Cotton yarns modified with three-dimensional metallic Ni conductive network and pseudocapacitive Co-Ni layered double hydroxide nanosheet array as electrode materials for flexible yarn supercapacitors

Abstract

A novel cotton/Ni/Co-Ni layered double hydroxide (CT/Ni/Co-Ni LDH) hybrid yarn electrode material is prepared by constructing a three-dimensional (3D) metallic Ni conductive network on the cotton fiber surface for rapid electron transportation and subsequently in situ growing Co-Ni LDH nanosheet array layers with high electroactive surface area for faradic reactions. The 3D porous electrode structure facilitates the effective electrolyte ion diffusion. The obtained yarn electrode material shows a high areal capacitance of 1.26 F/cm² (121571.1 C/cm²) at a scan rate of 5 mV/s, as well as a stable electrochemical performance under various mechanical deformations. An all-solid-state yarn supercapacitor device is further assembled based on the obtained yarn electrode materials, which exhibits a high energy density of 9.3 μWh/cm² at a power density of 43.99 μW/cm².

Introduction

In order to meet the increasing demands of flexible and wearable electronic devices, developing high-performance flexible energy storage devices with the properties including small volume, light weight and low cost has aroused general interests [1,2]. Compared with batteries, supercapacitors, also named electrochemical capacitors or ultracapacitors, storage the energy through the electrical double layer mechanism and faradic reactions, which are safe, easy maintenance, environmental friendliness and high power density. They are regarded as one of the most promising energy storage devices [[2], [3], [4], [5]].

In recent years, developing flexible supercapacitors with promising electrochemical and mechanical performances has been a research frontier in energy storage devices [2,6,7]. Among various materials that can be considered as a power source, yarn supercapacitors, also called fiber supercapacitors, have gained widely attraction because they facilitate to be integrated into textiles [[8], [9], [10], [11], [12], [13]]. Moreover, their small volumes can ensure the mechanical flexibility and high capacitance. Taking advantage of their potential in assembling into various structures, various novel energy textiles are believed to be designed to power flexible and wearable electronic devices. Tremendous advances in yarn supercapacitors have been achieved in recent years. Carbon materials with high conductivity were widely used to prepare yarn supercapacitors [14]. However, carbon-based yarns, such as carbon nanotube (CNT) yarns and their composite yarns, suffer from high cost and complicated preparation process. More importantly, their compact electrode structure leads to low effective surface area, ineffective ion diffusion and inappropriate pore size, resulting in an unsatisfied electrochemical performance. The specific capacitance of pure CNT composite yarn electrode only achieved 5 F/g [15]. Recently, metal yarns with high conductivity were involved in the yarn electrode materials [[16], [17], [18]]. In order to endow metal yarns with energy storage properties, their fiber surface is selectively modified with carbon nanomaterials or/and pesudocapacitor materials. Coating ultrathin graphene sheets on the Au fibers formed an Au/graphene electrode, which exhibited an areal capacitance of 0.726 mF/cm² through the electrical double layer capacitance (EDLC) mechanism [18]. NiO/Ni(OH)₂ nanoflowers encapsulated in 3D interconnected poly(3,4-ethylenedioxythiophene) (PEDOT) were fabricated on the Cu-Ni alloy wires. The NiO/Ni(OH)₂/PEDOT electrode delivered a specific capacitance of 404.1 mF/cm² [19]. Unfortunately, the high mass density and solid nature of metal yarns result in ineffective loading mass of active materials and high weight fraction of metal fibers in metal yarn-based electrodes, limiting their actual electrochemical performance.

The daily clothing is an ideal platform for flexible and wearable electronic devices [1,20]. Despite the recently reported carbon and metal based yarn electrode exhibited good electrochemical performance, they are not suitable for the knitting or weaving processes due to their low strength, limited length, low production, high cost and complicated preparation process [13,21,22]. Considering these above issues, the common textile yarn materials are more reasonable to be chosen to prepare flexible electrodes with satisfied electrochemical and mechanical performances [23]. The cotton (CT) yarn fabricated by numerous cotton fibers is a highly flexible and porous material, which possesses attractive characteristics, such as low cost, large-scale production, appropriate surface functional group, high porosity, excellent flexibility and knittability. Some previous reports have demonstrated its potential in flexible energy storage devices [24,25]. However, the insulated nature and low electroactive surface area are the main issues blocking its application for electrochemical energy storage. Conductive carbon nanomaterials including carbon nanotubes (CNTs) and graphene have been involved in the fabrication of cotton-based yarn electrodes [24,26]. It is unfortunate that the resulting yarn electrodes usually exhibited high liner resistivity because of the poor contacts of CNTs and incomplete reduction of graphene [26]. Moreover, due to the restacking of graphene sheets and low specific surface area of CNTs on the cotton fiber surfaces, the limited carbon effective surface area resulted in low electrolyte ion-accessible surface area. These two issues lead to low EDLC capacitance. Further coating with pseudocapacitor materials such as MnO₂ and polypyrrole (PPy), the areal capacitances are still far from the demands in the practical application due to the low conductivity and small electroactive surface area. The recently reported yarn supercapacitor fabricated by coating cotton yarns with PPy nanotubes exhibited a low areal capacitance of 74 mF/cm² [27]. Therefore, it is an enormous challenge to design and prepare high-performance textile-based yarn electrode materials with the following characteristics: lightweight, flexible, porous, cost effective, large-scale production, excellent mechanical robust and good electrochemical performance.

In the present work, we designed and fabricated a hybrid textile yarn electrode material by electroless plating metallic Ni layers on the cotton fiber surface to construct a 3D conductive network for rapid electron transportation and subsequently in situ growing Co-Ni LDH nanosheet array layers with high electroactive surface area for faradic reactions. The hierarchically porous electrode structure facilitates the effective electrolyte ion diffusion. Due to the synergistic effects among these components, this yarn electrode material achieved a high areal capacitance of 1.26 F/cm² at 5 mV/s and a stable cycling performance of 82.5% of initial capacitance after 1200 cycles. An all-solid-state symmetric supercapacitor with a two-ply structure was fabricated by using two CT/Ni/Co-Ni LDH yarn electrodes, which delivered an energy density of 9.3 μWh/cm² at a power density of 43.99 μW/cm².