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number of ions could be the charge source. Typically, the triboelectric charge density is not able to reach the level of maximum surface charge density (MSCD), which is deter- mined by the breakdown electric field of air, due to the lim- ited potential differences between two conventional fric- tion surfaces. This means most polymer surfaces can han- dle extra charges besides the portion acquired from tri- boelectrification. One intuitive sense to introducing addi- tional charges is directly injecting charged ions into de- sired surfaces. Through electrostatic absorption, the in- jected charged ions can remain in the dielectric polymers for years [70]. The most common method to conduct ion injection is plasma treatment. Reactive ion etching (RIE) is a widely used technique to increase the surface roughness of polymer by generating NW morphology with argon and oxygen as the plasma gas [71]. During this process, cor- responding argon and oxygen ions are spontaneously in- jected into the triboelectric polymer, resulting in chemical modulation of surface states. From this point of view, all polymer NWs that made from RIE will experience chemi- cal modification by ion injection. In addition to the plasma treatment, Wang et al. employed ionized-air gun to controllably implanting extra charges to the fluorinated ethylene propylene (FEP) surface and systematically studied the MSCD of FEP polymer [70]. Compared to other charge implantation methods such as ICP and corona discharging [72], this hand-sized ionized- air gun is simple and convenient. Ionized-air gun can produce ions with both polarities via triggering the discharge of air inside the gun. The polarity of ion can be manipulated by either squeezing or releasing the trigger. Negative charges (CO3− , NO3− , NO2− , O3− and O2− ) were selected in this work since FEP has strong tendency to gain electrons (Fig. 5(a)). In order to achieve the MSCD for high-performance TENG, the metal electrode that attached to the back of polymer layer was grounded. Under this circumstance, the injected negative charges are able to drive equal amount of electron away from the metal electrode to ground, leaving positive charges in the electrode. As a consequence, the electrical field of the double layer charge will be confined only within the FEP film. Since the dielectric strength of FEP is two orders of magnitudes larger than that of air, the grounded connection could allow a much larger charge density compared to the electrode-free FEP film, in which the maximum charge density will be limited by the breakdown voltage of the air. To investigate the MSCD of FEP film, the charge density on the charge-injected polymer surface was instantaneously monitored by a Coulomb meter, which could measure the charge flow between the bottom electrode and ground (inset of Fig. 5(b)). As shown in Fig. 5(b), every period of ion injection induced a charge transferring process with an area density of ∼40 μC/m2, indicating the same amount of accumulated negative charge on FEP surface. After 17 times of injection, the charge density was saturated at ∼630 μC/m2, which was the MSCD of this 50 μm-thick FEP film. This increment of charge density was subsequently investigated by assembling the ion-injected FEP film into a conventional contact-separation TENG with an Al plate as the counter electrode. The short-circuit charge density (σSC ) generated by this TENG should equal to the surface charge density (σ0) on the FEP film. Fig. 5(c) presented the recorded σSC of TENG after each ion injection. With no injection, the initial σSC was only about 50 μC/m2. One ion injection process could increase σSC to ∼100 μC/m2 . After that, σSC was elevated gradually by multiple ion injection processes and ultimately arrived at ∼240 μC/m2 at the fifth injection. Further increasing the ion injection times only resulted in very small amount of σSC enhancement. After the ninth injection, the charge transfer suddenly behaved differently, as marked in the red dashed box. The first pressing process induced a σSC of ∼260 μC/m2. However, when Al was released and separated from FEP film, the σSC dropped to ∼230 μC/m2, leading to the upward shift of the base line. In the following contact and separation process, the σSC was maintained at the same level. This abrupt decrease of σSC was attributed to the air breakdown during the first releasing process, which was caused by a voltage drop between Al plate and FEP polymer. The presence of this voltage drop was verified by the numerically calculated potential distribution across the TENG (Fig. 5(d)). As shown in the bottom portion of Fig. 5(d) a voltage drop (Vgap) of ∼300 V existed in the air gap. When this Vgap exceeded the breakdown voltage of air, the positive ions in the air discharging corona will partially screen the negative charge on the FEP surface, resulting in the reduced σSC . As discussed above, the initial short-circuit charge density (σSC -I ) from the first pressing motion would be larger than that from the remaining operation circles (σSC -R ) after the ninth ion injection. Their relationship was summarized in Fig. 5(e), where two regions can be clearly recognized. At region I, the Vgap caused by the FEP charge was smaller than the air breakdown voltage, where the σSC -R was linearly increased with the enlargement of σSC -I . When σSC -I reached the MSCD of ∼240 μC/m2 , the σSC -R started to deviate from σSC -I due to the air breakdown (region II of Fig. 5(e)). In this region, larger σSC-I led to smaller σSC-R, implying that the discharging of the air was more intensive with higher σSC -I . During this ion injection process, the MSCD of tribo- electric polymers can be measured. In this work, the rela- tionship between the MSCD (σmax) and film thickness (d) of FEP was studied both theoretically and experimentally. As demonstrated by the calculated results in Fig. 5(f) and (g), when d decreased from tens of microns to hundreds of nanometers, the MSCD was enhanced by more than two orders of magnitudes, approaching 50 mC/m2 . Experimen- tally recorded MSCD of three FEP films with the thicknesses of 50, 75 and 125 μm followed the same trend of the calcu- lated curve (Fig. 5(g)). The good agreement of experimental and simulated results confirmed the conclusion that a thin- ner dielectric film is favorable for the TENG development via the improvement of surface charge density, which was consistent with other report [15]. In addition to the MSCD investigation, this ion injection approach can substantially enhance the output of TENG by maximizing the surface charge density. As shown in Fig. 5(c), ion injection could enable ∼5 times larger charge density on FEP surface. In principle, proportionally- enhanced open circuit voltage (Voc) and short circuit Y. Yu, X. Wang / Extreme Mechanics Letters ( ) – 7PDF Image | Chemical modification of polymer surfaces for advanced triboelectric nanogenerator development
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