© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 6, FEBRUARY 15, 2001
LETTERS Graphite Nanofibers as an Electrode for Fuel Cell Applications Carol A. Bessel,*,† Kate Laubernds,‡ Nelly M. Rodriguez,*,‡ and R. Terry K. Baker*,‡ Department of Chemistry, Northeastern UniVersity, Boston, Massachusetts 02115, and Department of Chemistry, VillanoVa UniVersity, VillanoVa, PennsylVania 19085 ReceiVed: September 14, 2000; In Final Form: December 8, 2000
The potential of graphite nanofiber supported platinum catalysts as an electrode for fuel cell applications was investigated using the electrochemical oxidation of methanol at 40 °C as a probe reaction. Various types of graphite nanofibers were used and the behavior of supported platinum particles on these materials compared to that when the metal was dispersed on Vulcan carbon (XC-72). Catalysts consisting of 5 wt % platinum supported on “platelet” and “ribbon” type graphite nanofibers, which expose mainly edge sites to the reactants, were found to exhibit activities comparable to that displayed by about 25 wt % platinum on Vulcan carbon. Furthermore, the graphite nanofiber supported metal particles were observed to be significantly less susceptible to CO poisoning than the traditional catalyst systems. This improvement in performance is believed to be linked to the fact that the metal particles adopt specific crystallographic orientations when dispersed on the highly tailored graphite nanofiber structures.
Introduction The unique properties of graphite nanofibers (GNFs)1,2 have generated intense interest in the application of these new carbon materials toward a number of applications including selective absorption,3 energy storage,4-6 polymer reinforcement,7 and catalyst supports.8-12 Recent studies performed in this laboratory on graphite nanofiber supported metal particles have raised the possibility that the conductive substrate material may be exerting electronic perturbations as well as geometric constraints on the dispersed crystallites.13 The current investigation was undertaken to explore the physiochemical effects of GNF-supported platinum particles on the electrocatalytic oxidation of methanol when compared with a traditional support medium, Vulcan carbon.14-16 * To whom correspondence should be addressed:
[email protected] (CAB),
[email protected] (NMR),
[email protected] (RTKB). † Villanova University. ‡ Northeastern University.
The oxidation of methanol in acidic solution, typically catalyzed by carbon-supported platinum particles, produces 6 equiv of electrons:
CH3OH + H2O f CO2 + 6H+ + 6e-
(1)
This electrocatalytic oxidation has been examined extensively in fuel cell applications (direct methanol fuel cells, DMFC17-21) especially in regard to power sources for motor vehicle transportation.22 This reaction represents an excellent starting point for electrochemical studies using GNF-supported platinum catalysts as the commercial practicality of the DMFC supported platinum anode still suffers from a high percent weight loading of the expensive metal catalyst as well as self-poisoning by reaction byproducts. Three types of GNF materials were examined as platinum support media during electrocatalyzed methanol oxidation studies: possessing “platelet”, “ribbon”, and “herring-bone” structures, respectively, Figure 1. These nanofibers were grown
10.1021/jp003280d CCC: $20.00 © 2001 American Chemical Society Published on Web 01/19/2001
1116 J. Phys. Chem. B, Vol. 105, No. 6, 2001
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Figure 2. HRTEM micrograph showing the appearance of 5 wt % Pt on “platelet” graphite nanofibers.
Figure 1. Schematic representations of the “platelet”, “ribbon”, and “herring-bone” structured GNF as initially synthesized from metal catalyst particles.
via the catalytic decomposition of hydrocarbons or carbon monoxide over metal catalysts composed of copper, iron, nickel, or their bimetallic compounds.23,24 The materials consist of graphite sheets aligned in definite directions that are dictated by the catalytic entity selected for the growth process. Unlike conventional graphite materials and nanotubes where the basal plane is exposed, the structure of graphite nanofibers is one where only edge regions are revealed. “Platelet” graphite nanofibers possess a structure in which the graphite sheets are oriented in a direction perpendicular to the growth axis and since the layers maintain a minimum interlayer spacing of 0.355 nm, this well-ordered arrangement can act as a template for subsequent deposition of metals. The width of the nanofibers is controlled by the size of the catalyst particle responsible for their growth and can vary from between 2 and 100 nm in width, with lengths ranging from 5 to 100 µm.2 The BET (N2) surface areas of graphite nanofibers range from 80 to 200 m2/g, depending upon the particular structural conformation. Experimental Section GNF-supported platinum materials were demineralized by magnetically stirring a ca. 2 g sample of the “as-synthesized” graphite nanofibers23,24 in 200-400 mL of 1 M HCl for 2 days. This suspension was allowed to settle, the acid was decanted, and fresh acid solution was added. This procedure was repeated a minimum of 10 times for each fiber sample. After the final acid treatment, the fibers were vacuum filtered and washed with distilled water until the filtrate was neutral. The fibers were airdried. This demineralization procedure removes the metallic (or bimetallic) catalyst that was used to prepare the graphite nanofibers. In this way, only the subsequently added platinum metal can catalyze the methanol oxidation reaction. Using the incipient wetness method (in 95% ethanol) an appropriate amount of the platinum precursor salt (NH3)2Pt(NO2)2 was dissolved in ethanol and slowly added to the carbon sample. The supported catalyst was dried overnight at 110 °C in air, followed by calcination at 250 °C for 4 h, and finally, reduced at 300 °C for 20 h. The catalyst was cooled to room temperature in helium and prior to removal from the reactor was passivated in 2% air/helium for 1.0 h. This latter step
Figure 3. HRTEM micrograph showing the appearance of 5 wt % Pt on Vulcan carbon.
