Mechanical switches integrated sandwich triboelectric nanogenerator with high instantaneous output power

Triboelectric nanogenerator (TENG) has emerged as a promising energy harvester that converts random mechanical energy in the environment into electricity to enable self-powered system construction. However, the conventional TENG exhibits a large internal resistance resulting in a low output power across external loads for practical application. In this work, the travel mechanical switches were integrated into the TENG to boost output power, where the switches were turned on and off by the periodic motion between the triboelectric layers. Moreover, the triboelectric layers of TENG were developed as contact separation sandwich structures to work with mechanical switches, allowing the device to release the co-accumulated charges from the dual triboelectric layer to yield giant instantaneous discharge. Compared to a conventional TENG, the device offers ~2.8 times more transfer charge in one cycle and ~106 times more instantaneous power at an external load of 500 Ω, which renders the internal impedance match almost negligible. As a result, the high-power source demonstrates the ability to illuminate the commercial lamps in parallel (total power: 50 W). Such a unique TENG with high output power for low resistance load could greatly enrich the practical application fields of self-powered systems.


Introduction
The booming development of Internet of Things (IoT) technology is expected to involve over 100 billion sensor nodes connected to monitoring networks in various fields, such as healthcare, transportation, smart cities, industrial internet, smart grid, and so on [1][2][3].These sensors are distributed in natural environments and wireless connectivity, and therefore face the challenge of obtaining a sustainable and stable power supply.Batteries are common but might not be optimal for distributed sensors because of the limited lifespan and even potential environmental issues.In alternative ways, the widely distributed random and irregular mechanical energy around the sensors is a potential recyclable energy source, which can be exploited for energy harvesting and conversion [4].Some of the typical energy harvesting technologies including electromagnetic, piezoelectric, and triboelectric effects have been developed to convert mechanical energy into electrical energy [5].Compared with the former, the triboelectric nanogenerator (TENG) [6] is featured with low cost, light weight, and high energy conversion efficiency, which is more suitable for harvesting redundant energy from the environment towards large-scale deployment in the future [7][8].The fundamental theory of TENG is based on Maxwell's displacement current and generates a capacitive conduction current to an external load [9].Owing to the inherent capacitive impedance characteristics, the TENG is characterized by large impedances to the MΩ levels, while common electronic devices exhibit low impedance at the kΩ levels [10][11].Such a huge mismatch problem between TENG and electronic devices fails to achieve optimum power output from TENG.Previous work has proposed that it is possible to decrease the impedance of TENGs by incorporating electronic switch-based power management circuits, where the method of running the MOSFET and releasing the energy quickly achieves a direct energy supply to small impedance loads [12][13].However, an additional power source is required to control the electronic switch, and the electronic components themselves have resulted in considerable energy losses.Besides, an alternative way to decrease the internal resistance of the TENG is to integrate a travel mechanical switch, which can be turned on/off with the movement of the TENG to release energy instantaneously [14].Therefore, the mechanical switch strategy represents a promising energy management approach with the potential to address the energy supply problem for small impedance loads, while its integration strategy and working principle need to be further investigated.In this work, a sandwich structure TENG with integrated mechanical switches is proposed for mechanical energy harvesting and achieving a high power supply, especially for small impedance loads.The travel mechanical switches are turned on/off by the periodic motion between the triboelectric layers to replace continuous discharge (CD) into instantaneous discharge (ID), and the sandwich structure TENG works in conjunction with mechanical switches, enabling the co-accumulated charges from the dual triboelectric layer to be released and thus generating high output power.The new physical model of TENG is established for analyzing its unique output performance, while the output difference between this TENG and the conventional TENG is evaluated through relevant experimental tests.To demonstrate the energy supply capability of the designed TENG, the commercial high-power lamp with 10 units in parallel can be illuminated successfully.This work presents a novel TENG as high-power sustainable power source, which expands the scope of TENGs to be utilized in practical applications for the widespread deployment of self-powered electronics.

Structure design and fabrication
As shown in Figure 1a, the TENG was designed into a sandwich structure and its matched mechanical switches, being divided into upper and lower layers and media layer.The polymethyl methacrylate (PMMA) as a substrate of the upper and lower layers were accordingly attached with Cu electrode (E1 and E2) and polymer film, where the upper layer was polyamide (PA) film and the lower layer was polytetrafluoroethylene (PTFE) film.The media layer was composed of electrode layer and polymer film layer, whose electrode layer made of a steel plate (SP) was paired with the PTFE film of the lower layer, and the polymer film layer of PTFE was paired with the PA film of the upper layer, thus forming one group of triboelectric materials, respectively.Mechanical switches made of Cu electrodes located on one side of the upper and lower layers as well as the SP electrode layer of the media layer to form S1 and S2, respectively, with the three overlapped in the vertical plane.Eventually, the electrodes E1 and E2 were connected to the mechanical switches S2 and S1 via external circuits and resistance loads.All the above-mentioned components were fabricated, as shown in detail in Figure 1b.For experimental tests, the fabricated device was fixed on an optical platform and driven by a linear motor, which is presented in Figure 1c.Under a mechanical excitation, the media layer of TENG was forced into contact separation with the upper and lower layers, which simultaneously triggers the mechanical switches contacts to be open/closed.The output signals generated by TENG were acquired via a programmable electrometer (Keithley 6514 System Electrometer).With periodic mechanical excitation at 1 Hz, the designed TENG reached an ID output with a current IID of 0.82 mA for an external resistance load of 1 MΩ, which is far larger than a conventional contact separation TENG composed of PTFE and SP.The tested CD output with current ICD of 0.008 mA is shown in Figure 1d.

