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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Research on stretchable strain sensors is actively conducted due to increasing interest in wearable devices. However, typical studies have focused on improving the elasticity of the electrode. Therefore, methods of directly connecting wire or attaching conductive tape to materials to detect deformation have been used to evaluate the performance of strain sensors. Polyaniline (PANI), a p-type semiconductive polymer, has been widely used for stretchable electrodes. However, conventional procedures have limitations in determining an appropriate metal for ohmic contact with PANI. Materials that are generally used for connection with PANI form an undesirable metal-semiconductor junction and have significant contact resistance. Hence, they degrade sensor performance. This study secured ohmic contact by adapting Au thin film as the metal contact layer (the MCL), with lower contact resistance and a larger work function than PANI. Additionally, we presented a buffer layer using hard polydimethylsiloxane (PDMS) and structured it into a dumbbell shape to protect the metal from deformation. As a result, we enhanced steadiness and repeatability up to 50% strain by comparing the gauge factors and the relative resistance changes. Consequently, adapting structural methods (the MCL and the dumbbell shape) to a device can result in strain sensors with promising stability, as well as high stretchability.

Stretchable strain sensors are capable of acquiring data when in contact with human skin or equipment and are widely used in wearable applications. Most strain sensors have tensile properties of less than 20% and have limitations regarding body motion linkage, complex sensor structure, and motion nonreliability. To address these problems, we developed a high tension and high sensitivity sensor with a gauge factor over 40 and tensile stress about 50%. Polydimethylsiloxane (PDMS) was selected as the flexible substrate to ensure tensile strength, and polyaniline (PANI) was used to measure the resistance changes in the sensor. In particular, problems regarding poor uniformity of PANI on PDMS were resolved by surface treatment of the PDMS, wherein PANI polymerization was performed sequentially after forming a self-assembled monolayer (SAM) on the PDMS substrate. O2 plasma and (3-aminopropyl)triethoxysilane were used to form the SAM. It is expected that this sensor can obtain stable characteristics even under high tensile stress through the evenly formed PANI films on the surface-treated PDMS substrate and may be used in various flexible sensor applications.

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