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

Flexible electrochemical energy storage devices and related applications: recent progress and challenges

Given the escalating demand for wearable electronics, there is an urgent need to explore cost-effective and environmentally friendly flexible energy storage devices with exceptional electrochemical properties. However, the existing types of flexible energy storage devices encounter challenges in effectively integrating mechanical and electrochemical performances. This review is intended to provide strategies for the design of components in flexible energy storage devices (electrode materials, gel electrolytes, and separators) with the aim of developing energy storage systems with excellent performance and deformability. Firstly, a concise overview is provided on the structural characteristics and properties of carbon-based materials and conductive polymer materials utilized in flexible energy storage devices. Secondly, the fabrication process and strategies for optimizing their structures are summarized. Subsequently, a comprehensive review is presented regarding the applications of carbon-based materials and conductive polymer materials in various fields of flexible energy storage, such as supercapacitors, lithium-ion batteries, and zinc-ion batteries. Finally, the challenges and future directions for next-generation flexible energy storage systems are proposed.

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.

Manipulating the Crystallization of Tin Halide Perovskites for Efficient Moisture-to-Electricity Conversion

Manipulating the crystallization of perovskite in thin films is essential for the fabrication of any thin-film-based devices. Fabricating tin-based perovskite films from solution poses difficulties because tin tends to crystallize faster than the commonly used lead perovskite. To achieve optimal device performance in solar cells, the preferred method involves depositing tin perovskite under inert conditions using dimethyl sulfoxide (DMSO), which effectively retards the formation of the tin-bromine network, which is crucial for perovskite assembly. We found that under ambient conditions, a DMSO-based tin perovskite salt solution resulted in the formation of a two-phase system, SnBr4(DMSO)2 and MABr, whereas a dimethylformamide-based solution resulted in the formation of vacancy-ordered double perovskite MA2SnBr6. Humidity is known to solvate MABr to form the solvated ions, and so we used the two-phase system for the application in moisture to electricity conversion. The importance of the presence of the scaffold can be seen with the negligible power output from the vacancy-ordered double perovskite obtained with MA2SnBr6. We have fabricated a device with two-phase system that can generate an open-circuit potential of 520 mV and a short-circuit current density of 30.625 μA/cm2 at 85% RH. Also, the device charges a 10 μF capacitor from 150 mV at 51% RH to 500 mV at 85% RH in 6 s at a rate of 52.5 mV/s. Moreover, the output can be scaled by connecting devices in series and parallel configurations. A 527 nm green LED was powered by connecting five devices in series at 75% RH. This indicates a potential for utilizing these moisture-to-electricity conversion devices in powering low-energy requirement devices.

Advances in Energy Harvesting Technologies for Wearable Devices

The development of wearable electronics is revolutionizing human health monitoring, intelligent robotics, and informatics. Yet the reliance on traditional batteries limits their wearability, user comfort, and continuous use. Energy harvesting technologies offer a promising power solution by converting ambient energy from the human body or surrounding environment into electrical power. Despite their potential, current studies often focus on individual modules under specific conditions, which limits practical applicability in diverse real-world environments. Here, this review highlights the recent progress, potential, and technological challenges in energy harvesting technology and accompanying technologies to construct a practical powering module, including power management and energy storage devices for wearable device developments. Also, this paper offers perspectives on designing next-generation wearable soft electronics that enhance quality of life and foster broader adoption in various aspects of daily life.

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.

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.

Green moisture-electric generator based on supramolecular hydrogel with tens of milliamp electricity toward practical applications

Moisture-electric generators (MEGs) has emerged as promising green technology to achieve carbon neutrality in next-generation energy suppliers, especially combined with ecofriendly materials. Hitherto, challenges remain for MEGs as direct power source in practical applications due to low and intermittent electric output. Here we design a green MEG with high direct-current electricity by introducing polyvinyl alcohol-sodium alginate-based supramolecular hydrogel as active material. A single unit can generate an improved power density of ca. 0.11 mW cm-2, a milliamp-scale short-circuit current density of ca. 1.31 mA cm-2 and an open-circuit voltage of ca. 1.30 V. Such excellent electricity is mainly attributed to enhanced moisture absorption and remained water gradient to initiate ample ions transport within hydrogel by theoretical calculation and experiments. Notably, an enlarged current of ca. 65 mA is achieved by a parallel-integrated MEG bank. The scalable MEGs can directly power many commercial electronics in real-life scenarios, such as charging smart watch, illuminating a household bulb, driving a digital clock for one month. This work provides new insight into constructing green, high-performance and scalable energy source for Internet-of-Things and wearable applications.

