Enhanced electrochemical performance of carbon quantum dots-polyaniline hybrid

Highlights

• A photoassisted CV method was used to prepare net-like CQDs-PANI hybrid nanowires.

• CQDs improved conductivity and alleviated volume change of the CQDs-PANI hybrid.

• The CQDs-PANI hybrid exhibited a high capacitance of 738.3 F g⁻¹ at 1.0 A g⁻¹.

• Flexible supercapacitor based on CQDs-PANI hybrid displayed high energy density.

Abstract

Carbon quantum dots-polyaniline (CQDs-PANI) hybrid was developed as supercapacitor electorde material by incorporating CQDs into PANI. PANI nanowires were grown vertically on carbon fiber substrate to form an interconnected network structure. Meanwhile, CQDs were uniformly distributed in the interior and on the surface of well-established net-like PANI nanowires. High specific capacitance of 738.3 F g⁻¹ at 1.0 A g⁻¹ was obtained for CQDs-PANI compared to that of 432.5 F g⁻¹ for pure PANI. The capacitance retention after 1000 cycles of CQDs-PANI and PANI is 78.0% and 68.0% at 5.0 A g⁻¹, respectively. The high capacitance and reasonable cycle stability were ascribed to the incorporation of CQDs into PANI, which improved the conductivity and alleviated the volume change of the CQDs-PANI electrode during the charge/discharge process. In addition, a flexible solid-state CQDs-PANI supercapacitor was constructed using carbon paper as current collector and polyvinyl alcohol gel electrolyte, exhibiting the stable capacitive performance at planar and bending state. The specific capacitance, energy and power density were determined to be 169.2 mF cm⁻², 33.8 μWh cm⁻² and 0.3 mW cm⁻² at a potential window of 1.2 V and a current density of 1.0 mA cm⁻². CQDs-PANI presented the promising application in flexible energy-related device.

Introduction

Supercapacitors or electrochemical capacitors, with high power density, fast charge-discharge rate and long life span, have attracted considerable attention in recent years [1]. Based on the charge storage mechanisms, supercapacitors are divided into two types, which includes electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs based on carbon materials of high surface area usually display perfect cycling stability, but low specific capacitance due to the limited electrochemically available surface area [2], [3]. By comparison, pseudocapacitors based on conducting polymers and transition metal oxides present high specific capacitance, while poor cycle life [4], [5], [6], [7]. Therefore, many research works involve the combination of the superior pseudocapacitance of conducting polymers and the good stability of carbon materials to improve the rate capability and cycling stability of the conducting polymer-based supercapacitors [8], [9], [10], [11], [12], [13].

Polyaniline (PANI) is a promising electrode material due to its low cost, ease of synthesis, relatively high theoretical capacity, as well as high doping/dedoping rate during charge/discharge processes [14], [15], [16]. However, the high resistance during cycling and poor cycling stability of PANI as a result of the volumetric swelling and shrinking during ion doping-dedoping process hinder the practical application of supercapacitors [17], [18], [19], [20]. Highly active carbon, titanium nitride and quantum dot materials have been widely investigated for photoelectrochemical applications [21], [22], [23]. In order to improve the conductivity of PANI-based electrode materials, tremendous efforts have been devoted to prepare carbon/PANI hybrid materials [24], [25], [26], [27], [28]. For example, a carbonaceous shell (∼5 nm) was deposited onto PANI surface via a hydrothermal reaction to enhance the cycling stability of PANI electrodes [29]. The carbonaceous shell-coated PANI achieved a capacitance retention of 95% after 10000 cycles in 1 M H₂SO₄ electrolyte. However, the specific capacitance of the electrodes was only 189.73 F g^-1. PANI was introduced onto the carbon fibers (CFs) surface via vapor deposition polymerization method. The CFs/PANI hybrid exhibited a maximum specific capacitance of 264 F g⁻¹ [30]. Another study prepared the hybrid films of chemically converted graphene (CCG) and PANI nanofibers by vacuum filtration the mixed dispersions of both components. The supercapacitor device based on the CCG/PANI nanofibers hybrid film showed a specific capacitance of 210 F g^-1 at a current density of 0.3 A g^-1, and it achieved ∼79% capacitance retention after 800 cycles at 3 A g^-1 [31]. Reduced graphene oxide (RGO)/PANI hybrid film was fabricated through solution processing. RGO/PANI hybrid film (906 S cm^-1) shows much higher electrical conductivity than the pristine PANI (580 S cm^-1) and RGO (46.5 S cm^-1) [32]. Additionally, the RGO/PANI hybrid film exhibited a specific capacitance of 431 F g^-1 at a current density of 0.45 A g^-1 with the capacitance retention of 74% after 500 charge/discharge cycles. RGO/PANI hybrid was prepared by directly coating the PANI on the surface of RGO sheets via an in situ polymerization process. The RGO/PANI hybrid exhibited a specific capacitance of 361 F g⁻¹ at a current density of 0.3 A g⁻¹, with ∼82% of initial capacitance retained after 1000 cycles [33]. In most of the previous works, either the fabrication methodology is complicated or the capacitive performance of PANI-based hybrid materials is low. Therefore, electrodeposition of PANI is the preferred methodology over chemical techniques, as the former has been shown to exhibit higher specific capacitance than chemically formed PANI [34]. In addition, among various carbon materials, carbon quantum dots (CQDs) with exceptional conducting properties that arise from the quantum confinement and edge effects [35], [36], [37] can realize the goal of improving the conductivity of PANI-based electrode materials. However, there is limited information concerning the fabrication of CQDs-PANI hybrid by electrodeposition method and application as supercapacitors electrode materials.

In this work, a novel composite electrode material of carbon quantum dots-polyaniline/carbon fibers (CQDs-PANI/CFs) was fabricated by directly photoelectrodepositing the CQDs and PANI on the surface of CFs. The CQDs were interconnected with PANI to promote electrons transportation, which could efficiently improve the conductivity of PANI and enhance its Faradic process. The unique hybrid-structured CQDs-PANI is expected to exhibit high specific capacitance, excellent rate capability and cycle stability for supercapacitor application.