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simplification and clear illustration. At the aligned position, the two parts of the sliding pair completely match because of the same central angle. Upon con- tact between the PTFE film and the metal strips, charge transfer takes place. Due to the difference on tribo- electric polarities, electrons are injected from metal into the surface of PTFE. Since the two sliding surfaces are completely aligned, triboelectric charges with op- posite polarities are fully balanced out, making no electron flow in the external circuit (Figure 11bI). Once a relative sliding occurs as a result of rotation, tribo- electric charges on the mismatched areas cannot be compensated. The negative ones on the PTFE will drive free electrons on the back electrode to the sliding electrode through the external circuit due to electro- static induction, neutralizing positive triboelectric charges on the sliding electrode and leaving positively induced charges behind on the back electrode (Figure 11bII). The flow of the induced electrons lasts until the mismatch between the two sliding surfaces reaches the maximum when the positive triboelectric charges are fully screened by induced electrons (Figure 11bIII). As the relative rotation continues, the PTFE film will come into contact with an adjacent metal strip (Figure 11bIV). Thus, the induced electrons will flow back in an opposite direction until the fully aligned position is restored (Figure 11bI). Therefore, in a cycle of electricity generation, AC electric output is generated. Since all of the sliding units are electrically connected in parallel, output current from different units are synchronized to constructively add up. To characterize the electric output, Isc and Voc of a rotating TENG with 6 strip units were measured at a rotation rate of 1000 r/min, corresponding to an equivalent linear rotational velocity of 1.33 m/s. An approximately linear relationship between the linear rotational velocity and the current amplitude can be derived from the results shown in Figure 11c. The linear relationship between the velocity and output current can be explained by the increasing frequency. The Voc stays stable at different rotation rate because the voltage is only a function of percentage of mismatch as shown in Figure 11d. For output charge, a higher current frequency produces a larger amount of accu- mulative charges within the same period of time, resulting in a larger equivalent direct current. As shown in Figure 11e, the equivalent direct current increases from 3.7 to 38.9 μA when the linear rotational velocity increases from 0.13 to 1.33 m/s. Single-Electrode-Based TENG. The TENGs presented in the last few sections must have two electrodes in order to form a closed circuit for the electrons to flow. Such a configuration is possible for a working alone system, but it introduces difficulty in engineering de- sign for specific applications such as harvesting energy from a rotating tire. In this section, we introduce a single-electrode-based TENG that is more practical and feasible design for some applications such as finger-tip- driven TENG.2932 To obtain a more quantitative understanding about the TENG, we employed finite element simulation to calculate the electric potential distribution in the TENG and the charge transfer be- tween the ITO electrode and ground using COMSOL. The constructed model is based on a skin patch and a PDMS film on the ITO electrode with the dimensions of 45 mm 65 mm and thickness of 1 mm for skin, 0.5 mm for PDMS film, 1 mm for ITO electrode, as depicted in Figure 12a. The ITO electrode was con- nected with the ground. The triboelectric charge den- sities on the skin and PDMS film were assumed to be þ10 and 10 μC/m2, respectively. Figure 12b shows the calculated results of the electric potential distribu- tion in the TENG under the different separation dis- tances of 0.5, 10, 30, and 60 mm. When the skin and PDMS fully contact with each other, the electric poten- tials on both the skin and PDMS approach zero. The electric potential difference was found to increase dra- matically with increasing separation distance. When they are separated by 60 mm, the electric potential on the skin surface reaches 1.2 104 V, while the electric potential on the PDMS is still close to zero, which is associated with contact with the ITO electrode/ground. As illustrated in Figure 12c, the amount of the total charges on the ITO electrode was plotted as a function of separation distance between the skin and PDMS. It increases with increasing separation distance, reveal- ing that charge transfer decreases with increasing distance. These results are consistent with the mea- sured change in charge quantity when the contacted skin was removed from the PDMS film, as shown in Figure 12d. The working principle of the fabricated TENG is schematically shown in Figure 12e by the coupling of contact electrification and electrostatic induction. In the original position, the surfaces of skin and PDMS fully contact with each other, resulting in charge trans- fer between them. According to the triboelectric series, electrons were injected from the skin to the PDMS since the PDMS is more triboelectrically negative than skin, which is the contact electrification process. The produced triboelectric charges with opposite polarities are fully balanced/screened, leading to no electron flow in the external circuit. Once a relative separation between PDMS and skin occurs, these triboelectric charges cannot be compensated. The negative charges on the surface of the PDMS can induce positive charges on the ITO electrode, driving free electrons to flow from the ITO electrode to ground. This electrostatic induc- tion process can give an output voltage/current signal if the distance separating the touching skin and the bottom PDMS is appreciably comparable to the size of the PDMS film. When negative triboelectric charges on the PDMS are fully screened from the induced positive charges on the ITO electrode by increasing the WANG VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX O www.acsnano.org REVIEWPDF Image | Triboelectric Nanogenerators as New Energy Technology
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