Nanocellulose-based Materials for the Solar Cell, Wearable Sensors, and Supercapacitors

Table of Contents:

Introduction

With the development of world economy and the population explosion, we have been facing the issues of energy shortage and environmental pollution in the past decades. The traditional fossil resources have limited reserves and are not renewable, which are not conductive to the sustainable development of the world. To solve this problem, recently the 'green' comprehensive utilization of bioresources has attracted great attention from government and researchers (Gaurav et al. 2017, Zhang et al. 2018). Some conventional materials are expected to be replaced by the natural bioresources for the eco-effkient and environment-friendly materials, products, and devices.

Cellulose, composed of (3-1,4-anhydro-D-glucopyranose units, is the most abundant organic polymer in nature (Fu et al. 2020, Cannon and Anderson 1991). It is the main component of most biomass like wood, cotton, and other plant fibres (Kotelev et al. 2017). Cellulose holds the merits of renewability, sustainability, and biodegradability etc.; therefore, it has been regarded as the potential material for replacing the conventional fossil resource in widespread applications, such as energy conversion, energy storage, and sensing.

Recently, a nanostructured cellulose, also known as nanocellulose, has drawn great attention from researchers (Xue et al. 2017, Chen. Yu, et al. 2018b). Nanocellulose is isolated from the cellulose fibres at nanoscale via mechanical and/ or chemical methods; however, nanocellulose has extraordinary physicochemical properties that are absent in the original cellulose fibres. Therefore, nanocellulose and nanocellulose-based composites are becoming the study focus in the fields of ‘green’ energy and sensing (Sabo et al. 2016).

In this chapter, the production, characterization, and applications of the nanocellulose and/or nanocellulose-based composite are briefly introduced. Furthermore, our group’s recent works on the various nanocellulose-based functional materials (i.e., nanopaper, hydrogel, and carbon aerogel) for solar cells (Chen, Song, et al. 2018a), wearable sensors (Shao et al. 2018), and supercapacitors (Wang et al. 2019) are discussed in detail.

Nanocellulose

Nanocellulose is a form of nanostructured cellulose fibre and has lots of advantages, such as large specific area (100-200 girr2), adjustable aspect ratios (100-150), high tensile strength (7.5-7.7 GPa), high Young’s modulus (110-220 GPa), low coefficient of thermal expansion, superior gas-barrier, high chemical resistance, and easy surface functionalization (Chen and Hu 2018, Chen, Yu, et al. 2018b, Thomas et al. 2018, Fang et al. 2019). Typical sources of nanocellulose are wood, cotton, wheat/ rice straw, bamboo, algae, and bacteria, etc. The obtained cellulose fibres from these materials can be further disintegrated into nanocellulose through mechanical and/or chemical treatment. Generally, nanocellulose can be classified into three types: cellulose nanofibrils (CNFs), cellulose nanocrystals (CNCs), and bacterial nanocelluloses (BNCs). The synonyms, formation processes, and average size of these nanocellulose materials are compared and presented in Table 3.1. The nanocellulose

TABLE 3.1

Three Types of Nanocellulose Materials (Klemm et al. 2011)

Type of Nanocellulose

Synonyms

Formation

Average Size

Cellulose nanofibrils (CNFs)

Nanofibrillated cellulose, cellulose nanofibres, nanofibrils, microfibrils

Delamination of wood pulp by mechanical pressure before and/or after chemical or enzymatic treatment

Diameter, 5-60 nm: length, several micrometres

Cellulose nanocrystals (CNCs)

Nanocrystalline cellulose, crystallites, whiskers, nanowhiskers, rod-like cellulose microcrystals

Acid hydrolysis of cellulose from various sources

Diameter, 5-70 nm; length, 100-250 nm (from plant). 100 nm to several micrometres (from cellulose of tunicates, algae, and bacteria)

Bacterial nanocelluloses (BNCs)

