Research Article pubs.acs.org/acscatalysis
Hierarchical Nanostructured Pt8Ti-TiO2/C as an Efficient and Durable Anode Catalyst for Direct Methanol Fuel Cells Jakkid Sanetuntikul, Kriangsak Ketpang, and Sangaraju Shanmugam* Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1 Sang-Ri, Hyeongpung-Myeon, Dalseong-gun, Daegu 711-873, Republic of Korea S Supporting Information *
ABSTRACT: A catalyst for the electrochemical oxidation of methanol in direct methanol fuel cells (DMFCs) comprising Pt8Ti intermetallic nanoparticles dispersed in carbon nanorods (Pt8Ti-TiO2/ C) is presented. The catalyst consists of Pt8Ti and rutile TiO2 nanoparticles dispersed in nitrogendoped carbon hierarchical nanostructures. The Pt8Ti-TiO2/C catalyst showed a 50 mV positive onset potential and 10 times higher specific activity than a commercial Pt/C catalyst. Using a halfcell experiment, we show that Pt8Ti intermetallic nanoparticles greatly enhance the methanol oxidation activity and durability in comparison to a Pt/C commercial catalyst. More importantly, a DMFC anode constructed with Pt8Ti-TiO2/C catalyst showed 4.6 times higher power density than a commercial Pt/C catalyst at 0.35 V and 333 K. Additionally, the Pt8Ti-TiO2/C catalyst displayed superior durability in comparison to the Pt/C catalyst. Pt8Ti-TiO2/C showed an electrochemical surface area decay of 23% at the end of 3000 CV cycles, whereas the Pt/C catalyst showed a more rapid decay of 90% at the end of 3000 CV cycles. The excellent stability of the Pt8Ti-TiO2/C catalyst during the accelerated durability stability test (AST) can be attributed to the stability of the rutile TiO 2 support, which is chemically resistant in the acidic electrolyte medium. The chronoamperometry and AST durability results confirmed that the Pt8Ti-TiO2/C hierarchical catalyst exhibited better stability than the pure Pt/C catalyst, suggesting that Pt8Ti-TiO2/C could be a promising anode catalyst in DMFCs. KEYWORDS: methanol oxidation, Pt8Ti-TiO2, durability, direct methanol fuel cells, anode catalyst
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INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) using H2 gas as fuel have been extensively studied over the last two decades and have received enormous attention as a promising power source system for portable applications due to their high energy efficiency and low air pollution.1,2 Direct methanol fuel cells (DMFCs) are one type of PEMFC that can use a small organic molecule liquid as a fuel instead of H2 gas.3 DMFCs have been shown to be promising for use as power sources because of their many advantages, such as the use of liquid fuel, quick refueling, low cost of methanol, and ease of design for various applications, especially for portable applications.4 Electrocatalysts play an important role in driving fuel cell reactions, with platinum (Pt) electrodes shown to be the most active electrodes among the pure metal catalysts used for electrochemical reactions. Pt nanoparticles dispersed on a carbon support (Pt/C) are usually used as the major catalyst in fuel cells and in many other electrochemical applications. The high cost and low CO-poisoning tolerance are the most important technical limitations of the Pt/C catalyst. Thus, the design of highly active electrode materials that can effectively reduce the methanol oxidation reaction (MOR) overpotential is highly desirable.3 Among the available strategies for improvement of MOR activity and reduction of Pt content in the catalysts, the preparation of alloys of Pt with base metals (PtM) appears to be a promising approach.4 Currently, transitionmetal−Pt alloy catalysts have been shown to be the best choice © XXXX American Chemical Society
for achieving reduced cost, improved catalytic activity, and enhanced durability. For example, a high-performance electrocatalyst with improved MOR activity was developed on the basis of PtFe nanoparticles supported on ordered mesoporous carbon (OMC) prepared via a modified polyol method.5 The PtFe/OMC catalyst exhibits higher specific activity due to the highly homogeneous dispersion of PtFe on mesoporous OMC and provides lower onset potential in MOR than the Pt/C catalyst. In another example, a Pt-Ti intermetallic catalyst was prepared by using sodium naphthalenide as a reducing agent in tetrahydrofuran to form Pt3Ti nanoparticles with ordered Pt3Ti, showing superior oxidation