3.1. Microstructural characterization of hybrid PDA/CNF films
Figure 1(a) schematically illustrates the preparation of PDA/CNF composite membranes by PDA self-polymerization. Dopamine (DA) polymerizes and self-assembles under weakly alkaline conditions and can be adsorbed on the material surface by the synergistic action of covalent and non-covalent bonds (Cheng et al., 2019). Based on this principle, we used PDA to modify CNFs to prepare composite membrane materials. First, a 2 mmol/L DA Tris solution was prepared at pH = 8.5. The PDA/CNF precursor solution was prepared by mixing the CNF and DA solutions under magnetic stirring. Afterwards, the obtained PDA/CNF solution was washed and suction filtered to form a membrane, which was dried for further use (detailed synthesis process and TENG fabrication are provided in the experimental section). Figures 1(b)-(d) show the TEM images of CNFs before and after modification. Figure 1(e) shows the SEM image of the modified CNF composite membrane.
To verify the effect of PDA on the CNFs, FT-IR, XRD, and XPS were used for functional group characterization and element mapping of the films. As shown in Fig. 2(a), the PDA/CNF retained the characteristic absorption peak of CNF. The wide absorption peak at 3388 cm− 1 is attributed to the O-H stretching vibration in CNFs, the absorption peak at 2896 cm− 1 belongs to the C-H stretching vibration of -CH2-, and the 890 cm− 1 β-characteristic absorption peak corresponds to the glycosidic bond (Kaynak et al., 2018). Compared with the CNF films, new absorption peaks appeared in the PDA/CNF films at 1630 cm− 1 and 1524 cm− 1, corresponding to the C = O-NH and N-H bending vibration peaks, respectively. It was confirmed that PDA successfully modified the CNFs, and amination reactions also occurred between the amino and hydroxyl groups of the CNFs in PDA.
Figure 2(b) shows the XRD diffractograms of the CNFs before and after PDA modification. The peaks at 2θ = 16.3° and 22.35° are diffraction peaks of cellulose I, indicating that the CNF before and after modification still retains the crystalline structure of cellulose. The relative crystallinity of cellulose was calculated according to the Segal formula. The crystallinity of pure cellulose is 72.07%, and that of PDA/CNF is 70.49%, showing slight degradation. PDA is an amorphous polymer that contains a large number of hydroxyl groups. Some hydroxyl groups on the surface of CNFs are replaced by PDA, which reduces the total crystallinity.
XPS was used to analyze the composition and chemical structures of the samples. The peaks of the unmodified CNFs, shown in Fig. 2(c), appear at 531.20 and 285.62 eV, corresponding to O1s and C1s in the sample, respectively (Wang et al., 2016). After modification, the CNFs show a new peak at 399.85 eV, corresponding to N1s (Nie et al., 2020). These results were further confirmed by the high-resolution carbon spectra of carbon (C1s) and nitrogen (N1s) and the distribution of chemical bonds. Figure 2(d) and (e) showed four carbon bonds of the CNF films, namely C-C/C-H (C1, 284.42 eV), C-O-C/C-O-OH (C2, 285.80 eV), O-C-O (C3, 286.45 eV), and O = C-O (C4, 287.83 eV). Compared to that in the CNF film, the proportion of carbon bonds in the PDA/CNF film changed, and the proportion of O-C-O bonds increased from 25.90–37.52%, which is attributed to the dehydration condensation of hydroxyl groups of CNFs and the phenolic hydroxyl group of PDA connected by O-C-O bonds. Consistent with the conclusions reported in the literature, catechol in PDA tends to react with hydroxyl groups to cause dehydration and generate charge transfer. The increase in the number of O-C-O bonds proved successful chemical grafting between the CNFs and PDA. In addition, as shown in the high-resolution nitrogen spectrum of N1s (Fig. 2(f)), new peaks appeared at 398.58 and 399.50 eV, corresponding to primary and secondary amines, respectively. These results strongly confirm the successful modification of the CNFs.
Table 1
Proportions of Elements on the CNF and PDAA-CNF Films
Sample | C1s(%) | O1s(%) | N1s(%) | O/C(%) |
CNFs | 64.79 | 35.21 | 0 | 54.34 |
PDA-CNFs | 62.18 | 33.60 | 4.22 | 54.37 |
Figures 2(g)-(i) show the surface roughness of the CNF films before and after modification. The roughness of the PDA/CNF films increased significantly, corresponding to the structure of the composite film in Fig. 1(e), and the introduction of micro/nanostructures increased the effective contact area of the films. Fan et al. (Fan et al., 2012) confirmed the enhancement of the nano/microstructure on the electrical output performance of TENGs. They believed that the enhancement of structured films is due to the larger effective friction area of the surface with complex structures, which can generate more surface charges during friction.
