Elsevier

Electrochimica Acta

Volume 191, 10 February 2016, Pages 165-172
Electrochimica Acta

Coherent polyaniline/graphene oxides/multi-walled carbon nanotubes ternary composites for asymmetric supercapacitors

https://doi.org/10.1016/j.electacta.2016.01.076Get rights and content

Abstract

A coherent polyaniline (PANI)/graphene oxides (GOs)/multi-walled carbon nanotubes (MWCNTs) composite was prepared by in-situ solution polymerization as a positive electrode of supercapacitors. The orderly growth of PANI nano-dots on GOs led to the formation of the nano-ravines that can enhance ions diffusion efficiency. MWCNTs surrounded by PANI connected all components, and thus the conductivity with the increasing electron transfer rate was improved. The results showed that the electrode exhibited the outstanding electrochemical performances with the specific capacitance up to 696 F g−1 at 20 mV s−1. The KOH-activated GOs/MWCNTs were used as a negative electrode to assemble an asymmetric supercapacitor (ASC). The ASC possessed an extended working potential (1.6 V), a good rate capability (58% capacitance retention even after the current density being increased by 10 times), an excellent cycling stability (89% capacitance retention after 3000 cycles), and a decent average energy and power density (69 W h/kg and 6.4 kW/kg).

Introduction

With the rapid development of energy crisis and living standards, efficient and convenient energy storage devices such as batteries and supercapacitors (SCs) have attracted intensive attention in recent decades [1], [2], [3], [4], [5], [6], [7], [8]. In comparison with batteries, SCs, also called electrochemical capacitors, are more intriguing on account of their higher power density, longer cycle life, lower impact on the environment and fewer application restrictions [5], [6], [9], [10]. According to energy storage mechanisms, SCs are always classified into double electric layer capacitors (DELCs) and pseudocapacitors [6]. The former is known for quick charging-discharging, long cycle life and superior rate capability. However, they also hold a main weakness of low energy density since storing or releasing charge only depends on physically absorbing or desorbing ions at the electrode and electrolyte interface. Carbon based materials are always employed as their electrode materials [3]. Pseudocapacitors, storing charge through chemical redox reaction on the surface of the electrode, can tremendously enhance energy density, but the cycling stability and power density are simultaneously sacrificed. Conducting polymer and transition metal oxide/hydroxides are their representatives [7], [11], [12].Therefore, fabrication of hybrid materials based devices is an available way to solve these problems.

The asymmetric supercapacitor (ASC) is a kind of important hybrid devices, which are assembled by a negative electrode as a source of power density and a positive electrode as a source of energy density [13]. Although the high-performance ASC needs preeminent cathode and anode materials, considerable efforts were made to explore cathode materials because the energy density of an ASC is mostly ascribed to the cathode [14]. Hence, to seek an outstanding positive electrode is the critical factor for improving the energy density of devices, and the energy density can be calculated by the Eq. (1):E = 1/2CsV2In which, Cs is the specific capacitance and V is the operating voltage [14]. Some factors, such as theoretical value of Cs, specific surface area (SSA), conductivity, have significant influences on Cs of an electrode material. In addition, organic electrolytes and the above-mentioned ASC are two major ways to extend the operating voltage. However, organic electrolytes are toxic and expensive.

Here, we take advantage of polyaniline (PANI), graphene oxides (GOs), and multi-walled carbon nanotubes (MWCNTs) to construct smart ternary composites as a positive electrode, looking forward to exhibiting excellent electrochemical performances. Thereinto, PANI, one of the well-known conducting polymers, has been extensively studied as the electrode materials of amazing supercapacitors because it was manifested to have a prominent theoretical specific capacitance, good doping reversibility, wide operating potential, abundant redox states, ease of synthesis, controllable morphology, light weight, and low cost [7]. However, neat PANI commonly possessed relatively low SSA and conductivity, leading to much lower realistic specific capacitance and poor cycling stability that are stumbling blocks in advanced applications [15]. GOs, composed of few-layers of graphites, are always acquired through chemical oxidation and exfoliation of graphite so as to exist many oxygen-containing functional groups. GOs possess very large SSA and nice solution processability [16]. They are proper to load PANI to enlarge SSA of a composite, but their conductivity are poor and reduced GOs are easily agglomerative [17]. MWCNTs consist of graphitic central tubes with nanometric diameters [18]. Their favorable conductivity are beneficial for supercapacitors [19], [20].

In this paper, we prepared the PANI/GOs/MWCNT (PAGM) ternary composites as the positive electrode of the ASC through in-situ solution polymerization. The PAGM displayed preferable electrochemical performances of low resistance and high specific capacitance up to 696 F g−1 at 20 mV s−1 compared with binary composites. Additionally, KOH was not only used as an activator but also flocculant to produce activated GOs/MWCNTs (A-GM) with uniform and poriferous structures. A-GM was used as the negative electrode material to assemble ASC, which exhibits an extended working potential (1.6 V), a good rate capability (58% capacitance retention even after the current density being increased by 10 times), an excellent cycling stability (89% capacitance retention after 3000 cycles), and a decent average energy and power density (69 W h kg−1 and 6.4 kW kg−1). It could contribute a few tips for supercapacitor in advanced applications.

Section snippets

Preparation of GOs, GOs/MWCNTs, MWCNTs solutions

GOs were prepared by the reported method [21], [22]. MWCNTs were purchased from Guangzhou Wei chemical co., LTD and purified by diluted acid before use. In order to acquire homogeneous dispersion liquids, 50 mg mixtures (weight ratios of GOs: MWCNTs = 3:2) were dispersed into 50 mL ultra-pure water by fully stirring and ultrasonication. For enhancing dispersion and avoiding aggregation, 0.5 g polyvinyl pyrrolidone (PVP) was added into the mixture with continuous stirring to dissolve them. Similarly,

Synthesis and characterization of PAGM

Two key factors immensely affect the electrochemical properties of the composites: (1) the good dispersion of the supporting materials such as GOs and MWCNTs; (2) the uniform growth of PANI on GOs/MWCNTs. Hence, PVP, a kind of nonionic surfactant, was introduced into the process of synthesis, as shown in Fig. 1, to enhance dispersion and restrict the polymerization rate. For the PAG composites shown in Fig. 1b, PANI nano-dots orderly paved on the surfaces of GOs. From the insets of Fig. 1b,

CONCLUSIONS

We have acquired the ternary composites based on GOs providing enough SSA, MWCNTs enhancing conductivity, and PANI supplying abundant energy density by a simple solution polymerization. This composites displayed neat micro-structure that uniform PANI nano-dots were absorbed on the surfaces of GOs and MWCNTs, and the modified MWCNTs by PANI were equably interlaced between GOs. Therefore, the composite exhibited low resistance and high specific capacitance up to 696 F g−1 at 20 mV s−1. In addition,

Acknowledgement

This work was financially supported by National Natural Science Foundation (51272071, 21401049) and Hubei Provincial Department of Science & Technology (2014CFA096), China.

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