Fabrication and electrocatalytic performance of highly stable and active
platinum nanoparticles supported on nitrogen-doped ordered mesoporous
carbons for oxygen reduction reaction†
Shou-Heng Liu,‡
aMin-Tsung Wu,
abYing-Huang Lai,
cChien-Chang Chiang,
abNingya Yux
aand Shang-Bin Liu*
abReceived 8th April 2011, Accepted 1st June 2011 DOI: 10.1039/c1jm11488c
A facile synthesis route for the preparation of N-doped ordered mesoporous carbons (NOMCs) containing well-dispersed, highly stable Pt nanoparticles (NPs) is reported. The synthesis of these mesostructured Pt-NOMC materials invokes pyrolysis of co-fed carbon sources and Pt precursors in 3-[2-(2-aminoethylamino)ethylamino]propyl-functionalized mesoporous SBA-15 silicas, which served simultaneously as N sources and hard templates. It was found that the dispersion of Pt NPs increases with increasing N content in the Pt-NOMC nanocomposites, leading to higher electrocatalytic activity during oxygen reduction reaction (ORR) and methanol-tolerant stability compared to typical commercial electrocatalyst (Pt/XC-72). The superior electrochemical performances observed for the synthesized Pt-NOMCs have been attributed to the dispersion and unique nanostructure of Pt NPs particularly in the presence of pyridinic-N atoms in the mesoporous carbon supports.
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) have been appreciated as one of the next-generation power sources for light-duty vehicles1and stationary or portable applications as an alternative to conventional power sources, such as internal combustion engines and secondary batteries.2–4In the past few decades, considerable efforts have been devoted to R&D in fuel cell-related areas, however, two major challenges remain to be resolved for future practical applications of PEMFCs, namely their costs and durability.2 For the former, the progressive increase in prices associated with the limited amounts in the reserve of noble metals (e.g., Pt),5which have been recognized as the most active catalysts for methanol oxidation (MOR) and oxygen reduction (ORR) reactions, is certainly one of the major obstacles for practical commercialization and utilization of PEMFCs. As such, research aimed at decreasing further the
amount of Pt in the electrocatalysts, while increasing their mass specific activities, are of great importance. Thus, catalyst support materials with desirable textural and electrochemical properties, such as high surface areas and electrical conductivities, should be helpful to the dispersion of Pt as well as to the reaction kinetics occurring at both the anode and cathode.6On the other hand, in terms of the durability of the electrocatalysts,7–11 structural stability and surface properties of the catalyst supports as well as the methodologies invoked in incorporating Pt catalyst onto the support are crucial for the dispersion and stability of the metal nanoparticles (NPs) during DMFC operations in view of toler-ances for CO-poisoning and methanol crossover at anode and cathode, respectively.12–14
Nanostructured carbon materials, which possess high surface areas, tailorable textural structures, and good electronic conductivities, and corrosion resistances, are ideal supports for Pt-based electrocatalysts.2–4 Indeed, aside from the most common commercially available Vulcan XC-72 activated carbons, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and ordered mesoporous carbons (OMCs) have been suggested to be pertinent supports for electrocatalysts in PEMFCs at both anode and cathode. Nevertheless, owing to their chemically inert nature, the majority of nanostructured carbons normally lack desirable functional groups on their surfaces, consequently, making the supported Pt NPs more vulnerable for aggregation and deactivation during electrocatatlytic reactions. To circum-vent these problems, incorporation of heteroatoms (e.g., N, B, and S) on to the carbon supports so as to modify their surface and physicochemical properties have been investigated.15,16
aInstitute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box
23-166, Taipei, 10617, Taiwan. E-mail: [email protected]; Fax: +886-2-23620200; Tel: +886-2-23668230
bDepartment of Chemistry, National Taiwan Normal University, Taipei,
11677, Taiwan
cDepartment of Chemistry, Tunghai University, Taichung, 40704, Taiwan
† Electronic supplementary information (ESI) available: See DOI: 10.1039/c1jm11488c
‡ Present Address: Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan
x Present Address: Department of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 40704, China
