Self-operating transpiration-driven electrokinetic power generator with an artificial hydrological cycle

Autonomous energy scavenging from the ambient environment, or self-energy management, has attracted increasing attention because it could solve the energy problem of abundant Internet of things (IoT) devices. In recent years, several energy harvesters that generate electricity using water have been invented due to their simplicity, sustainability, and eco-friendliness. Until now, the devices have required periodic supplementation of water for continuous electricity generation, which hinders their practical use. Here, we built an artificial hydrological cycle in a transpiration-driven electrokinetic power generator (TEPG) to continuously and autonomously generate electric power. The TEPG, composed of carbon-coated cotton fabric, generates electricity by using a few drops of water (0.2 mL); the electric power originates from the potential difference in the asymmetrically wetted device and the pseudostreaming current. However, after only one hour, the TEPG stops generating electricity, as water inevitably evaporates from the device. For continuous self-operation, we utilized calcium chloride (CaCl2), a typical deliquescent chemical, to collect water vapor from the surrounding environment and continuously supply water to the TEPG. In the range of 15-60% relative humidity (RH), CaCl2 successfully compensates for the water loss by evaporation and maintains the electrical power generation in the closed system. In addition, CaCl2 enhances the generated voltage (0.74 V) and current (22.5 mA) by supplying additional Ca2+ ions to the carbon surface and reducing the resistance of the device, respectively. The developed self-operating transpiration-driven electrokinetic power generator (STEPG) is stable enough to light a light-emitting diode (LED) for a week and charge a commercialized supercapacitor (5 F) to 1.6 V for 8 days.