Platinum Nanoparticles Modified with Alkylamine Derivatives as an

Feb 14, 2014 - The potential cycle test demonstrates that the PA modification of Pt NPs .... Recent advancements in Pt and Pt-free catalysts for oxyge...
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Platinum Nanoparticles Modified with Alkylamine Derivatives as an Active and Stable Catalyst for Oxygen Reduction Reaction Keiko Miyabayashi,*,†,‡ Hiroki Nishihara,† and Mikio Miyake*,† †

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Graduate School of Engineering, Shizuoka University, 3-5-1 Johoku, Naka, Hamamatsu, Shizuoka 432-8561, Japan

‡

S Supporting Information *

ABSTRACT: Platinum nanoparticles (NPs) protected with octylamine (OA) as well as partially replaced by controlled ratios of alkylamine with pyrene group (PA) were successfully synthesized by a two-phase liquid reduction method of Pt(IV). The NPs without any pretreatment to remove the introduced organic-protected agents have been well-characterized and applied as supported catalysts on carbon black for oxygen reduction reaction (ORR). The modification of the Pt NP surface with OA and PA significantly improved the electrocatalytic activity such as area specific and mass specific activities, whose values increased by an increase in PA ratios. The potential cycle test demonstrates that the PA modification of Pt NPs enhances the stability of the catalyst and sustains high area and mass specific activities of ORR.

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INTRODUCTION Electrocatalysts for fuel cells, such as the polymer electrolyte fuel cell (PEFC), essentially consist of Pt nanoparticles (NPs) on carbon supports,1,2 whose high cost still hinders prevalence, irrespective of the tremendous efforts to advance highperformance catalysts. Recent distinguished development in nanotechnology has led to feasible synthesis of strictly controlled Pt NPs, such as size-controlled NPs,3 shapecontrolled NPs,4−7 composition-controlled alloy NPs,7−12 and core−shell NPs.13,14 These sophisticated NPs are often synthesized by Pt(II) or Pt(IV) reduction in the presence of organic compounds, followed by deposition of the prepared NPs on a support. When the surfaces of NPs are covered by organic compounds, they are usually removed by acid and/or heat treatment to obtain a clean surface for use as catalysts.7,11,12,14 Several groups have applied structurecontrolled NPs prepared by advanced nanotechnology as model catalysts for oxygen reduction reaction (ORR) to gain further understanding of high performance catalysts.3,4,7−14 Some of these catalysts surpass ORR electrocatalytic activity of the 2017 DOE target; however, their stability is below target. Various catalysts have been synthesized to enhance stability, such as Pt NPs with narrow size distribution to suppress the growth of NPs by Ostwald ripening,15 Pt NPs coated with carbon or silica to prevent the aggregation of NPs,16,17 and Pt NPs decorated by Au cluster catalyst to control the dissolution of Pt.18 Although the stability of these catalysts has been enhanced, their activity was still below target. A catalyst with both high activity and high stability is desirable for practical fuel cell application. We have successfully applied Pt NPs covered with organic modification agents as model catalysts for hydrogenation of olefins to evaluate the activities of different facets.19 Recently, a significant improvement of ORR by the © 2014 American Chemical Society

modification of Pt NP surfaces with molecules or ions has been reported.20−23 However, few reports have addressed improvement in electrocatalytic performance, including stability, of Pt NP catalysts modified with organic compounds. Here we report highly active and stable Pt NP catalysts with surface organic modification agents of octylamine (OA) and alkylamine with pyrene group (PA) for ORR. The ratio of introduced OA/PA on Pt NP surfaces can be changed by tuning the charged ratio of OA/PA during synthesis by a twophase liquid reduction method of Pt(IV). The NPs were used as ORR catalysts without any pretreatment to remove the introduced organic-protective agent. Such a strategy to modify the surface of NP catalysts by different organic agents with controlled ratios will be widely applied in designing highly active and stable catalysts, particularly moderate cost electrocatalysts for PEFC.

