Int. J. Electrochem. Sci., 7 (2012) 8326 - 8336
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org
Functionalized Mesoporous Carbons as Platinum
Electrocatalyst Supports for Applications in Fuel Cells
Shou-Heng Liu*, Jyun-Ren Wu
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan
*
E-mail: [email protected]
Received: 21 June 2012 / Accepted: 27 July 2012 / Published: 1 September 2012
Mesoporous carbons (MCs) were synthesized by an organic-organic self-assembly process and surface-modified by the conventional acid-oxidation, H2O2 oxidation and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) grafted methods. Fabrication of Pt nanoparticles (NPs) supported on MC (Pt/MC) and modified MC (Pt/MC-HNO3, Pt/MC-H2O2 and Pt/MC-AEPTMS) were further performed. These resultant catalysts were characterized by a variety of different spectroscopic and analytical techniques such as Fourier transformation infrared spectroscopy (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analysis. Pt NPs were found to be aggregated on the Pt/MC-HNO3 catalysts. For Pt/MC-AEPTMS catalysts, Pt NPs (ca. 2 nm) supported uniformly on surface of modified MC which however has a low electrical conductivity. Among three surface-modified methods, the H2O2 treatment method was a simply controllable way for surface modification of MC which possesses desirable electrical conductivity, well-dispersed and nanosized Pt (ca. 3 nm). The Pt/MC-H2O2 samples were found to have superior electrocatalytic activity for oxygen reduction reaction in comparison with synthesized Pt/MC, HNO3, Pt/MC-AEPTMS and the typical commercial electrocatalyst (Pt/XC-72).
Keywords: Platinum, Mesoporous carbon, Surface modification, Oxygen reduction reaction.
1. INTRODUCTION
Highly dispersed noble metal (Pt) nanoparticles (NPs) supported on conductive materials with high surface areas, such as carbon blacks [1-5], carbon nanotubes [6-9], graphene [10,11] and mesoporous carbons (MCs) [12-16] are desirable for anodic/cathodic electrocatalysts in direct methanol fuel cells (DMFCs) and proton-exchange membrane fuel cells (PEMFCs). Generally, surface functionalization of carbon supports may increase the surface binding sites, avoid metal NPs aggregation, and improve the dispersion of metal NPs. Nevertheless, it is also certainly accompanied
with some problems, such as irregular distribution of the surface functional groups, structural damage, and partial loss in electrical conductivity of the carbon supports.
In the previous studies, we have developed a novel route to fabricate well-dispersed and highly stable mono- (Pt) and bi-functional (PtCo)NPs (ca. 2-3 nm) supported on MCs based on the nano-casting method [17-22]. The synthesized Pt and PtCo/MC catalysts were found to possess superior electrocatalytic performances compared to most commercially available cathodic electrocatalysts (Pt on XC-72 activated carbons). However, the synthesis routes invoked in above-mentioned electrocatalysts were still limited by the ineffectiveness in material cost and preparation time, which further hinder their practical industrial applications. In recent years, fabrication of MCs by cross-linking phenolic resins in the presence of various self-assembled block-copolymer templates, followed by pyrolysis at moderate temperature, have been extensively studied [23-25]. The resultant MCs were found to possess high surface areas with ordered mesopores and structure matrices abundant with hydroxyl groups that facilitate further surface functionalization by dispersion/loading of catalysts in a controllable fashion. In the present study, a simple procedure was developed to synthesize MC by one-step self-assembly approach. Moreover, the synthesized MC materials were treated with various chemical modifications, including H2SO4/HNO3, H2O2 oxidation and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) functionalization. After incorporating Pt NPs, the resultant Pt/MC and Pt/modified MC samples were examined as electrocatalysts for oxygen reduction reaction (ORR) at cathode by electrochemical techniques, aiming to improve the electrocatalytic activity.
2.EXPERIMENTAL 2.1. Catalyst Preparation
The MC samples were synthesized by dissolving 3.2 g of phloroglucinol (98%, Acros) and 5.0 g of F127 tri-block copolymer (Sigma) in 36.0 g of ethanol and water mixture (1:1 vol%). After the complete dissolution of the solid ingredients under stirring at room temperature (298 K), 0.4 g of HCl (37 wt%) was added into the solution as a catalyst, then, the mixture solution was further stirred for 2 h. Subsequently, 5.0 g of formaldehyde (37 wt%) was slowly introduced dropwisely into the above solution. The resultant solution was kept for 24 hr after which two separate layers were readily observed. After discarding the upper solution layer, the lower polymer-rich layer was cured at 373 K for 24 hr, followed by a gradual carbonization treatment (1 K/min) under vacuum to 1123 K and maintained at the same temperature for additional 3 h. Surface modifications of MC were performed in three ways. (1) Acid oxidation of MC was carried out by refluxing MC samples in a mixed acid solution (H2SO4 : HNO3 in volume ratio of 1) at room temperature for 4 hr. (2) MC samples were also refluxed with H2O2 at 353 K for 4 hr. Then, both of treated MC samples were washed with deionized water, filtered and dried at room temperature. (3) The MC samples were functionalized with AEPTMS by a postgrafting method. Typically, MC samples were refluxed in toluene solution containing AEPTMS at 383 K for 24 hr under an N2 flow. The products were washed with toluene and dried at
333 K overnight. Pt NPs were deposited on the surface-modified MC by using wet chemical reduction method [26]. In brief, ca. 0.1 g of H2SO4/HNO3, H2O2 and AEPTMS-treated samples were added in a solution containing a desired amount of 0.01 M hydrogen hexachloroplatinate hexahydrate (H2PtCl6． 6H2O, Acros). The mixture was stirred for 30 min and an excessive 0.1 M sodium boronitride (NaBH4, Aldrich) solution was added into the mixture dropwisely. After stirring for 1 hr, the solid was recovered by centrifugation, extensively washed with H2O and dried in air at 333 K for 24 hr. The obtained samples were denoted as Pt/MC-HNO3, Pt/MC-H2O2 and Pt/MC-AEPTMS, respectively.
