Moisture adsorption-desorption full cycle power generation

Hydrovoltaic Effects from Mechanical-Electric Coupling at the Water-Solid Interface

The natural water cycle on the Earth carries an enormous amount of energy as thirty-five percent of solar energy reaching the Earth's surface goes into water. However, only a very marginal part of the contained energy, mostly kinetic energy of large volume bulk water, is harvested by hydroelectric power plants. Natural processes in the water cycle, such as rainfall, water evaporation, and moisture adsorption, are widespread but have remained underexploited in the past due to the lack of appropriate technologies. In the past decade, the emergence of hydrovoltaic technology has provided ever-increasing opportunities to extend the technical capability for energy harvesting from the water cycle. Featuring electricity generation from mechanical-electric coupling at the water-solid interface, hydrovoltaic technology embraces almost all dynamic processes associated with water, including raining, waving, flowing, evaporating, and moisture adsorbing. This versatility in dealing with various forms of water and associated energy renders hydrovoltaic technology a solution for fossil fuel-caused environmental problems. Here, we review the current progress of hydrovoltaic energy harvesting from water motion, evaporation, and ambient moisture. Device configuration, energy conversion mechanism mediated by mechanical-electric coupling at various water-solid interfaces, as well as materials selection and functionalization are discussed. Useful strategies guided by established mechanisms for device optimization are then covered. Finally, we provide an outlook on this emerging field and outline the challenges of improving output performance toward potential practical applications.

Wetting Behavior-Induced Interfacial transmission of Energy and Signal: Materials, Mechanisms, and Applications

Wetting behaviors can significantly affect the transport of energy and signal (E&S) through vapor, solid, and liquid interfaces, which has prompted increased interest in interfacial science and technology. E&S transmission can be achieved using electricity, light, and heat, which often accompany and interact with each other. Over the past decade, their distinctive transport phenomena during wetting processes have made significant contributions to various domains. However, few studies have analyzed the intricate relationship between wetting behavior and E&S transport. This review summarizes and discusses the mechanisms of electrical, light, and heat transmission at wetting interfaces to elucidate their respective scientific issues, technical characteristics, challenges, commonalities, and potential for technological convergence. The materials, structures, and devices involved in E&S transportation are also analyzed. Particularly, harnessing synergistic advantages in practical applications and constructing advanced, multifunctional, and highly efficient smart systems based on wetted interfaces is the aim to provide strategies.

Radiative cooling assisted self-sustaining and highly efficient moisture energy harvesting

Harvesting electricity from ubiquitous water vapor represents a promising route to alleviate the energy crisis. However, existing studies rarely comprehensively consider the impact of natural environmental fluctuations on electrical output. Here, we demonstrate a bilayer polymer enabling self-sustaining and highly efficient moisture-electric generation from the hydrological cycle by establishing a stable internal directed water/ion flow through thermal exchange with the ambient environment. Specifically, the radiative cooling effect of the hydrophobic top layer prevents the excessive daytime evaporation from solar absorption while accelerating nighttime moisture sorption. The introduction of LiCl into the bottom hygroscopic ionic hydrogel enhances moisture sorption capacity and facilitates ion transport, thus ensuring efficient energy conversion. A single device unit (1 cm2) can continuously generate a voltage of ~0.88 V and a current of ~306 μA, delivering a maximum power density of ~51 μW cm-2 at 25 °C and 70% relative humidity (RH). The device has been demonstrated to operate steadily outdoors for continuous 6 days.

Ambient Moisture-Driven Self-Powered Iontophoresis Patch for Enhanced Transdermal Drug Delivery

Iontophoretic transdermal drug delivery (TDD) devices are known to enhance the transdermal transport of drugs. However, conventional transdermal iontophoretic devices require external power sources, wired connections, or mechanical parts, which reduce the comfort level for patients during extended use. In this work, a self-powered, wearable transdermal iontophoretic patch (TIP) is proposed by harvesting ambient humidity for energy generation, enabling controlled TDD. This patch primarily uses moist-electric generators (MEGs) as its power source, thus obviating the need for complex power management modules and mechanical components. A single MEG unit can produce an open-circuit voltage of 0.80 V and a short-circuit current of 11.65 µA under the condition of 80% relative humidity. Amplification of the electrical output is feasible by connecting multiple generator units in series and parallel, facilitating the powering of certain commercial electronic devices. Subsequently, the MEG array is integrated with the TDD circuit to create the wearable TIP. After 20 min of application, the depth of drug penetration through the skin is observed to increase threefold. The effective promotion effect of TIP on the transdermal delivery of ionized drugs is corroborated by simulations and experiments. This wearable TIP offers a simple, noninvasive solution for TDD.

