Hygroscopic Materials

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 autonomous smart environmental interactions. Therefore, our current special issue, " Hygroscopic Materials," highlights the research progress pertaining to the strengths of hygroscopic materials, while simultaneously circumventing their drawbacks, thus providing motivation to focus on this emerging and prospective field. This special issue contains 22 articles (2 Perspectives, 10 Research Articles, and 10 Reviews), including strategies to avoid moisture ingress in perovskite solar cells, improve CO2 capture under humid environments, sorption-based AWH, personal heat management, hydrovoltaic devices, moisture-responsive materials, and soft robotics, all of which were contributed by world-leading experts involved with fundamental research on understanding the hygroscopicity nature and applied research on extending the application scenarios of hygroscopic materials. We believe that this special issue provides insights and first-hand knowledge for the development of hygroscopic materials. i) Sorption-based AWH Harvesting water directly from ambient moisture using hygroscopic materials is effective for alleviating water scarcity and mitigating imbalanced water distribution worldwide. A comprehensive analysis by Prof. Jia Zhu and co-workers (article number 2209134) shows that water-stressed communities are characterized by high solar flux, low income, and long distance from drinking water services, thus providing solid prerequisites for employing such a low-cost and off-grid water harvesting technique. Among the AWH sorbents, hydrogels and MOFs are promising candidates with exciting research progress on performance and interesting sorption mechanisms. In the breakthrough on hydrogels, a milestone work with unprecedented water uptake of 1.79 and 3.86 g g−1 at 30% and 70% RH is realized by Prof. Evelyn Wang and co-workers (article number 2211783) through extremely high salt loading via maximized swelling. In fact, the abovementioned values are a new record for all AWH sorbents. Prof. Guihua Yu and co-workers (article number 2207786) focus on improving water uptake kinetics to enable rapid water extraction via material and device optimizations, which results in 7.9 to 19.1 L kg−1 of daily water acquisition. In terms of progress in MOFs, Prof. Monique A. van der Veen and co-workers (article number 2210050) deliver precise information regarding the structural characteristics of MOFs that results in a significant increase in water uptake at 10–30% RH. Meanwhile, Dr. Christian Serre and co-workers (article number 2211302) introduce polymorphism in Al-based MOFs, as another promising approach, for improving the performance of adsorption-driven air conditioning, which exhibits an extremely high coefficient (0.63) of performance for cooling and ultralow driving temperature of 60 °C. The mechanism and design strategies of MOF water harvesters, along with their ability to capture carbon, are demonstrated comprehensively by Prof. Hong-Cai Zhou and co-workers (article number 2209073). In addition, other emerging and cost-effective hygroscopic sorbent biopolymers are introduced and summarized by Prof. Wenshuai Chen and co-workers (article number 2209479). ii) Personal heat management Water is a suitable cooling agent because of its high heat capacity. Meanwhile, hygroscopic materials are another promising approach for atmospheric water-harvesting-based evaporative cooling. Prof. Peng Wang and co-workers (article number 2209460) summarize the material design and engineering optimization of AWH-based evaporation cooling under different application scenarios, including food storage, buildings, photovoltaic panels, and electronic device cooling. Textile cooling is vital, particularly in hot and humid environments, where human sweat does not evaporate easily. Prof. Po-Chun Hsu and co-workers (article number 2209825) provides a detailed discussion and analysis of moisture-responsive textiles based on flap opening and closing, yarn/fiber deformation, and sweat-evaporation regulation strategies for personal thermoregulation. iii) Hydrovoltaic devices Harvesting energy from moisture using hygroscopic materials necessitates the development of hydrovoltaic devices. Although these devices can power small appliances, the detailed mechanism of power generation is yet to be elucidated, particularly for devices with continuous power generation, and the application scope is confined to power generation. The two research advances presented herein attempt to address these limitations. Prof. Jun Yao and co-workers (article number 2300748) propose a generic air-gen effect and a "leaky capacitor" model to describe the process by which electricity is harvested inside hydrovoltaic devices for sustainable power output, and to predict current behaviors based on consistent experimental results. Prof. Swee Ching Tan and co-workers (article number 2208081) extend the application scope of hydrovoltaic devices to humidity-regulated hierarchical information encryption and displays. By patterning hygroscopic hydrogels with different hygroscopicities and incorporating encoding methods, their developed device successfully delivers different types of information in certain humidity ranges. In addition, three comprehensive reviews pertaining to the development of hydrovoltaic devices are presented herein. Prof. Wanlin Guo and co-workers (article number 2211165) pay attention to how to improve device performance and prolong device operation time, and also provide a brief summary regarding moisture batteries and water-splitting systems based on hygroscopic materials. Prof. Il-Doo Kim and co-workers (article number 2301080) categorize hydrovoltaic devices into diffusion, streaming potential, and capacitive types for a detailed introduction, whereas Prof. Liangti Qu and co-workers (article number 2209661) focus on hygroscopic materials and structural optimization for devices. iv) Moisture-responsive materials and soft robotics Hygroscopic materials are promising candidates for humidity-sensing and actuation applications. Among them, supramolecular nanostructures have received increasing attention because of their noncovalent nature, which guarantees rapid response, high reversibility, and prompt recovery during sensing events. Prof. Paolo Samorì and co-workers (article number 2208766) summarize the most enlightening strategies related to the use of supramolecular nanostructures for humidity sensing, where detailed operating principles and sensing mechanisms are presented. In addition to supramolecular nanostructures, liquid crystal polymers with a network structure (LCNs) are promising for use in soft robotics as they are responsive to diverse external stimuli and can undergo rapid, programmable, and complex shape morphing. However, when a liquid crystal