Soft multi-layer actuators integrated with the functions of electrical energy harvest and storage

Soft multi-layer actuators are smart, lightweight, and flexible, which can be used in a wide range of fields such as artificial muscles, advanced medical devices, and wearable devices. The research on the actuation property of the soft actuators has made significant progress, paving the way for the controllable motions of the actuators. However, compared with the intelligence and adaptability of life in nature, these actuators still have the problem of insufficient intelligence. The phenomenon is reflected in a lack of continuous supply of energy. Therefore, it has become a development trend to combine functions such as energy harvesting, storage, and conversion with actuators to build intelligent actuators. This concept presents a synopsis of the advancements made in soft actuators that have been coupled with the capabilities of electrical energy harvesting and storage. The design concepts and typical applications of this soft smart actuators are introduced in detail. Finally, the future research directions and applications of smart actuators are prospected from our perspective.

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.

Ion-Diode-Like Heterojunction for Improving Electricity Generation from Water Droplets by Capillary Infiltration

Water-droplet-based electricity generators are emerging hydrovoltaic technologies that harvest energy from water circulation through strong interactions between water and nanomaterials. However, such devices exhibit poor current performance owing to their unclear driving force (evaporation or infiltration) and undesirable reverse diffusion current. Herein, a water-droplet-based hydrovoltaic electricity generator induced by capillary infiltration with an asymmetric structure composed of a diode-like heterojunction formed by negatively and positively charged materials is fabricated. This device can generate current densities of 160 and 450 µA cm-2 at room temperature and 65 °C, respectively. The heterojunction achieves a rectification ratio of 12, which effectively suppresses the reverse current caused by concentration differences. This results in an improved charge accumulation of ≈60 mC cm-2 in 1000 s, which is three times the value observed in the control device. When the area of the device is increased to 6 cm2 , the current increases linearly to 1 mA, thus demonstrating the scale-up potential of the generator. It has been proven that the streaming potential originates from capillary infiltration, and the presence of ion rectification. The proposed method of constructing ion-diode-like structures provides a new strategy for improving generator performance.

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.

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.

Self-chargeable supercapacitor made with MXene-bacterial cellulose nanofiber composite for wearable devices

The development of wearable electronics is restricted by the developments of supporting energy storage devices, especially flexible supercapacitors. Nowadays, miniaturized supercapacitors based on MXenes due to their obvious advantages in the specific capacity have received extensive attention. The energy existing in the surrounding environment has been used to directly charge energy storage devices. However, the hybrid wearable electronics integrated supercapacitors are mechanically connected through metal wires leading to non-compact devices. Thus, it is urgent to develop a general and universal method to fabricate high-performance robust MXene-based flexible electrodes with high electrical conductivity and apply them to self-chargeable supercapacitors and compact wearable devices. Herein, the bacterial cellulose (BC) nanofibers are used as a crosslinking agent to connect two-dimensional MXene nanosheets through the hydrogen bond, which greatly improves the mechanical strength of MXene-bacterial cellulose (MXene-BC) composite films (Young's modulus reaching 6.8 GPa). The supercapacitors made with the electrodes of MXene-BC composite films (BC content is 10%) present high capacitance behavior (areal capacitance up to 346 mF cm^-2) because the introduction of BC nanofibers increases the interlayer spacing of MXene nanosheets, providing more storage space for the ions in the electrolyte. Then, a self-chargeable supercapacitor is proposed based on the combination of a zinc-air (Zn-air) battery and a supercapacitor. The self-chargeable supercapacitor can realize self-charging after dropping a drop of electrolyte solution into the Zn-air battery. The charging voltage of a single self-chargeable supercapacitor can reach 0.6 V after adding artificial sweat as the electrolyte. Finally, a smart wristband with the function of self-charging is proposed, which can absorb the sweat generated by the human for self-chargeable supercapacitors to drive the pedometer integrated within the smart wristband to work. The proposed self-chargeable supercapacitors are simple and effective, not restricted by the use environment, providing a promising way for self-powered wearable electronics.