Entangled Mesh Hydrogels with Macroporous Topologies via Cryogelation for Rapid Atmospheric Water Harvesting

Sorption-based atmospheric water harvesting (SAWH) is a promising technology to alleviate freshwater scarcity. Recently, hygroscopic salt-hydrogel composites (HSHCs) have emerged as attractive candidates with their high water uptake, versatile designability, and scale-up fabrication. However, achieving high-performance SAWH applications for HSHCs has been challenging because of their sluggish kinetics, attributed to their limited mass transport properties. Herein, a universal network engineering of hydrogels using a cryogelation method is presented, significantly improving the SAWH kinetics of HSHCs. As a result of the entangled mesh confinements formed during cryogelation, a stable macroporous topology is attained and maintained within the obtained entangled-mesh hydrogels (EMHs), leading to significantly enhanced mass transport properties compared to conventional dense hydrogels (CDHs). With it, corresponding hygroscopic EMHs (HEMHs) simultaneously exhibit faster moisture sorption and solar-driven water desorption. Consequently, a rapid-cycling HEMHs-based harvester delivers a practical freshwater production of 2.85 Lwater kgsorbents -1 day-1 via continuous eight sorption/desorption cycles, outperforming other state-of-the-art hydrogel-based sorbents. Significantly, the generalizability of this strategy is validated by extending it to other hydrogels used in HSHCs. Overall, this work offers a new approach to efficiently address long-standing challenges of sluggish kinetics in current HSHCs, promoting them toward the next-generation SAWH applications.

Hierarchical Bilayer Polyelectrolyte Ion Paper Conductor for Moisture-Induced Power Generation

Harvesting energy from air water (atmospheric moisture) promises a sustainable self-powered system without any restrictions from specific environmental requirements (e.g., solar cells, hydroelectric, or thermoelectric devices). However, the present moisture-induced power devices traditionally generate intermittent or bursts of energy, especially for much lower current outputs (generally keeping at nA or μA levels) from the ambient environment, typically suffering from inferior ionic conductivity and poor hierarchical structure design for manipulating sustained air water and ion-charge transport. Here, we demonstrate a universal strategy to design a high-performance bilayer polyelectrolyte ion paper conductor for generating continuous electric power from ambient humidity. The generator can produce a continuous voltage of up to 0.74 V and also an exceptional current of 5.63 mA across a single 1.0 mm-thick ion paper conductor. We discover that the sandwiched LiCl-nanocellulose-engineered paper promises an ion-transport junction between the negatively and positively charged bilayer polyelectrolytes for application in MEGs with both high voltage and high current outputs. Moreover, we demonstrated the universality of this bilayer sandwich nanocellulose-salt engineering strategy with other anions and cations, exhibiting similar power generation ability, indicating that it could be the next generation of sustainable MEGs with low cost, easier operation, and high performance.

Polyelectrolyte Hydrogel-Functionalized Photothermal Sponge Enables Simultaneously Continuous Solar Desalination and Electricity Generation Without Salt Accumulation

Technologies that can simultaneously generate electricity and desalinate seawater are highly attractive and required to meet the increasing global demand for power and clean water. Here, a bifunctional solar evaporator that features continuous electric generation in seawater without salt accumulation is developed by rational design of polyelectrolyte hydrogel-functionalized photothermal sponge. This evaporator not only exhibits an unprecedentedly high water evaporation rate of 3.53 kg m-2 h-1along with 98.6% solar energy conversion efficiency but can also uninterruptedly deliver a voltage output of 0.972 V and a current density of 172.38 µA cm-2 in high-concentration brine over a prolonged period under one sun irradiation. Many common electronic devices can be driven by simply connecting evaporator units in series or in parallel without any other auxiliaries. Different from the previously proposed power generation mechanism, this study reveals that the water-enabled proton concentration fields in intermediate water region can also induce an additional ion electric field in free water region containing solute, to further enhance electricity output. Given the low-cost materials, simple self-regeneration design, scalable fabrication processes, and stable performance, this work offers a promising strategy for addressing the shortages of clean water and sustainable electricity.

