Recent advances in wood-based electrode materials for supercapacitors

In the last few years, renewable, inexpensive, biocompatible, and biodegradable wood and lignocellulose materials have triggered considerable research interest for application in various functional devices. However, there is still a lack of a new perspective to deeply understand the structure-property-function relationships of micro-/nanostructures and components of natural wood, so as to explore the potential of wood assembly of nano-energy materials with different dimensions. Herein, the recent progresses in the design and construction of free-standing supercapacitor (SC) electrodes containing wood-derived materials are reviewed. We begin with an illustrated introduction to the hierarchical structure and characteristics of natural wood with a "top-down" view, which includes the cell walls, cellulose microfibers, nanocellulose, and cellulose supramolecules. Next, the fabrication, structure, and properties of lignocellulose are highlighted. We focus on the structure-property-function relationships between the hierarchical micro- and nanoscale structure of wood and electroactive materials. The focus then turns to a summary of the recent advances in wood-based free-standing SC electrodes with a unique point that is ever out of the spotlight, including one-dimensional integrated fibers, two-dimensional flexible films/papers, three-dimensional porous hydrogels/aerogels, and ultra-thick electrodes. Finally, we put forward our perspectives on the challenges to further promoting the development of this emerging field in the future.