## 4 Conclusions  
In summary, the direct-current fluid-flow-based triboelectric nanogenerator was demonstrated to be a simple and effective mechanism for mechanical-to-electrical energy conversion from flowing water. The falling of a water droplet from a pipe can generate a direct current in the external circuit by contacting a separated electrode. This is different from a conventional single-electrode TENG that has the distinct characteristics of AC. Experimental results prove that the device can produce DC outputs with an open-circuit  
Figure 7: Application of DC–FluTENG. (**a**) Power densities and (**b**) charge densities and energy densities of the device at various working conditions, including load resistance and water–flow rate. (**c**) The measured voltage of various capacitors charged by DC–FluTENG under a water–flow rate of 113 mL/min. (**d**) The dependence of output voltage of DC–FluTENG on the water–flow rate. (**e**) Photograph of DC-FluTENG as a power source in a self-powered IoT sensing system. (**f**) Photograph of DC-FluTENG as a direct power source for driving 50 LEDs in series without a rectifier or energy storage units.  
voltage of 35 V and a short-circuit current of 3.7 \(\upmu\)A. The maximum instantaneous power can reach 57.6 \(\upmu\)W at a load resistance of 10 M\(\Omega\). The type and flow rate of water droplets and the material of electrodes are investigated as the factors affecting the output performance of DC-FluTENG. More importantly, the ability to generate DC output enables the DC-FluTENG to directly drive electronic devices, such as directly lighting up 50 LEDs connected in series, without requiring a rectifier. Moreover, the produced energy can be stored directly in a capacitor and used to power a commercial temperature and humidity IoT sensors device. Taking advantage of direct current outputs, both portability and energy utilization efficiency can be improved. Furthermore, the output voltage and the flow rate have a great proportional relationship, with a high sensitivity of about 48.4 mV/cc-min and linearity of R\({}^{2}\) = 0.998. Based on these results, this work highlights a promising potential for applications in harvesting hydrokinetic energy and self-powered sensors systems.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app12052724/sl](https://www.mdpi.com/article/10.3390/app12052724/sl)]([https://www.mdpi.com/article/10.3390/app12052724/sl](https://www.mdpi.com/article/10.3390/app12052724/sl)). Figure S1: (a) Open-circuit voltage and (b) short-circuit current of DC-FluTENG under various water flow rates in a range from 113 to 563 mL/min, Figure S2: Open-circuit voltage and short-circuit current of DC-FluTENG by using different (a,b) types of water and (c,d) materials of the metal electrode, respectively. Measured at a water flow rate of 113 mL/min. Figure S3: Application of DC-FluTENG to directly drive electronics. (a) DC-FluTENG integrated into an IoT sensors application. (b) Experiment setup for charging a capacitor directly without a rectifier. Video S1: Charge a capacitor without a rectifier. Video S2: Power an IoT sensors application without a rectifier. Video S3: Light up light-emitting diodes (LEDs) without a rectifier.  
Conceptualization, Q.T.N.; investigation, Q.T.N. and C.P.V.; methodology, Q.T.N. and T.H.N.; writing--original draft preparation, Q.T.N.; software, C.P.V.; validation, C.P.V.; writing--review and editing, C.P.V. and T.H.N.; supervision, K.K.A.; funding acquisition, K.K.A.; project administration, K.K.A. All authors have read and agreed to the published version of the manuscript.  
This research was supported by Basic Science Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, South Korea (NRF-2020R1A2B5B03001480).  
Not applicable.  
The authors confirm that the data supporting the finding of this study is available within the article.  
The authors declare no conflict of interest.  
References