Structural design and working mechanism of MDC-TENG
The structure of microstructure-designed DC-TENG (MDC-TENG) with rationally patterned electrode is presented in Fig. 1a-c, which possesses multiple fine friction electrodes (FEs) and interlaced charge collected electrodes (CCEs). All of the individual FEs keep a tiny distance with the adjacent CCEs, and there is a very narrow gap existing between the CCEs and the friction layer (Fig. 1b). It can be seen from the scanning electrons microscopy (SEM) image of the MDC-TENG sample (Fig. 1c) that each friction electrode is about 250 µm in width, and the CCE (~100 µm in width) is located between two FEs. The distance between two adjacent FEs is about 1000 µm, so is the distance between two adjacent CCEs. All of the FEs and CCEs are embedded into the acrylic substrate. When the FE rubs with the friction layer (polytetrafluoroethene, PTFE), electrons transfer from FE to friction layer due to triboelectrification effect, and then a direct current is produced due to the air breakdown between the CCE and the charged friction layer (Fig. 1d, detailed mechanism of DC-TENG is shown in Supplementary Fig. 1 and Note 1).
The reason why the friction electrode width can be miniaturized to microscale dimension in this work can be explained by Fig. 1e. Prior to the atomic-scale contact of two materials, their respective electron cloud keeps separate without overlap (Fig. 1e1). When the slide block slides forward and contacts with the friction layer, the electron transition from M1 to D1 occurs, and the energy potential barrier difference between two materials becomes lower (Fig. 1e2), which is called Wang transition model and has been confirmed by the atomic force Kevin probe microscopy recently4,28,29. Thus, few electrons would transit from M2 to D1 owing to their lower potential difference when M2 overlaps D1 (Fig. 1e3). This assumption has been confirmed by the output performance of DC-TENGs with different widths of FE, as shown in Supplementary Fig. 2. It can be seen that, for the same length and sliding distance of FE, the output charges and short-circuit currents of all the DC-TENGs are not significantly different with the FE width decreasing from 10 mm to 0.25 mm. Namely, the friction electrode width has little effect on the σtriboelectrification in formula (2) due to the high triboelectrification efficiency of the sliding TENG4.
To improve the utilization rate of friction electrode and efficiency of TENG, we provide a method to replace a single large-scale friction electrode by using patterned multiple micron-size friction electrodes, interlaced with micron-size CCEs in two adjacent friction electrodes (M1 and M2). The working mechanism can be described in Fig. 1f. The initial stage (Fig. 1f1) and CE process between M1 and D1 (Fig. 1f2) are similar to those in Fig. 1e1 and 1e2, respectively. However, with the slider moving forward, electrostatic breakdown will occur between CCE and D1 (fundamental mechanism is shown in Supplementary Note 1), and electrons will transfer from D1 to CCE, and then to M1/M2 via external circuit, causing the energy potential barrier difference to form again between M2 and D1. Thus, the contact electrification process will take place again between M2 and D1 due to the Wang transition model, and the following CCE can collect the generated charges. Consequently, combining electrode microstructural design with electrostatic breakdown effect, electron transition of CE occurs twice within the same width of electrode compared with Fig. 1e. Based on the above mechanism, an MDC-TENG with more elaborately-designed patterned electrodes is prepared and an ultrahigh effective surface charge density of ~5.4 mC m-2 is achieved with 50 friction electrodes.
Fig. 1g shows the development of effective surface charge density in AC-TENG and DC-TENG9, 18, 21-27, 30, 31, respectively. After the unremitting efforts of researchers in recent years, many approaches, e.g., high-vacuum18, ion injection30, charge pumping23, 24 and charge excitation25, 26, were carried out (Supplementary Note 2). The limitation factors in formula (1) were broken gradually, and charge density of AC-TENG was successfully improved from less than 0.05 mC m-2 to over 2.3 mC m-2.18, 21, 23-26, 30, 31 As for DC-TENG, its typical charge density of 0.4 mC m-2 was reported in 2019, which is relatively low compared to the AC-TENG in the same period9. However, in this work, the charge density of 5.4 mC m-2 is not only over 10-fold higher than that of DC-TENG reported in 2019, but also over 2 times than that of the state-of-the-art of various-type TENGs (Fig. 1g).
