Ionic liquid-based molecular design for transparent, flexible, and fire-retardant triboelectric nanogenerator (TENG) for wearable energy solutions
Graphical Abstract
A new type of ionic liquid-based molecular design for our TENG device structure containing dual-positive ions and forming the electrical double layer (EDL) due to the rapid movement of internal ions during friction. It was intended that the dual ILs including Li+ and EMIM+ positive ions can easily transport through the bi-continuous structure of fire-resistant polymer backbone upon the mechanical surface friction as depicted in the right image.
Introduction
Recently, significant developments in electronic technology have followed the general trends of miniaturization, functionality, and portability, and research has been conducted to adapt existing energy storage systems into self-powered alternative energy devices. Many studies have focused on improving power generation by developing new materials and microstructures for wearable displays and sensors [1], [2], [3], [4], [5], [6], [7], [8], [9]. Remarkable progress has been made to date in addressing the issues of replacing the traditional battery-based operation of wearable systems. Conventional liquid batteries are the most commercialized portable power source, but are difficult to incorporate into devices that increasingly rely on small size and flexibility. The biggest obstacle to the commercialization of wearable electronics has been the issue of power supply, which has required frequent battery replacement or heavy battery weights. Battery replacement is a major challenge in wearable devices because of the limited battery lifespan, the inconvenience of having to replace batteries, and the additional cost. This is less of a problem if the number of batteries is small, but the situation changes rapidly as the number of batteries to be replaced increases [10], [11], [12], [13], [14], [15], [16]. Gradually, the trend of developing wearable electronics has been toward low power consumption, allowing the energy harvested from the usual environment of the device to directly power the device by designing self-powered systems [17], [18], [19], [20]. New technologies that can harvest energy from the environment are an emerging field of nano/micro-energy that is expected to be applied to small-scale materials and technology for new energy systems [21], [22].
Collecting mechanical energy from human motion and its connection to the energy storage component such as the capacitor and other storage device is considered an attractive approach to meet the growing demand for wearable and light power supplies. Hence, a study of triboelectric nanogenerators (TENG) was extensively performed, with particular focus on triboelectric materials and various surface treatment methods of materials proposed to improve the output [23]. TENG follow a mechanism in which two different triboelectric materials come into contact and then separate or slide against each other, and electron transfer occurs between their surfaces. TENG based on the combined effect of contact triboelectrification and electrostatic induction have seen rapid development as a sustainable power source owing to their various advantages such as simple structure (basically, two contact materials and electrodes), low weight, flexibility, and high energy conversion. Since the first report by Wang et al. in 2012, there has been an explosive development in applications such as wearable electronics. Recently, advances in new triboelectric materials, nanotechnology, and mechanical design have increased the instantaneous power of TENG by several times [24], [25], [26].
However, considerable care must be taken to select appropriate triboelectric materials for high output generation. This is because the permittivity, surface morphology, and general inherent properties of materials can affect the static surface charge density produced by physical contact. Various polymer-based TENG have been developed for stable high triboelectric output, but have been largely excluded from material selection due to the flammability, firmness, and low permittivity of the polymer itself. In particular, the fire-retardancy and low-flammability of triboelectric materials and devices is a critically important for designing the wearable devices and system working at the severe temperature condition. So, the fire-retardant characteristics offered by our TENG makes it a path-breaking device for the fire fighters and for lifesaving wearable applications [27], [28], [29]. There has been a problem of obsolescence in which production of triboelectric output decreases over time, and various issues have been raised regarding the difficulty and safety of incorporating non-transparent and non-bending materials into wearable devices. To overcome the material constraints of polymers, we propose a strategy of incorporating high-permittivity particles or conventional ionic liquid (IL) into triboelectric dielectric polymer materials [30], [31], [32], [33]. The IL-based polymer (i.e., ion- gel) used to overcome the aforementioned problems is an aggregation of polymer networks interconnected to IL, and it has been gradually drawing attention due to its promising characteristics such as safety, bio-compatibility, and transparency [34]. It also offers a strategy of integrating high capacitance and low leakage current into the triboelectric dielectric material to overcome the material constraints. These fascinating properties make ion-gel a promising material in various fields of research.
Here, we introduce a dual cation-containing gel friction layer for high-performance transparent, flexible, fire-retardant TENG devices and potential application in high-density tactile sensors [33], [35]. In addition, experiments showing the non-flammable characteristics of durability and stability tests that may occur during heat treatment were conducted to differentiate them from other polymer materials [27]. Therefore, the proposed dual cation-containing ion-gel TENG may be an energy solution for wearable electronic systems and may also represent an ideal platform for strain- and pressure-sensitive e-skin [36].
