Preparation and characterization of coaxial halloysite/polypyrrole tubular nanocomposites for electrochemical energy storage

Abstract

Halloysite nanotubes/polypyrrole (HNTs/PPy) nanocomposites with coaxial tubular morphology for use as electrode materials for supercapacitors were synthesized by the in situ chemical oxidative polymerization method based on self-assembled monolayer amine-functionalized HNTs. The HNTs/PPy coaxial tubular nanocomposites were characterized with transmission electron microscope (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), electrical conductivity measurement at different temperatures, cyclic voltammetry (CV), and galvanostatic charge–discharge measurements. The coaxial tubular nanocomposites showed their greatest conductivity at room temperature and a weak temperature dependence of the conductivity from 298 K to 423 K. A maximum discharge capacity of 522 F/g after correcting for the weight percent of the PPy phase at a current density of 5 mA cm⁻² in a 0.5 M Na₂SO₄ electrolyte could be achieved in a half-cell setup configuration for the HNTs/PPy composites electrode, suggesting its potential application in electrode materials for electrochemical capacitors.

Introduction

Growing demands for the generation of power sources with transient high-power density have stimulated great interest in electrochemical capacitors in recent years. It has been accepted that supercapacitors are the best candidates to provide the high power and long durability needed for new energy devices, such as hybrid peak-power sources in electric vehicles, backup sources for various electrical devices, and uninterrupted power supplies [1], [2]. The materials studied for supercapacitor applications include three main types: (1) carbon; (2) metal oxides; and (3) conducting polymers [3], [4], [5]. Conducting electroactive polymers remain a subject of intense investigation of many research groups worldwide. Among them, polypyrrole, first synthesized in 1916 [6], is one of the most widely investigated conducting polymers because of its good thermal and environmental stability, and good electrical conductivity, which are favorable for various applications, such as metallization of dielectrics [7], batteries [8], antistatic coatings, shielding of electromagnetic interference [9], sensors [10], actuators [11], and microactuators [12]. To solve problems, such as brittleness, low elongation, and poor processibilities, strategies have been developed, such as preparing composites and copolymers, reforming the monomers of these conducting polymers, and blending with some commercially available polymer that offers better mechanical and/or chemical properties. The conducting polymer composites related to supercapacitors containing transition metal oxides such as RuO₂, MnO₂, TiO₂, In₂O₃ and carbon materials, and carbon materials have been synthesized and widely studied [13], [14], [15], [16], [17], [18], [19]. In these composites, the transition metal oxides or carbon materials used are conductive or semi-conductive.

Halloysite, which was first described by Berthier in 1826 as a dioctahedral 1:1 clay mineral of the kaolin group, can adopt a variety of morphologies, the most common of which is the elongated tubule [20]. Recently, halloysite has gained growing interest in the synthesis of complex structures as an economically available nanotubular raw material [21], [22], [23], [24], [25], [26].

At the present time, few approaches for preparing HNTs/PPy coaxial tubular nanocomposite have been reported. In this paper, low-costing natural halloysite nanotubes were used to form HNTs/PPy nanocomposites with well-defined coaxial tubular morphology for the first time. Furthermore, the halloysite nanotubes are insulating and only used as the supports for the polypyrrole coatings here.