Development of a new hybrid CNT-TEPA@poly(3,4-ethylenedioxythiophene-co-3-(pyrrol-1-methyl)pyridine) for application as electrode active material in supercapacitors

Highlights

• A novel hybrid CNT-TEPA@poly(3,4-ethylenedioxythiophene-co-3-(pyrrol-1-methyl) pyridine) was synthesized.

• The characterization of the hybrid suggested the formation of covalent bonds between the CNT and the copolymer.

• The hybrid electrochemical versatility was tested in both aqueous and organic electrolyte.

• The SC presented 81.3%, and 77.5% of capacitance retention after 5000 cycles in aqueous and organic media, respectively.

Abstract

We report the synthesis of a CNT-TEPA@PEPy hybrid material by the copolymerization of EDOT and Py analogs units covalently bonded to TEPA modified carbon nanotubes (CNTs). The chemical structure and morphology of the hybrid were compared to those of the unmodified CNTs using FTIR, Raman and XPS spectroscopies. The hybrid was applied as the electrode active material in symmetric supercapacitors (SCs) which were evaluated in aqueous and organic (acetonitrile) electrolytes to study the versatility of the as-synthesized CNT-TEPA@PEPy. The capacitances values of the cells obtained from the hybrid-based SCs are 56% and 115% higher than those of the cells prepared with the unmodified CNT in organic and aqueous electrolytes, respectively. The full cells assembled with CNT-TEPA@PEPy exhibited excellent cyclability with 81.3% and 77.5% of capacitance retention after 5000 cycles in aqueous and organic media, respectively. The device properties indicate the feasibility of the applied synthetic approach, showing that the prepared hybrid is a promising material for use in highly cyclable supercapacitors.

Introduction

Conjugated polymers (CPs) are materials that present alternate conjugated double bonds in their structures, which helps the migration of electrons along the polymeric chains and the extension of this π-conjugated system is directly related to the electronic conduction of the polymer [1,2]. Among these CPs, polythiophene (PPt), polypyrrole (PPy), and polyaniline (PANI) have chemical and electrochemical stability, can form thin films and are capable of storing charge in all their volume [3]. However, some CP may present insolubility against organic solvents, electrochemical instability in solutions with varying pH, etc [4]. The copolymerization between different monomers is a promising method to obtain a material with improved properties compared to the origin homopolymers [5,6]. The CPs align mechanical properties such as flexibility, elasticity, and processability with electrical and optical characteristics, allowing their application in the field of transistor [7], batteries [8], diodes [9], biomedical [10] and biological sensors [11], fuel cells [12], dye-sensitized solar cell [13,14], supercapacitors (SCs) [[15], [16], [17]], etc.

Among the energy storage technologies, SCs have stood out due to their high-power density, high efficiency, moderate energy density and long cycling life [18]. While in the electric double-layer capacitors (EDLCs) there is a pure electrostatic charge accumulation at the interface of the electrode material and the electrolyte, pseudocapacitors [[19], [20], [21]] are a class of SCs that can be formed by electroactive materials like transition metal oxides and/or conducting polymers that contribute with additional redox processes. The transition metal oxides show high capacitance but have a higher cost compared to CPs. On the other hand, CP can provide a full electrochemical window due to the different polymeric materials that can be used, showing high capacitance and energy density values. The main problem with these materials is the low cyclability due to the degradation of the molecular structure of CP [18].

Carbon nanotube (CNT) is also a material that has been studied for several applications since the work of Iijima in 1991 [22]. This interesting material has been the subject of intense research due to its optical, electrical, thermal and mechanical properties and the large length/diameter aspect ratio [23]. Among other applications, CNTs can be used as electrodes for SCs due to their chemical stability, low resistivity, and mesoporous structure [[24], [25], [26], [27]]. However, due to their moderate surface area, the capacitance regarding the electric double layer is relatively low. One strategy to overcome this issue is to increase the charge storage by adding faradaic contributions to the charge storage mechanism leading to pseudocapacitance [18], which in the case of CNT it is achieved by surface modifications such as the introduction of functional groups that can be done by electrochemical oxidation of the CNTs [28] or chemical processes [29].

Hybrids and/or composites materials are a class of materials where two or more substances together come to exhibit unique properties, not observed in their individual components. The hybrid materials stand out and can be defined as the combination of inorganic components (metal ions, graphene, carbon nanotubes, salts, oxides, etc.) and organic components (organic groups or molecules, ligands, polymers, etc.) which result in the synergistic enhancement of their functional properties [30]. The CNTs and CPs combination are interesting because they have unique electrochemical properties, leading the research in new hybrids essential for obtaining a material that has specific applications [31]. The use of these materials in energy conversion and storage technologies has an important highlight due to the great worldwide demand, which makes the research and development in this area increasingly necessary. Several studies using CNT and other carbon materials and CP have been developed for application in organic photovoltaic (OPV) [32] and SCs [[33], [34], [35]].

The CNT-conjugated polymers composites/hybrids can deliver higher capacitance than the neat CNT and also overcome the low cyclability of the CPs [36]. There are many approaches in the literature to modify the CNT surface with CPs. One of these methods consists of wrapping the CP polymer chains around the pristine CNT, oxidized-CNT or after the adsorption of surfactant molecules in the surface of the CNT, without any chemical bonding between the CNT and the CP [34,37,38]. On the other hand, the second approach is based on a chemical modification of the CNT surface leading to chemically bounded CPs to the CNT [39]. The chemical bond between the materials was found to facilitated electronic transport at the material interface CNT/CP and reduced the electrical percolation threshold facilitating their application in electric/electrochemical devices [35,40,41].

Besides the electrode material, the electrolyte plays an important role in device performance. There are different electrolytes with a different electrochemical potential window that can be used. The aqueous electrolytes are important and conventional systems, mainly due to their low cost, non-toxicity, and non-flammability. However, its stability window is low, usually around 1.0 V, with water decomposition at higher potentials [18,42]. Organic electrolytes, on the other hand, are more stable and may exceed a 2.0 V electrochemical window in most cases. However, these electrolytes have disadvantages such as their cost, drying to make them moisture free and their flammability and the production of gases that can cause explosions [18,42].

In this study, the poly(3,4-ethylenedioxythiophene-co-3-(pyrrol-1-methyl)pyridine) (P(EDOT-co-PyMP)) denominated here by PEPy was bonded into CNT, forming a novel hybrid with the same content between the polymer and the CNT. The work describes the chemical synthesis of the hybrid and its structural characterization by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG) and transmission electron microscopy (TEM). The material was electrochemically characterized by cyclic voltammetry and galvanostatic charge and discharge cycles. Also, the hybrid was applied as the active electrode material in SCs to demonstrate the advantages of this new material. The SCs were constructed with a symmetrical two-electrode design, using organic (acetonitrile (ACN)) and aqueous medium both in lithium perchlorate (LiClO₄) 0.5 M as the electrolyte.