Hygroscopic Materials

Abstract

Water in its gaseous form, i.e., moisture, is pervasive in the atmosphere. Almost all materials naturally exhibit either weak or strong affinity to moisture via physical adsorption within porous structures or chemical absorption through spontaneous hydration reactions, which is a property known as hygroscopicity. Thus, we can safely infer that we live in a “hygroscopic materials” world, where moisture is ubiquitous and affects our daily lives. The most common scenario lies in adding moisture-proof barriers for hygroscopic products before usage, e.g., salts, sugar, cellulose, and wood. In materials science research, special care for moisture–material interactions must be considered; for example, the hygroscopic nature of different types of chemicals, including salts, metal halides, acids, and bases, requires stringent storage in dry cabinets and glove boxes. Perovskite solar cells (PSCs), which are emerging and promising candidates for solar energy conversion, are highly sensitive to moisture because of the hygroscopic nature of functional metal halides (spontaneous hydration reactions); this results in either compromised device efficiency or the requirement for an additional passivation layer. Despite their unprecedented chemical and structural diversity, metal–organic frameworks (MOFs) have suffered long-term from a lack of hydrolytic stability (structural degradation under high humidity and temperature), which precluded their practical use until the recent development of more robust MOF architectures. In the era of carbon neutrality and to satisfy the demands of the Paris Agreement, direct air CO2 capture is a highly promising technique for CO2 concentration enrichment, which allows CO2 to be converted into useful carbon-based chemicals subsequently. However, the ubiquity of moisture is a significant hindrance because competition between CO2 and moisture sorption results in sorbent deactivation, particularly at high relative humidity (RH) levels and low CO2 concentrations (≈400 ppm) under ambient conditions. Hygroscopic materials also offer unique advantages. By leveraging their hygroscopic properties, diverse applications based on the moisture sorption process have recently been developed. One straightforward example is dehumidification for personal heat management, particularly in high-RH regions where the apparent temperature (feel-like temperature) is much higher than the air temperature. Another prospective application is atmospheric water harvesting (AWH) using hygroscopic materials, particularly in water-starved communities that are distant from rivers, wells, and other original water resources, or in regions where sanitation infrastructures cannot be constructed easily owing to geographical restrictions. Additionally, AWH is promising for mitigating uneven water distribution worldwide. In addition to water harvesting, power generation during the moisture sorption process, named hydrovoltaic devices is another revolutionary and sustainable alternative to renewable energy. Although the current power density of hydrovoltaic devices is not comparable to those of other types of renewable energy sources, these devices can power small appliances, such as calculators, clocks, and light-emitting diodes, for domestic usage. Finally, moisture-responsive materials can not only function as humidity sensors via either resistance change or colorimetric change, but can also serve as a switch for actuation. Moisture-responsive soft materials, such as hydrogel composites, can be used to develop physically intelligent soft robots and devices that can react to environmental or external humidity changes to change their shape in a programmable manner to achieve different tasks autonomously. Many plants exhibit humidity-driven shape-morphing behaviors, e.g., digging/burrowing their seeds under the soil and utilizing day/night humidity change cycles. Robots and devices inspired by these plants can enable novel bioinspired systems with auton