Gel-Based Triboelectric Nanogenerators for Flexible Sensing:Principles,Properties,and Applications

The rapid development of the Internet of Things and artificial intelligence technologies has increased the need for wearable,portable,and self-powered flexible sensing devices.Triboelectric nanogenerators(TENGs)based on gel materials(with excellent conductivity,mechanical tunability,environmental adaptability,and biocompatibility)are considered an advanced approach for developing a new generation of flexible sensors.This review comprehensively summarizes the recent advances in gel-based TENGs for flexible sensors,covering their principles,properties,and applications.Based on the development requirements for flexible sensors,the working mechanism of gel-based TENGs and the characteristic advantages of gels are introduced.Design strategies for the performance optimization of hydrogel-,organogel-,and aerogel-based TENGs are systematically summarized.In addition,the applications of gel-based TENGs in human motion sensing,tactile sensing,health monitoring,environmental monitoring,human-machine interaction,and other related fields are summarized.Finally,the challenges of gel-based TENGs for flexible sensing are discussed,and feasible strategies are proposed to guide future research.

Multiscale cellulose-based fireproof and thermal insulation gel materials with water-regulated forms

Different forms of construction materials(e.g.,paints,foams,and boards)dramatically improve the quality of life.With the increasing environmental requirements for buildings,it is necessary to develop a comprehensive sustainable construction material that is flexible in application and exhibits excellent performance,such as fireproofing and thermal insulation.Herein,an adjustable multiform material strategy by water regulation is proposed to meet the needs of comprehensive applications and reduce environmental costs.Multiform gels are constructed based on multiscale cellulose fibers and hollow glass microspheres,with fireproofing and thermal insulation.Unlike traditional materials,this multiscale cellulose-based gel can change forms from dispersion to paste to dough by adjusting its water content,which can realize various construction forms,including paints,foams,and low-density boards according to different scenarios and corresponding needs.

Bioinspired Adaptive Gel Materials with Synergistic Heterostructures

In nature, many biological soft tissues with synergistic heterostructures, such as sea cucumbers, skeletal muscles and cartilages, exhibit high functionality to adapt to complex environments. In artificial soft materials, hydrogels are similar to biological soft tissues due to the unique integration of ＂soft and wet＂ properties and elastic characteristics. However, currently hydrogel materials lack their necessary adaptability, including narrow working temperature windows and uncontrollable mechanics, thus restrict their engineering application in complex environments. Inspired by abovementionedbiological soft tissues, researchers have increasingly developed heterostructural gel materials as functional soft materials with high adaptability to various mechanical and environmental conditions. This article summarizes our recent work on high-performance adaptive gel materials with synergistic heterostructures, including the critical design criteria and the state-of-the-art fabrication strategies of our gel materials. The functional adaptation properties of these heterostructural gel materials are also presented in details, including temperature, wettability, mechanical and shape adaption.

Cellulose-based functional gels and applications in flexible supercapacitors

In order to resolve the global crisis of fossil energy shortage and climate warming,the development of efficient energy storage devices is a significant topic at present.Supercapacitors as the novel type of energy storage devices have the unique advantages,including the fast charging/discharging behaviors,high-energy/power density,and stable cycling performance.Compared with traditional supercapacitors,flexible supercapacitors are environmen-tally friendly,light weight,small size and high safety.Therefore,flexible supercapacitors have a wide application prospect in emerging electronic devices.Due to its flexibility,biocompatibility,and structure designability,cellu-lose and its gel materials are gradually used as electrodes,separators and electrolytes in flexible supercapacitors.Several construction processes at molecular scale for high-performance cellulose gels are summarized.Meanwhile,this review covers the recent progress of developing the flexible supercapacitors and all-in-one supercapacitors based on cellulose functional gels.We finally discussed the potential challenges and opportunities for cellulose and its derived materials in new-style flexible supercapacitors and other electronic devices.

Novel mechanical behavior of periodic structure with the pattern transformation

Abstract In periodic cellular structures, novel pattern transformations are triggered by a reversible elastic instability under the axial compression. Based on the deformation-triggered new pattern, periodic cellular structures can achieve special mechanical properties. In this paper, the designed architecture materials which include elastomer matrixes containing empty holes or filled holes with hydrogel material are modeled and simulated to investigate the mechanical property of the periodic materials. By analyzing the relationship between nominal stress and nominal strain of periodic material, and the corresponding deformed patterns, the influence of geometry and shapes of the holes on the mechanical property of architecture material is studied in more details. We hope this study can provide future perspectives for the deformation-triggered periodic structures.

