Synthesis and Characterization of Platinum Supported on Surface-Modified Ordered Mesoporous Carbons by Self-assembly and Their Electrocatalytic Performance towards Oxygen Reduction Reaction

Synthesis and characterization of platinum supported

on surface-modified ordered mesoporous carbons

by self-assembly and their electrocatalytic performance

towards oxygen reduction reaction

Shou-Heng Liu

*

, Jyun-Ren Wu

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan

a r t i c l e i n f o

Received 1 May 2012 Received in revised form 8 August 2012

Accepted 23 August 2012

Available online 20 September 2012

Keywords: Platinum

Ordered mesoporous carbon Self-assembly

Surface modification Oxygen reduction reaction

a b s t r a c t

Ordered mesoporous carbons (OMCs) were fabricated by an organiceorganic self-assembly process. Surface-modified OMCs were also prepared via the conventional acid-oxidation, H2O2 oxidation and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) grafted routes. Pt nanoparticles (NPs) supported on OMC (Pt/OMC) and modified OMC (Pt/OMC-H2SO4, Pt/OMC-H2O2and Pt/OMC-AEPTMS) were synthesized and charac-terized by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), transmission electron microscopy (TEM) analysis. It was found that acid-oxidation (H2SO4/ HNO3) method led to formation of a much wider Pt distribution with mean particle size of 6.8 nm. Unlike Pt/OMC-H2SO4samples, Pt NPs (ca. 2.0 nm) were supported uniformly on AEPTMS-modified OMC with low electrical conductivity. Among three surface-modified methods, the H2O2treatment method was an easily controllable way for surface modifi-cation of OMC which possesses desirable electrical conductivity, well-dispersed and nanosized Pt (ca. 3 nm). Accordingly, the Pt/OMC-H2O2samples were observed to have superior electrocatalytic activity for oxygen reduction reaction as compared to synthesized Pt/OMC, Pt/OMC-H2SO4, Pt/OMC-AEPTMS and the commercial electrocatalysts (Pt sup-ported on XC-72).

Copyrightª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Conductive nanocarbon materials with high surface areas, such as carbon blacks[1e5], carbon nanotubes[6e8], and ordered mesoporous carbons (OMCs) [9e13], were investi-gated for the use as anodic/cathodic supports in direct methanol fuel cells (DMFCs) and proton-exchange membrane fuel cells (PEMFCs). To reduce the cost of DMFCs/PEMFCs, the total amount of noble metal (Pt) loaded

on carbon supports needs to be decreased. To achieve this goal, the average Pt nanoparticles (NPs) size has to be reduced and aggregation of Pt NPs should be avoided. Thus, catalyst support materials with desirable textural and elec-trochemical properties, such as high surface areas, surface binding sites and electrical conductivities, should be favor-able for the dispersion of Pt as well as to the reaction kinetics occurring at both the anode and cathode. Generally, surface functionalization of carbon supports may increase the

* Corresponding author. Tel.:þ886 7 381 4526x5152; fax: þ886 7 3830674. E-mail address:[email protected](S.-H. Liu).

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surface binding sites, avoid metal NPs aggregation, and improve the dispersion of metal NPs. Nonetheless, it is also definitely accompanied with some problems, such as uneven distribution of the surface functional groups, struc-tural damage, and partial loss in electrical conductivity of the carbon supports.

In our earlier studies, a novel route to fabricate well-dispersed and highly stable mono (Pt) and bi-functional (PtCo) NPs (ca. 2e3 nm) supported on OMCs and N-doped OMCs has been developed based on the nano-casting method

[14e17]. The synthesized Pt and PtCo/OMC catalysts were found to possess superior electrocatalytic performances compared with a commonly commercial cathodic electro-catalyst (JohnsoneMatthey; 20 wt% Pt on Vulcan XC-72 acti-vated carbon; denoted as JM-Pt/C hereafter). However, the synthesis routes involved in above-mentioned electro-catalysts were still hindered by the difficulties concerning preparation time and cost which further limit their practical applications. During the past few years, synthesis of OMCs by using organic resins in the presence of various self-assembled surfactant templates, followed by pyrolysis at suitable temperature, has been reported[18e20]. The resulting OMCs were observed to possess ordered mesopores with high surface areas and unique surface properties abundant with hydroxyl groups which are favorable for surface modification/ functionalization and dispersion/loading of metal NPs in a controlled way.