3.2. Electrical characterization of hybrid PDA/CNF films based TENGs
As a typical contact-separation TENG, the working principle of PDA/CNF-TENG is the coupling of electrification and electrostatic induction. In TENG operation, the potential difference between the material surfaces determines the triboelectric charge density of the materials (Diaz&Felix-Navarro, 2004). Because the potential of organic substances mainly depends on the electronic affinity of their functional groups, chemical functionalization is an important way to adjust the potential of the material surface. The amino group, as a strong electron-donating functional group has proven to be an effective and direct method to improve the triboelectric properties of materials(Nie et al., 2020; Wang et al., 2016; Zhang et al., 2019). As shown in Fig. 3(b), when the two triboelectric layers of the PDA/CNF and FEP films were not in contact, there was no charge on the electrodes surface. When the two triboelectric layers were in contact with each other, the surface of the PDA/CNF film induced a positive electrostatic charge, whereas the FEP film induced a negative electrostatic charge. Because FEP has strong electron capture ability and NH functional group has strong positive induction, PDA / CNF film provides electrons to FEP during contact, as shown in Fig. 3(a). Once the two triboelectric layers are separated, a potential difference appears between them. Electrons are driven from the upper electrode to the lower electrode to balance the potential difference. When the two layers reach a certain distance, the electrons stop flowing and reach an equilibrium. Subsequently, the PDA/CNF and FEP films squeeze each other, resulting in electrons returning from the lower electrode to the upper, thus balancing the electric field. The above steps constitute the motion cycle.
The triboelectric output performance of TENG mainly depends on the friction material. To determine the best concentration and thickness of PDA/CNF film obtain the best output performance, a series of PDA/CNF films with different concentration (wt.% = 1.0%-5.0%) and thickness (0.05-0.08mm) were prepared. Using FEP as the negative triboelectric layer, the PDA/CNF film (4.0 cm × 4.0 cm) was tested at a working frequency of 2 Hz. As shown in Figs. 3(c)-(d), the output values for open-circuit voltage and short-circuit current of the pure CNF-TENG were 64V and 2.5µA, respectively. The triboelectric properties of PDA/CNF-TENG were significantly improved, and the maximum output values for Voc and Isc of 4.0% PDA/CNF reached 185 V (a 189% increase) and 15.7 µA (a 528% increase), respectively. By studying the influence of the different contents of PDA from 0 wt% to 2.0 wt% on the electrical performance, it was found that Voc of the PDA/CNF increased significantly from 95 to 185 V, and Isc increased from 8.0 to 15.7 µA. As shown in the Fig. 3(d), the variation trends of Isc and Qsc are similar to that of Voc. The improvement in the output performance is attributed to the increase in the number of amino groups and surface roughness caused by the increase in the amount of PDA, which increased the triboelectric polarity and effective contact area of the PDA/CNF. However, as the amount of PDA continues to increase, the triboelectric properties of PDA/CNF-TENG decreased.
In addition to the concentration of PDA, the thickness of PDA/ CNF film also has influence on the output performance of TENG. Figures 3(f)-(g) shows that when the film thickness increased from 0.05 to 0.07 mm, Voc, Isc and Qsc were relatively stable; However, when the film thickness increased to 0.08 mm, the output values decreased slightly. When the film thickness was 0.06 mm, PDA/CNF-TENG reached the output peak. When the thickness increased, the surface charge of PDA/ CNF film increased, and the output performance increased with the increase of film thickness. With the further increase of film thickness, the performance decreases, which may be the result of the weakening of electrostatic induction between the friction layer and the conductive electrode (Han et al., 2020b).
As a promising power source for electronic devices, it is necessary to evaluate the triboelectric properties of PDA/CNF-TENG under different working conditions. The working conditions changes in operating frequency, force and relative humidity. As shown in Figs. 4(a)-(c), when the working frequency of the relative contact separation movement between the two electrodes increased from 1 to 2.5 Hz, Voc increased from 166 to 205 V, Isc increased from 7.6 to 20.0 µA, and Qsc increased from 4.1 to 5.2 nC·cm− 2. The output performance of TENG was positively correlated with the operating frequency. This is because at different operating frequencies, the charge transfer rate increases at higher frequencies, which makes the external electrons reach the flow balance faster, shortens the duration of the current peak, and leads to an increase in short-circuit current. As shown in Figs. 4(d)-(f), when the working force between the two electrodes increased from 10 to 50 N, Voc increased from 99 to 156 V, the variation trends of the Isc, Qsc are similar to the Voc. This is because PDA endows the CNFs with rough nanostructures on the surface. When the working force is small, the rough surface prevents close contact with the FEP film. With an increase in the working force, the PDA/CNF film deforms slightly and fills the gap with the FEP film, increasing the contact area, thus improving the output performance of the TENG (He et al., 2018; Mi et al., 2018) .
