Electro-synthesis of novel nanostructured PEDOT films and their application as catalyst support
© Zhou et al; licensee Springer. 2011
Received: 8 February 2011
Accepted: 27 April 2011
Published: 27 April 2011
Poly(3,4-ethylenedioxythiophene) (PEDOT) films doped with nitric and chlorine ions have been electrochemically deposited simply by a one-step electrochemical method in an aqueous media in the absence of any surfactant. The fabricated PEDOT films were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results indicate that the hierarchical structured PEDOT film doped with nitric ions displays a 'lunar craters' porous morphology consisting of PEDOT nano-sheets with a thickness of less than 2 nm. The effect of counter ions on the electro-polymerization, the electrochemistry, and the morphology of the polymer film was studied. Compared with PEDOT film doped with nitric acid, PEDOT film deposited in the presence of chlorine ions shows irregular morphology and less electrochemical activity. The specific nanostructure of the polymer was further studied as catalyst support for platinum nanoparticles to methanol electro-oxidation.
Poly(3,4-ethylenedioxythiophene) (PEDOT) is an important π-conjugated conducting polymer, which is currently being investigated for use in many fields , such as antistatic and anticorrosion materials, artificial muscles, electrode materials in batteries, super-capacitors, display devices, and biosensors. Although various 1D or 2D PEDOT nanomaterials, such as nanofibers, nanospheres, nano-tubes, and nanorods [2–6], have been prepared and studied, there are few reports on the more complicated hierarchical structure of this functional polymer. PEDOT film is one of the main applied forms of this functional material. For example, PEDOT films are recently studied as catalyst support for Pt or Pd nanoparticles for either electro-oxidation of methanol or ethanol [7, 8], which can be potentially used in direct methanol fuel cell (DMFC) or sensors to some chemicals, such as nitrite, bromate, oxygen, hydrogen peroxide . As many applications of the PEDOT material are related to its microstructure and electrochemical activity, studies on the film surface control and physical chemical properties are very important.
Electrochemical polymerization is a convenient method to prepare PEDOT, and generally a film yields on the surface of the anode. The electro-synthesis can be done both in organic and aqueous solutions. Due to the low solubility of EDOT in water at room temperature, previous research on the electro-polymerization of PEDOT was usually done in an organic solution, such as acetonitrile and propylene carbonate [10, 11]. Recently, there are growing interests in the study of electro-polymerization of PEDOT films in an aqueous solution [12–19]. It is known that the properties of conducting polymers are strongly dependent on their morphologies and their physical and chemical structures. Polypyrrole micro-containers have been synthesized electrochemically by direct oxidation of pyrrole monomer in an aqueous solution [20, 21]. Surfactants were often used to control the morphology of the PEDOT film with electrochemical methods recently [12–17]. Sodium dodecylsulphate (SDS) has also been used in the electro-synthesis of PEDOT with different morphologies (globular and fibrous) . PEDOT/PSS (poly(4-styrene sulfonate)) film with microstructures of micro-rings/arrows and bubbles has been prepared with the cyclic voltammetry method and the influences of applied potentials and surfactant (PSS) on the morphology of the resulted film were also investigated . PEDOT micro-cups with diameters in the range of 1 to 4 mm were generated by direct oxidation of EDOT in the aqueous solution of LiClO4 and tri (ethylene glycol (EG)) on the ITO electrode with a PSS/PDDA multilayer coating .
The conductive polymers, such as PAni or PEDOT have been studied as catalyst or its support for hydrogen and methanol fuel cell applications [22–24]. Pt supported on PAni-based nano-tubes and nanofibers showed an excellent electrochemical activity for methanol oxidation, respectively [23, 24]. In this work, a three-dimensional (3D) nanostructured PEDOT was fabricated through a simply one-step electrochemical route in an aqueous solution in the absence of any surfactant. Microscopic studies indicated that the films are comprised of fine electroactive polymer sheets with the edge thickness of less than 2 nm, which is only slightly larger than the well known graphene sheets. Moreover, a lunar crater porous morphology was observed on one surface of the films. To the best of our knowledge, this is the first report concerning the deposition of such nanostructured PEDOT films on Pt and its application as catalyst support for methanol oxidation.
