploited to develop a self-heating/self-sensing platform to carry out direct-current annealing experiments in high (-8 bar) vacuum, while simultaneously evaluating the interfacial heat transport across a Ti3C2Tz/SiO2 interface. At room temperature, the thermal boundary conductance (TBC) of this interface is found, on average, to increase from 10 to 27 MW m-2 K-1 upon current annealing up to the breakdown limit. In situ heating X-ray diffraction and X-ray photo-electron spectroscopy reveal that the TBC values are mainly affected by interlayer and interface spacing due to the removal of absorbents, while the effect of surface termination is negligible. This study provides key insights into understanding energy transport in MXene nanostructures and other 2D material systems.

Lithium-CO

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.

Electrocatalytic reactions offer alternative routes for the conversion and storage of energy. For instance, the electrocatalytic reduction of carbon dioxide could lead to energy-rich syngas, methanol, or even hydrocarbons. Layered transition metal dichalcogenide nanomaterials, such as MoS2, have emerged as electrocatalysts towards CO2 reduction with high efficiency and low overpotentials. However, the underlying chemical and electronic states defining the catalytic activity have not yet been completely defined. These key states can be probed for both metals and ligands in selected MX2 (M=W, Mo; X=S, Se) using synchrotron-based X-ray absorption spectroscopy (XAS). In recent work we performed in-situ XAS measurements at Mo K-edge and S K-edge for MoS2 nanosheets. Measurements at the S K-edge showed drastic changes at the pre-edge and edge indicating changes in number of available unoccupied states and Zeff respectively. On the other hand, only a small shift of the Mo K-edge was observed. Metal d-states are found to be the active site for catalysis, yet the ligand K-edge presents a better method to examine catalytic processes. Since the pre-edge reflects a transition to metal-ligand hybridized states, its intensity and position gives direct insight into the activity and changes of d-states. Given the vast compositional space of metal and ligand for MX2, the knowledge of how d-state activity will serve to pave the way to a better study of metal and chalcogenide combinations to improve the electrocatalytic performance. Figure 1

Li–O2 batteries have recently emerged to meet nowadays elevated electric energy demands. Redox mediators (RMs) for solution-inducing decomposition of discharge products are one approach to increase energy efficiency and reduce high overpotentials in these batteries. However, multiple obstacles hinder their usage such as redox shuttling, capacity fading, electrolyte degradation, etc. Herein, we present a new chemistry based on a combination of LiNO3, TEGDME and an ionic liquid that enables LiI (1 M) to lower the charge potential (3.5V) with a long cycle life of 270 cycles. 0.1 M LiI increases the cyclability up to 500 with a slightly increased charge potential (~4V) for a fixed capacity of 1000 mAh/g. Up to 100 cycles, this battery system retained ~95% Li2O2 capacity with a ~0.8 V charge-discharge polarization gap. The addition of LiNO3 to the electrolyte provides a protective solid electrolyte interface (SEI) on anode that works in synergy with the LiI RM. Moreover, we found that this electrolyte blend results in domain formation of ionic and neutral species enhancing the discharge and charge processes. Finally, DFT calculations provide a better understanding of the role of the anode SEI layer and the Li2O2 decomposition promoted by the LiI during charge on the cathode.

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.

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 in situ differential electrochemical mass spectrometry experiment shows the onset overpotential of 31 mV for this catalyst, which is the ...

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.

Recently, two-dimensional 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 T i 3 C 2 T z, the first and most studied MXene, using density functional theory and the phonon Boltzmann transport equation and quantified the effect of surface termination (bare, fluorine, and oxygen) on its lattice thermal conductivity. We found that the thermal conductivity of fluorine-terminated T i 3 C 2 T z (108 W/m K) is approximately one order of magnitude higher than its oxygen-terminated counterpart (11 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.

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

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 Ti

Two-dimensional materials, including graphene, transition metal dichalcogenides and their heterostructures, exhibit great potential for a variety of applications, such as transistors, spintronics, and photovoltaics. While the miniaturization offers remarkable improvements in electrical performance, heat dissipation and thermal mismatch can be a problem in designing electronic devices based on two-dimensional materials. Quantifying the thermal expansion coefficient of 2D materials requires temperature measurements at nanometer scale. Here, we introduce a novel nanometer-scale thermometry approach to measure temperature and quantify the thermal expansion coefficients in 2D materials based on scanning transmission electron microscopy combined with electron energy-loss spectroscopy to determine the energy shift of the plasmon resonance peak of 2D materials as a function of sample temperature. By combining these measurements with first-principles modeling, the thermal expansion coefficients (TECs) of single-layer and freestanding graphene and bulk, as well as monolayer MoS_{2}, MoSe_{2}, WS_{2}, or WSe_{2}, are directly determined and mapped.

