A Green Triboelectric Nano-Generator Composite of Degradable Cellulose, Piezoelectric Polymers of PVDF/PA6, and Nanoparticles of BaTiO3

In this paper, a kind of green triboelectric nano-generator based on natural degradable cellulose is proposed. Different kinds of regenerated cellulose composite layers are prepared by a blending doping method, and then assembled with poly(tetrafluoroethylene) (PTFE) thin films to form tribioelectric nanogenerator (TENG). The results show that the open circuit output voltage and the short circuit output current using a pure cellulose membrane is 7.925 V and 1.095 μA. After adding a certain amount of polyamide (PA6)/polyvinylidene fluoride (PVDF)/barium titanate (BaTiO3), the open circuit output voltage peak and the peak short circuit output current increases by 254.43% (to 20.155 V) and 548.04% (to 6.001 μA). The surface morphology, elemental composition and functional group of different cellulose layers are characterized by Scanning Electronic Microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and tested by the electrochemical analyze. Moreover, after multiple assembly and rectification processing, the electrical output performance shows that the peak value of open-circuit output voltage and the peak value of short circuit output current increases by 132.06% and 116.13%. Within 500 s of the charge-discharge test, the single peak charge reached 3.114 V, and the two peak charges reached 3.840 V. The results demonstrate that the nano-generator based on cellulose showed good stability and reliability, and the application and development of natural biomaterials represented by cellulose are greatly promoted in miniature electronic sensing area.


Introduction
In nature, there are different kinds of energy, where mechanical energy has the characteristics of high energy density, diverse expressions and wide distribution, which is the preferred choice for energy harvesting and conversion. In previous studies, it has been proved that nano-generators can effectively convert mechanical energy into electrical energy [1][2][3][4], and the conversion efficiency depends on the degree of coupling between friction effect and electrostatic induction. Moreover, nano-generators have the characteristics of high output voltage, small size, light weight, low cost and good safety [5][6][7][8][9]. They are widely used in the fields of mobile electronic devices, sensing systems and biomedicine [10][11][12][13]. Now, it becomes the research focus in energy collection and conversion.

The Preparation of Different Types of Composite Cellulose Layers
[Bmim]Cl (5 g) is added to q beaker, and then stirred at 85 • C for 10 min. Then, α-cellulose (0.5 g) weighed by using an analytical balance, is added into the [Bmim]Cl, and the mixture is stirred at a low speed for 60 min at a temperature of 85 • C. A certain amount of PA6, PVDF and BaTiO 3 are added Sensors 2020, 20, 506 3 of 14 and stirred at a low speed for 30 min at a temperature of 85 • C to prepare different kinds of cellulose substrate friction layers. Under room temperature and humidity conditions, the prepared friction layer solution is coated on a certain size of glass plate and stands for 1 h. After that, the membrane is placed in distilled water for 20 min. Each of the friction layer films, which named of Cel, Cel + PA6, Cel + PA6 + BaTiO 3 , Cel + PVDF, Cel + PVDF + BaTiO 3 , is removed on a glass plate.
The nano-generator device based on degradable cellulose consists of six layers of top electrode (Al 1 ), different cellulose layer, PTFE layer, bottom electrode (Al 2 ). Due to the good mechanical properties of PMMA materials, it is used as the upper and lower encapsulation layers. The specific structure is shown in Figure 1a above. The electron microscope image of cellulose film with upper friction layer is shown on the right. Under high magnification, it can be seen that the surface of cellulose film is full of pore structure, which is convenient for ion transmission and also conducive to the attachment of granular PA6, PVDF and BaTiO 3 . EDS analyze results show that the presence of C, N and O elements proves the main composition of cellulose, and the presence of a small amount of Cl element is caused by incomplete phase exchange. Finally, the molecular formula of cellulose film is shown in Figure 1a.
The nano-generator device based on degradable cellulose consists of six layers of top electrode (Al1), different cellulose layer, PTFE layer, bottom electrode (Al2). Due to the good mechanical properties of PMMA materials, it is used as the upper and lower encapsulation layers. The specific structure is shown in Figure 1a above. The electron microscope image of cellulose film with upper friction layer is shown on the right. Under high magnification, it can be seen that the surface of cellulose film is full of pore structure, which is convenient for ion transmission and also conducive to the attachment of granular PA6, PVDF and BaTiO3. EDS analyze results show that the presence of C, N and O elements proves the main composition of cellulose, and the presence of a small amount of Cl element is caused by incomplete phase exchange. Finally, the molecular formula of cellulose film is shown in Figure 1a.

Operating Principle
In this paper a nano-generator based on the piezoelectric contact and electrostatic induction is proposed. The plate area is 30 mm × 30 mm, the external force is 10 N, and the frequency is 1 Hz. The upper and lower friction layers contact or rub each other, and the inner surfaces of the two friction layers are charged with the same amount of different charges as shown in Figure 1b (the specific working mechanism see the supporting information).
Different kinds of regenerated cellulose layers are prepared by blending doping method, and assembled with PTFE to form TENG. After rectification treatment, the capacitor is supplied with power to test its charge and discharge performance within 1000 s. After that, power is supplied to microcontroller, which is used to drive the alarm of a SFM-27 buzzer and light up the LCD screen (see Figure 1). .