Robustly and Intrinsically Stretchable Ionic Gel-Based Moisture-Enabled Power Generator with High Human Body Conformality

Direct harvesting of energy from moist air will be a promising route to supply electricity for booming wearable and distributed electronics, with the recent rapid development of the moisture-enabled electricity generator (MEG). However, the easy spatial distortion of rigid MEG materials under severe deformation extremely inconveniences the human body with intense physical activity, seriously hindering the desirable applications. Here, an intrinsically stretchable moisture-enabled electricity generator (s-MEG) is developed based on a well-fabricated stretchable functional ionic gel (SIG) with a flexible double-network structure and reversible cross-linking interactions, demonstrating stable electricity output performance even when stretched up to 150% strain and high human body conformality. This SIG exhibits ultrahigh tensile strain (∼600%), and a 1 cm × 1 cm SIG film-based s-MEG can generate a voltage of ∼0.4 V and a current of ∼5.7 μA when absorbing water from humidity air. Based on the strong adhesion and the excellent interface combination of SIG and rough fabric electrodes induced by the fabrication process, s-MEG is able to realize bending or twisting deformation and shows outstanding electricity output stability with ∼90% performance retention after 5000 cycles of bending tests. By connecting s-MEG units in series or parallel, an integrated device of "moisture-powered wristband" is developed to wear on the wrist of humans and drive a flexible sensor for tracking finger motions. Additionally, a comfortable "moisture-powered sheath" based on s-MEGs is created, which can be worn like clothing on human arms to generate energy while walking and flexing the elbow, which is promising in the field of wearable electronics.

The emerging chemistry of self-electrified water interfaces

Water is known for dissipating electrostatic charges, but it is also a universal agent of matter electrification, creating charged domains in any material contacting or containing it. This new role of water was discovered during the current century. It is proven in a fast-growing number of publications reporting direct experimental measurements of excess charge and electric potential. It is indirectly verified by its success in explaining surprising phenomena in chemical synthesis, electric power generation, metastability, and phase transition kinetics. Additionally, electrification by water is opening the way for developing green technologies that are fully compatible with the environment and have great potential to contribute to sustainability. Electrification by water shows that polyphasic matter is a charge mosaic, converging with the Maxwell-Wagner-Sillars effect, which was discovered one century ago but is still often ignored. Electrified sites in a real system are niches showing various local electrochemical potentials for the charged species. Thus, the electrified mosaics display variable chemical reactivity and mass transfer patterns. Water contributes to interfacial electrification from its singular structural, electric, mixing, adsorption, and absorption properties. A long list of previously unexpected consequences of interfacial electrification includes: "on-water" reactions of chemicals dispersed in water that defy current chemical wisdom; reactions in electrified water microdroplets that do not occur in bulk water, transforming the droplets in microreactors; and lowered surface tension of water, modifying wetting, spreading, adhesion, cohesion, and other properties of matter. Asymmetric capacitors charged by moisture and water are now promising alternative equipment for simultaneously producing electric power and green hydrogen, requiring only ambient thermal energy. Changing surface tension by interfacial electrification also modifies phase-change kinetics, eliminating metastability that is the root of catastrophic electric discharges and destructive explosions. It also changes crystal habits, producing needles and dendrites that shorten battery life. These recent findings derive from a single factor, water's ability to electrify matter, touching on the most relevant aspects of chemistry. They create tremendous scientific opportunities to understand the matter better, and a new chemistry based on electrified interfaces is now emerging.

Biopolymers for Hygroscopic Material Development

The effective management of atmospheric water will create huge value for mankind. Diversified and sustainable biopolymers that are derived from organisms provide rich building blocks for various hygroscopic materials. Here, a comprehensive review of recent advances in developing biopolymers for hygroscopic materials is provided. It is begun with a brief introduction of species diversity and the processes of obtaining various biopolymer materials from organisms. The fabrication of hygroscopic materials is then illustrated, with a specific focus on the use of biopolymer-derived materials as substrates to produce composites and the use of biopolymers as building blocks to fabricate composite gels. Next, the representative applications of biopolymer-derived hygroscopic materials for dehumidification, atmospheric water harvesting, and power generation are systematically presented. An outlook on future challenges and key issues worthy of attention are finally provided.

Interfacial Assembly of 2D Graphene-Derived Ion Channels for Water-Based Green Energy Conversion

The utilization of sustained and green energy is believed to alleviate increasing menace of global environmental concerns and energy dilemma. Interfacial assembly of 2D graphene-derived ion channels (2D-GDICs) with tunable ion/fluid transport behavior enables efficient harvesting of renewable green energy from ubiquitous water, especially for osmotic energy harvesting. In this review, various interfacial assembly strategies for fabricating diverse 2D-GDICs are summarized and their ion transport properties are discussed. This review analyzes how particular structure and charge density/distribution of 2D-GDIC can be modulated to minimize internal resistance of ion/fluid transport and enhance energy conversion efficiency, and highlights stimuli-responsive functions and stability of 2D-GDIC and further examines the possibility of integrating 2D-GDIC with other energy conversion systems. Notably, the presented preparation and applications of 2D-GDIC also inspire and guide other 2D materials to fabricate sophisticated ion channels for targeted applications. Finally, potential challenges in this field is analyzed and a prospect to future developments toward high-performance or large-scale real-word applications is offered.