Working mechanism
The fundamental working mechanism of the designed TENG derives from the conjunction of contact electrification and electrostatic induction [9].When two triboelectric materials come into contact, an equivalent charge with opposite polarities is resulted on the surfaces, and the next relative movement of the two materials will induce the continuous flow of electric charges between the back electrodes to maintain the electrostatic equilibrium.As illustrated in Figure 2a. in the initial stage, the surface charge of Q1 is defined in the PA film of the upper layer, and the surface charge of Q2 is defined in the PTFE film of the lower layer.Since the SP in the media layer operates as both a triboelectric layer and an inductive electrode, the charge is denoted here as Q1-Q2.In the next stage shown in Figure 2b, as the media layer gradually approaches the upper layer, the charges are transferred between the electrodes of E1 and E2 to equalize the resulting potential difference until coming into contact with the upper layer.
The output characteristic at this stage is a CD output with a total transferred charge of -Q2.When being in the stage of Figure 2c, the media layer contacts the upper layer, and the mechanical switch S1 is connected.Thus the charges of Q1-Q2 distributed in the electrodes SP will be released to E1 immediately, where the output characteristic is an ID output with a high current IID.In the stage shown in Figure 2d, the charges are similarly transferred between the electrodes of E1 and E2 as the media layer gradually approaches the lower layer, where the total charge transfer is Q1 with a CD output.Back to the initial stage, the media layer contacts the lower layer once again, while simultaneously opening the mechanical switch S2 so that the charge Q1-Q2 accumulated at electrode E2 is released to electrode SP with high current ID output.

Output characterization
Figure 3a displays the transfer charge curve of the designed TENG in one cycle, which is well match the working mechanism of each stage, as shown in Figure 2.Among them, the transferred charges of stages 1 and 3 were 390 nC and 405 nC, while stages 2 and 4 were 185 nC, 206 nC, respectively.The output of stages 1 and 3 verify that the released charges were co-accumulated from the dual triboelectric layer.Statistically, its total transferred charge through the external resistance reached 1186 nC.For comparison, the total transfer charge of a conventional TENG was also measured to be 423 nC for one cycle, as shown in Figure 3c.It can be concluded that the designed TENG is 2.8 times larger than conventional TENG in terms of transfer charge, which effectively enhances the output energy of the device.The physical model based on the classic capacitor models is established here to further investigate the output characteristics of the designed TENG.Originally, the conventional contact separation TENG is equivalent to a series connection circuit of a variable capacitor and an alternating current voltage source, which produces a CD output during contact separation movement.The equivalent physical model is illustrated in Figure 4a and the V-Q-x relationship of conventional TENG can be given by [15]: where CT is the capacitance of the TENG located between the two electrodes, Q is the transferred charges, V is the load voltage, and Voc is the open-circuit voltage.x is the distance between the PTFE and the top electrode, σ is the surface charge density.S is the surface area of PTFE.d is the thickness of PTFE.Ɛr is the relative dielectric constant of PTFE.When the TENG is connected to an external resistance load R.
The output current can be expressed as: Unlike the conventional TENG for CD output, the CD mode and ID mode co-exist in the designed TENG.Since the mechanical switch will be opened in a certain position, the ID mode is equivalent to a series connection circuit of a constant capacitor and a voltage source, shown in Figure 4b.Among the dielectric materials of TENG, the upper layer of PA film with surface charge density σ1, thickness d1, and relative dielectric constant Ɛr1, respectively.The middle layer of PTFE with surface charge density -σ1, thickness d2, and relative dielectric constant Ɛr2, respectively.The lower layer of PTFE film with surface charge density -σ2, thickness d1, and relative dielectric constant Ɛr2, respectively.x is defined as the distance between the PTFE and PA.At the initial stage, the separation reaches x=xmax with the mechanical switch S2 connected, thus allowing the co-accumulated charges of Q1-Q2 to be released instantaneously.Before the switch is opened, the V-Q-x relationship of TENG can be given by: where CT is the constant capacitance of the TENG, which can be expressed as: When the switch is turned on, the generated current in this process can be expressed by: In this stage, the output voltage will be a fixed value independent of the external resistance.The output signals rise suddenly from zero to the peak, and then fall slowly to zero with exponential decay over time, like in Figure 1d.At stage 2, the switch S2 is turned off and the V-Q-x relationship of TENG with CD mode can be given by: where CT is the variable capacitance of the TENG, which can be expressed as: The generated current in this process can be expressed by: Accordingly, the stage 3 is similar to the stage 1 while the stage 4 is similar to the stage 2, and will not be repeated here.To fully investigate the overall energy output characteristics of the designed TENG, the output voltages, currents, and peak powers of the TENG were systematically measured with a series of external resistances (500 Ω to 1 GΩ).As a comparison, the output characteristics of conventional TENG were also measured and the results are shown in Figure 5.For the conventional TENG, the output voltage value grew as the resistance value increased and became saturated with the value of 270 V.While the current value exhibited the opposite trend with saturation in the low resistance region.Therefore, the conventional TENG resulted in maximum peak power at a load resistance of 50 MΩ with a peak power of 1.39 mW.For the designed TENG, whose ID mode was triggered at a fixed position, and the output voltage was a relatively constant value of about 800 V with independent of the external resistor.And the output current in ID mode dramatically jumps as the load resistance decreased.Consequently, the maximum instantaneous power of the designed TENG reached 1310 W at an external load of 500 Ω, this value is 10 6 times larger than the conventional TENG.

