Preparation of High-performance Covalently Bonded Polyaniline Nanorods/Graphene Supercapacitor Electrode Materials using Interfacial Copolymerization Approach

Abstract

In this article, the polyaniline nanorods/graphene sheet composites (G-PANI) with covalent bond were synthesized through interfacial copolymerization of aniline and functionalized graphene (G-PPD) obtained by in situ reduction of graphene oxide and functionalization with para-phenylenediamine (PPD). During the interfacial polymerization, the monopolymerization of aniline and copolymerization of aniline and G-PPD are carried out simultaneously, which produce a mixture of PANI nanorods and G-PANI. Not only the homopolymer possesses one-dimensional structure, but also the graphene surface was covered evenly by the PANI nanorods. As comparison with conventional synthesis methods of PANI, the interfacial polymerization can produce large-scale one-dimensional PANI nanorods, which can form a loose crisscross aggregation. Even when it is grafted onto graphene sheet, the nanorod morphology is remained. The cyclic voltammetry (CV) and charge-discharge tests confirm that the G-PANI has higher electrochemical activity and higher capacitive properties over those of neat PANI nanorods. At the current density of 1 mAcm⁻², the specific capacitance of G-PANI and PANI nanorods are 909 Fg⁻¹ and 772 Fg⁻¹, respectively. Especially, the specific capacitance of G-PANI has 62% enhancement over that of PANI nanorods at high current density of 50 mAcm⁻². The remarkable improved capacity performance can be attributed to the enhanced mass transfer process brought from the loose nanostructure aggregation of G-PANI, as well as the reduced charge transfer resistance caused by the highly conductive graphene and strong interfacial interaction between PANI and graphene via covalent bond.

Introduction

As one kind of energy storage devices, the supercapacitors have attracted more and more attentions owing to their fast charge-discharge speed, high power density, high energy density, and good stability, in recent years. Based on the various energy storage mechanisms, the supercapacitors include electrochemical double layer capacitors (EDLC), which can store moderate energy and possess high reliability, and faradic pseudo-capacitors, which exhibit high specific capacitance due to their multiple charge storage mechanisms including redox reaction, adsorption and intercalation of ions, etc. [1] Polyaniline (PANI), as a typical pseudo-capacitor material has been studied widely because of its high capacitance, ease in preparation, good processability and low cost. [2], [3] As a widely accepted fact, the PANI nanostructure possesses better electrochemical performance over those conventional PANI materials owing to the fast redox reactions brought by the large interfacial area with electrolytes and short diffusion length for ions and probably also increased accessible redox sites. [4] Variable PANI nanostructures including nanotubes, nanowires, nanoparticles and nanorods ware synthesized and utilized as supercapacitor electrodes, which exhibit improved capacitive properties. Li [5] synthesized PANI nanowires arranged electrodes with specific capacity of 1142 F g⁻¹. Ma [6] prepared PANI nanotubes using a template-free method with D-tartaric acid (d-TA) as the dopant. The PANI nanotubes electrode, with [d-TA]/[aniline] molar ratio of 1:1, exhibits larger specific capacitance (as high as 625 F g⁻¹ at 1 A g⁻¹) and higher capacitance retention (77% of its initial capacitance after 500 cycles) in 1 M H₂SO₄ aqueous solution. Besides of PANI nanostructure, the mutli-phase PANI composites with carbon nanostructure are another approach to improve the capacitive performance of PANI. Though the carbon nanostructures including carbon nanotubes, fullerene and graphene possess capacitive feature, they are not used as capacitor electrode solely owing to their relative lower capacitive performance and high cost. Usually, the carbon nanostructures can be added in other capacitor materials to play the role of secondary electrode material and to enhance the capacitive properties by increasing the electrical conductivity, a very important parameter which influent the capacitive behavior of composites. It is believed that with addition of highly conductive carbon nanostructures, the charge transfer resistance of composites can be decreased, which is very important for the supercapacitor materials with low electrical conductivity or the electrode material with variable conductivity during the charge-discharge process. For example, the conducting polymer can be switched to their low conductive state (dedoped state) when discharge. Different carbon nanostructures including fullerene, single-wall carbon nanotubes and multi-wall carbon nanotubes, graphene, carbon nanofibers, carbon microsphere, and mesoporous carbon, have been introduced to PANI matrix by various methods (chemical or electrochemical polymerization with or without covalent bond) to improve its capacitive properties. [7] So the highly conductive carbon nanostructures are helpful for reducing the interfacial and charge transfer resistance of conducting polymer based supercapacitors, especially during their discharge process, as well as enhancing their capacitive performances.

In most of cases, with addition of various carbon nanostructures into composites, the PANI with special nanostructures are also employed at the same time. The synergic effect of carbon nanostructures and PANI nanostructures give a performance boost for PANI/carbon nanostructure composite supercapacitors. Wang synthesized PANI nanorods modified with sulfonated carbon nanotubes via in situ oxidative polymerization of aniline in the HClO₄ solution. With 76.4 wt.% PANI loading, a maximum specific capacitance of 515.2 F g⁻¹ was obtained. [8] Similar work was done by Xu using in situ polymerization of aniline on the surface of functional multi-wall carbon nanotubes (MWCNTs) to prepare a hierarchical nanocomposite of vertical PANI nanorods aligned on the MWCNTs with enhanced capacity performance. [9] Li synthesized vertically aligned PANI nanowhishers on the external surface of ordered mesoporous carbon, which exhibits 470 F g⁻¹ specific capacitance with 40 wt.% PANI content.[10] Also the PANI nanowires, nanofiber and nanoparticle were tethered onto graphene and graphene oxide sheets by in situ polymerization method. In Xie's work, a remarkable specific capacitance (1130YF g⁻¹) of graphene nanosheets/PANI nanofibers was achieved, attributing to great improvement of charge transfer reaction brought by the PANI nanofiber, which is homogeneously coated on the surface of graphene nanosheets according to the author's description. [11] We can make a conclusion that combining different dimensional nanomaterials of PANI and carbon materials is a promising approach for preparation of high-performance supercapacitor materials.

With consideration of facile preparation of PANI one-dimensional nanostructure with interfacial polymerization method [12], [13], [14], we employ interfacial copolymerization of aniline and functionalized graphene at the interface of two separated oil/water phases to prepare PANI nanorodes/graphene (G-PANI) electrode materials. Owing to the fast reaction mechanism and two competitive reactions of homopolymerization and copolymerization in the interfacial polymerization process, the composites may exhibit various types of morphologies, e.g., PANI nanorods and G-PANI multi-phase nanostructure. To study the influence of their structures and morphologies on the supercapacitive performance will be interested. Herein, we report the interfacial copolymerization of PANI nanorods/graphene sheet composites and demonstrate their electrochemical and capacity properties.