A Flexible and Knittable Fiber Supercapacitor for Wearable Energy Storage with High Energy Density and Mechanical Robustness

Abstract

A flexible and knittable symmetric all-solid-state fiber supercapacitor (ASSFS) was fabricated by using two polypyrrole (PPy) microcone arrays coated stainless steel yarns (SSY) as fiber electrodes and current collectors. The microcone arrays and interconnected network structure of PPy could increase transport path for ions/electrons and enlarge the real contact area between PPy and electrolyte, leading to superior specific capacitance. In addition, the fiber electrode has a high tensile strength of ∼1200 MPa and elongation at break is 12.5%. The outstanding mechanical performance of the fiber electrode is attributed to the high mechanical properties of the SSY and the protection of PPy coating. The resultant ASSFS exhibits maximum specific capacitance of 42.7 F/cm 3 , energy density of 3.8 mWh/cm 3 and power density of 388 mW/cm 3 (normalized to the whole device). The specific capacitance maintains 100% at different bending angles and 93.5% of its initial value after 1000 bending cycles. As a demonstration, a fabric and a bracelet knitted with five ASSFSs connected in series can light a LED. Flexible supercapacitors are of particular concern owing to the fast development of microelectronics and wearable energy storage devices which have showed tantalising prospects for stretchable circuitries , sportswear, artificial electronic skin and bio-implantable systems. 1-6 Among various flexible supercapacitors, 1D fiber su-percapacitors have received considerable attention because they not only retain merits of traditional supercapacitors such as high power density, stable cycling life and fast charge/discharge rate, but also possess good portability, light weight and high flexibility. 7-13 Moreover , compared with other flexible supercapacitors (paper-like su-percapacitors, textile supercapacitors, hydrogel/aerogel supercapac-itors, etc.), these fiber supercapacitors can be readily incorporated conventional textile fabrics and used as flexible power sources with diverse shapes, which render more design diversity for wearable energy storage. 12-14 Although considerable efforts have been made to develop fiber supercapacitors, fabrication of fiber supercapacitors with outstanding capacitive and mechanical performance to meet the practical requirements of wearable energy storage remains a challenge. Fiber supercapacitors are usually constructed by two fiber electrodes in three structure configurations: parallel, 15,16 twisted, 17,18 and coaxial 19,20 configuration. Since electrode materials play a vital role for supercapacitors, the core challenge is to fabricate fiber electrodes with satisfactory performance. Generally, fiber electrodes can be achieved by coating electrochemically active materials (polypyrrole (PPy), polyaniline (PANI), multiwalled carbon nanotubes (MWC-NTs), reduced graphene oxide (rGO) and MnO 2 , etc.) on the surface of fibrous support. As a typical conducting polymer, PPy has been considered an ideal electrochemically active material owing to its excellent electrochemical properties, remarkable environmental stability and ease of fabrication. 21-24 A series of recent reports have focused on improving the capacitive performance of PPy-based electrodes for energy storage. Su et al. investigated the influence of dopants on the performance of PPy coated MWCNT. 25 Kim et al. prepared a hollow PPy film with macro-porous structure to increase energy storage ability. 26 PPy can be prepared via two methods: (1) chemical polymerization by the use of a chemical oxidant; (2) electropolymer-ization at a conductive substrate/electrode through the application of an external potential. Compared with chemical polymerization, the z E-mail: zanglimin0705@163.com; yangchao_chem@163.com electropolymerization method is more controllable by adjusting the external potential/current and reaction time. More importantly, PPy can be formed directly on the electrode surface with a more ordered arrangement of molecular chains by electropolymerization, which is conducive to improving its capacitive performance. 27 Both noncon-ductive and conductive fibrous supports have been utilized to carry out the electropolymerization of pyrrole to fabricate fiber electrodes so far. For nonconductive fibrous supports (such as plastic fibers and natural fibers), it needs to prior convert them to be conductive by some strategies, including dip-coating of carbonaceous materials, electro-less plating of metal, metal spraying, and so on. 28 However, such complex fabrication process induces high-cost and time-consuming. Directly use of conductive fibrous supports is regarded as a more facile approach. 29-34 In light of superior mechanical strength and excellent electrical conductivity, metallic wires are promising conductive supports which can have both the function of fibrous structural supports and current collectors. However, the flexibility and knittability of the metallic wires-based fiber supercapacitors are insufficient for wear-able energy storage due to their inborn rigidity. In addition, metallic wires often have low surface area which limits the deposited amount of PPy. In this work, a commercial stainless steel yarn (SSY) was used as fibrous structural support to fabricate fiber electrode. The SSY is produced by twisting lots of 316 L stainless steel monofilaments together into a neat three-ply yarn, which succeeds the advantages of 316 L stainless steel, including high electrical conductivity, excellent mechanical strength as well as outstanding heat, acid and alkali resistance. It is notable that SSY possesses super flexibility and can be woven to fabrics just like common cotton/plastic yarn. Moreover, SSY has much higher surface area than that of the stainless steel wire with the same diameter, which is helpful for depositing more amount of PPy. In order to promote the specific capac-itance, PPy microcone arrays were electrochemically synthesized on the surface of SSY by template-free method. The electrochem-ical properties of the electrode were investigated. Then a symmetric all-solid-state fiber supercapacitor (ASSFS) based on two parallel fiber electrodes was assembled using poly(vinyl alcohol)/sulfuric acid (PVA/H 2 SO 4) as gel electrolyte. Compared with previously reported fiber supercapacitors, our proposed device is easily manufactured and exhibits higher capacitive and mechanical performance, indicating it is an ideal energy storage device for flexible and wearable electronics.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.122.230.132 Downloaded on 2018-05-19 to IP