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J. Phys. Chem. C 2008, 112, 8372–8380
Temperature Dependence of Oxygen Reduction Activity at Carbon-Supported PtXCo (X ) 1, 2, and 3) Alloy Catalysts Prepared by the Nanocapsule Method Hiroshi Yano,† Jung Min Song,† Hiroyuki Uchida,‡ and Masahiro Watanabe*,† Clean Energy Research Center, UniVersity of Yamanashi, Takeda 4, Kofu 400-8510, Japan, and Interdisciplinary Graduate School of Medicine and Engineering, UniVersity of Yamanashi, Takeda 4, Kofu 400-8511, Japan ReceiVed: December 22, 2007; ReVised Manuscript ReceiVed: March 13, 2008
Monodispersed PtXCo (atomic ratio, X ) 1, 2, and 3) alloy nanoparticles supported on carbon black (CB, 50 wt % metal loading) were prepared by the nanocapsule method. The average particle diameters (d) measured by scanning transmission electron microscopy ranged from 1.9 (X ) 3) to 2.4 nm (X ) 1). The alloy composition was found to be well-controlled to the projected value among the supported particles. Oxygen reduction reaction (ORR) activity and H2O2 yield at Nafion-coated PtXCo/CB catalysts were examined in O2-saturated 0.1 M HClO4 solution at 30 to 100 °C by using the multichannel flow double electrode (MCFDE) method. In the temperature range from 30 to 80 °C, the value of apparent rate constant kapp (per real active surface area) for the ORR at X ) 3 (d ) 1.9 nm) was the largest, by a factor of 2.2 compared with that of Pt/CB (50 wt % Pt, d ) 2.6 nm), i.e., the mass activity of Pt was expected to be enhanced by 3.3 times due to an advantage of higher specific surface area. The Pt3Co/CB also exhibited the highest stability for an immersion test in air-saturated H2SO4 at 70 °C. 1. Introduction Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential cogeneration systems. However, both the anode and the cathode of PEFCs usually require platinum or its alloys as the catalyst, having a high activity at low operating temperatures 0.35 V. Thus, the Nafion coating on Pt is the major reason for triggering H2O2 production. It has been proposed that strongly adsorbed species such as organic adsorbates or halide anions on Pt could induce the H2O2 production,15,43–45 since the dissociation of adsorbed O2 might be blocked by them. Sulfonate groups in Nafion could be possible species strongly adsorbed on the Pt surface. Such a specific adsorption of SO3- on Pt(bulk) electrode was observed by in situ FTIR.46 For the Nafion-Pt3Co/CB, the adsorption of SO3- could be weakened by the modified electronic structure at the Pt skin layer.15 However, at the temperature ranger higher than 60 °C for X ) 2 and ca. 80 °C for X ) 1 and 3, the P(H2O2) increased and reached the same value as that for Nafion-Pt/ CB. Such changes in both P(H2O2) and SA with the temperature described above suggest that the surface of PtXCo particles becomes more “Pt-like”. We will discuss this in detail later.
Carbon-Supported PtXCo Alloy Catalysts
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3.4. ORR Activity. The kinetically controlled current IK at a given potential E is determined from the hydrodynamic voltammograms in the M-CFDE by using the following equation
1/I ) 1/IK + 1/IL ) 1/IK + 1/{1.165nF[O2]w(UmD2x12/h)1/3} (2) where n is the number of electrons transferred, F is the Faraday constant, [O2] is the O2 concentration in the bulk of the electrolyte solution, w is the width of the working electrode, Um is the mean flow rate of the electrolyte solution, D is the diffusion coefficient of O2, x1 is the length of the working electrode in the electrolyte flow direction, and h is the halfchannel height. An example of I-1 vs Um-1/3 plots for the ORR on Nafion-PtCo/CB electrode is shown in Figure S2 in the Supporting Information. Linear relationships between I-1 and Um-1/3 were seen at all of the potentials of 0.80, 0.76, and 0.70 V at all the electrodes. By extrapolating Um-1/3 to 0 (infinite flow rate), the value of IK was calculated. As we reported previously, the value of IK is dependent on the change in the [O2], which decreases with elevating the operation temperature. Since the contribution of two-electron reduction to produce H2O2 to the overall ORR was very low, we can calculate an apparent rate constant kapp at a constant overpotential η from the standard potential E° (η ) E - E°) over the whole operating temperature region from 30 to 100 °C, in the same manner as our previous works,15,33,34,36
IK/(4FS) ) -kapp[H+][O2] [H+]
(3)
H+
where is the bulk concentration of (0.1 M) and S is the active surface area of PtXCo at each temperature, electrochemically determined with CVs in Figure 5. Figure 8 shows Arrhenius plots for the kapp per real surface area at various electrodes. Since both the E° and E[RHE(t)] shift to less positive values in a different manner, the corrected potential E is applied so as to keep a constant overpotential for the ORR at each temperature.15,36 In the low-temperature region (30-60 °C for PtCo/CB, 30-70 °C for Pt2Co/CB, and 30-80 °C for Pt3Co/CB), linear relationships between log kapp and 1/T are observed, corresponding to the following Arrhenius equation.
kapp)Z exp(-εa/RT)
(4)
The value of kapp (area specific value) on the PtXCo/CB is larger than that on the Pt/CB by a factor of 1.5 (X ) 1), 1.6 (X
Figure 8. Arrhenius plots of the apparent rate constant kapp for the ORR at Nafion-PtXCo/CB with X ) 1 (O), 2 (∆), and 3 (0) electrodes. Dotted lines indicate the least-squares fitting data for the Nafion-Pt/CB electrode. The overpotential of -0.485, -0.525, and -0.585 V vs E° corresponds to 0.80, 0.76, and 0.70 V vs RHE at 30 °C, respectively.