Electrochemistry is central to applications in the field of energy storage and generation. However, it has advanced far more slowly over the last two decades, mainly because of a lack of suitable and affordable catalysts. Here, we report the synthesis of highly crystalline layered three-dimensional (3D) molybdenum disulfide (MoS2) catalysts with bare Mo-edge atoms and demonstrate their remarkable performance for the hydrogen evolution reaction (HER). We found that Mo-edge-terminated 3D MoS2 directly grown on graphene film exhibits a remarkable exchange current density (18.2 μA cm–2) and turnover frequency (>4 S–1) for HER. The obtained exchange current density is 15.2 and 2.3 times higher than that of MoS2/graphene and MoS2/Au catalysts, respectively, both with sulfided Mo-edge atoms. An easily scalable and robust growth process on a wide variety of substrates, along with prolonged stability, suggests that this material is a promising catalyst in energy-related applications.

Lithium–oxygen batteries allow oxygen to be reduced at the battery’s cathode when a current is drawn; in present-day batteries, this results in formation of Li2O2, but it is now shown that another high energy density material, namely LiO2, with better electronic conduction can be used instead as the discharge product, if the electrode is decorated with iridium nanoparticles. Nonaqueous lithium–air batteries have a much superior theoretical gravimetric energy density compared to conventional lithium ion batteries, and thus have the potential for making long-range electric vehicles a reality. Batteries based on sodium and potassium superoxides have recently been reported, but thermodynamically unstable lithium superoxide (LiO2), with its potential high energy density, has proved more problematic. This paper demonstrates that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable cathode material — reduced graphene oxide decorated with iridium nanoparticles. A battery based on this new lithium–oxygen chemistry was demonstrated through 40 cycles before failure, achieving high efficiency and good capacity. Batteries based on sodium superoxide and on potassium superoxide have recently been reported1,2,3. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research4,5,6,7,8 into the lithium–oxygen (Li–O2) battery because of its potential high energy density. Several studies9,10,11,12,13,14,15,16 of Li–O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime17. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form18 because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li–O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

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.

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.

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 Mo1- x Wx S2 with different alloying concentrations to determine the TEC. Significant changes in the TEC are seen as a function of chemical composition in Mo1- x Wx 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.

Although far less studied, lithium-CO2 (Li-CO2) batteries are attractive energy storage systems for fulfilling the demand for the future large-scale applications such as electric vehicles and grid systems due to their higher specific energy density (~1876 Wh/kg) compared to those of commonly used lithium-ion (~265 Wh/kg) and lead-acid (~30-40 Wh/kg) batteries. However, the major challenges with these batteries are the low cyclability and poor reversibility of discharge products (e.g., Li2CO3 and carbon) during the battery cycling. An ideal system must operate in carbon neutral conditions in order to reversibly balance the electrochemical reactions during discharge and charge processes. In this work, using molybdenum disulfide nanoflakes as a cathode catalyst combined with an ionic liquid and dimethyl sulfoxide hybrid electrolyte, we have obtained a Li-CO2 battery with a long cycle life while maintaining a carbon neutrality in the system. The battery shows a superior cycle life of 500 for a fixed 500 mAh/g capacity per cycle and a remarkable deep discharge capacity of 60,000 mAh/g, which are by far the best cycling stability and highest capacity reported in Li-CO2 batteries, respectively. The presented Li-CO2 battery system demonstrates for the first time that C-O bond making and breaking chemical transformations can be used in energy storage systems with a long cycle life, in addition to the widely studied alkali metal (Li, Na, K) – oxygen bond making and breaking transformations. Theoretical calculations are used to provide insight into the reaction mechanisms for the reversible formation and decomposition of the products during the discharge and charge processes. The achievement of a reversible, long cycle life Li-CO2 battery helps to improve Li-CO2 chemical performance and energy storage systems.

