Ultra‐Robust and High‐Performance Rotational Triboelectric Nanogenerator by Bearing Charge Pumping

As an emerging technology to convert environmental high‐entropy energy into electrical energy, triboelectric nanogenerator (TENG) has great demands for further enhancing the service lifetime and output performance in practical applications. Here, an ultra‐robust and high‐performance rotational triboelectric nanogenerator (R‐TENG) by bearing charge pumping is proposed. The R‐TENG composes of a pumping TENG (P‐TENG), an output TENG (O‐TENG), a voltage‐multiplying circuit (VMC), and a buffer capacitor. The P‐TENG is designed with freestanding mode based on a rolling ball bearing, which can also act as the rotating mechanical energy harvester. The output low charge from the P‐TENG is accumulated and pumped to the non‐contact O‐TENG, which can simultaneously realize ultralow mechanical wear and high output performance. The matched instantaneous power of R‐TENG is increased by 32 times under 300 r/min. Furthermore, the transferring charge of R‐TENG can remain 95% during 15 days (6.4 × 106 cycles) continuous operation. This work presents a realizable method to further enhance the durability of TENG, which would facilitate the practical applications of high‐performance TENG in harvesting distributed ambient micro mechanical energy.


Introduction
[11] Different from the traditional energy conversion devices, the TENG shows the remarkable merits in easy fabrication, [12][13][14][15] abundant material selection [16][17][18] and high conversion efficiency for harvesting environmental micro-nano energy in low frequency, [19] micro amplitude [20] and weak input power. [21]Nevertheless, the durability of TENG is limited by the ineluctable mechanical wear between two direct-contact tribo-layers, which would shorten service time and limit the practical application of TENG.
[27][28] However, these methods are a certain degree of compromise between output performance and mechanical wear. [29,30]Achieving and keeping the excellent performance in a long operation cycle is full of the challenges due to the insufficient contact electrification and charge dissipation. [31,32]][35] The rolling ball bearing is an essential and indispensable component in rotating mechanism, which can also be employed as the pumping generator of the charge pumping system.Therefore, it is expected to eliminate the mechanical wear and improve the durability without sacrificing the output performance for the rotating TENG by the bearing charge pumping strategy.
Here, an ultra-robust and high-performance rotational TENG (R-TENG) by bearing charge pumping is proposed.The R-TENG composes of a pumping TENG (P-TENG), an output TENG (O-TENG), a voltage-multiplying circuit (VMC), and a buffer capacitor.The P-TENG is designed with freestanding mode based on a rolling ball bearing, which can also act as the rotating mechanical energy harvester.The output low charge from the P-TENG is accumulated and pumped to the non-contact O-TENG, which can simultaneously realize ultralow mechanical wear and high output performance.The matched instantaneous power of R-TENG is increased by 32 times under 300 r/min.Furthermore, the transferring charge of R-TENG can remain 95% during 15 days (6.4 9 10 6 cycles) continuous operation.This work presents a realizable method to further enhance the durability of TENG, which would facilitate the practical applications of high-performance TENG in harvesting distributed ambient micro mechanical energy.
As an emerging technology to convert environmental high-entropy energy into electrical energy, triboelectric nanogenerator (TENG) has great demands for further enhancing the service lifetime and output performance in practical applications.Here, an ultra-robust and high-performance rotational triboelectric nanogenerator (R-TENG) by bearing charge pumping is proposed.The R-TENG composes of a pumping TENG (P-TENG), an output TENG (O-TENG), a voltage-multiplying circuit (VMC), and a buffer capacitor.The P-TENG is designed with freestanding mode based on a rolling ball bearing, which can also act as the rotating mechanical energy harvester.The output low charge from the P-TENG is accumulated and pumped to the noncontact O-TENG, which can simultaneously realize ultralow mechanical wear and high output performance.The matched instantaneous power of R-TENG is increased by 32 times under 300 r/min.Furthermore, the transferring charge of R-TENG can remain 95% during 15 days (6.4 3 10 6 cycles) continuous operation.This work presents a realizable method to further enhance the durability of TENG, which would facilitate the practical applications of high-performance TENG in harvesting distributed ambient micro mechanical energy.

