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Flexible Sustainable Supercapacitors

S. Siva Shalini, R. Balamurugan, I. Ajin and A. Chandra Bose*

Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamil Nadu, India

Abstract

The intense growth in renewable energy sources and the global drive towards meeting the net-zero target have encouraged the development of long-cycle life- sustainable electrochemical energy storage devices. Despite many ongoing research and advancements, challenges still remain in commercial applications such as boosting power and energy density, improving scalable fabrication techniques, developing safety, increasing cycling lifetime, and securing comfortable wearing experiences. For wearable and flexible applications that demand high capacitance, high power ratings, long life cycles, stretchability, portability, and durability, flexible supercapacitors are a very promising development. Applications for smart textiles include communications devices, thermoregulated clothing, heat storage, thermoelectric energy harvesting, electroluminescence applications, and energy storage. For the above-mentioned applications, sustainable production of the power source is crucial. There are many ways that energy storage can be accomplished. Supercapacitors offer better performance in addition to a wide range of applications, including energy harvesting devices, hybrid cars, and smartphone components. In this chapter, we systematically summarize the developments in flexible fabric-type energy storage devices, as well as their hybrid fabrics for energy storage and harvesting in flexible wearable electronics.

Keywords: Flexible electrodes, sustainability, energy storage, biomass derived, portable hybrid supercapacitors, superior electrochemical performance

1.1 Introduction

The development of energy storage and conversion devices using fossil fuels (coal, oil, gas, etc.) holds certain drawbacks such as skin diseases, excess pollution, etc. Recently, with the continuous increase in the world's population and the endless modernization of technologies, an intelligence era has arrived [1]. The history and advancements of supercapacitor are shown in Figure 1.1. Right now, wearable/portable electronics have obtained a fast growth in various fields such as sports, environmental, biomedical, and electronics applications [2]. One of the upcoming technologies with a significant potential influence in textile electronics, which offers wearable, flexible, and comfortable electrical systems [3, 4]. Their primary requirements are flexible energy conversion and storage systems that exhibit superior mechanical durability, high safety, low weight, and outstanding electrochemical performance [5]. Flexible supercapacitors are a desirable option because of their strong flexibility, increased safety, and nearly constant performance under a range of mechanical deformations [6]. However, they are required to possess a high energy density, long cycle life, and an excellent electrical conductivity [7].

Figure 1.1 History and advancement of supercapacitors.

The components of a flexible supercapacitor (FSC) usually include a flexible electrode with superior electrochemical qualities, a separator in a flexible assembly, and an electrolyte that is compatible [8]. In this chapter, we systematically summarized starting from the electrode materials to the assembly of flexible supercapacitors, with a focus on various types of electrode materials for flexible supercapacitors, their improving electrochemical performance, fabrication of the flexible electrode, and frequently used electrolytes [9].

1.2 Flexible Electrodes

The performance of flexible supercapacitors is significantly determined by the electrode. In particular, the flexible supercapacitors are better in overall performance, including their lifetime, power density, and specific capacitance [10]. The materials used for the electrodes affect the energy density and flexibility. Numerous nanomaterials, including conductive polymers, metal compounds, and carbon materials have played a key role in the development of flexible, smart, and self-sustaining FSCs [11]. The electrode materials for FSCs are categorized based on the energy storage mechanisms. The pseudocapacitor includes transition metal compounds, and conductive polymers have unique benefits and drawbacks to the use of flexible supercapacitors, and the electric double layer capacitor (EDLC) uses carbonaceous materials [12].