In this study, a facile method was developed to fabricate OMC by one-step self-assembly approach, in turn, the synthesized OMC materials were treated with various chem-ical modifications as shown inFig. 1, including H2SO4/HNO3, H2O2 oxidation and 3-[2-(2-aminoethylamino)ethylamino] propyltrimethoxysilane (AEPTMS) functionalization. After depositing Pt NPs, the resultant Pt/OMC and Pt/modified OMC samples were analyzed by various spectroscopic techniques and also examined as electrocatalysts for oxygen reduction reaction (ORR) at cathode by electrochemical measurements, intending to improve the electrocatalytic ORR activity.

2.

Experimental method

The OMC samples were synthesized similar to the procedure reported in the earlier study[20]. Typically, ca. 3.2 g of phlor-oglucinol (98%, Acros) and 5.0 g of F127 tri-block copolymer (Sigma) were dissolved in 36.0 g of ethanol and water mixture (1:1 vol%). Upon complete dissolution of the solid compo-nents, 0.4 g of HCl (37 wt%) was introduced into the solution, followed by stirring for 2 h. Then, 5.0 g of formaldehyde (37 wt %) was added dropwisely into the above-mentioned solution. The resultant solution was maintained still for at least 24 h until two separate layers were clearly observed. Then, the lower polymer-rich layer was cured at 373 K for 24 h after disposing of the upper solution layer. Finally, a carbonization treatment (1 K/min) was performed under vacuum from room temperature to 1123 K and kept at the same temperature for extra 3 h. Three methods for surface modifications of OMC were carried out including: (1) Acid oxidation of OMC was performed by refluxing OMC samples in a mixed acid solution (H2SO4: HNO3in volume ratio of 1) at room temperature for 4 h; (2) OMC samples were also refluxed with H2O2at 353 K for 4 h. Then, both of treated OMC samples were washed with deionized water, filtered and dried at room temperature; (3) The OMC samples were functionalized with AEPTMS by a post-grafting method[21]. In general, OMC samples were refluxed in toluene solution containing AEPTMS at 383 K for 24 h under an N2 flow. The products were washed with toluene and dried at 333 K overnight. Pt NPs were deposited on the surface-modified OMC by using wet chemical reduction method [22]. 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 drop by drop.

Fig. 1e Scheme of preparation of Pt on modified OMC.

After stirring for 1 h, the solid was recovered by centrifuga-tion, extensively washed with H2O and dried in air at 333 K for 24 h. The obtained samples were denoted as Pt/OMC-H2SO4, Pt/OMC-H2O2and Pt/OMC-AEPTMS, respectively.

2.2. Characterization methods

The weight percentage of platinum in all samples was deter-mined by energy dispersive X-ray analysis (EDX, JEOL JEM-2100F). The X-ray diffraction (XRD) analysis was recorded on a PANalytical (X’Pert PRO) diffractometer using CuKa radiation (l ¼ 0.1541 nm). Nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. The BrunauereEmmetteTeller (BET) method was used to determine the specific surface area of the samples in the rela-tive pressure (P/P0) range from 0.05 to 0.2. The total pore volume was calculated from the volume of N2adsorbed at the P/P0of 0.99. Pore size distribution curves were obtained by the Bar-retteJoynereHalenda (BJH) method from the adsorption branch. Fourier transform infrared (FTIR) spectra were collected on a Bio-rad 165 spectrometer with 4 cm1resolution using KBr pellets at room temperature. Elemental analyses (EA) were conducted using a CHN elemental analyzer (Heraeus varioIII). The transmission electron microscopy (TEM) images of samples were obtained at room temperature using an elec-tron microscope (JEOL JEM-2100F) operating at a 200 kV accel-eration voltage. The electrical conductivity (s) of a sample was measured by the previously reported method[17,23].

