Cellulose-based thermoelectric composites: A review on mechanism, strategies and applications.

The ever-increasing demand for energy and environmental concerns have driven scientists to look for renewable and eco-friendly alternatives. Bio-based thermoelectric (TE) composite materials provide a promising solution to alleviate the global energy crisis due to their direct conversion of heat to electricity. Cellulose, the most abundant bio-polymer on earth with fascinating structure and desirable physicochemical properties, provides an excellent alternative matrix for TE materials. Here, recent studies on cellulose-based TE composites are comprehensively summarized. The fundamentals of TE materials, including TE effects, TE devices, and evaluation on conversion efficiency of TE materials are briefly introduced at the beginning. Then, the state-of-the-art methods for constructing cellulose-based TE composites in the forms of paper/film, aerogel, liquid, and hydrogel, are highlighted. TE performances of these composites are also compared. Following that, applications of cellulose-based TE composites in the fields of energy storage (e.g., supercapacitors) and sensing (e.g., self-powered sensors) are presented. Finally, opportunities and challenges that need investigation toward further development of cellulose-based TE composites are discussed.

Highly stretchable, fast self-healing nanocellulose hydrogel combining borate ester bonds and acylhydrazone bonds.

Self-healing hydrogels have received considerable attention as a promising material for flexible electronic devices given their mechanical durability and structurally tunable properties. In this study, a highly stretchable self-healing hydrogel with dual cross-linking network was developed via borate ester bonds generated by polyvinyl alcohol and borax, and acylhydrazone bonds formed by aldehyde nanocellulose with adipic acid dihydrazide-modified alginate. Compared with the single network hydrogel composed of polyvinyl alcohol and borax, the introduction of dynamic acylhydrazone bonds greatly increases the flexibility of the hydrogel. The elongation rate increased from 480 % to approximately 1440 %, and the self-healing efficiency increased from 84.6 % to 92.7 % after healing for 60 min at ambient temperature without any stimulus. Moreover, the longer the self-healing time, the more evident the self-healing effect of the acylhydrazone bonds. In addition, electrical measurements confirmed a wide working strain range (ca.1000 %), durability, and reliability. Once assembled as a strain sensor, the hydrogel is able to monitor both large and subtle human motions. Besides, this hydrogel exhibited desirable biocompatibility, as demonstrated by in vitro cytotoxicity towards NIH 3T3 cells. These integrated properties make this nanocomposite hydrogel a promising candidate for future applications as green, flexible, and smart sensors.