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EXPERIMENTAL SECTION

Chemicals. Hydrogen hexachloroplatinate hexahydrate (≥98.5%, H2PtCl6·6H2O), tetraoctylammonium bromide (≥97.0%, TOAB), acetone (≥99.5%), dichloromethane (≥99.5%), toluene (≥99.5%), methanol (≥99.8%), and 2-propanol (≥99.9%) were obtained from Wako Pure Chemicals Co., Ltd. Octylamine (≥98%, OA) was purchased from Tokyo Chemical Industry Co., Ltd. Hydrogen perchlorate (62.2%, Ultrapure) was obtained from Kanto Chemical Co., Inc. Nafion (5 wt %) was purchased from Aldrich. All chemicals were used as purchased without further purification. Milli-Q pure water (18.2 MΩ cm) was used for preparation of Pt and NaBH4 aqueous solutions and electrochemical measurements. Alkylamine with pyrene group (8-(pyrene-1-ylmetoxy)octane-1-amine, PA) was Received: November 7, 2012 Revised: February 12, 2014 Published: February 14, 2014 2936

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(ALS, 720C) at room temperature in 0.1 M HClO4 solution. The Pt catalysts cast on the GC, Pt wire, and reversible hydrogen electrode (RHE) were used as working, counter, and reference electrodes, respectively. The working electrode was pretreated by applying cycled potential between 0.05 and 1.2 V with a scan rate of 200 mV s−1 under Ar (>99.9999%) atmosphere until reproducible CVs were obtained (ca. 30 cycles). The electrochemical active surface area (ECSA) was determined by the peak area corresponding to hydrogen adsorption observed on the CV with a scan rate of 20 mV s−1. The ORR was evaluated after purging with oxygen (>99.99995%) for 30 min; a gentle oxygen flow was maintained over the solution during the measurements. The ORR measurement was conducted by rotating the electrode at five different speeds (400, 900, 1600, 2500, and 3600 rpm) and by applying a potential of 0.2−1.2 V versus RHE in the positive direction at a rate of 10 mV s−1. The data were presented after iR-drop correction. For comparison, 30% Pt/CB (Tanaka Kikinzoku Kogyo K. K., TEC10 V30E) was used as a reference catalyst, and the electrochemical measurements were conducted following the procedures described above. The ECSA values for OA-Pt/CB and OA/PA-Pt/CBs were estimated from the adsorbed hydrogen peak area between 0.06 and 0.45 V versus RHE after correction of the electrical double layer and by adopting a value of 210 μC cm−2 for the adsorption of hydrogen as a monolayer. The stability of the catalyst was estimated from ECSA change during continuous square wave potential cycling between 0.6 (3 s hold) and 1.0 V (3 s hold) in an Arsaturated solution of 0.1 M HClO4. After a 10 000 potential cycle test, ORR measurement was conducted to estimate the catalytic activity.

