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.

The demand for flexible power sources with high energy density and durability has increased rapidly with the development of flexible and wearable electronic devices. Metal–air batteries are considered as the most promising candidates for these applications due to their excellent theoretical energy densities. In particular, rechargeable zinc–air and lithium–air batteries have attracted much attention because of their potential to offer high energy density while maintaining a long operational life. Although significant progress has been made in enhancing the electrochemical performance of these batteries, many technical challenges still remain to achieve the mechanical flexibility required for wearable electronic devices while maintaining high performance. This article describes the most recent advances and challenges in the development of flexible zinc–air and lithium–air batteries. We start with an overview of the latest innovations in the exploration of various battery configurations to effectively accommodate stresses and strains associated with the use of flexible electronic devices. This is followed by a detailed review of the advancements made in the design of flexible battery components: the metal electrode, the electrolyte membrane, and the air electrode. Furthermore, the effects of operating conditions on battery performance characteristics and durabilities are discussed, including the effect of the operating temperature and the contaminants commonly encountered in ambient air (e.g., carbon dioxide and moisture). Finally, challenges facing the development of a new generation of flexible metal–air batteries are highlighted, together with further research directions and perspectives.

Flexible Zn– and Li–air batteries: recent advances, challenges, and future perspectives

Aqueous metal-air batteries: Fundamentals and applications

Advances and challenges in lithium-air batteries

Metal–air batteries (MABs), which possess exceptionally high energy density and exhibit other ideal features such as low cost, environmental benignity and safety, are regarded as promising candidates for the next generation of power sources. The performance of MABs and the challenges involved in these systems are primarily related to metal electrodes. In the present work, different types of MABs are overviewed from the perspective of the metal electrodes. Most metal electrodes that have been studied in recent years are reviewed, among which Zn, Al, Mg and Fe are highlighted. The advantages and disadvantages of each system are presented, and recent advances that address challenges such as corrosion, passivation and dendrite growth are introduced. In addition, investigations focused on revealing interactions between the metal electrodes and electrolytes or exploring electrolytes to improve the performance of metal electrodes are also discussed. Finally, a general perspective on the current situation of this field and on future research directions is provided.

Recent advances and challenges in divalent and multivalent metal electrodes for metal–air batteries

A mixed-pH dual-electrolyte microfluidic aluminum–air cell with high performance

http://hub.hku.hk/bitstream/10722/241582/1/content.pdf

A Hydrogel Template Synthesis of TiO2 Nanoparticles for Aluminium-ion Batteries ☆

https://spiral.imperial.ac.uk/bitstream/10044/1/57644/6/JPS-Alair.pdf

A high specific capacity membraneless aluminum-air cell operated with an inorganic/organic hybrid electrolyte

A Low-Cost Mechanically Rechargeable Aluminum–Air Cell for Energy Conversion Using Low-Grade Aluminum Foil

A novel facile and cost-effective synthesis method for anatase TiO2 nanoparticles has been developed by using poly-acrylic acid hydrogel as template at room temperature. The newly developed synthesis method avoids the use of hazardous reagents and/or hydrothermal steps, and enables production of highly active TiO2 nanoparticles from low cost inorganic titanium sources. The synthesized TiO2 nanoparticles have been studied in several applications including dye-sensitized solar cells as a photoanode as well as in organics degradation of methyl orange in aqueous media. Good photocatalytic performances were obtained in both applications.

/pdf/polymeric-templating-synthesis-of-anatase-tio2-nanoparticles-5dvqqe3oen.pdf

Binbin Chen

Papers

A mixed-pH dual-electrolyte microfluidic aluminum–air cell with high performance

A Hydrogel Template Synthesis of TiO2 Nanoparticles for Aluminium-ion Batteries ☆

A high specific capacity membraneless aluminum-air cell operated with an inorganic/organic hybrid electrolyte

A Low-Cost Mechanically Rechargeable Aluminum–Air Cell for Energy Conversion Using Low-Grade Aluminum Foil

Polymeric Templating Synthesis of Anatase TiO2 Nanoparticles from Low‐Cost Inorganic Titanium Sources