Continuous Energy Harvesting from Ubiquitous Humidity Gradients using Liquid-Infused Nanofluidics

Magneto-Controlled Tubular Liquid Actuators with Pore Engineering for Liquid Transport and Regulation

Liquid manipulation using tubular actuators finds diverse applications ranging from microfluidics, printing, liquid transfer to micro-reactors. Achieving flexible and simple regulation of manipulated liquid droplets during transport is crucial for the tubular liquid actuators to perform complex and multiple functions, yet it remains challenging. Here, a facile tubular actuator for directional transport of various liquid droplets under the control of an externally applied magnetic field is presented. The surfaces of the actuator can be engineered with submillimeter-sized through-hole pores, which enables the liquid droplet to be easily modulated in the transport process. Furthermore, the liquid actuator with featured through-hole pores is expanded to function as a switch in an integrated external electric circuit by magnetically controlling the motion of a conductive liquid droplet. This work develops a strategy for regulating liquid droplets in the tubular actuation systems, which may inspire ideas for designing functional liquid actuators with potential applications in microfluidics, microchemical reaction, liquid switch, and liquid robotics.

Biomimetic Superwetting Fabric for Evaporation-Induced Body Sweat and Heat Management and Electricity Generation

Solar optothermal evaporation of water possesses the potential for thermal regulation and electricity generation, which are desirable for regulating body perspiration and heat as well as improving electrical output and strain sensing. However, ordinary fabrics exhibit poor evaporation capacity and antifouling performance due to limited adsorption capacity and internal hydrophilicity. Moreover, conventional evaporation-driven generators show a low power supply without widely practical use due to limited and fluctuating evaporation rates. Herein, an antifouling cooling fabric with an evaporation-driven electricity performance is obtained by constructing Janus channels on the superomniphobic fabric. Sweat can be easily eliminated from inside to outside through Janus channels by efficient evaporation, and the green liquid metal ink (CGM/LMP-rGO@PPy) cotton fabric shows a thermal conductivity of 0.18 W m-1 K-1, suggesting a comfortable dry and cooling sense. Meanwhile, the fabric can stably output a potential of 302.20 mV when seawater flows through the ionic channels at an evaporation rate of 1.58 mL h-1 with one sun power density. In addition, the multifunctional fabric demonstrates strain sensing at high electrical conductivity for body motion monitoring. This work would offer a prospect for intelligent textile construction and energy harvesting by water evaporation.

Microphase-Separated Elastic and Ultrastretchable Ionogel for Reliable Ionic Skin with Multimodal Sensation

Bioinspired artificial skins integrated with reliable human-machine interfaces and stretchable electronic systems have attracted considerable attention. However, the current design faces difficulties in simultaneously achieving satisfactory skin-like mechanical compliance and self-powered multimodal sensing. Here, this work reports a microphase-separated bicontinuous ionogel which possesses skin-like mechanical properties and mimics the multimodal sensing ability of biological skin by ion-driven stimuli-electricity conversion. The ionogel exhibits excellent elasticity and ionic conductivity, high toughness, and ultrastretchability, as well as a Young's modulus similar to that of human skin. Leveraging the ion-polymer interactions enabled selective ion transport, the ionogel can output pulsing or continuous electrical signals in response to diverse stimuli such as strain, touch pressure, and temperature sensitively, demonstrating a unique self-powered multimodal sensing. Furthermore, the ionogel-based I-skin can concurrently sense different stimuli and decouple the variations of the stimuli from the voltage signals with the assistance of a machine-learning model. The ease of fabrication, wide tunability, self-powered multimodal sensing, and the excellent environmental tolerance of the ionogels demonstrate a new strategy in the development of next-generation soft smart mechano-transduction devices.

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.

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.

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.

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.

Wafer-Level, High-Performance, Flexible Sensors Based on Organic Nanoforests for Human-Machine Interactions

High-performance flexible sensors are essential for real-time information analysis and constructing noncontact communication modules for emerging human-machine interactions. In these applications, batch fabrication of sensors that exhibit high performance at the wafer level is in high demand. Here, we present organic nanoforest-based humidity sensor (NFHS) arrays on a 6 in. flexible substrate prepared via a facile, cost-effective manufacturing approach. Such an NFHS achieves state-of-the-art overall performance: high sensitivity and fast recovery time; the best properties are at a small device footprint. The high sensitivity (8.84 pF/% RH) and fast response time (5 s) of the as-fabricated organic nanoforests are attributed to the abundant hydrophilic groups, the ultra-large surface area with a huge number of nanopores, and the vertically distributed structures beneficial to the transfer of molecules up and down. The NFHS also exhibits excellent long-term stability (90 days), superior mechanical flexibility, and good performance repeatability after bending. With these superiorities, the NFHS is further applied as a smart noncontact switch, and the NFHS array is used as the motion trajectory tracker. The wafer-level batch fabrication capability of our NFHS provides a potential strategy for developing practical applications of such humidity sensors.

