Conductive polyaniline composite films from aqueous dispersion: Performance enhancement by multi-walled carbon nanotube

Highlights

• Water-borne conductive MWNT/polyaniline films were synthesized.

• The conductivity showed a 2 order of magnitude increase.

• The films showed high capacitance and better cyclability.

Abstract

The intractable multi-walled carbon nanotube (MWNT) was dispersed in aqueous solutions of conductive polyaniline (cPANI) by externally doping of MWNT/polyaniline (PANI) employing an organic polymeric phosphate acid as the dopant. Free-standing composite films were obtained from the MWNT/cPANI aqueous dispersion after drying. In comparison with the cPANI, the conductivity of MWNT/cPANI composites showed a 2 order of magnitude increase with the maximum value 1.2 S cm⁻¹, at the MWNT loading of 1.0 wt% because of the doping effect of MWNT on PANI. Under the strong π–π interactions between PANI and MWNT which were discussed by FT-IR and UV–vis analysis, the redox performance of the MWNT/cPANI composites was significantly modified compared with the pure cPANI. In addition, the capacitive behaviors of the MWNT/cPANI films varied with the difference of the MWNT loading due to the morphology variation resulted from the established interactions. The enhanced specific capacitance with the highest value of 371 F g⁻¹ at a current loading of 0.5 A g⁻¹ was obtained for the MWNT/cPANI films containing MWNT content of 1.0 wt%, as well as the better cyclability.

Introduction

Among numerous conductive polymers, polyaniline (PANI) is considerably attractive because of easy synthesis, environmental stability and high controllable conductivity, which has been widely used in light weight battery electrodes [1], energy storage devices [2], [3], light-emitting diodes [4] and anticorrosion coatings [5]. To further enhance electrical and mechanical properties over pure conducting PANI (cPANI), introduction of carbon nanotube (CNT) showing high conductivity into cPANI matrix has recently attracted much attention [6], [7]. Because of the strong molecular adhesion among them, the main challenge is to obtain processable CNT/PANI in the emeraldine salt form, which is considerably required for the fabrication of flexible electrochemical devices.

Soluble conducting polyanilines can actually be obtained by incorporation of functional groups on the backbone and doping of PANI by soluble polymeric acid [8], [9]. Once cPANI becomes soluble, the intractable CNTs can be dissolved into the formed solutions stabilized by the solubilized cPANI chains [10]. By using of the anionic surfactant as a common stabilizer as well as an additional dopant, the composite of cPANI/single-walled carbon nanotube (SWNT) prepared by interfacial polymerization was successfully extracted from the aqueous phase to the organic xylene phase, forming a processable organic dispersion [11]. With the increasingly environmental concerns, synthesis of water-soluble conductive polyaniline/CNT composites from environment-benign processes becomes inevitable. Whereas micelle–CNT hybrid template directed synthesis method is a feasible route to aqueous dispersion of CNT/cPANI composites [12], post elimination of the surfactant from the system is generally needed during practical applications. Therefore, the chemical modification method by hydrophilization of cPANI has been a favorably used route to prepare the water-borne CNT/cPANI composites, where the tedious removal of insulating surfactants is not required. In Haddon group, by covalent attachment of poly(m-aminobenzene sulfonic acid) to single-walled carbon nanotube, the nanotube-sulfonic cPANI compound without the presence of surfactant was made water-soluble [13], which showed a conductivity of 5.6 × 10⁻³ S cm⁻¹. In addition, via noncovalent functionalization of multi-walled carbon nanotube (MWNT) by sulfonic cPANI, the nanotube/cPANI composite also showed good water-soluble behaviors [14]. However, the conductivity of these nanotube/sulfonated cPANI composites was difficult to be further increased because of the presence of the electron-withdrawing sulfonic group on the aromatic rings of PANI. In fact, cPANI can also be well dispersed in water by externally doping PANI with a suitable polymeric acid, aside from the style of synthesizing water-soluble self-doped cPANI. Employing acidic phosphate diester carrying long hydrophilic tail as the dopant, stable aqueous solution of cPANI was successfully prepared by Wang et al. [15], [16], [17]. However, the conductivity of the free-standing films obtained was less than 0.1 S cm⁻¹, due to the presence of high concentrations of insulating polymeric components in the system. For the electrochemical performance enhancement, addition of some compounds showing high conductivity such as CNTs into the cPANI matrix is a facile approach.

We report here a water-dispersible conducting MWNT/cPANI composite with enhanced electrochemical performance than pure cPANI. The conducting MWNT/PANI composite was prepared based on the in-situ polymerization of aniline and MWNTs followed by doping with acidic phosphate diester carrying long hydrophilic tail in water, as shown in Fig. 1. By using a solution casting method, the free-standing flexible MWNT/cPANI composite films were available. On the other hand, CNTs and conducting polymers are considered as the main materials for the supercapacitor electrode aside from metal oxides [18], [19]. Therefore, combination of MWNT and cPANI may endow the composites surprisingly high capacitance, thus opening many new possibilities for their prospective technological applications.