synthesized by Gabriel reaction of 1-((8-bromooctyloxy)methyl) pyrene (see the Supporting Information). Synthesis of Pt Nanoparticles Modified with Octylamine (OA-Pt NPs). Platinum NPs were prepared via a modified Brust method.24 A mixture of aqueous solution containing H2PtCl6·6H2O (10 mM, 30 mL) and toluene (70 mL) was stirred for 30 min at room temperature, and TOAB was added as a phase transfer reagent in toluene (60 mM, 10 mL). A toluene solution of OA (150 mM, 10 mL) and an aqueous solution of NaBH4 (200 mM, 30 mL) were added to the solution. The reaction mixture was vigorously stirred for 12 h. The organic layer was separated and then concentrated to ca. 5 mL by a vacuum evaporator. The crude NP solution was added dropwise into ethanol (30 mL) with stirring for 10 min. After standing for 2 h, the solution with a dark brown precipitate was decanted and filtrated to remove the excess OA. The filtered NPs were further purified by redispersion in dichloromethane (3 mL) and then precipitated by adding 30 mL of ethanol and methanol. Synthesis of Pt Nanoparticles Modified with Octylamine (OA) and Alkylamine with Pyrene Group (PA) with Different OA/PA Ratios (OA/PA-Pt NPs). The OA/PA-Pt NPs were synthesized in a similar manner to that of OA-Pt NPs, where different molar ratios of OA and PA (9/1, 8/2, and 7/3) were prepared by tuning the added OA and PA. An aqueous H2PtCl6·6H2O solution (10 mM, 30 mL) and toluene (80 mL) containing TOAB (60 mM, 10 mL toluene) were mixed and stirred for 30 min at room temperature. A toluene solution of OA (107, 108, and 135 mM, 10 mL) and an aqueous solution of NaBH4 (200 mM, 30 mL) were added to the solution. After vigorous stirring for 30 min, a toluene solution of PA (45.9, 27.0, and 15.0 mM, 10 mL) was then added to the solution to prepare Pt NPs with OA/PA at molar ratios of 7/3, 8/2, and 9/1, respectively. The reaction mixture was stirred overnight. The organic layer was separated and then concentrated to ca. 5 mL by a vacuum evaporator. The crude NPs were recovered and purified in a manner similar to that described of OA-Pt. Preparation of Carbon-Supported Catalysts (OA-Pt/CB, OA/ PA-Pt/CBs). Carbon black (Vulcan XC72) (CB) was stirred in a 6 M HCl solution. After decantation of the HCl solution, the precipitate was thoroughly washed with Milli-Q water. The recovered CB was filtered and dried overnight at room temperature. The treated CB (5.0 mg) was dispersed in acetone (10 mL) under ultrasonic irradiation. The dichloromethane solution of OA-Pt or OA/PA-Pt NPs (Pt concentration was adjusted to 10 mM) was added to the CB solution and stirred overnight. The OA-Pt NPs and OA/PA-Pt NPs supported on CB were filtered and washed with chloroform and then dried overnight. OA-Pt/CB and OA/PA-Pt/CBs were obtained. Preparation of Electrode. A glassy carbon (GC) (diameter, 5 mm; area, 0.196 cm2) was polished with 0.05 μm alumina and washed with Milli-Q water twice under sonication. A GC electrode with a Pt catalyst was prepared as follows: OA-Pt/CB or OA/PA-Pt/CBs (1 mg) was dispersed in 2-propanol (1 mL) by ultrasonic treatment at 0 °C. The aliquot of the catalyst solution was cast on a GC electrode and settled in saturated 2-propanol atmosphere overnight for the electrode to dry. The Pt content of the catalyst solutions was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) before casting on the GC. Each electrode was prepared to contain 14 μg cm−2 Pt. Nafion solution (10 μL, 5 wt %) was cast (4.5 μL) on the catalyst layer of GC electrode after dilution with ethanol (990 μL). Characterization. The size and size distributions of OA-Pt NP and OA/PA-Pt NPs were characterized by transmission electron microscopy (TEM) operated at 100 kV (Hitachi, H7100). Adsorption spectra of OA/PA-Pt NPs were measured in a dichloromethane solution using a UV−vis spectrometer (Shimadzu Co., MultiSpec1500). The Pt content of OA-Pt NP, OA/PA-Pt NPs, OA-Pt/CB, and OA/PA-Pt/CB was determined by ICP-AES (Shimadzu Co., ICPS7100). The OA-Pt NP, OA/PA-Pt NPs, OA-Pt/CB, and OA/PA-Pt/ CBs were dissolved in hot aqua regia (1 mL), which was followed by dilution to optimal concentration for ICP-AES analysis. The electrochemical measurements followed procedures reported in the literature.25 Cyclic voltammograms (CVs) of OA-Pt/CB and OA/PAPt/CBs cast on GC were obtained using an electrochemical analyzer

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RESULTS AND DISCUSSION OA and PA were adopted as modification agents in the present study because alkylamine is a typical protective agent in the preparation of small and monodispersed Pt NPs by a liquid reduction method. The structural formula of PA is shown in the inset of Figure 1. To evaluate the amount of introduced PA on

Figure 1. UV−vis spectra of PA, OA/PA(9/1)-Pt NP, OA/PA(8/2)Pt NP, and OA/PA(7/3)-Pt NP. The lower right inset shows an enlarged view of the absorption band attributed to pyrene group, and the upper right inset shows structural formula of PA.