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Among them, N-containing carbons have received considerable attentions due to: first, the strong electron donor nature of N which should promote enhancement in p bonding, leading to improved stability, electron transfer rate, and hence durability of the carbon supports during electrocatalytic processes.17,18 Second, the presence of N should enhance affinity between the Pt and the carbons, facilitating nucleation sites to promote a high dispersion of Pt NPs on the surfaces of the supports.19,20 In general, N-doping can be achieved by either in situ doping, i.e., direct synthesis of nanostructured carbons invoking N-contain-ing precursors21–27 or ‘‘post-doping’’, namely post-synthesis treatment of carbons by N-containing chemicals.28,29It should be noted that the latter normally involves severe surface modifica-tion treatments of the carbons in order to attain a desirable amount of incorporated N. As such, post-synthesis N-doping is typically accompanied by degradation of surface properties and/ or collapse of ultra-fine nanostructures of the carbons.18
Previously, we have developed a facile route to synthesize well-dispersed mono-(Pt)30and bi-functional (PtRu)31NPs supported on OMCs based on the pyrolysis of co-fed primary carbon sources (e.g., furfuryl alcohol and trimethylbenzene) and Pt/Ru precursors (e.g., Pt/Ru acetylacetonates, which also serve as the secondary carbon sources) in the presence of a mesoporous silica hard template, such as SBA-15. Such strategy abstains from conventional post-synthesis impregnation of noble metal on carbons, which normally results in uncontrollable growth and undesirable loss of metal particles after reduction treatment at elevated temperatures. More importantly, by pyrolyzing the co-fed carbon sources with metal precursors followed by carbon-ization treatment at elevated temperature (>1073 K), strong interactions between the metal NPs and carbon support may be anticipated, leading to Pt- and Pt/Ru-OMC nanocomposites with highly stable noble metal NPs studded on the interior pore walls of the OMCs. As a result, these nanocomposites were found to possess superior electrocatalytic performance and long-term durabilities during MOR compared to most commercially available anodic catalysts (e.g., Pt or Pt/Ru on Vulcan XC-72). On the basis of earlier studies on the effects of N-doping to the Pt/C electrocatalysts,32–35 the objective of this work aims to develop a facile synthesis route for N-doped Pt-OMC nano-composites (hereafter denoted as Pt-NOMC-x, where x repre-sents the total N content in wt%) using 3-[2-(2-aminoethylamino) ethylamino]propyl-functionalized SBA-15 as the hard template containing the primary N source. The Pt-NOMC-x catalysts so fabricated were characterized by a variety of different spectros-copy and analysis techniques and utilized as electrocatalysts for DMFC at cathode; their catalytic activities and durabilities during ORR were also evaluated.
2.
Experimental
2.1. Catalyst preparationThe siliceous SBA-15 mesoporous template was synthesized according to the procedures reported earlier.36The 3-[2-(2-ami-noethylamino)ethylamino]propyl-functionalized SBA-15 mate-rials were prepared by co-condensating 3-[2-(2-aminoethyl-amino)ethylamino]propyltrimethoxysilane (TA; Acros) with tetraethyl orthosilicate (TEOS; Acros), as described previously.37
Typically, 5.7 g of Pluronic 123 was dissolved in a mixture of 37% HCl solution (24.4 g) and water (169.3 g) at room temperature, followed by adding TEOS. The resultant solution was equili-brated at 313 K for 4 h to pre-hydrolyze TEOS, and then TA was slowly added into the solution. The molar composition of the mixture was (1 z) TEOS: z TA: 6.1 HCl: 0.017 P123: 165 H2O,
where z varied from 0 to 0.4. The resulting mixture was stirred at 313 K for 20 h and then transferred into a polypropylene bottle and heated at 373 K under static condition for 24 h. The solid product was recovered by filtration and dried at room temper-ature overnight. The template of P123 was removed from the as-synthesized material by refluxing in ethanol. The final material was filtered, washed several times with water and ethanol, and dried at 373 K. The mesoporous templates are denoted as SBA-15-Ny, where y denotes the molar percentage of the TA/(TEOS + TA). Subsequent direct replication of SBA-15-Ny material into Pt-NOMC-x was accomplished by adopting our strategy repor-ted earlier.30,31Typically, ca. 0.5 g of SBA-15-Ny was dehydrated at 333 K for 12 h under vacuum.
A certain amount of platinum acetylacetonate (Pt(CH (COCH3)2)2, denoted as Pt(acac)2; 98%, Acros) was dispersed in
furfuryl alcohol (FA; 98%, Acros) and trimethylbenzene (TMB; 98%, Acros) under ultrasonication. Oxalic acid (98%, Acros) was used as the acid catalyst for polymerization of FA solution. The mixture solution was infiltrated in SBA-15 by incipient wetness impregnation at room temperature, followed by polymerization at 333 K then at 353 K each for 12 h in air. The resultant composite was treated at 423 K for 3 h, ramped to 573 K with a heating rate of 1 K min1, then to 1073 K with a heating rate of 5 K min1and maintained at that temperature for 3 h. The above carbonization procedure was performed under vacuum. Finally, the resultant black powders were leached with HF (1 wt%) aqueous solution for at least 24 h to remove the silica template, washed with distilled water and alcohol, and then dried at 373 K to obtain the Pt-NOMC-x samples.
