Energy and water are of fundamental importance for our modern society, and advanced technologies on sustainable energy storage and conversion as well as water resource management are in the focus of intensive research worldwide. Beyond their traditional biological applications, hydrogels are emerging as an appealing materials platform for energy- and water-related applications owing to their attractive and tailorable physiochemical properties. In this review, we highlight the highly tunable synthesis of various hydrogels, involving key synthetic elements such as monomer/polymer building blocks, cross-linkers, and functional additives, and discuss how hydrogels can be employed as precursors and templates for architecting three-dimensional frameworks of electrochemically active materials. We then present an in-depth discussion of the structure-property relationships of hydrogel materials based on fundamental gelation chemistry, ultimately targeting properties such as enhanced ionic/electronic conductivities, mechanical strength, flexibility, stimuli-responsiveness, and desirable swelling behavior. The unique interconnected porous structures of hydrogels enable fast charge/mass transport while offering large surface areas, and the polymer-water interactions can be regulated to achieve desirable water retention, absorption, and evaporation within hydrogels. Such structure-derived properties are also intimately coordinated to realize multifunctionality and stability for different target devices. The plethora of stimulating examples is expounded with a focus on batteries, supercapacitors, electrocatalysts, solar water purification, and atmospheric water harvesting, which showcase the unprecedented technological potential enabled by hydrogels and hydrogel-derived materials. Finally, we study the challenges and potential ways of tackling them to reveal the underlying mechanisms and transform the current development of hydrogel materials into sustainable energy and water technologies.

Hydrogels and Hydrogel-Derived Materials for Energy and Water Sustainability.

Smart window refers to the on-demand window that can dynamically modulate light transmittance. It is recognized as a promising technology to economize building energy A smart window that dynamically modulates light transmittance is crucial for building energy efficiently, and promising for on-demand optical devices. The rapid development of technology brings out different categories that have fundamentally different transmittance modulation mechanisms, including the electro-, thermo-, mechano-, and photochromic smart windows. In this review, recent progress in smart windows of each category is overviewed. The strategies for each smart window are outlined with particular focus on functional materials, device design, and performance enhancement. The advantages and disadvantages of each category are summarized, followed by a discussion of emerging technologies such as dual stimuli triggered smart window and integrated devices toward multifunctionality. These multifunctional devices combine smart window technology with, for example, solar cells, triboelectric nanogenerators, actuators, energy storage devices, and electrothermal devices. Lastly, a perspective is provided on the future development of smart windows. Smart Windows

Smart Windows: Electro-, Thermo-, Mechano-, Photochromics, and Beyond

The 2019 coronavirus outbreak (COVID-19) is affecting over 210 countries and territories, and it is spreading mainly by respiratory droplets. The use of disposable surgical masks is common for patients, doctors, and even the general public in highly risky areas. However, the current surgical masks cannot self-sterilize in order to reuse or be recycled for other applications. The resulting high economic and environmental costs are further damaging societies worldwide. Herein, we reported a unique method for functionalizing commercially available surgical masks with outstanding self-cleaning and photothermal properties. A dual-mode laser-induced forward transfer method was developed for depositing few-layer graphene onto low-melting temperature nonwoven masks. Superhydrophobic states were observed on the treated masks' surfaces, which can cause the incoming aqueous droplets to bounce off. Under sunlight illumination, the surface temperature of the functional mask can quickly increase to over 80 °C, making the masks reusable after sunlight sterilization. In addition, this graphene-coated mask can be recycled directly for use in solar-driven desalination with outstanding salt-rejection performance for long-term use. These roll-to-roll production-line-compatible masks can provide us with better protection against this severe virus. The environment can also benefit from the direct recycling of these masks, which can be used for desalinating seawater.

Reusable and Recyclable Graphene Masks with Outstanding Superhydrophobic and Photothermal Performances.

Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576, Singapore Center for Intelligent Sensors and MEMS, National University of Singapore, Singapore, 117608, Singapore Hybrid-Integrated Flexible (Stretchable) Electronic Systems Program, National University of Singapore, Singapore, 117608, Singapore NUS Suzhou Research Institute (NUSRI), Suzhou, 215123, China NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore, 117456, Singapore

Progress in wearable electronics/photonics—Moving toward the era of artificial intelligence and internet of things

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, w...

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges.

The past few years have witnessed a rapid development of solar-driven interfacial evaporation, a promising technology for low-cost water desalination. As of today, solar-to-steam conversion efficiencies close to 100% or even beyond the limit are becoming increasingly achievable in virtue of unique photothermal materials and structures. Herein, the cutting-edge approaches are summarized, and their mechanisms for photothermal structure architecting are uncovered in order to achieve ultrahigh conversion efficiency. Design principles to enhance evaporation performance and currently available salt-rejection strategies for long-term desalination are systematically investigated. The guidelines to utilize every component in solar desalination systems for simultaneous in situ energy generation are also revealed. Finally, opportunities and challenges for future works in this field are also discussed and concluded.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201903478

Structure Architecting for Salt-Rejecting Solar Interfacial Desalination to Achieve High-Performance Evaporation With In Situ Energy Generation.

Defect Engineering in Manganese‐Based Oxides for Aqueous Rechargeable Zinc‐Ion Batteries: A Review

Water scarcity is a ubiquitous problem with its magnitude expected to rise in the near future, and efforts to seek alternative water sources are on the rise. Harvesting water from air has intrigued enormous research interest among many groups with Scientific American listing this technology as the second most impactful technology that can bring about a massive change in people's lives. Though desalination offers a huge prospect in mitigating water crisis, its practicality is limited by exorbitant energy requirement. Alternatively, the air above sea water is moisture rich, with the quantity of vapor increasing at the rate of 0.41 kg m-2 . Herein, a method to sustainably harvest water from this moisture rich zone is demonstrated by employing a nanoporous superhygroscopic hydrogel, which is capable of absorbing water from highly humid atmospheres by over 420% (highest) of its own weight. The desorption process from the hydrogel, occurring at 55 °C (lowest), is triggered by natural sunlight (A.M 1.5) thereby ensuing an external energy-less water harvesting approach. The hydrogel exhibits excellent stability even after 1000 absorption/desorption cycles. Through multiple absorption/desorption cycles, it is possible to harvest over 10 L water per kg of hydrogel daily.

Solar Energy Triggered Clean Water Harvesting from Humid Air Existing above Sea Surface Enabled by a Hydrogel with Ultrahigh Hygroscopicity.

Water and energy are intimately intertwined, and it is high time to put forth integrated approaches to address the challenges and opportunities of the water–energy nexus. Herein, a novel fluidic photothermal structure integrated with a brine-drenching induced electricity generator is reported for simultaneous clean water and power production. The fluidic photothermal structure is capable of sustaining solar-thermal distillation systems (e.g. interfacial and multi-stage configurations) at high energy conversion efficiency while completely preventing salt formation from occurring in the long-term desalination process, by taking advantage of unidirectional brine fluid transportation. The brine-drenching induced electricity allows immediate power generation with a piece of wet fabric only. Furthermore, the fabric-based energy generator exhibits remarkable programmable properties, and we have also demonstrated that power generation is also achievable with different water sources such as human sweat, groundwater and rainwater, which opens up exciting applications in various fields of energy harvesting and water detection systems, and provides a promising solution for sustainable powering of wearable devices and electronics.

Yaoxin Zhang

Papers

Structure Architecting for Salt-Rejecting Solar Interfacial Desalination to Achieve High-Performance Evaporation With In Situ Energy Generation.

Defect Engineering in Manganese‐Based Oxides for Aqueous Rechargeable Zinc‐Ion Batteries: A Review

Solar Energy Triggered Clean Water Harvesting from Humid Air Existing above Sea Surface Enabled by a Hydrogel with Ultrahigh Hygroscopicity.

Manipulating unidirectional fluid transportation to drive sustainable solar water extraction and brine-drenching induced energy generation

Food-derived carbonaceous materials for solar desalination and thermo-electric power generation