Operating Principle
In this paper a nano-generator based on the piezoelectric contact and electrostatic induction is proposed. The plate area is 30 mm × 30 mm, the external force is 10 N, and the frequency is 1 Hz. The upper and lower friction layers contact or rub each other, and the inner surfaces of the two friction layers are charged with the same amount of different charges as shown in Figure 1b (the specific working mechanism see the supporting information). Different kinds of regenerated cellulose layers are prepared by blending doping method, and assembled with PTFE to form TENG. After rectification treatment, the capacitor is supplied with power to test its charge and discharge performance within 1000 s. After that, power is supplied to microcontroller, which is used to drive the alarm of a SFM-27 buzzer and light up the LCD screen (see Figure 1).

FT-IR/XRD Characterization Analysis
By comparing the FT-IR patterns of different types of friction layers (Figure 2a), it can be seen that each material has similar functional group peak positions, FT-IR graph shows a similar trend. However, the sharpness of each peak is different, because C-H, C-F and N-F bonds are introduced before doping. This causes a change in the content, which is manifested by a change in the sharpness of the corresponding peak position and a certain shift in the peak position.

FT-IR/XRD Characterization Analysis
By comparing the FT-IR patterns of different types of friction layers (Figure 2a), it can be seen that each material has similar functional group peak positions, FT-IR graph shows a similar trend. However, the sharpness of each peak is different, because C-H, C-F and N-F bonds are introduced before doping. This causes a change in the content, which is manifested by a change in the sharpness of the corresponding peak position and a certain shift in the peak position. Figure 2b shows the XRD scan of different types of friction layers. After adding PA6, PVDF and BaTiO3, the XRD pattern of the diffraction peak angle of each friction layer was changed. Specifically, after dissolution and regeneration, the friction layer of pure cellulose shows a diffraction peak position at 19.63° and 22.78°, which is a typical cellulose II type. After the addition of PA6 and PVDF, a new diffraction peak appears at the positions of 9.28° and 9.91°. After the addition of BaTiO3, new diffraction peaks appear at around 32.61°, 39.93°, 46.49°, and 52.05°. In addition, the sharpness of the diffraction peak and the change of the peak width are caused by an increase in the crystallinity of the friction layer after the addition of the new substance, which is manifested by an increase in the intensity of the diffraction peak. The diffraction peaks of each friction layer are shifted rightward or leftward. This is because during the stirring process, the ions are replaced during the phase exchange, resulting in a change in the intermolecular and hydrogen bonding forces along with the change of the lattice size and it is represented by the diffraction peak shift. After adding different substances, the XRD diffraction peak sharpness change, peak shift and new peak position appear to change the internal structure and physical properties of the friction layer, which affect the output performance of the nano-generator.

CV (Cyclic Voltammetry) Test Analysis
The electrochemical testing of the cellulose-based friction film was performed by a two-electrode system. The prepared friction film (size: 10 mm × 10 mm) is the intermediate layer of the capacitor. The electrolyte used was a LiCL solution of 1 mol L −1 . The scan rate is set to 20-500 mV s −1 , and the scan potential window is set to 0-1 V. The calculation Equation (1) of specific capacitance is as follows, and the calculation results are shown in Table 1: where m is the mass of the active substance on the electrode, s is the voltage scanning rate, V Δ is the potential drop in the whole cycle and V0 is the lowest voltage in the cycle.  Figure 2b shows the XRD scan of different types of friction layers. After adding PA6, PVDF and BaTiO 3 , the XRD pattern of the diffraction peak angle of each friction layer was changed. Specifically, after dissolution and regeneration, the friction layer of pure cellulose shows a diffraction peak position at 19.63 • and 22.78 • , which is a typical cellulose II type. After the addition of PA6 and PVDF, a new diffraction peak appears at the positions of 9.28 • and 9.91 • . After the addition of BaTiO 3 , new diffraction peaks appear at around 32.61 • , 39.93 • , 46.49 • , and 52.05 • . In addition, the sharpness of the diffraction peak and the change of the peak width are caused by an increase in the crystallinity of the friction layer after the addition of the new substance, which is manifested by an increase in the intensity of the diffraction peak. The diffraction peaks of each friction layer are shifted rightward or leftward. This is because during the stirring process, the ions are replaced during the phase exchange, resulting in a change in the intermolecular and hydrogen bonding forces along with the change of the lattice size and it is represented by the diffraction peak shift. After adding different substances, the XRD diffraction peak sharpness change, peak shift and new peak position appear to change the internal structure and physical properties of the friction layer, which affect the output performance of the nano-generator.