As shown in Figs. 4(g)-(i), with the increase of relative humidity, the output performance of PDA / CNF-TENG decreased in varying degrees. When the relative humidity increases from 60–90%, the Voc of TENG decreased from 180V to 50V, and the variation trend of Isc and Qsc were similar to that of voltage. PDA / CNF contain hydrophilic OH groups. With the increase of relative humidity, water molecules will be physically adsorbed on hydroxyl groups through single hydrogen bonds, forming a barrier layer on the surface of the material that hinders the formation of induced charges (Bi et al., 2013), which leads to a sharp decline in the output performance of PDA / CNF-TENG at high humidity.
To investigate the relationship between the output performance and load resistance, impedance matching was performed at a load resistance of 1000 Ω to 80 GΩ. When the PDA/CNF-TENG was connected to an external load, the open-circuit voltage increased with an increase in load resistance, and the short-circuit current decreased with an increase in load resistance (Fig. 5(a)). The output power density of the equipment is calculated using the following formula:
where P is the power, U is the output voltage, A is contact area and R is the external load resistance.
We then plotted the dependence of power density on the external load (Fig. 5(b)). The maximum power density (56.25 µW·cm− 2) was achieved with a load resistance of 80 MΩ, which is sufficient to power many portable or wearable electronic devices. Compared with other CNF-based TENGs, the PDA/CNF-TENG exhibited a good output performance. A comparison of the output performance of the CNF-based TENG is presented in Table 2 and Fig. 5(c). TENGs can collect energy from various types of mechanical movement. As a demonstration, our equipment was used to supply power to LEDs, and 158 LED bulbs were successfully lit (Fig. 5(e)). As shown in Fig. 5(f), the equipment showed good cycling stability and maintained approximately 99% of its initial value after 10000 consecutive cycles.
Table 2
Output performance Comparison of CNF-based TENGs
Positive materials | Negative materials | Voltage (V) | Current(µA) | Power density (µW/cm2) | References |
CNF-phosphorene | PET | 5.2 | 1.8 | 1.06 | (Cui et al., 2017) |
PA | PFOTES-CNF | 28.5 | 9.3 | 1.35 | (Nie et al., 2021) |
Alc-S5-CNF | PVDF | 7.9 | 5.13 | 18.2 | (Roy et al., 2020) |
CNF-SIO2 | PET | 58.2 | 6.3 | 60 | (Mi et al., 2018) |
AEAPDMS-CNF | FEP | 155 | 17.5 | 22 | (Nie et al., 2020) |
CNF-PEI/Ag | FEP | 286.5 | 4.0 | 43 | (Zhang et al., 2019) |
CMF-CNF | FEP | 21.9 | 0.73 | 7.68 | (He et al., 2018) |
PDA-CNF | FEP | 205 | 20 | 56.25 | This study |
3.3. Application for human motion detection
Various human motions are detected by attaching PDA/CNF-TENG to different body parts for biomechanical motion energy harvesting in a self-powered manner. Figures 6(a)-(c) show the output voltage versus time for walking, skipping, and running when the TENG was attached to the sole of the foot. The output voltage shows a regular rhythm when walking and jumping, because when the foot touches the ground and lifts the foot, the two friction layers are contacted and separated, forming a regular output voltage signal. Voltage sinks during running are stable, repetitive signals. It is worth noting that the voltage response is synchronized with low frequency and high frequency, and the output voltage is positively correlated with the pressure. The output voltage of 100V during jumping is significantly greater than the output voltage of walking, indicating that the material has good pressure response characteristics. Figures 6(d)-(f) show the electrical signals of attaching TENG to the hand to monitor clapping, finger clicking, palm grip and releasing. It can be seen from Fig. 6 that the output voltages of clapping, clicking and griping are about 80 V, 70 V and 130 V. The electrical signals induced by clapping are not stable, while pressing and fisting present regular electrical signals. Specifically, the output voltage also exhibits stable, repeatable, and recoverable signals when monitoring various human motions, indicating great potential in wearable electronics.