Self-Sustained Programmable Hygroelectronic Interfaces for Humidity-Regulated Hierarchical Information Encryption and Display

The emerging moisture-driven energy generation (MEG) technology opens up new possibilities for humidity-responsive materials, devices, and interdisciplinary opportunities in fields like information security. However, such potential remains untapped. Here, an original MEG structure with a hygroionic energy-conversion route by selective coating of ionic hygroscopic hydrogels on a carbon black surface is reported. The hygroionic route features a process in which the scavenged energy is stored in the electrical double layers formed at the interfaces between the ionic hydrogel and the carbon nanoparticles. The resultant electrical field developed across the hydrogel-coated wet carbon and the rest of the dry carbon area is thus durably lasted. Based on this unique structure, hygroelectronic information interfaces (HEII) for humidity-regulated information encryption and display are put forward by devising hydrogel patterns on a carbon platform. Further by tuning the hygroscopicity of the ionic hydrogels and incorporating encoding methods (e.g., Morse code), it is demonstrated that the HEII platform is programmable to carry different information in certain humidity ranges. Unlike those conventional anti-counterfeiting methods that optically reveal the hidden information once the required stimulus is provided, the new HEII serves as a hierarchical solution for high-security encryption and display.

Moisture-Enabled Electricity from Hygroscopic Materials: A New Type of Clean Energy

Water utilization is accompanied with the development of human beings, whereas gaseous moisture is usually regarded as an underexploited resource. The advances of highly efficient hygroscopic materials endow atmospheric water harvesting as an intriguing solution to convert moisture into clean water. The discovery of hygroelectricity, which refers to the charge buildup at a material surface dependent on humidity, and the following moisture-enabled electric generation (MEG) realizes energy conversion and directly outputs electricity. Much progress has been made since then to optimize MEG performance, pushing forward the applications of MEG into a practical level. Herein, the evolvement and development of MEG are systematically summarized in a chronological order. The optimization strategies of MEG are discussed and comprehensively evaluated. Then, the latest applications of MEG are presented, including high-performance powering units and self-powered devices. In the end, a perspective on the future development of MEG is given for inspiring more researchers into this promising area.

Harvesting Energy from Atmospheric Water: Grand Challenges in Continuous Electricity Generation

Atmospheric water is ubiquitous on earth and extensively participates in the natural water cycle through evaporation and condensation. This process involves tremendous energy exchange with the environment, but very little of the energy has so far been harnessed. The recently emerged hydrovoltaic technology, especially moisture-induced electricity, shows great potential in harvesting energy from atmospheric water and gives birth to moisture energy harvesting devices. The device performance, especially the long-term operational capacity, has been significantly enhanced over the past few years. Further development; however, requires in-depth understanding of mechanisms, innovative materials, and ingenious system designs. In this review, beginning with describing the basic properties of water, the key aspects of the water-hygroscopic material interactions and mechanisms of power generation are discussed. The current material systems and advances in promising material development are then summarized. Aiming at the chief bottlenecks of limited operational time, advanced system designs that are helpful to improve device performance are listed. Especially, the synergistic effect of moisture adsorption and water evaporation on material and system levels to accomplish sustained electricity generation is discussed. Last, the remaining challenges are analyzed and future directions for developing this promising technology are suggested.

Hydrovoltaic Electricity Generator with Hygroscopic Materials: A Review and New Perspective

The global energy crisis caused by the overconsumption of nonrenewable fuels has prompted researchers to develop alternative strategies for producing electrical energy. In this review, a fascinating strategy that simply utilizes water, an abundant natural substance throughout the globe and even in air as moisture, as a power source is introduced. The concept of the hydrovoltaic electricity generator (HEG) proposed herein involves generating an electrical potential gradient by exposing the two ends of the HEG device to dissimilar physicochemical environments, which leads to the production of an electrical current through the active material. HEGs, with a large variety of viable active materials, have much potential for expansion toward diverse applications including permanent and/or emergency power sources. In this review, representative HEGs that generate electricity by the mechanisms of diffusion, streaming, and capacitance as case studies for building a fundamental understanding of the electricity generation process are discussed. In particular, by comparing the use and absence of hygroscopic materials, HEG mechanism studies to establish active material design principles are meticulously elucidated. The review with future perspectives on electrode design using conducting nanomaterials, considerations for high performance device construction, and potential impacts of the HEG technology in improving the livelihoods are reviewed.