CV (Cyclic Voltammetry) Test Analysis
The electrochemical testing of the cellulose-based friction film was performed by a two-electrode system. The prepared friction film (size: 10 mm × 10 mm) is the intermediate layer of the capacitor. The electrolyte used was a LiCL solution of 1 mol L −1 . The scan rate is set to 20-500 mV s −1 , and the scan potential window is set to 0-1 V. The calculation Equation (1) of specific capacitance is as follows, and the calculation results are shown in Table 1:

IdV
(1) where m is the mass of the active substance on the electrode, s is the voltage scanning rate, ∆V is the potential drop in the whole cycle and V 0 is the lowest voltage in the cycle. Figure 3a-h are CV curves of different types of friction layers at a scan rate of 20-500 mV s −1 . It can be seen from each CV curve that when the sweep speed is lower than 200 mV s −1 , each friction layer has an obvious redox reaction peak. This is most obvious in the friction layer with the addition of PA6. After adding PVDF and BaTiO 3 , it has a certain inhibitory effect on the redox peak. I pc /I pa ≈ 1 is obtained by calculation, which indicates that the whole cycle process is close to stable. However, due to the internal resistance of each friction layer, a small amount of leakage current is generated, which is slight deviation from the ideal state. At the same time, the smooth and non-virtual CV curve also shows that each friction layer has good electrochemical stability. As the sweep speed increases, the redox peak gradually shifts until it disappears, because the scan rate is too fast to react.
Sensors 2020, 20, x FOR PEER REVIEW 5 of 14 Figure 3a-h are CV curves of different types of friction layers at a scan rate of 20-500 mV s −1 . It can be seen from each CV curve that when the sweep speed is lower than 200 mV s −1 , each friction layer has an obvious redox reaction peak. This is most obvious in the friction layer with the addition of PA6. After adding PVDF and BaTiO3, it has a certain inhibitory effect on the redox peak. Ipc/Ipa ≈ 1 is obtained by calculation, which indicates that the whole cycle process is close to stable. However, due to the internal resistance of each friction layer, a small amount of leakage current is generated, which is slight deviation from the ideal state. At the same time, the smooth and non-virtual CV curve also shows that each friction layer has good electrochemical stability. As the sweep speed increases, the redox peak gradually shifts until it disappears, because the scan rate is too fast to react.
The corresponding specific capacitance value is obtained by calculating the area of the CV curve. Figure 3i and Table 1 show that the specific capacitance of the friction layer changes greatly after the addition of PA6, PVDF and BaTiO3. Specifically, after adding PA6, the specific capacitance of the friction layer is greatly increased. At 20 mV s −1 , the specific capacitance increased from 66.225 mF cm −2 to 77.896 mF cm −2 , which is increase by 1.176 times. At a sweep speed of 500 mV s −1 , the specific capacitance increased from 28.156 mF cm −2 to 39.212 mF cm −2 , which is increase by 1.393 times. Due to the certain redox reaction and the internal ion migration rate of each friction layer, the specific capacitance cannot increase with the increase of the scanning rate as the sweep speed increases, and the capacitance value of each friction layer decreases. On this basis, the friction layer is greatly reduced in specific capacitance after the addition of BaTiO3. At 20 mV s −1 sweep speed, the specific capacitance is reduced from 77.896 mF cm −2 to 67.252 mF cm −2 , which is reduced to 0.863 times. At 500 mV s −1 sweep speed, the specific capacitance decreased from 39.212 mF cm −2 to 22.654 mF cm −2 , which is reduced to 0.578 times. After adding PVDF, the specific capacitance of the friction layer is reduced. At 20 mV s −1 , the specific capacitance decreases from 66.225 mF cm −2 to 65.509 mF cm −2 , which is reduced to 0.989 times. At a sweep speed of 500 mV s −1 , the specific capacitance decreases from 39.212 mF cm −2 to 27.078 mF cm −2 , which is reduced to 0.962 times. On this basis, after adding BaTiO3, the friction layer ratio is reduced at low sweep speed. At 20 mV s −1 , the specific capacitance decreases from 66.225 mF cm −2 to 65.293 mF cm −2 , which is reduced to 0.997 times. At 500 mV s −1 , the specific capacitance increases from 27.078 mF cm −2 to 29.488 mF cm −2 , which increases by 1.089 times. The corresponding specific capacitance value is obtained by calculating the area of the CV curve. Figure 3i and Table 1 show that the specific capacitance of the friction layer changes greatly after the addition of PA6, PVDF and BaTiO 3 . Specifically, after adding PA6, the specific capacitance of the friction layer is greatly increased. At 20 mV s −1 , the specific capacitance increased from 66.225 mF cm −2 to 77.896 mF cm −2 , which is increase by 1.176 times. At a sweep speed of 500 mV s −1 , the specific capacitance increased from 28.156 mF cm −2 to 39.212 mF cm −2 , which is increase by 1.393 times. Due to the certain redox reaction and the internal ion migration rate of each friction layer, the specific capacitance cannot increase with the increase of the scanning rate as the sweep speed increases, and the capacitance value of each friction layer decreases. On this basis, the friction layer is greatly reduced in specific capacitance after the addition of BaTiO 3 . At 20 mV s −1 sweep speed, the specific capacitance is reduced from 77.896 mF cm −2 to 67.252 mF cm −2 , which is reduced to 0.863 times. At 500 mV s −1 sweep speed, the specific capacitance decreased from 39.212 mF cm −2 to 22.654 mF cm −2 , which is reduced to 0.578 times. After adding PVDF, the specific capacitance of the friction layer is reduced. At 20 mV s −1 , the specific capacitance decreases from 66.225 mF cm −2 to 65.509 mF cm −2 , which is reduced to 0.989 times. At a sweep speed of 500 mV s −1 , the specific capacitance decreases from 39.212 mF cm −2 to 27.078 mF cm −2 , which is reduced to 0.962 times. On this basis, after adding BaTiO 3 , the friction layer ratio is reduced at low sweep speed. At 20 mV s −1 , the specific capacitance decreases from 66.225 mF cm −2 to 65.293 mF cm −2 , which is reduced to 0.997 times. At 500 mV s −1 , the specific capacitance increases from 27.078 mF cm −2 to 29.488 mF cm −2 , which increases by 1.089 times.
Since the friction layer specific capacitance is related to its internal charge capacity and ion permeability, after adding PA6, the internal area of the friction layer increases, and the charge capacity increases, which cause the specific capacitance rises. After adding PVDF and BaTiO 3 , the specific capacitance of PVDF and BaTiO 3 -based cellulose films are lower than that of cellulose composite films. On the one hand, PVDF can easily obtain electrons by friction and is used as a negative material. Although its piezoelectric performance can make it a dopant, it can improve the performance of friction generators. But because it is easy to get electrons, the specific capacitance will be lower than that of pure cellulose film. On the other hand, it is because during the electrochemical test, the sample film is not squeezed without generating a dipole moment and producing a piezoelectric effect, and then its specific capacitance is reduced.