current densities for MOR.6 The current state-of-the-art electrocatalyst for DMFCs is Pt−Ru/C because of the reduced catalyst cost, improved performance, and good CO tolerance in comparison with pure Pt/C. However, dissolution of Ru is still a significant issue for DMFCs.7,8 Recently, Pt-intermetallic compounds with metals such as Au,9 Zn,10 Bi,11 Ti,12 and Ni13 have received great attention because of their improved activity and good corrosion resistance. The enhanced electrochemical formic acid oxidation current was observed for a Bi-modified Pt electrode by Kang et al.11 Intermetallic PtPb nanoparticles exhibit 10 times higher Received: July 4, 2015 Revised: November 3, 2015
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Figure 1. (a) X-ray Rietveld refinement profiles for Pt8Ti including the TiO2 phase, recorded at room temperature. Observed data points are shown as black dots, and the solid red line is the calculated profile. The bottom trace shows the difference (blue curve), and the tick marks denote the expected peak positions for Pt8Ti (top, pink) and TiO2 (bottom, light blue). (b) SEM image, (c) TEM image, and (d) HR-TEM image showing lattice fringes of Pt8Ti and TiO2 particles.
major Pt8Ti phase (77.3 vol %) as well as a minor TiO2 phase (22.7 vol %) were identified. A scanning electron microscopy (SEM) image of the composite nonwoven mat showed perfect fibers without any beads (Figure S2 in the Supporting Information). The morphology of the pyrolyzed sample shows the presence of nanorod structures with an average diameter of 180 nm. Furthermore, transmission electron microscopy (TEM) images reveal that the nanorod surfaces show good nanoparticle dispersion (Figure 1c). The nanoparticles are crystalline with an average particle size of approximately 14 nm. The size distribution of Pt8Ti nanoparticles was measured by counting 200 particles in the TEM image and fitting the obtained size data using the Lorentzian peak function, as shown in Figure S3 in the Supporting Information. According to SEM and TEM morphology analyses, Pt8Ti nanoparticles are well-dispersed on the carbon support, with dark spots indicating the presence of Pt atoms attributed from Pt8Ti nanoparticles and TiO2 nanoparticles appearing as the brighter contrast spots due to the lower atomic number of Ti atoms in the TEM image (Figure 1d). The elemental mapping analysis of Pt8Ti-TiO2/C shows the uniform distribution of carbon, platinum, titanium, oxygen, and nitrogen, ensuring that the Pt8Ti and TiO2 nanoparticles are uniformly distributed in the carbon without much aggregation, as depicted in Figure S4 in the Supporting Information. The well-resolved lattice fringes with a d-spacing value of 0.224 nm correspond to the (031) Pt8Ti plane (JCPDS 031-0936). The image also shows another particle with a lattice distance of 0.324 nm, corresponding to the (002) plane of orthorhombic TiO2 (JCPDS 039-0809). X-ray photoelectron spectroscopy (XPS) measurements were used to evaluate the composition of Pt8Ti-TiO2/C nanorods. The XPS data were fitted using a Gaussian−Lorentzian function. The peak at 284.4 eV is attributed to C 1s of the
specific mass activity than a PtRu/C catalyst for formic acid oxidation.14 It has been theoretically predicted that intermetallic Pt8Ti could be a potential electrode material,15 but as of now, to the best of our knowledge, the use of Pt8Ti nanoparticles as an electrocatalyst has not been explored. Here, for the first time, we describe a novel method for the fabrication of Pt8Ti intermetallic nanoparticles dispersed in nitrogen-doped carbon, as well as their use as an electrocatalyst for the methanol oxidation reaction and in DMFCs as an anode catalyst. The novelty of the current approach relies on the synthesis of Pt8Ti and TiO2 nanoparticles in a simple approach that embeds them in a porous nitrogen-doped carbon nanorod. The presence of TiO2 particles imparts electrochemical stability to the catalyst. We show that Pt8Ti intermetallic nanoparticles greatly enhance the MOR activity and durability in comparison to the commercial Pt/C catalyst. A single-cell DMFC using a Pt8TiTiO2/C anode catalyst displayed higher or comparable maximum power density relative to a state-of-the-art Pt alloy catalyst at 298 and 333 K. To the best of our knowledge, the direct synthesis of Pt8Ti intermetallic nanoparticles has not been reported to date.