Electro-deposition of PEDOT
The electrochemical deposition of PEDOT was carried out on a CHI1202 Electrochemical Analyzer (CH Instruments). All solutions were prepared in distilled water and all potentials reported are referenced to saturated calomel electrode (SCE). An electrolyte of 0.1 M KNO3 (or 0.1 M KCl) + 0.01 M EDOT was used for the deposition of PEDOT doped with nitric ions (denoted as PEDOT-NO3) (or doped with chlorine ions (denoted as PEDOT-Cl)) on platinum electrode. A three-electrode electrochemical cell was used for the electrochemical measurements, where the counter electrode was a Pt foil and the reference electrode was a SCE. The charge consumed was monitored and was considered as a measure of the mass of the deposited PEDOT. The polymer was removed carefully from the electrode and dispersed in EG under ultrasonic irradiation to prepare the PEDOT suspensions.
Electrocatalyst fabrication and the methanol oxidation
Pt particles in EG were prepared in situ by a microwave assisted EG reduction method. 0.1 g H2PtCl6 and 0.1 g NaOH were dissolved in 5 mL EG. The glass bottle was placed in the center of a microwave oven and heated for 13 s to prepare the Pt nanoparticle containing solution. The fabricated black solution was very stable and was mix with the above PEDOT suspensions. Seventy microliters of the Pt containing solution was added dropwise to the above PEDOT suspension with strong ultrasonic irradiation to prepare the electrocatalyst composites. After centrifugation and washing with ethanol, the material was dispersed in 0.05 wt% Nafion solutions and sonicated for 20 min to prepare the catalyst ink. Eight microliters of such catalyst solution was added to the surface of the glass carbon (GC) electrode (Φ = 3 mm) and dried in air. Prior to the electrochemical measurement, the catalyst covered electrode was soaked in the electrolyte solution (1 M CH3OH + 0.5 M H2SO4) for 10 min.
Morphology and spectrum of the materials
Transmission electron microscopy (TEM) characterization was performed on a Philips CM12 operating at an accelerated voltage of 120 kV and scanning electron microscopy (SEM) analyses were performed on a Zeiss ULTRA plus. Raman spectroscopy of the products was performed on an Invia Renishaw Raman using a He-Ne laser at 633 nm wavelength.
Results and discussion
Electropolymerization of PEDOT
The conductive polymers are always prepared in their oxidation form with doping ions. The doping ions in the PEDOT have been reported to have great influence on the electrical properties of the film [12, 13]. For example, the PEDOT films doped with Cl- have much less electrical conductivity than those doped with NO3 - . Here we studied the effect of the doping ions (Cl- and NO3 -) on the electro-polymerization behavior, the morphology, and the electrochemical properties of the resulted PEDOT polymer film.
Methanol oxidation on Pt-PEDOT catalyst
PEDOT films with a flower-like nanostructure on Pt electrodes have been electrochemically generated simply by a one-step electrochemical method in an aqueous media in the absence of any surfactant. Inorganic counter ions have great effect on the electrochemistry and morphology of the electropolymerized PEDOT films. The morphology of the prepared PEDOT film doped with nitric ions shows porous hierarchical nanostructure based on fine nanosheets which develop large specific surface area with high electrochemical activity. Electrocatalysts comprised the PEDOT film and Pt nanoparticles showed high catalyst performance to methanol electro-oxidation.
direct methanol fuel cell
scanning electron microscopy
saturated calomel electrode
transmission electron microscopy.
The authors are grateful for access to the characterization facilities in the Australian Microcopy & Microanalysis Research Facility at the Australian Key Centre for Microscopy and Microanalysis, University of Sydney. C.Z. acknowledges the award of an APA scholarship. Z.L. and X.D. would like to thank the Australian Research Council (ARC) for the financial supports (DP0773977 and DP0772551).
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