Metal-air batteries are greatly considered as an advanced solution for future energy requirements due to their remarkably high-energy density. For instance, the theoretical energy density of Li-air batteries (3500 Wh/kg) is about one order of magnitude higher than that of commonly used Li-ion batteries (~400 Wh/kg). In terms of cruising-speed range on passenger cars, the performance of Li-air batteries is comparable to that of internal combustion engines. Thus far, the research on the Li-air battery has been limited to the short life study of lithium oxygen (Li-O2) batteries where the O2, as the oxidant in Li–air cells, needs to be extracted from air to eliminate the side reactions of N2, CO2 and water with lithium or cell components. Hence although oxygen is an abundant and freely available element, its use will require on-site purification, which is energetically expensive and reduce the energy density of practical cells. In this presentation, I will overview our recently developed Li-air battery system that operates with the actual air components (N2, O2, CO2 and H2O) with a superior cycle life and energy efficiency that can operate up to 700 continuous cycles with the capacity of 500 mAh/g [1]. This Li-air battery benefits from a novel protected Li-anode, an air cathode based on MoS2 catalyst and a mixture electrolyte including DMSO and EMIM-BF4 ionic liquid. Various characterizations including differential electrochemical mass spectroscopy (DEMS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy on the cathode reveal that the only discharge product in our system is lithium peroxide (Li2O2) with no evidence of side products such as lithium hydroxide (LiOH) and lithium carbonate (Li2CO3). Nuclear magnetic resonance (NMR) results also show the stability of the electrolyte. Density functional theory calculations suggest that the combination of MoS2 cathode and ionic liquid in the electrolyte provides a kinetically favorable system for Li2O2 formation. The study of this Li-air system is a key step toward the development of next generation lithium batteries with higher energy density and cycle life for transportation and grid scale applications. [1] A. Salehi-Khojin et al., A lithium–oxygen battery with a long cycle life in an air-like atmosphere, Nature, 555 (7697), 502, 2018.

Lithium-air (Li-O2) batteries -comprising of an air (O2) cathode and a lithium anode- are able to deliver significantly higher amounts of theoretical energy densities compared to conventional Li-ion batteries. Therefore, these batteries have been extensively investigated lately to become the practical substitution for Li-ion batteries especially in transportation and electric vehicles. Performance of a Li-O2 battery is fundamentally governed by two main reactions occurring at the cathode: oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during charge. In order to improve the performance of Li-O2 batteries, seeking a bifunctional catalyst which operates favorably in both ORR and OER reactions, is a challenging issue. The role of various materials has been investigated in OER-ORR reduction in both aprotic and non-aprotic media, including: noble metals, metal oxides, transition metals, transition metal carbides, and etc. Among these catalysts, transition metal dichalcogenides (TMDCs) such as MoS2 indicated a superior performance towards a bifunctional catalytic activity for ORR and OER in aprotic electrolytes. Herein we report for the first time the catalytic activity of a number of novel two dimensional TMDCs with transition metals of groups V-IX and the three heavier chalcogens (S, Se, Te) in ORR-OER in an aprotic medium with Li salt. We have synthesized these materials via chemical vapor transport (CVT) and the nanoflakes accompanied by different characterization techniques such as: Raman, XPS, UPS, DLS, AFM and etc. We performed cyclic voltammetry experiements to comparatively study the TMDCs catalytic activity in ORR-OER reactions, their turnover frequency and so on. These catalysts outperformed all of the reported catalysts in aprotic media with Li salts during ORR and OER. A number of them exhibited an excellent bifunctional behavior such as: NbS2, VS2 and VSe2. Density functional theory (DFT) calculations were also performed on a number of these catalysts in order to understand their catalytic activity during ORR and OER in ionic liquid electrolyte. We believe these materials possess great potential to enhance the catalytic activity in core electrochemical reactions especially in Li-O2 battery applications.

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.

The effects of choline chloride on the hydrogen evolution reaction, formic acid electrooxidation, and carbon dioxide conversion are investigated on a platinum cathode. We find that choline chloride suppresses hydrogen formation, but by contrast formic acid electrooxidation is enhanced and the overpotential of carbon dioxide reduction is reduced dramatically. We also find that choline chloride can be used to extend the window where electrochemical experiments can be performed. These results demonstrate that monolayers of species such as choline can have significant effects that need to be explored in more detail.

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.

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all-2D electronic circuitry. This study employs seed-free consecutive chemical vapor deposition processes to synthesize high-quality lateral MoS

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.