EIS and GCD Test Analysis
Electrochemical alternating current impedance spectroscopy (EIS) is an effective method for studying electrochemical processes. The AC impedance characteristics of cellulose-based friction film materials can reflect various kinds of information such as ion diffusion and charge transfer. The equivalent circuit parameters (Figure 4a) C dl and ionic conductivity σ are calculated by Equations (2) and (3), as follows: where ω is the highest angular frequency of the arc, L is the thickness of the sample, and A is the surface area of the ionic electrolyte film. The EIS curves of different types of friction layers are conducted at 10 5 Hz-10 −2 Hz (Figure 4a). The EIS graph curve is roughly divided into three sections: high, medium and low. The three sections are associated with the parameters of Re, W 0 , R 1 and C dl in the equivalent circuit. The semi-arc of the high frequency band is remarkable, indicating that each friction layer has stable and good electrochemical characteristics, and the arcs are different in size due to different materials. The equivalent resistance R e is obtained by the intersection of the high-frequency arc area and the left side of the real axis, and R e reflects the overall internal resistance of the friction layer. When other conditions are constant, the thickness of the friction layer increases and R e increases after adding Sensors 2020, 20, 506 7 of 14 PA6, PVDF and BaTiO 3 . The mid-band is ideally a small 45 • line segment, indicating the diffusion of ions into the pore structure of the friction layer. The slope changes due to the Warburg impedance region. It can be seen from the difference of the slope of the EIS curve that the difficulty of charge transfer in each friction layer system is different, and the slope of the EIS curve of the Cel friction layer is small, indicating that it has a large impedance charge transfer capability. The slope of the EIS curve of the friction layers after adding PA6, PVDF and BaTiO 3 increased, which indicates that it has a small impedance value. W 0 combines the high frequency and low frequency to obtain the overall transmission impedance value R ct . The charge transfer resistance R ct reflects the difficulty of transferring charge (electrons and ions) into the friction layer. The size can be obtained from the high frequency band. The difficulty of transfer is inversely related to the size. The small diameter can cause a small transfer resistance, and there is a certain relationship with the frequency band in the test segment. The ideal low frequency band is a straight line parallel to the vertical axis, but due to the continuous current impact, there will be a certain leakage resistance R 1 in the driver. Because it is small, it is usually ignored. The slope of the overall curve can reflect the speed of the ion diffusion rate of the whole process, and the conductivity σ can be obtained by formula calculation.
Sensors 2020, 20, x FOR PEER REVIEW 8 of 14 since the ion migration transmission rate cannot be consistent with the current density increase amplitude. As the current density increases, the frictional capacitance of each friction layer first decreases sharply and then tends to be stable. In addition, after adding PA6, PVDF, and BaTiO3, the internal arrangement structure of each friction layer is different and lead to different permeability, so the electrochemical parameters are different at each current density. The energy density E is calculated according to Equation (5), and the result is plotted as Figure 4c. It can be found that the energy density has a similar change trend with the specific capacitance. Since this experiment uses a potential-based charge and discharge test method. By simplifying the calculation formula, it is known that the current density is the larger than the energy density, so it tends to decrease as the current density increases. Among them, at the current density of 1 A g −1 , after adding PVDF, the energy density reaches 8.85 Wh Kg −1 (up to 112.78% of the pure cellulose friction layer). It can be found that the specific capacitance and energy density decrease after adding PA6, PVDF and BaTiO3. Although the number of internal conductive particles increases, but the permeability decreases, and the ion rate of transmission also decreases at low current densities. It is manifested that the specific capacitance and energy density are reduced at this current density.

