This report describes the synthesis of a layered molybdenum disulfide (MoS2)–tungsten disulfide (WS2) heterostructure onto fluorine doped tin oxide covered glass substrates using a combination of chemical bath deposition and RF sputtering techniques. FESEM images revealed that the MoS2–WS2 heterostructure surface consisted of a cauliflower structured array of grains with spherical structures. The vertically aligned atomic layers were explored by transmission electron microscopy images for MoS2–WS2 heterostructure. Hydrogen evolution reaction (HER) kinetics show overpotentials of 151 and 175 mV @ 10 mA/cm2 with Tafel slope values of 90 and 117 mV/decade for pristine MoS2 and WS2 electrocatalysts, respectively. Improved electrocatalytic activity for HER was established with overpotential 129 mV @ 10 mA/cm2 and Tafel slope 72 mV/decade for the MoS2–WS2 heterostructure. The MoS2–WS2 heterostructure electrocatalyst showed robust continuous HER performance over 20 h in an acidic solution. This improved electrochemical performance emerges from the elevation of electron–hole separation at the layer interfaces and sharing of active edge sites through the interface. This study provides the basis to develop new applications for transition-metal dichalcogenides heterostructures in future energy conversion systems.

Since the ability of ionic liquid (IL) was demonstrated to act as a solvent or an electrolyte, IL-based electrolytes have been widely used as a potential candidate for renewable energy storage devices, like lithium ion batteries (LIBs) and supercapacitors (SCs). In this review, we aimed to present the state-of-the-art of IL-based electrolytes electrochemical, cycling, and physicochemical properties, which are crucial for LIBs and SCs. ILs can also be regarded as designer solvents to replace the more flammable organic carbonates and improve the green credentials and performance of energy storage devices, especially LIBs and SCs. This review affords an outline of the progress of ILs in energy-related applications and provides essential ideas on the emerging challenges and openings that may motivate the scientific communities to move towards IL-based energy devices. Finally, the challenges in design of the new type of ILs structures for energy and environmental applications are also highlighted.Polymers 2020, 12, 918 2 of 37 and structural stability of the ionic species are extremely crucial and responsible for the efficient outputs in energy storage devices. In the light of this fact, high current (i.e., automotive applications) operating devices are required to have storage devices that have higher power density and faster ion transport properties. Previous studies have suggested that one of the most favorable approaches to simultaneously progress the safety, along with energy and power densities, is the incorporation of ILs into the electrolyte system [11].In past decades, room temperature ionic liquids (RTILs) have been acknowledged with noteworthy attention due to their excellent miscellaneous properties, such as thermal and chemical stability, tunable structure over the wide range of operating temperature, a broad electrochemical stability window, high ionic conductivity in the range of 10 −3 -10 −2 S cm −1 at room temperature, and non-flammability as a potential candidate for EES devices, such as LIBs [12], electric double-layer SCs [13][14][15], proton exchange membrane fuel cells, and solar cells [16][17][18][19]. Using ILs as an alternative to organic electrolytes has the advantage of improving the ions' mobility, as well as eliminating the hazards associated within the organic electrolytes. Additionally, ILs often have comparable zero or negligible vapor pressure at normal temperatures due to their high thermal stability. Because of the above statements, ILs are widely used as solvents or electrolytes for energy storage applications in recent times [7,18,[20][21][22][23][24][25][26][27].Typically, ILs are organic salts, also defined as molten salts, which have a lower melting point (<100 • C) with a wide degree of variation. Moreover, they are comprised of organic cations, such as an pyridinium (PY) [28], imidazolium (Im) [29][30][31][32], pyrrolidinium (PYR) [33][34][35][36], ammonium [37], and sulfonium [38], derivatives joined inorganic or organic anions, such as BF 4 − [39,40], PF 6 − [30], triflate (...

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