Novel electrode design for energy harvesting devices based on liquid-solid contact and enhancement of effective interface area

Electrical double layer (EDL) modulation in liquid-solid interfaces has been extensively studied as an easy and effective approach to harvest triboelectric energy from micromechanical wasted energy. Owing to the fluidity of liquids, the liquid-solid contact area in an EDL-based device, for example, a liquid droplet sandwiched between two substrates, can be modulated to a large extent by simply squeezing or releasing the two substrates. However, conventional EDL-based technology cannot fully exploit this advantage, and the relevant devices suffer from poor efficiency. Herein, we demonstrate a simple yet highly effective EDL-based energy harvesting device with a novel electrode design composed of a hydrophilic inner circular electrode and hydrophobic outer ring electrode allowing the best use of the liquid-solid interface, i.e., large modulation of the contact area. The contact area variations in the top and bottom substrates contribute in an opposite manner to the electric generation in a conventional device. However, the new electrodes are designed to ensure that the contact area variations in the two substrates contribute cooperatively to the energy generation. The new device increases the output power by approximately 30-fold compared to that of conventional devices. Moreover, the optimal design for higher power generation considering internal resistance is systematically investigated and theoretically verified. The feasibility of these devices is demonstrated by the instantaneous lighting of LEDs and long-term stability tests. This new concept can expand the possibility of EDL-based energy-harvesting devices.