TABLE 2: Preparation Condition of Suspensions and Resulting wPt-Co and wCB by Pipetting the Suspension with 12.5 µL/cm2 on an Au Substrate catalsyts
CPtCo/CBa (g/L)
wPtCob (µg/cm2)
wPtc (µg/cm2)
wCBd (µg/cm2)
PtCo/CB Pt2Co/CB Pt3Co/CB
0.80 0.85 0.85
4.77 5.00 4.93
3.62 4.32 4.45
5.23 5.62 5.69
a Amount of PtXCo/CB catalysts in the suspension. b Amount of PtXCo attached on the Au substrate. c Amount of Pt attached on the Au substrate. d Amount of CB attached on the Au substrate.
) 2), and 2.2 (X ) 3), respectively. In addition to such an enhanced specific activity, additional advantages of PtXCo/CB were higher specific surface area SA (with smaller d) and lower Pt content per mass of metal loaded on the CB. For example, the SA of Pt3Co/CB was about 1.4 times (factor of 2.6/1.9) higher than that of the commercial Pt/CB, and the Pt content in the alloy was 90 wt %. Taking the amount of Pt contained in each working electrode (Table 2) into account, the highest mass activity of Pt (A/gPt) was obtained on Pt3Co/CB, which was 3.3 times higher than that of commercial Pt/CB. An apparent activation energy a on each alloy was found to be 40 kJ/mol, which is comparable to that on the Pt/CB electrode of 38 kJ/mol. This indicates that the high ORR activities at the PtXCo alloys are ascribed to the large pre-exponential factor in the rate constant. Our recent EC-XPS study indicates that the modified electronic state at the Pt skin layer formed on the alloy may increase the coverage of adsorbed oxygen species.47 However, the kapp values of the PtXCo rather decrease with elevating temperature above 60 (at PtCo/CB), 70 (at Pt2Co/CB), and 80 °C (at Pt3Co/CB), and settle at almost the same value as the Pt/CB electrode at each the temperatures. It is also noted that the temperature for losing the high ORR activity corresponds well with that of reduction in SA (Figure 5) and that of increasing the H2O2 production activity (Figure 7). These results indicate that the Pt skin layer formed on the PtCo, Pt2Co, and Pt3Co alloy nanoparticles by the potential cycle can stand at temperatures 80 °C, these alloy catalysts behaved as “bulk Pt-like” ones with respect to the ORR activity and H2O2 yield. This is certainly ascribed to a further dealloying of the Co component in a hot acid solution, resulting in a thick Pt layer, the electronic state of which is no longer modified by the underlying alloy. Among the alloy catalysts examined, the Pt3Co/CB showed the highest stability against dissolution of Co in air-saturated H2SO4 at 70 °C. Relatively high stability of our PtXCo/CB catalysts is certainly attributed to the high alloying level, well-controlled composition, and particle size in nanosize space through the synthesis. The present research can contribute to find a clue for designing catalysts for high-performance PEFCs. Acknowledgment. This work was supported by the fund for “Leading Project” of the Ministry of Education, Science, Culture, Sports and Technology of Japan, and KAKENHI (19760489). Supporting Information Available: Supporting Information contains electrochemical data for all the alloys. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750. (2) Toda, T.; Igarashi, H.; Watanabe, M. J. Electroanal. Chem. 1999, 460, 258. (3) Toda, T.; Honma, I. Trans. Mater. Res. Soc. Jpn. 2003, 28, 215. (4) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. J. Electrochem. Soc. 1995, 142, 1409. (5) Mukerjee, S.; Srinivasan, S. J. Electroanal. Chem. 1993, 357, 201. (6) Min, M.; Cho, J.; Cho, K.; Kim, H. Electrochim. Acta 2000, 45, 4211. (7) Neergat, N.; Shukla, A. K.; Gandhi, K. S. J. Appl. Electrochem. 2001, 31, 373. (8) Stamenkovic, V.; Schmidt, T. J.; Markovic, N. M., Jr.; Ross, P. N. J. Phys. Chem. B 2002, 106, 11970. (9) Stamenkovic, V.; Schmidt, T. J., Jr.; Ross, P. N.; Markovic, N. M. J. Electroanal. Chem. 2003, 554-555, 191. (10) Drillet, J.-F.; Ee, A.; Friedemann, J.; Ko¨tz, R.; Schnyder, B.; Schmidt, V. M. Electrochim. Acta 2002, 47, 1983. (11) Yang, H.; Vogel, W.; Lamy, C.; Alonso-Vante, N. J. Phys. Chem. B 2004, 108, 11024. (12) Paffett, M. T.; Berry, J. G.; Gottesfeld, S. J. Electrochem. Soc. 1988, 135, 1431. (13) Thamizhmani, G.; Capuano, G. A. J. Electrochem. Soc. 1994, 141, 968.
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