Nanocellulose-based conductive materials and their emerging applications in energy devices - A review

Highlights

• Nanocelluloses are promising materials for soft and sustainable energy devices due to its unique properties.

• Recent progress on the preparation of nanocellulose-based conductive materials is reviewed.

• The advantages of nanocellulose in the applications of supercapacitors, lithium ion batteries and solar cells are discussed in detail.

Abstract

Nowadays, the demand for sustainable energy devices (e.g. lithium ion batteries, supercapacitors, solar cells) has increased rapidly in recent decades. Nanocelluloses (NCs) from plants or bacteria have shown promising potential as their excellent physical, mechanical and optical properties, which are important for fabricating high-performance energy devices. NC materials and their applications in different areas have been extensively reviewed in literature. However, those reviews focused on more broad properties and applications of NC materials but the discussion on the energy applications are far from comprehensive. Technically, NCs are not electrically conductive; however, conductivity is essential for some core components of an energy device. Therefore, various chemical or physical modification approaches have been developed to prepare conductive NC-based materials. Because of the excellent physical properties of NC materials as well as the rapidly increasing demands on renewable materials based energy devices, enormous research efforts have been devoted to the NC-based conductive materials and energy devices. This paper is a comprehensive review focusing on the recent progress of fabricating conductive NC materials and the energy devices, including supercapacitors, lithium ion batteries and solar cells.

Introduction

We live in a constantly growing global society that requests an enormous amount of new energy devices to meet the needs of accelerating technology development. Plastics, inorganic semiconductors and other petrochemical-based products remain fundamental for these needs. The incredible demand of raw materials for production of these devices inevitably leads to drastic environmental problems. One is a colossal amount of waste energy devices cannot naturally degrade, the other is a rapid exhaustion of scarce natural elements, such as indium and gallium, leads to resource depletion [1]. Moreover, issues such as greenhouse gas emission, embodied energy, toxicity, and the sustainability of the materials are becoming increasingly significant to society. In order to minimize the negative environmental impacts and to create a sustainable future, low-cost and energy-efficient carbon-based green materials are being increasingly explored as candidates to replace some conventional materials in the fabrication of energy devices. One of the most important classes of carbon-based materials is made from natural resources, such as cellulose, starch, chitosan, collagen, soy protein and casein. Among them, cellulose is the most promising natural polymer. As the skeletal component of plants, it is an almost inexhaustible green material with an annual production of about 1.5 trillion tons. Natural cellulose based materials (eg. cotton, wood, hemp) have been used by human for thousands of years and continue to be used in worldwide industries involving paper and textiles. Reliable stability, rich surface chemistry and low cost of cellulose, allow the wide use in many applications, including emerging energy areas. Cellulose based products with never seen before functionality, durability, and uniformity are constantly being investigated [2]. Furthermore, it also enables fabrication of flexible, conformable and very thin devices due to its nanosize and soft nature.

In the last several decades, a nano-structured cellulose, also known as nanocellulose, was isolated from cellulose at the nanoscale giving rise to remove most of the defects associated with the hierarchical structure [3], [4], [5]. Nanocelluloses with some distinct properties are becoming more and more rampant in research communities. In this review, we use the term nanocelluloses (NCs) to refer to cellulose nanofibrils (CNF), cellulose nanocrystals (CNC) and bacterial cellulose (BC) that have at least one dimension in nanoscale [5]. Mechanical treatment, acid hydrolysis, and enzymatic hydrolysis are three typical separation approaches to isolate the nanocelluloses [3], [4], [5]. The nanofibrils are always isolated from wood-based fibers by mechanical treatment including high pressure homogenizers, grinders, cryocrushing, high intensity ultrasonic treatment, and microfluidization [5]. Crystalline and rigid nanoparticles can also be processed by acid hydrolysis [6]. Compared to regular cellulose substrates, much smaller NCs have higher mechanical strength, better optical transparence, and smoother surface. These properties combined with their low cost, light weight, flexibility, and environmental friendliness, make NCs are promising candidates for fabrication of novel composites and energy devices. There are also several challenges along with the NCs production, such as reducing the cost and energy consumption of the isolation processes, and controling the size and properties of NCs [5]. However, the interests in NCs and their modification for energy applications have been exponentially increasing due to the excellent properties. NCs usually cannot be directly used in fabrication of most energy devices due to their electrically non-conductive nature. Therefore, making NCs based conductive materials by combining with conductive materials (e.g. conductive polymers, metallic particles, conductive carbon materials) are extensively investigated. These composites usually possess the advantages of both NCs and the conductive materials, which can meet the most needs for energy applications.

NCs have been chemically and physically modified into flexible substrates or various types of smart paper for energy storage devices, such as rechargeable lithium ion batteries (LIBs) and supercapacitors (SCs). These two energy storage devices have become vital and dominant power sources for applications ranging from portable electronics to electric vehicles, hybrid electric vehicles, and even huge energy-storage systems [7]. The NCs based flexible battery or supercapacitor is a novel device that can be applied in wearable and flexible electronics. NCs also are promising candidates for the fabrication of next-generation solar cell. As a low cost substrate with excellent flexibility and printing compatibilities, NCs have attracted much attention in a wide-variety of applications. They have great potential to be used in consumer products such as sportswear, or for military uses in light weight armor concepts [8]. For these devices, flexibility, lightweight, and durability are crucial for proper function. Rapid development involving the applications of NCs has occurred throughout the last decade.

Although several early review articles have discussed cellulose or paper based conductive materials and their diverse applications [2], [9], [10], [11], [12], [13], [14], most of them focused on the regular cellulose fiber (micrometer to millimeter sized) paper rather than nanocelluloses. Thereinto, Hu and Cui summarized their excellent works in expolring paper for emerging energy applications and the challenges [12]. Sharifi et al. also discussed the paper-based devices for energy applications, including fuel cells, lithium-ion batteries and alkaline batteries [11]. Ummartyotin and Manuspiya reviewed the application of conductive cellulose in lithium based battery [9]. However, it has been well known that smaller NCs have much higher mechanical strength, better optical transparence, and smoother surfaces compared to regular cellulose fibers. These unique properties have been exploited to further advance energy applications. Recently, Zhu et al. comprehensively reviewed the woody materials (e.g. cellulose, NC, lignin, hemicellulose) for the applications in energy storage, green electronics, biological devices and bioenergy [15]. In Kim et al.'s review, it also mentioned the application of NCs in energy storage devices [16]. However, based on our opinion, those reviews focused on more broad properties and applications of NC materials but the discussion on the energy applications are far from comprehensive. Since the significant advantages of NC materials over microsized cellulose fibers as well as the rapid increased demands in renewable materials based energy devices, this review will specifically focus on the NC-based conductive materials and their emerging applications in fabrication of energy devices, including flexible supercapacitors, lithium ion batteries and solar cells.