Surface Modification of Silicon Nanowires with Siloxane Molecules for High-Performance Hydrovoltaic Devices

Hydrovoltaic devices (HDs) based on silicon nanowires (SiNWs) have attracted significant attention due to their potential of high output power and good compatibility with Si-based photovoltaic devices for integrated power systems. However, it remains a major challenge to further improve the output performance of SiNW HDs for practical applications. Here, a new strategy to modify the surface of SiNWs with siloxane molecules is proposed to improve the output performance of the SiNW HDs. After modification, both the open-circuit voltage (Voc) and short-circuit current density (Jsc) of n-type SiNW HDs can be improved by approximately 30%, while the output power density can be greatly increased by over 200%. With siloxane modification, Si-OH groups on the surface of typical SiNWs are replaced by Si-O-Si chemical bonds that have a weaker electron-withdrawing capability. More free electrons in n-type SiNWs are liberated from surface bound states and participate in directed flow induced by water evaporation, thereby improving the output performance of HDs. The improved performance is significant for system integration applications as it reduces the number of required devices. Three siloxane-modified SiNW HDs in series are able to drive a 2 V light-emitting diode (LED), whereas four unmodified devices in series are initially needed for the same task. This work provides a simple yet effective strategy for surface modification to improve the output performance of SiNW HDs. Further research into the effect of different surface modifications on the performance of SiNW HDs will greatly promote their performance enhancement and practical applications.

Nanofluid-Guided Janus Membrane for High-Efficiency Electricity Generation from Water Evaporation

Harvesting electricity from widespread water evaporation provides an alternative route to cleaner power generation technology. However, current evaporation power generation (EPG) mainly depends on the dissociation process of certain functional groups (e.g., SO3 H) in water, which suffers from low power density and short-term output. Herein, the Janus membrane is prepared by combining nanofluid and water-grabbing material for EPG, where the nanoconfined ionic liquids (NCILs) serve as ion sources instead of the functional groups. Benefiting from the selective and fast transport of anions in NCILs, such EPG demonstrates excellent power performance with a voltage of 0.63 V, a short-circuit current of 140 µA, and a maximum power density of 16.55 µW cm-2 while operating for at least 180 h consistently. Molecular dynamics (MD) simulation and surface potential analysis reveal the molecular mechanism, that is, the diffusion of Cl- anions during evaporation is much faster than that of cations, generating the voltage and current across the membrane. Furthermore, the device performs well in varying environmental conditions, including different water temperatures and sources of evaporating water, showcasing its adaptability and integrability. Overall, the nanofluid-guided Janus membrane can efficiently transform low-grade thermal energy in evaporation into electricity, showing a competitive advantage over other sustainable applied approaches.

Power Generation from the Interaction of a Carbon Foam and Water

Understanding the interaction mechanisms between the surface of carbon-based materials and water is of great significance for the development of water-based energy storage and energy conversion devices. Herein, a self-supporting electric generator is demonstrated based on water adsorption on the surface of the carbon foam (CF) that works with various water resources, including deionized (DI) water, tap water, wastewater, and seawater. It is revealed that the dissociation of oxygen-containing groups on the surface of CF after water molecule adsorption leads to a reduction of the surface potential of the CF. Through surface modulation techniques such as reduction and oxidation, a balance has been uncovered between the oxygen content and conductivity for the high-performance CFs. The generator can generate an open-circuit voltage of approximately 0.6 V in natural seawater with a power density of up to 0.77 mW g-1. A high voltage of more than 2 V can be achieved easily by assembling components connected in series to drive electronic devices, such as a light-emitting diode (LED). This work demonstrates a simple and low-cost method for electricity harvesting, offering an additional option for self-powered devices.

Modified Wood Fibers Spontaneously Harvest Electricity from Moisture

With the rapid development of modern society, our demand for energy is increasing. And the extensive use of fossil energy has triggered a series of problems such as an energy crisis and environmental pollution. A moisture-enabled electric generator (MEG) is a new type of energy conversion method, which can directly convert the ubiquitous moisture in the air into electrical energy equipment. It has attracted great interest for its renewable and environmentally friendly qualities. At present, most MEGs still have low power density, strong dependence on high humidity, and high cost. Herein, we report the development of a high-efficiency MEG based on a lignocellulosic fiber frame with high-power-density, all-weather, and low-cost characteristics using a simple strategy that optimizes the charge transport channel and ion concentration difference. The MEG devices we manufactured can generate the open-circuit voltage of 0.73 V and the short-circuit current of 360 μA, and the voltage can still reach 0.6 V at less than 30% humidity. It is possible to drive commercial electronic devices such as light-emitting diodes, electronic displays, and electronic calculators by simply connecting several electric generators in series. Biomass-based moisture-enabled electric generation has a low cost, is easy to integrate on a large scale, and is green and pollution-free, providing clean energy for low-humidity or high-electricity-cost areas.