1.3 Electrode Materials

EDLCs uses carbonaceous electrode materials that consist of graphene, carbon nanotubes, activated carbon, and carbon nanofibers [13]. They serve as a viable candidate for constructing flexible supercapacitor electrodes due to their excellent stability, electrical conductivity, huge surface area, and strong mechanical performance [14]. However, their usage is constrained by the very low specific capacitance of these carbonaceous materials reduces the energy density. Surface modification techniques like heteroatom doping, surface activation, and exfoliation were utilized to significantly increase the capacitance [15]. Transition metal compounds are used as a pseudocapacitive electrode material because they are affordable, have a high theoretical specific capacitance, easy to handle, and chemically stable. These compounds include transition metal hydroxides, carbides, oxides, sulfides, and nitrides [16]. However, most transition metal oxides have short cycle lives and limited rate capabilities due to their poor reversibility and poor electrical conductivity. The Mn+1XnTx MXenes have hydrophilic surfaces, a high surface-to-volume ratio, and strong electrical conductivity. X can be carbon or nitrogen, and Tx is the surface termination. The electrochemical capacities can be improved by adding quantum dots to nanostructured transition metal oxide [17].

Conductive polymers such as polypyrrole (PPy), polythiophene, poly-aniline (PANI), and their derivatives are an excellent choice for pseudo-capacitors. Certain benefits of pseudocapacitive materials include their remarkable environmental stability, ease of synthesis, and electrical conductivity [18]. In this regard, the specific capacities of PPy, poly (3,4-ethylenedioxythiophene), and PANI are 480 F g-1, 210 F g-1, and 1284 F g-1, respectively. The polymerization procedure, dopant concentration, structural morphology, and ionic diffusion length all have an impact on conductive polymers specific capacitance [19]. However, the doping and dedoping processes that occur during the charge and discharge course lead the conductive polymers to expand and contract, which causes mechanical deterioration and poor cycling stability [20]. In addition, recently developed innovative electrode materials include POMs (polyoxometalates), MOF (metal-organic framework), and BP (black phosphorus) [21]. Owing to its porous structure, significant surface area, and abundance of active redox sites that contain metal, MOF is one of them that is attracting the most attention as a supercapacitor electrode material [22].

1.4 Modifying Techniques to Enhance Electrochemical Performance

To ensure rate performance and high capacity, the ideal electrode materials for flexible supercapacitors need to have a large ion accessible surface area and rapid electron transfer kinetics [23]. Therefore, flexible supercapacitor electrode materials can achieve superior electrochemical performance by increasing their surface area, mass loading, reducing the path length for the electrolyte ion diffusion, and improving the ion transport [24]. Many strategies, like building diverse electrode material microstructures and creating composites with a well-balanced composition, are used to improve the electrochemical performance of the flexible electrode materials [25].

1.5 Flexible Supercapacitors

Xiaojing Lv et al. fabricated a flexible laterally configured electrochromic supercapacitor through a fully solution process. Figure 1.2a schematically illustrates the structure of flexible electrodes, and Figure 1.2b shows the detailed step-by-step fabrication process of the flexible laterally configured electrochromic supercapacitor device. For transparency and flexibility, polyethylene plastic film (PET) was used as a substrate. The supercapacitor's current collectors need high electrical conductivity. However, PET is a resistive material. For inducing high electrical conductivity, the conducting layer of the AgNWs/Aa-PDA complex and PEDOT:PSS was produced using spraying in the sequence method. On the above conducting layer, poly (3,4-propylenedioxythiophene) (magenta electrochromic polymer) and the gel electrolyte PMMA/PC/LiBF4/BMIMOTF/ACN were cast sequentially to fabricate the asymmetric device PProDOT/PEDOT:PSS. It demonstrated strong cyclic stability, retaining 85.65% of its initial areal capacitance even after 1000 cycles of mechanical deformation, and a maximum areal capacitance of 2.15 mF/cm2 at a current density of 0.05 mA/ cm2. Therefore, the electrochromic supercapacitor device with a flexible laterally configuration shows the potential for use in wearable and portable displays like smart watches and electronic paper [26].

Figure 1.2 (a) Schematic illustration of the structure and (b) the detailed fabrication process of the flexible laterally configured electrochromic supercapacitor...