Pt NPs, UV−vis spectra of OA/PA-Pt NPs were measured (Figure 1). The absorption bands attributed to pyrene group, which appear at 234−343 nm, indicate successful modification of the Pt NP surface with PA. The amounts of PA and OA introduced on the Pt NPs were determined by ICP-AES (to estimate wt % of Pt) and UV−vis spectra (to estimate PA amount). The wt % of Pt, OA, and PA is summarized in Table S1 (see the Supporting Information), indicating successful tuning of the introduced OA/PA ratio by the charged ratio. Figure 2 shows a schematic model of an OA/PA(7/3)-Pt NP to illustrate the introduced OA/PA on the Pt NP surface, which 2937

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Figure 2. Schematic image of OA/PA(7/3)-Pt NP. One organic protective molecule occupies 1.32 surface Pt atoms.

Figure 3. TEM images and size distributions of Pt NPs of (a) OA-Pt/CB, (b) OA/PA(9/1)-Pt/CB, (c) OA/PA(8/2)-Pt/CB, (d) OA/PA(7/3)-Pt/ CB, and (e) 30% Pt/CB to show that the Pt NPs of the prepared catalysts were homogeneously dispersed on CB and demonstrated narrow size distributions.

CB without aggregation, which is better than the commercial Pt/CB catalyst (30% Pt/CB; TEC10 V30E; average diameter, 2.6 nm; Figure 3e). The CVs of OA-Pt/CB and OA/PA-Pt/CBs on the GC electrodes were measured in 0.1 M HClO4, as shown in Figure 4a. The CV of commercial 30% Pt/CB on a GC electrode was measured for comparison. The OA-Pt/CB and OA/PA-Pt/CBs showed higher ECSA values (76.7−77.8 m2 gPt−1) compared with that of 30% Pt/CB (63.7 m2 gPt−1) (Table 1). This result infers that the OA and PA may weakly adsorb on Pt and may not interfere with H+ adsorption at electrode surface.26 Such

was estimated on the basis of the data shown in Table S1 (see the Supporting Information). The TEM images of Pt NPs supported on CB and the estimated size distributions of Pt NPs are shown in Figure 3. The average diameters of Pt NPs, which were estimated by measuring over 200 Pt NPs from different areas of the TEM grid, were 2.8, 2.4, 2.5, and 2.5 nm for OA-Pt/CB, OA/PA(9/ 1)-Pt/CB, OA/PA(8/2)-Pt/CB, and OA/PA (7/3)-Pt/CB, respectively (Table 1). No change in Pt NP size was observed before and after support on CB. The TEM images show that Pt NPs modified with OA and PA homogeneously dispersed on 2938