2.3. Electrocatalytic performance

The electrocatalytic activities of various samples were measured at room temperature in a conventional three-electrode glass cell. A glassy carbon three-electrode with geomet-rical area of 0.196 cm2_{was used as a working electrode. A} saturated Ag/AgCl electrode and a Pt wire were used as reference and counter electrodes, respectively, in the three electrode configuration. All potentials in this work are refer-enced to the reversible hydrogen electrode (RHE). The catalyst ink was prepared by ultrasonically mixing ca. 5 mg catalyst with 2.5 mL deionized water for 0.5 h. To prepare the working electrode, ca. 20mL of the ink was deposited onto the glassy carbon electrode, and then dried in air at 333 K for 1 h. About 20mL of 5% Nafion_{(DuPont) solution was applied onto the} catalyst layer to make sure good adhesion of the catalyst onto the glassy carbon electrode.

Electrochemical performance of Pt/OMC, various modified Pt/OMC and commercial JM-Pt/C samples were carried out on a potentiostat/galvanostat (CHI Instruments, 727D) equipped with an RDE system (Pine Instrument, AFMSRCE). The stable cyclic voltammetry (CV) curves were recorded after scanning for ten cycles in the potential region from 0 to 1.2 V in 0.1 M H2SO4solution under purging N2condition in order to obtain the background curve, clean and activate the working elec-trode surface. ORR activities of all catalysts were measured by a linear sweep voltammetry (LSV) technique. The polarization curves were evaluated by using an RDE system (rotating speed of 1600 rpm) in the potential range from 0.1 to 1.0 V vs. RHE (the scan rate of 5 mV s1) under oxygen saturated 0.1 M H2SO4 electrolyte.

3.

Results and discussion

Various spectroscopic and analytical methods were used to investigate the physicochemical properties of the pristine OMC, Pt/OMC, and Pt/modified OMC samples. As can be seen inFig. 2a, the small-angle XRD pattern of OMC samples show a main (100) diffraction peak at 2q ¼ w0.8_{, indicating the}

presence of an ordered mesoporous structure with a two-dimensional (2-D) hexagonal symmetry analogous to that of CMK-3[24]. However, upon loading of Pt onto the pristine OMC and modified OMC samples, less resolved (100) peak was observed, suggesting that the presence of Pt metals may lead to the distortion of a long-range structural ordering of meso-porosity. In particular, it is noted that surface modifications of OMC by H2SO4/HNO3 and AEPTMS may lead to remarkable influence on the overall structure and the physical properties of the catalyst since the feature peaks (100) was almost not observed. As shown inFig. 2b, the large-angle XRD pattern of OMC samples indicating a broad diffraction peak at ca. 24.6is attributed to C (002). Upon incorporating Pt NPs onto OMC and modified OMC samples, the large-angle XRD patterns show individual (111), (200), (220), and (311) diffraction peaks at 2q ¼ 39.8_{, 46.2}_{, 67.8}_{, and 81.3}_{, respectively, in line with}

those of Pt metal (Pt(0)) NPs having a face-centered cubic (fcc) structure. Consequently, the Pt average sizes of Pt/OMC, Pt/ OMC-H2SO4, Pt/OMC-H2O2and Pt/OMC-AEPTMS samples can

Pt/OMC-AEPTMS 1 2 3 4 5 6 7 8 Pt/OMC-H2O2 Pt/OMC-H2SO4 Pt/OMC OMC (degree)

a

b

Fig. 2e (a) Small- and (b) large-angle powdered XRD patterns of various samples.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 9 9 4e1 7 0 0 1

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