Single Open Circuit Voltage/Short Circuit Current Test
The performance of different types of friction layer and PTFE film is tested, and the extrusion frequency was 1 Hz. The open circuit output voltage and short circuit current values of different types of friction layers in the 50 s are tested experimentally. By eliminating the cluttered data and fitting the curve, the open circuit output voltage curve of a single different type of friction layer is shown in Figure 5, and the short circuit output current curve is shown in Figure 6. It can be seen that the peak output voltage of the pure cellulose friction layer is 7.925 V, and the peak value of the shortcircuit output current is 1.095 μA. After adding PA6, the output voltage increased to 14.279 V (up 180.17%) and the output current increased to 2.917 μA (up 266.30%). After adding PVDF, the output voltage increased to 15.755 V (up 198.80%) and the output current increased to 3.239 μA (up 295.63%). Due to the different electronegativity of different materials, the electrical output performance is different. The greater different in electronegativity between the two materials, the better the electrical output performance of the nano-generator. Therefore, the electrical output performance after PVDF is better than that with PA6. After adding BaTiO3 based on the addition of PA6, the output voltage was further increased to 18.798 V (up 131.65%), and the output current was increased to 5.129 μA (up 192.59%). After adding BaTiO3 to the PVDF, the output voltage is further increased to 20.155 V (up 127.93%), and the output current is increased to 6.001 μA (up 185.30%). This is because BaTiO3 is a ferroelectric compound with high dielectric constant and low dielectric loss, and has a strong charge storage capacity.
By using its high dielectric constant property, strong charge storage capacity and piezoelectric effect, its incorporation into the original friction layer can increase its dielectric constant and further improve the output performance of the nano-generator. At the same time, since the BaTiO3 crystal has no symmetry center, the charge distribution changes when the pressure is applied, the dipole moment is generated, and the piezoelectric effect is generated. The piezoelectric effect further The electric double layer capacitor C dl reflects the internal charge capacity. As can be seen from Table 2, the value increases after adding PA6 and PVDF. After the addition of BaTiO 3 , the C dl value further increases. The ionic conductivity σ is calculated by Equation (2). Combined with Table 2, it can be found that the σ value increases after adding PA6, PVDF and BaTiO 3 . After adding PA6, PVDF and BaTiO 3 , the increase of C dl and σ indicates that the charge capacity and the charge transfer rate of the cellulose layer increase. Based on strategies to develop efficient TENG through increasing dielectric constant, optimization of internal resistance, and surface modification, the internal resistance of the cellulose friction film changes, and the electrical performance of the friction generator will be improved [34]. Thus it is estimated that the electrical output performance of TENG after assembly also increases. Specific capacitance C and energy density E can be obtained by using Equations (4) and (5), where m is the mass of active substances on the electrode, I is the charging current, ∆V is the charging potential difference, and ∆t is the charging time: Sensors 2020, 20, 506 GCD curves of different types of friction layers at different current densities (1 A g −1 , 5 A g −1 , 10 A g −1 , see Figure 2). Due to the internal resistance of each friction layer, the voltage drop at different current densities is obtained by processing the GCD curve ( Figure 2). It can be seen that the voltage drop increases with the increase of current density. Among them, at low current density (<5 A g −1 ), the voltage drop of the friction layer all increases after adding PA6, PVDF and BaTiO 3 . The specific capacitance C is calculated by Equation (4), and the result is plotted as Figure 4b. It can be seen that since the ion migration transmission rate cannot be consistent with the current density increase amplitude. As the current density increases, the frictional capacitance of each friction layer first decreases sharply and then tends to be stable. In addition, after adding PA6, PVDF, and BaTiO 3 , the internal arrangement structure of each friction layer is different and lead to different permeability, so the electrochemical parameters are different at each current density. The energy density E is calculated according to Equation (5), and the result is plotted as Figure 4c. It can be found that the energy density has a similar change trend with the specific capacitance. Since this experiment uses a potential-based charge and discharge test method. By simplifying the calculation formula, it is known that the current density is the larger than the energy density, so it tends to decrease as the current density increases. Among them, at the current density of 1 A g −1 , after adding PVDF, the energy density reaches 8.85 Wh Kg −1 (up to 112.78% of the pure cellulose friction layer). It can be found that the specific capacitance and energy density decrease after adding PA6, PVDF and BaTiO 3 . Although the number of internal conductive particles increases, but the permeability decreases, and the ion rate of transmission also decreases at low current densities. It is manifested that the specific capacitance and energy density are reduced at this current density.

