Flexible fiber-shaped supercapacitors: Design, fabrication, and multi-functionalities
Graphical abstract
Introduction
Sustainable development and efficient utilization of energy on a broad scale necessitate the rapid progress and upgrading of energy harvesting, conversion and storage systems. Among them, supercapacitors (SCs), also named as ultracapacitors or electrochemical capacitors, are robustly developing energy storage devices. They are capable of storing higher energy than conventional capacitors and delivering energy at a much higher rate than batteries and fuel cells [1], [2], and thus have attracted extensive research attention. In many advanced applications, such as in powering portable and wearable electronics, SCs are required to be flexible, smart, and miniaturized, besides the high energy and high power prerequisites [3], [4], [5], [6], [7]. Compared with conventional three-dimensional (3D) bulky SCs and two-dimensional (2D) planar SCs, one-dimensional (1D) fiber-shaped SCs (FSCs) have small size, light weight and good flexibility, and can be readily woven into breathable fabrics/textiles and integrated into power systems with other fiber based energy devices. Therefore, FSCs are particularly promising in meeting the ever-increasing demands from wearable electronics.
Similar to all other SCs, FSCs can be grouped into three types according to their charge storage mechanisms: Electrical Double Layer Capacitors (EDLCs), pseudocapacitors, and asymmetric capacitors (Fig. 1a-c) [10], [11]. An EDLC stores energy by electrostatically accumulating positive and negative charges separately at the interface of electrode and electrolyte. The charge absorption capability is generally 0.17–0.20 electrons per atom at accessible surface [12]. This is a non-Faradic process, i.e. no charge transportation between the active material and the electrolyte theoretically. Thus, electrode materials with large accessible specific surface area (SSA) and high electrical conductivity are required. A pseudocapacitor stores electrochemical energy through fast and reversible redox reactions occurred at the interface of electrolyte and electrode material. This is Faradic in origin, and yields a charge absorption capability of ~2.5 electrons per atom at accessible surface [12], [13]. Theoretical capacitance limit and geometric configuration of pseudocapacitive materials are important for the overall electrochemical performance of a pseudocapacitor. An asymmetric supercapacitor combines an electrostatic electrode (power source) and an electrochemical electrode (energy source) in the same cell [9], [14], [15]. Presently, most asymmetric capacitors are fabricated by following two combinations: (i) a capacitive carbon electrode and a pseudocapacitive electrode, and (ii) a capacitive electrode and a lithium insertion electrode. When a battery-like electrode is used in the cell, the asymmetric supercapacitor is more frequently called as a hybrid supercapacitor. In general, EDLCs have a high power density and long cycle life, pseudocapacitors have a high energy density, and the asymmetric capacitors try to get both high power density and high energy density.
In achieving high capacitive performance, the electrode materials are critically important. For EDLCs, typical electrode materials are carbon and carbon derivatives, including activated carbon (AC), porous carbon, ordered mesoporous carbon (OMC), carbon nanoparticle, carbon nanotube (CNT), carbon nanofiber, carbon microfiber (CMF), graphene, and reduced graphene oxide (RGO), etc. With respect to pseudocapacitors, their electrode materials are generally metal oxide/hydroxide/nitrides (such as MnO2, Ni(OH)2, TiN, etc.) and electrically conducting polymers (polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly(3,4-ethylenedioxythiophene) (PEDOT), etc.). For all electrode materials, the geometric shape and lattice structure are essential in determining the electrochemical performance.
Along with the electrochemical performance, other two driving forces in the development of FSCs are flexibility and multi-functionality. Since most electrode materials are not flexible, the FSCs are usually fabricated by attaching their electrode materials on flexible 1D scaffolds, such as carbonaceous fibers, metal wires, synthetic polymer fibers (SPFs) and natural fibers. Multi-functionalities are realized by using multifunctional scaffolds or by integrating FSCs with other functional devices.
Considering the diversification of approaches being used and the rapid progress being made in the field of flexible FSCs, it is meaningful and necessary to give a comprehensive review on this topic with a detailed classification to identify the existing benchmarks and understand the future trend. Therefore, in this review paper, we introduce the basic device structures and key parameters of FSCs first, and then review their electrochemical performance and multi-functionalities according to the device structures and fiber scaffolds. The flexibility tests like bend, knot, weave/knit, twist/torsion and even arbitrary entanglement will be discussed throughout the context.
Section snippets
Device structures
The fundamental components of a FSC are the same as a traditional capacitor, which consists of two opposite electrodes (each including current collectors and active materials) separated by an electrolyte and an ionically conducting but electrically insulating separator (Fig. 1a-c). An encapsulation casing is employed sometimes to prevent the device from environmental damage. With respect to FSCs, the key issue is to fabricate fiber-shaped electrodes (FSEs) and assemble all components into a
Carbonaceous fibers
Carbonaceous materials were employed in the SCs from the early stage due to their large SSA, chemical inertness and low cost, and remain the most popular electrode materials in commercial EDLCs of today. With the advancement of science and technology, new forms of electrochemical active carbons are discovered, such as OMC, CNT, graphene and so on. Moreover, some of the nanosized carbons are spun into 1D fibers/yarns with remarkable electrochemical activity and mechanical performance.
Multifunctional FSCs
Although the real application of FSCs are still wrestling with many issues, one efficient approach to make the FSCs more adaptable and compatible for practical utilization is extending their functions beyond the energy storage. Several prospective trials along this direction are discussed below.
Integration of FSCs
Integration is an efficient strategy to develop high performance power systems [222], and could be used to realize the multifunctionalities in FSCs. In such a case, the FSCs can provide basic energy or power function, and have energy managing capabilities as well.
Applications
The application of FSCs is on its infant stage. Only proof-of-concept examples were demonstrated in the literature. FSCs were used as an energy source to power energy consumption devices like LED, electronic ink display and commercial MP3 (Fig. 17a-c) [21], [30], [233]. Several FSCs were assembled in series to power an UV photodetector without any external bias voltage (Fig. 17d), immediate and considerable changes in the current were observed upon UV irradiation (Fig. 17e) [153]. Another
Conclusions and outlook
FSCs have attracted intensive research attention in the past few years due to their unique merits of lightweight, flexibility and good electrochemical performance. Considerable progress have been made in developing advanced electrode materials, assembly modules and multifunctional FSC based devices. In this review, FSCs derived from four kinds of fiber scaffolds, carbonaceous fibers, metal wires, SPFs and natural fibers, are discussed. New functions beyond energy storage are also summarized.
In
Acknowledgments
We appreciate the support from Khalifa University Internal Research Funds (KUIRF 210064 and KUIRF 8431000007).
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