Applications
For the demonstration of the capability of designed TENG as a high-power source, the commercial lamp with high power (220 V, 5 W) was utilized as an example for practical application.As shown in Figure 6a, the 10 lamps were connected in parallel as an external load with a total power of 50 W, especially in small internal resistance.Then the loads were directly connected to the designed TENG for visual observation.Under mechanical excitation of the linear motor with 1 Hz, the device successfully derived 10 lamps to illuminate together, as shown in the photograph in Figure 6b.Nevertheless, the conventional TENG could not accomplish such an application, even though just one lamp will never be illuminated.
The designed TENG has been demonstrated as a superior power supply capability for high power loads, and solving the impedance mismatch problem of conventional TENGs due to its original high internal resistance.Such a novel strategy paves the way for exploiting TENG technology towards broad practical applications in new generation IoT.

Conclusion
A sandwich structure TENG (Triboelectric nanogenerator) with integrated mechanical switches has been developed, as a high-power supply source for self-powered electronics.The combination of coaccumulated charges from the dual triboelectric layer and instantaneous charges released by mechanical switches allows TENG to generate a large instantaneous current under periodic motion.Experimental results show that the TENG transfers up to 2.8 times more charge than a conventional TENG in one cycle, which is consistent with the established theoretical model.Meanwhile, the output performance of the TENG is measured with various load resistances and reaches an instantaneous power of up to 1310 W at a small load of 500 Ω.As an application demonstration, the commercial high-power lamps with 10 units connected in parallel (total power: 50 W) can be successfully illuminated by this TENG, but conventional TENG can' t be done.This work is dedicated to developing a high-power sustainable power supply solution and solving the impedance mismatch problem that allows TENG technology to be widely deployed in practical self-powered applications.

Figure 1 .
Figure 1.Structure design of the TENG.(a) Sandwich structure TENG with integrated mechanical switches.Photograph of (b) the fabricated TENG and (c) the experiment platform.(d) The current output of the designed TENG and conventional TENG.

Figure 2 .
Figure 2. Schematic illustration of the working mechanism of the designed TENG.

Figure 3 .
Figure 3. Transferred charge of (a) the designed TENG and (b) the conventional TENG in one cycle.

Figure 4 .
Figure 4. Schematic illustration of the theoretical model.(a) Equivalent physical model of conventional TENG.(b) Equivalent physical model of designed TENG.To fully investigate the overall energy output characteristics of the designed TENG, the output voltages, currents, and peak powers of the TENG were systematically measured with a series of external resistances (500 Ω to 1 GΩ).As a comparison, the output characteristics of conventional TENG were also measured and the results are shown in Figure5.For the conventional TENG, the output voltage value grew as the resistance value increased and became saturated with the value of 270 V.While the current value exhibited the opposite trend with saturation in the low resistance region.Therefore, the conventional TENG resulted in maximum peak power at a load resistance of 50 MΩ with a peak power of 1.39 mW.For the designed TENG, whose ID mode was triggered at a fixed position, and the output voltage was a relatively constant value of about 800 V with independent of the external resistor.And the output current in ID mode dramatically jumps as the load resistance decreased.Consequently, the maximum instantaneous power of the designed TENG reached 1310 W at an external load of 500 Ω, this value is 10 6 times larger than the conventional TENG.

Figure 5 .
Figure 5.Comparison of output characteristics between designed TENG and conventional TENG.Dependence of the (a) output voltage, (b) current, and (c) peak power on the external load resistances.

Figure 6 .
Figure 6.Application for energy supply to high power loads.Photographs of (a) the demonstration platform and (b) the designed TENG as a power source for commercial high-power lamps illumination.