Raindrop falling, which is one kind of water motions, contains large amount of mechanical energy. However, harvesting energy from the falling raindrop to drive electronics continuously is not commonly investigated. Therefore, a self-cleaning/charging power system (SPS) is reported, which can be exploited to convert and store energy from falling raindrop directly for providing a stable and durable output. The SPS consists of a hydraulic triboelectric nanogenerator (H-TENG) and several embedded fiber supercapacitors. The surface of H-TENG is amphiphobic, enabling the SPS self-cleaning. The fiber supercapacitor which uses α-Fe 2 O 3 /reduced graphene oxide composite possesses remarkable specific capacitance, excellent electrical stability, and high flexibility. These properties of the fiber supercapacitor make it suitable for a wearable power system. A power raincoat based on the SPS is demonstrated as application. After showering by water flow, which simulates falling raindrops, for 100 s, the power raincoat achieves an open-circuit voltage of 4 V and lights a light-emitting diode for more than 300 s. With features of low cost, easy installation, and good flexibility, the SPS harvesting energy from the falling raindrop renders as a promising sustainable power source for wearable and portable electronics.

become a more serious issue. [1-3] Efficient conversion of ambient neglected energy into electricity has been a research hotspot from last decade. [4-18] In particular, the triboelectric nanogenerator (TENG) based on coupling the contact electrification (CE) and electrostatic induction possesses incomparable superiority in harvesting low frequency and micro mechanical energy. [19-21] Various environmental based mechanical energy including human motion, [22-30] rotating motion, [31,32] vibration, [33-38] water drop, [39,40] sea wave, [41-44] and wind, [45-56] can be harvested effectively with TENGs. Moreover, the self-powered systems without external power supply, [57-59] can be built based on TENG, [60-68] which can be applied to environmental surveillance, [60-62] chemical detection, [62-64] tracking system, [65,66] liquid volume monitoring, [67] electronic skin, [68] etc. In conventional TENGs, a pulsed alternating current (AC) output is obtained by changing the distance or contact area periodically between the materials with opposite electrostatic charges. [69-73] In order to obtain direct current (DC) output, some necessary rectification methods are needed, such as power management circuits, [47,74-76] rectifier bridge, [77-82] and electric brushes, [83,84] which will reduce the portability and energy utilization efficiency. [85] Moreover, the pulsed AC output leads to a high crest factor (the ratio between the peak and the root-mean square value of output), which means the output is unstable. [86,87] These inherent output characteristics of the conventional TENGs can seriously affect its widespread applications in driving electronics and charging energy storage devices where DC output is most appropriate. In this case, the novel strategies and technologies for converting mechanical energy directly into DC electric energy are essential. Lately, various types of DC triboelectric nanogenerators (DC-TENGs) with new frictional materials and different structures are introduced. [88-91] The concept of DC-TENG as illustrated in Figure 1, describes that the mechanical energy is harvested and converted into constant DC electric power, which is suitable to power electronic equipment or charge energy storage devices. In the future, this promising research field will also attract more and more scientific efforts. The progress and development of DC-TENG is summarized in this paper, which begins with the development and working mechanisms of the insulator-based DC-TENG by phase coupling and dielectric breakdown. The DC voltage output of various insulator-based DC-TENGs can reach up to kV, and its crest factor is close to 1. Thereafter, the semiconductor-based DC-TENG with lower internal impedance and higher output current density is introduced. The charge transfer process in

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