The composites based on carbon nanotubes (CNTs) and conducting polymers (CPs) are promising materials for supercapacitor devices due to their unique nanostructure that combines the large pseudocapacitance of the CPs with the fast charging/discharging double-layer capacitance and excellent mechanical properties of the CNTs. Here, we report a new electrochemical method to obtain polypyrrole (PPY)/single-walled carbon nanotube (SWCNT) composites. In the first step, the SWCNTs are covalently functionalized with monomeric units of pyrrole by esterification of acyl chloride functionalized SWCNTs and N-(6-hydroxyhexyl)pyrrole. In the second step, the PPY/SWCNTs composites are obtained by copolymerizing the pyrrole monomer with the pyrrole units grafted on SWCNTs surface using controlled potential electrolysis. The composites were further characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The results showed good electrochemical charge storage properties for the synthesized composites based on PPY and SWCNTs covalently functionalized with pyrrole units making them promising electrode materials for high power supercapacitors.
Currently, electrochemical capacitors are attracting much attention for use in high power energy-storage devices [
Especially polyaniline (PANI) and polypyrrole (PPY) have been considered as the most promising materials for this application due to their excellent capacity for energy storage, easy synthesis, higher conductivity, and lower cost than many other conducting polymers [
However, conducting polymer films suffer from mechanical and chemical instability in life cycle tests and low conductivity in the reduced or neutral states. These are directly related to the life cycle of conducting polymer-based supercapacitors [
Due to their unique nanostructure that combines the redox charge storage mechanism of conducting polymers with the high surface area and nanoporosity of carbon nanotube (CNT) films, recent research has sought to combine carbon nanotubes and conductive polymers for use as composite electrodes in supercapacitors [
These composites structures were commonly prepared by electrogeneration of a polymer on electrodes already modified by adsorbed CNTs coatings [
This work focuses on a new fabrication route to prepare PPY/CNTs composite electrodes for use as electrochemical capacitors, that is, the monomer copolymerization with the pyrrole units grafted on single-walled carbon nanotubes (SWCNTs) surface by galvanostatic electropolymerization. Pyrrole functionalized CNTs have been synthesized by chemical binding of carboxylic functionalized CNTs, commercially available, and N-(6-hydroxyhexyl)pyrrole. The hydroxyhexyl chain served as a flexible spacer to facilitate the chemical coupling between pyrrole radical cations in the polymerization process. This method affords the obtaining of composite materials with good electrochemical charge storage properties and, in the same time, leads to a simple and reproducible formation of organic film with precise spatial resolution over surfaces, whatever their size and geometry.
The morphology and electrochemical properties of the PPY/CNTs composites were characterized in order to evaluate their possible application for pseudocapacitors.
Carboxylic acid functionalized single-walled carbon nanotubes (4-5 nm × 0.5–1.5
Pyrrole functionalized CNTs have been synthesized by chemical binding of carboxylic functionalized CNTs, commercially available, and N-(6-hydroxyhexyl)pyrrole via ester formation.
N-(6-hydroxyhexyl)pyrrole synthesis is presented in Scheme
Both compound
The chemical binding of carboxylic functionalized SWCNTs and N-(6-hydroxyhexyl)pyrrole is presented in Scheme
The composite films were synthesized electrochemically via codeposition from a solution containing both the functionalized CNTs and the pyrrole monomer. The pyrrole functionalized CNTs showed a good solubility in propylene carbonate; so the electropolymerization has been performed (galvanostatic method, at different current densities) from a propylene carbonate solution containing the pyrrole functionalized CNTs (0.4 mg mL−1), 0.05 M pyrrole, and 0.1 M LiClO4 as supporting electrolyte.
IR absorption spectra of CNTs in the spectral range 500–4000 cm−1 are shown in Figure
FTIR spectra of CNTs-COOH and pyrrole functionalized CNTs.
SEM showed that the surface morphology of the studied films differs remarkably between the PPY/CNTs and
SEM morphology for PPY ((a)–(c)) and PPY/CNTs composite films ((d)–(f)) obtained galvanostatically at 0.1 mA cm−2 for 20 min.
Figure
CVs in 0.1 M LiClO4 in propylene carbonate of the PPY/CNTs composite films compared with the pure polymeric ones galvanostatically obtained at different charge densities.
A more detailed estimation of the electrochemical properties for the
Nyquist ((a)-(b)) and Bode ((c)-(d)) diagrams for PPY/CNTs nanocomposite films compared to the pure polymeric one (obtained galvanostatically at different charge densities) at open circuit potentials in 0.1 M LiClO4 solution in propylene carbonate.
The capacitances of the electrode materials were calculated, according to the equation
Capacitance evaluation for
One can observe one order of magnitude higher capacitance values for PPY/CNTs film in respect of
Capacitance and real impedance values at 0.01 Hz for pure polymeric and composite films.
Modified electrode | Slope of |
|
|
Relative standard deviation (%) |
---|---|---|---|---|
PPY/ |
12.646 | 73.5 | 0.079 | 4.23 |
PPY/CNTs |
10.807 | 47.8 | 0.093 | 4.57 |
PPY/CNTs |
4.4154 | 15.3 | 0.226 | 6.26 |
PPY/CNTs |
2.2193 | 10.2 | 0.451 | 6.73 |
The real impedance at low frequencies, where the capacitive behaviour dominates, is an indication of the combined resistance of the electrolyte and the film including both electronic and ionic contributions. The values of the real impedance at 0.01 Hz are also given in Table
The capacitance and real impedance values at 0.01 Hz presented in Table
Also, it is worth mentioning that this fabrication route provides composite coatings that are stable in organic and aqueous solvents. Our studies showed that, after an extensive washing procedure with solvents as acetonitrile or ethanol using sonication, the PPY/CNTs films could not be removed from the Pt electrode surface.
A new route is reported here for the fabrication of PPY/CNTs composite electrodes on the basis of pyrrole electrocopolymerization with pyrrole units grafted on CNTs surface. The covalent functionalization of CNTs with pyrrole units was carried out by the esterification of acyl chloride functionalized SWCNTs with N-(6-hydroxyhexyl)pyrrole. The electropolymerizable pyrrole group was chemically bound to the SWCNT backbone by using a hydroxyhexyl chain that served as a flexible spacer to facilitate the chemical coupling between pyrrole radical cations in the polymerization process. Future work will be dedicated to the optimisation of the polymerization process using appropriate spacers to assure an optimal accessibility of the pyrrole radical cations in the polymerization process.
PPY/CNTs composite films of various thicknesses can be grown by galvanostatic polymerisation onto platinum electrodes. SEM results confirmed the presence of well-distributed, networked nanotubes that are individually coated with PPY, thus forming a highly porous composite structure. Compared with similarly grown pure PPY films, the composite films exhibit better electrochemical responses. The capacitance per geometric electrode surface area (F cm−2) of the PPY/CNTs composite films increases relative to the electropolymerization charge, reaching 0.226 F cm−2, while the capacitance of similarly grown
This work was supported by CNCS-UEFISCDI, project PN II-RU, no. 15/05.08.2010, code TE_153.