## 3 Results and Discussion
### Application of DC-FluTENG  
DC-FluTENG is proposed as a simple mechanical-to-electrical energy conversion from harvesting hydrokinetic energy. Indeed, DC power can be produced due to the continuous flow of water droplets through a pipe and a metal electrode. The dependences of power density on the flow rate and loading resistance are represented in Figure 7a. At the optimal load resistance of 10 M\(\Omega\), the power density increases from 0.327 to 5.76 W/m\({}^{2}\) when increasing the flow rate from 113 to 676 mL/min, respectively. Obviously, by increasing the water-flow rate to 6 times, the output power density increases more than 17 times, which proves a considerable influence of water flow rate on the power generation performance of the device. As the power density of DC-FluTENG is much higher than those of 1.74 mW/m\({}^{2}\) in [43], 27.86 mW/m\({}^{2}\) in [44], 170 mW/m\({}^{2}\) in [45], and 590 mW/m\({}^{2}\) in [24], it declares a dramatic enhancement in output performance compared to other droplet-based TENGs, particularly AC-type devices. The transferred charge density (\(J_{tc}\)) and electrical energy density (\(J_{ce}\)) regarding the water flow rate are shown in Figure 7b, in which \(J_{tc}\) increases from 7.8 to 14.95 mC/m\({}^{2}\) and \(J_{ce}\) increases from 3.1 to 377.9 mJ/m\({}^{2}\) when the flow rate increases from 113 to 676 mL/min, respectively. These results can demonstrate the potential application of DC-FluTENG as an energy harvester. Moreover, the ability to generate DC output enables the TENG to directly drive electronic devices. DC-FluTENG is integrated into a self-powered IoT sensors device and acts as a power source, as described in Supplementary Materials Figure S3a. DC power generated by the TENG can be used to directly charge an energy storage unit, such as a capacitor, without requiring a rectifier. The actual experiments are demonstrated in Supplementary Materials Figure S3b and Video S1. The charging behaviors plotted in Figure 7c were examined by using different capacitors from 1 to 100 \(\mu\)F. It can be observed that larger capacitances have to take more time to achieve the saturation voltage. Under a droplet flow rate of 113 mL/min, to attain a voltage of about 2 V, a 100 \(\mu\)F capacitor needs more than 270 s, while a 10 \(\mu\)F capacitor can reach 2 V within 20 s. However, at the same period, a 1 \(\mu\)F capacitor can be charged to 10 V. This is important to find out the optimal load capacitance in achieving the maximum energy storage. To demonstrate the application of the device as a power source in IoT sensing systems, a commercial Internet of Things (IoT) device, namely Solar-Powered BLE Beacon of Cypress Semiconductor Corporation company, is used. This IoT device could be powered by DC-FluTENG instead of the solar part which is removed. The room temperature and humidity signals are transmitted to the smartphone via Bluetooth after charging in the capacitor within 20 s; here, a 1 \(\mu\)F capacitor was utilized. The actual experimental operation is shown in Figure 7e and Supplementary Materials Video S2. In addition, DC-FluTENG not only can charge the energy storage unit directly but also power electronics directly. In this work, light-emitting diodes (LEDs) can be utilized to verify the application of the TENG. As shown in the inset image of Figure 7f, 50 LEDs in series can be illuminated by connecting directly with the DC-FluTENG without a rectifier, thus indicating another advantage of the TENG in practical applications. It can also be demonstrated in Supplementary Materials Video S3, the actual experimental operation. These results prove that DC-FluTENG can be effectively utilized to directly drive electronics; thus, it shows a high potential application value. Another important aspect, DC-FluTENG, which is based on different water-flow rates, shows a greatly varied output voltage; hence, the output voltage can be applied as a self-powered sensor for active water-flow control.