2.4. Energy-harvesting performance of lignin/polycaprolactone nanofiber-based triboelectric nanogenerator
Figures 4a–4d present the output voltage (V ) signals of the LP-TENGs composed of the KLP, HILP, and HOLP NF mats under different tapping conditions of applied force (F ) and frequency (f ). Teflon tape was selected as the tribonegative material because of its low surface energy. The KLP- and HILP-TENGs exhibited increases inV signals with increasing F (Figures 4a and4b ). On the other hand, no discernible effect on Vsignals was observed for the HOLP-TENG. The maximum output positive voltage (V max) of the HILP-TENG increased from 22 to 60 V and further to 96 V as the F increased from 3 to 6 and 9 N, respectively. However, those of the HOLP-TENG were just 12, 16, and 15 V when the F were 3, 6, and 9 N, respectively. Similarly, theV signals of all LP-TENGs increased as the f increased from 3 to 9 Hz (Figures 4c and 4d ). This is because of a decrease in contact time (t ), corresponding to an increase in the tapping speed, thus leading to an increase in V (= RI= Rq /t , where R is an external load and q is the surface charge).61 Note that this behavior can be explained because the LP-TENGs were evaluated at the short-circuit condition.
The superior energy-harvesting performance of the HILP-TENG was attributed to an increase in electron exchange between tribo-positive and -negative materials because of the increased distinction in their electrostatic potential. When tribo-positive and -negative materials with different electrostatic potential come into contact, electrons are hopped from the high-electrostatic-potential material to the low-electrostatic-potential one.8, 62 Because of the electron transfer, a potential difference between two dissimilar materials is generated. The higher surface electrostatic potential of tribopositive HILP NF mat led to a further increase in the surface electrostatic potential difference against the tribonegative Teflon film, thus resulting in a greater potential difference, thereby exhibiting higher V signals.
Cyclic tests by repetitive tapping on the KLP-, HILP-, and HOLP-TENGs were conducted (Figure 4e ). 100,000 tapping cycles were performed with fixed F of 9 N and f of 9 Hz. The KLP-TENG exhibited a slight increase in V max from V= ~ 15 to ~ 30 V during the test while the HILP-TENG showed a significant increase inV max from V = ~ 30 to ~ 80 V (Figure S4a ). This gradual increase inV over hundreds of thousands of cycles originates from the unique morphological structure of the nonwoven NF mat, as discussed in other previously-reported studies.63 The NFs are continuously compressed during the repetitive tapping, which contributes to an increase in the actual contact area between NFs, thus facilitating the gradual rise in V.
The energy-harvesting efficiency η , defined as η =V max F ‒1S ‒1, where S is the projected contact surface area,10 of the HILP-TENGs and previously-reported tribopositive biopolymer-based TENGs are compared inFigure 4f and Table 2 . Note that other tribopositive biopolymer-based TENGs that did not provide specific V max,F , or S values are not described here. The HILP-TENG in this study exhibited an outstanding η value of 4133.33 V N‒1 m‒2 despite a relatively smallS (see the red star in Figure 4f ), demonstrating its great potential as an eco-friendly and sustainable high performance energy harvester.
Figure 5 shows the industrial potential of HILP-TENG as a kinetic energy harvester. The output performance of the HILP-TENG was evaluated by exploring the variations in the peak-to-peak voltage (V ptop) and current (I ptop) as varying the external load (R ) from 105 to 109 Ω (Figures 5a and 5b ). Repetitive tapping was conducted under tapping conditions of F = 9 N and f = 9 Hz. As the Rincreased, the V ptop increased up to 75 V while the I ptop decreased to 0.3 μA, which aligns with the principles outlined in Ohm’s law. The resultant maximum output power (P ) and output power density (PS ‒1) atR = 2×107 Ω, were 392 μW and 157 mW m‒2, respectively.
Figures 5c and S4b depict the current (I ) signals obtained from HILP- and HOLP-TENGs under tapping conditions ofF = 9 N and f = 9 Hz, allowing to quantify of the surface charge accumulation occurring in the HILP- and HOLP-TENGs. For the HILP-TENG, the magnitudes of the positive and negative surface charges during a single tapping (q pos andq neg, respectively) were almost the same with showing q pos = 2.42×10–8 C andq neg = – 2.42×10–8 C. The similarity in the magnitudes of q pos andq neg indicates that the electrons engaged in the pressing motion were equally involved in the subsequent release motion during repetitive tapping of the HILP-TENG. For the HOLP-TENG, it exhibited lower magnitudes in both q pos andq neg values measured to be 1.93×10–8 and 1.92×10–8 C, respectively.
To further investigate the effect of the surface charge of HILP and HOLP on the surface potential distribution of the different triboelectric pairs, we also performed COMSOL Multiphysics simulations (Figure 5d ). The surface charge density (σ 0) was calculated as:
, (1)
where q and S are the charge resulting from the accumulation of current (Figures 5c and S4b ) and the size of the TENG, respectively. The transferred charge density (σ ’) upon the contact of the tribo-positive and -negative materials can be expressed as a function of σ 0as:64
, (2)
where d gap, d PTFE, andε PTFE are the gap distance, thickness, and dielectric constant of the PTFE (Teflon) film, respectively. Then, the electric potential (V p) generated by the TENG can be obtained as:
, (3)
where ε is the vacuum permittivity (8.85 × 10-12 F m‒1). As presented in Eq. (3) , V p is proportional toq . Based on the results obtained from COMSOL with the presented parameters (Table S2 ), it can be predicted that the HILP-TENGs demonstrate a higher potential difference between the top and bottom layers than the HOLP-TENGs, which agrees well with the experimental results.
The utilization of a full-wave bridge rectifier circuit along with capacitors enabled the accumulation of output voltage (Figure 5e ). Given tapping conditions of F = 9 N and f = 9 Hz, the electrical energy could be effectively stored in various capacitors having capacitances of 0.22, 2 and 22 μF. The 0.22, 2, and 22 μF capacitors were charged to 0.28, 0.25 and 0.01 V, respectively. The HILP-TENGs also demonstrated the capability of running 34 individual LEDs by repetitive tapping (Figure 5F and Movie S1 ), showing the potential of the HILP-TENG for self-powered electronic devices.
To demonstrate the development of TENGs without petroleum-based materials, we also fabricated the HILP-HOLP-TENG. In contrast to the HILP-TENG, the HILP-HOLP-TENG employed the HOLP NF mat as a tribonegative material instead of using Teflon film (cf. Figures 1 and S1a ). Then, its energy-harvesting performance was evaluated (Figures S4c , 5G , and Movie S2 ). For the cyclic tapping test with tapping conditions of F = 9 N and f = 9 Hz, the V max gradually increased from 15 to 35 V (Figure S4c ), showing a reasonable energy-harvesting performance even without using Teflon film. Furthermore, we also demonstrated that when a user engages in activities such as walking or running, the HIILP-HOLP-TENG can produce Vsignals. This remarkable capability of the HIILP-HOLP-TENG demonstrates their potential for practical energy generation from everyday human motion.