Phyto-inspired sustainable and high-performance fabric generators via moisture absorption-evaporation cycles

Collecting energy from the ubiquitous water cycle has emerged as a promising technology for power generation. Here, we have developed a sustainable moisture absorption-evaporation cycling fabric (Mac-fabric). On the basis of the cycling unidirectional moisture conduction in the fabric and charge separation induced by the negative charge channel, sustainable constant voltage power generation can be achieved. A single Mac-fabric can achieve a high power output of 0.144 W/m2 (5.76 × 102 W/m3) at 40% relative humidity (RH) and 20°C. By assembling 500 series and 300 parallel units of Mac-fabrics, a large-scale demo achieves 350 V of series voltage and 33.76 mA of parallel current at 25% RH and 20°C. Thousands of Mac-fabric units are sewn into a tent to directly power commercial electronic products such as mobile phones in outdoor environments. The lightweight (300 g/m2) and soft characteristics of the Mac-fabric make it ideal for large-area integration and energy collection in real circumstances.

Electric-Eel-Type Bi-Ionic Gradient Battery

Electric eels generate electricity with a discharge voltage of up to 860 V under ionic gradients, providing a fascinating example to inspire viable and flexible power sources. However, hitherto reported eel-related devices are strictly restricted by complicated fabrication and environmental energy input. Herein, an electric-eel-type bi-ionic gradient battery (BGB) is performed by cationic and anionic polyelectrolyte hydrogels featuring simplified units and self-energy supply. Benefiting from ionic bonds with opposite charges in the polymer chain, bianion gradients as well as ion selective migration pathways are synchronously constructed and integrated units are enabled. As a result, an open-circuit voltage of 0.54 V and a short-circuit current density of 13 μA cm-2 are generated by a BGB unit. Moreover, a voltage output up to 60 V is derived from integrated BGB devices, demonstrating the potential to drive wearable and implantable electronics. In this case, these artificial electric systems could overcome the great challenges of environmentally friendly, biocompatible, low-cost, and soft power sources, providing in-depth insights into the development of clean and sustainable power generation technologies.

Sorption-Based Atmospheric Water Harvesting: Materials, Components, Systems, and Applications

Freshwater scarcity is a global challenge posing threats to the lives and daily activities of humankind such that two-thirds of the global population currently experience water shortages. Atmospheric water, irrespective of geographical location, is considered as an alternative water source. Sorption-based atmospheric water harvesting (SAWH) has recently emerged as an efficient strategy for decentralized water production. SAWH thus opens up a self-sustaining source of freshwater that can potentially support the global population for various applications. In this review, the state-of-the-art of SAWH, considering its operation principle, thermodynamic analysis, energy assessment, materials, components, different designs, productivity improvement, scale-up, and application for drinking water, is first extensively explored. Thereafter, the practical integration and potential application of SAWH, beyond drinking water, for wide range of utilities in agriculture, fuel/electricity production, thermal management in building services, electronic devices, and textile are comprehensively discussed. The various strategies to reduce human reliance on natural water resources by integrating SAWH into existing technologies, particularly in underdeveloped countries, in order to satisfy the interconnected needs for food, energy, and water are also examined. This study further highlights the urgent need and future research directions to intensify the design and development of hybrid-SAWH systems for sustainability and diverse applications.

Low-grade wind-driven directional flow in anchored droplets

Low-grade wind with airspeed Vwind < 5 m/s, while distributed far more abundantly, is still challenging to extract because current turbine-based technologies require particular geography (e.g., wide-open land or off-shore regions) with year-round Vwind > 5 m/s to effectively rotate the blades. Here, we report that low-speed airflow can sensitively enable directional flow within nanowire-anchored ionic liquid (IL) drops. Specifically, wind-induced air/liquid friction continuously raises directional leeward fluid transport in the upper portion, whereas three-phase contact line (TCL) pinning blocks further movement of IL. To remove excessive accumulation of IL near TCL, fluid dives, and headwind flow forms in the lower portion, as confirmed by microscope observation. Such stratified circulating flow within single drop can generate voltage output up to ~0.84 V, which we further scale up to ~60 V using drop "wind farms". Our results demonstrate a technology to tap the widespread low-grade wind as a reliable energy resource.