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RESULTS AND DISCUSSION
The crystalline structure of catalyst samples was examined by X-ray diffraction (XRD). The crystal structure of the catalyst was confirmed by using the general structure analysis system (GSAS) powder profile refinement program.16 Figure 1a shows the XRD pattern, which can be readily indexed to the Pt8Ti tetragonal crystal system with the I4/mmm space group (space group No. 139, JCPDS 98-010-5818).15,17 Additionally, the XRD pattern also shows a good fit for the presence of rutile TiO2, as characterized by the diffraction peak at 27.40°.18 The phase fractions were analyzed by XRD profile matching; a 7322
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Figure 2. HR XP spectra of (a) Pt 4f and (b) N 1s of Pt8Ti-TiO2/C.
Figure 3. (a) MOR cyclic voltammograms at a sweep rate of 50 mV s−1. (b) Chronoamperometric responses of Pt8Ti-TiO2/C (27 μgPt cm−2) and Pt/C (56 μgPt cm−2) in a 0.5 M HClO4 + 0.5 M methanol solution at 0.5 V vs RHE.
oxidized (intermediate) products are further oxidized during the backward scan (Ib).10 Thus, the ratio of the forward to the backward peak (If/Ib) can be used to compare the selectivity of the catalysts. A higher If/Ib value indicates a more selective methanol oxidation electrocatalyst. The If/Ib value is 0.88 for Pt8Ti-TiO2/C, while that of Pt/C is approximately 0.72, implying that the Pt8TiTiO2/C catalyst exhibits better CO-poisoning tolerance than the Pt/C catalyst. The onset potential (the potential where the current density increased to ∼4.8 mA mgPt−1) values for Pt8Ti-TiO2/C and Pt/ C catalysts were 0.34 and 0.39 V vs RHE, respectively. A 50 mV positive onset potential shift was observed for Pt8Ti-TiO2/C relative to Pt/C, indicating that even a small amount of Ti incorporated in Pt shifts the anodic onset potential, resulting in significantly enhanced methanol oxidation kinetics;21,22 the additional contribution of the effect of the N-doped carbon in the Pt8Ti-TiO2/C nanorod catalyst makes Pt8Ti-TiO2/C an attractive support material, because it can provide specific anchoring sites for the deposition and dispersion of the catalyst.23,24 The nitrogen-doped carbon support was likely forming during the pyrolysis of polyacrylonitrile. Another effect resulting from the presence of metal species may accelerate the formation of oxygen-containing groups, which are an important factor in the MOR process.25,26 The presence of Ti may modify the d-band center of the Pt metal. According to density functional theory (DFT) calculations, on alloying with Ti, the d-band center shifts up to the Fermi level in comparison to that of the parent Pt metal due to the combination of second metal overlayers with the Pt surface, resulting in an increased absorbate binding energy and promotion of the CO-oxidation performance of the intermetallic surface.21,26,27 The electrochemical active surface area (ECSA) was calculated using the H2 adsorption−desorption region (0−0.3 V vs RHE) in 0.5 M HClO4 at a scan rate of 50 mV s−1 (Figure
Pt8Ti-TiO2/C catalyst. The Pt 4f spectrum was deconvoluted into two peaks. The binding energy centered at 71.2 eV (Pt 4f5/2) is attributed to the characteristic signature of metallic Pt, and a small peak with a binding energy value of 72.3 eV is attributed to the formation of oxide Pt2+ (Figure 2a). A positive shift (0.4 eV) of the Pt 4f5/2 peak in comparison to the pure Pt/C (70.8 eV) peak was observed for the Pt8Ti alloy.4 This positive shift is in good agreement with the results of Ross et al. for the Pt−Ti intermetallic phase.12 The positive