Flexible fiber-shaped supercapacitors: Design, fabrication, and multi-functionalities

Abstract

Fiber-shaped supercapacitors (FSCs) have excellent electrochemical properties and flexibility, can function in the forms of individual fibers or integrated textiles, and thus are the most promising energy storage devices for future portable and wearable electronics. Considerable research effort and significant progress have been made in the past few years, and a comprehensive review of the recent development and achievement is becoming immediately necessary. Since the main driving forces in developing FSCs are flexibility and multi-functionalities, in addition to high capacitive performance, herein we review the FSCs according to their flexible scaffolds and functionalities, with special focuses on the material selection, assembly method, and the electrochemical and mechanical performance. The future trends, prospects and challenges are also presented.

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.