Hollow nitrogen-containing core/shell fibrous carbon nanomaterials as support to platinum nanocatalysts and their TEM tomography study
© Zhou et al; licensee Springer. 2012
Received: 14 October 2011
Accepted: 2 March 2012
Published: 2 March 2012
Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.
Keywordscarbon nanofiber N-doping core/shell polyaniline catalyst support methanol oxidation TEM tomography
The supporting materials of electrochemical catalysts have been shown to have great effects on the electrochemical performance of the catalysts . Many kinds of carbon nanomaterials, such as carbon black, carbon nanotubes (CNTs) and graphenes, have been studied as supports for platinum (Pt)-based electrochemical catalysts [2–5]. Conductive polymers are another type of catalyst supports and are believed to provide both electron and proton conductions in the catalyst layer of the electrode of a fuel cell [6–9]. It was concluded that polyaniline (PANI) in the catalyst in the anode was beneficial in the absorption of water and formation of an active oxy-compound Pt-OHads, which would promote oxidizing CO to CO2 and Langmuir absorption of methanol . Recently, nitrogen (N)-doped carbon materials seemed promising catalyst supports through the effects of N-doping on surface physicochemical properties, electron transfer and nanostructures of the supports and catalysts, which exhibited higher catalytic activity and durability [11–15]. Two methods have been usually employed for the synthesis of N-doped carbon nanomaterials: by direct N-doping during preparation of the carbon nanomaterials and treatment of the carbon composites with N-containing precursors [11–19]. PANI could be a good candidate for such precursors owing to the facile post-treatment for its carbonization [14, 15, 18–20]. Also, it exhibits a strong interaction with aromatic graphenes in carbon nanomaterials, which facilitates easy fabrication of uniform nanostructured carbon/PANI composites. Moreover, Pt catalysts supported by CNT/PANI composites have been demonstrated to have excellent electrochemical activity for methanol oxidation [21–24]. Thus, it is expected that the electrochemical catalytic performance of catalysts supported by carbon/PANI composites can be further improved by N-doping of the composite supports followed by carbonization. In this work, a simple method to prepare N-doped carbon nanofibers (CNFs) by carbonization of preformed PANI was developed. It was found that the carbonization treatment of the CNF/PANI composites greatly increased the electrochemical catalytic activity for methanol oxidation in fuel cells. Transmission electron microscope (TEM) tomography was used to examine the dispersion of Pt nanoparticles in the catalyst supports.
In situ polymerization to prepare CNF/PANI core/shell composites
CNF (Pyrograf Products Inc., Cerdaville, OH, USA) was first treated with a conventional chemical oxidation method using concentrated nitric acid to remove the metal impurities. The CNF/PANI nanocomposite was prepared using the following procedure. A given amount of carbon nanofiber powder obtained above and 1 ml aniline were put into 50 ml 1 mol/L HCl solution (weight ratio between CNF and aniline was 2:3). Then, 2.5 g FeCl3.6H2O in 20 ml solution was added into the mixture. The mixture was constantly stirred for 24 h. After filtration, the precipitate was washed with 1 M HCl, water and, finally, ethanol. The product was dried at 80°C for 12 h and stored in a desiccator before characterization. The CNF/PANI composite was treated at 900°C in an oven for 5 min to carbonize the PANI. The heat-treated and untreated CNF/PANI supports in this study are denoted as CNF/HPANI and CNF/PANI, respectively.
Nanocatalyst fabrication and the methanol oxidation
Pt particles for dispersion on the catalyst supports were prepared by a method of in situ reduction of a Pt salt with formic acid. H2PtCl6.H2O (0.2 g) was dissolved in 10 ml distilled water. CNF/HPANI catalyst support was dispersed in the distilled water by sonication for 20 min. Then, an appropriate amount of H2PtCl6.H2O solution was added dropwise to the above support suspension with sonication. After adding formic acid, the suspension was heated in a water bath at 70°C for 3 h with ultrasonication to complete the reduction of Pt. After centrifugation and washing with water and ethanol, the material was dispersed in 0.05 wt.% Nafion solution and sonicated for 20 min to prepare the catalyst ink. An amount (3.5 μL) of this catalyst solution was added to the surface of the glassy carbon (GC) electrode (3 mm in diameter) and dried in air. For comparison, the catalyst with CNF/PANI composite as the support was also prepared under the same experimental conditions. Energy dispersive spectroscopy (EDS) analysis indicated that the Pt content in both catalysts was approximately 30 wt.%. Prior to the electrochemical measurement, the catalyst-covered electrode was soaked in the electrolyte solution (0.5 M H2SO4) for 10 min. The electrochemical activity of the catalyst with respect to methanol oxidation was tested in 1 M CH3OH + 0.5 M H2SO4.