Confined Ionic-Liquid-Mediated Cation Diffusion through Layered Membranes for High-Performance Osmotic Energy Conversion

Ion-selective membranes act as the core components in osmotic energy harvesting, but remain with deficiencies such as low ion selectivity and a tendency to swell. 2D nanofluidic membranes as competitive candidates are still subjected to limited mass transport brought by insufficient wetting and poor stability in water. Here, an ionic-liquid-infused graphene oxide (GO@IL) membrane with ultrafast ion transport ability is reported, and how the confined ionic liquid mediates selective cation diffusion is revealed. The infusion of ionic liquids endows the 2D membrane with excellent mechanical strength, anti-swelling properties, and good stability in aqueous electrolytes. Importantly, immiscible ionic liquids also provide a medium, allowing partial dehydration for ultrafast ion transport. Through molecular dynamics simulation and finite element modeling, that GO nanosheets induce ionic liquids to rearrange, bringing in additional space charges, which can be coupled with GO synergistically, is proved. By mixing 0.5/0.01 m NaCl solution, the power density can achieve a record value of ≈6.7 W m^-2 , outperforming state-of-art GO-based membranes. This work opens up a new route for boosting nanofluidic energy conversion because of the diversity of the ILs and 2D materials.

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.

Application of nanoarchitectonics in moist-electric generation

The consumption of energy is an important resource that cannot be ignored in modern society. Non-renewable forms of energy, such as coal, natural gas, and oil, have always been important strategic resources and are always facing a crisis of shortage. Therefore, there is an urgent need for green renewable forms of energy. As an emerging green energy source, the moist-electric generator (MEG) has been studied in recent years and may become an energy source that can be utilized in daily life. Along with the advancement of technological means, nanoarchitectonics play an important role in MEG devices. This review aims to provide a comprehensive summary of the fundamentals of the MEG from the perspective of different material classifications and to provide guidance for future work in the field of MEGs. The effects of various parameters and structural designs on the output power, recent important literature and works, the mechanism of liquid-solid interactions at the nanoscale, and the application status and further potential of MEG devices are discussed in this review. It is expected that this review may provide valuable knowledge for future MEG research.

An Asymmetric Hygroscopic Structure for Moisture-Driven Hygro-Ionic Electricity Generation and Storage

The interactions between moisture and materials give rise to the possibility of moisture-driven energy generation (MEG). Current MEG materials and devices only establish this interaction during water sorption in specific configurations, and conversion is eventually ceased by saturated water uptake. This paper reports an asymmetric hygroscopic structure (AHS) that simultaneously achieves energy harvesting and storage from moisture absorption. The AHS is constructed by the asymmetric deposition of a hygroscopic ionic hydrogel over a layer of functionalized carbon. Water absorbed from the air creates wet-dry asymmetry across the AHS and hence an in-plane electric field. The asymmetry can be perpetually maintained even after saturated water absorption. The absorbed water triggers the spontaneous development of an electrical double layer (EDL) over the carbon surface, which is termed a hygro-ionic process, accounting for the capacitive properties of the AHS. A peak power density of 70 µW cm^-3  was realized after geometry optimization. The AHS shows the ability to be recharged either by itself owing to a self-regeneration effect or via external electrical means, which allows it to serve as an energy storage device. In addition to insights into moisture-material interaction, AHSs further shows potential for electronics powering in assembled devices.

Ionic Hydrogel for Efficient and Scalable Moisture-Electric Generation

The progress of spontaneous energy generation from ubiquitous moisture is hindered the low output current and intermittent operating voltage of the moisture-electric generators. Herein a novel and efficient ionic hydrogel moisture-electric generator (IHMEG) is developed by rational combination of poly(vinyl alcohol), phytic acid, and glycerol-water binary solvent. Thanks to the synergistic effect of notable moisture-absorption capability and fast ion transport capability in the ionic hydrogel network, a single IHMEG unit of 0.25 cm² can continuously generate direct-current electricity with a constant open-circuit voltage of ≈0.8 V for over 1000 h, a high short-current density of 0.24 mA cm^-2 , and power density of up to 35 µW cm^-2 . Of great importance is that large-scale integration of IHMEG units can be readily accomplished to offer a device with voltage up to 210 V, capable of directly driving numerous commercial electronics, including electronic ink screen, metal electrodeposition setup, and light-emitting-diode arrays. Such prominent performance is mainly attributed to the enhanced moisture-liberated proton diffusion proved by experimental observation and theoretical analysis. The ionic hydrogel with high cost-efficiency, easy-to-scaleup fabrication, and high power-output opens a brand-new perspective to develop a green, versatile, and efficient power source for Internet-of-Things and wearable electronics.

Insights into Ionic Liquids: From Z-Bonds to Quasi-Liquids

Ionic liquids (ILs) hold great promise in the fields of green chemistry, environmental science, and sustainable technology due to their unique properties, such as a tailorable structure, the various types available, and their environmentally friendly features. On the basis of multiscale simulations and experimental characterizations, two unique features of ILs are as follows: (1) strong coupling interactions between the electrostatic forces and hydrogen bonds, namely in the Z-bond, and (2) the unique semiordered structure and properties of ultrathin films, specifically regarding the quasi-liquid. In accordance with the aforementioned theoretical findings, many cutting-edge applications have been proposed: for example, CO₂ capture and conversion, biomass conversion and utilization, and energy storage materials. Although substantial progress has been made recently in the field of ILs, considerable challenges remain in understanding the nature of and devising applications for ILs, especially in terms of e.g. in situ/real-time observation and highly precise multiscale simulations of the Z-bond and quasi-liquid. In this Perspective, we review recent developments and challenges for the IL research community and provide insights into the nature and function of ILs, which will facilitate future applications.