Multistage coupling water-enabled electric generator with customizable energy output

Constant water circulation between land, ocean and atmosphere contains great and sustainable energy, which has been successfully employed to generate electricity by the burgeoning water-enabled electric generator. However, water in various forms (e.g. liquid, moisture) is inevitably discharged after one-time use in current single-stage water-enabled electric generators, resulting in the huge waste of inherent energy within water circulation. Herein, a multistage coupling water-enabled electric generator is proposed, which utilizes the internal liquid flow and subsequently generated moisture to produce electricity synchronously, achieving a maximum output power density of ~92 mW m-2 (~11 W m-3). Furthermore, a distributary design for internal water in different forms enables the integration of water-flow-enabled and moisture-diffusion-enabled electricity generation layers into mc-WEG by a "flexible building blocks" strategy. Through a three-stage adjustment process encompassing size control, space optimization, and large-scale integration, the multistage coupling water-enabled electric generator realizes the customized electricity output for diverse electronics. Twenty-two units connected in series can deliver ~10 V and ~280 μA, which can directly lighten a table lamp for 30 min without aforehand capacitor charging. In addition, multistage coupling water-enabled electric generators exhibit excellent flexibility and environmental adaptability, providing a way for the development of water-enabled electric generators.

A Continuous Gradient Chemical Reduction Strategy of Graphene Oxide for Highly Efficient Evaporation-Driven Electricity Generation

Spontaneously harvesting electricity through a water evaporation process is renewable and environmentally friendly, and provides a promising way for self-powered electronics. However, most of evaporation-driven generators are suffering from a limited power supply for practical use. Herein, a high-performance textile-based evaporation-driven electricity generator based on continuous gradient chemical reduced graphene oxide (CG-rGO@TEEG) is obtained by a continuous gradient chemical reduction strategy. The continuous gradient structure not only greatly enhances the ion concentration difference between the positive and negative electrodes but also significantly optimizes the electrical conductivity of the generator. As a result, the as-prepared CG-rGO@TEEG can generate a voltage of 0.44 V and a considerable current of 590.1 µA with an optimized power density of 0.55 mW cm-3 when 50 µL of NaCl solution is applied. Such scale-up CG-rGO@TEEGs can supply sufficient power to directly drive a commercial clock for more than 2 h in ambient conditions. This work offers a novel approach for efficient clean energy harvesting based on water evaporation.

Advances and Challenges for Hydrovoltaic Intelligence

In recent years, excessive exploitation and rapid population growth have posed numerous challenges. The climate crisis is deepening because of the unabated use of fossil fuels and the ascendance of greenhouse gas levels, so there is still an urgent need to seek different clean energy sources and electricity generating methods with the purpose of adjusting energy structures and solving environmental problems. In the ubiquitous hydrologic cycle, at least 60 petawatts (10¹⁵ W) energy can be supplied, but little of it has yet been utilized. Nowadays, hydrovoltaic intelligence has emerged and exhibited an ecofriendly concept of electricity generation compared with traditional methods with the rise of nanoscience and nanomaterials. Hence, it provides the prospect of upgrading the mode of water energy use, constructing a renewable energy industry, and alleviating environmental issues. In this review, starting by introducing different types of hydrovoltaic effect mechanisms─energy harvesting based on drawing potential of liquids; energy harvesting based on water evaporation, and energy harvesting based on moisture adsorption─we summarize the fabrication processes, material classifications, intelligent applications, and representative advances in detail. Moreover, the future development trends of hydrovoltaic intelligence and the challenges for improvement in electrical output are further discussed.

High-Performance, Highly Stretchable, Flexible Moist-Electric Generators via Molecular Engineering of Hydrogels

Harvesting energy from ubiquitous moisture has emerged as a promising technology, offering opportunities to power wearable electronics. However, low current density and inadequate stretching limit their integration into self-powered wearables. Herein, a high-performance, highly stretchable, and flexible moist-electric generator (MEG) is developed via molecular engineering of hydrogels. The molecular engineering involves the impregnation of lithium ions and sulfonic acid groups into the polymer molecular chains to create ion-conductive and stretchable hydrogels. This new strategy fully leverages the molecular structure of polymer chains, circumventing the addition of extra elastomers or conductors. A centimeter-sized hydrogel-based MEG can generate an open-circuit voltage of 0.81 V and a short-circuit current density of up to 480 µA cm^-2 . This current density is more than ten times that of most reported MEGs. Moreover, molecular engineering improves the mechanical properties of hydrogels, resulting in a stretchability of 506%, representing the state-of-the-art level in reported MEGs. Notably, large-scale integration of the high-performance and stretchable MEGs is demonstrated to power wearables with integrated electronics, including respiration monitoring masks, smart helmets, and medical suits. This work provides fresh insights into the design of high-performance and stretchable MEGs, facilitating their application to self-powered wearables and broadening the application scenario.