shift may reflect the increased number of unfilled 5d states caused by alloying with Ti.11,18 The XPS also showed the presence of nitrogen peaks; therefore, we acquired the HR N 1s spectra as shown in Figure 2b. The N 1s spectra of Pt8Ti-TiO2/C nanorods can be deconvoluted into two peaks with binding energy values of 398.1 eV (N1) and 400.37 eV (N2), assigned to pyridinic-type and pyrrolic-type nitrogen functional groups, respectively19 (Table S1 in the Supporting Information). Furthermore, the Pt8Ti-TiO2/C electrocatalyst showed two peaks at 458.1 and 464.5 eV that can be attributed to the Ti4+ which appears due to TiO2 formation (Figure S5 in the Supporting Information).12,20 The carbon and nitrogen contents in the Pt8Ti-TiO2/C catalyst were found to be 38.52 and 1.38 wt %, respectively, as determined by C, H, and N elemental analysis (Table S2 in the Supporting Information). The electrochemical oxidation of methanol was first evaluated using cyclic voltammetry measurements with a potential window of 0−1.26 V vs RHE in a 0.5 M methanol + 0.5 M HClO4 solution at a sweep rate of 50 mV s−1. Figure 3a compares the MOR activities of Pt8Ti-TiO2/C and commercial Pt/C (20%Pt/C, Premtek); during the forward scan (If), the current increased sharply due to the dehydrogenation of methanol followed by oxidation of the absorbed methanol on the electrode sites; the methanol is not completely oxidized to CO2 during the forward scan, and the remaining incompletely 7323
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Figure 4. CV curves for (a) Pt/C (56 μgPt cm−2) and (b) Pt8Ti-TiO2/C (27 μgPt cm−2) during durability testing for 3000 cycles in N2-saturated 0.5 M HClO4. (c) ECSA based on hydrogen charge vs cycle number.
current density of 2.67 mAmgPt−1 corresponding to 8% decay at the end of the 10000 s time period. These results indicate that Pt8Ti-TiO2/C exhibits better activity and stability. Moreover, the presence of rutile TiO2 in the Pt8Ti-TiO2/C catalyst can prevent the decay of the current, because TiO2 exhibits excellent corrosion resistance in acidic, alkaline, and neutral media.18,29 Furthermore, we have compared the Pt8Ti-TiO2/C and Pt/C electrode stabilities using the cycling durability test by performing repeated potentiodynamic cycling for 3000 cycles with a potential range of 0.1−1.3 V vs RHE at a scan rate of 50 mV s−1 (Figure 4a,b). Figure 4c compares the normalized ECSAs of the two electrocatalysts as a function of cycle number. The fresh electrodes exhibit ECSA values of 58.55 ± 7.18 and 15.31 ± 3.87 m2 gPt−1 for Pt/C and Pt8Ti-TiO2/C, respectively. A similar ECSA value (61.56 m2 g−1) was observed for Pt/VX-72 catalyst by Yao et al.30 Both electrocatalysts, showing a decrease in surface area with increasing accelerated stability test (AST) cycle values. The Pt/C catalyst showed a more rapid decay of 90% at the end of 3000 CV cycles. In contrast, the Pt8Ti-TiO2/C catalyst showed a decay of 21% at the end of 3000 cycles. The significant decrease in the Pt/C ECSA is attributed to the corrosion of the carbon support and agglomeration of Pt nanoparticles. The observed AST result for the Pt/C catalyst is in good agreement with AST results reported by Yao et al. for a commercial Pt/XC-72 catalyst. After 3000 cycles, Yao et al. observed a huge loss (98%) in ECSA for Pt/VX-72 catalyst.30 The smaller decrease in ECSA for the Pt8Ti-TiO2/C catalyst during the potential CV cycling can be attributed to the stability of the rutile TiO2 support, which is chemically resistant in the acidic electrolyte medium.27 Both chronoamperometry and CV results confirmed that the Pt8TiTiO2/C hierarchical catalyst exhibited better stability than the pure Pt/C in a half-cell mode, suggesting that the Pt8Ti-TiO2/ C intermetallic catalyst could be a potential fuel cell electrocatalyst. The ultrahigh stability of the Pt8Ti-TiO2/C catalyst can be attributed to the strong metal−support interaction (SMSI) between Pt8Ti and TiO2 particles.28,31
