and droplet-based electricity generator (DEG). [9] However, inherent flaws exist in current approaches. Reverse electrowetting energy harvesting devices always need external voltages. [1] Triboelectric nanogenerator (TENG), [10,11] which was first invented in 2012 by Wang and coworkers, [12,13] has provided a passive energy harvesting approach. But the performance of TENG is limited by the low density and poor stability of surface charges on tribo-layers. High surface charge density could only be achieved in vacuum environment [14] or by utilizing external pumping or excitation sources. [11,15] The droplet energy harvesting efficiency of the conventional TENG was only 0.01%. [5] Recently, Z. K. Wang and coworkers have reported a water dropbased electric generator, DEG, [9] showing significantly enhanced energy harvesting efficiency to 2.2%. Nevertheless, the energy harvesting efficiency of DEG is still limited by the density and stability of charges generated by triboelectrification during drop impact. The maximum surface charge density of DEG displayed around 0.184 mC m −2 (49.8 nC for 2.7 cm 2). [9] The surface charges in DEG were superior stability compared to the conventional TENG, although the charge density still degraded in a harsh environment with 100% humidity. Moreover, the efficiency greatly dropped with increasing salt Strategies toward harvesting energy from water movements are proposed in recent years. Reverse electrowetting allows high efficiency energy generation, but requires external electric field. Triboelectric nanogenerators, as passive energy harvesting devices, are limited by the unstable and low density of tribo-charges. Here, a charge trapping-based electricity generator (CTEG) is proposed for passive energy harvesting from water droplets with high efficiency. The hydrophobic fluoropolymer films utilized in CTEG are pre-charged by a homogeneous electrowetting-assisted charge injection (h-EWCI) method, allowing an ultrahigh negative charge density of 1.8 mC m −2. By utilizing a dedicated designed circuit to connect the bottom electrode and top electrode of a Pt wire, instantaneous currents beyond 2 mA, power density above 160 W m −2 , and energy harvesting efficiency over 11% are achieved from continuously falling water droplets. CTEG devices show excellent robustness for energy harvesting from water drops, without appreciable degradation for intermittent testing during 100 days. These results exceed previously reported values by far. The approach is not only applicable for energy harvesting from water droplets or wave-like oscillatory fluid motion, but also opens up avenues toward other applications requiring passive electric responses, such as diverse sensors and wearable devices.

We use a combination of high-speed video imaging and electrical measurements to study the direct conversion of the impact energy of water drops falling onto an electrically precharged solid surface into electrical energy. Systematic experiments at variable impact conditions (initial height; impact location relative to electrodes) and electrical parameters (surface charge density; external circuit resistance; fluid conductivity) allow us to describe the electrical response quantitatively without any fit parameters based on the evolution of the drop-substrate interfacial area. We derive a scaling law for the energy harvested by such "nanogenerators" and find that optimum efficiency is achieved by matching the timescales of the external electrical energy harvesting circuit and the hydrodynamic spreading process.

Converting various types of ambient mechanical energy into electricity, triboelectric nanogenerator (TENG) has attracted worldwide attention. Despite its ability to reach high open-circuit voltage up to thousands of volts, the power output of TENG is usually meager due to the high output impedance and low charge transfer. Here, leveraging the opposite-charge-enhancement effect and the transistor-like device design, we circumvent these limitations and develop a TENG that is capable of delivering instantaneous power density over 10 MW/m2 at a low frequency of ~ 1 Hz, far beyond that of the previous reports. With such high-power output, 180 W commercial lamps can be lighted by a TENG device. A vehicle bulb containing LEDs rated 30 W is also wirelessly powered and able to illuminate objects further than 0.9 meters away. Our results not only set a record of the high-power output of TENG but also pave the avenues for using TENG to power the broad practical electrical appliances.