Comparison between sliding AC-TENG and MDC-TENG
To confirm the superiority of MDC-TENG over the traditional AC-TENG, the output performance of MDC-TENG and AC-TENG was carried out and shown in Fig. 2. Working principle of the sliding AC-TENG is presented in Fig. 2a. The slider contacts with the friction layer, generating opposite charges on the surface of electrode and friction layer due to the CE effect, respectively. When the slider continues to move forward, the relative displacement between two electrodes makes the potential difference between them, and the electrons flow in the external circuit to balance this potential difference (Fig. 2a). Fig. 2b-e show the effective surface charges, charge density, short current and current density of sliding AC-TENG (friction layer: PTFE) with different electrode lengths and sliding distances (electrode length: x mm, sliding distance: y mm, x = y in this test, electrode width: 10 mm). With the x and y increasing, output charges increase from 6 nC to 68 nC, accompanied with charge density ~ 0.12 mC m-2 (Fig. 2b, 2c). The short current Isc increases fast with the device size and sliding distance increasing, but the current density decreases from 510 μA m-2 to 170 μA m-2 (Fig. 2d, 2e).
The schematic of MDC-TENG is shown in Fig. 2f, where one MDC-TENG unit includes one FE and one CCE, electrode length (the distance from the first FE to the last CCE): x mm, sliding distance: y mm, MDC-TENG unit: n, x = y = n in this test, electrode width: 10 mm (the photograph of MDC-TENG with 20 units, shown in Supplementary Fig. 3). Fig. 2g-j show the effective surface charges, charge density, short current and current density of MDC-TENG (friction layer: PTFE) with different electrode lengths, sliding distances and different numbers of MDC-TENG unit. With the x, y and n increasing, charges of MDC-TENGs increase from 0.025 μC to 2.6 μC (Fig. 2g), which are much larger than that of AC-TENG at the same dimension (Fig. 2b). More importantly, charge density of MDC-TENGs rises from 0.5 mC m-2 to 5.2 mC m-2 (Fig. 2h), which are nearly 40-fold larger than those of AC-TENGs under the same condition. The high charge density of MDC-TENGs with n = 50 is due to the multiple electron transition through the repeated triboelectrification and discharge processes (Fig. 1f). Meanwhile, Isc increases fast with the increase of x, y and n (Fig. 2i), accompanied with the enhanced current density (Fig. 2j). Generally, the traditional AC-TENG improves the contact efficiency by reducing the contact area to achieve a high charge density4,21, and thus the charge density and current density (Fig. 2c, 2e) decrease with the size of AC-TENG device increasing. However, the enhanced charge density and current density with the enlarged size of MDC-TENG addresses this performance attenuation of AC-TENG, which is benefit to the application in large scale energy harvesting system for TENGs.
Performance of MDC-TENG under different vector motion parameters
As a vector motion device, the output performance of sliding mode MDC-TENG (with 20 MDC-TENG units, shown in Supplementary Fig. 3) under various vector motion parameters is shown in Fig. 3. As the slider slides back and forth on the PTFE surface within different distances, the output charge curve shows a stepped-like shape. When the sliding distance is 2 cm, the average charges of MDC-TENG is about 0.35 µC (Fig. 3a). The output charge at each movement process increases with the extension of sliding distance (Fig. 3b). The Isc of MDC-TENG is proportional to the sliding distance with a high linearity of ~0.99 (the inset in Fig. 3b). Within the sliding distance 10 cm, the output characteristic of MDC-TENG at various velocities (uniform motion) is shown in Fig. 3c and 3d. The average charge density maintains ~2.0 mC m-2 during the sliding velocity increasing from 0.02 to 0.16 m s-1, but the average Isc rises rapidly from 0.6 to 4.6 µA. The average Isc shows a good linear relationship with velocity (the inset in Fig. 3d). The detailed relationship between Isc and velocity is explained in the Supplementary Note 3. Moreover, the output performance of MDC-TENG under different accelerations is shown in Fig. 3e, 3f. The average charge density also maintains ~2.0 mC m-2 at different accelerations. Meanwhile, the Isc increases from 1.8 µA to 9.8 µA with the acceleration of slider increasing from 0.1 to 2.0 m s-2, respectively. The dI/dt is proportional to the sliding acceleration with a high linearity ~0.99 (the inset in Fig. 3b), whose relationship is calculated in the Supplementary Note 4. The output characteristic of MDC-TENG shows a good correlation with the vector motion parameters (e.g., distance, velocity and acceleration), which is the basis of motion vector sensor. In addition, the size of MDC-TENG can be further miniaturized, while the high output performance ensures the strength and anti-interference of sensing signals32. Thus, the MDC-TENG shows great potential in the applications on MEMS as the motion vector sensor unit.
