## 1 Introduction  
With the growth of the Internet of Things, the development of electronic devices follows a general trend of miniaturization, portability, and functionality, which require a very small power in microwatt to milliwatt level [1]. Thus, micro-/nano-technologies received extended attention from scientists, resulting in the invention of the nanogenerators, including the triboelectric nanogenerator, which was invented by [PERSON]'s group. The fundamental theory of TENG is based on the coupling effects of contact electrification (CE) and electrostatic induction [2]. In the last decades, TENG devices are rapidly developed with a high potential for harvesting ambient mechanical energy from the environment, such as human motion, vibrations, ocean waves, and wind energies. However, conventional TENGs exhibit alternating-current (AC) output characteristics due to the changing periodically of the distance or contact area between materials, resulting in the fact that triboelectric charges generated by CE cannot be accumulated continuously without using a rectifier unit. It would be inconvenient for directly powering electronic devices where direct-current (DC) power is required. In this case, a rectification method, such as bridge rectifiers [3; 4; 5; 6], electric brushes [7; 8], and power management circuits [9; 10; 11], is mandatory to obtain DC power from AC power. Nevertheless, a rectifier module in the electrical circuit could increase the complexity of the device, while the power consumedby the rectifier could reduce the energy utilization efficiency of TENGs. Therefore, direct-current triboelectric nanogenerators (DC-TENGs) have attracted considerable interest from researchers as a promising solution to solve the above problems. It believes that the ability to generate DC power enables DC-TENGs to have great potential for practical applications with advantages for the miniaturization of self-powered systems and the high efficiency of energy storage [12, 13, 14, 15]. In particular, numerous fluid-based triboelectric nanogenerators (FluTENGs) have been developed to harvest fluid flow energy, including airflow and water-flow energies [16, 17, 18]. Notably, FluTENGs relying on water-based CE have been demonstrated as a new approach for harvesting low-frequency and small-scale energy from moving water, such as waterdrops/raindrops [19, 20, 21, 22], water streams [23, 24, 25], and water waves energies [26, 27, 28]. Considering fluid-based DC-TENGs, [PERSON] et al. [8] proposed a direct-current rotary-tubular TENG based on liquid-dielectrics contact, in which produced electricity can directly charge the capacitor without a rectifier unit and act as a power source to drive a thermometer after a charging time. Similarly, [PERSON] et al. [15] reported a DC-TENG via water electrification and phase control that can generate DC power without any converters or batteries. [PERSON] et al. [29] proposed a liquid-liquid TENG that can produce DC power owing to the contact-separation between a water droplet and a conductive liquid membrane. [PERSON] et al. [30] reported a concept wherein DC electricity can be generated from a droplet falling into an electrolyte solution. This is mainly due to the dynamic wetting of a pair of copper electrodes immersed in the electrolyte. Additionally, droplet-based electricity generators proposed by [PERSON] et al. and [PERSON] et al. [31, 32] prove an important contribution of the metal electrode in the charge-transfer process between the insulating layer and the electrode when water is in contact with both, resulting in enhancing output performance of their TENGs. Moreover, [PERSON] et al. [33, 34] demonstrated that a unidirectional current can be generated because of the friction when water/oil flows through a metal pipe. [PERSON] et al. [35] also reported a DC-power-harvesting system by using streaming electrification and an opposite charge extractor. Furthermore, [PERSON] et al.