Water is not only the origin of life, but also contains lots of energy. Harvesting the kinetic energy from flowing water at high speed has traditionally changed the energy landscape. In spite of this, the energy stored in raindrops, rivers, and ocean waves remains largely unexplored owing to their low frequency motion that is not compatible with the state of art electromagnetic techniques. In recent years, several techniques such as triboelectric nanogenerator, hydrovoltaic technology, piezoelectric nanogenerator, and reverse electrodialysis, which mainly rely on the generation and transfer of interfacial charges, have been developed to harvest these untapped energies with the aim for practical applications in self-powered systems, wireless electronics, and large-scale energy needs.

Particularly, a novel droplet-based electricity generator featuring a transistor-like architecture was proposed to overcome the drawback of conventional counterparts limited by interfacial effects, which is capable of achieving an increase by several orders of magnitude in electrical output as well as energy conversion efficiency. Therefore, the novel droplet-based electricity generator offers a highly attractive platform to extract energy from abundant water as a sustainable power supply for electronic devices. Despite this exciting progress, this novel droplet-based electricity generator suffers from relatively low working frequency and unstable output performance under demanding conditions, which might hinder its practical applications.

In this thesis, guided by the design principle of the novel droplet electricity generator with a transistor-like architecture, I aim to develop robust droplet-based electricity generators through materials selection and optimization in fabrication technique, which allow for high and stable electrical performance towards harsh and demanding environments.

First, I summarize the latest progress in the development of water-based electricity generators and present the fundamentals of electricity generation at the water-solid interface, which lays a theoretical foundation for the design and fabrication of water-based electricity generators.

Second, inspired by the transistor-like droplet-based electricity generator, I report the design of superhydrophobic surface-based droplet electricity generator (SHS-DEG). This SHS-DEG device enables efficient energy harvesting of the impinging droplets at significantly higher frequency without compromising output performance stability, thereby overcoming the drawback of low-working frequency in current droplet-based electricity generators. Especially, the SHS-DEG device is capable of timely water droplet shedding without the formation of unwanted liquid film and demonstrates superior electrical performance under high impinging frequencies over 165 Hz.

Third, I present a monolithic integrated droplet-based electricity generator (MI-DEG) with high flexibility and robustness by virtue of a thermal fusion strategy. The MI-DEG is designed by the rational choice of polymer electrodes with similar chemical component to dielectric material, thus enabling the seamless interface fusion of electrodes and dielectric material for the formation of a transistor-like architecture. Such a design endows MI-DEG with excellent mechanical-tolerant capacity, thus sustaining stable electrical performance under long-term mechanical actions such as bending and abrasion without disrupting the integrity of MI-DEG. In addition, MI-DEG can be applied in some off-grid regions to act as a sustainable energy reservoir for powering electronics such as hygro-thermometer and environmental monitoring sensor.

In summary, robust droplet-based electricity generators are investigated systematically in this thesis. The marriage of superhydrophobic surface and transistor-like droplet electricity generator provides a design principle for efficient energy harvesting at high droplet impinging frequencies. Furthermore, the monolithic integrated droplet electricity generator is presented with excellent mechanical-tolerant capacity while sustaining stable electrical performance. The research work in this thesis offers new dimensions in the development and construction of robust droplet-based electricity generators towards harsh application scenarios. I envision that the design of robust droplet-based electricity generators can finally contribute to the attainment of ultimate Internet-of-Things society in a clean and sustainable manner.