J. Phys. Chem. C 2007, 111, 10329-10335
10329
Electrocatalytical Activity on Oxidizing Hydrogen and Methanol of Novel Carbon Nanocages of Different Pore Structures with Various Platinum Loadings Jun Jie Niu,* Jian Nong Wang, Li Zhang, and Yiqing Shi School of Materials Science and Engineering, Shanghai Jiao Tong UniVersity Shanghai, 200030, People’s Republic of China ReceiVed: January 25, 2007; In Final Form: May 9, 2007
Novel carbon nanocages (CNCs) with various pore structures were produced at different temperatures and were subsequently deposited with Pt particles by a simple liquid reduction method with chloroplatinic acid as the Pt source and ethylene glycol as the reducing agent. The chloroplatinic acid was separately dropped and induced an excellent Pt particle distribution with small size. The electrocatalytical activity of the CNCs with Pt loadings was analyzed by cyclic voltammograms (CVs) in H2SO4 and H2SO4/CH3OH solutions. Excellent electrocatalytical activity was presented compared with the commercial Pt/C catalyst. The specific surface area (SSA) and graphitic structure played an important role on the electrochemical activity. The Pt/ CNCs catalyst with CNCs fabricated at 900 °C possesses not only good catalytic characteristics but also high economic yield.
Introduction Recently, various porous carbon nanomaterials have attracted considerable attention due to their extensive applications in the fields of catalysis, energy storage, capacitors, and sorbents.1 The controllable pore size distribution in different ranges is important for various applications. Carbon nanomaterials with micropores are favorable in gas adsorption as an ideal filter and hydrogen carrier.2 In contrast, carbon nanomaterials with plenty of mesoporous structure are extremely important for applications in absorption and separation of many gases, molecules, or ions that are too large to enter the micropores.3 Proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), promising power sources used in electric vehicles and portable devices, are intensively studied due to the high power density, quick start up, rapid response, and low operating temperature.4 It was described that catalyst on carbon porous nanomaterials showed excellent electrochemical activity and were less susceptible to carbon monoxide poisoning than traditional catalyst systems in fuel cells.5,6 Nanosized platinum supported on carbon materials (e.g., Vulcan XC-72) with high specific surface area (SSA) is used extensively as an electrocatalyst to enhance the dispersion of platinum particles and thus to increase the utilization of noble metal catalyst.7 New supports such as porous carbon,8,9 carbon nanotubes,10 C60,11 and carbon spherules12 are widely used in electrocatalysts and show satisfactory electrochemical activity. However, the natural flaw of poor crystallinity for carbon blank and entangled morphology for carbon nanotubes prevents further improvement in electrocatalytical activity. Thus, there is a need for better carbon nanomaterials as catalyst support. Carbon nanocages (CNCs), as a mesoporous material with large specific surface area (SSA) and pore volume, are more suitable for catalyst support in fuel cells.13,14 Novel carbon nanocages as catalyst supports present a promising application in fuel cells. Up to date, a number of syntheses of porous carbon nanomaterials including carbonization activation, zeolite or * Corresponding author: tel +86-21-62932050; e-mail
[email protected].
nanostructured silica as a template, arc discharge, laser evaporation, thermal chemical vapor deposition, and metal catalytic reaction, have been widely published.15-19 However, deep discussion of the catalytic effect with surface area and crystal structure is seldom reported. Several Pt-loading methods have been reported including physical evaporation,20 electrochemical deposition,21 impregnation,22 intermittent microwave irradiation (IMI),23 surface ionexchange technique,24 etc. Obviously, size distribution and crystalline degree significantly affect the electrochemical activity of the catalyst. The aforementioned methods described Pt loadings on carbon-based materials with varying size distributions. However, the synthesis of highly dispersed platinum particles with adequate size control to increase the catalyst utilization still remains a challenge. Especially, the usually adopted electrodepostion of platinum particles is likely to cause cocurrent reduction of H+ during the process. The loading mass of Pt catalyst is not easy to estimate according to the deposition charge.25 Consequently, continuing efforts are underway to develop simple synthesis methods to generate nanosized Pt particles with greater uniformity. In a previous paper, we have successfully fabricated porous CNCs by a simple thermal pyrolysis of iron carbonyl and ethanol method.26 In this paper, novel carbon nanocages (CNCs) with varying pore structures were used as supports for Pt loadings and displayed good electrochemical activity through the analysis of cyclic voltammograms (CVs) in H2SO4 and H2SO4/CH3OH solutions. The graphitic structure and pore structures play an important role on Pt loadings and electrochemical activity. The SSA and pore volume of as-formed CNCs are continually decreased while the crystalline structure is enhanced with increasing growth temperature. The Pt loadings were processed by a simple and controllable separate reduction of chloroplatinic acid in ethylene glycol solution. The sample with high SSA will make a better Pt dispersion with small size, while the crystalline degree is reversely decreased. The Pt loadings on CNCs grown at 900 °C with an ideal graphite structure and SSA of ∼166 m2/g exhibited the highest eletrochemical activity.