The Structure and Working Principle
Figure 1 indicates the main structure and working principle of R-TENG.As depicted in Figure 1a, the R-TENG includes a P-TENG, an O-TENG, a VMC, and a commercial ceramic capacitor as the buffer capacitor.As indicated in Figure 1b, the P-TENG is adopted in freestanding mode based on a rolling ball bearing, which can also act as the rotating mechanical energy harvester.Eight pairs of interdigital copper electrodes are fixed on the exterior surface of the bearing outer ring.The O-TENG is coaxial with the P-TENG, which is fabricated in the sliding mode consisting of a rotor and a stator.Two copper films are attached at the surface of the rotor and stator as the electrode, respectively.The Kapton film is employed as the dielectric layer.The distance d between the rotor and stator can avoid the mechanical wear and increase the durability of the O-TENG.The detail fabricated process is indicated in Experimental Section. Figure 1c shows the detailed charge transfer process of the R-TENG.There are two symmetrical charge shuttling channels with the Q + and Q À sides between two electrodes of O-TENG and buffer capacitor.The capacitance of the buffer capacitor (C f ) keeps constant, while the O-TENG has the changeable capacitance due to the changing overlapping area between two electrodes.In the charge accumulation stage, the charges from P-TENG are injected into the O-TENG through the VMC.As depicted in Figure 1c (I), when the rotor rotates clockwise, the capacitance of the O-TENG increases with the increasing overlapping  area of the copper electrodes.The charges flow from the buffer capacitor to electrodes of the O-TENG driven by the potential difference.While the overlapping area decreases, as indicated in Figure 1c (II), the charges will flow back from the electrodes to the buffer capacitor with the decreasing capacitance of the O-TENG.with the continuous rotating of the rotor, the positive and negative charges can shuttle between the O-TENG and buffer capacitor, generating an alternative current in external circuit.Benefitted from the charge pumping strategy, the output charge of R-TENG enhances by 13 and 4 times compared with the P-TENG and O-TENG, respectively, as depicted in Figure 1d.At the same time, the maximum instantaneous power of R-TENG is correspondingly increased 32 and 15 times, respectively, as shown in Figure 1e. Figure 1f indicates the durability of the R-TENG.After 15 days (6.4 9 10 6 cycles) continuous operation, 95% output charge is retained, which demonstrates a great durability.2b,c, 78.88 V and 8.33 nC are achieved, respectively.The peak-peak value of the output U OC and Q SC keep constant, when the rotating rate rises from 120 to 300 r/ min, as shown in Figure 2d.The VMC is incorporated to enlarge the output voltage from the P-TENG and promote the charge injection from the P-TENG to O-TENG.The VMC is designed in six voltagemultiplying mode consisting of six capacitors (C M ) and diodes.The output characteristic of VMC is affected by capacitance, which is expected to provide a higher voltage with lower ripple.As shown in Figure S1, Supporting Information, the ripple of output voltage decreases with the increasing of capacitance.However, the output voltage of VMC is decreased with the C M = 4.7 nF.The reason is that the P-TENG cannot provide sufficient charges for the VMC with too large capacitance.Therefore, the capacitor of 2.2 nF is selected to form the VMC.With the charge injection from the P-TENG, the transferring charge and voltage of R-TENG increases firstly, and then tends to saturation under the 300 r/min, as shown in Figure 2e.With the d = 0.5 mm, C f = 2.2 nF, the saturation value of voltage and transferring charge reach to 537.71 V and 110.61 nC, respectively.The saturation value of the transferring charge keeps constant with the rising rotating rate, while the saturation time decreases, as shown in Figure 2f.It is worth nothing that the stable charge at 120 r/min is 103.12 nC, which is less than that of transferring charge at other rotating rates.At this rotating rate, the pumping charge from the P-TENG to the O-TENG is insufficient due to the charge loss on the VMC.The O-TENG and buffer capacitor can be equivalent to two capacitors in parallel.The transferring charge is dependent on the DC b , C f and V RC .As shown in Figure 2g, the transferring charge firstly rises with the enlarged C f , and then achieves a stable value, under the DC b = 0.17 nF.The buffer capacitor of 2.2 nF is chosen for another experiment.Figure S2a, Supporting Information, indicates the capacitance variation in C b with the different rotating angle of rotor.

