Evolution of flexible 3D graphene oxide/carbon nanotube/polyaniline composite papers and their supercapacitive performance

Abstract

Polyaniline (PANI) layers are deposited onto the freestanding, flexible reduced graphene oxide/carbon nanotube (RGO/CNT) papers for application as electrodes in supercapacitors. The morphology of PANI and thus the electrochemical performance of the papers are optimized by controlling the in situ anodic electropolymerization (AEP) process. The synergistic effect arising from the 3D PANI layer in the highly conductive RGO/CNT sandwich structure promotes the capacitive performance of RGO/CNT papers. The composite papers electropolymerized for 900 s deliver the most balanced rate capabilities of 229, 202, 181 and 155 F g⁻¹ at current densities of 0.2, 0.5, 1 and 2 A g⁻¹, respectively.

Introduction

There have been ever increasing demands for environmentally friendly, high performance energy storage systems to power portable devices and electric vehicles [1], [2], [3]. Supercapacitors, especially those containing flexible and freestanding electrodes, are among the most promising candidates for the above applications due to many advantages over other types of energy storage devices. They include higher power densities and shorter charging time than Li ion batteries (LIBs), higher energy densities than conventional capacitors, long cyclic lifetime and low maintenance costs [4], [5], [6], [7]. To realize high-performance flexible supercapacitors, other criteria also need to be satisfied, namely freestanding, binder-free electrodes with favorable mechanical strength and high capacitance [8].

A myriad of efforts have been dedicated to developing LIBs and supercapacitors consisting of flexible electrodes, based mainly on nanocarbon materials, such as carbon nanotubes (CNTs) and graphene nanosheets. They possess high electrical and thermal conductivities, excellent mechanical properties and extremely large specific surface area, all of which are important attributes to satisfy the functional requirements of flexible LIBs and supercapacitors. Nevertheless, CNT buckypapers have inherently low capacitance, limiting their practical applications when used alone. Although the theoretical surface area and capacitance of graphene are very high, i.e. 2965 m² g⁻¹ and 593 F g⁻¹, their experimental values are often disappointingly low, say 80 m² g⁻¹ and 120 F g⁻¹, respectively, because of the inevitable re-stacking and agglomeration of graphene oxide (GO) sheets during the film formation [4], [5], [6], [7], [9], [10]. Furthermore, a proper reduction process has to be chosen for a given electrolyte because the degree of reduction significantly affects the supercapacitive performance of reduced GO (RGO) papers [10].

In an effort to avoid the degradation of useful properties due to re-stacking of RGO sheets, CNTs were intercalated between them, so that open channels were created for the access of electrolyte and the electrical conduction was improved through the RGO film thickness direction by forming 3D networks [11]. Although the power and energy densities of hybrid nanocarbon papers were substantially enhanced, the specific capacitance remained far below that of graphene/conductive polymer or graphene/transitional metal oxide hybrid papers [12], [13], [14], [15]. Polyaniline (PANI) is a typical inherently conductive polymer (ICP) with good environmental stability, excellent electroactivity and unusual doping/de-doping chemistry. Three different processing methods, including (i) in situ chemical polymerization with the aid of acid and ammonia persulfate [16], [17], [18], (ii) physical mixing of graphene and PANI nanofillers [12], and in situ anodic electropolymerization (AEP) [14], have been explored to synthesize graphene/PANI hybrid electrodes. Among these methods, the in situ AEP offers the simplest and most efficient route to prepare flexible, freestanding graphene/PANI thin films or papers. They showed a high tensile strength of 12.6 MPa and a stable capacitance of 233 F g⁻¹, which outperformed other carbon-based flexible electrodes [14] but was lower than those of graphene/PANI powders or CNT/PANI hybrid papers. This finding is ascribed to the difficulties in penetrating PANI deposition into the inside of the graphene papers, resulting in significantly lower PANI contents in graphene/PANI papers than in the latter hybrids [12], [14].

Herein, we have prepared flexible, freestanding RGO/CNT/PANI hybrid papers via an in situ AEP method, creating open channels in the CNT-intercalated GO papers. The morphological evolution and the corresponding supercapacitive performance of the hybrid papers are investigated by controlling the AEP duration to identify the optimal conditions. The physical, chemical, and electrochemical properties of the hybrid papers are characterized to establish the correlation between the material characteristics and the supercapacitive performance of the electrodes made from the composite papers.