Reversible switching performance of water droplet-driven triboelectric nanogenerators using a magnetocontrollable lubricant-infused surface for sustainable power generation
Graphical Abstract
Introduction
Triboelectric nanogenerators (TENGs) have attracted increasing attention as renewable power sources that convert mechanical energy into electrical energy. Their working mechanism is based on the combination of triboelectrification and electrostatic induction. When two materials with diverse triboelectric polarities are contacted, electron or ion transfer induces a potential difference on their contact surfaces; as the cycle of contact and separation is repeated, electrons flow through an external load, generating a continuous electrical output. Various TENGs based on solid–solid triboelectrification have been explored due to their easy fabrication and low cost, as well as the wide range of available materials [1], [2], [3], [4], [5], [6], [7], [8].
Another type of TENGs, based on liquid–solid (LS) triboelectrification, is drawing attention since Wang group first reported a prototype [9]. In particular, water droplet-driven TENGs are highly useful because water energy sources exist abundantly everywhere in the forms of river/ocean waves and raindrops [10], [11], [12], [13]. The electric double layer (EDL) theory explains LS triboelectrification, where electron transfer is key [14]. Most previous studies focused on increasing the amount of electrical output from the perspective of power generation. In general, TENG has a high inner impedance which leads to low current output [15]. Various attempts have been made to enhance and optimize the power generation, such as the introduction of micro/nano hierarchical structures [16], surface functionalization [17], [18], the pre-injection of electrical charges [19], [20], and the development of various materials and electrode structures [21]. Despite all these efforts, the generated power is still too low to use LS-TENGs as efficient power sources.
To extend the applicability of TENGs, intelligent approaches are required in low power consumption. An example is achieving reversible switching controllability of the electrical output by external stimuli, which has wide potential applications in self-powered sensors and switches [22], [23], [24], [25], [26], [27]. When LS-TENGs have switchable LS interfacial states such as wettability driven by various external stimuli, they can generate reversible switching power. However, despite the necessity of this triboelectric switching ability, few studies have been conducted on the reversible switching of LS triboelectrification. As a rare example, a temperature-sensitive LS-TENG could reversibly regulate triboelectrification by thermal stimuli [28]. The reversible switching of the electrical output was enabled by utilizing polycaprolactone (PCL) as a thermosensitive tribomaterial; the interfacial wettability of PCL is reversibly changed according to its temperature, which induces a change in the surface structure and, consequently, the LS triboelectrification switching. The interfacial wettability and surface structure can be modulated also by other external stimuli, including stress [29], electric fields [30], and magnetic fields [31]. Among them, switching by magnetic fields has the advantage of fast, easy, and highly reversible responses. Yong group recently demonstrated that a magnetocontrollable lubricant-infused surface (MCLIS) has a switching capability between slippery and sticky wetting states depending on the applied magnetic field direction, which is also applicable to reversible LS triboelectrification switching [32].
The present study proposes a novel approach for the magnetocontrollable switching of LS triboelectrification by using an MCLIS as a triboelectric layer. The MCLIS was fabricated by aligning an array of magnetocontrollable microwires (MCMws) on a flat polydimethylsiloxane (PDMS) layer to provide triboelectric charges; the MCMws consisted of cobalt and PDMS, and they were coated with superhydrophobic nanoparticles. The MCLIS layer was completed by infiltrating fluorinated oil on this hierarchical structure, where the MCMws stood vertically or laid down (corresponding to the ON or OFF states, respectively) reversibly according to the direction of the applied magnetic field. When water droplets contacted the ON or OFF state surfaces, the MCLIS-based TENG (MCLIS-TENG) generated a voltage output of, respectively, ∼2 V (ON state) or < 0.5 V (OFF state). It is known that liquid-liquid triboelectrification has a much weaker charge transfer than liquid-solid triboelectrification [14]. Therefore, this switching behavior could be explained by the formation and destruction of a continuous lubricant layer on the MCMws, which strongly affects the number of charges induced by triboelectrification between the water droplets and MCMws. Our MCLIS-TENG showed high durability and reversibility in repeated cycles, with a stable switching electrical output; its switching characteristics were studied systematically by varying the Weber number, inclination angle, and lubricant thickness. Moreover, the MCLIS-TENG attained highly sustainable power generation by utilizing these switchable wetting states even under high humidity, where the performance of most LS-TENGs degrade due to surface wetting problems. In addition, the MCLIS-TENG device has been proposed as a self-powered sensor to detect the magnetic proximity.
Section snippets
Results and discussion
Fig. 1a illustrates the basic structure of the MCLIS-TENG, which consisted of a glass substrate, aluminum electrodes, an MCLIS film on a flat PDMS layer acting as a solid friction layer, and lubricant oil. The Al electrodes were deposited on the glass substrate in the freestanding mode. The basic units of the MCLIS film were the MCMws; they were built via a simple template-free method by using a mixture of PDMS and ferromagnetic cobalt particles. PDMS is the one of the commonly used negative
Conclusion
We have presented an original concept of reversible triboelectric switching characteristics stimulated by a magnetic field change. When the magnetic field direction changes over the MCLIS, its MCMws switch their alignment between the vertically standing and lying down states. Due to this phenomenon, the triboelectric power generation output of the proposed MCLIS-TENG differs nearly three times between these two states, which can be utilized as a triboelectric switching. The mechanism of
Materials
An Al tape, PDMS Base (Sylgard 184), a curing agent, and cobalt microparticles (average diameter of 2 µm, Sigma Aldrich) were used.
MCLIS preparation
The PDMS base, curing agent, and cobalt microparticles were mixed in a 1:1:2 wt ratio. After sufficient stirring, proceed by dividing into spin coating secondary steps. The first and second rounds were run for, respectively, 5 s at 500 rpm and 20 s at 2000 rpm. With a heat-resistant magnet (50 mm × 50 mm × 20 mm, superficial magnetic field intensity of 4500 Gs), a
CRediT authorship contribution statement
Soyeon Yun, Seunghyup Lee, Kijung Yong: Conception and design of study. Soyeon Yun, Suhyeon Cho, Seunghyup Lee: Investigation and Data curation. Soyeon Yun, Seunghyup Lee: Writing – original draft preparation. Soyeon Yun, Hyeon Woo Kim, Sung Beom Cho, Seunghyup Lee: Writing – review & editing. Seunghyup Lee, Kijung Yong: Supervision. Kijung Yong: Funding acquisition.
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.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government(MSIT) (NRF-2021R1A5A1084921, NRF-2021K1A4A8A02079226).
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