As indicated in
The DC b of the O-TENG reduces with the rising distance d, as depicted in Figure S2b, Supporting Information.The corresponding transferring charge also decreases with the increasing distance d at C f = 2.2 nF.In addition, the transferring charge decayed of R-TENG caused by charge dissipation is also measured after achieving saturation state.Without continuous charge injection from P-TENG, the transferred charge of the R-TENG slowly decay to 29% after 3200 s, as shown in Figure S3, Supporting Information.To investigated the relationship between the V RC and output characteristic of R-TENG, a Energy Environ.Mater.2024, 7, e12566 controllable voltage source is adopted to replace the P-TENG and VMC.As shown in Figure S4, Supporting Information, the transferring charge of R-TENG is linearly correlation with the power voltage.internal impedance of the R-TENG at the various rotating rates is also tested in detail.As indicated in Figure 2h, both the instantaneous and average power firstly rise and then decline, reaching the peak value at 100 MΩ.As depicted in Figure 2i, the maximum peak power is proportional to the rotating rate.When the rotating rate rises from 120 to 300 r/min, the peak instantaneous power rises from 36.54 to 75.96 lW, while the average power increase from 21.07 to 44.2 lW.The impedance characteristics of P-TENG and O-TENG are summarized in Figure S5, Supporting Information.In order to drive the electrical devices, the output impulsive alternating current electricity of R-TENG need to be managed with the rectification circuit.As shown in Figure 2j, the charging characteristics for different capacitors with the R-TENG have been studied experimentally.The charging time increases gradually with increasing of capacitance.The charging times are $ 64, $ 272, and $ 356 s for 4.7, 10, and 47 lF capacitors to 3 V, respectively.Figure S6, Supporting Information, compares the charging characteristics of P-TENG, O-TENG, and R-TENG for a 47 lF capacitor.It is observed that the R-TENG can charge the capacitor to 3 V within 356 s, while the 954 and 852 s are spent with the P-TENG and O-TENG, respectively.As a result, the rotating mechanical energy can be converted into electrical energy more efficiently by the R-TENG based on the bearing charging strategy.

The Durability of R-TENG
The durability of the R-TENG and P-TENG are investigated, as shown in Figure 3.The normalized charge output of the R-TENG indicates an excellent stability after achieving saturated state, as shown in Figure 3a.After 15 days (6.4 9 10 6 cycles) continuous operation under 300 r/ min, the output transferring charge can maintain 95% compared with initial state.The waveforms of the transferring charge indicate negligible fluctuation for 2 9 10 6 , 4 9 10 6 , and 6.4 9 10 6 cycles.Benefitting from the charge pumping strategy and non-contact operation, the R-TENG exhibits the excellent electric output stability.Furthermore, the scanning electron microscope (SEM) diagrams of the copper and Kapton film before and after 15 days are measured, as shown in Figure 3b.Due to the distance between the rotor and stator, the mechanical wear on the copper and Kapton surfaces are hardly observed.The surface of the copper and Kapton films are almost unchanged before and after 15 days.Figure 3c shows the output stability of the P-TENG during the 15 days.The peak-peak value of the U OC can maintain 96% output compared with initial state.The waveforms of the U OC indicate negligible fluctuation for 2 9 10 6 , 4 9 10 6 , and 6.4 9 10 6 cycles, which can ensure the sustained charge injection to the O-TENG.Figure S7, Supporting Information, depicts the SEM images of the internal surface of bearing outer ring before and after 15 days.The surface is almost unchanged.At the time, we compare the normalized output of the R-TENG with the reported works, as shown in Figure 3d.[38] The R-TENG and P-TENG exhibit excellent durability.

Potential Application of R-TENG
Figure 4 indicates the possible applications of R-TENG.The R-TENG, as shown in Figure 4a, can be employed to capture ambient micronano energy such as wind energy.The excellent durability and output performance will make the R-TENG promising served as a sustainable power source for the lower-power electronics like laptop, mobile phone, smartwatch, and so on.Figure 4b,c and Video S1, Supporting Information, show the application for powering a thermometer when the R-TENG is driven by the wind under 5 m/s. Figure 4b indicates the voltage curve of the 100 lF storage capacitor.When the voltage reaches 2.5 V, the switch S is switched on, and the thermometer can be started-up and run.Figure 4c shows the photograph of R-TENG powering the thermometer.Another intuitive demonstration, as depicted in Figure 4d and Video S2, Supporting Information, the R-TENG can directly light up 100 light-emitting diodes (LEDs) in series.With the excellent output stability, the R-TENG can harvest the distributed ambient micro mechanical energy with long-term service life, which has broad prospects as a sustainable supply source for the low-power sensor in the Internet of Things.

Conclusion
An ultra-robust and high-performance R-TENG by bearing charge pumping is proposed.The R-TENG composes of a P-TENG, an O-TENG, a VMC, and a buffer capacitor.The P-TENG is designed with freestanding mode based on a rolling ball bearing, which can also act as the rotating mechanical energy harvester.The output low charge from the P-TENG is accumulated and pumped to the non-contact O-TENG, which can simultaneously realize ultralow mechanical wear and high output performance.At the different rotating rates, the output performances of R-TENG are investigated.The saturation value of transferring charge can achieve 110.61 nC.When the rotating rate rises from 120 to 300 r/min, the maximum instantaneous power increase from 36.54 to 75.96 lW.The matched instantaneous power of R-TENG is increased by 32 times under 300 r/min.Furthermore, the R-TENG exhibits excellent durability.During 15 days (6.4 9 10 6 cycles) continuous operation, the peak-peak value of the U OC for the P-TENG can maintain 96% output compared with initial state, which can ensure the sustained charge injection to the O-TENG.The saturated transferring charge of R-TENG can maintain 95% compared with initial state.The waveforms of the transferring charge indicate negligible fluctuation for 2 9 10 6 , 4 9 10 6 , and 6.4 9 10 6 cycles.Benefitting from the noncontact state between the rotor and stator, the mechanical wear on the copper and Kapton surfaces are hardly observed.This work presents a realizable method to further enhance the durability of TENG, which would facilitate the practical applications of high-performance TENG in harvesting distributed ambient micro mechanical energy.