S6 in the Supporting Information). The obtained ECSA values were 58.55 ± 7.18 and 15.31 ± 3.87 m2 gPt−1 for the Pt/C and Pt8Ti-TiO2/C catalysts, respectively. At a given potential of 0.7 V, the mass activities of Pt8Ti-TiO2/C and Pt/C were 29.28 and 10.58 mA mgPt−1, respectively. The MOR mass activity of the Pt8Ti-TiO2/C catalyst showed a significantly improved value which was about 2.8 times higher than that of Pt/C at 0.7 V vs RHE. The enhancement of Pt8Ti-TiO2/C MOR activity was more evident in the specific activity values obtained by normalizing mass activity (mA mgPt−1) with the ECSA value (m2 gPt−1). The specific activity of Pt8Ti-TiO2/C was 0.191 mA cmPt−2, which is significantly higher than that of Pt/C (0.018 mA cmPt−2). The value for the Pt8Ti-TiO2/C hierarchical nanostructured catalyst was approximately 10 times higher than that of the Pt/C catalyst (Table S1 in the Supporting Information). A straightforward comparison of activity from the reported different research groups is difficult because of different experimental conditions such as scan rate, electrolyte type, and concentration used in their studies. We compared our results with those of Obradovic et al., and they reported a MOR mass activity of 4.6 mA mg−1 at 0.5 V versus a SCE electrode for Pt/Ru0.1Ti0.9O2 catalyst.28 In addition, they reported 9.1 mA mg−1 mass activity for Pt/C commercial catalyst, which is similar to the mass activity observed for the commercial Pt/C catalyst used in the present study. The mass activity of Pt8TiTiO2/C was 29.28 mA mgPt−1, which is much higher than that of the Pt/Ru0.1Ti0.9O2 catalyst reported in the literature.28 The higher mass and specific activity of Pt8Ti-TiO2/C intermetallic catalyst was further confirmed with chronoamperometry (CA) measurements. Figure 3b shows the CA response of Pt8TiTiO2/C and Pt/C catalysts in 0.5 M methanol + 0.5 M HClO4 at 0.5 V vs RHE for 10000 s. The catalysts showed slow current decay approaching the limiting current density until 10000 s. The Pt8Ti-TiO2/C catalyst showed an initial mass activity of 6.33 mAmgPt−1 (at 30 s) and reached a current density of 6.10 mAmgPt−1, corresponding to 4% decay, while Pt/C showed an initial current density of 2.90 mAmgPt−1 and reached a final 7324
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ACS Catalysis The Pt8Ti-TiO2/C nanorod catalyst methanol oxidation activity was evaluated using single-cell DMFC tests. Parts a and b of Figure 5 show the single-cell DMFC performances at 298
Figure 6. Polarization and power density curves of a direct methanol fuel cell operating at (a) 298 K and (b) 353 K using a Pt8Ti-TiO2/C (0.091 mgPt cm−2) and 20% PtRu/C (1:1, Premetek, 0.1 mgPt cm−2) anode catalyst. The cathode was 40% Pt/C (Johnson Matthey, 0.4 mgPt cm−2) with 2 M methanol and O2-feeding mode determined. Figure 5. Polarization and power density curves for direct methanol fuel cells using Pt8Ti-TiO2/C (0.091 mgPt cm−2) and Pt/C as anode (0.189 mgPt cm−2) catalysts. The cathode was 40% Pt/C (0.4 mgPt cm−2, Johnson Matthey) with 2 M methanol and O2-feeding mode operated at (a) 333 K and (b) 298 K.
open active catalyst with easy diffusion of oxidant and reactants, resulting in fast kinetics at the electrode catalyst sites.