Single Open Circuit Voltage/Short Circuit Current Test
The performance of different types of friction layer and PTFE film is tested, and the extrusion frequency was 1 Hz. The open circuit output voltage and short circuit current values of different types of friction layers in the 50 s are tested experimentally. By eliminating the cluttered data and fitting the curve, the open circuit output voltage curve of a single different type of friction layer is shown in Figure 5, and the short circuit output current curve is shown in Figure 6. It can be seen that the peak output voltage of the pure cellulose friction layer is 7.925 V, and the peak value of the short-circuit output current is 1.095 µA. After adding PA6, the output voltage increased to 14.279 V (up 180.17%) and the output current increased to 2.917 µA (up 266.30%). After adding PVDF, the output voltage increased to 15.755 V (up 198.80%) and the output current increased to 3.239 µA (up 295.63%). Due to the different electronegativity of different materials, the electrical output performance is different. The greater different in electronegativity between the two materials, the better the electrical output performance of the nano-generator. Therefore, the electrical output performance after PVDF is better than that with PA6. After adding BaTiO 3 based on the addition of PA6, the output voltage was further increased to 18.798 V (up 131.65%), and the output current was increased to 5.129 µA (up 192.59%). After adding BaTiO 3 to the PVDF, the output voltage is further increased to 20.155 V (up 127.93%), and the output current is increased to 6.001 µA (up 185.30%). This is because BaTiO 3 is a ferroelectric compound with high dielectric constant and low dielectric loss, and has a strong charge storage capacity.
By using its high dielectric constant property, strong charge storage capacity and piezoelectric effect, its incorporation into the original friction layer can increase its dielectric constant and further improve the output performance of the nano-generator. At the same time, since the BaTiO 3 crystal has no symmetry center, the charge distribution changes when the pressure is applied, the dipole moment is generated, and the piezoelectric effect is generated. The piezoelectric effect further enhances the piezoelectric performance of the friction layer and can also improve the output performance of the generator.
Sensors 2020, 20, x FOR PEER REVIEW 9 of 14 enhances the piezoelectric performance of the friction layer and can also improve the output performance of the generator.

Multiple Combined Open Circuit Voltage/Short Circuit Current Test
For the TENG composed of different types of cellulose-based friction layers, different numbers are assembled, and the electrical output performance is tested after rectification and compared with a single TENG output parameter. The open-circuit output voltage and short-circuit current value of the friction layer TENG based on Cel, Cel + PVDF + BaTiO3 in 50 s are tested. The cluttered data is eliminated and the curve is fitted. The output voltage curves of different combinations are shown in Figure 7, short circuit output current curve is shown in Figure 8. It can be seen that the peak output voltage of the pure cellulose friction layer is 10.329 V (up 130.34%) and the peak value of the shortcircuit output current is 1.227 μA (up 112.04%) compared to the single TENG output parameter. The output voltage of the friction layer based on Cel + PVDF + BaTiO3 is 25.872 V (up 128.37%), and the peak output current is 6.623 μA (110.37% increase). Sensors 2020, 20, x FOR PEER REVIEW 9 of 14 enhances the piezoelectric performance of the friction layer and can also improve the output performance of the generator.

Multiple Combined Open Circuit Voltage/Short Circuit Current Test
For the TENG composed of different types of cellulose-based friction layers, different numbers are assembled, and the electrical output performance is tested after rectification and compared with a single TENG output parameter. The open-circuit output voltage and short-circuit current value of the friction layer TENG based on Cel, Cel + PVDF + BaTiO3 in 50 s are tested. The cluttered data is eliminated and the curve is fitted. The output voltage curves of different combinations are shown in Figure 7, short circuit output current curve is shown in Figure 8. It can be seen that the peak output voltage of the pure cellulose friction layer is 10.329 V (up 130.34%) and the peak value of the shortcircuit output current is 1.227 μA (up 112.04%) compared to the single TENG output parameter. The output voltage of the friction layer based on Cel + PVDF + BaTiO3 is 25.872 V (up 128.37%), and the peak output current is 6.623 μA (110.37% increase).