Progress on nanostructured gel catalysts for oxygen electrocatalysis

Driven by the serious ecological problems,it is urgent to explore high-efficiency sustainable energy technologies.Oxygen electrocatalysis acts as important half-reactions in the emerging electrochemical energy techniques including electrolysis and batteries.Gel composites exhibit the merits of rich porous,superior hydrophilic,and large specific surface area,which can significantly improve the electrolyte penetration and boost the kinetics process of oxygen electrocatalysis.In this invited contribution,the advances and challenges of a novel gel materials for oxygen electrocatalysis are summarized.Starting from the structure-activity-performance relationship of gel materials,synthetic routes of nanostructured gel materials,namely,radical polymerization,sol-gel method,hydrothermal/solvothermal reactions,and ligand-substitution method,are introduced.Afterward,the gel composites are divided into polymer-based,metal-based,and carbon-based materials in turn,and their applications in oxygen electrocatalysis are discussed respectively.At the end,the perspective and challenges for advanced gel oxygen electrocatalysts are proposed.

Gelation of polymeric nanoparticles

We review how, starting from polymeric nanoparticles, to generate clusters of fractal morphology and to expand the entire space and interconnect to form gels, through either Brownian motion or intense shear-induced aggregation. In the case of Brownian motion-induced gelation, specific techniques developed to obtain uniform structure of gels under both reaction-limited and diffusion-limited cluster aggregation conditions have been described. In the case of intense shear-induced gelation as a newly developed technique, our focus is on its principle, theoretical development and advantages with respect to Brownian motion-induced gelation in practical applications. We consider gelation of both rigid and soft particles. As a physical process, the bonding between the particles within gels is owed to van der Waals attractions, thus being easily broken. However, in the case of soft particles that can coalesce upon contact, the coalescence can allow the particles to stick together forming permanent gels. In this case, the gel structure can be controlled by controlling the degree of coalescence. Techniques used to control the degree of coalescence have also been described.

Characterization and Electrochemical Properties of Nanostructured Zr-Doped Anatase TiO_2 Tubes Synthesized by Sol–Gel Template Route

A series of nanostructured Zr-doped anatase TiO_2 tubes with the Zr/Ti molar ratio of 0.01, 0.02, 0.03, and0.09 were prepared by a sol–gel technology on a carbon fiber template. The electrochemical performance of Zr-doped anatase TiO_2 as anodes for rechargeable lithium batteries was investigated and compared with undoped titania. Tests represented that after 35-fold charge/discharge cycling at C/10 the reversible capacity of Zr-doped titania（Zr/Ti = 0.03） reaches 135 m A h g^（-1）, while the capacity of undoped titania（Zr/Ti = 0） yielded only 50 m A h g^（-1）. Based on the results of the physicochemical investigation, three reasons of improving electrochemical performance of Zr-doped titania were suggested. According to the scanning electron microscopy and transmission electron microscopy, Zr^（4＋） doping induces a decrease in nanoparticle size, which facilitates the Li＋diffusion. The Raman investigations show the more open structure of Zr-doped TiO_2 as compared to undoped titania due to changing of the unit cell parameters, that significantly affects on the reversibility of the insertion/extraction process. The electrochemical impedance spectroscopy results indicate that substitution of Zr^（4＋） for Ti^（4＋） into anatase TiO_2 has favorable effects on the conductivity.

Dynamic covalent gels assembled from small molecules：from discrete gelators to dynamic covalent polymers

Dynamic covalent chemistry has emerged recently to be a powerful tool to construct functional materials.This article reviews the progress in the research and development of dynamic covalent chemistry in gels assembled from small molecules.First dynamic covalent reactions used in gels are reviewed to understand the dynamic covalent bonding.Afterwards the catalogues of dynamic covalent gels are reviewed according to the nature of gelators and the interactions between gelators.Dynamic covalent bonding can be involved to form low molecular weight gelators.Low molecular weight molecules with multiple functional groups react to form dynamic covalent cross-linked polymers and act as gelators.Two catalogues of gels show different properties arising from their different structures.This review aims to illustrate the structure-property relationships of these dynamic covalent gels.

Fabrication of NiO coated SiO_2 and SiO_2 coated NiO for the removal of Pb^(2+) ions

We have prepared silica,SiO2coated NiO and NiO coated SiO2by sol-gel method.The physicochemical properties of the desired materials were investigated by surface charge properties,scanning electron microscopy（SEM）,energy dispersive X-ray（EDX） spectroscopy,surface area measurements and X-ray diffraction（XRD） analyses.The point of zero charge（PZC） of the solid was determined by the salt addition method.In coated materials,two PZC values were noted representing the surface charge of their counterparts.The SEM image of SiO2coated NiO displays a uniform coating of silica on the surface of NiO whereas in case of NiO coated SiO2,a honeycomb like appearance was observed with highly porous structures.In the diffractograms of NiO,the characteristic peaks were suppressed in NiO coated silica however,no diffraction peak could be seen in SiO2coated NiO.Batch adsorption technique was applied for the removal of Pb2＋ions from aqueous solution.The sorption trend for Pb2＋ions was observed in the order of NiO coated SiO2〉 SiO2coated NiO 〉 NiO 〉 SiO2.This trend confirms that the coated materials have more sorption capacities than their parent counterparts.