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.

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