Structure optimization and output performance of MDC-TENG
To further optimize the output performance of MDC-TENG, the effect of the electrode distance between FE and adjacent CCE is studied, as shown in Fig. 4a-d. When the width of FE is 250 μm and CCE is 100 μm, the distance between FE and CCE ranges from 800 μm to 100 μm (Fig. 4a), accompanied with the whole length MDC-TENG device decreasing from 10 mm to 3.5 mm (MDC-TENG unit: 5, width of MDC-TENG: 10 mm). Within the slide distance ~10 cm, the output charges are about 0.5 µC for all MDC-TENGs with various electrode distances. The corresponding Isc ~0.67 µA and Voc ~33 V are also insusceptible with the decrease of electrode distance, indicating the potential of the miniaturization of MDC-TENG. On one hand, charge densities calculated by the friction area (10 cm2) of MDC-TENGs with the same electrode number but the decreasing electrode distance maintains ~0.5 mC m-2 (Fig. 4b). On the other hand, the decrease in electrode distance makes the whole area of MDC-TENG device become smaller. It is of particular important for the miniaturized TENG because the charge densities calculated by the MDC-TENG device area gradually rise with the electrode distance decreasing (Fig. 4e). In a word, the smaller electrode distance means the larger k value in unit area, resulting in higher output of MDC-TENG device. This is critical for the application of MDC-TENG in small electronic device systems or MEMS as the energy supply resource or sensor unit.
The fundamental mechanism of MDC-TENG is air breakdown in the gap between CCE and friction layer. Thus, the gap distance between CCE and friction layer is important for the output performance of MDC-TENG by increasing the σc, electrostatic breakdown in formula (3). We prepared the MDC-TENGs with the precisely controlled gap distance from 125 to 35 µm. Their corresponding SEM images are shown in Fig. 4f and their charge densities are shown in Fig. 4g. Taking PTFE as friction layer, when the gap is 125 µm within the sliding distance 10 cm, the charge density of MDC-TENG with 20 MDC-TENG units is just 0.49 mC m-2. With decreasing the gap distance to 35 μm, the charge density gradually increases to 2.0 mC m-2, indicating that the σc, electrostatic breakdown in formula (3) increases with the gap distance decreasing. Meanwhile, the output short-circuit current Isc (Supplementary Fig. 4) also significantly increases from 0.5 µA for 125 µm gap to 2.0 µA for 35 µm gap.
To clarify this significantly enhancement output, we analyze the potential distribution in the gap between CCE and PTFE by using COMSOL software at different gap distances. Due to the existance of charges on the PTFE surface (setting value: 120 µC m-2 from Fig. 2c), a huge electrostatic field generates between the CCE and the PTFE surface, as shown in Fig. S5. The simulated average strength of electrostatic field is shown in Fig. 4h and Supplementary Fig. 6, and the calculation method is shown in the Supplementary Note 5. With the decrease of gap, the electric field increases sharply and reach to 9.1 MV m-1 for the gap ~35 µm, resulting in easier air breakdown and more complete electron release process from PTFE surface to CCE. Thus, the MDC-TENG with smaller gap distance will show larger σc, electrostatic breakdown, as well as more effective charge density.
