## 3 Results and Discussion
### Factors Affecting the Output Performance  
To evaluate the factors affecting the output performance, the experiments are realized at different flow rates of water droplets, which are controlled in a range from 113 to 676 mL/min. Typical electrical output signals, including the Voc and Isc, are illustrated in Supplementary Materials Figure S1a,b, respectively. From these figures, we can see that the voltage and current signals have similar DC characteristics with all negative pulses. It is considered that a rising trend occurs for both magnitudes of voltage and current output as the flow rate increases. Following the changes in the average magnitudes of voltage and current output summarized in Figure 5a, we see that the magnitude of Voc increases from 8.1 to 35.1 V, while the value of Isc increases from 0.757 to 3.728 uA when  
Figure 4: Electricity–generation performance of control–TENG. (**a**) Schematic diagram of a control–TENG. (**b**) Open–circuit voltage and short–circuit current curves. (**c**) Comparison of output performance between DC–FluTENG and control–TENG. increasing the flow rate from 113 to 676 mL/min, respectively. Similarly, an identical trend is found for the transferred charges in short-circuit mode, where its value increases from 78 to 149.6 nC monotonically. This is mainly due to the increase of accumulative triboelectric charges generated by the water flow. More interestingly, generated electric energy can directly charge an energy storage unit without requiring a rectifier unit. The charging capability concerning the flow rate is studied by measuring the voltage developed across a 1 uF capacitor in the external circuit. The charging behaviors are plotted in Figure 5b, where the maximum measured voltage is obtained at a flow rate of 676 mL/min with a value of about \(-13.3\) V, corresponding to a maximum amount of 88.45 uJ energy stored. Additionally, the impact of the water-flow rate on the output performance is also investigated by applying various load resistance in the external circuit with a range from 10 k\(\Omega\) to 50 M\(\Omega\). The average voltage and instantaneous power are respectively represented in Figure 5c,d. At the same load resistance, the magnitudes of both the peak voltage and maximum instantaneous power are monotonically increasing with the flow rate. Meanwhile, for each applied flow rate, the highest values of power output are found at the optimal loading resistance of 10 M\(\Omega\). Consequently, the highest output power obtains a value of 57.6 uW regarding the power density of 5.76 W/m\({}^{2}\), as measured at the load resistance of 10 M\(\Omega\) under a flow rate of 676 mL/min. Another important aspect to examine is how material selection affects the output performance of the DC-FluTENG. The experiments are realized based on different types of water, such as DI water, tap water, and 0.3 M CuSO4 solution, and different materials of the electrode, such as silver, copper, and aluminum. Supplementary Materials Figure S2a,b displays the voltage and current curves when passing water droplets through contacting  
Figure 5: Impacts of water–flow rate on the output performance. (**a**) Dependence of Voc and Isc on the water flow rate. (**b**) Charging behavior of a 1 uF capacitor under a range of water flow rates from 113 to 676 mL/min. (**c**) Voltage and (**d**) Instantaneous power as a function of the water flow rate and load resistance. an aluminum electrode at a flow rate of 113 mL/min. It seems that both output the Voc and Isc produced by DI water are higher than those by two other types of water. This is mainly caused by the presence of electrolytes in the water droplet. In general, the water cannot be completely separated from the electrode; residual water always exists on the electrode surface. If the water contains electrolytes, a partial screening of the triboelectric charges on the surface due to positive charges, including dissolved ions that remain on the electrode surface, is required.