Bacterial cellulose, microbial cellulose, biocellulose

Bacterial synthesis

Diameter: 20-100 nm. different types of nanofibre networks

ТЕМ images of three types of nanocellulose

FIGURE 3.1 ТЕМ images of three types of nanocellulose.

materials have different names, and CNF. CNC, and BNC are most widely used in literature. The size of these nanocellulose materials is also very different. As can be seen in Figure 3.1, ТЕМ images present the morphology and especially dimensions of the cellulose-based nanostructured fibres. CNFs and CNCs have the similar diameter. but the rod-like CNCs are dramatically shorter than CNFs. BNCs also have the high aspect ratio like CNFs but different types of nanofibre networks. Besides, various technologies were utilized to prepare these nanocellulose materials.

CNFs are prepared mainly by mechanical fibrillation (Veigel et al. 2011). The equipment that has been commonly used includes high-pressure homogenizer, micro-jet machine, ball miller, freezing grinder, and ultrasonic grinder, etc. In 1983, Turbak produced the stable suspension of CNFs in water by a physical treatment of wood cellulose pulps for the first time. Although the obtained novel cellulose-based material was called microfibriHated cellulose (MFC), the treated cellulose fibres were in nanoscale. Mechanical fibrillation is an effective method to prepare CNFs.

However, the energy consumption is very high during mechanical processing, which limits the cost-effective productions of CNFs. To solve this problem, the chemical and/or biological technologies can be used to pretreat the cellulose pulps to destroy the original structure of cellulose fibres. This can decrease the energy consumption of post-mechanical treatment and thus the whole cost of CNFs production. For example, the 2, 2, 6, 6-tetramethylpiperidine-l-oxyl (TEMPO) mediated oxidation is able to break the interfibril hydrogen bonds in cellulose and convert C6 primary hydroxyls to carboxyls, which can reduce the energy needed for defibrillating cellulose by at least 2 orders of magnitude (Jiang and Hsieh 2016, Isogai et al. 2011).

Cellulose nanocrystals (CNCs) can be extracted from native cellulose fibres by acid hydrolysis. The most commonly used reagents are sulfuric acid and hydrochloric acid. Besides, phosphoric acid and hydrobromic acid can also be used for the preparation of CNCs. The Figure 3.2 illustrates the crystalline structure of native cellulose and its change during the fabrication of CNC and CNF (Hubbe et al. 2017). Cellulose is composed of crystalline domain and disordered (amorphous) regions. Strong acid hydrolysis can destroy and eliminate the amorphous regions of the cellulose but maintain its crystal domains. However, both crystalline and amorphous regions were contained for CNFs. The obtained CNCs are highly crystalline and relatively rigid rod-like nanoparticles, which have lower aspect ratios and flexibility when compared with CNFs.

As demonstrated by the schemes in Figure 3.3, bacterial nanocelluloses (BNCs) are purely synthesized by bacteria like Acetobacter xylinum in aqueous culture media in several days. During the production of BNCs, the bacterial body can generate glucose chains and extrude them out through tiny pore on the cell envelope. The glucan chain aggregated and formed the cellulose microfibre, which continued to aggregate as ribbons, i.e., BNCs. Compared to the CNFs and CNCs extracted from plant sources, the pure biosynthesis BNCs do not have the contaminants such as lignin and hemicellulose (Lin et al. 2013, Lin and Dufresne 2014). Therefore, the BNCs have excellent biocompatibility and exhibit potential applications in the fields of biomedicine and functional biomaterials.

Schemes showing the crystalline changes for the fabrication of CNC and CNF (Hubbe et al. 2017)

FIGURE 3.2 Schemes showing the crystalline changes for the fabrication of CNC and CNF (Hubbe et al. 2017).

Schemes and SEM images of BNCs (Shi et al. 2014)

FIGURE 3.3 Schemes and SEM images of BNCs (Shi et al. 2014).

 
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