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CONCLUSIONS We have successfully synthesized Pt8Ti intermetallic nanoparticle catalysts using the pyrolysis method. In comparison with a commercial Pt/C catalyst, Pt8Ti intermetallic nanoparticles dispersed in carbon nanorods (Pt8Ti-TiO2/C) greatly enhance the MOR activity because of the intermetallic nature of Pt8Ti and the durability due to the presence of corrosionresistant TiO2. When the Pt8Ti-TiO2/C catalyst was used in a single-cell DMFC experiment, it produced higher maximum power density at 298 and 333 K than the Pt/C-based anode catalyst under the same test conditions. This work demonstrated that Pt8Ti-TiO2/C hierarchical nanostructures are indeed a promising new family of catalysts for enhanced MOR in DMFCs.
and 333 K using Pt8Ti-TiO2/C nanorods as an anode catalyst in comparison with the performance of the Pt/C anode catalyst. At 0.35 V, the measured current densities are 7.4 and 2.4 mA cm−2 at 298 K and 35.2 and 7.7 mA cm−2 at 333 K for the Pt8Ti-TiO2/C and Pt/C catalysts, respectively. The Pt8TiTiO2/C anode catalyst showed much higher current densities at all temperatures than the Pt/C anode catalyst. At 0.35 V, the Pt8Ti-TiO2/C electrocatalyst showed 4.6 and 3 times higher power density than the commercial Pt/C catalyst at 333 and 289 K, respectively. The maximum power densities are 15.3 and 9.3 mW cm−2 at 298 K and 48.7 and 31.1 mW cm−2 at 333 K for Pt8Ti-TiO2/C and Pt/C catalysts, respectively. Under the same test conditions, Pt8Ti-TiO2/C anode catalyst exhibited 1.6 times higher maximum power density at both temperatures in comparison with the Pt/C-based anode catalyst. The singlecell DMFC results are in good agreement with the results observed for half-cell methanol oxidation. All performance measurements were carried out several times and were reproducible. In particular, the Pt8Ti-TiO2/C catalyst showed a higher performance than the commercial Pt/C. The single-cell DMFC performance of the Pt8Ti-TiO2/C hierarchical nanostructured catalyst was compared with that of the state-of-the-art PtRu/C (20 wt %) anode catalyst. For DMFC performance, a low catalyst loading of Pt8Ti-TiO2/C showed power density better than or comparable to that of the commercial PtRu/C at 298 and 353 K, indicating that Pt8TiTiO2/C could be a promising alternative DMFC anode electrocatalyst (Figure 6). This represents highly significant progress for the field of DMFCs, due to the achievement of an
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EXPERIMENTAL SECTION Synthesis of Pt8Ti Intermetallic Nanoparticles Dispersed in Carbon Nanorods (Pt8Ti-TiO2/C). We prepared the nonwoven-web-containing metal ions using the electrospinning method. The metal precursors (H2PtCl6·6H2O, 0.28 g, Aldrich, Korea) and titanium(IV) oxyacetylacetonate (TiOacac, 0.2 g, Aldrich, Korea) were completely dissolved in 6.00 g of N,N-dimethylformamide at 363 K. One gram of polyacrylonitrile (PAN, Mw = 150000 g mol−1, Aldrich, Korea) was completely dissolved in DMF at 363 K. The metal precursor and polymer solutions were then mixed together and stirred at 363 K until a clear homogeneous solution was obtained. The solution was electrospun using an electrospinning setup (NanoNC Ltd., Korea) with a traveling distance between the spinneret to collector of 10 cm, a voltage power supply of 18 kV, a volume feed rate of 1.0 mL h−1, and a rotating speed of 300 rpm under a humidity of 30% RH at 293−298 K. The catalyst was prepared by pyrolysis of the composite nonwoven 7325
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(SciTech, Korea). The polarization data were collected point by point, and 1 min was provided for the system to achieve a steady state. The reproducibility of the data was ascertained by repeating the experiments at least twice. All MEAs were evaluated in DMFCs under atmospheric pressure.