Section snippets
Introduction and characteristics of materials applied to TENG devices
The thermoset polymer matrix phase and the conductive IL phase are described herein. Thermal curing was performed by synthesizing diglycidyl ethers of bisphenol-A (DGEBA, KUKDO Chemical, Korea) epoxy resins, methyl tetrahydrophthalic anhydride (MeTHPA, KUKDO Chemical, Korea) thermal hardening agents, and N-benzyl dimethyl-amine (BDMA, Sigma Aldrich) catalysts to build a polymer network structure. For the ionic conducting phase, a tetraglyme (or tetraethylene glycol dimethyl ether) (G4, Sigma
Results and discussion
Fig. 1 visualize the proposed ionic liquid-based molecular design for our TENG device structure containing dual-positive ions, the electrical double layer (EDL) formation due to the rapid movement of internal ions during friction, and the important characteristics of the ion-gel layer using transparent substrates and electrodes [14], [37]. The combination of the bi-continuous polymer structure and the embedded ILs formed an ion-gel optimized for ion transporting, fast polarization,
Conclusions
In summary, a transparent and flexible high-performance TENG was successfully designed and demonstrated through the EDL effect by thermal ion-gel. Efficient dense EDLs were established through synthesized ion-gels, which significantly improved surface charge density, contributing to the high output power of TENG. In particular, ion-gel, a mixture of lithium salt and plasticizer, was found to play an important role in effectively transmitting ions by modifying polymer chains and electrochemical
CRediT authorship contribution statement
Y. Kim performed the experiment and analyzed the data. D. Lee synthesize the thermoset polymer ion-gel and optimized its condition for the best performance. J. Seong helped Y. Kim to continue the fabrication of triboelectric nanogenerator on a flexible and transparent platform. B. Bak carried out the finite element analysis for the simulation of triboelectric nanogenerator device at the moment of friction. U. Choi analyzed and interpreted the data. J. Kim conceived the idea and supervised all
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the research fund of Hanyang University (HY-2017-N).
Youngkyun Kim received his B. S. degree in Applied Physics and M.S. degree in Photonics and Nanoelectronics from Hanyang University (ERICA campus). For more than 2 years, Y. Kim has been studying the fundamentals and physics of diverse materials and relevant devices focusing on the flexible electronic applications such as energy harvesting, interactive soft electronics, e-skin, nanogenerators, wearable device and thin film transistor.
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Youngkyun Kim received his B. S. degree in Applied Physics and M.S. degree in Photonics and Nanoelectronics from Hanyang University (ERICA campus). For more than 2 years, Y. Kim has been studying the fundamentals and physics of diverse materials and relevant devices focusing on the flexible electronic applications such as energy harvesting, interactive soft electronics, e-skin, nanogenerators, wearable device and thin film transistor.
Dawoon Lee received his B. S. degree in Applied Physics. Currently, he is a Ph.D. student in the department of Photonics and Nanoelectronics at Hanyang University. His research interests are focused on polymeric ion-containing energy materials such as ionic liquid-based conductive electrolytes and electrochemical energy storage devices. He is also interested in 2D material, thin-film transistor, sensors for flexible and wearable devices.
Junsu Seong received his B. S. degree in Photonics and Nanoelectronics from Hanyang University. He is a Master’s student in Department of Photonics and Nanoelectronics at Hanyang University. His research interests are focused on pressure sensor array using triboelectric nanogenerator for wearable device and flexible electronics. He is also interested in blue energy harvesting, 3D printing, and IoT sensors.
Byeongwoo Bak received his B. S. degree in Applied Physics at Hanyang University. Currently, he is a Ph.D. student in the department of Photonics and Nanoelectronics at Hanyang University. His research interests are focused on micro LEDs and assembly techniques for micro LED display. He is also interested in thin film transistor, microfluidic system, electrohydrodynamic physics and wearable devices.
U Hyeok Choi received the B.S. degree in Materials Science and Engineering from the Yonsei University, Seoul, in 2006, and M.S. and Ph.D. degrees in Materials Science and Engineering from The Pennsylvania State University, University Park, in 2009 and 2012, respectively. He is currently an associate professor with the department of Polymer Science and Engineering, Inha University. His research interests include the development of polymeric ion-containing energy materials such as ionomeric lithium single-ion conductors, polymerized ionic liquids for electro-active devices, smart energy storage structural nanocomposites, and hybrid solid polymer electrolytes.
Jaekyun Kim received the B. S. and M. S. degree in Materials Science and Engineering from Korea University, Seoul, in 2001 and 2003, respectively. He received the Ph. D. degree in Electrical Engineering from The Pennsylvania State University, University Park, USA in 2010. Since 2017, he has been an associate professor in the department of Photonics and Nanoelectronics in Hanyang University (ERICA campus). His research interest spans from the soft energy devices and systems using an ion-containing polymer film to the nanoscale materials and devices for the display, sensor, light-emitting diode applications.