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for ORR. The Tafel plot obtained from the kinetic currents jk after mass transfer correction is shown in the inset in Figure 4b. The change in slopes at approximately 0.87 V may be due to the transition between Langmuir adsorption and Temkin adsorption of the reaction intermediate, or may be due to a change in the surface coverage of the oxygen species.28 OA-Pt/ CB and OA/PA-Pt/CBs have higher current densities compared with that of 30% Pt/CB. To evaluate the kinetically limited current (jk) depending on the ratios of OA/PA quantitatively, the ORR current values observed at different rotating speeds were analyzed by using a Koutecky−Levich plot (Supporting Information Figures S1 and S2).29 From the slope of the Koutecky−Levich plot, the number of electrons involving the ORR of OA-Pt/CB and OA/ PA-Pt/CBs was estimated to be four (Supporting Information Figure S2), which coincides with the theoretical value of ORR. The mass and area specific activities (denoted as jkm and jksp, respectively) were obtained by normalizing the kinetic current values to the Pt weight and ECSA, respectively (Figure 4c). The jkm value (A g−1) decreased in the order OA/PA(7/3)-Pt/ CB (518) > OA/PA(8/2)-Pt/CB (483) > OA/PA(9/1)-Pt/CB (352) > OA-Pt/CB (326) > 30% Pt/CB (208). The corresponding jksp value (μA cm−2) decreased in the same order OA/PA(7/3)-Pt/CB (673) > OA/PA(8/2)-Pt/CB (634) > OA/PA(9/1)-Pt/CB (464) > OA-Pt/CB (426)> 30% Pt/CB (332). The presented jksp and jkm values for OA-Pt/CB, OA/ PA-Pt/CBs, and 30% Pt/CB are a measured mean value for three electrodes. Evidently, both the specific activities increased with an increase in the ratio of PA. Thus, OA/PA(7/3)-Pt/CB

Table 1. Properties of OA-Pt/CB, OA/PA-Pt/CBs, and 30% Pt/CB Catalysts sample

av diameter of Pt NP/ nm

Pt contenta/ wt %

ECSA/m2 gpt−1

2.8 2.4

28.6 32.4

77.5 76.7

2.5

31.5

77.0

2.5

30.8

77.8

2.6

30.0

63.7

OA-Pt/CB OA/PA(9/l)-Pt/ CB OA/PA(8/2)-Pt/ CB OA/PA(7/3)-Pt/ CB 30% Pt/CB a

Determined by ICP.

high ECSA values for OA-Pt/CB and OA/PA-Pt/CBs may originate from the highly dispersed states of Pt NPs on CB, as can be seen in the TEM images in Figure 3.27 OA-Pt/CB and OA/PA-Pt/CB consist of monodispersed Pt particles, while the size distribution of Pt particles for the commercial catalyst was wide and showed tailing toward larger particle sizes. Presence of the large particles decreases the surface area of catalyst per unit mass, and therefore, we observed small ECSA for commercial Pt catalyst compared with our Pt one with OA and PA. The ORR polarization curves for OA-Pt/CB, OA/PA-Pt/ CBs, and 30% Pt/CB shown in Figure 4b were measured in O2saturated 0.1 M HClO4 solutions using rotating disk electrodes at 1600 rpm. OA-Pt/CB and OA/PA-Pt/CBs showed more positive onset potential than 30% Pt/CB. This result indicates high electrocatalytic activity of OA-Pt/CB and OA/PA-Pt/CBs

Figure 4. (a) Cyclic voltammograms for OA-Pt/CB, OA/PA(9/1)-Pt/CB, OA/PA(8/2)-Pt/CB, OA/PA(7/3)-Pt/CB, and 30% Pt/CB catalysts at 20 mV s−1. (b) ORR polarization curves for OA-Pt/CB, OA/PA(9/1)-Pt/CB, OA/PA(8/2)-Pt/CB, OA/PA(7/3)-Pt/CB, and 30% Pt/CB catalysts recorded at room temperature in an O2-saturated 0.1 M HClO4 aqueous solution with a sweep rate of 10 mV s−1 and a rotation rate of 1600 rpm. Inset shows Tafel plots for OA-Pt/CB, OA/PA-Pt/CBs, and 30% Pt/CB obtained from the polarization curves in the positive-going scans at 1600 rpm (b), which were normalized to the Pt electrochemical surface area. (c) Area (jksp) and mass (jkm) specific activity for these five catalysts at 0.9 V. 2939