web in a tubular quartz tube furnace (Wisd Laboratory Instruments, Korea). The pyrolysis process was carried out in an air atmosphere in which the composite nonwoven web was stabilized at 523 K for 1.5 h to remove the organic materials and the heating then continued to 1173 K for 3 h under an Ar/ H2 atmosphere (5% H2). Electrochemical Methanol Oxidation Studies. Electrochemical methanol oxidation was carried out by using cyclic voltammetry (CV) with a three-electrode system using a potentiostat (Bio-Logic, VSP, France). The three-electrode glass cell consisted of a glassy-carbon (3 mm, Φ), Pt-wire and Ag/AgCl (saturated KCl) electrodes acting as the working (0.07 cm2), counter and reference electrodes, respectively. All potentials reported in the RHE scale were converted from the SCE scale using a calibration (Figure S1). The working electrode was fabricated by dispersing 5 mg of catalyst in a 250 mL mixture of isopropyl alcohol, water, and 5 μL of Nafion ionomer (5%) by ultrasonication for 20 min. From this dispersion, the catalyst ink was coated on a clean glassy-carbon electrode with a catalyst loading of 0.3 mg cmgeo−2. Electrochemical methanol oxidation was carried out at room temperature in a 0.5 M methanol + 0.5 M HClO4 solution with a potential window of 0−1.26 V vs RHE at a sweep rate of 50 mV s−1. Calculation of Electrochemical Surface Area (ECSA). The Coulomb charge for hydrogen (QH) was used to calculate the active platinum surface of the electrodes, and the value of QH was calculated from the electrodesorption and electroadsorption of H2 on Pt sites by averaging these two values. The electrocatalyst ECSA was calculated using ECSA =
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01390. Additional experimental details and additional SEM, TEM, ECSA and DMFC tests using Pt−Ru/C (PDF)
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AUTHOR INFORMATION
Corresponding Author
*S.S.: e-mail,
[email protected]; fax, (+82) 53-785-6409. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (15BD-01) for financially supported.
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REFERENCES
(1) Debe, M. K. Nature 2012, 486 (7401), 43−51. (2) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.-I.; Iwashita, N. Chem. Rev. 2007, 107 (10), 3904−3951. (3) Ganesan, R.; Lee, J. S. Angew. Chem., Int. Ed. 2005, 44 (40), 6557−6560. (4) Zhou, Y.; Neyerlin, K.; Olson, T. S.; Pylypenko, S.; Bult, J.; Dinh, H. N.; Gennett, T.; Shao, Z.; O’Hayre, R. Energy Environ. Sci. 2010, 3 (10), 1437−1446. (5) Xiang, D.; Yin, L. J. Mater. Chem. 2012, 22 (19), 9584−9593. (6) Abe, H.; Matsumoto, F.; Alden, L. R.; Warren, S. C.; Abruña, H. D.; Di Salvo, F. J. J. Am. Chem. Soc. 2008, 130 (16), 5452−5458. (7) Piela, P.; Eickes, C.; Brosha, E.; Garzon, F.; Zelenay, P. J. Electrochem. Soc. 2004, 151 (12), A2053−A2059. (8) Chung, Y.; Pak, C.; Park, G. S.; Jeon, W. S.; Kim, J. R.; Lee, Y.; Chang, H.; Seung, D. J. Phys. Chem. C 2008, 112 (1), 313−318. (9) Xiao, S.; Xiao, F.; Hu, Y.; Yuan, S.; Wang, S.; Qian, L.; Liu, Y. Sci. Rep. 2014, 4, 4370. (10) Kang, Y.; Pyo, J. B.; Ye, X.; Gordon, T. R.; Murray, C. B. ACS Nano 2012, 6 (6), 5642−5647. (11) Kang, S. J.; Lee, J. Y.; Lee, J. K.; Chung, S. Y.; Tak, Y. S. J. Phys. Chem. B 2006, 110 (14), 7270−7274. (12) Beard, B. C.; Ross, P. N. J. Phys. Chem. 1986, 90 (1), 6811− 6817. (13) Liu, X.-J.; Cui, C.-H.; Gong, M.; Li, H.-H.; Xue, Y.; Fan, F.-J.; Yu, S.-H. Chem. Commun. 2013, 49 (77), 8704−8706. (14) Alden, L. R.; Han, D. K.; Matsumoto, F.; Abruna, H.; Disalvo, F. Chem. Mater. 2006, 18 (23), 5591−5596. (15) Taylor, R. H.; Curtarolo, S.; Hart, G. L. W. J. Am. Chem. Soc. 2010, 132 (19), 6851−6854. (16) Dreele, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory: Los Alamos, NM, 2004; Report LAUR 86-748, Vol. 748. (17) Pietrokowsky, P. Nature 1965, 206, 291. (18) Kim, S. H.; Jung, C. H.; Sahu, N.; Park, D.; Yun, J. Y.; Ha, H.; Park, J. Y. Appl. Catal., A 2013, 454, 53−58.