10.1021/jp0706432 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
10330 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Niu et al.
Figure 1. TEM images of the raw CNCs grown at temperatures of (a) 600, (b) 650, (c) 700, and (d) 900 °C.
Experimental Section CNC samples under different temperatures were synthesized by simple thermal pyrolysis of iron carbonyl [Fe(CO)5] and ethanol in a vertical electrical furnace, which can be found in our previous paper.26 Pt loadings on CNCs were processed as follows. A suspension of CNCs with various SSAs in ethylene glycol solution was heated to 140 °C in a reflux device. Subsequently, chloroplatinic acid solution with an accurate content was dropped under vigorous stirring in several steps. The time interval was about 15 min. The concentration of chloroplatinic acid in ethylene glycol was desired to be 2 mmol/ L. After 5 h of reaction, the as-received sample was washed thoroughly with deionized water for removing the residuals, filtered, and dried at 80 °C in a low-vacuum system. In addition, Pt/CNCs with the intended loadings of 45 wt % have been prepared by this procedure. The microstructures were characterized by means of transmission electron microscopy (TEM; JEM100 and JEM2010 instruments). The textural analysis was performed on a Belsorp measuring instrument (Belsorp-mini II, Japan, Inc.) with N2 physisorption at 77 K, up to a pressure of 1 bar. Prior to the experiment, the sample was degassed at 250 °C under nitrogen flow for at least 4 h. SSA and pore structure were derived by the Brunauer-Emmett-Teller (BET) equation. The crystalline structure was analyzed by X-ray diffraction (XRD, D8Advance, Bruker) with Cu KR radiation. Pt particle size was estimated by using the well-known Scherrer formula. The eletrocatalytical properties were evaluated in a conventional three-electrode cell by cyclic voltammograms (CVs) with the electrochemical interface system (SI1287, Solartron) at room temperature. The electrodes were prepared by mixing the Pt/CNCs with 5% Nafion 2-propanol solution, and the Pt loadings were controlled
to 0.4 mg/cm2. The working electrode was a circle glassy carbon (GC) material with a diameter of 3 mm. A platinum foil was used as the counterelectrode, and a saturated calomel electrode (SCE) was used as reference electrode. The Pt/CNCs were measured in 0.5 M H2SO4 aqueous solution and 0.5 M H2SO4/ 1.0 M CH3OH aqueous solutions between -0.24 and 1.0 V, respectively. The scanning rate was 50 mV/s in all the measurements. Results and Discussion Pore Structures of the Carbon Nanocages. Shown in Figure 1 are the TEM morphology images of the CNCs at different growth temperatures. As can be seen from the figure, plenty of hollow carbon nanocages with a relatively regular circle shape are uniformly distributed. The size of as-formed CNCs is clearly increased when the growth temperature is continually improved. The diameter of CNCs at low temperatures ranges within several nanometers, while the value is enhanced tens of nanometers at higher temperatures. Furthermore, the thickness of the CNC wall is simultaneously enlarged when the temperature is increased. Crystal variation of the graphitic peak with C (002) can be observed from the XRD data showed in Figure 5. As can be observed from Figure 5a, the intensity of the peak is gradually improved when the temperature is enhanced, indicating an increasing crystalline structure. As a result, the CNCs grown at 900 °C possess the best crystal degree compared to those at lower temperature. It is believed that the graphitic structure is continually increased when the growth temperature is improved. Pore structures of the samples with different growth temperatures were analyzed by N2 adsorption/desorption isotherms displayed in Figure 2a. As can be seen from the figure, all of the CNCs exhibited typical type IV isotherms with H1 hyster-
Electrocatalytic Activity of Carbon Nanocages
Figure 2. (a) N2 adsorption/desorption isotherms of the CNCs at various temperatures. Also shown are enlarged N2 adsorption/desorption isotherms at (b) low partial pressure (P/P0 < 0.3) and (c) high partial pressure (0.45 < P/P0 < 1.0). Solid symbols represent adsorption isotherms, and open symbols represent desorption isotherms.
esis.27,28 Large scales of mesopores (diameter 2-100 nm) and quantity of nanopores (diameter