Characterization and measurements
X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Siemens D5000, Siemens, AG, Munich, Germany) with Ni-filtered CuKα radiation (λ = 1.54 Å). Scanning electron microscopy (SEM) (Zeiss ULTRA plus, Carl Zeiss AG, Oberkochen, Germany)) was used to observe the morphology of the samples. Transmission electron microscopy (TEM) was done with the following instruments: routine imaging, Philips CM12 (120 kV) (Philips, Eindhoven, The Netherlands); high-resolution TEM (HRTEM) and EDS microanalyses, JEOL 2200FS (200 kV) (JEOL Pty. Ltd., Frenchs Forest, New South Wales, Australia ) and electron tomography, JEOL 1400 (120kV) (JEOL Pty. Ltd.). Fourier transform infrared (FTIR) attenuated total reflection spectra and UV-visible spectroscopy (UV-Vis) spectra were recorded on a Bruker IFS66V FT-IR (Bruker Optics, Melbourne, Victoria, Australia) and a Cary 5-UV-Vis spectrometer (Agilent Technologies, Santa Clara, CA, USA), respectively. Raman spectroscopy of the samples was recorded with an inVia Renishaw Raman (Renishaw Oceania Pty. Ltd., Melbourne, Victoria, Australia) using a He-Ne laser at 633 nm wavelength. The electrochemical characterizations were performed on a CHI1202A Electrochemical Analyzer (CH Instruments Inc., Austin, TX, USA). All the solutions were prepared in distilled water, and all potentials reported were referenced to a saturated calomel electrode (SCE). A three-electrode electrochemical cell was used for the measurements, where the counter electrode was a Pt foil and the reference electrode was a SCE.
The 2D experimental images for the 3D TEM (electron tomography) were recorded in a bright-field mode on a JEOL 1400 (120 kV) using a high-tilt sample holder. The tilt series were acquired with automatic rectification (corrections of focus and horizontal displacement) by using the SerialEM  softwareon a 1,350 × 1,040 pixel Erlangshen CCD camera (Gatan, Inc., Pleasanton, CA, USA). The tilt range was set from -65° to 65° with a basic increment of 1°, giving a total of 130 images. The tilt series data were treated for image processing and reconstruction using the IMOD software program from Boulder Laboratory for 3D Electron Microscopy of Cells and the University of Colorado .
Results and discussion
The location of the metal catalyst nanoparticles in the catalyst support has a great influence on their catalytic performance. From the SEM images, comparing with the CNF/PANI (Figure 6a), a much greater degree of Pt nanoparticle agglomeration is observed on the CNF/HPANI supports (Figure 6a, b); however, the catalytic performance of this carbonized material is much better than that of the CNF/PANI as shown in Figures 7 and 8. So, it is expected that the location of the Pt nanoparticles in the two supports may be different.
To study the accessibility of the inner tube of the CNF/HPANI-Pt catalyst support, EDS mapping of the distribution of fluorine (F) was done on the catalyst sample deposited on a copper grid in the TEM (JEOL 2200FS). From Figure S4, it can be observed that the inner tube of the catalyst support can be accessed by the Nafion electrolyte since a strong fluorine signal was detected within the inner tube. The Nafion solution was the only source of F amongst the reagents used. As the electrochemical oxidation of methanol always takes place at the interface between the Pt particles and the Nafion ionomer, these particles in the inner tube accessible to the Nafion ionomer electrolyte can still enable the catalytic oxidation of methanol. Moreover, the majority of the HPANI/CNF has an open-ended hollow structure, which means that the internal surface of the hollow tubes is readily accessible to the electrolyte and fuel. Thus, CNF/HPANI-Pt shows higher electrochemical catalytic performance to the oxidation of methanol than the CNF/PANI-Pt catalyst.
A facile process for the synthesis of CNF/PANI core/shell composite was developed. Carbonization of the composite produces core/shell carbon fibers with N-doping at the surface, which has been confirmed by XRD, FTIR, Raman and EDS studies. TEM tomography shows that the Pt nanoparticles were dispersed both on the inner surface and on the outer surface of the hollow N-doped carbon nanofibers. EDS mapping confirmed that the particles on the inner surface of the tube are still accessible by the Nafion ionomer electrolyte. Hence, carbonization provides significant improvements (approximately seven times) in their catalytic performance as catalyst supports for methanol electrochemical oxidation.
saturated calomel electrode
scanning electron microscopy
transmission electron microscopy.
The authors are grateful for the access to the characterization facilities at the Australian Microcopy and Microanalysis Research Facility at the Australian Centre for Microscopy and Microanalysis, University of Sydney. CZ acknowledges the award of an APA scholarship. ZL and XD would like to thank the Australian Research Council (ARC) for their financial supports.
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