Experimental Section
Fabrication of the P-TENG: The P-TENG composes of a rolling ball bearing and eight pairs of interdigital copper electrodes.The size of the rolling ball bearing is Φ 20 9 Φ 52 9 15 mm.The material of the rings is POM, while the ball is fabricated with glass.Eight pairs of interdigital copper films (60 lm thick) are fixed at the external surface of the outer ring as the electrodes.The width of each copper electrode is 8 mm.The distance between the adjacent electrodes is ~2.2 mm.
Fabrication of the O-TENG: The O-TENG includes a stator and a rotor.The size of the stator is Φ 82 9 120 mm.Various sizes (Φ 80.7 9 100 mm, Φ 80.5 9 100 mm, Φ 80.3 9 100 mm, Φ 80.1 9 100 mm, Φ 79.9 9 100 mm) of rotors are fabricated.The cylindrical stator and rotor are fabricated by CNC turning.The wall thickness of stator and rotor is 3 mm.One of the copper films (60 lm thick) is fixed on the inner surface of the stator as the electrodes.The other copper film (60 lm thick) is fixed on the external surface of the rotor as the electrodes.And then, the Kapton film that are 30 lm thick is glued on the top of the copper electrode.Half area of the stator and rotor are covered by copper electrodes.
Characterization and measurement: The SEM diagrams are measured by Nova NanoSEM 450 (FEI Company).

Figure 1 .
Figure 1.Main structure and working principle of R-TENG.a) Main structure of R-TENG.b) Sectional schematic of P-TENG and O-TENG.c) Working principle of R-TENG.d) Comparison of transferring charge of P-TENG, O-TENG, and R-TENG through charge pump strategy.e) Comparison of matched instantaneous power of P-TENG, O-TENG, and R-TENG.f) The charge retention of R-TENG during 15 days continuous operation.

Figure 2 .
Figure 2. Measuring method and output of P-TENG and R-TENG.a) The measuring method of the voltage and transferring charge.b) Output open-circuit voltage waveform of P-TENG at 300 r/min.c) Output short-circuit transferring charge waveform of P-TENG at 300 r/min.d) Relationship between the output U OC , Q SC and rotating rate.e) Voltage and charge saturation process of R-TENG through charge pump strategy by P-TENG under 300 r/ min rotating rate.f) Saturated transferring charge and saturation time with different rotating rate.g) Maximum transferring charge with the different C f .h) Output instantaneous and average power of R-TENG under various load resistances at 300 r/min.i) Relationship between the matched instantaneous, average power and rotating rate.j) Charging curve of different capacitors to 3 V at 300 r/min.

Figure 2 ,
the output characteristics of the P-TENG and R-TENG are tested by connected the driving shaft with a rotating motor.In all experiments, the rotating rate is 120 to 300 r/min.Figure 2a shows the measuring method of the voltage and transferring charge.The electrometer (Keithley 6514) is employed to measure the transferring charges.While the electrostatic voltmeter (Model 347 and Keithley 6514) is used to measure the voltage.The output open-circuit voltage (U OC ) and short-circuit transferring charge (Q SC ) of P-TENG are firstly measured.As indicated in Figure

Figure 3 .
Figure 3. Durability of the P-TENG and R-TENG.a) The transferring charge of R-TENG during 6.4 9 10 6 cycles continuous operation, the inset depicts the waveforms of the transferring charge for 2 9 10 6 , 4 9 10 6 , and 6.4 9 10 6 cycles.b) The SEM diagrams of the copper electrode and Kapton film in day-0 and day-14.c) Stability of the P-TENG in 6.4 9 10 6 cycles continuous operation, the illustration indicates the U OC of the P-TENG for 2 9 10 6 , 4 9 10 6 , and 6.4 9 10 6 cycles.d) Comparison of the stability with reported works.

Figure 4 .
Figure 4. Demonstrations of the R-TENG to power electronics and potential applications.a) Demonstrations of the R-TENG for various applications.b) Voltage curve of the storage capacitor for powering a thermometer.c) Photographs of R-TENG powering a thermometer.d) Directly power 100 LEDs in series by the R-TENG.