coincides with water, the actuation effect is lost, and the resulting situation becomes complicated. Prof. Arri Priimagi and co-workers (article number 2303740) review the existing literature pertaining to aquatic soft robotic applications using either hygroscopic or nonhygroscopic LCNs, discuss the challenges associated with LCNs, and provide possible approaches for their successful use in aquatic environments. Furthermore, they develop a new liquid-crystal elastomer for light-driven nonreciprocal self-oscillators and apply it to fluidic transportation and coupling (article number 2209683). v) Moisture-resistant perovskite solar cells (PSCs) The performance and stability of PSCs are significantly affected by moisture, owing to the hygroscopicity of metal halides. Developing moisture-resilient PSC with enhanced stability is a significant prerequisite for the further commercialization of this appealing and rapid extension technique. Prof. Stefaan De Wolf and co-workers (article number 2211317) present a detailed summary of an engineering approach for maximizing moisture resistance without jeopardizing the device performance by passivating the bulk of the metal halide film, introducing passivation interlayers at the top contact, exploiting hydrophobic charge transfer layers, and encapsulating finished devices with hydrophobic barrier layers. vi) Competitive sorption of water in CO2 capture Moisture is also a barrier to direct air or post-combustion CO2 capture processes owing to competitive sorption. Therefore, a more profound understanding of CO2 capture at the molecular level using multiple techniques is required. Prof. Jorge A. R. Navarro and co-workers (article number 2211317) investigate the CO2 adsorption mechanism at the microscopic scale in an iron (III) pyrazolate-based MOF, including the framework dynamics as well as thermodynamic information, by in situ variable-temperature and pressure X-ray diffraction. Prof. George Shimizu and co-workers (article number 2301730) highlight two representative porous solids (zeolite-13X and CALF-20) based on their superior selective gas sorption capability. The water adsorption of zeolite-13X is unaffected by CO2 because CO2 adsorption occurs prior to moisture adsorption. Meanwhile, hydrogen-bonding networks are not formed in CALF-20, thus resulting in retained CO2 adsorption capacity even at an RH level of ≈40%. Additionally, this perspective addresses challenges that require interdisciplinary effort to resolve. Aside from the above articles, Prof. Bettina Valeska Lotsch and co-workers (article number 2210613) investigate the role of water in the formation of Zr-porphyrin-based MOFs and highlight the importance of water as a decisive component in Zr MOF formation. Prof. Rahul Banerjee and co-workers (article number 2209919) develop high-stability porous organic polymers via tandem reversible/irreversible bond formation for electrocatalytic water splitting and achieve state-of-the-art performance. We would like to take this opportunity to express our heartfelt gratitude to all the authors who have contributed to this special issue for their invaluable insights and unique viewpoints—thank you very much for your tremendous support! Moreover, we thank the editorial team of Advanced Materials for their approval and huge support on our special issue. We especially acknowledge Dr. Babak Mostaghaci for introducing this idea in February 2022 and providing us with his kind help along the way. We acknowledge Dr. Flóra Kiss for inviting the tentative authors and for managing and processing the manuscripts. Please accept our sincere apologies for the invited papers that did not arrive before the end of this special issue. We appreciate your great effort and wish you the best for your paper submission. We realize that with only one issue, it is only possible to explore the surface of this booming area. Acknowledging this, we hope to use this collection to highlight the significant potential of hygroscopic materials to motivate scientists with diverse backgrounds (including, but not limited to, mechanical engineering, chemistry, materials science, and electrical engineering) to focus on this rapidly expanding field to conduct more interdisciplinary research. S.C.T on behalf of the authors would like to thank Dr. Evelyn N. Wang for her invaluable and pioneering contribution as the guest editor of this special issue until the date of her confirmation (December 22, 2022) by the United States Senate to serve as the Director of the Advanced Research Projects Agency-Energy (ARPA-E), which forbids her to be a corresponding author of this editorial. Shuai Guo is currently a third-year Ph.D. student in Prof. Swee Ching Tan's lab at the National University of Singapore. His research interests focus on photothermal and moisture-responsive materials, and their applications in the solar-water-energy nexus, including water harvesting, water purification, and energy generation. Stefaan De Wolf received his Ph.D. degree in 2005 from Katholieke Universiteit Leuven in Belgium, during which he was affiliated with Imec in Belgium. From 2005 to 2008, he was admitted to the AIST, Japan. In 2008, he joined the EPFL in Switzerland. In 2016, he joined the KAUST in Saudi Arabia, where he is a professor focusing on high-efficiency silicon, perovskite, and perovskite/silicon solar cells. Metin Sitti is the director of the Physical Intelligence Department at Max Planck Institute for Intelligent Systems in Stuttgart, Germany. He is also a professor at ETH Zurich and Koç University. He was a professor at Carnegie Mellon University (2002–2014) and a research scientist at UC Berkeley (1999–2002) in the USA. He received his Ph.D. from the University of Tokyo (1999). His research interests include physical intelligence, small-scale mobile robotics, and wireless medical devices. Christian Serre (53 y/o) is a CNRS research director at the head of a research institute dedicated to Porous Materials at Ecole Normale Supérieure and ESPCI within the frame of PSL University, Paris, France, a position he has held since 2016. His expertise lies in the design of functional metal–organic frameworks and related composites, and their potential applications in health, environment, and energy. He is also the co-founder of a startup in 2021, SquairTech, dedicated to indoor air quality. Swee Ching Tan is currently an associate professor in the Department of Materials Science and Engineering at the National University of Singapore. His research interests include developing hygroscopic materials and designing solar desalination systems for water-related applications such as water harvesting, dehumidification, and water purification. He is the founder of Ultra Dry Pte. Ltd., which is a spin-off company from the National University of Singapore based on the invention of a super hygroscopic material.