2.2. Characterization methods
The amounts of platinum in various samples were analyzed by energy dispersive X-ray analysis (EDX, JEOL JEM-2100F). All powdered X-ray diffraction (PXRD) patterns were recorded on a PANalytical (X’Pert PRO) diffractometer using CuK radiation ( = 0.1541 nm). Nitrogen adsorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. Fourier transform infrared (FTIR) spectra were collected on a Bio-rad 165 spectrometer with 4 cm-1 resolution using KBr pellets at room temperature. For the transmission electron microscopy (TEM), samples were first suspended in acetone (99.9 vol%) by ultrasonication, followed by deposition of the suspension on a lacey carbon grid, then the TEM images were obtained at room temperature using an electron microscope (JEOL JEM-2100F) operating at an electron acceleration voltage of 200 kV. The electrical conductivity (σ) of samples was conducted according to the method reported in the literature [27].
2.3. Electrocatalytic performance
The electrocatalytic measurements were performed in a single compartment glass cell with a standard three-electrode configuration. A glassy carbon electrode (diameter ca. 5 mm) was used as a working electrode and a saturated Ag/AgCl electrode and a platinum wire were used as reference and counter electrodes, respectively. In this study, all potentials are referred to the reversible hydrogen electrode (RHE). The glossy carbon thin-film electrode was prepared by the following steps: first, ca. 5 mg of Pt-loaded carbon sample was added into 2.5 mL deionized water, followed by ultrasonic treatment for 0.5 h. Then, ca. 20 μL of the resultant suspension mixture was withdrawn and injected onto the glassy carbon electrode, followed by drying in air at 333 K for 1 h. Finally, 20 μL of 1% Nafion (DuPont) solution was added as a binder under N2 environment. Electrocatalytic activity measurements of various Pt/MC and modified Pt/MC samples were performed on a potentiostat/galvanostat (CHI Instruments, 727D). Cyclic voltammetry (CV) experiments were performed to clean and activate the electrode surface. Prior to each CV measurement, the electrolytic solution was purged with high-purity N2 (99.9%) for at least 0.5 h to remove the dissolved oxygen, subsequently the experiment was conducted between 0 and 1.2 V under purging N2 condition. Oxygen reduction reaction was evaluated by a linear sweep voltammetry (LSV) technique. The 0.1 M H2SO4 electrolyte was saturated with ultrahigh purity oxygen for at least 0.5 h. T The polarization curves were
Various RDE voltammograms of various Pt/MC and Pt/modified-MC electrocatalysts in O2 saturated 0.1 M H2SO4 solution at room temperature under different rotating rates are illustrated in Fig. 6, and their corresponding Koutecky-Levich plots at 0.3 V vs. RHE are shown in Fig. 7. Accordingly, the number of electron (n) involved during ORR in the Pt/MC, Pt/MC-HNO3 and Pt/MC-H2O2 electrocatalysts were deduced to be 4.1, 3.9 and 4.2, respectively, close to the theoretical value (4.0) for reduction involving four-electron transfer. However, Pt/MC-AEPTMS had the n value of 2.5, which was also responsible for the worst ORR activity among all electrocatalysts. It is known that two-electron reductions during ORR lead to production of hydrogen peroxide radicals which attack the carbon support as well as the proton exchange membrane (PEM) electrolyte, resulting in an undesirable degradation of the membrane-electrolyte-assembly (MEA) of the fuel cells.
4. CONCLUSIONS
In summary, three surface-modified routes including H2SO4/HNO3, H2O2 oxidation and AEPTMS functionalization were studied by using self-assembly MC as carbon supports. Pt deposition on H2O2-treated MC surface using H2PtCl6 precursors by wet chemical reduction process was a controllable and simple way to optimize a cathodic catalyst during ORR. Thus, Pt/MC-H2O2 which possessed moderate electrical conductivity, well-dispersed and nanosized Pt (ca. 3 nm) was found to have surpassing ORR electrocatalytic activity as compared to Pt/MC, Pt/MC-HNO3, Pt/MC-AEPTMS and a commercial JM-Pt/C catalyst. These Pt/MC-H2O2 should render practical cost-down effective commercial applications in hydrogen energy related areas, for examples, as supported electrocatalysts for PEMFCs and DMFCs.
ACKNOWLEDGEMENTS
The financial support of the Taiwan National Science Council is gratefully acknowledged.
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