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PA(9/1)-Pt/CB (289), OA/PA(8/2)-Pt/CB (419), and OA/ PA(7/3)-Pt/CB (506) decreased by 18%, 13%, and 2% of their initial values, respectively. On the other hand, OA-Pt/CB (211) and commercial 30% Pt/CB (114) lost 35% and 46%, respectively, of their initial mass activities after the potential cycle test. Evidently, the decrease of mass specific activity was suppressed with an increase in the PA ratio. OA/PA(7/3)-Pt/ CB sustained high ORR activity even after the potential cycle test; i.e., the jkm and jksp values for OA/PA(7/3)-Pt/CB were 4.5 and 2.0 times to those of the commercial 30% Pt/CB, respectively. The improvement of the area specific activities of PA-modified catalysts compensates the decrease of mass specific activity by the rearrangement of the Pt NP surface. These findings prove that the modification of Pt NPs by PA significantly improved the durability of the catalysts. The stability of the catalysts was also evaluated on the basis of ECSA values during the accelerated potential cycle tests (Figure 6). The decrease of ECSA before and after the potential

exhibited the highest ORR activity among the OA- and OA/ PA-modified catalysts, i.e., the jkm and jksp values for OA/PA(7/ 3)-Pt/CB were 2.5 and 2.0 times those of the commercial 30% Pt/CB catalyst, respectively. The large surface area of Pt(110), whose catalytic activity for ORR in HClO4 is the highest among three low index surfaces, may be one of the reasons to realize improved ORR activity of the prepared catalysts, as suggested by the large hydrogen desorption peak area of Pt(110) (0.11 V) compared with that of Pt(100) (0.20 V) (see CV in Figure 4a). A strong interaction between Pt NPs and CB surface due to the presence of OA or OA/PA may be another reason for the improved catalytic activity of ORR because the electrical double layer between 0.45 and 0.55 V of the CV for the synthesized catalysts is narrow compared with that of 30% Pt/CB. The enhanced dispersion of Pt NPs protected with organic compounds on CB (see TEM images in Figure 3) may be another reason for the improved mass specific activity of ORR,27 as suggested by the high ECSA values for OA-Pt/CB and OA/PA-Pt/CBs (see Table 1). A change of adsorption kinetics of the reaction intermediates or polar components on the Pt surface by PA is another possible reason for the improved ORR activity of OA/PA-Pt/CBs because the Tafel slope values decreased as the PA ratio increased (Figure 4b, inset).28 Yeager et al. reported that organic compounds render the interfacial region less polar, resulting in preferable O2 adsorption on the Pt compared with other polar components of electrolyte.26,30 Currently, in situ surface-enhanced infrared spectroscopy measurements of OA-Pt/CB and OA/PA-Pt/CB under ORR conditions are in progress to reveal the role of organic compounds in catalytic activity. In addition, improved catalytic activity by modifying Pt NPs with an organic compound has been reported recently.22,23,31 A plausible role of organic modification agents is to prevent oxidation of the Pt surface, although the detailed mechanism remains unclear.22,23 The catalytic activities of the present OA-Pt/CB and OA/PAPt/CBs are high compared with those of para-substituted phenyl compounds-stabilized Pt NP catalysts reported by Zhou et al. In addition, the OA/PA-Pt/CBs have the advantage of high stability, as is discussed below. The changes in mass and area specific activity at 0.9 V for all investigated catalysts before and after an accelerated potential cycle test (10 000 square wave cycles between 0.6 and 1.0 V in 0.1 M HClO4) are summarized in Figure 5. After the potential cycles, the ORR mass specific activities (jkm (A gPt−1)) of OA/

Figure 6. Changes of ECSA values with cycle number for OA-Pt/CB, OA/PA(9/1)-Pt/CB, OA/PA(8/2)-Pt/CB, OA/PA(7/3)-Pt/CB, and 30% Pt/CB during potential cycle tests.