provided a protective oxide layer over the metal particle surfaces and was essential in order to prevent bulk oxidation of platinum. The impregnation procedure generated platinum particles throughout the carbon nanofibers and Vulcan carbon. The platinum catalysts were examined by high-resolution transmission electron microscopy with a JEOL 2000 EXII instrument (lattice resolution 0.18 nm). Suitable transmission specimens were prepared by ultrasonic dispersion of the respective samples in isobutanol following by application of a drop of the suspension to a holey carbon support grid. The electrocatalytic activity of the various carbon-supported platinum samples was studied using cyclic voltammetry at modified glassy carbon working electrodes (GCEs). This method of producing modified-GCE has been successfully used to electrochemically characterize a variety of solid-state catalysts25 and battery materials.26 Results and Discussion Inspection of the electron micrograph, Figure 2, reveals that the platinum crystallites dispersed on the GNF were relatively thin, highly crystalline faceted structures, which are characteristics that are generally associated with the establishment of a strong metal-support interaction.27 In contrast, metal particles supported on Vulcan carbon, Figure 3, were found to adopt a more dense globular morphology, suggesting that in this case there was a relatively weak interaction with the metal and support. A comparison of methanol oxidation currents (reaction yields) between the typical industrial standard, platinum supported on
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Figure 4. Comparison of methanol oxidation currents for platinum particles supported on various types of carbon supports.Conditions: 0.5 M methanol in 0.5 M H2SO4, 40 °C.
Vulcan carbon, and platinum supported on GNFs under zeroorder conditions is presented in Figure 4. The data demonstrate the importance of the nature of the support on the activity of the metal catalyst. Using carbon-modified working electrodes, a calibration curve was developed by measuring the anodic currents arising from methanol oxidation as a function of various Pt loadings on Vulcan carbon. Notably, while 5 wt % Pt on “herring-bone” GNF exhibited very poor electrocatalytic activity, extrapolation from the calibration curve indicated that a 5 wt % Pt loading on “platelet” GNF corresponds to approximately 24 wt % loading of Pt on Vulcan carbon. Samples containing 5 wt % Pt on “ribbon” GNF corresponded to similar improvements; extrapolations showed that these materials gave activities that were equivalent to about 25 wt % Pt loading on Vulcan carbon. While we have not yet determined the cause of the 400% improvement in the current yield observed for “platelet” and “ribbon” GNF-supports (when compared to the Vulcan carbon support), the fact that “herring-bone”-supported platinum exhibited very little electrocatalytic activity toward methanol oxidation indicates that the hydrophilic nature of the latter material3 may interfere with the performance of the metal catalyst in this process. In addition, direct methanol oxidation reactions are often plagued by the self-poisoning of the platinum catalyst by oxidation byproducts or residues.22 Although several poisons have been identified from mechanistic studies (CO, CO2, -COH, -CHO, C2O3H, organosulfur compounds, etc.),28-32 carbon monoxide is often cited as a leading cause of reduced catalyst cycling ability as it binds irreversibly to the platinum catalyst, reducing the number of sites available for methanol oxidation. As a measure of the effect of the carbon support on catalyst poisoning, the ratios of the peak current attributed to the oxidation of carbon monoxide formed during self-poisoning divided by the anodic peak current attributed to the oxidation of methanol have been measured for the various carbonsupported platinum catalysts. For the current industrial standard, 30 wt % Pt on Vulcan carbon, this poisoning ratio was 0.73 at 40 °C, while, under the same conditions, the 5 wt % Pt on platelets gave a ratio of 0.38 and the 5 wt % Pt on ribbons gave a ratio of 0.04.33 The conditions for poisoning studies were as follows: 1.0 M methanol in 0.5 M H2SO4, 40 °C, 100 mV/ s, cycle #2 was measured; Pt wire auxiliary electrode, saturated calomel reference electrode (SCE), and modified-glassy carbon working electrode (GCE). Modified GCEs were prepared according to a variation of the procedure described in ref 25:
J. Phys. Chem. B, Vol. 105, No. 6, 2001 1117 approximately 250 µg of carbon-supported platinum was transferred to a polished glassy carbon electrode then overcoated with poly(acrylic acid) (PAA, MWav ) 450 000). Cyclic voltammetry was performed under zero-order conditions such that