Multiple Combined Open Circuit Voltage/Short Circuit Current Test
For the TENG composed of different types of cellulose-based friction layers, different numbers are assembled, and the electrical output performance is tested after rectification and compared with a single TENG output parameter. The open-circuit output voltage and short-circuit current value of the friction layer TENG based on Cel, Cel + PVDF + BaTiO 3 in 50 s are tested. The cluttered data is eliminated and the curve is fitted. The output voltage curves of different combinations are shown in Figure 7, short circuit output current curve is shown in Figure 8. It can be seen that the peak output voltage of the pure cellulose friction layer is 10.329 V (up 130.34%) and the peak value of the short-circuit output current is 1.227 µA (up 112.04%) compared to the single TENG output parameter. The output voltage of the friction layer based on Cel + PVDF + BaTiO 3 is 25.872 V (up 128.37%), and the peak output current is 6.623 µA (110.37% increase).  According to the experimental results, compared with a single TENG, the open circuit output voltage is increased by about 1.273-1.321 times after assembly, and the short-circuit output current peak is increased by about 1.110 times. This is due to the large friction layer resistance and the device's own loss in rectifying process, resulting in the two after assembly open circuit output voltage and short circuit output current increase is small. After four assemblies, the output voltage of the open circuit based on the pure cellulose friction layer is 5.185 V (up 49.80% increase), and the peak value of the short-circuit output current is 0.767 μA (37.51% reduction). The output voltage of the friction layer based on Cel + PVDF + BaTiO3 is 12.755 V (50.71% reduction), and the peak output current is 3.947 μA (40.42% reduction). According to the experimental results, compared with a single TENG, the open circuit output voltage is reduced by 0.498-0.507 time, and the short-circuit output current peak is reduced by 0.375-0.405 time. After four assemblies, the open circuit output voltage and the short-circuit output current are greatly reduced mainly due to two aspects, namely, the power loss of the rectifying device and the asynchronous power of each TENG output. Subsequently, improving the output performance of the generator starts from reducing the power consumption of the rectifier  According to the experimental results, compared with a single TENG, the open circuit output voltage is increased by about 1.273-1.321 times after assembly, and the short-circuit output current peak is increased by about 1.110 times. This is due to the large friction layer resistance and the device's own loss in rectifying process, resulting in the two after assembly open circuit output voltage and short circuit output current increase is small. After four assemblies, the output voltage of the open circuit based on the pure cellulose friction layer is 5.185 V (up 49.80% increase), and the peak value of the short-circuit output current is 0.767 μA (37.51% reduction). The output voltage of the friction layer based on Cel + PVDF + BaTiO3 is 12.755 V (50.71% reduction), and the peak output current is 3.947 μA (40.42% reduction). According to the experimental results, compared with a single TENG, the open circuit output voltage is reduced by 0.498-0.507 time, and the short-circuit output current peak is reduced by 0.375-0.405 time. After four assemblies, the open circuit output voltage and the short-circuit output current are greatly reduced mainly due to two aspects, namely, the power loss of the rectifying device and the asynchronous power of each TENG output. Subsequently, improving the output performance of the generator starts from reducing the power consumption of the rectifier According to the experimental results, compared with a single TENG, the open circuit output voltage is increased by about 1.273-1.321 times after assembly, and the short-circuit output current peak is increased by about 1.110 times. This is due to the large friction layer resistance and the device's own loss in rectifying process, resulting in the two after assembly open circuit output voltage and short circuit output current increase is small. After four assemblies, the output voltage of the open circuit based on the pure cellulose friction layer is 5.185 V (up 49.80% increase), and the peak value of the short-circuit output current is 0.767 µA (37.51% reduction). The output voltage of the friction layer based on Cel + PVDF + BaTiO 3 is 12.755 V (50.71% reduction), and the peak output current is 3.947 µA (40.42% reduction). According to the experimental results, compared with a single TENG, the open circuit output voltage is reduced by 0.498-0.507 time, and the short-circuit output current peak is reduced by 0.375-0.405 time. After four assemblies, the open circuit output voltage and the short-circuit output current are greatly reduced mainly due to two aspects, namely, the power loss of the rectifying device and the asynchronous power of each TENG output. Subsequently, improving the output performance of the generator starts from reducing the power consumption of the rectifier bridge and ensures the phase synchronization of the integrated TENG output, which reduces the internal resistance of the friction layer.

Charge and Discharge Test
Different number of cellulose based nano-generators is used to supply power to the 47 µF capacitor, and the charge and discharge performance in 1000 s was tested. The first 500 s is used for power supply, and the power-down characteristics are observed after 500 s to facilitate the design of the subsequent energy storage circuit. Figure 9a,b,c are based on the Cel, Cel + PVDF + BaTiO 3 friction layer, assembled single, two, four TENG charge and discharge patterns. It can be seen that the charging peak of a single Cel-based friction layer generator in the 500 s reaches 1.965 V, and the power is reduced to 1.672 V, and the power failure rate is 14.91%. The charging peak value of the Cel + PVDF + BaTiO 3 friction layer generator reaches 3.114 V, and the power is reduced to 2.628 V, and the power failure rate is 15.61%. The charging peak of two Cel-based friction layer generators reaches 2.554 V, and the power is reduced to 2.177 V, and the power-down rate is 14.76%. The charging peak value of the Cel + PVDF + BaTiO 3 friction layer generator reaches 3.840 V, and the power is reduced to 3.254 V, and the power failure rate is 15.26%. The charging peak of four Cel-based friction layer generators reaches 1.291 V, and the power is reduced to 1.098 V, and the power-down rate is 14.69 %. The charging peak value of the Cel + PVDF + BaTiO 3 friction layer generator reaches 1.903 V, and the power is reduced to 1.612 V, and the power failure rate is 15.18%. According to the test results, compared with the single TENG charge and discharge, the two TENG charging rates and peaks are improved at the same time. The charging peaks based on the Cel, Cel + PVDF + BaTiO 3 friction layer increases to 129.96% and 123.33%, respectively. Four TENG charging rates and peak values are significantly reduced by 49.46% and 50.45%, respectively. This corresponds to the open circuit output voltage after assembly in the previous test. In addition, for TENG consisting of the same friction layer, the power-down rate of TENG after twice and four times charge is basically the same. Among them, the combined power-down rate has decreased, indicating the stability of multiple combined TENG. The offset rate also ensures subsequent powering of the MCS-52 microcontroller, which in turn drives the SFM-27 buzzer alarm and illuminates the 1602 LCD.
Sensors 2020, 20, x FOR PEER REVIEW 11 of 14 bridge and ensures the phase synchronization of the integrated TENG output, which reduces the internal resistance of the friction layer.