2.2. Characterization method
X-ray diffraction (XRD) patterns of all samples were recorded on a PANalytical (Philips X’Pert PRO) instrument using Cu-Ka radiation (l ¼ 0.1541 nm). The contents of Pt in various Pt-NOMC-x catalysts were determined by a thermal gravity analyzer (TGA, Netzsch TG209) at a rising temperature rate of 10 K min1from 303 to 1123 K under a continuous air flow with a flow rate of 30 mL min1. Elemental analyses (EA) were carried out using a CHN elemental analyzer (Heraeus varioIII). X-ray photoelectron spectra (XPS) of samples were measured at the wide-range beamline of National Synchrotron Radiation Research Center in Taiwan; the incident angle of the photon beam was 45from the surface normal. Emitted photoelectrons were collected with an electron analyzer oriented 10 from the surface normal in an angle-integrated mode. Collected spectra were numerically fitted with Gaussian-broadened Lorentzian functions after Shirley background subtraction with a third-order polynomial to each side of a peak in all fits. Nitrogen adsorption isotherms were measured at 77 K on a volumetric adsorption analyzer (Quantachrome, Autosorb-1). The disper-sion of platinum on various samples were measured by hydrogen chemisorption at 323 K on a chemisorption analyzer
12490 | J. Mater. Chem., 2011, 21, 12489–12496 This journal is ª The Royal Society of Chemistry 2011
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in Figs. S6–S10, ESI† and their corresponding Koutecky–Levich plots at 0.3 V vs. RHE are shown in Fig. 6. Accordingly, the number of electron (n) involved in the Pt-NOMC-x electro-catalysts during ORR were deduced to be 3.4–3.8 (see Table 3), resembling to the theoretical value (4.0) for reduction involving four-electron transfer. Unlike two-electron reduction, the above results therefore exclude the existence of hydrogen peroxide radicals, whose presence prone to attack the carbon support as well as the proton exchange membrane (PEM) electrolyte, leading to an undesirable degradation of the membrane-elec-trolyte-assembly (MEA) of the fuel cells.
Other crucial issues regarding to the stability of a cathodic electrocatalyst are the tolerance for methanol crossover and its durabilities.56–58The electrocatalytic stability of the Pt-NOMC-2.2 catalyst was further evaluated by continuous LSV tests performed both in O2 saturated 0.1 M H2SO4 and in 0.1 M
H2SO4/1.0 M CH3OH mixture solution. As shown in Fig. 7,
regardless of the presence of methanol, only a marginal decrease
in current density was observed after 50 repeated cycles, indi-cating that the Pt-NOMC nanocomposite material fabricated herein indeed is a highly stable catalyst with a superior electro-catalytic activity during ORR even in the presence of the meth-anol in the electrolyte.
4.
Conclusions
In summary, a series of mesostructured Pt-NOMC nano-composites with different N content as well as well-dispersed and highly stable Pt nanoparticles were prepared by using N-con-taining organo-functionalized SBA-15 as the primary N sources and hard templates. The resultant Pt-NOMCs so fabricated were thoroughly characterized by a variety of different spectroscopic and analytical techniques and were tested for applications as electrocatalysts during ORR for DMFC at cathode. It was found that the presence of pyridinic-N species, whose concentration increases with increasing N content in the Pt-NOMC materials, is favorable for the Pt metal dispersion, and hence, for the higher electrocatalytic activity observed for the supported nano-composite catalysts. The stability of the Pt NPs in mesostruc-tured Pt-NOMC electrocatalysts is also found to be closely related to the fact that they are partial embedded in the carbon matrix consisting of a hexagonal array of nanorods. Conse-quently, the Pt-NOMC-2.2 supported catalyst with a Pt and N content of ca. 10 and 2.2 wt%, respectively, was found to exhibit the optimal electrocatalytic activity, durability, and methanol-tolerance stability during oxygen electroreduction surpassing that of a commercial JM-Pt/C catalyst containing 20 wt% Pt. Thus, the Pt-NOMC nanocomposites so fabricated should render future practical and cost-effective applications in hydrogen-energy related areas, for example, as electrocatalysts for PEMFCs and DMFCs.
Acknowledgements
Financial support of this work from the National Science Council, Taiwan (Contract No.: NSC98-2113-M-001-017-MY3) is gratefully acknowledged.
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