QH [Pt ] × 0.21
where QH is the hydrogen charge (mC cm−2), [Pt] represents the platinum loading (mg cm−2) in the electrode, and 0.21 (mC cm−2) represents the charge required to oxidize a monolayer of H2 on bright Pt. MEA Fabrication and DMFC Performance Evaluation. The single-cell DMFC performance of the synthesized catalyst was evaluated by fabricating membrane electrode assemblies (MEA). Gas diffusion layers (GDL, SGL, thickness 0.27 mm) were used as the backing layers. For the cathode catalyst layers, 40 wt % Pt/C catalyst (Johnson Matthey) was mixed with 30 wt % Nafion ionomer and isopropyl alcohol. The slurry was then ultrasonicated for 30 min. The resultant slurries were coated onto the GDL. For the fabrication of the anode catalyst layer, the synthesized catalyst was dispersed in isopropyl alcohol and 30 wt % of Nafion and the slurry was coated in a similar manner. Catalyst loading on both the anode and cathode was kept at 1.0 mg cm−2. The DMFC active area was 5 cm2. A thin layer of Nafion ionomer was applied to the catalyst surface of both electrodes. MEAs were obtained by sandwiching the membrane (Nafion-117) between the cathode and anode followed by hot compaction under a pressure of 30 kg cm−2 at 130 °C for 2 min. MEAs were coupled with Teflon gas-sealing gaskets and placed in single-cell test fixtures with a parallel serpentine flow field machined on graphite plates. The DMFC was maintained by passing 2.0 M methanol and oxygen gases to its anode and cathode sides, respectively, at a flow rate of 300 sccm set by a mass flow controller. After equilibration, measurements of cell potential as a function of current density were conducted galvanostatically using a fuel cell test station 7326
DOI: 10.1021/acscatal.5b01390 ACS Catal. 2015, 5, 7321−7327
Research Article
ACS Catalysis (19) Shanmugam, S.; Osaka, T. Chem. Commun. 2011, 47 (15), 4463−4465. (20) Wang, M.; Guo, D.; Li, H. J. Solid State Chem. 2005, 178 (6), 1996−2000. (21) He, Q.; Shyam, B.; Nishijima, M.; Yang, X.; Koel, B.; Ernst, F.; Ramaker, D.; Mukerjee, S. J. Phys. Chem. C 2013, 117 (3), 1457−1467. (22) Levy, O.; Hart, G. L. W.; Curtarolo, S. J. Am. Chem. Soc. 2010, 132 (13), 4830−4833. (23) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332 (6028), 443−447. (24) Pylypenko, S.; Borisevich, A.; More, K. L.; Corpuz, A. R.; Holme, T.; Dameron, A. a.; Olson, T. S.; Dinh, H. N.; Gennett, T.; O’Hayre, R. Energy Environ. Sci. 2013, 6 (10), 2957−2964. (25) Liu, Z.; Jackson, G. S.; Eichhorn, B. W. Energy Environ. Sci. 2011, 4 (5), 1900−1903. (26) Pang, C. L.; Lindsay, R.; Thornton, G. Chem. Rev. 2013, 113 (6), 3887−3948. (27) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L. J. Mol. Catal. A: Chem. 1997, 115 (3), 421−429. (28) Obradovic, M. D.; Lacnjevac, U. C.; Babic, B. M.; Ercius, P.; Radmilovic, V. R.; Krstajic, N. V.; Gojkovic, S.Lj. Appl. Catal., B 2015, 170−171, 144−152. (29) Zhao, X.; Zhu, J.; Liang, L.; Liao, J.; Liu, C.; Xing, W. J. Mater. Chem. 2012, 22 (37), 19718−19725. (30) Yao, C.; Li, F.; Li, X.; Xia, D. J. Mater. Chem. 2012, 22 (32), 16560−16565. (31) Shanmugam, S.; Gedanken, A. J. Phys. Chem. C 2009, 113 (43), 18707−18712.
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DOI: 10.1021/acscatal.5b01390 ACS Catal. 2015, 5, 7321−7327