In order to further improve the output performance of MDC-TENG, the structure factor k in formula (3) which is related to the number of MDC-TENG units is introduced into the microstructural design of MDC-TENG. Using the PTFE as the friction layer, the charge density and Isc increase linearly with the number of MDC-TENG unit under the sliding distance of 10 cm (Fig. 4i, 4j), so the k value can be simply considered to be equal with the unit number n. The charge density of MDC-TENG with k = 5 is ~0.5 mC m-2, but increases to 5.4 mC m-2 with k = 50. The same trend is also observed in open-circuit voltage Voc curves of MDC-TENG device (Fig. S7). This phenomenon is consistent with the previous analysis: the increased efficiency of contact electrification enhances the effective surface charge density, resulting in the number of electron transfers from PTFE to CCE increasing with the adding number of MDC-TENG units within a certain sliding distance.
Based on the above discussion, a record high of charge density of TENG is achieved by microstructural design with the rationally patterned electrodes, and the output performance can be further enhanced by the optimization of microstructure, e.g., electrode distance, gap distance, and electrode quantity.
Application of MDC-TENG for driving electronic device
As a direct-current nanogenerator, MDC-TENG could directly drive electronic devices or charge capacitor without bridge rectifier. To achieve continuous DC output, a rotary mode MDC-TENG was prepared. Its schematic diagram (stator: MDC-TENG) is shown in Fig. 5a. PTFE film is attached on the rotator surface. The stator is a rotary type MDC-TENG device, whose structure is similar to the sliding mode MDC-TENG (Fig. 1a) with interlaced CCEs and FEs. Fig. 5b shows the output charges of the rotary MDC-TENG at different rotation rates. With the rotation rate rising, output charge reaches to 20 µC within a short time. For example, it only takes 0.3 s to output 20 µC charges at the rotation rate of 600 r min-1. When the rotary MDC-TENG works stably, the output current is about 65 µA at 600 r min-1 (Fig. 5c), accompanied with a typical DC curve (the crest factor is close to 1). Three commercial LED bulbs (each rated power: 7 W) can also be directly driven with high brightness and no flash due to the high DC output (Supplementary Movie 1).
As an energy harvester device, the harvesting energy of MDC-TENG can also be stored in capacitors or batteries for the subsequent utilization of electronic device. The charging curves of different capacitors charged by the rotary MDC-TENG at 600 r min-1 are presented in Fig. 5d, and the detailed circuit is shown in Supplementary Fig. 8. It just takes 0.7 s, 2.0 s and 4.2 s to charge 10 µF, 22 µF and 47 µF capacitors to 5 V, respectively. In addition, as an energy source, the MDC-TENG can also drive the electronic device and charge energy storage device at the same time. The self-powered system is built by integrating MDC-TENG with the commercial capacitor (660 µF) as the energy storage part and the commercial thermo-hygrometer (rated working current: 55 µA) as the energy consumption unit, whose circuit is shown in Supplementary Fig. 9. The voltage of capacitor (660 µF) monitored by voltmeter at different MDC-TENG working conditions is shown in Fig. 5e. At initial stage, MDC-TENG is out of work, the capacitor powers the hygrometer alone, resulting in the decrease in capacitor voltage. When the MDC-TENG begins to work, the voltage of capacitor rises because MDC-TENG provides additional energy, which not only offsets the consumption of hygrometer, but also charges the capacitor. This indicates the excellent output performance of MDC-TENG. However, when the MDC-TENG stops working, the voltage turns to reduce due to the consumption of thermo-hygrometer. Taking advantages of the direct current and high output, the MDC-TENG can directly drive the electronic devices (e.g., thermo-hygrometer) without any auxiliary electronic components, as shown in Fig. 5f and Supplementary Movie 2, and the corresponding circuit is shown in Supplementary Fig. 8. The output energy of MDC-TENG can direct drive small electronic devices or charge the energy storage device in a short time, showing its great potential in the application of harvesting mechanical energy.