A review of humidity gradient-based power generator: Devices, materials and mechanisms

Humidity gradient-based power generators (HGPGs), which can convert moisture potential into electric power, offer a promising avenue due to the wide and abundant distribution of air humidity in surrounding environment. Considering the air relative humidity is between 100% and 60% at 25 °C, the enthalpy difference, 3.94 kJ·kg−1, is noteworthy. HGPGs show broad application prospects in distributed power supply and self-powered electronics such as humidity sensors and human respiration monitoring. In this review, recent research on HGPGs since 2015 with the humidity gradient along in-plane and in-thickness direction was summarized. In-plane HGPGs have the advantage of long-term stable power generation (> 150 h), while in-thickness HGPGs can realize the higher current density (> 550 μA·cm−2). However, existing HGPGs are still less efficient, less than 5% of the ideal value. Feasible enhancement methods were summarized, including structural modification through connection design and multi-component assembly, and material modification through doping of high-valent metal cations and introducing of oxygen-containing functional groups. In particular, the addition of hydrogel in HGPG assembly can provide a stable humidity gradient under various environmental conditions, while the performance is significantly affected by the type and amount of hydrogel. Furthermore, mechanism research of energy conversion is far from reaching a consensus. The current theoretical analyses focused on the necessary formation conditions of ion pairs, the separation reasons of positive and negative ions and the influencing factors of their directional movements. Molecular dynamics simulations such as Ab initio method allow the visualization of microscopic conversion processes, which are considered a suitable method for illustrating how HGPGs work in the future. This review will serve interested readers as a shortcut and reference for understanding the development process, and stimulate further discussion on fundamentals and optimizations of humidity-based power generators.