Charge and Discharge Test
Different number of cellulose based nano-generators is used to supply power to the 47 μF capacitor, and the charge and discharge performance in 1000 s was tested. The first 500 s is used for power supply, and the power-down characteristics are observed after 500 s to facilitate the design of the subsequent energy storage circuit. Figure 9 a,b,c are based on the Cel, Cel + PVDF + BaTiO3 friction layer, assembled single, two, four TENG charge and discharge patterns. It can be seen that the charging peak of a single Cel-based friction layer generator in the 500 s reaches 1.965 V, and the power is reduced to 1.672 V, and the power failure rate is 14.91%. The charging peak value of the Cel + PVDF + BaTiO3 friction layer generator reaches 3.114 V, and the power is reduced to 2.628 V, and the power failure rate is 15.61%. The charging peak of two Cel-based friction layer generators reaches 2.554 V, and the power is reduced to 2.177 V, and the power-down rate is 14.76%. The charging peak value of the Cel + PVDF + BaTiO3 friction layer generator reaches 3.840 V, and the power is reduced to 3.254 V, and the power failure rate is 15.26%. The charging peak of four Cel-based friction layer generators reaches 1.291 V, and the power is reduced to 1.098 V, and the power-down rate is 14.69 %. The charging peak value of the Cel + PVDF + BaTiO3 friction layer generator reaches 1.903 V, and the power is reduced to 1.612 V, and the power failure rate is 15.18%. According to the test results, compared with the single TENG charge and discharge, the two TENG charging rates and peaks are improved at the same time. The charging peaks based on the Cel, Cel + PVDF + BaTiO3 friction layer increases to 129.96% and 123.33%, respectively. Four TENG charging rates and peak values are significantly reduced by 49.46% and 50.45%, respectively. This corresponds to the open circuit output voltage after assembly in the previous test. In addition, for TENG consisting of the same friction layer, the power-down rate of TENG after twice and four times charge is basically the same. Among them, the combined powerdown rate has decreased, indicating the stability of multiple combined TENG. The offset rate also ensures subsequent powering of the MCS-52 microcontroller, which in turn drives the SFM-27 buzzer alarm and illuminates the 1602 LCD.

Conclusions
In summary, this paper proposes an environmentally-friendly triboelectric-piezoelectric nanogenerator based on degradable cellulose, which utilizes cellulose that can be extracted from many natural vegetation sources and organisms. By utilizing the characteristics of cellulose-IL dissolution regeneration, regenerated cellulose film is obtained as a friction and piezo-layer. Then, it is assembled with a PTFE film to form TENG that generates electricity by extrusion friction, and tested for electrical output properties. The results show that the peak value of open circuit output voltage increases by 2.544 times to 20.155 V, and the peak value of short circuit output current increases by 5.480 times to 6.001 μA. After two assemblies, the open-circuit output voltage is increased to a maximum value by 1.321 times, and the short-circuit output current peak is increased to a maximum value by 1.161 times. After four assemblies, the open circuit output voltage and short circuit output current are greatly

Conclusions
In summary, this paper proposes an environmentally-friendly triboelectric-piezoelectric nano-generator based on degradable cellulose, which utilizes cellulose that can be extracted from many natural vegetation sources and organisms. By utilizing the characteristics of cellulose-IL dissolution regeneration, regenerated cellulose film is obtained as a friction and piezo-layer. Then, it is assembled with a PTFE film to form TENG that generates electricity by extrusion friction, and tested for electrical output properties. The results show that the peak value of open circuit output voltage increases by 2.544 times to 20.155 V, and the peak value of short circuit output current increases by 5.480 times to 6.001 µA. After two assemblies, the open-circuit output voltage is increased to a maximum value by 1.321 times, and the short-circuit output current peak is increased to a maximum value by 1.161 times. After four assemblies, the open circuit output voltage and short circuit output current are greatly reduced. This is due to the large internal resistance of the friction layer and the loss of the device itself during the rectification process and the phase frequency of the TENG output are not synchronized, resulting in a small increase in the output voltage and short-circuit output current after assembly. Within 500 s, the charge peak of single charge, two reaches and four charge reaches 3.114 V, 3.840 V, and 1.903 V, respectively. This ensures long-term storage of electrical energy and subsequent supply of power to the electronics. The test shows that the proposed cellulose-based nano-generator device has good stability and reliability, and can be widely used in the field of energy harvesting conversion storage, which greatly promotes the application and development of natural biomaterials in the field of TENG and other electronic sensing devices.