Ongoing efforts to develop this energy have led to the creation of a variety of novel technologies based on photovoltaic, [12,13] piezoelectric, [14,15] triboelectric, [16,17] and thermoelectric principles. [18,19] Devices based on these technologies are readily incorporated into self-powered, battery-free electronic systems. This also acts to eliminate the harm to the environment caused by the use of conventional batteries. [20,21] As recyclable resource, water is not only indispensable to life but also constitutes the largest energy carrier on Earth (Figure 1a). As 71% of the Earth's surface is covered by water and 35% of solar energy incident on the Earth is absorbed by water, petawatts of energy are received in this way. [22] As the water resource is clean and sustainable, the energy contained in even a small fraction of the water on Earth can meet the current global energy demand if this energy can be efficiently harvested. Terrestrial water exists in vapor and liquid form including moisture, rain, clouds, lakes, rivers, and oceans. It also forms the basis for biological systems. Even though there is a long history of electricity generation from running water, most technologies only utilize the gravitational and kinetic energy in liquid water. [23] Such systems are inherently incompatible with the development of self-powered devices. The development of nanomaterials has enabled technology for the extraction of electrical energy from moisture, leading to The exploration of the utilization of sustainable, green energy represents one way in which it is possible to ameliorate the growing threat of the global environmental issues and the crisis in energy. Moisture, which is ubiquitous on Earth, contains a vast reservoir of low-grade energy in the form of gaseous water molecules and water droplets. It has now been found that a number of functionalized materials can generate electricity directly from their interaction with moisture. This suggests that electrical energy can be harvested from atmospheric moisture and enables the creation of a new range of self-powered devices. Herein, the basic mechanisms of moisture-induced electricity generation are discussed, the recent advances in materials (including carbon nanoparticles, graphene materials, metal oxide nanomaterials, biofibers, and polymers) for harvesting electrical energy from moisture are summarized, and some strategies for improving energy conversion efficiency and output power in these devices are provided. The potential applications of moisture electrical generators in self-powered electronics, healthcare, security, information storage, artificial intelligence, and Internet-of-things are also discussed. Some remaining challenges are also considered, together with a number of suggestions for potential new developments of this emerging technology.

Harvesting energy from ambient moisture and natural water sources is currently of great interest due to the need for standalone self-powered nano/microsystems. In this work, we report on the development of a cost-effective nanogenerator based on a carbon paper-Al 2 O 3 nanoparticle layer-carbon paper (CAC) sandwich structure, where the 3D Al 2 O 3 layer is deposited via vacuum filtration. This type of device can produce an open-circuit voltage (U OC ) of up to 4 V and a short-circuit current (I SC ) of ∼18 μA with only an 8 μL water droplet applied. To our knowledge, this is the highest voltage yet reported from a single moisture/water-induced electricity nanogenerator using solid oxides and carbon-based materials. A remarkable output power of 14.8 μW can be reached with an optimized resistive load. An LED with a working voltage of 3−3.2 V can operate for a short time with the power from a single CAC device exposed to one 8 μL water droplet. Furthermore, a CAC generator adsorbing as little as 2 μL water droplets every 3 min can also give a U OC of 3.63 V. We show that CAC devices provide a robust electrical output over more than 200 wet−dry cycles without any deterioration in performance. These units demonstrate much promise as cost-effective electricity generators for harvesting energy from natural sources like rainwater, tap water, snow runoff, and dew. The response time of CAC devices can be as fast as 10−100 ms, making them ideal for applications as self-powered water detectors. The generation of power in this device arises from the streaming current. To assist in the optimization of these devices, we have analyzed how their response is related to such factors as layer thickness, time interval between application of water droplets, and the volume of each water droplet.

This study reports the concept of a water/moisture-induced hygroelectric generator based on the direct contact between magnesium (Mg) alloy and oxidized carbon nanofibers (CNFs). This device generates an open-circuit voltage up to 2.65 V within only 10 ms when the unit is placed in contact with liquid water, which is higher than the reduction potential of magnesium. The average peak short-circuit current density is ∼6 mA/cm 2 , which is among the highest values yet reported for water-induced electricity generators. Our results indicate that galvanic corrosion occurs at the interface between the CNF and Mg electrode, but the device can still generate electricity because of the high contact resistance caused by the work function difference between Mg and CNF and the surface oxidation. The oxidized CNF is shown to absorb water/moisture and get reduced, leading to a capacitive discharging effect to provide enhanced signal amplitude and sensitivity. These devices are found to be highly sensitive to small quantities of water, and their high output voltage and current make them useful for the detection of water vapor in the human breath as well as changes in ambient humidity. The Mg/ CNF systems thus provide a new technology for use in the fabrication of self-powered water/moisture sensors and the development of portable electric power generators.