Moisture electricity generation: Mechanisms, structures, and applications

With the increasing concern of energy crisis and global warming, the whole globe is in an urgent need to develop clean energy that comes from renewable sources and does not harm the environment to fulfill the carbon neutralization and green earth commitments. Water is the most abundant substance on earth and has been historically used as the major energy carriers in watermill, water wheel, and hydroelectricity. Moisture electricity generation is another emerging technology that can convert low-grade energy in the widely-accessible moisture to electricity simply by the integration of moisture, electrodes, and deliberately-designed hygroscopic films. Recent research on moisture electricity generators (MEGs) led to the creation of a series of self-powered sensors and in some occasions they have replaced conventional batteries to power miniaturized devices. In this review, the basic mechanisms of MEGs are firstly clarified, and three categories of them, i.e., gradient structure, homogeneous structure, and heterogeneous structure depending on the structure of hygroscopic films, are then introduced. Furthermore, recent advances in the fabrication, characteristics, and performance of MEGs are summarized, and MEGs with continuous or transient output that could be applied in self-powered sensors and power sources are discussed. Finally, some remaining challenges and our perspectives on MEGs are highlighted.