cycle test of OA-Pt/CB and OA/PA-Pt/CBs (16−20%) was small compared with that of the commercial 30% Pt/CB (27%). To estimate the degree of degradation by aggregation of Pt NPs, the TEM images of catalysts and the estimated size distributions of Pt NPs after the potential cycle tests are shown in Figure 7. The average diameters were 4.1, 4.0, 3.7, and 3.9 nm for OA-Pt/CB, OA/PA(9/1)-Pt/CB, OA/PA(8/2)-Pt/CB, and OA/PA (7/3)-Pt/CB, respectively, which are small compared with that of the commercial 30% Pt/CB (4.6 nm). Various mechanisms such as agglomeration, Ostwald ripening, and Pt dissolution have been discussed about fuel cell catalyst degradation related to a decrease of ECSA.32 Suppression of the agglomeration of Pt NPs may be the most plausible explanation for the reduced particle growth of OA/PA-Pt/CBs because a limited number of irregularly shaped (e.g., L and T shaped) NPs, which should be formed by agglomeration, were noticeable in the TEM images of OA/PA-Pt/CBs after a 1000 cycle test (Figure S1 in the Supporting Information). These irregularly shaped NPs were observed in the TEM images of the commercial 30% Pt/CB and OA-Pt/CB in the early periods of the cycle tests. These results indicate that the pyrene group anchored Pt NPs on the carbon support.

Figure 5. Area (jksp) and mass (jkm) specific activities after accelerated potential cycle test for OA-Pt/CB, OA/PA(9/1)-Pt/CB, OA/PA(8/ 2)-Pt/CB, OA/PA(7/3)-Pt/CB, and 30% Pt/CB catalysts at 0.9 V. Initial activities (dotted lines) of these five catalysts are shown for comparison. 2940

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Figure 7. TEM images and size distributions of Pt NPs for (a) OA-Pt/CB, (b) OA/PA(9/1)-Pt/CB, (c) OA/PA(8/2)-Pt/CB, (d) OA/PA(7/3)Pt/CB, and (e) 30% Pt/CB after 10 000 potential cycle test.

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CONCLUSIONS We have successfully modified a Pt NP surface with a controlled ratio of OA and PA by convenient two-phase liquid reduction procedures. The modified Pt NP catalysts supported on CB indicate that the electrocatalytic activity toward ORR was significantly improved, and the activity values increased by an increase of PA ratio. The large Pt(110) surface area of OAand OA/PA-modified Pt NP catalysts may realize improved ORR activity, as suggested by the large hydrogen desorption peak area of Pt(110) (at 0.11 V) compared with that of a commercial catalyst. High ORR electrocatalytic activity by OA/ PA-Pt/CBs seems to originate from a change of the adsorption kinetics of reaction intermediates and/or concentration of oxygen adjacent to the Pt NP surface. The results of accelerated potential cycling tests suggest that the activity of PA-modified Pt NP catalysts sustains high specific value, and a decrease in ECSA values for OA/PA(7/3)-Pt/CB was significantly suppressed compared with that of the commercial 30% Pt/CB. The narrow size distribution of Pt NPs suppressed NP growth derived from Ostwald ripening, and the modification of Pt NPs by PA prevented migration of NPs on CB. The approach to modify the Pt NP surface with different organic compounds with controlled ratios reported here may have an economic advantage in which it may provide a means to use inexpensive organic reagents and facilitate preparation under mild conditions. A demonstrated strategy will open a new approach for further evolution of Pt-based catalysts for PEFC.

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1000 potential cycle test. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +81-761-51-1540. Fax: +81-761-51-1149. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the program for Development of PEFC Technologies Aiming for Practical Application/Base Technology/Analysis of Morphology, Electrochemical Reaction and Mass Transfer for MEA Materials (No. 10000806-0) from NEDO, Japan.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of PA; estimated wt % composition and numbers at Pt NP surface of Pt, OA, and PA for OA/PA-Pts; ORR polarization curves of OA/PA(7/3)-Pt/CB at different rotating speed; Koutecky−Levich plot of OA/PA-Pt/CB, OA/PA-Pt/ CBs, and 30% Pt/CB; and TEM images of the catalysts after a 2941

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dx.doi.org/10.1021/la402412k | Langmuir 2014, 30, 2936−2942