Ionic liquid-based molecular design for transparent, flexible, and fire-retardant triboelectric nanogenerator (TENG) for wearable energy solutions

Highlights

• Ionic liquid-based molecular design for high-performance and mechanically durable TENG film.

• Flame-retarding, transparent and flexible epoxy-based ion-gel films.

• Transparent and flexible triboelectric nanogenerators (TENGs) producing an output voltage and current of ~150 V and ~45 μA from an external mechanical stimulus.

Abstract

Transparent and flexible triboelectric nanogenerator (TENG) represent an efficient and invisible energy solution for generating eco-friendly electricity from mechanical human motion for wearable electronic devices and systems. In addition to boosting the output performance of TENG, the molecular design relying on non-flammable materials and anti-ignition invulnerability should be considered when designing TENG devices, to ensure the safety of personnel working under extreme temperature conditions. However, the requirement for non-flammability of conventional transparent triboelectric materials in wearable applications remains either unmet or almost unexamined to date. Here, we propose bi-continuous and flame-retarding epoxy-based ion-gel films that retain mechanical flexibility, optical transparency, and fast ionic polarization for high-performance and deformable TENG. It is found that our transparent and flexible TENG devices produce an output voltage and current as high as approximately 150 V and 45 μA, respectively, from an external mechanical stimulus while also retaining their fire retardancy and low flammability. This TENG is not flammable even after 20 s of trying, whereas conventional triboelectric materials were completely burned by the fire under the same conditions. Therefore, we propose that our synergistic design of triboelectric ion-gel films, including fire-retardant epoxy-based dual cation-incorporated ionic liquid, represents a significant step toward a high-performance, durable, and transparent wearable energy solution.

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].