Water Reactivity in Electrified Interfaces: The Simultaneous Production of Electricity, Hydrogen, and Hydrogen Peroxide at Room Temperature

Hygroelectric cells deliver hydrogen, hydrogen peroxide, and electric current simultaneously at room temperature from liquid water or vapor. Different cell arrangements allowed the electrical measurements and the detection and measurement of the reaction products by two methods each. Thermodynamic analysis shows that water dehydrogenation is a non-spontaneous reaction under standard conditions, but it can occur within an open, non-electroneutral system, thus supporting the experimental results. That is a new example of chemical reactivity modification in charged interfaces, analogous to the hydrogen peroxide formation in charged aqueous aerosol droplets. Extension of the experimental methods and the thermodynamic analysis used in this work may allow the prediction of interesting new chemical reactions that are otherwise unexpected. On the other hand, this adds a new facet to the complex behavior of interfaces. Hygroelectric cells shown in this work are built from commodity materials, using standard laboratory or industrial processes that are easily scaled up. Thus, hygroelectricity may eventually become a source of energy and valuable chemicals.

Humidity-Tolerant Moisture-Driven Energy Generator with MXene Aerogel-Organohydrogel Bilayer

Free-standing and film-type moisture-driven energy generators (MEGs) that harness the preferential interaction of ionized moisture with hydrophilic materials are interesting because of their wearability and portability without needing a water container. However, most such MEGs work in limited humidity conditions, which provide a substantial moisture gradient. Herein, we present a high-performance MEG with sustainable power-production capability in a wide range of environments. The bilayer-based device comprises a negatively surface-charged, hydrophilic MXene (Ti₃C₂T_x) aerogel and polyacrylamide (PAM) ionic hydrogel. The preferential selection on the MXene aerogel of positive charges supplied from the salts and water in the hydrogel is predicted by the first-principle simulation, which results in a high electric output in a wide relative humidity range from 20% to 95%. Furthermore, by replacing the hydrogel with an organohydrogel of PAM that has excellent water retention and structural stability, a device with long-term electricity generation is realized for more than 15 days in a broad temperature range (from -20 to 80 °C). Our MXene aerogel MEGs connected in series supply sufficient power for commercial electronic components in various outdoor environments. Moreover, an MXene aerogel MEG works as a self-powered sensor for recognizing finger bending and facial expression.

Controllable Transition Metal-Directed Assembly of [Mo2O2S2]2+ Building Blocks into Smart Molecular Humidity-Responsive Actuators

Smart molecular actuators have become a cutting-edge theme due to their ability to convert chemical energy into mechanical energy under external stimulations. However, realizing actuation at the molecular level and elucidating the mechanisms for actuating still remain challenging. Herein, we design and fabricate a novel nanoscaled polyoxometalate-based humidity-responsive molecular actuator {Bi₈Mo₄₈} through the assembly of [Mo₂O₂S₂]²⁺ units, transition metals, and flexible phosphonic acid ligands. {Bi₈Mo₄₈} exhibits a semi-flexible cage-like architecture with oxygen-rich surfaces and highly negative charges 72-. The nanoscaled molecular actuator shows reversible expansion and contraction behavior under humidity variations due to lattice expansion and contraction induced by hydrogen bonding and solvation interactions between {Bi₈Mo₄₈} and water molecules. Molecular dynamics simulation was further employed to study these processes, which provides a fundamental understanding for the mechanism of humidity actuation at the molecular level.

A Machine-Learning-Enhanced Simultaneous and Multimodal Sensor Based on Moist-Electric Powered Graphene Oxide

Simultaneous multimodal monitoring can greatly perceive intricately multiple stimuli, which is important for the understanding and development of a future human-machine fusion world. However, the integrated multisensor networks with cumbersome structure, huge power consumption, and complex preparation process have heavily restricted practical applications. Herein, a graphene oxide single-component multimodal sensor (GO-MS) is developed, which enables simultaneous monitoring of multiple environmental stimuli by a single unit with unique moist-electric self-power supply. This GO-MS can generate a sustainable moist-electric potential by spontaneously adsorbing water molecules in air, which has a characteristic response behavior when exposed to different stimuli. As a result, the simultaneous monitoring and decoupling of the changes of temperature, humidity, pressure, and light intensity are achieved by this single GO-MS with machine-learning (ML) assistance. Of practical importance, a moist-electric-powered human-machine interaction wristband based on GO-MS is constructed to monitor pulse signals, body temperature, and sweating in a multidimensional manner, as well as gestures and sign language commanding communication. This ML-empowered moist-electric GO-MS provides a new platform for the development of self-powered single-component multimodal sensors, showing great potential for applications in the fields of health detection, artificial electronic skin, and the Internet-of-Things.