Nanostructured paper for flexible energy and electronic devices
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ntroduction For thousands of years, cellulose paper has been a major medium for displaying and transmitting information in many parts of the world. Its chemical and mechanical stability under atmospheric conditions and ability to absorb ink readily remain unmatched by other materials used in large abundance. Cellulose, the major component of paper, can be obtained from plants and represents one of the most abundant organic materials on earth. In the past decade, research on nanostructures of cellulose has increased dramatically due to the potential applications in electronics, biosensors, and energy storage devices. 1–4 Large-scale, energy-efficient production of nanofibrillated cellulose (NFC) has recently become possible by employing various physical, chemical, and enzymatic pretreatment methods before the homogenization step.1 In parallel, the development of nanostructured inorganic materials in the form of nanocrystals, nanowires, and nanotubes provides a list of functional inks for integration into paper.5–9 Cellulose by itself is usually limited in functionalities. However, the three-dimensional (3D) hierarchical structures formed by cellulose fibers at different length scales, combined with the ability to accommodate other functional materials, open up many opportunities for applications in electrical, electrochemical, and optical devices.
The focus of this article is to present recent progress in the development of energy and electronics devices fabricated using wood fiber cellulose as the building block in conjunction with other nanomaterials. The first part of this article will focus on the hierarchical structure of wood cellulose, as well as the fabrication and properties of paper. In particular, regular cellulose fibers with a diameter of ∼20 μm and nanocellulose fibers with a diameter of ∼20 nm will be discussed in detail. The second part will focus on the recent development of conductive paper for energy devices, particularly for ultracapacitors and batteries. The last part will focus on the development of transparent nanocellulose paper and its potential applications in electronics and optoelectronic devices.
Cellulose: The building block The cell wall of wood has a fascinating 3D hierarchical structure designed to maximize the stability and durability of the trees. The wood fiber is made up of crystalline cellulose nanofibrils (around 40 wt% of the wood), random amorphous hemicellulose (around 25 wt% of the wood), and organic “glue” lignin (around 30 wt% of the wood) that cross-link different polysaccharide in wood to form a strong and durable structure.10 At the molecular scale, the cellulose polymer molecules have a linear chain structure consisting of glucose repeating units with
Guangyuan Zheng, Department of Chemical Engineering, Stanford University; [email protected] Yi Cui, Department of Materials Science and Engineering, Stanford University; [email protected] Erdem Karabulut, KTH Royal Institute of Technology, Sweden; [email protected] Lars Wågberg, KTH Royal Institute of Technology, Sweden; wagberg
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